Optical sensing of flammable substances using porous silicon microcavities

A porous silicon multilayer, constituted by a Fabry–Pèrot cavity between two distributed Bragg reflectors, is exposed to vapor of several organic species. Different resonant peak shifts in the reflectivity spectra, ascribed to capillary condensation of the vapor in the silicon pores, have been observed. Starting from experimental data, the layer liquid volume fractions condensed in the sensing stack have been numerically estimated. Values ranging between 0.27 (for ethanol) and 0.33 (for iso-propanol) have been found. Time-resolved measurements show that the solvent identification occurs in less then 10 s.     

Optical sensors for vapors, liquids, and biological molecules based on porous silicon technology

The sensing of chemicals and biochemical molecules using several porous silicon optical microsensors, based both on single-layer interferometers and resonant-cavity-enhanced microstructures, is reported. The operation of both families of sensors is based on the variation of the average refractive index of the porous silicon region, due to the interaction with chemical substances either in vapor or liquid state, which results in marked shifts of the device reflectivity spectra. The well established single-layer configuration has been used to test a new chemical approach based on Si-C bonds for covalent immobilization of biological molecules, as probe, in a stable way on the porous silicon surface. Preliminary results on complementary oligonucleotide recognition, based on this technique, are also presented and discussed. Porous silicon optical microcavities, based on multilayered resonating structures, have been used to detect chemical substances and, in particular, flammable and toxic organic solvents, and some hydrocarbons. The results put in evidence the high sensitivity, the reusability, and the low response time of the resonant-cavity-enhanced sensing technique. The possibility of operating at room temperature, of remote interrogation, and the absence of electrical contacts are further advantages characterizing the sensing technique.     

一种新型光学传感器的理论和实验研究

当多孔硅处于有机物蒸汽环境时,由于自身的多孔结构和巨大的比表面积,有机物蒸汽分子将迅速地吸附到多孔硅的表面,并在多孔硅的孔内发生毛细冷凝作用,这将引起多孔硅层有效折射率的变化,从而导致多孔硅微腔反射谱透射峰峰位的变化.本文主要利用Bruggeman介电常数近似理论与光子晶体传输矩阵的方法,建立了多孔硅微腔的理论传感模型.使用光学实验装置对多孔硅微腔进行了传感实验,证明多孔硅微腔可以实现对有机物蒸汽分子种类的检测,且分辨率较高,响应时间和恢复时间短,可重复性好.     

金刚石膜在多孔硅发光中的应用

采用微波等离子体化学气相沉积（MPCVD）法成功地在多孔硅上沉积出均匀、致密的金刚石膜。光致发光测量表明,金刚石膜可以有效稳定多孔硅的发光波长和发光强度,具有明显的钝化效应。金刚石膜的这个特点再加上高硬度特性使金刚石膜成为多孔硅的一种潜在的钝化膜。     

多孔硅基电致发光研究进展

综述了多孔硅电致发光研究进展.阐述了多孔硅液相和全固态电致发光体系及其发光特点,并详细介绍了多孔硅复合体系的制备方法和发光特性.     

Stabilization and operation of porous silicon photonic structures from near-ultraviolet to near-infrared using high-pressure water vapor annealing

The effects of high-pressure water vapor annealing (HWA), electrochemical oxidation, and substrate resistivity on the properties of porous silicon Bragg mirrors and photoluminescent cavities have been investigated. The photonic structures treated by HWA show very good stability upon ageing in air whereas as-formed structures exhibit significant drifts in their optical properties. Using HWA with lightly doped porous silicon, the structure transparency is enhanced sufficiently to enable the possible photonic operation in the near-ultraviolet. However, the index contrast achievable with these structures is very low in the visible and near-infrared. Useful index contrasts in this range can be achieved with either lightly doped porous silicon treated by electrochemical oxidation and HWA or heavily doped porous silicon treated by HWA.     

