Effects of substrate defect density and annealing temperature on the nature of Pt clusters vapor deposited on the basal plane of highly oriented pyrolytic graphite

Abstract

The influence of substrate defect sites on the morphology of Pt vapor deposited on the basal plane of highly oriented pyrolytic graphite (HOPG(bp)) has been investigated in situ (in ultrahigh vacuum (UHV)) by X-ray photoelectron spectroscopy (XPS), and ex situ by high-resolution scanning electron microscopy (HRSEM), and scanning tunneling microscopy (STM). Defects were introduced on selected sections of HOPG(bp) surfaces via Ar + bombardment by employing specially designed masks. The effect of such defects on the electronic and structural nature of the Pt clusters formed was examined by evaporating the metal simultaneously on both damaged and nominally pristine sections of HOPG(bp). HRSEM and STM images of Pt deposits formed at 300 K on pristine HOPG(bp) were found to display three-dimensional fractal-type aggregates. Image analysis of such Pt clusters gave a fractal dimension D L=1.7 for Pt on Pt, using area ( A)/perimeter ( P) relations, and D L=1.5 for Pt on HOPG(bp), based on the count box method. Possible origins of this difference include higher cluster edge atom diffusion rates for Pt in the first monolayer (ML) and/or small cluster mobilities as in the model investigated by Meakin [Phys. Rev. Lett. 51 (1983) 1119.]. This overall behavior was unlike that observed for Pt vapor deposited on sections of Ar +-bombarded HOPG(bp) surfaces, which yielded instead clusters roughly circular in shape, ca. 1 nm in diameter, and one to two atomic layers high. Such disparate morphologies, however, are in agreement with models proposed in the literature which predict formation of highly ramified structures for diffusion limited aggregation (DLA) on substrates with low density of defect sites and small round clusters when the defect density is increased. Also studied in this work were the effects of post-deposition thermal annealing on the nature of the deposits.