Browsing by Author "Karppinen, Maarit, Prof., Aalto University, Department of Chemistry, Finland"

Filter results by typing the first few letters
Now showing 1 - 6 of 6
  • Atomic layer deposition of complex thin films
    (2016) Ahvenniemi, Esko
     School of Chemical Technology | Doctoral dissertation (article-based)
    Atomic layer deposition (ALD) is an advanced method for fabricating thin films on various substrate chemistries and architectures. It is employed commercially in semiconductor industry where typically thin films of binary oxides are employed in high-tech devices. Simple binary films have many useful properties, but to exploit the whole potential of the nanoscale devices, studies on more complex materials are required. Ternary and quaternary compounds possess several potentially exciting properties, but so far the studies on these complex ALD-fabricated thin films have been relatively scarce. Device thicknesses measured in nanometers can bring astonishing advantages for a number of frontier applications such as solid oxide fuel cells. For example, thin film cathodes benefit in massively lowered operation temperatures due to the exclusion of the bulk properties of the material. In this dissertation, ALD processes were developed for SrCoO3-d and (La,Sr)CoO3-d thin films to complete the set of ALD processes for one of the best cathode materials, (La,Sr)(Co,Fe)O3-d, and to gain deeper understanding of the growth of complex oxide films in general. Moreover, the dissertation studies concerned a relatively new class of materials, i.e. hybrid atomic/molecular layer deposition (ALD/MLD) fabricated inorganic-organic hybrid thin films, where different metal cations and organic molecules are mixed together creating materials which combine the best properties from the both worlds. When this dissertation research was initiated, ALD/MLD processes had been developed only for Al, Ti and Zn metal constituents, and practically only diols has been used as organic constituents. In this dissertation, di- and tricarboxylic acids are presented as highly reactive organic precursors even with the stable but vastly applied β-diketonate precursors to deliver copper, cobalt, manganese and calcium based hybrid ALD/MLD thin films with exciting structural features.
  • Atomic Layer Deposition of Thermoelectric ZnO Thin Films
    (2013) Tynell, Tommi
     School of Chemical Technology | Doctoral dissertation (article-based)
    Thermoelectric energy harvesters hold great potential for reducing our dependence on conventional energy sources by making use of untapped heat sources and converting them into electricity. For the technology to be able to make a significant impact though, more efficient materials need to be developed. This is a huge challenge because of the interdependence of the physical properties that affect the performance of a thermoelectric material, and nanostructuring might be the only way to overcome this obstacle. This thesis presents research on the effect of inorganic dopants and inorganic-organic hybrid superlattice structures on the thermoelectric properties of ZnO thin films. Atomic layer deposition (ALD) was used to fabricate the thin films in this study due to the suitability of the technique for the deposition of precisely controlled nanostructures. Zinc oxide is one of the most promising thermoelectric oxide materials, especially when doped with for instance Al or Ga, but it is somewhat held back by the lack of a stable p-type ZnO material. The effects of aluminum and phosphorus doping on the thermoelectric properties of ALD-grown ZnO thin films were investigated in this work, and both were found to increase the carrier concentration of ZnO, with Al turning out to be the better dopant in terms of improving thermoelectric performance. An attempt was also made to induce p-type conductivity in ZnO through thermal treatment of the P-doped films, but a deterioration of the electrical properties of the films was observed instead. The fabrication of super lattice structures of organic layers within ZnO with a combination of the atomic and molecular layer deposition (MLD) techniques was successfully demonstrated for three different organic precursors: hydroquinone, 4-aminophenol and 4,4'-oxydianiline. All of the organic molecules were found to have an effect on the electrical and thermoelectric properties of ZnO, the magnitude of which varied noticeably between the different organic constituents despite their relatively similar structures. The observed effects from the introduction of the hybrid superlattice structures amounted to only small changes in the thermoelectric power factor of ZnO, and these changes were not cumulative with the effects of Al doping when hybrid superlattices were combined with Al doping. The net effect from the organic layers on the thermoelectric performance of ZnO is predicted to be greater than implied by the slight changes in power factor due to the expected decrease in thermal conductivity resulting from phonon inhibition by the organic layers.
