Formation Of Nanowires Research Articles

Исследование особенностей эпитаксиального роста GaAs на подложках Si, модифицированных фокусированными ионными пучками

In this paper, we present a study of the effect of the silicon substrate modification by focused ion beams on subsequent growth of GaAs layers by molecular beam epitaxy. We demonstrate that when samples exposed to the ion irradiation at various accelerating voltages and ion beam passes are annealed in the absence of the arsenic flux, an increase in the depth of the modified Si substrate areas occurs. At the same time, crystallization of gallium accumulations during annealing in the arsenic flux leads to the filling of holes formed during the ion bombardment. We reveal that the growth of GaAs on substrates with areas modified at an accelerating voltage of 30 kV and subjected to subsequent annealing in the arsenic flux at a temperature of 600 °C is accompanied by the formation of nanowires, the density of which increases within areas with a large number of ion beam passes. The results of the conducted research can be used for the development of technological approaches to the formation of GaAs epitaxial layers on Si substrates.

Study of GaAs epitaxial growth on Si substrates modified by focused ion beams

In this paper, we present a study of the effect of the silicon substrate modification by focused ion beams on subsequent growth of GaAs layers by molecular beam epitaxy. We demonstrate that when samples exposed to the ion irradiation at various accelerating voltages and ion beam passes are annealed in the absence of the arsenic flux, an increase in the depth of the modified Si substrate areas occurs. At the same time, crystallization of gallium accumulations during annealing in the arsenic flux leads to the filling of holes formed during the ion bombardment. We reveal that the growth of GaAs on substrates with areas modified at an accelerating voltage of 30 kV and subjected to subsequent annealing in the arsenic flux at a temperature of 600oC is accompanied by the formation of nanowires, the density of which increases within areas with a large number of ion beam passes. The results of the conducted research can be used for the development of technological approaches to the formation of GaAs epitaxial layers on Si substrates. Keywords: molecular beam epitaxy, monolithic integration, gallium arsenide, silicon, focused ion beams.

Flow Effects on the Morphology of Silicon Materials Produced in a Gas Phase Reaction of SiCl<sub>4</sub>

Chemical reaction formula does not provide information on size and morphology of formed solids. In this talk, we introduce our recent results on process-dependent changes in size and morphology of silicon solids produced via zinc reduction of SiCl4. Silicon whiskers, microparticles and nanowires form in the reaction, in which zinc vapor reduces SiCl4 and zinc droplets catalyse crystallization of silicon. Flow of gases in a reactor and temperature of substrate affect solidification processes of silicon. Effects of there factors on the formation of silicon whiskers and nanowires are discussed using both experimental results and numerical analysis.

In situ analysis of the nucleation of O- and Zn-polar ZnO nanowires using synchrotron-based X-ray diffraction.

The selection of the polarity of ZnO nanowires grown by chemical bath deposition offers a great advantage for their integration into a wide variety of engineering devices. However, the nucleation process of ZnO nanowires and its dependence on their polarity is still unknown despite its importance for optimizing their morphology and properties and thus to enhance the related device performances. To tackle this major issue, we combine an in situ analysis of the nucleation process of O- and Zn-polar ZnO nanowires on O- and Zn-polar ZnO single crystals, respectively, using synchrotron radiation-based grazing incidence X-ray diffraction with ex situ transmission and scanning electron microscopy. We show that the formation of ZnO nanowires obeys three successive phases from the induction, through nucleation to growth phases. The characteristics of each phase, including the nucleation temperature, the shape and dimension of nuclei, as well as their radial and axial development are found to depend on the polarity of ZnO nanowires. A comprehensive description reporting the dominant physicochemical processes in each phase and their dependence on the polarity of ZnO nanowires is presented, revisiting their formation process step-by-step. These findings provide a deeper understanding of the phenomena at work during the growth of ZnO nanowires by chemical bath deposition and open the perspective to develop a more accurate control of their properties at each step of the formation process.

