Simultaneous force and conduction measurements in atomic force microscopy

We used an ultrahigh-vacuum atomic force microscope (AFM) to measure lateral forces and conductivity simultaneously as a function of the applied normal force for nanometer-sized elastic contacts. Metal-coated or bare Si AFM tips are used on cleaved NbSe2 or graphite surfaces. Results are used to compare Various means of obtaining the tip-sample contact area (A(0)). We find that simple continuum models can give a reasonable description of the mechanical behavior of the contact. Specifically, the Maugis-Dugdale model [D. Maugis, J. Colloid Interface Sci. 150, 7-53 (1992)] provides a good basis for describing the elastic contact between an AFM tip and a smooth sample. The theoretical variation in contact radius with load is in good agreement with the experimental variation in friction ford, conductivity, and lateral tip-sample contact stiffness. To find the contact area Ao, the best approaches appear to be either to fit the applied force data to an appropriate continuum model (provided the contact is elastic) or to measure the lateral tip-sample contact stiffness. In principle we show that conduction AFM methods can be used to find Ao for Ohmic contacts. However, the uncertainty in the conduction properties of available AFM tips means that at present the absolute value of A(0) cannot be found with confidence. In this regard the use of metal-coated tips can often be misleading for conduction and mechanical measurements because metal wears rapidly off all or some of the tip apex.