Design and plasma deposition of dispersion-corrected multiband rugate filters

Inverse Fourier transform method has been commonly used for designing complex inhomogeneous optical coatings. Since it assumes dispersion-free optical constants, introducing real optical materials induces shifts in the position of reflectance bands in multiband inhomogeneous minus (rugate) filters. We propose a simple method for considering optical dispersion in the synthesis of multiband rugate filter designs. Model filters designed with this method were fabricated on glass and polycarbonate substrates by plasma-enhanced chemical vapor deposition of silicon oxynitrides and SiO2/TiO2 mixtures with precisely controlled composition gradients.     

Single and Double Fabry–PÉrot Structure Based on Porous Silicon for Chemical Sensors

This paper presents sensing of chemicals using porous silicon as optical sensor fabricated by periodically modulating the porosity of silicon to produce multilayered structures. Single and double Fabry-Perot (FP) structure is designed by using electrochemical anodical etching technique. The operation of chemical sensor is based on the change of effective refractive index of the porous silicon medium, induced by condensation of solvent vapors around the pillars. Resonant wavelengths of single and double multilayer with a microcavity have presented a red shift when exposed to vapor of solvents. On the other hand, resonant wavelength of double FP structure sandwiched with a diffusion layer has presented different optical response when exposed to vapor of solvents. This structure actually has two stop bands with two resonant wavelengths. While of the beginning first and second stop band and resonant wavelength shift together to the infrared region continuously, after awhile second stop band stopped but the first stop band continued to shifting to the infrared region. Time dependence of optical response of proposed structure can be used for identification chemicals.     

Controllable and Facile Fabrication of Gold Nanostructures for Selective Metal‐Assisted Etching of Silicon

A method with the combination of organic‐vapor‐assisted polymer swelling and nanotransfer printing (nTP) is used to manufacture desirable patterns consisting of gold nano‐clusters on silicon wafers for Au‐assisted etching of silicon. This method remarkably benefits to the size control and regional selection of the deposited Au. By tuning the thickness of the Au films deposited on the polydimethylsiloxane (PDMS) stamps, along with the swelling of PDMS stamps in acetone atmosphere, the Au films are cracked into diverse nanostructures. These nanostructures are covalently transferred onto silicon substrates in a large scale and enable to accelerate the chemical etching of silicon. The etched areas are composed of porous structures which can be readily distinguished from the surroundings on optical microscope. PDMS stamps and the Au clusters provide the control over the feature of the etched areas and the porous silicon, respectively. The silicon surfaces with patterned porous features offer a platform for exploiting new functional templates, for example, they present a diversity of antireflective and fluorescent performance.     

Stress characterization of Si by near-field Raman microscope using resonant scattering

We have developed a tapping-mode scanning near-field optical Raman microscope (SNORM) with a caved and pyramidical probe, using resonant Raman scattering, and have measured the stress distribution of Si. The peak frequency shifts to a lower frequency by 0-0.5 cm(-1) in the area covered by silicon dioxide, whereas it shifts to a higher frequency by 0-0.3 cm(-1) in the area uncovered by silicon dioxide, showing that the areas covered and uncovered by silicon dioxide are under tensile and compressive stresses, respectively. It has been found that compressive stresses of about 0.69 GPa/cm2 are concentrated on the corner of the area uncovered by silicon dioxide. The comparison of stress distributions measured with and without the cantilever shows that the SNORM we developed has a spatial resolution of at least less than 250 nm.     

Fabrication of human IgG sensors based on porous silicon interferometer containing Bragg structures

A simply modified biosensor based on protein A-modified distributed Bragg reflectors (DBR) porous silicon (PSi) chip for the detection of human immunoglobin G (IgG) are developed. The fabrication, optical characterization, and surface derivatization of DBR PSi are investigated. The sensor system studied consist of multi-layer of porous silicon modified with protein-A. The sensor is operated by the measurement of the reflection peak in the white light reflection spectrum. Molecular binding is detected as a shift in wavelength of reflection peaks.     

