Amorphous Nanowires Research Articles

Thermomechanical Behavior of Molded Metallic Glass Nanowires.

Metallic glasses are disordered materials that offer the unique ability to perform thermoplastic forming operations at low thermal budget while preserving excellent mechanical properties such as high strength, large elastic strain limits, and wear resistance owing to the metallic nature of bonding and lack of internal defects. Interest in molding micro- and nanoscale metallic glass objects is driven by the promise of robust and high performance micro- and nanoelectromechanical systems and miniature energy conversion devices. Yet accurate and efficient processing of these materials hinges on a robust understanding of their thermomechanical behavior. Here, we combine large-scale thermoplastic tensile deformation of collections of Pt-based amorphous nanowires with quantitative thermomechanical studies of individual nanowires in creep-like conditions to demonstrate that superplastic-like flow persists to small length scales. Systematic studies as a function of temperature, strain-rate, and applied stress reveal the transition from Newtonian to non-Newtonian flow to be ubiquitous across the investigated length scales. However, we provide evidence that nanoscale specimens sustain greater free volume generation at elevated temperatures resulting in a flow transition at higher strain-rates than their bulk counterparts. Our results provide guidance for the design of thermoplastic processing methods and methods for verifying the flow response at the nanoscale.

Electrochemically synthesized amorphous and crystalline nanowires: dissimilar nanomechanical behavior in comparison with homologous flat films.

The effects of constrained sample dimensions on the mechanical behavior of crystalline materials have been extensively investigated. However, there is no clear understanding of these effects in nano-sized amorphous samples. Herein, nanoindentation together with finite element simulations are used to compare the properties of crystalline and glassy CoNi(Re)P electrodeposited nanowires (ϕ ≈ 100 nm) with films (3 μm thick) of analogous composition and structure. The results reveal that amorphous nanowires exhibit a larger hardness, lower Young's modulus and higher plasticity index than glassy films. Conversely, the very large hardness and higher Young's modulus of crystalline nanowires are accompanied by a decrease in plasticity with respect to the homologous crystalline films. Remarkably, proper interpretation of the mechanical properties of the nanowires requires taking the curved geometry of the indented surface and sink-in effects into account. These findings are of high relevance for optimizing the performance of new, mechanically-robust, nanoscale materials for increasingly complex miniaturized devices.

Interwoven heterostructural Co3O4–carbon@FeOOH hollow polyhedrons with improved electrochemical performance

This work displays well-defined Co3O4–carbon hollow polyhedrons that can serve as a growth platform for amorphous FeOOH nanowires, forming interwoven hollow polyhedrons that exhibit improved reversibility and high capacity as anode material for lithium ion batteries.

Shape anisotropy in zero-magnetostrictive rapidly solidified amorphous nanowires

The magnetic behavior of zero-magnetostrictive rapidly solidified amorphous nanowires has been investigated in order to understand their magnetic bistability. The study has been performed both experimentally – based on inductive hysteresis loop measurements – and theoretically, by means of micromagnetic simulations. Experimental hysteresis loops have shown that the amorphous nanowires display an axial magnetic bistability, characterized by a single-step magnetization reversal when the applied field reaches a critical value called switching field. The simulated loops allowed us to understand the effect of shape anisotropy on coercivity. The results are key for understanding and controlling the magnetization processes in these novel nanowires, with important application possibilities in new miniaturized sensing devices.

Defect-related photoluminescence properties of as-synthesized and annealed NiO nanostructures via hydrothermal method

A hydrothermal-based synthesis technique was used to produce both amorphous and nano-grained NiO nanowires. The scanning electron microscopy images showed the morphologies before and after annealing treatment at 350 and 400°C for 2h. The microstructural features of the mixed amorphous and local crystalline in as-synthesized NiO and polycrystalline in annealed NiO were characterized by X-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy, respectively. The corresponding elemental compositions were determined via energy-dispersive X-ray. In addition, annealing-dependent effects on the photoluminescence spectra were observed in at least two different energy band regions, e.g., at ~395, 660, and 680nm. The photoluminescence properties, including intensity and energy, of the individual NiO nanowires varied significantly with microstructural changes arising from nickel and oxygen defects from annealing the as-synthesized amorphous NiO. The microstructural and optical origin of as-synthesized amorphous and annealed nano-grained NiO nanowires is discussed on the basis of the aforementioned results.

