Coaxial multi-interface hollow Ni-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-ZnO nanowires tailored by atomic layer deposition for selective-frequency absorptions

In this work atomic layer deposition (ALD) was employed to fabricate coaxial multi-interface hollow Ni-Al2O3-ZnO nanowires.The morphology,microstructure,and ZnO shell thickness dependent electromagnetic and microwave absorbing properties of these Ni-A12O3-ZnO nanowires were characterized.Excellent microwave absorbing properties with a minimum reflection loss (RL) of approximately-50 dB at 9.44 GHz were found for the Ni-Al2O3-100ZnO nanowires,which was 10 times of Ni-A12O3 nanowires.The microwave absorption frequency could be effectively varied by simply adjusting the number of ZnO deposition cycles.The absorption peaks of Ni-Al2O3-100ZnO and Ni-A12O3-150ZnO nanowires shifted of 5.5 and 6.8 GHz towards lower frequencies,respectively,occupying one third of the investigated frequency band.The enhanced microwave absorption arose from multiple loss mechanisms caused by the unique coaxial multi-interface structure,such as multi-interfacial polarization relaxation,natural and exchange resonances,as well as multiple internal reflections and scattering.These results demonstrate that the ALD method can be used to realize tailored nanoscale structures,making it a highly promising method for obtaining high-efficiency microwave absorbers,and opening a potentially novel route for frequency adjustment and microwave imaging fields.     

Fabrication and photoconductivity of macroscopically long coaxial structured Ag/Ag2S nanowires with different core-to-shell thickness ratios

Macroscopically long core/shell structured Ag/Ag(2)S coaxial nanowires and Ag(2)S nanowires have been fabricated using the solid-state ionics method for Ag nanowires, combined with a subsequent gas-solid reaction, and characterized at different spatial scales. The photoconductive properties of such samples are investigated by performing transport measurements with 532 nm laser illumination ON/OFF cycles under different bias. A significant change in the photoconductivity from negative to positive has been observed in the coaxial structured Ag/Ag(2)S nanowires when the Ag(2)S layer thickness increases to a certain level. Such behaviors are ascribed to two photoconductive mechanisms in the Ag core and the Ag(2)S shell, respectively. These results indicate a promising approach to fabricate nanoscale photoswitches with different dark resistances and photoinduced currents based on the Ag/Ag(2)S coaxial nanowires for various optoelectronic applications.     

Enhanced Cu₂S/CdS coaxial nanowire solar cells by piezo-phototronic effect

Nanowire solar cells are promising candidates for powering nanosystems and flexible electronics. The strain in the nanowires, introduced during growth, device fabrication and/or application, is an important issue for piezoelectric semiconductor (like CdS, ZnO, and CdTe) based photovoltaic. In this work, we demonstrate the first largely enhanced performance of n-CdS/p-Cu(2)S coaxial nanowire photovoltaic (PV) devices using the piezo-phototronics effect when the PV device is subjected to an external strain. Piezo-phototronics effect could control the electron-hole pair generation, transport, separation, and/or recombination, thus enhanced the performance of the PV devices by as high as 70%. This effect offers a new concept for improving solar energy conversation efficiency by designing the orientation of the nanowires and the strain to be purposely introduced in the packaging of the solar cells. This study shed light on the enhanced flexible solar cells for applications in self-powered technology, environmental monitoring, and even defensive technology.     

Synthesis of GaPO4-GaN Coaxial Nanowires

GaPO₄-GaN coaxial nanowires were synthesized by two-step chemical vapor deposition method using H₂ and NH₃ as reactant gas in turn at 950°C. The morphology and microstructures of the GaPO4-GaN coaxial nanowires were studied by scanning elctron microscopy (SEM), X-ray diffraction (XRD) and transmission lectron microscopy (TEM). The nanowires have an average diameter of ~15 nm and length of hundreds of anometers. The core is GaPO₄ crystal and the outer shell is GaN crystal. The formation mechanism was iscussed and the key factors controlling the growth are temperature and the concentration of reactant gases. hese coaxial nanowires may have potential application for piezoluminescence nano-devices, and the two-step ynthetic technique could be used to grow rationally other 1D GaN-based nanowire heterostructures.     

