charge transport in organic electronic ......1.2.1 molecular rectifiers.....3 1.3 organic...
TRANSCRIPT
CHARGE TRANSPORT IN ORGANIC ELECTRONIC DEVICES: FROM NANO TO MACRO-SCALE
BY
ZACHARY ALAN LAMPORT
A Dissertation Submitted to the Graduate Faculty of
WAKE FOREST UNIVERSITY GRADUATE SCHOOL OF ARTS AND SCIENCES
in Partial Fulfillment of the Requirements
for the Degree of
DOCTOR OF PHILOSOPHY
Physics
May 2018
Winston-Salem, North Carolina
Copyright Zachary A. Lamport 2018
Approved By:
Oana D. Jurchescu, Ph.D., Advisor
Mark E. Welker, Ph.D., Chair
Martin Guthold, Ph.D.
Jed C. Macosko, Ph.D.
Timo Thonhauser, Ph.D.
ii
Acknowledgements
There are so many people that I would like to thank for helping me get to where I am today. First on the list is my advisor and mentor, Dr. Oana Jurchescu. She has had an incredibly positive influence on each facet of my life, starting with allowing me to join her group, to learn, and especially to grow under her direction. Her patience, kindness, and understanding made a world of difference to me as I struggled through difficult situations in my life. I am particularly grateful for her entrusting so many wonderful undergraduate students to me which has simply been an awesome experience for me to learn as a teacher and mentor for my own students. One of the remarkable things about Oana is that she can tell when someone will fit in well with our group, and through this she has put together what is certainly the best, in more ways than one, group at Wake Forest. And of course, I must thank the undergraduate students with whom I have been so very fortunate to work. First among them was Andy Vuong, and I feel that my thanks to him almost requires an apology, as I was still very much learning how to teach my own students, however he deserves my gratitude for all his hard work. Next was Katrina Barth, who worked with me for all four years of her undergraduate degree and I truly cannot say enough about. It has been a great pleasure to watch, and hopefully to help, her growth through the years. Her undying enthusiasm and cheerfulness made each day she was in the lab seem a little brighter, and for that alone I must thank her. Beyond those qualities, her determination and dedication to her work was inspiring to see and I know she is destined for great things both during and after her graduate studies at Duke. The work shown in Chapters 2 and 5 would not have been possible without her, especially impressive considering those are two radically different types of experiments. Following her, Ben Scharmann joined our group and I couldn’t have been happier to have him as a student. His sense of humor and, just generally being a character, made him a welcome addition to an already somewhat eccentric mixing of personalities in our lab group. He has also taken on the responsibility of training a new undergraduate himself which is quite impressive along with his other duties. But of course, he has done tremendous work in the two years that he has worked with me on the molecular rectifier project and the work shown in Chapter 3 shows the fruits of his labors, and his future success at the Georgia Tech campus in Shenzhen, China is assured. The last is Robby Bradford, and unfortunately he and I haven’t had much time together in the lab as Ben was his main instructor, but I can see that he is a quick study and look forward to working with him over this summer. I would be remiss if I did not also thank the students that had come before me. Eric Chapman, in his dual role as lab manager and graduate student, has been a tremendous resource for those of us in the department as well as a genuinely good person to talk to. He has always been willing to help with any question or problem that I had, both in research and in my TA work, and for that I am
iii
forever grateful. Dr. Jeremy Ward was a great influence on me as he spent so much time teaching me early in my degree, but also he taught me how to teach, which has been invaluable when put into practice with my own students. He had also set a terrific example in how to be an efficient researcher and how to make the most of the time that you have. Dr. Katelyn Goetz was also instrumental in my early career as she taught me everything that I know about the single-crystal growth and fabrication process, for which I am eternally grateful. She was also just a generally fun person to be around in our lab and office and her effect on the group can still be seen today. Dr. Yaochuan Mei provided a sterling example of how creativity in the lab, and the willingness to try everything, can result in fantastic advances in our research. Last but far from least is Dr. Peter Diemer, who after graduating has stayed with our group and really kept the whole place together. Pete has been a great friend through the 6 years that I have known him, and it has been a great experience getting to know both him and his family. His willingness to support me in both research and personally will never be forgotten and I don’t know if I could ever express how much he has helped over the years. And of course, I must thank the students that are still with us, first being Andrew Zeidell. He has become a great friend to me over these past three years and it has been a pleasure getting to know him both inside and out of the lab. He has continued on with the tradition of keeping a sense of humor around the lab and office (whether it is appreciated or not!), but also has an enviable determination with which he pursues his research projects. Then came Colin Tyznik and Hamna Haneef, who, even though they joined the group together, couldn’t be more different. I’ve greatly appreciated conversations with Colin about, well, nearly everything but it has been awesome to find someone who shares my love of fantasy books and will listen to me expound on one series or another. I’m glad to see that he is taking his research in a different direction than the FET which we have come to know and love, because I had done much the same thing and I like to see our group expand its focus. It has been great getting to know Hamna, whose (evil?) sense of humor has made these past two years a ton of fun. I only hope that she is more patient than I was as she is working on her research into single crystal semiconductors. And the last member to join our group is Matthew Waldrip, who has only been with us for about a year but has already been a great addition to the group and I look forward to great things from him in the future. Then I also must thank my brother Brendan and my parents Bert and Janet. They were with me every step of the way, providing inspiration throughout and, often enough in the case of my mom, a much-needed sounding board. There is absolutely no way that I would have made it to where I am today without their constant love and encouragement and the depth of my gratitude cannot be accurately expressed.
iv
Table of Contents
List of figures .................................................................................................................... vi
List of tables.................................................................................................................... viii
List of abbreviations ........................................................................................................ ix
Abstract ...............................................................................................................................x
Chapter 1 Introduction................................................................................................1
1.1 Electronic Function through Chemistry .......................................................1 1.2 Molecular Electronics ..................................................................................2
1.2.1 Molecular Rectifiers.........................................................................3 1.3 Organic Field-effect Transistors ..................................................................7
1.3.1 OFET Structure and Operation ........................................................8 1.3.2 FET Analysis .................................................................................13 1.3.3 Charge Injection in OFETs ............................................................15 1.3.4 Contact Modification .....................................................................19 1.3.5 Contact Resistance in OFETs ........................................................20
1.4 Thesis Outline ............................................................................................23 References ..............................................................................................................25
Chapter 2 Fluorinated Benzalkylsilane Molecular Rectifiers ...............................33
2.1 Introduction ................................................................................................34 2.2 Experimental ..............................................................................................36
2.2.1 Molecular Structures and Analysis ................................................36 2.2.2 Device Fabrication .........................................................................39 2.2.3 Surface Analysis ............................................................................40 2.2.4 Ab Initio Calculations ....................................................................43 2.2.5 Electrical Characterization .............................................................44
2.3 Results and Discussion ..............................................................................44 2.4 Conclusions ................................................................................................50 References ..............................................................................................................52
Chapter 3 Molecular Diodes from Self-Assembled Monolayers on Silicon ..........56
3.1 Introduction ................................................................................................57 3.2 Experimental ..............................................................................................60
3.2.1 Device Fabrication .........................................................................60
v
3.2.2 Electrical Characterization .............................................................62 3.2.3 Cyclic Voltammetry .......................................................................62
3.3 Results and Discussion ..............................................................................63 3.4 Conclusions ................................................................................................68 References ..............................................................................................................69
Chapter 4 Organic Thin Films with Charge Carrier Mobility Exceeding that of Single Crystals ..............................................................................72
4.1 Introduction ................................................................................................73 4.2 Experimental ..............................................................................................76
4.2.1 Single Crystal Growth....................................................................76 4.2.2 OFET Fabrication and Electrical Characterization ........................77 4.2.3 Theoretical Calculations ................................................................78 4.2.4 Structural Characterization ............................................................78
4.3 Results and Discussion ..............................................................................79 4.4 Conclusions ................................................................................................89 References ..............................................................................................................91
Chapter 5 Organic Thin-Film Transistors with Charge-Carrier Mobilities of 20 cm2/Vs, Independent of Gate Voltage ...........................................96
5.1 Introduction ................................................................................................97 5.2 Experimental ............................................................................................100
5.2.1 Device Fabrication .......................................................................100 5.2.2 Device Characterization ...............................................................101 5.2.3 Grazing Incidence X-Ray Diffraction ..........................................101
5.3 Results and Discussion ............................................................................102 5.4 Conclusions ..............................................................................................112 References ............................................................................................................114
Curriculum Vitae ...........................................................................................................119
vi
List of Figures
1.1 Device structures on the two relevant length scales ....................................1 1.2 Electrical characteristics for a molecular rectifier .......................................5 1.3 Mechanism of rectification ..........................................................................6 1.4 OFET device structures................................................................................7 1.5 Operation of an OFET from linear to saturation ..........................................9 1.6 Transfer characteristics of an OFET ..........................................................11 1.7 Injection in a p-type OFET ........................................................................16 1.8 Gated-TLM measurement ..........................................................................22
2.1 Chemical structures of fluorinated benzalkylsilanes .................................36 2.2 General synthesis of fluorinated benzalkylsilanes .....................................39 2.3 Contact angle measurements......................................................................41 2.4 AFM measurement on silicon ....................................................................43 2.5 Device structure of molecular rectifiers and electrical measurements ......45 2.6 Representative measurements on SAMs ....................................................47 2.7 Rectification ratio vs. dipole moment ........................................................48
3.1 Chemical and device structures .................................................................61 3.2 Average J-V curves ....................................................................................63 3.3 Histograms of measured rectification ratios ..............................................65 3.4 J-V characteristics of native SiO2 ..............................................................66 3.5 Cyclic voltammetry measurements ............................................................67
4.1 Chemical structure of TMS-BT .................................................................74 4.2 PVT setup and device structure .................................................................76 4.3 Electrical characteristics of single-crystal OFETs of TMS-BT .................80 4.4 Histogram of single-crystal OFET mobilities ............................................81 4.5 Band structure of TMS-BT ........................................................................82 4.6 DFT calculations of TMS-BT ....................................................................83 4.7 GIXD on single crystals .............................................................................84 4.8 Illustration of device structure in reference to crystal structure ................85 4.9 Thin-film OFET and electrical characteristics ...........................................86 4.10 XRD on single crystals and thin films .......................................................87 4.11 Thin-film XRD assuming (001) texture .....................................................88 4.12 Thin-film XRD assuming (02-1) texture ...................................................88
5.1 Chemical structure of diF-TES ADT and electrical characteristics of thin-
film OFETs ..............................................................................................102 5.2 Alternate OFET characteristics ................................................................103
vii
5.3 Mobility vs. gate voltage..........................................................................104 5.4 Mobility and contact resistance vs. deposition rate .................................105 5.5 GIXD measurements and molecular orientation of diF-TES ADT .........106 5.6 Device structure and contact resistance illustration .................................107 5.7 AFM measurements of Au deposited at slow and fast rates ....................108 5.8 Fast Fourier transforms of AFM images ..................................................109 5.9 KPFM measurements of PFBT-treated Au at slow and fast rates ...........110 5.10 Chemical structure of C16IDT-BT and electrical characteristics of thin-film
OFETS .....................................................................................................111
viii
List of Tables
2.1 Properties of SAMs composed of compounds 1-9 ....................................42
ix
List of Abbreviations
Abbreviation Meaning
SAM Self-assembled monolayer
HOMO Highest occupied molecular orbital
LUMO Lowest unoccupied molecular orbital
R Rectification ratio
MO Molecular orbital
EF Fermi level
OPV Organic photovoltaic
OLED Organic light-emitting diode
OFET Organic field-effect transistor
µ Field-effect mobility
DOS Density of states
WF Work function
CT Charge transfer
KPM Kelvin probe microscopy
RC Contact resistance
RCh Channel resistance
TLM Transmission line method
DFT Density functional theory
XRD X-ray diffraction
EGaIn Eutectic gallium-indium
AFM Atomic force microscopy
TMS-BT 7,14-bis(trimethylsilylethynyl) benzo[k]tetraphene
PVT Physical vapor transport
GIXD Grazing incidence x-ray diffraction
diF-TES ADT 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene
SKPM Scanning Kelvin probe microscopy
x
Abstract
The electrical properties of devices based on an organic compound result from the structure of the molecules, their solid-state packing, efficiency of charge injection from the electrodes, and the fabrication procedures. The length scales of interest can also vary widely, ranging from a few nanometers in the case of charge transport through single molecules or two-dimensional molecular ensembles, to tens of micrometers in devices focusing on thin films or molecular crystals. The work outlined in this thesis examines the characteristics of electronic devices at both extremes by incorporating organic molecules in molecular rectifiers and organic field-effect transistors (OFETs).
We successfully designed and fabricated molecular rectifiers based on self-assembled monolayers and identified relevant structure-function relationships. We elucidate the dependence of the rectification behavior on molecular length and structure, and found that the degree of rectification is enhanced in shorter molecules and linearly dependent on the strength of the molecular dipole moment. We further developed compounds that, when included into the molecular diodes, rectified current by as much as three orders of magnitude depending on their structure. This performance is on par with that of the best molecular rectifiers obtained on a metallic electrode, but it has the advantage of lower cost and more efficient integration with current silicon technologies, which may yield hybrid systems that can expand the use of silicon towards novel functionalities governed by the molecular species grafted onto its surface.
We then explored charge transport in OFETs using the organic semiconductor 7,14-bis(trimethylsilylethynyl)benzo[k]tetraphene (TMS-BT). We produced thin-film OFETs which exhibited more efficient electronic transport than single crystal devices of the same material, in spite of the inherent presence of grain boundaries. We explained these findings in terms of charge transport anisotropy and electronic trap formation at the interface between the semiconductor and dielectric. We further reduced aggressively the contact resistance in small molecule and polymer OFETs by varying the metal deposition rate, which resulted in over 5 times improved charge carrier mobility compared with the best reported devices with identical composition and structure. The obtained contact resistance normalized over the channel width was 500 Ωcm, and the corresponding devices exhibited charge carrier mobilities of 19.2 cm2/Vs for 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT) and 10 cm2/Vs for indacenodithiophene-co-benzothiadiazole copolymer (C16IDTBT), with minimal dependence on the gate voltage.
1
Chapter 1 Introduction
1.1 Electronic Function through Chemistry
The synthetic versatility of organic chemistry offers a seemingly limitless variety of
electronic properties and possible device configurations based on covalent bonds and van
der Waals interactions. The reactivity, bonding mechanisms, band gap, and surface energy,
among other properties, can, in principle, be tuned at will by altering the chemical structure.
The work incorporated in this thesis focused on the study of various electrical properties
of such molecules. In each of the compounds discussed here, the feature of interest is the
presence of delocalized, π-conjugated electrons which allows access to the frontier electron
orbitals. Some electronic materials investigated here can covalently bond to a substrate in
order to examine their properties on the molecular scale, i.e. single molecule charge
transport, see for example Figure 1.1a. Other compounds form molecular crystals such that
the π-electrons are delocalized across multiple molecules. This aggregation gives rise to
a) b)
Figure 1.1. (a) Example device structure on the molecular scale, (b) Example device structure
spanning the nanoscale to the microscale.
2
interesting properties, which are highly sensitive to the solid-state packing both at the nano-
scale, and the long-range order up to micrometers, see Figure 1.1b. In the first example,
the necessary presence of a σ-bonded alkyl chain allows the exploitation of a molecular
imbalance and the creation of electronically asymmetric devices, as discussed briefly in
Section 1.2 and more in depth in Chapters 2 and 3. The second example is fundamental to
the formation of intrinsic organic semiconductors, the use of which is examined broadly in
Section 1.4, and more specifically in Chapters 4 and 5. In summary, this work will study
molecular electronic devices and organic electronic devices. In the first class, electronic
transport takes place through a single molecular layer, whereas in the second charges move
through an organic semiconductor film consisting of an aggregate of molecules, thus it is
a function of both intramolecular and intermolecular interactions.
1.2 Molecular Electronics
As commercial electronics research has focused on increasing the density of devices on a
microchip, the ultimate goal for the downsizing of electronics is the use of a single
molecule as an active component. Moore’s law, the empirical relation stating that the
density of transistors doubles approximately every two years, is nearing the limits of the
capabilities provided by current, silicon-based electronics. As a fledgling technology,
research into molecular electronics has grown in many directions since the first
experimental proof-of-concept in 1995 [1], but has yet to be adopted in real-world
applications. The most prevalent molecular-scale electronic device in the literature is the
rectifier, with examples of both single-molecule devices [2–7] as well as molecular
ensembles [8–15]. While the behavior of single-molecule devices is by no means
irrelevant, here the focus is on molecular ensembles as this represents a more application-
3
driven technology. The main structure of interest is the self-assembled monolayer (SAM),
which is a single layer of molecules covalently bonded to a substrate. This type of
architecture is relatively straightforward to work with since, regardless of the deposition
method, a single molecular layer will form due to the carefully designed chemical
structures. The compounds are formulated to bond to a surface, but once bonded to the
substrate there are no available bonding sites remaining for additional molecules on top.
Any remaining molecules will only be held to the surface through electrostatic interactions
and can be easily removed through physical means, leaving behind the strongly bound
layer. Once formed, the difficulty therein lies with electrically contacting a single
molecular layer without causing irreparable damage.
1.2.1 Molecular Rectifiers
Conduction through a nanometer-scale organic insulator occurs through a tunneling
mechanism which can be approximated by a simplified Simmons model [16–18]:
𝐽𝐽 = 𝐽𝐽0𝑒𝑒−𝛽𝛽𝛽𝛽 (1.1)
where J is the current density, J0 is the extrapolated zero-distance tunneling current, β is
the tunneling decay constant, and d is the distance between electrodes. This mechanism,
however, assumes that tunneling is the only effect by which charges can transfer from one
electrode to the other. In the landmark publication by Aviram and Ratner [19], it was
suggested that molecular-scale junctions can facilitate charge transport of a more complex
variety. In their example, electron donor and electron acceptor units were connected by a
σ-bonded tunneling bridge to keep separate the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO). The highly anisotropic
4
electronic levels were hypothesized to allow a different amount of current at opposing
biases, an effect known as rectification with the figure of merit being the rectification ratio
R:
𝑅𝑅 = 𝐽𝐽𝑓𝑓𝑓𝑓𝛽𝛽𝐽𝐽𝑟𝑟𝑟𝑟𝑟𝑟
(1.2)
with Jfwd and Jrev the forward and reverse bias current density, respectively. An example of
rectifier J-V characteristics is provided on a linear scale in Figure 1.2a and a logarithmic
scale in Figure 1.2b.
The first experimental evidence of such an effect was introduced by the Metzger
group [9, 20], however it soon became clear that this molecular design needed
improvement to approach the degree of rectification available in macroscale, silicon-based
diodes. A significant step forward came in the form of a rectifier utilizing a single
molecular orbital in the metal-insulator-metal structure [21]. If there exists a frontier
electronic level, either the HOMO or LUMO, near the Fermi levels of the two associated
electrodes, this level can participate in charge transport across the junction. In the example
from Kornilovitch, Bratkovsky, and Williams [21], they achieve rectification through a
single molecular orbital (in this case the LUMO) through molecular asymmetry, where the
MO is located to one side of the molecule as can be seen in Figure 1.3a. At reverse bias,
Figure 1.3b, most of the applied potential is dropped over the insulating chain resulting in
a reduced total conductivity. At forward bias, Figure 1.3c, the applied potential allows
conduction through the MO before tunneling through the insulating chain. It is also
possible that at one bias polarity the MO falls between the contact Fermi levels, resulting
in the MO participating in transport, and at the opposite bias it falls outside of the contact
5
Fermi levels, resulting in entirely tunneling current. Lastly, rectification can occur due to
differences at the contacts, including asymmetric electrode work functions, resembling a
Schottky diode [11], as well as differences in the molecule/electrode interaction [22, 23],
rectification in real devices is usually a combination of these effects. The literature contains
many examples of monolayer rectifiers, but only a few have reached 3 orders of magnitude:
a donor-acceptor pair self-assembled on Au reached R = 3000 [24], a molecular
heterojunction showed a maximum of R = 1000 [25], and 6.3 × 105 using a two-level
ferrocene compound in conjunction with a Schottky diode [26]. Chapters 2 and 3 will
present a series of examples which can be described by this transport model.
Figure 1.2. Rectifier J-V characteristics on (a) Linear scale and (b) Logarithmic scale.
a) b)
6
Figure 1.3. Energy level diagrams showing the mechanism of rectification in a two-
component molecule and slightly offset contact Fermi levels with a dotted line representing
tunneling and a solid line representing hopping at (a) Zero bias, (b) Reverse bias, and (c)
Forward bias.
a)
b)
c)
7
1.3 Organic Field-Effect Transistors
The field of organic electronics has exploded in popularity since its humble beginnings in
1960 with the identification of the organic semiconductor [27, 28], with many
commercially available products already on the market. The structure of organic
semiconductors departs quite heavily from the covalently-bonded inorganic
semiconductors, where the former are either molecular crystals formed of small molecules
or conjugated polymers and held together by weak van der Waals bonds. The
semiconducting properties of these materials arise from the conjugated π-orbitals present,
which facilitate electron delocalization and charge transport between molecules. It is
important to note that organic electronics was never meant to replace the existing silicon-
based technologies but to supplement by enabling applications inaccessible to rigid
electronics. The weak bonding of these organic materials allows for low-temperature
solution processing, opening the realm of soft plastics [29–36], textiles [37], and paper
[38–40] for use as substrates. These qualities have led to the introduction of various organic
photovoltaics (OPVs) [41–43], organic light-emitting diodes (OLEDs) [44, 45], various
pressure and gas sensors [46, 47], as well as the organic field-effect transistor (OFET) [48,
49]. Since the earliest OFETs were presented in the 1980s [50–53], device performance
has radically improved due to a better understanding of the chemical and solid-state
structures of the semiconductor, the effects of the dielectric layer, and the injection
processes at the contacts. In this section, the OFET is introduced and the components,
physics, materials, methods, and non-ideal characteristics are detailed.
8
1.3.1 OFET Structure and Operation
The OFET is comprised of 5 different electrically active layers assembled on a substrate:
the organic semiconductor, the gate dielectric, and 3 electrodes, namely the gate, source,
and drain electrodes. These layers are generally arranged in one of 4 device architectures
(Figure 1.4 a-d): bottom gate, bottom contact (BGBC), bottom gate, top contact (BGTC),
top gate, bottom contact (TGBC), and top gate, top contact (TGTC). These 4 structures are
divided into two categories, coplanar (BGBC and TGTC) and staggered (BGTC and
TGBC). Here, coplanar refers to the source, drain, and conducting channel being located
a) b)
c) d)
Figure 1.4. OFET device structures: (a) Bottom gate, bottom contact, (b) Bottom gate,
top contact, (c) Top gate, bottom contact, (d) Top gate, top contact.
9
on the same plane, and in the staggered structures the conducting channel is offset from the
plane of the source and drain contacts.
The operation of an OFET relies on the application of two potentials, the gate-
source voltage (VGS) and the drain-source voltage (VDS), with the source electrode held at
ground. For the following discussion, the semiconductor is assumed to be p-type, where
the majority charge carriers are holes, though for n-type, where the charge carriers are
electrons, the polarities are simply reversed. With no VGS being applied, there is no charge
accumulation at the semiconductor-dielectric interface, i.e. the device is turned “off.” At
the simplest level, an applied VGS polarizes the dielectric causing the accumulation of
charge carriers at the semiconductor-dielectric interface: the transistor turns “on.” An
applied VDS forces these accumulated charge carriers from the source to the drain electrode
where the drain current (ID) is measured. The charge density in the transistor channel and,
thus, the current, are modulated by the magnitude of the field applied, VGS, hence the “field-
effect” terminology. In a real device, a small, negative VGS is sometimes required first to
fill charge traps at the semiconductor-dielectric interface before free charge carriers can
accumulate in the conduction channel. This trap-filling potential is known as the threshold
voltage (VTh) and can result from sources like crystal defects, impurities, and interfacial
roughness. In addition, small amounts of dopants in the semiconductor and surface dipoles
can result in a positive threshold voltage wherein the device is already “on” at VGS = 0V
and requires a positive VGS to reach the “off” state.
Figure 1.5a shows a typical evolution of the drain current with increasingly negative
drain-source voltage where each curve is measured at a fixed, negative, gate-source
voltage, this type of measurement is known as the “output characteristics” or “transport.”
10
When VDS < |VGS – VTh| the device is in the linear regime, Figure 1.5b; the gate-source
voltage dictates the charge density in the conduction channel, thus the drain current will
increase linearly with the drain-source voltage and the device acts as a gate voltage-
controlled variable resistor. However, as VDS increases and the magnitude approaches that
of VGS, the shape of the conduction channel changes due to the two interacting potentials.
At the critical point where VDS = |VGS – VTh|, the area near the drain electrode is depleted of
free charge carriers and the channel becomes pinched off, Figure 1.5c. As the drain-source
voltage further increases, the competing effects of the increasing potential forcing charges
from source to drain and the growing depletion zone near the drain cause a saturation of
ID, Figure 5d, aptly named the saturation regime.
b)
d)
c)
a)
Figure 1.5. (a) Ideal OFET output characteristics, (b-d) BGBC structures with the
conduction channel in purple, (b) In the linear regime, (c) At pinch-off, (d) In the
saturation regime.
