Pseudo-wetting behaviour of nanostructures induced by STM

The behavior of nanoscale liquid metal droplets has recently received renewed research interest following some exciting new observations by Sutter and Sutter [1] under high vacuum conditions. In the present paper, we consider whether similar conditions can be generated to observe liquid metal droplet phenomena in a scanning probe architecture, and moreover, to examine what the consequences of such a phenomenon might be. Strong interactions between tip and sample in tunnelling microscopies can be deliberately invoked by lowering the tunnel gap impedance. Indeed, nanoscale features may be created this way, usually exhibiting a temporal stability suggesting that the approach could be exploited for ultra high density data storage. Occasionally however, unstable features may form, and their decay characteristics can be related to local dynamics and kinetics. In real liquids, one such evolutionary mode involves the phenomenon of wetting, and the formation of thin precursor films. Here, it is demonstrated that a similar process may occur for the decay of a nanoscale mound of [presumed] Au atoms onto an Au(l 11) substrate. The mound is thought to be created by a 'jump-to-contact' process when the gap impedance, Z_g, is deliberately lowered by reducing the tip-surface displacement. Resultant features have a diameter of circa 30nm, and heights of up to lOnm. They appear stable when scanned repeatedly at gap impedances higher than 10MΩ, however if Z_g is lowered below 500kΩ, then morphology can alter dynamically, and a thin layer of material, only two atomic widths thick, is seen to emanate from the periphery. Relaxation in the nanostructure is observed. Interestingly, the observations agree qualitatively with classical wetting phenomena observed on microscale droplets of involatile liquids on solid surfaces. Favourable comparisons may also be drawn in the light of recent molecular-dynamics simulations, and also Monte-Carlo methods.