Abstract
It has recently been shown that ‘sub-atomic’ contrast can be observed during NC-AFM imaging of the Si(111)-7×7 substrate with a passivated tip, resulting in triangular shaped atoms [Sweetman et al. Nano Lett. 2014, 14, 2265]. The symmetry of the features, and the well-established nature of the dangling bond structure of the silicon adatom means that in this instance the contrast cannot arise from the orbital structure of the atoms, and it was suggested by simple symmetry arguments that the contrast could only arise from the backbonding symmetry of the surface adatoms. However, no modelling of the system has been performed in order to understand the precise origin of the contrast. In this paper we provide a detailed explanation for ‘sub-atomic’ contrast observed on Si(111)-7×7 using a simple model based on Lennard-Jones potentials, coupled with a flexible tip, as proposed by Hapala et al. [Phys. Rev. B 2014, 90, 085421] in the context of interpreting sub-molecular contrast. Our results show a striking similarity to experimental results, and demonstrate how ‘sub-atomic’ contrast can arise from a flexible tip exploring an asymmetric potential created due to the positioning of the surrounding surface atoms.
Highlights
Recent developments in low temperature scanning probe instrumentation [1], coupled with specific experimental techniques utilising the in situ functionalisation of scanning probe tips with single molecules [2], and operation in the constant-height imaging mode, have resulted in an explosion of interest in highresolution imaging and force mapping of atomic and molecular structures using non-contact atomic force microscopy (NCAFM)
Pretation of sub-molecular resolution imaging has been the explicit consideration of deflection in the tip–sample junction [5,6,7], which can result in contrast enhancement [6], and unwanted distortions and potential artefacts [5,7,8,9,10]
We show that the triangular features observed experimentally arise naturally from the exploration of an asymmetric potential by a flexible tip and do not require consideration of the detailed electronic structure of the surface
Summary
Recent developments in low temperature scanning probe instrumentation [1], coupled with specific experimental techniques utilising the in situ functionalisation of scanning probe tips with single molecules [2], and operation in the constant-height imaging mode, have resulted in an explosion of interest in highresolution imaging and force mapping of atomic and molecular structures using non-contact atomic force microscopy (NCAFM). Suppressing the chemical bonding between tip and sample enables the stable exploration of the repulsive part of the tip–sample force regime, which has allowed outstanding resolution to be obtained during imaging of planar organic molecules [3,4]. Modelling using computationally inexpensive empirical potentials has produced a surprisingly good agreement with experimental data, and allows for simulated images to be computed with a comparable size and resolution to experiment, which is essential for meaningful qualitative comparisons. We show that the triangular features observed experimentally arise naturally from the exploration of an asymmetric potential by a flexible tip and do not require consideration of the detailed electronic structure of the surface. We highlight the limitations of the model when chemical interactions become important at close approach, and explore the qualitative variation in contrast observed between force and Δf images depending on oscillation amplitude
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