First experimental observations

Theory [1] shows that the noise due to surface diffusion can be distinguished at the background of the fundamental thermal and shot noises. This conclusion is supported by experimental indications on oxygen diffusion on Ni manifested in STM current fluctuations [2], The first quantitative analysis of the STM current fluctuation spectra, along the lines discussed above, has been published in [8], where the ex-situ oxygen diffusion on stepped Si(l 11) was studied. The method used followed exactly the ideas discussed above: the fluctuations of the tunneling current were monitored with the tip stationary over the surface and the feedback loop suspended. The time-current correlation function was analyzed with a frequency spectrum analyzer with account taken for high- and low-frequency cut-offs imposed by electronics. The very high sensitivity to the presence of oxygen atoms (the results are compared to the noise spectra on the clean, oxygen-free Si) was found, which allowed the authors to attribute the fluctuation pattern to the current spikes induced by surface diffusion. The mode of fluctuations observed seemed to follow the Fourier transform of Eq.(13) with a definite logarithmic divergence at low frequencies (Eq.(15)), indicative of isotropic diffusion. The problem of exact evaluation of Q did not emerge here: after obtaining tc ~ D the authors focused not on the absolute values of D but on the evaluation of the activation energy of surface diffusion using the Arrhenius plots (logic versus 1/7).

Similar in-situ data in electrolytes have not been published so far, though preliminary results [9] on the diffusion events in the region of the so-called „frizzy" steps on Ag(lll) [1] have been reported. Signature of \/jco was found, indicative of fast anisotropic diffusion of silver surface atoms along the step edges.

A number of methods exist for the study of surface diffusion ex-situ [11-13]. The STM noise method, discussed above, has its own advantages (or shortcomings) as the local probe method. However, for the electrochemical interface it seems to be unique. (The impedance measurements [14] may not always be unambiguously interpreted, and nor do they give the „local" information about the surface.) We thus expect that the STM noise method will be widely used for in-situ study of surface diffusion, as long as the resolution of the high frequency noise measurements improves. Then the maps of local adatom diffusivity on metal electrodes will become a reality.

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