Preparation of Thin-Film Alumina for Catalytic Activity Studies

Abstract

An atomic-scale understanding of electronic, morphological and chemical structure of materials is a necessary prerequisite for tailoring nanostructured materials for catalytic applications. Scanning tunneling microscopy (STM) and spectroscopy (STS) are surface sensitive tools than that can be used to systematically probe morphological and electronic structure that can affect catalytic properties. STS, which can be used in tandem with STM, can give information on the densities of both filled and unfilled states at the nanometer scale by probing the local density of states (DOS) underneath the tip. This mapping is accomplished by varying the applied voltage and measuring the tunneling current while holding the tip at a constant position over an area of interest in the ST micrograph. A current-to-voltage (I-V) spectrum providing information regarding the chemical environment of a single atom can thus be produced. Figure 1 shows a diagram showing the band gap (E_g) between the conduction (E_c) and valence (E_v) band edges of metal clusters adsorbed onto a conductive support. Electrons (injected from the tip to the surface) occurs between the Fermi levels of the tip and sample, with electrons tunneling out of the more negative source. Fully metallic clusters exhibit no band gap (denoted by the length of the plateaus at the zero current); but an increase is observed with a decrease in size as the admetal clusters adopt a more non-metallic character. The length of the plateaus (in eV) is an effective band gap measurement of the supported adclusters.