David 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 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.

The commercial viability of nanomaterials strongly depends on the development of low-cost, highperformance technologies that are capable of producing nanoparticles from the inexpensive solid feedstock. The plasma arc technology that was developed by our scientific group meets this requirement and provides the means for producing different nanomaterials. The purpose of this work was to apply the scanning electron microscopy (SEM) for the study of a morphology and size distribution of nanomaterials produced by plasma arc synthesis. Besides the

SEM, the X-ray diffraction, EDX and cathodoluminescence imaging were also applied for identifying existing crystalline phases and their composition. Cd and ZnO nanoparticles were deposited by the arc plasma synthesis. The choice of Cd nanoparticles was motivated by an

ongoing project devoted to the synthesis of CdS nanoparticles by direct sulfidation of metallic Cd. As for ZnO, this material in the nanocrystalline form has attracted great interest due to its wide application, which ranges from bio-medical purposes to optical and electronic usage. It was established that the synthesized Cd nanoparticles had a spherical shape and a hexagonal close packed crystal structure with a = 0.297 nm and c = 0.561 nm. Depended on the applied plasma power their diameters ranged from tens of nanometers up to 200 nm. The spherical shapes of Cd nanoparticles indicated that before crystallization they were in a molten state and the surface tension affected their shape during solidification. The composition and morphology of ZnO nanomaterials were greatly influenced by synthesis parameters. At a high plasma output power, the growth rate was significantly increasing causing the formation of stoichiometric ZnO nanoparticles mixed with oxygen-deficient and even pure Zn nanoparticles. At certain plasma parameters, the formation of ZnO tetrapods was observed. This

type of ZnO nanomaterials is known to exhibit superior optical properties. The results of this study showed a high potential of the developed plasma synthesis technology for producing different nanomaterials with an increased yield.

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. 

A vast amount of nanoparticles has been developed and proposed for the local hyperthermia of cancer during the last decades, but only a few of them correspond to the mandatory requirements of having therapeutic range Curie temperature (T_C= 41-45⁰C), high-rate crystallinity and “strong” magnetic properties, strictly controlled homogeneity and dispersion of the nanoparticles, good biocompatibility and harmless decomposition products. Among them are the nickel-copper (Ni-Cu) and silver doped lanthanum manganite (AgxLa1-xMnO3) nanoparticles. The developed research showed that the materials obtained at lower than usual temperatures using microwave enhanced synthesizes and annealing can be successfully used for local hyperthermia revealing high magnetic properties. Mixtures of Nanoparticles with liquid crystals can play an important role as effective and non-toxic material for modalities of the Curie point controlled strongly localized magnetic hyperthermia with a precisely tunned prolonged release of nanoparticles into tissue cells.Therefore, the preparation of mixtures of liquid crystals with a transition temperature from the nematic to the dispersed phase in the range of 41-45⁰C, and their comprehensive study is an important practical task. This problem is complicated by the fact that the transition temperature to the dispersed phase depends essentially on the concentration of nanoparticles in the mixture. The reported research deals with preparation of mixtures of Ni-Cu and AgxLa1-xMnO3 nanoparticles and compositions of 12 commercially available liquid crystals.

It has been shown, that microwave heating can substantially reduce the temperature and increase the capacity of synthesis of nanoparticles. It has also been shown, that low toxic mixtures of magnetic nanoparticles and liquid crystal compositions with a transit temperature from the nematic to the dispersed phase in the range of 41-45 ⁰C can be composed using different widely used commercially available liquid crystals. Unfortunately, all tested mixtures with nickel-copper (Ni-Cu) nanoparticles showed a high tendency to corrosion and, if affected by a medium or strong magnetic field, to rapis agglomeration and precipitation. To overcome the both above obstacles, nanoparticles with coated or modified surface should be investigated. The reported research deals with preparation of mixtures of liquid crystal compositions with the boron nitride encapsulated Ni_0.7-Cu_0.3 nanoparticles and carbon supported phosphatized Cu-Ni nanoparticles, investigating their corrosion and precipitation in magnetic field, and testing their acute toxicity to white rats using a long-term monitoring of their behavioral characteristics and physiological parameters.Experiments showed that corrosion and agglomeration rates, as well as the acute toxicity of  the mixtures are significantly reduced in comparison to the mixtures with uncoated nanoparticles. It is important, that the heating capacity of all mixtures remained the same. Additional experiment should be done using different coating technics and toxicity testing methods.