Study of Morphological and Optical Properties of Porous Silicon

Porous silicon layers have been prepared from non-polished p-type silicon wafers of (100) orientation. Scanning electron microscopy and fluorescence spectroscopy have been used to characterize the morphological and optical properties of porous silicon. The influence of varying the anodizing current density on the morphological and optical properties of porous silicon has been investigated. SEM micrographs show that by increasing the anodizing current density in the electrochemical process two peculiar surface morphologies are obtained. The surface morphology in the central region of the sample consists of solid cells delimited by trenches and the trenches bottom is covered by polyhedral pores, and the surface morphology in the periphery contains polyhedral and current-line-oriented pores. The fluorescence spectrum peak at the anodizing current density of 93 mA/cm² gets the maximum intensity and is blue shifted.     

Distributed Bragg reflector enhancement of electroluminescence from a silicon nanocrystal light emitting device

We demonstrate distributed Bragg reflector (DBR) enhanced electroluminescence from a silicon nanocrystal-based light emitting device. An a-Si/SiO₂ superlattice containing silicon nanocrystals serves as the intrinsic layer in an n-i-n device that is embedded in a DBR cavity consisting of alternating layers of silicon and silicon dioxide. The entire structure, including DBR, superlattice and contact layers, is deposited by plasma-enhanced chemical vapor deposition. The photoluminescence, electroluminescence (EL) and optical output power are measured and compared to a reference device. The DBR is found to enhance the peak EL intensity by a factor of 25 and the external quantum and power conversion efficiencies by a factor of 2.     

Manipulation of liquid droplets using amphiphilic, magnetic one-dimensional photonic crystal chaperones

The controlled manipulation of small volumes of liquids is a challenging problem in microfluidics, and it is a key requirement for many high-throughput analyses and microassays. One-dimensional photonic crystals made from porous silicon have been constructed with amphiphilic properties. When prepared in the form of micrometre-sized particles and placed in a two-phase liquid such as dichloromethane/water, these materials will accumulate and spontaneously align at the interface. Here we show that superparamagnetic nanoparticles of Fe(3)O(4) can be incorporated into the porous nanostructure, allowing the materials to chaperone microlitre-scale liquid droplets when an external magnetic field is applied. The optical reflectivity spectrum of the photonic crystal displays a peak that serves to identify the droplet. Two simple microfluidics applications are demonstrated: filling and draining a chaperoned droplet, and combining two different droplets to perform a chemical reaction. The method provides a general means for manipulating and monitoring small volumes of liquids without the use of pumps, valves or a microfluidic container.     

Optical characterization of nanocrystals in silicon rich oxide superlattices and porous silicon

We propose to analyze ellipsometry data by using effective medium approximation (EMA) models. Thanks to EMA, having nanocrystalline reference dielectric functions and generalized critical point (GCP) model the physical parameters of two series of samples containing silicon nanocrystals, i.e. silicon rich oxide (SRO) superlattices and porous silicon layers (PSL), have been determined. The superlattices, consisting of ten SRO/SiO₂ layer pairs, have been prepared using plasma enhanced chemical vapor deposition. The porous silicon layers have been prepared using short monopulses of anodization current in the transition regime between porous silicon formation and electropolishing, in a mixture of hydrofluoric acid and ethanol. The optical modeling of both structures is similar. The effective dielectric function of the layer is calculated by EMA using nanocrystalline components (nc-Si and GCP) in a dielectric matrix (SRO) or voids (PSL). We discuss the two major problems occurring when modeling such structures: (1) the modeling of the vertically non-uniform layer structures (including the interface properties like nanoroughness at the layer boundaries) and (2) the parameterization of the dielectric function of nanocrystals. We used several techniques to reduce the large number of fit parameters of the GCP models. The obtained results are in good agreement with those obtained by X-ray diffraction and electron microscopy. We investigated the correlation of the broadening parameter and characteristic EMA components with the nanocrystal size and the sample preparation conditions, such as the annealing temperatures of the SRO superlattices and the anodization current density of the porous silicon samples. We found that the broadening parameter is a sensitive measure of the nanocrystallinity of the samples, even in cases, where the nanocrystals are too small to be visible for X-ray scattering. Major processes like sintering, phase separation, and intermixing have been revealed as a function of annealing of the SRO superlattices.     

