Porous Silicon Photonic Crystals Research Articles

Hybrid Systems Based on Porous Silicon Photonic Crystals, Liquid Crystals, and Quantum Dots

Hybrid Systems Based on Porous Silicon Photonic Crystals, Liquid Crystals, and Quantum Dots

Gold-coated porous silicon as a SERS substrate for near-infrared excitation: Off- and on-resonant conditions

Porous silicon, being one of the most popular SERS-active substrates, was in this work utilized for near-infrared 1064 nm excitation which minimizes the radiation-induced degradation of molecules. The production of porous silicon photonic crystals and deposition of gold nanoparticles, as the two possible methods for quenching the crystal silicon band-gap photoluminescence, were thoroughly discussed. For the off-resonant laser excitation conditions, gold nanoparticles were grown on porous silicon by the immersion plating procedure. For the on-resonant conditions, gold nanorods with the appropriate aspect ratio were synthesized by a seed-mediated protocol and deposited on the porous silicon surface. The capability of these metal-dielectric substrates toward SERS enhancement was evaluated using crystal violet dye and the advantages and drawbacks of both methods were addressed in detail. The obtained detection limits for the crystal violet dye were in the range between 10−7 and 10−8 M. So far, gold-coated porous silicon SERS-active substrates for near-infrared 1064 nm excitation have not been reported, both for the off- and on-resonant regimes.

One-dimensional porous silicon photonic crystals for chemosensors: Geometrical factors influencing the sensitivity

One-dimensional photonic crystals with microcavity were prepared to be used as optical chemosensors. The experimental reflectance of the as-etched devices immersed in solvent solutions appear greater than the theoretical one. The fitting procedure of the experimental data using the transfer matrix method together with the Looyenga effective medium approach reveals that the deviation of the experimental data occurs because of the expansion-contraction effect of the high and low porosity layers, respectively, caused by the presence of the solvent into the porous matrix and is proportional to the solvent effective refractive index. Due to this phenomenon, the sensitivity of the as-etched photonic crystals increases about two times but is not stable because of the aging effect. To avoid this effect, the photonic crystal structures were passivated by thermally silicon oxide growing, gold thin film and titanium oxide deposition. After thermal treatment in an air environment, the optical response achieves its stabilization even after long storage times, thus showing the importance of the passivation procedure. Among them, the passivation by TiO2 shows better performance and lower loss in sensitivity in comparison to the as-etched devices.

Optical and structural characterization of one-dimensional porous silicon photonic crystals made in single and double electrochemical cells: Study on the back contact effect

One-dimensional porous silicon photonic crystals with microcavity were fabricated using single and double electrochemical cells and the same electrochemical parameters. Devices with better optical quality can be achieved when liquid acids are used as back contacts for current flow. Optical analysis of them shows that devices with less rough interfaces and layers with negligible in-depth inhomogeneities are formed. However, the etch rate and porosity depend strongly on the acid strength, the diffusion of the acid species from the liquid bulk to the interface, and the anodization temperature. Strong acids promote a higher etch rate. Thinner films with lower porosity and rougher interfaces are observed for devices fabricated in substrates with aluminum or platinum as solid ohmic contacts compared to devices fabricated in unmetallized substrates, as shown by optical analysis, such that devices with aluminum as the ohmic contact have about 80% of the reflectance, while with platinum shows 94%.

Thickness and porosity characterization in porous silicon photonic crystals: The etch-stop effect

One-dimensional photonic crystals and microcavities were fabricated using porous silicon via anodization. The study investigated the effect of an etch-stop layer, which is included between two adjacent porous layers to resolve the issue of HF diffusion during pore formation. Although proposed as a solution, no systematic investigation had been conducted on this parameter until now. Our study found that the etch-stop layer induces a redshift in the photonic bandgap, regardless of the electrolyte solution used. The fitting analysis of both single and multilayer structures revealed that the redshift resulted from an increase in the silicon etch rate and a decrease in porosity due to the enhancement of HF diffusion through the porous structure. Furthermore, an analysis of oxidation as a function of temperature demonstrated that the inclusion of the etch-stop layer led to the formation of porous layers with varying microstructures. This effect was particularly pronounced in devices where the first upper layer had high porosity. The thermal treatment caused a photonic bandgap inhibition, which was linked to the contraction-expansion phenomena of the low and high porosity layers, respectively. This resulted in the partial violation of Bragg's law. Overall, our study provides insights into the influence of etch-stop layers on the optical properties of porous silicon-based photonic structures.