11
Figures 1.6a and 1.6b show in black the drain current on a logarithmic scale as a
function of gate-source voltage with drain-source voltage held constant in the linear and
saturation regimes, respectively. In blue, Figure 1.6a shows the drain current on a linear
scale and Figure 1.6b shows the square-root of drain current on a linear scale. This type of
measurement is known as the “transfer” characteristics. The field-effect mobility, µ, a
measure of how quickly charge carriers move in response to an external electric field, is
calculated from the slope of the red line in Figure 1.6a, 𝜕𝜕𝐼𝐼𝐷𝐷𝜕𝜕𝜕𝜕𝐺𝐺𝐺𝐺
, for the linear regime, and
from the slope of the red line in Figure 1.6b, 𝜕𝜕𝐼𝐼𝐷𝐷𝜕𝜕𝜕𝜕𝐺𝐺𝐺𝐺
, for the saturation regime, as detailed in
Section 1.3.2. The mobility µ has units of cm2/Vs, as a reference the mobility of amorphous
silicon is approximately 1 cm2/Vs and that of single-crystal silicon is 100-1000 cm2/Vs,
depending on the conditions. Also shown in Figure 1.6b is the threshold voltage, VTh, which
is usually calculated by finding the intercept of 𝐼𝐼𝐷𝐷 = 0 and the red line, although there are
Figure 1.6. Evolution of ID at fixed, negative, VDS as a function of increasingly negative VGS
in (a) The linear regime and (b) The saturation regime.
a) b)
12
many different methods of calculating VTh [54]. The threshold voltage is a result of trap
states being filled by the applied VGS and the density of trap states increases with decreasing
temperature due to the reduction in thermal energy required to eject charges from a trap.
Therefore, VTh is dependent on temperature, allowing an estimation of the density of
interfacial trap states, Nit, through measurements at varying temperature and application of
Equation 1.3:
𝑁𝑁𝑖𝑖𝑖𝑖 =𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑𝑘𝑘𝐵𝐵𝑞𝑞
𝜕𝜕𝑉𝑉𝑇𝑇ℎ𝜕𝜕𝜕𝜕
(1.3)
where kB is Boltzmann’s constant, T is the temperature, q is the elementary charge, and
Cdiel is the gate dielectric capacitance per unit area [55]. In addition to the threshold voltage,
another indication of charge traps can be seen in the quantity known as the sub-threshold
swing or inverse sub-threshold slope (S), the inverse slope of the orange line in Figure 1.6b.
S is a measure of how fast ID increases with VGS at fixed VDS and has units of V/decade, or
the amount that VGS must increase to cause a 10-fold increase in ID. S depends on the
interfacial trap states, and the gate dielectric as defined in Equation 1.4:
𝑆𝑆 =𝑘𝑘𝐵𝐵𝜕𝜕𝑇𝑇𝑇𝑇(10)
𝑞𝑞𝑁𝑁𝑖𝑖𝑖𝑖𝑞𝑞2
𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑+ 1 . (1.4)
A smaller value of S indicates that the device will have a sharp turn-on and the theoretical
lower limit of S at room temperature (T = 300K) is 60 mV/dec as the first term in Equation
1.4 approaches zero.
Another signature of the existence of traps is device hysteresis, where there is a
significant difference in current characteristics in the forward voltage sweep and the
reverse voltage sweep. This difference can occur when charges become trapped and then
13
released on the forward and reverse sweeps, respectively, and also in the presence of highly
polar dielectrics. A series of extremely useful methods for characterizing the trap states in
an OFET have emerged whereby the trap density as a function of energy in the band gap
(trap density of states, or trap DOS) is calculated from the standard transistor measurements
[56, 57].
1.3.2 FET Analysis
To analyze the electrical properties of an OFET, an assumption called the gradual channel
approximation is made. Here the electric field between the source and gate electrode is
much larger than the electric field between the source and drain electrode. This is generally
accomplished by ensuring that the gate dielectric thickness, d, and the channel length, L,
satisfy the relation 𝐿𝐿𝛽𝛽≥ 10, which becomes more relevant in devices with very short
channel lengths [58]. With this relation in place, the gate-source voltage dictates the charge
accumulation in the semiconductor, these charges reside at the semiconductor-dielectric
interface, and the potential distribution can be approximated as one-dimensional across the
channel. The total charge density that is accumulated at the semiconductor-dielectric
interface is given by, with x the distance across the channel:
𝑄𝑄 = −𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑𝑉𝑉𝑖𝑖𝑡𝑡𝑖𝑖𝑡𝑡𝑑𝑑(𝑥𝑥), 𝑉𝑉𝑖𝑖𝑡𝑡𝑖𝑖𝑡𝑡𝑑𝑑(𝑥𝑥) = 𝑉𝑉𝐺𝐺𝐺𝐺 − 𝑉𝑉(𝑥𝑥) (1.5)
where Vtotal(x) is the total potential in the channel resulting from an applied VDS and VGS.
However as mentioned earlier, a small potential, VTh, is required to fill traps before mobile
charge carriers are accumulated and thus the more relevant expression for mobile charge
carriers becomes:
𝑄𝑄𝑚𝑚𝑡𝑡𝑚𝑚𝑖𝑖𝑑𝑑𝑟𝑟 = −𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑(𝑉𝑉𝐺𝐺𝐺𝐺 − 𝑉𝑉(𝑥𝑥) − 𝑉𝑉𝑇𝑇ℎ). (1.6)
14
Then, as these charges are mobile along the x-direction between the source and drain
electrodes, the current ID through the drain is defined as:
𝐼𝐼𝐷𝐷 = 𝜇𝜇𝑄𝑄𝜇𝜇𝐸𝐸𝑥𝑥, 𝐸𝐸𝑥𝑥 = −𝑑𝑑𝑉𝑉𝑑𝑑𝑥𝑥
(1.7)
where Ex is the electric field in the direction of current flow, W is the width of a contact or
the channel width, and µ is the charge-carrier mobility. Substituting Equation 1.6 into
Equation 1.7 gives:
𝐼𝐼𝐷𝐷𝑑𝑑𝑥𝑥 = 𝜇𝜇𝜇𝜇𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑(𝑉𝑉𝐺𝐺𝐺𝐺 − 𝑉𝑉(𝑥𝑥) − 𝑉𝑉𝑇𝑇ℎ)𝑑𝑑𝑉𝑉 (1.8)
and, assuming that the charge-carrier mobility µ does not vary with applied potential,
integrating Equation 1.8 along the channel from x = 0 to the channel length L, and from V
= 0 to VDS as such:
𝐼𝐼𝐷𝐷𝑑𝑑𝑥𝑥𝐿𝐿
0= 𝜇𝜇𝜇𝜇𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑 (𝑉𝑉𝐺𝐺𝐺𝐺 − 𝑉𝑉(𝑥𝑥) − 𝑉𝑉𝑇𝑇ℎ)𝑑𝑑𝑉𝑉
𝜕𝜕𝐷𝐷𝐺𝐺
0
results in:
𝐼𝐼𝐷𝐷,𝑑𝑑𝑖𝑖𝑙𝑙 =𝜇𝜇𝐿𝐿𝜇𝜇𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑 (𝑉𝑉𝐺𝐺𝐺𝐺 − 𝑉𝑉𝑇𝑇ℎ)𝑉𝑉𝐷𝐷𝐺𝐺 −
12𝑉𝑉𝐷𝐷𝐺𝐺2 . (1.9)
Equation 1.9 is valid in the linear regime, however the drain current in the linear and
saturation regimes will follow different relations due to the pinch-off at saturation which
results in an effective maximum drain-source voltage of 𝑉𝑉𝐷𝐷𝐺𝐺 = 𝑉𝑉𝐺𝐺𝐺𝐺 − 𝑉𝑉𝑇𝑇ℎ. Substituting this
into Equation 1.9 yields for the saturation drain current:
𝐼𝐼𝐷𝐷,𝑠𝑠𝑡𝑡𝑖𝑖 =𝜇𝜇2𝐿𝐿
𝜇𝜇𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑(𝑉𝑉𝐺𝐺𝐺𝐺 − 𝑉𝑉𝑇𝑇ℎ)2. (1.10)
15
Then, taking the derivative with respect to VGS and rearranging Equations 1.9 and 1.10
gives:
𝜇𝜇𝑑𝑑𝑖𝑖𝑙𝑙 =𝐿𝐿
𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑𝜇𝜇𝑉𝑉𝐷𝐷𝐺𝐺𝜕𝜕𝐼𝐼𝐷𝐷𝜕𝜕𝑉𝑉𝐺𝐺𝐺𝐺
(1.11)
𝜇𝜇𝑠𝑠𝑡𝑡𝑖𝑖 =2𝐿𝐿
𝜇𝜇𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑𝜕𝜕𝐼𝐼𝐷𝐷𝜕𝜕𝑉𝑉𝐺𝐺𝐺𝐺
2
(1.12)
which are the two relations used to find the charge-carrier mobility in the linear and
saturation regimes, respectively. In a perfect device the values for the µlin and µsat are
identical, however in devices which have a significant barrier to injection, µlin is
substantially lower than µsat. Any resistance at the contacts will result in a reduced effective
VDS, and in the linear regime, the potential between source and drain electrodes is already
quite small compared to the saturation regime. Consequently, the linear regime is affected
to a larger degree by contact resistance, the causes of which are discussed in Section 1.3.5.
1.3.3 Charge Injection in OFETs
Choosing the proper contacts for an OFET is of the utmost importance to achieving optimal
device performance. The contact interface between the source electrode and the
semiconductor is the site of carrier injection, and so it plays a significant role in device
performance. This is especially true when downscaling devices, as contact effects dominate
at small transistor channel size [59]. The goal is to obtain an ohmic contact, or one that
results in a negligible voltage drop and should be able to supply as much current as needed
and act as a charge carrier reservoir. There are three dominant models of charge injection:
thermionic emission, field emission, and hopping through gap states. Thermionic injection
16
occurs when charge carriers have sufficient energy to overcome the injection barrier, and
is applicable as long as the applied voltage is not large or the contact is heavily doped [60].
If these conditions apply, then field emission is the more appropriate model. The energy
barrier at the interface becomes thin enough that charges are able to quantum-mechanically
tunnel through to the semiconductor conduction band, even without necessarily having
energy greater than the injection barrier. Lastly, defects can induce gap states between WF
and the HOMO/LUMO level; the charge carrier can hop through these states into the
semiconductor.
Contact injection is largely determined by the alignment between the work function
of the contact, WF, and the HOMO/LUMO levels of the semiconductor, Figure 1.7. The
injection barrier ΦB is the difference between WF and the HOMO for p-type transistors (or
WF and the LUMO for n-type transistors) and in general should be minimized. Indeed,
Figure 1.7. An illustration of the energetics at the semiconductor/dielectric interface.
17
since most organic semiconductor materials are intrinsically ambipolar, the alignment of
WF with the HOMO/LUMO levels can determine if a device is p-type, n-type, or ambipolar.
In p-type devices WF of the electrode aligns with the HOMO of the organic semiconductor,
in n-type with the LUMO, and ambipolar transport requires that WF lies in between the
HOMO and LUMO levels.
Alignment of the contact work function and the semiconductor HOMO/LUMO
levels is not trivial. Although the work function of a metal is equal to its Fermi energy EF
in vacuum, work function is a surface property and is subject to environmental effects. The
electron cloud of a bare metal extends slightly into vacuum, creating an electric field
oriented away from the metal. Any charge entering or leaving the metal will have to
overcome the potential of this electric field, an effect that the work function accounts for.
When a new material is brought near to or in contact with the metal, the electron cloud of
the new material pushes back the electron cloud of the metal. This changes the magnitude
of the electric field at the surface and therefore the work function. This is known as the
push-back or pillow effect [61]. Additionally, if the two materials have different Fermi
energies, then they will establish a common Fermi energy (i.e. thermal equilibrium) by
exchanging carriers. A depletion or accumulation zone (depending on the relative Fermi
energies) forms in the semiconductor creating an additional electric field. This is
commonly known as band bending [62–65]. This new electric field also alters the work
function, shifting the injection barrier. Since the Fermi energy of an intrinsic
semiconductor is positioned between the HOMO and LUMO energies, attempting to match
a metal with a work function close to one of these bands will cause significant charge
transfer to occur to establish equilibrium. This in turn shifts the HOMO and LUMO levels
18
and an energy barrier remains. This effect is known as Fermi level pinning [66, 67]. With
these and other effects, it is impossible to predict the energy level alignment at the
contact/semiconductor interface given only the contact work function and semiconductor
HOMO/LUMO levels, making the contact selection difficult.
Metallic contacts are often used due to their chemical stability, reliable processing,
and well-known properties. These include gold, silver, copper, platinum, calcium, and
aluminum, among others, and are generally deposited through thermal or electron-beam
evaporation in high vacuum. In the pursuit of all-organic or printable devices, several
groups have experimented with using organic metals as the electrodes or as a surface
treatment to other metallic electrodes [68]. One example is poly(3,4-
ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), a solution-processable
macromolecular salt [69]. Depending on the processing conditions, PEDOT:PSS can
exhibit high conductivities (up to 3065 S/cm) while remaining sufficiently transparent for
solar cell applications [70]. Another class of compounds that falls into the organic metal
family are charge-transfer (CT) complexes, which are formed through the combination of
electron donor and acceptor compounds and were first discovered in 1973 [71–73]. CT
complexes have garnered considerable attention due, in part, to the relative ease of work
function modification through chemical substitution, as exemplified by the transition from
p-type, to n-type, and finally to ambipolar behavior on the same semiconductor through
manipulation of the donor and acceptor constituents [74]. One issue that arose with the use
of CT complexes, however, is their low solubility in common organic solvents. One
approach to overcome this challenge was introduced by Hiraoka, et al. through the use of
“double-shot inkjet printing,” where the more soluble donor and acceptor components were
19
individually deposited and the CT complex was allowed to form on the substrate [75]. π-
conjugated carbon in the form of carbon nanotubes or graphene is also gaining considerable
interest as an electrode material in OFETs due to the reduced injection barrier stemming
from the interfacial morphology as well as possibly aiding in the growth of an overlaid
organic semiconductor [76–82].
1.3.4 Contact Modification
Chemical modifications of metallic contacts can create a more favorable interface for
charge injection into the organic semiconductor. Metal oxides such as molybdenum oxide
and titanium oxide were used to modify the work function by a large degree in either
direction [67, 83, 84]. Charge transfer salts [68, 71] and SAMs [85–90] can also shift the
work function of the source and drain electrodes, and can also impact the semiconductor
morphology [90, 91]. The most convenient method of work function tuning for metallic
contacts is through SAMs, usually of polar, thiol-based small molecules [66]. The SAM
adheres spontaneously to the metal surface as the thiol group forms a covalent bond with
metals. The presence of the SAM modifies the local electric field at the surface of the
electrode by creating a dipole at the interface. The electric field of this dipole, combined
with the polar dipole of the SAM itself, change the work function of the contact. The sign
and magnitude of the SAM dipole determines whether the work function is shifted up or
down. In general, alkane terminals decrease the work function while halogenated terminals
increase the work function. The work function can be further fine-tuned using a blend of
SAMs [92]. The surface potential, and therefore the work function, resulting from a SAM
assembled on an electrode can be measured through non-contact methods such as
ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe microscopy (KPM).
20
1.3.5 Contact Resistance in OFETs
It is important to consider the non-idealities that may be present in real OFETs to aid in
device design as well as to ensure that any extracted quantities are physically meaningful.
There exist two main contributions to the overall device resistance: charge injection from
the contacts into the organic semiconductor layer, and charge transport in the
semiconducting channel [93]. With the advent of new semiconducting materials with high
intrinsic mobilities and correspondingly low channel resistances, RCh, the impact of contact
resistance, RC, became much more significant. In addition, the focus on device downscaling
has exacerbated this issue as the channel resistance decreases yet the contact resistance
remains constant with smaller channel length. For an OFET to function as intended, the
resistance due to the contacts must be substantially lower than the resistance due to the
channel. If this is not the case, the voltage drop at the contacts results in a smaller effective
potential across the channel. While the injection barrier is the main source of contact
resistance RC, additional factors can have a significant effect. The bulk resistance RC,bulk,
or the resistance due to the depleted semiconductor between the contacts and the
conducting channel (as opposed to the contact interface resistance RC,int, or the resistance
to charge injection from contacts into the semiconductor) can be reduced through device
geometry. In a staggered device geometry, Figures 1.4b and c, injection occurs along the
surface parallel to the channel; as the gate voltage is increased, more of this surface
participates in injection due to increased channel conduction, effectively reducing RC. In
contrast, coplanar devices, Figures 1.4a and d, inject directly into the channel along the
surface perpendicular to the channel. This eliminates RC,bulk, but the injecting surface is
limited to the thickness of the conduction channel, typically only a few nanometers. High
21
mobility also aids in carrier injection, reducing RC,int, while simultaneously decreasing the
bulk transport resistance RC,bulk [60]. However, contact resistance still remains dominant in
high mobility devices. Generally, long channel-length (50-100 µm) devices with mobilities
around 1 cm2/Vs will not display significant contact effects as long as RC is less than a few
kΩcm, however as mobilities increase past 5 cm2/Vs, RC must be less than 1 kΩcm. At
small channel length, the restriction on these resistance values is even greater, especially
at high frequencies [94].
A large and non-ohmic contact resistance can result in quite non-ideal electrical
measurements, easily seen as an “S” shape in the ID1/2 vs. VGS curves where the slope will
abruptly increase and then fall back to a steady state, leading to an overestimation of the
field-effect mobility if the value for 𝜕𝜕𝐼𝐼𝐷𝐷𝜕𝜕𝜕𝜕𝐺𝐺𝐺𝐺
is extracted at this peak. This phenomenon has
captured the attention of many and is attributed to a large injection barrier at the contacts
[95, 96]. The injection barrier then disappears with increasing VGS as the greater potential
aids in the extraction of charges. This effect can also be seen when the contact resistance
begins higher than the channel resistance, before dropping with increasing VGS. Chapter 5
focuses on the contact resistance of OFETs and a new method for minimizing the resistance
at injection.
Several techniques have been developed to measure contact resistance both directly
and indirectly. An example of an indirect method to determine the resistance due to the
injecting contacts is the gated transmission line method (gated TLM), which relies on the
assumption that the organic semiconductor is uniform and isotropic, and thus the channel
resistance scales linearly with the channel length. This technique uses linear regime transfer
22
measurements on many devices of differing channel length to evaluate the total resistance
of the device Rdevice. This value is given by the series combination of the contact resistance
(independent of the channel length, L), and the channel resistance (directly proportional to
the channel length) and follows Equation 1.13:
𝑅𝑅𝛽𝛽𝑟𝑟𝑟𝑟𝑖𝑖𝑑𝑑𝑟𝑟 = 𝑅𝑅𝐶𝐶 + 𝑅𝑅𝐶𝐶ℎ(𝐿𝐿). (1.13)
Plotting the width-normalized resistance vs. channel length results in a straight line with
the y-intercept equal to RCW, Figure 1.8. To minimize the errors due to the fact that each
device turns on at a different voltage and determine the correct value for the device
resistance, the drain current must be measured at a particular value of VGS called the
overdrive voltage (𝑉𝑉𝐺𝐺𝐺𝐺 − 𝑉𝑉𝑇𝑇ℎ = 𝑉𝑉𝑡𝑡𝑟𝑟𝑟𝑟𝑟𝑟𝛽𝛽𝑟𝑟𝑖𝑖𝑟𝑟𝑟𝑟). The use of the overdrive voltage ensures
consistency between devices which may have differing VTh, so that the magnitude of the
drain current is taken at the same distance away from the threshold voltage. The drain-
Figure 1.8. Example Gated TLM graph showing the determination of the contact resistance.
23
source voltage is then divided by ID to obtain the total device resistance which, multiplied
by the channel width as a normalization factor, is plotted with many devices as shown in
Figure 1.8.
A more direct method of measuring contact resistance uses scanning Kelvin probe
microscopy (SKPM) which in this case uses a metal-coated atomic force microscope
(AFM) tip to measure the surface potential across the channel of a device in operation [97,
98]. By measuring the drain current and the voltage drop at the contacts, the contact
resistance can be extracted. It is important to note that this technique can only be used in
certain device configurations where the contacts and the channel are accessible with the
SKPM tip.
1.4 Thesis Outline
Chapter 2 presents 9 newly developed compounds and details their incorporation into a
molecular-scale metal-insulator-metal structure designed to rectify current. The
compounds are each allowed to form SAMs on silicon wafers and contacted using a
eutectic gallium-indium (EGaIn) probe tip. The measured electrical properties of each
compound, along with the dipole moment calculated using density functional theory
(DFT), revealed a structure-function relationship. Depending on the molecular length, the
degree of rectification was positively correlated with the strength of the associated dipole.
Chapter 3 expands upon the work outlined in Chapter 2 and introduces an additional 8
newly-synthesized compounds using the same device structure. The new experimental
design resulted in the highest rectification ratio obtained in SAMs formed on silicon, up to
a maximum of 2635.
24
In Chapter 4, the device under study is the OFET, where the organic components contain
larger conjugated structures than the rectifiers above. The organic semiconductor here is
processed from the vapor phase in the form of a single crystal where electrical
measurements reveal significant anisotropy and a maximum mobility of 0.3 cm2/Vs. The
combination of DFT and X-ray diffraction (XRD) measurements revealed the main
direction for hole transport in the plane of the measurement, although not necessarily
parallel to the conduction channel. Through solution processing, thin-film OFETs were
produced, resulting in an enhancement of the high-mobility crystallographic direction
parallel to the conduction channel and an associated increase in mobility.
Chapter 5 details the production of thin-film OFETs, also referred to as organic thin-film
transistors (OTFTs), with aggressively reduced contact resistances, and associated
increased mobilities, through modifications to the contact deposition rate. Using two model
semiconductors, the small molecule 2,8-difluoro-5,11-bis(triethylsilylethynyl)
anthradithiophene (diF-TES ADT) and the copolymer indacenodithiophene-co-
benzothiadiazole (C16IDT-BT), contact resistances of 500 Ωcm and 200 Ωcm were
obtained, respectively. The reduced injection barrier in these devices resulted in record-
high charge carrier mobility of 19.2 cm2/Vs for diF-TES ADT, and 10 cm2/Vs in C16IDT-
BT.
Together, these results represent significant developments in the fields of both molecular
electronics and organic electronics, achieving record performance in silicon-based
molecular rectifiers as well as small-molecule OFETs.
25
References
[1] C. Joachim, J. K. Gimzewski, R. R. Schlittler, and C. Chavy, “Electronic transparence of a single C60 molecule,” Phys. Rev. Lett., Vol. 74, p. 2102, 1995.
[2] B. Xu and N. J. Tao, “Measurement of Single-Molecule Resistance by Repeated Formation of Molecular Junctions,” Science (80-. )., Vol. 301, p. 1221, 2003.
[3] D. J. Wold and C. D. Frisbie, “Formation of Metal−Molecule−Metal Tunnel Junctions: Microcontacts to Alkanethiol Monolayers with a Conducting AFM Tip,” J. Am. Chem. Soc., Vol. 122, p. 2970, 2000.
[4] D. J. Wold and C. D. Frisbie, “Fabrication and Characterization of Metal−Molecule−Metal Junctions by Conducting Probe Atomic Force Microscopy,” J. Am. Chem. Soc., Vol. 123, p. 5549, 2001.
[5] I. Díez-Pérez, J. Hihath, Y. Lee, L. Yu, L. Adamska, M. A. Kozhushner, I. I. Oleynik, and N. Tao, “Rectification and stability of a single molecular diode with controlled orientation,” Nat. Chem., Vol. 1, p. 635, 2009.
[6] M. Elbing, R. Ochs, M. Koentopp, M. Fischer, C. von Hänisch, F. Weigend, F. Evers, H. B. Weber, and M. Mayor, “A single-molecule diode,” Proc. Natl. Acad. Sci. U. S. A., Vol. 102, p. 8815, 2005.
[7] M. L. Perrin, E. Galán, R. Eelkema, J. M. Thijssen, F. Grozema, and H. S. J. van der Zant, “A gate-tunable single-molecule diode,” Nanoscale, Vol. 8, p. 8919, 2016.
[8] J. W. Baldwin, R. R. Amaresh, I. R. Peterson, W. J. Shumate, M. P. Cava, M. a. Amiri, R. Hamilton, G. J. Ashwell, and R. M. Metzger, “Rectification and nonlinear optical properties of a Langmuir-Blodgett monolayer of a pyridinium dye,” J. Phys. Chem. B, Vol. 106, p. 12158, 2002.
[9] R. M. Metzger, B. Chen, U. Höpfner, M. V. Lakshmikantham, D. Vuillaume, T. Kawai, X. Wu, H. Tachibana, T. V. Hughes, H. Sakurai, J. W. Baldwin, C. Hosch, M. P. Cava, L. Brehmer, and G. J. Ashwell, “Unimolecular electrical rectification in hexadecylquinolinium tricyanoquinodimethanide,” J. Am. Chem. Soc., Vol. 119, p. 10455, 1997.
[10] G. J. Ashwell, W. D. Tyrrell, and A. J. Whittam, “Molecular rectification: Self-assembled monolayers in which donor-(π-bridge)-acceptor moieties are centrally located and symmetrically coupled to both gold electrodes,” J. Am. Chem. Soc., Vol. 126, p. 7102, 2004.
[11] S. Lenfant, C. Krzeminski, C. Delerue, G. Allan, and D. Vuillaume, “Molecular rectifying diodes from self-assembly on silicon,” Nano Lett., Vol. 3, p. 741 2003.
[12] S. Lenfant, D. Guerin, F. Tran Van, C. Chevrot, S. Palacin, J. P. Bourgoin, O. Bouloussa, F. Rondelez, and D. Vuillaume, “Electron transport through rectifying self-assembled monolayer diodes on silicon: Fermi-level pinning at the molecule-
26
metal interface.,” J. Phys. Chem. B, Vol. 110, p. 13947, 2006.
[13] L. Luo, L. Balhorn, B. Vlaisavljevich, D. Ma, L. Gagliardi, and C. D. Frisbie, “Hopping Transport and Rectifying Behavior in Long Donor–Acceptor Molecular Wires,” J. Phys. Chem. C, Vol. 118, p. 26485, 2014.
[14] C. A. Nijhuis, W. F. Reus, and G. M. Whitesides, “Molecular rectification in metal-SAM-metal oxide-metal junctions,” J. Am. Chem. Soc., Vol. 131, p. 17814, 2009.
[15] L. Yuan, N. Nerngchamnong, L. Cao, H. Hamoudi, E. del Barco, M. Roemer, R. K. Sriramula, D. Thompson, and C. A. Nijhuis, “Controlling the direction of rectification in a molecular diode,” Nat. Commun., Vol. 6, p. 6324, 2015.
