Aleksandre Jishiashvili

The vapor growth of ZnO microcrystals was studied using the Scanning Electron Microscopy and Energy Dispersive Spectroscopy. The microcrystals were grown by annealing ZnO, CuO and ammonium chloride powders. As a result, the ZnCl₂, ZnO and CuCl vapor was formed and condensed on Si substrate, which was gradually heated up to 350^oC. It was established that the growth proceeded in two stages. Beginning from 240^oC, the CuCl-ZnCl₂ eutectic droplet with a low melting point was formed. With time, the droplet was oversaturated with ZnO, and ZnO's solid nuclei were precipitating from it. They served as seeds for the formation of hexagonal ZnO microrods, which were growing along the c-axis ([0001] direction) by the slow, thermodynamically driven Vapor–Solid mechanism. As a result, the rod-like ZnO microstructures were produced on Si substrate. The second stage of growth started when the substrate temperature reached 300^oC. At this temperature, the secondary nucleation took place on the prism surfaces of ZnO microrods, causing brunched structures. The ZnO brunches were growing again along the c-axes, forming elongated 1D type microcrystals. In contrast to the slow growth of primer ZnO microrods, at elevated temperatures the ZnO brunches were growing significantly faster. This kinetically driven process caused the vanishing of the fast growing (0001) plains, resulting in the tapering of ZnO microcrystal tips

The purpose of this work was to study the ability of an NH3 + HCl gaseous mixture to produce copper-based nanomaterials. A gas mixture containing both reactants can be easily produced by heating the solid powder of NH4Cl at temperatures exceeding 340 °C. The vertical quartz reactor with CuO source on its bottom was first evacuated and then heated up to 750 °C. After reacting with gas mixture the volatile species were formed and condensed on the Si substrate placed in the cold zone above the CuO source. The obtained nanomaterials were analyzed by the transmission and scanning electron microscopy, X-ray diffraction and energy dispersive spectroscopy. Nanomaterials with different shapes and compositions were synthesized depending on the Si substrate temperature, which was changed in the range of 250 – 500 °C. These materials include CuO, CuCl, and CuCl2 nanocrystals and dendrite-like structures. The most interesting synthesized nanostructures were the elongated surface nanobubbles with the average wall thickness of 80 nm and heights up to tens of micrometers. They were formed of CuCl, which is strongly hygroscopic and degrades in the atmosphere. The surface bubbles may be blown out due to the decomposition of two chemicals that are formed in the reactor, namely, Cu2OCl2 and CuCl2. The detailed thermochemical analyses of reactions led us to the conclusion that the thermal decomposition of CuCl2, accompanied by the release of Cl2, is the most probable reason for the formation of surface bubbles. It was shown that the chemical activity of the NH3 + HCl gaseous mixture can be farther increased by introducing hydrazine (N2H4) vapor into the reactor, which easily decomposes to nitriding (NH, NH2) and reducing (H2, atomic H) species.

The vapor growth of ZnO microcrystals was studied using the Scanning Electron Microscopy and Energy Dispersive Spectroscopy. The microcrystals were grown by annealing ZnO, CuO and ammonium chloride powders. As a result, the ZnCl2, ZnO and CuCl vapor was formed and condensed on Si substrate, which was gradually heated up to 350oC. It was established that the growth proceeded in two stages. Beginning from 240oC, the CuCl-ZnCl2 eutectic droplet with a low melting point was formed. With time, the droplet was oversaturated with ZnO, and ZnO's solid nuclei were precipitating from it. They served as seeds for the formation of hexagonal ZnO microrods, which were growing along the c-axis ([0001] direction) by the slow, thermodynamically driven Vapor–Solid mechanism. As a result, the rod-like ZnO microstructures were produced on Si substrate. The second stage of growth started when the substrate temperature reached 300oC. At this temperature, the secondary nucleation took place on the prism surfaces of ZnO microrods, causing brunched structures. The ZnO brunches were growing again along the c-axes, forming elongated 1D type microcrystals. In contrast to the slow growth of primer ZnO microrods, at elevated temperatures the ZnO brunches were growing significantly faster. This kinetically driven process caused the vanishing of the fast growing (0001) plains, resulting in the tapering of ZnO microcrystal tips.

