Quantifying STM-tip induced directed hopping of Sb2 dimers on Si(001): A kinetic Monte Carlo study

Abstract

© 2015 American Vacuum Society. In this study, kinetic Monte Carlo simulations of a lattice gas model were performed to investigate the experimentally observed enhanced directed diffusion of Sb2 ad-dimers on Si(001) during asymmetric scanning - one that uses invasive and noninvasive scanning tunneling microscope (STM) settings for leftward and rightward scanning, respectively. The authors model the invasive scanning by postulating an attractive interaction between the STM tip and the adsorbates. This is done by lowering (raising) the activation barrier by an adjustable energy difference ΔE for hopping of Sb2 dimers toward (away from) the STM tip site. Effectively, ΔE is a measure of the severity of the STM tip's impact on surface kinetics. Additionally, they explore the effect of varying ΔE on the activation barrier for hopping of Sb2 dimers orthogonal to the Si dimer rows by setting its value to 0.1-0.4 eV. Experimentally determined to be 1.2 eV, the authors found that for very small ΔE (ΔE < 0.1 eV) the activation barrier could be underestimated by no more than 0.03 eV, and as much as 0.13 eV for large ΔE (ΔE = 0.4 eV). Next, they ask if this model could induce asymmetry in the hopping frequency under asymmetric scanning, that is, STM runs in the invasive mode when moving from right to left, and noninvasive mode in the opposite direction. The authors found that indeed there is a net movement of Sb2 dimers from right to left, or along the direction of the invasive scan. Moreover, they found that this directed motion becomes slightly more pronounced as the scanning speed decreases. These twin observations were explained by noting the asymmetry in the occupation probability of the sites immediately to the left and right of the STM tip - the site trailing the tip, or the site to its right, is more likely to be occupied compared than the one leading it. In this sense, according to their model, the STM tip gently drags the adsorbates it comes in contact with but falls short of precise, active manipulation even for large ΔE (ΔE = 0.4 eV).

Source or Periodical Title

Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films

ISSN

7342101

Document Type

Article

This document is currently not available here.

Share

COinS