Straight Nanowires Research Articles

Cu-Ni Bimetallic Nanowires with Various Structures Originating from Ni Reduction Kinetics.

Bimetallic nanowires play important roles in the fields of electronics and mechanics. However, their structure types and morphological control methods are limited, especially for systems with low lattice mismatch. Herein, for a Cu-Ni bimetallic system with lattice mismatch ratio less than 2.5%, a novel preparation approach of various Cu-Ni nanowires dominated by Ni(II) reduction kinetics is presented. With the increase of Ni(II) reduction rate, the core-shell Cu@Ni straight nanowires, the asymmetric Cu-Ni nanocurves, and asymmetric Cu-Ni nanocoils can be prepared, respectively. The formation of Cu-Ni nanowires with different structures can be divided into the growth of Cu nanowires and the deposition of Ni. The regulatory effects were revealed by establishing a kinetic model for Ni(II) reduction. For the novel Cu-Ni asymmetrically distributed nanocurves and nanocoils, the formation mechanism was proposed by considering the Cu nanowire bending due to the rearrangement of surface ligand and bending-induced symmetry breaking of Ni(II) reduction.

Evidence of two-dimensional lateral quantum confinement in self-formed core–shell InGaN nanowires on Si (111) emitting in the red

The proof of strong two-dimensional lateral quantum confinement in the In-rich core of red-light emitting self-formed core–shell InGaN nanowires is given. The nanowires are directly grown on Si (111) by plasma-assisted molecular beam epitaxy. After the initial InGaN nucleation, straight nanowires with quantum-size core radius determined by x-ray diffraction, transmission electron microscopy, and energy dispersive x-ray mappings develop. Detailed comparison of the photoluminescence from the core, the In contents of the core and shell, and the core radius with theoretical modeling reveals a parabolic confinement potential with large ground state quantum confinement energies of electrons and holes. Such strong lateral quantum confinement in a vertical quantum wire active region is ideal for the performance of optoelectronic devices, in particular of our reported red InGaN light emitting diode with high brightness and color stability.

Bending and reverse bending during the fabrication of novel GaAs/(In,Ga)As/GaAs core–shell nanowires monitored by in situ x-ray diffraction

We report on the fabrication of a novel design of GaAs/(In,Ga)As/GaAs radial nanowire heterostructures on a Si 111 substrate, where, for the first time, the growth of inhomogeneous shells on a lattice mismatched core results in straight nanowires instead of bent. Nanowire bending caused by axial tensile strain induced by the (In,Ga)As shell on the GaAs core is reversed by axial compressive strain caused by the GaAs outer shell on the (In,Ga)As shell. Progressive nanowire bending and reverse bending in addition to the axial strain evolution during the two processes are accessed by in situ by x-ray diffraction. The diameter of the core, thicknesses of the shells, as well as the indium concentration and distribution within the (In,Ga)As quantum well are revealed by 2D energy dispersive x-ray spectroscopy using a transmission electron microscope. Shell(s) growth on one side of the core without substrate rotation results in planar-like radial heterostructures in the form of free standing straight nanowires.

Dynamics of chiral domain walls in bent cylindrical magnetic nanowires

Cylindrical magnetic nanowires (NWs) constitute a viable component of 3D nanoscale magnetic devices and engineering their response to external stimuli is necessary for their future functionalization. Here, by means of micromagnetic simulations, we study the dynamical response of vortex–antivortex and Bloch point domain walls under the action of an applied magnetic field in curved arc-shaped NWs varying the saturation magnetization value. Our results provide evidence that, in the range considered in this work, the curvature has no influence on the critical diameters, delimiting different domain wall types. However, it has a relevant effect on the domain wall dynamics. Specifically, the vortex–antivortex domain wall oscillates back and forth while rotating around the nanowire, and the frequency and amplitude can be tuned by curvature and applied field. On the contrary, Bloch point domain wall dynamics does not show any oscillatory behavior, and the domain wall is rapidly expelled from the nanowire with velocities similar to that of the straight cylindrical nanowires. These results allow engineering magnetic response of cylindrical nanowires with curvature.

