Surface Of Silicon Nanowires Research Articles

A bottom-gate silicon nanowire field-effect transistor with functionalized palladium nanoparticles for hydrogen gas sensors

The highly sensitive operation of a bottom-gate silicon nanowire (SiNW) field-effect transistor (FET)-based hydrogen (H2) sensor is demonstrated by controlling the working regime of the sensor. It is observed that the deposition of palladium (Pd) nanoparticles on the SiNW surface for the selective absorption of H2 can result in a significant enhancement of the electrostatic properties, such as the subthreshold swing and on-current, of the SiNW FET-based H2 sensor. By comparing the experimental results with the numerical simulation, we conclude that the improvement of the electrostatic properties of the sensor is due to the coupling effect between the electrostatic potentials in the Pd nanoparticle and bottom gate. Based on these results, highly sensitive detection of H2 gas could be achieved in the subthreshold regime where the gating effect induced by absorbed H2 gas is the most effective.

Controlling the Sensing Properties of Silicon Nanowires via the Bonds Nearest to the Silicon Nanowire Surface.

Controlling the sensing properties of a silicon nanowire field effect transistor is dependent on the surface chemistry of the silicon nanowire. A standard silicon nanowire has a passive oxide layer (native oxide), which has trap states that cause sensing inaccuracies and desensitize the surface to nonpolar molecules. In this paper, we successfully modified the silicon nanowire surface with different nonoxide C3 alkyl groups, specifically, propyl (Si-CH2-CH2-CH3), propenyl (Si-CH═CH-CH3), and propynyl (Si-C≡C-CH3) modifications. The effect of the near surface bond on the sensor sensitivity and stability was explored by comparing three C3 surface modifications. A reduction of trap-states led to greater sensor stability and accuracy. The propenyl-modified sensor was consistently the most stable and sensitive sensor, among the applied sensors. The propenyl- and propynyl-modified sensors consistently performed with the best accuracy in identifying specific analytes with similar polarity or similar molecular weights. A combination of features from different sensing surfaces led to the best rubric for specific analytes identification. These results indicate that nonoxide sensor surfaces are useful in identifying specific analytes and that a combination of sensors with different surfaces in a cross-reactive array can lead to specific analytes detection.

Observation of Au nanoparticles on the surface of silicon nanowire grown by rapid thermal chemical vapour deposition

The size evolution of gold (Au) nanoparticles (NPs) on the sidewall surface of silicon (Si) nanowires (NWs) has been investigated by thermal treatments, using high-angle annular dark field scanning transmission electron microscopy. The Si NWs grown at 550°C by rapid thermal chemical vapour deposition have been observed to be surrounded by Au NPs with less than 5 nm diameter and ∼1012 cm−2 density on the whole Si NW surface. To explore the size change of Au NPs, the Au NPs on the Si NW were annealed ex situ at the temperature range of 700–900°C for 20 min. The sizes of NPs for samples annealed at 700, 800 and 900°C represent Gaussian distribution with the average size of 4, 6 and 7 nm, respectively, while at high temperatures above 900°C, they change to a bimodal distribution. It is suggested that the surface diffusion rate of Au NPs on Si NW is much lower than that on the Si substrate because of the substitutional diffusion mechanism.

Water evaporation phenomena on micro and nanostructured surfaces

Augmentation of a microevaporator performance has been investigated experimentally to provide high quality vapor flow. Silicon, Silicon dioxide (SiO2), Silicon nanowires (SiNW), silicon pillars (P–Si), silicon pillars covered by silicon dioxide (P–SiO2) and silicon pillars with nanowires etched on the top (P–SiNW) are considered as evaporation surfaces. These surfaces are fabricated based on deep reactive ion etching (DRIE) as well as electrochemically etched nanowires. Two regimes (no flooded evaporation regime and flooded evaporation regime) are called for evaporation based on different applications. Experiments are repeated three times to ensure repeatability of the observations. Results show that in the case of no flooded regime, evaporation rate are significantly affected by three mechanisms; water spreading between micro and nanostructures, shape and thickness of water droplets on the surface and dynamic behavior of evaporation. In this regime, the P–SiO2 surface has the highest performance among the other surfaces. However, in the case of the flooded regime, the nucleation sites of boiling are very important to achieve maximum rate of evaporation. In this regime the P–SiNW surface is the most efficient surface.

