Defended Dissertations in 2013

Kęstutis Prušinskas
Author: Kęstutis Prušinskas
Dissertation title: Investigation of the autocatalytic Cu(II) reduction processes in the systems with natural polyhydroxylic compounds as ligands
Fields of science: Physical sciences, Chemistry (03 P)
Scientific supervisor: Prof. habil. Dr. Eugenijus Norkus
Defence of the dissertation: 2013-01-11
The aim of the work is to investigate the electroless copper plating processes in the systems using natural polyhydroxylic ligands - saccharose and glycerol - as Cu(II) ligands as well as to determine the impact of ultrasound different copper plating systems of different types.
Main tasks:
  • To calculate and to analyze formation and distribution of the copper(II) among complexes with polyhydroxylic compounds - saccharose and glycerol - in the alkaline medium.
  • To investigate the influence of solution pH on electroless copper deposition process for the systems with formaldehyde as reducing agent.
  • To measure the roughness of copper surfaces electrolessly deposited from the systems with ligands saccharose and glycerol.
  • To perform studies of formaldehyde anodic oxidation the studies on electrolessly deposited copper coatings.
  • To determine the influence of ultrasonic irradiation on the processes electroless copper plating systems. 
Novelty and actuality of the work

With the development of modem industry and technology, the growth in consumption of environmental resources rises heavily, while there is also the problem of various wastes. In order to reduce the negative environmental impact, industry is looking for the possibilities to use renewable sources of energy and materials, also looking for technologies which reduces the harmful environmental impact of products, it is searching for products that would make it easier to recycle. This work investigates polyhydroxylic alcohols - glycerol and saccharose - as an alternative ligand for electroless copper plating systems using formaldehyde as reducing agent. Natural polyhydroxylic alcohols are environmentally friendly, easily bio-degradable materials, but have not been studied enough for suitability for electroless copper plating systems. In order to increase of the efficiency, the electroless copper plating was investigated in ultrasonic environment, to observe how ultrasound impacts the electroless copper plating rate and properties of the coatings obtained.

  • The calculations of copper(II)-saccharose and copper (Il)-glycerol complex formation equilibria showed that saccharose and glycerol provides sufficient capability to form complexes and prevent formation of copperCu(II) hydroxide and can be used as ligands in alkaline electroless copper plating solutions.
  • The autocatalytic reduction of Cu(II) starts from the pH > 12 when the saccharose as Cu(II) ligand was used. Under optimum operating conditions the copper deposition rate can reach 2.2 µm h-1 (pH 12.75). The mixed potential of copper electrode has relatively negative values (-600 mV - 750 mV) and the calculated overpotential values (-520 mV - 570 mV) are exceptionally negative for systems with Cu(II) ligand saccharose. Received copper coating roughness factor reaches quite high values (Rf = 25).
  • The voltamperic measurements have shown, that the current density of formaldehyde anodic oxidation range from 1.7 to 5.8 mA cm-2 on electrolessly coated copper electrode (ligand saccharose), and the dependence of anodic current density on pH is directly correlated with copper plating rate and surface roughness factor.
  • The studies have shown that glycerol can be used as ligand for electroless copper plating systems. The autocatalytic Cu (II) reduction starts from the pH > 13. Under optimal conditions, the copper deposition rate can reach 3.8 µm h-1. The mixed potential of Cu electrode has relatively positive values (-250 mV - 350 mV); the overpotential values are low (-55 - 180 mV). Roughness factors of copper coatings obtained in the systems with ligand glycerol are relatively low (2.3 to 4.3), in some cases close to the electrochemically deposited copper coatings.
  • The current density of formaldehyde anodic oxidation on electrolessly coated copper electrode (using glycerol as ligand) range from 1.5 to 7.4 mA cm-2 depending on pH of the solution and glycerol concentrations. It was found that the shape of dependences of current density of anodic formaldehyde oxidation on pH is similar to dependences of overpotential and surface roughness factor for electroless copper plating system with glycerol as Cu(II) ligand.
