Defended Dissertations in 2010

Artūr Šimukovič
Author: Artūr Šimukovič
Dissertation title: Investigation DC, microwave characteristics and noise of SiGe and A3B5 heterojunction bipolar transistors
Fields of science: Physical sciences, Chemistry (03P), Semiconductor physics P265
Scientific supervisor: Dr. Paulius Sakalas
Defence of the dissertation: 2010-09-14
Tasks of the work
The main task of this work is an investigation of dc, microwave and noise characteristics in the relevance frequency range of 1 -30 GHz of Si/SiGe and InGaP/GaAs HBTs. Noise simulations and modeling have been performed to investigate the shot noise source cross - correlation, impact ionization noise modeling and temperature dependences of dc, rf and noise parameters.
Practical value
HBT modeling in device level was performed with transport equation based on hydrodynamic (HD) and drift - diffusion (DD) models [4]. Compact modeling (CM) was performed with HICUM/L2 v2.23 [5] one of the standard Compact Modeling Council confirmed industry bipolar transistor models. Radio frequency (RF) circuit sensibility, high - speed and low noise performance are dependent on their compound semiconductor device properties. Noise performance is especially critical parameter for the low noise amplifiers. The more accurate models of transistors are for the more accurate design of circuit performance is obtained. Intelligent optimization design circle decreases expenses and increases product competitive ability.
Scientific novelty
The shot noise sources cross - correlation in HBTs was investigated. Compact model HICUM simulations were performed to seek the importance of the shot noise source cross - correlation on advanced SiGe and AIIIBV based HBTs high frequency noise characteristics. SiGe HBTs DC, RF and noise characteristics were investigated at biases of impact ionization region. Chynoweth relation based impact ionization noise in CM was found a sufficient way to characterize noise performance of SiGe based HBTs biased at II region. Temperature dependence of dc, rf and noise performance of SiGe HBTs dc, rf was investigated. It was shown that SiGe HBTs, especially LEC can operate in extreme temperature environment (4 K to 500 K). It was shown that CM can capture dc, rf and noise behavior in the practical (- 25°C - 200°C) temperature range.
  • Compact model HICUM simulation of InGaP/GaAs and SiGe HBTs noise was compared with measurements and show a good agreement for the frequency rangebelow ~ 10 GHz. Noise figure deviations beyond 10 GHz and higher current densities are affected by base and collector shot noise cross - correlation power. Derived analytical noise model with shot noise sources cross - correlation showed very good agreement with measurement data.
  • Investigation of base and collector shot noise showed an importance of shot noise cross - correlation at high frequencies. Implemented correlated shot noise model solution in CM (HICUM L.2 v.2.3) via Verilog-A code was verified against CED SiGe and LEC SiGe HBTs. Performed simulations with this topical model exhibit a good agreements with measured data in practical frequency range (f< fT)
  • Chynoweth law describe well weak impact ionization in SiGe HBT (without device degradation, at M-1<<1). MC method was used to calculate electron multiplication factor. Simulations of LEC SiGe HBT with HICUM with implemented impact ionization model were in a perfect agreement with measured data in impact ionization bias range.
  • LEC SiGe HBTs are capable to operate at 4 - 423 K ambient temperature range. At cryogenic temperatures a rapidly growing dc gain and cutoff frequency was observed. Hydrodynamic and compact model describes SiGe DC and noise characteristics only in the limited 300 - 423 K temperature range. The main noise sources of LEC SiGe HBT at 300 - 423 K temperature range are collector shot noise and base thermal noise.
Gediminas Molis
uthor: Gediminas Molis
Dissertation title: Investigation of the terahertz pulse generation from the narrow band gap semiconductor surfaces
Fields of science: Physical sciences, Physics (02 P), Semiconductor physics (P265)
Scientific supervisor: Prof. Habil. Dr. Arūnas Krotkus
Defence of the dissertation: 2010-06-22
Main goals
  • Development of a THz Time domain spectroscopy system and the optimization of its optical and optoelectronic components.
  • The search for the best THz surface emitter by the comparison various semiconductor materials and different THz surface emission mechanisms.
The objectives of the study
  • Using photoconductive emitters and detectors made in Semiconductors Physics Institute, make THz generation and detection system from scratch. Adapt it for THz radiation surface emitters investigation experiments.
  • Using the THz TDS system, investigate various surface emitters and compare them.
  • Explore THz pulse amplitude dependences on intrinsic carriers’ concentration and azimuthal angle for several different THz surface emitter materials.