[16] J. G. Simmons, “Generalized Formula for the Electric Tunnel Effect between Similar Electrodes Separated by a Thin Insulating Film,” J. Appl. Phys., Vol. 34, p. 1793, 1963.
[17] C. Joachim and M. a Ratner, “Molecular electronics: some views on transport junctions and beyond,” Proc. Natl. Acad. Sci. U. S. A., Vol. 102, p. 8801, 2005.
[18] C. A. Nijhuis, W. F. Reus, and G. M. Whitesides, “Mechanism of rectification in tunneling junctions based on molecules with asymmetric potential drops,” J. Am. Chem. Soc., Vol. 132, p. 18386, 2010.
[19] A. Aviram and M. A. Ratner, “Molecular rectifiers,” Chem. Phys. Lett., Vol. 29, p. 277, 1974.
[20] A. S. Martin, J. R. Sambles, and G. J. Ashwell, “Molecular rectifier,” Phys. Rev. Lett., Vol. 70, p. 218, 1993.
[21] P. E. Kornilovitch, A. M. Bratkovsky, and R. S. Williams, “Current rectification by molecules with asymmetric tunneling barriers,” Phys. Rev. B, Vol. 66, p. 165436, 2002.
[22] C. Van Dyck and M. a. Ratner, “Molecular Rectifiers: A New Design Based on Asymmetric Anchoring Moieties,” Nano Lett., Vol. 15, p. 1577, 2015.
[23] C. Van Dyck and M. A. Ratner, “Molecular Junctions: Control of the Energy Gap Achieved by a Pinning Effect,” J. Phys. Chem. C, Vol. 121, p. 3013, 2017.
[24] G. J. Ashwell, B. Urasinska, and W. D. Tyrrell, “Molecules that mimic Schottky diodes,” Phys. Chem. Chem. Phys., Vol. 8, p. 3314, 2006.
[25] J. A. Smerdon, N. C. Giebink, N. P. Guisinger, P. Darancet, and J. R. Guest, “Large Spatially Resolved Rectification in a Donor-Acceptor Molecular Heterojunction,” Nano Lett., Vol. 16, p. 2603, 2016.
[26] X. Chen, M. Roemer, L. Yuan, W. Du, D. Thompson, E. del Barco, and C. A. Nijhuis, “Molecular diodes with rectification ratios exceeding 105 driven by electrostatic interactions,” Nat. Nanotechnol., Vol. 12, p. 797, 2017.
[27] R. G. Kepler, “Charge carrier production and mobility in anthracene crystals,”
27
Phys. Rev., Vol. 119, p. 1226, 1960.
[28] O. H. LeBlanc, “Hole and Electron Drift Mobilities in Anthracene,” J. Chem. Phys., Vol. 33, p. 626, 1960.
[29] M. Medina-Sánchez, C. Martínez-Domingo, E. Ramon, and A. Merkoçi, “An Inkjet-Printed Field-Effect Transistor for Label-Free Biosensing,” Adv. Funct. Mater., Vol. 24, p. 6291, 2014.
[30] Y. Xu, R. Gwoziecki, I. Chartier, R. Coppard, F. Balestra, and G. Ghibaudo, “Modified transmission-line method for contact resistance extraction in organic field-effect transistors,” Appl. Phys. Lett., Vol. 97, p. 63302, 2010.
[31] T. Kubo, R. Häusermann, J. Tsurumi, J. Soeda, Y. Okada, Y. Yamashita, N. Akamatsu, A. Shishido, C. Mitsui, T. Okamoto, S. Yanagisawa, H. Matsui, and J. Takeya, “Suppressing molecular vibrations in organic semiconductors by inducing strain,” Nat. Commun., Vol. 7, p. 11156, 2016.
[32] C. Reese, W.-J. Chung, M. Ling, M. Roberts, and Z. Bao, “High-performance microscale single-crystal transistors by lithography on an elastomer dielectric,” Appl. Phys. Lett., Vol. 89, p. 202108, 2006.
[33] W. Xie, K. Willa, Y. Wu, R. Häusermann, K. Takimiya, B. Batlogg, and C. D. Frisbie, “Temperature-independent transport in high-mobility dinaphtho-thieno-thiophene (DNTT) single crystal transistors,” Adv. Mater., Vol. 25, p. 3478, 2013.
[34] D. Ji, L. Jiang, X. Cai, H. Dong, Q. Meng, G. Tian, D. Wu, J. Li, and W. Hu, “Large scale, flexible organic transistor arrays and circuits based on polyimide materials,” Org. Electron., Vol. 14, p. 2528, 2013.
[35] Y. Zang, F. Zhang, D. Huang, X. Gao, C. Di, and D. Zhu, “Flexible suspended gate organic thin-film transistors for ultra-sensitive pressure detection,” Nat. Commun., Vol. 6, p. 6269, 2015.
[36] H. T. Yi, M. M. Payne, J. E. Anthony, and V. Podzorov, “Ultra-flexible solution-processed organic field-effect transistors,” Nat. Commun., Vol. 3, p. 1259, 2012.
[37] K. Cherenack and L. Van Pieterson, “Smart textiles: Challenges and opportunities,” J. Appl. Phys., Vol. 112, p. 091301, 2012.
[38] T. Minari, Y. Kanehara, C. Liu, K. Sakamoto, T. Yasuda, A. Yaguchi, S. Tsukada, K. Kashizaki, and M. Kanehara, “Room-Temperature Printing of Organic Thin-Film Transistors with π-Junction Gold Nanoparticles,” Adv. Funct. Mater., Vol. 24, p. 4886, 2014.
[39] U. Zschieschang and H. Klauk, “Low-voltage organic transistors with steep subthreshold slope fabricated on commercially available paper,” Org. Electron., Vol. 25, p. 340, 2015.
[40] P. J. Diemer, A. F. Harper, M. R. Niazi, A. J. Petty, J. E. Anthony, A. Amassian, and O. D. Jurchescu, “Laser-Printed Organic Thin-Film Transistors,” Adv. Mater. Technol., Vol. 2, p. 1, 2017.
28
[41] B. Kippelen and J.-L. Brédas, “Organic photovoltaics,” Energy Environ. Sci., Vol. 2, p. 251, 2009.
[42] J. Yu, Y. Zheng, and J. Huang, “Towards high performance organic photovoltaic cells: A review of recent development in organic photovoltaics,” Polymers, Vol. 6, p. 2473, 2014.
[43] G. J. Hedley, A. Ruseckas, and I. D. W. Samuel, “Light Harvesting for Organic Photovoltaics,” Chem. Rev., Vol. 117, p. 796, 2017.
[44] K. J. Baeg, M. Binda, D. Natali, M. Caironi, and Y. Y. Noh, “Organic light detectors: Photodiodes and phototransistors,” Adv. Mater., Vol. 25, p. 4267, 2013.
[45] J. Mei, Y. Hong, J. W. Y. Lam, A. Qin, Y. Tang, and B. Z. Tang, “Aggregation-induced emission: The whole is more brilliant than the parts,” Adv. Mater., Vol. 26, p. 5429, 2014.
[46] B. Adhikari and S. Majumdar, “Polymers in sensor applications,” Prog. Polym. Sci., Vol. 29, p. 699, 2004.
[47] L. Torsi, M. Magliulo, K. Manoli, and G. Palazzo, “Organic field-effect transistor sensors: a tutorial review,” Chem. Soc. Rev., Vol. 42, p. 8612, 2013.
[48] J. W. Ward, Z. A. Lamport, and O. D. Jurchescu, “Versatile organic transistors by solution processing,” ChemPhysChem, Vol. 16, p. 1118, 2015.
[49] H. Klauk, “Organic thin-film transistors,” Chem. Soc. Rev., Vol. 39, p. 2643, 2010.
[50] K. Kudo, M. Yamashina, and T. Moriizumi, “Field Effect Measurement of Organic Dye Films,” Jpn. J. Appl. Phys., Vol. 23, p. 130, 1984.
[51] A. Tsumura, H. Koezuka, and T. Ando, “Macromolecular electronic device: Field‐effect transistor with a polythiophene thin film,” Appl. Phys. Lett., Vol. 49, p. 1210, 1986.
[52] H. Koezuka, A. Tsumura, and T. Ando, “Field-effect transistor with polythiophene thin film,” Synth. Met., Vol. 18, p. 699, 1987.
[53] M. Madru, G. Guillaud, M. A. Sadoun, M. Maitrot, C. Clarisse, M. L. Contellec, J.-J. André, and J. Simon, “The first field effect transistor based on an intrinsic molecular semiconductor,” Chem. Phys. Lett., Vol. 142, p. 103, 1987.
[54] D. Boudinet, G. Le Blevennec, C. Serbutoviez, J.-M. Verilhac, H. Yan, and G. Horowitz, “Contact resistance and threshold voltage extraction in n-channel organic thin film transistors on plastic substrates,” J. Appl. Phys., Vol. 105, p. 84510, 2009.
[55] J. Smith, R. Hamilton, Y. Qi, A. Kahn, D. D. C. Bradley, M. Heeney, I. McCulloch, and T. D. Anthopoulos, “The Influence of Film Morphology in High-Mobility Small-Molecule:Polymer Blend Organic Transistors,” Adv. Funct. Mater., Vol. 20, p. 2330, 2010.
[56] W. L. Kalb and B. Batlogg, “Calculating the trap density of states in organic field-
29
effect transistors from experiment: A comparison of different methods,” Phys. Rev. B, Vol. 81, p. 35327, 2010.
[57] D. Oberhoff, K. P. Pernstich, D. J. Gundlach, and B. Batlogg, “Arbitrary Density of States in an Organic Thin-Film Field-Effect Transistor Model and Application to Pentacene Devices,” IEEE Trans. Electron Devices, Vol. 54, p. 17, 2007.
[58] L.-L. Chua, P. K. H. Ho, H. Sirringhaus, and R. H. Friend, “High-stability ultrathin spin-on benzocyclobutene gate dielectric for polymer field-effect transistors,” Appl. Phys. Lett., Vol. 84, p. 3400, 2004.
[59] D. Natali, J. Chen, F. Maddalena, F. García Ferré, F. Di Fonzo, and M. Caironi, “Injection Length in Staggered Organic Thin Film Transistors: Assessment and Implications for Device Downscaling,” Adv. Electron. Mater., Vol. 2, p. 1600097, 2016.
[60] C. Liu, Y. Xu, and Y. Y. Noh, “Contact engineering in organic field-effect transistors,” Mater. Today, Vol. 18, p. 79, 2015.
[61] M. Knupfer and G. Paasch, “Origin of the interface dipole at interfaces between undoped organic semiconductors and metals,” J. Vac. Sci. Technol. A Vacuum, Surfaces, Film., Vol. 23, p. 1072, 2005.
[62] W. Schottky, “Halbleitertheorie der Sperrschicht,” Naturwissenschaften, Vol. 26, p. 843, 1938.
[63] W. Schottky, “Zur Halbleitertheorie der Sperrschicht- und Spitzengleichrichter,” Z. Phys., Vol. 113, p. 367, 1939.
[64] N. F. Mott, “Note on the contact between a metal and an insulator or semi-conductor,” Math. Proc. Cambridge Philos. Soc., Vol. 34, p. 568, 1938.
[65] N. F. Mott, “The Theory of Crystal Rectifiers,” Proc. R. Soc. A Math. Phys. Eng. Sci., Vol. 171, p. 27, 1939.
[66] D. Natali and M. Caironi, “Charge Injection in Solution-Processed Organic Field-Effect Transistors: Physics, Models and Characterization Methods,” Adv. Mater., Vol. 24, p. 1357, 2012.
[67] S. Choi, C. Fuentes-Hernandez, C.-Y. Wang, T. M. Khan, F. A. Larrain, Y. Zhang, S. Barlow, S. R. Marder, and B. Kippelen, “A Study on Reducing Contact Resistance in Solution-Processed Organic Field-Effect Transistors,” ACS Appl. Mater. Interfaces, Vol. 8, p. 24744, 2016.
[68] R. Pfattner, C. Rovira, and M. Mas-Torrent, “Organic metal engineering for enhanced field-effect transistor performance,” Phys. Chem. Chem. Phys., Vol. 17, p. 26545, 2015.
[69] M. Halik, H. Klauk, U. Zschieschang, T. Kriem, G. Schmid, W. Radlik, and K. Wussow, “Fully patterned all-organic thin film transistors,” Appl. Phys. Lett., Vol. 81, p. 289, 2002.
30
[70] Y. Xia, K. Sun, and J. Ouyang, “Solution-processed metallic conducting polymer films as transparent electrode of optoelectronic devices,” Adv. Mater., Vol. 24, p. 2436, 2012.
[71] K. P. Goetz, D. Vermeulen, M. E. Payne, C. Kloc, L. E. McNeil, and O. D. Jurchescu, “Charge-transfer complexes: new perspectives on an old class of compounds,” J. Mater. Chem. C, Vol. 2, p. 3065, 2014.
[72] S. Horiuchi, T. Hasegawa, and Y. Tokura, “Molecular Donor–Acceptor Compounds as Prospective Organic Electronics Materials,” J. Phys. Soc. Japan, Vol. 75, p. 51016, 2006.
[73] J. Ferraris, D. O. Cowan, V. Walatka, and J. H. Perlstein, “Electron Transfer in a New Highly Conducting Donor-Acceptor Complex,” J. Am. Chem. Soc., Vol. 95, p. 948, 1973.
[74] Y. Takahashi, T. Hasegawa, Y. Abe, Y. Tokura, and G. Saito, “Organic metal electrodes for controlled p- and n-type carrier injections in organic field-effect transistors,” Appl. Phys. Lett., Vol. 88, p. 73504, 2006.
[75] M. Hiraoka, T. Hasegawa, T. Yamada, Y. Takahashi, S. Horiuchi, and Y. Tokura, “On-Substrate Synthesis of Molecular Conductor Films and Circuits,” Adv. Mater., Vol. 19, p. 3248, 2007.
[76] Y. Cao, S. Liu, Q. Shen, K. Yan, P. Li, J. Xu, D. Yu, M. L. Steigerwald, C. Nuckolls, Z. Liu, and X. Guo, “High-performance photoresponsive organic nanotransistors with single-layer graphenes as two-dimensional electrodes,” Adv. Funct. Mater., Vol. 19, p. 2743, 2009.
[77] W. H. Lee, J. Park, S. H. Sim, S. B. Jo, K. S. Kim, B. H. Hong, and K. Cho, “Transparent flexible organic transistors based on monolayer graphene electrodes on plastic,” Adv. Mater., Vol. 23, p. 1752, 2011.
[78] S. Pang, H. N. Tsao, X. Feng, and K. Mullen, “Patterned graphene electrodes from solution-processed graphite oxide films for organic field-effect transistors,” Adv. Mater., Vol. 21, p. 3488, 2009.
[79] S. Lee, G. Jo, S. J. Kang, G. Wang, M. Choe, W. Park, D. Y. Kim, Y. H. Kahng, and T. Lee, “Enhanced charge injection in pentacene field-effect transistors with graphene electrodes,” Adv. Mater., Vol. 23, p. 100, 2011.
[80] F. Cicoira, N. Copped, S. Iannotta, and R. Martel, “Ambipolar copper phthalocyanine transistors with carbon nanotube array electrodes,” Appl. Phys. Lett., Vol. 98, p. 1, 2011.
[81] M. A. McCarthy, B. Liu, and A. G. Rinzler, “High Current, Low Voltage Carbon Nanotube Enabled Vertical Organic Field Effect Transistors,” Nano Lett., Vol. 10, p. 3467, 2010.
[82] H. Chen and X. Guo, “Unique role of self-assembled monolayers in carbon nanomaterial-based field-effect transistors,” Small, Vol. 9, p. 1144, 2013.
31
[83] D. Kumaki, T. Umeda, and S. Tokito, “Reducing the contact resistance of bottom-contact pentacene thin-film transistors by employing a MoO[sub x] carrier injection layer,” Appl. Phys. Lett., Vol. 92, p. 13301, 2008.
[84] Y.-H. Lou, M.-F. Xu, L. Zhang, Z.-K. Wang, S. Naka, H. Okada, and L.-S. Liao, “Origin of enhanced electrical and conducting properties in pentacene films doped by molybdenum trioxide,” Org. Electron., Vol. 14, p. 2698, 2013.
[85] J. W. Ward, R. Li, A. Obaid, M. M. Payne, D.-M. Smilgies, J. E. Anthony, A. Amassian, and O. D. Jurchescu, “Rational Design of Organic Semiconductors for Texture Control and Self-Patterning on Halogenated Surfaces,” Adv. Funct. Mater., Vol. 24, p. 5052, 2014.
[86] S. K. Park, T. N. Jackson, J. E. Anthony, and D. A. Mourey, “High mobility solution processed 6,13-bis(triisopropyl-silylethynyl) pentacene organic thin film transistors,” Appl. Phys. Lett., Vol. 91, p. 63514, 2007.
[87] H. Wang, J. Mativetsky, Y. Ren, E. D. Gomez, C. Jaye, J. Schwartz, D. a. Fischer, and Y.-L. Loo, “Fluorinated and Hydrogenated Self-Assembled Monolayers (SAMs) on Anodes: Effects of SAM Chemistry on Device Characteristics of Polymer Solar Cells,” Org. Electron., Vol. 15, p. 3333, 2014.
[88] Y. Xu, K. Baeg, W. Park, A. Cho, E. Choi, and Y. Noh, “Regulating charge injection in ambipolar organic field-effect transistors by mixed self-assembled monolayers,” ACS Appl. Mater. Interfaces, Vol. 6, p. 14493, 2014.
[89] W. S. Hu, Y. T. Tao, Y. J. Hsu, D. H. Wei, and Y. S. Wu, “Molecular Orientation of Evaporated Pentacene Films on Gold: Alignment Effect of Self-Assembled Monolayer,” Langmuir, Vol. 21, p. 2260, 2005.
[90] D. J. Gundlach, J. E. Royer, S. K. Park, S. Subramanian, O. D. Jurchescu, B. H. Hamadani, a J. Moad, R. J. Kline, L. C. Teague, O. Kirillov, C. A. Richter, J. G. Kushmerick, L. J. Richter, S. R. Parkin, T. N. Jackson, and J. E. Anthony, “Contact-induced crystallinity for high-performance soluble acene-based transistors and circuits,” Nat. Mater., Vol. 7, p. 216, 2008.
[91] J. W. Ward, M. a. Loth, R. J. Kline, M. Coll, C. Ocal, J. E. Anthony, and O. D. Jurchescu, “Tailored interfaces for self-patterning organic thin-film transistors,” J. Mater. Chem., Vol. 22, p. 19047, 2012.
[92] B. De Boer, A. Hadipour, M. M. Mandoc, T. Van Woudenbergh, and P. W. M. Blom, “Tuning of metal work functions with self-assembled monolayers,” Adv. Mater., Vol. 17, p. 621, 2005.
[93] D. J. Gundlach, L. Zhou, J. A. Nichols, T. N. Jackson, P. V. Necliudov, and M. S. Shur, “An experimental study of contact effects in organic thin film transistors,” J. Appl. Phys., Vol. 100, p. 24509, 2006.
[94] H. Klauk, “Will We See Gigahertz Organic Transistors?,” Adv. Electron. Mater., p. 1700474, 2018.
[95] E. G. Bittle, J. I. Basham, T. N. Jackson, O. D. Jurchescu, and D. J. Gundlach,
32
“Mobility overestimation due to gated contacts in organic field-effect transistors,” Nat. Commun., Vol. 7, p. 10908, 2016.
[96] T. Uemura, C. Rolin, T. H. Ke, P. Fesenko, J. Genoe, P. Heremans, and J. Takeya, “On the Extraction of Charge Carrier Mobility in High-Mobility Organic Transistors,” Adv. Mater., Vol. 28, p. 151, 2016.
[97] M. Ando, S. Heike, M. Kawasaki, and T. Hashizume, “Trapped charge mapping in crystalline organic transistors by using scanning Kelvin probe force microscopy,” Appl. Phys. Lett., Vol. 105, p. 193303, 2014.
[98] L. C. Teague, O. D. Jurchescu, C. A. Richter, S. Subramanian, J. E. Anthony, T. N. Jackson, D. J. Gundlach, and J. G. Kushmerick, “Probing stress effects in single crystal organic transistors by scanning Kelvin probe microscopy,” Appl. Phys. Lett., Vol. 96, p. 203305, 2010.
33
Chapter 2 Fluorinated Benzalkylsilane Molecular
Rectifiers
The introduction of electrical components on the molecular scale introduces a host of new
challenges, however the simplicity of the self-assembled monolayer (SAM) alleviates
many of these issues through the ease of fabrication and the lack of a necessary alignment.
We examine the electrical properties of nine new alkylated silane SAMs of the form
(EtO)3Si(CH2)nN=CHPhX where n = 3 or 11 and X = 4-CF3, 3,5-CF3, 3-F-4-CF3, 4-F, or
2,3,4,5,6-F, and explore their rectification behavior in relation to their molecular structure.
The electrical properties of the films were examined in a metal/insulator/metal
configuration, with a highly-doped silicon bottom contact and a eutectic gallium-indium
liquid metal (EGaIn) top contact. The junctions exhibit high yields (> 90%), a remarkable
resistance to bias stress, and current rectification ratios (R) between 20 and 200 depending
on the structure, degree of order, and internal dipole of each molecule. We found that the
rectification ratio correlates positively with the strength of the molecular dipole moment
and it is reduced with increasing molecular length.
This work was adapted from Zachary A. Lamport, Angela D. Broadnax, David Harrison, Katrina J. Barth, Lee Mendenhall, Clayton T. Hamilton, Martin Guthold, Timo Thonhauser, Mark E. Welker, and Oana D. Jurchescu, “Fluorinated Benzalkylsilane Molecular Rectifiers,” Sci. Rep., Vol. 6, p. 38092, 2016.
34
2.1 Introduction
Molecular electronics has been an area of great interest as device technology closes
on the limits of the often-mentioned Moore’s law, with consumer-available
technology already in the tens of nanometers range. The idea of molecular
electronics was proposed as a means to surpass the challenges present in
downscaling existing technologies[1–3]; however, commercially viable examples
have yet to be introduced [4–8]. Since 1974, when Aviram and Ratner proposed the
first rectification mechanism for a molecular structure containing a donor-acceptor
pair separated by a σ-bonded tunnelling bridge, the field has witnessed spectacular
growth [9]. A rectifier is a device which allows a large amount of current to pass
through at one bias while restricting current flow at the opposite bias. While the
above-mentioned theoretical work extrapolated the inorganic solid state p-n junction
miniaturized to the size of a single molecule and relied on the manipulation of the
molecule’s energy levels, several different mechanisms for enhanced rectifications
have subsequently been proposed [10–13]. Further experimental work has also
demonstrated that the donor-acceptor pair may not always be necessary, and that
systems with a single π-bonded component and a σ-bonded insulating chain can
provide a more consistent, as well as a greater degree of, rectification [14–16].
Significant research effort was dedicated toward the examination of both single-
molecule and molecular assemblies as rectifiers, with clear advantages exhibited by
both [17–20]. Single molecule devices allow for a direct comparison with the
theoretical calculations due to the lack of complexity in such systems. Nevertheless,
Nijhuis et al. produced convincing arguments for the observed rectification in
35
ferrocene-terminated alkanethiolate molecules assembled on surfaces in monolayer
patterns [21–23]. Other self-assembled monolayers and Langmuir-Blodgett films
provide ease of fabrication due to the molecule’s penchant for assembling on a well-
chosen surface. This allows for the formation of many devices on a single substrate
simultaneously and without the difficult alignment task present in single-molecule
structures. The research to improve the rectifying behaviour of molecular diodes,
however, has proceeded mostly by trial and error and a relation between the
molecular structure and resulting strength of rectification has not been clearly
demonstrated.
We have developed nine new fluorinated benzalkylsilane molecules of varying
internal molecular dipole due to the imposed length and termination and studied their
electrical properties in relation to their chemical structure. We incorporated the self-
assembled monolayers (SAMs) composed of these molecules into molecular diodes and
obtained high yields and reproducible rectification ratios as high as 200, with good bias
stress stability. We calculated the molecular dipole using density functional theory (DFT)
and we found that the rectifying behavior is stronger in the case of short molecules and is
enhanced by the internal dipole. Our results provide evidence that the molecular structure
impacts the electrical properties of molecular diodes through the internal dipole
characteristic to each molecule and may promote a rational design of compounds that can
function as high-performance molecular rectifiers.
