Atomic force microscope resolves the polarity of chemical bonds in the molecules

Researchers from the Institute of Physics and the University of Regensburg (Germany) introduced a new method of atomic force microscopy (Atomic Force Microscopy = AFM), which allows to resolve the polarity of individual chemical bonds in a single molecule. The possibility of the detailed resolution of the charge distribution in the chemical bonds within a molecule significantly advances our current possibilities to study the charge transfer at the atomic and molecular level. For example, the detailed knowledge of the charge distribution on molecular level could help to design solar cells with better functionality.

So far, the charge distribution was mostly studied by so-called Kelvin probe force microscope (Kelvin Probe Force Microscopy = KPFM), which detects change in the local work function (i.e. the energy that is needed for removal of just one electron from the surface of a solid) at the atomic level. However, for achieving the sub-molecular resolution of the charge distribution it is necessary to bring the probe close to the investigated surface. Unfortunately, the proximity of the scanning probe near the surface induces undesired artifacts in KPFM measurements. These artifacts then prevent precise determination of the charge distribution within a single molecule.

To solve this problem scientists have proposed a new measurement setup that allows to remove the aforementioned artifacts. In an article published in the journal Physical Review Letters, scientists have described a new method of charge distribution detection, which is based on the use of the Kelvin probe force spectroscopy (Kelvin probe force spectroscopy = KPFS) for two different voltages applied to the scanning probe. The new KPFS method overcomes the shortcomings of the original KPFM method and it allows mapping the distribution of charge in the individual chemical bonds within the same molecule. Scientists have demonstrated the potential of this new method by the detection of the bond polarization on two very similar molecules. Namely, they investigated trimeric perfluoroortho-phenylenemercury (F₁₂C₁₈Hg₃) and its derivate, which contains hydrogen instead of fluorine atoms (H₁₂C₁₈Hg₃) on periphery of the molecule. Consequently, different electronegativity of hydrogen (H) or fluorine (F) gives rise to different bond polarization between carbon and hydrogen atoms (C-H) and fluorine (C-F), respectively, as shown on Fig. 1.

Fig.1 (a) atomically resolved atomic force microscope image of molecules, trimeric perfluoroortho-phenylenemercury (F₁₂C₁₈Hg₃) and its hydrogen-terminated counterpart (H₁₂C₁₈Hg₃) on the surface of Cu (111). (b) The charge distribution simulated using quantum mechanical calculations. (c) Image of the sub-molecular charge distribution of the studied molecules obtained by the new KPFS method showing the different polarity of the chemical C-H and C-F bonds, respectively.

The new method was published in the article in the journal Physics Review Letters [1], and it received a lot of attention soon. For example, the work was a subject of paper published in the journal Physics [2], which highlights the most important papers published in scientific journals of the American Physical Society. The importance of the work is underlined by positive reviews issued by Scientific Publishing IOP Publishing [3] and the Royal Society of Chemistry [4] on their websites.

References:
[1] F. Albrecht, J. Repp, M. Fleischmann, M. Scheer, M. Ondráček, P. Jelínek
Probing Charges on the Atomic Scale by Means of Atomic Force Microscopy
Phys. Rev. Lett. 115 (2015) 076101-1 - 076101-5.
[2] http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.115.076101
[3] http://physicsworld.com/cws/article/news/2015/aug/19/imaging-the-polarity-of-individual-
chemical-bonds
[4] http://www.rsc.org/chemistryworld/2015/08/afm-microscopy-charge-distribution-chemical-
bonds