molecular electronics where builders meet chiselersramu/msnt505/lec_notes/kuila...sample preparation...
TRANSCRIPT
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Molecular Electronics
• Self-assembly of molecules on metal and semiconductor surfaces– New possibilities for nanoscale devices– Eliminates machinery required to manipulate
objects with nm resolution
• Nanowires as interconnects for interfacing nanoscale devices to the microelectronic systems
•100 x 10-6 m (100 µm)
• 10 x 10-6 m (10 µm)
• 1 x 10-6 m (1 µm)
•100 x 10-9 m (100 nm)
• 10 x 10-9 m (10 nm)
• 1 x 10-9 m (1 nm)• 1 x 10-10 m (1 Å)
Transistor based Devices , 1960
Visible LightIntegrated Circuits, 1990
---Predicted Scaling Barrier---Mesoscopic PhysicsBiomolecules, Molecular AssembliesMoleculesAtoms
Scale
BuildingUp
(Chemists)
Scaling Down(Engineers)
Where Builders Meet Chiselers
Nanostructures
• Nanotechnology is still very much in infant stages
• Characterization of the nanoscale sytems is necessary– Knowledge of electrostatic interaction can
provide a powerful insight into electronic properties
“Plenty of Room at the Bottom”
• R. Metzger “ Electrical Rectification by a Molecule: The Advent of UnimolecularElectronic Devices” Acc. Chem. Res. 1999, 32, 950-957
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Roughness analysis- unannealed gold- area II
Profile along the grains
Roughness of Au/Ti/Si Substrates
• It depends on preparation.– ~ 2:00 nm ( e-beam evaporation at Purdue)– H2-Flame annealing reduces roughness to
0.7-0.8 nm
• Please see also the hand-out distributed today (4/21).
Sample Preparation
Scan Size: 1.7 µm X 1.7 µm
500 nm
1) Au substrates are flame-annealed and cleaned. This procedure produces large flat Au(111) grains.
2) The surface potentials of the bare Au substrates are measured prior to SAM deposition.
3) SAMs are then grown on the annealed Au substrates.
4) The surface potential of the SAMs are measured using EFM techniques (discussed above). The surface potential measurements are referenced to a bare Au reference sample.
Non-Contact Scans of BM Coated Au
Howell 00
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• Au substrates flame-annealed to produce large flat Au (111) grains
• SAMs prepared by placing Au (111) in ~ 1 mM solution of organic thiols for 12-18 hrs., followed by rinsing with solvent and drying in air or in a dry box
• SAMs characterized by ellipsometry (thickness) and RAIR (orientation, etc.) techniques
Preparation and Characterization of SAMs
DDT ODT
Alkanethiols
(Dodecanethiol) (Octadecylthiol)
Symmetric Non-Symmetric
XYLTMXYL
(Xylyldithiol) (Tetramethyl-xylyl-dithiol)
BM(Benzyl mercaptan)
Molecules Under Investigation
PMBM
(Pentamethylbenzylmercaptan )
Characterization of SAMs
• G. Whitesides• D. Allara; R. Nuzzo
• Hand-out [ Reflectance Absorption IR Spectroscopy (RAIRS) or IR-Reflectance Absorption Spectroscopy (IR-RAS)]
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References
• See the cross references on I-V studies in our book chapter– Heath & Ratner ( Physics Today, 2003)
• Scientific American, 2000 and 2001• Mark Reed, James Tour, Charles Leiber• IBM papers• HP papers ( Stan Williams)
Normal Vibrations & Vibrational Spectroscopy
• A non-linear molecule has 3N-6 normal vibrations ( or normal modes of vibration) – N is the # of atoms
• A linear molecule – 3N-5 normal vibrations• A fundamental transition will be IR active, if the
excited normal mode belongs to the same representation as any one or several of the Cartesian coordinates
• For Raman, the integral containing polarizabilitytensor has to be non zero.
