Image formation and contrast inversion in noncontact atomic force microscopy imaging of oxidized Cu(110) surfaces

Abstract

Joint experimental and theoretical investigation of image formation in noncontact atomic force microscopy (NC-AFM) of the $c$(6 $\ifmmode\times\else\texttimes\fi{}$ 2) and $p$(2 $\ifmmode\times\else\texttimes\fi{}$ 1) phases of the Cu(110):O surface is presented. We proposed previously that the $c$(6 $\ifmmode\times\else\texttimes\fi{}$ 2) reconstruction of the Cu(110):O surface may serve as a reference system allowing chemical identification of the tip apex atom during the course of NC-AFM experiments. The identification is possible due to the fact that two most likely possible contrasts that could be observed after intermittent contacts of the tip with the surface were found which can be attributed to the tip being either terminated by Cu or O atoms. In this paper the idea of exploiting the Cu(110):O surface in NC-AFM studies is further developed. Specifically, (i) we show that there must be an image contrast inversion when the $c$(6 $\ifmmode\times\else\texttimes\fi{}$ 2) surface reconstruction is scanned depending on the tip-surface distance: at the usual imaging conditions, at tip-sample distances of 2--5 \AA{}, the previously reported contrast is observed; however, an opposite contrast is observed for larger separations with one of the two tip terminations. (ii) We study in detail also the image contrast formation of the $p(2\ifmmode\times\else\texttimes\fi{}1)$ surface, which is another common surface reconstruction, and show that tip identification for it is not possible. (iii) Finally, we discuss here possible effects of the actual tip atomic structure on the NC-AFM image. In particular, we show that the O-terminated tip will remain such even after picking up a Cu atom from the surface, experimentally a frequently observed process. Hence this type of the tip modification would only affect secondary features in the image.