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Division of Materials Physics

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The division carries out research 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

At the 500-kV ion implanter accelerator KIIA, the broken high-voltage transformer of the main power supply was replaced. Causes of the interfering corona were located and corrected for. Previously, the corona prevented reaching of the nominal maximum voltage of 500 kV under the specified environmental conditions. The energy calibration of the high voltage was performed using an accurate HV probe together with known nuclear reaction resonances. Chlorine compounds were successfully tested in generation of ion beams of various non-volatile elements.

At the 5-MV tandem accelerator TAMIA, test runs continued through the year 2012 along with the experiments with proton and heavy-ion beams. The new injector platform performs as designed, allowing very good matching of ion optics to the tandem. At the high-energy side, the NMR-probe system at the analyzing magnet was replaced with a Hall-probe system. For the whole accelerator, the extensive logging of the beam parameters facilitates beam optimization and tracking of the causes of possible faults. The performance of the new control and automation systems together with the sophisticated operator interface was demonstrated in calibration measurements that showed very good stability of the beam energy. The first heavy-ion beams for the TOF-ERDA experiments were also run successfully. Analysis of the TOF-ERDA data gave further support for the improved stability of the tandem.

In the laboratory for nanomaterials projects related to the study of structural, thermal and electromagnetic properties of metallic nanoclusters and nanocrystalline films built up of the clusters were continued. The main activity was focused on unconventional magnetism in surface rich nanomaterials. A representative case is ultra-pure gold clusters and nanocrystalline films produced by our cluster deposition facility FaNaDe. It was demonstrated that even such a well-known noble element as gold can be imparted with unusual magnetic properties (viz., ferromagnetism) when produced in low-dimensional forms. The unusual magnetism was found to differ dramatically from the bulk gold diamagnetism and was shown to be due to so-called surface magnetism and large surface-to-volume atomic ratio favored by the nano-particulate morphology. New findings open an exiting way for modification of magnetic properties and enabling new functionalities in low-dimensional materials via surface nanostructuring.

Research facility development work was continued by the installation of a Cryogen-Free Dilution Refrigerator in a new electromagnetically shielded room. The facility has been tested and the lowest terminal temperature was demonstrated to be 10mK. Variable temperature measurement system on the base of the refrigerator has been developed and is being heavily engaged in our internal projects, as well as offered for external users.

Laboratory of electronic structure and laboratory of microtomography

During the year 2012 experimental research was carried out both using the laboratory x-ray scattering and tomography equipment and at large scale synchrotron and neutron research facilities. Many of the research projects were multidisciplinary and carried out in international collaboration.

The highlight of the year was the successful evolutionary biology research on teeth development using X-ray microtomography. Interesting results were also obtained on the hierarchical structures of natural polymer based materials and Norway spruce phloem. Neutron scattering was utilized in studies of the two-dimensional crystal structures of surface layer proteins of bacterial cell wall. This study was carried out at Institut Laue Langevin (Grenoble).

The experimental spectroscopic investigations concentrated mostly on oxide materials. Non-resonant inelastic x-ray scattering studies were performed for various LaAlO/SrTiO heterostructures to understand the interfacial electronic rearrangements. The debated structural and simultaneous electronic (metal-insulator) phase transition in vanadium oxide was studied by Compton-profile measurements.

The experimental work was accompanied by various computational activities, which strengthen the fundamental understanding of electronic structure and its response properties. Work on both solid-state systems as well as molecular clusters relevant to liquid and aerosol structures was continued. One important milestone was reached by the publication of a program package specifically designed for x-ray properties of atoms and molecules.

Synchrotron radiation based medical research was carried out in international collaboration with physicists and medical doctors. The primary goal was to develop new high-resolution imaging methods to provide deeper understanding of the structure and function of organs, especially in functional lung imaging and breast cancer research fields.

