ARTIFACTS IN NONCONTACT MODE FORCE MICROSCOPY - THE ROLE OF ADSORBED MOISTURE

Ordered layers of polystyrene spheres have been deposited on mica subs trates. Analysis in air by atomic force microscopy, in the non-contact mode, has revealed image artefacts at vacancy sites and at edges of l ayers. These artefacts cannot be ascribed to the effects of simple con volution of the tip shape with the geometry of the surface features. I nstead it is proposed that the origin must be with the interaction. De tailed first-principles calculation of synthetic images, on the assump tion of a pure dispersion interaction arising from pairwise summation of R(-6) terms, is found to reproduce the expected contours of constan t average force gradient, but does not explain the artefacts. Addition al results from measurements of force versus distance show that menisc us forces, due to adsorbed moisture, have the appropriate attributes t o explain the artefacts - e.g., strength, localisation and hysteresis. The meniscus force has been taken into account; the results from an a pproximate model give contours of force gradient which are in qualitat ive agreement with those at artefact locations. It is concluded that t he better-than-expected spatial resolution obtained in the non-contact mode can be due to the effects of the meniscus force.