Patterned Ga2O3 nanowires synthesized by CVD method for High-performance self-powered ultraviolet photodetector

High-performance and energy-efficient ultraviolet photodetectors play a critical role in civilian and military fields. Here, patterned growth of highly crystalline straight Ga2O3 nanowires on p-type GaN epitaxial film wafer substrate is achieved by bottom-up technology using an economical chemical vapor deposition method. A self-powered ultraviolet photodetector based on GaN/Ga2O3 nanowires p-n junction for broadband UV detection is constructed by combining spin coating Poly(methyl methacrylate) as an insulating layer and magnetic sputtering an Au thin film as top transparent conductive electrode. This nanowires-film structure device exhibits excellent performance under zero bias voltage, including considerable responsivity of 960 mA W−1, high detectability of 3.82 × 1013 Jones, ultrahigh ultraviolet/visible inhibition ratio (R254 nm/R400 nm) of 7.92 × 103, fast rise/decay time of 4/12 ms and a high photo-to-dark current ratio greater than 104. The self-powered property under zero bias is attributed to the built-in electric field formed between the p-GaN film and the n-Ga2O3 nanowires, which creates a driving force for the rapid separation of photo-generated carriers. The high-performance mainly credits to the device's excellent UV light utilization efficiency and unique interfacial engineering of the p-n junction. This work provides a new avenue for the application of Ga2O3 nanowires in high-performance p-n junction self-powered photodetectors that can operate in harsh environments where it is inconvenient to transmit electric power.

Large-Area Superconducting Nanowire Single-Photon Detectors for Operation at Wavelengths up to 7.4 μm.

The optimization of superconducting thin-films has pushed the sensitivity of superconducting nanowire single-photon detectors (SNSPDs) to the mid-infrared (mid-IR). Earlier demonstrations have shown that straight tungsten silicide nanowires can achieve unity internal detection efficiency (IDE) up to λ = 10 μm. For a high system detection efficiency (SDE), the active area needs to be increased, but material nonuniformity and nanofabrication-induced constrictions make mid-IR large-area meanders challenging to yield. In this work, we improve the sensitivity of superconducting materials and optimize a high-resolution nanofabrication process to demonstrate large-area SNSPDs with unity IDE at 7.4 μm. Our approach yields large-area meanders down to 50 nm width, with average line-width roughness below 10%, and with a lower impact from constrictions compared to previous demonstrations. Our methods pave the way to high-efficiency SNSPDs in the mid-IR band with potential impacts on astronomy, imaging, and physical chemistry.

Influence of reaction temperature on phase assemblage and morphologies of SiC nanowires by a carbothermal reduction method

3C-SiC nanowires were synthesized using Si, SiO2, and active carbon as raw materials at different reaction temperatures without additional metal catalysts. The influence of reaction temperature on the phase assemblage and morphologies of the products were investigated by XRD and SEM. The experimental results indicate that a suitable reaction temperature is essential for the final products. When the reaction temperature was not high enough (1400 and 1450∘C), the raw materials were not reacted completely, and a small amount of targeted nanowires were formed. When reaction temperature increased to 1500∘C, the nanowires were mainly composed of 3C-SiC phase, and they were straight, curved, and needle-shaped. The straight nanowires ranged from several to tens of microns, but the diameters were not uniform. The vapor-solid mechanism dominantly governed the formation of SiC nanowires.

Improved omnidirectional polarisation‐insensitive optical absorption and photoelectrochemical water splitting using aperiodic and tapered slanted, kinked and straight silicon nanowires

Improved omnidirectional polarisation‐insensitive optical absorption and photoelectrochemical water splitting using aperiodic and tapered slanted, kinked and straight silicon nanowires

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.

