Spectroscopic Determination of Electron and Hole Effective Masses in a Nanocrystalline Semiconductor Film

Spectroelectrochemical techniques may be used to determine the absolute energy of the valence and conduction band edges of a transparent nanocrystalline semiconductor electrode. Such determinations have been made for ZnO (wurtzite) and TiO 2 (anatase) electrodes constituted from nanocrystallites possessing average radii close to, and substantially larger than, the radius of a bound exciton in the corresponding bulk semiconductor. Electrodes constituted from crystallites whose radii are close to that of a bound exciton exhibit an onset for band gap absorption that is significantly blue-shifted. Those constituted from crystallites whose radii are substantially larger than that of a bound exciton exhibit an absorption onset characteristic of the bulk material. Knowing the absolute energies of band edges, the observed increase in band gap energy for electrodes constituted from confined nanocrystallites may be partitioned between the conduction and valence bands. A subsequent analysis permits determination of the effective electron and hole mass in a nanocrystalline film. Most notably, it has proved possible to determine, for the first time, a value for the effective hole mass in TiO 2 (anatase) of (0.8 ± 0.2) m e .