Spatial carrier confinement in core-shell and multishell nanowire heterostructures

We have derived an analytical effective-mass model and employed first-principles density functional theory to study the spatial confinement of carriers in core-shell and multishell structured semiconductor nanowires. The band offset effect is analyzed based on the subband charge density distributions, which is strongly dependent upon the strain relaxation. First-principles calculation results for spatially confined Si/Ge and GaN/GaP nanowires indicate accumulation of a Ge-core hole gas and a GaN-core electron gas, respectively, in agreement with experimental observations.     

Structure and photoluminescence properties of ZnS nanowires sheathed with SnO2 by atomic layer deposition

Influence of the thermal annealing atmosphere on the photoluminescence properties of ZnS-core/SnO₂-shell coaxial nanowires was investigated. ZnS nanowires were synthesized by a two-step process: the thermal evaporation of ZnS powders and the atomic layer deposition of SnO₂. Transmission electron microscopy and X-ray diffraction analyses reveal that two crystalline ZnS phases: one with a zinc blende structure and the other with an wurtzite structure coexist in the cores whereas the SnO₂ cores in the as-prepared coaxial nanowires are amorphous. The SnO₂ shells are found to be crystallized by thermal annealing. Photoluminescence (PL) measurements at room temperature show that the green emission of the ZnS/SnO₂ coaxial nanowires is enhanced in intensity by thermal annealing regardless of the annealing atmosphere. The PL emission is more significantly enhanced in intensity by annealing in a reducing atmosphere than in an oxidative atmosphere since Au_Zn ⁻ is more easily generated in the ZnS cores in the former atmosphere.     

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.     

Fabrication and optical emission of TiO2-sheathed ZnO nanowires

The ZnO nanowires were synthesized by using vapor-liquid-solid mechanism and then the ZnO nanowires were sheathed with TiO2 by metal organic chemical vapor deposition. The coaxial nanowires were 30-200 nm in diameter and up to 0.2 microm in length. Transmission electron microscopy and X-ray diffraction analysis results showed that the ZnO cores and TiO2 shells of the core-shell nanowires had wurtzite and amorphous structures, respectively. Photoluminescence measurement showed that TiO2 coating increased and decreased the near-band edge (NBE) and deep-level emissions of the ZnO nanowires in intensity, respectively. However, it appeared that subsequent annealing was undesirable since it decreased the NBE emission in intensity.     

Surface roughening of an aluminum 6016 alloy during bending and hemming

Hang-on parts of modern passenger cars such as doors and hoods are mainly manufactured by a two-step hemming process during assembly, whereby the edge of the already deep-drawn, blanked and flanged outer panel is further bent to a hem. This hem ties together outer and inner panel of hang-on part. Owing to the bending processes during hemming, large amounts of strain occur on the outer side of the hemming rope, causing local roughening of the formed sheet surface, which may become critical in terms of the quality of the coated component. In this study, the surface roughening during bending and hemming of a typical aluminum sheet material for hang-on parts is examined. In a first step, the roughening process is determined as a function of the bending angle and plastic strain during bending and hemming. Then hemming was further investigated to describe surface roughening as a function of the flanging radius and pre-strain level. The results of the plate bending test and hemming test at identical bending angles and plastic strain levels showed different levels of surface roughening, caused by the non-congruent size and local position of the forming zone in both bending methods.     

Interface structure of deposited GaSb on GaAs (001): Monte Carlo simulation and experimental study

The growth of GaSb thin films by MBE on GaAs (001) is investigated experimentally, using TEM, and theoretically, using KMC simulations. The atomic scale mechanisms inherent to the growth are discussed and described in the KMC model in which the strain is introduced through an elastic energy term based on a valence force field approximation. We observe that the first two monolayers of the deposited films form strained three-dimensional clusters, but further deposition induces film relaxation and rough 3D growth with valley formation presenting (111) facets with unstable bottoms. We show that the roughening morphology and creation of grooves during growth are in agreement with experimental TEM observations.     

