SiNW Arrays Research Articles

Complementary metal oxide semiconductor compatible silicon nanowires-on-a-chip: fabrication and preclinical validation for the detection of a cancer prognostic protein marker in serum.

An integrated translational biosensing technology based on arrays of silicon nanowire field-effect transistors (SiNW FETs) is described and has been preclinically validated for the ultrasensitive detection of the cancer biomarker ALCAM in serum. High-quality SiNW arrays have been rationally designed toward their implementation as molecular biosensors. The FET sensing platform has been fabricated using a complementary metal oxide semiconductor (CMOS)-compatible process. Reliable and reproducible electrical performance has been demonstrated via electrical characterization using a custom-designed portable readout device. Using this platform, the cancer prognostic marker ALCAM could be detected in serum with a detection limit of 15.5 pg/mL. Importantly, the detection could be completed in less than 30 min and span a wide dynamic detection range (∼10(5)). The SiNW-on-a-chip biosensing technology paves the way to the translational clinical application of FET in the detection of cancer protein markers.

Optical trapping enhancement from high density silicon nanohole and nanowire arrays for efficient hybrid organic–inorganic solar cells

The SiNH arrays exhibit excellent light-absorbing structures and higher optical trapping as compared to SiNW arrays, resulting in leading performance power conversion efficiency of 11.25% in the hybrid organic–inorganic solar cells.

Quantitative analysis of photons' decaying pathways in Si nanowire arrays for highly efficient photoelectrochemical solar hydrogen generation

We quantitatively investigated excitons' decaying pathways of photon-excited SiNW arrays to determine the reason for their low performances. It is demonstrated that excitons decay through both carriers' separation and energy transfer due to Si's indirect-band-gap feature, and these two pathways could be regulated through surface modification.

Influence of Fabrication Parameter on the Nanostructure of SiNWs under HF/AgNO<sub>3</sub>/Fe(NO<sub>3</sub>)<sub>3</sub> Etching System

Large-area SiNWs has been successfully fabricated through one-step metal-assisted chemical etching process at room temperature. The effects of key fabrication parameters (AgNO3 concentration, Fe (NO3)3 concentration, and etching time) on the nanostructure SiNWs were carefully investigated by SEM, TEM, respectively. The results show that AgNO3 concentration and Fe (NO3)3 concentration play important roles to the lengths and arrangements of SiNWs arrays in one-step MACE. The morphological transition of Si surfaces from solid nanowires to porous nanowires can be found with increasing AgNO3 concentration and Fe (NO3)3 concentration, which indicates that the re-dissolved Ag+ would work as the main oxidative species for oxidizing the silicon substrate and forming SiNWs in the HF/ AgNO3/ Fe (NO3)3 etching system. The length of SiNWs is increased with increasing AgNO3 concentration from 0.005 mol/L to 0.02 mol/L and etching time, the chemical polish phoenomenon can be observed when the Fe (NO3)3 concentration increases to 0.5 mol/L. A novel mechanism is proposed to explain the formation of SiNWs in HF/ AgNO3/ Fe (NO3)3 solution.

Molecular doping applied to Si nanowires array based solar cells

Solution-based processing is a rapidly growing area in the electronics and photonics field due to the possibility of reducing fabrication costs of materials for solar cells, transistors, memory and many other devices. Moreover thanks to its intrinsic nature it provides the possibility to perform conformal processing on structured surfaces. Most of the solution-processing work has so far been devoted to organic materials, but in this work an approach focused on nanostructured silicon is presented. The idea consists in the immersion of a silicon wafer, with Si nanowires grown on top, in a chemical bath containing dopant precursors molecules diluted in a solvent. The molecules deposit from the liquid all over the exposed surfaces and work as a dopant source for the Si nanowires during successive thermal annealing. Doping levels of 1×1019cm−3 are controllably obtained without structural damage and hetero-interfaces creation. The Si-NWs array used presents density of 2×1010cm−2, average length of 500nm and diameters up to 70nm. The doped Si-NWs are then integrated in complete solar cells which have been electrically characterized. It is found that the molecular doping method applied to the SiNW arrays provides higher short circuit current and fill factor than the reference samples.

Broadband absorption enhancement in elliptical silicon nanowire arrays for photovoltaic applications.

Semiconducting nanowire arrays have emerged as a promising route toward achieving high efficiencies in solar cells. Here we propose a perpendicular elliptical silicon nanowire (PEE-SiNW) array for broadband light absorption in thin film silicon solar cells. Simulation results reveal that light absorption enhancement is originated from the split of the principal modes as well as the excitation of high order modes caused by the asymmetry of the elliptical nanowires and the enhanced mode coupling between adjacent elliptical nanowires attained by the appropriate arrangement of nanowires. An ultimate efficiency of 29.1% is achieved for the optimal PEE-SiNW array, which is 16.4% higher than that of the circular SiNW array with the same fill fraction.

