无机化学(7.2) The Crystalline Solid State (Part 2).pdfVIP

Molecular Obitals in Solids Band Formation Overlap of 2 AO’s gives 2 MO’s Overlap of n AO’s gives n MO’s Solids have very large values of n, sometimes multiples of NA Band = many closely spaced MO’s of nearly continuous energy This would be called an ‘s band’. A similar model can be constructed for the p-orbitals and d-orbitals. One-dimensional model The Bonding Picture in Solids Molecular Orbitals in Solids • Band gap – separation between bands in which no MOs exist • Valence band – highest energy band containing electrons (HOMO) • Conduction band – the band immediately above the valence band in energy (LUMO) holes Insulator Conductor w/ no Voltage Conductor with applied Voltage Metals and Insulators holes Insulator Conductor w/ no Voltage Conductor with applied Voltage Insulator = large energy gap between Valence and Conduction bands Electrons can’t move through material Electron motion is what allows conduction of electricity and heat Conductor = partly filled Valence and Conduction bands (Most Metals) Little energy required for electron movement Hole = electron vacancy that can also “move” in partially occupied bands Semiconductors Temperature Effect on Conductors (Metals) – Vibrations increase as temperature increases – Vibrations interfere with electron movement, slow conductance (increase resistance) Semiconductors = full Valence Band, Empty Conductance Band, close together – Energy gap 2 eV – Si, Ge are common pure substances that are semiconductors – At low temperature they are insulators (not as good as true insulators) – At higher tem