36
2.2 Experimental
2.2.1 Molecular Structures and Analysis
The examined molecules were fluorine-substituted benzaldehyde imine-terminated
trialkoxysilanes, designed such that each contains a triethoxysilane group that
ensures the attachment on the Si/SiO2 surface, an alkyl chain with either 3 or 11 CH2
groups, a phenyl group to facilitate the formation of an “intermolecular top-link” to
enhance ordering, and a fluorine-containing head-group providing different amounts
Figure 2.1. Chemical structures of molecules 1-9
37
and orientations of fluorine atoms [24]. The chemical structures are included in
Figure 2.1, as follows: (E)-1-(4-(trifluoromethyl)phenyl)-N-(11-(triethoxysilyl)
undecyl)methanimine (molecule 1), (E)-1-(3,5-bis(trifluoromethyl) phenyl)-N-(11-
(triethoxysilyl)undecyl)methanimine (molecule 2), (E)-1-(4-fluorophenyl)-N-(11-
(triethoxysilyl)undecyl) methanimine (molecule 3), (E)- 1-(3-fluoro-4-
trifluoromethyl)phenyl)-N-(11-(triethoxysilyl)undecyl)methanimine (molecule 4),
(E)-1-(4-(trifluoromethyl)phenyl)-N-(3-(triethoxysilyl)propyl)methanimine
(molecule 5), (E)-1-(3,5-bis(trifluoromethyl)phenyl)-N-(3-(triethoxysilyl)propyl)
methanimine (molecule 6), (E)-1-(4-fluorophenyl)-N-(3-(triethoxysilyl)propyl)
methanimine (molecule 7), (E)- 1-(3-fluoro-4-trifluoromethyl)phenyl)-N-(3-
(triethoxysilyl)propyl)methanimine (molecule 8) and (E)-1-(perfluorophenyl)-N-(3-
(triethoxysilyl)propyl) methanimine (molecule 9). (E)-1-(perfluorophenyl)-N-(3-
(triethoxysilyl)undecyl) methanimine (the long chain analogue of molecule 9) was
also investigated; its properties, however, varied significantly from sample to
sample, as well as for the same film, and therefore we decided not to include it in
our analysis. The molecules were synthesized from amino trialkoxysilanes by using
the condensation of commercially available amino alkyl triethoxysilanes with
substituted benzaldehydes to prepare the precursors, following established
procedures, Figure 2.2 [25, 26]. We will refer to molecules 1, 2, 3, and 4 as the “long
molecules” (their lengths are between 2 and 2.2 nm) and molecules 5 through 9 as
the “short molecules” (their lengths are between 0.9 and 1.1 nm). The length was
evaluated using Spartan software, measured from the Si atom to the most distal atom
of the head-group for the lowest energy conformer of each molecule [27]. The
38
molecules were synthesized from amino trialkoxysilanes by using standard
procedures, see Figure 2.2. With the exception of one imine the yields for all of these
reactions were in the 68-87% range and the only purification of the products required
after the reaction was filtration and removal of residual solvent under high vacuum.
The new compounds were characterized by 1H and 13C nuclear magnetic resonance
(NMR) and elemental analysis or high-resolution mass spectrometry. The proton
nuclear magnetic resonance (1H NMR) spectra were obtained using a Bruker Avance
300 MHz spectrometer operating at 300.1 MHz or a Bruker Avance 500 MHz
spectrometer operating at 500.1 MHz. 13C NMR spectra were obtained using a
Bruker Avance 300 MHz spectrometer operating at 75.5 MHz. 1H and 13C NMR
spectra were referenced to the residual proton or carbon signals of the respective
deuterated solvents. All elemental analyses were performed by Atlantic Microlabs
Inc., Norcross, GA. High-resolution mass spectrometry was performed at the Mass
Spectrometry Facility at Northwestern University, Evanston, IL.
All reactions were carried out under an atmosphere of nitrogen.
Aminoundecyltriethoxysilane was purchased from Gelest and used as received.
Deuterated solvents were purchased from Cambridge Isotope Laboratories and dried
over molecular sieves. Aminopropyltriethoxysilane and the benzaldehydes were
purchased from Aldrich Chemical Company and used as received. Anhydrous
sodium sulfate and HPLC grd hexane were purchased from Fisher/Acros and used
as received.
39
2.2.2 Device Fabrication
The SAMs were deposited on highly-doped silicon wafers with native SiO2 formed
at their surface by either solution or vapor-based techniques. The substrates were
cleaned in hot acetone, then hot isopropanol (IPA), followed by a 10-minute
exposure to UV-ozone and subsequent rinse in DI water before being dried in a
stream of nitrogen. The UV-ozone step serves to both remove organic contaminants
and to increase the density of hydroxyl groups on the surface. The monolayers were
formed in a nitrogen (<0.1 ppm H2O, <0.1 ppm O2) glovebox. We found that for the
long SAMs (molecules 1, 2, 3, and 4) self-assembly from a 4 mMol solution in
chloroform at 30º C provided the films of highest quality. The short SAMs
(molecules 5 through 9) were amenable to deposition by vapor treatment, where the
silicon wafer was placed into a sealed jar along with 11 µL of the pure SAM
compound at 30º C. Both methods typically took 18 to 24 hours. To remove the
SiRO
OROR
(CH2)n
N
+
O H
R3
R4
R5
R2
R1H
R3
R4
R5
R2
R1SiRO
OROR
(CH2)n
NH2Na2SO4
dry solvent
1 R = Et, n = 11, R3 = CF3, R1, 2, 4, 5 = H, 73%
2 R = Et, n = 11, R2 = R4 = CF3, R1, 3, 5 = H, 75%
3 R = Et, n = 11, R3 = F, R1, 2, 4, 5 = H, 46%
4 R = Et, n = 11, R3 = CF3, R2 = F, R1, 4, 5 = H, 93%
5 R = Et, n = 3, R3 = CF3, R1,2,4,5 = H, 82%
6 R = Et, n = 3, R2 = R4 = CF3, R1,3,5 = H, 85%
7 R = Et, n = 3, R3 = F, R1,2,4,5 = H, 87%
8 R = Et, n = 3, R3 = CF3, R2 = F, R1, 4, 5 = H, 87%
9 R = Et, n = 3, R1-R5 = F, 68%
40
excess adsorbed molecules on the monolayer surface, the samples were thoroughly
rinsed in chloroform followed by IPA and then immediately dried in a stream of
nitrogen.
The conical-tip EGaIn contact, which was used as a top soft contact for our
molecular rectifiers, was created using a Micromanipulator probe holder, and a 0.5
mm diameter Micromanipulator probe tip. EGaIn was placed onto a sacrifical copper
strip into which the probe tip was slowly lowered and then raised with the aid of the
contact angle goniometer.
2.2.3 Surface Analysis
In order to evaluate the quality of the SAM and the best method for assembly, water
contact angle measurements were conducted using a Ramé-Hart Model 200 Contact
Angle Goniometer; the results are displayed in Table 2.1 and in Figure 2.3. A high
value of the contact angle generally coincides with a higher degree of order and/or
a more dense film [28]. Indeed, the contact angles measured for all the SAMs were
around 70°. These values agree with the measurements performed on other
fluorinated SAMs deposited on SiO2 [29]. For reference, we have also measured the
contact angle on untreated substrates, and that is displayed in the same figure. It can
be observed that the contact angle for bare native SiO2 is < 10º.
We employed atomic force microscopy (AFM) measurements to characterize
the surface roughness prior to SAM deposition. We measured a sample of 1 cm x 1
cm and imaged a surface area of approximately 2 x 2 µm in several spots using a
Nanoscope IIIA AFM (Veeco Instruments), see Figure 2.4. This surface is of similar
41
quality to that obtained using template stripped or flip-chip laminated metallic
electrodes [22, 30]. The roughness value is over an order of magnitude smaller than
the height of the molecules studied here, including the shortest one, and therefore it
is largely inconsequential with regard to the assembly of a film, allowing for the
formation of a highly ordered film when the processing parameters are carefully
tuned. A graphical illustration of this can be seen in Figure 2.4c, where we
schematically show the assembly of a long and a short molecule studied here and
the respective difference in the length scales of the substrate and molecules.
Figure 2.3. Contact angle measurements conducted on monolayers of molecules 1-9 assembled
on native SiO2. The contact angle of clean native SiO2 is shown for reference in panel 10.
42
The work function measurements on highly doped silicon were performed
using a Trek model 325 electrostatic voltmeter configured for Kelvin probe
measurements and calibrated using highly-ordered pyrolytic graphite (HOPG) as a
standard. The calculations followed from Equation 2.1:
𝜙𝜙 = −𝑒𝑒−(𝑉𝑉𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 − 𝑉𝑉𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻) + 𝜙𝜙𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻 (2.1)
where e is the elementary charge, 𝑉𝑉𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 is the measured signal from the substrate
(Si with a native layer of SiO2), 𝑉𝑉𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻 is the measured signal from HOPG, and
𝜙𝜙𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻 is the work function of HOPG (𝜙𝜙𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻 = 4.48 𝑒𝑒𝑉𝑉) [31]. Four samples were
evaluated, in at least 8 spots, and the results were consistent: we obtained a work
function of 𝜙𝜙𝑆𝑆𝑆𝑆 = 4.65 ± 0.05 𝑒𝑒𝑉𝑉 for the highly doped Si substrates with a thin
layer of native oxide on their surface.
Table 2.1: Contact angles, average rectification ratios, and dipole moments of molecules 1-9
Molecule Contact Angle (°) R μ (D)
1 74 23 ± 8 4.50
2 72 82 ± 24 4.64
3 70 36 ± 13 2.71
4 79 80 ± 25 5.29
5 66 183 ± 41 4.42
6 74 159 ± 38 4.59
7 77 34 ± 19 2.48
8 78 192 ± 42 5.40
9 70 104 ± 10 3.21
43
2.2.4 Ab Initio Calculations
We performed ab initio calculations at the density functional theory level, using
VASP [32, 33]. We used the standard projector augmented wave (PAW)
pseudopotentials provided by VASP along with the Perdew-Burke-Ernzerhof (PBE)
exchange-correlation functional [34, 35]. All calculations were done with a kinetic-
energy cutoff of 400 eV and an energy convergence criterion of 10-4 eV.
Calculations were performed in orthorhombic unit cells with a vacuum of at least 2
nm between periodic images. All structures were relaxed until the maximum force
on any atom was less than 10 meV/angstrom. Dipole moments were calculated using
Figure 2.4. (a) Isometric view of a single AFM scan, (b) general line profile from AFM scan,
and (c) schematic representation, to scale, of the SAM on a device substrate showing the
significant difference in length for both the long (left) and short (right) molecules compared to
the surface roughness.
a) c)
b)
44
VASP's built-in functionality to calculate and correct for the dipole moment in all
directions (i.e. the IDIPOL and LDIPOL tags).
2.2.5 Electrical Characterization
The electrical measurements were taken on metal/SAM/metal structures using a
eutectic gallium-indium (EGaIn) conical-tip top contact in which the silicon
substrate was held at ground and the bias was applied to the EGaIn contact in
ambient. In this configuration, the forward bias condition corresponds to a positive
potential applied at the fluorinated end of the molecules. The eutectic liquid metal
EGaIn was utilized as a soft top-contact because its use is non-damaging,
reproducible, and well-studied in this context [22, 23, 36–43]. A schematic of the
device structure used in this study is included in Figure 2.5a. This figure, however,
denotes an ideal structure of the device, while we are aware that our SAM layers
may exhibit a lower degree of order, different orientation with respect to the
substrate, and possible step-edges due to substrate imperfections, impurities, etc.
2.3 Results and Discussion
In Figure 2.5b we show the dependence of the current density (J) on the applied
voltage (V) for one film of molecule 3, in 25 different spots. The relative standard
deviation for these measurements is < 1.4. Measurements on different spots on the
films are consistent, suggesting that our SAMs are uniform. In addition, for a
particular SAM to be considered viable as a rectifier, it must be able to withstand
many repetitive measurements with minimum changes in the electrical
45
Figure 2.5. (a) Schematic of device structure showing a SAM sandwiched between the
native SiO2 bottom contact and EGaIn top contact. (b) Current density versus the applied
voltage on 25 different spots in a 1 cm x 1 cm area on a film consisting of molecule 3. (c)
Bias stress measurements on molecule 6 over 50 consecutive measurements in a single
spot.
a)
b)
c)
46
characteristics. This could involve a variability and instability in the measured
output characteristics or, in the worst-case scenario, a “breaking” of the monolayer,
indicating that there is now an irreversible conductive path between the two
electrodes. The good operational stability, as clearly seen in Figure 2.5c for molecule
5, where we show the results of 50 consecutive measurements, indicates that these
monolayers are robust to bias stress (the relative standard deviation is less than 0.21).
The difference between Figure 2.5b and Figure 2.5c at low voltages originates from
the fact that charging effects may occur when repeated measurements are taken on
the same spot on the film, causing noise and asymmetries in the measured current.
We note, however, that applying higher voltages may result in irreversible damage
of the SAM film. The breakdown field in these SAMs range from approximately 20
MV/cm for molecule 9 to 50 MV/cm for molecule 5.
Figure 2.6 shows typical current-voltage (J-V) characteristics for each of the
9 molecules, taken during the forward and reverse bias conditions. We measured at
least 10 devices for each type of SAM, in at least 25 spots each, and obtained similar
results. As expected, the magnitude of the measured current depends on the
molecular structure, with the long molecules generally exhibiting lower
conductivities and less pronounced asymmetries between the forward and reverse
bias conditions.
We define the rectification ratio, R, as the ratio between the J value during
the forward bias measurement, (V = +2V) and the J value during the reverse bias
measurement, (V = -2V):
𝑅𝑅= 𝐽𝐽(𝑉𝑉= +2𝑉𝑉)𝐽𝐽(𝑉𝑉=−2𝑉𝑉)
(2.2)
47
In Table 2.1 we list the R values obtained for the nine molecules. In order to
relate the electrical properties with the molecular structure, we determined the
electrical dipole of each molecule using DFT calculations; results are listed in Table
2.1. In Figure 2.7 we plot the dependence of R on the molecular dipole for all the
molecules investigated in this study. Results obtained for short molecules are
included in red, while those for long ones are shown in grey. First, note the high
value of R obtained for molecule 5 and 8, R5 = 183 ± 41 and R8 = 192 ± 42. These
values are on par with some of the highest reported in the literature, with a
thiophene-1,1-dioxide oligomer exhibiting an R of ~200, a ferrocene-alkanethiolate
with an R =150, and ferrocene placed on a monolayer of β-cyclodextrin
demonstrating an R of 170 [15, 38, 44]. We note, however, that larger values can
Figure 2.6. Representative current density versus applied voltage curves films of molecules 1-9.
48
also be obtained [45, 46]. Nevertheless, our molecules have the additional advantage
of being very low-cost since the precursors are common organic molecules, being
easy to synthesize, and requiring minimal purification steps. This figure also denotes
that a stronger rectification is achieved in the case of short molecules, in agreement
with previous reports [47, 48]. Another key point that can be observed from Figure
2.7 is that for molecules of similar length, the rectification ratio correlates positively
with the internal dipole moment of each molecule. This trend can originate from the
fact that the molecular dipole creates a local electric field proportional to the
magnitude of the dipole moment. This field contributes to the total net field
experienced by the molecule under external bias, modifying the shape of the
Figure 2.7. Rectification ratio vs. the dipole moment of each molecule. In red we show the
results corresponding to the short molecules and in grey for the long molecules.
49
tunnelling barrier, and, consequently, of the current magnitude. Although a detailed
interpretation of this dependence is beyond the scope of this experiment, the results
suggest that for molecules of similar length, the rectification increases with the
molecular dipole, regardless of structural details of each compound. The trend is
monotonic for the case of short molecules, and we note that there is a spread in our
data for the long molecules. On one hand, this may originate from variations in
molecular orientation and the degree of order of the SAMs on the surfaces, which
results from the competition between intermolecular interactions of the SAM
molecules and molecule-substrate interactions. We attempted to characterize the
ordering of the SAMs through polarized modulated-infrared reflection absorption
spectroscopy (PM-IRRAS) and X-ray reflectivity measurements, both of which
were unsuccessful due to the small size of the molecular layers. On the other hand,
local molecular torsions and changes in the conformation were shown to
significantly impact the value of the rectification ratio [49, 50]. Several other factors
may contribute in the observed asymmetries. First, the two contacts are not perfectly
symmetric: a strong, Si-O covalent bond is established between the SAM and the
bottom contact, while the top contact interacts with the SAM via a weak, van der
Waals bond established between the end-group of the SAM and the Ga2O3 layer
formed at the surface of the EGaIn electrode [22]. The strength of this bond, and
therefore the rate of charge tunnelling, may vary with changing the chemistry of the
SAM fluorinated functional group. In addition, these contacts have slightly different
work-functions (𝜙𝜙Si= 4.65 ± 0.05 eV, as determined from Kelvin probe
measurements described above, while 𝜙𝜙EGaIn = 4.1-4.2 eV [51]), which impact the
50
injection of charge carriers. Second, additional dipole moments may be formed
through band-bending when physical contact is established between the SAM and
electrodes. Nevertheless, the dependence is clearly distinguished, and this suggests
that tailoring the internal molecular dipole via molecular design may be a powerful
tool in controlling the rectification ratios in molecular diodes. This result may seem
to contradict that of Yoon and collaborators, who showed that the rectification
behaviour of molecular diodes is independent of their molecular dipole [12]. In that
study, however, the molecules had various lengths, and therefore the dependence
may have been masked. Further studies, focusing on the examination of properties
of different classes of compounds—including compounds with dipole moments in
opposite directions—as well as studies on SAMs of different anchoring and terminal
groups would clarify the generality and limitations of this method.
2.4 Conclusions
We have designed and measured nine new rectifying self-assembled monolayers of
different lengths and polar terminations. We found that these fluorinated
benzalkylsilane molecules exhibit high yield when incorporated in molecular
diodes, coupled with robust rectification ratios ranging from 20-200, depending on
their structure. In addition, they show good uniformity over large surface areas and
excellent bias stress stability. We found an increase in the rectification strength with
enhancing the internal molecular dipole of each molecule. Our results suggest that
the electrical properties of molecular diodes can be controlled by tailoring the
molecular structures of the constituent molecules. This is a significant step toward
51
controlling the performance of molecular rectifiers through manipulating specific
structure-property relationships for use in macroscopic electronics as diodes, half-
wave rectifiers, AC/DC converters, or other charge-restricting elements.
52
References
[1] C. P. Collier, “Electronically Configurable Molecular-Based Logic Gates,” Science, Vol. 285, p. 391, 1999.
[2] M. A. Reed, J. Chen, A. M. Rawlett, D. W. Price, and J. M. Tour, “Molecular random access memory cell,” Appl. Phys. Lett., Vol. 78, p. 3735, 2001.
[3] J. E. Green, J. W. Choi, A. Boukai, Y. Bunimovich, E. Johnston-Halperin, E. DeIonno, Y. Luo, B. A. Sheriff, K. Xu, Y. S. Shin, H.-R. Tseng, J. F. Stoddart, and J. R. Heath, “A 160-kilobit molecular electronic memory patterned at 10(11) bits per square centimetre,” Nature, Vol. 445, p. 414, 2007.
[4] H. Rascón-Ramos, J. M. Artés, Y. Li, and J. Hihath, “Binding configurations and intramolecular strain in single-molecule devices,” Nat. Mater., Vol. 14, p. 1, 2015.
[5] A. T. Haedler, K. Kreger, A. Issac, B. Wittmann, M. Kivala, N. Hammer, J. Köhler, H.-W. Schmidt, and R. Hildner, “Long-range energy transport in single supramolecular nanofibres at room temperature,” Nature, Vol. 523, p. 196, 2015.
[6] D. Xiang, X. Wang, C. Jia, T. Lee, and X. Guo, “Molecular-Scale Electronics: From Concept to Function,” Chem. Rev., Vol. 116, p. 4318, 2016.
[7] E. Lörtscher, “Wiring molecules into circuits,” Nat. Nanotechnol., Vol. 8, p. 381, 2013.
[8] R. M. Metzger, “Unimolecular Electronics,” Chem. Rev., Vol. 115, p. 5056, 2015.
[9] A. Aviram and M. A. Ratner, “Molecular rectifiers,” Chem. Phys. Lett., Vol. 29, p. 277, 1974.
[10] H. B. Akkerman and B. de Boer, “Electrical conduction through single molecules and self-assembled monolayers,” J. Phys. Condens. Matter, Vol. 20, p. 13001, 2007.
[11] R. M. Metzger, “Unimolecular electrical rectifiers,” Chem. Rev., Vol. 103, p. 3803, 2003.
[12] H. J. Yoon, C. M. Bowers, M. Baghbanzadeh, and G. M. Whitesides, “The Rate of Charge Tunneling Is Insensitive to Polar Terminal Groups in Self-Assembled Monolayers in Ag TS S(CH 2 ) n M(CH 2 ) m T//Ga 2 O 3 /EGaIn Junctions,” J. Am. Chem. Soc., Vol. 136, p. 16, 2014.
[13] S. Lenfant, C. Krzeminski, C. Delerue, G. Allan, and D. Vuillaume, “Molecular rectifying diodes from self-assembly on silicon,” Nano Lett., Vol. 3, p. 741, 2003.
[14] P. E. Kornilovitch, A. M. Bratkovsky, and R. S. Williams, “Current rectification by molecules with asymmetric tunneling barriers,” Phys. Rev. B, Vol. 66, p. 165436, 2002.
[15] N. Nerngchamnong, L. Yuan, D.-C. Qi, J. Li, D. Thompson, and C. A. Nijhuis,
53
“The role of van der Waals forces in the performance of molecular diodes,” Nat. Nanotechnol., Vol. 8, p. 113, 2013.
[16] D. K. Aswal, S. Lenfant, D. Guerin, J. V. Yakhmi, and D. Vuillaume, “Self assembled monolayers on silicon for molecular electronics,” Anal. Chim. Acta, Vol. 568, p. 84, 2006.
[17] H. B. Akkerman, P. W. M. Blom, D. M. de Leeuw, and B. de Boer, “Towards molecular electronics with large-area molecular junctions,” Nature, Vol. 441, p. 69, 2006.
[18] G. J. Ashwell, W. D. Tyrrell, and A. J. Whittam, “Molecular rectification: Self-assembled monolayers in which donor-(π-bridge)-acceptor moieties are centrally located and symmetrically coupled to both gold electrodes,” J. Am. Chem. Soc., Vol. 126, p. 7102, 2004.
[19] M. Elbing, R. Ochs, M. Koentopp, M. Fischer, C. von Hänisch, F. Weigend, F. Evers, H. B. Weber, and M. Mayor, “A single-molecule diode,” Proc. Natl. Acad. Sci. U. S. A., Vol. 102, p. 8815, 2005.
[20] I. Díez-Pérez, J. Hihath, Y. Lee, L. Yu, L. Adamska, M. A. Kozhushner, I. I. Oleynik, and N. Tao, “Rectification and stability of a single molecular diode with controlled orientation,” Nat. Chem., Vol. 1, p. 635, 2009.
[21] A. Batra, P. Darancet, Q. Chen, J. S. Meisner, J. R. Widawsky, J. B. Neaton, C. Nuckolls, and L. Venkataraman, “Tuning rectification in single-molecular diodes,” Nano Lett., Vol. 13, p. 6233, 2013.
[22] C. A. Nijhuis, W. F. Reus, and G. M. Whitesides, “Molecular rectification in metal-SAM-metal oxide-metal junctions,” J. Am. Chem. Soc., Vol. 131, p. 17814, 2009.
[23] C. A. Nijhuis, W. F. Reus, and G. M. Whitesides, “Mechanism of rectification in tunneling junctions based on molecules with asymmetric potential drops,” J. Am. Chem. Soc., Vol. 132, p. 18386, 2010.
[24] M. Halik, H. Klauk, U. Zschieschang, G. Schmid, C. Dehm, M. Schütz, S. Maisch, F. Effenberger, M. Brunnbauer, and F. Stellacci, “Low-voltage organic transistors with an amorphous molecular gate dielectric,” Nature, Vol. 431, p. 963, 2004.
[25] E. Y. Ladilina, V. V. Semenov, Y. A. Kurskii, O. V. Kuznetsova, M. A. Lopatin, B. A. Bushuk, S. B. Bushuk, and W. E. Douglas, “Carbofunctional fluorine-containing triethoxysilanes: synthesis, film forming and properties,” Russ. Chem. Bull., Vol. 54, p. 1160, 2005.
[26] J. C. Hicks, R. Dabestani, A. C. Buchanan, and C. W. Jones, “Spacing and Site Isolation of Amine Groups in 3-Aminopropyl-Grafted Silica Materials: The Role of Protecting Groups,” Chem. Mater., Vol. 18, p. 5022, 2006.
[27] SPARTAN, Wavefunction Inc. 18401 Von Karman Avenue, Suite 370, Irvine, CA 92612 USA.
54
[28] D. L. Allara and R. G. Nuzzo, “Spontaneously organized molecular assemblies. 1. Formation, dynamics, and physical properties of n-alkanoic acids adsorbed from solution on an oxidized aluminum surface,” Langmuir, Vol. 1, p. 45, 1985.
[29] K. P. Pernstich, S. Haas, D. Oberhoff, C. Goldmann, D. J. Gundlach, B. Batlogg, a. N. Rashid, and G. Schitter, “Threshold voltage shift in organic field effect transistors by dipole monolayers on the gate insulator,” J. Appl. Phys., Vol. 96, p. 6431, 2004.
[30] M. Coll, L. H. Miller, L. J. Richter, D. R. Hines, O. D. Jurchescu, N. Gergel-Hackett, C. a Richter, and C. a Hacker, “Formation of Silicon-Based Molecular Electronic Structures Using Flip-Chip Lamination,” J. Am. Chem. Soc., Vol. 131, p. 12451, 2009.
[31] J. W. Ward, M. a. Loth, R. J. Kline, M. Coll, C. Ocal, J. E. Anthony, and O. D. Jurchescu, “Tailored interfaces for self-patterning organic thin-film transistors,” J. Mater. Chem., Vol. 22, p. 19047, 2012.
[32] G. Kresse and J. Furthmüller, “Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set,” Phys. Rev. B, Vol. 54, p. 11169, 1996.
[33] G. Kresse and D. Joubert, “From ultrasoft pseudopotentials to the projector augmented-wave method,” Phys. Rev. B, Vol. 59, p. 1758, 1999.
[34] P. E. Blöchl, “Projector augmented-wave method,” Phys. Rev. B, Vol. 50, p. 17953, 1994.
[35] J. P. Perdew, K. Burke, and M. Ernzerhof, “Generalized Gradient Approximation Made Simple,” Phys. Rev. Lett., Vol. 77, p. 3865, 1996.
[36] R. C. Chiechi, E. A. Weiss, M. D. Dickey, and G. M. Whitesides, “Eutectic Gallium–Indium (EGaIn): A Moldable Liquid Metal for Electrical Characterization of Self-Assembled Monolayers,” Angew. Chemie Int. Ed., Vol. 47, p. 142, 2008.
[37] C. A. Nijhuis, W. F. Reus, J. R. Barber, M. D. Dickey, and G. M. Whitesides, “Charge transport and rectification in arrays of SAM-based tunneling junctions,” Nano Lett., Vol. 10, p. 3611, 2010.
