Imaging problems on insulators: What can be learnt from NC-AFM modelling on CaF2?

By combining theoretical modelling with experimental data on CaF2, we studied the interactions responsible for atomic resolution in this system; we discuss the general significance of these results for imaging other insulators. Theoretical modelling was used to calculate the tip–surface interactions in noncontact atomic force microscopy (NC-AFM) imaging of a charged and neutral CaF2 (111) surface. The modelling predicts that both the Ca and F sublattices can be imaged depending on the nature of potential from the tip. However, the theoretical scanlines of the surface are characteristic for each sublattice, and a method for determining the sublattice imaged in future experiments is suggested. It was found that atomic resolution was independent of the nature of the background force, and imaging problems with other insulators are likely to be due to surface roughness. PACS: 68.37.Ps; 68.35.Dv; 61.50.Ah; 61.72.Bb Difficulties in noncontact atomic force microscopy (NCAFM) imaging have so far prevented atomic resolution from being achieved on surfaces of many insulators, most notably MgO and alumina. These difficulties are usually associated with blunt tips, surface charging or surface roughness after cleavage. However, the mechanism behind the imaging instabilities is not well understood, and it is not clear how these factors interfere with stable AFM operation. Recent experiments [1] have successfully demonstrated atomic resolution on the (111) surface of CaF2, and, as a wide gap insulator, bulk CaF2 represents a good example for comparison with other, as yet, unimaged insulating surfaces. The normalized frequency change [2] of 38.6 for those experiments also implies that a large macroscopic van der Waals force due to a blunt tip or significant electrostatic forces due to tip and surface charging are present. Both explanations are feasible, since AFM studies [3] of CaF2 have already demonstrated that charging due to cleavage could be significant and tip ∗Corresponding author. (E-mail: [email protected]) NC-AFM 2000 – Third International Conference on Non-Contact Atomic Force Microscopy, July 16–19, 2000 in Hamburg, Germany charging and bluntness due to sputtering is a common preparation problem [4]. By studying theoretically the effects of a blunt tip and charging on NC-AFM imaging of the CaF2 (111) surface, some general conclusions about the importance of these effects in imaging of other insulators can be made. Several other factors also make CaF2 an attractive system to study theoretically. Structurally, the CaF2 (111) surface offers more interesting physical features than other flat insulators, such as NaCl. The surface has been demonstrated to be fluorine terminated [1], and, as can be seen from Fig. 1, the outermost fluorine layer protrudes out from surface in a similar manner to the bridging oxygens seen on the TiO2 surface or As atoms in the InAs surface. A common previous assumption in NC-AFM imaging has been that protruding atomic layers are imaged as bright, and it is important to establish whether this “intuition” holds theoretically. The experimental study itself demonstrated a general problem in NC-AFM imaging, in that the defect and the sublattice seen as bright in images were unable to be chemically identified. It also suffered from a technical problem which exaggerated the apparent contrast measured on the surface. This combination of instrumental problems and lack of information obtained from experimental images motivates the use of theoretical modelling to try and extract more information about the surface and tip–surface interaction and to aid in interpreting experimental images. Fig. 1. Fluorine-terminated CaF2 (111) surface. 1: calcium layer; 2: outermost fluorine layer; 3: lower fluorine layer. The layers are separated by