Porous Si Nanowires Research Articles

Cu and Ni Co-Doped Porous Si Nanowire Networks as High-Performance Anode Materials for Lithium-Ion Batteries.

Due to its extremely high theoretical mass specific capacity, silicon is considered to be the most promising anode material for lithium-ion batteries (LIBs). However, serious volume expansion and poor conductivity limit its commercial application. Herein, dealloying treatments of spray dryed Al-Si-Cu-Ni particles are performed to obtain a Cu/Ni co-doped Si-based anode material with a porous nanowire network structure. The porous structure enables the material to adapt to the volume changes in the cycle process. Moreover, the density functional theory (DFT) calculations show that the co-doping of Cu and Ni can improve the capture ability towards Li, which can accelerate the electron migration rate of the material. Based on the above advantages, the as-prepared material presents excellent electrochemical performance, delivering a reversible capacity of 1092.4 mAh g-1 after 100 cycles at 100 mA g-1. Even after 500 cycles, it still retains 818.7 mAh g-1 at 500 mA g-1. This study is expected to provide ideas for the preparation and optimization of Si-based anodes with good electrochemical performance.

Molecular dynamics simulation of hollow and porous amorphous silicon nanowires coated with amorphous aluminum oxide

Silicon causes structural changes and residual stresses during the process of lithium-ion penetration. Aluminum oxide coatings with an optimal thickness can prevent the structural variations and failure of the anode, though it will affect the ionic conductivity. In this research, the mechanical and electrochemical properties of the aluminum oxide-coated silicon nanowires are investigated by the molecular dynamics simulations. The results of the simulations show that the capacity increases, and the dimensional stability is maintained to a suitable extent by applying the void space or creating porosity in the coated nanowires. Furthermore, the volumetric changes for the base sample decrease from 56.3% to 10.25% and 19.1% for porous and hollow Si nanowires samples, respectively. Nevertheless, the capacity is increased from 1015.3 to 1063.8, 1219.6, and 1573.3 mAhg−1 for porous, closed, and open-hole samples, respectively. The radial distribution of the residual stresses and elemental mapping are also measured to investigate the effect of porosity and void space on the amorphous nanowires.

Chemically modified surface of silicon nanostructures to enhance hydrogen uptake capabilities

Hydrogen is an active substitute for current commercial fuels as a green energy source. However, the progress toward hydrogen economy is stalled due to a lack of efficient and economical storage systems usually restrained by the material used. Some specific issues still need to improve in recent practical hydrogen storage methods: low surface area, limited capacity to attract gas adsorbate, high-temperature medium for hydrogen desorption, and extreme reactivity towards Oxygen and air. The paper presents a study on the surface modification of silicon nanostructures (SiNSs) synthesized by chemical etching, followed by structure optimization to enhance the hydrogen storage capacity. The standard anodization method prepares porous silicon (PS) while metal-assisted chemical etching fabricates Si and Porous Si nanowires (PSiNWs) on Si and PS substrates. Morphological features like surface area and porous nature were evaluated using SEM, AFM, and BET to interpret the gas-surface interaction. The modified surface area and average roughness of SiNSs increase maximum to 437 m2 g−1 and 501 nm, respectively. The PS shows the highest gas-surface interaction (up to 86.6%) and surface energy due to uniform pore distributions and high surface area. The NMR and FTIR investigate the hydrogen termination on the surface, whereas TG-DSC indicates the desorption of surface hydrides at ∼290 °C. A rank matrix is constructed considering various adsorption-desorption and fabrication parameters to select optimized SiNS, whereas PS is selected as a suitable candidate for hydrogen storage.

