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Research summary

We explore the cornupia of new physics that can be accessed with the latest developments in the materials sciences, in particular low-dimensional (two-dimensional) new materials, in combination with cutting edge microscopy. Graphene, a single layer of carbon atoms, is currently at the center of interest because it is a particularly outstanding new material that also promises a wide range of new applications. It features an extraordinary electronic structure, a record mechanical stability, highest current carrying capabilities and thermal conductivity, the largest surface area per volume as well as a relatively inert surface.

Freely suspended mono-layer graphene is the thinnest possible membrane that is conceivable with currently known materials. Yet, it is remarkably stable under high-energy electron irradiation, and thus opens unprecedented opportunities also for electron microscopic studies. The graphene membrane structure and its defects are of outstanding interest for science and applications of this promising new material. Static deformations, topological defects, various vacancy configurations, substitutional dopants or the two-dimensional equivalent of dislocations have been studied by transmission electron microscopy (TEM). The formation and evolution of defects under electron irradiation is observed in real time with atomic resolution. High-energy electron irradiation provides a continuous "randomization" of some atoms, which then allows new insights into the complicated bonding behaviour in carbon materials. Further, graphene membranes can serve as a perfect sample support for transmission electron microscopy. Its contribution to the TEM image signal can be filtered out completely and adsorbed atoms and molecules on the graphene sheet can be imaged as if they were suspended in free space.

The remarkable developments in electron microscopy over the past few years, in particular the correction of lens aberrations⁠ and reduction of electron energies, have enabled the direct imaging of the exact atomic structure even in materials made of light elements and of low dimensionality. We can now study these materials with unprecedented precision and follow dynamic processes in in-situ experiments. Exploring new avenues in this direction is one part of our research focus.

 

Research Highlights

Towards two-dimensional all-carbon heterostructures

We showed that by using focused ion irradiation, three types of structures can be defined within a graphene layer in a single processing step: Pristine graphene, two-dimensional amorphous carbon, and empty areas.  Moreover, we showed that the amorphous areas are more reactive than defect-free graphene. This opens a simple route for creating all-carbon heterostructures to be used in fields ranging from nanoelectronics and chemical sensing to composite materials.  Our publication in Nano letters by J. Kotakoski, C. Brand, Y. Lilach, O. Cheshnovsky, C. Mangler, M. Arndt and J. C. Meyer can be found at http://pubs.acs.org/doi/10.1021/acs.nanolett.5b02063

Beam-driven Silicon-Carbon Bond Inversions for controlled relocation of Silicon impurities in Graphene

In this study we demonstrate that 60-keV electron irradiation drives the displacement of threefold-coordinated Si dopants in graphene by one lattice site at a time. First principles simulations reveal that each step is caused by an electron impact on a C atom next to the dopant.  Our results indicate a route for a nondestructive and atomically precise structural relocation of the impurity.

This work was carried out in a collaboration by D. Kepaptsoglou and Q. Ramasse from the SuperSTEM Laboratory (UK), R. Zan and U. Bangert from the University of Manchester (UK), T. C. Lovejoy and O. L. Krivanek from Nion Co. (US), and T. Susi, J. Kotakoski, C. Mangler, P. Ayala, J. C. Meyer from the University of Vienna (AT). The Physical Review Letters paper can be found at http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.115501

Imaging the atomic-level random walk of a point defect in graphene

The diffusion of defects in solids is behind many microstructural changes in solids. Here, we stimulate and follow the migration of a divacancy through graphene lattice using a scanning transmission electron microscope operated at 60 kV. The beam-activated process happens on a timescale that allows us to capture a significant part of the structural transformations and trajectory of the defect.  The low voltage combined with ultra-high vacuum conditions ensures that the defect remains stable over long image sequences, which allows us for the first time to directly follow the diffusion of a point defect in a crystalline material. This work was carried out by J. Kotakoski, C. Mangler and J. C. Meyer and our publication in Nature communications can be found at http://www.nature.com/ncomms/2014/140529/ncomms4991/full/ncomms4991.html

A new potential route to circumvent radiation damage

We introduce a new approach to circumvent the radiation damage problem by a statistical treatment of large, noisy, low-dose data sets of non-periodic configurations. On the basis of simulated data, we demonstrate that high-resolution images can be reconstructed from very low dose exposures of repeatedly occurring structures (e.g. defects, functional groups, small molecules). The only prerequisite is that there is a finite set of different configurations that appear repeatedly on a large area, imaged at very low dose where no damage is expected to occur. The idea and proof of concept was published in Ultramicroscopy and is accessible and is accessible under http://www.sciencedirect.com/science/article/pii/S0304399113003100

