Thick Nanowires Research Articles

Self-heating oxidized suspended Pt nanowire for high performance hydrogen sensor

In this work, joule-self-heating of a suspended Pt nanowire, under oxygen ambient, is utilized to form PtOx/Pt nanostructure to develop a new type of 1-dimensional sensor device architecture for detection of hydrogen at room temperature. An optimum design space for the sensor is elucidated with the initial Pt nanowire thickness ranging between 10 nm and 80 nm. Single PtOx/Pt nanowire sensor, based on optimum metal (Pt) to metal-oxide (PtOx) junction can detect down to 100 ppm H2 with ultra-low power consumption of ∼45 nW. This study also investigates hydrogen sensing characteristics of an array of oxidized Pt nanowires, which enhances the response with the maximum sensitivity of ∼936%, compared to single PtOx/Pt nanowire sensor sensitivity of ∼13% for 100 ppm hydrogen. Nanowire array sensor provides the minimum detection limit of 500 ppb (∼18.1%) with very fast response (∼25 s) and recovery (∼108 s) time. A phenomenological model is proposed to elaborate the nature of oxidation of joule-self-heated Pt nanowire in oxygen ambient.

Silicon-catalyzed growth of amorphous SiO x nanowires by continuous-wave laser ablation of SiO in high-pressure gas

Continuous-wave laser ablation of an SiO target was performed to grow nanowires (NWs) in high-pressure (up to 0.90 MPa) Ar, N2, and O2 gas. In Ar gas, the fraction of the NWs in deposits was changed significantly by the Ar pressure. As Ar pressure increased, thicker NWs with diameters of up to 80 nm were grown. The NWs were amorphous SiO x and attached with sphere-like nanoparticles (NPs) at their tips. The tip NPs were composed of crystalline Si covered with thin amorphous SiO x layers. Similar NW growth occured in N2 gas. However, only amorphous NPs were generated in O2 gas. On the basis of liquid-like molten SiO x NPs containing a portion of a Si phase formed by a disproportionation reaction of SiO x , we discuss the nucleation and growth of the NWs through supersaturation and anisotropic precipitation of SiO x .

Designing Diameter-Modulated Heterostructure Nanowires of PbTe/Te by Controlled Dewetting.

Heterostructures consisting of semiconductors with controlled morphology and interfaces find applications in many fields. A range of axial, radial, and diameter-modulated nanostructures have been synthesized primarily using vapor phase methods. Here, we present a simple wet chemical routine to synthesize heterostructures of PbTe/Te using Te nanowires as templates. A morphology evolution study for the formation of these heterostructures has been performed. On the basis of these control experiments, a pathway for the formation of these nanostructures is proposed. Reduction of a Pb precursor to Pb on Te nanowire templates followed by interdiffusion of Pb/Te leads to the formation of a thin shell of PbTe on the Te wires. Controlled dewetting of the thin shell leads to the formation of cube-shaped PbTe that is periodically arranged on the Te wires. Using control experiments, we show that different reactions parameters like rate of addition of the reducing agent, concentration of Pb precursor and thickness of initial Te nanowire play a critical role in controlling the spacing between the PbTe cubes on the Te wires. Using simple surface energy arguments, we propose a mechanism for the formation of the hybrid. The principles presented are general and can be exploited for the synthesis of other nanoscale heterostructures.

A Micromagnetic Protocol for Qualitatively Predicting Stochastic Domain Wall Pinning

Understanding dynamically-induced stochastic switching effects in soft ferromagnetic nanowires is a critical challenge for realising spintronic devices with deterministic switching behaviour. Here, we present a micromagnetic simulation protocol for qualitatively predicting dynamic stochastic domain wall (DW) pinning/depinning at artificial defect sites in Ni80Fe20 nanowires, and demonstrate its abilities by correlating its predictions with the results of focused magneto-optic Kerr effect measurements. We analyse DW pinning configurations in both thin nanowires (t = 10 nm) and thick nanowires (t = 40 nm) with both single (asymmetric) and double (symmetric) notches, showing how our approach provides understanding of the complex DW-defect interactions at the heart of stochastic pinning behaviours. Key results explained by our model include the total suppression of stochastic pinning at single notches in thick nanowires and the intrinsic stochasticity of pinning at double notches, despite their apparent insensitivity to DW chirality.

