Mechanical Vertical Manipulation of Selected Single Atoms by Soft Nanoindentation Using Near Contact Atomic Force Microscopy

A near contact atomic force microscope operated at low-temperature is used for vertical manipulation of selected single atoms from the $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$ surface. The strong repulsive short-range chemical force interaction between the closest atoms of both tip apex and surface during a soft nanoindentation leads to the removal of a selected silicon atom from its equilibrium position at the surface without additional perturbation of the $(7\times 7)$ unit cell. Deposition of a single atom on a created vacancy at the surface is achieved as well. These manipulation processes are purely mechanical, since neither bias voltage nor voltage pulse is applied between probe and sample. Differences in the mechanical response of the two nonequivalent adatoms of the $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$ with the load applied is also detected.

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Figure 1

Sequence of topographic images showing two mechanical single atom vertical manipulation processes performed successively over the selected atomic positions of the $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$ surface marked with a circle in (a) and (b), respectively, by using NC-AFM. As a result, two vacancies were created at the selected atomic positions (c). Image size is $66\times 66{\AA }^2$. Cantilever oscillation amplitude was $665\mathrm{\AA }$ and frequency shift reference value was $-1\mathrm{H}\mathrm{z}$ for all the images.

Figure 2

Sequence of topographic images showing the deposition of a single atom (c) as a result of applying a soft nanoindentation process over a vacancy (b) previously created by removing the central adatom marked with a circle in (a). Line profiles taken along the diagonal of the $(7\times 7)$ unit cell performed at the same position over each image are also shown. The line profiles are offset for clarity. Image size is $47\times 47{\AA }^2$. Cantilever oscillation amplitude and frequency shift reference values were $257\mathrm{\AA }$ and $-4\mathrm{H}\mathrm{z}$, respectively, for all the images.

Figure 3

Plot showing the evolution of the oscillation amplitude of the cantilever (open symbols) and the variation, respect to the initial value, of the normalized frequency shift $\gamma$ (solid symbols) when approaching the sample towards the tip during the removal process of a Si corner adatom (squares) and a Si central adatom (circles and spheres) of the $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$ reconstruction due to soft nanoindentation processes. Initial normalized frequency shift value for both curves was $-4.6\mathrm{f}\mathrm{N}\sqrt{m}$.