Nickel and indium core-shell co-catalysts loaded silicon nanowire arrays for efficient photoelectrocatalytic reduction of CO_(2) to formate

Developing an efficient artificial photosynthetic system for transforming carbon dioxide and storing solar energy in the form of chemical bonds is one of the greatest challenges in modern chemistry.However,the limited choice of catalysts with wide light absorption range,long-term stability and excellent selectivity for CO_(2) reduction makes the process sluggish.Here,a core-shell-structured nonnoble-metal Ni@In co-catalyst loaded p-type silicon nanowire arrays(SiNWs)for efficient CO_(2) reduction to formate is demonstrated.The formation rate and Faradaic efficiency of formate over the Ni@In/SiNWs catalyst reach 58μmol h^(-1) cm^(-2) and 87% under the irradiation of one simulated sunlight(AM 1.5 G,100 mW cm^(-2)),respectively,which are about 24 and 12 times those over the pristine SiNWs.The enhanced photoelectrocatalytic performance for CO_(2) reduction is attributed to the rational combination of Ni capable of effectively extracting the photogenerated electrons and In responsible for the selective activation of CO_(2).

Effects of source-drain underlaps on the performance of silicon nanowire on insulator transistors

The effects of source-drain underlaps on the performance of a top gate silicon nanowire on insulator transistor are studied using a three dimensional(3D) self-consistent Poisson-Schrodinger quantum simulation. Voltage-controlled tunnel barrier is the device transport physics. The off current, the on/off current ratio, and the inverse subthreshold slope are improved while the on current is degraded with underlap. The physics behind this behavior is the modulation of a tunnel barrier with underlap. The underlap primarily affects the tunneling component of drain current. About 50% contribution to the gate capacitance comes from the fringing electric fields emanating from the gate metal to the source and drain. The gate capacitance reduces with underlap, which should reduce the intrinsic switching delay and increase the intrinsic cut-off frequency. However, both the on current and the transconductance reduce with underlap, and the consequence is the increase of delay and the reduction of cut-off frequency.

Silicon nanowire formed via shallow anisotropic etching Si-ash-trimming for specific DNA and electrochemical detection

A functionalized silicon nanowire field-effect transistor （SiNW FET） was fabricated to detect single molecules in the pM range to detect disease at the early stage with a sensitive, robust, and inexpensive method with the ability to provide specific and reliable data. The device was designed and fabricated by indented ash trimming via shallow anisotropic etching. The approach is a simple and low-cost technique that is compatible with the current commercial semiconductor standard CMOS process without an expensive deep reactive ion etcher. Specific electric changes were observed for DNA sensing when the nanowire surface was modified with a complementary captured DNA probe and target DNA through an organic linker （--OCH2CH3） using organofunctional alkoxysilanes （3-aminopropyl） triethoxysilane （APTES）. With this surface modification, a single specific target molecule can be detected. The simplicity of the sensing domain makes it feasible to miniaturize it for the development of a cancer detection kit, facilitating its use in both clinical and non-clinical environments to allow non-expert interpretation. With its novel electric response and potential for mass commercial fabrication, this biosensor can be developed to become a portable/point of care biosensor for both field and diagnostic applications.

Vacancy effect on the doping of silicon nanowires:A first-principles study

The influence of vacancy defect on the doping of silicon nanowires is systematically studied by the first-principles calculations. The atomic structures and electronic properties of vacancies and vacancy-boron （vacancy-phosphor） com- plexes in H-passivated silicon nanowire with a diameter of 2.3 nm are explored. The results of geometry optimization indicate that a central vacancy can exist stably, while the vacancy at the edge of the nanowire undergoes a local surface reconstruction, which results in the extradition of the vacancy out of the nanowire. Total-energy calculations indicate that the central vacancy tends to form a vacancy-dopant defect pair. Further analysis shows that n-type doping efficiency is strongly inhibited by the unintentional vacancy defect. In contrast, the vacancy defect has little effect on p-type doping. Our results suggest that the vacancy defect should be avoided during the growth and the fabrication of devices.

