Fluorescence of a merocyanine dye adsorbed on ZnO crystals

Abstract

An organic merocyanine dye is deposited on well-defined ZnO single crystal surfaces by sublimation in ultrahigh vacuum (UHV). Various surface pretreatments such as annealing in UHV or oxygen, argon ion bombardment, or cleavage in UHV are applied before dye deposition. The optical absorption and fluorescence of the dye as well as the spectrally sensitized photoconductivity are measured for dye coverages between 1013 and 1015 cm-2. Solution and vapor phase spectra of the dye are measured for comparison.The variations of absorption and fluorescence bands with increasing dye coverage can be related to a growing degree of association between dye molecules. At submonolayer coverage the derived quantum yields for fluorescence and sensitized photoconductivity depend strongly on the ZnO surface preparation. There is no observable competition between fluorescence and photoconductivity. The yield of both is limited by nonradiative decay processes. A structural model for the dye-surface interaction is proposed, and the rate constants of various processes are estimated.