Time-Dependent Tunneling Spectroscopy for Studying Surface Diffusion Confined in Nanostructures

Abstract

By confining a diffusion atom in a nanometer region using surface potential heterogeneity, we have successfully employed a time-dependent tunneling spectroscopy to quantitatively study its random motion. A hopping rate in the range of 1-10 4 Hz, 3 orders of magnitude faster than those accessible by the existing diffusion methods based on scanning tunneling microscopy, was demonstrated for single Cu atoms diffusing in the faulted half unit cell of Si111-7 7. Our technique is potentially useful to detect fast diffusion processes such as H quantum diffusion at atomic scale. DOI: 10.1103/PhysRevLett.94.036103 PACS numbers: 68.35.Fx, 68.35.Md, 68.37.Ef, 68.47.Fg Surface diffusion is an important subject of study in physics, chemistry, biology, and materials science. It plays a vital role in chemical and surface catalytic reactions, selfassembly, crystal growth, and thin film epitaxy [1][2][3][4][5]. Observation and understanding of surface diffusion phenomenon in the past largely depend on the development of techniques [1,4]. Among the techniques, scanning tunneling microscopy (STM) [6] is advantageous in directly providing the atomistic mechanisms of diffusion [3][4][5][7][8][9][10] but is severely limited to slow diffusion rates from 10 ÿ19 to 10 ÿ14 cm 2 =s, as compared to the wide range from 10 ÿ19 to 10 ÿ5 cm 2 =s covered by the collection of different techniques [4]. Thus, processes in quantum diffusion of light atoms and in many other systems of practical interest with fast diffusion rates remain inaccessible by the STM technique.Limited by the electronic feedback loop response, the diffusion rate that can be probed by the widely used frameby-frame imaging [11] and atom-tracking [12] methods is in the slow regime. In atom tracking for which no full frame imaging is required, the fastest measurable hopping rate is about 10 Hz. With a state-of-art design [7,9,10], STM imaging can now be performed at a rate of 10 frame=s, again leading to observation of a maximum hopping rate of only 10 Hz (equivalent to 10 ÿ14 cm 2 =s for nearest neighbor site hopping). Using a new concept of measuring diffusion by the bypass of the feedback loop, we demonstrate in this Letter a new STM-based method that can expand the range by at least 3 orders of magnitude faster, to a hopping rate on the order of 10 4 Hz (equivalent to 10 ÿ11 cm 2 =s). Further expansion of our method will become possible if faster preamplifier and data acquisition systems are used. This leads to significant overlaps in measurable diffusion rates with other techniques while retaining the atomic resolution. The extension will not only enable STM study of diffusion in the fast regime to facilitate direct comparison with results obtained by other techniques, but also enable the study of processes such as H quantum diffusion for which cooling never slows it down [1]. As shown by Gomer and his co-workers with field emission microscopy, the H quantum diffusion measured for a large number of systems [13][14][15][16] falls in the range of 10 ÿ13 to 10 ÿ10 cm 2 =s...

Initial stages of the adsorption of Ge atoms on theSi(111)−(7×7)surface

“…We observed many fuzzy triangular patterns in both FHUC and UHUC of the surface. These noisy patterns are similar to the cases of single Pb atoms adsorbed on Si͑111͒-͑7 ϫ 7͒ surfaces at room temperature 21 and single noble metal atoms 22,23 and Sn atoms 24 adsorbed on the same surfaces at lower temperature. They are likely to originate from a single Ge atom hopping frequently from site to site within the half-unit cell, similar to the activity of noble metal atoms.…”

“…The details of this technique have been described in a previous work. 23 The inset of Fig. 3 shows a typical time-dependent tunneling current trace recorded at a bias of +2.0 V above a corner Si adatom site with feedback loop turned off.…”

Mechanical gate control for atom-by-atom cluster assembly with scanning probe microscopy

“…Three HUCs with Au atoms appear brighter than the bare HUCs. This is attributable to the topographic as well as the electronic structure modifications induced by Au atoms that are diffusing much faster than the scanning speed of the SPM tip, similar to what was observed in a previous study 21 . Atom gating performed over the position marked by the cross in Fig.…”

Dynamics of copper atoms on Si(111)-7×7 surfaces

“…Among noble-metals, gold atoms have been found to alloy with silicon atoms on 7 · 7 surfaces to form several reconstructions, including 5 · 2, 6 · 6, a À p 3 · p 3 and b À p 3 · p 3 reconstructions, depending on the gold coverage and the temperature of substrate [7][8][9]. Silver atoms prefer to exist as single atoms on 7 · 7 surfaces and evaporate easily at 450 K [10,11]. In earlier works, copper atoms were reacted with silicon (1 1 1)-7 · 7 surfaces in an annealing process to form a 'quasi-5 · 5' incommensurate structure using numerous surface analytical techniques [12][13][14][15][16].…”