Defended Dissertations in 2015

Rita Mažeikaitė
Author: Rita Mažeikaitė
Dissertation title: Search for synthetic methods of compounds containing thiophene, indole and pyrazole framework
Fields of science: Physical sciences, Chemistry (03P)
Scientific supervisor: dr. Linas Labanauskas
Defence of the dissertation: 2015-06-26
Compounds containing thiophene, indole and pyrazole framework often possess important properties that are widely applied in practice. Substituted thiophene derivatives containing electron-withdrawing substituents and pyrazole complexes with metals represent classes of active optical materials, such as organic light emitting diodes. Moreover, thiophene, indole and pyrazole frameworks can be often found in various biologically active compounds. For example, some time ago, a non-selective histone deacetylase inhibitor (HDAC) Panobinostat (LBH-589) bearing indole moiety and hydroxamic acid functionality was developed. Synthesis of functionally substituted thiophene, indole and pyrazole derivatives is a highlighting task, since this modification opens possibility for construction of new heterocyclic systems.
In these investigations we put our attention on the development of efficient synthetic methods for preparation of 2,3-disubstituted thiophenes, substituted 3-[4-(piperazin-1-yl)butyl]-IH-indoles and 3-substituted pyrazoles. First, we envisioned three main problems associated with the synthesis of these classes' compounds and their plausible solutions:
I. 2,3-Disubstituted  thiophenes  are usually  synthesized  from  3-halogeno-2-substituted thiophene derivatives obtained by modification of the 2m! thiophene ring position  of corresponding  3-substituted  thiophenes.  But  it  should be  noted  that  this methodology  is not  very selective and sometimes  modification  at the 5th position  takes  place. Having an ortho orienting group at 3rd position of thiophene ring, a selective modification  of the second position becomes possible by direct lithiation. Synthesis of 2,3-disubstituted  thiophenes  from 2-halogenothiophene  derivatives by  protecting  the 5th position and then selective introduction of nitro functional group at 3n.t  position,  looks much more acceptable and allows further modification of both 2nd and 3th positions of thiophene  ring.
2. 3-[(Piperazin-l-yl)alkyl]-IH-indole derivatives are often synthesized by modification of mono-substituted piperazine with IH-indol-3-yl-alkyl halides, mesylates or tosylates. But availability of mono-substituted piperazine derivatives is limited and their synthesis is rather complicated. Our main  focus was in 3-[4-(piperazin-1-yl)butyl]-lH-indole derivatives, because of not only for their structural similarity to Panobinostat (LBH- 589), but also the synthesis of 3-[(piperazin-l-yl)alkyl]-IH-indoles with longer  than propyl chain is not fully investigated. Thus, we chased commercially available 4-(lH­ indol-3-yl)butanoic acid as starting material for the synthesis of 3-[4-(piperazin-1- yl)butyl]-lH-indole. And by further modification of free NH group of piperazine ring, higher variety of substituted 3-[4-(piperazin-1-yl)butyl]-lH-indole  derivatives can be synthesized.
3. Electrophilic substitution ofpyrazole ring usually occurs at the 4th position. Thus, one of the most common approaches for the synthesis of 3-substituted pyrazoles is based on formation of the pyrazole ring via condensation reaction of substituted 1,3-dicarbonyl derivatives with hydrazine. Unfortunately, this method usually generates mixture of regioisomers and cause problems with isolation of products. Moreover, only limited number of dicarbonyl compounds are readily available. Another approach for the synthesis of 3-substituted pyrazoles is by transition metal catalyzed cross-coupling reactions of 3-halogcno or 3-organometalic pyrazole derivatives. 3-Br(Cl or I)-IH­ pyrazole derivatives are commercial available but very expensive and availability of 3-halogen-lH-pyrazole derivatives with substituents at the 4th or 5th positions  is very limited due to their complicated synthesis. Pyrazoles readily fonn complexes with transition metals so before performing cross-coupling reactions of 3-halogeno-lH­ pyrazoles, it is necessary to protect free NH group. Thus, transition metal-catalyzed cross­ coupling reactions are usually carried out using N-alkyl-, N-ary! or N-benzylpyrazoles. We decided to find simple and effective method for the synthesis of 4- or 5-substituted 3-halo­ IH-pyrazoles protected by easy removable lert-butyloxycarbonyl (Boc), 2-ethoxyethyl­ (EtOEt) or 2-tetrahydro-2H-pyranyl (THP) groups, because it can open possibilities for the synthesis of substituted pyrazoles with free NH group.
