Macroporous Silicon Research Articles

Fabrication of Highly Ordered Macropore Arrays in p-Type Silicon by Electrochemical Etching: Effect of Wafer Resistivity and Other Etching Parameters

Macroporous silicon is a promising substrate in the field of optics, electronics, etc. In this paper, highly ordered macropore arrays were fabricated in p-type silicon wafers by electrochemical etching using a double-tank cell. The effect of the silicon resistivity, etching voltage and etching time on the pore morphology was investigated and the influence mechanism was analyzed. The pore diameter would decrease with the increase in the silicon resistivity and the decrease in the etching voltage, due to the increase in the space charge region width (SCRL). The pore depth would increase with the resistivity and the voltage. However, too high resistivity would cause insufficiency at the pore tips and too high voltage would cause pore splitting, which may cause a decrease in the pore depth. Then, the aspect ratio of 21 can be obtained on the silicon wafer with a resistivity of 50–80 Ω·cm at the etching voltage of 5 V with a maximum etching rate of about 0.92 μm/min.

Gold Nanorod-Coated Hydrogel Brush Valves in Macroporous Silicon Membranes for NIR-Driven Localized Chemical Modulation.

A two-dimensional array of microfluidic ports with remote-controlled valve actuation is of great interest for applications involving localized chemical stimulation. Herein, a macroporous silicon-based platform where each pore contains an independently controllable valve made from poly(N-isopropylacrylamide) (PNIPAM) brushes is proposed. These valves are coated with silica-encapsulated gold nanorods (GNRs) for NIR-actuated switching capability. The layer-by-layer (LBL) electrostatic deposition technique was used to attach the GNRs to the PNIPAM brushes. The deposition of GNRs was confirmed by dark-field optical microscopy, and the localized surface plasmon resonance (LSPR) of the deposited GNRs was analyzed using UV-Vis spectra. To evaluate the chemical release behaviors, fluorescein dye was employed as a model substance. The chemical release properties, like OFF-state diffusion through the valve, the ratio between ON-state and OFF-state chemical release, and the rapidness of chemical modulation of the valve, were investigated, varying the PNIPAM brush thickness. The results indicate that enhancing the thickness of the PNIPAM brush in our platform improves control over the chemical modulation properties. However, excessive increases in brush length may lead to entanglement, which negatively impacts the chemical modulation efficiency.

Fabrication of Macro-Porous Silicon Nitride Ceramics by Bonded EPS Ball Template and Gel-Casting Method

In the pursuit of advanced biomaterials suitable for orthopedic applications, macro-porous silicon nitride has emerged as a highly promising candidate due to its exceptional biocompatibility, mechanical strength, and potential to stimulate tissue regeneration. This describes the fabrication of macro porous silicon nitride ceramic materials using expanded polystyrene (EPS) particles as pore-forming agents and gel-casting molding for preform preparation. Pore size, porosity and compressive strength of as obtained samples have been varied according to the heating time of the EPS template and the solid content of Si3N4 slurry. Increasing the template heating time leads to a reduction in macropore size from 400 to 300 μm, accompanied by an increase in compressive strength from 12.1 to 15.3 MPa. With increasing in the solid content, the mean pore size of samples have been kept as constant of 320 μm, while the compressive strength increases from 2.79 MPa to 16.74 MPa, attributed to Si3N4 matrix densification. This developed approach offers a customizable strategy for advanced orthopedic biomaterials, addressing the need for reliable solutions in musculoskeletal regenerative medicine.

The X-Direction Scanning of Atomic Force Microscopy to Analyze Porous Silicon P-Type Si(100) Fabricated by Electrochemical Anodization Method

This work only investigated the x-direction scanning of atomic force microscopy, which can accurately measure porous silicon's width, depth, and roughness. Pores on p-type Si (100) surfaces fabricated by electrochemical anodization method with the variation of resistivity and current density, i.e., 0.001-0.005 Ω.cm (high dopant) and 1-10 Ω.cm (low dopant), and 4, 6, 8, and 10 mA/cm2, respectively. Macroporous silicon was obtained for both high and low dopants. Pore width, pore depth, and roughness of silicon increase with increasing the current density. Characteristics of porous silicon for high dopants are smaller than that for low dopants. It indicates that large amounts of dopant in silicon can slow the etching process.