It has been previously shown, that that the Ni-Cu nanoparticles synthsized at lower than usual temperatures using microwave enhanced synthesizes and annealing can be successfully used for local hyperthermia revealing high magnetic properties. Mixtures of Nanoparticles with liquid crystals can play an important role as effective and non-toxic material for modalities of the Curie point controlled strongly localized magnetic hyperthermia with a precisely tuned prolonged release of nanoparticles into tissue cells. It has also been shown that coating, encapsulating or modifying of nanoparticle surface can help to overcome the problems related to corrosion and agglomeration in a medium and strong magnetic field and reduce the acute toxicity of combinations to white rats tested using a long-term monitoring of their behavioral characteristics and physiological parameters. However, taking into account the growing restrictions on the use of laboratory animals and enhancing of measures for providing increased safety of use of manufactured nanoparticles, new methods of testing of acute toxicity should be developed and applied. The reported research deals with deals with preparation of mixtures of liquid crystal compositions with uncoated, coated, surface modified, phosphatized and encapsulated nanoparticles and testing their acute toxicity to chick embryos using visible light ovoscopy. Experiments showed that the acute toxicity of  the mixtures is maximally reduced through encapsulating in boron nitride. Additional experiments using behavioral monitoring of exposed white rats and chick embryos is necessary to assess the correlation between the results achieved using these two different methods of toxicity testing. 

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.

Carbon-coated ferromagnetic (Fe–Fe3C) nanocomposites have been synthesized using solidphase pyrolysis of metal-organic compounds. The structure, morphology and magnetic characteristics of these nanocomposites were investigated by electron microscopy, X-ray diffraction, Raman spectroscopy and magnetometry. The magnetic characteristics such as saturation magnetization and coercivity as well as the specific absorption rate (SAR) make these materials attractive for magnetic hyperthermia applications. Hysteresis loop of the (Fe–Fe3C) & C nanocomposites is of special interest as it shows almost square behavior, where Mr / M (200 Oe) = 0.75. The limitation of the magnetic field amplitude and frequency (H · f) ≤ 10.625 ×106 Oe / s makes this factor important that provides a high energy absorption even in case of low magnetic fields. The TEM image, hysteresis loop and heating saturations.Ag-doped lanthanum manganite and carbon-black hybrids have been prepared by physical mixing of modified carbon and Ag-doped LaMnO3 nanoparticles, followed by sintering at different temperatures. AgxLa1–xMnO3 nanoparticles were first synthesized via microwave enhanced chemical precipitation, and the carbon support was modified using graphitization, followed by HNO3 and ammonia treatments. Microwave assisting yielded in improved uniformity of nanoparticles and reduced time of synthesis, which can become important for practical use of lanthanum manganite

nanocomposites

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 signing and ratification of the Association Agreement between the European Union and Georgia marked the beginning of a new stage in the history of Georgia. It signaled the political and historical return of Georgia to the Community of European Countries, known as the European Union (EU). This seminal event will come to no avail if Georgia does not overcome the huge gap in economic and social development between Georgia and EU countries. Georgia must take her place in the EU at the same level of science and education standards, environmental protection, and standard of living as the rest of the EU countries [1, 2]. The global problem of rapidly increasing quantity of industrial and domestic waste is particularly acute in Georgia where a disproportionately large amount of toxic waste is concentrated in a relatively small area [3]. Recent discoveries by Georgian scientists and scientific institutions in the field of microwave enhanced processing of hazardous mining and metallurgical [4, 5], polymeric [4], agricultural, municipal, medical, radioactive waste and microwave assisted synthesis of nano-particles and nano-materials [6], brought to the possibility of semi-industrial and industrial processing. The lab-scale implementation of the developed and optimized equipment for producing highly demanded marketable products, like liquid and gaseous fuel, manganese oxide concentrate, manganese alloys and multi-composite compounds, metallic, semiconducting and polymeric nano-materials is reported. The advances of microwave enhances processing can form a scientific and technological basis for Georgia to move toward increased economic and social development, environmental safety and security establish a “Green Economics” and a Knowledge Based Society and achieve sustainable development. Innovative approaches and experimental microwave installation uses for waste processing and nano-particle synthesis place Georgia in the forefront of using microwave technology.