Electro- and Magneto-Transport Behavior of Template-Released Isolated Ferromagnetic Nanowires

The electrical and magnetic transport behavior of two assemblages of template-released isolated bundles of three Ni0.6Fe0.4 nanowires have been investigated using magnetoresistance (MR) measurements. The MR data were consistent with the theoretical predictions of anisotropic MR effect. Inhomogeneous magnetic structures were noticed at remanence upon removing the magnetic field. Where ~92% of the magnetic moments were aligned parallel to the nanowire’s long axis at remanence in longitudinal measurements for both assembalages of nanowires. In contrast, ~65% and ~35% of the magnetic moments were aligned away from the nanowire’s long axis at remanence in transverse measurements for both assemblages of wires, respectively. This indicates that there is a strong force competes with the shape anisotropy and this force can impose a magnetic alignment away from the nanowires long axis. The MR measurements showed: small and large features in the resistance profiles. The extracted field values from these features were compared with each other and with the switching fields reported in the literature that adopted the same and different investigative techniques. The angular dependence of the switching fields obtained from the large features was compared with the theoretical predictions of the curling model of domain reversal. An agreement and disagreement were, respectively, seen at small and high angles, indicating that the magnetization reversal within such wires might occur in a different complicated way. The results of this paper, predicts a difference between the magnetization reversal at the surface and the bulk of such relatively thick ferromagnetic nanowires, therefore, more studies are recommended to ascertain the results reported here.

1-D WO3@BiVO4 heterojunctions with highly enhanced photoelectrochemical performance

1-Dimentional WO3@ BiVO4 heterojunction thin films are prepared for the first time by combining a flame vapor deposition (FVD) process with spin coating method. A rapid, economical FVD system are developed, which incorporates a controllable wire feeder with precise feed rate for the growth of sub-stoichiometic tungsten oxide nanowires. These flame synthesized WO3 nanowires provide high interfacial contact area for the coating of BiVO4 shells. The WO3@BiVO4 heterojunction nanowire array utilizes the individual strengths of WO3 (good electron transport property) and BiVO4 (strong light absorption) and exhibits superior performance under back illumination. The optimal thickness of WO3 nanowires for best-performing nanostructured WO3@BiVO4 thin film is found in this study. This WO3@BiVO4 photoanode produces a photocurrent of up to 3.8 mA/cm2 under simulated solar light back illumination (100 mW/cm2) at 1.23 V vs. RHE (reversible hydrogen electrode). The present work offers a promising practical route for the preparation of 1-dimentional WO3 based composites photoanode for photoelectrochemical water splitting.

Transport properties of ultrathin YBa2Cu3O7−δ nanowires: A route to single-photon detection

We report on the growth and characterization of ultrathin YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) films on MgO (110) substrates, which exhibit superconducting properties at thicknesses down to 3 nm. YBCO nanowires, with thicknesses down to 10 nm and widths down to 65 nm, have been also successfully fabricated. The nanowires protected by a Au capping layer show superconducting properties close to the as-grown films, and critical current densities, which are only limited by vortex dynamics. The 10 nm thick YBCO nanowires without the Au capping present hysteretic current voltage characteristics, characterized by a voltage switch which drives the nanowires directly from the superconducting to the normal state. Such bistability is associated in NbN nanowires to the presence of localized normal domains within the superconductor. The presence of the voltage switch, in ultrathin nanostructures characterized by high sheet resistance values, though preserving high quality superconducting properties, make our nanowires very attractive devices to engineer single photon detectors.

A Nose for Hydrogen Gas: Fast, Sensitive H2 Sensors Using Electrodeposited Nanomaterials.

