Materials Physics

Research is carried out in the following fields: nanostructures, ion beam analysis, electronic structures, microtomography, computational materials physics, electronics and medical physics.

Ion beam analysis laboratory and laboratory for nanomaterials

The main task in 2013 at the 5-MV tandem accelerator TAMIA, was the installation and thorough testing of the upgraded 40-sample Accelerator Mass Spectrometry (AMS) ion source, together with the fast sequential injection system for carbon isotopes. This AMS setup allows routine measurements of radiocarbon at the accuracy level of 0.3%. Other uses of the tandem were irradiation and particle-detector energy calibration experiments with 1.5 to 9.0 MeV proton beams, ion beam analyses by PIXE with 3-MeV protons and by the TOF-ERDA method employing 50-MeV heavy ions (Cl, Br, or I). The implemented control and automation system with a dedicated operator interface has turned out to be an invaluable tool for optimization of the beam parameters, especially in radiocarbon AMS. Furthermore, the upgraded generating voltmeter allows for the stabilization of the accelerator terminal voltage to better than ±1 kV without the need of stabilizing slits. This implies better beam transmission and higher and more stable beam intensities at the target. This fact was demonstrated by the new record-high, stable terminal voltage of 5050 kV used with +9e bromide ion beam.

A gas recirculation system for the ion source of the 500-kV accelerator KIIA was installed and tested to save the precious 3He gas. Test runs with 4He and residual-gas N and O show that more than 10 µA beam can be easily extracted when running the source with recirculation only. Only routine servicing of the ion sources was required through the year. The main cause for overheating of both the anode power supply and the coil of the source magnet of the Penning ion source was found to be the excessive erosion of the exit aperture. Ion implantation experiments with ions ranging from 10-keV H3 molecular ions to 500-keV Ar we performed. Furthermore, extraction tests of multiple-charged Ar ions from the Penning ion source were successful, producing the following beam currents for various charge states: Ar+, 100 µA; Ar++, 10 µA; Ar3+, 1.5 µA, Ar4+, 50 nA.

During the year, the laboratory researchers continued investigation of the unexpected ferromagnetism observed in nanocrystalline materials built of nanoclusters at the FANADE facility. Gold as a representative model element fundamentally non-magnetic (meaning diamagnetic) in bulk was revealed to exhibit ferromagnetic properties when produced in the nanocrystalline form. The study is presented in more detail in the Research Highlights section.

Significant efforts of the laboratory have been focused on research collaboration within the Center of Excellence project in Atomic Layer Deposition (CoE ALD) funded by the Academy of Finland. Aiming at developing multifunctional materials, multiferroic bismuth ferrite films grown by ALD were found to exhibit a spin-glass behavior at high temperatures that makes the material of interest for practical applications. To enhance the lab performance in the research related to CoE, two postdocs have been hired and a new multiferroic characterization facility has been acquired from Radiant Technologies, Inc. As a user facility, FANADE has been used for fabrication of nanocluster samples provided to MIKES on a commercial basis (European Metrology Research Programme project MechProNO).

Recently installed dilution refrigerator was actively employed in studying electrical and transport properties of silicon-based mesoscopic structures. As a user facility, the cryogenic system was provided on pay-basis to researches from the Division of Geophysics and Astronomy, UH, and VTT, developing multiplexed SQUID based readout electronics for large scale detector arrays for space applications within the EU project "E-SQUID".

Fully supporting the general commitment to strengthen continuously the department capabilities in research and development, a new analytical facility has been purchased and installed in the accelerator building. The facility combines two techniques for surface and thin film analysis, viz., Secondary Ion/Neutral Mass-Spectrometry and X-ray Photoelectron Spectroscopy. While the former enables ultrahigh-sensitive surface analysis in the ppb range and depth profiling, the latter enables surface chemical state analysis. The facility will open new analytical perspectives in basic research for academics and in R&D for users from industry.

