Direct Force Measurement in Scanning Tunneling Microscopy.

A novel method for measuring tunneling forces with a scanning tunneling microscope is described. Results obtained from tunneling between a sharp tungsten tip and a highly oriented pyrolytic graphite substrate show that a maximum tip-sample force on the order of 10 sub (-6) N exists during the constant current mode of operation of the STM. These results are in agreement with a previous model where large contact area insulated by contact area insulated by contaminants between tip and substrate was suggested to give rise to large tip-sample interaction forces.