During the year 2012 the rotating anode based X-ray scattering equipment was remodeled. As a part of the European Synchrotron Radiation Facility (Grenoble) Upgrade Programme, the inelastic x-ray scattering spectroscopy beamline ID16 was shut down and the construction of an entirely new state-of-the-art beamline ID20 started. The Laboratory of Electronic Structure team has participated actively in the planning and commissioning phases. The new experimental stations will be available for users in 2013.

Computational materials science

In 2012 the computational materials research activities expanded significantly on the topic of nuclear reactor materials with the acceptance of two new Academy of Finland projects in this field. This is well in line with the recognition of the group being among the worldwide leading ones in this field. This is evident in e.g. evidenced by e.g. the group leader being member of the international committees of all three major conferences in this field (IBMM, REI and COSIRES) and chairing an OECD Nuclear Energy Agence committee on primary radiation damage. In the new projects, the group will combine its preexisting skills on nanoscale precipitates and interatomic potential development to tackle the major challenge of simulating interactions of dislocations with carbide and oxide precipitates in stainless steels.

In parallell with the new projects, the nanoscience projects continued to produce outstanding results. Work on magnetism and defects in graphene results in several publications in the top-ranking journals Nature Physics, Phys. Rev. Letters and Nano letters. Most of these works were done in collaboration with experimental groups worldwide, highlighting the value of doing experimental and simulation work in close conjunction with each other.

Electronics

Sensing and actuation methods combining ultrasonics, optics, electronics, and advanced signal processing for industrial and scientific applications were developed together with our collaborators. We built: 1 km space tether for space sailing, 1 GHz acoustic microscope for bone research, massless sound source for room acoustics, fast ultrahigh vacuum manometer for CLICK, laserultrasonic setups for osteoporosis detection and osteoarthritis therapy, ultrasound glove for emergency crews. We recorded: nanometer resolution videos showing viral DNA ejection and motion of a buried MEMS membrane, sleepiness by self-tests, fixation of orthopedic implants.

Medical physics

In 2010, Medical Physics research group at the Department of Physics established a Research Community (RC) called BNCT & Medical Imaging, BNCTMI, to strengthen its scientific output. The RC members come from the UH (Departments of Physics, Chemistry and Centre for Drug Research), Helsinki University Central Hospital (Medical Imaging Center, HUSLAB and Dept. of Oncology), VTT Technical Research Centre of Finland, STUK Radiation and Nuclear Safety Authority and private sector. The hospital subgroup is one of the strongest in Finland in the fields of medical physics research and research training.

One of the tasks of the RC is training of hospital physicists (HPs). In the spring of 2012, the research community BNCTMI received good grades from international evaluation of research and doctoral training at the University of Helsinki 2005-2010. “The RC has an excellent track record in training of hospital physicists (HPs). Majority of the PhD students get their scientific training parallel to their professional training for a HP degree. Due to the well recognized and appreciated profession of HPs, the RC is able to recruit talented young candidates and guide them also to research. The doctoral training follows good practices of HU, and the head of the RC represents a remarkable supervisor in the field of medical physics.” The evaluation report is available in:

 http://www.helsinki.fi/julkaisut/aineisto/rc_evaluation2012/hallinnon_julkaisuja_80_68_2012.pdf.  

In July 2012, the Head of VTT Technical Research Centre of Finland decided to close down the FiR 1 research reactor used for the BNCT treatments mainly due to socio-economic reasons. Thus, the BNCT clinical trials in Finland were stopped and over 20 years of multidisciplinary research in this field is now coming to an end. The clinical BNCT trials were started on malignant brain tumors in 1999. Since then, 249 patients with malignant brain or head and neck tumors have been treated at the facility. After January of 2012, some technical research has been performed at the reactor. Applicability of alanine dosimeters in mixed photon-neutron beam dosimetry was studied in collaboration with the University of Mainz (Mainz, Germany) and the National Physical Laboratory (NPL, Middlesex, UK). The photon beam spectrum was defined with ionization chamber measurements applying multiple build-up caps and spectrum unfolding code in collaboration with the University of Mainz and the National Tsing Hua University (HsinChu, Taiwan).