High thermoelectric figure of merit of porous Si nanowires from 300 to 700\u2009K

Thermoelectrics operating at high temperature can cost-effectively convert waste heat and compete with other zero-carbon technologies. Among different high-temperature thermoelectrics materials, silicon nanowires possess the combined attributes of cost effectiveness and mature manufacturing infrastructures. Despite significant breakthroughs in silicon nanowires based thermoelectrics for waste heat conversion, the figure of merit (ZT) or operating temperature has remained low. Here, we report the synthesis of large-area, wafer-scale arrays of porous silicon nanowires with ultra-thin Si crystallite size of ~4 nm. Concurrent measurements of thermal conductivity (κ), electrical conductivity (σ), and Seebeck coefficient (S) on the same nanowire show a ZT of 0.71 at 700 K, which is more than ~18 times higher than bulk Si. This ZT value is more than two times higher than any nanostructured Si-based thermoelectrics reported in the literature at 700 K. Experimental data and theoretical modeling demonstrate that this work has the potential to achieve a ZT of ~1 at 1000 K.

Incorporation of Pt Nanoparticles on the Surface of TeO₂-Branched Porous Si Nanowire Structures for Enhanced Room-Temperature Gas Sensing.

A new gas sensor working in room temperature, which is compatible with silicon fabrication technology is presented. Porous silicon nanowires (NWs) were synthesized by metal-assisted chemical etching method and then TeO₂ NWs branches were attached to their stem by thermal evaporation of Te powders in the presence of air. Afterwards TeO₂ branched porous Si NWs were functionalized by Pt via sputtering followed by low temperature thermal annealing. Scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy collectively confirmed successful formation of TeO₂ branched porous Si NWs functionalized by Pt nanoparticles. Their gas sensing properties in the presence of CO, C6H6 and C7H8 were tested at room temperature, for Si wafer, pristine porous Si NWs, pristine TeO₂ branched porous Si NWs, and Pt functionalized TeO₂ branched porous Si NWs sensors. Pt functionalized TeO₂ branched porous Si NWs have higher responses to all tested gases than the other sensors. The origin of high response is discussed in detail. This new room temperature gas sensor can open a new aperture for development of gas sensors with minimum energy consumption which are compatible with silicon fabrication technology.

Porous Si/SnO2 nanowires heterostructures for H2S gas sensing

In this work, a low-temperature (100 °C) gas sensor based on heterojunctions of porous Si and SnO2 nanowires (NWs) is presented. Porous Si was obtained from p-Si wafers by electrochemical etching, and SnO2 NWs were fabricated by a vapor-liquid-solid route. Different characterization techniques were used to verify the formation of porous Si/SnO2 NW heterojunctions. H2S gas sensing results showed enhanced gas sensing performance of the porous Si/SnO2 NW sensor in comparison with that of a porous Si sensor. The reasons for such enhancement are discussed in detail. This study demonstrates the promising effects of SnO2 NWs in combination with porous Si to realize low-temperature H2S gas sensors that are highly compatible with existing Si processing technology.

Spectroscopic Characterization of Silicon Wire‐Like and Porous Nanolayers in the Process of Metal‐Assisted Chemical Etching of Single‐Crystal Silicon

The preparation of single‐crystal silicon (c‐Si) nanowires and porous nanolayers using the two‐stage metal‐assisted chemical etching (MACE) process is studied. The structures are characterized in all stages of the process by spectroscopic ellipsometry and using the following two approaches: 1) with the complex pseudo‐dielectric function ε determined and analyzed and 2) with parameters of simulated multilayer structures determined in the effective medium approximation. With the structural parameters found from scanning electron microscopy (SEM) images taken into consideration, spectra of ψ and Δ are calculated and fitted to the experimental spectra to obtain the best convergence. The thicknesses and the fractions of the components (Si:Air) in the porous Si layers and Si nanowires fabricated in the study are determined. The two‐step MACE method, compared with the single‐step process, is more promising because of the ability to control the morphology of Si nanowires using a preset morphology of Ag nanoparticles (catalyst mask). The spectroscopic ellipsometry and SEM are nondestructive and informative methods of study that can be used to obtain samples with required parameters.

Unique Scattering Properties of Silicon Nanowires Embedded with Porous Segments.

