Aligned Si Nanowires Research Articles

Deep-reactive ion etching of silicon nanowire arrays at cryogenic temperatures

The pursuit of sculpting materials at increasingly smaller and deeper scales remains a persistent subject in the field of micro- and nanofabrication. Anisotropic deep-reactive ion etching of silicon at cryogenic temperatures (cryo-DRIE) was investigated for fabricating arrays of vertically aligned Si nanowires (NWs) of a large range of dimensions from micrometers down to 30 nm in diameter, combined with commonly used wafer-scale lithography techniques based on optical, electron-beam, nanoimprint, and nanosphere/colloidal masking. Large selectivity of ∼100 to 120 and almost 700 was found with resists and chromium hard masks, respectively. This remarkable selectivity enables the successful transfer of patterned geometries while preserving spatial resolution to a significant extent. Depending on the requirements by applications, various shapes, profiles, and aspect ratios were achieved by varying process parameters synchronously or asynchronously. High aspect ratios of up to 100 comparable to the best result by metal-assisted wet-chemical etching and sub-μm trenches by DRIE were obtained with NW diameter of 200 nm, at an etch rate of ∼4 μm/min without being collapsed. At the same time, low surface roughness values were maintained on the NW top, sidewall, and bottom surface of ∼0.3, ∼13, and ∼2 nm, respectively, as well as high pattern fidelity and integrity, which were measured using angle-resolved Fourier microscopy, combined atomic force, and scanning electron microscopy on selected NWs. This work establishes the foundation in the controllable development of Si nanoarchitectures, especially at sub-100 nm structures, for energy-harvesting and storage, damage-free optoelectronics, quantum, photovoltaics, and biomedical devices.

Aptamer conjugated Si nanowire network/poly(pyrrole)-NTA structure for electrochemical sensing

The current study describes the design, analytical characterization, and testing of aptamer sensors based on Si nanonets (Si NN) for thrombin electrochemical detection. Si nanonets are thin sheet arrays of randomly aligned Si nanowires with controlled and programmable properties that may be transported and integrated on a variety of substrates. Si nanonets were placed onto a commercial glassy carbon working electrode to create our aptasensor. Then they were functionalized by conductive pyrrole polymerization, which was followed by aptamer bioreceptor grafting. Electrochemical detection of thrombin was carried out after optimizing the electrode properties. On the one hand, cyclic voltammetry analyses demonstrated a linear association between the acquired current and the logarithm of thrombin concentration from 5 nmol L−1 to 2 µmol L−1. Impedance measurements, on the other hand, revealed a linear relationship between the charge transfer resistance as well as the impedance modulus. Thus, for the first time, Si nanonet thin sheets were used as constitutive nanoporous parts of electrodes capable of detecting thrombin at concentration limits of 5 nM, considered as a risk factor or presence of thrombosis. This demonstrates the concept of using this type of nanomaterial to detect molecules of biological interest.

DFT and experimental interpretations of silicon-based room-temperature NO2 sensors improving humidity independence

Insights toward detecting hazardous NO2 gas at room temperature with nearly humidity-independent sensing characteristics has been promisingly envisioned yet rare reports have been addressed. Here, the spatial configurations for favorable NO2 adsorption on the incorporated WOx/Si structures were studied using density functional theory (DFT) calculations. Results reveal that the more negative adsorption energy encountered for NO2 adsorption exists in exposed Si surfaces, which further show the largest magnitude of charge transfer (0.16 e), compared with WOx surfaces (0.08 e), and WOx/Si interfaces (0.02 e) evidenced from Bader charge analysis. To experimentally remedy the correlated sensing performances, incorporations of either dense or distributed WOx nanoparticles on aligned Si nanowires featuring z-scheme band structure were synthesized, where the coupled heterostructures with distributed WOx features display the environmentally stable gas-sensing characteristics by featuring trivial degradation of sensing response of 8.4% under high humidity of 83% compared with average response within humidity range of 24.5–55.2%, and such detection robustness is much superior than other oxide-based NO2-gas sensors in the literature. Examinations of band structures clarify that the rule of Si in such promising NO2 sensors behaves as a bridge that efficiently rectifies the hole flows for synergistically improving room-temperature detection.

