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

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Atomic layer deposition towards novel device applications
(2020) Marin, Giovanni
School of Chemical Technology | Doctoral dissertation (article-based)
Atomic layer deposition (ALD) is a gas-phase thin film deposition technique that has gained increasing popularity in the last 20 years because of its unique properties. It is based on self-limiting chemical reactions that ensure the layer-by-layer growth of the film. This unique growth mode is fundamental to the fine control of both film thickness and structure. The film grows conformally on the substrate, following the morphology of the surface. ALD can grow films at low temperature, making possible the use of temperature-sensitive substrates. A slightly modified technique called molecular layer deposition (MLD) utilises organic precur-sors to deposit fully organic films. Hybrid inorganic-organic materials can be deposited with a combination of ALD and MLD. The aim of this research was to utilise the unique characteris-tics of ALD/MLD in two different applications, thermoelectrics and barrier coatings. Thermoelectric devices were fabricated on flexible plastic, glass, and textile. Testing of the barrier properties of ALD-grown films were carried out on 3D printed plastic substrates. The conformality of the deposition is fundamental in both applications. The films needed to coat the single fibres within the textile substrate as well as the porous surface of the 3D printed plastic. The low deposition temperature made it possible to use cotton as well as various plas-tics as substrates. The fine control over the film thickness and structure, enabled the deposi-tion of inorganic-organic superlattice hybrid materials. Zinc oxide (ZnO) and hydroquinone (HQ) were chosen for the fabrication of the thermoelectric devices while aluminium oxide (AlOx) was the chosen barrier material. Hydroquinone was utilised as monomolecular layers within the ZnO matrix to lower thermal conductivity and enhance the thermoelectric perfor-mance. The ALD-deposited AlOx coating was shown to successfully lower the vacuum degassing of the 3D printed plastics compared to commercial sealants. These superior performances open the way to inexpensive and personalised, 3D printed, laboratory tools coated with ALD which pro-vide degassing protection to the vacuum environment. Thermoelectric devices were fabricated on several substrates (silicon, flexible plastic, flexible glass, and textile) using the n-type ZnO as thermoelectric. On textile, the device was made with both n-type (ZnO or ZnO-HQ) and p-type (poly(3,4-ethylenedioxythiophene) - PEDOT) components to improve performance. The ZnO-HQ superlattice outperformed the bare ZnO films, proving that the hybrid approach is worth pursuing to reduce thermal conductivity. The best device fabricated on textile, produced an open-circuit voltage around 150 mV at a ΔT of 20 °C with a power output in the order of pW. These numbers, although low, are paving the way for future application of the ALD/MLD in the fabrication of thermoelectric devices inte-grated into smart clothing.
Atomic/molecular layer deposited crystalline metal-organic thin films based on low-valent metals
(2023) Multia, Jenna
School of Chemical Technology | Doctoral dissertation (article-based)
Crystalline metal-organic thin films offer promising solutions for the diverse needs of miniature electronic devices, requiring functional or smart materials for sensing, energy storage, or gas capture. The semiconductor industry has widely employed atomic layer deposition (ALD) of thin films in applications that require ultra-thin and high-quality coatings on demanding surfaces. The advantage of the ALD method is the controllability of the thin film growth with atomic-level precision, as well as the uniformity of the resulting coating on large and complex structures. However, with the goal of continuously improving the performances of nanoscale devices, studies on more complex thin-film materials are needed. Thin-film materials with unique physical or chemical properties can be achieved by combining the organic linking molecules and metal counterparts, which is possible by atomic/molecular layer deposition (ALD/MLD). In this thesis, 19 different ALD/MLD processes based on low-valent metals together with aromatic organic counterparts were studied. The most important finding was that the ALD/MLD technique can be used to directly produce a variety of in-situ crystalline metal-organic thin-film materials, including materials that have not been previously fabricated by any other method. The research also provides valuable insight for designing such ALD/MLD processes: metal precursors with fewer and smaller ligands can promote the formation of an in-situ crystalline thin-film structure. A significant observation from the application point of view was that some of the in-situ crystalline thin-film structures discovered by the ALD/MLD technique can serve as guest-matrix structures and host intercalated metal ions or reversibly adsorb water molecules without disrupting the fundamental crystal structure. Currently, there are already 50 original ALD/MLD publications where thin-film structures show either in-situ or post-deposition treated crystallinity. Since the field of metal-organic thin films is still in its early stage, the ALD/MLD technique offers plenty of possibilities for developing novel and functional thin films.
