Lecture 1 oms
- 1. Organic Molecular Solids Prof. Allen M. Hermann Professor of Physics Emeritus University of Colorado Boulder, Colorado USA allen.hermann@colorado.edu
- 3. Lecture I. . Introduction Materials, crystal structures Prototypical Molecules, anthracene, naphthalene, etc. Molecular Solids Materials Preparation Electronic Properties Measurements
- 4. II. Insulators Charge Transport Theory, narrow bands Delocalized (Bloch) Wave Functions Localized Wave Functions Excitons Peirels Distortion (1D systems)
- 5. 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
- 6. 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
- 7. 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
- 8. 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
- 9. Lecture I. . Introduction Materials, crystal structures Prototypical Molecules, anthracene, naphthalene, etc. Molecular Solids Materials Preparation Electronic Properties Measurements
- 18. Bonds Chapter 5 of Solymar
- 19. 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
- 20. 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
- 21. 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
- 22. 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 )1 n m ( r B E m o C
- 23. Bond Types • Four types in total – Ionic – Covalent – Metallic – van der Waals
- 24. 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
- 25. 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
- 26. 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
- 27. 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
- 28. 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
- 39. Extreme Case – Nearly Ionic Bonds in Highly Conducting Complexes “Charge Transfer salts”
- 40. Discovery of Conducting Organic Crystals
- 44. 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.
- 45. • 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
- 51. S S S S S S S S
- 55. Conductivity s = enm n: number of carriers; m: mobility of the carriers
- 58. Van Der Pauw resistivity measurement
- 60. Hall effect
- 63. Drift Mobility from Time of Flight Measurements and TFT Structures
- 72. Some references to this material