Enhanced photovoltaic performance of inverted pyramid-based nanostructured black-silicon solar cells passivated by an atomic-layer-deposited Al2O3 layer.

Inverted pyramid-based nanostructured black-silicon (BS) solar cells with an Al2O3 passivation layer grown by atomic layer deposition (ALD) have been demonstrated. A multi-scale textured BS surface combining silicon nanowires (SiNWs) and inverted pyramids was obtained for the first time by lithography and metal catalyzed wet etching. The reflectance of the as-prepared BS surface was about 2% lower than that of the more commonly reported upright pyramid-based SiNW BS surface over the whole of the visible light spectrum, which led to a 1.7 mA cm(-2) increase in short circuit current density. Moreover, the as-prepared solar cells were further passivated by an ALD-Al2O3 layer. The effect of annealing temperature on the photovoltaic performance of the solar cells was investigated. It was found that the values of all solar cell parameters including short circuit current, open circuit voltage, and fill factor exhibit a further increase under an optimized annealing temperature. Minority carrier lifetime measurements indicate that the enhanced cell performance is due to the improved passivation quality of the Al2O3 layer after thermal annealing treatments. By combining these two refinements, the optimized SiNW BS solar cells achieved a maximum conversion efficiency enhancement of 7.6% compared to the cells with an upright pyramid-based SiNWs surface and conventional SiNx passivation.

Epitaxial growth of silver nanoislands on the surface of silicon nanowires in ambient air

We study the epitaxial growth of Ag nanoislands on the surface of silicon nanowires (SiNWs) in ambient air by annealing SiNWs fabricated by metal-assisted chemical etching using Ag particles as the catalysts. We demonstrate that the nucleation of Ag on the surface of SiNWs is realized either by the direct decomposition of AgmO or by the reaction between AgmO and Si depending on the annealing temperature; AgmO is an adspecies with a lower effective detachment barrier, commonly introduced in oxygen-induced Ag surface migration systems. Along with the formation of Ag nanoislands, a thin layer of SiOx is formed on the outside surface of Ag nanoislands. The ambient-air epitaxial growth of Ag islands on the surface of SiNWs offers great flexibility in designing ideal metal contacts and Schottky barrier formation, and studying the electronic, magnetic and optical properties of the Si/Ag system. We also believe that SiNWs decorated by Ag nanoislands have many potential applications in plasmonic photovoltaic cells, surface-enhanced Raman spectroscopy detection and electronic devices.

Fluorescent biosensor for alkaline phosphatase based on fluorescein derivatives modified silicon nanowires

A fluorescent sensor for alkaline phosphatase (ALP) was realized by covalently immobilizing fluorescein molecules onto the surface of silicon nanowires (SiNWs) and phosphorylation of the immobilized fluorescein group. Based on the fluorescence enhancement, the new type of sensor exhibited good selectivity and sensitivity to ALP, and displayed a linear relationship between the fluorescence intensity and the ALP concentration in the range of 0.0175–0.3U/mL. A single nanowire-based sensor was successfully employed to monitor the ALP activity and examine the inhibition effect of levamisole on the ALP activity. The fluorescence images of the single nanowire-based sensor showed a high spatial resolution. The present sensor paves a way to directly assay the activity of intracellular ALP by inserting a single nanowire-based sensor into a cell.

Improved performance of silicon nanowire/cadmium telluride quantum dots/organic hybrid solar cells

We fabricated silicon nanowire/cadmium telluride quantum dots (CdTe QDs)/organic hybrid solar cells and investigated their structure and electrical properties. Transmission electron microscope revealed that CdTe QDs were uniformly distributed on the surface of the silicon nanowires, which made PEDOT:PSS easily filled the space between SiNWs. The current density–voltage (J–V) characteristics of hybrid solar cells were investigated both in dark and under illumination. The result shows that the performance of the hybrid solar cells with CdTe QDs layer has an obvious improvement. The optimal short-circuit current density (Jsc) of solar cells with CdTe QDs layer can reach 33.5mA/cm2. Compared with the solar cells without CdTe QDs, Jsc has an increase of 15.1%. Power conversion efficiency of solar cells also increases by 28.8%. The enhanced performance of the hybrid solar cells with CdTe QDs layers are ascribed to down-shifting effect of CdTe QDs and the modification of the silicon nanowires surface with CdTe QDs. The result of our experiments suggests that hybrid solar cells with CdTe QDs modified are promising candidates for solar cell application.

