Dynamic Force Microscopy and Spectroscopy in Vacuum

Abstract

Dynamic force microscopy (DFM) operated in ultrahigh vacuum, which is often also called noncontact atomic force microscopy (NC-AFM), is able to image the atomic structure of surfaces, including observation of point defects independent from the sample’s conductivity. Within the last ten years, a variety of materials, including conductors, semiconductors, and insulators, have been imaged down to the atomic scale. Atomic arrangements and individual defects such as vacancies or impurity atoms have been observed, and chemically different species have been distinguished due to their different energy dissipation properties in DFM. In these atomic-scale images, the contrast is mainly due to short-range interatomic bonding forces, repulsive atom—atom forces, or van der Waals forces. However, in many experiments, electrostatic forces play a significant role in contrast formation, and electrostatic charge distributions can be imaged with high resolution. For example, the charge distribution around charged monoatomic vacancies can be mapped, as well as the charge distribution around individual doping atoms in semiconductors. At higher doping levels, the charge clouds around individual doping atoms overlap, and DFM images the resulting modulation in the surface potential of the sample. Finally, using dynamic force spectroscopy, the relative contributions of long-ranged electrostatic and van der Waals forces as opposed to short-ranged chemical bonding forces can be distinguished and analyzed site-specifically.