Transition from the tunneling regime to point contact studied using scanning tunneling microscopy

Using scanning tunneling microscopy, we have studied local surface modifications induced by point contact of a tunnel tip with metallic surfaces. Two distinct types of topographical modification are found which correlate directly with the chemical condition of the tip as determined by tunneling spectroscopy. Observation of the dynamics of the transition from the tunneling regime to contact permits an evaluation of the gap distance prior to point contact. At gap spacings of s≲3 Å a significant decrease in the apparent tunnel barrier height is observed just before touching. Contact was found to initiate with the intimate contact of several atoms only. The implications of these results for lithography on a nanometer scale, and on the investigation of the mechanical properties of surfaces over ranges of a few nanometers are discussed.