Electric-field control of a single-atom polar bond

by M. Omidian, S. Leitherer, N. Néel, M. Brandbyge, J. Kröger, Phys. Rev. Lett. (akzeptiert, April 2021)
Department of Experimental Physics I / Surface Physics

"What I want to talk about is the problem of manipulating and controlling things on a small scale ... What would the properties of materials be if we could really arrange the atoms the way we want them? … Atoms on a small scale behave like nothing on a large scale, for they satisfy the laws of quantum mechanics …
Now, you might say, ``Who should do this and why should they do it?´´ … but I know that the reason that you would do it might be just for fun. But have some fun! Let's have a competition between laboratories. Let one laboratory make a tiny motor which it sends to another lab which sends it back with a thing that fits inside the shaft of the first motor … " — These are the thoughts of famous physicist and Nobel laureate Richard Feynman, exposed at the APS meeting in Pasadena in 1959. They represent the driving motivation of research activities in the group of Experimental Physics I. One example of these activities has recently been published in the prime journal for physics research – Physical Review Letters – and shall be outlined here.

Controlling processes at the atomic and molecular scale by means of the macroscopic world is at the heart of  nanotechnology. The combination of atomic force microscopy (AFM) experiments at the Technical University of Ilmenau and non-equilibrium transport calculations at the Technical University of Denmark has achieved an important step toward that  goal.

In brief, a golden AFM tip was used to approach and contact a graphene sheet epitaxially grown on a SiC surface (figure 1). The chemical bond between the single Au atom terminating the tip apex and a C atom of the graphene lattice was altered by the polarity of the voltage applied across the junction. A positive voltage orients the electric field from the surface to the tip and yields a strong chemical bond. The strength is sufficient to anchor the graphene sheet robustly to the tip and partly delaminate it from the surface upon tip retraction. In contrast, a negative voltage that orients the electric field in the junction cavity from the tip to the surface gives rise to a weak chemical bond. It is unable to sustain the mechanical load imposed by the graphene sheet, which, thus, is easily released.

Figure 1: (A) Sketch of the experimental setup comprising a golden AFM tip oscillating at its resonance frequency of ~ 30 kHz with a quality factor of ~ 50000 and an amplitude of 50 pm together with the graphene-covered SiC(0001) surface. (B) AFM topograph of the graphene honeycomb mesh with indicated C atom positions. Position von C-Atomen.

Experimentally, the strong Au-C bond is unveiled by a characteristic loop behaviour of the resonance frequency changes (Δf) of the oscillating AFM probe as a function of the tip-surface distance (figure 2A). Approaching the tip leads to a variation Δf (solid line in figure 2A) that is typical for a Lennard-Jones-type tip-surface interaction. At point A, retraction of the tip is initiated. The Δf evolution for tip retraction (dashed line) clearly deviates from the approach data, which signals the gradual delamination of the graphene sheet that is tightly attached to the tip. At point R, graphene is released to the surface and retraction and approach Δf data coincide for distances beyond that point. The Δf variation for reversed voltage polarity is markedly different in that approach and retraction data virtually coincide, i.e., the Au-C bond is too weak for graphene detachment. The width δ of the Δf loop can be tailored by the applied voltage U (figure 2B), which shows that the single-atom bond can be tuned to lift the graphene sheet by more than 600 pm from the surface.

Figure 2: (A) Changes in the resonance frequency Δf of the force probe with tip displacement z toward (from left to right) and away from (from right to left) the surface. For positive voltage a pronounced loop behaviour of Δf is observed. The loop spans from point A where tip retraction with anchored graphene starts to point R where the graphene sheet is released. (B) Loop width δ as a function of the voltage U.

The physical mechanism underlying the experimentally observed effect was unraveled by experts in the field of non-equilibrium Green’s function methods and density functional theory. The electric field in the tipsurface cavity is able to move charge in the polar Au-C bond (figure 3A) and thereby control its strength. An electric field pointing toward the tip (figure 3B) depletes electron charge at the Au atom and accumulates electron charge at the C atom leading to a strengthened bond, whilst the opposite field direction (figure 3C) weakens the bond.

Figure 3: Simulations quantify the electron transfer from the Au to the C atom as a function of the bias voltage U (A) and show the field-dependent accumulation and depletion of electron densities at the atomic reaction partners (B), (C). The vertical arrow marks the orientation of the electric field.

What is the impact of the presented work? Most importantly, the combination of prototypical contact geometries and state-of-the-art simulations can unveil novel phenomena that extend or even change our understanding of physics at the atomic scale. From the nanotechnological point of view the findings show that a user in the macroscopic world can tune a chemical bond at the single-atom level with an appropriate voltage. The field of applications is wide. For instance, in future experiments flakes of two-dimensional (2D) materials will be transported to and deposited onto other 2D materials in the delineated manner. Making use of the weak van der Waals coupling between the stacked materials and the concomitant facile control of translational and rotational degrees of freedom, the twist angle of the stacking will finely be tuned by the described method. Stackings of 2D materials offer a panoply of fascinating condensed-matter phases that are determined by the twist angle. We will move on in the quantum design of matter and try to best realize the ideas envisioned by Richard Feynman in 1959.