Solid Nanowires Research Articles

Anomalous solid-like necking of confined water outflow in hydrophobic nanopores

<h2>Summary</h2> Intrusion of liquid into hydrophobic nanopores has yielded unusual fluid and transport properties that classical theories cannot fully describe, and its extrusion yet remains largely mysterious. Here, we report that the outflow of confined water from hydrophobic nanopores exhibits an anomalous necking phenomenon followed by its breaks, similar to that of solid metal nanowires under uniaxial tension. Molecular dynamics simulations reveal that this solid-like behavior attributes to strong cohesions of water molecules and nanoporous confinements associated with hydrogen-bonding networks and dipole alignments. The extracted elastic modulus and failure strength are quantitatively described by establishing solid-like mechanics scaling laws of outflow deformation and strength. Quasistatic experiments on hydrophobic nanopores/water systems were performed and confirmed the scaling laws of both outflow deformation and strength in remarkable agreement with theoretical predictions. This study offers a facile route for probing mechanical and structural properties of nanoconfined liquid by leveraging its unprecedented solid-like outflow behavior.

Bottom-up nanoscale patterning and selective deposition on silicon nanowires

We demonstrate a bottom-up process for programming the deposition of coaxial thin films aligned to the underlying dopant profile of semiconductor nanowires. Our process synergistically combines three distinct methods—vapor–liquid–solid nanowire growth, selective coaxial lithography via etching of surfaces (SCALES), and area-selective atomic layer deposition (AS-ALD)—into a cohesive whole. Here, we study ZrO2 on Si nanowires as a model system. Si nanowires are first grown with an axially modulated n-Si/i-Si dopant profile. SCALES then yields coaxial poly(methyl methacrylate) (PMMA) masks on the n-Si regions. Subsequent AS-ALD of ZrO2 occurs on the exposed i-Si regions and not on those masked by PMMA. We show the spatial relationship between nanowire dopant profile, PMMA masks, and ZrO2 films, confirming the programmability of the process. The nanoscale resolution of our process coupled with the plethora of available AS-ALD chemistries promises a range of future opportunities to generate structurally complex nanoscale materials and electronic devices using entirely bottom-up methods.

1D NiHPO4 nanotubes prepared using dissolution equilibrium as bifunctional electrocatalyst for high-efficiency water splitting

In this work, one-dimensional NiHPO 4 nanotubes are successfully fabricated on nickel foam by hydrothermal reaction, in which a dissolution equilibrium between phosphates is controlled by tuning the proportion of the mixed solvent and amounts of KOH. As the dissolution equilibrium is broken, the morphology of NiHPO 4 transfers from solid nanowires to hollow nanotubes. The resulting 1D NiHPO 4 nanotubes exhibit good electrocatalytic activity and stability in oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Notably, a water-splitting voltage of 1.62 V at a current density of 10 mA cm −2 is obtained in an electrolyzer setup assembled using 1D NiHPO 4 nanotubes as cathode and anode, demonstrating NiHPO 4 nanotubes are promising catalysts for overall water splitting. Moreover, the revealed mechanism of forming tube morphology can be extended to fabricate other metal phosphates with hollow structures. • Ni foam-supported NiHPO 4 nanotubes catalyst is prepared without any template. • The mechanism of evolving NiHPO 4 nanotubes based on dissolution equilibrium is proposed. • NiHPO 4 nanotubes shows good bifunctional performance for overall water solitting • The present work provides a strategy to explore efficient electrocatalysts.

LixCu alloy nanowires nested in Ni foam for highly stable Li metal composite anode

Porous metal architectures are widely adopted as three-dimensional conducting scaffolds for constructing Li metal composite anodes, whereas their macropores hinder their practical application due to limited surface area and large pore size of few hundred micrometers. In this work, a network of LixCu solid solution alloy nanowires is in situ formed via infiltrating molten Li-Cu alloy into Ni foam and subsequent cooling treatment, whereby a three-component composite anode consisting of Li metal, LixCu alloy, and Ni foam is fabricated. The LixCu nanowires nested as secondary frame split the macropores into micropores, enlarging the active surface area and inducing uniform Li deposition significantly. The lithiophilicity of the alloy wires and the shrunken void size built by the hierarchical architecture can further tune the nucleation and growth behavior of Li. The multiscale synergetic effect between the primary and secondary scaffold guarantees the composite anode sheet with extraordinarily long-term cycling stability even under high current rates.