Effects of skeleton pore size on the microstructure and electromagnetic absorbing property of the SiC nanowires/SiC composites

SiC nanowires exhibited great electromagnetic absorbing property while decorated in porous ceramic skeletons. However, the ceramic skeletons used in former researches always had multi-mode pore structure, so the effects of skeleton pore size on the properties of SiC nanowires could not be researched independently. In this work, porous SiC ceramics with single-mode pore distribution and controllable pore size were applied to prepare SiC nanowires/SiC composites. While the skeleton pore size was decreased from 13.94 μm to 2.18 μm, the SiC nanowires turned from straight to curved. Compared with the straight nanowires, the curved ones presented high-density stacking faults and large surface area, endowing the composites with superior electromagnetic absorbing property.

The growth behaviors and high controllability of GaN nanostructures on stripe-patterned sapphire substrates

The growth behaviors of multi-dimensional GaN nanostructures including simultaneous epitaxial growth of films and horizontal nanowires on the sapphire substrates were observed by making substrates per-patterned, leading to some interesting phenomena, thereby helping us understand the characteristics of GaN growth more comprehensively and clearly. The nonpolar a-plane GaN films grown on the Au-coated areas were formed by many triangular pyramids. And there are two kinds of GaN horizontal nanowires in the Au-free areas containing straight-growing nanowires and U-shaped nanowires. The detailed mechanism of those growth phenomena is proposed: when Au catalysts and Ga are sufficient, the growth of GaN is mainly controlled by vapor-solid (VS) mechanism, gradually growing into films. While when Au catalysts are insufficient, the growth of GaN mainly follows vapor-liquid-solid (VLS) mechanism. The difference between the growth of straight and U-shaped nanowires reflects that ±polarity has an important impact on the growth direction of nanowires, making the nanowires take an U-turn during the growth. In addition, the GaN nanostructures show a high degree of controllability. The width and length of nanowires, the continuity of films, even the presence of Au particles at the end of nanowires can be controlled, which provides a simple and convenient synthesis strategy.

Latex/AgNPs: Synthesis, and Their Antibacterial Activity

Silver nanoparticles (Ag NPs) coated by an aqueous emulsion of copolymer were elaborated in a short time using polyol process and microwave (MW) heating. This approach is based on reducing AgNO3 powder in the presence of two main components; ethylene glycol as a dispersed medium and an industrial copolymer (latex) as protecting agent. The structural, morphological and optical properties of the prepared silver nanoparticles have been investigated by UV–Vis spectroscopy, Infrared Spectroscopy (IR), X Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). It was found that all these properties are strongly correlated to the latex copolymer concentration and the MW heating process. The absorption band was determined by UV Visible spectra, showing several bands (localized surface Plasmon band) in the visible region at wavelengths between 410 and 430 nm. The TEM analysis of the obtained products indicates the presence of straight nanowires and dispersed, spherical silver nanoparticles with well-controlled size ranging from 7 to 20 nm. The X Ray Diffraction showed that silver nanoparticles synthesized exhibit face centered cubic (fcc) structure. The infrared spectra show the strong coordination between the surface of silver particles and molecules of latex by O–H and C=O groups. Antimicrobial activity of latex/Ag NPs against E. coli, Staphylococcus aureus, and Pseudomonas aeruginosa has been also demonstrated. The results indicated that latex/AgNPs may damage the structure of bacterial cell membrane.

Focused-Electron-Beam Engineering of 3D Magnetic Nanowires

Focused-electron-beam-induced deposition (FEBID) is the ultimate additive nanofabrication technique for the growth of 3D nanostructures. In the field of nanomagnetism and its technological applications, FEBID could be a viable solution to produce future high-density, low-power, fast nanoelectronic devices based on the domain wall conduit in 3D nanomagnets. While FEBID has demonstrated the flexibility to produce 3D nanostructures with almost any shape and geometry, the basic physical properties of these out-of-plane deposits are often seriously degraded from their bulk counterparts due to the presence of contaminants. This work reviews the experimental efforts to understand and control the physical processes involved in 3D FEBID growth of nanomagnets. Co and Fe FEBID straight vertical nanowires have been used as benchmark geometry to tailor their dimensions, microstructure, composition and magnetism by smartly tuning the growth parameters, post-growth purification treatments and heterostructuring.