The development of advanced imaging tools is important for the investigation of the fundamental properties of nanostructures composed of single or multiple nanomaterials. However, complicated preparation processes and irreversible alterations of the samples to be examined are inevitable in most current imaging techniques. In this work, we developed a simple method based on polarization-resolved light scattering measurements to characterize the structural and optical properties of complex nanomaterials. In particular, we examined a single Si nanowire embedded with porous Si segments, in which the porous Si could not be easily distinguished from solid Si by scanning electron microscopy. The dark-field optical images and polarization-resolved scattering spectra showed unique optical features of porous and solid Si. In particular, the porosity, diameter, and number of porous Si segments in the single Si nanowire were identified from the scattering measurements. In addition, we performed systematic optical simulations based on the effective medium model in individual porous and solid Si nanowires. A good agreement between the simulation and measurement results enabled the estimation of the structural parameters of the nanowires, such as diameter and porosity. We believe that our method will be useful for analyzing the structural and optical properties of nanomaterials prior to using complicated and uneconomical imaging tools.

Self-formed silver nanoparticles on freestanding silicon nanowire arrays featuring SERS performances.

Herein, the universal luminescence characteristics of porous Si nanowire arrays were exploited using a wide range of doping types and concentrations; we found that the dual-band photoluminescence intensities were correlated with the formation rates of Si nanowires with porous features; however, these intensities exhibited no evident dependence on the doping conditions. Furthermore, we demonstrated a facile and reliable transfer method implementing the freestanding Si nanowire arrays while maintaining the robust photoluminescence behaviors under bending conditions. The fabrication protocol, involving lateral etching locally at the nanowire ends, enabled the controlled formation of uniform and large-area transferred nanowires with vertical regularity. Without the additional deposition of Ag nanoparticles, these transferred Si nanowire films inherently possessed SERS sensing capability with a relative enhancement factor over 1.8 times that of the Si nanowires with electroless-deposited Ag nanoparticles, which could practically emerge as a functional design for the integration of practical biochip devices.

Impact of pore anisotropy on the thermal conductivity of porous Si nanowires

Porous materials display enhanced scattering mechanisms that greatly influence their transport properties. Metal-assisted chemical etching (MACE) enables fabrication of porous silicon nanowires starting from a doped Si wafer by using a metal template that catalyzes the etching process. Here, we report on the low thermal conductivity (κ) of individual porous Si nanowires (NWs) prepared from MACE, with values as low as 0.87 W·m−1·K−1 for 90 nm diameter wires with 35–40% porosity. Despite the strong suppression of long mean free path phonons in porous materials, we find a linear correlation of κ with the NW diameter. We ascribe this dependence to the anisotropic porous structure that arises during chemical etching and modifies the phonon percolation pathway in the center and outer regions of the nanowire. The inner microstructure of the NWs is visualized by means of electron tomography. In addition, we have used molecular dynamics simulations to provide guidance for how a porosity gradient influences phonon transport along the axis of the NW. Our findings are important towards the rational design of porous materials with tailored thermal and electronic properties for improved thermoelectric devices.

Carbon-coated Si particles with self-supported Si nanowires as anode material for lithium-ion batteries

Although extremely high specific capacity has been envisioned for Si anode materials, the application of Si in lithium-ion batteries (LIBs) is still hampered due to the rapid capacity fading mainly caused by the huge volume changes during the cycling. In this study, carbon-coated Si particles with self-supported Si nanowires were prepared using low-cost raw materials by a versatile and scalable method and investigated as anode materials in LIBs. The single Si particle with size of about 30 ∼ 40 μm is composed of a solid Si core and numerous porous Si nanowires radially distributed on the Si core. This unique architecture can effectively accommodate the volume expansion of Si, release the mechanical stress, prevent the aggregation of Si nanowires and keep the structural integrity. Utilized as anode materials for LIBs, the as-prepared carbon-coated Si particles show improved cycle stability and high initial coulombic efficiency than the raw metallurgical Si particles.