  • The ultrasonic irradiation effects differently electroless copper plating systems. UG changes copper plating rates - depending on the copper plating system and ultrasonic frequency - copper deposition rate could fall (system with reducer Co(II), ligand pentaethylenehexsaamine), may increase to 1.44 times (system with reducing agent formaldehyde, ligand EDTA) or may increase 2-5 times up to 10.9 µm h-1 (system with reducing agent formaldehyde, ligand saccharose), Usually the coatings formed in ultrasound environment are more compact, smoother.
  • The data obtained suggest, that in the case of electroless copper plating it is difficult to predict and explain apparently the effects and mechanisms of ultrasound actions, including impact of characteristics of ultrasonic irradiation. Therefore it remains an area for further research.
Paulius Ragulis
Author: Paulius Ragulis
Dissertation title: Development, research and application of wide band resistive sensors
Fields of science: Physical sciences, Physics (02 P), Semiconductor physics (P 265)
Scientific supervisor: habil. Dr. Žilvinas Kancleris
Defence of the dissertation: 2013-09-14
The main goal of the thesis was to create the resistive sensor implemented in a double-ridged waveguide with flat frequency response. The resistance of the sensor should be lower than 1 kΩ and voltage standing wave ratio (VSWR) lower than 1.5. The characteristics of these sensors were investigated experimentally. By connecting the resistive sensor to the wide band horn antenna microwave pulse power density (electric field strength) measurements in free space were performed.
Main objectives:
  • To calculate electromagnetic field components in a sensing element of the RS implemented in WR90, WRD250 and WRD840 waveguides by using finite - difference time - domain method finding optimal electro physical parameters: dimensions and resistivity of the SE.
  • To investigate experimentally the characteristics of the manufactured resistive sensors with found optimal parameters, also performing measurements of the microwave pulse power density (electric field strength) in free space.
  • To demonstrate the practical use of the resistive sensors in the investigations of the electromagnetic susceptibility of the electronic devices.
Results and conclusions
  • Using finite-difference time-domain method the optimal parameters of the sensing elements mounted in WR90, WRD250 and WRD840 waveguides were determined.
  • Using finite-difference time-domain method for the modelling of the resistive sensors, it was noticed that by changing transverse dimensions or specific resistance of the sensing element, the character of the dependence of the relative electric field strength inside the sensing element on frequency can be changed. This, in turn, has allowed us to optimize the frequency response of the resistive sensor.
  • Accounting for the horn antenna characteristics when choosing the parameters of the sensing element, allows us to develop a measurement unit, consisting of the horn antenna and resistive sensor, for measuring microwave pulse power density (electric field strength) in free space with flat frequency response.
  • Sensitivity variation ±9.1% and ±25% has been found experimentally for resistive sensors implemented in WR90 and WRD250 waveguides, respectively, in the pass band of the corresponding waveguide.
  • Designed and manufactured resistive sensors implemented in WRD250 waveguide measure microwave pulses from a few watts, and their dynamic range is 50 dB.
  • The dependence of the threshold electric field for the upset level (L3) on frequency, pulse duration and repetition rate has been determined for three different type internet routers. It was found that older generation 100 Mb/s speed router is less susceptible to HPM radiation in comparison with wireless and 1 Gb/s routers.
  • Observed dependence of the threshold electric field for upset level (L3) on pulse duration and repetition rate for internet routers can be clarified neither by thermal heating nor by parasitic charging models. In order to prove one of the considered models, the measurements with shorter then τ=0.7µs high power microwave pulses is desirable.
  • Experimental investigations of the susceptibility of the media converters strongly confirms the assumption that metallic enclosure can act as a resonator cavity at some frequency, resulting in a strong increase of susceptibility level of the device.