• Ref. F. A. Cotton; Ch-10: “Chemical Applications of Group Theory” Second Edition, Wiley-InterScience, New York [ Relevant pages distributed as handouts ( 4/21)]
Vibrational Spectra
• Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives by G. Varsanyi ( John Wiley & Sons, New York)
• Hand-outs
Vibrational Spectroscopy ( IR) of Molecules on Metal Surfaces
• Chemisorption –may involve major rearrangement of the bonding pattern
• Metal-Surface Selection rules – high electron mobility of electrons (dielectric behavior) has an important influence as the electrons are able to screen centers of charge in electric fields– Vibrational modes with a component of dynamic dipole
moment perpendicular to the surface can be observed
Ref. F. M. Hoffman, “Infrared Reflection-Absorption Spectroscopy of Adsorbed Molecules” Surf. Sci. Rep. 1983, 3, 107-192.
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RAIRS or IR-RAS
• Grazing angle incidence necessary to have more interaction of light w the surfcae
• Signal is quite weak; need a lot of scans– Signals proportional to ( # of scans)1/2
• You do not see all the peaks as in regular-IR• Remember that normal IR ( solution, solid, or
gas) is quite strong [ using KBr windows or ATR)
• We have both RAIR and ATR accessories at IfM
DDT ODT
Alkanethiols
(Dodecanethiol) (Octadecylthiol)
Symmetric Non-Symmetric
XYLTMXYL
(Xylyldithiol) (Tetramethyl-xylyl-dithiol)
BM(Benzyl mercaptan)
Molecules Under Investigation
PMBM
(Pentamethylbenzylmercaptan )
1112
1260 15
96
0
0.002
0.004
0.006
0.008
0.01
0.012
1000 1200 1400 1600 1800 2000 2200
Wavenumber (cm-1)
Abs
orba
nce
RAIR Spectrum of Benzylthiol on Au
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RAIR Spectrum of Xylyl-dithiol (XYL) SAM on Au 4-Pyridinethiol Derivatives (4-PySHD) and 4-PySHD coordinated to MTPP
N
(CH2)nSH
When n = 0; 4-Pyridinethiol
n = 0, 1, 2, etc.
M
N
N N
N
N
(CH2)n-SAu
M = Zn, Co, Ni, Mn
n = 0, 1, 2,3Preliminary ESP Measurements4-PySH 30 ± 50mV 4-PySH-CoTPP 130± 50mV
RAIR Spectrum of PySH–CoTPP SAM on Au Primer: A Cell Up Close
Cell membrane: Lipids (structure), Proteins (gateways)
bR
Cell wall: Rigid, Permeable
PurdueNanoscience
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Percent CoveragePurdueNanoscience
HOPG30% Coverage
Au40% Coverage
Crittenden & Reifenberger
Suggested Papers for Reading
• K. Vijayamohanan & M. Aslam “ Applications of Self-Assembled Monolayers for Biomolecular Electronics” Appl. Biochem. Biotech., 2001, 96, 25-39.
• J. F. Fang et al. “Self-Assembled Rigid Monolayers of 4’-Substituted-4-mercaptobiphenyls on Gold and Silver Surfaces” Langmuir, 2001, 17, 95-106.
Nanostructures
• Nanotechnology is still very much in infant stages
• Characterization of the nanoscale sytems is necessary– Knowledge of electrostatic interaction can
provide a powerful insight into electronic properties
• AFM is capable of measuring piconewtonforces with nm resolution
Experimental Set-Up
Photo DiodeLaser Topo Lock-In EFM Lock-In
Ref Ref
Feedback Control
Piezotube
Sample
AFM Tip
+
Out Out
InIn
Piezo Vibrator
1ω
oω
To Computer
The EFM lock-in measures the amplitude of the ω 1 component: ]Vy)(x,[VVdzdC
Amp tipsoω1 −−=
t)sin(ωV 1o
TipV
t)sin(ωV oamp
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PurdueNanoscience Experimental Set-Up
The EFM lock-in measures the amplitude of the ω 1 component: ]Vy)(x,[VVdzdC
Amp tipsoω1 −−=
Nano-Scale Charge Transfer in Au/Organic Interfaces
+ + + +
Au Substrate
Molecule Dipoles
AFM Tip
Debasish Kuila
Langmuir, 2002, 18, 5120-25
+
Contact potential difference (CPD)
• CPD exists when crystalline objects are placed in intimate contact to form a junction– Results from the equilibrium of both the
temperature and the chemical potential throughout the junction
-600 -400 -200 0 200 400 600
0
100
200
300
400
500
600
700
800
900
340 mV
Tip over Ni
Tip over Au
Ele
ctro
stat
ic F
orce
Mag
nit
ud
e (A
. U.)
