STM investigations of self-assembled bismuth nanostructures and ultra-fine gold nanparticles.

Abstract

In-situ scanning tunneling microscopy (STM) has been utilized to investigate the growth of bismuth nanorods (single/multi- layer, straight/branched), ultra-thin Bi nanowires, Bi superstructures, and ultra-fine Au nanoparticles (NPs) on various substrates. When deposited on MoS2(0001), before the height exceeds the critical thickness, Bi form Bi(110) nanobelts (nanoribbons). Straight Bi nanorods can be obtained at low Bi flux and deposition amount, while at high Bi flux, multi-layer branched nanostructures form. A structural transformation from Bi(110) to Bi(111) was observed when the Bi(110) film thickness exceeds 8-Bi(110) monolayer. Other measurements such as scanning electron microscopy (SEM) and low energy electron diffraction (LEED) were used to characterize the orientation distribution of Bi nanobelts. In addition, Bi nanostructures deposited on highly-oriented pyrolytic graphite (HOPG) were studied by low temperature scanning tunneling spectroscopy (LT-STS). Thickness dependent local density of states (LDOS) on Bi(110) layers with different thickness was observed, which may result from the structural relaxation and transformation from Black-P like Bi(110) to bulk-like one. Using a molecular layer 3,4,5,10-perylene tetracarboxylic dianhydride (PTCDA) on MoS2(0001) as a template, ultra-thin Bi nanowires can be synthesized. Bi first grow into NWs with single atomic layer thickness and aligned orientation and then develop into 4- or 6-layer Bi (110) NWs at larger deposition amounts. The NWs grow along three directions of the ordered molecular layer. Due to the side wall passivation by PTCDA, the growth of width of NWs is greatly depressed and hence NWs with large length-to-width ratio (LWR) can be obtained. Using LEED and STM, three structural phases were revealed when Bi deposited on Ru(0001), with Bi coverage ranged from sub-monolayer (ML) to a few ML. A loosely rectangular superlattice (2 × v3) formed at the initial growth stage. After more Bi was deposited, a hexagonal (v7 × v7)R19.1° superlattice was observed. When Ru(0001) was saturated with this (v7 × v7)R19.1°-Bi, it acts as a buffer layer and the surface becomes rather inert. With additional Bi deposited, Bi(110) thin film is formed on this inert substrate. Using PTCDA as a surfactant layer, size-tunable ultra-fine Au NPs can be synthesized on MoS2. The PTCDA overlayer can greatly increase the nucleation density of Au NPs and prevent fine NPs from aggregating into larger particles. Molecular scale STM images show that Au atoms nucleate and grow into NPs underneath the PTCDA layer and lift the molecules to the top of the NPs. Moreover, by annealing the sample, PTCDA molecules can desorb from the MoS2 surface first and then desorb from the top of Au NPs at a higher temperature. By controlling the deposition amount of Au, the size of Au NPs can be tuned. In addition, interaction of Au NPs with PTCDA was investigated in-situ by X-ray photoelectron spectroscopy (XPS), and charge transfer from Au NPs to PTCDA was observed, which indicates that these Au NPs may have new chemical properties.