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Description
Frequency-modulated atomic force microscopy (fm-AFM) is well suited to probe forces between the tip and surface. Vertical forces can be inferred by sampling the frequency shifts while varying the tip-sample distance above a point on the sample. In our work, we explore a custom-synthesized molecule: tetrakis(iodomethyl)germane (Ge(CH2I)4; TIMe-Ge), which consistently presents a near-normal-facing CH2I group upon deposition on depassivated Si(100) surface. At 4K, the normal-facing CH2I group remains stationary in one of three energy-minimized rotamers. The group can be manipulated into each of the three rotamers by using the attractive force between the tip and the CH2I.
We employ the methodology used by Ternes, et al.[1] to measure the horizontal force required to switch between rotamers. In this work, we iteratively took frequency shift measurements along lines parallel to the sample surface at sequentially smaller tip sample separations. The line direction is off-center from the TIMe-Ge molecule so that the normal-facing CH2I group is manipulated into a new rotamer. The frequency shift data can then be used to determine the potential energy landscape at each point along this line. By taking a partial derivative along the direction of the horizontal line, horizontal forces can be inferred at each point, including the point at which the CH2I group rotates. By demonstrating the ability to measure this force for a single bond, this work highlights the utility of fm-AFM as a technique to investigate molecular rotors and other machines.
1. Ternes, M., Lutz, C. P., Hirjibehedin, C. F., Giessibl, F. J. & Heinrich, A. J. The Force Needed to Move an Atom on a Surface. Science 319, 1066–1069 (2008).
| Keyword-1 | fm-AFM |
|---|---|
| Keyword-2 | Scanning Tunneling Microscopy |
| Keyword-3 | Molecular Rotor |