Raimonda Celiešiūtė
Author: Raimonda Celiešiūtė
Dissertation title:
Fields of science: Physical sciences, Chemistry (03P)
Scientific supervisor: dr. Rasa Pauliukaitė
Defence of the dissertation: 2015-03-23
Investigations of electrochemical (bio)sensors where intensively developed over the last few decades. Electrochemical methods have  advantages in comparison with the other methods because of several reasons: i) the immediate conversion of the (bio)chemical event to understandable electrical signal, ii) they are simple for everyday use, e.g. widely used glucometers and iii)  they  are  relatively  cheap,  because  of  the  possibility  to  employ  cheap materials  such as glass,  carbon as well  as to use micro- or even nanoquantities of  expensive  materials  used  for  modifications   of  the electrode.  Various traditional  methods  were  being  applied  for  the  analysis  with  (bio)sensors including  cyclic voltammetry   (CV),  fixed  potential   amperometry   (FPA), electrochemical  impedance spectroscopy (EIS), etc. including combination of several   different   methods.  Sensitivity,   stability   and   electrochemical properties  of the (bio)sensors depend on electrode surface modifications, the biorecognition   element  and  its  immobilisation.  Electrode surface  can  be modified   with   various  materials   such  as  mediators,  polymers,   etc.;  the modification strategy most depends on required properties of the (bio)sensor in the   particular   case.   The   examples   of   such   materials could   be   metal nanoparticles,   conductive  polymers,  carbonaceous  materials.  Surface modifications with micro- and nanostructures performed using such methods as  lithography,  laser  technologies  enables  an  enhancement  of  the stability, sensitivity,  selectivity  and  other properties  as well  as reduction  the use  of expensive materials to micro- or even nanoscale, which is still important in the development of electrochemical (bio)sensors.
The aim of this research: to form various electrode platforms by modifying surfaces with micro- and nanostructures and to test their applicability to the development of electrochemical (bio)sensors.
Rokas Kondrotas
Author: Rokas Kondrotas
Dissertation title: Electrochemical deposition of Cu-Zn-Sn precursor and formation of Cu2ZnSnSe4 solar cell
Fields of science: Physical sciences, Chemistry (03P)
Scientific supervisor: prof. dr. Remigijus Juškėnas
Defence of the dissertation: 2015-04-24
Growing world population, emerging economies, higher living standards account for the increasing global primary energy consumption. The major part of generated energy is supplied from fossil fuel (coal, natural gas, oil). However, C02 is the by-product of burning process of the fossil fuel, which is thought to be one the main cause of the global warming [1). European Union has set the task to reduce C02 emission and increase power generation from renewable energy sources by 20% in 2020. Naturally, in order to minimize C02 emission alternative (renewable) energy sources have to be implemented on a wide scale. As of 2013 renewable energy sources (wind, solar, hydro etc.) accounted only for 5.3 %of global power generation [2). Although alternative energy market share is small, it is rapidly growing and over 2012-2013 power generation from renewable sources increased 16.3%. [2]. Solar power was the fastest  growing  branch  of  all  renewable  energy  sources  -  33% of growth, accordingly.
Sun is the major source of energy for the planet Earth. Sun provides an enormous amount of energy, it is estimated that average power density provided by the sun per year is 342 W/m2 [3). If one calculates amount of energy received to the whole earth, one would obtain 148 PWh. As a comparison, global primary energy consumption  was  19.9 TWh in 2013 that means the amount of sun energy, which reaches Earth surface is 7400 times greater than actually required. This shows a huge potential of solar power.
