Porous Silicon Samples Research Articles

Optimizing of porous silicon morphology for synthesis of silver nanoparticles

A group of porous silicon (PSi) samples with different surface morphologies prepared at different etching current densities were investigated as a substrate for formation of silver nanoparticles (AgNPs). Simple dipping process of PSi in silver nitrite (AgNO3) with concentrations of 10−3, 10−2, and 5 × 10−2 M was employed to synthesize AgNPs.The p-type PSi was prepared by electrochemical etching process at different values of current densities. Three different forms of PSi morphology; meso, macro, and pillar with different roughness values 1.21, 177, 89.5, and 38.7 nm were prepared with varying the current densities 30, 60, 90, and 120 mA/cm2 respectively. The structural and surface morphology properties of samples before and after dipping in AgNO3 were studied through analyzing of scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD). The results show that the AgNPs sizes depend on the surface morphology and roughness of PSi. The AgNPs prepared by dipping process does not depend on the porosity of the porous layer. For meso pore-like structures the sizes of AgNPs were in the range from 0.52 to 4.09 μm for a low surface roughness, and from 0.52 to 6.83 nm for high surface roughness. For the case of macro pore-like structure which possesses the highest pore sizes and surface roughness, the AgNPs sizes vary from 0.61 to 3.42 μm. The minimum AgNPs size of about 190 nm was obtained from porous surface of pillar form.

Effect of current density and etching time on photoluminescence and energy band gap of p-type porous silicon

Porous silicon samples were formed on p-type silicon wafer by electrochemical method. Metal porous silicon structure was used to obtain the rectifying behavior. Current density and etching time was studied which affect the photoluminescence and energy band gap. Three different groups were prepared using different current density and varying etching time. The optical properties for p-type porous silicon were investigated by photoluminescence (PL) spectroscopy. Findings from study showed that the porous silicon has band gap energy in the range from 1.81 to 2.07 eV. The band gap energy also increased with increasing current density or increasing etching time. The PL peaks showed a steady red color shift from 500 to 800 nm with increasing etching time.

Measurement of Optical Constants of Wet Porous Silicon Using In Situ Photoconduction

Most techniques used to measure the optical constants of porous silicon (PSi) are based on dried samples, which may be contaminated by the environment, and require sensible setups as well as meticulous sample preparation, which could alter the PSi characteristics. Here, we show a method of determination of wet PSi optical constants using the analysis of the photocurrent-thickness curves acquired in situ in hydrofluoric acid during PSi formation. In particular, interference patterns in these curves allow the determination of the refractive index of the electrolyte-impregnated PSi. The photocurrent is proportional to the optical transmission through PSi. Processing only one sample in a single experiment was sufficient for the determination of the optical constants. The method shown here does not require any sophisticated sample preparation or handling, or high-precision optical instruments, while preserving fresh PSi layers, even for high porosities and for arbitrarily layer thicknesses. The method may be of interest in the fields of optical constant measurement, optical sensing using liquid media in PSi, or in situ optical monitoring of the evolution of wet PSi in various conditions. Here, PSi samples of different porosities were used with three different illuminations (633, 543, and 405 nm).

Properties of fluorescence based on the immobilization of graphene oxide quantum dots in nanostructured porous silicon films

The fluorescence of graphene oxide quantum dots (GOQDs) that are infiltrated into porous silicon (PSi) is investigated. By dropping activated GOQDs solution onto silanized PSi samples, GOQDs are successfully infiltrated into a PSi device. The results indicate that the intensity of the fluorescence of the GOQD-infiltrated multilayer with a high reflection band located at its fluorescence spectra scope is approximately double that of the single layer sample. This indicates that the multilayer GOQD-infiltrated PSi substrate is a suitable material for the preparation of sensitive photoluminescence biosensors.

Boron Doped Ultrananocrystalline Diamond Films on Porous Silicon: Morphological, Structural and Electrochemical Characterizations

Boron doped ultrananocrystalline diamond (BDUND) films were grown and characterized on porous silicon (PS) substrates. PS samples were prepared from n-type monocrystalline silicon wafers (100) with 1-20 Ω.cm of resistivity, by electrochemical etching, using HF-acetonitrile solution as electrolyte. BDUND films were grown by Hot Filament Chemical Vapor Deposition using CH4, H2 and Ar. The doping process consisted of an additional hydrogen line, passing through a bubbler containing B2O3 dissolved in methanol, with boron/carbon ratio of 20000 ppm in solution. Raman spectroscopy and X-Ray diffraction were used to evaluate the quality of the films. Scanning electron microscopy was used for morphological characterization, and confirmed that the films covered the pores without filling them. Electrochemical response and capacitance behavior of the electrodes were explored, by cyclic voltammetry. Samples presented high capacitance, confirming that BDUND/PS electrodes are promising for application as electrochemical capacitors.