Atomic/Molecular Layer Deposition of an All-Solid-State Thin-Film Battery Based on Organic Electrode Materials
(2018) Nisula, Mikko
School of Chemical Technology | Doctoral dissertation (article-based)
All-solid-state Li-ion batteries in thin-film format are currently the most promising concept for the energy storage needs of miniature electronic devices. Their applicability is however restricted by their inherently poor energy and power densities. By using 3D substrates, the effective surface area and thus the energy density could be markedly increased. Atomic layer deposition (ALD) is one of the few methods capable in producing conformal layers on such complex structures. As the basic research on new ALD processes for Li-containing thin films is only in early stage, the true impact of the ALD technique in the Li-ion battery field is yet to be demonstrated. The aim of this theses was to advance the field with the introduction of novel deposition processes for each of the active components of a thin-film Li-ion battery. An ultimate goal was to manufacture a fully functional all-solid-state thin-film battery. For each material, the process design needs to take into account the fundamental difficulties related to lithium-based ALD chemistries. Additionally, by avoiding the use of metal components other than Li in the materials, the environmental impact of the newly designed and fabricated thin-film battery could potentially be reduced. For the solid electrolyte, a novel thermal-ALD process was developed for one of the most promising thin-film battery electrolyte material, i.e. lithium phosphorus oxynitride (LiPON). Due to its complex composition, it had been considered highly challenging compound for the ALD synthesis. In this thesis, the key innovation was the use of a novel ALD precursor, diethyl phosphoramidate. In combination with lithium bis(trimethylsilyl)amide, the quaternary target material could be deposited with a simple binary ALD process. The conformality on high-aspect-ratio substrates was confirmed and the measured ionic conductivity value is among the highest reported for ALD-grown solid electrolytes. For the electrode materials, a completely new approach was demonstrated. By utilizing combined atomic/molecular layer deposition (ALD/MLD) technique the range of available electrode materials was broadened to those based on conjugated carbonyl systems. Based on their fully organic backbones, such lithium organic electrode materials should be less harmful than their inorganic counterparts. The negative electrode material, lithium terephthalate, was known as one of the top performing organic electrode materials, whereas a completely novel material, dilithium-1,4-benezenediolate (Li2Q), was developed to function as the positive electrode. Excellent rate performance was demonstrated for both materials; in particular, charge/discharge times as low as 0.25 s were observed for Li2Q. Moreover, these materials were combined into an all-solid-state thin-film battery that was able to undergo extended charge/discharge cycling.
Atomic/Molecular Layer Deposition of Photoresponsive Azobenzene-Containing Thin Films
(2023) Khayyami, Aida
School of Chemical Technology | Doctoral dissertation (article-based)
Owing to their rapid and reversible photoisomerization, azobenzenes are efficient molecular photoswitches that could potentially provide effective control over various properties of their host surroundings. This unique ability has opened up a wide array of potential applications spanning across diverse fields, from electronics and medicine to environmental monitoring and security. This thesis aims to unveil new possibilities for the application of the atomic/molecular layer deposition (ALD/MLD) thin-film technique to incorporate fully functional azobenzene moieties into an inorganic matrix. Several new ALD/MLD processes with azobenzene-4, 4'-dicarboxylic acid (AzoBDC) as the source of the azobenzene moiety were developed. Three types of novel thin-film materials were fabricated: amorphous zinc-azobenzene hybrids, zinc oxide:azobenzene superlattice (SL) structures, and metal-organic framework (MOF) type structures with azobenzene as the linker. In zinc-azobenzene hybrids (Zn-O-C14H9N2-O4-), diethyl zinc (DEZ) was used as the source of zinc in combination with AzoBDC. The fabrication route developed for the Zn-AzoBDC hybrid films was then combined with the DEZ/H2O ALD process for zinc oxide thin films to grow SL structures where single azobenzene layers are sandwiched between thin crystalline ZnO blocks. The ratio of the ALD-ZnO and MLD-(Zn-O-C14H9N2-O4-) cycles was varied between 199:1 and 1:1. The photoreactivity was confirmed for all the films upon 360 nm irradiation, and the kinetics of the resultant trans−cis photoisomerization was found to somewhat depend on the SL structure. Crystalline photoresponsive MOFs were prepared by using FeCl3, Ca(thd)2, Li(thd), Na(thd), or Sr(thd)2 as the metal precursor in combination with AzoBDC. Isomerization of azobenzene units within the strictly crystalline, highly porous environment of Fe-AzoBDC, Li-AzoBDC, and Ca-AzoBDC films was confirmed despite their different crystal structures. Furthermore, the gas absorption and release capability of the films was demonstrated. The ALD/MLD synthesis of Fe-AzoBDC MOFs was also modified by using in parallel with AzoBDC terephthalic acid (TPA) as another organic precursor to obtain structures with two types of organic linkers. These films were deposited by mixing the FeCl3 + AzoBDC and FeCl3 +TPA deposition cycles on a 1:1 frequency to deposit Fe-AzoBDC–Fe-TPA films. It was shown the dilution of the photoactive AzoBDC linkers with the nonactive TPA linkers increased the stability of the crystal structure.