Tip Voltage (mV)
PurdueNanoscience Elimination of the Electrostatic Force
cpdtip VV =
When two metals are in contact, their Fermi levels will coincide due to thermodynamic equilibrium. By connecting this system to a bias voltage source, the electrostatic potential can be eliminated.
Contact Potential Difference Test2FE1FE
Tip Sample
{ }}1φ 2φ eVcpd
21 φφ −=
1FE2FE
{
}{ 2φ1φ
tipV
VACE
Howell 00
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Macroscopic Kelvin Probe
• A device that measures the CPD between a sample and a reference electrode ( w known WF, Work Function)
• Two electrodes composed of different metals to form a parallel plate capacitor– Diameters > separation of the plates
connected in series w a current meter and a voltage source
Atomic Force Microscopy (AFM)• Measures forces by detecting the motion of a
spring like probe known as cantilever– Long thin micro-machined beams of Si with a
base containing a tiny tip attached at its end ( radius ~ 10 nm)
• High lateral resolution of force measurements is due to the small diameter of the tip’s apex.
• Interaction between the tip and the sample cause the cantilever beam to deflect– different forces (Magnetic, van der Waals,
electrostatic, adhesion) can be measured simultaneously
Measurement of Electrostatic Interaction
• A conducting tip is biased with a controlled voltage– Modifies the tip-sample potential difference which causes
a deflection of the cantilever
• Controlling the tip-sample potential difference, the electrostatic force emanating from a sample’s surface can be measured as a function of position
• EFM ( Electrostatic Force microscope) – a modified AFM
References:1. Stephen W. Howell, Ph.D. Thesis, Purdue U, May 20012. Langmuir, 2002, 18, 5120-25
Nano-Scale Charge Transfer in Au/Organic Interfaces
+ + + +
Au Substrate
Molecule Dipoles
AFM Tip
Debasish Kuila
Louisiana Tech University
Langmuir, 2002, 18, 5120-25
+
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Electrostatic Surface Potential (ESP) of Organic Thiols on Au using AFM
ESP Measurements
• SAMs of Aliphatic and Aromatic thiols
• ESPs of Symmetric vs. Non-symmetric Systems
• Theoretical Calculations (Preliminary)
• Summary
Electrostatic Surface Potential (What and Why)
Self-Assembled Monolayer of Molecules on Au
+ + + + + + + +- - - - - - - -
E
V
VSAM (wrt Au)
0Au
=
Why Measure Surface Potential
• Insight into electronic properties of SAMs
• A diagnostic feature for the molecule (s)
• Better models to I-V
• Potential Chemical Sensors
• Potential Chemical FETs for nanoelectronic devices
Experimental Set-Up
Photo DiodeLaser Topo Lock-In EFM Lock-In
Ref Ref
Feedback Control
Piezotube
Sample
AFM Tip
+
Out Out
InIn
Piezo Vibrator
1ω
oω
To Computer
The EFM lock-in measures the amplitude of the ω 1 component: ]Vy)(x,[VVdzdC
Amp tipsoω1 −−=
t)sin(ωV 1o
TipV
t)sin(ωV oamp
Measuring the Electrostatic Force
Force Detector
y)(x,VS
Voltage Control
)ω,2ω,(ωF 11otot
t)sin(ωVV 1oTip +
Vibrator
The electrostatic forces acting on the cantilever due to the tip-sample capacitance is:
The potential difference between the tip and substrate is:
t))sin(V(Vy)(x,VV 1oTipS ω+−=
t)sin(V]Vy)(x,[VdzdC
1oTipS ω−−Howell 00
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11
-400 -200 0 200 400 600
0
100
200
300
400
500
Mag
nit
ud
e of
Ele
ctro
stat
ic F
orce
(a.
u.)