Hydrogen gas (H2) is odorless and flammable at concentrations above 4% (v/v) in air. Sensors capable of detecting it rapidly at lower concentrations are needed to "sniff" for leaked H2 wherever it is used. Electrical H2 sensors are attractive because of their simplicity and low cost: Such sensors consist of a metal (usually palladium, Pd) resistor. Exposure to H2 causes a resistance increase, as Pd metal is converted into more resistive palladium hydride (PdHx). Sensors based upon Pd alloy films, developed in the early 1990s, were both too slow and too insensitive to meet the requirements of H2 safety sensing. In this Account, we describe the development of H2 sensors that are based upon electrodeposited nanomaterials. This story begins with the rise to prominence of nanowire-based sensors in 2001 and our demonstration that year of the first nanowire-based H2 sensor. The Pd nanowires used in these experiments were prepared by electrodepositing Pd at linear step-edge defects on a graphite electrode surface. In 2005, lithographically patterned nanowire electrodeposition (LPNE) provided the capability to pattern single Pd nanowires on dielectrics using electrodeposition. LPNE also provided control over the nanowire thickness (±1 nm) and width (±10-15%). Using single Pd nanowires, it was demonstrated in 2010 that smaller nanowires responded more rapidly to H2 exposure. Heating the nanowire using Joule self-heating (2010) also dramatically accelerated sensor response and recovery, leading to the conclusion that thermally activated H2 chemisorption and desorption of H2 were rate-limiting steps in sensor response to and recovery from H2 exposure. Platinum (Pt) nanowires, studied in 2012, showed an inverted resistance response to H2 exposure, that is, the resistance of Pt nanowires decreased instead of increased upon H2 exposure. H2 dissociatively chemisorbs at a Pt surface to form Pt-H, but in contrast to Pd, it stays on the Pt surface. Pt nanowires showed a faster response to H2 exposure than Pd nanowires operating at the same elevated temperature, but they had a surprising disadvantage: The resistance change observed for Pt nanowires was exactly the same for all H2 concentrations. Electron surface scattering was implicated in the mechanism for these sensors. Work on Pt nanowires lead in 2015 to the preparation of Pd nanowires that were electrochemically modified with thin Pt layers (Pd@Pt nanowires). Relative to Pd nanowires, Pt@Pd nanowires showed accelerated response and recovery to H2 while retaining the same high sensitivity to H2 concentration seen for sensors based upon pure Pd nanowires. A new chapter in H2 sensing (2017) involves the replacement of metal nanowires with carbon nanotube ropes decorated with electrodeposited Pd nanoparticles (NPs). Even higher sensitivity and faster sensor response and recovery are enabled by this sensor architecture. Sensor properties are strongly dependent on the size and size monodispersity of the Pd NPs, with smaller NPs yielding higher sensitivity and more rapid response/recovery. We hope the lessons learned from this science over 15 years will catalyze the development of sensors based upon electrodeposited nanomaterials for gases other than H2.

Magnetization dynamics of Ni80Fe20 nanowires with continuous width modulation

A systematic investigation of the magnetization reversal and the dynamic behaviors of uncoupled ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ nanowires (NWs) with artificial continuous width modulation is presented. In contrast with the single resonance mode observed in the homogeneous NWs from the broadband ferromagnetic resonance spectroscopy, the NWs with continuous width modulation display three to five distinct resonance modes with increasing wire thickness in the range from 5 to 70 nm due to the nonuniform demagnetizing field. The highest frequency mode and the frequency difference between the two distinct highest modes are shown to be markedly sensitive to the NW thickness. Interestingly, we found that these modes can be described in terms of the quantization of the standing spin waves due to confined varied width. In addition, the easy axis coercive field for the width modulated NWs is much higher than homogeneous NWs of the same thickness when less than 70 nm. Our experimental results are in good qualitative agreement with the micromagnetic simulations. The results may find potential applications in the design and optimization of tunable magnonic filters.

Achieving Remarkable Activity and Durability toward Oxygen Reduction Reaction Based on Ultrathin Rh-Doped Pt Nanowires.

The research of active and sustainable electrocatalysts toward oxygen reduction reaction (ORR) is of great importance for industrial application of fuel cells. Here, we report a remarkable ORR catalyst with both excellent mass activity and durability based on sub 2 nm thick Rh-doped Pt nanowires, which combine the merits of high utilization efficiency of Pt atoms, anisotropic one-dimensional nanostructure, and doping of Rh atoms. Compared with commercial Pt/C catalyst, the Rh-doped Pt nanowires/C catalyst shows a 7.8 and 5.4-fold enhancement in mass activity and specific activity, respectively. The combination of extended X-ray absorption fine structure analysis and density functional theory calculations reveals that the compressive strain and ligand effect in Rh-doped Pt nanowires optimize the adsorption energy of hydroxyl and in turn enhance the specific activity. Moreover, even after 10000 cycles of accelerated durability test in O2 condition, the Rh-doped Pt nanowires/C catalyst exhibits a drop of 9.2% in mass activity, against a big decrease of 72.3% for commercial Pt/C. The improved durability can be rationalized by the increased vacancy formation energy of Pt atoms for Rh-doped Pt nanowires.