The influence on magnetic property of nickel nanoparticles deposited on the silicon nanowires arrays at low temperature

Abstract There are a large number of paramagnetic defects on the surface of as-grown silicon nanowires (SiNWs) in contrast to H-terminated silicon (Si). Herein, SiNWs arrays were fabricated by chemical etching, Nickel nanoparticles (Ni NPs) were deposited on the surface of SiNWs arrays by the electroless plating. The influence of paramagnetic defects on the magnetic property of Ni/SiNWs was investigated and compared to Ni/Si. The paramagnetic defects of as-grown SiNWs and H-terminated Si were studied by ESR and FTIR spectra. The diameter distribution of Ni NPs on the surface of SiNWs arrays and Si was probed by SEM and fitted by the lognormal probability density function. The results reveal that the diameters of Ni NPs are 35.09 ± 0.53 and 34.92 ± 0.72 nm respectively. The magnetic properties of M–H hysteresis loops for Ni/SiNWs and Ni/Si were measured from 5 to 400 K. With the temperature increasing, the saturation magnetization of Ni/SiNWs and Ni/Si decreases gradually due to the thermal activation effect. Overall temperature range (5–400 K), the saturation magnetization of Ni/Si follows the modified Bloch's law. However, the data of Ni/SiNWs are only valid for high temperature range (50–400 K). At low temperature (5 K), there is an abrupt increase and the experimental data deviated from the modified Bloch model. This deviation at low temperature is associated with surface paramagnetic defects of SiNWs.

Stem-loop DNA-assisted silicon nanowires-based biochemical sensors with ultra-high sensitivity, specificity, and multiplexing capability.

A class of stem-loop DNA-assisted silicon nanowires (SiNWs)-based fluorescent biosensor is presented in this report. Significantly, the sensor enables rapid and sensitive detection of DNA targets with a concentration as low as 1 pM. Moreover, the large planar surface of SiNWs facilitates simultaneous assembly with different DNA strands, which is favorable for multiplexed DNA detection. On the other hand, the SiNWs-based sensor is highly efficacious for detecting heavy metal ions. Mercury ions (Hg(2+)) of low concentrations (e.g., 5 pM) are readily identified from its mixture with over 10 kinds of interfering metal ions, even in real water samples. Given that SiNWs can be fabricated in a facile, reproducible and low-cost manner, this kind of SiNWs-based high-performance sensor is expected to be a practical analytical tool for a variety of biological and environment-protection applications.

Enhanced photovoltaic performance of organic/silicon nanowire hybrid solar cells by solution-evacuated method.

A method has been developed to fabricate organic-inorganic hybrid heterojunction solar cells based on n-type silicon nanowire (SiNW) and poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) hybrid structures by evacuating the PEDOT:PSS solution with dip-dropping on the top of SiNWs before spin-coating (solution-evacuating). The coverage and contact interface between PEDOT:PSS and SiNW arrays can be dramatically enhanced by optimizing the solution-evacuated time. The maximum power conversion efficiency (PCE) reaches 9.22% for a solution-evacuated time of 2min compared with 5.17% for the untreated pristine device. The improvement photovoltaic performance is mainly attributed to better organic coverage and contact with an n-type SiNW surface.

C@SiNW/TiO2 core-shell nanoarrays with sandwiched carbon passivation layer as high efficiency photoelectrode for water splitting.

One-dimensional heterostructure nanoarrays are efficiently promising as high performance electrodes for photo electrochemical (PEC) water splitting applications, wherein it is highly desirable for the electrode to have a broad light absorption, efficient charge separation and redox properties as well as defect free surface with high area suitable for fast interfacial charge transfer. We present highly active and unique photoelectrode for solar H2 production, consisting of silicon nanowires (SiNWs)/TiO2 core-shell structures. SiNWs are passivated to reduce defect sites and protected against oxidation in air or water by forming very thin carbon layer sandwiched between SiNW and TiO2 surfaces. This carbon layer decreases recombination rates and also enhances the interfacial charge transfer between the silicon and TiO2. A systematic investigation of the role of SiNW length and TiO2 thickness on photocurrent reveals enhanced photocurrent density up to 5.97 mA/cm2 at 1.0 V vs.NHE by using C@SiNW/TiO2 nanoarrays with photo electrochemical efficiency of 1.17%.

