Growth Direction Of Nanowires Research Articles

Synthesis and growth twinning of γ-Al2O3nanowires by simple evaporation of Al–Si alloy powder

In this study, single-crystalline γ-Al2O3 nanowires were synthesized by simple evaporation of Al–Si alloy source powder at 1070 °C under different working pressures. The structures and growth mechanisms of the nanowires were examined. Under a low working pressure of 0.5 torr, uniform nanowires with a catalyst particle were synthesized by a vapor–liquid–solid mechanism and grew along a [2-20] direction. Twinning occurred parallel to nanowire growth direction and a twin angle of 15.5°/164.5° was identified. As the pressure increased, zigzag nanowires without a catalyst grew along a [−1−11] direction alternatively via a vapor–solid route. Twinning also formed with an angle of 15.5°/164.5°. Perpendicular twinning planes to the growth direction induced the deflected feature of the nanowires.

Phonon Transport and Thermoelectricity in Defect-Engineered InAs Nanowires

ABSTRACTThere have been reports of improvements in the thermoelectric figure of merit through the use of nanostructured materials to suppress the lattice thermal conductivity. Here, we report on a fundamental study of the combined effects of defect planes and surface scattering on phonon transport and thermoelectric properties of defect-engineered InAs nanowires. A microfabricated device is employed to measure the thermal conductivity and thermopower of individual suspended indium arsenide nanowires grown by metal organic vapor phase epitaxy. The four-probe measurement device consists of platinum resistance thermometers and electrodes patterned on two adjacent SiNx membranes. A nanowire was suspended between the two membranes, and electrical contact between the nanowire and the platinum electrodes was made with the evaporation of a Ni/Pd film through a shadow mask. The exposed back side of the device substrate allows for characterization of the crystal structure of the suspended nanowire with transmission electron microscopy (TEM) following measurement. The 100-200 nm diameter zincblende (ZB) InAs nanowire samples were grown with randomly spaced twin defects, stacking faults, or phases boundaries perpendicular to the nanowire growth direction, as revealed by transmission electron microscopy (TEM) analysis. Compared to single-crystal ZB InAs nanowires with a similar lateral dimension, the thermal conductivity of the defect-engineered nanowires is reduced by fifty percent at room temperature.

Reversible phase transformations in a shape memory alloy In–Tl nanowires observed by in situ transmission electron microscopy

Shape memory alloy In–21at.% Tl nanowires were prepared by a mechanical injection method. Transmission electron microscopy observations revealed that the nanowires are composed of polycrystals. At room temperature, the nanowires were characterized as martensite with dense twins, and the twining plane was identified as {101} of the face-centered tetragonal structure. The nanowire growth direction was found to be primarily along <110>. By in situ heating, reversible phase transformations between the martensite and the parent phase were observed.

Why self-catalyzed nanowires are most suitable for large-scale hierarchical integrated designs of nanowire nanoelectronics

Nanowires are grown by a variety of mechanisms, including vapor-liquid-solid, vapor-quasiliquid-solid or vapor-quasisolid-solid, oxide-assisted growth, and self-catalytic growth (SCG) mechanisms. A critical analysis of the suitability of self-catalyzed nanowires, as compared to other nanowires, for next-generation technology development has been carried out. Basic causes of superiority of self-catalyzed (SCG) nanowires over other nanowires have been described. Polytypism in nanowires has been studied, and a model for polytypism has been proposed. The model predicts polytypism in good agreement with available experiments. This model, together with various evidences, demonstrates lower defects, dislocations, and stacking faults in SCG nanowires, as compared to those in other nanowires. Calculations of carrier mobility due to dislocation scattering, ionized impurity scattering, and acoustic phonon scattering explain the impact of defects, dislocations, and stacking faults on carrier transports in SCG and other nanowires. Analyses of growth mechanisms for nanowire growth directions indicate SCG nanowires to exhibit the most controlled growth directions. In-depth investigation uncovers the fundamental physics underlying the control of growth direction by the SCG mechanism. Self-organization of nanowires in large hierarchical arrays is crucial for ultra large-scale integration (ULSI). Unique features and advantages of self-organized SCG nanowires, unlike other nanowires, for this ULSI have been discussed. Investigations of nanowire dimension indicate self-catalyzed nanowires to have better control of dimension, higher stability, and higher probability, even for thinner structures. Theoretical calculations show that self-catalyzed nanowires, unlike catalyst-mediated nanowires, can have higher growth rate and lower growth temperature. Nanowire and nanotube characteristics have been found also to dictate the performance of nanoelectromechanical systems. Defects, such as stacking faults, dislocations, and nanopipes, which are common in catalyst-mediated nanowires and nanotubes, adversely affect the efficiency of nanowire (nanotube) nanoelectro-mechanical devices. The influence of seed-to-seed distance and collection area radius on the self-catalyzed, self-aligned nanowire growths in large arrays of seeds has been examined. A hypothesis has been presented for this. The present results are in good agreement with experiments. These results suggest that the SCG nanowires are perhaps the best vehicles for revolutionary advancement of tomorrow’s nanotechnology.