Microfabricated porous silicon backbone for stable neural interfaces

To fabricate a porous silicon–polymer hybrid neural electrode with the capability of adaptive stiffness, a critical microfabrication process was developed and the porous silicon (PSi) backbone of the neural electrode was prepared for mass production on 8-in. wafers via microelectromechanical systems (MEMS) technologies. Surface characteristics of the microfabricated PSi backbone were determined using scanning electronmicroscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectrum (FTIR). Moreover, acute cytotoxicity of the PSi backbone was assessed by seeding a mouse fibroblast cell line (L929) on the surface. After 2 days of culture, morphology of cells was observed using a fluorescence microscope, and relative cell viability was also used to quantitatively evaluate the cytotoxicity. Compared to Si samples, relative cell attachment of PSi samples was 192.77±27.19% due to the nano featured surface providing more suitable sites for cell adhesion, indicating that the microfabricated PSi backbone was cytocompatible.

Photoluminescence properties of porous silicon formed on plasma-processed substrates

Measurements have been made of the photoluminescence spectra of samples of porous silicon after they were processed with a compression plasma flow in a nitrogen atmosphere. A significant increase of the photoluminescence of the test samples is detected by comparison with porous silicon obtained by the usual technique.

Investigations of the composition of macro-, micro- and nanoporous silicon surface by ultrasoft X-ray spectroscopy and X-ray photoelectron spectroscopy

The features of atomic and electron structure and phase composition of the surface in the samples of macro-, micro- and nanoporous silicon and their changes with depth were investigated with the use of X-ray photoelectron spectroscopy, ultrasoft X-ray emission spectroscopy and scanning electron microscopy. Analysis of the X-ray emission spectra applying the simulation method allowed to follow the trends in the changes of the composition for the investigated samples starting from the surface and into the bulk and to show the differences in the phase composition between the samples of porous silicon with different pores size.

Study on morphological and structural properties of silver plating on laser etched silicon

The immersion-plating method for the preparation of silver nanoparticles is highly selected because it has the virtue of simplicity and lower cost. Silver nanoparticles features deposition onto a porous silicon (PSi) layer from aqueous solutions of two silver salt (AgNO3) concentration 10−2M, and 5×10−3M were studied. The PSi was prepared by laser assisted etching technique for illumination laser intensity of 5 to 20mW/cm2. The average pore sizes were 0.66, 1.45, 1.71, and 1.79μm respectively. Structural and morphological characterizations of PSi samples before and after dipping were studied through analyzing of scanning electron microscope (SEM) and X-Ray Diffraction (XRD) pattern. These measurements showed that silver nanoparticles were in different sizes and distribution. This difference depends on the illumination laser Intensity used in the etching process. It was found that silver planes, grain size, specific surface area, and nucleation sites of silver nanoparticles strongly depended on PSi morphology and silver salt concentrations.

Morphological evolution of the porous silicon surface for different etching time and current density in hf-ethanol solution

Porous silicon samples were obtained by the anodization etching process of a n-type silicon wafer. The pores formation was investigated taking into account the anodization time. Scanning Electron Microscopy and Optical Profilometry were used to characterize the morphology, pore size, pore depth, thickness, roughness, surface area and also the morphological evolution of porous silicon layer. The formed porous silicon layer showed a tendency to increase the pore size, pore depth, roughness and surface area as a function of the etching time. The porous silicon was obtained at different etching times and at a fixed current density. The experimental results showed that the porosity of the porous silicon prepared under different anodization times can be controlled.

Structural, optical and electrical characterization of nanostructured porous silicon: Effect of current density