High-light: Magnetism in Nanocrystalline Gold

Never dying and ever growing attention to gold as a precious noble metal is contributed nowadays by nanoscience and nanotechnology striving for developing new applications, devices and products employing new nano-motif that would offer properties not available in the bulk. In focus are novel electronic, optical, catalytic, tribological, antibacterial, etc. properties of dispersed or finely divided gold, and nanostructured surfaces with large surface-to-volume ratio. Enabling a variety of new exiting and marketable techniques ranging from sensing to applications in biomedicine and forensic sciences, nanostructuring allows reducing considerably the amount of used gold (and, hence, costing) without degrading or sometimes even enhancing the desired functionality. Until recently, magnetism in gold was of little interest. Our researches have shown that while bulk gold is diamagnetic, nanostructured gold can be imparted with unconventional magnetic properties. Gold nanocrystalline films produced by cluster deposition in the aggregate form that can be considered as a crossover state between a nanocluster and a continuous film was found to exhibit ferromagnetic-like hysteretic magnetization with temperature dependence indicative of spin-glass-like behavior1. Imparting gold with ferromagnetic properties, especially remanence, would add more functionalities to gold based applications and devices gaining sensitivity to external magnetic fields. This would provide a sensing option to magnetic signal generating stimuli and more degrees of freedom in control and manipulating via magnetization switching, e.g., control over local catalytic properties, applications in biomedicine and therapeutics highly demanding with respect to biocompatibility (precise drug delivery and cancer targeting, magnetic resonance imaging and hyperthermia, magnetically activated gold nanoparticle-based sensors).

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Reference: V. Tuboltsev, A. Savin, A. Pirojenko, and J. Räisänen, Magnetism in Nanocrystalline Gold, ACS Nano 7(8)(2013)6691.

Laboratory of electronic structure and laboratory of microtomography

During the year 2013 experimental research was carried out both at synchrotron research facilities and using the laboratory x-ray scattering and microtomography setups. The highlight of the year was the X-ray Raman spectroscopy study on the microscopic structure of water at sub- and supercritical conditions. These experiments were carried out at the European Synchrotron Radiation Facility (ESRF).

As a part of the ESRF Upgrade Programme, a novel state-of-the-art beamline (ID20) for inelastic x-ray scattering spectroscopy was inaugurated at ESRF, and the Laboratory of Electronic Structure team participated actively in the commissioning phase. Already several experiments were carried out using the new facility. For example, plasmons in transition metal dichalcogenides and chemical reactions in acetic acid were studied.

New computational materials physics research topics on advanced materials were started via the recruitment of two new post-doctoral researchers. Their research concentrates on third generation photovoltaic solid state and composite materials (intermediate band and dye-sensitized solar cell systems, respectively). The latter is carried out within a consortium involving Department of Chemistry and Universities of Aalto and Oulu. The other computational research topics were continued both in the gas and the liquid phases, combining experiments and theory, and continuing the collaboration in the field of systems related to aerosol particles and atmospheric chemistry.

The x-ray microtomography equipment at the Division of Materials Physics was developed further by adding to it another microfocus x-ray tube for x-ray scattering experiments. The combination of these two methods opens up new possibilities for structural studies of complex materials. The first applications included studies on variation of nanostructure inside meteorite samples and clay-polymer composites.

During the year structural studies of natural polymer based materials were continued mainly using the laboratory x-ray scattering equipment in co-operation with Department of Food and Environmental Sciences and Aalto University. Bacterial cell wall structure was studied using small-angle x-ray scattering at Max IV laboratory, Lund, in co-operation with Department of Chemistry and Department of Veterinary Biosciences.

Synchrotron radiation based medical research was carried out in international collaboration with physicists and medical doctors, especially in functional lung imaging and breast cancer research fields. The primary goal was to develop new high-resolution imaging methods to provide deeper understanding of the structure and function of organs. In addition to the ongoing bronchial hyperresponsiveness studies at the ESRF, functional lung imaging was applied to study acute lung injury and ventilation strategies during mechanical ventilation.

Computational materials science

In 2013 the computational materials research activities achieved several exciting high-visibility results on one-dimensional nanostructures. One of these dealt with the ion irradiation of single gold nanowires. In collaboration with a UK group doing experimental work on exactly the same system, we found that ion irradiation of a nanowire can lead to sputtering yields of more than 1000, which are an order of magnitude higher than those of the same material in any regular bulk material. In fact, this sputtering yield also constitutes the world record in sputtering yields for any kind of ion on any kind of solid in the nuclear collision regime. Our computer simulations showed that the reason is an explosive emission of material in the two open surface dimensions of a nanowire [1].