硅基单结太阳能电池的制备技术、缺陷及其性能的研究

首先综述了硅基单结太阳能电池的分类、制备方法及进展,介绍了化学气相沉积法、液相外延法(LPPE)、金属诱导结晶法(MIC)、磁控溅射法以及分子束外延法等各种硅基太阳能电池的制备方法,阐述了各种制备工艺的优缺点。其次,总结了单晶硅、多晶硅以及非晶硅太阳能电池在组织结构、缺陷方面的研究现状。最后,对硅基太阳能电池的机械、电学、光学以及光电性能等方面的研究进展做了论述。     

AC impedance spectroscopy of porous silicon thin films containing metallic cations

Thin porous silicon (PS) films were prepared by HF/HNO₃ vapor etching on silicon wafers. The infiltration of metallic cations inside the porous silicon matrix followed by slow heating in air leads to an interesting electrical and optical physical phenomena. Al³⁺, Cu⁺, K⁺, Li⁺ metallic cations as chloride or as nitrate solutions are infiltrated inside the silicon porous matrix. After annealing in air at 500 °C during 2 h a structure was achieved in order to measure the Nyquist diagram corresponding to the cations embedded in PS/silicon. The real and imaginary parts of the whole structure depend on the voltage bias and on the frequency. Those signify that the junction is a Schotky-barrier junction.

From the variation of Ln(σT) versus absolute temperature T, where σ is the conductivity, we have deduced the activation energy of the metallic impurities in the [100 °C–400 °C] temperature range. We have found that the activation energies are of about 0.19 eV, 0.25 eV, 0.49 eV and 0.71 eV for Cu⁺, K⁺, Al³⁺ and Li⁺ cations respectively. These kinds of structures are suitable for gas sensing, optical sensing or for the fabrication of fuel cell membranes.     

Vapor-Phase Synthesis of Molecularly Imprinted Polymers on Nanostructured Materials at Room-Temperature

Molecularly imprinted polymers (MIPs) have recently emerged as robust and versatile artificial receptors. MIP synthesis is carried out in liquid phase and optimized on planar surfaces. Application of MIPs to nanostructured materials is challenging due to diffusion-limited transport of monomers within the nanomaterial recesses, especially when the aspect ratio is >10. Here, the room temperature vapor-phase synthesis of MIPs in nanostructured materials is reported. The vapor phase synthesis leverages a >1000-fold increase in the diffusion coefficient of monomers in vapor phase, compared to liquid phase, to relax diffusion-limited transport and enable the controlled synthesis of MIPs also in nanostructures with high aspect ratio. As proof-of-concept application, pyrrole is used as the functional monomer thanks to its large exploitation in MIP preparation; nanostructured porous silicon oxide (PSiO₂) is chosen to assess the vapor-phase deposition of PPy-based MIP in nanostructures with aspect ratio >100; human hemoglobin (HHb) is selected as the target molecule for the preparation of a MIP-based PSiO₂ optical sensor. High sensitivity and selectivity, low detection limit, high stability and reusability are achieved in label-free optical detection of HHb, also in human plasma and artificial serum. The proposed vapor-phase synthesis of MIPs is immediately transferable to other nanomaterials, transducers, and proteins.     

Investigations of the optical absorption spectra of porous silicon layers on the Silicon backing by the nondestructive photoacoustic method

This paper presents a series of experimental photoacoustic spectra of porous silicon layers on the crystalline silicon and their numerical analysis performed in the proposed two layer model. The goal of the analysis was to calculate the optical absorption coefficient spectra of porous silicon from the photoacoustic spectra of the porous silicon layer on the silicon backing. The character of the observed optical absorption band associated with the porous silicon was revealed.     

Waveguide properties of optical structures fabricated by oxidation of porous silicon

Results of an investigation of the optical properties of channel waveguides fabricated by oxidation of porous silicon are described. The waveguide parameters are estimated and the existence of optical anisotropy is established. The effective refractive index of the dominant quasi-TM waveguide mode is measured. The results suggest that a buffer layer exists between the waveguide and the silicon substrate. It is hypothesized that a second refractive index peak exists within this layer. Pis’ma Zh. Tekh. Fiz. 23, 86–89 (May 26, 1997)