  • Investigate the dependences of the THz signal amplitude generated from various semiconductors surfaces on the excitation laser wavelength.
The novelty of the study
  • THz pulses generations from various semiconductors surfaces have been thoroughly investigated.
  • By using the THz excitation spectroscopy technique, the intervalley separation energies in the conduction band of InxGa1-xAs (x=0; 0.2; 0.53; 1) and InSb samples ware measured for the first time.
The defended propositions of the dissertational research
  • Best surface emitter when using 800nm wavelength Ti:Sapphire femtosecond laser pulses for excitation is p-InAs (p≤1017 cm-3).
  • THz radiation in the InAs surface mainly is generated because the photo-excited charge carriers move into the bulk with different speeds and electrical field is formed and latter part of the optical pulse is rectified due to an optical non-linearity that is induced by this field.
  • THz radiation amplitude, generated in the semiconductor surface, depends on the excitation pulse wavelength; it has maximum from which position it is possible to calculate the intervalley energy separation in the conduction band of the material.
Jan Devenson
Author: Jan Devenson
Dissertation title: InAs/AlSb short wavelenght quantum cascade lasers
Fields of science: Physical sciences, Physics (02P), Semiconductor physics (P265)
Scientific consultants: Habil. Dr. Alexei Baranov, Prof. Dr. Gintaras Valušis
Defence of the dissertation: 2010-10-26
Quantum cascade lasers (QCLs) are considered now as standard light sources for many chemical sensing applications in the mid-infrared above 4 µm. Some spectroscopic applications require injection semiconductor lasers emitting at shorter wavelengths in the vicinity of 3 µm. This spectral region is, in principle, accessible both for interband diode lasers and quantum cascade lasers (QCLs) operating at room temperature (RT). It deserves to note that performances of diode lasers rapidly degrade above 3 µm due to fundamental limitations such as increasing influence of nonradiative Auger recombination. High quality QCLs based on the InP technology are suited for  λ > 3.8 µm but the extension their operation range towards shorter wavelengths is still a challenge due mainly to material limitations. Using adequate materials high performance QCLs operating at wavelengths as short as 3 µm or even below can be developed.
The InAs/AlSb material system seems to be at present the most promising for the development of short wavelength QCLs thanks to the high conduction band offset of 2.1 eV and the large Γ-L distance of 0.73 eV in InAs. Another advantage of this system is the small electron mass in InAs, which is favorable to obtain QCLs with high gain and low threshold. However, InAs/AlSb material system was not used for development of short wavelength QCLs mainly due to the difficulties associated with the epitaxial growth as there are no common atoms at the well/barrier interfaces. The first short wavelength antimonide-based QCL emitting near 4.5 µm at room temperature has been developed by NANOMIR group of the IES (Institut d’Electronique du Sud) in University Montpellier II in France and it was a starting point of my work at the same laboratory. The advantage of this new material system for development of short wavelength QCLs was obvious, however to fully exploit it variety of investigations on this material properties and
Importance for application
The mid-infrared range, sometimes called the fingerprint region of the electromagnetic spectrum, is of enormous scientific and technological interest since many molecules have their fundamental rotational-vibrational absorption bands in this range. The MIR absorption spectrum is very specific to the structure of a particular molecule, allowing highly selective detection. In addition, since these absorption lines are very strong (several orders of magnitude stronger than the overtone and combination bands in the NIR), concentrations in the parts-per-billion (ppb) to parts-per-trillion (ppt) ranges can be detected using relatively compact laser-based sensors. Fast, sensitive, and selective chemical sensors are needed in numerous applications. In industrial process control they are used for detection of contamination in semiconductor fabrication lines and for plasma monitoring, in law enforcement for drug and explosive detection, in automotive industry for engine exhaust analysis, in environmental science for pollution monitoring, in medical diagnostics for exhaled breath analysis, and in homeland security for detection of chemical airfare agents.
Another interesting feature of the mid-infrared are the atmospheric transmission windows between 3-5 µm and 8-12 µm which enable free-space optical communications, remote sensing, and thermal imaging. High power lasers in the 3-5 µm range will also enable the development of infrared counter-measures for homeland security.
Especially, application of intersubband transition based lasers is indispensable in a hot test environment such as the oil boring or other applications where temperature may reach 400 K.
