Invited

Can dynamic force microscopy really "see" intermolecular bonds?

A.M. Sweetman¹, S.P. Jarvis², M.A. Rashid¹, J. Leaf¹, A. Allen¹, S. Taylor¹, J.L. Dunn¹, Hongqian Sang³, Lev Kantorovich⁴, and P. Moriarty¹

¹School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UK

²Physics Department and Materials Science Institute, Lancaster University, Lancaster, LA1 4YB, UK

³Institute for Interdisciplinary Research Jianghan University Wuhan 430056 China

⁴Department of Physics, King's College London, Strand, London WC2R 2L, UK

Since Gross et al.'s pioneering observation of the internal "architecture" of a pentacene molecule in 2009 [1], there has been a rapid growth in ultrahigh resolution imaging of single molecules using dynamic force microscopy [2]. Image interpretation is, however, always a challenge with any type of scanning probe technique as, very often, the precise role of the probe can be difficult to 'deconvolve' from the experimental data. I shall discuss a series of ultrahigh vacuum, low temperature force microscopy results (based around the qPlus sensor [3]) for a variety of molecules adsorbed on both metal and semiconductor surfaces: NTCDI [4,5] and biisonicotinic acid (which are both largely planar), and C₆₀ (which is, of course, very far from planar) [6].

Although the biisonicotinic acid data (see Fig. 1) were acquired with a CO-terminated tip, as per the standard approach in the dynamic force microscopy community, high resolution imaging of both NTCDI and C₆₀ involved imaging using a tip which, to the very best of our knowledge, was terminated by an NTCDI or C₆₀ molecule respectively. Intramolecular resolution was possible for tips that interacted both very strongly (~ 2 nN) and very weakly (~ 50 pN) with the Si(111)-(7×7) surface, demonstrating that an inert probe is not a prerequisite for intramolecular imaging. This has important implications for the continuing development of 'internal' imaging of adsorbed molecules on semiconductors.

Figure 1: (L-R) Experimental constant height frequency shift image of a monolayer of biisonicotinic acid on Au(111); geometry-optimized structure of the biisonicotinic assembly (determined using the CP2K DFT code [7]); constant height image simulated using the analytical model developed by Hapala et al. [8]

[1] L. Gross et al., Science 325, 1110-1114 (2009)

[2] N. Pavliček and L. Gross, Nature Rev. Chem. 1, 0005 (2017)

[3] F.J. Giessibl, Rev. Mod. Phys. 75 949 (2003)

[4] A. Sweetman et al., Nature Comm. 5, 3931 (2014)

[5] A. Sweetman et al., Phys Rev B 90, 165425 (2014)

[6] S.P. Jarvis et al., Phys Rev B 92, 241405(R) (2015)

[7] https://www.cp2k.org/

[8] P. Hapala et al., Phys Rev B 90, 085421 (2014)