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

One dimensional nanomaterials (nanotubes, nanowires, nanofibers, nanobelts etc.) are considered as the main building blocks of modern nanodevices which can serve as transient materials for the future quantum dot based advanced nanocircuits and devices. Except laser ablation, the synthetic methods for producing one dimensional (1D) oxides, nitrides, phosphides usually rely on the use of corresponding vapor flows and in most cases need relatively high temperatures exceeding ~600°C. The purpose of this work was to develop a hydrazine-based simple and efficient new technology for fabricating new 1D nanomaterials (nitrides, oxynitrides, oxides, phosphides) of Ge, In, Ga and Ge-In, Ge-Zn, In-Ga, In-Zn, In-Ga-Zn systems. Another purpose was to investigate the properties of the emerging novel nanomaterials in order to evaluate their application potential in different nanodevices. The composition and structure of 1D nanomaterials were analyzed by X-ray diffraction, Energy Dispersive Spectroscopy, Transmission and Raster Electron Microscopy. The hydrazine (N2H4) vapor was chosen as a chemically active ambient due to its low pyrolysis temperature and ability to form active radicals and reducing agents like hydrogen. For producing oxide and oxinitride nanowires (NWs) the hydrazine was diluted with water (up to 3 mol.%). 1D nanomaterials were grown in the vertical quartz reactor. The solid source materials were placed on its heated bottom. After evaporation of sources and chemical reactions the NWs were growing on the substrates located at some distance above the source. In contrast to traditional growth methods with dynamic gas flows, the static pressure of hydrazine (~10 Torr) was preserved during the whole nanowire growth time (0.5 – 5.0 hours). Using this technology following 1D nanomaterials were synthesized: Ge3N4, In2Ge2O7, InN, InP, InP:Zn, InxGa1-xP. The initial experiments clearly show that during the NW growth processes the hydrazine vapor can serve a variety of purposes.

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.

Germanium Nitride Nanowires (Ge₃N₄) were synthesized by the developed hydrazine (N₂H₄)-based technology. Annealing of germanium or Ge source in the vapor of N₂H₄+3 mol.% H₂O caused the formation of volatile GeO molecules in the hot zone. These molecules were transferred to the Si substrate, which was placed in the cold zone of a reactor. After interacting with hydrazine decomposition products (NH₃, NH₂, NH, H₂, H) and water, Ge₃N₄ nanowires and nanobelts were produced on the Ge source in the temperature range of 500–520 ºC. The growth temperature of Ge₃N₄ nanowires in hydrazine vapor was by 350 ºC lower than the temperature reported in the literature. The possible chemical reactions for the synthesis of these nanostructures were evaluated. The results of this work clearly demonstrate that the hydrazine vapor has a high chemical activity and can be successfully used for the low temperature production of nitride nanomaterials.

The synthesis of nitride nanomaterials is a complicated process, which needs the application of high temperatures, close to 1000°C and active environment. Recently we have developed the new technology for the fabrication of nitride nanomaterials. The technology employs a highly active hydrazine (N₂H₄) vapor diluted with 3-10 mol.% H₂O. It was established, that at a high water content in hydrazine, the germanium oxinitride was produced, while at 3 mol.% water content the pure crystalline Ge₃N₄ nanowires were formed. The morphology of nanomaterials was strongly affected by the substrate temperature. At 320°C the spherical particles of nitride were produced having diameters in the range of 80-140 nm. At elevated temperatures, close to 450°C the nanowires were growing. The micrometer size a- Ge₃N₄ single crystalline blocks were formed at a substrate temperature of 500°C.

