Surface dislocation nucleation mediated deformation and ultrahigh strength in sub-10-nm gold nanowires

The plastic deformation and the ultrahigh strength of metals at the nanoscale have been predicted to be controlled by surface dislocation nucleation. In situ quantitative tensile tests on individual 〈111〉 single crystalline ultrathin gold nanowires have been performed and significant load drops observed in stress-strain curves suggest the occurrence of such dislocation nucleation. High-resolution transmission electron microscopy (HRTEM) imaging and molecular dynamics simulations demonstrated that plastic deformation was indeed initiated and dominated by surface dislocation nucleation, mediating ultrahigh yield and fracture strength in sub-10-nm gold nanowires.      

Mechanical elasticity of vapour-liquid-solid grown GaN nanowires

Mechanical elasticity of hexagonal wurtzite GaN nanowires with hexagonal cross sections grown through a vapour-liquid-solid (VLS) method was investigated using a three-point bending method with a digital-pulsed force mode (DPFM) atomic force microscope (AFM). In a diameter range of 57-135?nm, bending deflection and effective stiffness, or spring constant, profiles were recorded over the entire length of end-supported GaN nanowires and compared to the classic elastic beam models. Profiles reveal that the bending behaviour of the smallest nanowire (57.0?nm in diameter) is as a fixed beam, while larger nanowires (89.3-135.0?nm in diameter) all show simple-beam boundary conditions. Diameter dependence on the stiffness and elastic modulus are observed for these GaN nanowires. The GaN nanowire of 57.0?nm diameter displays the lowest stiffness (0.98?N?m(-1)) and the highest elastic modulus (400 ± 15?GPa). But with increasing diameter, elastic modulus decreases, while stiffness increases. Elastic moduli for most tested nanowires range from 218 to 317?GPa, which approaches or meets the literature values for bulk single crystal and GaN nanowires with triangular cross sections from other investigators. The present results together with further tests on plastic and fracture processes will provide fundamental information for the development of GaN nanowire devices.     

Growth mechanism and properties of InGaN insertions in GaN nanowires

We demonstrate the strong influence of strain on the morphology and In content of InGaN insertions in GaN nanowires, in agreement with theoretical predictions which establish that InGaN island nucleation on GaN nanowires may be energetically favorable, depending on In content and nanowire diameter. EDX analyses reveal In inhomogeneities between the successive dots but also along the growth direction within each dot, which is attributed to compositional pulling. Nanometer-resolved cathodoluminescence on single nanowires allowed us to probe the luminescence of single dots, revealing enhanced luminescence from the high In content top part with respect to the lower In content dot base.     

Three-dimensional GaN/AlN nanowire heterostructures by separating nucleation and growth processes

Bottom-up nanostructure assembly has been a central theme of materials synthesis over the past few decades. Semiconductor quantum dots and nanowires provide additional degrees of freedom for charge confinement, strain engineering, and surface sensitivity-properties that are useful to a wide range of solid state optical and electronic technologies. A central challenge is to understand and manipulate nanostructure assembly to reproducibly generate emergent structures with the desired properties. However, progress is hampered due to the interdependence of nucleation and growth phenomena. Here we show that by dynamically adjusting the growth kinetics, it is possible to separate the nucleation and growth processes in spontaneously formed GaN nanowires using a two-step molecular beam epitaxy technique. First, a growth phase diagram for these nanowires is systematically developed, which allows for control of nanowire density over three orders of magnitude. Next, we show that by first nucleating nanowires at a low temperature and then growing them at a higher temperature, height and density can be independently selected while maintaining the target density over long growth times. GaN nanowires prepared using this two-step procedure are overgrown with three-dimensionally layered and topologically complex heterostructures of (GaN/AlN). By adjusting the growth temperature in the second growth step either vertical or coaxial nanowire superlattices can be formed. These results indicate that a two-step method allows access to a variety of kinetics at which nanowire nucleation and adatom mobility are adjustable.     

Measurement of the bending strength of vapor-liquid-solid grown silicon nanowires

The fracture strength of silicon nanowires grown on a [111] silicon substrate by the vapor-liquid-solid process was measured. The nanowires, with diameters between 100 and 200 nm and a typical length of 2 microm, were subjected to bending tests using an atomic force microscopy setup inside a scanning electron microscope. The average strength calculated from the maximum nanowire deflection before fracture was around 12 GPa, which is 6% of the Young's modulus of silicon along the nanowire direction. This value is close to the theoretical fracture strength, which indicates that surface or volume defects, if present, play only a minor role in fracture initiation.     

