MEASUREMENT AND MANIPULATION OF VAN-DER-WAALS FORCES IN ATOMIC-FORCE MICROSCOPY

An understanding of the interaction between tip and sample in atomic-force microscopy is needed to interpret atomic-force-microscope (AFM) images. In contact mode, the strength of the van der Waals (vdW) force sets image resolution; in noncontact mode, local gradients in the vdW force are imaged. By immersing tip and sample in an appropriate fluid, we can decrease the vdW forces and even change their sign. Selecting a fluid that leads to a small repulsive vdW force can greatly improve image resolution and eliminates problems caused by the well-known tip-snapping instability. To measure the vdW interactions produced by different fluids, we have developed ways to calibrate the spring constant and sharpness of AFM tips and to measure accurately the Hamaker constant of vdW interactions. These techniques show that the AFM can be used for local force measurements with an accuracy approaching that of surface-force apparatuses. As an example, we have observed the crossover from nonretarded to retarded vdW forces between a silicon nitride tip and a mica sample in air.