Introducing ytterbium acetate to luminescent CsPbCl3 nanocrystals for enhanced sensitivity of Cu2+ detection

The Yb(OAc)3 promoted the formation of CsPbCl3 nanowires (NWs). The NWs exhibited boost in conductivity and the adsorbed AcO− on the surface of NWs enhanced the adsorption capacity of Cu2+, which enabled NWs with high detection sensitivity of Cu2+.

Copper mesh supported nickel nanowire array as a catalyst for the hydrogen evolution reaction in high current density water electrolysis.

Hydrogen generation by water splitting using various renewable energy sources will play an important role in the sustainable green energy supply of the future. Unfortunately, wide industrial adoption of this process is currently impeded by the necessity to use noble metal based electrolysis catalysts. In this study, a low-cost and highly efficient water electrolysis catalyst active in the hydrogen evolution reaction (HER) taking place in alkaline medium is developed. The catalyst preparation procedure consists of electrodeposition of a rough nickel layer onto a smooth copper mesh, followed by the growth of hierarchical Ni nanowires on its surface. The rough nickel layer provides plenty of active sites for nanowire formation, resulting in a synergetic effect between copper and nickel in the copper mesh supported nickel nanowire array, which effectively enhances the HER electrocatalytic performance of this novel material. The catalyst demonstrated an HER overpotential as low as 317 mV in 1 M KOH electrolyte at a current density of 1 A cm-2. The copper substrate's superior electrical conductivity to that of nickel is responsible for the excellent HER performance of the catalyst at high current density, making it a promising candidate to replace highly expensive noble metal based electrodes.

Controlling morphology in electrosprayed methylcellulose nanowires via nanoparticle addition: coarse-grained modeling and experiments.

Electrospray deposition (ESD) has shown great promise for manufacturing micro- and nanostructured coatings at scale on versatile substrates with complex geometries. ESD exhibits a broad spectrum of morphologies depending upon the properties of spray fluids. Among them are nanowire forests or foams obtained via the in-air gelation of electrospray droplets formed from methylcellulose (MC) solutions. In this study, we explored MC ESD loaded with nanoparticles of various shapes and uncovered the effects of particle fillers on morphology evolution using coarse-grained simulations and physical experiments. Utilizing electrostatic dissipative particle dynamics, we modeled the electrohydrodynamic deformation of particle-laden MC droplets undergoing in-flight evaporation. The simulations quantitatively predict the suppression of droplet deformation as the size or concentration of spherical nanoparticles increases. While small particles can be readily encapsulated into the nanowire body, large particles can arrest nanowire formation. The model was extended to nanoparticles with complex topologies, showing MC nanowires emerging from particle edges and vertices due to curvature-enhanced electric stress. In all cases, strong agreements were found between simulation and experimental results. These results demonstrate the efficacy of the coarse-grained model in predicting the morphology evolution of electrosprayed droplets and lay the groundwork for employing MC nanowires for developing nanostructured composites.

Preparation, microstructure and properties of Al2O3–ZrO2–C slide plate material in presence of nanoscale oxides

Low carbon Al2O3–ZrO2–C refractories with nano-oxides are prepared at 1500 °C in graphite embedded condition using tabular alumina, reactive alumina powder, zirconia-mullite, zirconia-corundum and flake graphite as well as nano-ZrO2, nano-TiO2 and nano-Al2O3 powders as additives. The effects of nano-ZrO2, nano-TiO2 and nano-Al2O3 powders as single additive on the properties and microstructure of Al2O3–ZrO2–C material are investigated. The results show that nano-oxides take the catalytic effect in SiC nanowires (NWs) formation and growth, leading to increasing in length-diameter (L/D) ratio of SiC NWs. Samples with nano-oxides addition possess improved sinterability, thermal shock resistance and oxidation resistance.

