Development of individual semiconductor nanowire for bioelectrochemical device at low overpotential conditions

In this work we report the bioelectrochemical study using an individual indium tin oxide (ITO) nanowire (ITO-NW) electrode modified with glucose oxidase enzyme (GOx), in which the enzymatic activity and the biocatalytic activity was evaluated. The main objective is to show that at low overpotential condition, semiconductor NW can be used as an electron donor during biocatalytic process. We demonstrate the possibility of immobilizing an ITO-NW electrode on gold contacts deposited on top of a microchip (oxidized Si wafer). A protective polymer layer containing an aperture over the sample area was photolithographically deposited over the microchip to isolate the metallic contacts. For H₂O₂ reduction during the biocatalysis at ITO-NWs surface, with η  50 mV, normal linear behavior is not observed and an exponential current is evident, similar to n–p semiconductor junction behavior. These results can open new tools for studying redox enzymes at the single-molecule level, and the device described here is very promising as a candidate for further exploration in bioelectrochemical devices, such as biofuel cells and biosensors.     

Self-catalytic solution for single-crystal nanowire and nanotube growth

Vast majority of nanowires is grown by the chemical vapor deposition (CVD), molecular beam epitaxy (MBE), metal-organic CVD (MOCVD), or the laser ablation method via the vapor-liquid-solid (VLS) route. Others are grown via the oxide-assisted route. In this investigation a self-catalytic synthesis route based on VLS formalism and suitable for the CVD, MBE, MOCVD, or the laser ablation method has been described. Various issues pertaining to growth kinetics, nanowire alignment, diameter distribution, and nanotube formation have been addressed. The strength of the self-catalytic route has been highlighted. As this route does not make use of foreign element catalytic agents to mediate the synthesis, it suffers from difficulties. Attempts have been made to elucidate means to overcome these difficulties. Attempts have also been made to explain the means to separate the nanowires thus produced from the substrate/scaffold, and to control their physicochemical characteristics.     

Synthesis and characterization of axial heterojunction inorganic-organic semiconductor nanowire arrays

The end-to-end P-N heterojunction nanowire arrays combined organic (poly[1,4-bis(pyrrol-2-yl)benzene], BPB) and inorganic (CdS) molecules have been successfully designed and fabricated. The electrical properties of P-N heterojunctions of organic-inorganic nanowire arrays were investigated. The diode nature and rectifying feature of P-N heterojunction nanowire arrays were observed. The rectification ratio of the diode increased from 29.9 to 129.7 as the illumination intensity increased. The material exhibits a new property, which is an improvement in the integration of the physical and chemical properties of the two independent components.     

Architecture of CuS/PbS heterojunction semiconductor nanowire arrays for electrical switches and diodes

CuS/PbS p-n heterojunction nanowires arrays have been successfully synthesized. Association of template and DC power sources by controllable electrochemistry processes offers a technique platform to efficiently grow a combined heterojunction nanowire arrays driven by a minimization of interfacial energy. The resulting p-n junction materials of CuS/PbS show highly uniform 1D wire architecture. The single CuS/PbS p-n heterojunction nanowire based devices were fabricated, and their electrical behaviors were investigated. The independent nanowires exhibited a very high ON/OFF ratio of 1195, due to the association effect of electrical switches and diodes.     

First-principles studies of SnS2 nanotubes: a potential semiconductor nanowire

First principles calculations are used to predict the stability and electronic structures of SnS(2) nanotubes. Optimization of several structures and their corresponding strain energies confirm the stability of SnS(2) nanotube structures. Band structure calculations show that SnS(2) nanotubes could have moderate band gaps regardless of their chirality. It suggests that SnS(2) nanotubes would be well-suited to use as semiconductor wires in nanoelectronic devices if they are synthesized. Adsorption of NH(3) onto SnS(2) is also investigated and discussed with regard to potential sensor application.     