Thermal stability of silicon nanowires:atomistic simulation study

Using the Stillinger Weber （SW） potential model,we investigate the thermal stability of pristine silicon nanowires based on classical molecular dynamics （MD） simulations.We explore the structural evolutions and the Lindemann indices of silicon nanowires at different temperatures in order to unveil atomic-level melting behaviour of silicon nanowires.The simulation results show that silicon nanowires with surface reconstructions have higher thermal stability than those without surface reconstructions,and that silicon nanowires with perpendicular dimmer rows on the two （100） surfaces have somewhat higher thermal stability than nanowires with parallel dimmer rows on the two （100） surfaces.Futher-more,the melting temperature of silicon nanowires increases as their diameter increases and reaches a saturation value close to the melting temperature of bulk silicon. The value of the Lindemann index for melting silicon nanowires is 0.037.

Preparation of high-purity straight silicon nanowires by molten salt electrolysis

Silicon nanowires of high purity and regular morphology are of prime importance to ensure high specific capacities of lithium-ion batteries and reproducible electrode assembly process.Using nickel formate as a metal catalyst precursor,straight silicon nanowires(65–150 nm in diameter)were directly prepared by electrolysis from the Ni/SiO2 porous pellets with 0.8 wt%nickel content in molten CaCl2 at 900℃.Benefiting from their straight appearance and high purity,the silicon nanowires therefore offered an initial coulombic efficiency of 90.53% and specific capacity of 3377 m Ah/g.In addition,the silicon nanowire/carbon composite exhibited excellent cycle performance,retaining 90.38%of the initial capacity after 100 cycles.Whilst further study on the charge storage performance is still ongoing,these preliminary results demonstrate that nickel formate is an efficient and effective metal catalyst precursor for catalytic preparation of high purity straight silicon nanowires via the molten salt electrolysis,which is suitable for large-scale production.

Ultra-low thermal conductivity of roughened silicon nanowires:Role of phonon-surface bond order imperfection scattering

The ultra-low thermal conductivity of roughened silicon nanowires(SiNWs)can not be explained by the classical phonon-surface scattering mechanism.Although there have been several efforts at developing theories of phonon-surface scattering to interpret it,but the underlying reason is still debatable.We consider that the bond order loss and correlative bond hardening on the surface of roughened SiNWs will deeply influence the thermal transport because of their ultra-high surface-to-volume ratio.By combining this mechanism with the phonon Boltzmann transport equation,we explicate that the suppression of high-frequency phonons results in the obvious reduction of thermal conductivity of roughened SiNWs.Moreover,we verify that the roughness amplitude has more remarkable influence on thermal conductivity of SiNWs than the roughness correlation length,and the surface-to-volume ratio is a nearly universal gauge for thermal conductivity of roughened SiNWs.

Ultra-Low Breakdown Voltage of Field Ionization in Atmospheric Air Based on Silicon Nanowires

Classic field ionization requires extremely high positive electric fields, of the order of a few million volts per centimeter. Here we show that field ionization can occur at dramatically lower fields on the electrode of silicon nanowires （SiNWs） with dense surface states and large field enhancement factor. A field ionization structure using SiNWs as the anode has been investigated, in which the SiNWs were fabricated by improved chemical etching process. At room temperature and atmospheric pressure, breakdown of the air is reproducible with a fixed anode-to-cathode distance of 0.5 μm. The breakdown voltage is -38 V, low enough to be achieved by a batterypowered unit. Two reasons can be given for the low breakdown voltage. First, the gas discharge departs from the Paschen＇s law and the breakdown voltage decreases sharply as the gap distance falls in μm range. The other reason is the large electric field enhancement factor （β） and the high density of surface defects, which cause a highly non-uniform electric field for field emission to occur.

Surface effects on the thermal conductivity of silicon nanowires

Thermal transport in silicon nanowires （SiNWs） has recently attracted considerable attention due to their potential applications in energy harvesting and generation and thermal management. The adjustment of the thermal conductivity of SiNWs through surface effects is a topic worthy of focus. In this paper, we briefly review the recent progress made in this field through theoretical calculations and experiments. We come to the conclusion that surface engineering methods are feasible and effective methods for adjusting nanoscale thermal transport and may foster further advancements in this field.

