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  • Hildén, Timo (Helsingin yliopisto, 2015)
    Gas Electron Multiplier (GEM) detectors are special of position sensitive gas filled detectors used in several particle physics experiments. They are capable of sub- millimeter spatial resolution and energy resolution (FWHM) of the order of 20%. GEM detectors can operate with rates up to 50 kHz/mm2, withstand radiation excellently and can be manufactured up to square meter sizes. This thesis describes the Quality Assurance (QA) methods used in the assembly of 50 GEM detectors for the TOTEM T2 telescope at the LHC at CERN. Further development of optical QA methods used in T2 detector assembly lead into development of a unique large-area scanning system capable of sub-µm resolution. The system, its capability and the software used in the analysis of the scans are described in detail. A correlation was found between one of the main characteristics of the detector, the gas gain, and the results of the optical QA method. It was shown, that a qualitative estimation of the gain can be made based on accurate optical measurement of the microscopic features of the detector components. Ability to predict the performance of individual components of the detectors is extremely useful in large scale production of GEM based detectors.
  • Ihalainen, Toni (Helsingin yliopisto, 2016)
    Quality control methods and test objects were developed and used for structural magnetic resonance imaging (MRI), functional MRI (fMRI) and diffusion-weighted imaging (DWI). Emphasis was put on methods that allowed objective quality control for organizations that use several MRI systems from different vendors, which had different field strengths. Notable increases in the numbers of MRI studies and novel MRI systems, fast development of MRI technology, and international discussion about the quality and safety of medical imaging have motivated the development of objective, quantitative and time-efficient methods for quality control. The quality control methods need to be up to date with the most modern MRI methods, including parallel imaging, parallel transmit technology, and new diffusion-weighted sequences. The methods need to be appropriate to those organizations that use MRI for quantitative measurements, or for the participation in multicenter studies. Two different test object methods for structural MRI were evaluated in a multi-unit medical imaging organization, these were: the Eurospin method and the American College of Radiology (ACR) method. The Eurospin method was originally developed as a part of European Concerted Action, and five standardized test objects were used to create a quality control protocol for six MRI systems. Automatic software was written for image analysis. In contrast, a single multi-purpose test object was used for the ACR method, and image quality for both standard and clinical imaging protocols were measured for 11 MRI systems. A previously published method for fMRI quality control was applied to the evaluation of 5 MRI systems and was extended for simultaneous electroencephalography (EEG) and fMRI (EEG fMRI). The test object results were compared with human data that were obtained from two healthy volunteers. A body-diameter test object was constructed for DWI testing, and apparent diffusion coefficient (ADC) values and level of artifacts were measured using conventional and evolving DWI methods. The majority of the measured MRI systems operated at an acceptable level, when compared with published recommended values for structural and functional MRI. In general, the measurements were repeatable. The study that used the test object revealed information about the extent of superficial artifacts (15 mm) and the magnitude of signal-to-noise ratio (SNR) reduction (15%) of the simultaneous EEG fMRI images. The observations were in accordance with the data of healthy human volunteers. The agreement between the ADC values for different methods used in DWI was generally good, although differences of up to 0.1 x10^-3 mm^2/s were observed between different acquisition options and different field strengths, and along the slice direction. Readout-segmented echo-planar imaging (EPI) and zoomed EPI in addition to efficient use of the parallel transmit technology resulted in lower levels of artifacts than the conventional methods. Other findings included geometric distortions at the edges of MRI system field-of-view, minor instability of image center-of-mass in fMRI, and an amplifier difference that affected the EEG signal of EEG fMRI. The findings showed that although the majority of the results were within acceptable limits, MRI quality control was capable of detecting inferior image quality and revealing information that supported clinical imaging. A comparison between the different systems and also with international reference values was feasible with the reported limitations. Automated analysis methods were successfully developed and applied in this study. The possible future direction of MRI quality control would be the further development of its relevance for clinical imaging.
