Properties of metals

Metals (75% of elements)• Lustrous (reflect light)

• (almost) all solids

• malleable & ductile

• good conductors of heat and electricity

• oxides are basic, ionic solids

• aqueous cations (n+)

Bonding in metalsFree Electron ModelMetals are positive ions in “sea” of nearly free electronsElectrons bond metal ions together but are free to roam the crystal lattice.

Explains high electrical and thermal conductivity

Bonding in MetalsBand Theory•Atomic orbitals (AO) mix to form molecular orbitals (MO). •Start with 2 AO, end with 2 MO•Start with n AOs, end up with n MOs•In metals energy difference between orbitals in valence band is small.•Orbital form a continuous “band” of allowed energy states.

Conduction and InsulationMetalValence electrons do not fill available orbitals (not enough electrons)

Insulator or semiconductorValence band is full (or completely empty). Energy gap separates valence band from empty orbitals.

Band GapsInsulators: The energy gap is > 3.0 eV (= 290 kJ/mol) in insulators

Semiconductors: The energy gap is between 0.05 and 3.0 eV in semiconductors

(kJ/mol)

SEMICONDUCTORSAdd impurities (dopants) to semi-conductorIf impurities donate extra electrons, then the semiconductor is n-type

e.g. P impurities in Si.

If impurities accept electrons, then the semiconductor is p-type e.g. B impurities in Si.

n-type: negative charge carriers (electrons).p-type: apparent positive charge carriers (holes).

SiliconProperties:

shiny, silvery graybrittlepoor thermal conductorsemiconductor

Uses:alloy (with Al, Mg)silicone polymers

electronics, solar cells:very pure silicon (<1ppb) is required.

Zone refining

to get pure Si

A diode is a semiconductor with a p-type material bonded to an n-type material.

Solar cells (photovoltaics) and light emitting diodes (LEDs) are both diode devices.

Diodes

When no current flows

Diodes

A diode allows current to flow in only one direction

Electrons can flow from n-type to p-type under forward bias

In a solar cell, light excitation makes current flow in the opposite direction

Current flows when the diode is forward biased

Light Emitting Diode

When electrons combine with holes, light is emitted.

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.

LED Materials

Wavelength Color Material and structure of LEDs

700 red GaP:ZnO/GaP

660 red Ga0.65Al0.35As/GaAs

630 red GaAs0.35P0.65:N/GaP

610 orange GaAs0.25P0.75:N/GaP

590 yellow GaAs0.15P0.85:N/GaP

565 green GaP:N/GaP

555 green GaP/GaP

LEDs: Light Emitting Diodes

More energy efficient than incandescent lighting

LEDs producing visible light are typically made from doped Aluminum-Gallium-Arsenide (AlGaAs)

Where are LEDs used?