Browsing by master's degree program "Magisterprogrammet i materialforskning"

Halide perovskites are a promising materials class for solar energy production. The photovoltaic efficiency of halide perovskites is remarkable but their toxicity and instability have prevented commercialization. These problems could be addressed through compositional engineering in the halide perovskite materials space but the number of different materials that would need to be considered is too large for conventional experimental and computational methods. Machine learning can be used to accelerate computations to the level that is required for this task. In this thesis I present a machine learning approach for compositional exploration and apply it to the composite halide perovskite CsPb(Cl, Br)3 . I used data from density functional theory (DFT) calculations to train a machine learning model based on kernel ridge regression with the many-body tensor representation for the atomic structure. The trained model was then applied to predict the decomposition energies of CsPb(Cl, Br)3 materials from their atomic structure. The main part of my work was to derive and implement gradients for the machine learning model to facilitate efficient structure optimization. I tested the machine learning model by comparing its decomposition energy predictions to DFT calculations. The prediction accuracy was under 0.12 meV per atom and the prediction time was five orders of magnitude faster than DFT. I also used the model to optimize CsPb(Cl, Br)3 structures. Reasonable structures were obtained, but the accuracy was qualitative. Analysis on the results of the structural optimizations exposed shortcomings in the approach, providing important insight for future improvements. Overall, this project makes a successful step towards the discovery of novel perovskite materials with designer properties for future solar cell applications.

In this thesis, sputtering of several low- and high-index tungsten surface crystal directions are investigated. The molecular dynamics study is conducted using the primary knock-on atom method, which allows for an equal energy deposition for all surface orientations. The energy is introduced into the system on two different depths, on the surface and on a depth of 1 nm. Additionally to the sputtering yield of each surface orientation, the underlying sputtering process is investigated. Amorphous target materials are often used to compare sputtering yields of polycrystalline materials with simulations. Therefore, an amorphous surface is also investigated to compare it's sputtering yield and process with crystalline surface orientations. When the primary knock-on atom was placed on the surface all surface orientations had a cosine shaped angular distribution with little variation in the sputtering yield for most of the surface orientations. Linear collision sequences were observed to have a large impact on the sputtering yield when the energy was introduced deeper inside the material. In these linear collision sequences the recoils are traveling along the most close packed atom rows in the material. The distance from the origin of the collision cascade to the surface in the direction of the most close packed row is therefore crucial for the sputtering yield of the surface. Surface directions with high angles between this direction and the surface normal hence show a reduction in the sputtering yield. The amorphous material had a little lower sputtering yield than the crystalline materials when the primary knock-on atoms was placed on the surface whereas the difference rose into several orders of magnitude when the energy was given at 1 nm. It is impossible for linear collision sequences to propagate long distances in the amorphous material and therefore the angular distribution in both cases is cosine shaped. The amorphous material has no long range order and was therefore unable to reproduce the linear collision sequences, which are characteristic for the crystalline materials. The difference in the sputtering yield was hence up to several orders of magnitude as a result when the energy was introduced at 1 nm depth.