An interfacial and bulk charge transport model for dye-sensitized solar cells based on photoanodes consisting of core-shell nanowire arrays

Dye-sensitized solar cells (DSSCs) based on ordered photoanode morphologies, such as nanotubes and nanowires, are widely gaining attention because these geometries are believed to enhance interfacial charge transfer and bulk charge transport. Unfortunately, experimental results have yet to show substantial improvement to conversion efficiency over nanoparticle-based DSSCs. A model is developed to characterize the performance of an idealized photoanode based on an ordered array of transparent conductive nanowires coated with an anatase titania shell. The role of the interfacial electric field in nanowire-based DSSCs is explored computationally by turning electron migration ON or OFF. The results show that back-reaction rates are most strongly influenced by the electric field. These electron loss mechanisms can be reduced by several orders of magnitude, leading to improvements in short-circuit current, open-circuit voltage, and fill factor.     

Perpendicular growth of catalyst-free germanium nanowire arrays

High yields of single-crystalline Ge nanowires (NWs) were synthesised in the vapour phase of a high boiling point organic solvent without the need for metal catalyst particles. High density, perpendicular arrays of Ge NWs were subsequently grown from ITO coated substrates. The approach represents a convenient route toward orientated arrays of catalyst-free Ge NWs.     

Nucleotide-directed growth of semiconductor nanocrystals

We engineer colloidal quantum dot nanocrystals through the choice of biomolecular ligands responsible for nanoparticle nucleation, growth, stabilization, and passivation. We systematically vary the presence of, and thereby elucidate the role of, phosphate groups and a multiplicity of functionalities on the mononucleotides used as ligands. The results provide the basis for synthesis of nanoparticles using precisely controlled synthetic oligonucleotide sequences.     

Solution-liquid-solid growth of semiconductor nanowires

The serendipitously discovered solution-liquid-solid (SLS) mechanism has been refined into a nearly general synthetic method for semiconductor nanowires. Purposeful control of diameters and diameter distributions is achieved. The synthesis proceeds by a solution-based catalyzed-growth mechanism in which nanometer-scale metallic droplets catalyze the decomposition of metallo-organic precursors and crystalline nanowire growth. Related growth methods proceeding by the analogous vapor-liquid-solid (VLS) and supercritical fluid-liquid-solid (SFLS) mechanisms are known, and the relative attributes of the methods are compared. In short, the VLS method is most general and appears to afford nanowires of the best crystalline quality. The SLS method appears to be advantageous for producing the smallest nanowire diameters and for variation and control of surface ligation. The SFLS method may represent an ideal compromise. Recent results for SLS growth are summarized.     

Thermodynamic and kinetic size limit of nanowire growth

The universal models of nucleation thermodynamics and growth kinetics were established for nanowire growth upon metal-catalyst-assisted thermal chemical vapor transport on the basis of vapor-liquid-solid (VLS) mechanism. The thermodynamic and kinetic size limit of nanowire growth was deduced from the proposed model. Theoretical predictions are in agreement with experimental data.     

Mass transport in nanofluidic devices

Nanofluidics is a recent appearing research field, introduced in 1995 as an analogue of the field of microfluidics, and has been becoming popular in the past few years. The proximity of the channel dimension, the Debye length, and the size of biomolecules such as DNA and proteins gives the unique features of nanofluidic devices. Of various unique properties of the nanofluidics, mass transport in nanochannel plays determining roles in fundamental reaches and practical applications of nanofluidic device. Thus, much work including numerical and experimental researches has been performed to investigate the mass transport behaviors in nanofluidic devices. This review summarizes the fabrication technologies for nanofluidic devices, the mass transport behaviors in nanochannel, and their applications in bioanalysis. The main focus will be laid on the effects of nanochannel size and surface charge on mass transport including electrokinetic transport of charged analytes, diffusion of electric neutral molecules, ionic current rectification, concentration polarization, nonlinear electrokinetic flow at the micro-nanofluidic interfaces.     

Mass transport to tubular electrodes

The mathematical treatment of mass transport in tubular electrodes has been examined for the EC process when a reversible electrode charge transfer is followed by a reversible chemical reaction. The transition from fast chemical reaction to slow chemical reaction is investigated. The system of convective-diffusion equations, representing the physical phenomena, are solved using properties of the integral transform and numerical computation of the integral equation. The effects of the chemical equilibrium parameter, the axial velocity of the flow and the potential scan rate on theoretical current-potential curves are elucidated and a simple procedure to determine the chemical reaction rate constant is presented.     

