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Unformatted text preview: MO Theory of Solids and Proper4es of Metals Chapter 12, 23 Supplementary Reading on Metals Bonding in Metals 1 Bonding in metals Atoms (metals) held together by metallic bonds Recall: Electron -sea model of bonding does NOT explain metal proper9es. How does MO bonding theory help us understand the electrical proper9es of materials? Bonding in Metals 2 Molecular Orbital picture of bands BAND THEORY Note: bonding and an4bonding character Bonding in Metals 3 Band theory Atomic orbitals (AO) mix to form molecular orbitals (MO) Start with 2 AO's, end with 2 MO's Start with n AO's, end with n MO's Extending MO theory to metals In metals, the energy difference between individual orbitals is very small, so they "blend together" Orbitals form a con4nuous bans of allowed energy states where electrons can be located. Bonding in Metals 4 Band structure of a metal Bonding in Metals 5 Conduc4on and Insula4on METAL: Conductor Valence electrons do not fill available orbitals (not enough electrons) Insulator or semiconductor Valence band is full (or completely empty). Energy gap separates valence band from empty orbitals. Bonding in Metals 6 Metals, semiconductors, insulators Good electrical conductors valence electrons do not fill all available orbitals in a band (not enough electrons) Metals Energy gap exists, but is small can increase conduc4vity by chemical modifica4on (doping) or increasing temperature Semiconductors Energy gap is very large and valence band is completely full (or empty) large energy gap separates valence band from available empty orbitals, making it a poor conductor Insulators Bonding in Metals 7 Conductors (examples) What kinds of materials are conductors? Most metals (most elements) Some network covalent solids (graphite) Some polymers (those with conjugated double bonds) Some ceramics (transi4on metal oxides with unusually low or high posi4ve oxida4on states, e.g. V2+ or Cu3+) Bonding in Metals 8 Semiconductors Semiconductors have a gap between the valence band and conduc4on band of ~50 to 300 J/mol The conduc4vity of semi-conductors can be increased with T, or applied fields. Among elements, only silicon, germanium, and graphite (carbon) are semiconductors. They all have 4 valence electrons. Inorganic semiconductors (like GaAs) tend to have an average of 4 valence electrons (3 for Ga, 5 for As). Spotlight on semiconductors One of the most common uses of semiconductors is in electronic devices (e.g. "silicon chip") How exactly does a silicon chip work? It has to exhibit electrical conduc4vity to process informa4on, but it is a semiconductor with a bandgap. How can this be? Need to develop strategies for increasing the electrical conduc9vity of a semiconductor Bonding in Metals 10 Silicon Proper4es: shiny, silvery gray brifle Poor thermal conductor SEMI-METAL Uses: alloy (with Al, Mg) Silicone polymers Electronic applica4ons for these
applica4ons very pure silicon (<1ppb) is required. Zone refining to get pure Si 11 Bonding in Metals Semiconductors What happens when we inten9onally add impuri9es (dopants) to a semiconductor? Add impuri9es that donate extra electrons: n-type Add impuri9es that accept electrons: p-type Which represents an n-type semiconductor? p-type semiconductor? Bonding in Metals 12 Examples What elements would you use to dope Si to make it an n-type semiconductor? What elements would you use to dope Si to make it a p-type semiconductor? Classify each of the following as n-type, p-type, or not doped B-doped Si P-doped Si Te-doped GaAs GaAs Bonding n Metals i As-doped Ge GaAs Si-doped 13 Applica4ons of Semiconductors: Silicon Chips Solar Energy Conversion LED (Light Emilng Diodes) Bonding in Metals 14 Solar Cells: conver9ng light into electricity Photoconduc9vity: absorp4on of electron increases conduc4vity Semiconductor devices Bonding in Metals 15 Semiconductor devices: Diodes A diode is a semiconductor with a p-type material bonded to an n-type material. A diode allows current to flow in only one direc4on. Charge build-up at junc9on creates Deple9on Zone No current flows Apply an external field (baWery) to get current to flow Electrons can flow from n-type to p-type under forward bias Bonding in Metals 16 Semiconductor devices Light-emiYng diodes When electrons combine with holes, light is emiWed. The energy of light (E = h) is the same as the band gap energy Eg The band gap energy depends on the material used to make the diode. Wavelength nm 700 660 630 610 590 565 555
Bonding in Metals Color red red red orange yellow green green Material and structure of LEDs GaP: Zn-O/GaP GaAI0.35 As/GaAs GaAs0.35PO.65: N/GaP GaAs0.25Po.75: N/GaP GaAs0.15PO.85: N/GaP Gap: N/GaP GaP/GaP
17 Semiconductor devices Light-emiYng diodes LEDs small light bulbs that produce useful light from the flow of electrons through a semiconductor diode More energy efficient than incandescent ligh4ng LEDs producing visible light are typically made from doped Aluminum-Gallium- Arsenide (AlGaAs) Changing the dopants changes the size of the deple4on zone and the color the visible light produced EXAMPLE Bonding in Metals 18 Insulators The energy band gap: They are not electrical conductors. What kinds of materials are insulators? Bonding in Metals 19 Inorganic, typically non-metallic solids Ceramics Crystalline: Oxides (Al2O3, ZrO2,BeO) Carbides (SiC, Ca2C) Nitrides (BN) Silicates (SiO2 mixed with metal oxides) Aluminosilicates (Al2O3 + SiO2 + metal oxides: Mica, Talc, Pofery, Clay) Amorphous (glasses) Bonding in Metals 20 Proper9es of Ceramics Typically hard and briWle Less dense than metals, lighter More elas4c than metals Resist corrosion and wear, don't deform Stable at high temperatures High mel4ng Can be covalent network and/or ionic Usually electrical insulators Piezoelectric effect: ability of a material to generate electricity in response to a mechanical stress; grill lighter or convert an electrical impulse into a mechanical response; quartz watch Bonding in Metals 21 Superconduc4vity Superconductors show no resistance to the flow of electricity Superconduc4ng behavior starts below the superconduc4ng transi4on temperature, Tc. Meissner effect: Bonding in Metals 22 Superconductors Much research has been done in the search for a high-temperature superconductors. Bonding in Metals 23 Superconductors The development of higher and higher temperature superconductors will have a tremendous impact on modern culture. Superconduc9ng Ceramic Oxides Bonding in Metals 24 Applica4ons MRI , NMR (magne4c resonance imaging) Levita4ng trains Cell phone frequency filter Bonding in Metals 25 ...
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- Summer '07