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  • Vesanto, Veli-Heikki (Helsingin yliopisto, 2012)
    While spaceborne satellite data has been extensively used to extract biophysical forest characteristics through reflectance features and indices, there are still many questions regarding seasonal changes in reflectance. Boreal forests have already seen changes in growth patterns from climate change, and the large scale monitoring of these forests is becoming more important. Understanding seasonal changes in reflectance in the boreal region will allow for the monitoring of seasonal biophysical changes using satellite imagery. For this monitoring to be possible the satellite imagery needs to be preprocessed and atmospherically corrected to create a time series of hemispherical-directional reflectance factors. The red edge is the abrupt change in reflectance between 680 and 740 nm seen in vegetation spectra. The red edge inflection point is the wavelength, at which the slope is steepest in the red edge. The red edge inflection point is sensitive to plant chlorophyll content and has been extensively used for estimating vegetation biophysical parameters including: leaf-area index, biomass and plant health levels. Hyperion is a narrowband imaging spectrometer aboard the Earth Observer-1 satellite. Hyperion captures data across 242 spectral bands covering a spectral range of 356 to 2577 nm resulting in a nominal spectral range of 10 nm. While the high spectral resolution of Hyperion makes it possible to calculate the REIP, there is no consensus on how this should be done, with different methods producing conflicting results. This study explains the preprocessing and atmospheric correction of a seasonal time series of five Hyperion EO-1 images (Provided courtesy of the USGS) from Hyytiälä, Southern Finland (61° 51 N, 24° 17 E). The time series ranges from 31.5.2010 to 12.8.2010, covering much of the growing season and the seasonal changes in reflectance. The first derivative, four-point linear interpolation, Lagrangian interpolation, and fifth-order polynomial fitting methods for calculating the REIP are looked at to determine their applicability for Hyperion imagery using this time series. Hyperion data requires considerable preprocessing before atmospheric correction can be done. In this study the preprocessing covered: destriping, desmiling, atmospheric correction and finally geocorrection. Atmospheric correction was done using both FLAASH and ATCOR, both of which are MODTRAN based absolute atmospheric correction algorithms. The final atmospherically corrected HDRF images were evaluated using in situ handheld spectrometer reference measurements of a grass field in the area. An average RMSE value of around 3% was achieved with both algorithms. The corrected Hyperion images were also compared against two MODIS products, which also showed good agreement. The aerosol retrieval however did not work with either algorithm, on any scene. The use of a sun photometer for aerosol level estimation was also not effective. Due to the dynamics of the red edge and expected seasonal red edge inflection point trends, the fifth-order polynomial fitting method was seen as the best method for calculating the red edge inflection point. The red edge inflection point did not correlate strongly with leaf area index overall, however there was a strong correlation with individual plots. A strong correlation was observed between Hyperion red edge inflection point and understory red edge inflection point, both overall and for individual plots.
  • Lindqvist, Hannakaisa (Helsingin yliopisto, 2013)
    The atmosphere of the Earth contains a rich variety of small particles that contribute to numerous local and global meteorological phenomena, such as visibility, water circulation, and climate change. Of particular relevance for this thesis, they scatter and absorb solar light and, therefore, are significant to the radiative balance of the Earth-atmosphere system. Likewise, they influence remote-sensing observations and can be monitored through the radiation they scatter. The interaction between radiation and the individual particles cannot be accurately established without modeling the physics of scattering and absorption. In the case of spherical particles, such as water droplets in low-altitude clouds, this is straightforward because an analytical solution exists, but solid-form particles are not spherical. They need models for their morphology, and dedicated computational methods to solve their scattering properties. Radiatively most important classes of nonspherical atmospheric particles are ice crystals in high-altitude clouds and dust aerosol particles, including both volcanic and mineral dust. Both scattering and absorption are generally known to be sensitive to the physical properties of the particles --- size, shape, and composition --- and yet, these ice and dust particles are currently modeled with overly simplistic and not properly validated approaches in many key applications, including climate models. In the thesis, light scattering by atmospheric ice and dust particles is investigated using morphologically faithful models that take into account the shape, internal structure, and material inhomogeneity. Detailed models are developed for small ice crystals, whose shapes have been controversial due to lack of knowledge, and for volcanic dust particles, whose porosity greatly impacts their scattering properties: an effect that is usually ignored. As a major highlight, the thesis presents a paradigm change in modeling scattering by mineral dust by introducing a first-ever model that has been derived directly from the observed shape and inhomogeneous composition of individual, micrometer-scale mineral dust particles. For ice crystals, the thesis introduces a fully automatic shape classification algorithm, which greatly facilitates the analysis of ice cloud microphysical data and, hence, the computation of cloud radiative effect. Selected results of the thesis are already in use in multidisciplinary applications; in atmospheric sciences, the results have led to more accurate radiative impact estimations and simulations of the entire climate.