Rasa Suzanovičienė
Author: Rasa Suzanovičienė
Dissertation title: Investigation of carrier kinetics in semiconductors by terahertz radiation pulses
Fields of science: Physical sciences, Physics (02 P), Semiconductor physics P265
Scientific supervisor: Prof. Habil. Dr. Arūnas Krotkus
Defence of the dissertation: 2010-11-12
Main goals
  • To develop an optical pump - terahertz probe technique for measuring various characteristic electron times: relaxation time the energy relaxation time and momentum relaxation time.
  • To investigate electron lifetimes in GaAs - MnAs alloys and GaAs crystals implanted with various amount of Au ion.
  • To measure photoconductivity of narrow - gap semiconductors after excitation with femtosecond laser pulses, and to estimate its relation with hot electron energy relaxation and nonparabolicity of conduction band.
  • To investigate reflection and transmission spectrum of InSb in the terahertz range; to determine plasma frequency and electron damping time from these data.
The novelty of the study
Investigating of the optically exited hot electron energy relaxation times, terahertz (THz) radiation where used for the first time. By comparing experimental and theoretical data obtained by optical pump - terahertz probe measurements and their theoretical simulation, of epitaxial Cd0,2Hg0,8Te and In0,5Ga0,5As layers with different composition, it has been found that the energy relaxation times in both samples are about 1 ps. By using the same optical pump - THz probe technique, electron life times were measured in Ga1-xMnxAs samples with different Mn compositions.
THz - time domain spectroscopy (THz-TDS) has been successfully used for characterizing InSb crystals in THz frequency range. Both THz reflection and transmission spectra in different temperature range were measured. Electron momentum relaxation times and plasma frequency where obtained in a wide temperature range. It should be noted that for the semiconductor investigation, THz-TDS in reflection geometry where not used before. It is necessary to take into account the energy dependence of the effective mass in the non-parabolic conduction band of InSb.
  • The optical pump - terahertz probe experimental technique for measuring electron lifetime, electron energy relaxation time in semiconductors was investigated.
  • Increasing Mn concentration in GaMnAs 0.06 % electron lifetime increases about 100 times. This increase of electron lifetime is related to sudden decrease of electron mobility and change in polarity of emitted terahertz pulse at the surface of the semiconductor, in the cause of evidence of the electrical state of material defect.
  • It is estimated that rise in terahertz radiation, in narrow - gap semiconductors gets slower until few picoseconds. This phenomenon related to excess energy photoelectron relaxation. The electron energy relaxation time about 1 picosecond were estimated for Cd0,2Hg0,8Te and In0,5Ga0,5As.
  • The InSb crystals were investigated in the various temperatures. It is found that InSb became transparent in the temperatures below 150 K. The radiation frequency, at with transparency is evidenced, allow electron impulse relaxation time temperature dependency to be estimated.
  • The minimum of the InSb reflection spectrum were observed. This minimum is related with the electron plasma frequency. The value of plasma frequency in the room temperature, can only be described including contribution of electron distribution in non - parabolic conduction band.
Virginijus Bukauskas
Author: Virginijus Bukauskas
Dissertation title: Applications of scanning probe microscopy for development and investigation of gas sensitive nanosystems integrated with the ultra-thin metal oxide films
Fields of science: Technological sciences, Material engineering (08T)
Scientific supervisor: Assoc. Prof. Dr.  Arūnas Šetkus
Defence of the dissertation: 2010-08-20
Aim of the work. The aim of this work is to apply the methodologies of the SPM for characterization of the local point and local area properties in the gas sensitive MO films with the nanoscaled thickness that can be used for nanosystems and hybrid (combined) materials in novel types of chemical detectors.
Scientific novelty. Novelties of this work include the following results in the research area of gas sensitive materials and material science:
The special Veeco SPM TUNA method was originally adapted for the investigation of physical properties of ultra-thin MO films.
An original method based on the SPM probe controlled electrical current was proposed for the formation of nanosystems with various electrical properties on the surfaces of thin MO films.
It was proved that the specific oxygen transport in the MO based nanosystem results in the growth of the nanoobjects on the surfaces of MO films and the formation of these objects is sensitive to the composition of the atmosphere.
It was proved that a dependence of the magnitude and the kinetics of the response to gas on film thickness is related to the structural changes of MO films rather than to the diffusion of gases into the film.
It was shown that the surface properties in the nanoscaled areas of the MO films can be individually and intentionally modified by both the SPM driven nanoobject growth and deposition of the multimolecular 3D objects with/without the biochemicaly active centers.