Tailoring transport and dielectric properties by surface passivation of silicon nanowires with Polyacrylic acid/TiO2 nanoparticles composite

We have presented the composite device including silicon nanowires (SiNWs), Polyacrylic acid (PAA) and Titanium dioxide nanoparticles (TiO2 NPs). SiNWs and TiO2 NPs were synthesized by metal assisted electroless chemical etching (MACE) and co-precipitation method respectively. Solution containing PAA and TiO2 NPs in DI water was spun on already grown vertical SiNW arrays. We have investigated the transport and dielectric properties of p-type SiNWs/PAA/TiO2 NPs (p-SPT) and n-type SiNWs/PAA/TiO2 NPs (n-SPT) composite devices. Presence of PAA/TiO2 NPs on the surface of SiNWs have increased electrical current in p-SPT device than that of n-SPT device. Ohmic like conduction was dominant at lower bias voltages followed by space charge limited current (SCLC) with traps at intermediate voltages. The calculated values of trap densities (Ht) were 7.73×1011 cm−3 and 5.34×1011 cm−3 for p-SPT device and n-SPT device respectively. Similarly p-SPT device shows higher real part of dielectric constant (ε′) and AC conductivity (σac) ∼15 times and ∼85 times respectively than that of n-SPT device. Increment in electrical and dielectric properties can be attributed to the presence of hydrophilic materials (PAA/TiO2 NPs) which may results in enhancement of acceptor like states.

Improved antireflection properties and optimized structure for passivation of well-separated, vertical silicon nanowire arrays for solar cell applications

Large-area, well-separated, and vertically aligned silicon nanowire (SiNW) arrays with excellent antireflection properties were fabricated through a combination of anodic aluminum oxide template and metal-assisted chemical etching, followed by supercritical drying. Less than 1% reflectance was achieved over the wavelength range of 200–600nm, and 23% reduction in average reflectance was observed over the 200–1000nm range, compared with the conical-frustum structure array by natural drying. Furthermore, the well-separated SiNW arrays considerably facilitated the conformal coating of the plasma-enhanced chemical vapor deposited amorphous silicon layer on the SiNW surface, which could result in effective passivation of surface states. Therefore, such well-separated and vertically aligned SiNW arrays are highly promising for solar cell application.

Preparation of silicon nanowires by in situ doping and their electrical properties

Individual silicon nanowires (SiNWs) doped by in situ dopant incorporation during growth of the SiNWs was investigated. Electrical charge transport measurements were taken before and after removing the material from the boron-doped SiNW surfaces by wet chemical etching. AFM (atomic force microscopy) images showed the radial cross section of the ∼10nm thick material that was cut off from the SiNW surface. The electrical transport characteristics after each etching cycle revealed the radial core–shell distribution of the activated dopants. The carrier concentration close to the surface of the boron-doped SiNWs was a factor of 6 higher than the value in the core. Using a simple, rapid and reliable technique, the natural distribution of the surface dopants in the SiNWs, which was distinct from many top-down fabricated NWs, explained the enhancement in the charge transport properties of these NWs. This could yield robust properties in ultra-small devices that are often dominated by random dopants fluctuations.

Volatile organic solvents sensing using silicon nanowires array fabricated by spontaneous electrochemical reaction

In order to prevent overexposure to volatile organic solvents (VOCs), a simple sensing device with high sensitivity is needed in the common workplace. Meanwhile, silicon nanowires carrying the high surface-bulk ratio as a unique characteristic could be used for VOCs sensing. This study utilised spontaneous electrochemical reaction, which involves etching of silicon wafer in a silver nitrate and hydrofluoric acid based solution, to modify etching times for silicon nanowires fabrication. This research discovered that the length of nanowire increased and VOCs sensing sensitivity improved as the etching time increased. Furthermore, diethyl ether had the optimal performance on the effect of silicon nanowires array on VOCs sensing. The study also discussed the effect of VOCs molecules and electron distribution on silicon nanowires surface, to further investigate the impact on sensing sensitivity.

