2. Articole
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Item Synthesis and properties of Al-doped ZnO thin films for photovoltaics [Articol](Institute of Electrical and Electronics Engineers Inc., 2018-11-27) Potlog, Tamara; Lungu, Ion; Botnariuc, Vasile; Gorceac, Leonid; Raevschi, Simion; Taku, Miyake; Worasawat, Suchada; Mimura, HidenoriPolycrystalline undoped and Al-doped ZnO thin films were deposited by spray pyrolysis using oxygen and argon atmospheres. The influence of postdeposition vacuum annealing on the structure, morphology and optical properties was studied by X-ray diffraction, atomic force microscopy and UV–VIS spectroscopy. The thermal vacuum annealing induces structural and morphological changes, especially in Al-doped ZnO thin films synthetized in oxygen atmosphere. Further on, the electrical conductivity in dependence of the nature of gas atmosphere and Al concentration is discussed.Item Effect of carrier gas and vacuum annealing on the physical properties of ZnO thin films doped with gallium by chemical spray pyrolysis [Articol](Editura "Tehnica-UTM", 2018-05-24) Potlog, Tamara; Lungu, Ion; Botnariuc, Vasile; Raevschi, Simion; Worasawat, Suchada; Mimura, Hidenori; Gorceac, LeonidStructural, optical and electrical properties of zinc oxide thin films doped with 2% Ga, annealed in the vacuum at the same temperature and obtained onto glass substrates by the chemical spray pyrolysis method in the different gas atmospheres were investigated by X-ray Diffraction (XRD), UV-VIS spectrophotometry and Hall measurements, respectively. XRD studies revealed that all films were polycrystalline in nature, with a hexagonal wurtzite crystal structure and a predominant (002) c-axis orientation. All ZnO:Ga thin films had higher than 80 % transmittances in the visible region. Doping with 2% Ga led to a decrease in the optical band gap indifferent of the nature of the carrier gas. The synthesis of ZnO thin films with 2% Ga doping and annealing in the same conditions in Ar and O2 atmospheres led to the lower conductivity with electron concentration of 1016 cm−3.Item Caracterizarea structurală a nanoformațiunilor de Ga2O3 obținute prin tratament termic al monocristalelor Ga2O3 [Articol](Universitatea Tehnică din Moldova, 2018-05-24) Untilă, Dumitru; Sprincean, Veaceslav; Caraman, Mihail; Cojocaru, Ala; Lupan, Oleg; Tighineanu, Ion; Palachi, Leonid; Caraman, IulianaPrin tratament termic (TT), în atmosferă normală, al monocristalelor β-Ga2S3, cu rețea cristalină monoclinică, la temperaturi din intervalul 970-1170 K, timp de 3-6 ore, suprafața acestora se acoperă cu un strat omogen de β-Ga2O3, cu rețea cristalină monoclinică. La interfața monocristal-oxid se formează nanocristalite de Ga2O3 și Ga2S3. Structura cristalină și dimensiunile medii ale cristalitelor sunt analizate prin măsurări XRD, SEM și Raman.Item Absorbția optică și fotoluminescența compozitului Ga2S3-Ga2O3 [Articol](Universitatea Tehnică din Moldova, 2018-05-24) Сaraman, Iuliana; Evtodiev, Igor; Untilă, Dumitru; Dmitroglo, Liliana; Caraman, Mihail; Evtodiev, Silvia; Palachi, LeonidÎn această lucrare sunt studiate absorbția optică și fotoluminescența cristalelor de Ga2S3, obținute prin metoda CVT în atmosferă de I2, și a compozitului Ga2S3–Ga2O3, obținut prin tratament termic al monocristalelor de Ga2S3 în atmosferă normală, la temperatura 1073K. S-a determinat că în rezultatul tratamentului termic de lungă durată (12 ore) suprafața cristalelor de Ga2S3 se acoperă cu un strat granular de Ga2O3. Din măsurări ale reflexiei difuze, lățimea benzii interzise a stratului de Ga2O3 de pe suprafața monocristalului Ga2S3 a fost aproximată ca fiind egală cu 4,47 eV. La 300K, marginea benzii de absorbție a cristalelor de Ga2S3 este formată din trei sectoare în care au loc tranziții optice directe cu lățimea benzii interzise egală cu 3,020 eV, 3,178 eV și 3,312 eV, iar la 80K - cu 3,196 eV, 3,302 eV și 3,422 eV. Spectrul de FL a cristalelor de Ga2S3 conține doar o singură bandă în regiunea roșu a spectrului, ce se interpretează ca emisie radiativă a stratului de Ga2S3, iar spectrul de FL al compozitului Ga2S3–Ga2O3 pe lângă banda roșie conține și o bandă în regiunea violet–albastru a spectrului, ce se identifică ca emisie radiativă în cristalele de oxid din compozitul Ga2O3-Ga2S3.Item Oxygen-dependent formation of VO2 thin films by metalorganic aerosol deposition [Articol](Universitatea Tehnică din Moldova, 2018-05-24) Belenchuk, Alexandr; Shapoval, Oleg M.; Zasavitsky, E.; Vatavu, Sergiu; Kiritsa, Arcady; Moshnyaga, V.We report on the growth and properties of vanadium dioxide (VO2) on amorphous and oriented substrates by the low-cost and industry-oriented method of metalorganic aerosol deposition (MAD). X-ray diffraction and temperature- dependent Raman spectroscopy confirm formation of single phase tin films. Abrupt change of resistance by 5×103 times and steep drop of infrared transmission from 49 to 10% at 1700 nm pave the way for application of MAD technique in fabrication of VO2-based optoelectronic and thermochromic devices, such as “smart windows”.Item Peculiarities of Seebeck coefficient of semi-metallic bismuth wires in high magnetic field [Articol](2019) Condrea, E.; Muntyanu, F.; Nicorici, ValentinaThe effect of high magnetic field on the transport properties of semi-metallic bismuth has been a subject of increasing interest in the last years because a lot of unexpected findings detected far above the quantum limit of the electrons[1-3]. The presented investigations of the magnetotransport measurements of Bi wires complement the series of recently published experimental results on bulk Bi in high magnetic field. The design of measurements in magnetic field was diversified by using uniaxial