Mechanism of droplet motion and in-plane nanowire formation with and without electromigration

Au deposition on Si(110) at temperature higher than 633 K leads to the formation of elongated liquid nano-droplets. In this work we show that under an electric field, the droplets move along the [1–10] axis towards the positive electrode by electromigration. During the motion, they dissolve the substrate forming a hole underneath and a nanowire behind them. We develop a model highlighting that the driving force of these processes is bound to the respect (i) of the Au-Si bulk chemical equilibrium and (ii) of the correct balance of surface/interfacial energies. The motion takes place by means of sudden jumps with length of the order of a few μm and by small readjustments of the order of some nm of the droplet front and back edges. With respect to experiments without electric field, electromigration changes the droplet motion by widening the droplet dissolution front and by facilitating droplet jumps.

Study of the initial stage of GaAs growth on FIB-modified silicon substrates

In this work, we studied the effect of the deposition thickness, growth rate, arsenic flux, and implantation dose on the morphology of the GaAs nanostructures grown on modified Si areas. It is shown that an increase in the growth rate at the initial stages of the growth process leads to the transition of the growth regime from layered-like to one-dimensional with the formation of nanowires. Studies of the effect of As4 pressure have shown that a change in the equivalent As4 flux in the range of 3.7 - 5.0 ML/s does not lead to any significant change in the structure of the GaAs layer in the modified areas. An increase in the implantation dose during processing with a focused ion beam led to disordering of the directions of the grown nanowires due to the degradation of the substrate crystal structure.

Morphology transformation of InGaN nanowires grown on Si substrate by PA-MBE

The influence of the growth time on the structural properties of InGaN nanowires grown on Si substrate by plasma-assisted molecular beam epitaxy are studied. Under appropriate other growth conditions, the growth for 2h leads to the formation of separated nanowires, whereas the growth for 2h 30min and 3h leads to the formation of nanostructures such as nano-umbrellas. The separated NWs exhibit a photoluminescence spectrum with maxima at about 590 nm, whereas the nano-umbrellas show two pronounced photoluminescence lines at 421 and 619 nm.

Mechanistic Understanding of Formation of Ultrathin Single-Crystalline Pt Nanowires

Engineering anisotropic nanostructures of isotropic FCC metals (Au, Pt, or Pd) requires a symmetry-breaking mechanism. In wet chemical synthesis methods, a judicious selection of capping agents can...

Quantitative analysis of metastable wurtzite phase into the self-catalyzed GaP NWs

In this letter, we report the growth of the self-catalyzed GaP nanowires with a high concentration of wurtzite phase by molecular beam epitaxy. Formation of rotational twins and wurtzite polymorph in vertical nanowires was observed by the developed a complex approach based on the transmission electron microscopy and X-ray diffraction method. Microstructural analysis performed by high resolution transmission electron microscopy and micro-Raman spectroscopy gives us insights on the nanowire formation mechanism and vibrational properties of nanowires with mixed crystal phase. We obtained wurtzite polytype segments with thicknesses lying in the range from several tens up to 500 nm. The results of the work open new perspectives for high phase purity phosphide NWs synthesis and its fast investigation with XRD technique using a laboratory X-Ray source.

Thermal oxidation impact on the optoelectronic and hydrogen sensing properties of p-type copper oxide thin films

The paper presents the results of the investigation of the post-process annealing effect on selected properties of copper oxide thin films deposited by magnetron sputtering. It was found that the prepared thin films were nanocrystalline with the crystallite size smaller than 20 nm. Annealing at temperatures of 200 °C, 300 °C and 350 °C caused metallic copper oxidation to copper(I) oxide and then to copper(II) oxide. As-deposited and annealed at 200 °C thin films were composed of Cu and Cu2O. Copper(I) oxide transformation into CuO started at 300 °C, while the thin film annealed at 350 °C consisted of a single CuO phase. Scanning electron microscopy imaging revealed that annealing of the as-deposited thin films caused gradual formation of cupric oxide nanowires. Optimum annealing temperature for nanowires growth was 350 °C. X-ray photoelectron spectroscopy showed that the surface of the as-deposited thin films was oxidized to Cu2O and CuO, while in the case of annealed films it was completely oxidized to CuO. Thermal treatment also contributed to the increase in coating transparency in the infrared range. The transmission coefficient at 1500 nm of the thin film annealed at 350 °C was equal to 68. The resistivity of the as-deposited copper oxide thin film was equal to 1.9•10−3 Ωcm, while the highest resistivity of the annealed thin films was equal to 32 Ωcm for the coating annealed at 200 °C. A further increase in the annealing temperature resulted in a slight decrease in the resistivity value. Based on Seebeck coefficient measurements, it was found that all coatings were characterized by p-type conductivity. Additionally, sensing properties toward hydrogen were examined. The best sensor response exceeding 13 was observed for the thin film annealed at 200 °C.