Further proof of the unique morphologies of water-soluble poly(2-isopropyl-2-oxazoline)-block-poly(DL-lactide) and poly(2-isopropyl-2-oxazoline)-block-poly(L-lactide) (PiPOx-b-PDLLA and PiPOx-b-PLLA) nanoparticles was obtained via Fluorescence Spectroscopy. Additionally, loading and release studies were carried out with hydrophobic curcumin molecules to outline the potential of the amphiphilic block copolymers in drug delivery applications. To study the morphology of the nanoparticles, absorption and emission spectra of pyrene were measured in water dispersions of the nanoparticles at several concentrations. The obtained I1/I3, I337/I333.5 and partitioning constant (Kv) values were compared to corresponding data from a control core/shell nanoparticle poly(ethylene glycol)-block-poly(DL-lactide) (PEG-b-PDLLA). Of the three different amphiphilic polymers, PEG-b-PDLLA showed the smallest and PiPOx-b-PDLLA the highest Kv value. This indicates, that PiPOx-b-PDLLA core is less hydrophobic and looser compared to the dense cores of PEG-b-PDLLA and PiPOx-b-PLLA, making it capable of encapsulating the greatest amount of pyrene. In the loading and release studies, the nanoparticles were loaded with curcumin and placed in dialysis against PBS Tween® 80 solution. Curcumin content of the samples was monitored over a week by measuring the emission spectra of curcumin. PiPOx-b-PDLLA showed greater potential as a drug delivery agent: It formed more stable nanoparticles, showed higher loading capacities, higher encapsulation efficiencies and slower release rates. Flash nanoprecipitation method (FNP) was also used to prepare the same nanoparticles with and without encapsulated curcumin. In addition to the encapsulation efficiencies, sizes of the nanoparticles were determined via dynamic light scattering (DLS). PiPOx-b-PLLA forms the smallest nanoparticles with lowest encapsulation efficiencies, thus agreeing well with the higher density of PLLA core. All three investigated amphiphilic copolymers formed stable nanoparticles in water at room temperature. On the contrary, stability of the nanoparticles was found to be poor in saline solutions at body temperature. Mixing PEG-b-PDLLA with PiPOx-b-PLA in a ratio of 20:80 w-% increased the stability of the nanoparticles in physiological conditions simultaneously uncovering the thermoresponsive character of the PiPOx-blocks. Turbidity measurements of PEG-b-PDLLA mixed with PiPOx-b-PDLLA in ratio of 20:80 w-% showed slight decrease in transmittance at the 30 °C, which corresponds to the cloud point of PiPOx-b-PDLLA in PBS solution. However, it remains unclear, whether the increased stability is due to the PEG-b-PDLLA mixing in the same micelles with PiPOx-b-PDLLA, thus hindering the aggregation of the nanoparticles upon the cloud point of the PiPOx-blocks.

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.

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.

Ion beams have been the subject of significant industry interest since the 1950s. They have gained usage in many fields for their ability to modify material properties in a controlled manner. Most important has been the application to semiconductor devices such as diodes and transistors, where the necessary doping is commonly achieved by irradiation with appropriate ions, allowing the development of the technology that we see in everyday use. With the ongoing transition to ever smaller semiconductor devices, the precision required of the manufacturing process correspondingly increases. A strong suite of modeling tools is therefore needed to advance the understanding and application of ion beam methods. The binary collision approximation (BCA) as a simulation tool was first introduced in the 1950s. It allows the prediction of many radiation-related phenomena for single collision cascades, and has been adopted in many experimental laboratories and industries due to its efficiency. However, it fails to describe chemical and thermodynamic effects, limiting its usefulness where ballistic effects are not a sufficient description. Parallel to BCA, the molecular dynamics (MD) simulation algorithm was developed. It allows a more accurate and precise description of interatomic forces and therefore chemical effects. It is, however, orders of magnitude slower than the BCA method. In this work, a new variant of the MD algorithm is developed to combine the advantages of both the MD and the BCA methods. The activation and deactivation of atoms involved in atomic cascades is introduced as a way to save computational effort, concentrating the performed computations in the region of interest around the cascade and ignoring surrounding equilibrium regions. By combining this algorithm with a speedup scheme limiting the number of necessary relaxation simulations, a speedup of one order of magnitude is reached for covalent materials such as Si and Ge, for which the algorithm was validated. The developed algorithm is used to explain the behavior of Ge nanowires under Xe ion irradiation. The nanowires were shown experimentally to bend towards or away from the ion beam, and computational simulations might help with the understanding of the underlying physical processes. In this thesis, the high-fluence irradiation of a Ge nanowire is simulated and the resulting defect structure analyzed to study the bending, doubling as a second test of the developed algorithm.