Micromagnetic modeling of magnetic domain walls in curved cylindrical nanotubes and nanowires

We investigate the effect of curvature on the energy and stability of domain wall configurations in curved cylindrical nanotubes and nanowires. We use micromagnetic simulations to calculate the phase diagram for the transverse wall (TW) and vortex wall (VW) states in tubes, finding the lower energy configuration and the metastability region where both types of walls can exist. The introduction of curvature shifts the range for which the TW is the ground state domain wall to higher diameters and increases the range of metastability. We interpret this behavior to be primarily due to the curvature-induced effective Dzyaloshinskii–Moriya term in the exchange energy. Furthermore, we demonstrate qualitatively the same behavior in solid cylindrical nanowires. Comparing both tubes and wires, we observe how while in tubes curvature tends to suppress the transformation from the TW to VW, in wires it promotes the transformation of the VW containing the Bloch point into the TW. These findings have important implications in the fundamental understanding of domain walls in 3D geometries and the design of future domain wall devices.

Controlling solid–liquid–solid GeSn nanowire growth modes by changing deposition sequences of a-Ge:H layer and SnO2 nanoparticles

Alloying Ge with Sn is one of the promising ways for achieving Si compatible optoelectronics. Here, GeSn nanowires (NWs) are realized via nano-crystallization of a hydrogenated amorphous Ge (a-Ge:H) layer with the help of metal Sn droplets. The full process consists of three steps: (1) SnO2 nanoparticle (NP) reduction in a hydrogen plasma to produce Sn catalyst; (2) a-Ge:H deposition at 120 °C and (3) annealing. GeSn alloys with rich morphologies such as discrete nanocrystals (NCs), random, and straight NWs were successfully synthesized by changing process conditions. We show that annealing under Ar plasma favors the elaboration of straight GeSn NWs in contrast to the conventional random GeSn NWs obtained when annealing is performed under a H2 atmosphere. Interestingly, GeSn in the form of discrete NCs can be fabricated during the deposition of a-Ge:H at 180 °C. Even more, the synthesis of out-of-plane GeSn NWs has been demonstrated by reversing the deposition sequence of SnO2 NPs and a-Ge:H layer.

Influence of the size effect on the generation and transport of charge carriers of phthalocyanines

In this work, on the surface of glass substrates with an ITO coating, solid phthalocyanine films with grain sizes of 40–50 nm were obtained by thermal evaporation in a vacuum, and nanowires with a cross-sectional area of ~ 5–7 nm and a thickness of 100 nm were synthesized by the method of gradient-temperature vapor deposition. The absorption spectra were measured in the visible range of the spectrum. It was found that the observed broadening of the bands and the shift of the peaks in the Q- and B-ranges in the absorption spectra of phthalocyanine nanowires are associated with the fact that the molecules in the nanowires are more densely packed compared to the films obtained by thermal evaporation, as a result of which their optical properties are complicated by intermolecular interactions. ITO/MPc/Al cells were prepared for photoelectrophysical measurements. The I–V characteristics of solid films and nanowires are measured. It is shown that in nanowires there is an increase in the maximum value of the I–V characteristic of phthalocyanines by 1.5 times and the open-circuit voltage by 0.1 V. The impedance spectra are measured. It was found that in nanowires, as compared to solid films, an increase (more than 10 times) in the effective mean free path of the charge, a decrease in the recombination resistance (by almost two orders of magnitude), and an increase in conductivity are observed. The results obtained can be used to improve the electrical transport characteristics of composite solar cells, organic light-emitting devices, and to develop sensitive elements for photoelectronic devices.

The surface effects on solid and hollow nanowires under diametral loading

The size effects of energetic surfaces on the elastic behavior of solid and hollow nanowires are investigated under diametral loading for the first time. In order to model the diametral compression test on nanowires, the structural version of the locally-exact theory is developed for solid and hollow nanowires (LETNW) by considering energetic surfaces attached at the boundaries of bulk materials and modeled using the Gurtin-Murdoch model. The developed analytical solutions employ Fourier series representations of displacement fields in the cylindrical coordinate system that exactly satisfy the governing differential equations, facilitating rapid convergence of full-field stress distributions. The solution's accuracy is validated against the in-house finite-element (FE) program which is currently the computational standard, where excellent agreement is obtained. The surface effects on the local stresses are then illustrated by varying the surface parameters, loading angle, dimension of nanowires, as well as residual surface stress. The efficiency of the LETNW is attributed to its analytical nature and thus consumes much less execution time relative to the FE implementation. What's more, instabilities associated with the solution of stress fields are observed with surface properties in certain crystallographic orientations for both LETNW and FE approaches. Contrary to the LETNW technique, the bifurcation phenomenon is shown to depend on the finite-element mesh refinement. The locally-exact theory provides an alternative and independent way of identifying surface bifurcations in solid and hollow nanowires and hence is a good tool in assessing the finite-element method as well as other approaches’ predictive capability for this class of materials.