Practical value. Based on the results of this study, the long term drift of the MO gas sensors is partly explained by the oxygen redistribution in the thin films under external voltage and the fundamental aspects valuable for an improvement of the sensor stability are described. Fundamental aspects of original method for nano-tuning of ultra-thin MO films in selected areas are described and the principles for formation of multi-component nano-assembly of solid state and hybrid material sensors is proposed.
General conclusions
  • Restructurisation of SnOx films effects electrical response of the film to gas. Higher resistance response to gas of the SnOx films with thickness < 4 nm and > 7 nm caused by higher ratio of surface area exposed to gas molecules and conductive cross- section of the film. Changes in the microstructure and thickness of the SnOx film do not produce a related change in the time constant of the response to gas, i.e. time constant is independent on SnOx film thickness (up to 40 nm). The response time depends only on the rates of the surface chemical reactions.
  • According to the experimental results on investigation of properties of thin SnOx films morphology, electrical parameters and electrical response to gas, there were made gas sensors for detecting volatile components of propionic acid.
  • Electrical currents and changes of the currents in time that appear in nanosystems because of the charge transfer through or above the barrier depends on: 1) properties of the nanosystem, 2) changes in the system caused by external factors of measurement system, such as electric field and mechanical effect and 3) interaction of nanosystem with surrounding atmosphere (air or gas).
  • It was created original method for formation of nanostructures of various conductivity on the surface of thin metal oxide films by SPM electric treatment, when negative bias is applied on the surface. Dimensions of formed nanostructures depend on electrical field magnitude and time it was applied during the formation, and also on the atmosphere composition above the metal oxide surface.
  • Combination of various Scanning probe microscopy methods were applied to investigate mechanical, electrical properties and structure of relatively thick continuous hybrid layers and individual components of noncontinuous hybrid films by making special hybrid samples.
Kristina Brazdžiuvienė
Author: Kristina Brazdžiuvienė
Dissertation title: Studies of conducting polymer-modified electrodes and their application for electroanalysis
Fields of science: Physical sciences, Chemistry (03P)
Scientific supervisor: Prof. Habil. Dr. Albertas Malinaukas
Defence of the dissertation: 2010-11-17
The aim of this work - to investigate patterns of electrochemical oxidation of ascorbic acid on the polyaniline and poly(N-methylaniline) modified electrodes, in order to develop sensitive sensors for ascorbate.
The tasks of the work:
  • To modify Pt electrodes layers of PANI and PNMA, using a variety of electrochemical coating techniques and conditions, and monomer concentrations.
  • To characterize obtained polymer coatings (electrochemical activity and electrochemical behavior in sulfuric acid and buffer solutions).
  • To use modified electrodes for oxidation of ascorbic acid in buffer solutions.
  • To investigate the possibility of using PANI and PNMA modified electrodes as amperometric ascorbate sensors.
Scientific novelty and practical value of the work.
Scientific novelty: a detailed study of various factors affecting aniline and N- methylaniline electrochemical polymerization and the resulting properties of PANI and PNMA layers was carried out for this purpose. Comparative study of modified electrodes in solutions of different acidity was performed and it was shown that PNMA had a better redox activity in slightly acidic and neutral solutions compared to polyaniline. The nature of amperometric response of modified electrodes to ascorbate was investigated and autocatalytic mechanism of ascorbate electrooxidation on PANI modified electrode was suggested. Using PANI and PNMA modified electrodes, prototypes of amperometric ascorbate sensors have been developed and their comparative studies were carried out.
  • Polymer morphology studies have shown, that under the potentiodynamic conditions complete coverage of the surface is obtained and the structure is influenced by Elim. The structure of PANI is papillae like, consisting of small globules, while the morphology of PNMA is a sponge-like, and even though the thickness of PNMA under the same covering conditions is thinner than that of PANI, the coating looks more fluffy, due to the by-products inclusion into the bulk of polymer.
  • It has been ascertained that poly(N-methylaniline) shows better redox properties in slightly acidic or neutral solutions compared to those of PANI, due to slow oxidation of aniline in an alkaline media.
  • The autocatalytic reaction mechanism has been proposed for anodic oxidation of ascorbate in nearly pH-neutral buffered solutions: protons liberated during ascorbate oxidation cause a local pH drop within a thin polyaniline layer, turning it into a proton- doped conducting form which shows electrocatalytic activity towards oxidation of ascorbate. The proposed mechanism explains the ability of polyaniline to electrocatalyze the anodic oxidation of ascorbate even in pH-neutral solutions, where polyaniline is present in its undoped and non-conducting form.