Self-Assembled Au Nanoparticle Monolayers on Silicon in Two- and Three-Dimensions for Surface-Enhanced Raman Scattering Sensing.

Gold nanoparticle/silicon composites are canonical substrates for sensing applications because of their geometry-dependent physicochemical properties and high sensing activity via surface-enhanced Raman spectroscopy (SERS). The self-assembly of gold nanoparticles (AuNPs) synthesized via wet-chemistry on functionalized flat silicon (Si) and vertically aligned Si nanowire (VA-SiNW) arrays is a simple and cost-effective approach to prepare such substrates. Herein, we report on the critical parameters that influence nanoparticle coverage, aggregation, and assembly sites in two- and three-dimensions to prepare substrates with homogeneous optical properties and SERS activity. We show that the degree of AuNP aggregation on flat Si depends on the silane used for the Si functionalization, while the AuNP coverage can be adjusted by the incubation time in the AuNP solution, both of which directly affect the substrate properties. In particular, we report the reproducible synthesis of nearly touching AuNP chain monolayers where the AuNPs are separated by nanoscale gaps, likely to be formed due to the capillary forces generated during the drying process. Such substrates, when used for SERS sensing, produce a uniform and large enhancement of the Raman signal due to the high density of hot spots that they provide. We also report the controlled self-assembly of AuNPs on VA-SiNW arrays, which can provide even higher Raman signal enhancement. The directed assembly of the AuNPs in specific regions of the SiNWs with a control over NP density and monolayer morphology (i.e., isolated vs nearly touching NPs) is demonstrated, together with its influence on the resulting SERS activity.

Light Trapping of Inclined Si Nanowires for Efficient Inorganic/Organic Hybrid Solar Cells.

Light/matter interaction of low-dimensional silicon (Si) strongly correlated with its geometrical features, which resulted in being highly critical for the practical development of Si-based photovoltaic applications. Yet, orientation modulation together with apt control over the size and spacing of aligned Si nanowire (SiNW) arrays remained rather challenging. Here, we demonstrated that the transition of formed SiNWs with controlled diameters and spacing from the crystallographically preferred <100> to <110> orientation was realized through the facile adjustment of etchant compositions. The underlying mechanism was found to correlate with the competing reactions between the formation and removal of oxide at Ag/Si interfaces that could be readily tailored through the concentration ratio of HF to H2O2. By employing inclined SiNWs for the construction of hybrid solar cells, the improved cell performances compared with conventional vertical-SiNW-based hybrid cells were demonstrated, showing the conversion efficiency of 12.23%, approximately 1.12 times higher than that of vertical-SiNW-based hybrid solar cells. These were numerically and experimentally interpreted by the involvement of excellent light-trapping effects covering the wide-angle light illuminations of inclined SiNWs, which paved the potential design for next-generation optoelectronic devices.

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.

Photoelectrical Properties Investigated on Individual Si Nanowires and Their Size Dependence

Periodically ordered arrays of vertically aligned Si nanowires (Si NWs) are successfully fabricated with controllable diameters and lengths. Their photoconductive properties are investigated by photoconductive atomic force microscopy (PCAFM) on individual nanowires. The results show that the photocurrent of Si NWs increases significantly with the laser intensity, indicating that Si NWs have good photoconductance and photoresponse capability. This photoenhanced conductance can be attributed to the photoinduced Schottky barrier change, confirmed by I–V curve analyses. On the other hand, electrostatic force microscopy (EFM) results indicate that a large number of photogenerated charges are trapped in Si NWs under laser irradiation, leading to the lowering of barrier height. Moreover, the size dependence of photoconductive properties is studied on Si NWs with different diameters and lengths. It is found that the increasing magnitude of photocurrent with laser intensity is greatly relevant to the nanowires’ diameter and length. Si NWs with smaller diameters and shorter lengths display better photoconductive properties, which agrees well with the size-dependent barrier height variation induced by photogenerated charges. With optimized diameter and length, great photoelectrical properties are achieved on Si NWs. Overall, in this study the photoelectrical properties of individual Si NWs are systematically investigated by PCAFM and EFM, providing important information for the optimization of nanostructures for practical applications.

Morphology-Graded Silicon Nanowire Arrays via Chemical Etching: Engineering Optical Properties at the Nanoscale and Macroscale.