Efficient and reliable detection of rhodamine B using SERS from silver-decorated photonic crystal silicon nanoscale pores

Porous silicon photonic crystal (PSi PhC) decorated with silver nanoparticles (AgNPs) is shown to provide a new substrate for the improvement of SERS. AgNPs are obtained through immersion of PSi PhC samples in AgNO3 solutions and successive thermal annealing. The nanocomposite material generated by adding AgNPs inside the silicon nanoscale pores, integrates the ability of metal surfaces to amplify Raman scattering signals and an enlarged surface area. Besides, PhC structure formed by multilayer PSi increases interaction time of light and matter of the multilayer structure. The experimental results show that the AgNPs-decorated PSi PhC has the largest Raman intensity in comparison with other SERS substrates based on Si. The enhancement of Raman signals also reduces the threshold of the detection down to below 10-10 M. The analytical enhancement factor of AgNPs/PSi PhC SERS substrate for the detection of Rhodamine B (RhB) reaches 1010. This proposed AgNPs/PSi PhC SERS substrate could serve as a potential candidate for detecting RhB in foodstuffs at very low concentrations.

Локальное лазерное окисление фотонных кристаллов на основе пористого кремния

The process of thermal oxidation of porous silicon photonic crystals by laser treatment has been studied experimentally and theoretically. The temperature of laser heating was calculated by solving of the non-stationary thermal conductivity equation. The degree of silicon oxidation was estimated experimentally by measuring the spectral shift of the reflection curve of photonic crystals. We demonstrate the controllable shift of the photonic band gap of porous silicon photonic crystals made by means of laser irradiation.

Select and Rapid Detection of Methylene Blue on Porous Silicon Photonic Crystals Covered with Silver Nanoparticles by SERS

Select and Rapid Detection of Methylene Blue on Porous Silicon Photonic Crystals Covered with Silver Nanoparticles by SERS

Denoising of Fluorescence Image on the Surface of Quantum Dot/Nanoporous Silicon Biosensors.

In the process of biological detection of porous silicon photonic crystals based on quantum dots, the concentration of target organisms can be indirectly measured via the change in the gray value of the fluorescence emitted from the quantum dots in the porous silicon pores before and after the biological reaction on the surface of the device. However, due to the disordered nanostructures in porous silicon and the roughness of the surface, the fluorescence images on the surface contain noise. This paper analyzes the type of noise and its influence on the gray value of fluorescent images. The change in the gray value caused by noise greatly reduces the detection sensitivity. To reduce the influence of noise on the gray value of quantum dot fluorescence images, this paper proposes a denoising method based on gray compression and nonlocal anisotropic diffusion filtering. We used the proposed method to denoise the quantum dot fluorescence image after DNA hybridization in a Bragg structure porous silicon device. The experimental results show that the sensitivity of digital image detection improved significantly after denoising.

Chemical sensor based on the colorimetric response of porous silicon photonic crystal

In this article, the authors performed computer simulations and experimentally investigated the application of the colorimetric approach based on CIE1931 color space for characterization of the optical response of porous silicon (PS) based photonic crystals (PC). The colorimetric parameters as dominant wavelength (λdom) and excitation purity (EP) were determined and compared with conventional PC parameters as Bragg peak position λ0 and FWHM. The colorimetric parameters were obtained for PC with different sets of porosity values of bi-layers and the position of Bragg peak λ0(air). The PS sensor structures were fabricated by the method of electrochemical anodization of Si wafer. The colorimetric sensitivity of the PC sensors is higher for structure with smaller refractive indexes. Colorimetric parameters characterize the modification of the entire complex spectrum of the optical sensor as opposed to the conventional parameter as the Bragg peak position. The time dependences of colorimetric parameters on PC parameters during adsorption/desorption of ethanol are determined. The colorimetric approach for sensor characterization is preferable to develop the rapid not dispersive sensor systems as artificial tongue/nose with coordinate-dependent parameters of the spectrum.

Detection of Ultra-Low Concentration of Methylene Blue by Porous Silicon Photonic Crystals Covered With Siver Nanoparticles as Efficient Sers Substrate

In this work, porous silicon photonic crystals (PSi PhCs) covered by silver nanoparticles (AgNPs) were prepared as surface-enhanced Raman scattering (SERS) substrate to detect methylene blue (MB) at low concentrations. The limit of MB detection in water by the SERS substrate is evaluated to be 10\(^{-10}\) mol/L. The SERS signal intensities of 446 cm\(^{-1}\) and 1623 cm\(^{-1}\) peaks in SERS spectra of MB are fit in exponential functions of concentrations ranging from 10\(^{-4}\) to 10\(^{-10}\) mol/L. These results show that the AgNPs on PSi PhCs substrates could be applied in environmental sensing.

