Theoretical study of single-atom extraction using STM

Based on the discrete variational method with the local-density-functional approximation, we chose cluster models to simulate the extraction of a single Al atom from Al(111) sample surface by a scanning-tunneling microscopy W tip with and without external bias voltages. Our cluster calculations, which can deal with the detailed geometry of the sample and the tip (especially with an active site), can provide useful and relatively reliable results: (1) The “chemical interactions” between the sample and the tip play an important role in single-atom extraction processes; e.g., an Al atom can be extracted as the tip-sample separation becomes 10 a.u. (5.3 Å) without any external fields. (2) The polarity and the value of the external bias (near the threshold) are other important factors; e.g., at the tip-sample separation 12 a.u., an Al atom can be extracted with the threshold field (0.6 V/Å, positive bias to the sample), and the extracted Al atom can be put back with a negative bias to the sample. (3) The W atom on the tip cannot be extracted.