Difference between staying and diffusing Si adsorbates on the Si(1 1 1)7 × 7 surface

We investigate adsorbed Si atoms on the Si(1 1 1)7 × 7 surface deposited from an scanning tunneling microscope (STM) tip. In our experiments at room temperature, two types of Si adsorbates on the surface are observed, “staying” Si adsorbate that remains at the same position, and “diffusing” Si one that is detected as a noise-like feature on an STM image. Molecular orbital calculations are performed to distinguish difference of these Si adsorbates. Stable positions, transition states, diffusion barriers and electron density iso-surfaces for extra Si atoms on Si cluster models are obtained. From these results, we propose new assignments to these Si adsorbates: the staying and diffusing Si adsorbates correspond to two Si atoms and a single Si atom, respectively.