Tip Voltage (mV)
Experimental Procedure
Null Voltage
V1
EFM Probe
Sample
Howell 00
Au
VSAM
VAu
EFM Probe
Electrostatic Surface Potential of Molecules
Au
VTip
VTip
Howell 00
PurdueNanoscience
Evac
LUMO
HOMO
EF
Evac
φM
Molecule Metal
qVbi
Energy Structure of a Molecule Bonded to a Metal
EFo
qVmol
qVmol is the potential of the molecule wrt the metal.Howell 00
-600 -400 -200 0 200 400 600
0
100
200
300
400
500
600
700
800
900
340 mV
Tip over Ni
Tip over Au
Ele
ctro
stat
ic F
orce
Mag
nit
ude
(A. U
.)
Tip Voltage (mV)
Elimination of the Electrostatic Force
cpdtip VV =
When two metals are in contact, their Fermi levels will coincide due to thermodynamic equilibrium. By connecting this system to a bias voltage source, the electrostatic potential can be eliminated.
Contact Potential Difference Test2FE1FE
Tip Sample
{ }}1φ 2φ eVcpd
21 φφ −=
1FE2FE
{
}{ 2φ1φ
tipV
VACE
Howell 00
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φSAMφSAM
Tip Over Au Inferred surface potential Between SAM and Au
Comparing the Electrostatic Surface Potential of SAMs and Au Reference Samples
Since the work function of the tip is the same for both measurements, the surface potential of the SAM coated Au can be referenced to the bare Au substrate.
gap
EF
tip Au
φtip φAu
}eV1
Tip Over SAM/Au
gap
EF
tip Au
φtip φAu
eV2
SAM gap
EF
Au Au
φAu
eVSAM
SAM
φAu
φtip=eV1+ φAu φtip=eV2+ φSAM VSAM = (V1-V2)= -(φAu-φSAM)/e
Howell 00
PurdueNanoscience
Evac
EFo
LUMO
HOMO
EF
Evac
φm
Isolated Molecule Isolated Metal
φmolI. P.
qVbi
qVbi = φm - φmol
Energy Structure for Isolated Systems
Howell 00
Au
VSAM
VAu
EFM Probe
Electrostatic Surface Potential of Molecules
Au
VTip
VTip
Howell 00
Sample Preparation
Scan Size: 1.7 µm X 1.7 µm
500 nm
1) Au substrates are flame-annealed and cleaned. This procedure produces large flat Au(111) grains.
2) The surface potentials of the bare Au substrates are measured prior to SAM deposition.
3) SAMs are then grown on the annealed Au substrates.
4) The surface potential of the SAMs are measured using EFM techniques (discussed above). The surface potential measurements are referenced to a bare Au reference sample.
Non-Contact Scans of BM Coated Au
Howell 00
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DDT ODT
Molecules for Initial Studies
(Dodecanethiol) (Octadecylthiol)
Howell100 ± 20 mV 230 ± 30 mV
DDT ODT
Theoretical Calculations (preliminary)
(Dodecanethiol) (Octadecylthiol)
100 ± 20 mV 230 ± 30 mV
MRS Proceedings, 2000, # D9.38
SAM on a Au-substrate ESPs of Alkanethiols ( lit. & Purdue results)
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Symmetric Non-Symmetric
XYL TMXYL
(Xylyldithiol) (Tetramethyl-xylyl-dithiol)
BM(Benzyl mercaptan)
Molecules Under Investigation
PMBM(Pentamethylbenzylmercaptan )
1112
1260 15
96
0
0.002
0.004
0.006
0.008
0.01
0.012
1000 1200 1400 1600 1800 2000 2200
Wavenumber (cm-1)
Abs
orba
nce
RAIR Spectrum of Benzylthiol on Au
-1.0 -0.5 0.0 0.5 1.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
XYL+Au
Au
Mag
nit
ud
e of
EL
ectr
osta
tic
For
ce (
a.u
.)
