Porous Silicon Microcavities Research Articles

Porous silicon optical microcavity for chemical sensing application using tris-(8-hydroxyquinoline) aluminum (Alq3)

The present paper reports the study of a porous silicon based microcavity for a potential chemical sensing application using tris-(8-hydroxyquinoline) aluminum (Alq 3 ) molecules. Porous silicon based planar microcavity was first designed and fabricated using the electrochemical etching technique. Photoluminescence emission of a single porous silicon layer after immersion in an Alq 3 solution was first carried out in order to verify that the Alq 3 molecules were bound to the porous surface. A wide green band centered at 519 nm, typical of a nano-structured Alq 3 film, was observed. Reflectivity measurements of the porous silicon microcavity were then performed for different aluminum concentrations of the Alq 3 solution. The microcavity device showed a good sensibility for the Alq 3 molecules and an important shift of the microcavity photonic resonance was observed. This device might be considered for a potential aluminiun sensing application.

Innovative design for optical porous silicon gas sensor

A simple system for the detection of analytes in the atmosphere employing a freestanding nanostructured porous silicon optical microcavity was designed and built. The system is based on the measurement of optical transmittance changes as a function of the incidence angle of a monochromatic laser beam. Using a matrix formalism and effective medium theories we calculate the optimal conditions for the system so as to obtain the best sensitivity of the sensor. We tested the system over a detection area of the porous silicon microcavity of 100 μm 2 with isopropyl alcohol vapor diluted in a N 2 stream. With the system proposed we were able to detect changes in the concentration of isopropyl alcohol as small as 1 ppm, which is equivalent to a change in the effective refractive index of about 3 × 10 −6. The response time is very fast, lower than 0.5 s.

Detection and light enhancement of glucose oxidase adsorbed on porous silicon microcavities

AbstractPorous silicon (PSi) structure is used as support material to detect protein infiltration and to induce fluorescence and second harmonic light enhancement from glucose oxidase (GOX). Functionalization and protein infiltration is monitored by specular reflectometry. Optical response enhancement of PSi microcavity structures compared to PSi single layers or Bragg mirrors is observed, when GOX is impregnated. Penetration of organic molecules along the PSi microcavity structure is demonstrated by energy dispersive X‐ray profile. Enhanced fluorescence emission of GOX when adsorbed on PSi microcavity is evidenced by multi‐photon microscopy (MPM). Second harmonic light generation is observed at some particular pores of PSi and subsequent resonance enhancement of the signal arising from the GOX adsorbed within the pores is detected. Our work evidences an improved device functionality of GOX‐PSi microcavities due to strongly confined and localized light emission within these structures. This opens the way towards the application of PSi microcavity structures as amended biosensors based on their locally enhanced optical response. The second main achievement lies in the novelty of the used techniques. In contrast to the specular reflectometry used to monitor the macroscopic optical response of PSi structures, MPM presents a valuable alternative microscopic technique probing individual pores. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoinduced fluorescence enhancement and energy transfer effects of quantum dots porous silicon

AbstractWe investigate photoinduced fluorescence enhancement (PFE) and energy transfer effects of surfactant capped Cd/Se core and CdSe/ZnS core/shell quantum dots (QD) that emit at 630‐650 nm adsorbed in and onto porous silicon (PSi) single layer films and microcavity devices. Evidence is presented for fluorescence resonance energy transfer (FRET) from the QD (emitter) to mesoPSi nanostructures (acceptor) that photoluminess between 650‐850 nm. We also observe a strong PFE of QD emission adsorbed into meso porous silicon (mesoPSi) under continuous irradiation with 532 nm excitation light. New insight into the mechanism of PFE is gleaned from studies of PSi pore size and surface chemistry including native, H2O2, UV‐ozone, and thermal oxides, and aminosilane coated thermal oxide mesoPSi. These treatments alter the photoluminescent characteristics of the mesoPSi and QD. The PFE effect on QD PL is largest on thermally oxidized microcavities. PFE is considerably weaker for CdSe QD adsorbed onto macroPSi, silicon wafer surfaces and especially for ZnS capped QD suggesting the PFE effect requires a close interaction of the QD core with oxidized substrate. It is plausible that mesoPSi (10‐20 nm dia) mediates a more intimate contact compared to macro or planar Si wafer. The PFE effect dynamically increases the QD PL by passivating nonradiative state through oxidation and possibly through increasing the lifetime of nonradiative trapped state by coupling to surface vibrations. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Three-dimensional spatial resolution of the nonlinear photoemission from biofunctionalized porous silicon microcavity

Infiltration of biomacromolecules into porous silicon photonic architectures results in biofunctionalized structures with unique properties. Characterization of their optical response and performance optimization in biomacromolecular detection and biophotonic application require a combination of optical and structural studies. Nonlinear optical microscopy is applied to study porous silicon microcavities with and without infiltrated glucose oxidase. The infiltrated protein acts as an internal two-photon-excited fluorescence emitter and second harmonic generator, enabling the in-depth visualization of the porous structure. Enhanced second harmonic generation and fluorescence emission by the porous silicon structure is experimentally associated with the defect layer.