[38] K. S. Wimbush, W. F. Reus, W. G. Van Der Wiel, D. N. Reinhoudt, G. M. Whitesides, C. A. Nijhuis, and A. H. Velders, “Control over rectification in supramolecular tunneling junctions,” Angew. Chemie - Int. Ed., Vol. 49, p. 10176, 2010.
[39] C. A. Nijhuis, W. F. Reus, A. C. Siegel, and G. M. Whitesides, “A molecular half-wave rectifier,” J. Am. Chem. Soc., Vol. 133, p. 15397, 2011.
[40] W. F. Reus, M. M. Thuo, N. D. Shapiro, C. A. Nijhuis, and G. M. Whitesides, “The SAM, not the electrodes, dominates charge transport in metal-monolayer//Ga2O3/gallium-indium eutectic junctions,” ACS Nano, Vol. 6, p. 4806, 2012.
55
[41] H. J. Yoon, K.-C. Liao, M. R. Lockett, S. W. Kwok, M. Baghbanzadeh, and G. M. Whitesides, “Rectification in Tunneling Junctions: 2,2′-Bipyridyl-Terminated n -Alkanethiolates,” J. Am. Chem. Soc., Vol. 136, p. 17155, 2014.
[42] X. Lefèvre, F. Moggia, O. Segut, Y. Lin, Y. Ksari, G. Delafosse, K. Smaali, D. Guérin, V. Derycke, D. Vuillaume, S. Lenfant, L. Patrone, and B. Jousselme, “Influence of Molecular Organization on the Electrical Characteristics of π-Conjugated Self-Assembled Monolayers,” J. Phys. Chem. C, Vol. 119 p. 5703, 2015.
[43] L. Jiang, C. S. S. Sangeeth, A. Wan, A. Vilan, and C. A. Nijhuis, “Defect Scaling with Contact Area in EGaIn-Based Junctions: Impact on Quality, Joule Heating, and Apparent Injection Current,” J. Phys. Chem. C, Vol. 119, p. 960, 2015.
[44] B. Capozzi, J. Xia, O. Adak, E. J. Dell, Z.-F. Liu, J. C. Taylor, J. B. Neaton, L. M. Campos, and L. Venkataraman, “Single-molecule diodes with high rectification ratios through environmental control,” Nat. Nanotechnol., Vol. 10, p. 522, 2015.
[45] L. Yuan, R. Breuer, L. Jiang, M. Schmittel, and C. A. Nijhuis, “A Molecular Diode with a Statistically Robust Rectification Ratio of Three Orders of Magnitude,” Nano Lett., Vol. 15, p. 5506, 2015.
[46] K. Garg, C. Majumder, S. K. Nayak, D. K. Aswal, S. K. Gupta, and S. Chattopadhyay, “Silicon-pyrene/perylene hybrids as molecular rectifiers,” Phys. Chem. Chem. Phys., Vol. 17, p. 1891, 2015.
[47] L. Luo, L. Balhorn, B. Vlaisavljevich, D. Ma, L. Gagliardi, and C. D. Frisbie, “Hopping Transport and Rectifying Behavior in Long Donor–Acceptor Molecular Wires,” J. Phys. Chem. C, Vol. 118, p. 26485, 2014.
[48] M. Mativetsky, Y. Loo, and P. Samor, “Elucidating the nanoscale origins of organic electronic function by conductive atomic force,” J. Mater. Chem. C, Vol. 2, p. 3118, 2014.
[49] B. Cui, W. Zhao, H. Wang, J. Zhao, H. Zhao, D. Li, X. Jiang, P. Zhao, and D. Liu, “Effect of geometrical torsion on the rectification properties of diblock conjugated molecular diodes,” J. Appl. Phys., Vol. 116, p. 73701, 2014.
[50] J. M. Mativetsky, G. Pace, M. Elbing, M. A. Rampi, M. Mayor, and P. Samorì, “Azobenzenes as light-controlled molecular electronic switches in nanoscale metal-molecule-metal junctions,” J. Am. Chem. Soc., Vol. 130, p. 9192, 2008.
[51] R. C. Chiechi, E. A. Weiss, M. D. Dickey, and G. M. Whitesides, “Eutectic Gallium–Indium (EGaIn): A Moldable Liquid Metal for Electrical Characterization of Self-Assembled Monolayers,” Angew. Chemie Int. Ed., Vol. 47, p. 142, 2008.
56
Chapter 3 Molecular Diodes from Self-Assembled
Monolayers on Silicon
Silicon-based electronics have been the subject of intense study driven by the
commercialization of the digital age. Unfortunately, the technology has nearly reached the
stage where the current modes of thinking have been outpaced by the demand for smaller
devices than doped silicon and similar materials can account for. To continue the trend of
shrinking length scales, the field of molecular electronics has bloomed. Research efforts
are aimed at understanding structure-property relationships in order to develop molecules
with predictable and stable electronic properties. Here, we demostrate efficient current
rectification in a new class of compounds that form self-assembled monolayers on silicon.
Integration of molecular electronics with silicon may yield hybrid systems that can expand
the use of silicon towards novel functionalities governed by the molecular species grafted
onto its surface. The new molecules that we introduce here consist of one alkyl chain and
a benzene termination with varius substituents and result from a simple, yet robust and
high-yield synthetic procedure. We found that when incorporated in molecular diodes,
these coumpounds can rectify current by as much as three orders of magnitude depending
on their structure. This performance is on par with that of the best molecular rectifiers
obtained on a metallic electrode, but it has the advantage of lower cost and more efficient
integration with current technologies.
This work was adapted from Zachary A. Lamport, Angela D. Broadnax, Ben Scharmann, Andrew DelaCourt, Hui Li, Scott M. Geyer, Mark E. Welker, and Oana D. Jurchescu, To be submitted, 2018.
57
3.1 Introduction
Electronic devices are ubiquitous to our lives and the quest for ever-smaller and better
devices, where commercially available silicon-based technologies have reached their
physical limits, has spurred the rise of molecular electronics. The field of molecular
electronics began in 1974 with the pivotal paper by Aviram and Ratner where it was
suggested that a single molecule consisting of one electron donor and one acceptor unit
connected by a σ-bonded tunneling bridge could rectify current, thus forming a molecular-
scale diode [1]. Since then, different strategies were proposed to obtain these molecular-
scale devices in multilayers, self-assembled monolayers (SAMs), and a variety of single-
molecule techniques. In 1993, Martin, et al. showed that the rectification behavior in a
donor-acceptor system is due to the organic layer itself, rather than work function mismatch
between contacts resulting in a Schottky diode [2]. Although the type of structure used in
this work differed slightly from the one proposed by Aviram and Ratner, their result was
extremely important in showing that the electrical properties of several molecular layers
can result in macroscopically significant measurements. Using the same zwitterionic
structure, Metzger, et al. proved that rectification is possible not only through multilayered
films but also through an organic monolayer [3]. In 2002, Chabinyc, et al. demonstrated
that the donor-acceptor structure is not a prerequisite for current rectification, heralding the
introduction of a new class of molecular rectifiers [4]. There have also been many examples
of single-molecule devices, including mechanically-controlled break junctions [5],
scanning tunneling microscopy (STM) [6, 7], and conducting probe atomic force
microscopy (CP-AFM) [8, 9]. The field has seen an explosion of interest as the fabrication
and analysis techniques have become more sophisticated, including the realization of a
58
memory circuit based on molecular junctions [10]. One of the drawbacks of working with
single molecular layers is the difficulty in establishing electrical contact to an already-
deposited layer, as an evaporated metal has the tendency to form filaments through to the
surface underneath, destroying the original, intended functionality [11]. A solution to this
issue was presented by Akkerman, et al., who introduced a multilayered top electrode
composed of the conducting polymer poly(3,4-ethylenedioxythiophene):
polystyrenesulfonate (PEDOT:PSS) and evaporated gold, where the soft contact of spin-
coated PEDOT:PSS precludes the formation of conducting filaments through the SAM
[12]. Chiechi, et al. pioneered the use of eutectic gallium-indium (EGaIn), a soft contact
that allowed to reproducibly obtain high-yield molecular devices [13]. The molecular
rectifiers based on SAMs take advantage of the spontaneous assembly of the molecular
layer on the substrate via chemisorption from solution or gas phase, a process that is both
simple and efficient.
Most current molecular rectifiers use thiol-terminated SAMs, which can only bond
to metallic substrates. This is due to the fact that the bond energy associated with the newly
formed Au-S covalent bond is very small compared to that of the Si-C or Si-O-C
characteristic in the case of monolayers on silicon, thus allowing the thiolate to migrate
between different Au sites. This molecular diffusion results in enhanced order, a property
which is not easily obtained in molecular layers that bind to silicon [14]. Nevertheless, the
procedures commonly used for metal deposition (i.e. thermal or electron-beam
evaporation) typically yield films with surface roughness that can match or even exceed
the lengths of the SAMs, particularly in the case of the short-chained compounds. In that
case, the measured electrical characteristics can be significantly altered by the
59
overwhelmingly greater conductivity of a metal-metal junction. To circumvent this issue,
the template-stripping method was developed wherein metal is deposited onto a silicon
wafer and, through the use of a release layer, the metal layer is inverted and transferred to
a different substrate [15]. While this process has proven effective for fabrication of high
performance Au-molecule-metal junctions, its complexity adds significantly to the costs of
production of these devices. Transition to silicon substrates could eliminate this issue since
a typical roughness of a silicon substrate without any post-processing is lower than 0.1 nm
[16], thus allowing its use without additional steps beyond cleaning. Another benefit to the
use of silicon as a substrate is the breadth of knowledge available from decades of work in
the commercial semiconductor industry and easier integration with current technologies
for hybrid devices [17]. The group of Wheeler provided a series of examples where devices
have been reliably assembled on the nanometer scale through chemically etching a
multilayered silicon-silicon oxide structure and bridging the gap with coated Au
nanoparticles [18, 19]. While, indeed, this work exploited the thiolates diffusion discussed
earlier, it also showed that the sub-10 nm scale can be achieved in silicon-based devices.
Following these studies, Ashwell, et al. used a similar procedure with amino-terminated
compounds to form molecular-scale devices which eschewed the metal nanoparticle used
in earlier studies, showing that compounds which covalently bonded to silicon are viable
options [20, 21]. Lenfant, et al. then gave an example of a sequentially-assembled
monolayer on silicon with an e-beam evaporated top contact, where a single molecular
orbital participated in charge transport leading to current rectification, but unfortunately
the rectification was very modest (maximum rectification ratio of 37 and yield of 50-70%)
[22]. Au-thiol-silicon molecular devices have been fabricated by using the flip-chip
60
lamination technique, which represented a significant step forward towards the
development of silicon-based molecular junctions [17]. Although there are convincing
reasons to work with molecular electronics on silicon, the vast majority of work has
focused on thiolates for the reasons outlined above, which can only be assembled on
metals. Here, we report on a series of newly developed molecules which can efficiently
assemble on silicon substrates to form high-performance molecular rectifiers. The
benzalkylsilane molecules consist of a 3-carbon alkyl chain and methanimine group
connected to a benzene ring with various substituents, and result from a very simple, and
high-yield synthetic procedure. We obtained a maximum rectification ratio of R = 2635 in
(E)-1-(4-cyanophenyl)-N-(3-(triethoxysilyl)propyl)methanimine, a value that rivals some
of the best molecular diodes on metallic surfaces. The ease of synthesis and straightforward
assembly on the surface of silicon makes these compounds compatible with current
electronic devices, and may expand the use of silicon technologies towards novel
functionalities.
3.2 Experimental
3.2.1 Device Fabrication
The eight different compounds, labelled 1-8, consisted of the same base structure: a
triethoxysilane, a 3-carbon alkyl chain, and a methanimine group attached to a benzene
ring. The substituents on the benzene ring were different for each system, as follows: 1: 4-
cyano, 2: 2,4-methoxy, 3: 2,6-difluoro, 4: 3-fluoro 4-cyano, 5: 3,5-difluoro, 6: 4-
dimethylamino, 7: 4-methoxy, 8: 4-nitro, see Figure 3.1a. SAMs comprised of these
61
molecules were constructed on 1 cm × 1 cm wafers of highly-doped silicon (≤0.005 Ωcm).
The silicon wafers were cleaned first by immersion in hot acetone for 10 minutes followed
by a rinse of fresh acetone and isopropyl alcohol (IPA), then a 10-minute immersion in hot
IPA followed by a rinse of fresh IPA and dried in a stream of nitrogen. They were then
subjected to a 10-minute UV/Ozone treatment, rinsed thoroughly with deionized water,
and dried in a stream of nitrogen. The samples were then introduced to a nitrogen glovebox
(<0.1 ppm H2O, <0.1 ppm O2) and submerged in a 4 mMol solution of one of compounds
1-8 in chloroform for 18-24 hours. Following the SAM deposition, the samples were
removed from the glovebox, rinsed thoroughly with fresh chloroform and IPA, and dried
Figure 3.1. (a) The general chemical structure of compounds 1-8. For compounds 1: R2, R3,
R5, R6 = H, R4 = CN; 2: R2, R3, R5 = H, R4, R6 = OCH3; 3: R3, R4, R5, = H, R2, R6 = F; 4: R2,
R3, R6 = H, R4 = CN, R5 = F; 5: R2, R4, R6 = H, R3, R5 = F; 6: R2, R3, R5, R6 = H, R4 =
N(CH3)2; 7: R2, R3, R5, R6 = H, R4 = OCH3; 8: R2, R3, R5, R6 = H, R4 = NO2, (b) Device
structure of the molecular rectifiers showing the liquid metal EGaIn probe tip, the SAM
under test, and the highly-doped silicon substrate.
a) b)
62
in a stream of nitrogen. The samples were immediately measured using an EGaIn probe tip
as the top electrode. The probe was constructed by depositing a small amount of EGaIn
onto a sacrificial copper strip into which a 0.5 mm diameter tungsten probe was lowered
and then gently raised using a Micromanipulator.
3.2.2 Electrical Characterization
Figure 3.1b shows the molecular rectifier structure where the bottom electrode was highly-
doped silicon upon which SAMs of compounds 1-8 were assembled, and the top electrode
was a cone of the liquid metal EGaIn. A bias was applied to the EGaIn electrode with the
silicon electrode held at ground, and the current was measured at forward and reverse bias,
of up to +2 V and -2 V. The figure of merit is the rectification ratio R, which is the ratio of
current density at forward bias compared to the current density at negative bias:
𝑅𝑅 =𝐽𝐽𝑉𝑉𝑓𝑓𝑓𝑓𝑓𝑓 = + 2 𝑉𝑉𝐽𝐽(𝑉𝑉𝑟𝑟𝑟𝑟𝑟𝑟 = − 2 𝑉𝑉)
(3.1)
where the current density is obtained by dividing the measured current by the area of the
EGaIn probe tip.
3.2.3 Cyclic Voltammetry
In order to determine the position of the highest occupied molecular orbital (HOMO),
cyclic voltammetry measurements were carried out using 0.1 M tetrabutylammonium
tetrafluoroborate in acetonitrile, a Pt counter-electrode, and an Ag/AgCl reference. The
system was calibrated using the Fc/Fc+ redox potential (0.6 V vs. Ag). The scans were
taken at a scan rate of 20 mV/s using the silicon wafer treated with the SAM under
examination as the working electrode.
63
3.3 Results and Discussion
In Figures 3.2a and 3.2b we show the average current density (J) as a function of applied
voltage (V) through SAMs of compound 1, on both a linear and a logarithmic scale. It can
be observed that charge transport is more efficient in the forward bias regime, similar to
the case of a solid-state diode. The average current density curve shown in Figure 3.2 was
obtained by taking the average current density from 100 J-V curves obtained on different
samples (multiple spots on one sample, as well as several SAM films).
The small standard deviation in current density points towards a great uniformity
in the electrical properties of the films, which is of paramount importance in any new
technology. Similar J-V curves were obtained on the other compounds, but with different
rectification strengths. It is also important to note that each compound rectified current in
the same direction, meaning that the current density at positive bias was always higher than
that at negative bias. At least 70 measurements resulting from a minimum of 5 samples
Figure 3.2. (a) The average J-V curve for compound 1 on a linear scale, showing the
diode-like rectification, (b) The average J-V curve for compound 1 on a logarithmic
scale where the scale bars show the standard deviation.
a) b)
64
were characterized for each type of SAM and the results are summarized in Figure 3.3,
where for each molecule we list the average and maximum rectification ratio obtained. The
highest rectification strength was obtained on compound 1, Rmax = 2635, with an average
of 1683 ± 458. To the best of our knowledge, this value represents the best rectification
ratio obtained from a SAM on silicon. In addition to the high performance showed in
devices, these compounds are synthetically simple to make, utilizing condensation
chemistry. For the rest of the molecules examined, the results are as follows: compound 2
Rmax = 1248, Ravg = 727 ± 323, compound 3 Rmax = 2118, Ravg = 711 ± 440, compound 4
Rmax = 1418, Ravg = 619 ± 230, compound 5 Rmax = 653, Ravg = 264 ± 162, compound 6 Rmax
= 933, Ravg = 235 ± 162, compound 7 Rmax = 590, Ravg = 221 ± 106, compound 8 Rmax =
367, Ravg = 212 ± 77. The variability in the properties of these compounds is by no means
a linear relationship, as not only does the rectification depends on the relative molecular
orbital’s energetic position and the physical asymmetry of each molecule, but also the local
electric field, the molecular disorder, and the molecular density. For example, although
compounds 2 and 3 show fairly large, and similar, standard deviations it is probable that
they arise from different sources. Compound 2 is asymmetric along the axis of the
molecule, which can introduce an intramolecular tilt thereby increasing disorder. However,
compound 3 is symmetric, but has a longer span in the plane of the substrate due to the
width of the molecule, which can decrease the molecular density on the surface.
To elucidate the contribution of the difference in work functions of the silicon and
EGaIn electrodes to current rectification we also evaluated a blank sample, in which the
EGaIn was in direct contact with the silicon wafer; the resulting J-V curve is shown in
Figure 3.4. The work functions of the EGaIn and silicon electrodes are 4.1-4.2 eV and 4.65
65
eV, respectively, raising the possibility of a Schottky mechanism when comparing forward
to reverse bias [16, 23]. However, as the electrical characteristics in Figure 3.4 show, there
is negligible current rectification in this sample, and therefore the observed rectification is
solely the result of the presence of the molecule.
Below we discuss the mechanism through which these compounds rectify current.
The asymmetries between the positive and negative biases result from the difference
between current resulting from strictly tunneling and current arising from tunneling plus
hopping transport, similar to the case of molecular rectifiers on Au, see Figure 1.3 in
Chapter 1 [24, 25]. At reverse bias, charges can only tunnel from one electrode to the other,
with the SAM acting as a tunneling barrier. On the contrary, in the forward bias regime,
the HOMO of the π-conjugated tailgroup is accessible between the two electrodes resulting
in charges moving through a hopping mechanism, as well as tunneling through the
remaining σ-bonded alkyl chain. Due to the considerable inefficiency of tunneling
Figure 3.3. Histograms showing the measured rectification ratios of compounds 1-8.
66
transport and its dependence on the length of the tunneling barrier, the abbreviated
tunneling plus hopping mechanism results in a higher measured current. Note, however,
that the rectification properties could also arise from molecular asymmetry, where the
position of the relevant molecular orbital is located closer to one end of the molecule [24].
Even if the molecular orbital is accessible at both bias polarities, the larger voltage drop
across the σ-bonded tunneling barrier results in different electrostatic conditions on the
conjugated portion of the molecule at equal and opposite biases [24]. In reality, the
rectification through SAMs is a combination of these two mechanisms, though it is also
affected by disorder in the monolayer, the type of bond between the molecules themselves
and the contacts, and any potential work function mismatch between the contacts.
Figure 3.4. J-V characteristics on a Si:SiO2:EGaIn junction, showing that the choice of
electrodes results in negligible rectification. The voltage sweep was taken from -1 V to 1
V due to the current reaching the compliance of the instrument at higher voltages.
67
To understand the differences in rectification strengths observed in Figure 3.3, we
conducted cyclic voltammetry measurements on the two compounds which gave the
highest rectification ratios, 1 and 2, the results are shown in Figure 3.5. The onset of
oxidation gives a value for the HOMO level of compound 1 of E1,HOMO = -6.3 eV (see
Figure 3.5a), and E2,HOMO = -6.1 eV for compound 2 (see Figure 3.5b). This difference of
ΔE = 0.2 eV plays a critical role in the current rectification as the electrostatic asymmetry
of these compounds is the defining factor here, and a difference in HOMO level values
results in a difference in the potential barrier to charge transport. Of course, as mentioned
above, the physical separation of the metallic contacts and the molecular orbital will play
a role as the system is highly asymmetric. The strong electron withdrawing cyano group in
compound 1 effectively pulls the HOMO level closer to the end of the molecule near the
EGaIn contact, resulting in the orbital being exposed to the greatest potential at positive
bias. This significant spatial asymmetry could result in an easier charge injection at positive
bias which causes the extreme asymmetry in current density at opposing biases.
Figure 3.5. (a) Cyclic voltammetry measurement on compound 1, (b) Cyclic
voltammetry measurement on compound 2.
a) b)
68
3.4 Conclusions
We have demonstrated efficient molecular rectifiers assembled on silicon substrates. The
new compounds rectify current to varying degrees, with the highest recorded rectification
ratio showing an average of nearly 1700 and a maximum of 2635. To date, this is the
highest rectification ratio obtained on a molecular junction fabricated on silicon. We have
identified the likely causes behind the large differences in electrical properties between the
compounds, laying the groundwork for future studies to enhance their efficacy. Not only
do they contribute superior performance, but they are relatively simple to produce and
manufacture. This work represents substantial progress towards the use of molecular-scale
devices in commercially-viable electronics.
69
References
[1] A. Aviram and M. A. Ratner, “Molecular rectifiers,” Chem. Phys. Lett., Vol. 29, p. 277, 1974.
[2] A. S. Martin, J. R. Sambles, and G. J. Ashwell, “Molecular rectifier,” Phys. Rev. Lett., Vol. 70, p. 218, 1993.
[3] R. M. Metzger, B. Chen, U. Höpfner, M. V. Lakshmikantham, D. Vuillaume, T. Kawai, X. Wu, H. Tachibana, T. V. Hughes, H. Sakurai, J. W. Baldwin, C. Hosch, M. P. Cava, L. Brehmer, and G. J. Ashwell, “Unimolecular electrical rectification in hexadecylquinolinium tricyanoquinodimethanide,” J. Am. Chem. Soc., Vol. 119, p. 10455, 1997.
[4] M. L. Chabinyc, X. Chen, R. E. Holmlin, H. Jacobs, H. Skulason, C. D. Frisbie, V. Mujica, M. A. Ratner, M. A. Rampi, and G. M. Whitesides, “Molecular Rectification in a Metal−Insulator−Metal Junction Based on Self-Assembled Monolayers,” J. Am. Chem. Soc., Vol. 124, p. 11730, 2002.
[5] M. A. Reed, C. Zhou, C. J. Muller, T. P. Burgin, and J. M. Tour, “Conductance of a Molecular Junction,” Science (80-. )., Vol. 278, p. 252, 1997.
[6] B. Xu, “Measurement of Single-Molecule Resistance by Repeated Formation of Molecular Junctions,” Science (80-. )., Vol. 301, p. 1221, 2003.
[7] C. Joachim, J. K. Gimzewski, R. R. Schlittler, and C. Chavy, “Electronic transparence of a single C60 molecule,” Phys. Rev. Lett., Vol. 74, p. 2102, 1995.
[8] D. J. Wold and C. D. Frisbie, “Formation of Metal−Molecule−Metal Tunnel Junctions: Microcontacts to Alkanethiol Monolayers with a Conducting AFM Tip,” J. Am. Chem. Soc., Vol. 122, p. 2970, 2000.
[9] D. J. Wold and C. D. Frisbie, “Fabrication and Characterization of Metal−Molecule−Metal Junctions by Conducting Probe Atomic Force Microscopy,” J. Am. Chem. Soc., Vol. 123, p. 5549, 2001.
[10] J. E. Green, J. W. Choi, A. Boukai, Y. Bunimovich, E. Johnston-Halperin, E. DeIonno, Y. Luo, B. A. Sheriff, K. Xu, Y. S. Shin, H.-R. Tseng, J. F. Stoddart, and J. R. Heath, “A 160-kilobit molecular electronic memory patterned at 10(11) bits per square centimetre,” Nature, Vol. 445, p. 414, 2007.
[11] B. de Boer, M. M. Frank, Y. J. Chabal, W. Jiang, E. Garfunkel, and Z. Bao, “Metallic Contact Formation for Molecular Electronics: Interactions between Vapor-Deposited Metals and Self-Assembled Monolayers of Conjugated Mono- and Dithiols,” Langmuir, Vol. 20, p. 1539, 2004.
[12] H. B. Akkerman, P. W. M. Blom, D. M. de Leeuw, and B. de Boer, “Towards molecular electronics with large-area molecular junctions,” Nature, Vol. 441, p. 69, 2006.
[13] R. C. Chiechi, E. A. Weiss, M. D. Dickey, and G. M. Whitesides, “Eutectic
70
Gallium–Indium (EGaIn): A Moldable Liquid Metal for Electrical Characterization of Self-Assembled Monolayers,” Angew. Chemie Int. Ed., Vol. 47, p. 142, 2008.
[14] A. Vilan, O. Yaffe, A. Biller, A. Salomon, A. Kahn, and D. Cahen, “Molecules on Si: Electronics with chemistry,” Adv. Mater., Vol. 22, p. 140, 2010.
[15] M. Hegner, P. Wagner, and G. Semenza, “Ultralarge atomically flat template-stripped Au surfaces for scanning probe microscopy,” Surf. Sci., Vol. 291, p. 39, 1993.
[16] Z. A. Lamport, A. D. Broadnax, D. Harrison, K. J. Barth, L. Mendenhall, C. T. Hamilton, M. Guthold, T. Thonhauser, M. E. Welker, and O. D. Jurchescu, “Fluorinated benzalkylsilane molecular rectifiers,” Sci. Rep., Vol. 6, p. 38092, 2016.