Saulius Tumėnas
Author: Saulius Tumėnas
Dissertation title: Optical response of ZnMgRE quasicrystals
Fields of science: Physical sciences, Physics -02 P, Semiconductor physics P265
Scientific supervisor: Dr. Vytautas Karpus
Defence of the dissertation: 2013-09-25
The main goal of the dissertation was to reveal the electronic structure of ZnMgRE (RE = Y, Ho, Er) quasicrystals by investigations of their optical response. The dissertation comprises experimental X-ray diffraction (XRD) and optical spectroscopy studies of the quasicrystals, a construction of their electron subsystem model, and a theoretical description of their optical response.
The XRD studies were aimed at a determination of the reciprocal quasicrystalline- lattice vectors, which define the atomic field acting on an electron subsystem.
The optical spectroscopy studies were carried out by the combined spectroscopic ellipsometry and reflectance spectroscopy technique, based on a suggested anchor- window method. High-accuracy ZnMgRE optical conductivity σ(ω) spectra were recorded in the wide, 0.01-6 eV, spectral range.
The model of ZnMgRE electron energy spectrum, previously suggested for an in­terpretation of experimental ZnMgRE photoemission spectra, was developed. The nearly-free-electron gas model of independent intersections was formulated in the extended zone presentation. A scheme of the theoretical σ(ω) calculations was exten­ded to account for various positions of the Fermi level with respect to a pseudogap.
The experimental ZnMgRE optical conductivity spectra was reproduced in detail by theoretical calculations performed within the framework of the suggested electron energy spectrum model. The set of electron energy spectrum parameters determined from an analysis of the optical data predicts actually the same structure of the Fermi level-vicinity electron energy spectrum, as was previously predicted from an analysis of photoemission data.
The electron subsystem in ZnMgRE quasicrystals maintains the nearly free elect­ron gas character. The energy spectrum of electrons in a vicinity of the Fermi level is determined by the Fermi surface intersections with (222100) and (311111) families of Bragg planes.
The optical response of ZnMgRE quasicrystals, as of other metallic compounds, is determined by the intraband Drude-type and interband optical transitions. The intra­band transitions contribute to the total optical conductivity with the relative spectral weight of about 10 %. The Drude relaxation times are of about 0.14-0.4 10-14 s. The intersections of the Fermi surface with Bragg planes lead to an essential increase of the optical mass in quasicrystals, as compared to the usual crystalline metals. The ZnMgRE optical mass is of the order of 10 m0.
The interband ZnMgRE optical conductivity is predominantly due to the optical transitions across 222100 and 311111 pseudogaps. Their relative spectral weight is of about 80 %. An influence of the low structure-factor Sg pseudopotentials on the optical response of ZnMgRE quasicrystals was revealed. The relative spectral weight of the low- Sg contribution is of about 10 %.
Aidas Aleknavičius
Author: Aidas Aleknavičius
Dissertation title: Investigation of composite laser active elements with thin doped layers
Fields of science: Technology sciences, material engineering (08T)
Scientific supervisor: Dr. Andrejus Michailovas
Defence of the dissertation: 2013-09-18
Main objectives and tasks
  • To analyze and elaborate a concept of composite laser element and its application possibilities.
  • To investigate generation and amplification properties of proposed active laser element experimentally.
  • To develop theoretical models describing optical properties of proposed laser element.
  • To confirm developed models experimentally.
  • To find factors limiting applicability of proposed laser element.
Novelty of this work
  • The use of composite materials expands technological possibilities in various areas. Laser technologies are not exceptions. Improvements in manufacturing of transparent ceramics allow producing high quality laser elements of custom shape, size and composition. This area is still not investigated widely.
  • A laser element of novel type was introduced in this work, expecting it to enable of producing high quality laser beam with a power of more than 50 watts. Realization of such laser would increase competitive ability of Lithuanian lasers in global market, especially in industrial area.
