STM evidence for anisotropic diffusive decay of nanofeatures on AU (111) surfaces

Controlled STM tip crashes have been made on Au thin films having extensive atomically flat areas. The films were prepared on heated mica substrates by sputtering. Initially an Ag film 0.8 μm thick is deposited and then a 0.4 μm thick layer of Au is laid on top. The total area covered is of the order of 50 cm². XRD shows a dominant (111) line with the (200) line undetectable; this implies very good epitaxy though there may be stacking faults. Except at the edges of the film atomically flat regions were prevalent. Nanoscale features were written on the film by controlled crashing of a W tip mounted in a piezo tripod of an STM. The features were subsequently scanned using the same tip. Initially a written feature appears as an indent reflecting the macroscopic geometry of the tip. Over a period of some tens of minutes the feature is seen to change and the following are observed: (i) monatomic steps develop on the faces of the dent, (ii) the contour of the step initially follows the dent geometry, (iii) the contours slowly change shape and adopt an alignment along the high symmetry directions of the Au (111) surface, (iv) the step contours remain aligned but shorten as the feature fills in by surface diffusion. A coherent explanation may be achieved by postulating the removal of atoms from the crash site to neighboring parts of the surface followed by surface diffusion processes which are anisotropic on an atomic scale. Also, the local (inhomogeneous) rate of diffusion is enhanced by virtue of the increased density of mobile surface adatoms released in the crash.