A journey from Order to Disorder

One of the most interesting questions in solid state theory is the structure of glass, which has eluded researchers since the early 1900's. Here, we present a direct, atomic-level structural analysis during a crystal-to-glass transformation, including all intermediate stages. We introduce disorder on a 2D crystal, graphene, gradually, utilizing the electron beam of a transmission electron microscope, which allows us to capture the atomic structure at each step. We identify three regimes of the disordered system, namely the crystal with point defects, individual crystallites separated by a vitreous network, and a fully vitreous random network. Our paper in Scientific Reports by F. Eder, J. Kotakoski, U. Kaiser and J. C. Meyer can be found at http://www.nature.com/articles/srep04060

Reshaping graphene via dual-probe microscopy

We showed that we can access the same point on both surfaces of a few-layer graphene membrane simultaneously, using a novel type of dual-probe scanning tunneling microscopy (STM) setup. With the two STM probes approaching the membrane from opposing sides, we were for the first time able to directly measure the deformations induced by one STM probe on a free-standing membrane with an independent second probe. We revealed different regimes of stability of few-layer graphene, and showed how the STM probes can be used as tools to shape the membrane in a controlled manner. Our work opens new avenues for the study of mechanical and electronic properties of two-dimensional materials.

See our publication in Nano letters at http://pubs.acs.org/doi/abs/10.1021/nl3042799. The research was carried out at the University of Vienna by F. Eder, J. Kotakoski, K. Holzweber, C. Mangler, V. Skakalova, and J. C. Meyer.

Mechanical and electronic properties of polycrystalline graphene samples

The most simple way to model a grain boundary in graphene is a periodic arrangement of dislocations. Unfortunately, this model does not correspond to the reality: High-resolution microscopy experiments have revealed meandering serpentlike boundaries containing a non-regular arrangement of carbon pentagons, heptagons and other polygons. We have developed an automated approach to generate realistic atomistic models of grain boundaries in graphene. Using this model, we have studied the mechanical properties of polycrystalline graphene. The work sheds light on the failure mechanism in graphene and clarifies the absence of an orientation dependence in the mechanical strength. This work was published in Physical Review B, see http://dx.doi.org/10.1103/PhysRevB.85.195447. The research was carried out at the University of Vienna by J. Kotakoski and J. Meyer.

With the same approach, the electronic properties of polycrystalline graphene were studied. Among a variety of other insights, a remarkably simple scaling behaviour of the electronic mobility on the average grain size was found. This work was published in Nano Letters (http://dx.doi.org/10.1021/nl400321r). The research was carried out by D. V. Tuan, T. Louvet, F. Ortmann and S. Roche at the Catalan Institute of Nanotechnology, CIN2 (ICN-CSIC) and Universitat Autónoma de Barcelona, and at the University of Vienna by J. Kotakoski and J. Meyer.

 

Research Highlights

(Projects from previous affiliations of J. Meyer:)

Transformations of carbon on graphene under extreme heat.

We showed a reorganization of carbon adsorbates by in situ atomic-resolution transmission electron microscopy (TEM) performed on specimens at extreme temperatures. By using graphene sheets at the same time as ultra-transparent TEM substrate and as in-situ heater, we can create a new nanocrystalline graphene layer on top of the existing membrane. The new layer displays domain sizes of 1-3 nanometer and open edges with predominantly armchair configuration.

See our Nano Letters article at http://pubs.acs.org/doi/abs/10.1021/nl203224z (subscription required). This research was carried out at the University of Ulm, Germany, by B. Westenfelder, J. C. Meyer, J. Biskupek, S. Kurasch, F. Scholz, C. E. Krill, and U. Kaiser.

Reactions of the inner surface of carbon nanotubes imaged at the atomic scale

In this study, we showed that in the presence of catalytically active atoms of rhenium inserted into nanotubes, the nanotube sidewall can be engaged in chemical reactions from the inside.

See the Nature Chemistry article at http://dx.doi.org/10.1038/NCHEM.1115. The research was carried out at the University of Nottingham, UK, and at the University of Ulm, Germany, by T. W. Chamberlain, J. C. Meyer, J. Biskupek, J. Leschner, A Santana, N. A. Besley, E. Bichoutskaia, U. Kaiser, and A. N. Khlobystov.

Analysis of charge transfer in nitrogen-doped graphene and hexagonal boron nitride

For the first time, we demonstrated that the charge density redistribution in chemical bonds can be analyzed from high accuracy high-resolution TEM measurements. This opens a new route to analyze the electronic configuration, and it is particularly suited for non-crystalline configurations such as point defects where a diffraction analysis is not possible.