In this work, an attempt has been made to fabricate porous silicon (PS) from p-type crystalline silicon (c-Si) wafers by using the electrochemical etching process at six different current densities (40, 60, 75, 100, 125 and 150mA/cm2) with constant time (30min). The influence of varying current density on morphological, structural, optical and electrical properties of PS samples were analyzed by using SEM, AFM, XRD, FT-IR, PL and electrical (I–V) techniques, respectively. Microstructural images clearly showed that the average pore diameter and thickness increase with increase current densities up to 100mA/cm2 and decrease for 125mA/cm2. It could be related to breaking of pore walls and exposing to the next layer of c-Si. Further increase the current density about 150mA/cm2, the average pore diameter increase as in the case of first layer (40–100mA/cm2) of c-Si wafer. The result is reflected in PL emission band (at 708nm) and the intensity of the emission band shifted towards red region. The X-ray diffraction pattern confirm the formation of porous silicon as appeared as a broad peak at 2θ=69.3° belongs to (400) reflection. The FTIR study supports the X-ray diffraction analysis that shows the vibrational bands of S–H2 and Si–O–Si at 2109cm−1, 915cm−1 and 615cm−1 and 1107cm−1, respectively. The I–V characteristic of PS exhibited rectifying behavior with different values of ideality factor (η) and barrier height (ϕb). It is concluded from the experimental results that the formed pores developed up to 100mA/cm2 in the top layer of c-Si and the formed pores exposed to the next layer of c-Si when increase the high electrochemical etching process (above 100mA/cm2).

CONTROL CAPABILITY OF ELECTROLYTIC CONCENTRATION ON REFRACTIVE INDEX AND DIELECTRIC CONSTANT OF POROUS SILICON LAYERS

Porous Silicon (PS) samples have been prepared by electrochemical anodization of p-type silicon wafer by varying HF concentrations in the electrolytic solution. The structural, surface morphological, optical and surface composition analysis of the prepared samples were done by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Photoluminescence (PL) and Fourier transform infrared (FTIR) spectroscopy studies respectively. The grain sizes of PS were determined by XRD study. The porosity of PS samples was estimated by using the parameters obtained from the SEM images by the geometrical method. The porosity of the samples was found to vary between 11% and 84% due to the variation in HF concentration in the electrolytic solution. The refractive index and dielectric constant values of PS as a function of porosity were determined by Effective Medium Approximation methods. Strong visible emission peak at 498 nm, with no apparent shift with respect to variation in etching parameter, is observed in Photoluminescence study. The surface bonding and their vibration modes of the PS were determined by transmission FTIR spectroscopy.

Detection of Organic Vapors Based on Photoluminescent Bragg-Reflective Porous Silicon Interferomete.

Novel photoluminescent Bragg-reflective porous silicon, exhibiting dual optical properties, both the optical reflectivity and photoluminescence, was developed and used for sensing organic vapors. Photoluminescent Bragg-reflective porous silicon samples were prepared by an electrochemical etch of n-type silicon under the illumination. The etching solution consisted of a 3:1 volume mixture of aqueous 48% hydrofluoric acid and absolute ethanol. The typical etch parameters for the generation of photoluminescent Bragg-reflective porous silicon involved a periodic square wave current with 50 repeats. The surface of photoluminescent Bragg-reflective porous silicon was characterized by a FT-IR spectroscopy. Both reflectivity and photoluminescence were simultaneously measured under the exposure of organic vapors. The shift of reflection band to the longer wavelength and the quenching of photoluminescence under the exposure of various organic vapors were observed.

Variations of the optical characteristics of nano-, meso-, and macroporous silicon with time

Time variations in the composition and optical properties of porous silicon samples with different pore sizes are discussed. Substrate orientation, type of conduction, and composition of etching solution were varied to produce nano-, meso-, and macroporous silicon and multilayer porous structures. A correlation of the photoluminescence (PL) intensity of the samples after long-term storage in air and the intensity of the IR absorption band (616 cm−1) assigned to the Si-Si bonds is demonstrated. The nanoporous silicon exhibits the highest PL intensity after 120-day storage.

Behavior of porous silicon crystallite size analized by Raman spectroscopy and phonon confinement model

Porous silicon samples, obtained by electrochemical process in a solution containing HF-Ethanol were prepared varying etching time and current density. Using the phonon confinement model the crystallite size was estimated on the porous layer through the best fit of the Raman spectrum of first order. The experimental and theoretical points were better adjusted in order to obtain more accurate results in relation to previous works. For the samples with varying the anodization time and keeping constant the current density the crystallite size had an oscillating behavior and tendency to decrease as a function of the anodization time. Varying current density, keeping constant the anodization time, was observed an increase in the crystallite size as a function current density.

Human aortic endothelial cell morphology influenced by topography of porous silicon substrates.