  • By the use of experimentally confirmed theoretical models, thermo-optical properties of thin active disk with undoped layer on top, was investigated. Influence of undoped layer on thin active disk was not investigated till now.
  • Additionally, an original method to rate induced optical aberrations which allows unambiguous comparison of different aberrations was presented. Further development of this method has a potential to be applied in designing and analysis of laser systems.
  • A concept of composite laser element with thin active layer was introduced. This concept combines properties of several types of laser active elements: shape of the element is similar to slab type laser elements; end-pumping scheme is usually used for rod type laser elements; thermal management is similar to thin-disk laser elements. Such kind of laser element is possible to realize due to transparent ceramic material manufacturing technologies which allow to make a laser element of desired shape and composition (in the sense of doping).
  • Experimental results show, that proposed laser element attributes to have high thermally induced aberration despite its similarity to thin-disk laser element. From this point of view this element has no advantage over common rod type laser element.
  • A method to rate different optical phase distortions induced by optical elements based on calculations of resulting M2 parameter of an affected diffraction limited laser beam was proposed. This method enabled to estimate the influence of undoped layer on top of thin active layer. Results show that adding undoped layer deteriorates propagation properties of the reflected diffraction limited laser beam.
  • Wave front aberrations arise mostly due to edge effects which can be reduced by using pump beams with Gaussian intensity profiles. Other way for reduction of aberrations is to increase the size ratio of pump and laser beams, but one has to keep in mind that power efficiency will be reduced.
  • Analysis of simplified numerical models showed that main reason for aberrations in proposed active element is the undoped medium between thin active layers. Regardless of advantages of monolithic structure, withdrawal of undoped filling between thin active layers should provide lower optical aberration maintaining similar absorption and amplification properties. In case of mastering a thin-disk bonding to a heat sink technology it could be a further way to investigate the proposed multi-disk concept.
  • Generally, a pump beam is asymmetric in respect of its waist plane when it is formed by optical system after out-coupling from a large core diameter fiber. This can be important when laser element is relatively long. A simple model to calculate the variation of intensity distribution as the beam propagates after passing optical system was presented.
Jonas Reklaitis
Author: Jonas Reklaitis
Dissertation title: Development of Mössbauer spectroscopy for magnetic nanomaterials and dynamics of macromolecules
Fields of science: Physics -02 P
Scientific supervisor: habil. Dr. Dalis Baltrūnas
Defence of the dissertation: 2013-06-14
The main tasks:
Classical Mössbauer spectroscopy has been applied to studies of iron nanowires and thus the following tasks were set:
  • preparation of different anodic alumina oxide matrices filled with iron nanowires;
  • investigation of the effectiveness of different electrolytes used in nanowire deposition;
  • exploration of the aging effects on chemical composition of nanowires;
  • investigation of the influence of annealing parameters on the chemical  compo­sition.
By expanding the possibilities of Mössbauer spectroscopy the following tasks were set:
  • to design the proper upgrades for the classical Mössbauer spectrometer, so that it could be used to detect scattered radiation;
  • to show the possibilities of the method by analyzing thermal movement of the carbon atoms in a polystyrene sample.
Novelty of the work
  • A new type of the Rayleigh scattering Mössbauer radiation spectrometer is created, which allows the analysis of materials not containing Mössbauer nucleus.
  • It has been determined that iron nanowires do not have a core-shell structure inside the anodic aluminum oxide template.
  • The aging of iron nanowires was analyzed with a complex use of the Mössbauer spectroscopy, the
  • conversion electron Mössbauer spectroscopy and X-ray diffraction.
  • The Mössbauer spectroscopy and X-ray diffraction were applied to determine a chemical composition of the annealed nanowires, encapsulated inside anodic aluminum oxide templates.
  • RSMR spectrometer is equipped with high resolution detector, which allowed to decrease observed linewidth from 1.3 keV to 0.3 keV and increse sensitivity from 1 % to 40 %.