Elastocapillary Force Induced Alignment of Large Area Planar Nanowires.

Achieving large scale precise positioning of the vapor-liquid-solid (VLS) nanowires is one of the biggest challenges for mass production of nanowire-based devices. Although there have been many noteworthy progresses in postgrowth nanowire alignment method development over the past few decades, these methods are mostly suitable for low density applications only. For high density applications such as transistors, both high yield and density are required. Here, we report an elastocapillary force-induced nanowire-aligning method that is extremely simple, clean, and can achieve single/multiple nanowire arrays with up to 98.8% yield and submicron pitch between the nanowires.

High-performance and broadband photodetection of bicrystalline (GaN)1-x(ZnO)x solid solution nanowires via crystal defect engineering

Crystal defect engineering is widely used as an effective approach to regulate the optical and optoelectronic properties of semiconductor nanostructures. However, photogenerated electron-hole pair recombination centers caused by structural defects usually lead to the reduction of optoelectronic performance. In this work, a high-performance photodetector based on (GaN)1-x(ZnO)x solid solution nanowire with bicrystal structure is fabricated and it shows excellent photoresponse to ultraviolet and visible light. The highest responsivity of the photodetector is as high as 60, 86 and 43 A/W under the irradiation of 365 nm, 532 nm and 650 nm, respectively. The corresponding response time is as fast as 170, 320 and 160 ms. Such wide spectral responses can be attributed to various intermediate energy levels induced by the introduction of various structural defects and dopants in the solid solution nanowire. Moreover, the peculiar bicrystal boundary along the axial direction of the nanowire provides two parallel and fast transmission channels for photo-generated carriers, reducing the recombination of photo-generated carriers. Our findings provide a valued example using crystal defect engineering to broaden the photoresponse range and improve the photodetector performance and thus can be extended to other material systems for various optoelectronic applications.

Direct Synthesis of Oxynitride Nanowires through Atmospheric Pressure Chemical Vapor Deposition.

Binary and ternary oxynitride solid alloys were studied extensively in the past decade due to their wide spectrum of applications, as well as their peculiar characteristics when compared to their bulk counterparts. Direct bottom-up synthesis of one-dimensional oxynitrides through solution-based routes cannot be realized because nitridation strategies are limited to high-temperature solid-state ammonolysis. Further, the facile fabrication of oxynitride thin films through vapor phase strategies has remained extremely challenging due to the low vapor pressure of gaseous building blocks at atmospheric pressure. Here, we present a direct and scalable catalytic vapor–liquid–solid epitaxy (VLSE) route for the fabrication of oxynitride solid solution nanowires from their oxide precursors through enhancing the local mass transfer flux of vapor deposition. For the model oxynitride material, we investigated the fabrication of gallium nitride and zinc oxide oxynitride solid solution (GaN:ZnO) thin film. GaN:ZnO nanowires were synthesized directly at atmospheric pressure, unlike the methods reported in the literature, which involved multiple-step processing and/or vacuum operating conditions. Moreover, the dimensions (i.e., diameters and length) of the synthesized nanowires were tailored within a wide range.

Solving the inverse Knudsen problem: Gas diffusion in random fibrous media

About a century ago, Knudsen derived the groundbreaking theory of gas diffusion through straight pipes and holes, which since then found widespread application in innumerable fields of science and inspired the development of vacuum and related coating technologies, from academic research to numerous industrial sectors. Knudsen's theory can be straightforwardly applied to filter membranes with arrays of extended holes for example, however, for the inverse geometry arrangement, which arises when solid nanowires or fibers are arranged into porous interwoven material (like in carpets or brushes) the derivation of an analytical theory framework was still missing. In this paper, we have identified the specific geometric and thermodynamic parameters that determine the gas diffusion kinetics in arrays of randomly oriented cylinders and provide a set of analytical expressions allowing to comprehensively describe the gas transport in such structures. We confirmed analytical solutions by Monte Carlo simulations. We specify our findings for an atomic layer deposition process, the diffusion of trimethyaluminium molecules into a carbon nanotube array, but highlight the applicability of our derivation for other fields comprising gas diffusion membranes, composite materials, fuel cells and more.

