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Utilizing tuning forks [1-3] as sensors for noncontact atomic force microscopy (NC-AFM) allows extending the capabilities of the technique to cryogenic temperatures. In the presentation, the structure of chiral clusters of (+)-(R)-2-aza[6]helicene on Cu(111) surfaces is investigated at 5K with combined scanning tunneling microscopy (STM) and tuning fork-based NC-AFM. The studies are based on analysis of simultaneously obtained current, frequency shift and dissipation images of the molecules. That allows for investigation of both the cluster structure and the interplay between the different signals in combined STM/NC-AFM operation. We could identify that the clusters are formed by six molecules as mutual dipole-dipole interactions combined with hydrogen bonding contribute to their stabilization. The chirality of the clusters is forced by the chiral structure of (+)-(R)-2-aza[6]helicene. The results prove that the tuning fork-based NC-AFM is capable of high resolution imaging large adsorbates at low temperatures.
[1] F.J. Giessibl, Appl. Phys. Lett. 76, 1470 (2000). [2] M. Ternes et al., Science 319, 5866 (2008). [3] B.J. Albers et al., Nature Nanotechnology 4, 307 (2009). |
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