Improved the Sensitivity and Limit of Detection of Surface Alloying SERS Sensors by Controlling Mixing Ratio of Trimetallic (Ag-Au-Pd) Nanoparticles

In this study, three different types of hybrid structures SERS sensors made from different mixing ratio of trimetallic (Ag-Au-Pd) nanoparticles of [Au1: Ag1: Pd1] , [Au1: Ag2: Pd1] , [Au1: Ag2: Pd2] in surface alloying forms deposited on macro porous silicon (macroPsi) layer were created and extensively tested. By using a simple and fast immersion process, trimetallic (Ag-Au-Pd) nanoparticles were created utilizing ion reduction of numerous metallic salts on a macro porous silicon (macroPsi) layer. The macroPsi layers were created using a laser assisted etching (LAE) technique for 15 minutes with a constant density of approximately (28mA/c.m2). At a constant concentration (10-3 M) of HAu.Cl4, Ag.NO3 and Pd.Cl2 to synthesize Au-NPs, Ag-NPs and Pd-NPs, immersion processes with various mixing ratios were performed. The hybrid structures of SERS sensors were examined using XRD, FE-SEM, and Raman microscopes. The sensors were tested at different concentrations from 10-6 to 10-12 of MB target molecules. The SERS trimetallic sensors demonstrated a considerable reliance on hot spot zones among trimetallic nanoparticles. Higher enhancement factor with lower detection limit of Raman signal was obtained from [Au1: Ag2: Pd2] hybrid structures SERS sensor of about 1.5×1010 and 10-14 respectively, due to extra ordinary specific surface area and high surface density forming trimetallic nanoparticles . The variations of mixing ratio of trimetallic (Ag-Au-Pd) provide an effective pathway to develop the sensors performance towards detection of lower concentrations.

Morphology and Optical Characteristics of TiO2 Nanofilms Grown by Atomic-Layer Deposition on a Macroporous Silicon Substrate

Morphology and Optical Characteristics of TiO2 Nanofilms Grown by Atomic-Layer Deposition on a Macroporous Silicon Substrate

Hydrogen gas sensor based on NiO decorated macroporous silicon heterojunction

Hydrogen gas sensor based on NiO decorated macroporous silicon heterojunction

Control of chemical release in hydrogel microvalve array for localized chemical stimulations

Chemical stimulation is considered as one of the most promising therapeutic solutions for degenerated retina due to its ability to mimic a natural chemical signaling pathway. However, the majority of demonstrated platforms actuate chemicals using bulky external setups. Hence, developing high-resolution chemical release platforms with remote-controlled actuation mechanism is of great interest to the next generation of chemical prosthetics. Moreover, the effectiveness of chemical stimulation depends on several parameters, such as the chemical dosage, pulse duration, and spread radius. Therefore, to engineer the chemical release profile, the actuation mechanism of these localized chemical release platforms needs to be thoroughly studied to facilitate modifications. In this study, we have presented a high-resolution chemical release platform based on a macroporous silicon membrane structure. Chemical release ports defined in the membrane pores can be remotely actuated through thermo-responsive poly(N-isopropylacrylamide) (PNIPAAm) hydrogel with near-infrared (NIR) light. As plasmonic heating elements, gold nanorod (GNR) was incorporated within PNIPAAm hydrogel. The pore size of the hydrogel was tuned by varying tetrahydrofuran content in the hydrogel polymerization solution. We have shown that the chemical release properties through membrane pores can be controlled as required using this approach. This insight can be used to engineer the relevant chemical release parameters, thereby creating an efficient chemical stimulation platform.

Localized flow control by photothermal actuation of pNIPAAm hydrogel brushes in a macroporous silicon membrane

Localized flow control by photothermal actuation of pNIPAAm hydrogel brushes in a macroporous silicon membrane

On the applicability of the Maxwell Garnett effective medium model to media with a high density of cylindrical pores

The optical properties of dielectric materials with subwavelength cylindrical pores are commonly described by effective medium models. We compare the Maxwell Garnett and the Bruggeman effective medium models for porous silicon with simulations and experiments for the case of polarization orthogonal to the pore axis. The Maxwell Garnett model matches the results of the simulations even up to very high porosities. An experimental study of the effective permittivity of macroporous and mesoporous silicon is conducted by analyzing the Fabry-Pérot oscillations in the long-wavelength limit. These experimental results are also consistent with the Maxwell Garnett model. We advocate using this model for media with cylindrical pores in the future.