Thermal stability and surface passivation of Ge nanowires coated by epitaxial SiGe shells

Epitaxial growth of a highly strained, coherent SiGe alloy shell around a Ge nanowire core is investigated as a method to achieve surface passivation and carrier confinement, important in realizing nanowire devices. The high photoluminescence intensity observed from the core-shell nanowires with spectral features similar to that of bulk Ge indicates effective surface passivation. Thermal stability of these core-shell heterostructures has been systematically investigated, with a method demonstrated to avoid misfit strain relaxation during postgrowth annealing.     

Anisotropy effect on strain-induced instability during growth of heteroepitaxial films

The use of misfit strain to improve the electronic performance of semiconductor films is a common strategy in modern electronic and photonic device fabrication. However, pursuing a favorable higher strain could lead to mechanical instability, on which systematic and quantitative understandings are yet to be achieved. In this paper, we investigate the anisotropy effects on strain-induced thin-film surface roughening by phase field modeling coupled with elasticity. We find that compared with films grown along {111} and {100} surfaces, the instability of {110} film occurs at a much lower strain. Our simulations capture the evolution of interface morphology and stress distribution during the roughening process. Similar characterizations are performed for heteroepitaxial growth from a surface pit. Finally, from 3D simulations, we show that the surface roughening pattern on {110} film exhibits a clear in-plane orientation preference, consistent with experimental observations.     

Surface stress effects on the critical buckling strains of silicon nanowires

The objective of this paper is to quantify how nanoscale surface stresses impact the critical buckling strains of silicon nanowires. These insights are gained by using nonlinear finite element calculations based upon a multiscale, finite deformation constitutive model that incorporates nanoscale surface stress and surface elastic effects to study the buckling behavior of silicon nanowires that have cross sectional dimensions between 10 and 25 nm under axial compressive loading. The key finding is that, in contrast to existing surface elasticity solutions, the critical buckling strains are found to show little deviation from the classical bulk Euler solution. The present results suggest that accounting for axial strain relaxation due to surface stresses may be necessary to improve the accuracy and predictive capability of analytic linear surface elastic theories.     

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.     

Grazing incidence reciprocal space mapping of partially relaxed SiGe films

Epitaxially grown lattice mismatched semiconductor structures are increasingly important for microelectronic and optoelectronic applications. Recently, a great deal of research has been carried out on strain relaxation mechanisms in lattice mismatched epitaxial films. Here, we describe triple-axis x-ray diffraction measurements that were performed to study strain relaxation mechanisms and dislocation formation in Si_1–x Ge _x alloys grown on (0 0 1) Si substrates. At low growth temperature (T_g 600°C) and small lattice mismatch (>2%), two different mechanisms of strain relaxation are observed, depending on the growth temperature and the magnitude of the strain. At Higher growth temperatures or larger lattice mismatch, strain relaxation occurs initially by surface roughening. Subsequently, 60° misfit dislocations nucleate in regions of high strain. At smaller lattice mismatch or lower growth temperature, the surface of the film does not roughen and the 60° misfit dislocations are formed primarily by Frank–Read multiplication. Triple-axis x-ray diffraction reciprocal space maps taken at grazing incidence on very thin epitaxial films can easily distinguish between these two mechanisms. Here, the lattice planes perpendicular to the interface are measured, whereas conventional diffractometry looks either at the planes parallel to the wafer surface or at planes having components both parallel and perpendicular to the surface. In the grazing incidence geometry, thickness broadening of the x-ray peak is eliminated, since the film is essentially infinitely thick parallel to the surface.     

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.     

