Quantum Confined Stark Effects in ZnO Quantum Dots Investigated with Photoelectrochemical Methods

The optical absorption behavior of ZnO quantum dots has been investigated as a function of particle size in the quantum confined regime, between 4 and 9 nm in diameter, by using photoelectrochemical methods. Thin films of quantum dots, with 18 different sizes, were prepared on conducting substrates where the Fermi level could be controlled potentiostatically simultaneously as absorption measurements were performed. While raising the Fermi level into the conduction band, the dominant effect is a decrease in absorption as a consequence of increased electron population in the conduction band. This is a potentiostatic analogue to the Burstein-Moss shift for degenerate semiconductors. For applied potentials in an interval of 0.2 eV below the conduction band edge, the absorption does, however, increases instead of decreases. This absorption increase was found to be caused by a transition into states located within the band gap, which are introduced as a consequence of the applied potential. The magnitude of this effect is for the smallest particles (4 nm) approximately 9% compared to the magnitude of the Burstein-Moss bleaching. The effect decreases with increased particle size and essentially disappears for particles approaching 9 nm. The phenomenon is analyzed in terms of the Stark effect where the consequence of the applied potential is a buildup of an electric field within the particles, breaking the symmetry and splitting the energy levels in the conduction band. The gradual disappearance of the effect for the growing particles gives the extent of the quantum confinement effects of this phenomenon. The size-dependent absorption probability is analyzed and gives important information concerning the nature of both the perturbed states above the conduction band edge and the formation of the subband edge states.