  • A new type RSMR spectrometer was developed, which due to favorable time scale of scattering process is suitable for the studies of complicated dynamics of biomolecules (not containing mossbauer nucleus).
  • Iron nanowires get deposited faster into AAO templates, in new aqueous elec­trolyte, composed of FeS04, MgS04 ir citric acid. 52 ± 2 nm/min and 11, 8 ± 0,6 nm/min respectively.
  • Metalic iron nanowires encapsulated inside AAO template (pore diameter ranges from 8 nm to 30 nm) transforms to Fe(II) and Fe(III) compounds, forming hercynite FeAl204 and
  • (FexAl1-x)203 (x ≤ 0.15) when annealing inside alumina for 1.5 h at temperatures up to 600 °C.
  • The annealing temperature, at which metalic nanowires encapsualted inside AAO template disappears, is lower for the templates with smaller pore diameter.
Renata Česūnienė
Author: Renata Česūnienė
Dissertation title: Adsorption of chromium complex dye and copper(II) ions by activated carbons
Fields of science: Physical sciences, Chemistry (03P)
Scientific supervisor: Dr. Audronė Gefenienė
Defence of the dissertation: 2013-12-30
The aim of the work was to investigate the peculiarities of the simultaneous and consecutive (successive) adsorption of chromium complex dye and copper(II) ions by activated carbons under static and dynamic conditions.
Novelty and actuality of the work:
Adsorption process of chromium complex dye and copper(II) ions by basic activated carbons using consecutive and simultaneous adsorption methods, to our knowledge, is not investigated. In the case of consecutive adsorption, the possibility to remove the copper(II) ions by activated carbons used for the adsorption of metal complex dye has been investigated. The influence of the adsorbent surface modification occuring during the dye removal for the adsorption of copper(II) ions was examined. The search for optimal conditions for the removal of metal complex dye and copper(II) from single and binary solutions has been carried out. No data was found in the literature about the investigation of the system “activated carbon-aqueous solution of chromium complex dye and copper(II) chloride”. The work is actual from the practical point of view, because the industrial wastewater is usually the mixture of various materials - organic and inorganic, so it is important to clarify their mutual influence on the efficiency of water treatment. It is important to find out the opportunities and options for the removal of chromium complex dye and heavy metal, hence the copper(II) ions, by the universal adsorbent activated carbons.
  • The investigation using potentiometric titration and pH drift methods has shown that the Norit PK 1-3 and Norit RB 0.8CC activated carbons are materials with basic properties: the amount of basic functional groups is higher than that of acidic, according basicity the basic adsorption sites according basicity may be divided in two different groups.
  • During the process of consecutive or simultaneous adsorption the value of solution pH increases. The change in solution pH decreases with the decrease of initial solution pH and the increase of initial copper(II) concentration.
  • The adsorption capacity of activated carbons saturated with chromium complex dye towards copper(II) ions is comparable to that of unsaturated ones and it depends on the initial dye solution pH, saturation time as well as on the initial pH and concentration of Cu(II) solution.
  • Under the conditions of sufficient hydroxide ions concentration in the solution the adsorption process of chromium complex dye and copper(II) is accompanied by the formation of insoluble copper(II) compounds on the surface of activated carbon and in the bulk of the solution. The amount, the form and the distribution of the precipitate in the adsorption system depends on the initial copper(II) concentration and the initial pH of the solution.
  • The precipitate of the copper(II) compounds serves as the additional adsorption sites for chromium complex dye, their formation enhances the removal efficiency of copper(II) and the dye especially in the case of simultaneous dynamic adsorption.
  • The optimal conditions for the removal of copper(II) ions and chromium complex    dye from single and binary solutions are as follows: Lanasyn Navy solution concentration 0.05-0.1 mM, and initial pH 5-6, 1-3 mmol/L copper(II) concentration and temperature of 40° C.