Porous Si nanowires for highly selective room-temperature NO2 gas sensing

We report the room-temperature sensing characteristics of Si nanowires (NWs) fabricated from p-Si wafers by a metal-assisted chemical etching method, which is a facile and low-cost method. X-ray diffraction was used to the the study crystallinity and phase formation of Si NWs, and product morphology was examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). After confirmation of Si NW formation via the SEM and TEM micrographs, sensing tests were carried out at room temperature, and it was found that the Si NW sensor prepared from Si wafers with a resistivity of 0.001–0.003 Ω.cm had the highest response to NO2 gas (Rg/Ra = 1.86 for 50 ppm NO2), with a fast response (15 s) and recovery (30 s) time. Furthermore, the sensor responses to SO2, toluene, benzene, H2, and ethanol were nearly negligible, demonstrating the excellent selectivity to NO2 gas. The gas-sensing mechanism is discussed in detail. The present sensor can operate at room temperature, and is compatible with the microelectronic fabrication process, demonstrating its promise for next-generation Si-based electronics fused with functional chemical sensors.

Enhanced photoelectrochemical performance of Si nanowires by etching a single-crystal Si(100) wafer

Si photoelectrodes with two different morphology, including the porous Si (PSi) and Si nanowires (SiNWs), have been prepared via a facile wet-chemical etching method. The SiNW displays acceptable stability and photocurrent density (J) of 1.1 mA∙cm−2 at 1.23 V versus (vs.) reversible hydrogen electrode (RHE) under solar irradiation, which are superior to the PSi and attributed to the improved surface area and light harvesting capability. Electrochemical impedance technique is employed to analyze the mechanism underlying the differences in PEC performance. A prototype PEC bio-sensor is fabricated using a SiNW electrode for detection of glutathione (GSH), leading to acceptable detect limit of 0.81 μmol L−1 (μM) and sensitivity of 61.32 mA cm−2 M−1. Further, it is found that both the PEC stability and photoresponse of SiNW electrodes could be enhanced efficiently by depositing thin Mo layers. Our studies demonstrated the prospect of Si as an electrode material for applying in the PEC or bio-sensing fields.

Highly Porous Silicon Nanowires Made with Solvent‐Mediated Wet Chemical Etching and Their Thermoelectric Applications

AbstractPorous silicon (Si) nanostructures with heavily doping concentrations possess many intriguing properties different from bulk Si, but the fabrication remains greatly challenging for the large‐quantity production. Therefore, in this study, the heavily doped Si nanowires with highly porous features were prepared using a co‐solvent mediated metal‐assisted chemical etching (MaCE). Unlike the conventional MaCE method with sole water as solvents, the mixed co‐solvents, with the addition of highly volatile methanol at ambient temperature are found to facilitate the significant sidewall etching through the evaporation of solvents when etching reaction proceeds. In addition, thermoelectric performances of incorporating such porous Si nanowires as inclusions with (BiSb)2Te3 particles as matrix were investigated. The beneficial effects, examined from three various amounts of Si nanowires dispersed in (BiSb)2Te3 matrix, are originated from the supplement of increased carrier concentrations. Meanwhile, the increase of thermal conductivity is not obvious due to the existence of porous Si nanowires, thus leading to the improved figure of merit (ZT) value, which is more than 1.3 times higher than that of pure (BiSb)2Te3 samples.

In Situ and Ex Situ TEM Study of Lithiation Behaviours of Porous Silicon Nanostructures.

In this work, we study the lithiation behaviours of both porous silicon (Si) nanoparticles and porous Si nanowires by in situ and ex situ transmission electron microscopy (TEM) and compare them with solid Si nanoparticles and nanowires. The in situ TEM observation reveals that the critical fracture diameter of porous Si particles reaches up to 1.52 μm, which is much larger than the previously reported 150 nm for crystalline Si nanoparticles and 870 nm for amorphous Si nanoparticles. After full lithiation, solid Si nanoparticles and nanowires transform to crystalline Li15Si4 phase while porous Si nanoparticles and nanowires transform to amorphous LixSi phase, which is due to the effect of domain size on the stability of Li15Si4 as revealed by the first-principle molecular dynamic simulation. Ex situ TEM characterization is conducted to further investigate the structural evolution of porous and solid Si nanoparticles during the cycling process, which confirms that the porous Si nanoparticles exhibit better capability to suppress pore evolution than solid Si nanoparticles. The investigation of structural evolution and phase transition of porous Si nanoparticles and nanowires during the lithiation process reveal that they are more desirable as lithium-ion battery anode materials than solid Si nanoparticles and nanowires.

