SPECTROSCOPIC DETERMINATION OF THE FLAT-BAND POTENTIAL OF TRANSPARENT NANOCRYSTALLINE ZNO FILMS

Ethanolic ZnO dispersions have been characterized by optical absorption spectroscopy, transmission electron microscopy, and X-ray diffraction spectroscopy. Freshly prepared dispersions contain spherical crystallites (hexagonal wurtzite), having an average diameter of 2 nm, and show confinement effects. Dispersions aged at room temperature for 5 days contain spherical crystallites, having an average diameter of 13 nm, and show no confinement effects. Transparent nanocrystalline films (thickness 4 mum) were formed on a conducting glass (SnO2) substrate by sintering 13-nm crystallites in air at 450-degrees-C for 3 h. Incorporation in an electrochemical cell, as the working electrode, permits potentiostatic control of the Fermi level within these films. On applying a potential more negative than the flatband potential, electrons accumulate in the ZnO conduction band. No absorbance which could be assigned to free conduction band electrons was observed between 300 and 800 nm. Charge carrier behavior was monitored by measuring the Burstein shift at wavelengths shorter than 385 nm. The potential at which a Burstein shift of a given magnitude was observed exhibits the expected Nernstian shift of 0.06V per pH unit for a metal oxide semiconductor in an 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.