Enhancing the performance of superconducting nanowire-based detectors with high-filling factor by using variable thickness

Current crowding at bends of superconducting nanowire single-photon detector (SNSPD) is one of the main factors limiting the performance of meander-style detectors with large filling factors. In this paper, we propose a new concept to reduce the influence of the current crowding effect, a so-called variable thickness SNSPD, which is composed of two regions with different thicknesses. A larger thickness of bends in comparison to the thickness of straight nanowire sections locally reduces the current density and reduces the suppression of the critical current caused by current crowding. This allows variable thickness SNSPD to have a higher critical current, an improved detection efficiency, and decreased dark count rate in comparison with a standard uniform thickness SNSPD with an identical geometry and film quality.

Strain-mediated bandgap engineering of straight and bent semiconductor nanowires.

Accurate simulation of semiconductor nanowires (NWs) under strain is challenging, especially for bent NWs. Here, we propose a simple yet efficient unit-cell model to simulate strain-mediated bandgap modulation in both straight and bent NWs. This is with consideration that uniaxlly bent NWs experience continuous compressive and tensile strains through their cross-sections. A systematic investigation of a series of III-V and II-VI semiconductors NWs in both wurtzite and zinc blende polytypes is performed using hybrid density functional theory methods. The results reveal three common trend in bandgap evolution upon application of strain. Existing experimental measurements corroborate with our predictions concerning bandgap evolution as well as direct-indirect bandgap transitions upon strain. By examining the variation of previous theoretical studies, our result further highlights the significance of geometrical relaxtion in NW simulation. This simplified model is expected to be applicable to investigations of the electronic, optoelectronic, and sensorial properties of all semiconductor NWs.

Thermoelectric Power of Silver Telluride Thin Films and its Thermal Conductivity Applications

The hydrothermal technique was used to create straight single crystal silver telluride nanowires with a diameter of around 200 nm and a length of up to micrometers of decades. There has been no template or surfactant used in the process. As-synthesized products are high purity and well-crystallized, confirmed by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectrum, transmission electron microscopy (TEM), and a high-resolution SAED pattern. Differential scanning calorimetry was used to observe the reversible structural phase shift from the low-temperature monoclinic structure to the high-temperature face-centered cubic structure. Furthermore, the dramatic drop in electrical current in a single nanowire at the phase transition temperature is revealed, paving the way for future research into the manufacturing of one-dimensional nanoscale devices. Silver telluride (Ag2Te) has large thermoelectric coefficients and it was tested by using resistor graph and calculated the values of it, thermal conductivity and Seebeck coefficient were discussed with respect to the temperature of thin films. Semiconductors were superior thermoelectric material due to higher ratio of electrical and thermal conductivities. Therefore, the AgTe thin films deposited on indium tin oxide (ITO) substrates were employed, thermoelectric power and thermal conductivity measurements, respectively.

High-efficiency polarization-insensitive superconducting nanowire single photon detector