Dopant-free GaN/AlN/AlGaN radial nanowire heterostructures as high electron mobility transistors

We report the rational synthesis of dopant-free GaN/AlN/AlGaN radial nanowire heterostructures and their implementation as high electron mobility transistors (HEMTs). The radial nanowire heterostructures were prepared by sequential shell growth immediately following nanowire elongation using metal-organic chemical vapor deposition (MOCVD). Transmission electron microscopy (TEM) studies reveal that the GaN/AlN/AlGaN radial nanowire heterostructures are dislocation-free single crystals. In addition, the thicknesses and compositions of the individual AlN and AlGaN shells were unambiguously identified using cross-sectional high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM). Transport measurements carried out on GaN/AlN/AlGaN and GaN nanowires prepared using similar conditions demonstrate the existence of electron gas in the undoped GaN/AlN/AlGaN nanowire heterostructures and also yield an intrinsic electron mobility of 3100 cm(2)/Vs and 21,000 cm(2)/Vs at room temperature and 5 K, respectively, for the heterostructure. Field-effect transistors fabricated with ZrO(2) dielectrics and metal top gates showed excellent gate coupling with near ideal subthreshold slopes of 68 mV/dec, an on/off current ratio of 10(7), and scaled on-current and transconductance values of 500 mA/mm and 420 mS/mm. The ability to control synthetically the electronic properties of nanowires using band structure design in III-nitride radial nanowire heterostructures opens up new opportunities for nanoelectronics and provides a new platform to study the physics of low-dimensional electron gases.     

Diameter-dependent electromechanical properties of GaN nanowires

The diameter-dependent Young's modulus, E, and quality factor, Q, of GaN nanowires were measured using electromechanical resonance analysis in a transmission electron microscope. E is close to the theoretical bulk value ( approximately 300 GPa) for a large diameter nanowire (d=84 nm) but is significantly smaller for smaller diameters. At room temperature, Q is as high as 2,800 for d=84 nm, significantly greater than what is obtained from micromachined Si resonators of comparable surface-to-volume ratio. This implies significant advantages of smooth-surfaced GaN nanowire resonators for nanoelectromechanical system (NEMS) applications. Two closely spaced resonances are observed and attributed to the low-symmetry triangular cross section of the nanowires.     

The controlled growth of GaN nanowires

This paper reports a scalable process for the growth of high-quality GaN nanowires and uniform nanowire arrays in which the position and diameter of each nanowire is precisely controlled. The approach is based on conventional metalorganic chemical vapor deposition using regular precursors and requires no additional metal catalyst. The location, orientation, and diameter of each GaN nanowire are controlled using a thin, selective growth mask that is patterned by interferometric lithography. It was found that use of a pulsed MOCVD process allowed the nanowire diameter to remain constant after the nanowires had emerged from the selective growth mask. Vertical GaN nanowire growth rates in excess of 2 mum/h were measured, while remarkably the diameter of each nanowire remained constant over the entire (micrometer) length of the nanowires. The paper reports transmission electron microscopy and photoluminescence data.     

Role of five-fold twin boundary on the enhanced mechanical properties of fcc Fe nanowires

The role of 5-fold twin boundary on the structural and mechanical properties of fcc Fe nanowire under tension is explored by classical molecular dynamics. Twin-stabilized fcc nanowire with various diameters (6-24 nm) are examined by tension tests at several temperatures ranging from 0.01 to 1100 K. Significant increase in the Young's modulus of the smaller nanowires is revealed to originate from the central area of quinquefoliolate-like stress-distribution over the 5-fold twin, rather than from the surface tension that is often considered as the main source of such size-effects found in nanostructures. Because of the excess compressive stress caused by crossing twin-boundaries, the atoms in the center behave stiffer than those in bulk and even expand laterally under axial tension, providing locally negative Poisson's ratio. The yield strength of nanowire is also enhanced by the twin boundary that suppresses dislocation nucleation within a fcc twin-domain; therefore, the plasticity of nanowire is initiated by strain-induced fcc→bcc phase transformation that destroys the twin structure. After the yield, the nucleated bcc phase immediately spreads to the entire area, and forms a multigrain structure to realize ductile deformation followed by necking. As temperature elevated close to the critical temperature between bcc and fcc phases, the increased stability of fcc phase competes with the phase transformation under tension, and hence dislocation nucleations in fcc phase are observed exclusively at the highest temperature in our study.     

Doping of Si into GaN nanowires and optical properties of resulting composites

Doping of Si into GaN nanowires has been successfully attained via thermal evaporation in the presence of a suitable gas atmosphere. Analysis indicated that the Si-doped GaN nanowire is a single crystal with a hexagonal wurtzite structure, containing 2.2 atom % of Si. The broadening and the shift of Raman peak to lower frequency are observed, which may be attributed to surface disorder and various strengths of the stress. The band-gap emission (358 nm) of Si-doped GaN nanowires relative to that (370 nm) of GaN nanowires has an apparent blue shift (approximately 12 nm), which can be ascribed to doping impurity Si.     