Facile fabrication of ultrahigh-sensitivity surface-enhanced Raman scattering sensor using porous silicon photonic crystals prepared on backside silicon wafer

Photonic crystals (PhCs) with rough surfaces can considerably enhance the formation of electromagnetic hotspots, which significantly promote surface-enhanced Raman scattering (SERS) activity. In this study, porous silicon PhCs with rough surfaces were synthesized. Deposition of gold nanoparticles on the substrate yielded a SERS substrate with good activity and reproducibility, consequently achieving the ultratrace detection of Rhodamine 6 G (R6G), with a minimum concentration of 10−15 M. The SERS activity of this substrate was simulated via finite element modeling. The experimental and theoretical results confirmed that the optical characteristics of the “black silicon,” created by inducing a rough surface, and the enhancement of the effective Raman susceptibility, or the time period of the light–matter interaction owing to photonic crystals, played an important role in the enhancement of the overall SERS activity. Furthermore, we demonstrated that the rough-surface porous silicon SERS substrate could be employed to selectively detect ultratrace military explosives containing picric acid (PA) with increased sensitivity. Accordingly, the detection limits of PA and the probe molecule R6G in the Raman spectra were 0.79 nM and 0.24 fM, respectively. This study suggests that the porous silicon PhCs with rough surfaces are promising candidates for ultratrace chemical sensing.

Near-infrared photoluminescent hybrid structures based on freestanding porous silicon photonic crystals and PbS quantum dots

Near-infrared photoluminescent hybrid structures based on freestanding porous silicon photonic crystals and PbS quantum dots

Non-invasive SERS serum detection technology combined with multivariate statistical algorithm for simultaneous screening of cervical cancer and breast cancer.

Surface-enhanced Raman scattering (SERS), as a rapid, reliable and non-destructive spectral detection technology, has made a series of breakthrough achievements in screening and pre-diagnosis of various cancerous tumors. In this paper, high-performance gold nanoparticles/785 porous silicon photonic crystals (Au NPs/785 PSi PhCs) active SERS substrates were specially designed for serum testing, and realized highly sensitive detection of serum from healthy people, patients with cervical cancer and breast cancer. Based on the SERS spectra of the three groups of serum, the significant differences between the healthy group and cancer group at 1030cm-1 and 1051cm-1 were analyzed, and the similar but different serum SERS spectra of cervical cancer and breast cancer patients were compared. In addition, the spectral difference detected by SERS technology combined with a multivariate statistical algorithm was used to distinguish three kinds of serum. The serum SERS spectral sensitive bands were extracted by recursive weighted partial least squares (rPLS), and the three classification diagnosis models were established by combining orthogonal partial least squares discriminant analysis (OPLS-DA), linear discriminant analysis (LDA) and principal component analysis support vector machine (PCA-SVM) for synchronous classification and discrimination of the three groups of serum. The diagnostic results showed that the overall screening accuracy of three models were 93.28%, 97.77% and 94.78%, respectively. These above results confirmed that the Au NPs/785 PSi PhCs can realize super-sensitive detection of serum, and the established diagnostic model has great potential for pre-diagnosis and simultaneous screening of cervical cancer and breast cancer.

Improvement of SERS for detection of ultra-low concentration of methyl orange by nanostructured silicon decorated with Ag nanoparticles

We studied the effect of nanostructured silicon integrated with Ag nanoparticles (NPs) on surface enhanced Raman scattering (SERS) to detect methyl orange (MO). Ag NPs are deposites on the two types of porous silicon, single-layer porous silicon (s-PSi) and porous silicon photonic crystals (PSi PhCs). Their characteristics were confirmed by scanning electron microscopy (SEM) and reflectance. SERS activity of the substrates was compared by using MO as the probe molecules. The results with SERS experiments showed that SERS activity of Ag NPs/PSi PhCs was much stronger than that of Ag NPs/s-PSi with 6.8-fold improvement. The limit of detection of MO was 10−9 M and the linear range was from 10−9 to 10-3 M.

1D porous silicon photonic crystals comprising Tamm/Fano resonance as high performing optical sensors

We theoretically discuss a new design for the sensing and detection of chemical and biomedical fluids based on the change in their indices of refraction in the refractive index range 1−1.5. The transfer matrix method in cylindrical coordinates and the Bruggeman's effective medium approximation represent the theoretical overview of our study. The metallic cap in front of a ternary porous silicon photonic crystal induces Tamm / Fano states inside the photonic band gap that take part of the resonant peaks in conventional defected PCs. Analysis shows that the spectral positions of these states are very sensitive to changes in refractive index of the void fluids. In particular, the proposed sensor could provide higher performance than many previous 1D photonic crystals sensors. At a void refractive index of 1.1, our design exhibits a sensitivity of 6770 nm/RIU and a quality factor of 662. Therefore, the detection and sensing procedures of chemical and biomedical fluids could be investigated in a rather much efficient, stable and novel technique that has not been applied before.