Development of Li-Organic Battery Materials with Atomic/Molecular Layer Deposition
(2021) Heiska, Juho
School of Chemical Technology | Doctoral dissertation (article-based)
Li-ion battery (LIB) is a piece of the puzzle for solving the present climate and energy crisis. Hence, the adaptation of LIBs is rapidly increasing, which has evoked severe concerns about the abundance, sustainability, and recyclability of the electrode raw materials. Cycle life is one of the main parameters regarding the sustainability of the LIBs, and it can be improved by protecting the active materials with suitable coatings. The scope of this thesis was twofold: (i) to challenge new organic electrode (OE) materials as more sustainable alternatives to current electrode materials, and (ii) to develop new Li-organic protective coating materials for the LIB electrodes. The gas-phase atomic/molecular layer deposition (ALD/MLD) thin-film technique provides unique advantages for both applications, and it was comprehensively utilized in this thesis. The focus was on developing new Li-organic ALD/MLD processes, which are still scarce in the literature. The overall goal was to show the applicability, benefits, and uniqueness of ALD/MLD technology to deposit various Li-organic materials for potential battery applications. In addition to five new ALD/MLD processes for OE anode materials, an empirical method for simplifying source temperature's optimization in ALD/MLD based on thermogravimetric analysis was developed. The OE anode materials showed similar intrinsic electrochemical performance metrics as previously measured with conventional electrodes with bulk materials. As the thin-film electrodes deposited from the gas phase do not contain any additives that are usually mixed with the active material, they can be considered a perfect model for studying the electrode materials' intrinsic properties. The post-mortem X-ray analysis of the cycled electrodes showed amorphization of the active material whenever the charge-discharge voltage plateau was lost. The performance decay is proposed to be caused by the excess lithiation of the aromatic core and subsequent side reaction. Pioneering work with three new ALD/MLD processes for possible Li-organic protective coatings was accomplished in this thesis. One of the processes utilized CO2 for the first time in the ALD/MLD process to deposit lithium alkyl carbonates. The lithium alkyl carbonates are an exciting group of materials because they are abundant in the organic part of the solid electrolyte interphase (SEI), forming naturally in the LIBs. This development allows the depositions of artificial SEI coatings. Furthermore, films of dilithium-1,4-benzenedisulfonate were deposited, diffraction patterns reported, and performance evaluated. The performance still needs to be improved, and the Li-organic coatings applied on conventional electrodes, but the groundwork is now laid down.
