Two-Dimensional Nanoscale Self-Assembly on a Gold Surface by Spinodal Decomposition

The structure formation upon spinodal decomposition of a two-dimensional model system, a Au adatom gas on a Au(111) surface, was observed in situ by scanning tunneling microscopy (STM). A thermodynamically unstable state was prepared by applying microsecond voltage pulses to the STM tip in an electrochemical system, causing the random dissolution of Au atoms from the uppermost monolayer. Interconnected, labyrinthine island patterns were formed at Au coverages between 0.4 and 0.9 monolayer with dominating length scales ${\lambda }_{\mathrm{m}}$ of the order of a few nanometers.

Figure 1

(a) In situ STM image of the evolution of a labyrinthine, interconnected island pattern upon the application of a $0.7\mu \mathrm{s}$, 5.2 V pulse between the STM tip and the Au(111) surface. The position of the STM tip, where the pulse was applied, is indicated by an arrow. The slow scan direction was directed downwards. The island pattern significantly coarsened during the imaging at a potential of ${\varphi }_{\mathrm{A}\mathrm{g}/\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}}=0.4\mathrm{V}$. (b) Linewise density autocorrelation function of the STM image in (a), 1 s after the pulse.

Figure 2

(a) Solid line: dominant wavelength ${\lambda }_{\mathrm{m}}$, expected from Cahn and Hilliard theory for the spinodal decomposition of a 2D Au adatom gas. The circles summarize experimental results for the dominating wavelengths, following pulses with different duration and hence differing Au coverage. (b)–(d) Surface morphology following 5 V pulses of $2.8\mu \mathrm{s}$ (b), $2.1\mu \mathrm{s}$ (c), and $0.7\mu \mathrm{s}$ (d) duration. Au densities are $\theta =0.5$, $\theta =0.7$, and $\theta =0.8$. Irrespective of the surface coverage, the dominating wavelength of the structure amounts to about 4 nm.

Figure 3

Slow dissolution of Au (a) upon application of a 15 ms, 2.4 V pulse to the STM (tip position during the pulse marked by an arrow) and (b) upon three subsequent 20 ms potentiostatically controlled potential steps to ${\varphi }_{\mathrm{A}\mathrm{g}/\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}}=0.9\mathrm{V}$. Well separated holes with diverse diameters, indicative of nucleation and growth processes formed on the surface.