Abstract
Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) images of graphene reveal either a triangular or honeycomb pattern at the atomic scale depending on the imaging parameters. The triangular patterns at the atomic scale are particularly difficult to interpret, as the maxima in the images could be every other carbon atom in the six-fold hexagonal array or even a hollow site. Carbon sites exhibit an inequivalent electronic structure in HOPG or multilayer graphene due to the presence of a carbon atom or a hollow site underneath. In this work, we report small-amplitude, simultaneous STM/AFM imaging using a metallic (tungsten) tip, of the graphene surface as-grown by chemical vapor deposition (CVD) on Cu foils. Truly simultaneous operation is possible only with the use of small oscillation amplitudes. Under a typical STM imaging regime the force interaction is found to be repulsive. Force–distance spectroscopy revealed a maximum attractive force of about 7 nN between the tip and carbon/hollow sites. We obtained different contrast between force and STM topography images for atomic features. A honeycomb pattern showing all six carbon atoms is revealed in AFM images. In one contrast type, simultaneously acquired STM topography revealed hollow sites to be brighter. In another, a triangular array with maxima located in between the two carbon atoms was acquired in STM topography.
Highlights
Graphene has been widely studied because of its potential use in future nanoelectronics, as it provides unprecedented mobility of charge carriers at room temperature [1]
scanning tunneling microscopy (STM) topography in Figure 1a was obtained in the forward scan in constant-current mode and the other two (Figure 1b,c) are the forward and backward scans of the force, respectively
The same area scanned with the same parameters except for the opposite sample bias of 500 mV resulted in an inversion of STM contrast, showing the honeycomb structure where C atoms are higher in contrast than the hollow sites
Summary
Graphene has been widely studied because of its potential use in future nanoelectronics, as it provides unprecedented mobility of charge carriers at room temperature [1]. Very high conductivity at room temperature and a half-integer quantum Hall effect suggest the presence of relativistic charge carriers with vanishing mass [2]. Graphene has been investigated by using scanning tunneling microscopy (STM) and atomic force microscopy (AFM) by various groups [3]. The interaction of graphene with its substrate affects the STM measurements and that casts doubts on its electronic structure. Having the possibility to make simultaneous STM and AFM measurements, on the same area would be useful in understanding the mechanisms of such interactions. A variety of methods are used to prepare graphene.
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