Multilayered ZnO-based thin films to control heat and electrical transport properties
(2021) Krahl, Fabian
School of Chemical Technology | Doctoral dissertation (article-based)
Interfaces between materials can have properties that differ greatly from the bulk state. In classical materials only a tiny fraction of atoms are at the interface while the vast majority is in the bulk of the material. The capability to engineer materials with an artificially high amount of interfaces opens up a pathway to amplify the interface effects and tailor the material properties by controlling the amount of interfaces. This approach to engineer materials step by step or layer by layer also allows for a controlled combination of very different materials into a hybrid material that would not form naturally and which can show fundamentally different and new properties. In this thesis atomic layer deposition (ALD), molecular layer deposition (MLD) and pulsed laser deposition (PLD) are utilized to engineer ZnO-based thin films with high interface densities. The films are analysed with x-ray reflectivity (XRR), x-ray diffraction (XRD) and transmission electron microscopy (TEM) in regards to their internal structure. Time domain thermoreflectance (TDTR) is utilized to measure the thermal conductivity, the electrical properties are measured with a hall measurement setup. The latter is the focus in layered thin films of polycrystalline ZnO and amorphous InGaZnO4 in which a considerable increase in the charge carrier concentration following the interface density could be demonstrated. The interfaces between a ZnO matrix, ZnO-benzene and AlOx layers are studied in detail in a hybrid ZnO/ZnO-benzene/AlOx system in which this work demonstrates, that these layers in ZnO can be as thin as a single atom/molecule, yet still form distinctive layers. However, these very thin layers of ZnO-benzene and AlOx are found to have little impact on the crystal growth of ZnO, but can act as effective barriers for ZnO crystal growth when 10 or more consecutive ALD/MLD cycles are utilized for each AlOx/benzene layer respectively. Finally the thermal conductivity in ZnO/benzene thin films is characterised, the database for the thermal conductivity in that system is significantly extended and thermal conductivities for irregularly layered structures are reported for the first time in ZnO/ZnO-benzene hybrid thin films. Analysis with multivariate data analysis of the database confirms that the interface density has the most pronounced effect on the thermal conductivity.
Novel iron-based oxide, hybrid and superlattice thin films - Versatility of atomic/molecular layer deposition
(2018) Tanskanen, Anne
School of Chemical Technology | Doctoral dissertation (article-based)
Atomic and molecular layer deposition (ALD and MLD) thin-film techniques may potentially yield novel materials not attained by other synthesis techniques. In particular, the superior feature common for both ALD and MLD is the precise control of the layer thicknesses which makes them well-suited for the fabrication of nanoscale thin films and superlattice (SL) structures. The aim of this work was to develop ALD/MLD processes for different iron oxide-based materials that could exhibit interesting magnetic, optical and redox properties. The work involved three types of thin film materials: iron oxides, iron-organic hybrids and oxide-hybrid superlattices. The requirements for the iron precursors were strongly exclusionary in order to be applicable to all the three process types. The suitable precursors found were cyclopentadienyl iron(II) dicarbonyl dimer and iron(III) chloride, which reacted with water to form iron oxide thin films. In addition to the common magnetite phase, a simple ALD process for the extremely rare ε-Fe2O3 phase could be developed. The ε-Fe2O3 phase is known for its enormous coercive field (20 kOe) and multiferroic properties. Stable iron-organic hybrid thin films were successfully deposited with hydroquinone, 4-aminophenol and terephthalic acid as the organic precursors. Among the new hybrid thin films, the crystalline iron terephthalate (Fe-TP) thin films were studied in more detail for their chemical structure. Iron was found to be at the trivalent state and bonded to the terephthalate entities so that the bonding for the carboxylates is of the bidentate chelation type. The SL structures were successfully deposited for both of the iron oxides. The optical properties and optical band gap values were determined for the plain ε-Fe2O3 and Fe-TP films and for their superlattices. In addition, the magnetic characteristics of ε-Fe2O3 thin films were analyzed. For the ε-Fe2O3 films the coercive field was determined to be 1.6 kOe, the transmittance was less than 50 % in the visible range and the indirect optical band gap was estimated to be 2.0 eV. The hybrid films were nearly transparent, having a band gap of 3.0 eV. In the SL thin films, a sudden increase in the band gap was observed when the ε-Fe2O3 layer thickness was decreased below ~2 nm, indicating towards a quantum confinement effect.