The Effect of Etching Temperature upon the Optical Reflactance on Silicon Nanowires Grown By Electroless Etching

THE EFFECT OF ETCHING TEMPERATURE UPON THE OPTICAL REFLACTANCE ON SILICON NANOWIRES GROWN BY ELECTROLESS ETCHING The antireflective nature of Silicon nanowires (SiNW’s) revealed by their black color and dull appearances have drawn the attention of researchers [1]. It may eliminate the need for antireflective coating on these nanostructures to reduce energy losses in solar cells [2]. SiNWs arrays prepared by electroless etching method can significantly suppress light reflection across the visible spectrum. Optical reflectance analysis on SiNWs fabricated at different temperatures (Fig. 1) show a significantly decrease in optical reflectivity down to 1.5% at 25°C.Enhanced optical reflectance may be achieved at lower etching solution temperatures. Low temperatures slow the kinetics reaction that would grew uniform and well aligned SINWs (Fig. 2a) contrary to randomly direction nanowires grow at high temperatures (Fig. 2b). A uniform growing of SiNWs could optimize nanowire spacing that would improve charge collection and enhance optical reflectance [3].In this experiment a Cary 100 UV-Visible Spectrometer was used for reflectance analysis and silicon (Si) wafers were dipped for 15 minutes at different temperatures (25°C, 35°C, 45°C, 55°C and 70°C) in an electroless solution prepared in a petri dish containing silver nitrate (AgNO3) and hydrofluoric acid (HF) [4].

Reflectance Studies in Silicon Nanowires Grown By Electroless Etching

One of the major energy loss mechanisms of solar cells is optical reflection. Utilization of nanostructures in solar cells may eliminate the need for antireflective coating [1]. The antireflective nature of Silicon nanowires (SiNW’s) revealed by their black color and dull appearances have drawn the attention of researchers [2]. Highly oriented SiNWs arrays prepared by electroless etching method can significantly suppress light reflection across a broad spectrum. Reflectivity studies on these show a significantly decrease in optical reflectivity down to 1.3% for 10 µm long SiNWs arrays [3]. Electroless etching is a top-down technique for growing SiNWs and does not need costly and expensive equipment to grow SiNWs as bottom-up methods usually do.The electroless process begins with the cleaning of silicon substrates. They were cleaned in acetone, methanol and deionized water, then blow-dried with N2. They are dipped in hydrofluoric acid (HF) 49% solution for 1 min to remove any remaining oxide.The silicon (Si) wafers are dipped for 45 minutes at room temperature in an electroless solution prepared in a petri dish containing silver nitrate (AgNO3) and hydrofluoric acid (HF) [4].The substrates are dipped and galvanic reactions disperse silver ions on their surface, losing silicon as SiF6. As a consequence of this etching process, the removal of Si via Ag ions results in vertically aligned arrays of SiNWs covered with silver dendrites. Varying the heights of the SiNWs(fig. 1). changes their optical reflectance. A target optical reflection would be 0.8.Fig 1. Cross sectional SEM images of the vertically standing Si NW arrays obtained by electroless etching.REFERENCES[1] Jin-Young Jung, Han-DonUm, Sang-WonJee, Kwang-TaePark, JinHoBang, Jung-HoLee, “Optimal design for antireflective Si nanowire solar cells,” Solar Energy Materials & Solar Cells, 2013.[2] Baris Ozdemir, Mustafa Kulakci, Rasit Turan and Husnu Emrah Unalan, “Effect of electroless etching parameters onthe growth and reflection properties ofsilicon nanowires,” IOP Science, 2011.[3] Hua Bao and Xiulin Ruan, "Optical absorption enhancement in disorderedvertical silicon nanowirearrays for photovoltaic applications," OPTICS LETTERS, 2010.[4] R. G. Mertens and K. B. Sundaram, “Recession and Characterization of Patterned Nanowires Grown by Electroless Etching of Silicon”. ECS Journal of Solid state Science and Technology, 2012.

Solid-phase crystallization of amorphous silicon nanowire array and optical properties

An amorphous silicon nanowire (a-SiNW) array perpendicular to a glass substrate can be successfully obtained through the metal-assisted chemical etching of amorphous silicon (a-Si) thin films. The solid-phase crystallization of a-SiNWs was carried out by thermal annealing in a forming gas in the temperature range from 600 to 900 °C. The effects of hydrogen in the film and the film morphology on the crystallization of a-SiNWs were investigated by Raman spectroscopy and transmission electron microscopy. A higher hydrogen concentration of a-SiNWs reduced the crystallization temperature, as in a-Si thin films. It was also revealed that the large surface area of the a-SiNW array affected the crystallization process. We also studied the optical property of the fabricated SiNW array and demonstrated its high potential as an active layer in solar cells.