Coherent charge transport in ferromagnet/semiconductor nanowire/ferromagnet double barrier junctions with the interplay of Rashba spin–orbit coupling, induced superconducting pair potential, and external magnetic field

By solving the Bogoliubov–de Gennes equation, the influence of the interplay of Rashba spin–orbit coupling, induced superconducting pair potential, and external magnetic field on the spin-polarized coherent charge transport in ferromagnet/semiconductor nanowire/ferromagnet double barrier junctions is investigated based on the Blonder–Tinkham–Klapwijk theory. The coherence effect is characterized by the strong oscillations of the charge conductance as a function of the bias voltage or the thickness of the semiconductor nanowire, resulting from the quantum interference of incoming and outgoing quasiparticles in the nanowire. Such oscillations can be effectively modulated by varying the strength of the Rashba spin–orbit coupling, the thickness of the nanowire, or the strength of the external magnetic field. It is also shown that two different types of zero-bias conductance peaks may occur under some particular conditions, which have some different characteristics and may be due to different mechanisms.

Phase slip and telegraph noise in δ-MoN nanowires

We have investigated the effect of the nanowire thickness on the superconducting resistive phase transition R(T) in δ-MoN nanowires. We have characterized the width of the transition in terms of thermally-activated phase-slip theory. A large increase in the width of the transition was found with the decrease of the nanowire thickness. Discrete phase-slip fluctuations also lead to the appearance of meta-stable resistive superconducting states in current-bearing superconducting wires, with spontaneous switching between them. We have investigated the effect of the bias current on the switching rate and the stability of different resistive states.

Two step polyol-solvothermal growth of thick silver nanowires

In this work a new method for the growth of thick silver nanowires (AgNWs) is presented. With this method, AgNWs are obtained in a first step by a modified polyol method and used, in a second step, as seeds for further growth in a solvothermal process. The variation of structure, morphology and dimensions of the obtained silver nanostructures was studied. It was found that the composition of solvent (ethylene glycol and isopropyl alcohol) used in the solvothermal process greatly influences the composition, morphology and dimensions of the final product. Several times longer and thicker silver nanowires, compared with the initial seeds, having diameters between 200 and 250nm and lengths between 10 and 25μm, can be obtained by controlling the solvent composition.

Superplastic behavior of silica nanowires obtained by direct patterning of silsesquioxane-based precursors

Silica nanowires spanning 10 μm-deep trenches are fabricated from different types of silsesquioxane-based precursors by direct e-beam patterning on silicon followed by release through deep reactive ion etching. Nanowire aspect ratios as large as 150 are achieved with a critical dimension of about 50 nm and nearly rectangular cross-sections. In situ bending tests are carried out inside a scanning electron microscope, where the etch depth of 10 provides sufficient space for deformation. Silica NWs are indeed observed to exhibit superplastic behavior without fracture with deflections reaching the full etch depth, about two orders of magnitude larger than the nanowire thickness. A large-deformation elastic bending model is utilized for predicting the deviation from the elastic behavior. The results of forty different tests indicate a critical stress level of 0.1–0.4 GPa for the onset of plasticity. The study hints at the possibility of fabricating silica nanowires in a monolithic fashion through direct e-beam patterning of silsesquioxane-based resins. The fabrication technology is compatible with semiconductor manufacturing and provides silica nanowires with a very good structural integrity.

Formation of Germanium-Carbon Core-Shell Nanowires by Laser Vaporization in High-Pressure Ar Gas without the Addition of Other Metal Catalysts

Germanium (Ge)-carbon (C) core-shell nanowires (NWs), 15 - 80 nm thick and <1 μm long, were grown using continuous-wave laser vaporization of Ge-graphite composite targets in high pressure (0.1 - 0.9 MPa) Ar gas. The NW core was crystalline Ge and the shell was amorphous C. The fraction of the NWs in deposits was changed significantly by the Ge content in the targets and had a maximum at the Ge content of 40 atomic %. With increasing Ar pressure, thicker NWs were grown. A strong correlation was evident between the two diameters of the NW and nanoparticle (NP) attached with the tip of the NW. The growth of the NWs can be explained by the formation of Ge-C liquid-like molten NPs having a specific range of size and composition and precipitation of Ge and C followed by phase separation.

Template-Assisted Electrochemical Synthesis of p-Type InSb Nanowires

We report on the growth of Sb-rich indium antimonide (InSb) nanowires fabricated by dc electrodeposition in the pores of a gold-coated nanoporous anodic alumina oxide (AAO) template. An explanation of the various mechanisms of mass transport during successive stages of the growth process is also presented with experimental results confirming these growth stages. The 100 nm thick InSb nanowires that were grown using an electrolyte of pH 1.7 were found to be rich in antimony (Sb). The electrical properties of a single InSb nanowire was investigated by connecting the nanowire in a field-effect-transistor type configuration. The nanowires showed p-type conduction with a hole concentration of ∼ 1.9 × 1016 cm− 3 and field effect hole mobility of ∼ 507 cm2V− 1s− 1. The device had a high on-off current ratio of the order of 103. Temperature-dependent transport measurements showed thermally activated Arrhenius conduction in the temperature range from 200–325 K, yielding an activation energy of 0.1 eV. The ability to obtain high density of p-type InSb nanowires without addition of any dopants opens up new opportunities for using these nanowires in fabrication of electronic devices.

