The electronic density of states of metal oxides can in many cases be measured by electrochemical techniques , such as chronopotentiometry or impedance spectroscopy. When small ions such as protons or Li ions are intercalated into a material, which is used as the working electrode in an electrochemical cell, electrons must be inserted from the back contact to maintain charge neutrality. These electrons will enter into previously unoccupied states and the Fermi level will shift upwards in energy as intercalation proceeds. Provided that the rigid band approximation holds, measuring the inserted charge during this process will give an image of the electronic densityof states over 1 to 2 eV from the band edge.
We have compared the so called electrochemical density of states obtained by this method to density functional calculations for a number of oxides. The electrochemical technique is able to give results in qualitative agreement with calculations. We have studied a number of metal oxides such as WO3, TiO2, V2O5, Sb:SnO2, In:SnO2, IrO2 as well as some NiO based coatings.
There are a number of questions that need to be better understood in order to make the technique fully quantitative, though. A limitation is that the number of ions that can be intercalated in a given material is restricted. In addition, the ions do not distribute uniformly in the coating but probably exhibit a gradient in concentration, due to slow kinetics of the process. In addition, the validity of the rigid band approximation is largely an open question. The technique will be useful for screening the density of states of a large amount of coatings in the laboratory. Then interesting materials and specimen can be selected for further studies by advanced techniques, such as photoelectron or X-ray spectroscopies at synchrotron facilities.
1. M. Strömme, R. Ahuja and G.A. Niklasson, Phys. Rev. Lett. 93 (2004) 206403
2014. 254- p.
Joint 12th Russia/CIS/Baltic/Japan Symposium on Ferroelectricity and 9th International Conference Functional Materials and Nanotechnologies, Riga, Latvia, Sept 29 - Oct 2, 2014