Sun emits energy  in a form  of  heat and  light. Depending on how energy is converted several types of solar energy technologies exits:
  • Solar  hot  water.  Sunlight  energy  and  heat  is  used  to  warm up  water through boiler system. Mostly implemented for domestic purposes.
  • Solar electricity. Vast amount of sunlight is concentrated in small area to heat water or salt up to their boiling/melting point. Then turbine s powered by steam are exploited to generate electricity. Mostly implemented in solar power plants.
  • Photovoltaic systems. Solar panels directly convert sunlight to  electricity based on photovoltaic effect in semiconductors. Solar  panels  are  used both for domestic purposes as well as for building solar power plants.
Photovoltaic systems comprises of solar panels, whereas solar panels are composed of solar cells. First modern solar cell was fabricated in 1954 at Bell's lab. It was silicon based solar cell and had 6% power conversion efficiency (PCE). At that time because of very high solar cell fabrication costs and low efficiency  it could not compete with  other  sources of energy.  In order to exploit photovoltaic energy  on mass scale  one  had to drastically  reduce production cost of solar cells. As a result, second generat ion solar cells have been invented. Second generation solar cell utilize less expensive materials, effectively  absorbs sunlight and cheaper brication methods are implemented, consequently   production  costs  are  significantly   reduced. Second generation solar cells are often referred as thin film (TF) solar cells, because the thickness of the absorber layer usually is only few micrometres thick.
Mainstream technologies of thin film solar cells are a-Si, CdTe and Cu(ln,Ga)Se 2 (CIGS). Although thin film solar cell technology had high growth expectations their part of photovoltaic energy market  is relatively  low- 16% [4]. Principally, the maturity of Si technology and China 's  cost reduction  of wafer based technologies suspended TF growth. However, the bright future is forecasted for thin film solar cell technology.
Looking at future perspectives,photovoltaic is a fast growing market and can achieve TW (1012 W) level capacity in 10-20 years. Current CdTe and CIGS technologies exploit Te and In metals, which are relatively rare in the earth's crust - 0.05 and 0.24 ppm, respectively [5]. Because of the scarcity of particular elements and that large part of indium is used for LCD production, substitute for CIGS has emerged - Cu2ZnSnSe4(CZTSe] and Cu2ZnSnSe4 (CZTS]. It comprises of earth-abundant (Zn, Sn- 75, 2.2 ppm), non-toxic, elements as well as possess similar physical and optical properties as ClGS, therefore is an ideal absorber for thin film solar cells. However, CZT5e(S) is a more complex system, relatively new material comprising of  4(5)  elements  and crystallization conditions have not yet been optimized as for example, for CIGS. As a result, record power conversion efficiency for CIGS based solar cells is 21.7% [6), whereas for CZT5Se -12.7% [7]. This indicates that CZT5e(5) is a complex system and the influence of technological parameters on CZTSe(S) formation mechanisms and its physical properties is not yet well understood.
Vaclovas Klimas
Author: Vaclovas Klimas
Dissertation title: Electrochemical synthesis and investigation of nanostructured anodic layers on iron
Fields of science: Physical sciences, Chemistry (03P)
Scientific supervisor: dr. Arūnas Jagminas
Defence of the dissertation: 2015-07-03
A rapid growth of nanostructured materials synthesis, investigations and applications in recent decades is related with unique, frequently non-characteristic for macrosubstances optical, electrical, mechanical, magnetic and catalytic properties as well as prospects of their effective application. At present, special attention is devoted to nanomaterials and their arrays capable of  effective usage for solar energy conversion to other kinds of energy and accumulation. Properly structured oxygen derivatives of trivalent iron due to their low-cost production, environment friendliness, chemical resistance, an attractive bandgap width, a high electrochemical potential and low lattice parameters, making it possible to accumulate high energy concentration in the mass unit, have been presently identified as prospective for solar cells construction and energy accumulation.