Highly sensitive covalently functionalised integrated silicon nanowire biosensor devices for detection of cancer risk biomarker

In this article we present ultra-sensitive, silicon nanowire (SiNW)-based biosensor devices for the detection of disease biomarkers. An electrochemically induced functionalisation method has been employed to graft antibodies targeted against the prostate cancer risk biomarker 8-hydroxydeoxyguanosine (8-OHdG) to SiNW surfaces. The antibody-functionalised SiNW sensor has been used to detect binding of the 8-OHdG biomarker to the SiNW surface within seconds of exposure. Detection of 8-OHdG concentrations as low as 1ng/ml (3.5nM) has been demonstrated. The active device has been bonded to a disposable printed circuit which can be inserted into an electronic readout system as part of an integrated Point of Care (POC) diagnostic. The speed, sensitivity and ease of detection of biomarkers using SiNW sensors render them ideal for eventual POC diagnostics.

Serotype-Specific Identification of Dengue Virus by Silicon Nanowire Array Biosensor

In this work, we demonstrated a silicon nanowire (SiNW) biosensing platform capable of simultaneously identifying different Dengue serotypes on a single sensing chip. Four peptide nucleic acids (PNAs), specific to each Dengue serotypes (DENV-1 to DENV-4), were spotted on different areas of the SiNW array surface, and the covalently immobilized PNA probes were then interacted with different Dengue serotypes target to establish the specificity of detection. Detection scheme is based on the changes in resistances due to accumulation of negative charges contributed by the hybridized DNA target. The results show that resistance changes only occur in regions where the Dengue target hybridizes with its complementary probe. What is more, a mixture of two different Dengue serotypes obtained from a one-step duplex RT-PCR was applied to the multiplex SiNW surface to validate SiNW capability to identify multiple Dengue serotypes on a single sensing platform. Through this study, we have established the multiplex SiNW biosensor as a promising device to detect multiple Dengue infections with high specificity.

The role of surface states in modification of carrier transport in silicon nanowires

We investigate transport properties of polyacrylic acid (PAA) capped n and p-type silicon nanowire (SiNW) arrays. PAA diluted with deionized water at different concentrations was spun directly on vertically grown SiNW arrays prepared by metal assisted electroless chemical etching. PAA provides mechanical support to electrical contacts and acts as a source of interface doping by creating acceptor like states (holes) on SiNWs surface. PAA capping results in increase in current in p-type SiNWs and decrease in current in n-type SiNWs. Schottky emission model fits current voltage (IV) characteristics of p-type SiNWs/PAA device. Ohmic like conduction at lower voltages followed by space charge limited current (SCLC) with and without traps is observed in p-type SiNWs, n-type SiNWs, and n-type SiNWs/PAA devices. Using SCLC model with exponential distribution of traps, the extracted trap density was 7.20 × 1011/cm3 and 6.0 × 1011/cm3 for p-type SiNWs and n-type SiNWs devices, respectively. Our findings also demonstrate that the carrier concentration in SiNWs depends not only on doping concentration but also depends significantly on density of surface states.

Label-Free Detection of Carbohydrate–Protein Interactions Using Nanoscale Field-Effect Transistor Biosensors

Carbohydrate-protein interactions play a significant role in cell communication, cell adhesion, cell trafficking, and immune responses. Many efforts have been made to demonstrate detection of carbohydrate-protein interactions. However, the existing methods are still tedious and expensive. Therefore, the detection of carbohydrate-protein interactions is of great significance, and new, efficient methods are required for fast and sensitive recognition testing. In this report, we, for the first time, developed the silicon nanowire (SiNW)-based biosensor capable of label-free electrical detection of carbohydrate-protein interactions with high selectivity and sensitivity by covalently immobilizing unmodified carbohydrates on the sensor surface. We fabricated new SiNW sensor chips with more SiNW arrays for potential detection of multiple analytes. In order to realize the immobilization of the unmodified carbohydrates on the SiNW surface, we used X-ray photoelectron spectra and fluorescence microscopy to verify the successful surface functionalization on the silicon surface. Furthermore, we demonstrated real-time detection of carbohydrate-protein interactions using the carbohydrate-modified SiNW sensor chips. The results show good specificity between galactose-lectin EC and mannose-Con A, which is in good agreement with that reported previously. Finally, the results also show that we are able to use the galactose-modified SiNW biosensor to detect lectin EC as low as 100 fg/mL, which is 4 orders of magnitude lower than that reported by other technologies. We believe that the developed SiNW biosensor paves a novel way for studying carbohydrate-protein interactions.