  • Poly(N-methylaniline) modified electrodes can be used for ascorbate (vitamin C) assay in pH-neutral solutions. The response of PNMA electrode for ascorbate is more than 320 times faster than for PANI based sensor. It has been shown that the fast response is due to a better redox activity of poly(N -methylaniline) in slightly acidic or neutral solutions compared to polyaniline.
  • It is shown that at optimal operating conditions modified PNMA and PANI electrodes can be used for ascorbate (vitamin C) testing in the pH- neutral solutions. It has been shown that polyaniline-modified electrodes can be used for ascorbate assay within a pH range of 5.5-7.2, operating potential window of 0.1-0.3 V versus Ag/AgCl, showing a linear range of response up to 0.6 mM, and a lower detection limit of 0.05 mM.
  • Oxidation of ascorbate using a PNMA modified glassy carbon rotating electrode occurred at more negative potentials than on unmodified electrode. Using the electrode potential from 0.2 to 0.3 V and the electrode rotation speed from 100 to 4000 rpm the catalytic current satisfies Koutecky-Levich equation conditions at a millimolar ascorbate concentration.
  • It has been shown that pt and glassy carbon electrodes modified by PNMA and PANI can be used for the determination of ascorbate concentration in juice.
Julija Uljanionok
Author: Julija Uljanionok
Dissertation title: Role of Cu(I) complexes in the electrochemical reduction of glycinate and maleate Cu(II) complexes
Fields of science: Physical sciences, Chemistry (03P)
Scientific supervisor: Prof. Habil. Dr. Arvydas Survila
Defence of the dissertation: 2010-01-21
Processes of electrochemical reduction of glycinate and maleate complexes of Cu(II) are the main subject of the research. The main difference between the two systems lies in the fact that, in contrast with glycinate system, a large amount of stabile Cu(I) complexes may be formed in maleate solutions. The major goal of the work consists in the comparative analysis of electrochemical characteristics of the systems focusing on the role of intermediates. To execute a purpose, the following tasks were scheduled:
  • determination of equilibrium characteristics of Cu(II) and maleic acid solutions;
  • derivation of the back-ground theoretical model accounting for step-wise charge transfer processes in the labile complex systems;
  • evaluation of the effect of the alkali metal cations on the reduction kinetics of Cu(II) glycinate complexes;
  • investigation of electrochemical reduction of Cu(II) maleate complexes;
  • analysis of mechanism and kinetics of Cu(I) generation in maleic acid system.
Novelty of scientific investigation:
  • new modifications based on the quantitative description of pH-metric and spectrophotometric data are proposed for the analysis of experimental data;
  • equilibrium characteristics of Cu(II) and maleic acid solutions containing supporting sulphate electrolyte are determined;
  • effect of the nature of supporting electrolyte on the reduction of Cu(II) glycinate complexes is established;
  • regularities of Cu(I) formation in the Cu|Cu(II), maleic acid system are analyzed;
  • peculiarities of pre-electrolysis of maleate solutions are investigated.
  • The equation derived for quantitative description of pH-metric data, was applied for the analysis of the entire titration curve. Protoniztion constants of maleate anions, obtained with fitting procedures, are as follows: logβ1H = 6.05, logβ2H = 7.48. It was established that the addition of 0.3 M K2S04 results in the diminution of stability constants (logβ1H = 5.75, logβ2H =7.30).
  • To obtain equilibrium characteristics of Cu(II) maleate complexes from spectrophotometric data, the method, based on the driven harmonic oscillator theory, was proposed. Stability constant of monoligand complex was found to be equal to 102.2M-1
  • Basic equations of formal electrochemical kinetics are applicable for interpretation of experimental data obtained for Cu|Cu(II), glycine system. According to the results of analysis, kinetic parameters of Cu(II) glycinate reduction depend on the nature of the supporting electrolyte. Exchange current density decreases and cathodic charge transfer coefficient increases in the sequence: Li+ - Na+ - K+ - Cs+.
  • Thermodynamic analysis shows that deep changes are possible in this system resulting in 90 % transform of Cu(II) into Cu(I). It follows from EQCM data, that the rates of Cu corrosion and Cu20 formation are estimated to be of the same order (nmol cm-2 s-1).
  • The theoretical model applied describes satisfactorily the steady-state voltammetric characteristics of Cu|Cu(II), maleic acid system, but it yields some contradictory results in the case of time-dependent processes.