  • Forsman, Pia (Helsingin yliopisto, 2008)
    This thesis focuses on the issue of testing sleepiness quantitatively. The issue is relevant to policymakers concerned with traffic- and occupational safety; such testing provides a tool for safety legislation and -surveillance. The findings of this thesis provide guidelines for a posturographic sleepiness tester. Sleepiness ensuing from staying awake merely 17 h impairs our performance as much as the legally proscribed blood alcohol concentration 0.5 does. Hence, sleepiness is a major risk factor in transportation and occupational accidents. The lack of convenient, commercial sleepiness tests precludes testing impending sleepiness levels contrary to simply breath testing for alcohol intoxication. Posturography is a potential sleepiness test, since clinical diurnal balance testing suggests the hypothesis that time awake could be posturographically estimable. Relying on this hypothesis this thesis examines posturographic sleepiness testing for instrumentation purposes. Empirical results from 63 subjects for whom we tested balance with a force platform during wakefulness for maximum 36 h show that sustained wakefulness impairs balance. The results show that time awake is posturographically estimable with 88% accuracy and 97% precision which validates our hypothesis. Results also show that balance scores tested at 13:30 hours serve as a threshold to detect excessive sleepiness. Analytical results show that the test length has a marked effect on estimation accuracy: 18 s tests suffice to identify sleepiness related balance changes, but trades off some of the accuracy achieved with 30 s tests. The procedure to estimate time awake relies on equating the subject s test score to a reference table (comprising balance scores tested during sustained wakefulness, regressed against time awake). Empirical results showed that sustained wakefulness explains 60% of the diurnal balance variations, whereas the time of day explains 40% of the balance variations. The latter fact implies that time awake estimations also must rely on knowing the local times of both test and reference scores.
  • Salonen, J Sakari (Helsingin yliopisto, 2012)
    Palaeoclimatic reconstructions from fossil proxies have provided important insights into the natural variability of climate in the late Quaternary. However, major challenges remain in ensuring the robustness of these reconstructions. Multiple factors may introduce variability and biases into the palaeoclimatic estimates. For example, quantitative reconstructions use diverse modern calibration data-sets, and a wide variety of numerical calibration methods. While the choice of calibration data-set and calibration method may significantly influence the reconstructions, the comparison and analysis of these data-sets and methods have received relatively little attention. Further challenges are presented by the validation of the prepared reconstructions and the identification of climatic variables which can be robustly reconstructed from a given proxy. In this work, summer temperature reconstructions are prepared based on late-Quaternary pollen sequences from northern Finland and northern Russia, covering the Holocene and the early part of the last glacial period (Marine Isotope Stages 5d c). The major aim of this work is to validate these reconstructions and to identify sources of bias in them. Reconstructions are prepared using a number of different calibration methods and calibration sets, to analyse the between-reconstruction variability introduced by the choice of calibration method and calibration set. In addition, novel regression tree methods are used to test the ecological significance of different climatic factors, with the aim of identifying parameters which could feasibly be reconstructed. In the results, it is found that the choice of calibration method, calibration data-set, and fossil pollen sequence can all significantly affect the reconstruction. The problems in choosing calibration data are especially acute in pre-Holocene reconstructions, as it is difficult to find representative calibration data for reconstructions from non-analogue palaeoclimates which become increasingly common in the more distant past. First-order trends in the reconstructed palaeoclimates are found to be relatively robust. However, the degree of between-reconstruction variability stresses the importance of independent validation, and suggests that ensemble reconstructions using different methods and proxies should be increasingly relied on. The analysis of climatic response in northern European modern pollen samples by regression trees suggests secondary climatic determinants such as winter temperature and continentality to have major ecological influence, in addition to summer temperature which has been the most commonly reconstructed variable in palaeoclimatic studies. This suggests the potential to reconstruct the secondary parameters from fossil pollen. However, validating the robustness of secondary-parameter reconstructions remains a major challenge for future studies.