Gas sensors employing nanowires attracted great interest during the last two decades due to their extremely high sensitivity, which for some gases reach even tens of ppb levels. The purpose of this work was to develop new technology for the growth of In2Ge2O7 nanowires. The second goal was to fabricate the nanowire networkbased gas sensors on these nanowires and examine their characteristics. The nanowires were synthesized using the vapour-solid growth mechanism realized in the gaseous ambient formed after pyrolytic decomposition of the moistened hydrazine (N2H4), containing water (3-10 mol.%). After pyrolysis, the ambient was simultaneously consisting of such oxidizing, reducing and nitriding species like O2, H2O, NH and NH2 radicals, atomic hydrogen, ammonia and hydrogen molecules. Annealing of In+Ge solid sources in such ambient caused the formation of volatile suboxide precursors (In2O and GeO) that were accomplishing the mass transfer to the substrate, located in the cold zone, and subsequent growth of nanowires. It was shown that the process temperature has a great influence on the composition and morphology of nanowires. At C, only indium oxide nanowires were formed that were growing from the In self°temperatures below 400 catalyst. The mixture of pure Ge and In2O3 nanowires were obtained when the substrate temperature was in C the In2Ge2O7 nanowires were synthesized, which were decorated with InN°C. At 450°the range of 400-420 nanocrystals. Scanning and transmission electron microscopy, together with X-ray diffraction and energy dispersive X-ray spectroscopy were used to study the composition, structure and morphology of synthesized nanowires. 

The vapor-phase synthesis of Cu-based nanomaterials using inorganic volatile Cu precursors is a key for controlling the composition, morphology and structure of copper containing nanomaterials. In this paper, we have shown that annealing of a solid Cu or CuO sources in the ambient of ammonium chloride and hydrazine decomposition products leads to the formation of volatile CuCl species. The mass transfer from source to the substrate, which was located in the “cold” zone of the reactor, was accomplished by these CuCl species. After condensation on Si substrate heated up to 400°C, they were interacting with hydrazine and ammonium chloride decomposition products forming, the agglomerated Cu microcrystals in case of Cu source. Different nanomaterials were synthesized when CuO was used as a source. These nanomaterials included Cu-based nanocrystals, nanowires and elongated microbubbles. Further investigations are planned to determine the composition and structure of these nanomaterials

The research is dedicated to microwave and conventional methods of solution combustion synthesis of the relatively new nanomaterial proposed for magnetic hyperthermia of cancer cells and preliminary assessment of the toxicity of developed materials based on the behavioral methods and techniques at the levels far below of commonly registered by means of usualy and widely applied assays for humans. Farther research is needed to optimize the methods of synthesis of silver doped lanthanum manganites with required characteristics.

The formation of crystalline nitride materials is a complicated task, which usually needs the high temperature processes and the application of active nitriding precursors. Previously, we have developed the hydrazine-based technology for producing such nitride nanomaterials as

germanium and indium nitrides. The purpose of this work was to further improve this technology by adding the ammonium chloride (NH4Cl) to hydrazine (N2H4), and to investigate the formation of Ge and boron nitrides using this technology. The germanium nitride was chosen as a model material, because its formation in a poor hydrazine was studied in details, allowing for the comparative study of differences between the

 hydrazine-assisted and hydrazine+ammonium chloride-based processes. The interest for the growth of BN nanomaterials was caused by its unique physical properties and the ability of hBN to form the layered 2D nanostructures. The grown nanomaterials were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS). The analysis of thermochemical reactions that involved the precursor existing in the reactor revealed that in the case of Ge and B sources the reactions that lead to the formation of Ge3N4 and BN are thermodynamically most favorable. The nanostructures containing BN layers were synthesized on Si substrate during 20 hours at 700°C. They were also subjected to Rapid Thermal Annealing. The formation of h-BN was confirmed by XRD and EDS analysis. The thickness of BN layers was well below 100 nm. As for germanium nitride, the α-Ge3N4 nanowires were synthesized on Ge substrate at 440 °C. This synthetic temperature is by 60 and 410 °C lower than the growth temperature of the same material using hydrazine or ammonia. The obtained results clearly demonstrated that in the vapor of a mixture of NH4Cl+ N2H4 the active nitriding precursors are formed, which enable the low temperature growth of nitride nanomaterials. 

