Author: VAIDAS PUDŽAITIS
Dissertation title: Surface enhanced infrared absorption spectroscopy of biomolecular layers and water at gold surface
Fields of science: Chemistry N 003
Scientific supervisor: prof. habil. dr. Gediminas Niaura
Defence of the dissertation: January 19, 2024
Author: KATSIARYNA CHARNIAKOVA
Dissertation title: Synthesis and characterization of functional coatings and nanoparticles
Fields of science: Chemistry N 003
Scientific supervisor: dr. Arūnas Jagminas
Defence of the dissertation: February 21, 2024
Author: GUSTAS LIAUGMINAS
Dissertation title: Nonlinear pulse formation in an optical fibers
Fields of science: Material Engineering T 008
Scientific supervisor: dr. Kęstutis Regelskis
Defence of the dissertation: May 15, 2024
Author: MATAS RUDZIKAS
Dissertation title: Investigation of optical and electrical characteristics in photovoltaic devices colored by the functional metal oxide coatings
Fields of science: Physics N 002
Scientific supervisor: dr. Arūnas Šetkus
Defence of the dissertation: May 29, 2024
SUMMARY: As solar energy continues to advance, it is accompanied by decreasing surface availability and its increasing costs, and alongside a seek of energy independence this drives an increasing demand for building-integrated photovoltaics. However, its expansion faces limitations due to the dominant PV colors in the market being blue or black, often not fitting with architectural ideas. Therefore, in this work cost-effective methodologies applying functional metal oxides for modification of solar cell color were analyzed, employing combined simulations with experiments while using commercial solar cells as a baseline. The simplest approach was deposition of ITO as a part of double layer anti-reflective coating on the top of the commercial silicon solar cell. It was proved that variation of ITO thickness from 0 to 240 nm allows modification of color from blue to brown resulting in relatively low current density loss up to 8%. Lower refractive index (1.8 at 632 nm) resulted in lower, blue-shifted and narrower reflection peaks in the surface reflection spectrum compared to a higher refractive index (2.07 at 632 nm). In the second part, Bragg reflector structure was optimized and then experimentally demonstrated using cheap sol-gel technique with only a 3 layered structure (TiO2/SiO2/TiO2), having 2-3 times higher color brightness compared to colored panels with commercial solar cells. Finally, coloring of kesterite/silicon 2 terminal tandem structures was investigated through digital modelling. The study revealed that ARC thickness variation of Al2O3 from 0 to 300 nm allows modification of color with an efficiency loss of less than 2% (relative) with color purity being low and hues restricted to brownish and purplish.
Author: TOMAS DAUGALAS
Dissertation title: Investigation of dependence of electrical properties on external mechanical force and electrical field in a structure with graphene sheet separating surfaces of conductors
Fields of science: Physics N 002
Scientific supervisor: dr. Arūnas Šetkus
Defence of the dissertation: June 26, 2024
SUMMARY: A promising technological direction for the next generation of electronic devices is based on combining bulk (3D) and two-dimensional (2D) materials. However, there is a lack of knowledge in one of the fundamental mechanisms of combined 2D-3D devices – charge transport perpendicular to the 2D material plane. This thesis presents a study of charge transfer perpendicular to a graphene plane, where a graphene monolayer is positioned between two conductive metallic planes and coupled by forces of electrostatic nature (van der Waals type contact). By creating external mechanical and electrical effects in the metal-graphene-metal (MGM) structure with the use of atomic force microscopy techniques, the atypical electrical characteristics of the system were measured as a function of applied external mechanical force. Using a physical model describing the system, the mechanisms underlying the atypical charge transfer were explained. In the second part of the thesis, the electrostatic interaction forces between the components of the MGM structure are investigated. The dependencies of the electrostatic forces on the applied external electric field were experimentally measured. The measured dependencies were described by a set of characteristic parameters following from a physical model of the system, thus evaluating the influence of the external voltage on the interaction forces between the components of the structure and determining the magnitude of the occurring internal electric field.
