Sinclair- Band Theory and Hopping Flashcards
General requirement for high conductivity in TMOs
Need mixed metal valence TMs. Like Cu2+/Cu3+
Methods of getting mixed valency in TMOs
Process in an oxidising or reducing atmosphere. Chemical doping
Oxidation method for mixed valency
Sample gains oxygen to become non-stoichiometric. Example NiO becomes NiO1+δ when processed at 1500C in air. Get Ni2+ (d8) and Ni3+ (d7). Goes from pale green insulator to black semiconductor. Right of d block tend to be oxidised
Reduction method for mixed valency
Sample loses oxygen to become non-stoichiometric. Example TiO2 goes to TiO2-δ when processed at 1500C in Ar (inert atmosphere). Get Ti3+ (d1) and Ti4+ (d0). Goes from white insulator to blue semiconductor. Left of d block tend to be resuced
Normal difference in radii allowed for doping
About +/-15%
What does aliovalent doping mean?
Dopant has different charge to host ion. Need for doping method for mixed valency
Acceptor doping for mixed valency
Dopant ion of lower charge. E.g Li+ doping of NiO turns it into a black semiconductor. Get mixed Ni2+ and Ni3+. Hole doping, p-type conductor
Donor doping for mixed valency
Dopant ion of higher charge. E.g Nb5+ doping of Ti site in StTiO3 can induce semiconductivity. Get mixed Ti3+ and Ti4+. Electron doping, n-type conductor
What is a hopping semiconductor?
One in which electrons or holes can hop between ions of the same type but different valence.
How is conductivity of hopping semiconductor influenced by temperature?
σ=nqμ still holds. Hopping is thermally activated so mobility of charge carrier is temperature dependent (contrast to band model where μ of charge carriers has little temperature dependence).
What happens when NiO heated in air?
Normally rocksalt structure. It picks up O2 molecules which adsorb on the surface, dissociate and pick up 2e- to form O2-. The electrons come from oxidation of Ni2+ to make Ni3+ (d7). To partially balance extra layer of O2- ions at surface Ni2+ ions migrate leaving cation vacancies throughout the crystal. Each O2- generated creates one Ni vacancy somewhere and two Ni3+ ions.
Ni(1-x)O becomes Ni2+(1-3x)Ni3+(2x)O
How does hopping conduction work in NiO after heating in air?
Electrons hop from Ni2+ to an adjacent Ni3+ (or holes hop in opposite direction. Electrons localised on Ni2+ so concept of conduction and valence bands doesn’t apply. Main process
Ni2+ -> Ni3+ + e-. Activation energy needed to cause hopping. Number of charge carriers, n, is [Ni3+]
NiO heated in air graph of lnσ vs 1/T
Straight line negative gradient. Slope -E/k
Why is it difficult to control degree of oxidation for NiO heated in air?
Depends on oxygen partial pressure, temperature and solid/gas equilibria/kinetics. Means n varies and therefore so does σ
How to control oxidation of NiO
Dope with Li+ to obtain Ni(1-x)LixO.
2Ni2+ -> Ni3+ + Li+. Compound becomes
Ni2+(1-2x)Ni3+(x)Li+(x)O
Means [Ni3+] and σ controlled by x so Li+. It forms a solid solution with rocksalt structure and Ni3+, Ni2+ and Li+ distributed at random over crystal octahedral sites. Fixed x at fixed field E means n constant. μ is lowish and Ea 0.2 to 1eV