Surface diffusion experiments with STM: equilibrium correlations and non-equilibrium low temperature growth

Abstract

Measurements of surface diffusion depend on the state of the system whether the state is equilibrium versus non-equilibrium. Equilibrium experiments carried out in 2-d overlayers measure the collective diffusion coefficient D(c) and can test theoretical predictions in two-dimensional statistical mechanics. Growth experiments typically carried out at low temperatures and/or high flux rates probe systems under non-equilibrium conditions where novel diffusion mechanisms can potentially exist. The use of STM to study both types of measurements is discussed. D(c) can be measured from the autocorrelation of time-dependent tunneling current fluctuations generated by atom motion in and out of the tunneling area. Controlled experiments as function of temperature, coverage and tip-surface separation confirm that the signal is diffusive. For growth experiments the unusually uniform height island (for Pb/Si(111) In/Si(111)) has revealed a novel and intriguing type of diffusion at low temperatures that accounts for the high degree of the self organization. By monitoring the evolution of the stable islands out of a mixture of stable and unstable islands the unusual role of the wetting layer surrounding the growing islands is revealed.

Characterization of epitaxially grown indium islands on Si(111)

“…We note that Tringides and Hupalo 33 have pointed out that there is an unusually high mobility of the In wetting layer, even for growth at temperatures lower than we investigate here. Further systematic investigation is required to fully understand the ramifications of this enhanced mobility for 3D island formation.…”

Microscopic Origin of the Apparent Activation Energy in Diffusion-Mediated Monolayer Growth of Two-Dimensional Materials

“…Previous work on surface diffusion has focused on such nucleation, growth and coalescence aspects, analyzing the time evolution of the monomer and island densities, characterizing the island size distributions, and describing the origin of the island capture numbers and capture zones. − Similarly, much effort has been invested on clarifying the differences and similarities between the tracer and collective diffusion coefficients, and describing them for various systems. − The tracer diffusivity, D T , which is the relevant transport coefficient when the particles can be followed, can be determined using three alternative formulations, e.g., eqs 1, 3, and 19 in ref . In this study, we present a fourth expression, describing D T in terms of the total hop rate, R h , defined as the sum of all distinct hop rates accessible to the adparticles, each multiplied by the corresponding hop multiplicity (the number of hops that share a given rate).…”

Determining molecule diffusion coefficients on surfaces from a locally fixed probe: Analysis of signal fluctuations

“…One approach was developed for the auto-correlation function (ACF) of an STM signal 16,17 and applied, on the basis of the corresponding power spectrum, to oxygen atom diffusion on Si(111), 15 and more recently to hydrogen atom diffusion on Si(111). 18 Another approach was followed in Ref. 19 by analyzing the distribution of peak widths in the signal, which correspond to residence times of single molecules under an STM tip.…”