Enhanced multiple exciton generation in amorphous silicon nanowires and films

ABSTRACTMultiple exciton generation (MEG) in nanometer-sized hydrogen-passivated silicon nanowires (NWs), and quasi two-dimensional nanofilms depends strongly on the degree of the core structural disorder as shown by the perturbative many-body quantum mechanics calculations based on the density functional theory simulations. Working to the second order in the electron–photon coupling and in the screened Coulomb interaction, we calculate quantum efficiency (QE), the average number of excitons created by a single absorbed photon, in the Si29H36 quantum dots (QDs) with crystalline and amorphous core structures, simple cubic three-dimensional arrays constructed from these QDs, crystalline and amorphous NWs, and quasi two-dimensional silicon nanofilms, also both crystalline and amorphous. Efficient MEG with QE ranging from 1.3 up to 1.8 at the photon energy of about 3Eg, where Eg is the electronic gap, is predicted in these nanoparticles except for the crystalline NW and crystalline film where QE ≃ 1. MEG in the amorphous nanoparticles is enhanced by the electron localisation due to structural disorder. Combined with the lower gaps, the nanometer-sized amorphous silicon NWs and films are predicted to have effective carrier multiplication within the solar spectrum range.

Direct growth, characterization, and optical properties of silica nanowires on cordierite monolith

We report the direct synthesis of amorphous silica nanowires on cordierite monolith. Our approach is based on a simple catalyst-assisted growth using Sn particles that are formed via the mechanochemical reaction of SnO2 (s) with Al (s, l) or Si (s) during the initial phase of the growth process. The silica nanowires have a diameter of approximately 20nm and become densely packed upon repeatedly branching and merging with each other. The silica nanowire assemblies grow up to 50µm in length. The microscopic images reveal that the Sn catalysts govern the growth by controlling the formation of the nuclei on their surface. Remarkably, the Sn catalysts induce the growth of multiple silica nanowire assemblies and the formation of Sn-rich amorphous (Si,Al)Ox nanotube/Al-doped orthorhombic Mg2SiO4 nanofiber as a minor product. The Sn-catalyzed silica nanowires show strong blue emission.

Fabrication of Amorphous Silver Nanowires by Helium Ion Beam Irradiation

Amorphous silver nanowires (a-Ag NWs) are fabricated from crystalline Ag NWs by using 5 MeV helium (He+) ion beam irradiation. At low dose (5 × 1015 ion/cm2), few defects are created in Ag NWs. As dose increases, more damage to the crystalline structure of Ag NWs is observed. Finally at high dose (8 × 1016 ion/cm2), the face-centered cubic structure of Ag NWs is transformed into the amorphous structure with similar morphology as Ag NWs. Phase transformation of crystalline Ag NWs upon irradiation with 5 MeV He+ ions is observed through high resolution transmission electron microscopy. Synthesis of large scale amorphous metal nanowires and metal nanowire alloy systems are discussed.

Optimizing thermoelectric property of antimony telluride nanowires by tailoring composition and crystallinity

In this work, an effect of the composition and microstructure changes on the electrical and thermoelectric properties of antimony telluride (SbxTey) nanowires was investigated to determine the optimum conditions that give the highest Seebeck coefficient. For a systematic study, a facile template-directed electrodeposition technique was utilized to prepare amorphous SbxTey nanowires with a well-controlled dimension and composition. The composition-dependent phase transition behavior of the nanowires, corresponding to the amorphous-to-crystalline solid-state transition, was investigated by measuring their electrical resistivity changes, where the electrical resistivity variation ratio (Δρ/ρ0 = (ρ − ρ0)/ρ0) was highly dependent on the Sb content. Temperature-dependent electrical properties (electrical resistivity, thermal activation energy (Ea), and field effect transistor (FET) mobility) of the amorphous and crystalline nanowires were also characterized before/after the phase transition. Material properties of the nanowires, such as crystallinity, grain size, and phase segregation as a function of the Sb content, were tailored by a post-annealing process. The tailored material properties successfully altered the electrical and thermoelectric properties of the nanowires. The highest Seebeck coefficient of 318 μV K−1 at the carrier concentration of 1.1 × 1019 cm−3 was observed in the Te-segregated Sb2Te3 (i.e., Sb32Te68) nanowires.

Wafer-level site-controlled growth of silicon nanowires by Cu pattern dewetting

An approach for the wafer-level synthesis of size- and site-controlled amorphous silicon nanowires (α-SiNWs) is presented in this paper. Microscale Cu pattern arrays are precisely defined on SiO2 films with the help of photolithography and wet etching. Due to dewetting, Cu atoms shrink to the center of patterns during the annealing process, and react with the SiO2 film to open a diffusion channel for Si atoms to the substrate. α-SiNWs finally grow at the center of Cu patterns, and can be tuned by varying critical factors such as Cu pattern volume, SiO2 thickness, and annealing time. This offers a simple way to synthesize and accurately position a SiNW array on a large area.

