Imaging surfaces in topographic mode (with constant frequency shift) has been the dominant method in dynamic scanning force microscopy (SFM). In principle it produces the three-dimensional surface profile, and therefore appears as a natural method to image nanosized clusters on surfaces. When imaging clusters that are of the same size or smaller than the tip apex, however, tip convolution caused by finite dimensions of the tip ruins the accuracy of the images. Utilizing imaging simulations we found that using dynamic SFM in the constant height mode can give a much more accurate description of the size and shape of the top facet of the cluster, and this was later confirmed by experiment [1].
Earlier simulations show that the success of constant height mode is dependent on a short-range interaction between the tip and a cluster. Thus we start new simulations on Pd clusters on MgO surface, which combine van der Waals interaction with a realistic short-range interaction, parameterized with atomistic simulations. From the obtained tip-sample forces we construct simulated constant frequency shift and constant height images utilizing the experimental imaging parameters. Our results show that the model explains the contrast differences between imaging modes of SFM, and reproduces the experimental results. We expect the method to increase knowledge of nanoclusters important in catalysis.
[1] C. Barth, O. H. Pakarinen, A. S. Foster and C. R. Henry,
Nanotechnology 17, S128-S136 (2006). |