Tutkielman kirjallisuusosuudessa on käyty läpi erilaisia kaupallisia biopolymeerejä, niiden synteesiä, käyttöä ja biohajoamista. Tutkielman pääpaino on erilaisten materiaalien biohajoamisessa ja näiden materiaalien kaupallisessa käytössä. Biohajoamisen evaluointiin tarkoitettuja standardeja, tutkimusmenetelmiä ja hyväksyntäkriteerejä on esitelty laajasti. Tutkimusosuudessa on valmistettu PLA:n ja PBAT:n seoksesta puukomposiitti ja materiaalin termomekaaniset ominaisuudet on karakterisoitu. Tavoitteena oli luoda biohajoava materiaali, jonka ominaisuudet ovat sellaisia, että sen kaupallinen hyödyntäminen kertakäyttömuovin korvikkeena on järkevää. Materiaalin mekaaniset ominaisuudet karakterisoitiin lopputuotteen kestävyyden, ja sulaominaisuudet kaupallisen tuotannon mahdollistamisen takia. Termomekaanisia analyysejä tehtiin materiaalin säilyvyyden ja lämpöominaisuuksien karakterisoimiseksi. Työssä on tutkittu myös puhtaan PLA/puukomposiitin biohajoamista meriympäristössä. Tutkimuksen tuloksena saatiin luotua riittävällä nopeudella biohajoava puukomposiitti, jonka mekaaniset ominaisuudet ovat riittäviä korvaamaan erilaisia kertakäyttöisiä muovituotteita ja joka on prosesoitavissa nykyisillä ekstruusiolaitteistoilla.

The complex physical mechanisms involved in the formation of vacuum arcs have been of interest for many decades. Vacuum arcs are relevant in many engineering disciplines, but the physics behind them is not yet fully understood. In recent years, there have been many experimental and computational studies focused on understanding aspects of vacuum arcs. This thesis focuses on further development of a simulation model to describe the physical processes starting from electron emission and leading to the formation of an ionized plasma. The FEMOCS code is extended to include plasma simulation based on previous work on ArcPIC. Emission of electrons and heating of the cathode is simulated using the finite element method, while plasma simulation is performed using the particle-in-cell method. We add evaporation of neutral atoms from the cathode, as well as ionization processes for multiple species of ions. Monte Carlo collisions for elastic, Coulomb, impact ionization, charge exchange and recombination collisions between particles are added. Direct field ionization of neutrals is included to account for ionization at high electric fields. A dynamic weighting scheme is described for adjusting superparticle weights during the simulation. Ion bombardment effects such as bombardment heating and sputtering are added to account for additional supply of neutrals resulting from energetic ions accelerated by the electric field. Finally, we add a circuit model for coupling to an external circuit. A static nanotip is simulated with different parameters to study local field thresholds leading to thermal runaway. We find that our simulations agree with experimental results. The most significant interactions contributing to initial formation of vacuum arcs are identified. We find most neutrals are created via evaporation rather than sputtering. The most important collision for plasma formation is impact ionization of neutrals into Cu+ ions, while higher-order ions are found to play a lesser role. Direct field ionization of neutrals is also found to be significant at high fields on the order of 10 GV/m.

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.

An optical data bus is a promising solution to provide a fast data transmission from room temperature to quantum devices at low temperatures, which would minimize the heat load into the cryogenic system compared to the conventional electrical cabling. Previously, a similar measurement setup was used to drive a Josephson junction array (JJA) with an optical pulse pattern generated with a mode-locked laser (MLL). A photodiode (PD) was used to convert optical signals to photocurrent signals to drive the JJA at low temperature. There was a long and non-ideal transmission line between the PD and the JJA at their operation temperature of 4 K. The experiments and simulations revealed that the non-idealities in the transmission line caused non-desired reflections. In this work the PD is integrated on a same chip as the JJA. The new on-chip integration provides shorter transmission line with less interfaces. The new compact transmission line promises less electrical signal reflections between the PD and the JJA. A custom-made MLL emitted an original optical single pulse to the optical pulse pair generator. The MLL was operated at a well-defined pulse frequency of 2.3 GHz to produce the single pulses in the desired frequency. An optical time delay circuit (OTD) was applied to the generated pulses in the MLL to time divide the pulses on the desired time delays. The generated optical pulse pair pattern was transmitted from room temperature to the PD in 4 K via an optical polarization maintaining fiber. The fiber was integrated on the top of the PD in 4 K, which was used to drive the JJA sample. The PD was biased with a reverse voltage and the JJA sample with a current. The amplification of optical twin pulses was varied during the measurements. We measured the DC voltage of the JJA sample and the DC photocurrent of the PD simultaneously. The measurement was repeated with several different manually defined optical time delayed twin pulses. The work included optimization of the optical setup. The optimization involved setting the reflective diffraction grating to the optimized position, which was used to filter away undesired wavelengths. The optical pulse pair method used in this work can be used to investigate the maximum speed of the data signals.