Unique Growth Pathway in Solution-Solid-Solid Nanowires: Cubic to Hexagonal Phase Transformation.

Solution–solid–solid (SSS) nanowires can be catalyzed by superionic Ag2S via ion diffusion. Here, we synthesize ZnS nanowires of the wurtzite crystal structure and heterostructures via a low-temperature growth pathway. Single-crystalline ZnS nanowires were produced by varying reaction time and temperature (120–200 °C) via thermal decomposition of a single-source precursor, Zn(DDTC)2. A phase transformation (zinc blende → wurtzite) was observed during the synthesis with a three-step growth pathway proposed. Temperature-controlled phase transformation facilitates oriented attachment into a 1D nanowire, followed by helical epitaxial and lateral growths during ripening. Additionally, the CdS–ZnS heterostructured nanowires can be obtained after introducing the Cd(DDTC)2 precursor. ZnS nanowires of defined diameters (5–10 nm) are served as backbones to grow heterostructures of ternary semiconductors with multicolor photoluminescence (450–800 nm). Structural and optical characterizations (PL, 2D PLE, and TCSPC) are investigated to confirm origins of broadband emission from multiple lifetimes (0.5–12 ns) for exciton recombination in heterostructures. Our study demonstrates this unique growth pathway for SSS nanowire synthesis under mild, facile, and atmospheric conditions.

Oscillations of As Concentration and Electron-to-Hole Ratio in Si-Doped GaAs Nanowires

III–V nanowires grown by the vapor–liquid–solid method often show self-regulated oscillations of group V concentration in a catalyst droplet over the monolayer growth cycle. We investigate theoretically how this effect influences the electron-to-hole ratio in Si-doped GaAs nanowires. Several factors influencing the As depletion in the vapor–liquid–solid nanowire growth are considered, including the time-scale separation between the steps of island growth and refill, the “stopping effect” at very low As concentrations, and the maximum As concentration at nucleation and desorption. It is shown that the As depletion effect is stronger for slower nanowire elongation rates and faster for island growth relative to refill. Larger concentration oscillations suppress the electron-to-hole ratio and substantially enhance the tendency for the p-type Si doping of GaAs nanowires, which is a typical picture in molecular beam epitaxy. The oscillations become weaker and may finally disappear in vapor deposition techniques such as hydride vapor phase epitaxy, where the n-type Si doping of GaAs nanowires is more easily achievable.

Effect of Arsenic Depletion on the Silicon Doping of Vapor–Liquid–Solid GaAs Nanowires

It is well known that chemical potential driving the vapor–liquid–solid growth of nanowires oscillates in synchronization with the monolayer growth. In III–V nanowires, this occurs due to depletion of group V atoms in a catalyst droplet. The amphoteric behavior of silicon doping, which often changes from n‐type in planar GaAs layers to p‐type in nanowires, is attributed to low arsenic concentrations. Herein, we present an analytical model which quantifies the doping oscillations over the monolayer formation cycle, and its impact on the electron‐to‐hole ratio for the silicon doping of GaAs nanowires. It is shown that arsenic depletion can easily double the amphoteric effect and strongly favor the tendency for p‐type doping. On a more general ground, the nanowire doping process appears highly sensitive to the chemical potential oscillations related to a restricted amount of material in a nanoscale catalyst.

Development of Growth Theory for Vapor–Liquid–Solid Nanowires: Wetting Scenario, Front Curvature, Growth Angle, Linear Tension, and Radial Instability

In this paper, we report that under wetting conditions (or modes) of nanowire (NW) growth, when a nonplanar crystallization front emerges under a catalyst droplet, a shift in the three-phase line (TPL) of the vapor–liquid–crystal interface occurs under thermodynamically stable conditions when the angle with respect to the droplet surface, termed the growth angle, is fixed. The growth angle of the NWs is determined not from a geometrical perspective but on the basis of the physical aspects of the processes occurring around the TPL, revealing a size dependence caused by the influence of linear tension of the three-phase contact of a vapor–liquid crystal. The observed radial periodic instability of the NWs is described according to the size dependence of the thermodynamic growth angle, which induces negative feedback in the system. Under the influence of linear tension and positive feedback, the tips or needles of NWs can be formed.

