Chemical and morphological characterization of Pd aggregates grown by plasma sputter deposition

Abstract

In this work, the synthesis of supported Pd aggregates by plasma sputter deposition is studied. A helicoidal metal wire is negatively biased with respect to the plasma potential, which results in sputtering of the metal by the argon ions present in the plasma. The wire bias voltage is one of the most important experimental parameters because it determines the ion flux bombarding the metal surface as well as the energy of these ions. Thus, it plays a major role in the sputtering process. In order to study this aspect, deposits have been synthesized at mbar argon pressure and 2 min deposition time, at various wire bias voltages from -100 to 0 V. The amount of Pd deposited is measured by x-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS) analyses and is found to increase linearly with the wire bias voltage (in absolute value) from 0.2 to 1.4 equivalent monolayers. In this study, it appears that XPS analysis is not suitable for quantification in the particular case of small dispersed aggregates, certainly because of a geometrical effect (apparent roughness) in recorded XPS line intensities. Nevertheless, from the XPS Pd peak shift with decreasing coverages, morphological information can be obtained. In this manner, the presence of small aggregates on the substrate surface has been detected. The morphology is more precisely studied by grazing incidence small angle x-ray scattering (GISAXS) and transmission electron microscopy (TEM) analyses. In the present metal coverage range, the deposits are composed of small ( nm) spaced-out spherical aggregates, well dispersed on the surface.