Author: ALGIMANTAS LUKŠA
Dissertation title: Investigation of the properties of graphene layers on insulating substrates: their dependence on growth conditions and interaction with humidity
Fields of science: Physics N 002
Scientific supervisor: dr. Arūnas Šetkus
Defence of the dissertation: June 26, 2024
SUMMARY: This thesis demonstrates the growth of nanocrystalline graphene layers using a plasma - enhanced chemical vapour deposition (PECVD) method. The aformentioned method allows the layer growth temperature to be reduced resulting in the lower temperature than the standard layer growth process. The layers were grown directly on a dielectric substrate without the use of an additional catalyst. The study shows that changing the growth parameters of nanocrystalline graphene changes the optical properties of these layers, which can be brought closer to graphene monolayer, graphite or amorphous carbon. Additionally, it was demonstrated that the characteristics of droplet drying on the surface of the layers depend on the growth parameters, which can be used for quick characterization of the layers. The applicability of the layers grown in this work was demonstrated by developing a laboratory sensor model capable of detecting a water droplet based on changes in resistance when the droplet is both on the surface of the layer and above it.
Author: PAULIUS ŠLEVAS
Dissertation title: Methods for the formation of structured light beams and their application for laser microprocessing
Fields of science: Physics N 002
Scientific supervisor: dr. Sergejus Orlovas
Defence of the dissertation: June 28, 2024
SUMMARY: The notion of structured light refers to its modulation in various degrees of freedom such as phase, amplitude, polarization, pulse length and frequency. Spatial structure of light can be conveniently controlled using diffractive optical elements, spatial light modulators, or geometrical phase elements. Among the shaped light fields, a class known as nondiffracting beams has contributed greatly to recent developments in high-power applications such as laser micromachining. The notorious Bessel and Airy beams exhibit characteristics such as self-healing and resistance to diffraction. However, the ability to further shape their transverse and longitudinal intensity distributions is also desirable. In this dissertation, the research focuses on the creation of spatially structured light beams having continuous intensity distributions. Methods to create complex light structures by the superposition of several nondiffractive beams are presented. Furthermore, the ability to use such beams in high-power laser setups is discussed and the performance of the selected beams for transparent material modification is presented. The results of this thesis showed how the structures of the superimposed beams depend on the parameters of the constituent beams and that the modifications inside transparent materials are determined by the geometry of the beam together with the lasing parameters.
Author: ROMUALD PETKEVIČ
Dissertation title: Development and application of laser metal particles sintering technology
Fields of science: Material Engineering T 008
Scientific supervisor: dr. Genrik Mordas
Defence of the dissertation: September 10, 2024
SUMMARY: This thesis uses for the first time a nozzle designed for laser metal deposition technology, the de Laval geometry of which is based on a bicubic parametric curve. With the help of the nozzle, the material particles reach supersonic speed, create a narrow stream of exiting particles and form a metal coating with the help of laser radiation. The developed technology allows not only the formation of metal coatings, but can also be applied to additive manufacturing. The characteristics of the nozzle, using it for additive manufacturing, would allow to ensure a higher resolution of the manufactured objects than the laser metal deposition systems currently on the market. The manufactured nozzle combines laser metal deposition technology and cold spray additive manufacturing technology, which uses supersonic gas flow. The goal of the doctoral work is to develop an advanced technology of laser deposition of metal particles, which would allow the deposition and sintering of metal powder particles and the formation of metal coatings. To achieve this goal, a system based on laser metal deposition technology has been developed, ensuring efficient formation of metal coatings; digital modeling was performed and, based on the obtained results, a de Laval nozzle designed for laser metal deposition technology was designed, manufactured and approved; characterized the chemical and physical properties of the CoCrMo powder used for the experiments, to ensure chemical homogeneity; the influence of laser metal particle deposition process parameters on the quality of the formed coating was determined. The dissertation consists of an introduction, three chapters, a summary of the results, lists of used literature and the author's publications on the topic of the dissertation. The introductory chapter discusses the research problem, the relevance of the work, describes the research object, formulates the purpose and tasks of the work, describes the research methodology, the scientific novelty of the work, the practical significance of the work results, and defensive statements. The first chapter reviews additive manufacturing technology, the main features of laser metal particle deposition technology, powder materials used for this technology and their production methods, and material feeding in the system. At the end of the chapter, conclusions are formulated and the tasks of the dissertation are clarified. The second chapter describes the materials and equipment used for the experiments, the methodology, the parameterization of the De Laval nozzle geometry, and the methods of studying the properties of the materials used in the work. The third chapter presents and analyzes the results of the numerical simulation of the De Laval nozzle, the tests and results of the CoCrMo powder used for the experiments, describes the experimental development of the laser metal particle deposition technology and the results of the investigation of the coatings formed with the help of the nozzle. The results of the dissertation are presented in 2 scientific articles, which are in the publications of the Clarivate Analytics Web of Science database. Research results have been published at 7 international scientific conferences in Lithuania.