  • To enhance the Cu(I) generation in maleic acid system, the pre-electrolysis procedure was applied. It was found that its effect depends on solution pH. Passivity of electrodes, observed at pH 3.9, gives rise to the reduction of H30+ ions and, as a consequence, to the accumulation of Cu(II) in the solution.
Živilė Stankevičiūtė
Author: Živilė Stankevičiūtė
Dissertation title: Studies of synthesis and cyclization reactions of alkylated 5-cyano-2-methylsulfanyl-4(3H)-pyrimidinones
Fields of science: Physical sciences, Chemistry (03P)
Scientific supervisor: Prof. Habil. Dr. Albertas Malinaukas
Defence of the dissertation: 2010-04-13
The goal of the work:
To investigate transformation of ethyl (E)-2-cyano-3-(S-methylisothioureido)-2- propenoate in to 5-cyano-2-methylsulfanyl-4(3H)-pyrimidinone, to investigate its O- and N-alkylation with 4-substituted ω-bromoacetophenones, cyclization and functionalisation reactions of O-alkylated derivatives.
Scientific novelty and practical value of the work.
It was refilled data about transformation of ethyl (E)-2-cyano-3-(S- methylisothioureido)-2-propenoate in acidic and alkaline media. It was found that boiling in glacial acetic acid proceeds selectively and leads to formation of ethyl 4-amino-2-methylsulfanylpyrimidine-5-carboxylate, while ring closure in alkaline media gives rise to a mixture of 5-cyano-2-methylsulfanyl-4(3H)-pyrimidinone (major product), small amounts of ethyl 4-amino-2-methylsulfanylpyrimidine-5-carboxylate and uncyclisized products of hydrolysis - ethyl 2-cyano-3-ureido-2-propenoates, separated as individual compounds. It was investigated alkylation of tridentate 5-cyano-2- methylsulfanyl-4(3H)-pyrimidinone with 4-substituted ω-bromoacetophenones in the presence of potassium carbonate and catalytic amount of potassium iodide in boiling acetonitrile. It was determined that the proportion of 0-, N1- and N3- alkylated products varied depending on the nature of substitute on the aromatic ring 4-position. For the first time 0-, N1- and N3-alkylated isomers were isolated from alkylation mixture. The main reaction product is the O-alkylated isomer with the N3- and N1-alkylation products respectively separated as minor components by fractional crystallization. It has been proposed a novel method for the synthesis of furo[2,3-d]pyrimidines by Thorpe-Ziegler condensation reaction of O-alkylated pyrimidinones. The synthesized compounds could be perspective analogues of the known pharmaceutical and agrochemical agents.
  • Cyclization of ethyl 2-cyano-3-(S-methylisothioureido)-2-propenoate under acidic conditions proceeds selectively and leads to formation of ethyl 4-amino-2- methylsulfanylpyrimidine-5-carboxylate while in alkaline media gives rise to a mixture of cyclization and hydrolysis products: 5-cyano-2-methylsulfanyl-4(3H)- pyrimidinone, small amounts of 4-amino-2-methylsulfanylpyrimidine-5- carboxylate and (Z)- and (E)-isomers of ethyl 2-cyano-3-ureido-2-propenoate. Optimal conditions for 5-cyano-2-methylsulfanyl-4(3 H)-pyrimidinone synthesis is 0.5 M NaOH at 50 °C, reaction in less alkaline media is favourable for formation of 3-ureidocompounds.
  • Alkylation of 4(3H)-pyrimidinones in the presence of potassium carbonate in boiling acetonitrile readily gives a mixture of 0-, N3- , N1-alkylated products (yield of separated components by fractional crystallization respectively: 13-57, 0.3-12 and 3-11 %). By 1H NMR spectral data, O-alkylated isomer is a major product, N3- and N1-alkylated products proportions varied depending on the nature of the substitute on the aromatic ring 4-position and decreasing in order: CH30 > CH3 > H > Br ≥ Cl > N02 .
  • For N1- , N3- and O-alkylated isomers structure estimation are important such spectral characteristics:
    1. In 1H NMR spectrums are typical XCH2 (X=0, N) and H-6 chemical shifts in a field: 5.99-6.11 and 8.93-8.96 for O-alkylated isomers, 5.66-5.82 and 8.63- 8.69 for N3-isomers, 5.69-5.86 and 8.58-8.61 ppm for N1-isomers respectively.
