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Browsing by study line "Inorganic Materials Chemistry"

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  • Lasonen, Valtteri (2022)
    Modern semiconductor devices require sophisticated patterning techniques that not only offer excellent resolution but also high throughput, low cost, and low number of errors. And because these devices require several patterning steps, even a slight improvement in a patterning technique can have a huge impact. New patterning technique that has a great potential to be used in many of these patterning steps is area-selective etching of polymers by catalytic decomposition. The catalytic effect can either be an intrinsic property of the underlying material, or materials can be catalytically activated/deactivated to achieve the desired pattern. This new technique is self-aligning and extremely simple, and therefore has a potential to significantly reduce the number of errors and cost, while having excellent resolution and throughput. In the literature review part of this thesis, we will have an overview of different aspects that must be considered when using polymers as thermocatalytically decomposable resists. Polymers are already widely used as resists in several patterning techniques due to an immense number of different polymers available, allowing almost endless possibilities to adjust the properties of the resist. Important polymer properties to consider include adequate gas permeability for the etching gases and the decomposition products, decomposition and degradation mechanisms, reflow, integrity during the patterning and the deposition processes, and adhesion to the substrate. Different catalysts and catalytic decomposition mechanisms of polymers as well as other carbon-containing compounds in different atmospheres are reviewed. Because area-selective etching of polymers is a new technique many challenges are still unknown. Therefore, this thesis is mainly aimed to give ideas and directions for the future research. In the experimental part, several metals and metal oxides were tested for their catalytic effect for decomposing poly(methyl methacrylate) (PMMA) in air and H2-atmosphere. Pt, Ti, and CeO2 were confirmed to have a catalytic effect in air, whereas SiO2 and Al2O3 showed no catalytic effect. In the H2-atmosphere, only Ti and Cu showed some promising catalytic effect, whereas SiO2, Al2O3, CeO2, Pt, W, Ni, and Co did not. Additionally, experiments were conducted to find out how thin CeO2 film has an adequate catalytic effect. And finally, the area-selectivity of this patterning technique was tested in the air atmosphere using CeO2 as a catalytic surface and Al2O3 as a non-catalytic surface.
  • Ojala, Juha (2022)
    Photocatalysis is a versatile method to use solar energy for chemical processes. Photocatalytic materials absorb light to generate energetic electron-hole pairs that can be used for redox reactions in production of hydrogen and other chemicals, degradation of pollutants, and many other applications. BiVO4 is a visible light absorbing oxide semiconductor with a band gap of about 2.4 eV, and it has received a lot of attention as a standalone photocatalyst and as a photoanode material. The literature part of this thesis explores how the electronic structure of semiconductors and the different processes in photocatalysis together affect the efficiency of the method. Semiconductor materials are classified based on their chemical composition and compared by selecting most researched materials as examples. Various strategies to improve the photocatalyst material properties are also discussed. Many strategies, such as nanostructured photocatalysts, benefit from deposition of semiconductor thin films. Atomic layer deposition (ALD), as a highly conformal and controllable chemical vapor deposition method, is an excellent choice for depositing semiconductors and various interfacial layers. The literature review also includes a survey of ALD processes for Bi2O3 and V2O5 and a thorough analysis of the existing BiVO4 ALD processes. From the selection of binary ALD processes, bismuth(III) 2,3-dimethyl-2-butoxide (Bi(dmb)3), tetrakis(ethylmethylamido)-vanadium(IV) (TEMAV), and water were chosen as precursors to develop a new ALD process for BiVO4. The binary processes were combined in various metal precursor ratios both completely mixed in supercycles and as nanolaminates, and the resulting films were annealed to crystallize the BiVO4. X-ray diffraction was used to characterize the crystalline phases of the films, and it was noticed that TEMAV reacts with Bi2O3 to make metallic bismuth, but it is reoxidized by annealing. Composition of the films was investigated with energy dispersive X-ray spectrometry and time-of-flight elastic recoil detection analysis (ToF-ERDA). Some sensitivity to process conditions was observed in the deposition, as the metal stoichiometry varied in unexpected manner between some sets of experiments. ToF-ERDA depth profiles also revealed that mixing of the nanolaminate layers was incomplete with annealing temperatures below 450 °C and with laminate layers over 10 nm in thickness. Scanning electron microscopy was used to study the morphology of the films and revealed a granular, non-continuous structure. The optical properties of the films grown on soda-lime glass were investigated with UV-vis spectrophotometry. The band gaps of the films were estimated to be 2.4–2.5 eV. The nanolaminate approach to depositing the films was deemed the best, as it avoids most of the reduction of bismuth by TEMAV. However, it is still not clear why this process is so sensitive to process conditions. This should be investigated to further optimize the film stoichiometry. The morphology of the films might be improved by using different substrates, but it is not a critical aspect of the process as there are methods to passivate the exposed substrate surface. Overall, this process has potential to deposit excellent BiVO4 films that are suitable for further research pertaining their photocatalytic properties and modifications such as nanostructured or doped photoanodes.