Enhanced photoconductivity and fine response tuning in nanostructured porous silicon microcavities

We used light confinement in optical microcavities to achieve a strong enhancement and a precise wavelength tunability of the electrical photoconductance of nanostructured porous silicon (PS). The devices consist of a periodic array of alternating PS layers, electrochemically etched to have high and low porosities – and therefore distinct dielectric functions. A central layer having a doubled thickness breaks up the symmetry of the one-dimensional photonic structure, producing a resonance in the photonic band gap that is clearly observed in the reflectance spectrum. The devices were transferred to a glass coated with a transparent SnO2 electrode, while an Al contact was evaporated on its back side. The electrical conductance was measured as a function of the photon energy. A strong enhancement of the conductance is obtained in a narrow (17nm FWHM) band peaking at the resonance. We present experimental results of the angular dependence of this photoconductance peak energy, and propose an explanation of the conductivity behaviour supported by calculations of the internal electromagnetic field. These devices are promising candidates for finely tuned photoresistors with potential application as chemical sensors and biosensors.

Doubly resonant porous silicon microcavities for enhanced detection of fluorescent organic molecules

The synthesis of porous silicon-based photonic structures for bio-sensing applications has been widely investigated in the last years. Thanks to its spongiform structure, porous silicon can efficiently host many organic molecules dispersed in solutions having proper chemical affinity. Fluorescent emission of organic-dyes embedded in porous silicon can be enhanced if a photonic structure like a Fabry–Pérot resonator is employed as a host solid matrix. In this work we present experimental evidence of a fluorescence enhancement effect obtained by means of doubly resonant microcavities tuned on both the excitation and the emission wavelengths. The use of doubly resonant cavities allows both a resonant excitation of dyes and a resonant amplification of the emission. We demonstrate that small concentrations of fluorescent dyes down to few picomoles can be detected. The bio-sensing capabilities of such a structure are tested on larger molecules of fluorescein-labelled protein A, yielding to an evident lowering of the detection limit by 2 orders of magnitude.

Biosensing and Protein Fluorescence Enhancement by Functionalized Porous Silicon Devices

Porous silicon (PSi) is a promising biomaterial presenting the advantage of being biocompatible and bioresorbable. Due to the large specific surface area and unique optical features, these microporous structures are excellent candidates for biosensing applications. Investigating device functionality and developing simple Si-based transducers need to be addressed in novel biological detection. Our work demonstrates that, among the various PSi configurations for molecular detection, PSi microcavity structure demonstrates the best biosensing performance, reflected through the enhanced luminescence response and the changes in the refractive index. For successful immobilization, molecular infiltration and confinement are the two key factors that are controlled by the pore size distribution of the PSi microcavities and by the surface modification obtained by silane-glutaraldehyde chemistry. Enhancement of the fluorescence emission of confined fluorescent biomolecules in the active layer of PSi microcavities was observed for a nonlabeled protein with a natural green fluorescence, the glucose oxidase enzyme (GOX). An increase in the fluorescence emission was also observed when functionalized PSi material was used to detect specific binding between biotin and a low concentration of labeled streptavidin. Evidence for the enzymatic activity of GOX in its adsorbed form is also presented. Use of smart silicon devices, enabling enhancement of fluorescence emission of biomolecules, offers easy-to-use biosensing, based on the luminescence response of the molecules to be detected.

Functionalization of nanostructured porous silicon microcavities for glucose oxidase detection

Abstract Stable porous silicon microcavities (PSiMcs) are designed in order to detect the confined glucose oxidase (GOX) at low concentration. A chemical pathway is proposed to functionalize and stabilize porous silicon structures that are primordial for further effective molecular confinement. The procedure avoids the preliminary thermal oxidation of the PSi surface and consists of a direct surface oxidation during the silanization process, an amino activation by a linker that also prevents further oxidation, and the binding of the GOX enzyme. Functionalization and protein confinement are monitored by the narrow resonance peaks in the near-infrared region (700–1000 nm) of the engineered meso- and macroporous microcavities. Protein penetration along the entire, large pore sized structure is demonstrated. The protein retains its native form within the properly functionalized PSi structures, which are ideal for molecular sensing also due to their high quality factor. A detection resolution of 25 nM GOX has been determined, thus demonstrating the high quality sensing of our functionalized porous silicon microcavity structures.