Gas sensors based on oxide-semiconductor nanowires have been a subject of extensive research because of their potential application in detecting several inflammable, toxic and odorless gases. Among them, In2O3 has been found to have a pronounced sensitivity to such gases as NO2, NH3, O3, Cl2, CO, H2, C2H5OH and other species. Sensing NO2 in the atmosphere has assumed great importance because of the serious problem of atmospheric air pollution caused by car exhaust and other sources. Recent developments showed that In2O3 nanowires doped with different atoms exhibit superior selectivity to NO2, H2S and some other gases with short response and recovery times [1 – 4]. In this work we describe the low-temperature synthesis of In2O3 nanowires, fabrication of a simple nanowire-network based gas sensor and its application for sensing NO2.

The germanium nitride and InP nanowires were grown using the pyrolytic decomposition products of hydrazine (N2H4), which was containing 3 mol.% H2O. In separate set of experiments the quartz microbalance was used to study the interaction of water containing hydrazine with Ge sample in the temperature range of 450-650°C. It was established that up to 500°C only water molecules interact with Ge, forming volatile suboxide GeO. At higher temperatures GeO molecules and nitrogen precursors, produced after decomposition of hydrazine, form crystalline Ge3N4 nanowires on the Ge surface. Analysis of thermo-chemical reactions reveal that in the presence of water molecules and nitrogen precursors the formation of nitride is thermodynamically favourable than the synthesis of germanium dioxide. When InP was annealed in hydrazine at 440°C the water molecules were producing volatile In2O. After reaching the Si substrate these molecules were interacting with phosphorus vapor producing InP nanowires

The InP based nanowires were produced by direct annealing of crystalline InP sources in hydrazine (N_2H_4) vapor and subsequent condensation of volatile spices onto the substrates. The morphology and sizes of nanowires showed strong dependence on the growth temperature. In the temperature range of 440 – 540 °C the morphology of InP nanostructures were changed from true nanowires with minimum diameters of ca. 25 nm formed at 440 °C, to faceted, several micrometer size large crystalline blocks of InP growing at 540 °C simultaneously with the rhombus decorated zigzag shaped InP nanowires with extended surfaces. The nanowires growth mechanism also varied with the temperature. In the range of 440 – 500 °C they were growing through the Vapor–Solid mechanism. At 540 °C the Vapor–Solid and Vapor–Liquid–Solid mechanisms coexisted forming large elongated blocks of indium phosphide together with zigzag shaped InP nanowires.

The tapered single-crystalline α-Ge_3N_4 nanowires were grown simultaneously on the surfaces of crystalline Ge source and Si substrate located at 3 mm above it. The growth was performed at 500 – 560 °C in the presence of hydrazine (N_2H_4) vapor containing 3 mol. % water. The nanowires were grown through the vapor–liquid–solid mechanism using Ge catalyst. Produced nanowires were tapered. However, the direction of taper was different for nanowires grown on Ge and Si. The difference in tapering was explained by differences in the fluxes of volatile GeO molecules at the beginning of growth process and at the stage of temperature stabilization. It was found that at the surface of Si substrate a part of GeO molecules was reduced to pure Ge due to the presence of hydrogen in the pyrolytic decomposition products of hydrazine. As a result the chain-like Ge nanostructures were formed together with Ge_3N_4 nanowires.

