Potential-Induced Strain Relaxation in Au Mono- and Bilayer Films on Pt(111) Electrode Surfaces

In situ scanning tunneling microscopy observations of 1–2 monolayer thick Au films on Pt(111) electrodes are presented, which show a complex, potential-dependent structural phase behavior of the Au film. Starting from a pseudomorphic structure at $<0.35{\mathrm{V}}_{\mathrm{A}\mathrm{g}\mathrm{/}\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}}$, a sequence of transitions into dislocation network structures occurs with increasing potential: $(1\times 1)\to$ “striped phase” $\to$ “hex phase” in the Au bilayer, $(1\times 1)\to$ striped phase in the Au monolayer. This is explained by a reduction of the in-plane stress in the Au surface layer(s) due to anion adsorption and a strain energy minimization as described by the Frenkel-Kontorova model.

Figure 1

Series of in situ STM images of a 1.3 ML Au deposit on Pt(111) in 0.1 M ${\mathrm{H}}_{\mathrm{2}}\mathrm{S}{\mathrm{O}}_{\mathrm{4}}$, recorded in the same surface area during a full potential cycle ($1800\AA \times 1800\AA$). (a) Initial surface at $+0.3\mathrm{V}$ ($t=0\mathrm{s}$), Au monolayer and bilayer islands are indicated by (i) and (ii), respectively. Subsequently, the potential was first stepwise increased to $+0.85\mathrm{V}$ and then back to $+0.2\mathrm{V}$. The images were recorded at (b) 0.45 V ($t=250\mathrm{s}$), (c) $+0.6\mathrm{V}$, ($t=565\mathrm{s}$), (d) $+0.7\mathrm{V}$, ($t=735\mathrm{s}$), (e) $+0.85\mathrm{V}$ ($t=1160\mathrm{s}$), (f) $+0.5\mathrm{V}$ ($t=1845\mathrm{s}$), (g) $+0.35\mathrm{V}$ ($t=2100\mathrm{s}$), and (h) $+0.30\mathrm{V}$ ($t=2400\mathrm{s}$). They show the emergence and disappearance of several dislocation network structures (dark lines) on the Au deposit (arrows, see text).

Figure 2

Quantitative analysis of the STM images as a function of potential during the potential sweep in positive (open symbols) and negative (solid symbols) directions. Circles, triangles, and squares denote monolayer ($1\times 1$) and “striped phase,” bilayer ($1\times 1$) and striped phase, and bilayer “hex phase,” respectively. Top diagram: Relative change of the area of second layer Au islands, showing an expansion of the Au lattice with increasing potential. Bottom diagram: Average local surface density of Au atoms in the mono- and bilayer, determined from a quantitative analysis of the line pattern (see text).