STM and XPS investigations of bismuth islands on HOPG

Abstract

The growth of bismuth thin films on highly oriented pyrolitic graphite (HOPG) was studied using ultra high vacuum (UHV) scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The locations of the main XPS peaks, and also the plasmon energy, are in good agreement with results recorded on bulk bismuth. The shape of the Bi 4f doublet, as well as the well developed Fermi edge, indicates the metallic character of the deposited film. One of the observed shoulders is identified with a quantum-well subband characteristic of thin bismuth films. There is no evidence for the existence of Bi–C bonds, consistent with weak bonding between the Bi islands and the HOPG. The height of the islands and their crystallographic orientation were investigated as a function of surface coverage. The Bi islands grow with the (110) plane parallel to the substrate. The observed heights (3, 5, 7, 9 ML) indicate that the preferred crystal structure involves paired layers on an intermediate mono-atomic Bi layer. There is evidence both for and against the Black Phosphorus like allotrope, and the nature of both the layer pairing and the intermediate layer are yet to be resolved. The islands exhibit stripes oriented along the < 1 1 ¯ 0 > axis of the Bi crystal, which is a fast growth direction due to the existence of strongly bonded zig-zag chains of atoms. The surface unit cell and the parameters of the rhombohedral bulk unit cell are estimated based on atomic resolution images. In the case of 2 ML stripes on top of a 5 ML base, the expansion of the outer atomic rows was estimated at 27%. Asymmetries in the growth of the islands are observed. Based on low coverage depositions at reduced substrate temperatures, it is proposed that there is a second fast growth direction corresponding to preferential attachment of atoms to one of the faces of the asymmetric, rhombic cross-section of the (110) crystal.