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Materials Physics

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During 2010 the Laboratory of Medical Physics was joined to the Division. In addition, computational physics was classified under laboratory of Materials Science Simulations. The division carries out presently research in the following fields: nanostructures, ion beam analysis, electronic structures, microtomography, computational materials physics, electronics and medical physics.

Laboratory for nanomaterials and ion beam analysis

The experimental research activities during 2010 were strongly influenced by the accelerator building renovation, which started mid-April. Both the 5-MV tandem accelerator (TAMIA) and the 500-kV ion implanter were routinely run until then. Prior to the renovation several projects involving ion- and cluster beams were carried out employing the particle accelerators and the Facility for Nanostructure Deposition (FANADE). After this all experimental activities were terminated till the end of the year.

During the renovation, the control and automation systems of TAMIA were redesigned. Almost all high-voltage power supplies were replaced. A time-shared fore-vacuum pumping system was designed to generate a laboratory-wide hydrocarbon-free high- and ultra-high vacuum. The development work concerning the low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) facilities continued, but the work needs to be finalized when the experimental facilities are again operational. The new Cryogen-Free Dilution Refrigerator System acquired in the laboratory for nanomaterials was received from the vendor, but its installation in the newly build shielded room will take place early 2011.

Laboratory of electronic structure and laboratory of microtomography

The use of synchrotron radiation as well as the versatile X-ray instrumentation in the laboratory stayed in the research focus. Synchrotron radiation was used in spectroscopy and scattering studies of gases at extreme temperatures, complex liquids, ultrathin polymer films, and strongly correlated materials like cuprate superconductors. A highlight of the year was the experimental determination of the valence-electron momentum density and renormalization factor of sodium. The new imaging opportunities via the new X-ray microtomography instrument capable of micron resolution have opened multidisciplinary projects in evolutionary biology, geology and paleontology. The imaging opportunities have also greatly expanded ongoing studies on the properties and hierarchical structures of wood and other renewable materials like paper. The experimental work is accompanied by strong computational activities which have played an important role in strengthening the fundamental understanding of electronic structure behind the macroscopic properties of materials via experiment-theory interaction.

Computational materials science

The studies of nanowires and graphene have increased dramatically in the last few years. In the simulation group, we expanded strongly our activities in both areas during 2010. We examined the formation of single and extended defects in graphene, and showed by comparison with aberration-corrected transmission electron microscopy experiments that electron irradiation damage in graphene can be fully described by atomistic simulations. We also initiated systematic studies of ion beam effects in Si nanowires. We found that damage production in the wires is strongly influenced by surface effects.

Electronics

Novel measurement methods and sensors combining ultrasonics, optics, laser-acoustics, electronics, and advanced signal processing for industrial and scientific applications have been developed.

Medical physics

Research is focused in new methods and applications for medical imaging and therapy. The research is being conducted by collaborators from HUCH, VTT, and STUK. The ongoing projects are:  boron neutron capture therapy (BNCT), dosimetry in diagnostics and treatments, medical imaging applications, mathematical modeling of physiological systems in clinical studies, and emerging solutions for dose and quality assurance in digital radiology.

Highlight of research

Atom-Scle Modifications in Graphene and White Graphene

The only known atomically thin two-dimensional materials are graphene and the hexagonal boron-nitride monolayer. They have a similar atomic structure, resembling a honeycomb or a chicken wire, but altogether different chemistry.

Graphene is a single layer of the familiar layered carbon material graphite, kept together by the sp2-hybridized covalent bonds between the carbon atoms, and exhibits semi-metallicity (or, in other words, is a zero-band gap semiconductor). Although many properties of graphene had been investigated theoretically as a prototype material for understanding the properties of graphite and carbon nanostructures, e.g., fullerenes and carbon nanotubes, it was a surprise in 2004 when this material was experimentally shown to exist also in reality. Due to this breakthrough discovery, Konstantin Novoselov and Andre Geim received the 2010 Nobel Prize in physics.

On the other hand, hexagonal boron nitride monolayer, the insulating counterpart of graphene sometimes called white graphene, is formed due to ionic bonding between the alternating boron and nitrogen atoms.

Discovery of these materials was significantly aided by the recent advances in the high-resolution transmission electron microscopy, especially via the introduction of aberration corrected devices, which allowed atomic resolution images at conditions, which are not destructive to the targets. The initial experimental observations displayed, along with the materials themselves, a set of atomic defects, which were either intrinsic or caused by the electron beam. Especially in the case of hexagonal boron nitride, the triangular vacancy structures caused much confusion in the field since they seemed to be unexplainable by the existing theories of damage production in this material.

Through a collaboration of University of Helsinki and the National Institute of Advanced Industrial Science and Technology (AIST), Japan, we have been able to explain the formation of these defects by primary knock-on’s between the electrons and the target atoms. The peculiar triangular shape of the defects is due to easier displacement of boron atoms in the system, which leads to vacancies terminated by nitrogen atoms. For graphene, the observed defects have been much less controversial. However, as we show in our recent review (written in collaboration with University of Strasbourg, France), the flexibility of carbon chemistry allows for a breadth of different defect structures to be formed.

As a result of collaboration of University of Helsinki and University of Ulm, Germany, we have recently found the lowest energy defect configurations in this material, which also introduce a band gap into this material. This may prove out to be an important step towards carbon-based electronics of the future. As the limiting case for the growing defect structures, we displayed the first-ever two-dimensional amorphous structure.

Kuva 1.
Triangular defects created into a hexagonal boron nitride monolayer by an electron beam with 80 kV acceleration voltage. Simulation result (left) and experimental result (right).

Kuva 2.
Example of a multi-vacancy defect structure in graphene created by an electron beam with 100 kV acceleration voltage. Scale bar is 1 nm.

References

Kotakoski, J., Jin, C., Lehtinen, O., Krasheninnikov, A.V. and Suenaga, K., 2010. Electron knock-on damage in hexagonal boron nitride monolayers. Phys. Rev. B, 81, 113404.

Banhart, F., Kotakoski, J. and Krasheninnikov A.V., 2011. Structural defects in graphene. ACS Nano, 5, 26-41.

Kotakoski, J., Krasheninnikov, A.V., Kaiser, U. and Meyer, J.C., 2011. From point defects in graphene to two-dimensional amorphous carbon. Phys. Rev. Lett., accepted for publication.