  • Terletskaia, Mariia (2023)
    In recent decades, more and more attention has been paid to solar energy because of the need to ensure “green” and sustainable future. Solar cells have been treated as one of the most promising technologies for solar energy utilization. Since conversion of sunlight into electricity mainly passes through the light absorbing material (absorber), its optoelectronic properties largely determine the cell performance. Among the existing absorbers, inorganic lead-free perovskites, like CsSnI3, are of great interest due to high potential efficiency, increased stability and the absence of toxic components. However, currently used fabrication techniques limit quality of the materials and their application in large-scale production. Atomic layer deposition (ALD) is a thin film fabrication technique which is now widely used in electronics and optoelectronics. Based on the principle of sequential saturated surface reactions, it is able to provide almost atomic level control over the thickness and composition of the film. Moreover, the principle ensures the formation of uniform films on large surfaces. Since precise composition control and scalability are of great importance for efficiency of perovskite solar cells, ALD acts as an excellent tool for production of this type of absorbers. The literature review of this thesis examines perovskites as absorber material for commercially efficient solar cells. The aim is to give the reader an overview of solar cell performance, currently available absorber materials and motivation for perovskites to become promising cost-efficient solution. Additionally, the most common fabrication techniques for perovskite structures are introduced together with limitations to emphasize the expediency of further experiments. The experimental part combines development of SnI2 thin film deposition in ALD reactor with a subsequent use of the technique in conversion to perovskite for future solar cell application. Unfortunately, the applicability of SnI2 ALD with proposed chemical process became doubted due to multitude issues that arose during the investigation. However, successful results on SnI2 pulsed chemical vapor deposition (pCVD) in the same ALD reactor supported feasibility of the chemical process. Application of the optimized pCVD technique for the conversion of CsI thin films, prepared by ALD, made it possible to obtain phase-pure CsSnI3 perovskite. In addition, conversion part demonstrates that use of SnI2 pCVD allows the formation of uniform and conformal perovskite thin films with promising band gap of 1.7 eV.
  • Heczko, Vilma (2021)
    Plasmonic catalysis utilises light energy to drive chemical reactions. Compared to conventional catalytic processes, which are run by high temperatures and pressures, light-driven processes can lower energy consumption and increase selectivity. Conventional plasmonic nanoparticles (Ag, Au) are relatively scarce and expensive, and therefore the use of materials with earth-abundant elements in plasmonic catalysis is widely pursued. Despite their good optical properties, plasmonic nanoparticles are often unsuitable catalysts. Hybrid catalysts, structures consisting of a light-harvesting plasmonic part and a catalytical centre of different material, have emerged as an opportunity to address these challenges and obtain desired properties. This thesis consists of two parts: In the first part, properties of plasmonic materials are described, and previous studies of hybrid catalysts with earth-abundant plasmonic materials are reviewed. Experimental work on plasmonic-catalytic nanohybrids, with TiN as the plasmonic part and Pd as the catalytic entity, is described in the second part. In this context, a Pd/TiN (Pd nanoparticles supported into TiN) catalyst was synthesised, characterised and applied to test catalytical reactions. Contrary to the hypothesis, light-induced rate enhancement was not observed in our current catalytical studies. These results call for further optimisation of synthesis and reaction conditions to prepare an earth-abundant, light-active catalyst.
  • Keränen, Laura (2021)
    Tutkielman kirjallisuusosassa tarkastellaan johtavien metalli-, oksidi- ja nitridikalvojen kasvattamista epitaksiaalisesti strontiumtitanaatille. Epitaksiaalisia kalvoja on kasvatettu fysikaalisilla kasvatusmenetelmillä, kuten laserpulssikasvatuksella, elektronisuihkuhöyrystyksellä ja sputteroimalla, sekä kemiallisilla kasvatusmenetelmillä, kuten atomikerroskasvatuksella, sooli-geeli-menetelmällä sekä metalliorgaanisella kemiallisella kaasufaasikasvatuksella. Useiden tekijöiden, kuten substraattien lämpötilan ja esikäsittelyn todettiin vaikuttavan kalvojen orientaatioon. Kokeellisessa osassa iridium- ja platinaohutkalvoja kasvatettiin (100)-orientoiduille strontiumtitanaattisubstraateille atomikerroskasvatuksella. Iridiumkalvojen lähtöaineina käytettiin iridiumasetyyliasetonaattia sekä happea tai otsonia ja vetyä. Platinakalvojen lähtöaineina käytettiin platina-asetyyliasetonaattia, otsonia ja vetyä tai metyylisyklopentadienyylitrimetyyliplatinaa ja happea. Kalvojen rakennetta ja tekstuuria tutkittiin θ-2θ- ja in plane -röntgendiffraktiolla. Osaa iridiumkalvojen poikkileikkauksista tutkittiin myös läpäisyelektronimikroskopialla. Iridiumkalvojen todettiin olevan vahvasti (100)-orientoituneita, mutta monikiteisiä. Platinan (h00)-piikkejä ei kyetty erottamaan substraatin (h00)-piikeistä, mutta vahvojen (111)-piikkien perusteella kalvot eivät olleet epitaksiaalisia. Kalvojen kuumentaminen lisäsi (111)-orientaatiota molemmissa metalleissa.
