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Section 1 oms

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  • 1. Organic Molecular Solids Prof. Allen M. HermannProfessor of Physics Emeritus University of Colorado Boulder, Colorado USA and Vice-President NanoTech Solutions, Inc. Lexington, KY USAallen.hermann@colorado.edu
  • 2. Course Outline
  • 3. Section I.. IntroductionMaterials, crystal structuresPrototypical Molecules, anthracene,naphthalene, etc.Molecular SolidsMaterials PreparationElectronic Properties Measurements
  • 4. II. InsulatorsCharge Transport Theory, narrow bandsDelocalized (Bloch) Wave FunctionsLocalized Wave FunctionsExcitonsPeirels Distortion (1D systems)
  • 5. III. Transient and Steady-state Photoconductivity inInsulators, Theory and Experiment Small-signal limitDrift MobilityTrapping (shallow and deep)IV. Effects of Finite Charge InjectionBoundary Conditions, Space Charge Limited CurrentsPulsed, Steady-state Electric Fields and LightExcitationsDispersive transport
  • 6. V. Organic ConductorsCharge-transfer ComplexesQuasi-one-dimensional and two-dimensionalmaterials, radical-ion saltsPolymersVI. Carbon-based nanostructures andSuperconductorsBuckyballs, Nanotubes, GrapheneOrganic Superconductors
  • 7. VII. ApplicationsElectrostatic Imaging and Xerographic materialsOrganic Light-emitting diodes ) OLEDS and ActiveMatrix OLEDS (AMOLEDS) for Display and LightingSolar CellsField-effect transistorsBatteriesPhoto-detectorsLuminescence for Land-mine SniffingLasersSwitchesE-Ink
  • 8. Section I. . Introduction Materials, crystal structures Prototypical Molecules,anthracene, naphthalene, etc. Molecular Solids Materials Preparation Electronic Properties Measurements
  • 9. ConductivityOf OrganicMaterials
  • 10. BondsChapter 5 of Solymar
  • 11. 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
  • 12. 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
  • 13. 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
  • 14. Mathematical• Mathematically A B E(r )  n  m – A and B are constants r r – The first term represents the repulsion and the second attraction• Minimum energy B m EC  m (  1) ro n – It must be negative, so m < n
  • 15. Bond Types• Four types in total – Ionic – Covalent – Metallic – van der Waals
  • 16. 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
  • 17. 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
  • 18. 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
  • 19. 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 700C
  • 20. 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
  • 21. Aromatic Hydrocarbon Bonds
  • 22. Conducting Organic Materials
  • 23. Extreme Case – Nearly Ionic Bonds in Highly Conducting Complexes “Charge Transfer salts”
  • 24. Discovery of Conducting Organic Crystals
  • 25. Materials Preparation Techniques
  • 26. S S S SS S S S
  • 27. Electronic Measurements
  • 28. Conductivity (Resistivity)
  • 29. Conductivity s = enmn: number of carriers; m: mobility of the carriers
  • 30. 4-probe resistivity measurement
  • 31. Van Der Pauw resistivity measurement
  • 32. Hall effect
  • 33. Drift Mobility from Time ofFlight Measurements and TFT Structures
  • 34. Some references to this material