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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Boosting electrocatalytic selectivity in carbon dioxide reduction:The fundamental role of dispersing gold nanoparticles on silicon nanowires

Carbon dioxide electrochemical reduction(CO_(2)RR)has been recognized as an efficient way to mitigate CO_(2)emissions and alleviate the pressure on global warming and associated environmental consequences.Gold(Au)is reported as stable and active electrocatalysts to convert CO_(2)to CO at low overpotential due to its moderate adsorption strength of^(*)COOH and^(*)CO.The request for improved catalytic performance,however,is motivated by current unsatisfied catalytic selectivity because of the side hydrogen evolution reaction.In this context,the design of Au based binary catalysts that can boost CO selectivity is of great interest.In the present work,we report that Au nanoparticles can be feasibly dispersed and anchored on silicon nanowires to form Au-Si binary nanomaterials.The Au-Si may stably drive CO_(2)RR with a CO Faraday efficiency of 95.6%at−0.6 V vs.RHE in 0.5 mol/L KHCO_(3)solution.Such selectivity outperforms Au particles by up to 61%.Controlled experiments illustrate that such catalytic enhancement can chiefly be ascribed to electronic effects of binary catalysts.Theoretical calculations reveal that spontaneously produced silicon oxide may not only inhibit hydrogen evolution reaction,but also stabilize the key intermediate^(*)COOH in CO formation.

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.

Multi-time scale photoelectric behavior in facile fabricated transparent and flexible silicon nanowires aerogel membrane

In recent years,transparent and flexible materials have been widely pursued in electronics and optoelectronics fields for usage as planar electrodes,energy conversion components and sensing units.As the most widely applied semiconductor material,the related progress in silicon is of great significance although with large difficulty.Herein,we report a one-step method to achieve flexible and transparent silicon nanowires aerogel membrane.A competitive carrier kinetics involving interfacial trapped carriers and the valence electrons transition is demonstrated,according to the photoelectric performance of a sandwiched graphene/silicon nanowires membrane/AI device,i.e.,rapidly positive photoresponse dominated by laser excited^ee-carriers generation(〜500 ms)and subsequent slow negative photocurrent evolution due to laser heating involved multi-levels process(>10 s).These results contribute to fabrication of silicon nanowire self-assembly structures and also the exploration of their optoelectrical properties in flexible and transparent devices.

On-demand production of hydrogen by reacting porous silicon nanowires with water

On-demand hydrogen generation is desired for fuel cells,energy storage,and clean energy applications.Silicon nanowires(SiNWs)and nanoparticles(SiNPs)have been reported to generate hydrogen by reacting with water,but these processes usually require external assistance,such as light,electricity or catalysts.Herein,we demonstrate that a porous SiNWs array,which is fabricated via the metal-assisted anodic etching(MAAE)method,reacts with water under ambient and dark conditions without any energy inputs.The reaction between the SiNWs and water generates hydrogen at a rate that is about ten times faster than the reported rates of other Si nanostructures.Two possible sources of enhancement are discussed:SiNWs maintain their high specific surface area as they don’t agglomerate,and the intrinsic strain of the nanowires promotes the reactivity.Moreover,the porous SiNWs array is portable,reusable,and environmentally friendly,yielding a promising route to produce hydrogen in a distributed manner.

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.

Single-electron transport through single and coupling dopant atoms in silicon junctionless nanowire transistor

We investigated single-electron tunneling through single and coupling dopant-induced quantum dots(QDs) in silicon junctionless nanowire transistor(JNT) by varying temperatures and bias voltages. We observed that two possible charge states of the isolated QD confined in the axis of the initial narrowest channel are successively occupied as the temperature increases above 30 K. The resonance states of the double single-electron peaks emerge below the Hubbard band, at which several subpeaks are clearly observed respectively in the double oscillated current peaks due to the coupling of the QDs in the atomic scale channel. The electric field of bias voltage between the source and the drain could remarkably enhance the tunneling possibility of the single-electron current and the coupling strength of several dopant atoms. This finding demonstrates that silicon JNTs are the promising potential candidates to realize the single dopant atom transistors operating at room temperature.

Gate-regulated transition temperatures for electron hopping behaviours in silicon junctionless nanowire transistors

We investigate gate-regulated transition temperatures for electron hopping behaviours through discrete ionized dopant atoms in silicon junctionless nanowire transistors.We demonstrate that the localization length of the wave function in the spatial distribution is able to be manipulated by the gate electric field.The transition temperatures regulated as the function of the localization length and the density of states near the Fermi energy level allow us to understand the electron hopping behaviours under the influence of thermal activation energy and Coulomb interaction energy.This is useful for future quantum information processing by single dopant atoms in silicon.