  • Atomic/molecular layer deposition of hybrid inorganic-organic thin films
    (2014) Sundberg, Pia
     School of Chemical Technology | Doctoral dissertation (article-based)
    The possibility to combine the best properties of the two constituents makes inorganic-organic hybrid materials as intriguing candidates for many high-end applications. However, the distinctly different material properties of the inorganic and organic constituents make the fabrication of hybrid materials challenging. For example when using solution deposition techniques, a solvent which works for the organic constituent does not necessarily work well for the inorganic one. Also when considering the potential applications for hybrids, many require the use of high-quality thin films. Atomic and molecular layer deposition (ALD and MLD) techniques are gas-phase deposition methods based on sequential surface-saturated reactions. By combining the two techniques it is possible to deposit high-quality hybrid thin films with accurate thickness and composition control. In this thesis novel inorganic-organic hybrid thin films were deposited using the combination of ALD and MLD. One of the main challenges when growing hybrid materials using the combined ALD/MLD technique is finding suitable precursors. When flexible linear organic precursors are used in ALD/MLD they may bend and hinder the growth process. In this work rigid aromatic amines, 4,4'-oxydianiline (ODA) and 4-aminophenol (AP) were used to improve the growth rates. Diethyl zinc (DEZ) and TiCl4, precursors commonly used in ALD, were selected as the inorganic precursors. Successful film growth was observed for all possible precursor combinations, but the growth rates achieved using AP as organic precursor were higher than those obtained using ODA, Ti-AP hybrid exhibiting almost ideal growth. As AP has two different functional groups, more control is achieved in the growth process whereas the  ether bond in ODA allows the molecule to bend. From the inorganic precursors TiCl4, which is stronger Lewis acid and has more ligands than DEZ, catalyzed the deposition reaction better. The cyclic growth process characteristic to ALD and MLD was utilized to make nano-structures consisting of oxides and hybrid materials. The material pairs investigated were TiCl4+ODA hybrid and TiO2, DEZ+AP hybrid and ZnO, and trimethylaluminum (TMA) + ethylene-1,2-diol (EG) and Al2O3. By varying the hybrid and oxide content in the Ti- and Zn-containing mixtures, it was possible to tune the degree of crystallinity, surface roughness, refractive index, mechanical properties and density in the formed films. The TMA-EG and Al2O3 nanolaminates were grown on polylactic acid substrates in order to fabricate a film more suitable for coating biomaterials in packaging applications. When compared to coatings made from pure Al2O3 or TMA+EG hybrid, the nanolaminate consisting of TMA+EG and Al2O3 layers withstood straining better. The strained nanolaminate was also better oxygen barrier than the strained pure coatings.
  • Layered thermoelectric materials : misfit cobalt oxides and oxyselenides
    (2015) Chou, Ta-Lei
     School of Chemical Technology | Doctoral dissertation (article-based)
    Because of the growing awareness of environmental and energy-related issues, harvesting waste heat by thermoelectric materials is getting increasing attention. A decent thermoelectric material is typically a heavily doped semiconductor with high thermopower and low thermal conductivity. To simultaneously achieve these characteristics, a complex structure is an essence. Inspired by the artificial superlattices various multi-layered compounds with a repetitive stacking of conducting and blocking layers which enables to decouple electrical and thermal transport are currently investigated as candidates to approach the ideal "phonon glass, electron crystal" scenario. Metal oxides feature advantages like availability and thermal stability in air at elevated temperatures yet poor carrier mobility without appropriate doping. Replacing oxygen with less electronegative anions, for instance chalcogens, would turn the framework more covalent and lead to readily improved electrical conduction. This dissertation presents several layered misfit cobalt oxides and oxyselenides as thermoelectrics. To begin with, double-interlayered phases [Sr2O2]0.52CoO2 and [Ca1.7(OH)2]0.58CoO2 were successfully made under compression and investigated by x-ray absorption spectroscopy (XAS) and thermogravimetry (TG) which unveil subtle local geometry, electronic structure and defects embedded in the lattice. Given that a mixed-anion oxide-chalcogenide framework might effectively benefit the electrical transport, some layered oxyselenides, such as BiOCuSe, Bi2MO4Cu2Se2 (M = Y or early rare earth element) and AE2CoO2Cu2Se2 (AE = alkaline earth element) which in common possess an anti-fluorite-type [Cu2Se2] sub-layer, were then investigated. Temperature-dependent extended x-ray absorption fine structure (EXAFS) is found to be a powerful probe to resolve the lattice characteristics of BiOCuSe upon finite doping. Meanwhile, Cu vacancies and alkaline-earth dopants are found to affect distinctively in the BiOCuSe matrix. Bi2MO4Cu2Se2 compounds, unfortunately, are metals with poor thermopower. The origin of metallicity is readily evidenced in the XAS analysis. Lastly the Sr2CoO2Cu2Se2 p-type semiconductor was visited. Computational work suggests there is no way to overlook the contribution from the oxide sub-layer at the valence band maximum (VBM). Replacing Sr with Ba, in contrast to the Ca-for-Sr substitution case, would more efficiently raise the thermoelectric power factor. It is quite convincing once an appropriate doping scheme is employed, either in the oxide or the selenide layers, the Sr2CoO2Cu2Se2 oxyselenides could be a promising option for intermediate-temperature thermoelectrics.
