Forces and Currents in Carbon Nanostructures: Are We Imaging Atoms?

Abstract

First-principles calculations show that the rich variety of image patterns found in carbon nanostructures with the atomic force and scanning tunneling microscopes can be rationalized in terms of the chemical reactivity of the tip and the distance range explored in the experiments. For weakly reactive tips, the Pauli repulsion dominates the atomic contrast and force maxima are expected on low electronic density positions as the hollow site. With reactive tips, the interaction is strong enough to change locally the hybridization of the carbon atoms, making it possible to observe atomic resolution in both the attractive and the repulsive regime although with inverted contrast. Regarding STM images, we show that in the near-contact regime, due to current saturation, bright spots correspond to hollow positions instead of atomic sites, providing an explanation for the most common hexagonal pattern found in the experiments. Fullerenes, nanotubes, graphene, and carbon nanoribbons are among the most promising materials for nanotechnological applications due to their unique mechanical and electronic properties [1,2]. Turning these expectations into real-world devices requires tools like the STM and the atomic force microscope (AFM) with true atomic resolution (FM-AFM) [3] to visualize, characterize, and manipulate these materials at the atomic scale. However, in spite of the apparent simplicity of the common honeycomb structure and the long-standing experimental research effort, we do not know yet something as fundamental as whether the maxima in the scanning probe microscopy images correspond to atoms or to the hollow sites [4][5][6][7]. Here, we use first-principles calculations to show that the rich variety of AFM and STM image patterns found in carbon nanostructures can be rationalized in terms of the chemical reactivity of the tip and the distance range explored in the experiments.The first STM images of the graphite(0001) surface did not display the expected honeycomb pattern but a hexagonal arrangement of bright spots (topographic maxima; see Fig. 1 in [8]) [9][10][11]. The accepted interpretation relies on a subtle electronic effect [12]. The Bernal stacking makes the two surface atoms inequivalent. C atoms, with a nearest neighbor right below in the second layer, do not contribute significantly to the density of states close to the Fermi level, and only the C atoms are imaged as bright spots at low bias voltages. Although a honeycomb pattern should be recovered for larger biases, the experimental evidence accumulated over the past 25 years [9][10][11][12][13] shows that STM images with a hexagonal pattern are overwhelmingly recorded over a broad range of bias voltages and distance operation conditions. Nevertheless, several groups have reported honeycomb patterns even for small bias voltage [13][14][15][16][17].The FM-AFM, relying on the forces between the tip and surface, is expected not to be so sensitive to electronic effects and to reflect the real atomic structure of the surface. However, the first reported ...

Atomic-Scale Variations of the Mechanical Response of 2D Materials Detected by Noncontact Atomic Force Microscopy

“…Measurements with a tuning fork using both inert (CO-terminated) and reactive (Ir-terminated) tips [12] were able to identify the atoms with both tips at any tip-sample distance. This atomic-scale resolution allowed the observation of an inversion from attractive to repulsive atomic contrast with decreasing tip-sample distance predicted theoretically for reactive tips [14]. Except for graphite [9,10], atomic resolution in weakly coupled G-based materials using cantilever AFM with large oscillation amplitudes has not been reported.…”

“…FM AFM experiments on carbon-based materials [6][7][8][9][10][11][12][13] show atomic contrast in Δf images and, depending on the setup, in the dissipation channel. While the origin of the dissipation is not well understood, the Δf contrast has been linked with the nature of the tip-sample interaction [14].G properties can be efficiently tuned by the interaction with metals [15,16]. The interaction strength varies widely from the strong coupling with Rh [17,18] and Ru [19] to the weak limit (Ir [20], Pt [21]), where G retains its unique electronic properties [22].…”

“…FM AFM experiments on carbon-based materials [6][7][8][9][10][11][12][13] show atomic contrast in Δf images and, depending on the setup, in the dissipation channel. While the origin of the dissipation is not well understood, the Δf contrast has been linked with the nature of the tip-sample interaction [14].…”

Understanding image contrast formation in TiO2with force spectroscopy

“…The same trends described here have also been reported in previous theoretical works. 25 The short-range van der Waals force calculated here (Fig. 9) and the macroscopic van der Waals component obtained from a fit of the experimental forces in Fig.…”

“…FS has been used to discriminate between the two ionic sublattices on several insulator surfaces, [18][19][20][21][22] to achieve single-atom chemical identification on semiconductors, 23 and to understand the nc-AFM contrast on carbon nanostructures. 24,25 In this work we combine site-specific force measurements and extensive first-principles calculations on TiO 2 (110)-1 × 1, aiming to clarify the origin of the observed nc-AFM contrast and to characterize the tip structures responsible for the protrusion and hole imaging modes. While many tip models could be compatible with forces and contrast on typical imaging distances, our data close to the force minima is only consistent with a tip apex contaminated with clusters of surface material.…”