Co-Ni Basic Carbonate Nanowire/Carbon Nanotube Network With High Electrochemical Capacitive Performance via Electrochemical Conversion.

In this work, the Co-Ni basic carbonate nanowires were in-situ grown on carbon nanotube (CNT) network through a facile chemical bath deposition method, which could be further converted into active hydroxide via cyclic voltammetry strategy. A series of carbonate nanowire/nanotube with different Co/Ni ratio revealed the different growth status of the nanowires on CNT network. The nanostructures of the as-synthesized samples were examined via powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) techniques. The Co/Ni ratio of the carbonate largely affected the size of the nanowires, that the low Co/Ni ratio was beneficial for thin nanowire formation and the nanowires loading on CNT network. Subsequently, the electrochemical performance of the Co-Ni basic hydroxides was studied in a three-electrode test system. The nanowires with low Co/Ni ratio 1/2 can form nanowire array on individual CNTs, which exhibited better electrochemical capacitive performance than the composite network with high Co/Ni ratio nanowires after electrochemical activation. The addition of Co enhanced the rate performance of the hydroxide/CNT, especially improved the long cycle stability largely compared to the rate performance of pure Ni converted hydroxide/CNT composite film reported by our previous research. This result is valuable for the design of inorganic electrochemical active composites based on conductive networks for energy conversion/storage applications.

The TiO2-ZnO Systems with Multifunctional Applications in Photoactive Processes-Efficient Photocatalyst under UV-LED Light and Electrode Materials in DSSCs.

The main goal of the study was the hydrothermal-assisted synthesis of TiO2-ZnO systems and their subsequent use in photoactive processes. Additionally, an important objective was to propose a method for synthesizing TiO2-ZnO systems enabling the control of crystallinity and morphology through epitaxial growth of ZnO nanowires. Based on the results of X-ray diffraction analysis, in the case of materials containing a small addition of ZnO (≥5 wt.%), no crystalline phase of wurtzite was observed, proving that a high amount of modified titanium dioxide can inhibit the crystallization of ZnO. The transmission electron microscopy (TEM) results confirmed the formation of ZnO nanowires for systems containing ≥ 5% ZnO. Moreover, for the synthesized systems, there were no significant changes in the band gap energy. One of the primary purposes of this study was to test the TiO2-ZnO system in the photodegradation process of 4-chlorophenol using low-power UV-LED lamps. The results of photo-oxidation studies showed that the obtained binary systems exhibit good photodegradation and mineralization efficiency. Additionally, it was also pointed out that the dye-sensitized solar cells can be a second application for the synthesized TiO2-ZnO binary systems.

Si doping-induced phase control, formation of p-type and n-type GaAs nanowires

We realized the control on the crystal structure of GaAs nanowires (NWs) by silicon (Si) doping and V/III ratio. This viewpoint has been clearly evidenced by Raman spectrum combined with high-resolution transmission electron microscopy (HRTEM). Undoped GaAs NWs generally show a mixture of zinc blende (ZB) and wurtzite (WZ) phases; the addition of Si dopant and high V/III ratio are beneficial for the nucleation of ZB structure. Field effect transistor (FET) devices were fabricated to show that Si-doped GaAs NWs exhibit p-type conductivity in the low V/III ratio of 25.8 and n-type conductivity in the high V/III ratio of 51.6. These results provide a clear route for achieving GaAs NWs phase control and different conductivity types, which is an essential step towards the fabrication of nanoscale devices.

