lecture 1 oms
TRANSCRIPT
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Organic Molecular Solids
Prof. Allen M. Hermann Professor of Physics Emeritus
University of Colorado
Boulder, Colorado USA
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Course Outline
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Lecture I. . Introduction Materials, crystal structures Prototypical Molecules, anthracene, naphthalene, etc. Molecular Solids Materials Preparation Electronic Properties Measurements
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II. Insulators
Charge Transport Theory, narrow bands
Delocalized (Bloch) Wave Functions
Localized Wave Functions
Excitons
Peirels Distortion (1D systems)
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III. Transient and Steady-state Photoconductivity in Insulators, Theory and Experiment Small-signal limit Drift Mobility
Trapping (shallow and deep)
IV. Effects of Finite Charge Injection
Boundary Conditions, Space Charge Limited Currents Pulsed, Steady-state Electric Fields and Light Excitations Dispersive transport
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VI. Carbon-based nanostructures and Superconductors Buckyballs, Nanotubes, Graphene
Organic Superconductors
V. Organic Conductors Charge-transfer Complexes Quasi-one-dimensional and two-dimensional materials, radical-ion salts Polymers
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VII. Applications Electrostatic Imaging and Xerographic materials Organic Light-emitting diodes ) OLEDS and Active Matrix OLEDS (AMOLEDS) for Display and Lighting
Solar Cells Field-effect transistors Batteries Photo-detectors Luminescence for Land-mine Sniffing Lasers Switches E-Ink
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VIII. Molecular Electronics and Nanoscience
Why Molecular Electronics Moore’s Law
Devices: Top-down and Bottom-Up Fabrication
Single Molecule Systems and Materials Many-Molecule Systems and Thin Films
DNA Computing
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Lecture I.
. Introduction Materials, crystal structures
Prototypical Molecules, anthracene, naphthalene, etc.
Molecular Solids Materials Preparation Electronic Properties
Measurements
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Conductivity Of Organic Materials
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Bonds
Chapter 5 of Solymar
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Introduction
• When two hydrogen atoms come close to each other
– They form a chemical bond, resulting in a hydrogen molecule (H2)
• When many silicon atoms come close – They form many chemical bonds, resulting in a crystal
• What brings them together? – The driving force is
To reduce the energy
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Interactions between Atoms
• For atoms to come close and form bonds, there must be an attractive force – Na gives up its 3s electron and becomes Na+
– Cl receives the electron to close its n = 3 shell and becomes Cl-
– The Coulomb attractive force is proportional to r-2
• In the NaCl crystal, Na+ and Cl- ions are 0.28 nm apart – There must be a repulsive force when the ions are too close to
each other
– When ions are very close to overlap their electron orbitals and become distorted, a repulsive force arises to push ions apart and restore the original orbitals
– This is a short-range force
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Equilibrium Separation
• There is a balance point, where the two forces cancel out (Fig. 5.1) – The energy goes to zero at infinite separation
– As separation decreases, the energy decreases, so the force is attractive
– At very small separation, the energy rises sharply, so the force is strongly repulsive
– The minimum energy point (Ec, or the zero force point) corresponds to the equilibrium separation ro
– The argument is true for both molecules in crystals
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Mathematical
• Mathematically
– A and B are constants
– The first term represents the repulsion and the second attraction
• Minimum energy
– It must be negative, so m < n
mn r
B
r
A)r(E
)1n
m(
r
BE
m
o
C
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Bond Types
• Four types in total
– Ionic
– Covalent
– Metallic
– van der Waals
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Metallic Bonds
• Each atom in a metal donates one or more electrons and becomes a lattice ion
– The electrons move around and bounce back and forth
– They form an “electron sea”, whose electrostatic attraction holds together positive lattice ions
– The electrostatic attraction comes from all directions, so the bond is non-directional
– Metals are ductile and malleable
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Covalent Bonds
• When two identical atoms come together, a covalent bond forms
• The hydrogen molecule – A hydrogen atom needs two electrons to fill its 1s shell
– When two hydrogen atoms meet, one tries to snatch the electron from the other and vice versa
– The compromise is they share the two electrons
• Both electrons orbit around both atoms and a hydrogen molecule forms
• The chlorine molecule – A chlorine atom has five 3p electrons and is eager to grab one more
– Two chlorine atoms share an electron pair and form a chlorine atom
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Group IV
• Carbon 1s22s22p2; Si 1s22s22p63s23p2; Ge 1s22s22p63s23p63d104s24p2
• Each atom needs four extra electrons to fill the p-shell – They are tetravalent
• sp3 hybridization – s shell and p shell hybridize to form four equal-energy dangling
electrons
– Each of them pairs up with a dangling electron from a neighbor atom
– There are four neighbor atoms equally spaced
– Each atom is at the center of a tetrahedron
– Interbond angle 109.4
– Covalent bond is directional
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Group IV
• At 0 K – All electrons are in bonds orbiting atoms – None can wander around to conduct electricity – They are insulators
• At elevated temperatures – Statistically, some electrons can have more enough energy to
escape through thermal vibrations and become free electrons – They are semiconductors
• The C–C bond is very strong, making diamond the hardest material known (Table 5.1) – Diamond has excellent thermal conductivity – It burns to CO2 at 700C
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van der Waals Bonds
• Argon has outer shell completely filled
• When argon is cooled down to liquid helium temperature, it forms a solid – The electrons are sometimes here and sometimes there, so the
centers of the positive charge (nucleus) and negative charge (electrons) are not always coincident
– The argon atom is a fluctuating dipole (instantaneous dipole)
– It induces an opposite dipole moment on another argon atom, so they attract each other
– Such attraction is weak, so the materials have low melting and boiling temperatures
– They are often seen in organic crystals
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Aromatic Hydrocarbon Bonds
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Conducting Organic Materials
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Extreme Case – Nearly Ionic Bonds in Highly Conducting Complexes
“Charge Transfer salts”
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Discovery of Conducting Organic Crystals
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Molecules as Electronic Devices: Historical Perspective
• 1950’s: Inorganic Semiconductors
• To make p-doped material, one dopes Group IV (14) elements (Silicon, Germanium) with electron-poor Group III elements (Aluminum, Gallium, Indium)
• To make n-doped material, one uses electron-rich dopants such as the Group V elements nitrogen, phosphorus, arsenic.
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• 1960’s: Organic Equivalents. – Inorganic semiconductors have their organic molecular
counterparts. Molecules can be designed so as to be electron-rich donors (D) or electron-poor acceptors (A).
– Joining micron-thick films of D and A yields an organic rectifier (unidirectional current) that is equivalent to an inorganic pn rectifier.
– Organic charge-transfer crystals and conducting polymers yielded organic equivalents of a variety of inorganic electronic systems: semiconductors, metals, superconductors, batteries, etc.
• BUT: they weren’t as good as the inorganic standards. – more expensive – less efficient
Molecules as Electronic Devices: Historical Perspective
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Materials Preparation Techniques
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S
S
S
S
S
S
S
S
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Electronic Measurements
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Conductivity (Resistivity)
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Conductivity s = enm
n: number of carriers; m: mobility of the carriers
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4-probe resistivity measurement
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Van Der Pauw resistivity measurement
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Hall effect
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Drift Mobility from Time of Flight Measurements and TFT
Structures
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Some references to this material
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