  • Porri, Paavo (2022)
    Ensuring adequate air quality is integral to healthy living. Since in modern societies the majority of time is spent indoors, understanding indoor air pollution and the means of air purification are of great importance. Adverse health effects are induced by volatile organic compounds (VOC) that originate from everyday activities and our surroundings. Photocatalysis is a radiation-activated chemical transformation that can be used to decompose organic pollutants into harmless constituents. However, existing air purification solutions employing photocatalysis often rely on UV light limiting the use of solar radiation. Titanium dioxide is a popular photocatalyst material, but it requires modification to its electronic properties to respond to visible light. An established approach is to introduce atoms of other dopant elements into the titania lattice. Atomic layer deposition (ALD) is a thin film deposition technique widely studied especially in metal and metal oxide research. Following from the principle of sequential saturation of the surface, control over the size and composition of the film may reach atomic level. Since the chemical configuration of a doped TiO2 film is of utmost importance to successful modification, ALD is an excellent tool to examine suitable photocatalytic TiO2 chemistries. Furthermore, thin solid films of catalytically active material would have a distinguished advantage for deployment in real-life settings over their powderous counterparts. The literature review of this thesis explores the semiconductor photocatalysis with an eye on its suitability to indoor air purification. The motivation is to give the reader a view on the air quality issue, the existing technological solutions and how a thin film photocatalyst could supplement the field. Titanium dioxide doping concepts are introduced to elucidate the rationale behind the experimental efforts. The experimental part describes a development project to deposit visible-light responding photocatalysts. Titanium dioxide thin films co-doped with nitrogen and zinc/fluorine were grown on steel plates. An in-house built reactor system was used to study acetaldehyde degradation under irradiation. Unfortunately, the reactor experienced a malfunction, rendering a large part of the results futile. Moreover, months of valuable time were lost in chasing a mirage of fallacious data. In the end an ALD grown photocatalyst responding to visible light could not be materialized.
  • Lehtinen, Miko (2022)
    Molecular hydrogen is considered as the primary alternative to replace fossil fuels for future energy supply. Hydrogen can be produced sustainably through electrocatalytic hydrogen evolution reaction which is a vital step in water electrolysis. So far, the efficiencies of electrochemical and photoelectrochemical water electrolysis systems are too low to satisfy the demands for hydrogen on a commercial scale. Plasmonic nanostructures containing a plasmonic and a catalytic component hold great promise for enhancing the performance of typical water electrolysis systems through plasmonic photocatalysis utilizing localized surface plasmon resonance excitation. Here, a novel plasmonic-catalytic u@AgPd nanorattle is synthesized, characterized, and investigated for plasmon-enhanced hydrogen evolution reaction to provide new insights into the design of light-assisted water electrolysis systems. The nanorattle exhibited significant improvements of performance towards hydrogen evolution reaction under 427 nm illumination, displaying a near 2-fold current increase and a decreased overpotential of 58 mV at a current density of 10 mAcm-2. The material is evidenced to plasmon-enhance the electrocatalytic performance through a combination of charge transfer and local heating mechanisms.
  • Pesonen, Leevi (2024)
    Rare earth trifluorides are a group of 17 compounds which have intriguing optical, electrical, and luminescence properties. However, realizing these properties in the form of thin films has had its challenges. Overall, research on the subject has been scarce. On the other hand, some rare earth fluoride thin films have found usage in for example optical filters in ultraviolet and infrared wavelengths.In this thesis a review of rare earth fluoride thin films and their deposition methods is made. Potential of the rare earth fluoride thin films is explored starting from the bulk properties of the rare earth fluorides which are compared to the published results for thin films. Additionally, the current status, challenges, and potential of rare earth fluoride thin films is discussed in the light of different deposition methods and their differences. In the experimental part of the thesis, deposition of holmium fluoride thin film by atomic layer deposition (ALD) is studied alongside its properties. In the HoF3 deposition, Ho(thd)3 (thd = 2,2,6,6- tetramethyl-3,5-heptanedionato) and niobium pentafluoride were used as precursors, latter of which was used as an ALD fluoride precursor for the first time.