Using seed particle composition to control structural and optical properties of GaN nanowires

The morphology, structure, and optical properties of gallium nitride (GaN) nanowires grown using metal-organic chemical vapor deposition (MOCVD) on r-plane sapphire using gold and nickel seed particles were investigated. We found that different seed particles result in different growth rates and densities of structural defects in MOCVD-grown GaN nanowires. Ni-seeded GaN nanowires grow faster than Au-seeded ones, and they do not contain the basal plane stacking faults that are observed in Au-seeded GaN nanowires. We propose that stacking fault formation is related to the supersaturation and surface energies in different types of seed particles. Room temperature photoluminescence studies revealed a blue-shifted peak in Au-seeded GaN nanowires compared to the GaN near-bandgap emission. The blue-shifted peak evolves as a function of the growth time and originates from the nanowire base, likely due to strain and Al diffusion from the substrate. Our results demonstrate that seed particle composition has a direct impact on the growth, structure, and optical properties of GaN nanowires and reveal some general requirements for seed particle selection for the growth of compound semiconductor nanowires.     

Orientation and temperature dependence of the tensile behavior of GaN nanowires: an atomistic study

Gallium nitride (GaN) is a high-temperature semiconductor material of considerable interest. It emits brilliant light and has been considered as a key material for the next generation of high frequency and high power transistors that are capable of operating at high temperatures. Due to its anisotropic and polar nature, GaN exhibits direction-dependent properties. Growth directions along [001], [1?10] and [110] directions have all been synthesized experimentally. In this work, molecular dynamics simulations are carried out to characterize the mechanical properties of GaN nanowires with different orientations at different temperatures. The simulation results reveal that the nanowires with different growth orientations exhibit distinct deformation behavior under tensile loading. The nanowires exhibit ductility at high deformation temperatures and brittleness at lower temperature. The brittle to ductile transition (BDT) was observed in the nanowires grown along the [001] direction. The nanowires grown along the [110] direction slip in the {010} planes, whereas the nanowires grown along the [1?10] direction fracture in a cleavage manner under tensile loading.     

Growth characteristics of GaAs nanowires obtained by selective area metal-organic vapour-phase epitaxy

GaAs nanowires were selectively grown by metal-organic vapour-phase epitaxy within a SiO(2) mask window pattern fabricated on a GaAs(111)B substrate surface. The nanowires were 100-3000?nm in height and 50-300?nm in diameter. The height decreased as the mask window diameter was increased or the growth temperature was increased from 700 to 800?°C. The dependence of the nanowire height on the mask window diameter was compared with a calculation, which indicated that the height was inversely proportional to the mask window diameter. This suggests that the migration of growth species on the nanowire side surface plays a major role. Tetrahedral GaAs grew at an early stage of nanowire growth but became hexagonal as the growth process continued. The calculated change in Gibbs free energy for nucleation growth of the crystals indicated that tetrahedra were energetically more favourable than hexagons. Transmission and scanning electron microscopy analyses of a GaAs nanowire showed that many twins developed along the [Formula: see text] B direction, suggesting that twins had something to do with the evolution of the nanowire shape from tetrahedron to hexagon.     

GaN nanowires: CVD synthesis and properties

The growth of GaN nanowires from Ga and NH₃ sources in the flow of Ar carrier gas using a chemical vapor deposition (CVD) system was systematically studied. The substrates used were Si(111) and Si(100). Fabricated nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). We investigated the influence of growth temperature, catalyst used, Ga amount, and the ratio of Ar and NH₃ flow rates on the morphology and properties of GaN nanowires. We found that the best results were obtained for a growth temperature of 950 °C. Optimal catalysts were Au and metallic Ni, while the use of nickel nitrate was found to lead to formation of SiO_x nanowire bunches in addition to GaN nanowires. For the optimal temperature and catalyst used, the influence of the Ga to N ratio on the nanowire growth was studied. It was found that different types of nanostructures are observed in relatively Ga-rich and in relatively N-rich conditions. Growth mechanisms of different types of nanowires, including the stacked-cone nanowires and the microscale structures formed by lateral growth under N-rich conditions, are discussed.     

Semiconductor Nanowire Light‐Emitting Diodes Grown on Metal: A Direction Toward Large‐Scale Fabrication of Nanowire Devices

Bottom‐up nanowires are attractive for realizing semiconductor devices with extreme heterostructures because strain relaxation through the nanowire sidewalls allows the combination of highly lattice mismatched materials without creating dislocations. The resulting nanowires are used to fabricate light‐emitting diodes (LEDs), lasers, solar cells, and sensors. However, expensive single crystalline substrates are commonly used as substrates for nanowire heterostructures as well as for epitaxial devices, which limits the manufacturability of nanowire devices. Here, nanowire LEDs directly grown and electrically integrated on metal are demonstrated. Optical and structural measurements reveal high‐quality, vertically aligned GaN nanowires on molybdenum and titanium films. Transmission electron microscopy confirms the composition variation in the polarization‐graded AlGaN nanowire LEDs. Blue to green electroluminescence is observed from InGaN quantum well active regions, while GaN active regions exhibit ultraviolet emission. These results demonstrate a pathway for large‐scale fabrication of solid state lighting and optoelectronics on metal foils or sheets.     

