New possibilities of imaging molecules with atomic force microscope

The sub-molecular resolution of individual molecules opens up entirely new possibilities in the study of physical and chemical properties of molecular nanostructures. However, it was possible to carry out these measurements only at very low temperatures close to absolute zero with specially modified microscope tips. The modification consists of a targeted location of just a single molecule (e.g. carbon monoxide) or a noble gas atom on the apex of the metal tip. The main obstacle to achieving sub-molecular contrast is the relatively weak signal-to-noise ratio detected during measurements. The presence of flexible particles on the apex of the tip results in a significant amplification of the signal, which allows us to achieve high-resolution images. However, such tips are stable only at very low temperatures, close to absolute zero. This condition has dramatically limited the use of this method in terms relevant to important chemical and biological processes, for which e.g. room temperature is essential.

A team of scientists from the Institute of Physics of the CAS and the University of Osaka introduced in the July issue of the journal Nature Communications a new method that allows you to achieve sub-molecular resolution even at room temperature with standard tips. Collaboration between two teams led to the optimization of key scanning parameters supported by theoretical calculations. The optimum measurement parameters enabled a significant enhancement of the detection signal even without the requirement of the special tip modification. This achievement pushes significantly the limits of the molecular resolution by means of the scanning probe microscopes. The possibility of imaging individual molecules on surfaces at ambient temperature represents an essential prerequisite for the study of catalytic reactions on solid surfaces.

Description

Fig.1 A) Experimental image with the sub-molecular resolution of PTCDA molecule on the silicon surface using an atomic force microscope at room temperature, B) calculated electron density distribution above a PTCDA molecule, which contributes to the formation of high-resolution AFM images, and C, D) optimized atomic structure of PTCDA molecule after deposition on the silicon surface obtained by quantum mechanical computer simulations (see [1]).

This work builds on previous research of scientists from Nanosurf laboratory of the Institute of Physics, which contributed significantly to the understanding of the mechanism leading to sub-molecular resolution of single molecules using scanning probe microscopes. Scientists from the Institute of Physics have formulated a new theoretical model that provides a deeper understanding of experimental measurements with the aid of computer simulations (see [2] and [3]). The importance of the theoretical model reflects a considerable number of citations which the papers received in less than one year after its publication (> 30). In addition, the purchase of low-temperature microscope in 2013 allowed the scientists from the Institute of Physics of the CAS to achieve experimental sub-molecular resolution. This achievement ranks the group among the few places in the world where it is possible to perform such experiments. The possibility of combining theoretical simulations and experimental cutting-edge technology gives very good preconditions for further research of physical and chemical properties of molecular nanostructures in the Institute of Physics of the Czech Academy of Sciences.

References:
[1] K. Iwata et al, "Chemical structure imaging of a single molecule by atomic force microscopy at room temperature”, Nature Communications 6, 7766 (2015)
doi:10.1038/ncomms8766
[2] P. Hapala et al, „Origin of high-resolution IETS-STM images of organic molecules with functionalized tips”, Phys. Rev. Lett. 113, 226101 (2014).
[3] P. Hapala et al, “Mechanism of high-resolution STM/AFM imaging with functionalized tips”, Phys. Rev. B 90, 085421 (2014).