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  • Salmi, Emma (Helsingin yliopisto, 2015)
    Corrosion is a major global challenge with both economical and technological impacts. The total world-wide costs of corrosion have been evaluated to rise to over 2 000 000 million euros annually. While several methods exist for corrosion protection, atomic layer deposited (ALD) coatings have an advantage in applications where thin, fully conformal, highly precise and well-defined coatings both in composition and thickness are needed. In this work the corrosion protection properties of ALD Al2O3 and Ta2O5 based coatings on low alloy steel were studied. The aim was to increase the general understanding on factors affecting the protective properties and failure mechanisms of the ALD coatings. The protective performance of ALD coatings on steel was improved by focusing on three topics: substrate pre-treatment, optimisation of the ALD coating architecture, and combination of the optimised ALD coatings with layers deposited by other methods. The substrate surface was found to significantly influence the ALD coating performance. Improved protective properties were found on steel samples that were mechanically polished to a lower surface roughness, and efficiently cleaned with H2-Ar plasma in addition to the traditional degreasing with an organic solvent. The smoother surface finish was concluded to be beneficial due to decreased defect formation after the coating deposition upon detachment of loose particles or mechanically fragile sites. The H2-Ar plasma removed organic residues from the steel surface, therefore improving the quality of the first layers of the ALD coating. The performance of the ALD coatings themselves was found to improve when Al2O3 and Ta2O5 were combined to produce Al2O3-Ta2O5 nanolaminate and AlxTayOz mixture coatings. In these coatings Al2O3 provided sealing properties and Ta2O5 the chemical stability, therefore resulting in coatings with better long-term performance than could be achieved with either material alone. Optimisation of the Al2O3-Ta2O5 nanolaminate and AlxTayOz mixture coating architectures further enhanced the protective properties. To further improve the coating-steel interface and to widen the application areas for the ALD based protective coatings, the optimised ALD coating processes were combined with layers deposited with other methods. Firstly, thin filtered cathodic arc sublayers were used to separate the ALD process from the steel surface. This enabled a more precise control of the coating-steel interface and led to improved durability of the ALD coatings. Secondly, pinhole defects in physical vapour and plasma-enhanced chemical vapour deposited hard coatings were sealed with ALD to afford coatings with both good corrosion protection performance and resistance against mechanical wear.
  • Pore, Viljami (Helsingin yliopisto, 2010)
    Photocatalytic TiO2 thin films can be highly useful in many environments and applications. They can be used as self-cleaning coatings on top of glass, tiles and steel to reduce the amount of fouling on these surfaces. Photocatalytic TiO2 surfaces have antimicrobial properties making them potentially useful in hospitals, bathrooms and many other places where microbes may cause problems. TiO2 photocatalysts can also be used to clean contaminated water and air. Photocatalytic oxidation and reduction reactions proceed on TiO2 surfaces under irradiation of UV light meaning that sunlight and even normal indoor lighting can be utilized. In order to improve the photocatalytic properties of TiO2 materials even further, various modification methods have been explored. Doping with elements such as nitrogen, sulfur and fluorine, and preparation of different kinds of composites are typical approaches that have been employed. Photocatalytic TiO2 nanotubes and other nanostructures are gaining interest as well. Atomic Layer Deposition (ALD) is a chemical gas phase thin film deposition method with strong roots in Finland. This unique modification of the common Chemical Vapor Deposition (CVD) method is based on alternate supply of precursor vapors to the substrate which forces the film growth reactions to proceed only on the surface in a highly controlled manner. ALD gives easy and accurate film thickness control, excellent large area uniformity and unparalleled conformality on complex shaped substrates. These characteristics have recently led to several breakthroughs in microelectronics, nanotechnology and many other areas. In this work, the utilization of ALD to prepare photocatalytic TiO2 thin films was studied in detail. Undoped as well as nitrogen, sulfur and fluorine doped TiO2 thin films were prepared and thoroughly characterized. ALD prepared undoped TiO2 films were shown to exhibit good photocatalytic activities. Of the studied dopants, sulfur and fluorine were identified as much better choices than nitrogen. Nanostructured TiO2 photocatalysts were prepared through template directed deposition on various complex shaped substrates by exploiting the good qualities of ALD. A clear enhancement in the photocatalytic activity was achieved with these nanostructures. Several new ALD processes were also developed in this work. TiO2 processes based on two new titanium precursors, Ti(OMe)4 and TiF4, were shown to exhibit saturative ALD-type of growth when water was used as the other precursor. In addition, TiS2 thin films were prepared for the first time by ALD using TiCl4 and H2S as precursors. Ti1-xNbxOy and Ti1-xTaxOy transparent conducting oxide films were prepared successfully by ALD and post-deposition annealing. Highly unusual, explosive crystallization behaviour occurred in these mixed oxides which resulted in anatase crystals with lateral dimensions over 1000 times the film thickness.