On the Structural and Magnetic Properties of B-site Ordered Double Perovkites
(2020) Tiittanen, Taneli
School of Chemical Technology | Doctoral dissertation (article-based)
Perovskites are one of the compound families that can be tailored to perform multiple scientiﬁcally and also technologically exciting and essential tasks. At the very early stage of perovskite history, right at the same time as the structure was solved, the ﬁrst application possibility was found in electronics. The introduction of BaTiO3 as a dielectric material greatly enhanced the performance of ceramic capacitors of the time. Not to a surprise, it is still used in the very same application, some 80 years after the discovery. Since then, perovskites have found their way to widespread and emerging applications, from capacitors to solid oxide fuel cells, piezoelectric transducers and actuators to second harmonic generators in lasers and to spintronics, not to forget phenomena purely of academic interest. This thesis aims to bring its portion to the pool of scientiﬁc data to be used to expand the knowledge of the perovskite structure and properties within. New double perovskites with tuned crystal structures were synthesised by introducing a set of elements from the Periodic Table and by applying a variety of solid-state chemistry synthesis methods. Not only synthesis and characterisation, but also multivariate analysis and prediction of new compounds were performed. The time and effort required to design, make and characterise each perovskite may be enhanced by utilising multivariate data analysis of the existing data. The main target of the multivariate data analysis was to ﬁnd new ferromagnetic B-site ordered double perovskite candidates. The proposed candidate compound should have ferromagnetic transition temperature within a reasonable temperature range in order to have an application without extensive or impractical effort to reach the operating temperature. Samples of two non-perovskite systems, (Sr,Ba)2FeSbO6 and Y2CuTiO6, were transformed into a variety of double perovskites with different crystal structures by applying synthesis techniques ranging from ambient pressure cation substitution to high-pressure high-temperature treatment. The SIMCA multivariate data analysis provided insight into ferromagnetic B-site ordered double perovskites. Agreement between the measured and predicted ferromagnetic transition tempera- tures were found as a result of the chosen multivariate analysis methods. Some non-perovskite compounds, previously synthesized but uncharacterised double perovskites and completely new stoichiometries were proposed as new ferromagnetic double perovskite candidates.
Photoluminescence and upconversion properties of lanthanide-based atomic and molecular layer deposited thin films
(2023) Ghazy, Amr
School of Chemical Technology | Doctoral dissertation (article-based)
Photoluminescence of trivalent lanthanide (Ln) ions is highly relevant to applications such as solar cells, light emitting devices and biological imaging. Organic molecules may potentially be utilized to enhance and tune the luminescence of Ln3+ ions when properly combined into Ln-organic hybrids. Such on-demand tailored materials could pave the way to various next-generation applications, especially if the materials could be produced in high-quality thin-film form. However, the conventional thin-film techniques of Ln-organic materials lack the ability to combine the well-controlled deposition and the tunability of the luminescence proper-ties to the needs of various applications. To address these challenges, efforts to apply the strongly emerging atomic/molecular layer deposition (ALD/MLD) method for lanthanide-organic thin films started recently. While ALD/MLD offers well controlled process development and growth of thin films, the tunability of the luminescence properties of such films has been critically difficult to achieve. In this thesis, the tunability issue was one of the central research questions, and it was addressed by developing a number of ALD/MLD processes with novel organic components. Within the scope of the thesis, the following organic precursors were inves-tigated for the first time in the context of ALD/MLD: pyridine-3-carboxylic acid (PDA), cytosine (Cyt), 1,3,5-triazine-2,4,6-triol (TZO), and 2-hyrdoxyquinoline-4-carboxylic acid (HQA). Among these, PDA and Cyt were found particularly interesting as they allowed the remarkable tuning of the absorption and excitation properties of the lanthanide-organic thin films by shifting the excitation wavelength from the typical 250 nm up to 365 nm. More careful selection during this work led to the development of Eu-HQA thin films, which can be excited through an exceptionally wide excitation wavelength range from ultraviolet light of 185 nm up to visible light of 400 nm. Luminescent thin films that can be excited with visible light are of particular interest to biological imaging applications. Therefore, the new Eu-HQA thin films were tested for the Förster resonance energy transfer mechanism, which is used in various bioimaging and detection techniques. As another way to tune the luminesce properties challenged in the thesis, different Ln3+ were combined into a single thin-film. Here, the combination of Eu3+, Tb3+, and Er3+ yielded interesting thin films with photoluminescence emissions that could be controlled between green, red and white light. On the other hand, using Er3+ and Ho3+ provided a promising means to achieve upconversion emission, through which near-infrared light can be converted to visible light. These latter results could provide a route to enhance the performance of solar cells that suffer from weak absorption of infrared light.