Remote Detection of HCN, HF, and Nerve Agent Vapors Based on Self-Referencing, Dye-Impregnated Porous Silicon Photonic Crystals.

A one-dimensional photonic crystal is prepared from porous silicon (pSi) and impregnated with a chemically specific colorimetric indicator dye to provide a self-referenced vapor sensor for the selective detection of hydrogen fluoride (HF), hydrogen cyanide (HCN), and the chemical nerve agent diisopropyl fluorophosphate (DFP). The photonic crystal is prepared with two stop bands: one that coincides with the optical absorbance of the relevant activated indicator dye and the other in a spectrally "clear" region, to provide a reference. The inner pore walls of the pSi sample are then modified with octadecylsilane to provide a hydrophobic interior, and the indicator dye of interest is then loaded into the mesoporous matrix. Remote analyte detection is achieved by measurement of the intensity ratio of the two stop bands in the white light reflectance spectrum, which provides a means to reliably detect colorimetric changes in the indicator dye. Indicator dyes were chosen for their specificity for the relevant agents: rhodamine-imidazole (RDI) for HF and DFP, and monocyanocobinamide (MCbi) for HCN. The ratiometric readout allows detection of HF and HCN at concentrations (14 and 5 ppm, respectively) that are below their respective IDLH (immediately dangerous to life and health) concentrations (30 ppm for HF; 50 ppm for HCN); detection of DFP at a concentration of 114 ppb is also demonstrated. The approach is insensitive to potential interferents such as ammonia, hydrogen chloride, octane, and the 43-component mixture of VOCs known as EPA TO-14A, and to variations in relative humidity (20-80% RH). Detection of HF and HCN spiked into the complex mixture EPA TO-14A is demonstrated. The approach provides a general means to construct robust remote detection systems for chemical agents.

Enhanced spontaneous emission from two-photon-pumped quantum dots in a porous silicon microcavity.

Photoluminescence (PL)-based sensing techniques have been significantly developed in practice due to their key advantages in terms of sensitivity and versatility of the approach. Recently, various nanostructured and hybrid materials have been used to improve the PL quantum yield and the spectral resolution. The near-infrared (NIR) fluorescence excitation has attracted much attention because it offers deep tissue penetration and it avoids the autofluorescence of the biological samples. In our study, we have shown both spectral and temporal PL modifications under two-photon excitation of quantum dots (QDs) placed in one-dimensional porous silicon photonic crystal (PhC) microcavities. We have demonstrated an up-to-4.3-fold Purcell enhancement of the radiative relaxation rate under two-photon excitation. The data show that the use of porous silicon PhC microcavities operating in the weak coupling regime permits the enhancement of the PL quantum yield of QDs under two-photon excitation, thus extending the limits of their biosensing applications in the NIR region of the optical spectrum.

Additive Combination of Spectra Reflected from Porous Silicon and Carbon/Porous Silicon Rugate Filters to Improve Vapor Selectivity.

Selectivity remains a challenge for rapid optical vapor sensing via light reflected from porous silicon photonic crystals. This work highlights a method to increase optical vapor selectivity of porous silicon rugate filters by analyzing additive spectra from two rugate filter substrates with different functionalities, an oxidized and carbonized surface. Individually, both porous silicon rugate filters demonstrated sensitivity but not selectivity toward the vapor analytes. However, differences in peak shift trends between the two substrates suggested differences in vapor affinities for the surfaces. By adding the two spectra, improvements to selectivity relative to the individual surfaces were observed even at low vapor pressures and for analytes of similar polarity, refractive index, and concentration. These results are expected to contribute toward optical vapor selectivity improvements in one-dimensional porous silicon photonic crystals.

Enhancement of spontaneous emission of semiconductor quantum dots inside one-dimensional porous silicon photonic crystals.

Controlling spontaneous emission by modifying the local electromagnetic environment is of great interest for applications in optoelectronics, biosensing and energy harvesting. Although the development of devices based on one-dimensional porous silicon photonic crystals with embedded luminophores is a promising approach for applications, the efficiency of the embedded luminophores remains a key challenge because of the strong quenching of the emission due to the contact of the luminophores with the surface of porous silicon preventing the observation of interesting light-matter coupling effects. Here, we experimentally demonstrate an increase in the quantum dot (QD) spontaneous emission rate inside a porous silicon microcavity and almost an order of magnitude enhancement of QD photoluminescence intensity in the weak light-matter coupling regime. Furthermore, we have demonstrated drastic alteration of the QD spontaneous emission at the edge of the photonic band gap in porous silicon distributed Bragg reflectors and proved its dependence on the change in the density of photonic states.