Morphology of self-catalyzed GaN nanowires and chronology of their formation by molecular beam epitaxy

GaN nanowires are synthesized by plasma-assisted molecular beam epitaxy on Si(111) substrates. The strong impact of the cell orientation relative to the substrate on the nanowire morphology is shown. To study the kinetics of growth, thin AlN markers are introduced periodically during NW growth. These markers are observed in single nanowires by transmission electron microscopy, giving access to the chronology of the nanowire formation and to the time evolution of the nanowire morphology. A long delay precedes the beginning of nanowire formation. Then, their elongation proceeds at a constant rate. Later, shells develop on the side-wall facets by ascending growth of layer bunches which first agglomerate at the nanowire foot.     

Nucleation and growth of GaN nanowires on Si(111) performed by molecular beam epitaxy

GaN nanowires (NWs) have been grown on Si(111) substrates by plasma-assisted molecular beam epitaxy (PAMBE). The nucleation process of GaN-NWs has been investigated in terms of nucleation density and wire evolution with time for a given set of growth parameters. The wire density increases rapidly with time and then saturates. The growth period until the nucleation of new nanowires is terminated can be defined as the nucleation stage. Coalescence of closely spaced nanowires reduces the density for long deposition times. The average size of the well-nucleated NWs shows linear time dependence in the nucleation stage. High-resolution transmission electron microscopy measurements of alternating GaN and AlN layers give valuable information about the length and radial growth rates for GaN and AlN in NWs.     

Monodisperse (In, Ga)N insertions in catalyst-free-grown GaN(0001) nanowires

Vertical stacks of (In, Ga)N insertions in GaN nanowires are grown by molecular beam epitaxy. The chemical composition and strain within the structure are probed by a combination of high-resolution x-ray diffraction, transmission electron microscopy, and geometrical phase analysis. The (In, Ga)N insertions are coherently strained. Finite-element simulations strongly support an ineffective [corrected] strain relaxation despite [corrected] the nanowire geometry, leading to high-quality (In, Ga)N/GaN nanowire heterostructures. An intense green photoluminescence emission is observed and attributed to an inter-well transition between the stacked (In, Ga)N insertions.     

Research on quantum efficiency and photoemission characteristics of exponential-doping GaN nanowire photocathode

Aimed at improving the actual photoemission performance of nanowire photocathode, an axial exponential-doping GaN nanowire photocathode is proposed. Based on two-dimensional continuity equation and finite difference method, the quantum efficiency of this exponential-doping GaN nanowire photocathode is obtained. The simulation results suggest that this structure of GaN nanowire photocathode can effectively obviate the difficulty in collecting the electrons escaping from side faces because a large part of carriers will escape from top surface under the built-in electric field. Besides, it is discovered that the optimal height of nanowires is 300 nm when the doping concentration of top surface is 1 × 10¹⁸ cm^?3 and that of back interface is 1 × 10¹⁹ cm^?3. Then, when the nanowires are arranged as array, the optimal light angle of incidence is approximately 60° by analyzing the electrons flow density of the array. By comparison of collection proportion of photoelectrons, the optimal nanowire spacing is 231 nm. This study demonstrates potential application value of exponential-doping GaN nanowire photocathode. The results can direct the preparation of this kind of photocathode.     

Controlled synthesis of AlN/GaN multiple quantum well nanowire structures and their optical properties

We report the controlled synthesis of AlN/GaN multi-quantum well (MQW) radial nanowire heterostructures by metal-organic chemical vapor deposition. The structure consists of a single-crystal GaN nanowire core and an epitaxially grown (AlN/GaN)(m) (m = 3, 13) MQW shell. Optical excitation of individual MQW nanowires yielded strong, blue-shifted photoluminescence in the range 340-360 nm, with respect to the GaN near band-edge emission at 368.8 nm. Cathodoluminescence analysis on the cross-sectional MQW nanowire samples showed that the blue-shifted ultraviolet luminescence originated from the GaN quantum wells, while the defect-associated yellow luminescence was emitted from the GaN core. Computational simulation provided a quantitative analysis of the mini-band energies in the AlN/GaN superlattices and suggested the observed blue-shifted emission corresponds to the interband transitions between the second subbands of GaN, as a result of quantum confinement and strain effect in these AlN/GaN MQW nanowire structures.