Role of potassium hydroxide in fouling and fireside corrosion processes in biomass-fired boilers
(2020) Blomberg, Tom
School of Chemical Technology | Doctoral dissertation (article-based)
Slagging, fouling and fireside corrosion in biomass fuelled boilers and gasifiers are major obstacles that diminish the efficiencies of the energy transformation processes in these systems. High chemical to thermal and thermal to electric conversion efficiencies require high surface temperatures in the equipment construction components. This leads to increased risks of shortening the service life times of the components, when biomass based fuels are used in place of fossil fuels. The inorganic part of biomass fuels are different from fossil fuels and therefore lead to different ash behaviour in the combustors. Sulphur contents in fossil fuels are typically much larger than in biomass fuels, whereas the potassium contents in bio-mass based fuels are typically larger than in fossil fuels. During combustion, potassium is initially released in the gas phase as elemental potassium, potassium hydroxide or potassium chloride. These compounds then react with SiO2(s,l,g), SO2(g)/SO3(g), HCl(g), and CO(g)/CO2(g) in the combustor and play a major role in the slagging, fouling and corrosion processes. Most of the published scientific work done so far to understand these processes conclude that KCl(s,l,g) is the most important potassium compound responsible for the slagging, fouling and corrosion problems. The importance of KOH(s,l,g) in these processes has gained a lot less attention. In this thesis, the possible effect of KOH(s,l,g) in the fouling and corrosion issues was studied. Categorizing biomass and fossil fuels based on the free potassium content (potassium not bound as chloride or sulphate) was found to separate the biomass based fuels from fossil fuels. Biomass has typically free potassium, whereas fossil fuels do not. This difference suggests that during combustion and gasification, the potassium in the gas phase may exist as KOH(g) in the flue gases near the heat transfer surfaces. In addition, a correlation of KOH(g) with the corrosion rates of different steels in a straw fired boilers was found. Furthermore, using in-situ electrochemical galvanic probe measurements in a full-scale biomass fired boiler showed that the electrochemical signal activity increases abruptly at ≈ 400 °C. This may indicate the melting temperature of the condensing layer, which is very close to the melting point of KOH (406 °C), suggesting that condensation of KOH(l) can happen before reaction with CO2(g) to form K2CO3(s). Finally, using laboratory exposures of KOH(s,l) and KCl(s,l) with Cr2O3(s) and Fe2O3(s) it was found out that the formation of K2CrO4(s) could be explained for both salts with similar mechanism: 4KOH(s,l) + Cr2O3(s) + 1½O2(g) → 2K2CrO4(s) + 2H2O(g). This result sheds light on the initial breakdown mechanism of the protective oxides on high temperature steels in biomass combustion and gasification atmospheres. Overall, the conclusion is that KOH(s,l,g) may be a major reaction intermediate taking part in the slagging, fouling and corrosion mechanisms in biomass fired combustors.
Strongly Correlated Oxides - Half-metallicity in Chromium-based Rutiles and Quantum Magnetism in Copper-based Double Perovskites
(2018) Mustonen, Otto
School of Chemical Technology | Doctoral dissertation (article-based)
Strongly correlated oxides are materials with significant electron-electron correlation effects arising from the Coulomb repulsion between localized electrons. While very difficult to model by theory, these materials exhibit a wide range of novel properties and phenomena of both fundamental and technological interest in solid state chemistry and physics. In this thesis two types of strongly correlated materials are investigated: half-metallic ferromagnets and quantum spin liquids. Chromium dioxide is an archetypical half-metallic ferromagnet. It is simultaneously ferromagnetic and metallic, a property of significance in spintronics applications. In this thesis, chemical substitution tuning was investigated in the CrO2-VO2 system as a route for discovering new half-metals. While density functional theory calculations predicted the V-for-Cr substituted phases to be half-metallic similar to CrO2, in practice the materials became antiferromagnetic. This revealed a significant potential pitfall in the search for new half-metals by entirely ab initio methods: the difficulty of correctly predicting oxidation states in materials with multiple transition metals. Quantum magnetism on spin-1/2 square lattices has been of significant theoretical and experimental interest in condensed matter physics for three decades. This is due to the discovery of high-temperature superconductivity in these materials in the 1980's. Specifically, a quantum disordered ground state has been predicted to emerge when nearest-neighbor and next-nearest neighbor interactions compete. In this dissertation, the first experimental evidence of such a state is presented. The perovskite compounds A2CuB''O6 are an ideal system for studying spin-1/2 square-lattice antiferromagnetism. A method for tuning magnetic exchange using diamagnetic d10/d0 cations on the B'' site was developed in this dissertation. This d10/d0 method allowed the tuning of the ground state into the quantum critical regime for the first time. Spin-liquid-like behavior was observed in the compound Sr2Cu(Te0.5W0.5)O6. A thorough investigation of the Sr2Cu(Te1-xWx)O6 system revealed the suppression of magnetic order in a wide composition range.