The composite nanowires comprising crystalline semiconductor cores surrounded with amorphous shells are considered as promising materials for the fabrication of nanowire based transistors, photovoltaics, gas sensors, catalysts for direct water splitting by sunlight etc. Recently we have developed the new pyrolytic technology and produced some core–shell 1D nanomaterials using the sublimation of products, formed after thermal annealing of InP + Zn source in the N2H4 + 3 mol. % H2O vapor [1 – 3]. The nanowires were synthesized from the gaseous phase on the Si substrate that was located in the “cold zone” just above the source.. The purpose of this work was to study the mechanism of the formation of shells in these composite nanowires. 

The development of new gas sensors and sensor materials is an important issue for different fields of human activities, including medicine, science, technology, environment protection etc. Nanowires are considered as one of the most suitable materials for the fabrication of modern gas sensors because of their unique physical and chemical properties, together with clearly manifested quantum features. One of the technical solutions for the fabrication of gas sensors is the formation of “nanowire network-based” sensor. In this work, we present the data on the growth of In₂O₃ nanowires using the pyrolytic technology, which was performed in the presence of hydrazine vapor diluted with 5 mol.% water. The nanowire network was deposited on the interdigitated gold electrodes with 5 mkm gapping that were deposited onto the glass substrate. It was found, that the fabricated gas sensor can detect ammonia at the level of hundred ppb-s.

The single crystal In₂O₃ and Ge₃N₄ nanowires were synthesized using the indium or germanium sources in new ambient comprising hydrazine decomposition products diluted with 3 mol.% water. This ambient was simultaneously containing oxidizing, nitriding and reducing active precursors. In spite of this only In₂O₃ nanowires were produced in case of In source, and only α-Ge₃N₄ nanowires were formed when Ge source was used. These active precursors provided formation of volatile suboxides of source materials, their flow to the Si substrate while a part of them was reduced to In or Ge catalyst droplets, and another part fed catalyst to grow the nanowiress trough the Vapor-Liquid-Solid mechanism. The growth temperature for In₂O₃ nanowires was lowered down to 420°C, while the Ge₃N₄nanowires were grown at 480°C which is by 370°C lower than the temperature indicated in the literature. The nanowires were characterized by a high crystallinity and the minimum thickness of 7 nm.

The hidrazine (N₂H₄) vapor was used to produce 1D nanomaterials. Hydrazine easily decomposes at the surfaces of semiconductors producing chemically active NH₂ species, because these surfaces serve as catalysts for the decomposition. This process is quite complicated and byproducts include H₂, NH₃, NH and other active species. The water content in hydrazine determines the composition of a final product. In₂O₃nanowires were used for the fabrication of nanowire network based gas sensors. It was able to detect the ammonia trases at the level of hundreds of ppb.The main result of this work: the developed hydrazine based pyrolytic technology is a viable method for producing oxide, nitride and phosphide 1D nanomaterials (nanowires, nanobelts, core-shell structures and nanotubes);

Indium phosphide nanowires (InP NWs) are accessible at 440 °C from a novel vapor phase deposition approach from crystalline InP sources in hydrazine atmospheres containing 3 mol % H2O. Uniform zinc blende (ZB) InP NWs with diameters around 20 nm and lengths up to several tens of micrometers are preferably deposited on Si substrates. InP particle sizes further increase with the deposition temperature. The straightforward protocol was extended on the one-step formation of new core-shell InP–Ga NWs from mixed InP/Ga source materials. Composite nanocables with diameters below 20 nm and shells of amorphous gallium oxide are obtained at low deposition temperatures around 350 °C. Furthermore, InP/Zn sources afford InP NWs with amorphous Zn/P/O-coatings at slightly higher temperatures (400 °C) from analogous setups. At 450 °C, the smooth outer layer of InP-Zn NWs is transformed into bead-shaped coatings. The novel combinations of the key semiconductor InP with isotropic insulator shell materials open up nteresting application perspectives in nanoelectronics.