  • Lauros, Johanna (Helsingin yliopisto, 2011)
    Atmospheric particles affect the radiation balance of the Earth and thus the climate. New particle formation from nucleation has been observed in diverse atmospheric conditions but the actual formation path is still unknown. The prevailing conditions can be exploited to evaluate proposed formation mechanisms. This study aims to improve our understanding of new particle formation from the view of atmospheric conditions. The role of atmospheric conditions on particle formation was studied by atmospheric measurements, theoretical model simulations and simulations based on observations. Two separate column models were further developed for aerosol and chemical simulations. Model simulations allowed us to expand the study from local conditions to varying conditions in the atmospheric boundary layer, while the long-term measurements described especially characteristic mean conditions associated with new particle formation. The observations show statistically significant difference in meteorological and back-ground aerosol conditions between observed event and non-event days. New particle formation above boreal forest is associated with strong convective activity, low humidity and low condensation sink. The probability of a particle formation event is predicted by an equation formulated for upper boundary layer conditions. The model simulations call into question if kinetic sulphuric acid induced nucleation is the primary particle formation mechanism in the presence of organic vapours. Simultaneously the simulations show that ignoring spatial and temporal variation in new particle formation studies may lead to faulty conclusions. On the other hand, the theoretical simulations indicate that short-scale variations in temperature and humidity unlikely have a significant effect on mean binary water sulphuric acid nucleation rate. The study emphasizes the significance of mixing and fluxes in particle formation studies, especially in the atmospheric boundary layer. The further developed models allow extensive aerosol physical and chemical studies in the future.
  • Kauppila, Tiina (Helsingin yliopisto, 2004)
  • Aalto, Pasi (Helsingin yliopisto, 2004)
  • Tommiska, Johanna (Helsingin yliopisto, 2008)
    Breast cancer is the most commonly occurring cancer among women, and its incidence is increasing worldwide. Positive family history is a well established risk factor for breast cancer, and it is suggested that the proportion of breast cancer that can be attributed to genetic factors may be as high as 30%. However, all the currently known breast cancer susceptibility genes are estimated to account for 20-30% of familial breast cancer, and only 5% of the total breast cancer incidence. It is thus likely that there are still other breast cancer susceptibility genes to be found. Cellular responses to DNA damage are crucial for maintaining genomic integrity and preventing the development of cancer. The genes operating in DNA damage response signaling network are thus good candidates for breast cancer susceptibility genes. The aim of this study was to evaluate the role of three DNA damage response associated genes, ATM, RAD50, and p53, in breast cancer. ATM, a gene causative for ataxia telangiectasia (A-T), has long been a strong candidate for a breast cancer susceptibility gene because of its function as a key DNA damage signal transducer. We analyzed the prevalence of known Finnish A-T related ATM mutations in large series of familial and unselected breast cancer cases from different geographical regions in Finland. Of the seven A-T related mutations, two were observed in the studied familial breast cancer patients. Additionally, a third mutation previously associated with breast cancer susceptibility was also detected. These founder mutations may be responsible for excess familial breast cancer regionally in Northern and Central Finland, but in Southern Finland our results suggest only a minor effect, if any, of any ATM genetic variants on familial breast cancer. We also screened the entire coding region of the ATM gene in 47 familial breast cancer patients from Southern Finland, and evaluated the identified variants in additional cases and controls. All the identified variants were too rare to