In search of nanoperfection: Experiment and Monte Carlo simulation of nucleation-controlled step doubling

Abstract

In order to make effective use of the extreme density of nanoscale elements that form spontaneously in self-assembling architectures, one must address the associated issue of minimizing defect creation during the formation of such structures. In this article we examine the competing roles that nucleation kinetics and two-dimensional growth processes play in nanostructure formation and defect minimization. We employ oxygen-induced step doubling of vicinal Ni(977) surfaces as our physical system, using elevated temperature scanning tunneling microscopy and Monte Carlo simulations to extract the desired details of interface evolution. Two interesting topological defect features are observed on the surface after doubling reaches its asymptotic limit: (i) “frustrated ends,” which form when two counter-propagating step-doubling events having a single step in common intersect, leaving a stable topological defect, and (ii) residual “isolated single steps,” which form when a single step is unable to partner with an adjacent step. This latter defect occurs when a single step is surrounded on both sides by previously doubled structures. In an attempt to understand and control these results, Monte Carlo simulations indicate that experimental control of the delicate and competing interplay of nucleation kinetics and two-dimensional growth kinetics is the key to the formation of more perfect interfaces. In this instance this corresponds to using a small initial oxygen exposure and reduced substrate temperature to achieve a doubled surface of higher perfection. Such optimized interfaces can act as templates for guiding the hierarchical assembly of nanowires and other nanoscale molecular assemblies.