APPLICATION OF LUMINESCENCE TECHNIQUES TO PROBE SURFACE-ADSORBATE INTERACTIONS ON OXIDE SINGLE-CRYSTALS
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Luminescence from semiconducting metal-oxide single crystals can provide important insights into the nature of the surface-adsorbate bond. This study examines cathodoluminescence from a zinc oxide single crystal and changes induced by adsorption and reaction of various oxygen-containing reagents on the (0001) Zn-polar plane. The electron gun on board the UHV surface analysis instrument was used to excite luminescence from the solid; emission spectra were measured with an optical system external to the chamber. Both the band gap emission (in the UV region at 380 nm) and the subband gap emission (in the visible region at 500 nm) characteristic of zinc oxide were monitored in cathodoluminescence from the single crystal. Adsorption at room temperature of methanol, formic acid, acetone, 2-propanol, methyl formate and benzaldehyde on the ZnO(0001) surface produced an irreversible increase of the band gap emission in each case. The UV intensity returned to its original level (i.e., to that of clean zinc oxide before adsorption) after annealing the surface to temperatures high enough to desorb and decompose organic adsorbates (700800 K). The increase in intensity of the UV emission was proportional to the amount of adsorbate on the surface, demonstrated by the correlation of the integrated C(b) peak intensity of the adsorbate in x-ray photoelectron spectroscopy with the change in the luminescence intensity. The luminescence peak intensity normalized by the surface coverage, referred to as the specific luminescence, decreased with increasing ionization potential of the reactant adsorbed. Failure to normalize the change in luminescence intensity for varying saturation coverages of different adsorbates produced the opposite trend. These results demonstrate that it is essential to calibrate surface coverage independently in order to relate the perturbation of the solid to the electronic properties of the adsorbate. This work demonstrates the application of luminescence to probe reactions on oxide surfaces, it illustrates the advantages of typical UHV surface analysis instruments for such experiments, and it considers the potential for quantitative determination of the extent of charge transfer between adsorbate and surface. 1993, American Vacuum Society. All rights reserved.
