Surface Of Silicon Nanowires Research Articles

Silver mirror reaction as a simple method for silicon nanowires functionalization

A cheap and facile wet-chemical ‘silver mirror’ reaction approach has been proposed for the silicon nanowires (SiNWs), formed by the metal-assisted wet-chemical etching, decoration with silver nanoparticles (AgNPs). Scanning electron microscopy studies show that AgNPs can be deposited uniformly on the surface of SiNWs by the variation of AgNO3 concentration and reaction time. Adopting the reaction conditions, a homogenous AgNPs shell is formed around the silicon nanowires. The resultant AgNPs@SiNWs nanostructures show significantly (up to 40%) higher light absorption in the near-infrared range (up to 1500 nm) in comparison to SiNWs without AgNPs. This approach opens a new perspectives for a cheap and fast preparation of AgNP shells around SiNWs. Our observation supports that synthesized nanostructures might have promising application potential in near-infrared photodetectors or as photocatalysts.

Gas sensing properties of silicon nanowires with different cross-sectional shapes toward ammonia: a first-principles study

Nanostructure-based sensors working based on the adsorption-desorption mechanism received much attention in a wide variety of applications as a way to achieve both ultrasensitive sensing and small dimensions. In this paper, the adsorption of ammonia molecule on silicon nanowire surface with square, circular, and triangular cross-sectional shapes, which has almost similar cross-sectional area (1.2 nm2), is studied. The effects of various adsorption configurations are considered on the sensing properties, utilizing first-principles ab initio density functional theory calculations. Structural properties such as bond lengths and bond angles are obtained along with electronic properties such as band structure, charge transfer, and projected density of states for several adsorption configurations. The most stable and most sensitive structure is calculated to be the triangular nanowire considering formation enthalpy as a stability factor. For this nanowire, adsorption energy, covalent bond length, and charge transfer between ammonia and nanowire are calculated to be 1.296 eV, 3.124 A, and 0.227 |e|, respectively. We also consider nanowires with fully hydrogen-passivated surface and nanowires with a dangling bond. It has been shown that nanowires with a dangling bond are more sensitive toward ammonia than fully passivated nanowires.

Ir‐Au Bimetallic Nanoparticle Modified Silicon Nanowires with Ultralow Content of Ir for Hydrogen Evolution Reaction

AbstractReducing the size and increasing the dispersity of the noble metals are effective ways to decrease the dosage and simultaneously increase their electrocatalytic activities for hydrogen evolution reaction (HER). The Ir−Au bimetallic nanoparticle modified silicon nanowires (Ir−Au−Si) with ultralow content of Ir are prepared and served to catalyze HER in the acid condition. The small Ir−Au nanoparticles are in‐situ grown and uniformly dispersed on the surface of silicon nanowires. Ir−Au−Si exhibits a significantly increased activity in HER catalysis compared with Ir−Si and Pt−Au−Si. Ir provides the active sites for HER, Au increases the electron transfer rate, and the silicon nanowires improve the speed of the hydrogen gas evolution. The optimal Ir−Au−Si‐2 shows a low Tafel slope of 24 mV dec−1 and high mass activity, which are better than those of Ir−Si and Pt−Au−Si catalysts.

Design of bio-inspired adhesive surface composed of hexanoyl group-modified gelatin and silicon nanowire.

To fabricate a soft-tissue adhesion surface by nanostructural control and functional modification, we prepared bio-inspired adhesive surfaces composed of hexanoyl group-modified gelatin (HxGltn) and silicon nanowire (SiNW). We synthesized the HxGltn with different modification ratios of 27 mol% (27HxGltn), 37 mol% (37HxGltn), and 50 mol% (50HxGltn) and processed two different types of SiNW plates (800 pitch and 1500 pitch) with 1400 nm in height and 400 nm in diameter (800/1400 SiNW and 1500/1400 SiNW, respectively. To enhance the adhesion to the intestine surface, we covalently immobilized HxGltn on the surface of SiNW plates. A significant increase in bonding strength was not observed when Gltn-, 27HxGltn-immobilized 800/1400 SiNW, and Gltn-, 27HxGltn, 37HxGltn, 50HxGltn-immobilized 1500/1400 SiNW were used. Furthermore, 37HxGltn- and 50HxGltn-immobilized 800/1400 SiNW only showed 5-fold higher bonding strength to intestine tissue compared with other samples. These 37HxGltn- and 50HxGltn-immobilized 800/1400 SiNW surfaces had potential for the application of soft tissue adhesion.