Effect of Temperature on the Morphology of Planar GaAs Nanowires (Simulation)

The self-catalyzed growth of planar GaAs nanowires is analyzed using the lattice kinetic Monte Carlo model. Vapor–liquid–solid nanowire growth on (111)A and (111)B substrates is investigated. The effect of temperature and the location of gallium droplets on the morphology and growth direction of planar GaAs nanowires is examined. The temperature range of the stable growth of planar GaAs nanowires on the GaAs(111)A surface is revealed. Special asymmetric arrangement of the droplets is shown to ensure unidirectional nanowire growth.

Synthesis and characterization of PbS nanowires doped with Tb3+ ions by using chemical bath deposition method

Crystalline lead sulfide (PbS) nanowires doped with terbium (Tb3+) ions were synthesized by the chemical bath deposition method at room temperature. The powder was obtained from an aqueous solutions using lead acetate dehydrate, terbium nitrate, thiourea, potassium hydroxide and ammonia. The terbium molar concentrations were varied in the deposition process to investigate the effect on the structural, optical, morphological and luminescent properties of PbS nanowires. The crystalline size was found to be dependent on the concentration of the Tb3+ ions used. The estimated average crystalline sizes were calculated from the X-ray diffraction and found to be 34, 33 and 37 nm for PbS: 0% Tb3+, PbS: 0.2% Tb3+ and PbS: 0.5% Tb3+, respectively. The scanning electron microscopy micrographs depict nanowire shape for the undoped as well as Tb-doped samples. The energy-dispersive X-ray and Auger electron spectroscopy analyses confirmed the presence of all the expected elements. The solid powder nanowires exhibited high absorptions in the UV–Vis regions. The band gap energies were estimated in the range of 1.99–2.46 eV. The absorption edge and the band gap energies of these PbS nanowires have shifted depending on the concentration of the dopant. The maximum luminescence intensity was obtained for PbS: 0.2% Tb3+ ions and luminescent quenching was observed for higher terbium concentrations.Graphic abstract

Iridium‐Chromium Oxide Nanowires as Highly Performed OER Catalysts in Acidic Media

AbstractThe oxygen evolution reaction (OER) severely restricts the efficiency of electrolyzers due to its sluggish kinetics. In particular, only a few OER electrocatalysts, such as expensive and scarce iridium (Ir)‐based materials, have been developed to function in acidic media. Therefore, the exploration of efficient and stable electrocatalysts with decreased precious metal content for the OER in acid is desired. Herein, chromium (Cr)‐incorporated IrOx solid solution nanowires not only exhibit high catalytic activity with a low overpotential of 250 mV at 10 mA cm−2, but also display robust catalytic performance without a decay in activity throughout 25 h of testing at 10 mA cm−2 in 0.5 M H2SO4. This improved catalytic performance for the Cr‐incorporated IrOx electrocatalysts is ascribed to an abundance of exposed active sites, tuned electronic states, optimized binding energy for intermediates on the catalyst surface and the consequent facile reaction kinetics.

Synthesis of silicon carbide nanomaterials by microwave heating: Effect of types of carbon nanotubes

One-dimensional silicon carbide nanomaterial (SiCNM) is the leading potential material for high temperature, high power and harsh environment components and devices. This is due to the outstanding properties of the one-dimensional SiCNMs such as high mechanical properties, high hardness, good chemical inertness and excellent electronic properties. In this paper, we reported the successful synthesis of one-dimensional SiCNMs from blend of SiO2 particles with two types of CNTs, namely MWCNTs and SWCNTs by using microwave heating and the effect of types of CNTs on the synthesis of one-dimensional SiCNMs. The result of x-ray diffraction, field emission scanning electron microscopy, high resolution transmission electron microscopy, energy dispersive x-ray spectroscopy, photoluminescence spectroscopy, Fourier transform infrared spectroscopy and thermo-gravimetric analysis revealed that high purity of β-SiC nanotube was obtained from blend of SiO2 particles and MWCNTs while solid SiC nanowire was synthesized from blend of SiO2 particles and SWCNTs and associated with the presence of residual of unreacted SiO2 particles. This clearly shows that types of one dimensional SiCNMs (hollow or solid) can be controlled by using different types of CNTs and thus this study proposed a high efficiency microwave heating method for the synthesis of one-dimensional SiCNMs with controllable morphology.