  • Pilvi, Tero (Helsingin yliopisto, 2008)
    Thin films of various metal fluorides are suited for optical coatings from infrared (IR) to ultraviolet (UV) range due to their excellent light transmission. In this work, novel metal fluoride processes have been developed for atomic layer deposition (ALD), which is a gas phase thin film deposition method based on alternate saturative surface reactions. Surface controlled self-limiting film growth results in conformal and uniform films. Other strengths of ALD are precise film thickness control, repeatability and dense and pinhole free films. All these make the ALD technique an ideal choice also for depositing metal fluoride thin films. Metal fluoride ALD processes have been largely missing, which is mostly due to a lack of a good fluorine precursor. In this thesis, TiF4 precursor was used for the first time as the fluorine source in ALD for depositing CaF2, MgF2, LaF3 and YF3 thin films. TaF5 was studied as an alternative novel fluorine precursor only for MgF2 thin films. Metal-thd (thd = 2,2,6,6-tetramethyl-3,5-heptanedionato) compounds were applied as the metal precursors. The films were grown at 175 450 °C and they were characterized by various methods. The metal fluoride films grown at higher temperatures had generally lower impurity contents with higher UV light transmittances, but increased roughness caused more scattering losses. The highest transmittances and low refractive indices below 1.4 (at 580 nm) were obtained with MgF2 samples. MgF2 grown from TaF5 precursor showed even better UV light transmittance than MgF2 grown from TiF4. Thus, TaF5 can be considered as a high quality fluorine precursor for depositing metal fluoride thin films. Finally, MgF2 films were applied in fabrication of high reflecting mirrors together with Ta2O5 films for visible region and with LaF3 films for UV region. Another part of the thesis consists of applying already existing ALD processes for novel optical devices. In addition to the high reflecting mirrors, a thin ALD Al2O3 film on top of a silver coating was proven to protect the silver mirror coating from tarnishing. Iridium grid filter prototype for rejecting IR light and Ir-coated micro channel plates for focusing x-rays were successfully fabricated. Finally, Ir-coated Fresnel zone plates were shown to provide the best spatial resolution up to date in scanning x-ray microscopy.
  • Ihanus, Jarkko (Helsingin yliopisto, 2010)
    The light emitted by flat panel displays (FPD) can be generated in many different ways, such as for example alternating current thin film electroluminescence (ACTFEL), liquid crystal display (LCD), light emitting diode (LED), or plasma display panel (PDP) technologies. In this work, the focus was on ACTFEL devices and the goal was to develop new thin film processes for light emitting materials in ACTFEL devices. The films were deposited with the atomic layer deposition (ALD) method, which has been utilized in the manufacturing of ACTFEL displays since the mid-1980s. The ALD method is based on surface-controlled self-terminated reactions and a maximum of one layer of the desired material can be prepared during one deposition cycle. Therefore, the film thickness can be controlled simply by adjusting the number of deposition cycles. In addition, both large areas and deep trench structures can be covered uniformly. During this work, new ALD processes were developed for the following thin film materials: BaS, CuxS, MnS, PbS, SrS, SrSe, SrTe, SrS1-xSex, ZnS, and ZnS1-xSex. In addition, several ACTFEL devices were prepared where the light emitting material was BaS, SrS, SrS1-xSex, ZnS, or ZnS1-xSex thin film that was doped with Ce, Cu, Eu, Mn, or Pb. The sulfoselenide films were made by substituting the elemental selenium for sulfur on the substrate surface during film deposition. In this way, it was possible to replace a maximum of 90% of the sulfur with selenium, and the XRD analyses indicated that the films were solid solutions. The polycrystalline BaS, SrS, and ZnS thin films were deposited at 180-400, 120-460, and 280-500 °C, respectively, and the processes had a wide temperature range where the growth rate of the films was independent of the deposition temperature. The electroluminescence studies showed that the doped sulfoselenide films resulted in low emission intensity. However, the emission intensities and emission colors of the doped SrS, BaS, and ZnS films were comparable with those found in earlier studies. It was also shown that the electro-optical properties of the different ZnS:Mn devices were different as a consequence of different ZnS:Mn processes. Finally, it was concluded that because the higher deposition temperature seemed to result in a higher emission intensity, the thermal stability of the reactants has a significant role when the light emitting materials of ACTFEL devices are deposited with the ALD method.
