SPECTROSCOPIC DETERMINATION OF THE FLAT-BAND POTENTIAL OF TRANSPARENTNANOCRYSTALLINE ZNO FILMS

Ethanolic ZnO dispersions have been characterized by optical absorptio n spectroscopy, transmission electron microscopy, and X-ray diffractio n spectroscopy. Freshly prepared dispersions contain spherical crystal lites (hexagonal wurtzite), having an average diameter of 2 nm, and sh ow confinement effects. Dispersions aged at room temperature for 5 day s contain spherical crystallites, having an average diameter of 13 nm, and show no confinement effects. Transparent nanocrystalline films (t hickness 4 mum) were formed on a conducting glass (SnO2) substrate by sintering 13-nm crystallites in air at 450-degrees-C for 3 h. Incorpor ation in an electrochemical cell, as the working electrode, permits po tentiostatic control of the Fermi level within these films. On applyin g a potential more negative than the flatband potential, electrons acc umulate in the ZnO conduction band. No absorbance which could be assig ned to free conduction band electrons was observed between 300 and 800 nm. Charge carrier behavior was monitored by measuring the Burstein s hift at wavelengths shorter than 385 nm. The potential at which a Burs tein shift of a given magnitude was observed exhibits the expected Ner nstian shift of 0.06V per pH unit for a metal oxide semiconductor in a n aqueous electrolyte solution. Calculation of the flatband potential was possible from the measured relationship between the Burstein shift and applied potential at several different pHs.