deformation directed along the wire axis. By combining high magnetic field and uniaxial strain, the electronic structure of the bismuth wires was modified; as a result, the quantum limit for light and heavy electrons could be changed in different ways. Measurements of the thermoelectric response (Seebeck coefficient) in quantizing magnetic field have revealed some oscillating instabilities in the magnetothermopower dependence in a magnetic field of 15 - 20 T. Observed correlation between a simultaneous shift in a magnetic field of the position of the anomalies and of the quantum limit of electrons, when modifying of the electronic structure under strain, allows us to attribute of unidentified peaks to the complex structure of the lowest Landau level of electrons when one of the lowest spinpolarized Landau sublevel of heavy electrons approaches and crosses the Fermi energy level. It should also be noted that a decrease in the resistance in higher fields with the apparent metallization of bismuth indicates possible changes in the mechanism of carrier scattering associated also with the Lifshitz Transition and with the substructure of the last Landau level of electrons.Item Obtaining highly conductive oxide single crystals for manufacturing nanotemplates [Articol](2019) Colibaba, Gleb; Monaico, E.; Rusnac, D.The present investigation is devoted to obtaining ZnO, In2O3 and Ga2O3 single crystals by chemical vapor transport (CVT). The thermodynamic analysis of the composition of CVT systems with ZnO, In2O3, Ga2O3 and various transport agents (TAs) is carried out for wide temperature range and for various density/composition of TA. The influence of the growth temperature and of the TA density/composition on the mass transport rate is investigated theoretically and experimentally. The possibility of increase in mass transport rate by several orders of magnitude at the presence of compound TAs is demonstrated. The application prospects of obtained single crystals as substrates for manufacturing nanoporous matrices (nanotemplates) by the electrochemical etching are analyzed. The fabrication of nanopore arrays with various morphology, which depends on the crystallographic orientation of substrates, is demonstrated for ZnOItem Phonons in twisted graphene [Articol](2019) Nika, DenisWhen two graphene layers are placed on top of each other, they can form a moire pattern – twisted graphene. In this case, one layer of carbon atoms is rotated relative to another layer by an arbitrary angle. In the talk, a brief review of recent theoretical and experimental results [1-3] on the engineering of phonon properties of twisted graphene by atomic plane rotations are provided. Angle-dependent specific heat and thermal conductivity of twisted graphene are discussed.Item Quantum nanostructures for terahertz devices and applications [Articol](2019) Sirkeli, Vadimerahertz (THz) waves refer to the electromagnetic radiation in the frequency range from 0.1 to 10 THz, which corresponds to the wavelengths from 3 mm to 30 μm, respectively. This spectral region, called also as “T-gap”, is important for many practical applications, including THz imaging, chemical and biological sensing, high-speed telecommunication, security and medical applications. Here we report the results of a numerical study of quantum transport in ZnO-based resonant-tunneling diodes (RTDs) and quantum cascade lasers (QCLs) with different design schemes. We found that by varying and optimizing constituent layer widths and doping level these quantum structures, high power performance of THz RTDs and QCLs can be achieved at room temperature. Moreover, it was established also that the ZnO-based terahertz sources can cover the spectral region of 5-12 THz, which is very important for THz imaging and detection of explosive materials, and which could be not covered by conventional GaAs-based terahertz devices.Item The interaction between Endothelial Cells and Gallium Nitride nanoparticles [Articol](2019) Braniste, Tudor; Andree, Birgit; Benecke, Nils; Raevschi, Simion; Plesco, Irina; Cebotari, Serghei; Haverich, Axel; Tighineanu, Ion; Hilfiker, AndresIn this study, human umbilical vein endothelial cells (HUVECs) were investigated in direct contact with Gallium Nitride (GaN/Fe) based nanoparticles. GaN is a compound semiconduc- tor material, with remarkable characteristics including piezoelectric properties, high thermal stability, radiation hardness and excellent chemical inertness, which make it promising for biomedical applications. There is, however, limited knowledge about the biocompatibility of nanostructured GaN and the impact of GaN nanoparticles on living cells. We report on growth and characterization of GaN/ZnFe2O4 multifunctional piezoelectric and magnetic nanoparticles as well as on their assimilation and interaction with HUVECs. Thin GaN layers were grown on ZnFe2O4 nanoparticles with sizes up to 100 nm, using Hydride Vapor Phase Epitaxy (HVPE). After GaN growth, the sacrificial core of nanoparticles was decomposed at high temperatures in hydrogen flow, the final composition of nanoparticles corresponding to GaN:Fe. The resulted nanoparticles were incubated with human umbilical vein endothelial cells in order to remotely influence the cells activity through nanoparticles. By cultivating cells in medium supplemented with different concentrations of nanoparticles, we show that HUVECs tolerate GaN nanoparticles. The obtained results show that, being uptaken by the cells, the GaN nanoparticles are deposited into vesicles and thus can be used as guiding elements for controlled transportation or designed spatial distribution of cells in a magnetic field, which represent a step forward towards application in cellular therapy.