    2. In l3C NMR spectrums are typical chemical shifts of XCH2 (X=0, N) and pyrimidine ring C-4 position in a field: 70.0-70.5 and 167.3-167.6 for O- alkylated isomers, 51.4-52.2 and 170.6-170.7 for N3-isomers, 58.9-59.5 and 163.0-163.1 ppm for N1-isomers respectively.
    3. In IR spectrum are observed valence vibrations absorption bands of CN and CO groups: 2226-2232 and 1686-1705 for O-alkylated isomers, 2224-2234, 1690-1698 and 1678-1685 for Mj-isomers, 2226-2229, 1684-1692 and 1652- 1623 cm'[1] for N3-isomers respectively.
  • New synthesis method for furo[2,3-d]pyrimidine system was proposed by cyclization of 4-[4'-R-phenacyloxy]-2-methylsulfonylpyrimidin-5-carbonitriles under Thorpe-Ziegler basic conditions. It has been determined that:
  1. The best yields of 5-amino-6-(4'-R-benzoyl)-2-methylsulfanylfuro[2,3- d]pyrimidines obtained in ethanol at 1.5 equivalent of sodium ethoxide.
  2. Cyclization in system CH3OH-CH3ONa (2 equivalents or more) leads to displacement reaction of 2-methylsulfanyl- into methoxygroup as byproduct.
  3. Cyclization in system K0H-H20-DMF occurs with side hydrolysis reaction of ether group.
  4. In aprotic solvents (CH3CN, C6H6) furo[2,3-d]pyrimidines not formed.
  • Acetylation, hydrazinolysis, hydrolysis and oxidation reactions was occurred for functionalization possibilities of 5-amino-6-(4'-R-benzoyl)-2-methylsulfanylfuro[2,3-d]pirimidines. 5-Acetylaminoderivatives are formed under acetylation in acetic anhydride. Oxidation reaction occurs with formation of 2-methylsulfonylcompound and Baeyer-Villiger oxidation of ketone group.
Skirmantas Keršulis
Author: Skirmantas Keršulis
Dissertation title: Magnetic and electrical field effects in polycrystalline La1-xSrxMnO3 films
Fields of science: Physical sciences, Physics (02P), Condensed matter: electronic structure; electrical, magnetic and optical properties; superconductors; magnetic resonance; relaxation; spectroscopy (P260)
Scientific supervisor: Assoc Prof. Dr. Nerija Žurauskienė
Defence of the dissertation: 2010-03-01
Aim of the work. The objective of this effort was to investigate electrical conductivity of La1-xSrxMnO3polycrystalline films in low and high magnetic, and at strong electrical fields, to investigate how magnetoresistance and electroresistance effects depend on film preparation conditions such as deposition temperature, composition and thickness of the films.
Practical value. The obtained results enable to suggest two prototypes of electronic devices. The first is contactless electrical current sensor operating at room temperature and intended for 10-30 A electrical current sensing. The second is fast fault current limiter consisting of polycrystalline and epitaxial film operating at liquid nitrogen temperatures and generating 3.5 dB attenuation after 0.5 ns and 8 dB after several tens of ns. The results of investigation were also used for the improvement of high pulsed magnetic field sensor by decreasing value of MR anisotropy in the range of magnetic field 0.5-2 T.
General conclusions 
  • It was obtained that La1-xSrxMnO3 films deposited on lucalox (99.9% A1203 + 0.1% MgO) substrates by PI MOCVD technique at Tsub =650—750 °C are polycrystalline, while at 825 °C are textured. The decrease of Tsub or thickness of the films results in decrease of dimensions of crystallites. The decrease of Tsub, thickness of the films or chemical composition x results in increase of the resistivity maximum pm and the shift of Tm to lower temperatures.
  • Magnetoresistance value of thin polycrystalline La1-xSrxMnO3 films grown on lucalox substrate can be changed by changing chemical composition, substrate temperature and film thickness. The MR in ferromagnetic phase increases with decrease of Tsub, x or thickness. At high temperatures (T>Tm) the MR decreases due to increased crystallites influence.
  • Magnetoresistance anisotropy at low magnetic fields (B< 1 T) is maximal close to Tm and increases with increase of x, dimensions of crystallites and film thickness at T>Tm. In high magnetic field the MRA does not depend on x and crystallite size and is negligible (<3%).
