Porous Silicon Array Research Articles

Comparing defect-assisted interfacial charge transfer in CdS/PSA and CdS/TiO2[sbnd]NTAs nanoheterojunctions for enhanced photodegradation and biosensing

Firstly, oxygen vacancy defects are intentionally introduced in the preparation process of TiO2 nanotube arrays (TiO2-NTAs) and porous silicon arrays (PSA) films through the anodic oxidation method after annealing treatment, as expected. Then, CdS/TiO2-NTAs and CdS/PSA heterostructures are successfully fabricated using the electrodeposition method, incorporating abundant surface vacancy defects. The crafted CdS/TiO2-NTAs nanoheterojunctions presented elevated photoelectrochemical (PEC) performances and sensing activities compared with that of CdS/PSA, validating by photodegradation and PEC biosensing tests, which mainly ascribed to the synergistic effect between the promoted carriers separation efficiency and boosted surface active sites induced by vacancies defects. Furthermore, a comparative and comprehensive interfacial charge transfer mechanism and qualitative dynamic processes for CdS/TiO2-NTAs and CdS/PSA type-II nanoheterojunctions are proposed, revealing an enlarged staggered band offset between CdS and TiO2-NTAs. The evaluation of this band offset was conducted using nanosecond time-resolved transient photoluminescence and steady photoluminescence spectra, respectively. Following this scheme, the photodegradation rate for CdS/TiO2-NTAs nanocomplex toward degradation of MO was 92.8% and 1.06 times than that of CdS/PSA; while the PEC biosensing performance for CdS/TiO2-NTAs showed a detection range 0 – 600 μM and limit of detection of 2.3 μM, with a sensitivity of 456 mA cm−2M−1.

Protein Identification and Quantification Using Porous Silicon Arrays, Optical Measurements, and Machine Learning.

We report a versatile platform based on an array of porous silicon (PSi) thin films that can identify analytes based on their physical and chemical properties without the use of specific capture agents. The ability of this system to reproducibly classify, quantify, and discriminate three proteins separately is demonstrated by probing the reflectance of PSi array elements with a unique combination of pore size and buffer pH, and by analyzing the optical signals using machine learning. Protein identification and discrimination are reported over a concentration range of two orders of magnitude. This work represents a significant first step towards a low-cost, simple, versatile, and robust sensor platform that is able to detect biomolecules without the added expense and limitations of using capture agents.

Silver-based SERS substrates fabricated using a 3D printed microfluidic device.

The detection of harmful chemicals in the environment and for food safety is a crucial requirement. While traditional techniques such as GC-MS and HPLC provide high sensitivity, they are expensive, time-consuming, and require skilled labor. Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical tool for detecting ultralow concentrations of chemical compounds and biomolecules. We present a reproducible method for producing Ag nanoparticles that can be used to create highly sensitive SERS substrates. A microfluidic device was employed to confine the precursor reagents within the droplets, resulting in Ag nanoparticles of uniform shape and size. The study investigates the effects of various synthesis conditions on the size distribution, dispersity, and localized surface plasmon resonance wavelength of the Ag nanoparticles. To create the SERS substrate, the as-synthesized Ag nanoparticles were assembled into a monolayer on a liquid/air interface and deposited onto a porous silicon array prepared through a metal-assisted chemical etching approach. By using the developed microfluidic device, enhancement factors of the Raman signal for rhodamine B (at 10-9 M) and melamine (at 10-7 M) of 8.59 × 106 and 8.21 × 103, respectively, were obtained. The detection limits for rhodamine B and melamine were estimated to be 1.94 × 10-10 M and 2.8 × 10-8 M with relative standard deviation values of 3.4% and 4.6%, respectively. The developed SERS substrate exhibits exceptional analytical performance and has the potential to be a valuable analytical tool for monitoring environmental contaminants.

