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Unformatted text preview: 1 Band to Band Transitions Band to Band Transitions Wide band gap semiconductors In these complexes the color comes from absorption of light that leads to excitation of an electron from a filled valence band to an empty conduction band. CdS (Cadmium Yellow) HgS (Vermillion) As 2 S 3 (Realgar) Energy Bands in Extended Solids Energy Bands in Extended Solids Molecules: Discrete Molecular Orbitals Extended Solids: Continuous Bands of MOs 2 1D Chain of Sodium Atoms 1D Chain of Sodium Atoms Energy Filled MOs Empty MOs Most Bonding Most Antibonding 1 Na atom 2 Na atoms 4 Na atoms Large # Na atoms Band of MOs Metals and Semiconductors Metals and Semiconductors Energy Metals Metals Partially filled band Electrons can easily move between filled and empty MOs. This leads to high electrical and thermal conductivity. Band Gap (E g ) Semiconductors Semiconductors Band gap separates the filled and empty states Electrons have to be excited across the band gap (or doping has to occur) in order to conduct electricity. 3...
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