  • Kinetic study of the simultaneous and consecutive adsorption has shown that the chromium complex dye and copper(II) ions adsorption is governed well by the pseudo-second order and intraparticle diffusion models. An increase in temperature leads to an increase in the rate of diffusion.
Domantas Peckus
Author: Domantas Peckus
Dissertation title: Ultrafast excition and charge carrier dynamics in nanostructured molecular layers
Fields of science: Physical sciences, physics (02P)
Scientific supervisor: Prof. Habil. Dr. Vidmantas Gulbinas
Defence of the dissertation: 2013-11-18
Due to their unique properties organic semiconductors may be used for various applications in organic optoelectronic devices: organic light emitting devices, organic optically or even electrically pumped lasers, organic field- effect transistors and organic photovoltaic cells. All these applications are possible because of conductivity of organic molecules. Despite high perspectives of organic semiconductors they are still upstaged by their inorganic counterpart. Continuous scientific and technological research is currently in progress in order to increase the competitiveness of organic semiconductors. Scientific research concentrates on ultrafast electrooptical processes in organic semiconductors. The most interesting processes in organic molecules are exciton diffusion, exciton recombination, charge transfer exciton generation and recombination, charge carrier generation and recombination, charge carrier mobility, charge carrier injection and extraction. In order to measure and analyze these processes, ultrafast optical and electrical techniques were used.
The main goal of this thesis is a detailed investigation of ultrafast exciton and charge carrier processes in pure organic semiconductors and their blends with fullerene derivatives. Measured organic or sliconorganic semiconductors were poly-di-n-hexylsilane, fluorenes F8BT and PSF-BT, merocyanine MD376. Fullerenes used in blends were C60 and its derivative PCBM. Ultrafast transient absorption, fluorescence, and integral mode photocurrent measurements were used for investigations. Integral mode photocurrent measurements were performed by two methods, by optical time-resolved electric field-induced second harmonic generation (TREFISH) method on a 1 ps-3 ns time domains and electrically by oscilloscope using time of flight (TOF) method on a 10 ns-100 µs time domain.
Each type of molecules was explored employing slightly different approach. Only ultrafast fluorescence methods were used for PDHS measurements, while for the investigation of fluorine and merocyanine semiconductors all the above mentioned methods were used.
The investigation of PDHS revealed that inorganic matrixes suppress nonemissive relaxation channel. PDHS nanocomposites can be used for improvement of neat PDHS films fluorescence properties.
The exploration of neat films of fluorenes F8BT and PSF-BT revealed exciton-exciton annihilation differences. The formation of intramolecular charge transfer state was proposed for PSF-BT neat films. Charge transfer scheme of the formation of long-lived charge pair state in PSF-BT/PCBM blend was presented.
The formation of charge transfer states was explored in neat merocyanine films and blends with fullerene derivatives. The formation of intermolecular charge transfer states was found in neat films and charge transfer state among merocyanine and fullerene derivatives was observed in merocyanine/fullerene blends. The scheme of generation of charge pairs and free charge carriers in merocyanine blends with fullerene derivatives was discussed in detail. The samples prepared by vacuum deposition technique shows better charge carrier generation and mobility properties then the ones prepared by solution processing technique.
Viktorija Nargelienė
Author: Viktorija Nargelienė
Dissertation title: Research and application of GaAs/AlxGa1-xAs heterostructures for microwave detection
Fields of science: Physical sciences, Physics (02P), Semiconductor physics (P 265)
Scientific supervisor: Prof. Dr. Algirdas Sužiedėlis
Defence of the dissertation: 2013-12-14
Main objectives
  • To grow GaAs/AlxGa1-xAs  structures using liquid phase epitaxy and molecular beam epitaxy growths method.
  • To characterize GaAs/AlxGa1-xAs epitaxial layers using photoluminescence and time correlated single photon counting techniques.
  • To fabricate planar asymmetrically necked microwave diodes, using epitaxial GaAs/Al0,25Ga0,75As layers having different doping profile of Al0,25Ga0,75As barrier layer.