Observation of Crystalline Oxidized Silicon Phase

AbstractSilica phases, SiOx forming at surfaces of various silicon crystals, e.g., Si wafers, nanowires, and nanoparticles via Si oxidation are key building blocks of diverse applications in the fields of electronics, medicine, and photonics for instance. The Si oxidation has been established to produce amorphous SiOx films, and the resulting oxide/silicon structures are prototypical junctions or contacts of amorphous and crystalline materials of which formation and properties have been extensively studied to understand and develop functionality of SiOx/Si in the applications. Here, observation of hitherto undiscovered phase of the SiOx/Si material, which is crystalline without traditional amorphous silica, is presented. The crystalline SiOx/Si structures are produced in vacuum environment via controlled oxidations of Si where O atoms are incorporated beneath the topmost Si layer. Concomitantly some Si atoms are detached from the crystal, and diffuse to the topmost surface, consistent with previous theoretical predictions, retaining still a pure Si‐surface type reconstruction and leading to a crystalline stack of Si/SiOx/Si. In addition to providing a well‐defined platform to studies of the SiOx/Si system, the found crystalline phase is also hypothesized to decrease amounts of disorder‐induced defects in the applications. Presented electrical characterization via carrier‐lifetime and capacitor measurements support the hypothesis.

Enhanced humidity resistance of porous SiNWs via OTS functionalization for rarefied NO2 detection

Porous silicon nanowires (SiNWs) is considered as one of promising candidates for high-performance NO2 sensor application in human health and quality life owing to its fast response and stable operation at room temperature. However, the development of SiNWs-based gas sensors for stable detection of ppb-level NO2 in practical high-humidity environments remains a challenge. In this study, octadecyltrichlorosilane (OTS) was utilized to modify partially insensitive SiNWs surface with the Si-O bond, yielding an immunity to high humidity in gas detection process. The modified OTS provides an umbrella-like barrier blocking water molecules attraction on the nanowire surface which disinhibits the main adsorption sites and sensing channels of the sensor. The OTS-modified porous SiNWs (OTS/SiNWs) sensor exhibited a practical capability to detect 5 ppb NO2 at humidity of 25%–55% RH and a response of 1.8% to 50 ppb NO2 at 75% RH at room temperature. Moreover, the fast response/recovery characteristics and good reversibility of the OTS/SiNWs sensor was observed even under high humidity conditions. In addition, the OTS/SiNWs sensor displayed excellent stability to humidity variations. The enhanced gas sensing performance is attributed to a marriage of porous structure of SiNWs and OTS. The simple modification for SiNWs-based gas sensor will broaden new avenues, not only in chemical trace detection, but also in high humidity health monitoring.

Nanostructured Silicon‐Based Heterojunction Solar Cells with Double Hole‐Transporting Layers

AbstractHybrid nanostructured silicon–organic solar cells have been pursued as a low‐cost solution for silicon photovoltaic devices. However, it is difficult for the organic semiconductor, typically poly(3,4‐ethylenedioxythiophene):polystyrene (PEDOT:PSS), to fully cover the nanostructured silicon surface due to the high surface tension of the polymer solution and the small size of the cavities in nanostructured silicon. As a result, the performance of the hybrid solar cells is limited by the defect‐induced surface recombination and poor hole extraction. In this work, an inorganic hole‐transporting layer, copper(I) thiocyanate (CuSCN), is introduced between silicon nanowire (SiNW) and PEDOT:PSS to improve the junction quality. The effect of CuSCN on as‐fabricated SiNW and tetramethylammonium hydroxide (TMAH)‐treated SiNW structures is examined, and it is shown that in both cases CuSCN can well cover the SiNW surface due to the easy penetration of its solution into the silicon nanostructure. As a result, the power conversion efficiency of the solar cells has been dramatically improved from 7.68% to 10.5% for as‐fabricated SiNW‐based‐hybrid cells, and from 10.75% to 12.24% for TMAH‐passivated SiNW‐based‐hybrid cells, suggesting that the double hole‐transporting layer approach can effectively improve the junction quality in hybrid organic‐nanostructured silicon‐based devices.