Label-free optical detection of peptide synthesis on a porous silicon scaffold/sensor.

Mesoporous porous silicon (PSi) microcavity sensors are used to conduct conventional solid-phase peptide synthesis. The sensor optical response provides a convenient means to monitor the synthesis reaction in a nondestructive manner. Measurements indicate that peptide synthesis occurs only when the PSi sensor/scaffold is amine-terminated using, for example, the amino silane or deprotected acid-labile Rink linker. Equivalent coupling efficiencies of the first amino acid to both amine terminations are observed. Kinetic studies indicate that coupling reactions are 90% complete in 1 h. Quantitative analysis of the optical response following the synthesis of homo-oligopeptides (4-mers) suggests that coupling efficiencies and/or optical thickness changes depend on the peptide length. The synthesis of the cell adhesive oligopeptide (RGD) was monitored by the optical sensor response and validated by the cell culture of primary dermal fibroblasts. Secondary ion mass spectrometry (SIMS) analysis successfully detected peptide on the silicon wafer adjacent to the PSi. Our findings suggest the potential to exploit the high surface area, efficient coupling, and intrinsic optical detection properties of PSi for label-free high-throughput screening.

Effects of organic adsorbent on the photoluminescence of porous silicon microcavities

The effects of organic absorbent on the refractive index and photoluminescence of porous silicon microcavities were theoretically investigated by using the Bruggeman effective medium approximation. Experimentally， porous silicon microcavities were fabricated by computer controlled electrochemical etching， and then interacted with the molecules of oil vapor from a rotary pump. We found that the narrow photoluminescence spectra of porous silicon microcavities were sensitive to the adsorption and de-adsorption of the oil vapor molecules， which leads to a 71nm red-shift and the intensity variation in photoluminescence spectrum. The experimental results are qualitatively consistent with the theoretical estimate.

Optical microsystems based on a nanomaterial technology

In this work, we present an optical sensor for quantitative determination of thealcohol content in hydro-alcohol mixtures, realized by using porous silicon (PSi)nanotechnology. The device is an oxidized PSi micro-cavity (PSMC) constituted by aFabry–Perot layer between two distributed Bragg reflectors. Due to the capillarycondensation, a red shift of the PSMC reflectivity spectrum is observed on exposure tovapour mixtures. The phenomenon is completely reversible. Moreover, to reducethe analysis time, we have designed the integration of the sensor in a thermallycontrolled lab-on-chip, by merging PSi and anodic bonding technologies. Numericalcalculations have been performed to study the thermal behaviour of the integrateddevice.

Determination of Ethyl Alcohol Content in Red Wines with an Optical Alcohol Meter Based on Nanostructured Silicon

A rapid analytical method for selectively monitoring ethyl alcohol in red wines without sample pretreatment, based on the use of an optical sensor, has been developed exploiting a porous silicon microcavity. The optical structure, realized by alternating layers of different porosities, was stabilized by thermal oxidation, and the resonant peak shift of the microcavity (projected at 600 nm) was monitored in the presence of more than 20 red wines. The resonant peak shows an increasing red shift depending on the wine alcoholic strength, which is ascribable to a change of refractive index due to physisorption/condensation of ethanol vapors inside the pores' structure. The linear response of the PS oxidized microcavity to the wine alcoholic strength insures the determination of the ethanol amount with a high accuracy and reliability. The calculated values differ by less than 0.5% to those obtained with the official method in accordance with the limits imposed by European laws. Moreover, a user-friendly interface, allowing the sensor to be used by unskilled persons, and portable packaging, able to ensure in situ measurements, have been developed.

Study on a vapor sensor based on the optical properties of porous silicon microcavities

In this paper, we set up a sensing model of PSMs (porous silicon microcavities) by applying the Bruggeman effective medium approximation theory and the transfer matrix method. In addition, we explain in detail the adsorption characteristics of porous silicon. Finally, using an experimental setup to measure the reflectivity spectrum of PSMs when the sensor is exposed to different organic vapors, the experimental results prove that it is a feasible optical sensor for the detection of organic species. Resolution of the PSMs sensor is high, response time and resume time is short and repetition is good.