  • Wang, Cong (Helsingin yliopisto, 2010)
    This thesis studies the intermolecular interactions in (i) boron-nitrogen based systems for hydrogen splitting and storage, (ii) endohedral complexes, A@C60, and (iii) aurophilic dimers. We first present an introduction of intermolecular interactions. The theoretical background is then described. The research results are summarized in the following sections. In the boron-nitrogen systems, the electrostatic interaction is found to be the leading contribution, as 'Coulomb Pays for Heitler and London' (CHL). For the endohedral complex, the intermolecular interaction is formulated by a one-center expansion of the Coulomb operator 1/rab. For the aurophilic attraction between two C2v monomers, a London-type formula was derived by fully accounting for the anisotropy and point-group symmetry of the monomers.
  • Kurtén, Theo (Helsingin yliopisto, 2007)
    Nucleation is the first step of the process by which gas molecules in the atmosphere condense to form liquid or solid particles. Despite the importance of atmospheric new-particle formation for both climate and health-related issues, little information exists on its precise molecular-level mechanisms. In this thesis, potential nucleation mechanisms involving sulfuric acid together with either water and ammonia or reactive biogenic molecules are studied using quantum chemical methods. Quantum chemistry calculations are based on the numerical solution of Schrödinger's equation for a system of atoms and electrons subject to various sets of approximations, the precise details of which give rise to a large number of model chemistries. A comparison of several different model chemistries indicates that the computational method must be chosen with care if accurate results for sulfuric acid - water - ammonia clusters are desired. Specifically, binding energies are incorrectly predicted by some popular density functionals, and vibrational anharmonicity must be accounted for if quantitatively reliable formation free energies are desired. The calculations reported in this thesis show that a combination of different high-level energy corrections and advanced thermochemical analysis can quantitatively replicate experimental results concerning the hydration of sulfuric acid. The role of ammonia in sulfuric acid - water nucleation was revealed by a series of calculations on molecular clusters of increasing size with respect to all three co-ordinates; sulfuric acid, water and ammonia. As indicated by experimental measurements, ammonia significantly assists the growth of clusters in the sulfuric acid - co-ordinate. The calculations presented in this thesis predict that in atmospheric conditions, this effect becomes important as the number of acid molecules increases from two to three. On the other hand, small molecular clusters are unlikely to contain more than one ammonia molecule per sulfuric acid. This implies that the average NH3:H2SO4 mole ratio of small molecular clusters in atmospheric conditions is likely to be between 1:3 and 1:1. Calculations on charged clusters confirm the experimental result that the HSO4- ion is much more strongly hydrated than neutral sulfuric acid. Preliminary calculations on HSO4- NH3 clusters indicate that ammonia is likely to play at most a minor role in ion-induced nucleation in the sulfuric acid - water system. Calculations of thermodynamic and kinetic parameters for the reaction of stabilized Criegee Intermediates with sulfuric acid demonstrate that quantum chemistry is a powerful tool for investigating chemically complicated nucleation mechanisms. The calculations indicate that if the biogenic Criegee Intermediates have sufficiently long lifetimes in atmospheric conditions, the studied reaction may be an important source of nucleation precursors.