Structural Principles of A-site ordered double perovskites: ferroelectric CaMnTi2O6 as a model system
(2024) Albrecht, Elisabeth Katharina
School of Chemical Technology | Doctoral dissertation (article-based)
Perovskites are a broad and versatile class of crystalline materials. Their unique structure combined with the countless possible ionic combinations makes them highly tunable and gives rise to many possible properties such as piezo-, pyro-, and ferroelectricity as well as ferromagnetism and superconductivity. Double perovskites even enhance the possibilities of tuning the material. Perovskites as functional materials are used in many applications such as sensors and energy storage and the demand for new materials with tailored properties is still rising. Nowadays, many high-performing functional materials, such as the ferroelectric, perovskite-like material lead zirconate titanate (PZT) are made of harmful or rare elements. Using computational methods to find more sustainable materials and experiments to synthesize and analyze their properties is a major task for today's materials scientists. For this thesis, first principles hybrid density functional theory is used to computationally approach A-site double perovskites in general and CaMnTi2O6 in particular. High-pressure/high-temperature synthesis is used to synthesize bulk samples of Ca2-xMnxTi2O6 with x = 0.2 to 1.0 and single crystal samples of CaMnTi2O6 which are analyzed by powder and single crystal X-ray diffraction together with Rietveld refinement, scanning electron microscopy, ferroelectric measurements, and Raman spectroscopy. The predictive power of a new tolerance factor for A-site double perovskites is studied. While it still can be used as a starting point, the new tolerance factor does not perform as well for A-site double perovskites as it does for B-site double perovskites and simple perovskites due to possible large size differences in A-site ions. Computational methods to predict and confirm the cation ordering and tilt system in A-site ordered double perovskites, demonstrated on CaMnTi2O6, prove to be a suitable addition to the new tolerance factor. Structural changes, as well as changes in the dielectric (ferroelectric) behavior, determine the centrosymmetric to non-centrosymmetric symmetry transition in Ca2-xMnxTi2O6 to be around x = 0.3 and x =0.4. A synthesis of CaMnTi2O6 single crystals can be realized at lower temperatures and at lower pressures than previously reported.
Transport-property tailored thin films for thermoelectrics through atomic/molecular layer deposition
(2023) Ghiyasi, Ramin
School of Chemical Technology | Doctoral dissertation (article-based)
Atomic layer deposition (ALD) creates a unique opportunity for effective materials nanostructuring. In this thesis, ALD, along with its other form, molecular layer deposition (MLD) and spin coating (SC), are utilized to alter the electrical, thermal, and structural aspects of thin films. Such alterations can be advantageous in various applications; however, thermoelectrics has been the subject of this study as a proof of concept. Thermoelectric energy harvesters are an intriguing group of materials capable of transforming thermal gradient to electrical potential and vice versa. The sequential nature of ALD and its independent deposition parameters provide tools for modifying the deposited films. Higher electrical and lower thermal conductivities are needed to achieve a better thermoelectric material. However, achieving this goal can be challenging due to the thermal conduction via electrons. The only option left is to suppress the thermal conductivity via phonons. The current research has illustrated that even a minimal change in deposition parameters, such as purge time after the metal precursor pulse, can improve thermoelectric performance through defect formation. Consequently, increase the carrier conduction via electrons and decrease the thermal conduction via phonons. Another approach to improving thermoelectric performance is to create interfaces inside the films. In the current study, this was carried out by growing superlattice films using a combination of ALD, MLD, and SC. Specifically, superlattice films of ZnO with different organics, i.e., p-phenylenediamine (PPD), hydroquinone (HQ), terephthalic acid (TPA), 4,4'-oxydianiline (ODA), and cellulose nanocrystals (CNCs) were prepared and studied. The results indicate that different organic compounds can have distinct effects on the inorganic matrix. This thesis focuses primarily on ZnO as an ideal n-type thermoelectric material. However, a p-type equivalent must be coupled with the n-type ZnO to complete the thermoelectric module. To address this issue, here, a versatile ALD process for SnO was developed in this study. The obtained films were analyzed and confirmed to be pure SnO films.