We report on a quick, simple, and cost-effective solution-phase approach to prepare centimeter-sized morphology-graded vertically aligned Si nanowire arrays. Gradients in the nanowire diameter and shape are encoded through the macroscale substrate via a “dip-etching” approach, where the substrate is removed from a KOH etching solution at a constant rate, while morphological control at the nanowire level is achieved via sequential metal-assisted chemical etching and KOH etching steps. This combined approach provides control over light absorption and reflection within the nanowire arrays at both the macroscale and nanoscale, as shown by UV–vis spectroscopy and numerical three-dimensional finite-difference time-domain simulations. Macroscale morphology gradients yield arrays with gradually changing optical properties. Nanoscale morphology control is demonstrated by synthesizing arrays of bisegmented nanowires, where the nanowires are composed of two distinct segments with independently controlled lengths and diameters. Such nanowires are important to tailor light–matter interactions in functional devices, especially by maximizing light absorption at specific wavelengths and locations within the nanowires.

Hierarchical ZnO/Si nanowire arrays as an effective substrate for surface-enhanced Raman scattering application

Hierarchical arrays of ZnO/Si nanowires were prepared through very convenient two-step solution method on silicon substrates. The tree-like nanowire structures showed high crystal quality, where well aligned Si nanowires were grown vertical to the substrate surface, whereas uniform assembly of branched ZnO nanowires was grown normal to the Si nanowires (backbones). Raman analysis indicated that among all the specimens having identical chemical compositions and dimensions but different forms (thin films or nanowires), Ag-decorated hierarchical arrays of ZnO/Si nanowires possessed the highest intensity peak in surface enhanced Raman scattering spectra. Such arrays demonstrated exceptional ability of detecting Rhodamine 6 G, limiting to a value as low as 1×10−8 mol/L and an enhancement factor up to 1.5 × 104. Both, experimental designing and band structure analysis indicated that electromagnetic effect of the localized surface plasmon resonance of Ag nanoparticles caused eight times enhancement, whereas chemical effect of hierarchical arrays of ZnO/Si nanowires played a more critical role in the high Raman sensitivity. Our results would be helpful to better understand the enhancement mechanisms and meanwhile, could be extended for further potential applications of hierarchical semiconducting nanowires.

Three-dimensional hybrid silicon nanostructures for surface enhanced Raman spectroscopy based molecular detection

Three-dimensional silver nanoparticles decorated vertically aligned Si nanowires (Si NWs) are effective surface-enhanced Raman spectroscopy (SERS) substrates for molecular detection at low concentration levels. The length of Si NWs prepared by silver assisted electroless etching is increased with an increase in etching time, which resulted in the reduced optical reflection in the visible region. These substrates were tested and optimized by measuring the Raman spectrum of standard dye Rhodamine 6G (R6G) of 10 nM concentration. Further, effective SERS enhancements of ∼105 and ∼104 were observed for the cytosine protein (concentration of 50 μM) and ammonium perchlorate (oxidizer used in explosives composition with a concentration of 10 μM), respectively. It is established that these three-dimensional SERS substrates yielded considerably higher enhancement factors for the detection of R6G when compared to previous reports. The sensitivity can further be increased and optimized since the Raman enhancement was found to increase with an increase in the density of silver nanoparticles decorated on the walls of Si NWs.

Correlating Light Absorption with Various Nanostructure Geometries in Vertically Aligned Si Nanowire Arrays

Exploring the interactions between light and nanostructures contributes greatly to understanding and engineering nanoscale optical phenomena related to device performance. However, this often involves a compromise between uniformity and scalability. Given that optical properties, and especially light absorption, are governed by the geometries of nanostructures, this study investigated the correlation between light absorption and vertically aligned silicon nanowire (v-SiNW) arrays synthesized using KrF stepper lithography. Controlled growth experiments of the v-SiNW arrays indicated that their geometrical parameters strongly influence their corresponding light absorption properties, as confirmed by reflection measurements and finite difference time domain (FDTD) simulations, which showed specific wavelength-dependent absorption. Moreover, the extent of tapering the v-SiNW arrays was modulating to achieve broad absorption of visible light resulting from the gradual change in diameter and to optimize their o...