[17] M. Coll, L. H. Miller, L. J. Richter, D. R. Hines, O. D. Jurchescu, N. Gergel-Hackett, C. a Richter, and C. a Hacker, “Formation of Silicon-Based Molecular Electronic Structures Using Flip-Chip Lamination,” J. Am. Chem. Soc., Vol. 131, p. 12451, 2009.
[18] S. M. Dirk, S. W. Howell, S. Zmuda, K. Childs, M. Blain, R. J. Simonson, and D. R. Wheeler, “Novel one-dimensional nanogap created with standard optical lithography and evaporation procedures,” Nanotechnology, Vol. 16, p. 1983, 2005.
[19] S. W. Howell, S. M. Dirk, K. Childs, H. Pang, M. Blain, R. J. Simonson, J. M. Tour, and D. R. Wheeler, “Mass-fabricated one-dimensional silicon nanogaps for hybrid organic/nanoparticle arrays,” Nanotechnology, Vol. 16, p. 754, 2005.
[20] G. J. Ashwell, L. J. Phillips, B. J. Robinson, B. Urasinska-Wojcik, C. J. Lambert, I. M. Grace, M. R. Bryce, R. Jitchati, M. Tavasli, T. I. Cox, I. C. Sage, R. P. Tuffin, and S. Ray, “Molecular bridging of silicon nanogaps,” ACS Nano, Vol. 4, p. 7401, 2010.
[21] G. J. Ashwell, L. J. Phillips, B. J. Robinson, S. A. Barnes, A. T. Williams, B. Urasinska-Wojcik, C. J. Lambert, I. M. Grace, T. I. Cox, and I. C. Sage, “Synthesis of covalently linked molecular bridges between silicon electrodes in CMOS-based arrays of vertical Si/SiO 2/Si nanogaps,” Angew. Chemie - Int. Ed., Vol. 50, p. 8722, 2011.
[22] S. Lenfant, C. Krzeminski, C. Delerue, G. Allan, and D. Vuillaume, “Molecular rectifying diodes from self-assembly on silicon,” Nano Lett., Vol. 3, p. 741, 2003.
[23] R. C. Chiechi, E. A. Weiss, M. D. Dickey, and G. M. Whitesides, “Eutectic Gallium–Indium (EGaIn): A Moldable Liquid Metal for Electrical Characterization of Self-Assembled Monolayers,” Angew. Chemie Int. Ed., Vol. 47, p. 142, 2008.
[24] P. E. Kornilovitch, A. M. Bratkovsky, and R. S. Williams, “Current rectification by molecules with asymmetric tunneling barriers,” Phys. Rev. B, Vol. 66, p. 165436, 2002.
71
[25] C. Krzeminski, C. Delerue, G. Allan, D. Vuillaume, and R. M. Metzger, “Theory of electrical rectification in a molecular monolayer,” Phys. Rev. B - Condens. Matter Mater. Phys., Vol. 64, p. 1, 2001.
72
Chapter 4 Organic Thin Films with Charge Carrier
Mobility Exceeding that of Single Crystals
The performance of organic field-effect transistors (OFETs) depends heavily upon the
intrinsic properties and microstructure of the semiconducting layer, the processes taking
place at the semiconductor/dielectric interface, and the quality of contacts. In this article,
we report on 7,14-bis(trimethylsilylethynyl) benzo[k]tetraphene single crystal and thin-
film OFETs and compare their properties. We find that the single crystals exhibit a
pronounced anisotropy in electrical characteristics, with a maximum field-effect mobility
of 0.3 cm2/Vs. Through density functional theory (DFT) calculations we identified the
main direction for hole transport, which was confirmed by X-ray diffraction (XRD)
measurements as parallel to the plane of the single crystal facet where the transport was
probed. By processing the material as a thin-film semiconductor, the content of high-
mobility direction probed within the transistor channel was enhanced. The control of film
morphology, coupled with a different design of the device structure allowed us to obtain
an order of magnitude higher charge carrier mobilities and a very small spread in device
performance.
This work was published as Zachary A. Lamport, Ruipeng Li, Chao Wang, William Mitchell, David Sparrowe, Detlef-M. Smilgies, Cynthia Day, Veaceslav Coropceanu, and Oana D. Jurchescu, “Organic thin films with charge-carrier mobility exceeding that of single crystals,” J. Mater. Chem. C, Vol. 5, p. 10313, 2017.
73
4.1 Introduction
Organic semiconductors have drawn considerable interest due to the inherent processing
flexibility afforded by their van der Waals intermolecular forces and the intrinsic synthetic
versatility of organic chemistry which allows for a near-limitless array of materials
depending, in principle, only on the desired function. The weakly bonded organic crystals
are amenable to manufacturing near room temperature from solution, allowing for a diverse
assortment of viable substrates. At the same time, these weak interactions result in a
reduced performance due, in part, to increased defect probability and stronger interactions
between the lattice and charge carriers. A strenuous experimental and theoretical effort into
material development produced significant improvements in organic device properties over
the past decades. For example, the charge-carrier mobility in organic field-effect transistors
(OFETs) has increased several orders of magnitude [1, 2]. The mobility of such a device,
however, is not simply a function of the intrinsic material properties but the film
microstructure, the quality of the contacts, and the processes at the
semiconductor/dielectric interface play a consequential role. In the example of a rigorously
studied organic semiconductor, diF-TES ADT (2,8-difluoro-5,11-bis(triethylsilylethynyl)
anthradithiophene), a change of dielectric from SiO2 to the polymer Cytop allowed an order
of magnitude increase in mobility due to a reduction in the trap density in the
semiconductor film [3]. In the same material, surface modifications with self-assembled
monolayers (SAMs) were used to tune both the film microstructure and device contact
resistance, which resulted in OFETs with mobilities up to several orders of magnitude
greater [4–9]. Fabricating devices using single crystals corresponded with an additional
order of magnitude increase in mobility [10]. OFETs made using semiconductors in a
74
single crystal form avoid many of the issues which arise in thin films because single
crystals possess a highly-ordered microstructure and grain boundaries are nonexistent.
These properties allow single-crystal OFETs to access the fundamental limits of charge
transport in a material, provided that the contacts are optimized [11], setting an upper limit
on the possible electrical characteristics for a new material when processed in a different
fashion [12–19]. While single-crystal organic semiconductors will often set benchmark
electrical properties for a compound, they are highly anisotropic, i.e. the charge-carrier
mobility is highly dependent on the crystallographic direction along which the
measurements are conducted [15, 20–22].
Here, we examine a compound that shows significant anisotropy in the ab-plane
such that the mobility varies by at least one order of magnitude depending on the direction
of measurement. We then tune the microstructure of the thin film to enhance the content of
the high-mobility molecular orientation with respect to the transistor channel, and by doing
Figure 4.1. Chemical structure of 7,14-bis (trimethylsilylethynyl) benzo[k]tetraphene.
75
so we consistently obtain mobilities greater than 1 cm2/Vs. The material of focus is the
nonlinear acene 7,14-bis(trimethylsilylethynyl) benzo[k]tetraphene (TMS-BT) (chemical
structure shown in Figure 4.1), which was synthesized following the procedures reported
elsewhere [23]. Nonlinear acenes have received considerable interest recently due to their
enhanced environmental stability as compared to the linear acenes, while maintaining the
favorable electronic properties which originally drew attention to the class of polycyclic
aromatic hydrocarbons [24–28]. For example, Zhang et al. produced a nonlinear acene that
exhibited a field-effect mobility as high as 6.1 cm2/Vs and identified the origin of the
additional stability present in this class of compounds [27]. They suggested that because a
nonlinear acene contains a greater number of aromatic Clar sextets than a linear acene, the
stability is increased in accordance with Clar’s sextet rule. In our system, single crystal
OFETs were fabricated on a SiO2 dielectric and mobilities between 0.0004 and 0.3 cm2/Vs
were obtained. To determine the source of this disparity in device performance, we
conducted density functional theory (DFT) calculations. This analysis confirmed the
presence of charge transport anisotropy and informed us on the direction of high mobility.
Using this information, we were then able to carefully tune processing parameters to
enhance the content of the high-mobility molecular orientation in thin films, as confirmed
by X-ray diffraction (XRD) data. By controlling the microstructure of the thin films and
implementing a device structure incorporating Cytop as the top-gate dielectric, we
consistently obtained mobilities greater than 1 cm2/Vs.
76
4.2 Experimental
4.2.1 Single Crystal Growth
The physical vapor transport (PVT) method was used to grow single crystals, as seen in
Figure 4.2a [29]. The quartz tube was cleaned using detergent followed by DI water, high-
purity acetone, then high purity isopropanol (IPA), and baked overnight at approximately
300°C. The starting material was then placed in the tube at the hottest part of the furnace
and left under flowing high-purity argon at room temperature overnight. Before bringing
the furnace to the sublimation temperature, the temperature was raised above 100°C for
two hours to remove any remaining water. TMS-BT was heated to approximately 195°C
under 50 mL/min high-purity argon flow and within a day of reaching the sublimation
temperature, thin, yellow-green platelets were obtained in the cold part of the tube, the
Figure 4.2. (a) A depiction of the physical vapor transport setup used to grow TMS-BT, (b)
Schematic representation of the single-crystal OFET device structure, (c) An image of TMS-BT
laminated between the source and drain contacts.
a)
b) c)
77
crystallization region. Once the crystals formed, the furnace temperature was reduced by
10°C every 20 minutes to room temperature to reduce thermal shock in the crystals upon
cooling the growth tube.
4.2.2 OFET Fabrication and Electrical Characterization
For single-crystal OFETs, a bottom-gate, bottom-contact structure was used where the
bottom gate consisted of highly-doped silicon with 200 nm of thermally grown SiO2 as the
gate dielectric. Source and drain contacts were patterned by photolithography or shadow
mask and deposited by e-beam evaporation (5 nm Ti/45 nm Au). No significant difference
was noted in the performance of the devices as a function of contact patterning method.
After metal evaporation, the substrates were cleaned in hot acetone, hot IPA, followed by
a 10-minute exposure to UV-ozone and rinsed with DI water. The contacts were then
treated with 2,3,4,5,6-pentafluorobenzenethiol (PFBT) using a 30mM solution in ethanol
for 30 minutes followed by a 5-minute sonication in ethanol. The single crystals were then
laminated by hand between the source and drain contacts to complete the OFET structure,
as seen in Figures 4.2b and 4.2c. The thin-film devices were prepared with a top-gate,
bottom-contact structure on a glass substrate. Source and drain contacts were processed
similar to the case of single-crystal devices. TMS-BT was spin-coated onto the samples
from a 10 mg/mL solution in mesitylene. The Cytop top-gate dielectric was then spin-
coated and baked at 100°C for 2 minutes on a hot plate to obtain an 800-850 nm thick layer,
onto which a silver top gate was evaporated. Electrical characterization was performed
using an Agilent 4155C Semiconductor Parameter Analyzer in ambient and dark.
78
4.2.3 Theoretical Calculations
The electronic structure of the TMS-BT crystal was obtained on the basis of experimental
crystal structure using the B3LYP functional and the 6-31G basis set. The Brillouin zone
was sampled using a uniform 10 × 4 × 4 k-point grid. The effective masses (mij) of holes
were calculated by means of Equation 1:
1𝑚𝑚𝑖𝑖𝑖𝑖
= 1ℏ2
𝜕𝜕2𝐸𝐸𝜕𝜕𝜕𝜕𝑖𝑖𝜕𝜕𝜕𝜕𝑖𝑖
(4.1)
Here E is the band energy, ℏ is Planck’s constant, k is the hole wave vector, and i and j are
the Cartesian coordinates in reciprocal space. The inverse effective mass tensor was
calculated by means of Sperling’s centered difference method with ∂k = 0.06/bohr. The
effective transfer integrals for holes were evaluated at the B3LYP/6-31G(d,p) level and
based on the fragment orbital approach combined with a basis set orthogonalization
procedure [30]. The periodic boundary conditions calculations were performed by means
of the Crystal 14 package [31] while the transfer integrals were obtained using the Gaussian
09 package [32].
4.2.4 Structural Characterization
X-ray diffraction (XRD) and grazing incidence X-ray diffraction (GIXD) were performed
at beamline G2 at the Cornell High Energy Synchrotron Source (CHESS). Both thin-film
and single-crystal samples were characterized on a Psi-Circle surface diffractometer [33]
using an X-ray energy of 11.2 keV (λ = 1.071 Å) from a Be single-crystal beam-splitting
monochromator. XRD was carried out in the θ-2θ mode from 2° to 20° with a step of 0.02°
and recorded using a 640-element linear diode-array detector. The diffraction peaks were
79
analyzed by fitting Gaussian distributions using Origin Pro software. Two molecular
orientations were identified in thin films and their relative content was calculated from the
ratio of integrated peak area and structure factor, according to ref. [8].
GIXD was performed on a surface diffractometer in powder mode, with the sample
being rotated 360° around the surface normal for each angle step of the data collection [34].
A set of Soller slits was mounted on the detector arm to provide an in-plane resolution of
0.16°. Scattered X-rays were collected with a linear diode array. The incident angle was
fixed at 0.15° and the in-plane scattering angle ν covered a range from 2 to 30° with a step
size of 0.1°. The diode array covered 10° of out-of-plane scattering angle δ such that a two-
dimensional scattering map I(ν,δ) of the sample was obtained in a single scan. The structure
was determined by fitting peak positions and parallel crystallographic planes [34, 35].
4.3 Results and Discussion
Included in Figures 4.3a and 4.3b are typical electrical measurements recorded on the
single-crystal OFETs. The evolution of drain current (ID) vs. drain-source voltage (VDS) at
varying gate-source voltages (VGS) shows linear behavior typical of Ohmic contacts before
the expected transition to the saturation regime in Figure 4.3a. These measurements
indicate weak or negligible contact effects in these devices. The gate-sweep measurement
of the drain current in the saturation regime at VDS = -40V is seen in Figure 4.3b, with the
square root of drain current in black corresponding to the left axis and the drain current on
80
a log scale in blue on the right axis, shows a fairly sharp turn-on with a subthreshold swing
of S = 1.3 V/dec a current on/off ratio greater than 105, a field-effect mobility of µ = 0.3
cm2/Vs, though the threshold voltage is quite high at VTh = -15V. This high threshold
voltage is a common characteristic of our single-crystal devices and this is most likely due
to the high density of trapping states at the surface of the SiO2 dielectric. We measured
over 30 single-crystal OFETs and a summary of the field-effect mobilities is included in
Figure 4.4. It can be observed that there is a spread of approximately three orders of
Figure 4.3. (a) The evolution of the drain current vs. drain-source voltage at varying gate
voltage, (b) A gate-source voltage sweep in the saturation regime at constant drain-source
voltage.
a)
b)
81
magnitude in measured mobilities, with a minimum of µmin = 0.0004 cm2/Vs, a maximum
of µmax = 0.3 cm2/Vs and an average of µavg = 0.03 cm2/Vs. This large variance in mobility
can be attributed to different factors including crystal and contact quality, as well as
anisotropy in the electrical properties along the different crystal directions as has been
reported in other systems [15, 20–22].
We conducted DFT calculations in order to elucidate the electronic properties of
TMS-BT and relate the results along the different crystallographic directions to the
measured spread in the electrical properties. The electronic structure calculations, Figures
4.5 and 4.6, suggest that the largest electronic couplings (46 meV) for holes occurs along
the a crystallographic axis. As a consequence, the smallest effective mass (𝑚𝑚1 = 1.01 𝑚𝑚0,
where 𝑚𝑚0 is the electron mass in vacuum) and the main direction for hole transport are also
identified with this direction. The calculations also reveal that the next smallest effective
mass component, which is found along [210] direction, is 2.38 𝑚𝑚0 suggesting that charge
Figure 4.4. Single-crystal OFET mobility statistics obtained for over 30 devices.
82
transport for holes has a quasi-two-dimensional character, as illustrated in Figure 4.6. XRD
measurements performed on single crystals of TMS-BT found a series of (00l) peaks (l =
1, 2, 3, 4) showing a (001) orientation of the crystals laminated on a silicon wafer substrate.
This assignment is supported by the GIXD intensity map which only shows reflections due
to a (001) texture (Figure 4.7). The observed texture shows that the ab-plane of the crystal
is parallel to the surface of the crystal, and therefore to the substrate, hence electrical
measurements on such laminated single crystals largely probe charge transport in the ab-
plane. From the known bulk structure [23], we conclude that the TMS-BT molecules are
essentially oriented along the crystal surface normal, as shown in Figure 4.8, where we
depict one possible orientation of the molecules with respect to the device structure.
Figure 4.5. Electronic band structures and densities of states of TMS-BT. The points of high
symmetry in the first Brillouin zone are in crystallographic coordinates: Γ = (0, 0, 0); X = (0.5,
0, 0); Y= (0, 0.5, 0); Z = (0, 0, 0.5); V = (0.5, 0.5, 0); U = (0.5, 0, 0.5); T = (0, 0.5, 0.5).
83
In this case, the high-mobility direction [100], determined by DFT (marked in red), is
perpendicular to the direction of charge transport (yellow). This crystal orientation with
respect to the source/drain contacts results in subdued electrical properties in the single-
crystal OFETs. Since we have not oriented the crystals with respect to the electrodes, the
large anisotropy in the electrical properties of this material in the ab-plane, which was
probed in OFET measurements, provides an explanation for the large spread in mobilities
seen in Figure 4.4. The highest mobilities likely were obtained when measuring along the
[100] direction and the lowest mobilities correspondingly measured nearly perpendicular
to this high-mobility direction. We do not exclude, however, the contributions from the
inherent differences in the crystal and contact quality.
t12= 46 meV
t34= 37 meV
t13= t24 = 10 meV
m1= 1.01 m0
m2= 2.38 m0
Figure 4.6. Illustration of dominant charge transport pathways for holes in TMS-BT crystals.
The smallest effective mass is observed along the stacking direction (a axis, indicated by a red
arrow), and the next smallest effective mass component is along the [210] direction (indicated
by a blue arrow). The numbers in the crystal structure label the molecules used for the electronic
coupling calculations.
84
In order to enhance the content of the high mobility [100] direction in devices, we
turn to processing. Controlling the solid-state packing by processing and/or post-
processing takes advantage of the weak intermolecular bonds characteristic to organic
crystals, which often allow to fine-tune the crystal structure by manipulating
solvent/substrate, solute/substrate, solvent/vapor, and solute/solvent interactions. Such
efforts have been a staple of the field of organic electronics and morphology modification
has been accomplished by many different techniques [7, 36–43]. By tuning the processing
conditions of TMS-BT layer in organic thin-film transistors (OTFTs), as well as the device
structure (nature of the constituent layers) and architecture (order in which the layers
Figure 4.7. Indexation of the grazing-incidence diffraction pattern of a single crystal. Laminated
single crystals show a predominant (001) texture.
85
are deposited), we were able to control the molecular orientation such that the content of
high-mobility orientation was enhanced along the direction where the transport was
measured. The structure of the OTFT can be found in Figure 4.9a, where the bottom-gate
dielectric SiO2 was replaced with a Cytop top-gate dielectric. Typical performance is
shown in Figure 4.9b. This particular device exhibits a more than three-fold increase in
mobility compared to the best single-crystal device, µ = 1 cm2/Vs. Measurements taken on
a large data set yielded an average mobility of µ = 0.95 ± 0.04 cm2/Vs, with a very small
spread. To identify the source of the increased mobility and consistency between multiple
films, we evaluated the structure of the TMS-BT films using XRD measurements. We
found that the films pack in the same polymorph as the single crystals (Figure 4.10a), but
Figure 4.8. An illustration of the crystal structure in reference to the fabricated single-crystal
OFETs, showing the fast-growth direction of the single crystals in yellow and the high mobility
direction in red.
86
consist of a mix of two orientations, (021) (Figure 4.10b), and (001) (Figure 4.10c) in a
ratio of 1:1.4. While, as expected, the unique molecular orientation in single crystals yields
to the presence of (00l) peaks only, thin films showed two peaks, (001) and (021). The
latter peak position is about 0.5° away from the (002) peak (Figure 4.10a inset), clearly
indicating the presence of the (021) orientation in thin films. GID maps, Figures 4.11 and
4.12, revealed a larger amount of mosaicity in the films as indicated by the arc-shaped
reflections, hence overlap of reflections was a problem. From our fits, we can state that
both the (001) and (021) orientations provide reflections compatible with the XRD data. In
a)
Figure 4.9. (a) Device structure of thin-film OFETs using Cytop as the gate dielectric, (b) Gate-
source voltage sweep for a typical thin-film device.
b)
87
addition, there may be a small distortion of the thin-film lattice compared to the bulk single-
crystal lattice which is commonly observed in triclinic materials.
In both of these orientations, the main direction for hole transport as determined by
DFT calculations lies in the plane of the thin film parallel to the substrate, as marked with
a red arrow in Figures 4.10b and 4.10c, and is therefore accessible in OTFT measurements.
Since a large number of grains are present in the films, and their alignment is randomly
Figure 4.10. (a) X-ray diffraction measurements of single crystals and thin films, including
insets showing the (001), (002), and (02𝟏𝟏) peaks, (b) a side-view of TMS-BT in (02𝟏𝟏)
orientation with the functional side-groups removed for clarity, (c) a top-view of TMS-BT in
(001) orientation. The red arrows correspond to the high-mobility direction as calculated by
DFT.
a)
b) c)
88
Figure 4.12. Indexation of a thin film diffraction pattern assuming the bulk structure and a
(02-1) texture.
Figure 4.11. Indexation of a thin film diffraction pattern assuming the bulk structure and a (001)
texture.
89
distributed, the probability of charge carriers accessing the high-mobility direction is
greatly increased and, consequently, the performance is consistently better in the thin-film
devices. On the contrary, the high-mobility direction can be accessed in single-crystal
devices only if that direction is first identified and the crystal is aligned such that that
particular crystalline axis is parallel with reference to the source and drain electrodes.
While the OTFTs obtained by spin coating yielded more consistent mobilities, the
additional improvements in performance are a result of device design. The staggered
contact geometry lowers the contact resistance, while the Cytop polymer top-gate dielectric
creates a very low density of traps. Collectively these modifications account for the
increase in charge-carrier mobility from 0.3 cm2/Vs, which was the best obtained in single
crystals to over 1 cm2/Vs in thin films [3, 44]. We had attempted to utilize this device
structure in our single-crystal OFETs in order to directly compare the different
semiconductor deposition methods using the same interface, but experienced significant
adhesion issues when laminating single crystals on the highly hydrophobic Cytop layer;
we do note, however that this has been accomplished in different systems [45].
4.4 Conclusions
Single-crystal field-effect transistors of TMS-BT on SiO2 dielectric exhibit a maximum
field-effect mobility of 0.3 cm2/Vs and a spread in performance of three orders of
magnitude. Through DFT calculations the high-mobility direction was identified to be
along the a crystallographic axis. XRD measurements conclude that the single crystals
form in a pure (001) orientation such that the high-mobility direction is in the plane of the
crystal surface. Through variations in thin-film processing, OTFTs of TMS-BT that contain
90
a mix of (001) and (021) orientations were produced. Both generated molecular
orientations contain the high-mobility direction in the plane of the device. By controlling
the film microstructure and replacing the bottom-gate SiO2 dielectric with a top-gate Cytop
dielectric we fabricated OFETs that consistently exhibit mobilities of 1 cm2/Vs. This work
gives an example of thin-film transistors that outperform single-crystal devices of the same
material though optimized film processing and device architecture. The results underline
the fact that single crystals provide the best performance of a material only if the high-
mobility direction is accessible, the device structure and dielectric/semiconductor-interface
are optimized, and the contacts are of high quality.
91
References
[1] H. Klauk, “Organic thin-film transistors,” Chem. Soc. Rev., Vol. 39, p. 2643, 2010.
[2] J. W. Ward, Z. A. Lamport, and O. D. Jurchescu, “Versatile organic transistors by solution processing,” ChemPhysChem, Vol. 16, p. 1118, 2015.
[3] P. J. Diemer, Z. A. Lamport, Y. Mei, J. W. Ward, K. P. Goetz, W. Li, M. M. Payne, M. Guthold, J. E. Anthony, and O. D. Jurchescu, “Quantitative analysis of the density of trap states at the semiconductor-dielectric interface in organic field-effect transistors,” Appl. Phys. Lett., Vol. 107, p. 103303, 2015.
[4] O. D. Jurchescu, B. H. Hamadani, H. D. Xiong, S. K. Park, S. Subramanian, N. M. Zimmerman, J. E. Anthony, T. N. Jackson, and D. J. Gundlach, “Correlation between microstructure, electronic properties and flicker noise in organic thin film transistors,” Appl. Phys. Lett., Vol. 92, p. 132103, 2008.
[5] R. J. Kline, S. D. Hudson, X. Zhang, D. J. Gundlach, A. J. Moad, O. D. Jurchescu, T. N. Jackson, S. Subramanian, J. E. Anthony, M. F. Toney, and L. J. Richter, “Controlling the Microstructure of Solution-Processable Small Molecules in Thin-Film Transistors through Substrate Chemistry,” Chem. Mater., Vol. 23, p. 1194, 2011.
[6] R. Li, J. W. Ward, D.-M. Smilgies, M. M. Payne, J. E. Anthony, O. D. Jurchescu, and A. Amassian, “Direct Structural Mapping of Organic Field-Effect Transistors Reveals Bottlenecks to Carrier Transport,” Adv. Mater., Vol. 24, p. 5553, 2012.
[7] J. W. Ward, M. a. Loth, R. J. Kline, M. Coll, C. Ocal, J. E. Anthony, and O. D. Jurchescu, “Tailored interfaces for self-patterning organic thin-film transistors,” J. Mater. Chem., Vol. 22, p. 19047, 2012.
