Direct force measurement in scanning tunneling microscopy

A novel force measurement using a scanning tunneling microscope as a forced oscillator is described. Results obtained from tunneling between a tungsten tip and a graphite substrate show that a maximum tip-sample force about 10−6 N exists during the constant current mode of operation. These results are in agreement with a previous model where large contact areas insulated by contaminants between tip and substrate were suggested as a cause of large tip-sample interaction forces. This method can achieve a force sensitivity of 10−8 N and for conductive substrates provide a simple, versatile alternative to existing methods of atomic force microscopy.