Amorphization and Directional Crystallization of Metals Confined in Carbon Nanotubes Investigated by in Situ Transmission Electron Microscopy.

The hollow core of a carbon nanotube (CNT) provides a unique opportunity to explore the physics, chemistry, biology, and metallurgy of different materials confined in such nanospace. Here, we investigate the nonequilibrium metallurgical processes taking place inside CNTs by in situ transmission electron microscopy using CNTs as nanoscale resistively heated crucibles having encapsulated metal nanowires/crystals in their channels. Because of nanometer size of the system and intimate contact between the CNTs and confined metals, an efficient heat transfer and high cooling rates (∼10(13) K/s) were achieved as a result of a flash bias pulse followed by system natural quenching, leading to the formation of disordered amorphous-like structures in iron, cobalt, and gold. An intermediate state between crystalline and amorphous phases was discovered, revealing a memory effect of local short-to-medium range order during these phase transitions. Furthermore, subsequent directional crystallization of an amorphous iron nanowire formed by this method was realized under controlled Joule heating. High-density crystalline defects were generated during crystallization due to a confinement effect from the CNT and severe plastic deformation involved.

Tissue-like Silicon Nanowires-Based Three-Dimensional Anodes for High-Capacity Lithium Ion Batteries.

Here, we report on the scalable synthesis and characterization of novel architecture three-dimensional (3D) high-capacity amorphous silicon nanowires (SiNWs)-based anodes with focus on studying their electrochemical degradation mechanisms. We achieved an unprecedented combination of remarkable performance characteristics, high loadings of 3-15 mAh/cm(2), a very low irreversible capacity (10% for the 3-4 mAh/cm(2) anodes), current efficiency greater than 99.5%, cycle stability (both in half cells and a LiFePO4 battery), a total capacity of 457 mAh/cm(2) over 204 cycles and fast charge-discharge rates (up to 2.7C at 20 mA/cm(2)). These SiNWs-based binder-free 3D anodes have been cycled for over 200 cycles, exhibiting a stable cycle life. Notably, it was found that the growth of the continuous SEI layer thickness, and its concomitant increase in resistivity, represents the major reason for the observed capacity loss of the SiNWs-based anodes. Importantly, these NWs-based anodes of novel architecture meet the requirements of lithium batteries for future portable, and electric-vehicle, applications.

Growth of tapered silica nanowires with a shallow U-shaped vapor chamber: Growth mechanism and structural and optical properties

Traditional chemical vapor deposition method modified with a shallow U-shaped vapor chamber has been used to synthesize tapered bamboo shoot-like (BS-like) amorphous SiO2 nanowires (NWs) on Si (100) substrates without catalyst. The key innovation of this approach lies in a creation of swirling flow of the reactant vapors during the growth, which leads to a harvest of tapered silica NWs with lengths up to several microns. The unique structures and corresponding luminescence properties of the BS-like NWs were studied and their relationship with the evaporated active reactants was explored. A thermodynamic model that considers the critical role of the vapor flow during the growth is proposed to understand the structural and optical features. The shallow U-shaped vapor chamber-aided approach may provide a viable way to tailor novel structure of NWs for potential applications in nano-devices.

Synthesis and Photoluminescence of Amorphous Silicon Oxycarbide Nanowires

Synthesis of amorphous SiCO nanowires was carried out by means of direct current arc discharge. Free-standing SiCO nanowires were deposited on the surface of a graphite crucible without any catalyst and template. The SiCO nanowires were analyzed by XRD, SEM, TEM, XPS, and FTIR. The SiCO nanowires were typically 20–100 μm in length and 10–100 nm in diameter as measured by SEM and TEM. The XPS and FTIR spectroscopy analysis confirmed that the Si atoms share bonds with O and C atoms in mixed SiCO units. The PL spectrum of the SiCO nanowires showed strong and stable white emissions at 454 and 540 nm. A plasma-assisted vapor-solid growth mechanism is proposed to be responsible for the formation of the SiCO nanowires.

Origin of high elastic strain in amorphous silica nanowires

An understanding of the origin of elastic strain is extremely important for both crystalline materials and amorphous materials. Owing to the lack of a long range order in their structure, it is arduous to dynamically study the elastic mechanism of amorphous materials experimentally at atomic scale compared with their crystalline counterparts. Here, the elastic deformation mechanism of amorphous silica nanowires (NWs) has been studied for the first time via in situ elastic tensile tests in a transmission electron microscope. Radial distribution functions (RDFs) calculated from the corresponding selected area electron diffraction patterns (SAEDPs) at different strains were used to reconstruct a structural model based on the reverse Monte-Carlo (RMC) method. The result interestingly indicates that the elastic strain of silica glass NWs can be mainly attributed to the elastic elongation of the bond length accompanied by a change in the bond angle distribution. This work is useful for understanding the high strength of amorphous materials.