A road for macroporous silicon stabilization by ultrathin ALD TiO2 coating

Ultrathin TiO2 layers deposited by ALD can successfully passivate macroporous silicon while keeping photocatalytic activity.

Absorption and Transmission of Macroporous Silicon and Nanowires

Absorption and Transmission of Macroporous Silicon and Nanowires

Formation and morphological evolution investigation of the porous silicon decorated with Pt particles

Platinum nanoparticles were prepared on silicon substrates at room temperature using electroless deposition with different durations from 1 to 40 min. The effect of deposition times on the morphology and composition of the deposited Pt particles and porous films was examined using scanning, transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Raman scattering. Moreover, atomic force microscopy was used to examine the structural properties and morphological characterization of the deposed particles. The wetting properties of the por-Si/Pt structure were investigated using the sessile drop method. The research indicates that the por-Si/Pt structure has uniformly distributed nonspherical self-assembled Pt particles with a height of up to 120 nm and a diameter of 200 nm. The concentration of these particles is 2.5 × 109 cm−2 and there is a mesoporous silicon layer with a thickness up to 580 nm. A uniform size distribution of the particles is achieved after 1 min of deposition. The conditions for creating the por-Si/Pt structure, characterized by a high Pt particle concentration and coverage, have been identified. The morphology parameters of the particles and layers depend on the duration of the deposition. The evolution of the formation of an array of platinum particles, meso- and macroporous silicon, including all stages of growth of the components of the Por-Si/Pt structure was established.

ZnO Deposition on Silicon and Porous Silicon Substrate via Radio Frequency Magnetron Sputtering

Nanostructured Zinc Oxide (ZnO) was deposited on silicon (c-Si) and macroporous silicon (m-PS) using a radio frequency (RF) reactive magnetron sputtering technique. Two RF powers of 60 and 80 W were selected for ZnO deposition on the substrates. Furthermore, the c-Si and m-PS substrate temperatures were kept at 500 and 800 °C, respectively. The morphological, structural, and optical characteristics of the samples were studied using scanning electron microscopy (SEM), an X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), and photoluminescence spectroscopy (PL). The SEM images revealed the formation of ZnO nanorods on the c-Si and ZnO nanostructures constituted by the assembly of nanorods. It has been found that the increasing RF sputtering power caused the rise in the residual stress. In addition, the increase in the deposition temperature caused an improvement in the arrangement of the crystals, which was attributed to the decrease in crystal defects.

Comparative Study of SERS-Spectra of NQ21 Peptide on Silver Particles and in Gold-Coated "Nanovoids".

The NQ21 peptide has relatively recently attracted attention in the biomedical sphere due to its prospects for facilitating the engineering of the HIV1 vaccine and ELISA test. Today, there is still a need for a reliable and fast methodology that reveals the secondary structure of this analyte at the low concentrations conventionally used in vaccines and immunological assays. The present research determined the differences between the surface-enhanced Raman scattering (SERS) spectra of NQ21 peptide molecules adsorbed on solid SERS-active substrates depending on their geometry and composition. The ultimate goal of our research was to propose an algorithm and SERS-active material for structural analysis of peptides. Phosphate buffer solutions of the 30 µg/mL NQ21 peptide at different pH levels were used for the SERS measurements, with silver particles on mesoporous silicon and gold-coated "nanovoids" in macroporous silicon. The SERS analysis of the NQ21 peptide was carried out by collecting the SERS spectra maps. The map assessment with an originally developed algorithm resulted in defining the effect of the substrate on the secondary structure of the analyte molecules. Silver particles are recommended for peptide detection if it is not urgent to precisely reveal all the characteristic bands, because they provide greater enhancement but are accompanied by analyte destruction. If the goal is to carefully study the secondary structure and composition of the peptide, it is better to use SERS-active gold-coated "nanovoids". Objective results can be obtained by collecting at least three 15 × 15 maps of the SERS spectra of a given peptide on substrates from different batches.