Works on formation of nanostructured anodic layers on iron and its alloys surface are scarce. The first of them was published in 2006, however recently increasing attention is paid to this process. Recent investigations of nanostructured iron anodic layers structure and properties support the prospects of such coatings application. On the basis  of analysis of the published works devoted to formation of anodic layers on iron one may state that the morphology of anodic coatings deposited in the ethylene glycol electrolytes with  NH4F  and  H2O additives  can  be  controlled,  yet  their  chemical  and  structural compositions remain unclear.
The published  data have shown that the most interesting photoelectic characteristics of anodic coatings formed are by far outstripped from the theoretical characteristics of hematite. Efforts to change the anodizing solutions or formation conditions have not yet brought any essential changes in this field. Therefore this study aims at compiling knowledge on the peculiarities of nanostructured layers formation on the iron and stainless steel surfaces in organic solvents with fluorides, the composition  and morphology of the anodic coatings as well as the reguliarities of their thermal transformation into hematite.
Tatjana Charkova
Author: Tatjana Charkova
Dissertation title: Synthesis and Investigation of Functionalized Alkyloligo (Ethylene Oxides)
Fields of science: Physical sciences, Chemistry (03P)
Scientific supervisor: dr. Olegas Eicher-Lorka
Defence of the dissertation: 2015-11-23
Self-assembled structures have attracted an enormous interest of different scientists. Self-assembled monolayers (SAMs) are the popular tool for tailoring the reactive properties of the surface. Due to their dense and stable structure, SAMs have been investigated for use in electrochemical sensors technology, electronics, nanodevices of any kind, and other areas. Besides, the close similarity of the SAMs with biomembranes enables the use of them as model systems. Such a system allows the preparation of ultrathin and high-resistance lipid layers on metals or semiconductors and the incorporation of receptor proteins into these insulating layers in order to design biosensors, bioelectronic or other biomimetic  devices. It also opens new paths to investigate membrane-related processes (cell adhesion, photosynthesis, respiration, drug­ protein interactions, etc.).
The stability, flexibility and other properties made tethered bilayer membranes (tBLMs) one of the most promising model  systems on a solid support, which can be successfully investigated with a multitude of surface-sensitive analytical techniques for much biotechnological manipulation.
Neutral oligo ethylene glycol (OEG) units are important components of specific­ binding SAM and might be incorporated with a number of functional groups; they also serve as a spacer arm for tethering the lipid layer on the solid substrate. Because the fonnation of the sulphur-gold bond is a commonly used chemisorption method for self­ assembly [8], oligo(ethylene oxide) thiols and sulphides arc the main building blocks for tBLMs. Such derivatives, functionalized with lipid, biotin and cholesterol groups are the most popular tools used in attachments, further development and investigation of artificial membranes.
Vadimas Dudoitis
Author: Vadimas Dudoitis
Dissertation title: Source Apportionment  of Fine and Carbonaceous Aerosol Particles in Urban and Background Environments
Fields of science: Physical Sciences, Physics (02P)
Scientific supervisor: Dr. Vidmantas Ulevičius
Defence of the dissertation: 2015-11-27
Fine and ultrafine particles play imporlant role in the climate systems, however the largest uncertainties of the anthropogenic radiative forcing occurs due to the underestimation of aerosol contribution. Thus, it is important to assess the spatial variability and pollution sources of atmospheric aerosol constituents.
In this work the receptor modelling (potential source contribution function (PSCF), chemical weighted trajectory (CWT) analysis and air mass trajectory cluster analysis) of aerosol particle number concentration (PNC) and black carbon (BC) sources was applied. The effects of air mass transport of atmospheric pollutants from regional biomass burning and vegetation fires in the South-Eastern Baltic Sea region were estimated. The loadings of aerosol constituents were investigated by aerosol chemical speciation monitor (ACSM) and aethalometer, particle number concentration and size distribution were depicted by condensational particle counter (CPC), scanning mobility particle sizer (SMPS) and aerodynamic particle sizer (APS), stable isotope mass spectrometry was conducted on collected filters.