Porous silicon has received much attention because of its optical properties and for its usefulness in cell-based biosensing, drug delivery, and tissue engineering applications. Surface properties of the biomaterial are associated with cell adhesion and with proliferation, migration, and differentiation. The present article analyzes the behavior of human aortic endothelial cells in macro- and nanoporous collagen-modified porous silicon samples. On both substrates, cells are well adhered and numerous. Confocal microscopy and scanning electron microscopy were employed to study the effects of porosity on the morphology of the cells. On macroporous silicon, filopodia is not observed but the cell spreads on the surface, increasing the lamellipodia surface which penetrates the macropore. On nanoporous silicon, multiple filopodia were found to branch out from the cell body. These results demonstrate that the pore size plays a key role in controlling the morphology and growth rate of human aortic endothelial cells, and that these forms of silicon can be used to control cell development in tissue engineering as well as in basic cell biology research.

Influence of applied current density on the nanostructural and light emitting properties of n-type porous silicon

Effects of current density on nanostructure and light emitting properties of porous silicon (PS) samples were investigated by field emission scanning electron microscope (FE-SEM), gravimetric method, Raman and photoluminescence (PL) spectroscopy. FE-SEM images have shown that below 60 mA/cm2, macropore and mesopore arrays, exhibiting rough morphology, are formed together, whose pore diameter, pore depth and porosity are about 265–760 nm, 58–63 μ m and 44–61%, respectively. However, PS samples prepared above 60 mA/cm2display smooth and straight macropore arrays, with pore diameter ranging from 900–1250 nm, porosity of 61–80% and pore depth between 63–69 μ m . Raman analyses have shown that when the current density is increased from 10 mA/cm2to 100 mA/cm2, Raman peaks of PS samples shift to lower wavenumbers by comparison to crystalline silicon (c-Si). The highest Raman peak shift is found to be 3.2 cm-1for PS sample, prepared at 90 mA/cm2, which has the smallest nanocrystallite size, about 5.2 nm. This sample also shows a pronounced PL, with the highest blue shifting, of about 12 nm. Nanocrystalline silicon, with the smallest nanocrystallite size, confirmed by our Raman analyses using microcrystal model (MCM), should be responsible for both the highest Raman peak shift and PL blue shift due to quantum confinement effect (QCE).

Study of growth of dot and column in porous silicon samples of various thicknesses prepared at a constant current density

Porous silicon is considered to be composed either of spherical shaped interconnected silicon quantum dots or combination of quantum dots and columns. This paper presents a study of a series of porous silicon films of various thicknesses, prepared at a 20mA current density by the electrochemical etching technique. The photoluminescence spectra of the series samples were monitored. Further, we used a photoluminescence fitting model by Singh and John (John–Singh) in its extended form by Elhouichet to estimate the percentage of dots and columns; their average diameters and corresponding variances. The shape of experimental photoluminescence spectra fits well with John–Singh model. As a result, the analytical curves drawn using the fitting parameters showed the decrease in mean crystallite diameter of columns and dot while increase in variance of column and decrease in variance of dots. Hence, more homogenous dots are formed. Thus, it results in the formation of a more ordered nanocrystalline structure with more porosity. It verified the quantum assumptions. The discrepancy in the PL behavior of a sample is well explained by the model.

Porous Silicon Based Humidity Sensor

Porous silicon (PS) is well known as a photovoltaic material. However, in the last couple of years, research has focused on the use of porous silicon as chemical-biological sensors. This paper discusses the use of PS as an optical humidity sensor. Photo-luminescence (PL) quenching measurements in a controlled humidity atmosphere (mixed Nitrogen gas and water vapor) were performed to test the sensor response towards the water vapor. Surface morphologies of the PS samples were characterized by a scanning electron microscopy (SEM) and structural properties were investigated via Fourier Transform Infrared (FTIR) spectroscopy. It was found that PS surface is very sensitive to the water vapor. The experimental results suggested that PS surface is a promising candidate material to be used as a humidity sensor.

White light from annealed porous silicon: Broadband emission from violet to the near infrared

We report white light emission under UV excitation from porous silicon (PS) samples annealed at different temperatures and surrounding atmospheres. The photoluminescence (PL) spectra show a broad emission from ultraviolet to near infrared. Annealed PS samples were studied by photoluminescence, FTIR and EPR spectroscopies. The PL spectra were found to include three main components centered at the violet, green and infrared. The FTIR experiments suggest the formation of a system composed by silicon nanocrystals embedded in a nonstoichiometric silicon oxide matrix (nc-Si/SiOx). The EPR measurements reveal the existence of the well-know Pb0 (g~2.007) and Pb1 (g~2.004) centres, and of a signal at g~1.9997. The combined PL, FTIR and EPR results suggest that violet and green emissions come from oxygen-excess defects in SiOx while the infrared emission is related to defects located at Si/SiOx interfaces.