The purpose of this work was to show the ability of hydrazine vapor to produce nitride, oxynitride and oxide 1D Nanomaterials. Hidrazine (N2H4) is a chemically very active reagent, which finds different applications. We used hydrazine for producing nitride nanomaterials by the pyrolysis at 500°C, in combination with Ge, InP and In source materials. It was found that the water content in hydrazine determines the composition of a final product. When the H2O content does not exceed 3 mol.%, the pure nitride nanowires of Ge3N4 and InN were produced. At elevated water concentrations the oxide and oxinitride nanomaterials were fabricated.

The main result of this work: the developed hydrazine based pyrolytic technology provides a simple way to fabricate different ID nanostructures (nanowires, nanobelts, nanocables, nanotubes etc.). The synthesized materials include Ge, Ge3N4, InN, In2Ge2O7, In2O3, Ga2O3, InP, Zn3(PO4)2.

The morphology of NWs strongly depends on the growth methods, process parameters and substrates. The purpose of this work was to study the shape and tapering of nitride NWs grown by vapor–liquid–solid (VLS) mechanism on Ge, Si and glass substrates, located in different zones of the pyrolytic reactor tube. The single-crystalline germanium nitride (α-Ge3N4) nanowires were chosen as model material. They were grown directly on the surface of a crystalline Ge source, together with glass and Si substrates located at 200 – 500 μm above it. During the first 15 min the temperature of Ge source was raised up to 500 – 585 °C and then stabilized, while the temperature of substrates above was kept by 100 – 150 °C lower. The nitration was carried out in the hydrazine (N2H4) vapor containing 3 mol. % of water. The mass transfer was performed by volatile GeO molecules formed by the interaction of Ge source and water. The nitration of GeO ensured the growth of Ge3N4 nanowires.

The Ge nanocrystals with tetragonal structure and maximum diameters of 12 nm were synthesized on the tips of Ge3N4 nanowires. These nanowires were grown by the Vapor-Liquid-Solid technology using the Ge self-catalyst. After the process was over, the cooling of a molten Ge droplet caused the formation of a solid natural oxide shell, which was covering the liquid Ge droplet core. The further solidification of a Ge core proceeded in a fixed volume. As is known, the Ge expands during solidification. Accordingly, the internal stress of 1.5 GPa appeared in the Ge core during its solidification. This resulted in the formation of tetragonal Ge particles with 12 atoms in the unit cell, instead of the diamond type Ge nanocrystals with 8 atoms per unitl cell. It was found that if the size of the droplet exceeded 12 nm, then it produced the amorphous Ge nanoparticle.

Gas sensors based on oxide-semiconductor nanowires have been a subject of extensive research because of their potential application in detecting several inflammable, toxic and odorless gases. Among them, In2O3 has been found to have a pronounced sensitivity to such gases as NO2, NH3, O3, Cl2, CO, H2, C2H5OH and other species. Sensing NO2 in the atmosphere has assumed great importance because of the serious problem of atmospheric air pollution caused by car exhaust and other sources. Recent developments showed that In2O3 nanowires doped with different atoms exhibit superior selectivity to NO2, H2S and some other gases with short response and recovery times. In this work we describe the low-temperature synthesis of In2O3 nanowires, fabrication of a simple nanowire-network based gas sensor and its application for sensing NO2. The simple gas sensor was fabricated on Si + SiO2 substrate by depositing Ti / Au interdigitated electrodes and connecting the electrodes by bridging nanowire networks, formed after drying the droplet of acetone with dissolved In2O3 nanowires. The fabricated gas sensor was able to detect the concentration of NO2 molecules down to 5 ppm level at 200°C, with quite short and stable response and recovery time

The nanowire-based gas sensors are considered as the most advanced technical solutions for increasing the sensitivity far below ppm levels. There is a great need for such sensors in the industry, medicine, environmental protection, etc. The purpose of this work was to develop new technology for the fabrication of indium oxide and indium nitride one-dimensional nanomaterials and to test the gas sensor properties fabricated on the bases of these nanomaterials.  In₂O₃, InN, InN and In₂Ge₂O₇ nanowires were synthesized using the developed hydrazine-based technology. After fabricating and testing of gas sensors, it was established that the gas sensor on In₂O₃ nanowires had the best characteristics because it allowed the reliable detection of CH₄ molecules at the level of 10 ppm

