Defended Dissertations in 2023


Dissertation title: Charge carrier generation and extraction in non-fullerene organic solar cells
Fields of science: Physics N 002
Scientific supervisor: prof., habil dr. Vidmantas Gulbinas
Defence of the dissertation: January 5, 2023

SUMMARY: In the search for alternative power generation sources, organic photovoltaics seem to be one of the most attractive options for certain niche applications. Recently, their power conversion efficiency has skyrocketed thanks to the introduction of a new generation of acceptor molecules. Nevertheless, the physical understanding of the underlying mechanisms remains poorly understood. This dissertation is devoted to the study of all important dynamic processes, such as the generation and extraction of free charge carriers in non-fullerene organic solar cells. Various transient measurement techniques and their improvements have been employed to study each step in the operation of organic solar cells over an exceptionally wide temporal range, from femto- to microseconds. In this work, the effects of spatial trapping of charge carriers in bulk heterojunction organic solar cells, as well as the dynamics of charge carrier extraction dynamics at close to real working conditions in organic solar cells, and the specifics of charge carrier generation in novel organic solar cells based on non-fullerene acceptors have been investigated. This has led to a better understanding of the physical processes in organic photovoltaic devices and contributed to their further improvement.

Dissertation title: Investigation of detection properties of planar microwave diodes based on A3B5 semiconductor compounds in millimeter–wavelength range
Fields of science: Electrical and electronic engineering T001
Scientific supervisor: prof., dr. Algirdas Sužiedėlis
Defence of the dissertation: February 10, 2023

SUMMARY:  Successful development of microwave technologies requires electromagnetic detectors capable of sensing high frequency signals at low levels of microwave power. Bulk barrier planar microwave diodes operating based on the major carrier phenomena are promising in high frequency electromagnetic radiation sensing applications. The dissertation aimed to develop and investigate new original planar microwave diodes with a lower spread of their electrical parameters and capable of detecting an electromagnetic signal in the millimeter wavelength range. The first chapter reviews the physical properties of microwave diode based detectors with quasi linear and non linear current voltage characteristics and microwave diodes with a two dimensional electron gas channel. Then, the application technologies for microwave detectors and methods for their investigation using appropriate probing systems are discussed. The second chapter covers the aspects of the development of planar semiconductor microwave diodes based on GaAs, AlGaAs, and AlGaAs/GaAs compounds. Next, the methodology for investigation methods of electrical parameters and detection properties of the microwave diodes is presented. The third chapter presents the results of experimental investigations of the electrical parameters and detection properties of planar dual microwave diodes based on a semi insulating or low resistivity GaAs substrate, including current voltage characteristics, detected voltage on power characteristics and dependence of voltage sensitivity on frequency in the millimeter wavelength range. The fourth chapter presents theoretical estimations and experimental investigations of the electrical and detection properties of bow tie type microwave diodes with partial gate above a two dimensional electron gas channel based on a selectively doped GaAs/AlGaAs heterostructure. The dissertation presents new developed planar microwave diodes, advanced techniques for investigating their properties, and ways for enhancing their detection properties. Five scientific papers were published on the topic of the dissertation: three papers in scientific journals included in the list of Clarivate Analytics Web of Science database with an impact factor, and two papers in conference proceedings included in the Clarivate Analytics Web of Science and Scopus databases. A European patent based on the results of the dissertation has been granted, and twelve reports, including the results of the dissertation, were presented at national and international scientific conferences.


Dissertation title: Development and application of advanced methods based on the application of a polyethylene naphthalate scintillation detector for recording and identification of ionizing radiation
Fields of science: Physics N 002
Scientific supervisor: dr. Artūras Plukis
Defence of the dissertation: May 5, 2023

SUMMARY: In this work we investigated new and advanced scintillation detectors capable of separating neutrons from the accompanying gamma particle radiation. Separation of particles is carried out by recording the light pulses (scintillations) generated by the detector and analyzing their shape.  Conclusions: 1. PEN is a suitable material for measuring the energy of various types of ionizing radiation particles in a wide energy range from 50 keV to 5.5 MeV. 2. The PEN material is suitable for use as a neutron flux detector, which can separate neutrons from a mixed gamma and neutron flux, the separation quality parameter FOM > 1 is obtained when the energy left in the ionizing radiation particle detector exceeds 200 keVee.  3. Thin PEN and Makrofol detectors with a FeB converter layer are suitable for recording the products of reactions caused by thermal neutrons in the converter layer.  4. The use of computer neural network models improves the separation of ionizing radiation particles when the amount of energy left by interacting ionizing radiation particles is low (<0.2 MeV), and the separation parameter FOM is less than 1.

