Aligned Silicon Nanowires Research Articles

Detection of Thiocholine for Pesticide Qualification Using Indium-Tin-Oxide-Coated Vertically Aligned Silicon Nanowires Extended Gate Field-Effect Transistor

Thiocholine is the hydrolysis of acetylthiocholine (ATCh) by acetylcholinesterase (AChE). The detection of thiocholine can be utilized to assess the activity and inhibitors of AChE, making it a valuable recognition element for constructing biosensors used in pesticide detection[1, 2]. The enzymatic hydrolysis of ATCh is catalyzed by AChE. First, high-aspect-ratio vertically aligned silicon nanowires (VASiNWs) were successfully fabricated through metal-assisted chemical etching (MACE). To investigate the impact of nanostructures on enhancing the sensitivity of the extended gate of the field-effect transistor (EGFET), the gate terminal was connected to 3-mercaptopropyl trimethoxysilane (MPTMS) modified indium-tin-oxide (ITO)-coated VASiNWs for the electrical detection of ATCh-catalyzed thiocholine, as illustrated in Figure 1. Various solutions of ATCh-catalyzed thiocholine were collected by applying different concentrations of ATCh (ranging from 2 mM to 10 mM) to the AChE-coated wells independently for 10 minutes. Subsequently, these solutions were introduced to the MPTMS modified ITO-coated VASiNWs EGFET, and the change in drain current was measured using a semiconductor parameter analyzer (HP 4145B). The thiocholine current demonstrated a sensitivity of 29.39 uA/concentration and a linearity of 0.88. Exploring the potential of pesticides to inhibit AChE hydrolysis represents a significant breakthrough, paving the way for the advancement of biosensors in pesticide detection. References Liu, G., et al., Sensitive electrochemical detection of enzymatically generated thiocholine at carbon nanotube modified glassy carbon electrode. Electrochemistry Communications, 2005. 7(11): p. 1163-1169. Pandey, P.C., et al., Acetylthiocholine/acetylcholine and thiocholine/choline electrochemical biosensors/sensors based on an organically modified sol–gel glass enzyme reactor and graphite paste electrode. Sensors and Actuators B: Chemical, 2000. 62(2): p. 109-116. Figure 1

Au Nanoparticles@Si Nanowire Oligomer Arrays for SERS: Dimers Are Best.

We report the synthesis of vertically aligned silicon nanowire (VA-SiNW) oligomer arrays coated with Au nanoparticle (NP) monolayers via a combination of colloidal lithography, metal-assisted chemical etching, and directed NP assembly. Arrays of SiNW monomers (i.e., isolated NWs), dimers, and tetramers are synthesized, decorated with AuNPs, and tested for their performance in surface-enhanced Raman spectroscopy. The ∼20 nm AuNPs easily enter within the ca. 40 nm gaps of the SiNW oligomers, thus reaching the hot spot region. At 785 nm excitation, the AuNPs@SiNW dimer arrays provide the highest Raman signal, in agreement with electromagnetic simulations showing a high electric field enhancement at the Au/Si interface within the dimer gap region.

TiO2 nanorods decorated Si nanowire hierarchical structures for UV light activated photocatalytic application

Water remediation techniques like photolysis have recently piqued the interest of many researchers due to water contamination resulting from heavy industrialization and urbanization. In the current work, as-synthesized TiO2 nanorod decorated vertically aligned silicon nanowire (SiNW) leads to a hierarchical morphological structure formation. The photocatalytic nature of the fabricated SiNW/TiO2 nanoheterojunction is examined by the dye degradation of textile pollutants like methylene blue (MB), rhodamine B (RhB), and eosin B (EB). The catalytic dye degradation investigations revealed that 4 h hydrothermal synthesis of TiO2 on the surface of SiNW (ST4) exhibited excellent catalytic behaviour. In the presence of H2O2 and UV irradiation, the ST4 nanoheterostructure can degrade 98.89% of the model pollutant methylene blue (MB) in 15 min, demonstrating remarkable photocatalytic performance. The direct Z-scheme heterojunction exhibited by the SiNW/TiO2 structure facilitates a more efficient charge transfer mechanism with higher reducing and oxidizing ability leading to enhanced photocatalytic behaviour. The degradation pathway examined by LC-MS studies demonstrated the complete breakdown of the organic MB dye molecules ultimately mineralizing into CO2, H2O, and other inorganic substances. The photocatalyst ST4 exhibited excellent reusability and stability after multiple cycles of dye degradation enabling its use in practical water purification purposes.