  • Blanquart, Timothee (Helsingin yliopisto, 2013)
    The atomic layer deposition process (ALD) is an alternative to the chemical vapour deposition (CVD) method that is universally appreciated for its unique advantages such as excellent repeatability, conformity and thickness control at the atomic level. ALD precursor chemistry has mainly been based on homoleptic compounds such as, but not limited to, metal halides, alkylamides or alkoxides. However, these precursors have drawbacks such as possible halide contamination and low thermal stabilities with respect to the alkylamides and alkoxides. Consequently, heteroleptic precursors were investigated as alternatives to the existing homoleptic counterparts, which have led to the development of several advantageous processes. Nevertheless, no thematic review dedicated to heteroleptic precursor and their properties exists and it seems that no coherent strategy has been adopted for the development of heteroleptic precursors. This thesis gives a brief description of ALD and presents studies on the deposition of thin films of groups 4 and 5 metal oxide films using ALD. A description of the general ALD properties of homoleptic precursors in addition to a review on the thermal ALD of groups 4 and 5 metal oxides from heteroleptic precursors is provided. Trends in the properties of heteroleptic ALD precursors based on a literature review and new experimental data are discussed. Several novel heteroleptic compounds were evaluated for the ALD of thin films of TiO2, ZrO2, Nb2O5 and Ta2O5. The characteristics of these processes were evaluated and the film properties of these oxides were investigated by means of various characterization approaches. The effects of oxygen source, water or ozone, on the film growth characteristics and properties of ZrO2, Nb2O5, and Ta2O5, were also investigated. Mixed alkoxide-alkylamide and alkoxide-amidinate titanium compounds are liquid at room temperature. They are highly volatile, have excellent reactivity towards water and have high thermal stability. The deposited films exhibit high purity and conformability on high aspect ratio substrates. The growth of thin films of ZrO2 from a heteroleptic alkylamide-guanidinate zirconium precursor was notable in that the films grew in the high permittivity cubic phase and the ozone-based process had a high growth rate. Thin films of VOx were deposited from the homoleptic vanadium alkylamide precursor,vanadium tetraethylmethylamide. It was found that the structure and oxidation state of the films could be tuned from an amorphous mixture of VO2 and V2O5 to crystalline VO2 or V2O5. This was accomplished by simply exposing the films to heat treatment in different atmospheres, namely air or N2. Finally, alkylamide-imide precursors were investigated for the ALD of Nb2O5 and Ta2O5 thin films. These precursors are liquid at room temperature, and exhibited high thermal stabilities compared with the earlier known niobium and tantalum ALD precursors. The alkylamide-imide precursors studied had high volatility and excellent reactivity towards both water and ozone. The deposited films were smooth, uniform, and contained only low amounts of impurities.
  • Vehkamäki, Marko (Helsingin yliopisto, 2007)
    Atomic layer deposition (ALD) is a method for thin film deposition which has been extensively studied for binary oxide thin film growth. Studies on multicomponent oxide growth by ALD remain relatively few owing to the increased number of factors that come into play when more than one metal is employed. More metal precursors are required, and the surface may change significantly during successive stages of the growth. Multicomponent oxide thin films can be prepared in a well-controlled way as long as the same principle that makes binary oxide ALD work so well is followed for each constituent element: in short, the film growth has to be self-limiting. ALD of various multicomponent oxides was studied. SrTiO3, BaTiO3, Ba(1-x)SrxTiO3 (BST), SrTa2O6, Bi4Ti3O12, BiTaO4 and SrBi2Ta2O9 (SBT) thin films were prepared, many of them for the first time by ALD. Chemistries of the binary oxides are shown to influence the processing of their multicomponent counterparts. The compatibility of precursor volatilities, thermal stabilities and reactivities is essential for multicomponent oxide ALD, but it should be noted that the main reactive species, the growing film itself, must also be compatible with self-limiting growth chemistry. In the cases of BaO and Bi2O3 the growth of the binary oxide was very difficult, but the presence of Ti or Ta in the growing film made self-limiting growth possible. The application of the deposited films as dielectric and ferroelectric materials was studied. Post-deposition annealing treatments in different atmospheres were used to achieve the desired crystalline phase or, more generally, to improve electrical properties. Electrode materials strongly influenced the leakage current densities in the prepared metal insulator metal (MIM) capacitors. Film permittivities above 100 and leakage current densities below 110-7 A/cm2 were achieved with several of the materials.