Dissertation title: Charge carrier transport in hybrid Perovskites
Fields of science: Physics N 002
Scientific supervisor: dr. Marius Franckevičius
Defence of the dissertation: May 31, 2023

SUMMARY: The accumulation of charge carriers in the perovskite layer under forward voltage causes the  electroluminescence overshoot effect, while the ion redistribution induced by the electric field minimizes the accumulation of charge carriers and the overshoot effect in perovskite solar cells. An unfavorable space charge distribution leads to a roll-off of the external quantum efficiency in hybrid perovskite light-emitting diodes, which can be avoided by a balanced injection current. Electric field-induced redistribution of ions of a single type in the perovskite layer causes complex multiphase dynamics of electroluminescence and current in MAPbI3 perovskite light emitting diodes. Electric field-induced luminescence quenching in MAPbI3 perovskites is determined by the direct effect of the electric field on the charge carrier distributions and the influence on the spatial redistribution of ions in the perovskite layer. There are potential barriers to lateral carrier movement in the polycrystalline hybrid perovskite films that affect carrier mobility, diffusion and recombination rates. 

Dissertation title: Research into variability of permanent magnet synchronous motor parameters by magnetic field modelling
Fields of science: Electrical and electronic engineering T 001
Scientific supervisor: prof. dr. Algirdas Baškys, 2017-2018, prof. dr. Voitech Stankevič 
Defence of the dissertation: June 15, 2023

SUMMARY: The dissertation deals with permanent magnet synchronous motors (PMSM) classical control theory and also by applying magnetic field theory approach. The purpose of the thesis is to create the mathematical model of the PMSM based on analytical equations of magnetic field accounting torque ripple, inductance distributions and back electromotive force, to develop the measurement system for direct measurement of magnetic flux density and to propose PMSM parameter estimation method. In order to solve the stated problem and reach the goal following main tasks of the thesis are formulated: create PMSM model based on the analytical equations of magnetic flux density; develop vector control method of PMSM and investigate it experimentally; develop PMSM parameter estimation method; develop and implement a magnetic flux density measurement system; perform experimental measurement of magnetic flux density of PMSM using the developed system. The dissertation consists of the introduction, three chapters, general conclusions, references, and the list of scientific publications of the author on the topic of the dissertation. The introduction discusses the research problem, the relevance of the dissertation, the object of the research, formulates the aim and tasks, de- scribes the research methodology, the scientific novelty of the dissertation, de- fended statements and the practical value of the research findings. At the end of the introduction, the dissertation and the structure of the dissertation are provided. The first chapter reviews the PMSM design, rotor and stator types, magnetic field analytical solutions for predicting magnetic flux in the PMSM and magnetic field measurement systems. The second chapter presents an analysis of the following topics of PMSM’s magnetic field distribution: the field created only by the rotor (open-circuit) and the field created only by the stator (armature reaction). Next, the effect of the stator slot on the magnetic field distribution is analysed. Finally, the enhanced PMSM model concept is stated based on magnetic field analysis. The third chapter presents the magnetic flux density measurement systems and compares the experimental results with the simulation results obtained using the proposed enhanced PMSM model. The main results of the dissertation were published in seven scientific publications: four in the Clarivate Analytics Web of Science database with an impact factor and three in other databases. The results were presented at eight scientific conferences. 

Dissertation title: Application of dosimeters and calibrated ionization chambers in optimization of exposure in nuclear medicine procedures
Fields of science: Physics N 002
Scientific supervisor: dr. Arūnas Gudelis
Defence of the dissertation: September 7, 2023

SUMMARY: In the thesis, the risk probability of tumour complications among Lithuanian health care specialists working in nuclear medicine was investigated and evaluated. The time interval was chosen for over 25 working years. The non-definition of activity meters of radiopharmaceutical preparations used by Lithuanian nuclear medicine departments was also carried out. Special attention is paid to the evaluation of exposure to workers and patients. During the work, the task was to investigate the evaluation of the whole body Hp(10) exposure received by nuclear medicine workers when working with automatic injectors and to perform a detailed analysis of the Hp(0,07) exposure of the hands when working with 99mTc and 18F radionuclides. A detailed analysis of the prescribed administered activity to patients was evaluated and the local diagnostic reference levels used in nuclear medicine were proposed and updated, considering the relevant assigned activities. 

