Step dynamics on Au(110) studied with a high-temperature, high-speed scanning tunneling microscope

Abstract

The dynamics of monoatomic steps on the Au(l 10) surface was studied with a scanning tunneling microscope from room temperature to 590 K. The time dependence of the position fluctuations of steps was measured as a function of temperature and kink density. The mean-square displacement of the position was found to be proportional to the square root of time. The proportionality constant exhibits Arrhenius behavior and varies linearly with the kink density. The step dynamics is dominated by the diffusion of geometrical kinks that cannot pass each other.PACS numbers: 68.35.Fx, 05.40.+J, 61.16.Ch Steps play a major role in many surface phenomena. For instance, they can act as nucleation sites for the growth of new layers and can provide preferred adsorption and reaction sites. The dynamics of steps is of crucial importance for mass transport in growth and erosion phenomena, as well as for surface phase transitions such as surface roughening, deconstruction, and faceting. A scanning tunneling microscope (STM) provides the means to study step dynamics on the atomic scale [1-4]. The atomic mechanism underlying the thermal movements of steps is not yet fully understood. Up to now, the STM has been used to investigate the so-called "frizziness" of steps on metal surfaces, for instance on Cu(001) and Ag(l 11) [1][2][3]. Frizziness is the phenomenon that a step appears rough in the STM due to an undersampling of the step in time. The results on the frizzy steps have been interpreted in terms of the thermal creation of kink pairs. In this scenario a pair of kinks of opposite direction is formed when one or more atoms either depart from or attach to a previously straight section of the step.In this Letter, we present a direct observation and a temperature-dependent statistical analysis of step dynamics on Au(llO) performed with a high-speed, hightemperature STM. The individual snapshot observations show that the step fluctuates due to the diffusion of preexisting kinks along the step and that thermal kink generation plays no role of importance in the step dynamics at the investigated temperatures. The dependence of the mean-square displacement of the step on time and kink density indicates that the kinks move due to the exchange of atoms between kink sites and adatom sites on the adjacent terraces. From the temperature dependence we derive the activation energy for the movement of a single kink.The experiments were performed in ultrahigh vacuum (p < 1 x 10 ~1 0 mbar) with a STM specially designed for use at high temperatures. This instrument has been used to image various metal and semiconductor surfaces with atomic resolution up to 750 K. The STM tip was prepared by electrochemical etching of a 0.25 mm diame-ter W wire and annealing in vacuum. The tip was further prepared in situ by field electron emission and Ar ion sputtering. The Au sample was chemically etched and mechanically polished. It was cleaned in situ by cycles of Ar ion sputtering and annealing to 550 K. The cycles were optimized to produce a sharp (1x2) low-en...

Nanoparticle Structure by Coherent X-ray Diffraction

“…23,25) The general result is an approximately spherical shape, upon which the steps are thermodynamically fluctuating at finite temperature, intercepted by flat facets. 25,26) As the temperature is increased, the area of the facets diminishes progressively. Facets can disappear altogether at a critical temperature called the roughening transition.…”

Surface Self-Diffusion by Vacancy Motion: Island Ripening on Cu(001)

“…Thus the diffusion-limited decay of islands seen on Ag(111) [1] and Cu(111) [2] is straightforward to understand in terms of adatom motion. To date, systems where attachment-detachment-limited step kinetics have been observed can be explained by the presence of a complex surface unit cell [7,13], which requires the coordinated motion of many adatoms for a single incorporation event, or by the presence of adsorbates [14]. No such mitigating factors appear to exist for clean Cu(001).…”

Nothing Moves a Surface: Vacancy Mediated Surface Diffusion

“…Many STM studies have been devoted to the mobility of surfaces. Most of these have focused on the motion of steps [1][2][3], islands [4], or adsorbates [5]. Recently it has been proposed that surface vacancies are responsible for mass transport between adatom islands on Cu(001) [6].…”