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Defended Dissertations in 2019


SANDRA STANIONYTĖ
Author: SANDRA STANIONYTĖ
Dissertation title: Growth of GaᵧIn₁₋ᵧAs₁₋ₓBiₓ layers by molecular beam epitaxy for optoelectronic applications
Fields of science:
Material Engineering (08T)
Scientific supervisor: 
dr. Vaidas Pačebutas
Defence of the dissertation:
February 14, 2019

SUMMARY:
Bismuth-containing compounds are attractive due to the possibility of usage as emitters or detectors operating in the near- and mid- infrared spectrum region, solar cells, components of spintronic devices and terahertz (THz) frequency systems. THz time-domain spectroscopy systems based on GaAs are currently the most common, but they are large in size and expensive. They could be replaced by more compact and rather cheaper systems, operating in the range of telecommunication wavelengths (1−1.5 µm), if the components – emitters and detectors – would be produced from GaᵧIn₁₋ᵧAs₁₋ₓBiₓ compound. Orienting to the development of mid-range infrared radiation detectors, GaᵧIn₁₋ᵧAs₁₋ₓBiₓ compounds can also be useful because it is known that it is possible to achieve compounds in which the optical absorption edge moves up to 6 μm. Optimization of the growth conditions of GaᵧIn₁₋ᵧAs₁₋ₓBiₓ layers is described in this dissertation. Bismide-based THz components for 1.55 µm spectroscopic systems were made and compact THz time-domain spectroscopy system was fabricated. GaᵧIn₁₋ᵧAs₁₋ₓBiₓ layers with the absorption edge up to 2.3 µm were grown in this dissertation; the investigation of the influence of post-annealing process on the properties of quaternary compounds has showed the good prospective of their applications in optoelectronic devices.
KAROLIS RATAUTAS
Author: KAROLIS RATAUTAS
Dissertation title: Laser-assisted formation of electro-conductive circuit traces on dielectric materials by electroless metal plating technique
Fields of science: 
Material Engineering (08T)
Scientific supervisor: 
dr. Gediminas Račiukaitis
Defence of the dissertation: 
March 04, 2019

SUMMARY:

In this thesis, laser-assisted selective chemical copper deposition on dielectrics, and its application for formation of electric circuit traces was investigated. Two approaches of selective plating methods were proposed and tested: laser direct structuring (LDS) of polypropylene with carbon nanotubes additive and the newly-developed technology Selective Surface Activation Induced by a Laser (SSAIL). Laser direct structuring is based on special additives called LDS additives mixed in whole volume of the polymeric part. Laser beam activates those additives, making them an active catalyst for electroless copper plating in the bath. Thus, only laser-written areas are deposited. In this work, new type of additives, multiwall carbon nanotubes were investigated. Experimental studies of activation process were performed with the sheet resistance measurements, scanning electron microscope analysis and Raman spectroscopy. SSAIL is a new technology developed during the PhD studies period. The method contains steps: laser excitation of the surface, wet chemical activation with a catalyst, rinsing and electroless copper deposition in the bath. Experimental results showed that the selective plating process is not caused only by surface roughness. Sheet resistance measurement, XPS analysis and microscope images have been used to analyse the process after the laser modification, activation and plating steps. Results of activation mechanism revealed that process works only after picosecond laser modification. XPS analysis has shown that PA6 after laser processing has reducing properties of metals ions.
EDGARAS MARKAUSKAS
Author: EDGARAS MARKAUSKAS
Dissertation title: Laser processes for monolithic interconnection formation in thin-film solar cells
Fields of science: 
Material Engineering (T008)
Scientific supervisor: 
dr. Gediminas Račiukaitis
Defence of the dissertation: 
May 30, 2019

SUMMARY:

Dissertation work aimed to investigate the laser processing of the P2 and P3 scribes in thin-film solar cells searching for optimal laser pulse duration, laser repetition rate, wavelength, and various scribing approaches based on the in-house developed novel direct electrical measurement technique of electrical performance of the thin-film solar cells. The dissertation work consists of an introduction, author’s publication list, literature review, methods, results, conclusions, references and a summary. In the thesis, a novel direct electrical measurement technique (LLST) was developed and investigated in CIGS and CZTSe thin-film solar cells. LLST measurements revealed the relationship between the P3 scribe conductivity and the used laser wavelength. Increasing the laser pulse repetition rate, increased heat accumulation in the cells, which was observed as the pronounced melting of the laser-scribed areas and increased scribe conductivity. Finally, it was shown that scribing with picosecond duration pulses at the 1 MHz pulse repetition rate could locally transform a CIGS absorber layer into a highly conductive compound suitable to act as a P2 interconnect.
ANDRIUS RIMKUS
Author: ANDRIUS RIMKUS
Dissertation title: Optical properties of InGaAs heterostructures
Fields of science: 
Physics (N002)
Scientific supervisor: 
dr. Bronislovas Čechavičius
Defence of the dissertation: 
May 31, 2019

SUMMARY:

The dissertation is dedicated to the study of InGaAs quantum dots (QDs), rings (QRings), and rods (QRods) of various morphology using modulation spectroscopy, photoluminescence (PL) and photoluminescence excitation (PLE) techniques. The first part of the work presents optical investigations of InAs QDs embedded in InGaAs/GaAs quantum well (QW). In addition, effect of stress-relieving InGaAs layer on QD optical properties and electronic structure has been revealed. Comparing experimental results and digital calculations, a detailed temperature-dependent analysis of the optical transitions, taking place in QWs and QDs, was performed. Second part of the work is devoted to the investigation of the electronic structure and optical transitions of InGaAs QRods embedded in the InGaAs QW using methods of photoreflection and PL. The influence of As source on InGaAs QRods and the surrounding InGaAs QW was experimentally investigated. Increase in PL intensity was shown, which is explained by the improved quantum localization of the electronic states of InGaAs QRods. Third part presents optical studies of InAs quantum ring (QRing) structures, which showed a significant energy blueshift of the interband transitions between the ground states of QRings. Mentioned blueshift is associated with the small size of the quantum ring (compared to the characteristic height of QDs). The reduction of the electron localization results in a ground state energy shift toward continuum.
MARIJONAS TUTKUS

Author: MARIJONAS TUTKUS
Dissertation title: Single-molecule fluorescence microscopy for protein dynamics studies
Fields of science: Physics (N002)
Scientific supervisor: prof. habil. dr. Leonas Valkūnas, dr. Gediminas Trinkūnas
Defence of the dissertation: July 5, 2019

SUMMARY:
Conformational dynamics of proteins are essential for their functioning objectives, which are possible due to the inherent flexibility of protein structure. Every protein, regardless of whether it is located in lipid membranes or found in the cytosol undergoes conformational dynamics. The mentioned classes of proteins are different regarding their native environment and involved in different aspects of cellular life. In this work we chose to investigate the following representatives from two different classes – transmembrane pigment-protein complexes from photosystem II of plants and water-soluble proteins interacting with nucleic acids – DNA Restriction endonucleases. Since conformational dynamics of protein are typically hidden in an ensemble type of measurements we revealed them with the aid of the single-molecule methods. Our results on single-molecule conformational dynamics studies of protein-pigment complexes and DNA-interacting proteins provided essential insights into each field of research. Also, both of our developed methods were successfully applied and will be useful for future studies of various DNA-interacting proteins and other pigment-protein complexes or their assemblies.