Organic Molecular Solids   Prof. Allen M. HermannProfessor of Physics Emeritus    University of Colorado   Boulder, Colora...
Course Outline
Section I.. IntroductionMaterials, crystal structuresPrototypical Molecules, anthracene,naphthalene, etc.Molecular SolidsM...
II. InsulatorsCharge Transport Theory, narrow bandsDelocalized (Bloch) Wave FunctionsLocalized Wave FunctionsExcitonsPeire...
III. Transient and Steady-state Photoconductivity inInsulators, Theory and Experiment Small-signal limitDrift MobilityTrap...
V. Organic ConductorsCharge-transfer ComplexesQuasi-one-dimensional and two-dimensionalmaterials, radical-ion saltsPolymer...
VII. ApplicationsElectrostatic Imaging and Xerographic materialsOrganic Light-emitting diodes ) OLEDS and ActiveMatrix OLE...
Section I.       . Introduction Materials, crystal structures   Prototypical Molecules,anthracene, naphthalene, etc.      ...
ConductivityOf OrganicMaterials
BondsChapter 5 of Solymar
Introduction• When two hydrogen atoms come close to each  other  – They form a chemical bond, resulting in a hydrogen mole...
Interactions between Atoms• For atoms to come close and form bonds, there must be  an attractive force   – Na gives up its...
Equilibrium Separation• There is a balance point, where the two forces cancel out (Fig.  5.1)   – The energy goes to zero ...
Mathematical• Mathematically                                            A B                                     E(r )  n ...
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...
Covalent Bonds• When two identical atoms come together, a covalent bond  forms• The hydrogen molecule   – A hydrogen atom ...
Group IV• Carbon 1s22s22p2; Si 1s22s22p63s23p2; Ge  1s22s22p63s23p63d104s24p2• Each atom needs four extra electrons to fil...
Group IV• At 0 K   – All electrons are in bonds orbiting atoms   – None can wander around to conduct electricity   – They ...
van der Waals Bonds• Argon has outer shell completely filled• When argon is cooled down to liquid helium temperature, it  ...
Aromatic Hydrocarbon Bonds
Conducting Organic Materials
Extreme Case – Nearly Ionic Bonds in Highly Conducting Complexes     “Charge Transfer salts”
Discovery of Conducting Organic Crystals
Materials Preparation    Techniques
S   S   S   SS   S   S   S
Electronic Measurements
Conductivity (Resistivity)
Conductivity s = enmn: number of carriers; m: mobility of the carriers
4-probe resistivity measurement
Van Der Pauw resistivity measurement
Hall effect
Drift Mobility from Time ofFlight Measurements and TFT         Structures
Some references to this material
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Section 1 oms

  1. 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. 2. Course Outline
  3. 3. Section I.. IntroductionMaterials, crystal structuresPrototypical Molecules, anthracene,naphthalene, etc.Molecular SolidsMaterials PreparationElectronic Properties Measurements
  4. 4. II. InsulatorsCharge Transport Theory, narrow bandsDelocalized (Bloch) Wave FunctionsLocalized Wave FunctionsExcitonsPeirels Distortion (1D systems)
  5. 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. 6. V. Organic ConductorsCharge-transfer ComplexesQuasi-one-dimensional and two-dimensionalmaterials, radical-ion saltsPolymersVI. Carbon-based nanostructures andSuperconductorsBuckyballs, Nanotubes, GrapheneOrganic Superconductors
  7. 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. 8. Section I. . Introduction Materials, crystal structures Prototypical Molecules,anthracene, naphthalene, etc. Molecular Solids Materials Preparation Electronic Properties Measurements
  9. 9. ConductivityOf OrganicMaterials
  10. 10. BondsChapter 5 of Solymar
  11. 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. 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. 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. 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. 15. Bond Types• Four types in total – Ionic – Covalent – Metallic – van der Waals
  16. 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. 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. 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. 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. 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. 21. Aromatic Hydrocarbon Bonds
  22. 22. Conducting Organic Materials
  23. 23. Extreme Case – Nearly Ionic Bonds in Highly Conducting Complexes “Charge Transfer salts”
  24. 24. Discovery of Conducting Organic Crystals
  25. 25. Materials Preparation Techniques
  26. 26. S S S SS S S S
  27. 27. Electronic Measurements
  28. 28. Conductivity (Resistivity)
  29. 29. Conductivity s = enmn: number of carriers; m: mobility of the carriers
  30. 30. 4-probe resistivity measurement
  31. 31. Van Der Pauw resistivity measurement
  32. 32. Hall effect
  33. 33. Drift Mobility from Time ofFlight Measurements and TFT Structures
  34. 34. Some references to this material

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