Dissertation title: The application of sum-frequency generation spectroscopy in studies of biomimetic systems
Fields of science: Chemistry N 003
Scientific supervisor: dr. Simona Strazdaitė
Defence of the dissertation: June 16, 2023

SUMMARY: Amyloid proteins which can form cytotoxic structures are related to incurable illnesses. Despite a significant amount of continuous research, there is a lack of knowledge explaining the molecular mechanisms of neurodegenerative disorders. The polymorphic structure of the amyloid fibril, as well as the range of binding and supporting forces, challenge in situ examination of amyloid diseases. The ability to identify the secondary structures of amyloid proteins and observe their interactions with phospholipids on fluid interfaces is highly relevant for many scientific fields. In order to understand protein aggregation and the interaction of protein aggregates with cell membranes, it is beneficial, to begin with, a simplified biomimetic system, e.g., a monolayer formed from one kind of lipid and a protein. Vibrational sum-frequency generation spectroscopy (VSFG) combined with Langmuir technique fits perfectly in studying such systems. With the advancements in techniques of surface-specific spectroscopy, VSFG has distinctively large surface specificity and submonolayer sensitivity and is an ideal optical probe to identify the secondary structure of surface-adsorbed proteins. This dissertation focuses on the adsorption of hen egg white lysozyme (HEWL) and the secondary structures of its aggregates at various interfaces using the VSFG method. The widely used phospholipid deuteration method was applied in this study, furthermore, its impact on the biomimetic system was assessed. The dissertation includes a wide range of interdisciplinary methods, which revealed the main features of biomimetic systems and ensured the implementation of the main goal of this study.

Dissertation title: Evaluation of the adsorption properties of composite materials for caesium, cobalt, and europium
Fields of science: Chemistry N 003
Scientific supervisor: dr. Galina Lujanienė
Defence of the dissertation: September 22, 2023

SUMMARY: This dissertation focuses on the synthesis of different composite materials as adsorbents for caesium, cobalt, and europium, their characterisation, adsorption studies, and the use of an adaptive neuro-fuzzy inference system to predict the adsorption capacity of composites. It was found that the muscovite mica clay-graphene oxide-maghemite-magnetite composite has a sufficiently high adsorption capacity for Cs(I) and Co(II). The composites Prussian blue-graphene oxide 2.1, Prussian blue-graphene oxide 2.2, and magnetite-Prussian blue-graphene oxide 3.2 adsorb Cs(I) better than magnetite-Prussian blue-graphene oxide 2.3. Among the chitosan-mineral composites, the chitosan-muscovite mica clay, chitosan-muscovite mica clay cross-linked by epichlorohydrin, and chitosan-montmorillonite modified by glycerol composites showed the highest adsorption capacity for Cs(I) and Co(II), and chitosan-zeolite for Eu(III). Based on experimental and literary data, the suggested mechanism for the adsorption of Cs(I) and Co(II) on muscovite mica clay-graphene oxide-maghemite-magnetite and of Cs(I), Co(II), and Eu(III) on chitosan-mineral composites is ion exchange, complexation, and electrostatic attraction. The mechanism of Cs(I) adsorption on Prussian blue-graphene oxide and magnetite-Prussian blue-graphene oxide composites is complexation, ion exchange, and ion trapping. The adaptive neuro-fuzzy inference system shoved good performance and generalisation ability to predict the adsorption capacity of composites.

