Electrochemically modulated diffraction - A novel strategy for the determination of conduction-band-edge energies for nanocrystalline thin-film semiconductor electrodes

Micropatterned titanium dioxide thin-film electrodes exhibit efficient diff raction in the presence of aqueous or nonaqueous electrolyte solutions. The diffraction efficiency can be modulated electrochemically. At the waveleng ths examined, the modulation is caused by changes in both real and imaginar y components of the refractive index. The index changes, in turn, are cause d by the addition of electrons to near-band-edge trap sites and by optical absorption by the trapped electrons. The onset potential for diffraction mo dulation provides a good measure of the potential of the electrode's conduc tion bandedge. Variable excitation wavelength measurements show that, after correction for absorption losses, the electrochemically induced changes in the proportion of light diffracted can be either positive or negative. The signs and the relative magnitudes of the wavelength-dependent changes are well described by a Kramers-Kronig analysis that assumes that changes in th e real component of the refractive index dominate the response. (C) 2000 Th e Electrochemical Society. S1099-0062(00)07-101-7. All rights reserved.