Enhanced flow boiling in silicon nanowire-coated manifold microchannels

An experimental study was carried out to investigate the heat transfer, pressure drop and flow instability characteristics associated with the flow pattern of deionized water during two-phase boiling in a silicon-based manifold microchannel heat sink coated with silicon nanowires (SiNWs) compared to a plain-wall device. The manifold microchannel device featured parallel transverse microchannels etched on a silicon substrate and longitudinal microchannels etched on a glass cover plate. Silicon nanowires were generated on the bottom and the sidewalls of the silicon microchannels. A closed-loop experimental system was constructed to demonstrate thermal and hydraulic performance. Experimental results were presented with mass fluxes ranging from 250 to 1250 kg/m2 s and subcooled inlet temperatures from 15 K to 65 K. Results for the SiNWs device showed an approximate 20% improvement in heat flux rejection compared to the plain-wall device under the same wall superheat conditions. A subcooled inlet temperature of 65 K associated with a mass flux of 1250 kg/m2 s is shown to be capable of dissipating an effective heat flux of 431.3 W/cm2 with a wall superheat of about 85 K. Overall, the SiNW coatings proved positive effects on enhancing the flow boiling heat transfer with slower pressure drop increase, meanwhile the three-dimensional manifold microchannel design is revealed to effectively mitigate flow instability during the entire single and two-phase flow regions. This indicates great potential in utilizing three-dimensional flows by integrating SiNWs surface structures in high heat flux cooling applications.

Investigations on mercury ion detection in aqueous solution by triglycine surface activated porous silicon nanowires

Porous silicon nanowires (PSiNWs) surfaces were grafted with organic functional groups. The obtained hybrid structure was tested as probe electrode for the electrochemical detection of mercury (Hg(II)) in solution. An electrochemical study of the Glycyl-Glycyl-Glycine-modified PSiNWs electrode is achieved in the presence of mercury ions. The recorded cyclic voltammograms show a quasi-irreversible process corresponding to the Hg(II)/Hg couple. These results demonstrate the potential role of peptides grafted on porous silicon nanowire in developing strategies for simple and fast detection of metal ions in solution.

Silicon nanowires (SiNWs) surface engineering potential for bioenergy

Silicon nanowires (SiNWs) surface engineering potential for bioenergy

Effective Removal of Surface Recombination Centers in Silicon Nanowires Fabricated by Metal-Assisted Chemical Etching

High-density silicon nanowires (SiNWs) were fabricated on p-Si substrates by metal-assisted chemical etching (MACE). A chemical cleaning with HF/piranha/HF was used to remove structural defects from the nanowire surface which are at the origin of surface recombination states. Surface photovoltage (SPV) spectroscopy, capacitance–voltage (C–V), and conductance measurements were used to study the SiNW properties and assess the effectiveness of the cleaning process. For the as-grown samples, the analysis of the SPV amplitude and phase spectra demonstrated high density of positively charged surface recombination centers. These centers increase the surface band bending and decrease the minority carrier lifetime and diffusion length. Si nanostructures at the SiNW surface introduce slow traps, as evidenced by the frequency dispersion at accumulation of the C–V curves and the increase of the diffusion length with frequency. All these features are effectively reduced when the SiNWs are subjected to HF/piranha/HF chemical cleaning. Thus, a cheap and effective way is demonstrated to reduce the surface state density and improve the electronic quality of the surface of SiNWs.

Pt nanoparticle/Si nanowire composites as an excellent catalytic counter electrode for dye-sensitized solar cells

Recently, much attention has focused on less platinum alternative materials serving as the counter electrode material for dye-sensitized solar cells, while platinum itself has a lot of room for improvement. Herein, we introduce the earth-abundant silicon and combine it with Pt nanoparticles as the counter electrode in dye-sensitized solar cells to enlarge surface area and enhance the activity of Pt. Si-H bonds can reduce metal ions and enable small metal nanoparticles grown on the surface of silicon nanowires, which can effectively prevent metal nanoparticles from agglomeration in the catalysis. The density functional theory (DFT) calculation shows that the adsorption energy of I atom on the Pt/Si interface is −0.8 eV, which is the optimal adsorption energy for triiodide reduction, indicating that Pt/Si is a perfect material for the counter electrode. The electrochemical characterizations and the photocurrent-voltage performance experimentally confirm that Pt/SiNW is a better counter electrode material than other metal/SiNW composites and Pt, which is in accordance with DFT calculations. The power conversion efficiency of device based on the Pt/SiNW electrode is higher than that of the device based on Pt (8.23% vs.7.93%).