Inorganic Nano Light-Emitting Transistor: p-Type Porous Silicon Nanowire/n-Type ZnO Nanofilm.

An inorganic nano light-emitting transistor (INLET) consisting of p-type porous Si nanowires (PoSiNWs) and an n-type ZnO nanofilm was integrated on a heavily doped p-type Si substrate with a thermally grown SiO2 layer. To verify that modulation of the Fermi level of the PoSiNWs is key for switchable light emitting, I-V and electroluminescent characteristics of the INLET are investigated as a function of gate bias (V g ). As the V g is changed from 0 V to -20 V, the current level and light-emission intensity in the orange-red range increase by three and two times, respectively, with a forward bias of 20 V in the p-n junction, compared to those at a V g of 0 V. On the other hand, as the V g approaches 10 V, the current level decreases and the emission intensity is reduced and then finally switched off. This result arises from the modulation of the Fermi level of the PoSiNWs and the built-in potential at the p-n junction by the applied gate electric field.

Thermal transport in porous Si nanowires from approach-to-equilibrium molecular dynamics calculations

We study thermal transport in porous Si nanowires (SiNWs) by means of approach-to-equilibrium molecular dynamics simulations. We show that the presence of pores greatly reduces the thermal conductivity, κ, of the SiNWs as long mean free path phonons are suppressed. We address explicitly the dependence of κ on different features of the pore topology—such as the porosity and the pore diameter—and on the nanowire (NW) geometry—diameter and length. We use the results of the molecular dynamics calculations to tune an effective model, which is capable of capturing the dependence of κ on porosity and NW diameter. The model illustrates the failure of Matthiessen's rule to describe the coupling between boundary and pore scattering, which we account for by the inclusion of an additional empirical term.

Functionalized porous Si nanowires for selective and simultaneous electrochemical detection of Cd(II) and Pb(II) ions

Thiol and amino-functionalized porous Si nanowires have been demonstrated to selectively detect Cd(II) and Pb(II) with high sensitivity using square wave anodic stripping voltammetry (SWASV) analysis. Through a typical Ag-assisted chemical etching approach, uniform and porous Si nanowires have been fabricated on heavily doped Si wafer. Continuously functionalized by (3-mercaptopropyl)trimethoxysilane (MPTMS) and (3-aminopropyl)triethoxysilane (APTES), the internal and external surfaces of porous Si nanowires have been decorated with thiol and amino groups, respectively. Inspired by the intrinsic properties of porous nanostructures, their electrochemical performances toward heavy metal ions have been further evaluated. The results indicate that Cd(II) and Pb(II) can be detected with high sensitivity owing to the strong complexing capacity of thiol and amino groups. Additionally, porous Si nanowires decorated with thiol groups present different stripping behaviors from those with amino groups toward the investigated heavy metal ions, realizing the selective and simultaneous detection of Cd(II) and Pb(II). Furthermore, simultaneous detection of Cd(II) and Pb(II) has also been demonstrated without any interference and their individual high sensitivity has been fundamentally preserved. Expectedly, porous Si nanowires as an effective modifier can be further extended to selectively detect other heavy metal ions through modifying with other functional organic groups.

Thermally activated tunneling in porous silicon nanowires with embedded Si quantum dots

Electronic transport properties of porous Si nanowires either with embedded Si quantum dots or with a percolative crystalline path are studied as a function of the temperature for the first time. We show that unlike bulk porous Si, the predesigned structure of the wires results in a single distinct conduction mechanism such as tunneling in the former case and variable range hopping in the latter case. We demonstrate that the geometry of the systems with a large internal surface area and high density of the Si quantum dots have a significant conduction enhancement compared to bulk porous silicon. These results can also improve the understanding of the basis of the different electronic transport mechanisms reported in bulk porous silicon.

Surface plasmon enhanced photoluminescence from porous silicon nanowires decorated with gold nanoparticles

About 8-fold photoluminescence enhancement is realized in porous Si nanowires via coupling with the surface plasmon of Au nanoparticles.