  • Hämäläinen, Jani (Helsingin yliopisto, 2013)
    Atomic layer deposition (ALD) is a chemical gas phase deposition method to grow thin films which are highly uniform and conformal over large and complex substrate areas. Film growth in ALD is precise, remarkably repeatable, and combined with unparalleled control of the film thickness. These inherent properties make ALD an attractive method to deposit thin films for advanced technological applications such as microelectronics and nanotechnology. One material group in ALD which has matured in ten years and proven to be of wide technological importance is noble metals. The purpose of this study was to investigate noble metal oxide film growth by ALD. The ALD of noble metal oxides has been very limited compared to the noble metal growth. Another aim was to examine noble metal film deposition at temperatures lower than required in the earlier ALD noble metal processes. In addition, the selection of noble metals that can be grown by ALD was expanded with osmium. The results of the study showed that oxides of iridium, rhodium, platinum, and palladium can be deposited from the common noble metal precursors using ozone as the reactant at temperatures below 200 °C. The development of ozone-based ALD noble metal oxide processes led further on to the low temperature deposition of noble metals by adding a reductive molecular hydrogen step after every oxidative ozone step. The noble metal deposition via noble metal oxide growth was achieved at lower temperatures than required with the common oxygen-based ALD noble metal processes. Film growth rates, resistivities, purities, and surface roughnesses resulting from the studied noble metal oxide and noble metal processes were reasonable. The processes showed some shortcomings but offer an alternative thermal ALD pathway to deposit noble metals and noble metal oxides compared to the oxygen-based ALD processes. Keywords: atomic layer deposition, ALD, noble metal oxide, noble metal, thin film, ozone
  • Aaltonen, Titta (Helsingin yliopisto, 2005)
  • Tolvanen, Antti (Helsingin yliopisto, 2010)
    Carbon nanotubes, seamless cylinders made from carbon atoms, have outstanding characteristics: inherent nano-size, record-high Young’s modulus, high thermal stability and chemical inertness. They also have extraordinary electronic properties: in addition to extremely high conductance, they can be both metals and semiconductors without any external doping, just due to minute changes in the arrangements of atoms. As traditional silicon-based devices are reaching the level of miniaturisation where leakage currents become a problem, these properties make nanotubes a promising material for applications in nanoelectronics. However, several obstacles must be overcome for the development of nanotube-based nanoelectronics. One of them is the ability to modify locally the electronic structure of carbon nanotubes and create reliable interconnects between nanotubes and metal contacts which likely can be used for integration of the nanotubes in macroscopic electronic devices. In this thesis, the possibility of using ion and electron irradiation as a tool to introduce defects in nanotubes in a controllable manner and to achieve these goals is explored. Defects are known to modify the electronic properties of carbon nanotubes. Some defects are always present in pristine nanotubes, and naturally are introduced during irradiation. Obviously, their density can be controlled by irradiation dose. Since different types of defects have very different effects on the conductivity, knowledge of their abundance as induced by ion irradiation is central for controlling the conductivity. In this thesis, the response of single walled carbon nanotubes to ion irradiation is studied. It is shown that, indeed, by energy selective irradiation the conductance can be controlled. Not only the conductivity, but the local electronic structure of single walled carbon nanotubes can be changed by the defects. The presented studies show a variety of changes in the electronic structures of semiconducting single walled nanotubes, varying from individual new states in the band gap to changes in the band gap width. The extensive simulation results for various types of defect make it possible to unequivocally identify defects in single walled carbon nanotubes by combining electronic structure calculations and scanning tunneling spectroscopy, offering a reference data for a wide scientific community of researchers studying nanotubes with surface probe microscopy methods. In electronics applications, carbon nanotubes have to be interconnected to the macroscopic world via metal contacts. Interactions between the nanotubes and metal particles are also essential for nanotube synthesis, as single walled nanotubes are always grown from metal catalyst particles. In this thesis, both growth and creation of nanotube-metal nanoparticle interconnects driven by electron irradiation is studied. Surface curvature and the size of metal nanoparticles is demonstrated to determine the local carbon solubility in these particles. As for nanotube-metal contacts, previous experiments have proved the possibility to create junctions between carbon nanotubes and metal nanoparticles under irradiation in a transmission electron microscope. In this thesis, the microscopic mechanism of junction formation is studied by atomistic simulations carried out at various levels of sophistication. It is shown that structural defects created by the electron beam and efficient reconstruction of the nanotube atomic network, inherently related to the nanometer size and quasi-one dimensional structure of nanotubes, are the driving force for junction formation. Thus, the results of this thesis not only address practical aspects of irradiation-mediated engineering of nanosystems, but also contribute to our understanding of the behaviour of point defects in low-dimensional nanoscale materials.
