Speed determination of single Sr adatoms moving within Si(111)-7×7 half unit cells

Abstract

In this paper we report on the motion of individual Sr adatoms within the limits of Si(111)-7×7 half unit cells (HUCs). The fast movement of the atom at the Si surface produces sharp signal fluctuations in scanning tunneling microscopy (STM) images resulting into noiselike patches. It is found that the length of the observed image streaks is a function of the scanning tip velocity. A Monte Carlo simulation implementing a model of independently moving Sr adatom and scanning tip, accounts for the observed STM image features quantitatively. Thus, by colleting STM images at various scanning speeds and matching simulated image features to the experimental observations, allows the average speed of Sr adatom within the limits of Si(111)-7×7 HUC to be estimated: 300 nm/s.

Electronic effects in the formation of apparently noisy scanning tunneling microscopy images of Sr onSi(111)−7×7

“…This indeed agrees with the experimental observations. 3 The migration paths for Sr in unfaulted HUC are found to be the same as in faulted HUC, albeit energies in unfaulted HUC sites are about 0.1 eV higher than analogous energies in faulted HUCs. Thus, the Sr atom should preferentially occupy faulted HUCs.…”

“…We have shown recently that the length of such streaks depends on the scanning tip velocity and therefore it allows the speed of the captured adatom to be deduced. 3 According to a simplistic picture, one would interpret the higher density of image streaks ͑or higher brightness in STM images͒ as a higher relative residence time of the adsorbed atom. Since this atom should move along low-energy diffusion paths at the PES, the experimental STM images should resemble the bright shapes of the PES in Fig.…”

“…As shown previously, at least 0.3 nm widening of image features should be assumed. 3 Another possible issue is that an influence of the scanning tip on the movement of the Sr atom cannot be completely excluded. Still, these issues cannot satisfactorily explain the striking difference between PES and STM images.…”

“…Sr adsorption on Si͑111͒ surface has been recently investigated by scanning tunneling microscopy ͑STM͒. 2,3 These experiments revealed highly mobile Sr adatoms at room temperature ͑RT͒ trapped within the 7 ϫ 7 half-unit cell ͑HUC͒ boundaries. Such motion originates fuzzy or apparently noisy patches in the STM images that have been found earlier for other adsorbates such as Si, Pb, Tl, Sn, Ag, and Y.…”

Surface diffusion of Pb atoms on the Si(553)-Au surface in narrow quasi-one-dimensional channels

“…Usually, the diffusion of atoms on low-index surfaces is two dimensional and rather isotropic, like in the case of various adsorbates on the Si(111) surface [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. However, the movement of atoms can become strongly anisotropic or even quasi-one-dimensional in the presence of a specific surface reconstruction or on stepped (vicinal) templates [23][24][25][26][27][28].…”