THEORY OF ATOM-TRANSFER BETWEEN AN ALUMINUM AND A TUNGSTEN ELECTRODE

The interaction energy for an Al atom between a W(110) and an Al(111) electrode is calculated within a model which includes electron correlation and multiple image effects. The results show that the Al atom will be spontaneously transferred from the Al surface to the W surface, if the W(110) surface is brought close enough to the Al(111) electrode. This suggests that transfer of Al atoms to the tip in scanning tunneling microscopy (STM) will occur, if an Al(111) surface is scanned with a tungsten tip, The calculated force gradients for the Al/W(110)-tip above the Al(111) sample surface resemble closely the experimental results of During and Zuger [Vacuum 41 (1990) 382] for an Ir tip on an Al film, indicating that material transport from the sample to the tip has occurred in these experiments. The force between tip and sample reaches its attractive minimum at 8 Angstrom tip-sample separation when the slightly positively charged Al tip atom is neutralized due to the proximity of the sample electrode and the image interaction ceases to be of dominant importance.