Gas sensors based on oxide-semiconductor nanowires have been a subject of extensive research because of their potential application in detecting several inflammable, toxic and odorless gases. Among them, In2O3 has been found to have a pronounced sensitivity to such gases as NO2, NH3, O3, Cl2, CO, H2, C2H5OH and other species. Sensing NO2 in the atmosphere has assumed great importance because of the serious problem of atmospheric air pollution caused by car exhaust and other sources. Recent developments showed that In2O3 nanowires doped with different atoms exhibit superior selectivity to NO2, H2S and some other gases with short response and recovery times. In this work we describe the low-temperature synthesis of In2O3 nanowires, fabrication of a simple nanowire-network based gas sensor and its application for sensing NO2. The simple gas sensor was fabricated on Si + SiO2 substrate by depositing Ti / Au interdigitated electrodes and connecting the electrodes by bridging nanowire networks, formed after drying the droplet of acetone with dissolved In2O3 nanowires. The fabricated gas sensor was able to detect the concentration of NO2 molecules down to 5 ppm level at 200°C, with quite short and stable response and recovery time.

Previously it was established that the Vapor-Liquid-Solid growth of Ge3N^T4 nanowire in the hydrazine vapor proceeds with the assistance of Ge boll-tip catalyst. In the present work we found that for the Ge catalyst sizes not exceeding 12 nm the post-growth cooling causes its crystallization in the high-pressure tetragonal form. At larger sizes of catalyst droplet the solidification proceeds with the formation of an amorphous Ge nanocrystal. We suppose, that during the cooling of a tip the amorphous Ge02 sheath around the Ge core is first solidified encapsulating liquid Ge droplet in a fixed volume and restricting its volume expansion during solidification (the volume of molten Ge increases by 6% when it crystallizes in the diamond structure 1). Upon further cooling the crystallizing Ge core embedded in GeCL sheath influences the high compressive pressure which is sufficient to form a tetragonal structure in droplets with sizes up to 12 nm.

The two types of single-crystalline alpha-Ge3N4 nanowires (NWs) were synthesized at 550degC by annealing the crystalline Ge sample in the hydrazine vapor containing 3 mol.% of water. The mass transfer was accomplished by volatile GeO molecules. The tapered NWs were grown by the vapor-liquid-solid method with ~8 nm Ge catalyst droplet surrounded by ~5 nm thick GeOx shell. NWs with uniform diameters were formed on the same sample by the oxide-assisted growth method. In the photoluminescence spectra of NWs the five peaks were observed in the blue-green region with energies close to the photoluminescence peaks of germanium suboxides.

Nanowires are considered as one of the most suitable materials for the fabrication of modern nanodevices because of their unique physical and chemical properties, together with clearly manifested quantum features. The Vapor-Liquid-Solid (VLS) mechanism of their growth is a well-established and popular method of their synthesis. Under some growth conditions, the tapering of VLS-grown nanowires is observed, which a technological drawback is. In this work, we have studied the reasons for the tapering of Ge3N4 nanowires grown by the self catalyzed VLS method. The technology was based on the application of active species, formed after pyrolytic decomposition of N2H4 3 mol.% water. These spaces were able to form volatile GeO molecules together with reducing NH radicals and hydrogen. It was found, that in Ge3N4 nanowires, grown on Ge source, the diameters were gradually decreased with growth time. The inverse tapering was observed in nanowires grown by the same VLS method on the Si substrate. It was established that during the growth process, which started at 350°C, the temperature was permanently raising with time, finally reaching the value of 500°C. This caused the time-dependent redaction of catalyst diameters for nanowires grown on Si substrate and their tapering. In contrast to this, for nanowires growing on Si substrate, the catalyst diameter was increased with time, due to more intense evaporation of GeO species and their reduction on the catalyst surface. As a result, the catalyst of nanowires that were growing on Si substrate was gradually increasing, thus increasing the diameter of Ge3N4 nanowires.

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