Stable Superwetting Surface Prepared with Tilted Silicon Nanowires

Large-scale uniform nanostructured surface with superwettability is crucial in both fundamental research and engineering applications.A facile and controllable approach was employed to fabricate a superwetting tilted silicon nanowires(TSNWs) surface through metal-assisted chemical etching and modification with low-surface-energy material.The contact angle(CA) measurements of the nanostructured surface show a large range from the superhydrophilicity(the CA approximate to 0°) to superhydrophobicity(the CA up to 160°).The surface becomes antiadhesion to water upon nanostructuring with a measured sliding angle(a) close to 0°.Moreover,the fluorinated TSNWs surface exhibits excellent stability and durability because strong chemical bonding has been formed on the surface.

Simulation of Chirped Pulse Propagation in Silicon Nanowires: Shape and Spectrum Analysis

In this paper, we simulate the propagation of chirped pulses in silicon nanowires by solving the nonlinear Schrodinger equation (NLSE) using the split-step Fourier (SSF) method. The simulations are performed both for the pulse shape (time domain) and for the pulse spectrum (frequency domain), and various linear and nonlinear effects changing the shape and the spectrum of the pulse are analyzed. Owing to the high nonlinear coefficient and a very small effective-mode area, the required length for observing nonlinear effects in nanowires is much shorter than that of conventional optical fibers. The impacts of loss, nonlinear effects, second- and third-order dispersion coefficients and the chirp parameter on pulse propagation along the nanowire are investigated. The results show that the sign and the value of the chirp parameter have important role in pulse propagation so that in the anomalous dispersion regime, the compression occurs for the up- chirped pulses, whereas the broadening takes place for the down-chirped pulses. The opposite situation happens for up- and down-chirped pulses propagating in the normal dispersion regime.

Enhanced Photocatalytic Degradation of Rhodamine B by Cu_2O Coated Silicon Nanowire Arrays in Presence of H_2O_2

Highly ordered Cu2O coated silicon nanowire arrays （SiNWAs） were fabricated as photocatalyst via depositing Cu nanoparticles on silver-assisted electroless-etched SiNWAs and subsequently annealing. The as-prepared samples have been characterized by scanning electron microscopy, X-ray diffraction and UV-VIS-NIR spectrophotometry. The photocatalytic properties of the Cu2O coated SiNWAs were investigated by degradation of Rhodamine B （RhB） under simulated solar light with a cut-off filter （λ 〉 420 nm）. The results indicated that H2O2 could greatly improve the photocatalytic properties of Cu2O coated SiNWAs, and exhibited strong synergy effect between them. The hybrid nanowire arrays will be promising photocatalytic materials in the field of energy and environment.

Strongly enhanced light trapping in a two-dimensional silicon nanowire random fractal array

We report on the unconventional optical properties exhibited by a two-dimensional array of thin Si nanowires arranged in a random fractal geometry and fabricated using an inexpensive,fast and maskless process compatible with Si technology.The structure allows for a high light-trapping efficiency across the entire visible range,attaining total reflectance values as low as 0.1%when the wavelength in the medium matches the length scale of maximum heterogeneity in the system.We show that the random fractal structure of our nanowire array is responsible for a strong in-plane multiple scattering,which is related to the material refractive index fluctuations and leads to a greatly enhanced Raman scattering and a bright photoluminescence.These strong emissions are correlated on all length scales according to the refractive index fluctuations.The relevance and the perspectives of the reported results are discussed as promising for Si-based photovoltaic and photonic applications.