  • Ullah, Mohammad Wali (Helsingin yliopisto, 2014)
    Gallium nitride (GaN) has emerged as one of the most important semiconductors in modern technology. GaN-based device technology was mainly pushed forward by invention of p-type doping and the successful fabrication of light emitting diodes (LEDs) and laser diodes (LDs). Intensive studies in the last 20 years on GaN have significantly advanced the understanding of the properties and have expanded the range of practical applications. Beside basic lighting, current applications of GaN include high-power and high temperature electronics, microwave, optoelectronic devices, and so on. The successful production of optical devices demands efficient tuning of charge carrier lifetime where defect engineering plays a vital role. During growth, varying the level of recombination centers is difficult, whereas ion irradiation can do this job efficiently on a final product. On the other hand, during doping, undesirable defects can also be produced and epitaxial GaN is known to have a highly defective structure. Thus, having both positive and negative aspects, it is very important to have a detailed understanding of irradiation-induced defects. To explain experimental findings, atomic level understanding is necessary, but it is not always possible to have an atomistic view of defect dynamics in experiments. Some damage build-up studies by single ions have been reported in the literature, but not many by molecular ions. In this thesis, the irradiation of GaN by single and molecular ions by the means of atomistic simulations was studied. Detailed analysis mainly of what kind of defects, their distribution, reason of defect formation and time evolution have been studied and compared with experiments. The irradiation response of both bulk and nano-structured GaN system were studied. For bulk studies, all projectiles were irradiated having the same energy per mass. The damage by molecular ions showed strong dynamic annealing. No non-linearity had been observed in the total number of point defects between single and molecular ions. On the other hand, molecular ions produce larger clusters of point defects than single ions. These large defect clusters can be one of the mechanisms of the experimentally observed faster carrier decay time for molecular projectiles. Defects were mostly concentrated at the surface and near surface regions, which is also evident from experiments. Comparison between a similar mass single ion and a molecular ion show that a single ion produced more defect clusters than molecular ions. This suggests that heavy ions are even more efficient than similar mass cluster ions to quench the carrier lifetime. Irradiation of a GaN nanowire (NW) reveals that a large surface to volume ratio promotes high density of surface defects. The experimentally observed yellow luminescence band is correlated with these defect induced surface states. Irradiation induced defects also expand the lattice parameters of the NW.