Palladium – silicon nanocomposites as a stable electrocatalyst for hydrogen evolution reaction

Pd nanoparticles grown in-situ on the surface of silicon nanowires (Pd-SiNW nanocomposites) were employed as electrocatalysts in hydrogen evolution reaction and exhibited satisfactory catalytic performance. Here, SiNWs served as the carrier with vast surface area. It is interesting to note that Pd-SiNW nanocomposites were more stable for hydrogen evolution reaction than the commercial Pt/C catalysts, which might be attributed to the three-dimensional crisscrossing structure of the composites and the strong interaction between Pd and Si. The strong interaction is due to the low lattice mismatch in the epitaxial growth of Pd particles on the surface of Si. Such special structure and interaction help to tightly hold Pd nanoparticles, which could avoid the loss of catalysts and keep the catalyst from being agglomerated and growing large.

Fabrication of silicon nanowire based solar cells using TiO2/Al2O3 stack thin films

ABSTRACTTo improve conversion efficiency of silicon nanowire (SiNW) solar cells, it is very important to reduce the surface recombination rate on the surface of SiNWs, since SiNWs have a large surface area. We tried to cover SiNWs with aluminum oxide (Al2O3) and titanium oxide (TiO2) by atomic layer deposition (ALD), since Al2O3 grown by ALD provides an excellent level of surface passivation on silicon wafers and TiO2 has a higher refractive index than Al2O3, leading to the reduction of surface reflectance. The effective minority carrier lifetime in SiNW arrays embedded in a TiO2/Al2O3 stack layer of 94 μsec was obtained, which was comparable to an Al2O3 single layer. The surface reflectance of SiNW solar cells was drastically decreased below around 5% in all of the wavelength range using the Al2O3/TiO2/Al2O3 stack layer. Heterojunction SiNW solar cells with the structure of ITO/p-type hydrogenated amorphous silicon (a-Si:H)/n-type SiNWs embedded in Al2O3 and TiO2 stack layer for passivation/n-type a-Si:H/back electrode was fabricated, and a typical rectifying property and open-circuit voltage of 356 mV were successfully obtained.

Enhanced vapour sensing using silicon nanowire devices coated with Pt nanoparticle functionalized porous organic frameworks.

Recently various porous organic frameworks (POFs, crystalline or amorphous materials) have been discovered, and used for a wide range of applications, including molecular separations and catalysis. Silicon nanowires (SiNWs) have been extensively studied for diverse applications, including as transistors, solar cells, lithium ion batteries and sensors. Here we demonstrate the functionalization of SiNW surfaces with POFs and explore its effect on the electrical sensing properties of SiNW-based devices. The surface modification by POFs was easily achieved by polycondensation on amine-modified SiNWs. Platinum nanoparticles were formed in these POFs by impregnation with chloroplatinic acid followed by chemical reduction. The final hybrid system showed highly enhanced sensitivity for methanol vapour detection. We envisage that the integration of SiNWs with POF selector layers, loaded with different metal nanoparticles will open up new avenues, not only in chemical and biosensing, but also in separations and catalysis.

CuInS2/SiNWs/Si composite material for application as potential photoelectrode for photoelectrochemical hydrogen generation

This work aims at developing a new composite material based on nanosized semiconducting CuInS2 (CIS) particles combined with silicon nanowires grown on a silicon substrate (SiNWs/Si) for photoelectrochemical (PEC)-splitting of water. The CIS particles were prepared via a colloidal method using N-methylimidazole (NMI) as the solvent and an annealing treatment. The SiNWs were obtained by chemical etching of silicon (100) substrates assisted by a metal. The CIS/SiNWs/Si composite material was obtained by deposition of an aliquot of a suspension of CIS particles onto the SiNWs/Si substrate, using spin coating followed by a drying step. The XRD pattern demonstrated that CuInS2 grows in the tetragonal/chalcopyrite phase, while SiNWs/Si presents a cubic structure. The SEM images show semi-spherical particles (∼10 nm) distributed on the surface of silicon nanowires (∼10 μm). The EIS measurements reveal n-type conductivity for CIS, SiNWs/Si and CIS/SiNWs/Si materials, which could favour the oxidation reaction of water molecules.