As the BNCT patient trials are discontinued and the FiR 1 research reactor will be run down in the near future, the new focus of the Medical Physics research community will be in diagnostics and therapies in the fields of Radiology, Nuclear Medicine, Clinical Neurophysiology and Oncology. 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 also focused on 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 demonstrated the feasibility of dopamine transporter tracer 123I β-CIT in preclinical research.

Highlights of research

Paramagnetism in irradiated and fluorinated graphene

Magnetism in various metal-free carbon systems, such as polymerized fullerenes and graphite, has been intensively studied during last years. The driving force behind these studies was not only to create technologically important, light, nonmetallic bio-compatible magnets with a Curie point well above room temperature, but also to understand a fundamental problem: the origin of magnetism in a system which traditionally has been thought to show diamagnetic behavior only. However, although many theoretical articles on the subject have been published, the experimental data are scarce and controversial.

The isolation of graphene, a sheet of carbon just one atom thick, by A. Geim and K. Novoselov (who received the Nobel Prize in Physics in 2010 for their discovery) has kindled new interest in magnetic carbon. First of all, graphene has a much simpler atomic structure than other carbon allotropes, which may help to unravel the origin of magnetism if graphene proves to exhibit magnetic behavior under certain conditions. Besides, unique electronic properties of this material may result in peculiar magnetic effects.

The magnetism observed in carbon systems was normally associated with defects, such as vacancies or impurity atoms which are non-magnetic by themselves, but give rise to local magnetic moments due to unusual chemical environment. Back in 2003, when graphene was not even manufactured, magnetism in graphene with defects with point defects was theoretically predicted [1,2].

Now this theoretical prediction has been verified [3] through the collaboration of Geim's group at the University of Manchester, UK, and Ion Beam Analysis Laboratory at the University of Helsinki. Point defects were created in graphene either by using ion irradiation (mostly vacancies) or by adding fluorine atoms (adatoms) to the surface of graphene. Both types of the defects were found to have localized magnetic moments. Magnetism of vacancies is associated with carbon dangling bonds, while adatoms give rise to local sp3 hybridization and effectively remove an electron from the sp2 electron system of graphene, as vacancies. The moments did not seem to interact with each other, giving rise to paramagnetic behavior. Thus the paper shed light on some of the controversial issues of graphene’s magnetism and sets limits for other graphitic compounds. Further research work will be carried out to study whether localized magnetic moments can be ferromagnetically coupled so that a real carbon magnet can be made from graphene.

References

1. P.O. Lehtinen, A.S. Foster, A. Ayuela, A. V. Krasheninnikov, K. Nordlund, and R. M. Nieminen, "Magnetic properties and diffusion of adatoms on a graphene sheet", Phys. Rev. Lett. 91 (2003) 017202.

2. P.O. Lehtinen, A.S. Foster, Y. Ma, A. V. Krasheninnikov, and R. M. Nieminen "Irradiation-induced magnetism in graphite: a density-functional study", Phys. Rev. Lett. 93 (2004) 187202.

3. R. R. Nair, M. Sepioni, I-Ling Tsai, O. Lehtinen, J. Keinonen, A. V. Krasheninnikov, T.Thomson, A. K. Geim and I. V. Grigorieva, "Spin-half paramagnetism in graphene induced by point defects" Nature Physics, in press.

Optical images of the pristine (a) and fully fluorinated (b) graphene laminates. (c) Atomic structure and spin density of single vacancy in graphene. Such defects produced by ion irradiation have localized magnetic moments. Optical images of the pristine (a) and fully fluorinated (b) graphene laminates. (c) Atomic structure and spin density of single vacancy in graphene. Such defects produced by ion irradiation have localized magnetic moments.

Space sailing soon: A one-kilometre-long electric sail tether was produced

The electric sail (ESAIL), invented by Dr. Pekka Janhunen at the Finnish Kumpula Space Centre in 2006, produces propulsion power for a spacecraft by utilizing the solar wind. The sail features electrically charged long and thin metal tethers that interact with the solar wind. Using ultrasonic welding, the Electronics Research Laboratory at the University of Helsinki successfully produced a 1 km long ESAIL tether. Four years ago, global experts in ultrasonic welding considered it impossible to weld together such thin wires. The produced tether proves that manufacturing full size ESAIL tethers is possible. The theoretically predicted electric sail force will be measured in space during 2013.