Detection of SARS-CoV-2 N protein using AgNPs-modified aligned silicon nanowires BioSERS chip.

The SARS-CoV-2 (COVID-19) pandemic had a strong impact on societies and economies worldwide and tests for high-performance detection of SARS-CoV-2 biomarkers are still needed for potential future outbreaks of the disease. In this paper, we present the different steps for the design of an aptamer-based surface-enhanced Raman scattering (BioSERS) sensing chip capable of detecting the coronavirus nucleocapsid protein (N protein) in spiked phosphate-buffered solutions and real samples of human blood serum. Optimization of the preparation steps in terms of the aptamer concentration used for the functionalization of the silver nanoparticles, time for affixing the aptamer, incubation time with target protein, and insulation of the silver active surface with cysteamine, led to a sensitive BioSERS chip, which was able to detect the N protein in the range from 1 to 75 ng mL-1 in spiked phosphate-buffered solutions with a detection limit of 1 ng mL-1 within 30 min. Furthermore, the BioSERS chip was used to detect the target protein in scarcely spiked human serum. This study demonstrates the possibility of a clinical application that can improve the detection limit and accuracy of the currently commercialized SARS-CoV-2 immunodiagnostic kit. Additionally, the system is modular and can be applied to detect other proteins by only changing the aptamer.

Trace level sensitivity and selectivity of room temperature NH3 gas sensors based on RGO/ZnO@SiNWs heterostructure

Ammonia (NH3) is one of the most common gaseous pollutants in various environmental and industrial processes which is accountable for various health issues, even for life-threatening situations. Due to this, high-precision detection of NH3 has become a top priority for various sectors so that healthy and safe environment could be ensured. Therefore, this work aims to develop a simple, reliable, and effective sensor with spatially distributed monitoring of NH3 gas wherein reduced graphene oxide (RGO) and Zinc oxide (ZnO) nanocomposite structure prepared using refluxing method were chosen to interact with NH3 gas. The vertically aligned silicon nanowires (SiNWs) were developed by using metal-assisted chemical etching (MACE) method. The RGO/ZnO nanostructure was then deposited on SiNWs to develop a selective and stable sensor for the trace level detection of NH3 gas with varying concentration from 0.01 ppm to 5 ppm at Room temperature (∼30 °C). The prepared materials were characterized using scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive spectroscopy (EDS), X-ray diffractometer (XRD), Raman spectrometer, X-Ray Photoelectron Spectrometer (XPS) and Ultraviolet–visible spectrophotometer which provide a thorough explanation about the developed nanostructures. The sensitivity of the sensor was observed to lie between 21 % at 0.01 ppm and 176 % at 5 ppm concentration of NH3 wherein the response/recovery time was measured to be 3 s/6 s and 5 s/12 s respectively. The fast response of the sensor for NH3 gas at low limit of detection (LOD) could be attributed due to the precise interaction of the analyte at the interface of RGO/ZnO@SiNWs Schottky heterostructure. Hence, the reported sensor can continuously be utilized to monitor the target analyte that is NH3 gas at low concentrations and reduces the chance of an unfortunate incident.