Alternate layer by layered self assembly of conjugated and unconjugated Salen based nanowires as capacitive pseudo supercapacitor

A novel electrosynthetic method has been introduced based on alternate layer-by-layered self-assembly of conjugated/unconjugated Salen-based nanowires as a capacitive pseudo-supercapacitor. For this purpose, a three-electrode system consisted of a glassy carbon (GC), Ag/AgCl (Sat’d Cl−) and a Pt rod as working, reference, and counter electrodes, respectively. The electrolyte included the same molar concentration (0.040 mol L−1) of each Salen monomer (as initial precursor), and KCl solution (as supporting electrolyte), besides using KOH solution (0.01 mol L−1, as basic-controlling reagent) inside acetone/water (4:1, V/V) as a solvent. The formation of this self-assembly nanowire was attributed to the control of the electrical conductivity of this polymer during formation of an organometallic complex with K+ as responsible complex forming agent. This novel nanowire then played role as a capacitive pseudo-supercapacitor. Based on the chrono—potentiometry, reproducible charge/discharge process for at least 5000 cycles was observed at a potential between − 2.00 and + 1.75 V (vs. Ag/AgCl). The capacity behavior of the polymer was also evidenced using electrochemical impedance spectroscopy. This synthesized polymeric nanowire was adopted as the acceptable pseudo-supercapacitor with real capacity equals to 3110 ± 6 (n = 3) C g−1. This study was considered as the first report at which the self—assembly of organometallic compounds as an efficient pseudo—supercapacitor was introduced.

Revisiting the Chemical Stability of Germanium Selenide (GeSe) and the Origin of its Photocatalytic Efficiency

AbstractRecently, germanium selenide (GeSe) has emerged as a promising van der Waals semiconductor for photovoltaics, solar light harvesting, and water photoelectrolysis cells. Contrary to previous reports claiming perfect ambient stability based on experiments with techniques without surface sensitivity, here, by means of surface‐science investigations and density functional theory, it is demonstrated that actually both: i) the surface of bulk crystals; and ii) atomically thin flakes of GeSe are prone to oxidation, with the formation of self‐assembled germanium‐oxide skin with sub‐nanometric thickness. Surface oxidation leads to the decrease of the bandgap of stoichiometric GeSe and GeSe1−x, while bandgap energy increases upon surface oxidation of Ge1−xSe. Remarkably, the formation of a surface oxide skin on GeSe crystals plays a key role in the physicochemical mechanisms ruling photoelectrocatalysis: the underlying van der Waals semiconductor provides electron–hole pairs, while the germanium‐oxide skin formed upon oxidation affords the active sites for catalytic reactions. The self‐assembled germanium‐oxide/germanium‐selenide heterostructure with different bandgaps enables the activation of photocatalytic processes by absorption of light of different wavelengths, with inherently superior activity. Finally, it is discovered that, depending on the specific solvent‐GeSe interaction, the liquid phase exfoliation of bulk crystals can induce the formation of Se nanowires.

Regular Nanowire Formation on Fe-Based Metal Glass by Manipulation of Surface Waves

We report the formation of a sole long nanowire structure and the regular nanowire arrays inside a groove on the surface of Fe-based metallic glass upon irradiation of two temporally delayed femtosecond lasers with the identical linear polarization parallel and perpendicular to the groove, respectively. The regular structure formation can be well observed within the delay time of 20 ps for a given total laser fluence of F = 30 mJ/cm2 and within a total laser fluence range of F = 30–42 mJ/cm2 for a given delay time of 5 ps. The structural features, including the unit width and distribution period, are measured on a one-hundred nanometer scale, much less than the incident laser wavelength of 800 nm. The degree of structure regularity sharply contrasts with traditional observations. To comprehensively understand such phenomena, we propose a new physical model by considering the spin angular momentum of surface plasmon and its enhanced inhomogeneous magnetization for the ferromagnetic metal. Therefore, an intensive TE polarized magnetic surface wave is excited to result in the nanometer-scaled energy fringes and the ablative troughs. The theory is further verified by the observation of nanowire structure disappearance at the larger time delays of two laser pulses.