Dissertation title: Growth and characterization of bismide alloys for infrared sensors
Fields of science: Material Engineering T 008
Scientific supervisor: dr. Renata Butkutė
Defence of the dissertation: September 8, 2023

SUMMARY: Significant progress has been made in the development of GaAsBi quantum structure growth technology, which can be used in a wide range of optoelectronic applications, including laser diodes, light-emitting diodes, photodetectors, spintronic devices, etc. Bismides have attracted considerable attention due to their unique physical properties, such as a large reduction in band-gap, spin-orbit splitting, and lower temperature sensitivity compared to traditional III-V semiconductor compounds. This thesis presents a comprehensive study of the growth of GaAsBi layers and quantum structures by MBE on GaAs substrates. The study focuses on optimizing the growth parameters to control the structural and optical properties of the material. The study successfully established the optimum growth conditions for the formation of 100 nm thick GaAsBi epitaxial layers with a Bi concentration of 10%, which can be used for the fabrication of GaAsBi detectors. In this work, the use of the analog alloy grading technique for the growth of parabolic GaAsBi quantum wells is demonstrated for the first time. It is shown that parabolic GaAsBi quantum wells exhibit more than 50 times higher photoluminescence intensity compared to rectangular multiple GaAsBi quantum wells. This study also determined the optimal growth parameters for the formation of parabolic quantum wells in both AlGaAs and GaAsBi laser diodes. A laser diode with a parabolic GaAsBi quantum well in the active region is demonstrated for the first time.

Dissertation title: Diversiform black silicon for bio-sensing
Fields of science: Physics N 002
Scientific supervisor: dr. Renata Karpicz
Defence of the dissertation: September 15, 2023

SUMMARY: Diversiform black silicon for bio-sensing The Thesis is dedicated to the design, computational analysis, fabrication, and testing of low-cost, stable, and scalable substrates based on diversiform black silicon sputtered with gold pseudo-layer for NIR surface-enhanced Raman spectroscopy (SERS) of living cells, organic molecules, and carbon-based nanomaterials. The black silicons were produced by either cryogenic or room temperature inductively coupled plasma reactive ion etching; gold pseudo-layer was magnetron-sputtered. The surface micro-structuring was tuned by the fabrication process variation, resulting in hydrophilic interfaces for living cell analysis or hydrophobic surfaces for analytical applications. A cone-like black silicon substrate coated with a 50-80 nm gold pseudo-layer showed excellent uniformity, an enhancement factor of 108, and was applied for living cell investigation. Computational simulation of the silicon-gold core-shell structures proved location of their optical resonance in 700-800 nm, adjustable by the gold layer thinning. It was validated by reducing gold thickness down to 25-50 nm and 10-25 nm for lace- and pillar-like black silicons, respectively. Implementation of black silicon-based SERS allowed revealing a hypochlorite-mediated biodegradation of carbon-based nanomaterials upon activation of neutrophils during cancer theranostics. These findings enable affordable, widespread SERS implementation in diverse applications, from nanomaterial science to biomedicine.

Dissertation title: Photophysical properties of boron dipyrromethene (BODIPY)-based fluorescent molecular sensors
Fields of science: Physics N 002
Scientific supervisor: dr. Aurimas Vyšniauskas
Defence of the dissertation: September 22, 2023

SUMMARY: This thesis presents how BODIPY fluorescence lifetime-based molecular rotors can help to study the changes in living cells. The photophysical properties of substituted derivatives were studied in varying environments to determine how different functional groups alter their sensitivity to viscosity, temperature, and polarity. The experiments were carried out using absorption and fluorescence spectroscopy. The thesis is based on five publications and is therefore divided into five chapters leading to four main statements of the dissertation. The study showed that β-substitution of the BODIPY core increases the conjugation of the derivatives and leads to a red shift in the absorption and fluorescence spectra. Moreover, substitution with a nitro group in the meso-phenyl position of the BODIPY core can be considered as a universal way to increase the viscosity sensitivity of the probe. Furthermore, substitution with an electron donating group enhanced the polarity sensitivity in BODIPY molecular rotors when placed at the β-position of the BODIPY core. In addition, it has been shown that some of the probes can be used in fluorescence lifetime imaging microscopy to visualise changes in the microenvironment in lipid droplets of living cells.