significantly contribute to breast cancer susceptibility. However, the role of ATM in cancer development and progression was supported by the results of the immunohistochemical studies of ATM expression, as reduced ATM expression in breast carcinomas was found to correlate with tumor differentiation and hormone receptor status. Aberrant ATM expression was also a feature shared by the BRCA1/2 and the difficult-to-treat ER/PR/ERBB2-triple-negative breast carcinomas. From the clinical point of view, identification of phenotypic and genetic similarities between the BRCA1/2 and the triple-negative breast tumors could have an implication in designing novel targeted therapies to which both of these classes of breast cancer might be exceptionally sensitive. Mutations of another plausible breast cancer susceptibility gene, RAD50, were found to be very rare, and RAD50 can only be making a minor contribution to familial breast cancer predisposition in UK and Southern Finland. The Finnish founder mutation RAD50 687delT seems to be a null allele and may carry a small increased risk of breast cancer. RAD50 is not acting as a classical tumor suppressor gene, but it is possible that RAD50 haploinsufficiency is contributing to cancer. In addition to relatively rare breast cancer susceptibility alleles, common polymorphisms may also be associated with increased breast cancer risk. Furthermore, these polymorphisms may have an impact on the progression and outcome of the disease. Our results suggest no effect of the common p53 R72P polymorphism on familial breast cancer risk or breast cancer risk in the population, but R72P seems to be associated with histopathologic features of the tumors and survival of the patients; 72P homozygous genotype was an independent prognostic factor among the unselected breast cancer patients, with a two-fold increased risk of death. These results present important novel findings also with clinical significance, as codon 72 genotype could be a useful additional prognostic marker in breast cancer, especially among the subgroup of patients with wild-type p53 in their tumors.
  • Kallio, Tarja (2000)
    People with addiction problems have been treated for many years with cognitive-behavioral methods. During treatment they are prepared to cope with situations where relapse to alcohol or drug use is common. According to Bandura's (1986) theory, a person's assessment of his coping ability in risk situations is connected with his actual behaving in these situations. There is no previous Finnish research on self-efficacy of people with alcohol problems, though this cocept is considered important in addiction treatment. The general aim of the study was to yield new insight into the treatment of substance abusers. Specific goals were to study 1) the connection of drinking related self-efficacy and post-treatment substance use of patients treated with cognitive-behavioral group-threrapy, 2) the impact of in-treatment change of drinking related self-efficacy on post tretment substance use, and 3) relapse-risk situations of Finnish substance abusers. The participants of the study were recruited from Järvenpää Addiction Hospital. The patients (N=100) were interviewed at the beginning and at the end of their in-patient treatment. Information was gathered by interview and by Situational Confidence Questionnaire, by General Self-Efficacy Scale and by Addiction Dependence Scale. Follow-up information was gathered after 6 and 12 months by post. The data was gathered and analyzed in 1996-1999. The methods of analyzis were percent tables, correlations, logistic regressions and t-tests. Analysis were made by SPSS-statistics program. According to the results, the patient's drinking related self-efficacy and general self-efficacy improved significantly during treatment. There was no connection between drinkinng related self-efficacy at the beginnig or at the end of treatment and post treatment substance abuse. Relapse-risk situations of Finnish substance abusers did not differ from previous research findings abroad. The results are discussed from the treatment point of view. The sources of this research were cognitive-behavioral theory books and english research articles from other countries.
  • 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.