  • Calsamiglia, John (Helsingin yliopisto, 2001)
  • Havukainen, Martti (Helsingin yliopisto, 1999)
  • Saxell, Sami (Helsingin yliopisto, 2009)
    Arguments arising from quantum mechanics and gravitation theory as well as from string theory, indicate that the description of space-time as a continuous manifold is not adequate at very short distances. An important candidate for the description of space-time at such scales is provided by noncommutative space-time where the coordinates are promoted to noncommuting operators. Thus, the study of quantum field theory in noncommutative space-time provides an interesting interface where ordinary field theoretic tools can be used to study the properties of quantum spacetime. The three original publications in this thesis encompass various aspects in the still developing area of noncommutative quantum field theory, ranging from fundamental concepts to model building. One of the key features of noncommutative space-time is the apparent loss of Lorentz invariance that has been addressed in different ways in the literature. One recently developed approach is to eliminate the Lorentz violating effects by integrating over the parameter of noncommutativity. Fundamental properties of such theories are investigated in this thesis. Another issue addressed is model building, which is difficult in the noncommutative setting due to severe restrictions on the possible gauge symmetries imposed by the noncommutativity of the space-time. Possible ways to relieve these restrictions are investigated and applied and a noncommutative version of the Minimal Supersymmetric Standard Model is presented. While putting the results obtained in the three original publications into their proper context, the introductory part of this thesis aims to provide an overview of the present situation in the field.
  • Soultanis, Elefterios (Helsingin yliopisto, 2016)
    This dissertation studies classical questions in the field of geometric analysis in the context of metric spaces. The dissertation is comprised of three research articles. The first is on the connection of quasiconformal maps and the quasihyperbolic metric. The remaining two concern notions of homotopy classes of Sobolev type maps between metric spaces, comparison with the manifold case, and the existence of minimizers of a p-energy in these homotopy classes. The unifying theme of all three articles is analysis on metric spaces. That is, all three papers deal with questions concerning maps between metric spaces. The particular type of metric spaces involved is generally referred to as PI-spaces. PI-spaces satisfy conditions allowing one to extend a large part of classical first order calculus, such as the theory of Sobolev maps and, á posteriori, differentiability of Lipschitz functions.
  • Holmström, Eero (Helsingin yliopisto, 2012)
    Understanding radiation effects in silicon (Si) is of great technological importance. The material, being the basis of modern semiconductor electronics and photonics, is subjected to radiation already at the processing stage, and in many applications throughout the lifetime of the manufactured component. Despite decades of research, many fundamental questions on the subject are still not satisfactorily answered, and new ones arise constantly as device fabrication shifts towards the nanoscale. In this study, methods of computational physics are harnessed to tackle basic questions on the radiation response of bulk and nanostructured Si systems, as well as to explain atomic-scale phenomena underlying existing experimental results. Empirical potentials and quantum mechanical models are coupled with molecular dynamics simulations to model the response of Si to irradiation and to characterize the created crystal damage. The threshold displacement energy, i.e., the smallest recoil energy required to create a lattice defect, is determined in Si bulk and nanowires, in the latter system also as a function of mechanical strain. It is found that commonly used values for this quantity are drastically underestimated. Strain on the nanowire causes the threshold energy to drop, with an effect on defect production that is significantly higher than in an another nanostructure with similar dimensions, the carbon nanotube. Simulating ion irradiation of Si nanowires reveals that the large surface area to volume ratio of the nanostructure causes up to a three-fold enhancement in defect production as compared to bulk Si. Amorphous defect clusters created by energetic neutron bombardment are predicted, on the basis of their electronic structure and abundance, to cause a deleterious phenomenon called type inversion in Si strip detectors in high-energy physics experiments. The thinning of Si lamellae using a focused ion beam is studied in conjunction with experiment to unravel the cause for the failure of the thinning method for very thin samples. Simulations predict a mechanism of erosion of the structure which is observed as catastrophic shrinkage of the sample in experiment. The results of the thesis contribute to the understanding of fundamental questions of radiation effects in Si as well as provide explanations to known experimental conundrums. At the same time, the results unambiguously indicate that further experimental testing is needed in order to ultimately evaluate the accuracy of the theoretical predictions.