(Invited) Development of Cell-Based Biosensor Platform for Monitoring the Effect of Functional Nanocarriers As Drug Delivery System on Breast Cancer Cells

With the rapid emerging research of nanotechnology and engineering in biomedical applications, scientists start to develop state-of-the-art diverse biosensing platforms. Here, in-vitro cell-based biosensor (CBB) was fabricated using nano/micro technologies to electrophysiologically observe the effects of drug treatment on human breast cancer cells. Indium tin oxide coated vertically aligned silicon nanowires (ITO-coated VA-SiNWs) were applied as a cell monitoring CBB platform to investigate the growth and adhesion of MCF-7 human breast cancer cells via impedimetric method. Monitoring was performed by measuring the non-Faradaic electrical impedance of the cell–APTMS-modified ITO-coated VA-SiNWs system using a precision LCR meter. Our APTMS-modified ITO-coated VA-SiNWs exhibited changes in impedance parameters during cell growth. It was observed that during MCF-7 cell growth, the impedance magnitude increased. By contrast, the cytolysis of the cells led to significant changes in impedance parameters. Impedimetric cell monitoring platform has the ability to observe MCF-7 cell growth and adhesion and investigate anomalous and non-electrogenic cell behavior in toxin and drug screenings. With the aid of CBB platform, the real-time electrical signals can be applied to monitor the therapeutic efficacy of the nanocarriers in drug delivery system and screening large libraries of potential drugs for cancer treatment. Moreover, our pilot study indicates that our CBB could be a feasible platform to reduce/replace the use of animals based on the “Three R principles” in animal-based research in the future. Figure 1

Chemical vapor etching of silicon wafer for the synthesis of highly dense and aligned sub-5 nm silicon nanowire arrays

We investigated the key chemical vapor etching parameters governing the morphology of Si nanowires. Highly aligned sub-5 nm Si nanowires can be achieved by controlling the oxidant gas concentration, reaction temperature, and hydrogen concentration.

Vertical nanowires enhanced X-ray radiation damage of cells

Cell behavior is affected by nanostructured surface, but it remains unknown how ionizing radiation affects cells on nanostructured surface. This paper reports an experimental investigation of X-ray radiation induced damage of cells placed on an array of vertically aligned silicon nanowires. X-ray photoelectrons and secondary electrons produced from nanowire array are measured and compared to those from flat silicon substrate. The cell functions including morphology, viability, adhesion and proliferation have been examined and found to be drastically affected when cells are exposed to X-ray radiation, compared to those sitting on flat substrate and those only exposed to X-ray. The enhanced cell damage on nanowires upon X-ray exposure is attributed to nanowire enhanced production of photoelectrons including Auger electrons and secondary electrons, which have high escaping probability from sharp tips of nanowires. The escaped photoelectrons ionize water molecules and generate hydroxyl free radicals that can damage DNAs of cells. An inference of this work is that the contrast in scanning electron microscopy is useful in assessing the effects of nanomaterials for enhanced X-ray radiation therapy.

Enhanced oxygen evolution based on vertically and well aligned silicon nanowires

Vertically and well aligned silicon nanowires (SiNWs) were fabricated, characterized and utilized to provide a high surface area and a high electrocatalytic activity for co-catalyst loading with a guiding 1-D charge transfer along the fabricated SiNWs. The impact of SiNWs structural features on the electrocatalytic properties of the fabricated samples and the cyclic voltammetry (CV) performance of Si and SiNWs electrodes were explored in 1.0 M KOH and 0.1 M H2SO4. The evaluation of oxygen evolution rection and the hydrogen evolution reaction were carried out using potentiostat/galvanostat with a three-electrode conventional system using a platinum wire as a counter electrode, Ag|AgCl|KClsat as a reference electrode, and a substrate having SiNWs nanostructure on its surface (catalysts) as a working electrode. For further investigations, we studied the oxygen evolution reaction (OER) performance of SiNWs and compared with that of Si electrodes based upon the electrochemical impedance spectroscopy (EIS). SiNWs achieved more efficient charge transport, owing to the direct conduction pathway and exhibited considerable stability as well as maximum amount of oxygen bubbles at a drive current of 708.6 μA/cm2 under a potential of 1.5 V in alkaline media.