Light trapping and surface plasmon enhanced high-performance NIR photodetector

Heterojunctions near infrared (NIR) photodetectors have attracted increasing research interests for their wide-ranging applications in many areas such as military surveillance, target detection, and light vision. A high-performance NIR light photodetector was fabricated by coating the methyl-group terminated Si nanowire array with plasmonic gold nanoparticles (AuNPs) decorated graphene film. Theoretical simulation based on finite element method (FEM) reveals that the AuNPs@graphene/CH3-SiNWs array device is capable of trapping the incident NIR light into the SiNWs array through SPP excitation and coupling in the AuNPs decorated graphene layer. What is more, the coupling and trapping of freely propagating plane waves from free space into the nanostructures, and surface passivation contribute to the high on-off ratio as well.

Air Heating Approach for Multilayer Etching and Roll-to-Roll Transfer of Silicon Nanowire Arrays as SERS Substrates for High Sensitivity Molecule Detection

SiNW array represents an attractive system for construction of high-performance energy, electronic, and sensor devices. To meet the demand for flexible devices as well as address the concern about the full use of the Si material, large-area transfer of the SiNW array from growth substrate is very desirable. Here, we report a simple air heating approach to achieve the multilayer etched SiNW array. This method allows the fabrication of up to a five-layer (while perfectly three-layer) cracked SiNW array on single-crystalline Si wafer via a templateless metal-assisted etching approach. Fractures could happen at the crack position when an appropriate pressure was applied on the SiNW array, facilitating the wafer-scale layer-by-layer transfer of the SiNW array onto a flexible substrate through a low-cost and time-efficient roll-to-roll (R2R) technique. Further releasing of the SiNW array to other receiving substrates was accomplished with the aid of a thermal release tape. After modification with sliver nanoparticles (AgNPs), the flexible SiNW array showed great potential as a high-sensitivity surface-enhanced Raman spectroscopy (SERS) substrate for detecting rhodamine 6G (R6G) molecule with concentration as low as 10(-9) M.

Effect of Dopants on Epitaxial Growth of Silicon Nanowires

We investigated the effects of dopants on epitaxial growth of Si NWs, with an emphasis on synthesizing vertical epitaxial Si NW arrays on Si (111) substrates. We found that addition of boron with a B:Si feed-in atomic ratio greater than 1:1000 improved the percentage of Si NWs grown along the vertical <111> direction to more than 90%, compared to 38% for i-Si NWs. We also demonstrated a stemmed growth strategy and achieved a 93% percentage of i-Si NW segments along the vertical <111> direction on Si substrates. In summary, our study opens up potential for using chemical synthesized vertical Si NW arrays integrated on Si substrates for large-scale applications.

Broadband Absorptance High Efficiency Silicon Nanowire Fractal Arrays for Photovoltaic Applications

ABSTRACTNanowire arrays have been proposed to enhance light trapping, increase efficiencies, and reduced material cost in photovoltaic solar cells. In this work we present a new crystalline silicon nanowire array structure, inspired by fractal geometry. The array structure is assumed to be an infinite 2D array in the x and y directions, and composed of vertically aligned SiNW suspended in air. Hexagonal fractal-like geometry is adapted in arranging cylindrical SiNW in these arrays. Full-wave finite element method 3D simulation is used to compute reflectance, transmittance and absorptance of the array for a normal incidence plane wave. The proposed fractal-like distribution of SiNW arrays yield broad absorption spectrum and enhanced efficiency while using less material. The efficiency of the proposed fractal-like SiNW arrays achieve ∼100% enhancement over that of the equivalent thickness flat c-Si film, and ∼18% enhancement over an equivalent height hexagonal array. The proposed optimized structures achieved a filling ratio ∼25%, which is ∼33% less than the corresponding hexagonal array.

Highly portable fluorescent turn-on sensor for sulfide anions based on silicon nanowires

A portable fluorescent sensor for S2− based on SiNW arrays was realized and used to monitor S2− in running water.