  • Lasa Esquisabel, Ane (Helsingin yliopisto, 2014)
    The world's energy demand and harmful green-house gas emissions are continuously increasing, while the fossil fuel reservoir may soon end. Currently, there is no clear alternative to the traditional energy production methods for a safe and clean future. Fusion could be part of the solution offering a green-house gas free, virtually endless, safe and large scale energy production. A major challenge for fusion is, however, to produce more energy than needed to achieve and maintain the fusion reaction. The most feasible fusion reaction is based on two hydrogen isotopes: deuterium and tritium, which fuse to produce a helium atom and a neutron. For these atoms to fuse, they must overcome the repulsive interaction between them, requiring extreme temperatures. Thus, the particles ionize forming gas plasma. On Earth, this condition can only be met by isolating the plasma from its environment, for instance, by using closed magnetic fields to form a torus-like shaped plasma, also known as tokamak. However, the plasma particles will interact with the reactor walls as their confinement is never perfect, the exhausted plasma must leave the reactor and impurities are introduced in the plasma boundary to control its characteristics. The plasma-wall interactions are especially intense at the divertor, where the plasma is designed to touch the wall. Understanding these processes is essential to develop safe, long-lasting materials and to avoid contaminating the plasma fuel. The main candidates as first wall materials in future fusion reactors are beryllium for the main wall, and tungsten and carbon for the divertor. Also, the materials may mix due to wall erosion, transport of the eroded particles and their deposition in a new location. Plasma-wall interactions can be studied in current experimental reactors or in linear plasma devices. However, this work is often insufficient to understand the underlying mechanisms. Further, the effects of plasma-wall interactions in materials develop in a wide range of time and length scales. Multi scale modelling is a tool that allows to overcome these challenges, improving the predictions for future fusion reactors. In this thesis, the plasma wall interactions taking place in a fusion reactors divertor have been studied by computational means. The interaction of pure and mixed divertor materials, with plasma and impurity particles were modelled. The work was mainly based on atomistic scale calculations, and a Kinetic Monte Carlo algorithm has also been developed to extend the results to macroscopic scales, enabling a direct comparison with experiments. First, deuterium irradiation of various W-C composites has been modelled, focusing on deuterium implantation, variations of the substrate composition and C erosion mechanisms. Carbon was preferentially eroded, varying the substrate's composition throughout the irradiation. The presence of carbon also affected the D implantation characteristics. As carbon became less likely to be an ITER first wall material, the present work focused on the tungsten-beryllium-deuterium system. The tungsten-beryllium mixing showed a strong dependence on irradiation energy and angle. Further, the presence of Be led to higher fuel implantation and W erosion was suppressed by mixed layer formation. The obtained yields were compared to Binary Collision Approximation results, in order to improve the description of the latter method. Furthermore, an unexpected and possibly harmful phenomenon has been addressed in this thesis: porous nano-morphology formation in tungsten by helium plasma exposure. First, the main characteristics and active mechanisms in the system were identified by atomistic simulations. Then, the porous morphology growth was modelled by implementing these processes in a Kinetic Monte Carlo code, resulting in rates that agreed with experimental findings. A morphology growth model was derived where the time dependence is driven by the evolution of the surface roughness, which is a stochastic process and thus evolves as the square root of time.
  • Parviainen, Stefan (Helsingin yliopisto, 2014)
    Modern society runs largely on electricity, and where there is electricity there are electric fields. As the boundaries of technology are pushed forward, stronger and stronger electric fields are either required, or appear due to unwanted effects. Examples of such applications, where very high electric fields are utilised include particle accelerators and atom probes. To further be able to improve on such techniques, it is necessary to gain a good understanding of the processes that are involved. Because it is often difficult, if not impossible, to observe these processes with high resolution in experiments, one needs to consider the use of atomistic simulations instead. This thesis provides an extension to classical molecular dynamics by describing an implementation where several electronic effects are considered when dealing with metal surfaces under high electric fields. These effects include the charging of surface atoms, field electron emission and the resulting resistive heating, as well as field evaporation of both neutral and charged atoms. In addition to the implementation details, the thesis also contains a brief background of the physics involved in these processes. Using the implementation, it is observed that a surface protrusion may grow on an initially flat surface in the presence of a near-surface void when a strong external electric field is applied. The growth is very rapid, resulting finally catastrophic breakage. This mechanism may explain the appearance of field emitters on otherwise pristine samples, and the instability of measured field emission currents. Simulations also reveal that high aspect ratio protrusions are subject to Rayleigh instability due to the temperature rise caused by field electron emission currents. As a result a large fraction of the protrusion can break off. The model also allows for the study of the trajectories of field evaporated ions from a surface, as they are accelerated in the electric field. From the simulations we see that even changes in the surface morphology on the atomic scale may result in aberrations in atom probe tomography experiments.