Photon-induced interfacial charge transfer mechanism of porous silicon/TiO 2 nanoparticles for photoelectrochemical performance

Abstract Highly ordered porous silicon array (PSA) films were fabricated by anodic oxidation of p-type silicon wafer. Using the methods of magnetron sputtering and thermal annealing, dispersed TiO 2 nanoparticles (NPs) were decorated onto the top of PSA films. Steady-state photoluminescence (PL) spectra and nanosecond time-resolved transient photoluminescence (NTRT-PL) spectra of as-fabricated nano-composites were studied under light irradiation with wavelengths of 266 nm and 400 nm, respectively. A distinct blue-shift in the NTRT-PL spectra is observed owing to the presence of Si 3+ and Ti 3+ cations, which is further confirmed by X-ray photoelectron spectroscopy (XPS). Such a phenomenon could be well explained by considering the competition mechanism between the photo-generated charge separation process and recombination associated with oxygen vacancy (OV). Our present research demonstrates a novel approach to investigate charge separation and transport in a dual-band gap photoelectrochemical (PEC) system. Following this scheme, UV and visible-light photocatalytic activities of TiO 2 /PSA heterojunction composites were evaluated through photodegradation of methyl orange (MO) in aqueous solution. Our current work is expected to provide a simple strategy for synthesising composite photocatalysts, with the hope that optimal sunlight harvesting would be achieved.

Synthesis of the ‘Carbon Nanotubes-porous Silicon’ Hybrid Material for Gas Sensors

The present work represents the approach to develop the novel functional material for gas micro- and nanosensors. The isolated porous silicon arrays have been synthesized using the electrostatic mask method in the electro-chemical chamber. The hybrid material ‘carbon nanotubes-porous silicon’ has been synthesized on the isolated porous silicon arrays with the CVD process. The nanocomposite structure has been synthesized by tin sputtering on the hybrid material ‘carbon nanotubes-porous silicon’. The morphology and electro-physical properties of the nanocomposite structures have been investigated. The test gas sensor based on the nanocomposite structures showed high sensitivity to NO2 molecule absorption at room temperature.

Fabrication of a Novel Micro-Nano-Hybrid Porous Silicon Array for Photonic Crystal Application

In this study, we present a novel technique for the fabrication of 2D hybrid photonic crystal structures based on ordered porous silicon array. Each unit of the array consists of a centric micro-sized macropore and six surrounding nano- pores. The fabrication process is based on photoelectrochemical etching, combined with steps of oxidation, CMP and TMAH etching. We investigate the absolute photonic band gap in such made 2D hybrid photonic crystal composed of triangular and honeycomb lattices. Our results indicate strongly that the photonic band gap is very sensitive to the structure parameters (especially to the diameter of nano-pores), and it can be an effective way to enlarge the absolute photonic band gap by modifying the pore diameters.

Growth and Photoluminescence of β-SiC Nanowires on Porous Silicon Array

Nonaligned and curly β-SiC nanowires (nw-SiC) were grown on porous silicon array (PSA) by a chemical vapor deposition method with nickel as the catalyst. The morphology, structure and the composition of the nw-SiC/PSA and the SiC-SiO2core-shell fibers which is the semi-product were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. Based on the experimental results a possible growth mechanism of nw-SiC was explained. Two broad photoluminescence peaks located around ~409 and ~494 nm were observed in nw-SiC/PSA in the PL measurement when utilizing 300 nm ultraviolet fluorescent light excited at room temperature. The excellent luminescent performances are ascribed to the quantum confinement effects in nw-SiC. The optical merits of nw-SiC/PSA made it a promising material in the fields of ultraviolet-blue emitting devices.

Rapid detection of illicit drugs in neat saliva using desorption/ionization on porous silicon

The ability to detect illicit drugs directly in oral fluids is of major interest for roadside, workplace and athlete drug testing. For example, roadside testing for popular drugs of abuse is being rolled out by law enforcement agencies following the introduction of legislation in several countries all over the world. This paper reports on the direct analysis of methamphetamine, cocaine and 3,4-methylenedioxymethamphetamine in oral fluids using a hydrophobic porous silicon array as a combined drug extraction and concentration medium. Analysis by laser desorption/ionization time-of-flight mass spectrometry (MS) identified these drugs with a sensitivity in line with the suggested confirmatory cut-off concentrations, and 300 times faster. In addition, MS imaging demonstrated good spot-to-spot reproducibility of the signal. Our analytical approach is compatible with multiplexing and is therefore suitable for high-throughput analysis of samples obtained from drug testing in the field. Furthermore, the application of this analytical technology is not limited to illicit drugs or oral fluids. Indeed, we believe that this platform technology could be applied to the high-throughput analysis of diverse metabolites in body fluids.