Stereoaligned Epitaxial Growth of Single-Crystalline Platinum Nanowires by Chemical Vapor Transport

Epitaxial Pt nanowire (NW) arrays are synthesized for the first time by a chemical vapor transport method by using a metal halide as a precursor. Here we report that the epitaxial growth direction of NWs can be steered by seed crystal morphology. Octahedral seeds grow into inclined NWs possessing six growth directions, whereas half-octahedral seeds grow into vertical and horizontal NWs. Interfacial energies between the seed material and the substrate are critical in determining the morphology of seed crystals. We also demonstrate that non-SERS-active Pt NWs can show strong surface-enhanced Raman scattering (SERS) spectra by placing them on Ag films. The active SERS observation would help to elucidate platinum-catalyzed chemical reactions.

Fabrication of Crystalline Semiconductor Nanowires by Vapor-Liquid-Solid Glancing Angle Deposition (VLS-GLAD) Technique

ABSTRACTVapor-liquid-solid (VLS) method has become one of the few and most powerful bottom-up single crystal nanowire growth techniques in nanotechnology due to its easy scalability from micro to nano feature sizes, high throughput, relatively low cost, and its applicability to various semiconductor materials. On the other hand, control of growth direction and crystal orientation of nanowires, which determine their electrical, optical, and mechanical properties, stand as major issues in VLS technique. In this study, we demonstrate a new vapor-liquid-solid glancing angle deposition (VLS-GLAD) fabrication approach to produce crystalline semiconductor nanowires with controlled geometry. VLS-GLAD is a physical vapor deposition nanowire fabrication approach based on selective deposition of nanowire source atoms onto metal catalyst nanoislands placed on a crystal wafer. In this technique, collimated obliquely incident flux of source atoms selectively deposit on catalyst islands by using “shadowing effect”. Geometrical showing effect combined with conventional VLS growth mechanism leads to the growth of tilted crystalline semiconductor nanowire arrays. In this study, we report morphological and structural properties of tilted single crystal germanium nanowire arrays fabricated by utilizing a conventional thermal evaporation system. In addition to the tilted geometry, by introducing substrate rotation, nanowires with various morphologies including helical, zig-zag, or vertical shapes can be fabricated. Engineering crystalline nanowire morphology by using VLS-GLAD have the potential of enabling control of optical, electrical, and mechanical properties of these nanostructures leading to the development of novel 3D nano-devices.

Texture analysis of GaAs nanowires

The technique of texture analysis by x-ray diffraction is demonstrated for GaAs nanowires grown by the vapor-liquid–solid process in a gas source molecular beam epitaxy system. This technique allows simultaneous and facile determination of nanowire growth direction and crystal structure over large sample areas and with improved statistics relative to scanning electron microscopy.

Cu2 S nanowires and MnS/Cu2 S nanojunctions derived from γ-MnS nanowires via selective cation-exchange reaction

Cu 2 S nanowires and MnS/Cu 2 S junctions have been derived on the basis of γ-MnS nanowires via cation-exchange reaction. Relationships between a selective cation exchange and the growth direction of the γ-MnS nanowires are identified by means of transmission electron microscopy. In the case of γ-MnS nanowires with [0001] growth direction, Cu + exchange with an initial γ-MnS nanowire results in a Cu 2 S nanowire. In contrast, for γ-MnS nanowires with [1-100] growth direction, Cu + exchange leads to a striped MnS/Cu 2 S junction. Mechanisms for this selective cation exchange are proposed.