  • Järvi, Tommi (Helsingin yliopisto, 2009)
    Nanotechnology applications are entering the market in increasing numbers, nanoparticles being among the main classes of materials used. Particles can be used, e.g., for catalysing chemical reactions, such as is done in car exhaust catalysts today. They can also modify the optical and electronic properties of materials or be used as building blocks for thin film coatings on a variety of surfaces. To develop materials for specific applications, an intricate control of the particle properties, structure, size and shape is required. All these depend on a multitude of factors from methods of synthesis and deposition to post-processing. This thesis addresses the control of nanoparticle structure by low-energy cluster beam deposition and post-synthesis ion irradiation. Cluster deposition in high vacuum offers a method for obtaining precisely controlled cluster-assembled materials with minimal contamination. Due to the clusters small size, however, the cluster-surface interaction may drastically change the cluster properties on deposition. In this thesis, the deposition process of metal and alloy clusters on metallic surfaces is modelled using molecular dynamics simulations, and the mechanisms influencing cluster structure are identified. Two mechanisms, mechanical melting upon deposition and thermally activated dislocation motion, are shown to determine whether a deposited cluster will align epitaxially with its support. The semiconductor industry has used ion irradiation as a tool to modify material properties for decades. Irradiation can be used for doping, patterning surfaces, and inducing chemical ordering in alloys, just to give a few examples. The irradiation response of nanoparticles has, however, remained an almost uncharted territory. Although irradiation effects in nanoparticles embedded inside solid matrices have been studied, almost no work has been done on supported particles. In this thesis, the response of supported nanoparticles is studied systematically for heavy and light ion irradiation. The processes leading to damage production are identified and models are developed for both types of irradiation. In recent experiments, helium irradiation has been shown to induce a phase transformation from multiply twinned to single-crystalline nanoparticles in bimetallic alloys, but the nature of the transition has remained unknown. The alloys for which the effect has been observed are CuAu and FePt. It is shown in this thesis that transient amorphization leads to the observed transition and that while CuAu and FePt do not amorphize upon irradiation in bulk or as thin films, they readily do so as nanoparticles. This is the first time such an effect is demonstrated with supported particles, not embedded in a matrix where mixing is always an issue. An understanding of the above physical processes is essential, if nanoparticles are to be used in applications in an optimal way. This thesis clarifies the mechanisms which control particle morphology, and paves way for the synthesis of nanostructured materials tailored for specific applications.
  • Jansson, Ville (Helsingin yliopisto, 2013)
    Neutron irradiation induces structural nano-scale changes in steels that in the long term cause degradation of the mechanical properties of the materials. These processes are important to understand to e.g. ensure the integrity of the steel wall of the reactor pressure vessel during the operational life-time of a nuclear power plant. In this thesis, some of the irradiation defects have been studied by using as a model alloy the iron-carbon (Fe-C) system, as iron and carbon are the basic elements in any steel. The interactions between C and vacancy (V) clusters and between C and self-interstitial atom (SIA) clusters have been studied using Molecular Dynamics simulation techniques. This way C-V clusters, such as C2V and CV2, able to trap large SIA clusters, have been identified and characterized. Using Object Kinetic Monte Carlo (OKMC), an model for the radiation-induced nanostructure evolution in Fe-C has been constructed. The model was validated by reproducing experimental data in terms of V and SIA cluster densities and mean sizes from irradiation experiments at low (340 K) and high operational temperature of light water reactors (560 K), as well as reproducing data from post-irradiation annealing up to 780 K. The new model has allowed a deeper understanding of the effect of carbon on the irradiation defect evolution. It was found that the effect of the immobile C-V complexes can be introduced using generic traps for SIA and vacancy clusters. These generic traps have a binding energy that depends on the size of the trapped cluster, which is supported by previously performed atomistic studies. Different trap regimes need to be used at low and high temperatures to account for the different populations of 1/2 <111> and <100> SIA loops at different temperatures, as observed in previous TEM studies. The traps are found to have an important function as nucleation points that promote the growth of larger clusters. The nanostructure evolution model, which is the main result of this thesis, is fully based on physical considerations and only uses a few parameters for calibration. The model is found to be capable of reproducing the experimental trends both at 340 K, 560 K and for annealing up to 700 K; thereby providing insight into the physical mechanisms of importance to determine the type of nanostructural evolution undergone by Fe alloys during irradiation.