Enhanced thermoelectric performance of vertically aligned silicon nanowires through the cold spot effect and charge carrier trapping effect of attached gold nanoparticles

The thermoelectric performance of vertically aligned silicon (Si) nanowires (NWs) with diameters of 100–150 nm fabricated through metal-assisted chemical etching is improved using electroless-deposited gold (Au) nanoparticles (NPs) (ф ≈ 10 nm). After the deposition of metal NPs, the thermal conductivity and Seebeck coefficient of the Si NWs decrease 5.78-fold and increased 2.71-fold, respectively. Despite the 3.23-fold increase in the electric resistivity of Si NWs through the deposition of Au NPs, the figure of merit of the Au NP-deposited Si NWs is enhanced by 1320% (0.444) compared to that of pristine Si NWs (0.0337). The power factor is enhanced 2.28-fold after Au NP deposition (0.175–0.400 mV·K−2m−1). Based on finite-element method simulations of Au NP-deposited Si NW, a cold spot is generated inside the Si NW by the attached Au NP. Moreover, charge carrier trapping at the interface between the Si NW and Au NP is anticipated due to interfacial Fermi-level pinning. Cold spot and charge carrier trapping effects are proposed as important factors for enhancing the thermoelectric performance of Au NP-deposited Si NWs.

Photo-induced catalytic removal of rhodamine-B by aligned silicon nanowires developed through metal assisted chemical etching

In this work, silicon nanowire (SiNW) is synthesized by conventional metal assisted chemical etching method with different etching time. The sample with the highest aspect ratio is confirmed from the field emission scanning electron microscopic (FESEM) study and is associated with SiNW etched for 27 min. The sample is further treated with hydrofluoric acid (HF) to make the sample hydrogen terminated (H-terminated). Along with FESEM, the as-prepared samples are characterized by X-ray diffraction (XRD), transmission electron microscope, Fourier transformed infrared spectroscopy (FTIR) and UV–Vis spectroscopic study. The FTIR study confirms the different bonding present in the sample. UV–Vis spectroscopic study confirms the change in the reflection of the sample due to HF treatment. XRD spectrum confirms the directional preference of the silicon substrate. Both the pure and H-terminated sample is employed to see their ability in removing poisonous textile dyes like rhodamine-B. It has been shown that both the samples can efficiently remove the dye, however the ability or removal efficiency came to a saturation just after one hour. Furthermore, unlike the previous reports, hydrogen termination in SiNWs could not make marked differences in the removal efficiency. Efforts have been made to explain the result, as well as the reaction kinetics has been studied by making a linear fit with the pseudo first order and second order kinetic plot. It has been shown that the reaction mostly followed the pseudo first order kinetics. • Silicon nanowires (SiNWs) are synthesized by MACE and modified with HF to obtain hydrogen-terminated SiNWs (H-SiNWs). • Both pure SiNWs and H-SiNWs show efficient photo-catalytic activity to remove Rhodamine B. • For the first time it is shown that SiNWs show better removal efficiency than the H-SiNWs.

Tailored Uniaxial Alignment of Nanowires Based on Off-Center Spin-Coating for Flexible and Transparent Field-Effect Transistors.

The alignment of nanowires (NWs) has been actively pursued for the production of electrical devices with high-operating performances. Among the generally available alignment processes, spin-coating is the simplest and fastest method for uniformly patterning the NWs. During spinning, the morphology of the aligned NWs is sensitively influenced by the resultant external drag and inertial forces. Herein, the assembly of highly and uniaxially aligned silicon nanowires (Si NWs) is achieved by introducing an off-center spin-coating method in which the applied external forces are modulated by positioning the target substrate away from the center of rotation. In addition, various influencing factors, such as the type of solvent, the spin acceleration time, the distance between the substrate and the center of rotation, and the surface energy of the substrate, are adjusted in order to optimize the alignment of the NWs. Next, a field-effect transistor (FET) incorporating the highly aligned Si NWs exhibits a high effective mobility of up to 85.7 cm2 V−1 s−1, and an on-current of 0.58 µA. Finally, the single device is enlarged and developed in order to obtain an ultrathin and flexible Si NW FET array. The resulting device has the potential to be widely expanded into applications such as wearable electronics and robotic systems.