Dissertation title: The Study of Anthropogenic Radionuclide Transport with Suspended Matter in the System Neman River – Baltic Sea
Fields of science: Physics N 002
Scientific supervisor: dr. Galina Lujanienė
Defence of the dissertation: September 29, 2023

SUMMARY: The study provides new data on the transport, distribution and flux of radionuclides (239,240Pu, 241Pu and 241Am) in the aquatic environment of the Neman River, the Curonian Lagoon and the Baltic Sea in the Lithuanian economic zone. The Neman River basin is a long-term source of 239,240Pu with an average annual rate of 62 MBq/year. Most of the activity is transported to the Curonian Lagoon by the particles > 1 µm in the Neman River. The suspended particles of 25 - 63 µm are responsible for the transport of 137Cs and natural radionuclides (7Be, 40K, 210Pb, 214Pb) with the water of the Neman River into the Curonian Lagoon. Nevertheless, particles with a size of 30 µm play an important role in the distribution of 137Cs, 40K and 214Pb, while suspended particles with a size of < 20 µm influence the transport of 210Pb in the Baltic Sea. The obtained data on the spatial distribution of 239,240Pu and 241Am in the Curonian Lagoon and in the Baltic Sea of the Lithuanian Economic Zone indicate a characteristic pattern supporting the use of these radionuclides to trace pollutants in the studied area.

Dissertation title: Optical engineering in terahertz imaging and 2D materials inspection
Fields of science: Physics N 002
Scientific supervisor: prof. habil.dr. Gintaras Valušis
Defence of the dissertation: September 29, 2023

SUMMARY: One of the essential reasons for the attractiveness of terahertz frequency (THz) radiation is the increasing use of THz imaging systems. In order to improve and expand their application capabilities, systems must be designed as compact and convenient-in-use as possible. Although most of the attention so far has been dedicated to the miniaturization of active components, passive optical components received significantly less attention. Aiming to reduce the dimensions of THz imaging systems, bulky optical components can be replaced by thin, flexible and compact diffractive optical elements. This thesis describes three options for developing alternative components. First of all, carbon-based materials were employed. A thin and flexible graphite zone plate with cross-shaped apertures was developed and studied. Also, the potential of graphene for creating compact optical elements have been investigated. It has been demonstrated that metasurfaces made of thin and flexible steel foil with inverted split-ring resonators can focus the THz beam in case of mechanical deformation and also are suitable for use in polarization resolved THz imaging. A nonparaxial THz imaging system has been developed using exclusively silicon-based optics via fabrication of thin Fresnel, Airy, Bessel or Fibonacci lenses. It has been shown that using silicon diffraction optics it is possible to generate structured THz light in the form of an Airy beam. This property enables imaging of objects in the presence of an obscure obstacle and opens a possibility to investigate properties of graphene layers. Digital THz holography is demonstrated, and a method to improve the quality of the obtained THz holographic images using a 2- and 4-step phase shift method is presented.
Dissertation title: Application of metallic and metal oxides nanostructures for proteins biosensing by ellipsometry and quartz crystal microbalance
Fields of science: Chemistry N 003
Scientific supervisor: dr. Ieva Plikusienė
Defence of the dissertation: October 6, 2023
SUMMARY: This work discusses metal and metal oxide nanostructure application for protein analyte biosensing in optical and/or piezoelectric methods. Out of all summarized, porous aluminium oxide, zinc oxide nanowires and thin gold films were identified to be in possession of suitable chemical properties, such as stability and biocompatibility. This work includes methods of nanostructure synthesis, functionalization and surface application towards biosensing. This work introduces utilised measurement methods of spectroscopic ellipsometry, total internal reflection ellipsometry and hybrid method of spectroscopic ellipsometry/quartz crystal microbalance with dissipation. Following research results were presented: analysed human serum albumin nonspecific adsorption into porous anodic aluminium oxide nanopores. Applied electrochemically formed zinc oxide nanowires for bovine serum albumin immobilization, quantitively analysed the diffusion impact for protein immobilization. Thin gold films were applied for SARS-CoV-2 spike protein and its’ α and β variant immobilization for immune complex formation with specific antibodies, immune complex formation kinetic parameters were quantitively determined. Hybrid spectroscopic ellipsometry/quartz crystal microbalance method was used to analyse interaction between SARS-CoV-2 nucleocapsid protein and specific antibodies, spatial arrangement of proteins in an immune complex was determined using a geometric model.