In another, almost directly opposite system, we found in collaboration with a group in Australia that very high energy ion irradiation can produce asymmetric voids due to a one-dimensional nanometric ion track deep inside solid germanium. These voids have a very peculiar bowtie shaped shape, an experimental observation that defied any simple explanation. By a long series of ion irradiation studies examining different track melting and resolidification scenarios, we were able to show that the bowtie shape can form when the balance between the density changes of the amorphous Ge and its resolidification rate is just right [2].

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Explosive emission of thousands of atoms from an Au nanowire due to 80 keV Xe ion irradiation. Each dot shows the position of one atom shortly after the ion hit the nanowire.

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Experimental (left) and computational (right) image of bowtie-shaped voids (whiter area) formed in amorphous germanium due to swift heavy ion irradiation. The almost exact match in the sizes of the voids gives confidence that our simulations give the correct track formation mechanism.

References:

G. Greaves, J. A. Hinks, P. Busby, N. J. Mellors, A. Ilinov, A. Kuronen, K. Nordlund, and S. E. Donnelly, Phys. Rev. Lett. 111, 065504 (2013).

M. Ridgway, T. Bierschenk, R. G. , B. Afra, M. D. Rodriguez, L. Araujo, A. P. Byrne, N. Kirby, O. H. Pakarinen, F. Djurabekova, K. Nordlund, M. Schleberger, O.Osmani, N. Medvedev, B. Rethfeld, W. Wesch, and P. Kluth, Phys. Rev. Lett. 110, 245502 (2013).

Electronics

Sensing and actuation methods combining ultrasonics, optics, electronics, and advanced signal processing for industrial and scientific applications were conceived and developed together with our collaborators. We did a lot of cool stuff:

Medical physics

In 2013, Medical Physics research group at the Department of Physics established a new Research Community (RC) called Medical Physics in Clinical and Preclinical research, to strengthen its scientific output. The hospital subgroup in RC is one of the strongest in Finland in the field of medical physics research. Quantitative imaging – clinical and preclinical - with various modalities is essential for accurate diagnostics, drug research and is the basis of treatment planning, extending from macrodosimetry to cell level microdosimetry. There are various preclinical and clinical problems motivating further research i.e. dosimetric calculations, in vivo dosimetry and imaging, reconstruction and study of new molecular carriers, which the RC have expertise needed for resolving the issues. The RC benefits HUCH by developing novel and improving methods for medical diagnosis and treatment. Within the past decade, HUCH has invested considerably in setting up and in testing with pilot-level studies several frontier methodologies at the intersection between radiology, oncology, paediatrics, physics, and neuroscientific fields of medicine. Presently, all these techniques are at the edge of becoming truly established and approved as part of everyday healthcare in HUCH. Additionally, the specialized training for potential hospital physicists ensures competent experts in the field of medical physics for HUCH. Collaboration between institutes of RC provides a very wide range of expertise, warranting multidisciplinary research with highest quality.

Main efforts of the digital radiology physical research and development are focused on radiation dosimetry, optimization of patient examinations and quality assurance methodology. Regarding ultrasound technology, we have studied the efficiency and cost-effectiveness of our current quality assurance protocol, and made preliminary studies to further extend it to the Doppler ultrasound. We continued validating the navigated transcranial magnetic stimulation (nTMS) technique for motor cortical mappings with epilepsy patients by comparing the nTMS results with invasive mapping results. The work with ultra-high resolution single-photon emission computed tomography at Viikki campus continued.

Medical physics researchers attended several international conferences during 2013 including the annual scientific meeting of the European Association of Nuclear Medicine (EANM) in Lyon, NACP Course on optimization of acquisition and post-processing in radiological examinations (Bergen, Norway), RSNA - 99th Scientific Assembly and Annual Meeting of the Radiological Society of North America (Chicago, USA).