Organic Molecular Solids

Prof. Allen M. Hermann Professor of Physics Emeritus

University of Colorado

Boulder, Colorado USA

allen.hermann@colorado.edu

Course Outline

Lecture I. . Introduction Materials, crystal structures Prototypical Molecules, anthracene, naphthalene, etc. Molecular Solids Materials Preparation Electronic Properties Measurements

II. Insulators

Charge Transport Theory, narrow bands

Delocalized (Bloch) Wave Functions

Localized Wave Functions

Excitons

Peirels Distortion (1D systems)

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

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

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

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

Lecture I.

. Introduction Materials, crystal structures

Prototypical Molecules, anthracene, naphthalene, etc.

Molecular Solids Materials Preparation Electronic Properties

Measurements

Conductivity Of Organic Materials

Bonds

Chapter 5 of Solymar

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

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

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

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

Bond Types

• Four types in total

– Ionic

– Covalent

– Metallic

– van der Waals

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

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

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

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

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

Aromatic Hydrocarbon Bonds

Conducting Organic Materials

Extreme Case – Nearly Ionic Bonds in Highly Conducting Complexes

“Charge Transfer salts”

Discovery of Conducting Organic Crystals

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.

• 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

Materials Preparation Techniques

S

S

S

S

S

S

S

S

Electronic Measurements

Conductivity (Resistivity)

Conductivity s = enm

n: number of carriers; m: mobility of the carriers

4-probe resistivity measurement

Van Der Pauw resistivity measurement

Hall effect

Drift Mobility from Time of Flight Measurements and TFT

Structures

Some references to this material