Author: KAROLIS STAŠYS
Dissertation title: Strategies for developing innovative mid-infrared light sources using molecular beam epitaxy
Fields of science: Material Engineering T 008
Scientific supervisor: dr. Jan Devenson
Defence of the dissertation: September 13, 2024
SUMMARY: The aim of this work was to develop new methods for molecular beam epitaxy to form quantum cascade lasers operating in the 3-15 µm wavelength range and lithography-free thermal emitters. The proposed new arsenic flux tuning methodology enabled the achievement of high-quality crystalline layers for quantum cascade laser structures and made the growth process of these structures easier. The formed lasers exhibited a particularly high operating temperature, reaching up to 108 ℃. Additionally, the foundations were laid for the technology of forming thermal emitters operating in this spectral range. The practical application of ENZ materials was demonstrated, along with the ability to control the emission peak position of thermal emitters by varying the doping of these materials. Thermal emitters based on ENZ exhibited exceptionally high emission peak stability with changing operating temperature. Furthermore, these thermal emitters, acting as plasmonic thermal radiation filters, exhibited a very narrow emission wavelength full-width at half maximum and polarization. In addition, during this work, components necessary for the production and testing of quantum cascade lasers were designed. The measured performance parameters of the devices were comparable to or even better than the results of other authors and were in agreement with theoretical models.
Author: ANTANAS NACYS
Dissertation title: New materials for low temperature fuel cells
Fields of science: Chemistry P 003
Scientific supervisor: dr. Loreta Tamašauskaitė-Tamašiūnaitė
Defence of the dissertation: September 13, 2024
SUMMARY: Nowadays everyday life is hard to imagine without phones, computers, cars, and other technologies that require the use of electricity, the energy sector becomes a vital area. The main fuel for energy production is still fossil fuel, but burning it releases gases that contribute to the greenhouse effect and ozone layer depletion, and pollutants that harm the environment (e.g., acid rain) and life (e.g., fume). This also significantly impacts climate change. Therefore, the European Union (EU) and other countries belonging to the Organization for Economic Co-operation and Development (OECD) are investing in the development of new technologies that will allow for more sustainable and cyclical energy production, thus preserving the environment while meeting societal needs. One such method of energy production is fuel cells, which are more efficient than internal combustion engines and more environmentally friendly. Fuel cells (FCs) are already being used in transportation, electronics, space, etc., but their wider application is limited by their cost, as the main components of FCs are electrodes (anode and cathode), which are made using precious metals. Currently, significant attention is being paid to the development of catalysts that would reduce the cost of FCs. For this purpose, research is being conducted and catalysts are being developed that either do not contain precious metals or contain significantly less of them. Various chemical, electrochemical, microwave, and other methods are used to synthesize materials in an attempt to obtain catalyst compositions with high electrocatalytic activity that can be used as anode or cathode materials in FCs. This dissertation focuses on the formation of new solid foam catalysts that can be used as anode materials in direct formic acid/formate fuel cells. To ensure high catalyst efficiency, Ni solid foams with a 3D structure and a large surface area were modified with a small amount of platinum. The electrocatalytic activity of the prepared catalysts was evaluated for formic acid (acidic) and formate (alkaline) oxidation reactions.