  • Smolander, Sampo (Helsingin yliopisto, 2006)
    This work develops methods to account for shoot structure in models of coniferous canopy radiative transfer. Shoot structure, as it varies along the light gradient inside canopy, affects the efficiency of light interception per unit needle area, foliage biomass, or foliage nitrogen. The clumping of needles in the shoot volume also causes a notable amount of multiple scattering of light within coniferous shoots. The effect of shoot structure on light interception is treated in the context of canopy level photosynthesis and resource use models, and the phenomenon of within-shoot multiple scattering in the context of physical canopy reflectance models for remote sensing purposes. Light interception. A method for estimating the amount of PAR (Photosynthetically Active Radiation) intercepted by a conifer shoot is presented. The method combines modelling of the directional distribution of radiation above canopy, fish-eye photographs taken at shoot locations to measure canopy gap fraction, and geometrical measurements of shoot orientation and structure. Data on light availability, shoot and needle structure and nitrogen content has been collected from canopies of Pacific silver fir (Abies amabilis (Dougl.) Forbes) and Norway spruce (Picea abies (L.) Karst.). Shoot structure acclimated to light gradient inside canopy so that more shaded shoots have better light interception efficiency. Light interception efficiency of shoots varied about two-fold per needle area, about four-fold per needle dry mass, and about five-fold per nitrogen content. Comparison of fertilized and control stands of Norway spruce indicated that light interception efficiency is not greatly affected by fertilization. Light scattering. Structure of coniferous shoots gives rise to multiple scattering of light between the needles of the shoot. Using geometric models of shoots, multiple scattering was studied by photon tracing simulations. Based on simulation results, the dependence of the scattering coefficient of shoot from the scattering coefficient of needles is shown to follow a simple one-parameter model. The single parameter, termed the recollision probability, describes the level of clumping of the needles in the shoot, is wavelength independent, and can be connected to previously used clumping indices. By using the recollision probability to correct for the within-shoot multiple scattering, canopy radiative transfer models which have used leaves as basic elements can use shoots as basic elements, and thus be applied for coniferous forests. Preliminary testing of this approach seems to explain, at least partially, why coniferous forests appear darker than broadleaved forests in satellite data.
  • Lunttila, Tuomas (Helsingin yliopisto, 2012)
    Almost all information on astrophysical objects is obtained through observation of electromagnetic radiation. The observed radiation has been altered in interactions with matter, and understanding the transport of radiation is a key prerequisite for understanding the physical conditions in the observed objects. The transport of radiation is described by the radiative transfer equation. Owing to its complex nature, solving the radiative transfer equation is difficult, and it is usually necessary to resort to numerical calculations. In this thesis, the focus is on the modelling of radiation transport in interstellar clouds. The dense gas and dust in interstellar clouds scatter, absorb, and emit radiation, and understanding the radiative transfer effects is crucial in the interpretation of observations. Four of the five articles that are contained in this thesis concern various applications of radiative transfer modelling. Two articles focus on the modelling of spectral line radiation. We study the use of OH Zeeman splitting observations in the determination of magnetic field strengths in molecular clouds. The role of magnetic fields in the process of star formation is still largely an open question with two competing models: the turbulence dominated scenario where magnetic fields are weak, and the ambipolar diffusion driven model with stronger magnetic fields. By combining magneto-hydrodynamical calculations with radiative transfer simulations, we show that the turbulence dominated scenario is consistent with the observed magnetic field strengths. Two articles concern the dust radiative transfer. We study the dust density distribution and grain properties in the dust envelope surrounding the carbon star IRC +10216. By modelling the surface brightness distribution of the scattered light in the dust envelope, we can infer the mass-loss history of the star and improve models of newly formed dust grains. In another article we use magneto-hydrodynamical calculations and radiative transfer simulations to study the reliability of cloud core mass estimates. Observations of dust thermal emission at the far-infrared and sub-millimetre wavelengths are commonly used to determine the masses of molecular cloud cores. By constructing synthetic observations of a model cloud and comparing the estimated masses to the true masses that are obtained directly from the cloud model, we can determine the robustness of mass estimates. Instead of focusing on the applications of radiative transfer modelling, one article describes new numerical methods for efficient radiative transfer simulations. We describe new algorithms for radiative transfer on hierarchical grids. The new algorithms, in particular the use of sub-iterations, are faster by a factor of several compared to the old methods.
