Ultrasensitive detection of lateral atomic-scale interactions on graphite (0001) via bimodal dynamic force measurements

Abstract

Enhanced sensitivity to lateral forces on the nominally flat and inert ͑0001͒ surface of graphite is demonstrated via room-temperature dynamic force microscopy using simultaneous excitation and FM detection of the lowest flexural and torsional cantilever resonance modes. The site-independent long-range tip-sample interaction causes no significant lateral force variations except near atomic steps but unprecedented sensitivity to short-range forces is achieved on flat terraces in the attractive range. Two-dimensional bimodal force vs distance maps confirm the stronger distance dependence of the torsional frequency shift compared to the flexural resonance shift. This agrees with model calculations based on theoretical expressions. The lateral force gradient is extracted from the measured torsional shift, and a lateral force variation in at most Ϯ20 pN is obtained by integrating this gradient parallel to the surface. A further integration reveals a potential-energy variation in the attractive force range of only 3 meV.

Measuring Electric Field Induced Subpicometer Displacement of Step Edge Ions

“…1(b). This is due to the fact that the sensitivity of the Áf TR signal to the site-dependent interaction (i.e., the short-range interaction on the flat surface) is extreme, as shown in our previous works [39].…”

“…Furthermore, the higher mechanical quality factor, compared to that of the flexural mode, can improve the force sensitivity itself [38]. Such extreme sensitivity to site-dependent interaction [39] allows us to measure the potential variations near the step edge of a prototype insulating ionic crystal LiF and investigate its changes with respect to an applied bias voltage. Our multiscale theoretical calculation concludes that the step ions are displaced by the electrostatic field by subpicometers.…”

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

“…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.The origin of the moiré contrast in G=Ir was explored with experimental Δf vs distance curves on the atop (highest) and fcc (lowest) areas [28]. These curves have minima that differ by ∼20% and are displaced by ∼1 Å. Density-functional theory (DFT) calculations with a fiveatom W tip that do not include any atomic relaxations provide very similar interaction energy curves that are only shifted by 0.40 Å.…”

“…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.…”

“…Dynamic AFM [1] in the frequency modulation (FM) mode [2] has resolved the true geometric structure of a broad range of materials [3][4][5]. 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].…”