Tungsten oxide nanowire growth by chemically induced strain

We have investigated the formation of tungsten oxide nanowires under different chemical vapor deposition (CVD) conditions. We find that exposure of oxidized tungsten films to hydrogen and methane at 900 degrees C leads to the formation of a dense array of typically 10 nm diameter nanowires. Structural and chemical analysis shows that the wires are crystalline WO3. We propose a chemically driven whisker growth mechanism in which interfacial strain associated with the formation of tungsten carbide stimulates nanowire growth. This might be a general concept, applicable also to other nanowire systems.     

Pattern and feature designed growth of ZnO nanowire arrays for vertical devices

An approach is demonstrated for growing aligned ZnO nanowire/nanorod arrays following a predesigned pattern and feature with controlled site, shape, distribution, and orientation. The technique relies on an integration of atomic force microscopy (AFM) nanomachining with catalytically activated vapor-liquid-solid (VLS) growth. The pattern and growth locations are defined by the catalyst distribution created by AFM, and the orientation is determined by the epitaxial growth on a single-crystal substrate. The technique opens a variety of possibilities of using nanowire arrays as sensor arrays, piezoelectric antenna arrays, nanolasers, photonic band gap crystal, biosensors, and field emitters with controlled density, location, shape, and distribution according to a designed pattern and feature.     

Near-surface transport of semiconductor nanoclusters upon cyclic photoexcitation

A mechanism for the directed motion of a semiconductor nanocluster along a polar substrate upon cyclic photoexcitation that alters the electron density distribution inside the particle is studied. A model that allows us to estimate the average velocity and optimize the system parameters (particle size, distance between the particle and the substrate, the average cycle duration, and charge distribution in the substrate) so as to ensure the maximum velocity is proposed. At the optimum parameters, the average velocity of directed motion can be quite high (~3 mm/s).     

An atomistic model and key parameters for devising single molecular nanowire sensors

Based on one impurity model Hamiltonian describing a nanowire upon adsorption of a molecule, we obtain an analytical formula of the conductance which is governed clearly by modulating key parameters. The formula shows that the conductance change in nanowire upon adsorption of a molecule is mainly controlled by three factors, electron hopping between adsorbed molecule and nanowire, chemical potential, and the change of atomic configurations of the nanowires near the adsorption site. Conductance is very sensitive to the choice of these key parameters; therefore, a proper nanowire system that renders matched chemical potential as well as hopping strength between the nanowire and the adsorbed molecule should be devised for the sensor applications. Our model calculations give similar conductance features to the conductance obtained by the first principle calculations for a singe-molecule-adsorbed molecular wire. It is worthy of note that the system can be in antiresonance, which is characterized by a quick drop in conductance when a molecule is adsorbed on the nanowires.     

Size-dependent orientation growth of large-area ordered Ni nanowire arrays

Large-area ordered Ni nanowire arrays with different diameters have been fabricated by the direct current electrodeposition into the holes of porous anodic alumina membrane. The crystal structure and micrograph of nanowire arrays are characterized by X-ray diffraction, field-emission scanning electron microscopy and high-resolution transmission electron microscopy. The results indicate that the growth orientation of Ni nanowires turns from [110] to [111] direction with increasing diameters of nanowires. The mechanism of the growth was discussed in terms of interface energy minimum principle. The size-dependent orientation of Ni nanowire arrays has the important significance for the design and control of nanostructures.     

Mass transport following impulsive optical excitation

Transition from reverse-saturable absorption to saturable absorption of the chloroaluminum phthalocyanine solution excited by a giant laser pulse is ascribed not just to the saturation of excited state absorption, but also to the outward migration of the solute molecules at the laser beam center. While the saturation of excited state absorption occurs within a single picosecond laser pulse, the beam center population decrease is sustained much longer than the pulse duration. We distinguish these two mechanisms with the Z-scan technique, utilizing picosecond pulses with pulse-to-pulse separations ranging from 0.1 to 5.0 s.     

Boron-assisted growth of silica nanowire arrays and silica microflowers for bendable capacitor application

Aligned-long silica nanowire arrays and microflowers were synthesized with boron as catalyst. Besides that parallel plate capacitors were fabricated using the silica nanowire mat as a dielectric. Their frequency response and mechanical properties were investigated.