  • Properties and Applications of A2B'B"O6 Perovskites: from Fuel Cells to Quasi-Low-Dimensional Magnetism
    (2014) Vasala, Sami
     School of Chemical Technology | Doctoral dissertation (article-based)
    Perovskite oxides ABO3 exhibit a wide range of properties which are of interest both in basic research and in applications. This is largely due to the possibility to incorporate almost any element of the periodic table into the perovskite structure. Partial substitution of elements is also possible, and can lead to chemical ordering of the elements: in case of B-site cation substitution, a so-called B-site ordered double perovskite A2B'B"O6 may form. Such double perovskites offer novel properties and applications due the cation order-disorder phenomenon and the combination of two different B-site cations. The aim of the present thesis was to gain a greater understanding of the structure, redox behavior and electronic and magnetic properties of A2B'B"O6 double-perovskite oxides, and the chemical and physical principles behind them. Among the possible applications for double perovskites, compounds of the type Sr2B'MoO6 have been considered as anode materials for solid-oxide fuel cells. In this work, Sr2B'MoO6 perovskites with B' = Mg, Mn, Fe, Co or Ni were examined. Of these, Sr2MgMoO6 was found to be redox stable, whereas the compounds with a transition metal at the B' site were not. However, partial substitution of Mg in Sr2MgMoO6 by a transition metal could provide materials with improved performance and good redox stability. Partial substitution of Mo in Sr2MgMoO6 by Nb or W was also studied. In both cases the compounds remained redox stable, but their electrical conductivities were impaired by the substitution. In case of the Nb-for-Mo substitution, strong correlations between oxygen-vacancy concentration and B-site cation ordering was found. Finally, a concept of using an all-ceramic composite material consisting of Sr2MgMoO6 and SrMoO4 as an SOFC anode was presented. The A2B'B"O6 perovskites also exhibit various magnetic properties, which are of interest in basic research. Compounds with the composition A2CuB"O6 are especially intriguing, as they may show low-dimensional and/or frustrated magnetic behavior depending on their composition. It is thus useful to examine this group of compounds as a whole, as they can provide information on magnetic properties in solids in general. In the present work two new compounds of this family, Sr2CuMoO6 and Sr2CuIrO6, were synthesized using a high-pressure synthesis method. In addition, magnetic ground state properties of the quasi-low-dimensional Sr2CuWO6 and Sr2CuMoO6 were determined. The Sr2CuB"O6 family of compounds were found to exhibit a transition from quasi-low-dimensional to a more typical antiferromagnetic behavior with increasing d-orbital occupancy of the B" cation. Similar transition was noted in a series of Ba2Cu(W1-xUx)O6 compounds with increasing x, with the distinction that both W and U are diamagnetic.
  • YBaCo4O7+δ and YMnO3+δ Based Oxygen-Storage Materials
    (2014) Parkkima, Outi
     School of Chemical Technology | Doctoral dissertation (article-based)
    The threatening exhaustion of the fossil fuel reserves sets demand for developing new solutions for energy production and greener industrial processes. Oxygen-storage materials are a key factor in several established (e.g. three-way catalyst) or novel (e.g. air separation) technologies that help to reduce emissions and lower energy consumption. The wide-range implementation of the applications requires new oxygen-storage materials with enhanced performance and good stability at a proper operation temperature. This thesis reports results of engineering of the properties of the two recently discovered oxygen-storage materials, YBaCo4O7+δ and YMnO3+δ, as well as a new catalytic application for YBaCo4O7+δ. The oxygen-storage capacity of YBaCo4O7+δ is outstandingly high but the compound suffers from poor phase stability at high temperatures. The effects of chemical substitutions to each cation site in YBaCo4O7+δ phase were systematically studied to reveal that the larger the substituent at the Y and Ba sites and accordingly the larger the crystal lattice is, the larger is the amount the phase can absorb oxygen but at the same time the poorer is the phase stability. On the other hand, all cation substitutions involving the redox-active cobalt atoms in YBaCo4O7+δ had a negative effect on the oxygen-storage capacity but some substitutions were highly beneficial for the stability. In particular, for the composition YBa(Co0.85Al0.075Ga0.075)4O7+δ the phase decomposition could be prevented without markedly reducing the oxygen-storage capacity. A new application possibility for YBaCo4O7+δ in industrially relevant catalytic oxidationreactions was also introduced in this thesis. The YBaCo4O7+δ catalyst showed an appreciably high activity (60 % conversion) compared to reference materials titanium and cobalt oxides (both < 20 %) in the oxidation of cyclohexene with H2O2. Calorimetric measurements showed that the reason for this high activity is in the interaction between the catalyst and H2O2. Oxygen absorption properties of another new oxygen-storage material candidate, YMnO3+δ, were also investigated in this thesis. The maximum amount of oxygen that could be loaded into the structure through a high oxygen pressure treatment was as high as δ = 0.4, and with careful adjustments of the annealing parameters essentially phase-pure samples were achieved at δ = 0.35 with a new crystal structure the nature of which was studied by electron microscopy. To see whether the isostructural RMnO3 compounds with the smaller rare earth (R) cations at the Y site can sustain oxygen-nonstoichiometry a sample series of R = Ho-Lu was synthesized and treated at various temperatures and oxygen partial pressures. Only the phase HoMnO3+δ was found to show oxygen hyperstoichiometry similar to the YMnO3+δ case.