  • The MR phenomenon in thin polycrystalline LSMO films deposited on lucalox substrate can be explained using modified Mott’s hopping model assuming that charge carriers transport mainly takes place in magnetically and structurally disordered grain boundaries (GBs). The MR dependence on magnetic field in these films in the temperature range between Tm and Tc can be explained using this model and taking into account both ferromagnetic and paramagnetic phases. Value of magnetic polaron J obtained as a fitting parameter increases exponentially with T and in the vicinity of Tm indicates that magnetically aligned clusters include approximately 10 manganese ions. The electroresistance in polycrystalline La1-xSrxMnO3 films depends on chemical composition, dimensions of crystallites and Tm. In the feromagnetic phase the ER is maximal for films having small crystallites which dimensions define higher amount of the disordered GB material in the films. At room temperature the ER is higher for films with large size crystallites. The threshold electrical field strength is highest for films having smallest crystallites. Carrier transport mechanism in the films can be described by Glasman-Matveev model: for films with small size crystallites the predominant charge carriers transport mechanism is inelastic tunneling through 3 localized states in GBs while for larger crystallites the tunneling through 2 states takes place.
  • The Tm of polycrystalline LSMO films shifts to higher temperatures with increase of electrical field strength. At ultrastrong electrical fields (>70 kV/cm) Joule heating of the films is observed during several tens of nanoseconds. The heating is inhomogeneous at T<Tm due to different heating effect on crystallites and grain boundaries.
  • At low temperatures the magnetic field is able to increase the ER phenomenon in films with small size crystallites. For films with larger crystallites only decrease of the ER with magnetic field is obtained.
  • The obtained results enable to suggest prototype of contactless electrical current sensor based on polycrystalline La1-xSrxMnO3 film operating at room temperature and intended for 10-30 A electrical current sensing. The fault current limiter consisting of polycrystalline and epitaxial La1-xSrxMnO3 film operating at liquid nitrogen temperatures and generating 3.5 dB attenuation after 0.5 ns and 8 dB after several ns also can be developed.
Rūta Araminaitė
Author: Rūta Araminaitė
Dissertation title: Study of electrocatalytic processes at Prussian blue modified glassy carbon electrode
Fields of science: Physical sciences, Chemistry (03P)
Scientific supervisor: Prof. Habil. Dr. Albertas Malinaukas
Defence of the dissertation: 2010-02-09
The main purpose of this work:
Study of electrochemical hydrogen peroxide and ascorbate reactions on electrodes modified by Prussian blue, with the aim to apply these electrodes in creation of sensors and biosensors.
  • To study of hydrogen peroxide and ascorbate electrochemical behavior on PB modified electrode in pH 5.5 and pH 7.3 phosphate solution applying cyclic voltamperometric (CV) analysis.
  • To study the cathodic reduction of hydrogen peroxide using rotating disk electrode.
  • To study the anodic oxidation of ascorbate using rotating disk electrode.
  • To study the stability of PB modified electrode during cathodic reduction of hydrogen peroxide.
  • Preliminary studies on the applicability of Prussian blue modified electrode for biosensors.
Scientific novelty of the work
For the first time, a detailed study of electrocatalytic reduction of hydroger peroxide and ascorbate oxidation has been provided with the use of a rotating disk electrode.
The kinetics of decomposition of PB modified electrode in the course of a cathodic reduction of hydrogen peroxide has been studied, and the influence of different factors to this process has been determined.
Prototypes of sensors and biosensors, for different analytes have been elaborate and tested.
  • Glassy carbon electrode, modified by Prussian blue, possesses electrocatalytic activity in reduction reactions of hydrogen peroxide and oxidation reactions of ascorbate. Catalysis of hydrogen peroxide cathodic reduction occurs effectively in wide range of electrode potential below the potential of Prussian blue reduction (approx. 0.2 V vs. Ag/AgCl). Oxidation of ascorbate on the electrode modified by Prussian blue occurs at electrode potentials, substantially (by 0.1-0.2 V) lower as compared to unmodified electrode.
  • At a low concentration of hydrogen peroxide (up to 0.3 mM), kinetic catalytic current appears to be about two times higher in pH 5.5 solution, as compared to pH 7.3 one, as determined with the use of a rotating disk PB modified electrode.
  • At a relatively high peroxide concentration, exceeding 0.6 mM, deviations from linearity of Koutecky-Levich equation have been observed. Based on this, a mechanism for hydrogen peroxide reduction at PB modified electrode has been proposed. In accordance with this mechanism, electron transfer appears to be rate- limiting step.
  • During electrocatalytic reduction of peroxide, PB layer undergo intense destruction processes. The calculated pseudo-first order degradation rate constants show a slower degradation in more acidic solutions.
  • With the use of PB modified electrode, prototypes of glucose biosensors were developed.