Superconducting nanowire single photon detector (SNSPD) has been widely used in many fields such as quantum communication due to its extremely high detection efficiency, low dark count rate, high count rate, and low timing jitter. Compared with conventional single-photon detectors with planar structure, SNSPD is typically made a periodical meandering structure consisting of parallel straight nanowires. However, owing to its unique linear structure, the detection efficiency of SNSPD is dependent on the polarization state of incident light, thus limiting SNSPD’s applications in unconventional fiber links or other incoherent light detection. In this paper, a polarization-insensitive SNSPD with high detection efficiency is proposed based on the traditional meandering nanowire structure. A thin silicon film with a high refractive index is introduced as a cladding layer of nanowires to reduce the dielectric mismatch between the nanowire and its surroundings, thereby improving the optical absorption efficiency of nanowires to the transverse-magnetic (TM) polarized incident light. The cladding layer is designed as a sinusoidal-shaped grating structure to minimize the difference in optical absorption efficiency between the transverse electric (TE) polarized incident light and the TM polarized incident light in a wide wavelength range. In addition, the twin-layer nanowire structure and the dielectric mirror are used to improve the optical absorption efficiency of the device. Our simulation results show that with the optimal parameters, the optical absorption efficiency of nanowires to both of the TE polarized incident light and TM polarized incident light has a maximum of over 90% at 1550 nm, and the corresponding polarization extinction ratio is less than 1.22. The fabricated device possesses a maximum detection efficiency of 87% at 1605 nm and a polarization extinction ratio of 1.06. The measured detection efficiency exceeds 50% with a polarization extinction ratio less than 1.2 in a wavelength range from 1505 nm to 1630 nm. This work provides a reference for high-efficiency polarization-insensitive SNSPD in the future.

One-dimensional twisted and tubular structures of zinc oxide by semiconductor-catalyzed vapor–liquid–solid synthesis

The exploration of unconventional catalysts for the vapor–liquid–solid synthesis of one-dimensional materials promises to yield new morphologies and functionality. Here, we show, for the model ZnO system, that unusual nanostructures can be produced via a semiconductor (Ge) catalyst. As well as the usual straight nanowires, we describe two other distinct morphologies: twisted nanowires and twisted nanotubes. The twisted nanotubes show large hollow cores and surprisingly high twisting rates, up to 9°/μm, that cannot be easily explained through the Eshelby twist model. A combination of ex situ and in situ transmission electron microscopy measurements suggest that the hollow core results from a competition between growth and etching at the Ge–ZnO interface during synthesis. The twisting rate is consistent with a softening of elastic rigidity. These results indicate that the use of unconventional, nonmetallic catalysts provides opportunities to synthesize unusual oxide nanostructures with potentially useful properties.

Adjusting the Morphology and Properties of SiC Nanowires by Catalyst Control.

We report on the growth of SiC nanowires on a single crystal Si substrate by pyrolysis of polycarbosilane and using two catalyst (Al2O3 and Ni) films with different thickness (2, 4, and 6 nm). The catalyst films were deposited on the Si substrate, and the SiC nanowires were grown according to two mechanisms, i.e., the oxide-assisted growth mechanism and vapor- liquid-solid mechanism. As a result, pearl-chain-like SiC nanowires and straight SiC nanowires were obtained. The prepared nanowires exhibited excellent photoluminescence properties, emission spectra displaying two emission peaks at 395 and 465 nm, and have good thermal stability below 1000 °C. The experimental results revealed the importance of the catalyst in controlling the morphology and properties of SiC nanowires.

SERS-based immunoassay and degradation of CA19-9 mediated by gold nanowires anchored magnetic–semiconductor nanocomposites

Here, straight upward Au nanowires (NWs) were successfully grown onto Fe3O4@TiO2 matrix through a seed-mediated strategy to intensively improve its photocatalysis and SERS performances, facilitating a peculiar recyclable surface-enhanced Raman spectroscopy (SERS)-based immunoassay of CA19–9 in liquid form based on visible light irradiation. Such immunoassay was also supported by a smart heterobifunctional cross-linking agent-mediated probe of anti-CA19–9/4-MBA without metal nanoparticles. A low limit of detection of 5.65 × 10−4 IUmL−1 and a wide linear range from 1000 to 0.001 IUmL−1 were demonstrated through repeated constructing the sandwich immunostructure with only one batch of nanocomposites. Moreover, the actual levels of CA19–9 for colorectal cancer patients were readily measured by the recyclable immunoassay, the results of which are principally consistent with the conventional CLIA detection. Thus, such a green strategy of visible light-induced recyclable immunoassay could be expected to have a potential practicability in the clinical diagnoses of cancer.