  • To fabricate rectifying microwave diodes using epitaxial GaAs/AlxGa1-xAs layers having different AlAs mole fraction x.
  • To investigate electrical properties of microwave diodes using current-voltage characteristics.
  • To investigate detection properties of microwave diodes in a wide frequency range from 8 GHz to 170 GHz.
Novelty and significance of the thesis
  • Detection properties of planar asymmetrically necked microwave diodes fabricated using selectively doped GaAs/Al0,25Ga0,75As  heterostructures were investigated in broad frequency range up to terahertz region. Dependence of detection properties on doping profile of Al0,25Ga0,75As barrier layer was estimated.
  • Detection properties of rectifying GaAs/AlxGa1-xAs microwave diodes in the millimeter frequency range were investigated. Dependence of detection properties on AlAs mole fraction x was estimated.
  • Influence of intervalley electromotive force in the detected signal of the point contact GaAs/Al0,25Ga0,75As microwave diode was evidenced.
Results and conclusions
  • Experimental investigation of photoluminescence in GaAs/AlxGa1-xAs heterostructures have shown, that quality of epitaxial structures grown by molecular beam epitaxy depends on growth temperature. Higher quality of GaAs was obtained in the heterostructures grown at 600 °C, while higher quality of AlxGa1-xAs was obtained in the heterostrucutres grown at 700 °C temperature.
  • Current-voltage characteristic of point-contact GaAs/AlxGa1-xAs microwave diode grown by liquid phase epitaxy was nearly ohmic at room temperature due to the graded heterojunction.
  • Planar asymmetrically necked GaAs/Al0,25Ga0,75As microwave diodes with δ-doped barrier layer has the lower differential resistance than planar asymmetrically necked GaAs/Al0,25Ga0,75As microwave diodes with homogeneously doped barrier layer due to higher electron mobility.
  • Detection of microwaves in point contact GaAs/Al0,3Ga0,7As microwave diode is mainly due to the rectification in heterojunction and intervalley electromotive force in n/n+- Al0,3Ga0,7As homojunction.
  • Planar asymmetrically necked GaAs/Al0,25Ga0,75As microwave diodes with δ-doped barrier layer has the larger voltage sensitivity than planar asymmetrically necked GaAs/Al0,25Ga0,75As microwave diodes with homogeneously doped barrier layer due to higher electron mobility.
  • Voltage sensitivity of planar GaAs/AlxGa1-xAs microwave diodes depends on AlAs mole fraction x Largest voltage sensitivity was obtained with diodes having AlAs mole fraction x = 0,2.
Ieva Matulaitienė
Author: Ieva Matulaitienė
Dissertation title: Vibrational spectroscopic study on the structure and interaction with solution components of monolayers with pyridinium functional group adsorbed on metal surface
Fields of science: Physical sciences, Chemistry (03P)
Scientific supervisor: Habil. Dr. Gediminas Niaura
Defence of the dissertation: 2013-12-27
The aims of this work is to get better insight into the molecular structure and interactions of monolayer formed from of N-(6-Mercapto)hexylpyridinium (MHP) molecules with inorganic ions, dodecylsulfate anion, and graphene oxide.
The main objectives of the work were as follows:
  • To establish stable monolayers of MHP molecules on Au and Ag electrodes;
  • To make assignments of MHP monolayer SERS bands based on isotopic substitution and quantum chemistry calculations;
  • To evaluate SERS marker bands of MHP structure and orientation with respect to the electrode surface;
  • To determine the effect of electrochemical potential on the molecular structure of MHP monolayer;
  • To investigate peculiarities of formation of ion pairs between the MHP monolayer and inorganic ions in solution;
  • To establish the detection limit of Cl04- ions in solution by using SERS method and MHP monolayer;
  • To determine peculiarities of interaction of graphene oxide with MHP monolayer;
  • To determine the effect of electrochemical potential on the structure of adsorbed graphene oxide.