Tip Voltage (V)
Electrostatic Surface Potential Measurements of Symmetric and Non-Symmetric Molecules
-800 -600 -400 -200 0 200 400 600 800
0
50
100
150
200
2501/2 ΞΨΛ+Αυ
Au
Mag
nit
ud
e of
Ele
ctro
stat
ic F
orc
e (a
. u
.)
Tip Voltage (mV)
Symmetric Non-Symmetric
Howell
BM + Au
Symmetric Non-Symmetric
XYL TMXYL
(Xylyldithiol) (Tetramethyl-xylyl-dithiol)
BM(Benzyl mercaptan)
Molecules Under Investigation
PMBM(Pentamethylbenzylmercaptan )
50 ± 30 mV16 ± 70 mV 235 ± 50 mV
150 ± 50 mV
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Chemisorption of Xylyldithiol on Au
Au onGlass
Xylyldithiol on Gold
Benzyl Mercaptan on Gold ( LANL2DZ basis set) Theoretical Calculations
• Currently underway in collaboration with Prof. Ramachandran
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• Measured ESPs of molecules w.r.t. bare Au
• ESPs of alkanethiols increase with chain length and the trend is similar to that reported in the literature
• Charge-transfer at the interface appears to be small and is dominated by the molecular structure
• Non-symmetric aromatic thiols have higher ESPs than
symmetric ones
• Theoretical work is underway to understand the
magnitude of these differences
Summary ESPs of Phenylthiols
SH SH SH
MPT BPT TPT-0.38 V -0.76 V -0.72 V
Book chapter & cross references
ESPs of TMXYL and the Charge-Transfer Complex
20±70 mV
-140±25 mV 30 ± 60 mV
• MRS Proceedings, 2000, #D9.38
• Langmuir, 2002, 18, 5120-25
• Encyclopedia of Nanoscience and Nanotechnology, “Nanoscale Charge Transfer in Metal-Molecule Heterostructures” 2004, Vol 1., pp. 683-698.– www.dekker.com
Additional Information
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ESP of N-terminal Peptides with different lengths
•
Gold substrate
NH
S S
O=CO
CH2O=C
OCH2
NH
S S
+
-
Alpha helical peptide
N terminal
C terminal
Few hundred mV –veESP
Chem. Phys. Lett1999, 315, 1-6
Book chapter & cross references
Molecule/Metal Heterostructure as a Sensor
Au
-+
+-
Symmetry: Small Net Dipole
Simple Physical Interpretation Based on Symmetry
Au
+
-
Non-Symmetry: Large Net Dipole
Mirror Plane Mirror Plane
Howell 2000
Experimental Set-Up
The EFM lock-in measures the amplitude of the ω 1 component: ]Vy)(x,[VVdzdC
Amp tipsoω1 −−=
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18
RAIR Spectrum of Xylyl-dithiol SAM on Au
Symmetric Non-Symmetric
XYL TMXYL
(Xylyldithiol) (Tetramethyl-xylyl-dithiol)
BT
(Benzyl mercaptan)
PurdueNanoscience Molecules Under Investigation
PMBT
(Pentamethylbenzylmercaptan )
50 ± 30 mV 16 ± 70 mV 235 ± 50 mV 150 ± 50 mV
Nonanedithiol
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
-1.5 -1 -0.5 0 0.5 1 1.5
Voltage (V)
Cu
rren
t (A
)
pad 1
pad2
pad 3
pad 3 meas 2
pad 4
S. Kadathur and D.B.Janes
I-V Measurements on NonanedithiolInterdigitated Au Fingers to Build Nano-
sensors
Pads for Probing Interdigited fingers
5.568µm
4.176µm
2.784µm
2.083µm
1.382µm
# of Square(*10^-3)
Space between Fingers
Choi, Janes, Santanam & Andres
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Acknowledgements
Support : DARPA/ARO, Indiana 21st Century Program
• S. Howell
• H. McNally
• B. Kasibhatla
• C. Kubaik
• D. Janes
• R. Reifenberger
• S. Datta
• T. Rakshit
• P. Damle
• P. Das
RAIR Spectrum of Xylyl-dithiol SAM on Au