Effect of Local Topography on Cell Division of Staphylococcus spp.

Surface engineering is a promising strategy to limit or prevent the formation of biofilms. The use of topographic cues to influence early stages of biofilm formationn has been explored, yet many fundamental questions remain unanswered. In this work, we develop a topological model supported by direct experimental evidence, which is able to explain the effect of local topography on the fate of bacterial micro-colonies of Staphylococcus spp. We demonstrate how topological memory at the single-cell level, characteristic of this genus of Gram-positive bacteria, can be exploited to influence the architecture of micro-colonies and the average number of surface anchoring points over nano-patterned surfaces, formed by vertically aligned silicon nanowire arrays that can be reliably produced on a commercial scale, providing an excellent platform to investigate the effect of topography on the early stages of Staphylococcus spp. colonisation. The surfaces are not intrinsically antimicrobial, yet they delivered a topography-based bacteriostatic effect and a significant disruption of the local morphology of micro-colonies at the surface. The insights from this work could open new avenues towards designed technologies for biofilm engineering and prevention, based on surface topography.

Real-time impedimetric detection of cardiac troponin I using ITO-coated vertically aligned silicon nanowires

This research focuses on developing a sensitive nanoelectronic device (NED) with impedimetric detection of human cardiac troponin I (cTnI) for the real-time diagnosis of acute myocardial infarction. The sensing area of NED was composed of indium tin oxide (ITO)-coated vertically aligned silicon nanowires (VASiNWs). ITO-coated-VASiNWs were examined by X-ray diffraction and scanning electron microscope for the identification of crystalline structure and morphological observation, respectively. After cTnI antibody (cTnIAb) modification, various concentrations of cTnI in the human blood sera were then impedimetrically detected by cTnIAb-modified-ITO-coated-VASiNWs via microfluidic systems. Our device exhibited a linear range from 1.76011 to 17601.1 ng/mL with good sensitivity for the detection of cTnI.

The SU-8 spin-coating on silicon nanowires formed by cryogenic dry etching

Arrays of vertically aligned silicon nanowires were fabricated by cryogenic dry etching. The post-processing technology was developed to full coating of arrays of NWs by SU-8 and release the top side of SiNWs. The Schottky diodes were fabricated on arrays of SiNWs with and without SU-8 by gold evaporation. The cryogenic dry etching leads to defect formation with Ea=0.28 eV and concentration lower 5⋅1012 cm−3 in near-surface area in silicon, and no defect are detected in bulk silicon. However, oxygen plasma treatment used to release top side of SiNWs leads to increase of its concentration by two order and formation of defect with Ea=0.39 eV, σ = 1⋅10−16 cm2 and a concentration of 5⋅1014 cm−3 in a bulk of SiNWs deeper than 1 μm.

Design and fabrication of wafer-scale highly uniform silicon nanowire arrays by metal-assisted chemical etching for antireflection films

We present a novel and facile scheme for fabricating wafer-scale arrays of highly uniform vertically aligned silicon nanowires (Si-NWs) in the diameter range of 50 to 200 nm. The key idea is to fabricate thin gold (Au) nanostructures using magnetron sputtering followed by de-wetting of Au and subsequent realization of wafer-scale and highly uniform Si-NWs arrays by metal-assisted chemical etching in low temperature (2 °C) without resorting to complex photolithography. The excellent properties of these Si-NWs arrays were also demonstrated numerically. A maximum exciton generation rate of 1.76 × 1024 and a maximum energy generation rate of 28 can be obtained from Si-NWs arrays with the diameter of 200 nm. The reflectivity of the Si-NWs arrays is declined with decreasing annealing temperature and is below 10% over a wide wavelength range at the annealing temperature of 200 °C. Our work provides a promising approach for constructing Si-NWs arrays with good photoelectric conversion performance.