Porous silicon microcavities fabricated using ion irradiation

By employing microcavity (MC) techniques, the broad spectral band and wide emission angle of photoluminescence from porous silicon (PSi) can be narrowed, directed and tuned in PSi microcavities. The microcavity structure is formed with a porous silicon active layer sandwiched between two Bragg reflectors. We have characterised the optical properties of the porous silicon multilayer structures. A number of Bragg reflectors and Fabry–Perot optical microcavities are fabricated with tuning of emission wavelengths over a wide range in heavily doped p-type silicon. The optical properties of these microstructures were investigated using reflectivity and photoluminescence measurements at different temperatures. This paper reports the ability of focussed 2 MeV proton beam irradiation to controllably blue shift the resonant wavelength of porous silicon microcavities in heavily doped p-type wafers. Using this process wafers are patterned on a micrometer lateral scale with microcavities tuned to different resonant wavelengths, giving rise to high-resolution full-colour reflection images over the full visible spectrum.

Fluorescent polymer-porous silicon microcavity devices for explosive detection

Conjugated polymers entrapped in porous silicon microcavity have been studied as optical sensors for low volatility explosives such as trinitrotoluene. The fluorescence spectra of entrapped polymers were modulated by the microcavity via a spectral “hole” that matches the resonance peak of the microcavity reflectance. Exposure of the porous silicon microcavity containing entrapped polymer to explosives vapor results in a red shift of the resonance peak and the spectral hole, accompanied by the quenching of the fluorescence. This multiplexed response provides multiple monitoring parameters, enabling the development of an optical sensor array for the detection of target explosive vapor.

Tunable silicon-based light sources using erbium doped liquid crystals

Tunable emission in the near infrared is demonstrated on a silicon platform. The building blocks for the tunable light sources consist of porous silicon microcavities infiltrated with erbium doped nematic liquid crystals. Erbium ions are the luminescence source, porous silicon microcavities narrow the emission band, and liquid crystals enable tuning of the peak wavelength. Greater than 10dB attenuation is achievable by thermal actuation with microcavities having a Q factor of 200. The bandwidth of the tunable emission is limited by the liquid crystal birefringence.

Porous silicon and aluminum co-gettering experiment in p-type multicrystalline silicon substrate

The lifetimes of non-equilibrium minority carriers, which bound with the diffusion length, are considered as two important parameters of the low-quality multicrystalline silicon (mc-Si) substrate. Its value defines the quality of the initial substrate. It is also subjected to change as a result of many high-temperature operations during the device fabrication. Therefore, it is necessary to incorporate certain processing steps that either improve or preserve the electronic quality of the mc-Si substrate. In this study, a novel porous silicon and aluminum co-gettering experiment has been applied as a beneficial approach to improve the electronic quality of the low-resistivity mc-Si substrates. Porous silicon layers were prepared by anodization of the n+ silicon region by a simple electrochemical etching process using an aqueous HF-based electrolyte, which leads to the creation of porous silicon microcavities. Besides making porous silicon and aluminum co-gettered samples, both phosphorous and aluminum alloy-gettered samples and reference samples were made. The gettering-induced lifetime enhancement in the test samples was monitored by measuring the lifetime/diffusion length of the test samples using two independent methods such as photoconductivity decay (PCD) measurement and the photocurrent generation method (PCM), respectively. The result in both the measurements has shown a reasonably good agreement with each other. Therefore, it is inferred that the applied co-gettering experiment has a synergetic effect to improve the lifetime of the mc-Si substrate.

Influence of the humidity conditions on the reflectivity spectrum of a porous silicon microcavity

We have designed and fabricated a porous silicon (PS) microcavity that shows a reflectivity resonance around 3 μm in between two spectral regions with high reflectivity values. The microcavity has been simulated following the photonic crystals formalism, which results in good agreement with the measured spectrum. The reflectivity spectrum of the microcavity has been analyzed under different humidity conditions. We demonstrate that the reflectivity resonance shifts to higher wavelengths and that the reflectivity decreases when the humidity increases. In addition, the reflectivity spectrum of the as-prepared device is recovered when the humidity returns to the initial laboratory conditions. Finally, the effect on the reflectivity spectrum of a condensed water layer at the surface of the microcavity is also analyzed.

Controlled blueshift of the resonant wavelength in porous silicon microcavities using ion irradiation

High-energy focused proton beam irradiation has been used to controllably blueshift the resonant wavelength of porous silicon microcavities in heavily doped p-type wafers. Irradiation results in an increased resistivity, hence a locally reduced rate of anodization. Irradiated regions are consequently thinner and of a higher refractive index than unirradiated regions, and the microcavity blueshift arises from a net reduction in the optical thickness of each porous layer. Using this process wafers are patterned on a micrometer lateral scale with microcavities tuned to different resonant wavelengths, giving rise to high-resolution full-color reflection images over the full visible spectrum.