  • Niskanen, Antti (Helsingin yliopisto, 2006)
    Atomic Layer Deposition (ALD) is a chemical, gas-phase thin film deposition method. It is known for its ability for accurate and precise thickness control, and uniform and conformal film growth. One area where ALD has not yet excelled is film deposition at low temperatures. Also deposition of metals, besides the noble metals, has proven to be quite challenging. To alleviate these limitations, more aggressive reactants are required. One such group of reactants are radicals, which may be formed by dissociating gases. Dissociation is most conveniently done with a plasma source. For example, dissociating molecular oxygen or hydrogen, oxygen or hydrogen radicals are generated. The use of radicals in ALD may surmount some of the above limitations: oxide film deposition at low temperatures may become feasible if oxygen radicals are used as they are highly reactive. Also, as hydrogen radicals are very effective reducing agents, they may be used to deposit metals. In this work, a plasma source was incorporated in an existing ALD reactor for radical generation, and the reactor was used to study five different Radical Enhanced ALD processes. The modifications to the existing reactor and the different possibilities during the modification process are discussed. The studied materials include two metals, copper and silver, and three oxides, aluminium oxide, titanium dioxide and tantalum oxide. The materials were characterized and their properties were compared to other variations of the same process, utilizing the same metal precursor, to understand what kind of effect the non-metal precursor has on the film properties and growth characteristics. Both metals were deposited successfully, and silver for the first time by ALD. The films had low resistivity and grew conformally in the ALD mode, demonstrating that the REALD of metals is true ALD. The oxide films had exceptionally high growth rates, and aluminium oxide grew at room temperature with low cycle times and resulted in good quality films. Both aluminium oxide and titanium dioxide were deposited on natural fibres without damaging the fibre. Tantalum oxide was also deposited successfully, with good electrical properties, but at slightly higher temperature than the other two oxides, due to the evaporation temperature required by the metal precursor. Overall, the ability of REALD to deposit metallic and oxide films with high quality at low temperatures was demonstrated.
  • Service, Robert (Helsingin yliopisto, 2012)
    The thesis consists of four papers in the area of mathematical biology and probability theory. Mathematical biology is an field of research which seeks understanding of biological phenomena through the application of existing and new mathematical methods. The motivating biological problem addressed in the first two papers of the thesis falls into the area of the mathematical theory of evolution, where an ecological model described by a dynamical system is equipped with a further mechanism, under which one (or more) of the species represented in the model is able to undergo evolution through mutation and natural selection. The present work examines when the possibility of displacement of a resident phenotype by a mutant of another phenotype is described in simple terms by a so-called optimisation principle. An optimisation principle is a numerical function, defined for phenotypes, that allows one to compare the potential to invade all potential environments set by some currently present phenotype simultaneously. The main result of the first paper gives a set of necessary and sufficient conditions for when an optimization principle exists. The third and fourth papers in the thesis deal from different viewpoints with topics connected to Poisson point processes.