Fabrication and gas sensing properties of vertically aligned Si nanowires

Abstract In this study, a peculiar configuration for a gas sensor consisting of vertically aligned silicon nanowires (VA-Si NWs) synthesized by metal-assisted chemical etching (MACE) is reported. Si NWs were prepared via a facile MACE method and subsequent thermal annealing. Etching was performed by generation of silver nanoparticles (Ag NPs) and subsequent etching in HF/H2O2 aqueous solution; the growth conditions were optimized by changing the process parameters. Highly vertically oriented arrays of Si NWs with a straight-line morphology were obtained, and a top–top electrode configuration was applied. The VA-Si NW gas sensor showed good sensing performance, and the VA-Si NWs exhibited a remarkable response (Rg/Ra = 11.5 ∼ 17.1) to H2 gas (10–50 ppm) at 100 °C which was the optimal working temperature. The formation mechanism and gas sensing mechanism of VA-Si NWs are described. The obtained results can suggest new approaches to making inexpensive, versatile, and portable sensors based on Si NWs having a novel top–top electrode structure that are fully compatible with well-developed Si technologies.

Vertically Aligned Si Nanowire Array—A Promising Anode Material for Li-Ion Battery

Vertically Aligned Si Nanowire Array—A Promising Anode Material for Li-Ion Battery

Graphene quantum dots modified silicon nanowire array for ultrasensitive detection in the gas phase

Si nanostructure-based gas detectors have attracted much attention due to their huge surface areas, relatively high carrier mobility, maneuverability for surface functionalization and compatibility to modern electronic industry. However, the unstable surface of Si, especially for the nanostructures in a corrosive atmosphere, hinders their sensitivity and reproducibility when used for detection in the gas phase. In this study, we proposed a novel strategy to fabricate a Si-based gas detector by using the vertically aligned Si nanowire (SiNW) array as a skeleton and platform, and decorated chemically inert graphene quantum dots (GQDs) to protect the SiNWs from oxidation and promote the carriers’ interaction with the analytes. The radial core–shell structures of the GQDs/SiNW array were then assembled into a resistor-based gas detection system and evaluated by using nitrogen dioxide (NO2) as the model analyte. Compared to the bare SiNW array, our novel sensor exhibited ultrahigh sensitivity for detecting trace amounts of NO2 with the concentration as low as 10 ppm in room temperature and an immensely reduced recovery time, which is of significant importance for their practical application. Meanwhile, strikingly, reproducibility and stability could also be achieved by showing no sensitivity decline after storing the GQDs/SiNW array in air for two weeks. Our results demonstrate that protecting the surface of the SiNW array with chemically inert GQDs is a feasible strategy to realize ultrasensitive detection in the gas phase.

Size-Dependent Surface Migration of Au Alloys on Si Nanowires at Different Cooling Rates.

Understanding metal alloy migration in metal-catalyzed nanowires growth is a prerequisite for improving its potential applications in the field of nanodevices. Here, we explored the surface migration of Au alloys in vertically aligned Si nanowires with different cooling rates. We varied the diameter of Au alloys by controlling the thickness of Au film as a catalyst for SiNW growth, and found that the behavior of Au alloys migration is dependent on size of Au alloys. In addition, the size-dependent migration mechanism of Au alloys was investigated at different cooling rates, which is related to different Au-Si eutectics.

Control over Alignment and Growth Kinetics of Si Nanowires through Surface Fluctuation of Liquid Precursor

Control over alignment and growth kinetics of vertically aligned Si nanowire (v-SiNW) arrays, which were grown using chemical vapor deposition (CVD) via a metal catalyst-assisted vapor–liquid–solid (VLS) mechanism, was demonstrated by introducing a homemade bubbler system containing a SiCl4 solution as the Si precursor. Careful control over the bubbler afforded different amounts of SiCl4 supplied to the reactor. By varying the dipping depth (Dd) and tilting angle (Ta) of the bubbler, the SiCl4 precursor concentration would fluctuate to different degrees. The different SiCl4 concentrations afforded the fine-tuning of v-SiNW array properties like alignment and growth kinetics. The degree of alignment of v-SiNWs could be increased with large amounts of SiCl4, which was caused by slight shallow depth or gentle tilting of the SiCl4 solution in the bubbler due to an increasing degree of fluctuation and fluctuation area. The ability to control alignment and growth kinetics of v-SiNW arrays could be employed in a...