Recently, we have developed the technology for the deposition of two-phase thin films comprising Ge nanocrystals embedded in the amorphous GeO₂ film (Ge:GeO₂ film). In this work, we show the possibility of preparing the rolled-up microtubes and micropores using this layer together with the sacrificial, water-soluble GeO₂ film. It was found, that due to the differences in the mechanical and thermal properties of Ge and GeO₂ phases, the internal compressive strain was built up in the two-phase film. To form the rolled-up microtubes, the sacrificial GeO₂ layer was deposited on the two-phased film. After water etching of the combined film, the microtubes were formed. By depositing the sacrificial layer under the two-phased film, the micropores were produced having lengths up to several tens of micrometer.

Nanoporous materials attract great interest due to their increased surface area and hence, their increased chemical activity. Our strategy for the formation of nanoporous films was based on the deposition of two-phased Ge/GeO₂ film and its subsequent etching. The films were deposited by magnetron sputtering of Ge target in the plasma of a mixture of O₂ and Ar gases at a total pressure of 2´10^-3 Torr. The partial pressure of oxygen was kept sufficiently low (8´10^-4 Torr) to ensure the formation of a phase-separated film comprising Ge nanocrystals, embedded in the amorphous GeO₂ film. The structure, morphology, and phase content of films were analyzed by SEM, TEM, and X-ray diffraction methods. It was shown that the nonhomogeneous internal strains are built-up in the film, which causes the formation of the nanoporous film after its etching in water

The amorphous Al2O3 films with embedded Ge or Si nanocrystals were produced by the magnetron sputtering of Al+Si(Ge) targets in a mixture of Ar+02. The value of the oxygen partial pressure was selected in such a way, that the Al target was sputtered in the "oxide" mode, while the neighboring Si (Ge) target was operating in the "metallic" mode. Annealing of these films using photon pulses with 6 kW power, cause formation of Ge or Si nanocrystals in the amorphous Ab03 matrix. The presence of Al2O3 and elemental Si(Ge) phases was proved by Auger spectroscopy and Transmission Electron Microscopy.

The films with regular and randomly distributed pores are challenging materials for membranes, photonic crystals, thermal insulation, surface protection, light coatings, and several other applications. The purpose of this work was to develop the technology for the deposition of phase-separated Ge:GeO₂ film using the magnetron sputtering of Ge target in the Oxygen deficient atmosphere. IR, SEM and X-ray diffraction confirmed the simultaneous presence of Ge and GeO₂ phases. As is well known, GeO₂ easily dissolves in water. After drying the water-treated phase-separated Ge:GeO­₂ films, the pores appeared with sizes in the range of 80-450 nm. The sizeable difference in pore sizes was attributed to the large deviations in Ge islands that were distributed in the amorphous GeO₂ matrices. It was concluded that the pore diameters decreased when the film deposition rate was increased due to the formation of smaller, densely distributed Ge dots. The increase in temperature caused the coalescence of neighboring Ge clusters, forming large Ge spots and larger pores. It was shown that the pore sizes and their distribution can be controlled by properly adjusting the process parameters of film deposition.  

