The height dependence of image contrast when imaging by non-contact AFM

Using the pair-wise summation of the Lennard-Jones potential, we have analyzed the height dependence of the image contrast in atomic force microscopy at atomic resolution when scanning in non-contact mode. When we study the images obtained as the surfaces of constant force gradient (fast feedback on frequency shift), we find that images remain qualitatively similar for the range of tip-sample distances analyzed. However, for a slow feedback mode, which is more commonly used because of mechanical instabilities that exist in the fast feedback regime, we have found a spectrum of artefacts that are dependent on the tip-sample distance. (C) 2000 Elsevier Science B.V. All rights reserved.