Artefacts in non-contact mode force microscopy: the role of adsorbed moisture

Ordered layers of polystyrene spheres have been deposited on mica substrates. Analysis in air by atomic force microscopy, in the non-contact mode, has revealed image artefacts at vacancy sites and at edges of layers. These artefacts cannot be ascribed to the effects of simple convolution of the tip shape with the geometry of the surface features. Instead it is proposed that the origin must be with the interaction. Detailed first-principles calculation of synthetic images, on the assumption of a pure dispersion interaction arising from pairwise summation of R−6 terms, is found to reproduce the expected contours of constant average force gradient, but does not explain the artefacts. Additional results from measurements of force versus distance show that meniscus forces, due to adsorbed moisture, have the appropriate attributes to explain the artefacts — e.g., strength, localisation and hysteresis. The meniscus force has been taken into account; the results from an approximate model give contours of force gradient which are in qualitative agreement with those at artefact locations. It is concluded that the better-than-expected spatial resolution obtained in the non-contact mode can be due to the effects of the meniscus force.