Decisive role of dopants in the optical properties of vertically aligned silicon nanowires prepared by metal-assist chemical etching

Silicon nanowires (SiNWs) were prepared by metal-assist chemical etching (MACE) from different doping types and resistivity of Si wafers, <100> orientation. The coexistence of Si nanocrystals (SiNCs) and SiNWs was revealed in both types of the semiconductor. From the p-type samples, heavily doped with boron (B), intense red-light emission was observed. High-resolution transmission electron microscopy (HR-TEM) images of this sample type shown numerous SiNCs in the porous surface of the SiNWs walls. These SiNCs were proposed to be responsible for the characterized optical properties of the material. The microscopic links between the photoluminescence behavior, the dopants, and the formation of SiNCs were presented and discussed.

Recent Advances in Structuring and Patterning Silicon Nanowire Arrays for Engineering Light Absorption in Three Dimensions.

Vertically aligned silicon nanowire (VA-SiNW) arrays can significantly enhance light absorption and reduce light reflection for efficient light trapping. VA-SiNW arrays thus have the potential to improve solar cell design by providing reduced front-face reflection while allowing the fabrication of thin, flexible, and efficient silicon-based solar cells by lowering the required amount of silicon. Because their interaction with light is highly dependent on the array geometry, the ability to control the array morphology, functionality, and dimension offers many opportunities. Herein, after a short discussion about the remarkable optical properties of SiNW arrays, we report on our recent progress in using chemical and electrochemical methods to structure and pattern SiNW arrays in three dimensions, providing substrates with spatially controlled optical properties. Our approach is based on metal-assisted chemical etching (MACE) and three-dimensional electrochemical axial lithography (3DEAL), which are both affordable and large-scale wet-chemical methods that can provide a spatial resolution all the way down to the sub-5 nm range.

Direct assembly of nanowires by electron beam-induced dielectrophoresis

Controllable self-assembly is an important tool to investigate interactions between nanoscale objects. Here we present an assembly strategy based on 3D aligned silicon nanowires. By illuminating the tips of nanowires locally by a focused electron beam, an attractive dielectrophoretic force can be induced, leading to elastic deformations and sticking between adjacent nanowires. The whole process is performed feasibly inside a vacuum environment free from capillary or hydrodynamic forces. Assembly mechanisms are discussed for nanowires in both one and two layers, and various ordered organizations are presented. With the help of moisture treatment, a hierarchical assembly can also be achieved. Notably, an unsynchronized assembly is observed in two layers of nanowires. This study helps with a better understanding of nanoscale sticking phenomena and electrostatic actuations in nanoelectromechanical systems, besides, it also provides possibilities to probe quantum effects like Casimir forces and phonon heat transport in a vacuum gap.

CO2 sensing behavior of vertically aligned Si Nanowire/ZnO structures

Metal oxide nanostructures are highly attractive for the fabrication of gas sensors due to their high surface‐to‐volume ratios. The design of novel structures has great importance to improve sensing behavior. Atomic layer deposited ZnO thin film on vertically aligned silicon nanowire (Si NW) arrays were used to understand the influence of etching time on CO2 sensing behavior of the SiNW/ZnO structure. The metal-assisted chemical etching method was used to realize SiNW arrays. The etching processes were performed for 5, 10, 30, and 60 min. The linear relationship between etching time and Si NW length was reported. 30 nm ZnO thin films were deposited on Si NWs by atomic layer deposition (ALD) method using diethyl Zinc and H2O as Zn and O2 sources, respectively. The CO2 sensing properties of the SiNW/ZnO structures were examined at ambient temperature and low CO2 concentration. The testing measurement results reveal that the sensor realized on un-etched Si does not exhibit any sensitivity. While the SiNW/ZnO was sensitive to CO2, the sensor sensitivity increase with the etching time due to the increase in the specific reactive surface. Moreover, the realized sensors have a relatively low response and recovery times, in the range of 2–7 s, by comparison to the reported ones in the literature.