Synthesis of Gallium-oxide nanoparticles and nanowires by using a thermal plasma

Gallium-oxide (Ga2O3) nanoparticles and nanowires were synthesized using a direct current arc thermal plasma. Gallium nitrate hydrate (Ga(NO3)3·xH2O) was used as a raw material for the synthesis of the Ga2O3 nanomaterials. (Ga(NO3)3·xH2O) was injected into the area of the thermal plasma in a liquid state with two different solvents, distilled water (H2O) and methyl alcohol (CH3OH). The as-synthesized Ga2O3 nanomaterials were characterized according to their crystallinity and morphology. In the cases of both the H2O and CH3OH solvents, the crystalline monoclinic β-Ga2O3 nanomaterials were synthesized in a reaction tube. Amorphous Ga2O3 nanoparticles were collected in a chamber in the case of the H2O solvent whereas amorphous Ga2O3 nanowires were found in the case of the CH3OH solvent. As a result, β-Ga2O3 nanoparticles, amorphous nanoparticles and nanowires were selectively synthesized by using a thermal plasma process, depending on the collection position of the product and the type of mixing solution.

Internal stress and its effect on mechanical strength of metallic glass nanowires

Size induced strength increase in crystalline nanowires is related to crystal defects at small length scales. But how this phenomenon manifests in metallic glasses, an amorphous solid without obvious intrinsic structural defects, still remains an open issue. Here we show that in the amorphous nanowires free of obvious structural defects or cracks, reduction in wire diameter generally leads to weakening of the strength compared to the bulk sample. Our analysis shows that the surface stress as well as surface induced internal stress that scales inversely with the wire diameter become significant. It is the complex interplay between the applied stress and the internal residual stresses induced by the nanoscale surface that affects the mechanical behavior in the amorphous nanowires at the small length scale regime. The role of the localized shear banding played in relation to the size effect is also discussed in the context of the deformation mechanisms in amorphous wires with different sizes.

Modeling the reduction of thermal conductivity in core/shell and diameter-modulated silicon nanowires

The thermal conductivity of a series of structural, diameter, and core/shell modulated silicon nanowires is computed by molecular dynamics simulations using the nonequilibrium method. The purpose of this study is to appraise the impact of surface roughness, amorphous parts integration, and diameter modulation on the thermal conductivity reduction. The addition of both amorphous regions and nanoconstrictions allows us to reach a thermal conductivity close to that of the amorphous pristine nanowires, while preserving a certain degree of crystallinity. This could result in interesting properties for thermoelectric applications.

Origin of magnetic bistability in rapidly solidified (Co0.94Fe0.06)72.5Si12.5B15 nearly zero magnetostrictive amorphous nanowires

Results on the analysis of the magnetoelastic and magnetostatic contributions in the total free energy of nearly zero magnetostrictive amorphous glass-coated nanowires prepared by means of rapid solidification are reported. The comparative study shows that both terms have values of the same order. However, the magnetostatic term is the most important one over a wide range of dimensions, being responsible for the domain structure formation and for the overall magnetic behavior of the investigated nanowires, which mainly consists in the appearance of an axial magnetic bistability. Even in the thinnest samples, the magnetostatic contribution is essential, in spite of an important magnetoelastic one, too. Calculated results agree with the experimental hysteresis loop data. The results are a key for understanding and controlling the magnetization processes in these novel nanowires.

Size effects on the fracture behavior of amorphous silica nanowires

Size effects on deformation behavior and fracture mechanism of amorphous silica nanowires (NWs) were studied by molecular dynamics simulation with reactive force field ReaxFF. We analyzed the evolution of voids during the tension deformation, and focused on the role of voids played in the fracture process of amorphous silica NWs. Our study shows that the smaller diameter NWs damage only from localized necking and the small growth of voids has no effect on its fracture behavior. While for the larger diameter NWs, there exists growth and coalescence of void inside the NWs and crack propagation on the surface of NWs. It is the combined action of these two mechanisms that leads to the final fracture. The evolution of the size of biggest void (BV) displays three distinct regions, which are corresponding to the different deformation stages of NWs. The voids begin to grow significantly after the yielding point, and the size of BV in the fracture stage increases much more quickly owing to the coalescence of voids. Moreover, the critical radius of BV at the start point of fracture stage increases from 2.6Å to 3.8Å with the increase of diameter of amorphous silica NWs.