Dissertation title: Numerical simulation of the light ion backscattering spectra in the particle channeling geometry
Fields of science: Physics N 002
Scientific supervisor: dr. Artūras Plukis
Defence of the dissertation: November 28, 2023

SUMMARY: Semiconductor structures are significantly affected by impurities and defects, with elastic backscattering spectroscopy being a vital method for defect evolution studies. The energy spectra of backscattered particles provide insights into atomic composition of the sample, impurity profiles, and crystalline defects. The energy of the detected particles depends on many factors, making the interpretation of experimental spectra practically impossible without the use of specialized software. To facilitate the process of interpreting spectra, two new open-source models were developed for numerical simulations of proton backscattering spectra from both amorphous and crystalline materials. These models allow for the utilization of various stopping powers, reaction cross-sections, physical processes, and more. The simulated spectra of protons, helium, and lithium ions closely match experimental results and those obtained using commercial tools for various optical coatings. The comparison of simulated backscattering spectra from Si, SiC, SiO2, and diamond crystals with experimental data showed excellent agreement. However, the modeling results are influenced by ion beam divergence and sample temperature. Additionally, modifications were made to the original channeling model in the GEANT4 environment, achieving deviations of less than 20% from experimental values for stopping powers in silicon, germanium, and gallium arsenide crystals when bombarded with 1–20 MeV proton and deuteron beams.

Dissertation title: Synthesis and characterization of effective nanoplatelet structure catalysts for electrochemical water splitting
Fields of science: Chemistry N 003
Scientific supervisor: dr. Arūnas Jagminas
Defence of the dissertation: December 1, 2023

SUMMARY: The aim of this work was to investigate the influence of amino acids on the activity and stability of hydrothermally synthesized molybdenum disulfide (MoS2) electrodes for the hydrogen evolution reaction, and to determine the nature of the most effective MoS2 active positions. The influence of amino acids on the hydrothermal synthesis of molybdenum disulfide was established for the first time. It was shown that the addition of glycine, cysteine or histidine amino acids to a solution of ammonium heptamolybdate and thiourea resulted in the hydrothermal synthesis of a nanoplatelet MoS2 coating with a relatively large amount of the metastable, metallic 1T-MoS2 phase. The increase in this metallic phase is attributed to the formation of amino acid fragments during the reaction, which intercalate into the molybdenum disulfide interlayers, stabilizing the metastable 1T-MoS2 phase. Due to a relatively high content of 1T-MoS2 phase, molybdenum sulfide electrodes with amino acids possess higher activity of hydrogen evolution reaction. A new method of increasing the activity of molybdenum disulfide electrodes by anodic pretreatment of synthesized MoS2 coatings has been investigated. It is shown that anodic pretreatment increases the relative amount of catalytically active bridging S22- groups, which are considered as the active positions for hydrogen evolution reaction. By treating MoS2 with a short anodic pulse, the activity of the electrode can be increased up to 30%.

Dissertation title: Effects of Pulsed Electric Field on the Structural and Techno-Functional Properties of Proteins
Fields of science: Chemistry N 003
Scientific supervisor: dr. Arūnas Stirkė
Defence of the dissertation: December 5, 2023

SUMMARY: The introduction emphasizes the significance of climate change and the need for sustainable practices, highlighting Pulsed Electric Field (PEF) as an effective method to enhance extraction yield, improve food safety, and extend shelf life. The thesis focuses on studying the impact of PEF treatment on micellar casein, as well as the interactions between BSA and soluble starch, and BSA and tea polyphenols (EGCG). Various analytical techniques, including FTIR, circular dichroism, Raman, UV-vis, and DLS, provide valuable insights into the effects of PEF on these protein systems. The study explored PEF treatment's influence on BSA/soluble starch glycation and casein micelles (CSMs) at different electric field strengths. Lower strengths (3.5-5.7 kV/cm) improved BSA/starch conjugates and emulsions, while higher strengths (>5.7 kV/cm) had adverse effects. Moderate electric field (MEF) treatment proved more efficient for modifying casein structure. Additionally, nanosecond PEF (nsPEF) treatment altered BSA, enhancing its interaction with EGCG, as evidenced by spectroscopic analysis and molecular docking. In conclusion, the thesis highlights PEF's ability to enhance BSA/starch conjugates and emulsions at lower strengths, while indicating limited effects at higher strengths. PEF was observed to modify CSMs through changes attributed to PEF-induced polarization and bond alterations. Furthermore, nsPEF treatment was found to modify BSA and improve its interaction with EGCG.