  • Pustorino, Gregory (2023)
    Bimetallic core-shell catalysts represent a new pathway to create highly selective and highly active catalysts. This can be done by using a relatively inactive metal as the core material and a more active metal as the shell material. The composition of both the core and shell structure can then be altered in order to tune the selectivity of the nanocatalyst. The synergistic effects of using bimetallic core-shell catalysts arise in part from the misfit strain that is encountered as a result of the difference in lattice spacings between the core and shell materials. The catalysts investigated here consist of an Au core and a Pd shell. Particles with four different Pd shell thicknesses were synthesized and the corresponding strain was measured. There is a 5.07% difference in the lattice spacings between Au and Pd, we therefore expect strain values to be near this amount. In this work, we directly measured the displacement fields that arise due to lattice mismatch in Au-Pd nanorods using High Resolution Scanning Transmission Electron Microscopy (HRSTEM) and 4D Scanning Transmission Electron Microscopy (4D-STEM). The strain was then calculated using three different analytical methods: Geometric Phase Analysis (GPA), Gaussian Peak Fitting, and nanodiffraction. These methods all measure the variations in local lattice parameters and plot these values for every pixel in the original STEM image, this results in a 2D strain map. These maps were then compared to see which produced the highest quality strain quantification.
  • Otaki, Miho (2019)
    In terms of nuclear waste management, the behavior of radionuclides with long half-lives, such as I-129, is of special concern especially for the final depository of nuclear waste. In addition, generally speaking, iodine is highly mobile and easily transferable to the natural environment. Furthermore, because iodine is an essential element for the synthesis of thyroid hormones, it accumulates in the human thyroid. Thus, radioactive iodine can also be the greatest potential danger of dose uptake for humans. Among many kinds of iodine species, it is rather challenging to separate iodate selectively from other anions and thus it is necessary to investigate new materials which can adsorb iodate efficiently for the removal of radioactive iodine. In this study, the iodate adsorption ability of hydrous zirconia has been investigated. Hydrous zirconia has been reported as an anion-exchanger, and because of its stability, this material is a promising candidate for selective iodate removal from radioactive waste solutions. White solid of hydrous zirconia was successfully synthesized with an amorphous structure. Its surface showed a character in between amphoteric and basic. The isotherm indicated that the material has a preference to adsorb iodate and the saturation value of adsorption was estimated to 1.8 mmol/g. The material showed lower uptakes as pH got higher. Among several competing anions tested, divalent sulphate ions suppressed the iodate adsorption to some extent due to higher affinity to the material surface. In a basic environment, boric acid also suppressed strongly the adsorption probably because of the formation of tetrahydroxyborate with hydroxide sites on the material surface. These suppressions of iodate adsorption became stronger as the concentration got higher. Post-heating at 400 °C resulted in the transformation of the material structure to tetragonal and a slight improvement of iodate adsorption rate. As the temperature of post-heating got higher, the structure became more monoclinic and showed the lower uptakes, which may be due to the loss of hydroxide sites. A column setup of the material with simulant of wastewater from Fukushima Daiichi Nuclear Power Plant has been operating and approximately 11,000 bed-volume of the solution has been gone through, but still, the column is yet to reach a 100% breakthrough. Based on the results presented in this study, it can be concluded that synthesized hydrous zirconia showed clear iodate preference and a possible high performance for the waste treatment from nuclear power plants.
  • Airola, KonstaPetteri (2022)
    Aluminium nitride is a piezoelectric material commonly used in piezoelectric microelectromechanical systems (MEMS) in the form of thin films deposited by sputtering. AlN-based devices are found in wireless electronics in the form of acoustic filters, but they also have prospective applications in a wide variety of sensor systems. To enhance the piezoelectric properties of AlN, some of the Al can be replaced with scandium, which is required for next-generation devices. However, addition of Sc makes both the deposition and patterning of the film more difficult. This work focuses on patterning of AlN and Sc0.2Al0.8N thin films with wet etching. Both materials are etched anisotropically, which in theory enables etching the materials with little deviation from the mask dimensions. However, in practise, undercutting at the mask edges occurs easily making the structures narrower compared to the etch mask. This work investigates and compares the mechanisms and etch rates of AlN and Sc0.2Al0.8N. Tetramethyl ammonium hydroxide was mostly used for etching, but also H3PO4 and H2SO4 were tested. Addition of 20 atom-% Sc lowered the etch rate of the material and resulted in more undercutting. The causes behind mask undercutting were examined by using 11 differently deposited etch masks, and the undercutting was minimized by optimizing the mask deposition, using thermal annealing, and optimizing the etching temperature. Finally, the work identifies and discusses the relevant factors in depositing and patterning the AlN, ScxAl1-xN and mask films.