Do we see the carbon atoms in carbon nanostructures?

Joined work of scientific teams from the Institute of Physics, Academy of Sciences of the Czech Republic, and the Autonomous University of Madrid (Universidad Autonóma de Madrid), Spain, notably advances our understanding of atomically resolved images of graphene and carbon nanostructures that can be acquired with scanning-probe microscopes. The results were published in the prestigious journal Physical Review Letters. The importance of the paper is stressed by the commentary that appeared in Viewpoint of Physics, a journal which highlights important researches published in the journals of American Physical Society. Prof. Eric I. Altman (Yale University, USA) emphasizes in his viewpoint on the paper of the Czech and Spanish scientists that the extensive computer simulations presented in the paper open new ways to optimize actual experiments that involve scanning probe microscopes (SPM). According to Prof. Altman, this work demonstrates that the theory and computer-assisted calculations have reached the stage of development at which we can accurately model the electronic, chemical and physical interactions responsible for the image formation in SPM. Such a combination of experiments with theoretical simulations introduces in a new era in SPM, which will be characterized by the possibility to design scanning-probe tips specifically optimized for the purpose of identifying atoms and investigating selected chemical and physical properties of materials on a particular type of surface.

Carbon-based materials, such as fullerene molecules, carbon nanotubes, and the graphene sheet itself, have been attracting the interest of physicists, chemists, and even biologists during the recent years due to their unique material properties and their potential for applications in nanotechnology. No wonder that the latest (2010) Nobel Prize in Physics was awarded to A. Geim and K. Novoselov for their groundbreaking experiments with graphene.

The ongoing research as well as future technological applications of carbon-based materials depend on profound understanding of the atomic-level physics and chemistry of these materials. The atomic and electronic structure of materials can be directly probed with SPM since the invention of these experimental techniques 30 years ago (Nobel Prize in Physics for G. Binnig and H. Rohrer in 1986). While achieving atomic resolution in the SPM images of graphite surface may be considered a routine task for experimentalists, it is very difficult to pinpoint the exact positions of the carbon atoms that form the surface. It is in particular unclear when the “bright” sites in the images (sites which yield the maximal tunneling current in STM or the maximal force in AFM) correspond to carbon atoms and when they correspond to interatomic hollow positions (see Fig. a-c).

The present work of the Czech and Spanish scientific team consisted in a detailed theoretical analysis of the mechanisms by which the images of carbon materials arise in the scanning tunneling microscope (STM) and the atomic force microscope (AFM). The scientists performed an extensive set of first-principles calculations, simulating the interaction of a structure composed of carbon atoms (graphite surface, the graphene monoatomic layer or a carbon nanotube) with different models of the atomically sharp tip that serves as the probe of the microscope. They calculated the force exerted by the carbon structure on the tip and the electric conductance of the tunneling barrier between the tip and the carbon structure. They in particular investigated how the force and conductance depend on the position of the tip with respect to the imaged structure. The results have shown that the diverse types of contrast found in experiments with graphite surface can be explained by differences in the chemical reactivity of the tip that was used to probe the surface. The authors have also demonstrated that the contrast seen in the images provided by a non-contact atomic force microscope on carbon-based structures is not caused by the van der Waals interaction, as it has been generally assumed until now, but rather a short-range force connected with the formation of a chemical bond between the carbon atoms and the atoms on the tip is responsible for the atomic contrast.

Fig. (a-c) Schematic representation of three possible types of atomic images which may be obtained for graphene when imaged with a scanning-probe microscope. Fig (d) Examples of atomic forces calculated in the computer simulations of atomic force microscopy, together with the resulting contrast in the microscopic images. Forces found at different sites on the surface of graphite and with different models of the tip are shown as functions of distance between the tip of the atomic probe and the surface.

The simulations were mostly carried out on a high-performance computer cluster SGI® Altix® ICE 8200 in the Institute of Physics, Academy of Sciences of the Czech Republic. The Czech part of the research team was support by the Czech Science Foundation (GAČR) and by the Academy of Sciences' Program for Supporting Projects of International Collaboration.

Reference:
Martin Ondráček, Pablo Pou, Vít Rozsíval, Cesar González, Pavel Jelínek, and Rubén Pérez, "Forces and currents in carbon nanostructures: Are we imaging atoms?" Physical Review Letters (Vol.106, No.17)
URL: http://link.aps.org/doi/10.1103/PhysRevLett.106.176101
DOI: 10.1103/PhysRevLett.106.176101

Eric.I. Altman “Proper carbon ID required”
Physics 4, 34 (2011)
URL: http://physics.aps.org/articles/v4/34
DOI: 10.1103/Physics.4.34

Contact persons:
Martin Ondráček
Pavel Jelínek