Author: RAIMONDA BOGUŽAITĖ
Dissertation title: Development of an electrochemical sensor based on polypyrrole and modification of its properties
Fields of science: Chemistry P 003
Scientific supervisor: dr. Vilma Ratautaitė
Defence of the dissertation: September 20, 2024
SUMMARY: Electrochemical sensors play an important role in various industries and applications in different areas, including environmental monitoring, healthcare, safety, and industrial processes. They can be used to detect and quantify specific chemical compounds or elements. The market for electrochemical sensors is expanding rapidly due to technological advancements, increased environmental regulations, and growing healthcare needs. Polypyrrole is a polymer often used to design electrochemical sensors because it combines electrical conductivity, electrochemical activity, and adaptability. This dissertation aimed to evaluate polypyrrole modification possibilities to develop an electrochemical sensor. The polypyrrole layer was modified with phenothiazine derivatives or molecular imprints. The thesis is based on five publications that discuss the modifications of the polypyrrole layer using phenothiazine derivatives such as methylene blue, azure A, thionine and other additives. The results have demonstrated that among the three phenothiazine derivatives, methylene blue is the most promising in the development of an electrochemical sensor, and the influence of polysaccharides such as heparin helps to improve the adhesion and durability of the layer. In the context of the COVID-19 pandemic, experiments were conducted to detect the spike protein of SARS-CoV-2. Analysis of the coefficients of the integrated Cottrell equation has proven useful in evaluating the adsorption of analytes or products on an electrode. The results demonstrated that polypyrrole modified by molecular imprinting can be applied to detect SARS-CoV-2 spike protein and the detection of methylene blue.
Author: MARYIA DROBYSH
Dissertation title: Electrochemical biosensors for COVID-19 diagnosis
Fields of science: Chemistry P 003
Scientific supervisor: prof., habil. dr. Arūnas Ramanavičius
Defence of the dissertation: September 23, 2024
SUMMARY: In this study, various electrochemical biosensor design strategies based on screen-printed carbon electrodes (SPCEs) were explored for coronavirus disease 2019 (COVID-19) diagnosis, including strategies-based on application of self-assembled monolayer (SAM) and conducting polymers. While SAM provided a conventional means of immobilizing sensing elements, conducting polymers appeared as a more promising due to their dual role in immobilization and variation of conductivity during electrochemical investigations. Electrochemical methods such as cyclic voltammetry, differential pulse voltammetry, electrochemical impedance spectroscopy, and pulsed amperometric detection (PAD) were evaluated assessed as a most suitable for the registration of analytical signal. Among these, PAD was determined as a rapid and effective technique in SPCE-based biosensing, noted for its straightforward operation and low limit of detection. Various severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) biocompounds were tested, demonstrating the efficiency of spike protein as a platform for detecting specific antibodies against SARS-CoV-2 from real serum samples. Additionally, the detection of the SARS-CoV-2 nucleocapsid protein was achieved using molecularly imprinted polymer (MIP)-based biosensors. The explored biosensors show potential for assessing status of immune system and diagnosis of ongoing COVID-19. The analysis highlighted the efficacy of integrating MIP within SPCE platform operating with PAD-based detection, offering advantages such as avoiding the application of additional redox mediators and eliminating the need for expensive biocompounds as sensing elements.