Vertical Si nanowire arrays fabricated by magnetically guided metal-assisted chemical etching

In this work, vertically aligned Si nanowire arrays were fabricated by magnetically guided metal-assisted directional chemical etching. Using an anodized aluminum oxide template as a shadow mask, nanoscale Ni dot arrays were fabricated on an Si wafer to serve as a mask to protect the Si during the etching. For the magnetically guided chemical etching, we deposited a tri-layer metal catalyst (Au/Fe/Au) in a Swiss-cheese configuration and etched the sample under the magnetic field to improve the directionality of the Si nanowire etching and increase the etching rate along the vertical direction. After the etching, the nanowires were dried with minimal surface-tension-induced aggregation by utilizing a supercritical CO2 drying procedure. High-resolution transmission electron microscopy (HR-TEM) analysis confirmed the formation of single-crystal Si nanowires. The method developed here for producing vertically aligned Si nanowire arrays could find a wide range of applications in electrochemical and electronic devices.

Studies of Silicon Nanowires with Different Parameters — By PECVD

One-dimensional nanostructures such as nanowires have a wide range of applications. Silicon is the best competitive material for the carbon nanotubes (CNTs). Carbon and silicon have some similar and peculiar properties. Silicon nanowires (SiNWs) were synthesized using plasma enhanced chemical vapor deposition (PECVD) on p-Si (111) wafer. Gold is used as a catalyst for the growth of the SiNWs. Based on our fundamental understanding of vapor–liquid–solid (VLS) nanowire growth mechanism, different levels of growth controls have been achieved. Gold catalyst deposited and annealed at different temperatures with different thicknesses (450[Formula: see text]C, 500[Formula: see text]C and 550[Formula: see text]C, 600[Formula: see text]C, 650[Formula: see text]C for 4[Formula: see text]min and 8[Formula: see text]min and 3[Formula: see text]nm, 5[Formula: see text]nm, 30[Formula: see text]nm Au thickness). SiNW grown by PECVD with different carrier gases varies with flow rate. We observed the different dimensions of Si nanowires by FESEM and optimized the growth parameters to get the vertical aligned and singular Si nanowires. Optical phonon of the Si nanowires and crystallinity nature were identified by Raman spectral studies.

Photocurrent transient variation in aligned Si nanowire field-effect transistors embedded with Au nanoparticles

Photocurrent transient variation caused by hot-electron transfer was detected in gold nanoparticles embedded in silicon nanowire field-effect transistors via their electrical response under illumination. The devices showed dramatic photocurrent transient variation at various illumination wavelengths (300, 500, 700, and 900 nm). The maximum transient variation of the source–drain current was about five-fold stronger with the gold nanoparticles than without. A finite-difference time-domain method was employed to determine the response wavelength range of the photocurrent transient variation. The distribution of the local electromagnetic field at the interface of the gold nanoparticles and the silicon nanowire was calculated. The weak hot-electron transfer for incident wavelengths below 500 nm was concentrated on the three-phase boundary of air, gold, and silicon, while there was a relatively strong hot-electron transfer present at the boundary of gold and silicon in the wavelength range from 700 to 900 nm.

Enhanced Photocurrents in the Vertically Aligned Hybrid CdTe-Si One-Dimensional Nanostructures with High Surface Area

One-dimensional hybrid CdTe nanostructures on Si nanowires in a wafer batch were synthesized by wet chemical processes. Si nanowires were vertically aligned by the employment of Ag nanoparticles as a catalyst of an electroless etching. The etched Si nanowires were utilized as sacrificial materials for the galvanic displacement reaction to Te nanostructures. The hybrid CdTe on Si nanostructures were created by the wet chemical reaction of the Te nanostructures prepared on aligned Si nanowires. The electrical and optical properties of the fabricated hybrid CdTe-Si nanostructures were characterized. Especially, the one-dimensional hybrid CdTe-Si nanostructures with high surface area demonstrated the enhanced optical current densities. The vertically aligned one-dimensional hybrid CdTe-Si nanostructures can propose an efficient route for the synthesis of hybrid hierarchical nanostructures with high electrical and optical properties in a wafer batch.