Epitaxial Directional Growth of Tin Oxide (101) Nanowires on Titania (101) Substrate

Highly aligned single crystal SnO2 (101) nanowires were epitaxially grown on TiO2 (101) substrates by thermal evaporation via Au-catalyzed vapor−liquid−solid (VLS) growth. The orientation relationship and interface structure between the nanowires and the substrate were determined by X-ray pole figure and high resolution transmission electron microscopy (HR-TEM) combined with the focused ion beam (FIB) lift-out technique. Epitaxially grown SnO2 (101) nanowires exhibited three angular growth directions ([101], [011], and [011]) with different inclination angles to the substrate due to a tetragonal crystal structure. An atomic stacking model was proposed to describe the angular growth of SnO2 (101) nanowires with ⟨101⟩ growth directions. The obtained results are expected to provide an understanding of the growth direction of nanowires and heteroepitaxial relationships between nanowires and substrate to synthesize the well-aligned SnO2 nanowires, which can be integrated into the electronic devices and lead t...

Composition and growth direction control of epitaxial vapor-liquid-solid-grown SiGe nanowires

The composition and growth direction of epitaxial SiGe alloy nanowires (NWs) grown via the Au-catalyzed vapor-liquid-solid technique can be controlled by varying growth conditions. These alloy NWs can adopt either Si-like or Ge-like characteristics. Si-like growth is characterized by Au-coated ⟨111⟩-oriented NWs for low pressure growth and Au-free ⟨112⟩-oriented NWs for higher pressure growth. Ge-like NWs always follow ⟨111⟩ and grow with Au-free sidewalls.

Large-Area Oblique-Aligned ZnO Nanowires through a Continuously Bent Columnar Buffer: Growth, Microstructure, and Antireflection

We demonstrate a novel defect-induced bending mechanism for a modified oblique-angle deposition (OAD) system, where different defect density was introduced to accommodate the mass difference between the shadowed and exposed surfaces, leading to continuous structural bending. Oblique angle sputtering and hydrothermal processes were employed for growth of inclined ZnO nanowire arrays on ZnO bent columns. Transmission electron microscopy images reveal that a dislocation network was introduced to accommodate the mass difference in bent columns, and the bending angle could be controlled by growth temperature. Nanowires were then grown along the tangent lines of the bent column tips. The bent column curvature and limited space determine the nanowire growth direction. The reflectance measurements demonstrate that the oblique-aligned ZnO nanowire arrays are an excellent candidate for antireflection coatings, showing the significant suppression of reflectance of 87.5% and 90.0% for polished Si under TE and TM polarization, respectively. The interference oscillations of reflectance show the optical anisotropy of oblique-aligned ZnO nanowire arrays, which is dependent on the angle range of nanowire direction.

The structure and growth direction of rare earth silicide nanowires on Si(100)

The growth direction and the structure of rare earth silicide nanowires grown on the Si(100) surface have been calculated from first principles. The energies of the optimum structures show that a structure related to the tetragonal bulk phase is more favorable than the hexagonal model and that growth parallel to the dimer rows is lower in energy than growth across the dimer rows.

Growth direction determination of a single RuO2 nanowire by polarized Raman spectroscopy

The dependence of band intensities in the Raman spectrum of individual single-crystal ruthenium dioxide (RuO2) nanowires on the angle between the plane of polarization of the exciting (and collected) light and the long axis of the nanowire, is shown to be a simple, complementary technique to high resolution transmission electron microscopy (HRTEM) for determining nanowire growth direction. We show that excellent agreement exists between what is observed and what is predicted for the polarization angle dependence of the intensities of the nanowires’ Eg (525 cm−1) and the B2g (714 cm−1) Raman bands, only by assuming that the nanowires grow along the (001) crystallographic direction, as confirmed by HRTEM.

Reactant-governing growth direction of indium nitride nanowires

Hexagonal wurtzite InN nanowires are grown via a vapor–liquid–solid (VLS) mechanismwith an Au catalyst. Microstructure characterizations of a large number of nanowiresdemonstrate that the growth direction of InN nanowires is governed by variableNH3 flux. InNnanowires at a NH3 flux of 10 standard cubic centimeters per minute (sccm) grow preferentially in a hexagonalclose-packed (hcp) direction, while those at 100 sccmNH3 flux favorthe hcp ⟨0001⟩ direction. A free energy minimization model is proposed to interpret this phenomenon.The first-principles calculations reveal that the oriented nucleus has the lowest energy at the lowerNH3 flux. In contrast,when NH3 fluxis high, the ⟨0001⟩ oriented nucleus has the lowest energy.