[8] J. W. Ward, R. Li, A. Obaid, M. M. Payne, D.-M. Smilgies, J. E. Anthony, A. Amassian, and O. D. Jurchescu, “Rational Design of Organic Semiconductors for Texture Control and Self-Patterning on Halogenated Surfaces,” Adv. Funct. Mater., Vol. 24, p. 5052, 2014.
[9] C.-H. Kim, H. Hlaing, J.-A. Hong, J.-H. Kim, Y. Park, M. M. Payne, J. E. Anthony, Y. Bonnassieux, G. Horowitz, and I. Kymissis, “Decoupling the Effects of Self-Assembled Monolayers on Gold, Silver, and Copper Organic Transistor Contacts,” Adv. Mater. Interfaces, Vol. 2, p. 1400384, 2015.
[10] O. D. Jurchescu, S. Subramanian, R. J. Kline, S. D. Hudson, J. E. Anthony, T. N. Jackson, and D. J. Gundlach, “Organic Single-Crystal Field-Effect Transistors of a Soluble Anthradithiophene,” Chem. Mater., Vol. 20, p. 6733, 2008.
[11] L. C. Teague, O. D. Jurchescu, C. A. Richter, S. Subramanian, J. E. Anthony, T. N. Jackson, D. J. Gundlach, and J. G. Kushmerick, “Probing stress effects in single crystal organic transistors by scanning Kelvin probe microscopy,” Appl. Phys. Lett., Vol. 96, p. 203305, 2010.
92
[12] V. Coropceanu, J. Cornil, D. A. da Silva Filho, Y. Olivier, R. Silbey, and J.-L. Brédas, “Charge Transport in Organic Semiconductors,” Chem. Rev., Vol. 107, p. 926, 2007.
[13] A. L. Briseno, S. C. B. Mannsfeld, M. M. Ling, S. Liu, R. J. Tseng, C. Reese, M. E. Roberts, Y. Yang, F. Wudl, and Z. Bao, “Patterning organic single-crystal transistor arrays,” Nature, Vol. 444, p. 913, 2006.
[14] A. L. Briseno, R. J. Tseng, M.-M. Ling, E. H. L. Falcao, Y. Yang, F. Wudl, and Z. Bao, “High-Performance Organic Single-Crystal Transistors on Flexible Substrates,” Adv. Mater., Vol. 18, p. 2320, 2006.
[15] M. E. Gershenson, V. Podzorov, and A. F. Morpurgo, “Colloquium: Electronic transport in single-crystal organic transistors,” Rev. Mod. Phys., Vol. 78, p. 973, 2006.
[16] V. Podzorov, “Organic single crystals: Addressing the fundamentals of organic electronics,” MRS Bull., Vol. 38, p. 15, 2013.
[17] T. Kubo, R. Häusermann, J. Tsurumi, J. Soeda, Y. Okada, Y. Yamashita, N. Akamatsu, A. Shishido, C. Mitsui, T. Okamoto, S. Yanagisawa, H. Matsui, and J. Takeya, “Suppressing molecular vibrations in organic semiconductors by inducing strain,” Nat. Commun., Vol. 7, p. 11156, 2016.
[18] T. Hasegawa and J. Takeya, “Organic field-effect transistors using single crystals,” Sci. Technol. Adv. Mater., Vol. 10, p. 24314, 2009.
[19] M. Mas-Torrent, M. Durkut, P. Hadley, X. Ribas, and C. Rovira, “High Mobility of Dithiophene-Tetrathiafulvalene Single-Crystal Organic Field Effect Transistors,” J. Am. Chem. Soc., Vol. 126, p. 984, 2004.
[20] J. Y. Lee, S. Roth, and Y. W. Park, “Anisotropic field effect mobility in single crystal pentacene,” Appl. Phys. Lett., Vol. 88, p. 252106, 2006.
[21] C. Reese and Z. Bao, “High-Resolution Measurement of the Anisotropy of Charge Transport in Single Crystals,” Adv. Mater., Vol. 19, p. 4535, 2007.
[22] V. C. Sundar, J. Zaumseil, V. Podzorov, E. Menard, R. L. Willett, T. Someya, M. E. Gershenson, and J. A. Rogers, “Elastomeric Transistor Stamps: Reversible Probing of Charge Transport in Organic Crystals,” Science (80-. )., Vol. 303, p. 1644, 2004.
[23] W. Mitchell, C. Wang, M. D’lavari, N. Blouin, and S. Tierney, “Non-linear acene derivatives and their use as organic semiconductors,” WO2012076092 A1, 14-Jun-2012.
[24] B. D. Rose, D. T. Chase, C. D. Weber, L. N. Zakharov, M. C. Lonergan, and M. M. Haley, “Synthesis, Crystal Structures, and Photophysical Properties of Electron-Accepting Diethynylindenofluorenediones,” Org. Lett., Vol. 13, p. 2106, 2011.
[25] K. N. Winzenberg, P. Kemppinen, G. Fanchini, M. Bown, G. E. Collis, C. M.
93
Forsyth, K. Hegedus, T. B. Singh, and S. E. Watkins, “Dibenzo [b,def] chrysene derivatives: Solution-processable small molecules that deliver high power-conversion efficiencies in bulk heterojunction solar cells,” Chem. Mater., Vol. 21, p. 5701, 2009.
[26] L. Zhang, A. Fonari, Y. Liu, A.-L. M. Hoyt, H. Lee, D. Granger, S. Parkin, T. P. Russell, J. E. Anthony, J.-L. Brédas, V. Coropceanu, and A. L. Briseno, “Bistetracene: An Air-Stable, High-Mobility Organic Semiconductor with Extended Conjugation,” J. Am. Chem. Soc., Vol. 136, p. 9248, 2014.
[27] L. Zhang, Y. Cao, N. S. Colella, Y. Liang, J. L. Brédas, K. N. Houk, and A. L. Briseno, “Unconventional, Chemically Stable, and Soluble Two-Dimensional Angular Polycyclic Aromatic Hydrocarbons: From Molecular Design to Device Applications,” Acc. Chem. Res., Vol. 48, p. 500, 2015.
[28] T. Okamoto, C. Mitsui, M. Yamagishi, K. Nakahara, J. Soeda, Y. Hirose, K. Miwa, H. Sato, A. Yamano, T. Matsushita, T. Uemura, and J. Takeya, “V-Shaped Organic Semiconductors With Solution Processability, High Mobility, and High Thermal Durability,” Adv. Mater., Vol. 25, p. 6392, 2013.
[29] R. A. Laudise, C. Kloc, P. G. Simpkins, and T. Siegrist, “Physical vapor growth of organic semiconductors,” J. Cryst. Growth, Vol. 187, p. 449, 1998.
[30] E. F. Valeev, V. Coropceanu, D. A. da Silva Filho, S. Salman, and J.-L. Brédas, “Effect of Electronic Polarization on Charge-Transport Parameters in Molecular Organic Semiconductors,” J. Am. Chem. Soc., Vol. 128, p. 9882, 2006.
[31] R. Dovesi, R. Orlando, A. Erba, C. M. Zicovich-Wilson, B. Civalleri, S. Casassa, L. Maschio, M. Ferrabone, M. De La Pierre, P. D’Arco, Y. Noël, M. Causà, M. Rérat, and B. Kirtman, “CRYSTAL14: A program for the ab initio investigation of crystalline solids,” Int. J. Quantum Chem., Vol. 114, p. 1287, 2014.
[32] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. O. Ogliaro, M. J. Bearpark, J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, N. J. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, “Gaussian 09, revision A. 2,” 2009.
[33] D. E. Nowak, D. R. Blasini, A. M. Vodnick, B. Blank, M. W. Tate, A. Deyhim, D.-M. Smilgies, H. Abruña, S. M. Gruner, and S. P. Baker, “Six-circle diffractometer with atmosphere- and temperature-controlled sample stage and area and line detectors for use in the G2 experimental station at CHESS,” Rev. Sci.
94
Instrum., Vol. 77, p. 113301, 2006.
[34] D.-M. Smilgies and D. R. Blasini, “Indexation scheme for oriented molecular thin films studied with grazing-incidence reciprocal-space mapping,” J. Appl. Crystallogr., Vol. 40, p. 716, 2007.
[35] D.-M. Smilgies, D. R. Blasini, S. Hotta, and H. Yanagi, “Reciprocal space mapping and single-crystal scattering rods,” J. Synchrotron Radiat., Vol. 12, p. 807, 2005.
[36] Y. Yuan, G. Giri, A. L. Ayzner, A. P. Zoombelt, S. C. B. Mannsfeld, J. Chen, D. Nordlund, M. F. Toney, J. Huang, and Z. Bao, “Ultra-high mobility transparent organic thin film transistors grown by an off-centre spin-coating method,” Nat. Commun., Vol. 5, p. 3005, 2014.
[37] G. Giri, E. Verploegen, S. C. B. Mannsfeld, S. Atahan-Evrenk, D. H. Kim, S. Y. Lee, H. A. Becerril, A. Aspuru-Guzik, M. F. Toney, and Z. Bao, “Tuning charge transport in solution-sheared organic semiconductors using lattice strain,” Nature, Vol. 480, p. 504, 2011.
[38] L. Yu, M. R. Niazi, G. O. Ngongang Ndjawa, R. Li, A. R. Kirmani, R. Munir, A. H. Balawi, F. Laquai, and A. Amassian, “Programmable and coherent crystallization of semiconductors,” Sci. Adv., Vol. 3, p. e1602462, 2017.
[39] A. D. Scaccabarozzi and N. Stingelin, “Semiconducting:insulating polymer blends for optoelectronic applications-a review of recent advances,” J. Mater. Chem. A, Vol. 2, p. 10818, 2014.
[40] A. F. Paterson, N. D. Treat, W. Zhang, Z. Fei, G. Wyatt-Moon, H. Faber, G. Vourlias, P. A. Patsalas, O. Solomeshch, N. Tessler, M. Heeney, and T. D. Anthopoulos, “Small Molecule/Polymer Blend Organic Transistors with Hole Mobility Exceeding 13 cm 2 V −1 s −1,” Adv. Mater., Vol. 28, p. 7791, 2016.
[41] Y. Han, G. Barnes, Y.-H. Lin, J. Martin, M. Al-Hashimi, S. Y. AlQaradawi, T. D. Anthopoulos, and M. Heeney, “Doping of Large Ionization Potential Indenopyrazine Polymers via Lewis Acid Complexation with Tris(pentafluorophenyl)borane: A Simple Method for Improving the Performance of Organic Thin-Film Transistors,” Chem. Mater., Vol. 28, p. 8016, 2016.
[42] P. J. Diemer, C. R. Lyle, Y. Mei, C. Sutton, M. M. Payne, J. E. Anthony, V. Coropceanu, J.-L. Brédas, and O. D. Jurchescu, “Vibration-Assisted Crystallization Improves Organic/Dielectric Interface in Organic Thin-Film Transistors,” Adv. Mater., Vol. 25, p. 6956, 2013.
[43] G. E. Purdum, N. Yao, A. Woll, T. Gessner, R. T. Weitz, and Y.-L. Loo, “Understanding Polymorph Transformations in Core-Chlorinated Naphthalene Diimides and their Impact on Thin-Film Transistor Performance,” Adv. Funct. Mater., Vol. 26, p. 2357, 2016.
[44] P. V. Necliudov, M. S. Shur, D. J. Gundlach, and T. N. Jackson, “Modeling of organic thin film transistors of different designs,” J. Appl. Phys., Vol. 88, p. 6594,
95
2000.
[45] W. L. Kalb, T. Mathis, S. Haas, A. F. Stassen, and B. Batlogg, “Organic small molecule field-effect transistors with CytopTM gate dielectric: Eliminating gate bias stress effects,” Appl. Phys. Lett., Vol. 90, p. 92104, 2007.
96
Chapter 5 Organic Thin-Film Transistors with Charge-
Carrier Mobilities of 20 cm2/Vs, Independent
of Gate Voltage
The field of organic electronics has focused on the development of high mobility
semiconductors to improve device performance and increase the viability of these devices
in the commercial marketplace. The charge-injecting contacts, however, have received
much less attention as the route to high-performing devices. In this study, we focus on the
deposition parameters of Au source and drain electrodes to greatly reduce the contact
resistance and vastly improve device performance in organic field-effect transistors
(OFETs). We use a well-studied small-molecule semiconductor in 2,8-difluoro-5,11-
bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT), and, by reducing the rate of
electrode deposition, reduce the contact resistance to 500 Ωcm and increase the field-effect
mobility to 19.2 cm2/Vs. To confirm that this method is suitable for use in OFETs beyond
small-molecule semiconductors, we produce devices using the polymer semiconductor
indacenodithiophene-co-benzothiadiazole (C16IDT-BT) and achieve a contact resistance of
200 Ωcm and field-effect mobilities up to 10 cm2/Vs.
This work was adapted from Zachary A. Lamport, Katrina J. Barth, Hyunsu Lee, Martin Guthold, Lee J. Richter, Iain McCulloch, John E. Anthony, and Oana D. Jurchescu, To be submitted, 2018.
97
5.1 Introduction
The promise to impact contemporary applications has sparked great interest in the study of
organic electronic and opto-electronic devices. The rich chemistry of organic materials,
low manufacturing cost and compatibility with flexible and stretchable substrates, provide
an opportunity to incorporate electronics in non-traditional areas such as clothing [1], paper
[2], flexible and rollable displays [3–5], or bio-integrated applications [6, 7]. Research
efforts have focused on manipulating the chemical structure and degree of order of the
semiconductor layer, understanding and controlling the processes taking place at device
interfaces, light manipulation, integration with biological systems and more. These efforts
have been largely successful, and as the field has seen radical improvements, an increasing
number of products are approaching the marketplace. For example, recently developed
solution-processable small-molecule and polymer semiconductors incorporated in organic
field-effect transistors (OFETs) have reached charge-carrier mobilities μ previously
reserved for inorganic materials [8–13]. But with this remarkable progress also come great
challenges. A direct consequence of enhancing the intrinsic mobility of the organic
semiconductor layer is that the contributions of the contact effects to the overall device
performance now can become significant. Understanding and reducing these contributions
is of the utmost importance since they limit the OFET performance, particularly in the
linear regime, where they would operate for applications such as the active matrix displays.
This issue becomes more severe as the channel dimensions are minimized, since the
channel resistance scales positively with the channel length, while the contact resistance is
independent of this variable. In addition, the development of new materials also hinges on
a correct evaluation of mobility. The equations adopted from silicon MOSFETs for the
98
characterization of OFET operation assume negligible contact resistance, and thus they fail
when the devices are severely limited by contacts. In this case it is thus impossible to access
the intrinsic properties of materials and to provide meaningful feedback for material design
[14–16].
The impact of contacts on OFET performance was recognized as early as 1996, by
Lin and co-workers [17], and several other groups have focused on this topic [18–26].
Recently, Klauk identified contact resistance as the single largest hurdle to overcome in
the pursuit of high-frequency OFETs [27]. The contact resistance results from the fact that
a small fraction of the voltage applied between the source and drain electrodes is necessary
to inject the charges from the electrode surface to the organic semiconductor layer. The
magnitude of this potential drop at the contact depends on several factors. The energetic
mismatch between the electrode work function and the electron affinity (n-type)/ionization
potential (p-type) of the organic semiconductor influences the injection process and
reduction of contact resistance by chemically tailoring this surface with self-assembled
monolayers (SAMs) has proven effective [28, 29]. Often, however, these modifications
not only alter the work function, but also the surface energy of the electrodes, therefore
impacting the morphology of the films deposited on these surfaces [30–32]. Charge
injection layers and contact dopants have also been proposed to enhance injection by
increasing the charge carrier concentration at the electrodes [21, 25, 33, 34]. The geometry
of the device can also play a role, with coplanar OFETs typically exhibiting higher contact
resistance than staggered structures [19]. For top-contact transistors, methods such as
nanotransfer printing or flip-chip lamination were introduced in order to avoid the
degradation of the semiconductor layer under the contact, which would lead to the
99
formation of parasitic resistances [35, 36]. In addition to these methods to reduce contact
resistance, the use of organic materials such as graphene, reduced graphene oxide, carbon
nanotubes or charge transfer salts has grown in popularity due not only to the favorable
work function but also a reduction in the injection barrier stemming from interfacial dipoles
[33, 37–44]. Recently, Uemura, et al. found that contact-annealing can greatly reduce the
contact resistance in bottom-gate, top-contact devices, and it can also eliminate the non-
ideal current-voltage curves arising from gated Schottky contacts [15]. Here, we reduced
aggressively the contact resistance in small molecule and polymer OFETs by varying the
metal deposition rate in conjunction with using a pentafluorobenzene thiol (PFBT)
treatment, which resulted in over 5 times improved charge carrier mobility compared with
the best previously reported devices with identical composition and structure. The obtained
contact resistance normalized over the channel width was as low as 200 Ωcm, and the
OFETs exhibited charge carrier mobilities of 19.2 cm2/Vs for 2,8-difluoro-5,11-
bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT) and 10 cm2/Vs for
indacenodithiophene-co-benzothiadiazole copolymer (C16IDTBT), with minimal
dependence on the gate voltage. To understand this drastic improvement in device
performance, we performed grazing incidence X-ray diffraction (GIXD) measurements on
the organic semiconductor films to evaluate if the contact deposition rate results in
variations in the film morphology and/or microstructure, and found no major differences
in the structure of the semiconductor layer. This result suggests that the improvements in
device performance originate only from the differences in the electrode properties. Indeed,
the metal grain size correlates negatively with the deposition rate, as confirmed by atomic
force microscopy (AFM) measurements, thus creating different environments for the SAM
100
attachment and also impacting its final structure. Evaluation of the PFBT/Au using
scanning Kelvin probe microscopy (SKPM) indicated that there exist local variations in
the work function of the electrodes fabricated using a low deposition rate, pointing to the
existence of regions with more efficient charge injection due to enhanced PFBT order, a
feature which is absent in the samples obtained via fast deposition.
5.2 Experimental
5.2.1 Device Fabrication
Bottom-contact, top-gate devices were fabricated on an insulating surface consisting of a
200 nm thermal silicon oxide. The wafers were cleaned by immersion in hot acetone for
10 minutes, then rinsed with fresh acetone and isopropyl alcohol (IPA), followed by
immersion in hot IPA for 10 minutes and an additional rinse using fresh IPA and dried in
a stream of nitrogen. Then the substrates were exposed to a UV-Ozone treatment for 10
minutes, rinsed thoroughly using deionized water and dried in a stream of nitrogen. The
source and drain contacts were patterned by shadow mask and consisted of a 5 nm titanium
adhesion layer deposited by e-beam evaporation at a rate of 1 Å/s followed by 40 nm of
thermally evaporated gold at varying deposition rates. These contacts were then treated for
30 minutes using a 30mM solution of room-temperature PFBT in ethanol followed by a 3-
minute sonication in fresh ethanol and a thorough ethanol rinse and dried in a stream of
nitrogen. The substrates were then brought into a nitrogen glovebox (<0.1 ppm O2, <0.1
ppm H2O) where the organic semiconductor layer was deposited immediately. A 16.5
mg/mL solution of diF-TES ADT in chlorobenzene was spin-coated at 1000 RPM for 80 s
and placed under vacuum for 90 minutes to remove additional solvent. C16IDT-BT was
101
spin-coated at 2000 RPM for 60 s from a 10 mg/mL solution in chlorobenzene before
annealing at 100°C for 10 minutes. Samples were then brought back into the glovebox to
apply the Cytop top-gate dielectric, which was spin-coated at 2000 RPM for 60 s and then
annealed at 55°C overnight. A 40 nm gold top gate was then applied using electron beam
evaporation at a rate of 1 Å/s.
5.2.2 Device Characterization
The transistor characterization measurements were carried out in the dark and under
ambient conditions using an Agilent 4155C Semiconductor Parameter Analyzer. AFM and
KPFM measurements were taken using an Asylum MFP-3D Bio AFM (Asylum Research,
USA) in ambient atmosphere. For AFM, a silicon cantilever (Nanosensors PPP-NCLR,
force constant: 21-98 N/m, resonance frequency: 146-236 kHz) was used in tapping mode
with a feedback setpoint of 500 mV, and 1 µm × 1 µm images were taken at a rate of 0.5
Hz. KPFM measurements used a silicon cantilever with a Ti/Ir coating (Oxford Instruments
ASYELEC.01-R2, force constant: 1.4-5.8 N/m, resonance frequency: 58-97 kHz) at a nap
height of 5 nm, and 20 µm × 20 µm images were taken at a rate of 1 Hz.
5.2.3 Grazing Incidence X-Ray Diffraction
GIXD was carried out at the Cornell High Energy Synchrotron Source (CHESS) on a
surface diffractometer in powder mode. The samples were rotated 360° for each step of the
measurement, using a set of Soller slits on the detector arm before a linear diode array,
resulting in an in-plane resolution of 0.16°. The in-plane scattering angle ν was varied from
2 to 30° with a step size of 0.1°, keeping the incident angle fixed at 0.15°. The linear diode
102
array had a 10° range in the out-of-plane scattering angle δ to form a two-dimensional
scattering map I(ν, δ) on each scan.
5.3 Results and Discussion
The chemical structure of diF-TES ADT is shown in Figure 5.1a and the electrical
characteristics of a device made using a Au deposition rate of 0.5 Å/s in Figures 5.1b and
c. In Figure 5.1b we show the evolution of the drain current (ID) as a function of the gate-
source voltage (VGS) in the saturation regime, with the drain-source voltage (VDS) held
constant at -40 V. The blue line corresponds to ID on a log scale (right axis) and the black
open circles correspond to the square root of ID (left axis). The red line serves as a visual
aid to show that the square root of ID follows a linear relation with VGS, as expected from
the gradual channel approximation, and indicates the section of the curve where the
mobility was calculated. Figure 5.1c shows the evolution of ID with VDS, where each line
Figure 5.1. (a) Chemical structure of diF-TES ADT, (b) Drain current as a function of gate-
source voltage in a diF-TES ADT device with L = 100 µm and W = 200 µm, in the saturation
regime, (c) Drain current as a function of drain-source voltage for the same device.
a) b) c)
103
is measured at a different VGS, and demonstrates linearity at low VDS and a clear transition
from the linear to saturation regime. Both these features are emblematic for low contact
resistances. This device exhibits a charge-carrier mobility of μsat = 19.2 cm2/Vs, a current
on/off ratio of Ion/Ioff = 5.9 ∙ 103, and a threshold voltage of VTh = 3.3 V. Larger Ion/Ioff ratios
are possible, as shown for example in Figure 5.2, where Ion/Ioff = 5 ∙ 107.
In order to make sure that the mobility is not overestimated, we also evaluated its
dependence on VGS. Mobility overestimation can occur in the case of gated Schottky
contacts, where there is a large injection barrier at the contacts which is overcome by an
increasing VGS. This relation causes a peak in the apparent mobility when the injection
barrier is overcome, before decreasing to a more realistic value [14–16]. As can be
observed in Figure 5.3, the mobility in our device first increases with increasing VGS,
followed by a plateau at higher VGS. Such a dependence has been observed in other high
mobility systems such as C10DNTT thin films or rubrene single crystals and was attributed
Figure 5.2. Drain current as a function of gate-source voltage in a diF-TES ADT device,
showing an on/off ratio greater than 107.
104
to the presence of electronic traps in the organic semiconductor layers [15, 45]. The
mobility evaluated in the linear regime for the device presented in Figure 5.1 was µlin =
16.0 cm2/Vs. The contact resistance has a greater effect on the effective device mobility in
the linear regime and the close correspondence recorded between the linear and saturation
mobilities hint to a low contact resistance, in agreement with the linear curves obtained in
the low-VDS range of the output characteristics in Figure 5.1c. A quantitative analysis of
the contact resistance and its effect on device properties will be provided later.
The evolution of the average mobility with the contact deposition rate is depicted
in Figure 5.4a. It can be observed here that the maximum average mobility, µsat,avg = 14.6
± 3.3 cm2/Vs, was obtained when a rate of 0.5 Å/s was used, decreasing to an average
mobility of µsat,avg = 3.24 ± 0.49 cm2/Vs at a rate of 3.0 Å/s. The lower values coincide
with those reported using the same methods, materials, device architecture, where devices
Figure 5.3. Mobility vs. gate-source voltage for the device in shown in Figure 5.1.
105
fabricated with a contact deposition rate of 2 Å/s resulted in an average mobility of µsat,avg
= 1.5 cm2/Vs and a maximum mobility of µsat,max = 3.14 cm2/Vs [46].
To understand the reason behind the improvements in mobility, we first performed
GIXD measurements on the diF-TES ADT films deposited on PFBT/Au. The results for
the rate of 0.5 Å/s are included in Figure 5.5a, while those for 2 Å/s are shown in Figure
5.5b. It can be observed that in both cases the series of (001) peaks can be distinguished,
confirming that the molecules are “edge-on” oriented, as illustrated in Figure 5.5c. These
findings are in agreement with earlier reports [30, 47, 48]. The dominant (001) orientation
is a result of the PFBT-treated Au acting as a templating mechanism, with the fluorine
atoms of the PFBT molecules interacting with the fluorine atoms of diF-TES ADT
molecules, and this orientation is the most favorable direction for charge transport.