Fast-response PN-photodetector based on V2O5-nanorods/macroporous Si heterojunction

The development of optoelectronic devices such as high-performance photodetectors is still needed. In this study, a heterojunction photodetector was fabricated by growing vanadium pentoxide nanorods (V2O5-NRs) on macroporous silicon (MacroPSi) using thermal evaporation technology. This was followed by 60 minutes of annealing at 550 °C in an oxygen-free environment. X-ray diffraction, field emission scanning electron microscopy, and ultraviolet-visible-near infrared spectroscopy were used to investigate the structural, morphological, and optical features of the produced V2O5-NRs. The performance of the photodetectors is measured by the (I-V) characteristics in the dark and in the light using two point probes connected to Keithley 6487 and a green LED (532 nm, 8 mW/cm2) with -5 V to +5 V for the baise voltage. The nonlinear I-V curve reveals the existence of Schottky barriers at the interfaces of the n-type V2O5-NRs and the MacroPSi substrate as the current runs vertically through the device. These curves are used to determine the photodetector's sensitivity and resposivity. The current device, V2O5-NRs/MacroPSi, demonstrated good values of sensitivity and responsivity equal to 20.8 and 5.2 A/W under 532 nm (8 mW/cm2) at 3 and 5 V, respectively. In addition, the rise and fall durations of the photodetector under the same light source at 5 V were estimated to be 0.24 and 0.22 seconds, respectively. In light of the obtained results, it's clear that the produced V2O5-NRs/MacroPSi heterostructure is a good contender for excellent performance as a photodetector in commercial photoelectronic device applications.

Marshmallow-like Macroporous Silicone Monoliths as Reflective Standards and High Solar-Reflective Materials

Marshmallow-like Macroporous Silicone Monoliths as Reflective Standards and High Solar-Reflective Materials

Biomimetic porous silicon oxycarbide ceramics with improved specific strength and efficient thermal insulation

Considering the challenge of aerodynamic heating, the development of high-performance insulating ceramic materials with lightweight and low thermal conductivity is crucially important for aerospace vehicles to achieve flight at high speed for a long time. In this work, macro-porous silicon oxycarbide (SiOC) ceramics with directional pores (DP-SiOC) (mean pore size of 88.1 μm) were prepared using polysiloxane precursors via freeze casting and photocrosslinking, followed by pyrolysis. The DP-SiOC samples were lightweight (density ∼0.135 g cm–3) with a porosity of 90.4%, which showed good shapability through the molding of polysiloxane precursors. The DP-SiOC samples also exhibited an ultra-low thermal conductivity of 0.048 W(m K)–1 at room temperature, which can also withstand heat treatment at 1200 °C for 1 h. In addition, scaffolds with triply periodic minimal surfaces (TPMS) were fabricated using digital light processing (DLP) printing, which was further filled with polysiloxane precursors for increasing the strength of DP-SiOC. The TPMS scaffolds filled with macro-porous SiOC ceramics (TPMS-DP-SiOC) showed good integration between TPMS and macro-pore structures, which had a porosity ∼75% and high specific strength of 9.73 × 103 N m kg–1. The thermal conductivity of TPMS-DP-SiOC samples was 0.255 W(m K)–1 at room temperature. The biomimetic TPMS-DP-SiOC ceramics developed in this study are likely used for thermal protection systems.

Kinetics of charge carriers in bilateral macroporous silicon

The kinetics of charge carriers in bilateral macroporous silicon with macroporous layers of equal thicknesses is calculated by the finite difference method. A diffusion equation for a monocrystalline substrate and macroporous layers is solved. The boundary conditions are defined at the boundaries between the monocrystalline substrate and the macroporous silicon layers on both sides. Stationary distribution of excess charge carriers in the bilateral macroporous silicon with the macroporous layers of equal thicknesses calculated by the finite difference method is set as the initial condition. Under stationary conditions, excess charge carriers are generated by light with the wavelengths of 0.95 µm and 1.05 µm. It is shown that at the counting times much longer than the relaxation time, all the distributions of the concentration of excess minority carriers generated by light with any wavelength approach the same distribution with exponentially decreasing value.

Wafer‐Scale Fabrication of Hierarchically Porous Silicon and Silica by Active Nanoparticle‐Assisted Chemical Etching and Pseudomorphic Thermal Oxidation (Small 22/2023)

Porous Silica In article number 2206842, Stella Gries, Patrick Huber, and co-workers present a novel approach based on silver nanoparticle-assisted chemical etching (MACE) of macroporous silicon for the synthesis of hierarchically porous silicon. The MACE process is mainly guided by a metal-catalyzed redox-reaction where silver nanoparticles drill mesopores into the silicon scaffold structure. The resulting porous silicon can be transformed structure-conserving by thermal oxidation to hierarchically porous silica glass.