V. Agarwal

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.

Photoluminescence studies of ZnO/porous silicon nanocomposites

This paper reports on the intense broadband photoluminescence (PL) emission from the ZnO/porous silicon nanocomposite films. The porous silicon (PS) samples were formed by electrochemical anodization on p-type (1 0 0) silicon wafer and ZnO thin films are deposited by the sol–gel spin coating technique in the pores of PS. The average pore size of PS samples is 30 nm. The glancing angle x-ray diffraction pattern of as-deposited and annealed films shows that the quality of (0 0 2) oriented ZnO nanocrystallites improves with annealing at moderate temperature and are polycrystalline in nature. The average crystallize size was found to be 40 nm. The surface topography of the ZnO/PS nanocomposite films has been studied using atomic force microscopy. The mechanism and interpretation of broadband PL from 400 to 900 nm of the nanocomposites are discussed using oxygen-bonding and native defects models for PS and ZnO, respectively. These nanocomposite films could be used as a source of broadband luminescence across most of the visible spectrum.

White light emission from chemically synthesized ZnO–porous silicon nanocomposite

White light emission across the extended visible region of the electromagnetic spectrum from the ZnO‐porous silicon (PS) nanocomposite is reported. Nanocrystallites of ZnO were grown inside the spongy structures of PS by the chemical route of sol‐gel spin coating. The property of the material arises from versatile interactions among the host structures of PS and ZnO. The origin of the observed extended white light emission from 1.4 to 3.3eV is discussed by developing a flat band energy diagram. (Some figures in this article are in colour only in the electronic version)

Enlargement of omnidirectional photonic bandgap in porous silicon dielectric mirrors with a Gaussian profile refractive index

For enhancing the omnidirectional photonic bandgap (OPBG), we report the fabrication of two different configurations of one-dimensional, wavelength scalable dielectric multilayer structures of porous silicon, consisting of a unit cell formed by varying the refractive index of the multilayers according to the envelope of a Gaussian function. As compared to the already reported OPBG of 88 nm (in the complete angular range of 0° to 89°), an enhancement up to 204 nm (2.3 times) was observed on stacking, six different Gaussian structures (balanced mirror) with only 8 periods each. An unbalanced mirror structure, consisting of the six similar Gaussian structures as the balanced mirror, but having different sequence of periods, (configuration with 13, 6, 5, 5, 6, and 13 periods for each Gaussian, respectively) was seen to demonstrate the OPBG of 252 nm (enhanced by 2.86 times). The total optical thickness of both the structures was kept to be the same. The omnidirectional nature of the PBG was verified experimen...

Optical characterization of polytype fibonacci and Thue-Morse quasiregular dielectric structures made of porous silicon multilayers

To investigate the reflection of light in quasiregular dielectrics, we study here the optical properties of porous-silicon-based Fibonacci and Thue–Morse heterostructures. The multilayered systems are fabricated in such a way that each element in the two-block substitutional sequence has a polytype structure. Both delta-like and traditional configurations are considered. The results for the optical reflectance are analysed. Numerical simulation for the transmittance of delta structures along the lines of the transfer matrix approach is also presented.

Doping concentration driven morphological evolution of Fe doped ZnO nanostructures

In this paper, systematic study of structural, vibrational, and optical properties of undoped and 1-10 at.% Fe doped ZnO nanostructures, synthesized adopting chemical precipitation route, has been reported. Prepared nanostructures were characterized employing an assortment of microscopic and spectroscopic techniques, namely Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray (EDX) Spectroscopy, X-ray Diffraction (XRD), Fourier Transform Infrared (FTIR), Micro-Raman Spectroscopy (μRS), and UV-visible and Photoluminescence (PL) spectroscopy. With Fe incorporation, a gradual morphological transformation of nanostructures is demonstrated vividly through SEM/TEM characterizations. Interestingly, the morphology of nanostructures evolves with 1–10 at. % Fe doping concentration in ZnO. Nanoparticles obtained with 1 at. % Fe evolve to nanorods for 3 at. % Fe; nanorods transform to nanocones (for 5 at. % and 7 at. % Fe) and finally nanocones transform to nanoflakes at...

Omnidirectional photonic bandgaps in porous silicon based mirrors with a Gaussian profile refractive index

We have designed and fabricated one dimensional photonic bandgap (PBG) structures from dielectric multilayers of porous silicon, with a periodic repetition of a unit cell consisting of 21 layers (95%) with the refractive index varying according to the envelope of a Gaussian function and another layer (5%) with a fixed refractive index. The structures can be designed to demonstrate the wavelength scalability within the visible as well as near infrared region. Three different structures have been stacked together to enhance the width of the PBG. The omnidirectional nature of the PBG was verified experimentally and theoretically up to 68° and 89° angles of incidence, respectively.

Noise mediated regularity of porous silicon nanostructures

Interaction of noise with nonlinear electrochemical kinetics involving the etching of porous silicon is studied experimentally. It is realized that by monotonically increasing the level of internal noise, one can tune the regularity of the spatial distribution of pores in silicon nanostructures. This regularity of the noise provoked structures is quantified using a spatial normalized variance technique in conjunction with the calculation of Hurst exponents. Our experimental results indicate the emergence of intrinsic coherence resonance. Consequently, there exists an optimal value of internal noise for which the spatial distribution of nanopores attain maximal regularity. This regularity of the pores can be useful for enhancing the optical response of porous silicon based devices.

Modification of optical and electrical properties of zinc oxide-coated porous silicon nanostructures induced by swift heavy ion

Morphological and optical characteristics of radio frequency-sputtered zinc aluminum oxide over porous silicon (PS) substrates were studied before and after irradiating composite films with 130 MeV of nickel ions at different fluences varying from 1 × 1012 to 3 × 1013 ions/cm2. The effect of irradiation on the composite structure was investigated by scanning electron microscopy, X-ray diffraction (XRD), photoluminescence (PL), and cathodoluminescence spectroscopy. Current–voltage characteristics of ZnO-PS heterojunctions were also measured. As compared to the granular crystallites of zinc oxide layer, Al-doped zinc oxide (ZnO) layer showed a flaky structure. The PL spectrum of the pristine composite structure consists of the emission from the ZnO layer as well as the near-infrared emission from the PS substrate. Due to an increase in the number of deep-level defects, possibly oxygen vacancies after swift ion irradiation, PS-Al-doped ZnO nanocomposites formed with high-porosity PS are shown to demonstrate a broadening in the PL emission band, leading to the white light emission. The broadening effect is found to increase with an increase in the ion fluence and porosity. XRD study revealed the relative resistance of the film against the irradiation, i.e., the irradiation of the structure failed to completely amorphize the structure, suggesting its possible application in optoelectronics and sensing applications under harsh radiation conditions.

ZnO-porous silicon nanocomposite for possible memristive device fabrication

Preliminary results on the fabrication of a memristive device made of zinc oxide (ZnO) over a mesoporous silicon substrate have been reported. Porous silicon (PS) substrate is employed as a template to increase the formation of oxygen vacancies in the ZnO layer and promote suitable grain size conditions for memristance. Morphological and optical properties are investigated using scanning electron microscopy (SEM) and photoluminescence (PL) spectroscopy. The proposed device exhibits a zero-crossing pinched hysteresis current-voltage (I-V) curve characteristic of memristive systems.