Dissertation title: Search, synthesis, and investigation of new framework electrode materials for aqueous Na-ion batteries
Fields of science: Chemistry N 003
Scientific supervisor: dr. Linas Vilčiauskas
Defence of the dissertation: December 15, 2023

SUMMARY: Electrochemical batteries are sought as one of the most attractive technologies for storing electrical energy. They have superior round-trip energy efficiency, low environmental footprint, easy scalability, and wide available power and energy range. Li-ion batteries have become the battery technology of choice, however, the highly volatile cost and supply of lithium as well as safety issues related to the use of highly flammable organic electrolytes requires the search and development of alternatives. Na-ion batteries, especially those employing aqueous electrolytes, are attracting increasing attention as potential candidates, especially suitable for large-scale applications. The aqueous aspect makes them significantly safer, non-flammable, low cost and environmentally friendlier with respect to comparable Li-ion technologies. The main goal of this work was to find and develop new framework electrode materials which would be suitable for the next generation aqueous Na-ion batteries in terms of their synthesizability, charge capacity, energy density, and stability. NaTi2(PO4)3, Na4Mn3(PO4)2(P2O7), Na3MnPO4CO3, Na3Fe2(PO4)3, Na4Fe3(PO4)2(P2O7), Na2-xFe[Fe(CN)6]⋅yH2O, Na7V4(PO4)(P2O7)4 and Na3VFe(PO4)3 were synthesized by solid-state, co-precipitation, sol-gel and hydrothermal methods. Additionally, materials were characterized by X-ray diffraction, scanning and transmission electron microscopies, thermal and elemental analysis as well as electrochemically investigated by cyclic voltammetry and galvanostatic charging/discharging.

Dissertation title: Search, synthesis, and investigation of new framework electrode materials for aqueous Na-ion batteries
Fields of science: Chemistry N 003
Scientific supervisor: dr. Evaldas Naujalis
Defence of the dissertation: December 21, 2023

SUMMARY: In this PhD thesis, the application of cryo-enrichment in gas chromatography is explored. It is a technique designed to enhance the separation and detection of volatile compounds in complex mixtures. By lowering the temperature of the chromatographic column to extremely low levels, this method reduces the volatility of compounds, leading to their efficient condensation onto the column's stationary phase. The primary focus of the thesis is the development of a custom cryo-enrichment module to improve system efficiency and sensitivity. Multiple prototypes were constructed and evaluated, yielding significant improvements for gas chromatography. Notably, peak resolution increased up to 2.2 times, and the most optimal outcomes were achieved when cooling 95 cm of the column length, resulting in a 20-fold sensitivity boost. However, limitations were observed, particularly related to the analyte working range, where some compounds were not retained adequately due to insufficient cooling while others were retained excessively due to inadequate heating. The module was tested in various applications, such as method calibration, impurities analysis, fungicide analysis, and measuring terpenes in Cannabis Sativa biomass. This research shows that cryo-enrichment in gas chromatography holds potential for improving analysis, but it also recognizes the necessity for more refinement and practical application.

Dissertation title: Applications of conductive polymers in sensors
Fields of science: Chemistry N 003
Scientific supervisor: dr. Inga Morkvėnaitė-Vilkončienė, dr. Rasa Pauliukaitė
Defence of the dissertation: December 22, 2023

SUMMARY: Nowadays, fast sensors for different substances are becoming very important. One such sensor is pH, which is important in chemical and biological processes, especially if they have several different levels of varying acidity. Other sensors detect pesticides hazardous to human health in foodstuffs or drinking water. One possible example is a sensor for atrazine, which has already been banned in Europe but its residues are still found. Glass electrodes, often used for pH determination, are unsuitable in some systems due to their size, shape, frequent need for calibration and long measurement time. In contrast, atrazine is routinely detected by mass spectrometry, ELISA or chromatography, which are unsuitable for rapid field testing due to the conditions of use. For these reasons, the development of new electrochemical pH and atrazine sensors is demanded. This thesis focuses on tracking the pH of a solution using a sensor that is more convenient and flexible than existing ones. This thesis aims to develop electrochemical pH and atrazine sensors using conductive polymers from natural monomers such as riboflavin, folate and the amino acid L-lysine. In this work, polyfolate is used for the first time in developing a pH sensor because it is a conductive, pH sensitive, biodegradable, non-hazardous compound. Such a sensor operates in the pH range between 6 and 9. A copolymer of folate and L-lysine is used for the first time in the development of an atrazine sensor due to the electrocatalytic effect of the polyfolate on atrazine and the ability of L-lysine to improve the binding of atrazine to the negatively charged polyfolate.