Single Electron Charging in Optically Active Nanowire Quantum Dots

We report optical experiments of a charge tunable, single nanowire quantum dot subject to an electric field tuned by two independent voltages. First, we control tunneling events through an applied electric field along the nanowire growth direction. Second, we modify the chemical potential in the nanowire with a back-gate. We combine these two field-effects to isolate a single electron and independently tune the tunnel coupling of the quantum dot with the contacts. Such charge control is a first requirement for opto-electrical single electron spin experiments on a nanowire quantum dot.

Asymmetric Doping in Silicon Nanostructures: The Impact of Surface Dangling Bonds

We investigate peculiar dopant deactivation behaviors of Si nanostrucures with first principle calculations and reveal that surface dangling bonds (SDBs) on Si nanostructures could be fundamental obstacles in nanoscale doping. In contrast to bulk Si, as the size of Si becomes smaller, SDBs on Si nanostructures prefer to be charged and asymmetrically deactivate n- and p-type doping. The asymmetric dopant deactivation in Si nanostructures is ascribed to the preference for negatively charged SDBs as a result of a larger quantum confinement effect on the conduction band. On the basis of our results, we show that the control of the growth direction of silicon nanowire as well as surface passivation is very important in preventing dopant deactivation.

Synthesis and characterization of GaN nanowires by ammoniating Ga 2O 3/Cr thin films deposited on Si(1 1 1) substrates

GaN nanowires have been successfully synthesized on Si(1 1 1) substrates by magnetron sputtering through ammoniating Ga 2O 3/Cr thin films at 950 °C. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), FT-IR spectrophotometer, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), and photoluminescence (PL) spectrum were carried out to characterize the microstructure, morphology, and optical properties of GaN samples. The results demonstrate that the nanowires are single-crystal GaN with hexagonal wurtzite structure and high-quality crystalline, have the size of 30–80 nm in diameter and several tens of microns in length with good emission properties. The growth direction of GaN nanowires is perpendicular to the fringe of (1 0 1) plane. The growth mechanism of GaN nanowires is also discussed in detail.

Catalytic growth of cubic phase ZnO nanowires with jagged surface

Cubic-phase ZnO nanowires with jagged surfaces were successfully synthesised through the Au-assisted chemical vapour deposition method. The nanowire has a fourfold symmetric shape with the diameter of tens of nanometres modulated along its axis. Especially, the interface between the catalyst droplet and the nanowire grows in epitaxial relationship. Thus, the authors believe that the property of Au particle may determine the crystalline structure and growth direction of the ZnO nanowires. The optical property of the products was also examined using photoluminescence spectroscopy.

Localisation of silicon nanowires grown by UHV-CVD in (111)-oriented apertures opened in Si (001)

In this paper, we have investigated an alternative method to obtain well-organized silicon nanowires grown by vapor-liquid-solid process. We have studied the influence of the droplet size on the growth direction of nanowires on plain wafers. By using self-organized Au droplets grown by molecular beam epitaxy on vicinal Si(111) surface with a 2° miscut towards the [11-2] crystallographic direction, we have obtained well organized gold droplets along the steps with small size dispersion. The growth of Si nanowires from these droplets allows defining a critical diameter for changing the Si nanowires growth direction. When the gold droplet diameter is larger than 100 nm, Si nanowires exhibit a <111> orientation and remain free of structural defects. The localisation of Si nanowires was performed in nanoscale silicon seeds formed by modified local oxidation of silicon process. This approach, based on a simple and reliable method, enable the organisation of Si nanowires with <111> orientation on Si(001) substrates.

Controlled III/V Nanowire Growth by Selective-Area Vapor-Phase Epitaxy

We report on the growth of surface-bound, vertically oriented one-dimensional III/V nanostructures, specifically GaAs and InAs nanowires, on lattice-matched and -mismatched substrates by selective-area vapor-phase epitaxy. Control of nanowire features and growth directions is achieved by tuning the growth conditions. Grown nanostructures are characterized by scanning and transmission electron microscopy, X-ray diffraction, and Raman spectroscopy.