Kinetically-Induced Hexagonality in Chemically Grown Silicon Nanowires

Various silicon crystal structures with different atomic arrangements from that of diamond have been observed in chemically synthesized nanowires.The structures are typified by mixed stacking mismatches of closely packed Si dimers.Instead of viewing them as defects,we define the concept of hexagonality and describe these structures as Si polymorphs.The small transverse dimensions of a nanowire make this approach meaningful.Unique among the polymorphs are cubic symmetry diamond and hexagonal symmetry wurtzite structures.Electron diffraction studies conducted with Au as an internal reference unambiguously confirm the existence of the hexagonal symmetry Si nanowires.Cohesive energy calculations suggest that the wurtzite polymorph is the least stable and the diamond polymorph is the most stable.Cohesive energies of intermediate polymorphs follow a linear trend with respect to their structural hexagonality.We identify the driving force in the polymorph formations as the growth kinetics.Fast longitudinal elongation during the growth freezes stacking mismatches and thus leads to a variety of Si polymorphs.The results are expected to shed new light on the importance of growth kinetics in nanomaterial syntheses and may open up ways to produce structures that are uncommon in bulk materials.

Conformal and continuous deposition of bifunctional cobalt phosphide layers on p-silicon nanowire arrays for improved solar hydrogen evolution

Vertically aligned p-silicon nanowire （SiNW） arrays have been extensively investigated in recent years as promising photocathodes for solar-driven hydrogen evolution. However, the fabrication of SiNW photocathodes with both high photoelectrocatalytic activity and long-term operational stability using a simple and affordable approach is a challenging task. Herein, we report conformal and continuous deposition of a di-cobalt phosphide （C02P） layer on lithography- patterned highly ordered SiNW arrays via a cost-effective drop-casting method followed by a low-temperature phosphorization treatment. The as-deposited C02P layer consists of crystalline nanoparticles and has an intimate contact with SiNWs, forming a well-defined SiNW@Co2P core/shell nanostructure. The conformal and continuous Co2P layer functions as a highly efficient catalyst capable of substantially improving the photoelectrocatalytic activity for the hydrogen evolution reaction （HER） and effectively passivates the SiNWs to protect them from photo-oxidation, thus prolonging the lifetime of the electrode. As a consequence, the SiNW@Co2P photocathode with an optimized C02P layer thickness exhibits a high photocurrent density of -21.9 mA·cm^-2 at 0 V versus reversible hydrogen electrode and excellent operational stability up to 20 h for solar-driven hydrogen evolution, outperforming many nanostructured silicon photocathodes reported in the literature. The combination of passivation and catalytic functions in a single continuous layer represents a promising strategy for designing high-performance semiconductor photoelectrodes for use in solar-driven water splitting, which may simplify fabrication procedures and potentially reduce production costs.

Progress in Silicon Nanowire-Based Field-Effect Transistor Biosensors for Label-Free Detection of DNA

Silicon nanowire （SiNW）, as one-dimensional semiconducting nanomaterial, has been incorporated into the filed-effect transistor （FET） devices to increase the efficacy and signal-to-noise in DNA sensing applications. Due to the advantages of high sensitivity, excellent selectivity, label-free detection, direct electrical readout, and minia- turization, SiNW FET-based DNA sensors have been regarded as an important tool in applications of molecular di- agnostics, DNA sequencing, gene expressions, and drug discovery. Here, we review the recent progress in SiNW- FET sensors for label-free electrical DNA detection. We first introduce the working principle of SiNW-FET DNA sensors, SiNW fabrication technologies, bio-functionalization on nanowire surface, and enhancement of device sen- sitivity. Then we sum up the applications of SiNW sensors in detection of DNA hybridization, infectious viruses, microRNA, genetic change （DNA mutation, DNA methylation, and DNA repair）, and protein-DNA interactions. We address several crucial points of sensing performance including sensitivity, selectivity, and limit of detection. Finally, the perspectives, challenges, and some solutions of the field are also discussed.