At the same time, our study shows for the first time a direct visualization of individual nitrogen dopants in mono-layer graphene and demonstrates a change in the electronic configuration of the nearest-neighbor carbon atom next to the dopant.

See the Nature Materials article at http://dx.doi.org/10.1038/nmat2941. The research was carried out at the University of Ulm, Germany, and at the Max Planck Institute for solid state research, Stuttgart, Germany, by J. C. Meyer, S. Kurasch, H.-J. Park, V. Skakalova, D. Künzel, A. Groß, A. Chuvilin, G. Algara-Siller, S. Roth, T. Iwasaki, U. Starke, J. Smet and U. Kaiser.

The first two-dimensional amorphous material

While crystalline two-dimensional materials have become an experimental reality during the past few years, an amorphous 2D material had not been reported before. Here, we created the two-dimensional euqivalent of amorphous carbon, by well controlled electron irradiation of graphene just above the knock-on damage threshold.

See our article in Physical Review Letters at: http://dx.doi.org/10.1103/PhysRevLett.106.105505. This project was carried out at the University of Helsinki, Finland, and at the University of Ulm, Germany, by J. Kotakoski, A. V. Krasheninnikov, U. Kaiser, and J. C. Meyer.

Creating carbon atomic chains from graphene

We formed single-atomic carbon chains by thinning a graphene constriction under the electron beam. The chains formed efficiently by self-organization during continuous removal of atoms from a graphene bridge. These new molecular structures may provide a novel element for all-carbon electronics.

See our article in the New Journal of Physics at http://dx.doi.org/10.1088/1367-2630/11/8/083019. This research was carried out at the University of Ulm by A. Chuvilin, J. C. Meyer, G. Algara-Siller, and U. Kaiser. A similar discovery was made by K. Suenaga and colleagues from AIST, Tsukuba, Japan, their article can be found here: http://link.aps.org/doi/10.1103/PhysRevLett.102.205501.

The graphene edge

The boundary of a 2-D material is a 1-D line of atoms. The 2009 paper in Science showed the first aberration-corrected high resolution images and videos of the open edge configurations in free-standing graphene and their dynamic rearrangement.

See the article and supplementary videos at http://dx.doi.org/10.1126/science.1166999. This work was carried out at the University of California at Berkeley and at the Lawrence Berkeley National Laboratory, Berkeley, USA by C. O. Girit, J. C. Meyer, R. Erni, M. D. Rossell, C. Kisielowski, L. Yang, C.-H. Park, M. F. Crommie, M. L. Cohen, S. G. Louie and A. Zettl.

Ultra-high resolution pattern definition on graphene

Using electron-beam induced deposition (EBID, also known as contamination lithography) we wrote smallest structures with a resolution down to 2.5nm (half-pitch) on top of a graphene membrane. Such patterns may provide a route to create nanometer-scale doping patterns, diffraction gratings, or etch masks in this novel electronic material.

The article in Applied Physics Letters can be found here: http://dx.doi.org/10.1063/1.2901147. The research was carried out at the University of California at Berkeley and at the Lawrence Berkeley National Laboratory, Berkeley, USA by J. C. Meyer, C. O. Girit, M. F. Crommie and A. Zettl.

The structure of suspended graphene sheets

Our studies by transmission electron microscopy revealed that even suspended graphene sheets are not perfectly flat: they exhibit intrinsic microscopic roughening such that the surface normal varies by several degrees and out-of-plane deformations reach 1 nm.

Our letter to Nature can be found at http://dx.doi.org/10.1038/nature05545. The research was carried out at the Max Planck Institute for solid state research, Stuttgart, Germany, the University of Manchester, UK, and the University of Nijmegen, Netherlands, by J. C. Meyer, A. K. Geim, M. I. Katsnelson, K. S. Novoselov, T. J. Booth and S. Roth.

The single-molecule torsional pendulum

A single-walled carbon nanotube served as a molecular-scale torsional spring and as mechanical support in this nano-electromechanical device (the scale bar on the image sequence to the right is ca. 500 nanometer.). We were able to determine the handedness of the carbon nanotube and observed the thermally induced vibrations in direct images. Devices of this type might serve as extremely sensitive nano-scale sensors.

Our report in Science can be found at http://dx.doi.org/10.112/science.1115067. This work was carried out at the Max Planck Institute for solid state research, Stuttgart, Germany, and the Universite de Montpellier, France, by J. C. Meyer, M. Paillet, and S. Roth.

Research Group Physics of Nanostructured Materials
Faculty of Physics

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