Thermal resistance measurement of In3SbTe2 nanowires

The thermal resistance along the thickness of In3SbTe2 crystalline nanowires was measured using the scanning thermal microscopy in 3ω mode. The nanowires were grown by metal organic vapor deposition, exploiting the VLS mechanism induced by Au metal‐catalyst nanoparticles and harvested on a SiO2/Si substrate. Two nanowires with different thickness (13 and 23 nm) were investigated. The thermal resistance of the nanowires was determined using two different approaches; the first one exploits the experimental data, whereas the second one is more sophisticated, since it involves a minimization procedure. Both methods led to comparable values of the thermal resistance along the transverse direction (thickness) of the nanowire. The obtained results were explained starting from the mean free path of phonons calculated in the In3SbTe2 bulk.

Flexible field-effect transistors using top-down fabrication of (111)-silicon nanowires and wafer-level transfer process for neural prostheses

In this paper, a simple fabrication method and a large-scale integration of silicon nanowires by combining top-down fabrication with wafer-level transfer process on a flexible substrate is presented. For applications in high-resolution neural prostheses, silicon-nanowire field-effect transistor (FET) switches on a flexible substrate is developed and integrated to microelectrode array (MEA). The fabrication process is developed, and results of (111)-silicon nanowire array and nanowire devices on flexible substrate are shown. The overall device thickness is sufficiently thin for high device flexibility. The quantitative current model is used to analyze the operation mechanism of silicon-nanowire FET. The proposed method can implement the width and thickness of silicon nanowires independently, which can result in higher performance characteristics. The electrical characteristics of the fabricated silicon-nanowire FET switch are measured. The current on/off ratio is above 1×107 and average RON is calculated to be 41kΩ, which shows similar results with the simulation. These results of proposed method show a strong potential for applications in high-resolution neural prostheses.

Luminous Efficiency of Ordered Arrays of GaN Nanowires with Subwavelength Diameters

We investigate the spontaneous emission from ordered arrays of GaN nanowires (NWs) with well-defined diameters, representing the building blocks for light-emitting as well as light-detecting devices integrated on Si substrates. The luminous efficiency of these arrays is observed to decrease by more than an order of magnitude when the NW diameter is increased from 120 nm to 240 nm. A detailed analysis of both steady-state and transient photoluminescence data reveals that this quenching is not caused by a corresponding decrease of the internal quantum efficiency. Hence, we examine the coupling of light into and out of the NW arrays by appropriate numerical simulations. While the change in absorbance is minor in the investigated diameter range, the extraction efficiency for thin NWs is enhanced by an order of magnitude as compared to thick NWs. This phenomenon primarily originates in the efficient coupling of the spontaneous emission to free space for subwavelength diameter NWs. Additionally, our results sho...

Morphological, microstructural, and photoluminescence characterization of heterogeneous/homogeneous TeO2 nanostructures based on effect of different N2 gas flow rates

TeO2 nanowires with diameters ranging from a few tens to hundreds of nanometers and lengths of a few tens of micrometers were prepared by thermal evaporation of Te powder with a Co catalyst. Initially, bead-like γ-TeO2 nanoparticles on the surface of pre-formed thick TeO2 nanowires were heterogeneously formed. Subsequently, uniform thin TeO2 nanowires without the γ-TeO2 phase were homogeneously synthesized by changing only the inlet N2 gas flow rates and keeping other process parameters constant. The morphological and microstructural evolution of the two different nanomaterials was examined by X-ray diffraction, energy-dispersive X-ray spectrometry, and scanning and transmission electron microscopy. The optical properties of the nanomaterials were also investigated using photoluminescence spectroscopy. The emission peaks in the PL measurement without the supply of N2 gas showed a synergetic effect between (1) bead-like orthorhombic γ-TeO2 nanoparticles that show two specific emission bands at 388 nm and 590 nm; and (2) uniform tetragonal TeO2 nanowires that show a characteristic emission band at 440 nm. In contrast, the pure tetragonal TeO2 nanostructures synthesized under a N2 gas flow rate of 2 standard liters per minute flow rate revealed a red-shifted emission band at 450 nm compared to heterogeneous structures of the same composition. The origins of the growth and emission mechanisms in the different TeO2 structures are also discussed.