Author: JUSTINAS JORUDAS
Dissertation title: Optical and electrical investigation of GaN-based HEMT and graphene structures for applications in THz detection
Fields of science: Physics N 002
Scientific supervisor: dr. Irmantas Kašalynas
Defence of the dissertation: September 24, 2024
SUMMARY: Recently, GaN has emerged as a material platform, promising to push the boundaries of electronic devices and THz detectors capabilities. GaN is a direct bandgap III-V group compound semiconductor and garnered significant attention over the last decades due to its exceptional breakdown field in tandem with high electron saturation velocity. Graphene, a two-dimensional carbon allotrope with atoms arranged in a hexagonal lattice and having zero bandgap has captivated the scientific community since its discovery due to its remarkable properties, such as unparalleled thermal conductivity and extraordinary electrical conductivity. Integrating graphene with GaN semiconductors offers the potential to enhance device performance and explore novel applications. This work is devoted to investigation of THz/IR-range optical and electrical properties of two different materials - GaN heterostructures and graphene structures for a possible integration in THz detection applications. Furthermore, integration of GaN and graphene presents a promising avenue for overcoming the limitations of current semiconductor technologies.
Author: MARIUS ČEPONIS
Dissertation title: Stellar populations in low mass systems
Fields of science: Physics N 002
Scientific supervisor: prof., dr. Vladas Vansevičius
Defence of the dissertation: September 27, 2024
SUMMARY: The dissertation examines the influence of the environment on the stellar populations in low-mass systems. The irregular dwarf galaxy Leo A and the star clusters in the Andromeda galaxy were studied. The observations of the Hubble Space Telescope and the ground-based Subaru Telescope were used for the research. Two new photometric methods have been developed for the study of stellar populations: an algorithm for reproducing the star formation history of dwarf galaxies and a procedure for determining the parameters of star clusters. The star formation history of the dwarf galaxy Leo A has been determined – most of the stars formed during the last 6 billion years in star formation bursts. It was shown that the star-forming region during the last ∼300 million years gradually receded. Also, ages, masses, and interstellar extinctions were determined for 854 star clusters in the Andromeda galaxy. These results suggest a burst of star formation activity ∼220 million years ago.
Author: DANIIL PASHNEV
Dissertation title: Study of THz-range optical properties of AlGaN/GaN heterostructures
Fields of science: Physics N 002
Scientific supervisor: dr. Irmantas Kašalynas
Defence of the dissertation: September 30, 2024
SUMMARY: THz frequency radiation have unique properties which makes it attractive for different applications. The 2D plasmons in semiconductor inversion layers are seen as a promising candidate for development of electrically tunable THz sources. Number of problems, such as low radiation efficiency, losses and damping, fabrication challenges, and others, have hindered the practical implementation of 2D plasmonic THz emitters, despite their huge theoretical potential. The III-nitride heterostructures are promising as these provide high breakdown voltage due to material wide bandgap and 2D plasmon excitation above liquid helium temperatures. In this thesis optical properties of AlGaN/GaN high electron mobility transistor (HEMT) structures were investigated in the terahertz range. The analytical model for a thin conductive layer on a dielectric substrate was developed and experimentally validated. The transmission amplitude and phase spectra of 2D plasmons excited in grating gated AlGaN/GaN HEMT structures were obtained and analyzed under external DC bias in the temperature range 77–300 K. The renormalization effect of Drude electron effective mass in AlGaN/GaN HEMT structures was observed via THz time-domain spectroscopy of 2D plasmon resonance and 2DEG conductivity.