      The nanocrystals embedded in insulating matrices are considered as prospective materials for the fabrication of different nanodevices including sensitive gas-sensors, light emitting diods, hard coatings etc.  The results of this work clearly show that germanium quantum dots embedded in GeO₂ can be fabricated by DC magnetron sputtering of Ge target in the mixture of Ar and oxygen. The IR bands at 880 and 680 cm ^{- 1} confirm the presence of GeO₂ phase and also show the existence of Ge-O-Ge bonds at the boundaries between Ge and GeO₂ phases. The optical absorption and Auger electron spectroscopy data prove that elemental Ge presents in the magnetron sputtered CeO_x film. TEM experiments show that crystallization takes place after Rapid Thermal Annealing (RTA) of films. Ge quantum dots with average size of 12 nm and different shapes are formed in GeO₂ matrix after 11 annealing cycles

The sublimation of GeO molecules formed by the solid phase reaction between a-Ge and a-GeO2 films (Ge+GeO2=2Ge) was studied using vacuum microbalance. The c-Si/Ge/GeO2, cSi/GeO2/Ge, c-Ge/GeO2 and c-Si/GeO/GeO2 structures, fabricated by the ion-plasma deposition were annealed in the temperature range of 650-725°C. The diffusion of GeO molecules through the amorphous GeO2 and Ge films was investigated. The calculated activation energy indicated that the sublimated molecules were (GeO)2 dimmers. Dimeric (GeO)2 molecules were formed directly in the reaction zone at the Ge/GeO2 interface and after diffusion through the overlayer they were sublimated without decomposition. The sublimation rate of (GeO)2 was influenced by the degree of crystallinity of the Ge film. The Arrhenius expressions were obtained for calculation of diffusivities and permeability. Examination of the diffusion and permeation data suggests, that the process of diffusion through the GeO2 or Ge films limits the evaporation rate of(GeO)2. © 2002 IEEE.

Auger electron spectroscopy (AES) was used to study the solid phase surface reactions and Ge diffusion process during the thermal treatment (500-750°C) of GeO₂/(111)GaAs structures. Different results were obtained for the A and B surfaces of (111)GaAs. The diffusivity data for A and B surfaces could be described by the following Arrhenius expressions: A-D(cm² s^-1)=1.16×10² exp(-3.54/kT); B-D(cm² s^-1)=2.16×10^-5 exp(-2.12/kT)

 According to modern understanding, high-quality surface passivation of GaAs can be achieved only after the removal of its thin native oxide layer, which covers the surface and forms the interface with a high interface defect density (>1012 cm2). This work was aimed at the development of such a technique, which can create an intermediate layer between GaAs and the outer insulating film. This layer, called the “interface control layer” (ICL), must form high-quality interfaces with both neighboring materials. A thin, ~20 nm thick, Ge layer was chosen to accomplish this task.

 The results reported in this paper clearly show that pyrolytic GeOxNy film can be used for high-quality surface passivation of GaAs and fabrication of an effective MIS transistor on this semiconductor.

One of the main directions of modern microelectronics concentrates on the development of high­ performance radiation-hardened electronic parts to be used in harsh environments. The ionizing radiation primarily affects the gate-insulator of MIS (Metal-Insulator-Semiconductor) devices. Our new approach is based on the assumption that the stability of MIS structures will increase if an insulating film will be substituted by an amorphous 0 and N doped Germanium film (GeOxNy) film) with a wide energy gap and a high resistivity. In such material a certain amount of free carriers exists and they have an anomalous low mobility. The movement of free carriers, whose relaxation times are significantly shorter, compensates the long relaxation time instabilities caused by irradiation. The developed new type film forms a high quality interface with Si (Nss=2´10 cm-2 eV-1), which is close to, the best Si-Si02 interface.

The results of our preliminary experiments prove that our approach has a sound physical basis. It can be used to develop an essentially new type of effective, simple and inexpensive technology for the high-quality surface passivation of Si, GaAs and other semiconductors. It can be also used for the fabrication of microwave MIS Integrated Circuits on GaAs with all inherent advantages of this type of devices.

 Zn and Ge diffusion in (n)GaAs from the zinc-silicate glass (ZSG) and Ge02 solid dopant sources was investigated. After pulse photon annealing of the ZSG/n-GaAs(lnP)/Ge02 system the p-n-n+ structures were formed during a single photon flash. Due to the amphoteric nature of Ge atoms their diffusion in the polar          (111) GaAs surfaces results in formation of a p-type layer in case of B-surface and n-layer - when A-surface is used.

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