Molecular dynamics evaluation of strain rate and size effects on mechanical properties of FCC nickel nanowires

Based on molecular dynamics method, an atomistic simulation scheme for damage evolution and failure process of nickel nanowires is presented, in which the inter-atomic interactions are represented by employing the modified embedded atom potential. Extremely high strain rate effect on the mechanical properties of nickel nanowires with different cross-sectional sizes is investigated. The stress–strain curves of nickel nanowires at different strain rates subjected to uniaxial tension are simulated. The elastic modulus, yield strength and fracture strength of nanowires at different loading cases are obtained, and the effect of strain rate on these mechanical properties is analyzed. The numerical results show that the stress–strain curve of metallic nanowires under tensile loading has the trend identical to that of routine polycrystalline metals, and the yield strain of nanowires is independent of the strain rate and cross-sectional size. Based on the simulation results, a set of quantitative prediction formulas are obtained to describe the strain rate sensitivity of nickel nanowires on the mechanical properties, and the resulting formulas of the Young’s modulus, yield strength and fracture strength of nickel nanowires exhibit a linear relation with respect to the logarithm of strain rate. Furthermore, some comprehensive correlation equations revealing both the strain rate and size effects on mechanical properties of nickel nanowire are proposed through the numerical fitting and regression analysis, and the mechanical behaviors observed in this study are consistent with those from the experimental and available numerical results.     

Controlling the abruptness of axial heterojunctions in III-V nanowires: beyond the reservoir effect

Heterostructure nanowires have many potential applications due to the avoidance of interface defects by lateral strain relaxation. However, most heterostructure semiconductor nanowires suffer from persistent interface compositional grading, normally attributed to the dissolution of growth species in the common alloy seed particles. Although progress has been made for some material systems, most binary material combinations remain problematic due to the interaction of growth species in the alloy. In this work we investigate the formation of interfaces in InAs-GaAs heterostructures experimentally and theoretically and demonstrate a technique to attain substantially sharper interfaces. We show that by pulsing the Ga source during heterojunction formation, In is pushed out before GaAs growth initiates, greatly reducing In carry-over. This procedure will be directly applicable to any nanowire system with finite nonideal solubility of growth species in the alloy seed particle and greatly improve the applicability of these structures in future devices.     

Synthesis and characterization of highly ordered cobalt-magnetite nanocable arrays

Magnetically tunable, high-density arrays of coaxial nanocables within anodic aluminum oxide (AAO) membranes have been synthesized. The nanocables consist of magnetite nanowires surrounded by cobalt nanotube sheaths and cobalt nanowires surrounded by magnetite nanotube sheaths. These materials are a combination of separate hard (Co) and soft (Fe3O4) magnetic materials in a single nanocable structure. The combination of two or more magnetic materials in such a radial structure is seen as a very powerful tool for the future fabrication of magnetoresistive, spin-valve and ultrafast spin-injection devices with nonplanar geometries. The nanocable arrays were prepared using a supercritical-fluid inclusion process, whereby the nanotube was first deposited onto the pore walls of the nanoporous membranes and subsequently filled with core material to form coaxial nanocables. In essence, this paper describes a technique for placing novel magnetic technologies into well-defined building blocks that may ultimately lead to new multifunctional devices, such as spin valves and high-density magnetic storage devices.     

Crystalline structures and misfit strain inside Er silicide nanocrystals self-assembled on Si(001) substrates

The morphology and crystalline structure of Er silicide nanocrystals self-assembled on the Si(001) substrate were investigated using scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). It was found that the nanowires and nanorods formed at 630?°C has dominant hexagonal AlB(2)-type structure, while inside the nanoislands self-organized at 800?°C the tetragonal ThSi(2)-type structure is prevalent. The lattice analysis via cross-sectional high-resolution TEM demonstrated that internal misfit strain plays an important role in controlling the growth of nanocrystals. With the relaxation of strain, the nanoislands could evolve from a pyramid-like shape into a truncated-hut-like shape.