Dependence of Performance of Si Nanowire Solar Cells on Geometry of the Nanowires

The dependence of performance of silicon nanowires (SiNWs) solar cells on the growth condition of the SiNWs has been described. Metal-assisted electroless etching (MAE) technique has been used to grow SiNWs array. Different concentration of aqueous solution containing AgNO3 and HF for Ag deposition is used. The diameter and density of SiNWs are found to be dependent on concentration of solution used for Ag deposition. The diameter and density of SiNWs have been used to calculate the filling ratio of the SINWs arrays. The filling ratio is increased with increase in AgNO3 concentration, whereas it is decreased with increase in HF concentration. The minimum reflectance value achieved is ~1% for SiNWs of length of ~1.2 μm in the wavelength range of 300–1000 nm. The performance and diode parameters strongly depend on the geometry of SiNWs. The maximum short circuit current density achieved is 35.6 mA/cm2. The conversion efficiency of solar cell is 9.73% for SiNWs with length, diameter, and wire density of ~1.2 μm, ~75 nm, and 90 μm−2, respectively.

Si-Nanowire-Array-Based NOT-Logic Circuits Constructed on Plastic Substrates Using Top–Down Methods

Si-nanowire (NW)-array-based NOT-logic circuits were constructed on plastic substrates. The Si-NW arrays were fabricated on a Si wafer through top-down methods, including conventional photolithography and crystallographic wet etching, and transferred onto the plastic substrates. Two field-effect transistors were fabricated on a single Si-NW array composed of five nanowires aligned in parallel and connected in series to form NOT-logic circuits. The excellent flexibility of the fabricated device was confirmed by bending-cycling tests. The voltage-transfer curve of the NOT-logic circuits showed an inverting operation with a logic swing of -92% and voltage gain of -2.5.

Silicon Nitride Passivation of Silicon Nanowires Solar Cell

Vertical aligned silicon nanowires were synthesized using chemical etching of silicon wafer. Influence of a silicon nitride layer on the top of the silicon nanowires solar cell has been investigated. The optical properties of a Si NWs array with and without silicon nitride passivation layer are examined in terms of optical reflection property. In the presence of silicon nitride layer, 1% reflection ratio in the spectral range (250-1000nm) is achieved. In addition, the solar cell characteristics have been significantly improved, which exhibits high short circuit current as well high efficiency. Based on the current-voltage measurements and morphology results, we show that the silicon nitride layer can passivate the defects generated by wet etching processes.

Control growth of silicon nanocolumns’ epitaxy on silicon nanowires

The epitaxial growth of Si nanocolumns on Si nanowires was studied using hot-wire chemical vapor deposition. A single-crystalline and surface oxide-free Si nanowire core (core radius*21 ± 5 nm) induced by indium crystal seed was used as a substance for the vapor phase epitaxial growth. The growth process is initiated by sidewall facets, which then nucleate upon certain thickness to form Si islands and further grow to form nanocolumns. The Si nanocolumns with diameter of 10-20 nm and aspect ratio up to 10 can be epitaxially grown on the surface of nanowires. The results showed that the radial growth rate of the Si nanocolumns remains constant with the increase of deposition time. Meanwhile, the radial growth rates are controllable by manipulating the hydrogen to silane gas flow rate ratio. The optical antireflection properties of the Si nanocol- umns' decorated SiNW arrays are discussed in the text.

Facile Method for Modification of the Silicon Nanowires and Its Application in Fabrication of pH-Sensitive Chips

A novel, facile, and effective method for modification of SiNWs or SiNW arrays has been developed. In this method, reaction between reductive Si-H bonds on the surface of SiNWs and the aldehyde group containing in organic molecules has been used for immobilization of organic molecules onto the surface of SiNW arrays. The method is time saving and can be operated at room temperature without any other complex reaction requirement. Fluorescence images, XPS, fluorescence spectra, and IR spectra were used for characterization of the modification. Through this method, a SiNW array-based pH sensitive chip was realized by covalently immobilizing 5-aminofluorescein molecules onto the surface of SiNW arrays with glutaraldehyde as linker molecules. Fluorescence intensity of the chip increased with increasing of pH value and a linear relationship between fluorescence intensity and pH values was acquired. In addition, the chip has been successfully used for real-time and in situ monitoring of extracellular pH changes for live HeLa cells and the result exhibited fine resolution of time and space.

Improved photovoltaic performance of silicon nanowire/organic hybrid solar cells by incorporating silver nanoparticles

Silicon nanowire (SiNW) arrays show an excellent light-trapping characteristic and high mobility for carriers. Surface plasmon resonance of silver nanoparticles (AgNPs) can be used to increase light scattering and absorption in solar cells. We fabricated a new kind of SiNW/organic hybrid solar cell by introducing AgNPs. Reflection spectra confirm the improved light scattering of AgNP-decorated SiNW arrays. A double-junction tandem structure was designed to manufacture our hybrid cells. Both short-circuit current and external quantum efficiency measurements show an enhancement in optical absorption of organic layer, especially at lower wavelengths.