Results   from  receptor   modelling   indicated   that  the  long-range   transport  of   air masses  had  increased  the  PNC  background  level  values  ( 1.00·104 cm-3) up  to  55% during the winter period in Vilnius city. In autumn and spring the estimated background level values (5.0·103 cm-3) were increased up to 130%. The background BC level in the South-Eastern Baltic Sea region was estimated to be 0.71 μg·m-3. Receptor modelling of BC concentration showed that during the whole study period the lowest concentrations had  been  observed  in  the  clusters  from  N  and  NW  and  had  varied  in  the  range  from 0.22 μg·m-3 to 0.45 μg·m-3 The increase of BC concentration was affected in winter by biomass burning up to 350% and in spring- by vegetation fires up to 80%.
It was proved that the combined aerosol particle sizing technique and stable carbon isotope ratio (ð13CTc) analysis were suitable for fossil fuel combustion and non-fossil emission source estimation. From the Bayesian isotope mixing model the fossil fuel source main contribution was indentified in the fine particle mode from 60% to I 00% with ð13CTc = -28.0%o and the non-fossil source main contribution - in the coarse particle mode from 60% to 100% with ð13CTc = -25.3%o.


Juozas Adamonis
Author: Juozas Adamonis
Dissertation title: Terahertz Optoelectronics  Components of Semiconductor Materials with Deep Defects
Fields of science: Physical Sciences, Physics (02P)
Scientific supervisor: Prof. habil. dr. Arūnas Krotkus
Defence of the dissertation: 2015-09-18
From the practical point of view the THz radiation is very attractive because of phenomena that could be observed in this frequency range. A vast amount of non-polar molecules  do not  react  to the THz field, while polar molecules  such as H20, N2,  02,  CO, S02, HCI etc. strongly interact with this field. Absorption spectra of various polar molecules have sufficient absorption peaks in the THz range originating from their rotational and vibrational states. These features are of interest in the case of the environment control, air pollution observation and gas detection. Besides, THz waves partly penetrate through clothing fabrics, polymers or paper, which allows one to perform the  THz imaging of objects, hidden behind these visually opaque materials. THz radiation has great potential in security as it can be used to detect hidden weapons. Drugs, plastic explosives and flammable liquids also have characteristic absorption peaks in the THz  frequency domain. All this enables utilizing the aforementioned radiation for detection of  these materials or ensuring safety in public gathering places.
Unfortunately, at the present time applications of THz systems are at the primary stage and they are still mostly used in the laboratory research. Semiconductor optoelectronic components and femtosecond lasers are widely employed in terahertz spectroscopy systems for spectroscopic measurements and  THz imaging. Most commonly THz systems are activated with Ti:Sapphire lasers (λ = 800 nm), whose photon energy quantum is slightly higher than the band gap of the GaAs crystal. Low temperature GaAs layers (LT GaAs) grown by means of molecular beam epitaxy have large defect concentrations, that act as electron traps. Other important features of these layers - large resistance, short carrier lifetime and high electron mobility - makes LT GaAs an attractive material for the THz component production. At present, both the LT GaAs and Ti:Sapphire laser technologies have reached a relatively mature level and their production is widely developed. Despite that, complex and bulky multi-stage optical pumping of Ti:Sapphire lasers limits a wider use ofTHz systems.