Micropatterned Arrays of Porous Silicon: Toward Sensory Biointerfaces

We describe the fabrication of arrays of porous silicon spots by means of photolithography where a positive photoresist serves as a mask during the anodization process. In particular, photoluminescent arrays and porous silicon spots suitable for further chemical modification and the attachment of human cells were created. The produced arrays of porous silicon were chemically modified by means of a thermal hydrosilylation reaction that facilitated immobilization of the fluorescent dye lissamine, and alternatively, the cell adhesion peptide arginine-glycine-aspartic acid-serine. The latter modification enabled the selective attachment of human lens epithelial cells on the peptide functionalized regions of the patterns. This type of surface patterning, using etched porous silicon arrays functionalized with biological recognition elements, presents a new format of interfacing porous silicon with mammalian cells. Porous silicon arrays with photoluminescent properties produced by this patterning strategy also have potential applications as platforms for in situ monitoring of cell behavior.

Direct-write patterning of microstructured porous silicon arrays by focused-ion-beam Pt deposition and metal-assisted electroless etching

Photoluminescent porous silicon (PSi) patterns of micrometer dimension were produced by the Pt-assisted electroless etching of Si in 1:1:2 methanol:HF:H2O2. Pt-containing squares with side lengths ranging from 1.25to20μm were defined by a focused-ion-beam-assisted maskless deposition of Pt from an organometallic precursor, trimethylmethylcyclopentadienyl platinum. The Pt-patterned Si samples were then etched to produce photoluminescent pixel arrays with high fidelity transfer of the Pt deposition pattern into luminescent pixels of varying size. The morphology of the PSi patterns was correlated with the spatial luminescence characteristics at the individual pixel level. Luminescent pixels with feature sizes down to ca. 1μm were largely confined to the areas initially coated with Pt, and the morphologies produced within any one set of equal-sized Pt squares were similar. For 5-μm pads and larger, the morphologies obtained were an admixture of a porous structure coexisting with deeper heavily etched crater regions. Only the porous areas were observed to emit, with the deeper crater areas being dark in a two-photon luminescence. The smaller 1.25- and 2.5-μm pads exhibited a common morphology, in which a brightly luminescent outer ring surrounds a weaker but still distinguishable luminescence in the center of the etched structure. These results are in contrast with the spatial luminescence patterns and morphologies for the millimeter-scale Pt pads [S. Chattopadhyay, X. Li, and P. W. Bohn, J. Appl. Phys. 91, 6134 (2002)], in which electroless etching and, thus, PSi formation is observed in the regions not initially coated with Pt.

Electrical Porous Silicon Microarray for dna Hybridization Detection

ABSTRACTThe sensitivity of Porous Silicon (PSi) to the presence of charged molecules and its large internal surface area represent two important properties that make this material and ideal candidate for electrical biosensor development. We have demonstrated the use of a macroporous silicon electrical sensor for label-free detection of DNA hybridization in real time as well as identification of organic solvents in liquid phase. Binding of DNA inside the PSi matrix induces a change in capacitance and conductance. Having demonstrated the suitability of macroporous silicon layers for real time detection of DNA hybridization on single devices, we have extended our findings to the fabrication of a microarray with individual device electrical addressing capabilities. On a crystalline p-type silicon wafer, process steps such as KOH etching and electrochemical dissolution are employed in selected regions to create a free-standing porous membrane for sensing applications. Individual electrical contacts are made on the front side of the wafer while the infiltration of the probe and target molecules is done from the back avoiding any direct interaction of the molecules with the contact sites. We will report on the design considerations of the electrical porous silicon array and the preliminary results obtained using synthetic DNA as a model molecule.