Figure 5.4. (a) Average field-effect mobility vs. contact deposition rate, (b) Width-
normalized contact resistance as a function of contact deposition rate.
a)
b)
106
Since no major differences were observed in the morphology of the film as a
function of contact deposition rate, we further focused on the quantitative analysis of the
changes in the contact resistance. The total device resistance Rdevice is given by the channel
resistance, RCh (a quantity which is proportional to the channel length), and the contact
resistance RC, as shown in equation 5.1. RC in a staggered structure, such as the ones studied
here and depicted in Figure 5.6a, has two main contributions, the interface resistance, Rint,
and the bulk resistance, Rbulk, which can be seen in Figure 5.6b. In general, Rint is a result
of the properties of the electrode surface, including the energy level mismatch between
electrode and semiconductor, and the presence of interfacial dipoles, whereas Rbulk governs
the transport of the injected charges through the organic semiconductor, from the
electrode/semiconductor interface to the accumulation layer. Thus, Rbulk strictly depends
on the conductivity of the semiconductor in the direction perpendicular to the channel and
Figure 5.5. GIXD measurements for diF-TES ADT on patterned Au deposited at (a) 0.5
Å/s and (b) 2 Å/s. (c) The (001) molecular orientation of diF-TES ADT.
a) b) c)
107
the thickness of the semiconducting layer. The relations between the Rdevice, RCh, Rint, and
Rbulk in the linear regime are as follows:
𝑅𝑅𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 = 𝑅𝑅𝐶𝐶ℎ(𝐿𝐿) + 𝑅𝑅𝐶𝐶 (5.1)
𝑅𝑅𝐶𝐶 = 𝑅𝑅𝑑𝑑𝑖𝑖𝑖𝑖 + 𝑅𝑅𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 (5.2)
We evaluated the contact resistance for the devices corresponding to each contact
deposition rate using gated transmission line method (gated TLM), based on equation 5.1,
and the results are displayed in Figure 5.4b. The gated TLM was conducted by first finding
VTh using the second-derivative method [49], and the value of ID in the linear regime was
then taken at an overdrive voltage of VGS -VTh = -40 V. It is clear that the increase in the
average field-effect mobility as a function of contact deposition rate is mirrored by the
inverse trend in contact resistance. The devices obtained using a fast deposition rate of 3
Å/s exhibit large contact resistance, RC = 3.1 kΩcm, which yields a mobility of µsat,avg =
3.24 ± 0.49 cm2/Vs. By slowing down the deposition process to a rate of 0.5 Å/s, we
Figure 5.6. (a) Schematic of the bottom-contact, top-gate device structure used in our devices,
(b) An illustration of the different sources of resistance in our devices.
a) b)
108
reduced the contact resistance by 6 times, to 500 Ωcm. Consequently, the mobility in these
devices is very high. This outcome suggests, along with the nearly identical GIXD results,
that the drastically improved device performance is a result of the enhanced charge
injection provided by the lower deposition rates, which yields lower contact resistance. The
resistance due to the distance between injection and the conduction channel, Rbulk, should
be unchanged between our devices fabricated using various contact deposition rates
because the semiconductor has been verified to have the same crystal structure. This
indicates that the main improvement in our devices lies in the resistance of the interface
between the contact and the semiconductor, or Rint.
Through AFM measurements, shown in Figures 5.7a and b, we found that the slow
deposition rate (0.5 Å/s in this case) yields a larger metal grain size than the faster rates
(here 2.5 Å/s), as confirmed by the 2D fast Fourier transforms, Figures 5.8a and b. When
the Au film formation is slow (similar to a simultaneous deposition and annealing), enough
Figure 5.7. (a) AFM image of Au deposited at 0.5 Å/s, (b) AFM image of Au deposited
at 2.5 Å/s.
a) b)
109
time and energy is allowed for each Au particle to reorganize to a more favorable energy
state before additional particles reach the substrates, effectively “trapping” those
underneath. This process resembles that of Uemura et al., but in that case the annealing
step was performed post-deposition, while here the deposition and annealing are
simultaneous [15]. Nevertheless, the result is the same: lowering of the contact resistance.
Interestingly, the RMS roughness of the two films are very similar (0.64 ± 0.04 nm for 0.5
Å/s, 0.69 ± 0.02 nm for 2.5 Å/s), which suggests that there is no change in the height
variation, but the in-plane variation is the determining factor in the mobility improvements.
To determine if this in-plane variation had any impact on the work function of PFBT-
treated Au films, and therefore on the injection barrier, we first conducted macroscale
Kelvin probe measurements and found no difference in the work function of the treated
Au, both 0.5 Å/s and 2.5 Å/s gave φAu, PFBT = 5.3 eV. Details on the determination of work
function based on Kelvin probe measurements were provided elsewhere [50]. To examine
the local features of the PFBT/Au surface potential, we performed scanning Kelvin probe
a) b)
Figure 5.8. 2D fast Fourier transforms to show the relative peak frequency in AFM images,
and therefore the distance between metal particles, on (a) 0.5 Å/s Au and (b) 2.5 Å/s Au.
110
microscopy (SKPM) measurements on the same samples, the results are displayed in
Figures 5.9a and b. The surface potential of the PFBT-treated Au deposited at 0.5 Å/s
exhibited local peaks, a feature that does not appear in the sample obtained via a fast
deposition rate. This observation indicated that while the average surface potential is very
similar for all samples, variations exist on small length scales, which are probably masked
when macroscopic measurements are carried out. The AFM and SKPM measurements
together point towards the larger grain size allowing the PFBT self-assembled monolayer
to achieve a longer-range order, and as a consequence the surface potential map displays
local maxima. These local maxima correspond to a larger work function, allowing more
efficient injection deeper into the highest occupied molecular orbital (HOMO) than the
surrounding area, and thus providing lower local resistance to injection.
Figure 5.9. KPFM images of PFBT-treated Au deposited at (a) 0.5 Å/s showing small regions of
higher surface potential and (b) 2.5 Å/s showing a more homogeneous surface potential
distribution.
a) b)
111
To discern whether this effect also applies to other systems, we fabricated an
additional set of several samples using the well-studied copolymer indacenodithiophene-
co-benzothiadiazole (C16IDT-BT), the structure of which is displayed Figure 5.10a. The
contact fabrication method that yielded the best performance in the small molecule OFETs
was adopted here, i.e. deposition rate of 0.5 Å/s. C16IDT-BT has been incorporated as the
semiconductor in many studies reaching a maximum charge carrier mobility of µ = 3.6
cm2/Vs [51]. Figures 5.10b and c show the evolution of the drain current at constant drain-
source voltage while varying the gate-source voltage, and the drain current as a function of
drain-source voltage with VGS held constant, respectively. This device exhibited a charge-
carrier mobility of 10 cm2/Vs, which is ~3x greater than the best mobility reported for this
material in this geometry and with Cytop as dielectric. Other device parameters include
Ion/Ioff =3 ∗ 104, S = 2.9 Vdec-1 and VTh = 7.4 V. These device properties are coupled with
a low contact resistance of 200 Ωcm, similar to the case of the small molecule device.
Figure 5.10. (a) Chemical structure of C16IDT-BT copolymer, (b) Drain current as a function of
gate-source voltage for an C16IDT-BT device, (c) Drain current as a function of drain-source
voltage.
a) b) c)
112
These results show that the reduced contact deposition rate has a strong, positive effect on
the device performance in polymer semiconductors as well.
5.4 Conclusions
In summary, we have fabricated a series of OFETs where we varied the bottom-contact
deposition rate between 0.5 Å/s and 3 Å/s and obtained massively improved charge-carrier
mobility when using a rate of 0.5 Å/s reaching a value of 19.2 cm2/Vs, along with a
precipitous drop in contact resistance. We conducted GIXD measurements and found no
difference in the microstructure of the films deposited over the substrates fabricated using
the various contact deposition rates. AFM measurements confirmed a larger grain size in
Au deposited at 0.5 Å/s, and KPFM measurements on the same surfaces exhibited local
maxima in the surface potential. These local maxima can provide channels of lower
resistance to injection into the semiconductor, drastically improving device performance.
To conclude, we have greatly improved the performance of our OFETs through
modification of the contact deposition rate in both small-molecule and polymer
semiconductors. The contact resistance of these devices is significantly decreased with
decreasing deposition rate, from 3.1 kΩcm characteristic to a fast rate, to 500 Ωcm or less
obtained at a rate of 0.5 Å/s. The obtained contact resistance is one of the lowest obtained
in OFETs, and here it results solely from a minor modification in the source and drain
deposition procedure. Our results underline the importance of minutious device design in
achieving high performance organic devices. The reduced contact resistance following this
procedure in conjunction with the use of other semiconductor deposition methods which
113
produce even higher quality semiconductor films could result in even greater enhanced
device performance.
114
References
[1] K. Cherenack and L. Van Pieterson, “Smart textiles: Challenges and opportunities,” J. Appl. Phys., Vol. 112, p. 091301, 2012.
[2] U. Zschieschang and H. Klauk, “Low-voltage organic transistors with steep subthreshold slope fabricated on commercially available paper,” Org. Electron., Vol. 25, p. 340, 2015.
[3] A. C. Arias, J. D. MacKenzie, I. McCulloch, J. Rivnay, and A. Salleo, “Materials and Applications for Large Area Electronics: Solution-Based Approaches,” Chem. Rev., Vol. 110, p. 3, 2010.
[4] A. Pierre, M. Sadeghi, M. M. Payne, A. Facchetti, J. E. Anthony, and A. C. Arias, “All-Printed Flexible Organic Transistors Enabled by Surface Tension-Guided Blade Coating,” Adv. Mater., Vol. 26, p. 5722, 2014.
[5] G.-J. N. Wang, A. Gasperini, and Z. Bao, “Stretchable Polymer Semiconductors for Plastic Electronics,” Adv. Electron. Mater., Vol. 4, p. 1700429, 2018.
[6] D. Khodagholy, T. Doublet, P. Quilichini, M. Gurfinkel, P. Leleux, A. Ghestem, E. Ismailova, T. Hervé, S. Sanaur, C. Bernard, and G. G. Malliaras, “In vivo recordings of brain activity using organic transistors,” Nat. Commun., Vol. 4, p. 1575, 2013.
[7] J. Rivnay, R. M. Owens, and G. G. Malliaras, “The Rise of Organic Bioelectronics,” Chem. Mater., Vol. 26, p. 679, 2014.
[8] S. Haas, Y. Takahashi, K. Takimiya, and T. Hasegawa, “High-performance dinaphtho-thieno-thiophene single crystal field-effect transistors,” Appl. Phys. Lett., Vol. 95, p. 022111, 2009.
[9] W. Zhang, J. Smith, S. E. Watkins, R. Gysel, M. McGehee, A. Salleo, J. Kirkpatrick, S. Ashraf, T. Anthopoulos, M. Heeney, and I. McCulloch, “Indacenodithiophene Semiconducting Polymers for High-Performance, Air-Stable Transistors,” J. Am. Chem. Soc., Vol. 132, p. 11437, 2010.
[10] D. Venkateshvaran, M. Nikolka, A. Sadhanala, V. Lemaur, M. Zelazny, M. Kepa, M. Hurhangee, A. J. Kronemeijer, V. Pecunia, I. Nasrallah, I. Romanov, K. Broch, I. McCulloch, D. Emin, Y. Olivier, J. Cornil, D. Beljonne, and H. Sirringhaus, “Approaching disorder-free transport in high-mobility conjugated polymers,” Nature, Vol. 515, p. 384, 2014.
[11] B. Sun, W. Hong, Z. Yan, H. Aziz, and Y. Li, “Record high electron mobility of 6.3 cm2 V−1 s−1 achieved for polymer semiconductors using a new building block,” Adv. Mater., Vol. 26, p. 2636, 2014.
[12] A. F. Paterson, N. D. Treat, W. Zhang, Z. Fei, G. Wyatt-Moon, H. Faber, G. Vourlias, P. A. Patsalas, O. Solomeshch, N. Tessler, M. Heeney, and T. D. Anthopoulos, “Small Molecule/Polymer Blend Organic Transistors with Hole
115
Mobility Exceeding 13 cm 2 V −1 s −1,” Adv. Mater., Vol. 28, p. 7791, 2016.
[13] J. W. Ward, Z. A. Lamport, and O. D. Jurchescu, “Versatile organic transistors by solution processing,” ChemPhysChem, Vol. 16, p. 1118, 2015.
[14] E. G. Bittle, J. I. Basham, T. N. Jackson, O. D. Jurchescu, and D. J. Gundlach, “Mobility overestimation due to gated contacts in organic field-effect transistors,” Nat. Commun., Vol. 7, p. 10908, 2016.
[15] T. Uemura, C. Rolin, T. H. Ke, P. Fesenko, J. Genoe, P. Heremans, and J. Takeya, “On the Extraction of Charge Carrier Mobility in High-Mobility Organic Transistors,” Adv. Mater., Vol. 28, p. 151, 2016.
[16] H. H. Choi, K. Cho, C. D. Frisbie, H. Sirringhaus, and V. Podzorov, “Critical assessment of charge mobility extraction in FETs,” Nat. Mater., Vol. 17, p. 2, 2017.
[17] Y. Y. Lin, D. J. Gundlach, and T. N. Jackson, “Contact Dependence of α-Sexithienyl Thin Film Transistor Characteristics,” MRS Proc., Vol. 413, p. 413, 1995.
[18] R. J. Chesterfield, J. C. McKeen, C. R. Newman, C. D. Frisbie, P. C. Ewbank, K. R. Mann, and L. L. Miller, “Variable temperature film and contact resistance measurements on operating n -channel organic thin film transistors,” J. Appl. Phys., Vol. 95, p. 6396, 2004.
[19] D. J. Gundlach, L. Zhou, J. A. Nichols, T. N. Jackson, P. V. Necliudov, and M. S. Shur, “An experimental study of contact effects in organic thin film transistors,” J. Appl. Phys., Vol. 100, p. 24509, 2006.
[20] T. J. Richards and H. Sirringhaus, “Analysis of the contact resistance in staggered, top-gate organic field-effect transistors,” J. Appl. Phys., Vol. 102, p. 094510, 2007.
[21] T. Minari, T. Miyadera, K. Tsukagoshi, Y. Aoyagi, and H. Ito, “Charge injection process in organic field-effect transistors,” Appl. Phys. Lett., Vol. 91, p. 53508, 2007.
[22] M. Marinkovic, D. Belaineh, V. Wagner, and D. Knipp, “On the Origin of Contact Resistances of Organic Thin Film Transistors,” Adv. Mater., Vol. 24, p. 4005, 2012.
[23] F. Ante, D. Kälblein, T. Zaki, U. Zschieschang, K. Takimiya, M. Ikeda, T. Sekitani, T. Someya, J. N. Burghartz, K. Kern, and H. Klauk, “Contact resistance and megahertz operation of aggressively scaled organic transistors,” Small, Vol. 8, p. 73, 2012.
[24] D. Natali and M. Caironi, “Charge Injection in Solution-Processed Organic Field-Effect Transistors: Physics, Models and Characterization Methods,” Adv. Mater., Vol. 24, p. 1357, 2012.
[25] S. Choi, C. Fuentes-Hernandez, C.-Y. Wang, T. M. Khan, F. A. Larrain, Y. Zhang, S. Barlow, S. R. Marder, and B. Kippelen, “A Study on Reducing Contact
116
Resistance in Solution-Processed Organic Field-Effect Transistors,” ACS Appl. Mater. Interfaces, Vol. 8, p. 24744, 2016.
[26] J.-L. Hou, D. Kasemann, J. Widmer, A. A. Günther, B. Lüssem, and K. Leo, “Reduced contact resistance in top-contact organic field-effect transistors by interface contact doping,” Appl. Phys. Lett., Vol. 108, p. 103303, 2016.
[27] H. Klauk, “Will We See Gigahertz Organic Transistors?,” Adv. Electron. Mater., p. 1700474, 2018.
[28] Y. Zhou, C. Fuentes-Hernandez, J. Shim, J. Meyer, A. J. Giordano, H. Li, P. Winget, T. Papadopoulos, H. Cheun, J. Kim, M. Fenoll, A. Dindar, W. Haske, E. Najafabadi, T. M. Khan, H. Sojoudi, S. Barlow, S. Graham, J.-L. Brédas, S. R. Marder, A. Kahn, and B. Kippelen, “A universal method to produce low-work function electrodes for organic electronics.,” Science, Vol. 336, p. 327, 2012.
[29] Y. Mei, D. Fogel, J. Chen, J. W. Ward, M. M. Payne, J. E. Anthony, and O. D. Jurchescu, “Interface engineering to enhance charge injection and transport in solution-deposited organic transistors,” Org. Electron., Vol. 50, p. 100, 2017.
[30] J. W. Ward, M. a. Loth, R. J. Kline, M. Coll, C. Ocal, J. E. Anthony, and O. D. Jurchescu, “Tailored interfaces for self-patterning organic thin-film transistors,” J. Mater. Chem., Vol. 22, p. 19047, 2012.
[31] S. K. Park, T. N. Jackson, J. E. Anthony, and D. A. Mourey, “High mobility solution processed 6,13-bis(triisopropyl-silylethynyl) pentacene organic thin film transistors,” Appl. Phys. Lett., Vol. 91, p. 63514, 2007.
[32] C.-H. Kim, H. Hlaing, J.-A. Hong, J.-H. Kim, Y. Park, M. M. Payne, J. E. Anthony, Y. Bonnassieux, G. Horowitz, and I. Kymissis, “Decoupling the Effects of Self-Assembled Monolayers on Gold, Silver, and Copper Organic Transistor Contacts,” Adv. Mater. Interfaces, Vol. 2, p. 1400384, 2015.
[33] C. Liu, Y. Xu, and Y. Y. Noh, “Contact engineering in organic field-effect transistors,” Mater. Today, Vol. 18, p. 79, 2015.
[34] C. Liu, G. Huseynova, Y. Xu, D. X. Long, W.-T. Park, X. Liu, T. Minari, and Y.-Y. Noh, “Universal diffusion-limited injection and the hook effect in organic thin-film transistors,” Sci. Rep., Vol. 6, p. 29811, 2016.
[35] Y.-L. Loo, R. L. Willett, K. W. Baldwin, and J. A. Rogers, “Interfacial Chemistries for Nanoscale Transfer Printing,” J. Am. Chem. Soc., Vol. 124, p. 7654, 2002.
[36] M. Coll, K. P. Goetz, B. R. Conrad, C. A. Hacker, D. J. Gundlach, C. A. Richter, and O. D. Jurchescu, “Flip chip lamination to electrically contact organic single crystals on flexible substrates,” Appl. Phys. Lett., Vol. 98, p. 163302, 2011.
[37] S. Pang, H. N. Tsao, X. Feng, and K. Mullen, “Patterned graphene electrodes from solution-processed graphite oxide films for organic field-effect transistors,” Adv. Mater., Vol. 21, p. 3488, 2009.
117
[38] S. Lee, G. Jo, S. J. Kang, G. Wang, M. Choe, W. Park, D. Y. Kim, Y. H. Kahng, and T. Lee, “Enhanced charge injection in pentacene field-effect transistors with graphene electrodes,” Adv. Mater., Vol. 23, p. 100, 2011.
[39] W. H. Lee, J. Park, S. H. Sim, S. B. Jo, K. S. Kim, B. H. Hong, and K. Cho, “Transparent flexible organic transistors based on monolayer graphene electrodes on plastic,” Adv. Mater., Vol. 23, p. 1752, 2011.
[40] Y. Cao, S. Liu, Q. Shen, K. Yan, P. Li, J. Xu, D. Yu, M. L. Steigerwald, C. Nuckolls, Z. Liu, and X. Guo, “High-performance photoresponsive organic nanotransistors with single-layer graphenes as two-dimensional electrodes,” Adv. Funct. Mater., Vol. 19, p. 2743, 2009.
[41] F. Cicoira, N. Copped, S. Iannotta, and R. Martel, “Ambipolar copper phthalocyanine transistors with carbon nanotube array electrodes,” Appl. Phys. Lett., Vol. 98, p. 1, 2011.
[42] F. Cicoira, C. M. Aguirre, and R. Martel, “Making contacts to n-type organic transistors using carbon nanotube arrays,” ACS Nano, Vol. 5, p. 283, 2011.
[43] Y. Takahashi, T. Hasegawa, Y. Abe, Y. Tokura, K. Nishimura, and G. Saito, “Tuning of electron injections for n-type organic transistor based on charge-transfer compounds,” Appl. Phys. Lett., Vol. 86, p. 63504, 2005.
[44] G. Eda and M. Chhowalla, “Chemically Derived Graphene Oxide: Towards Large-Area Thin-Film Electronics and Optoelectronics,” Adv. Mater., Vol. 22, p. 2392, 2010.
[45] V. C. Sundar, J. Zaumseil, V. Podzorov, E. Menard, R. L. Willett, T. Someya, M. E. Gershenson, and J. A. Rogers, “Elastomeric Transistor Stamps: Reversible Probing of Charge Transport in Organic Crystals,” Science (80-. )., Vol. 303, p. 1644, 2004.
[46] P. J. Diemer, Z. A. Lamport, Y. Mei, J. W. Ward, K. P. Goetz, W. Li, M. M. Payne, M. Guthold, J. E. Anthony, and O. D. Jurchescu, “Quantitative analysis of the density of trap states at the semiconductor-dielectric interface in organic field-effect transistors,” Appl. Phys. Lett., Vol. 107, p. 103303, 2015.
[47] D. J. Gundlach, J. E. Royer, S. K. Park, S. Subramanian, O. D. Jurchescu, B. H. Hamadani, a J. Moad, R. J. Kline, L. C. Teague, O. Kirillov, C. A. Richter, J. G. Kushmerick, L. J. Richter, S. R. Parkin, T. N. Jackson, and J. E. Anthony, “Contact-induced crystallinity for high-performance soluble acene-based transistors and circuits,” Nat. Mater., Vol. 7, p. 216, 2008.
[48] R. J. Kline, S. D. Hudson, X. Zhang, D. J. Gundlach, A. J. Moad, O. D. Jurchescu, T. N. Jackson, S. Subramanian, J. E. Anthony, M. F. Toney, and L. J. Richter, “Controlling the Microstructure of Solution-Processable Small Molecules in Thin-Film Transistors through Substrate Chemistry,” Chem. Mater., Vol. 23, p. 1194, 2011.
[49] D. Boudinet, G. Le Blevennec, C. Serbutoviez, J.-M. Verilhac, H. Yan, and G.
118
Horowitz, “Contact resistance and threshold voltage extraction in n-channel organic thin film transistors on plastic substrates,” J. Appl. Phys., Vol. 105, p. 84510, 2009.
[50] Z. A. Lamport, A. D. Broadnax, D. Harrison, K. J. Barth, L. Mendenhall, C. T. Hamilton, M. Guthold, T. Thonhauser, M. E. Welker, and O. D. Jurchescu, “Fluorinated benzalkylsilane molecular rectifiers,” Sci. Rep., Vol. 6, p. 38092, 2016.
[51] X. Zhang, H. Bronstein, A. J. Kronemeijer, J. Smith, Y. Kim, R. J. Kline, L. J. Richter, T. D. Anthopoulos, H. Sirringhaus, K. Song, M. Heeney, W. Zhang, I. McCulloch, and D. M. DeLongchamp, “Molecular origin of high field-effect mobility in an indacenodithiophene-benzothiadiazole copolymer,” Nat. Commun., Vol. 4, p. 2238, 2013.
119
Curriculum Vitae Education
2004 - 2008 Mt. Lebanon High School, Pittsburgh, PA
2008 - 2010 Wake Forest University
2010 - 2012 B.S. Physics, Pennsylvania State University
Minor in Mathematics
2012 - 2018 Ph.D. Physics, Wake Forest University, Winston-Salem, NC
Advisor: Dr. Oana D. Jurchescu
Patents
M. W. Welker, O. D. Jurchescu, and Z. A. Lamport, Invention Disclosure 08/30/2017, “Substituted Benzalkylsilane Molecular Rectifiers.”
Publications 1. Z. A. Lamport, H. F. Haneef, M. Waldrip, S. Anand, and O. D. Jurchescu,
Submitted, J. Appl. Phys. 2018
2. Z. A. Lamport, K. J. Barth, H. Lee, M. Guthold, L. J. Richter, I. McCulloch, J. E. Anthony, D. M. DeLongchamp, and O. D. Jurchescu, To be submitted, July 2018.
3. Z. A. Lamport, B. C. Scharmann, A. D. Broadnax, M. E. Welker, and O. D. Jurchescu, To be submitted, July 2018.
4. A. D. Broadnax, Z. A. Lamport, B. C. Scharmann, O. D. Jurchescu, and M. E. Welker, J. Organomet. Chem. Vol. 856, p. 23, 2018.
5. Z. A. Lamport, R. Li, C. Wang, W. Mitchell, D. Sparrowe, D.-M. Smilgies, C. Day, V. Coropceanu, and O. D. Jurchescu, J. Mater. Chem. C Vol. 39, p. 10313, 2017.
6. Z. A. Lamport, A. D. Broadnax, D. Harrison, K. J. Barth, L. Mendenhall, C. T. Hamilton, M. Guthold, T. Thonhauser, M. E. Welker, and O. D. Jurchescu, Sci. Rep Vol. 6, p. 38092, 2016.
7. J. W. Ward, Z. A. Lamport, and O. D. Jurchescu, ChemPhysChem Vol. 16, p. 1118, 2015.
8. P. J. Diemer, Z. A. Lamport, Y. Mei, J. W. Ward, K. P. Goetz, W. Li, M. M. Payne, M. Guthold, J. E. Anthony, and O. D. Jurchescu. Appl. Phys. Lett. Vol. 107, p. 103303, 2015.
120
Presentations 1. Organic thin-films with charge-carrier mobilities of 20 cm2/Vs, independent of gate
voltage Materials Research Society (MRS) Spring Meeting Oral Presentation in Phoenix, AZ (2018)
2. Organic Thin Films with Charge Carrier Mobility Exceeding that of Single Crystals American Physical Society (APS) March Meeting Oral Presentation in New Orleans, LA (2017)
3. Fluorinated Benzalkylsilane Molecular Rectifiers International Conference of Electroluminescence and Optoelectronic Devices (ICEL) Oral Presentation in Raleigh, NC (2016)
4. The effect of internal molecular dipole moment on the properties of molecular rectifiers Solar Energy Research Center (SERC) Conference Poster Presentation in Charlotte, NC (2016)
5. Fluorinated benzalkylsilane molecular rectifiers Materials Research Society (MRS) Spring Meeting Poster Presentation in Phoenix, AZ (2016)
6. Evaluation of the Density of Trap States at the Semiconductor-Dielectric Interface in Organic Field-Effect Transistors Electronic Materials Conference (EMC) Oral Presentation in Santa Barbara, CA (2014)