The novelty and significance of the work
The positive-charge-bearing self-assembled monolayer of N-(6- mercapto)hexylpyridinium (MHP) molecules was formed on Au and Ag electrodes. The molecular structure of monolayer was probed by surface-enhanced Raman spectroscopy (SERS). Based on quantum chemical calculations, isotopic substitution, immersion time- and temperature-dependent experiments, the specific SERS marker bands for structure and orientation of MHP molecules were evaluated. A band near 1083 cm-1 has been assigned to C-C stretching vibration of hydrocarbon chain in all-trans conformation. Based on dependence of relative intensity-electrode potential profiles on excitation wavelength the operation of charge transfer resonant Raman enhancement mechanism in Au/MHP system was suggested. It was demonstrated that MHP monolayer effectively attracts inorganic anions as well as dodecyl sulfate ion from aqueous solution. The detection limit for perchlorate anion was found to be 10-8 M. The downshift in frequency of totally symmetric stretching mode of anions due to the formation of ion pairs at interface was detected by SERS. The higher shift was found to be correlated with the large Gibbs dehydration energy of anion. The reduced graphene oxide was prepared at interface of Au modified by MHP and SERS was employed to probe in situ the potential induced changes in parameters of D and G bands. It was found that both D- and G-band wavenumbers linearly depends on the potential. The effect was explained in terms of changes in the C-C bond length induced by the electrochemical doping. The positive- charge-bearing MHP monolayer on Au and Ag surfaces might be utilized in various electrochemical applications.
  • N-(6-mercapto)hexylpyridinium (MHP) molecules form stable positive-charge-bearing self-assembled monolayers capable of attracting negatively charged molecules from the solution phase.
  • Raman markers for structure and orientation of MHP molecule have been established based on isotopic substitution, quantum chemical calculations, and adsorption time- and temperature-dependent studies. A band near 1083 cm-1 has been assigned to C-C stretching vibration of hydrocarbon chain in all-trans conformation.
  • As the electrode potential becomes more negative, changes in the orientation of MHP with respect to the surface takes place so that the methylene groups of hydrocarbon chain contact the metal and the relative intensity of the pyridinium ring mode ν8a increases. The dependence of relative intensity I(ν8a)/I(ν12) on the electrode potential and excitation wavelength was observed indicating on the operation of an additional charge transfer resonant Raman enhancement mechanism in Au/MHP system.
  • MHP monolayer electrostatically attracts NH2S03-, S042-, N03-, BF4-, C104-, and PF6- anions. The frequency of totally symmetric stretching vibration of adsorbed ions decreases comparing with the corresponding mode of ions in solution. The higher shift corresponds to the larger Gibbs dehydration energy of the anion.
  • MHP monolayer on gold electrode provides possibility to determine the concentration of C104- ions in solution at a level of 10-8 M by probing the totally symmetric stretching vibration at 933 cm-1. The dependence of SERS intensity on concentration is near linear in the concentration range from 10-8 to 10-3 M. The downshift in frequency of totally symmetric stretching mode for adsorbed ion depends on the solution concentration and decreases with increasing the concentration.
  • Interaction of MHP monolayer with dodecyl sulfate (DS-) anions results in formation of bilayer at the interface. DS- anions can be detected by PSRS method at 10-6 M solution concentration. The decrease in frequency and increase in relative intensity of symmetric stretching vibration of sulfate group for adsorbed DS- indicates that interaction of MHP and the anion takes place through the sulfate group.
  • MHP monolayer electrostatically attracts negatively charged graphene oxide. Adsorbed graphene oxide can be reduced electrochemically. The frequencies of both G and D bands of reduced graphene oxide linearly decrease as electrode potential becomes more negative with slopes of 5.9±0.3 and 4.7±0.3 cm-1/V, respectively. The effect was explained in terms of electrochemical doping induced changes in the C-C bonds length.