For generation  and registration of THz radiation compact  and potentially less expensive semiconductor and fiber lasers emitting in the near infrared region of 1 μm and 1.55 μm are being more increasingly used. On the other hand, a large drawback of such systems is lack of efficient optoelectronic components operating in this wavelength range. Thus, in this work photoconductive antennas made from LT GaAs epitaxial layers were examined as THz emitters and detectors activated with the laser wavelength longer than the bandgap  of the material (hvEg). In this case, photoconductivity  of LT GaAs layers is governed by sole electrons excited from the defect band. This carrier generation mechanism is very attractive for THz photomixers, which were also investigated in this work. Finally, in this work a compact, easily and quickly tunable narrow linewidth THz pulse generation system, which was activated with the picosecond fiber laser, has been demonstrated. This system is simpler and more attractive for the THz imaging and environmental control applications.
Bogdan Voisiat
Author: Bogdan Voisiat
Dissertation title: Formation of Frequency  Selective Surfaces Using Laser Ablation Methods and Characterisation of Their Properties
Fields of science: Technological Sciences, Material Engineering (08T)
Scientific supervisor: Dr. Gediminas Račiukaitis
Defence of the dissertation: 2015-09-29
In this PhD thesis the formation of frequency selective surfaces (FSS) using laser ablation techniques was investigated.  Two laser ablation methods for such structure formation was proposed and tested: direct laser ablation (DLA) that uses a single focused laser beam to ablate the material, and direct laser interference patterning (DLIP) that uses the periodic intensity pntlem formed by laser beam interference to directly structure the material surface. DLA was used  for fabrication  of THz band-pass  filters consisting of cross shaped apertures in 30 un thick molybdenum foil (Mo) and 0.5 ).lffi thick molybdenum film deposited on polyimide substrate (Mo\PI). The dashed raster laser scan strategy was proposed for faster and better quality apertures formation. The fabricated band-pass filter transmission performance was measured using THz time-domain spectroscopy. The results showed that Mo filter peak transmittance was in the range of 85-95% and transmittance ofMo\PI filter was 2 times smaller (30--40%) due to the dissipation in the PI substrate. It was found that band-pass filters with smaller than 1.5 THz resonant frequency was faster to produce on metal foil than on thin metal film.
The same optimized DLA method was used to form 10 mm and 5 mm focal lengths standard THz zone plates (TZP) and zone plates with integrated band pass filter (TZP-BPF) for the 580 GHz. The measured focusing performance showed that integrated filter performed additional filtering of the incident radiation, making the focusing performance of the zone plate more sensitive to the frequency of incident radiation.
DLA method was also used for multilevel phase zone plate (PZP) production on silicon substrate. Using the fastest silicon laser ablation regime it was possible to reach the highest ablation depth control resolution and smallest surface roughness. Using this regime the mullilevel phase zone plates with the I 0 mm focal length for 580 GHz radiation was produced and focusing perfom1ance  was measured.
The second laser fabrication method, called direct laser interference patterning (DLIP), was applied to pattern thin metallic films on the glass substrate and the nickel film on the silicon substrate. The 2D-grutings with a period of 5 )liD and consisting of circular holes were fabricated. It was found that when DLIP with high pulse energy was used to pattern large surface area the fanned periodic structures are not unifonn along the fabricated area. To find out the reason for this structure distortion, the DLIP optical setup simulation was performed using ZEMAX. The results showed that the non-uniform structure formation is caused due to the wave front distortions of the interfering beams. To compensate this distortions the special designed objectives was proposed and tested, that resulted up to 16 times larger area of the uniform periodic intensity distribution.
The three techniques to control the fabricated using DLIP periodic structure shape was proposed and investigated in this thesis: the manipulation of the each split beam phase, polarization  and using the sample sub-period shift method. It was found that the split beams polarization manipulation provides wider choice of the pattern shape modifications and it is easier to realize experimentally, than manipulation of the beams phase. Using the sub-period shift method it was possible to form custom shaped periodic elements in thin chromium film with the size smaller than the period of the interference intensity pattern. In order to demonstrate the frequency selective behavior of periodical structures made using DLIP, the computer modeling of transmittance of those structures was performed. The results showed that structure fabricated using DLIP exhibits resonant transmission characteristics in the inferred spectrum range.