Atomic-scale metal adhesion investigated by scanning tunneling microscopy

The interaction of a sharply pointed metal tip with a metal surface is investigated both theoretically and experimentally. By resorting to an effective potential approach, the characteristics of tip-sample forces are analyzed systematically in terms of known theory of bulk metal adhesion. Experiments probing the short-range adhesion interaction by means of a combination of force gradient sensing with tunneling microscopy are described. It is found that the concepts based on adhesion at a macroscopic level are not generally applicable in describing the observed tip-sample force gradient characteristics. These characteristics can, however, be explained in a semiquantitative way using effective interactions determined from a cluster calculation. It is also shown that the chemical information obtained by probing short-range interactions can be used to identify adsorbates on metal surfaces.