  • Leino, Aleksi (Helsingin yliopisto, 2015)
    Ions in the keV energy range are regularly used in the semiconductor industry for device fabrication. Irradiation with ions of higher energies can also induce favorable structural changes in the irradiated samples. Among these, irradiation effects of the so-called swift heavy ions (SHIs, heavy ions with specific energies in the 1 MeV / amu range) in electrically insulating materials are particularly interesting. Despite the wide range of existing applications (filters, printed circuit boards and geological dating) and application potential (fuel cells, cell mimicking membranes) of SHI irradiation, the mechanisms by which SHIs interact with insulators are still under debate. Modelling of SHIs is a very challenging task as, contrary to ions with lower energies, they mostly interact with electrons, inducing lots of electronic excitations. Incorporating the latter with atomistic dynamics is especially difficult in insulators, and the methods have not yet been fully established. SHIs can induce a cylindrical region of structural transformation known as an ion track. In crystalline silicon dioxide, a track consists of an amorphized region that is typically several microns long and has a radius of less than ten nanometers. Furthermore, it was recently found out that SHI irradiation can be used to induce a shape transformation in metal nanoclusters (NCs) that are embedded in amorphous silicon dioxide. Spherical NCs (radius 1-50 nm) elongate along the ion beam direction and are shaped into nanorods or prolate spheroids. The phenomenon can be exploited to produce large arrays of equally aligned nanoclusters within a solid substrate, which is difficult to achieve otherwise. In this thesis, ion track formation and the elongation of gold nanoclusters in silicon dioxide are studied using so called two-temperature molecular dynamics simulations. The structure of the tracks is studied and a mechanism is proposed for the nanoparticle elongation effect. The work presented here is a step towards the understanding of SHI related effects in a broader range of insulating materials for the SHI based applications.
  • Avchaciov, Konstantin (2015)
    Rapid technological development motivates the research community to find out new processing technologies for producing materials with unique properties. Production of new materials or improving the existing ones requires development of new methods, which sometimes involves processing under far-from-equilibrium conditions. Usually underlying physical processes, taking place during such processing, are not well known. Understanding them is the key to the enhancement of processing methods. This thesis covers only a tiny part of a huge number of problems existing in the material science. The studies done here could be useful for solving the problem of metallic glass brittleness and for further development of the ion beam technology for controlling magnetic properties of thin films. Discussion on the pathway of the self-propagating high-temperature synthesis in multilayer metallic films was also included in the thesis We studied the effect of ion irradiation on Cu64Zr36 metallic glass by means of molecular dynamics (MD). Low-energy ion irradiation of metallic glass produces damage zones, where the local structure of glass differs from the structure of pristine glass. These changes are independent on ion energy and flux due to the possibility of partial recovery during cascade relaxation. However, the size of the damage zones could be controlled by varying the dose of ions. This makes possible the production of either isolated damaged regions or homogeneously damaged glass. We showed that mechanical properties of the glass strongly depend on the local structure. In damage zones, the specific arrangement of the structural units is destroyed. This promotes the formation of many shear transformation zones in the damaged area, when a mechanical load is applied. It results in enhanced plasticity of the irradiated glass. Our research indicates that ion irradiation could be used for the softening of brittle metallic glasses for preventing formation of cracks in them. Atomistic studies of the reaction pathway in the multilayer Ru/Al films were done to interpret the results from X-ray diffraction measurements. The MD simulations covered the first nanoseconds; at this timescale, no information about the reaction pathway could be obtained from the experimental methods. The MD simulations demonstrated simultaneous Ru dissolution into liquid Al and $B2$ RuAl phase nucleation at the Ru/Al interface. Our results agreed well with experimental observations, and, moreover, helped to refine understanding of the X-Ray diffractograms. We also studied structural modifications of Pt/Co/Pt trilayer films, which could explain appearance of the perpendicular magnetic anisotropy (PMA) after irradiation with Ga+ ions. Molecular dynamics and Monte Carlo (MD-MC) simulations were performed to find changes in the long range order and short range order, associated with ion irradiation. The results of our simulations showed that the separated metal layers containing Pt and Co were intermixed and formed the new solid solution. This solution was chemically disordered at the nanosecond time scale. With combined MD-MC simulations, we showed that the transition between chemically ordered and disordered phases was possible. The fractions of the ordered phases grew monotonously with an ion dose due to the ongoing process of ion-beam mixing. However, assuming the correlation between the magnetic properties of the film and the fraction of the chemically ordered CoPt phase in it, this result cannot explain the disappearance of PMA at the high doses. The strain analysis showed that the Co layer exhibited tensile strain in the lateral direction at the moderate doses, where the PMA was detected. We explained the appearance of strains by changing of the thickness of the hcp-Co layer right inside the trilayer Pt/Co/Pt structure, which was possible due to its partial transformation to the fcc structure. Below the critical size of the hcp layer, the strain inside it appears, which, in our opinion, could cause the appearance of PMA at the moderate doses and the disappearance of PMA at the high doses.