An electric solar wind sail, a.k.a electric sail, consists of long, thin (25–50 micron) electrically conductive tethers manufactured from aluminium wires. A full-scale sail can include up to 100 tethers, each 20 kilometres long. In addition, the craft will contain a high-voltage source and an electron gun that creates a positive charge in the tethers. The electric field of the charged tethers will extend approximately 100 metres into the surrounding solar wind plasma. Charged particles from the solar wind crash into this field, creating an interaction that transfers momentum from the solar wind to the spacecraft. Compared with other methods, such as ion engines, the electric sail produces a large amount of propulsion considering its mass and power requirement. Since the sail consumes no propellant, it has in principle an unlimited operating time.

The electric sail is raising a lot of interest in space circles, but until now it has been unclear whether its most important parts, i.e. the long, thin metal tethers, can be produced. The research further shows that the cumulative effect of these displacements can be either ultra-smoothening, which may be useful for the surface treatment of surgical tools, or topographic pattern-forming instabilities, which can degrade materials. The outcome depends on the type of material, energetic particle, and irradiation conditions. 

“The team at the University of Helsinki is apparently the first one in the world to use ultrasonic welding to join wires together into a tether,” says the team leader, Professor Edward Hæggström from the Department of Physics.

A single metal wire is not suitable as an ESAIL tether, as micrometeoroids present everywhere in space would soon cut it. Therefore the tether must be manufactured from several wires joined together every centimetre [Figure 1]. In this way, micrometeoroids can cut individual wires without breaking the entire tether.

The tether factory has so far produced ultrasonic welds for one kilometre of aluminium tether

The Electronics Research Laboratory team started studying the production problem four years ago. At the time, the view of international experts in ultrasonic welding was that joining thin wires together was not possible. However, the one-kilometre-long tether produced now, featuring 90,000 ultrasonic welds, shows that the method works and that producing long electric sail tethers is possible.

The wire is produced with a fully automated tether factory, a fine mechanical device under computer control, developed and constructed by the team itself. [Figure 2].The tether factory at the Kumpula Science Campus in Helsinki, Finland, was integrated into a modified commercial ultrasonic welding device. Ultrasonic welding is widely in the electronics industry, but normally it is used for joining a wire to a base. “We have a challenging task, as keeping thin wires repeatedly in the precisely correct position is hard,” says Timo Rauhala who works in the laboratory.

Approximately three metres of tether is currently produced per hour. Its quality is verified optically with a real-time measurement that inspects the connection of every individual joint. In the future, the production speed is to be raised and the weld quality will be assured during the production process.

The products of the tether factory will soon see action in space. The first opportunity will be the ESTCube-1 satellite, an Estonian small satellite to be launched in March 2013. ESTCube-1 will deploya 15-metre long tether in space and measure the ESAIL force it is subjected to. This is ground-breaking as, so far, the theoretically predicted electric sail force has not yet been experimentally measured.

Next in turn will be the Aalto-1 small satellite from the Aalto University, to be launched in 2014, which will deploy a 100-metre long tether.

The deployed tethers are kept straight in space by the centrifugal force, the magnitude of which is five grams in a full-scale electric sail. The wire-to-wire welds of the ESAIL tether produced at the University of Helsinki will tolerate a pull of 10 grams.

More information:

 http://www.electric-sailing.fi  

 http://www.estcube.eu  

 https://wiki.aalto.fi/display/SuomiSAT/Etusivu  

Tether from several wires joined at approximately one-centimetre intervals. Photo by Henri Seppänen & Sergiy Kiprich Figure 1. Tether from several wires joined at approximately one-centimetre intervals. Photo by Henri Seppänen&SergiyKiprich