Controlled growth of MoS2 nanopetals on the silicon nanowire array using the chemical vapor deposition method

In order to get a physical/chemical insight into the formation of nanoscale semiconductor heterojunctions, MoS2 flakes are deposited on the silicon nanowire (SiNW) array by chemical vapor deposition (CVD). In this study, H2O2 treatment provides a favorable place where the formation of SiO bonds on the SiNW surfaces that play important roles (i.e., the nucleation centers, catalyst control centers or “seeds”) can dominate the growth of MoS2 on the SiNWs. Using this configuration, the effect of a change in the S/MoO3 mass ratio (MS/MMoO3) on the surface morphology of MoS2 is studied. It is shown that an increase in the value of MS/MMoO3 leads to the increased nucleation rate, increasing the size of MoS2 nanopetals. This study provides valuable scientific information for directly CVD-grown edge-oriented MoS2/SiNWs heterojunctions for various nanoscale applications, including hydrogen evolution reaction and electronic and optoelectronic devices.

Probing Intrawire, Interwire, and Diameter-Dependent Variations in Silicon Nanowire Surface Trap Density with Pump–Probe Microscopy

Surface trap density in silicon nanowires (NWs) plays a key role in the performance of many semiconductor NW-based devices. We use pump-probe microscopy to characterize the surface recombination dynamics on a point-by-point basis in 301 silicon NWs grown using the vapor-liquid-solid (VLS) method. The surface recombination velocity (S), a metric of the surface quality that is directly proportional to trap density, is determined by the relationship S = d/4τ from measurements of the recombination lifetime (τ) and NW diameter (d) at distinct spatial locations in individual NWs. We find that S varies by as much as 2 orders of magnitude between NWs grown at the same time but varies only by a factor of 2 or three within an individual NW. Although we find that, as expected, smaller-diameter NWs exhibit shorter τ, we also find that smaller wires exhibit higher values of S; this indicates that τ is shorter both because of the geometrical effect of smaller d and because of a poorer quality surface. These results highlight the need to consider interwire heterogeneity as well as diameter-dependent surface effects when fabricating NW-based devices.

Folate functionalized silicon nanowires with highly enhanced adhesion to cancer cells

AbstractIn this study, we implemented a functionalization process on the surface of silicon nanowires (SiNWs) which were fabricated by chemical vapour deposition (CVD) method in a low‐pressure CVD (LPCVD) chamber. Surface functionalization was carried out in three stages: hydroxylation of the surface with piranha solution, silanization with a solution of 3‐Aminopropyl triethoxysilane (APTES) in n‐hexane, and finally functionalization with folic acid as a detection agent. Optical spectroscopy and electron microscopy techniques were used to experimentally characterize materials. Surface functionalization was evaluated using Attenuated Total Reflectance Fourier‐Transform Infrared spectroscopy (FTIR‐ATR), Field Emission Scanning Electron Miscoscopy (FESEM), and Transmission Electron Microscopy (TEM). Cell adhesion to functionalized and non‐functionalized silicon nanowires was compared; a cell assay analysis on lung metastatic cells was conducted. Lung cancer metastatic QUDB cells were seeded on a SiNWs surface in RPMI1640 complete medium. To evaluate the adhesion of cancer cells to these surfaces FESEM imaging was used. The number of cancer cells trapped on the folic acid functionalized SiNWs was found to be approximately 3 times higher than that of the non‐functionalized ones.

Pencil-shaped silicon nanowire synthesis and photovoltaic application

Pencil-shaped silicon nanowires (SiNWs) were synthesized by colloidal lithography and inductively coupled plasma reactive ion etching for photovoltaic application. Light reflectance of below 10% could be obtained by the asymmetric pencil-shaped SiNW array. The pencil-shaped SiNW structure and properties were investigated and compared with metal catalyzed electroless etching (MCEE) and nanoimprinted SiNWs. Single-junction solar cells consisting of pencil-shaped n-SiNW substrates and p-Si shell layer grown by chemical vapor deposition were fabricated. A combination of two-step H2 annealing and back surface field formation was applied to improve the solar cell properties. Good crystal quality and surface of pencil-shaped SiNWs provided an excellent solar cell junction interface. The great light trapping of the pencil-shaped SiNW solar cell could enhance the conversion efficiency by more than 0.6% compared to the solar cells using MCEE and nanoimprinted SiNWs.