  • Riekki, Kirsi (Helsingin yliopisto, 2012)
    The self-organized growth of nanodots and size selection are studied using reaction kinetic model rate equations. Two independent numerical methods and a mesoscopic continuous model are used to solve and analytically predict the details of the stationary nanodot size distribution. The strongly reversible growth of kinetic origin is studied. The power-law distributions which are common in nature, display scaling of the size distribution with clearly defined scaling exponents. The stochastic simulation results and predictions of continuous model are in good agreement. The self assembly of nanodots, observed in experiments and enabling the industrial use of dots in electronics, arises from the strain in heteroepitaxial growth systems and leads to uniform size distributions. To model the size selection, the size dependent thermodynamical energy of the nanodot is included into the reaction kinetics. The resulting distribution is studied in detail to resolve the overshooting phenomenon in which the mean of the distribution exceeds the thermodynamically favored size. The physical origin of the overshooting is explained as a combination of the reaction kinetics and the thermodynamical energy. The skewness of the size distribution is found from the numerical data, and it is added into the continuous model as a parameter to obtain an analytical estimate of the mean size. The predictions of overshooting are calculated for two different types of growth; the 3D metal nanodots and semiconductor nanodots with double-well thermodynamical energy. The optimal, narrow size distributions are found, and external adatom flux from e.g. an external adatom source or ion beam assisted deposition improves the size selection by driving the size distribution to the narrowest location. Nucleation theory calculations of the thermodynamically stable distributions are performed, and the results are comparable to numerical and modelling results.
  • Lahtinen, Maarit (Helsingin yliopisto, 2013)
    Laccases (EC, benzenediol: oxygen oxidoreductase) are multicopper oxidases that can catalyze the oxidation of several, mainly different phenolic but also some inorganic substrates. Laccases selectively catalyze the one-electron oxidation of a phenolic substrate to a phenoxy radical, which can react further in non-enzymatic radical reactions. The phenolic subunits of lignin, one of the major components of wood, are natural substrates of laccases. In the presence of suitable small molecules, mediators, laccases can also catalyze the oxidation of the etherified (i.e. non-phenolic) subunits of lignin. The aim of this research is to increase the knowledge on the direct reactions of laccases and lignin, without mediators. Recently, this area has begun to garner increasing general interest as a result of the biorefinery concept, which aims to produce valuable raw material from sustainable resources. In addition, the most recent development of laccase mediators has been focusing on lignin-based phenolic molecules, which links these two areas, the laccase-mediator system and reactions without the mediator, directly to each other. Monomeric and dimeric lignin model compounds were used to evaluate the reactions and reactivity with laccases. Many of the model compounds represented the most common linkage-type in lignin, the beta-O-4 structure; thus, more efficient ways to synthesize these types of compounds were developed. Further, the oxidizabilities of the compounds, revealed by cyclic voltammetry, and the oxidation rates using the low- and the high-redox potential laccases from M. albomyces and T. hirsuta were compared in view of the theory that the reaction rate is dependent on the redox potential difference between the substrate and the laccase. However, it was found that the redox potential difference could not entirely explain the preferences of the studied laccases. The reaction products from the lignin model compounds were mainly formed as a result of 5-5 coupling and oxidation of the benzylic hydroxyl group to an aldehyde. The analysis was also performed as a function of time; for guaiacylic products the 5-5 coupling was the preferred reaction and these products were formed first. One model compound, vanillyl alcohol, was used to examine the effect of pH, enzyme dosage and temperature, all of which affected the product distribution. The observed predominating product was the 5-5 dimer, although according to computational evaluation, vanillin was the thermodynamically favored product, with a difference of 5.6 kcal mol-1. The transition states leading to the products seemed to affect the observed product distribution. In addition, the calculated pKa-values suggested that at the used pH range (4.5 7.5), rearomatization of the quinone intermediates could occur through deprotonation rather than through protonation. Finally, the M. albomyces laccase was tested in the presence of 1-allyl-3-methylimidazolium chloride, [Amim]Cl; an ionic liquid able to dissolve lignin. An expected decrease in enzyme activity was also found experimentally. The monolignol coniferyl alcohol was polymerized further, as expected, but the chemical structure of the formed dehydropolymer (DHP) was also affected by the presence of [Amim]Cl.