Electronic Properties Flashcards
Conductivity
ability to conduct electrical current electrons flow = electric current S m -1 sigma = N x Ze x u density of electrons x charge x velocity
Conductance
object property
conductance = conductivity x area/length
S
Sigma Values
insulator 104
superconductor = 1024
Bands
N infinity energy levels
no longer discrete
continuum with maxima and minima
Fermi Level
N elections, N orbitals
half full
HOMO - fermi level
LUMO - unoccupied
Simple Metal
high coordination number
low ionisation energy - positive ion
core electrons shield positive
electrons move independently
Thermal Excitation of Electrons
finite temperature
electrons near fermi level
excite electrons at the top of the fermi level
resistance increase with temperature
ion core neglected, appear bigger, repel more
electrons less efficient
energy dicipated as heat
Incandescence
electrons collide with nuclei
energy transferred to nucleus
core electrons excited
electrons fall back down = glowing
Nearly Free Electron Model
electrons attracted to the core energy decreases as they approach ion core = band gap filled band = insulator bigger band = better insulator
Conductor
fermi in unoccupied, LUMO
Insulator
fermi at top of valence, band gap
Semi-Conductor
fermi top of valence if 0 K, small band gap
Group 1
partially filled band
high density near fermi
conduction the
Group 2
filled band - should be an insulator
actually conducts
s and p orbitals overlap
fermi in unoccupied, LUMO
Superconductivity
electrons flow through attracted to ion cores, move slightly subsequent electrons follow first electron Cooper pairs increased conductivity
Quantum Tunnelling
current proportional to width of wall
insulator - particles too far apart
squeezed - conducts, quantum tunnelling, current flows
Transition Metals
d-orbitals highly directional, cannot use free electron model
s and d orbitals overlap
conduct - fermi in s orbital, d orbital filled
Conjugated Planes
planar pi-system, bands formed conduct at room temperature 0 K bad conductor, cannot excite electrons dope with electron donating atom enhances conductivity increase density of states near fermi
Covalent Solids
tetrahedral elements
s and p orbitals overlap
group 4 - gap decreases down the group
Semiconductor Junctions
0 K do not conduct
temperature increases so does conductivity
doped, increases conductivity
i-type HOMO full, LUMO empty
p-type HOMO full, piece of LUMO near fermi
n-type HOMO full, piece of HOMO near LUMO
Conducting Polymers
conjugated linear system electrons in resonance, flow bond lengths alternate electrons found near close electrons can add dopants