Author: EDITH FLORA JOEL
Dissertation title: Investigation of chitosan-graphene oxide nanocomposites and their application in environmental protection
Fields of science: Chemistry P 003
Scientific supervisor: dr. Galina Lujanienė
Defence of the dissertation: October 1, 2024
SUMMARY: The aim of this study was to provide a conceptual framework for the development of environmentally friendly composites and thin films with adjustable biodegradability that could serve to protect water systems and are capable of reducing the environmental burden of toxic waste by replacing plastics with non-toxic natural materials. Contaminants can be extracted and used in closed production processes. In order to achieve the objectives of the study, the following tasks were formulated: 1. The synthesis of nanoparticles, composites and films and their characterization. 2. Investigation of the mechanism of adsorption of Eu on GO-MGH-CS composites in batch experiments and comparison of the adsorption of Eu with Am and Pu in natural waters. 3. Application of the ANFIS model to predict the adsorption capacity of GO-MGH-CS composites for Eu+3. 4. Investigation of the antibacterial effect of GO-CS-Pt/copper/copper oxide composite films intended for use in food packaging on Escherichia coli. 5. Investigation of the properties of GO-CS-Pt/copper/copper oxide composite films in terms of UV radiation blocking, hydrophilicity, degree of swelling and solubility. 6. Investigation of the biodegradation of GO-CS-Pt copper/copper oxide composite films in soil and the migration rate of copper by double contact techniques in food simulants.
Author: VLADISLOVAS ČIŽAS
Dissertation title: Coexistence of high-frequency parametric and Bloch gain in doped GaAs/AlGaAs superlattices
Fields of science: Physics N 002
Scientific supervisor: prof., habil. dr. Gintaras Valušis
Defence of the dissertation: October 7, 2024
SUMMARY: Semiconductor superlattices - quantum structures comprising at least one electronic minibands is the main object of the research for the particular thesis. The formation of the minibands features possibility of superior charge transport control making the superlattices an attractive media for the exploration of different physical effects. Superlattices are predicted to perform as terahertz (THz) and sub-terahertz (sub-THz) frequency range generators employing parametric generation or scattering-assisted inversionless Bloch gain mechanisms. Employment of the superlattices as small, tunable and efficient THz frequency sources has been discussed for a very long time, still, up to now no experimental evidence of the stable gain has been recorded. The particular thesis will provide the reader introduction into the long-lasting theoretical investigation and experimental achievements, supplementing an understanding on the significance of the further presented results. Thesis present evidence of the first experimental observation of the dissipative parametric generation in the DC and microwave biased GaAs/AlGaAs superlattice. Introduction of the large-signal gain model contributing to the evidence on the coexistence of Bloch and parametric gain in biased semiconductor superlattices expands the previously achieved understanding on the generation processes. Finally, the first experimental evidence on the stable Bloch gain in electrically biased GaAs/AlGaAs superlattice is described.
Author: SIMAS MELNIKAS
Dissertation title: Investigation of dependence of Bragg and chirped mirrors design to spectral parameters and optical resistance
Fields of science: Material Engineering T 008
Scientific supervisor: dr. Ramutis Drazdys
Defence of the dissertation: October 23, 2024
SUMMARY: Advances in laser technology research allow for the development of laser systems with ever higher power, higher repetition rates and shorter pulses. Such systems are applied in scientific research, high-tech industry, medicine and other fields. Multi-layer interference coatings used in high-intensity laser systems must have low absorption and meet increasingly strict spectral and laser radiation resistance requirements. The aim of the dissertation is to expand the application possibilities of Bragg and broadband chirped mirrors in high-power ultrashort pulse laser systems by optimizing the coating structure and thus improving their resistance to intense laser radiation, spectral parameters and determining critical parameters for linear absorption control. The first part of the thesis describes the research that determines the extinction coefficients of dielectric multilayer coating materials for 1064 nm wavelength radiation. The absorption of multilayer coatings is also measured and compared with the modeled values based on the determined extinction coefficients. In the second part of the dissertation, a prototype of a broadband chirped mirror with a nanostructured porous layer is demonstrated. Also the parameters of the mirror, their stability over time and due to relative humidity change are investigated. In the third part of the dissertation, the dependence of the laser-induced damage threshold on the coating structure is investigated. The research described in the thesis allows to understand the dependence of the studied parameters on the structure of the coating and the materials used and provides the necessary knowledge for the prediction and optimization of the parameters