One-step growth of large-area silicon nanowire fabrics for high-performance multifunctional wearable sensors

Silicon nanowire(SiNW)fabrics are of great interest for fabricating high-performance multifunctional wearable sensors.However,it remains a big challenge to fabricate high-quality SiNW fabrics in a simple and efficient manner.Here we report,for the first time,one-step growth of large-area SiNW fabrics for multifunctional wearable sensors,by using a massive metal-assisted chemical vapor deposition(CVD)method.With bulk Sn as a catalyst source,numerous millimeter-long SiNWs grow and naturally interweave with each other,forming SiNw fabrics over 80 cm2 in one experiment.In addition to intrinsic electronic properties of Si materials,the SiNw fabrics also feature high flxibility,good tailorability and light weight,rendering them ideal for fabricating multifunctional wearable sensors.The prototype sensors based on the SiNW fabrics could efectively detect various stimuli including temperature,light,strain and pressure,with outstanding performance among reported multifunctional sensors.We further demonstrate the integration of the prototype sensors onto the body of a robot,enabling its perception to various environmental stimuli.The ability to prepare high-quality SiNW fabrics in a simple and eficient manner will stimulate the development of wearable devices for applications in portable electronics,Internet of Things,health care and robotics.

Integration of silicon nanowires in solar cell structure for efficiencyenhancement: A review

Silicon nanowires (SiNWs) are a one-dimensional semiconductor, which shows promising applications indistinct areas such as photocatalysis, lithium-ion batteries, gas sensors, medical diagnostics, drug delivery,and solar cell. From an implementation point of view, SiNWs are fabricated using either a topdownor bottom-up approach, and SiNWs are both optically and electronically active. SiNWs enhancesthe efficiency of the solar cell due to better electronic, optical, and physical properties that can becontrolled by tuning the physical dimensions of SiNWs. The SiNWs shows an inherent capability to beutilized in radial or coaxial p-n junction solar cells, to stipulate orthogonal photon absorption, antireflection,and enhanced carrier collection. This paper reviews property-control of SiNWs, theirvarious types of incorporation in a solar cell, and the reasons behind enhanced efficiency.

Enhanced photocatalytic activities of silicon nanowires/graphene oxide nanocomposite:Effect of etching parameters

Homogeneous and vertically aligned silicon nanowires(SiNWs)were successfully fabricated using silver assisted chemical etching technique.The prepared samples were characterized using scanning electron microscopy,transmission electron microscopy and atomic force microscopy.Photocatalytic degradation properties of graphene oxide(GO)modified SiNWs have been investigated.We found that the SiNWs morphology depends on etching time and etchant composition.The SiNWs length could be tuned from 1 to 42μm,respectively when varying the etching time from 5 to 30 min.The etchant concentration was found to accelerate the etching process;doubling the concentrations increases the length of the SiNWs by a factor of two for fixed etching time.Changes in bundle morphology were also studied as function of etching parameters.The SiNWs diameter was found to be independent of etching time or etchant composition while the size of the SiNWs bundle increases with increasing etching time and etchant concentration.The addition of GO was found to improve significantly the photocatalytic activity of SiNWs.A strong correlation between etching parameters and photocatalysis efficiency has been observed,mainly for SiNWs prepared at optimum etching time and etchant concentrations of 10 min and 4:1:8.A degradation of92%was obtained which further improved to 96%by addition of hydrogen peroxide.Only degradation efficiency of 16%and 31%has been observed for bare Si and GO/bare Si samples respectively.The obtained results demonstrate that the developed SiNWs/GO composite exhibits excellent photocatalytic performance and could be used as potential platform for the degradation of organic pollutants.

Ammonia sensing using arrays of silicon nanowires and graphene

Ammonia （NH3） is a toxic gas released in different industrial, agricultural and natural processes. It is also a biomarker for some diseases. These require NH3 sensors for health and safety reasons. To boost the sensitiv- ity of solid-state sensors, the effective sensing area should be increased. Two methods are explored and compared using an evaporating pool of 0.5 mL NH4OH （28% NH3）. In the first method an array of Si nanowires （Si NWA） is obtained via metal-assisted-electrochemical etching to increase the effective surface area. In the second method CVD graphene is suspended on top of the Si nanowires to act as a sensing layer. Both the effective surface area as well as the density of surface traps influences the amplitude of the response. The effective surface area of Si NWAs is 100 × larger than that of suspended graphene for the same top surface area, leading to a larger response in amp- litude by a factor of -7 notwithstanding a higher trap density in suspended graphene. The use of Si NWAs in- creases the response rate for both Si NWAs as well as the suspended graphene due to more effective NH3 diffu- sion processes.