Porous Silicon Matrix Research Articles

Nanostructured cobalt on porous silicon substrate: Structure and magnetic behaviour

AbstractDuring an anodization process, porous silicon (PS) consisting of pores with a diameter of about 40 nm and a depth from 5 µm to 40 µm has been produced. To achieve oriented channels in this mesoporous range, a p+‐type Si wafer was electrochemically etched in an aqueous electrolyte of HF. We report the formation, after the anodization step, of a cobalt nanostructure in a porous silicon matrix. Co nanocrystals on and in a porous silicon layer have been prepared by the UHV evaporation technique and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). This technique was performed to show the chemical element distribution within the channels. It is found that the deposition condition is an important factor for obtaining nano‐structures. Initial deposition leads to Co particle penetration in silicon pores whereas subsequent deposition results only in an increase of the thickness at the surface with no further penetration. Additional experiments were carried out by using the magneto‐optical Kerr effect to obtain information about the magnetic properties. The first results show that the magnetic response for layers <5 nm presents an important perpendicular component of magnetization whereas for thicker deposited layers (8 nm < t < 20 nm) the magnetic response seems to act as that of a thin film in which the squareness of the hysteresis loop decreases with increasing film thickness. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hybrid nanostructure of polypyrrole and porous silicon prepared by galvanostatic technique

Nanostructured porous silicon (PS) layer is prepared in a lightly doped p-type substrate (with pores < 5 nm) and used as a working electrode to deposit conducting polypyrrole (PPy) by the electrochemical oxidative polymerization technique in an organic liquid phase. Three distinguishable stages of PPy deposition are observed and recorded under constant applied current: nucleation of polymer at the pore bottom, unidirectional growth of PPy inside the pores, and polymerization outside the PS surface. The hybrid nanostrucutre of PS/PPy shows a significant improvement of electrical conductivity as opposed to the unmodified PS layer. The improved conductivity is observed in spite of the formation of insulating layer of silicon oxides as detected by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) measurements. Systematic study of fabrication and characterization of this organic–inorganic heterosystem, quantification of the PPy in the PS matrix, and the mechanism of filling the nanopores with polymer are presented and thoroughly discussed.

DBR PSi/PMMA composite materials for smart patch application

New nanostructured composite materials based on porous silicon (PSi) have been developed as a smart patch type for drug delivery applications. Distributed Bragg reflector (DBR) PSi films and caffeine-impregnated polymethyl methacrylate (PMMA) are used to produce flexible and stable composite materials in which the porous silicon matrix is covered with polymer. DBR PSi films and the composite films display a reflection at 520 and 555nm in the optical reflectivity spectrum, respectively. These composite materials are stable in aqueous buffer solutions without any degradation for 3 days. The release of caffeine and the shift of reflection peak were simultaneously measured by the use of UV–vis and reflectance spectrometer. The intensity of reflection peak from the composite film is exponentially decreased with a caffeine release. The blue shift of reflection peak results in the decrease of refractive index of composite materials during the caffeine release. The concentration of released caffeine exhibits an exponential relationship with a release time.

DBR 다공성 실리콘/고분자 Composite 재료-이중적 광학특성

광 발광성 (photoluminescence, PL)과 광 반사성(reflectivity)의 두 가지 광학적 성질을 동시에 가지고 있는 새로운 다층 DBR(distributed Bragg reflector) 다공성 실리콘 composite 필름을 개발하였다. 발광 효율이 높은 실리콘 고분자 폴리실올을 PMMA(polymethylmethacrylate)에 첨가하여 다층 DBR 다공성 실리콘 표면에 코팅을 한 composite 필름을 제조하였다. 이 composite 필름은 510 nm에서의 발광스펙트럼과 565 nm에서의 반사스펙트럼을 동시에 나타낸다. 이러한 composite 필름은 광학적 정보가 저장되어있고 물리적인 힘을 가하여도 그 광학적 정보를 잃지 않는 장점을 갖는다. 또한 다층 DBR 다공성 실리콘 필름은 쉽게 부서지는 단점 때문에 다루기가 어려운 반면 composite 필름은 고분자로 고형화 되어있으므로 기계적인 안정도를 증가시킬 수 있었다. DBR (distributed Bragg reflectors) PSi (porous silicon) composite films displaying dual optical properties, both optical reflectivity and photoluminescence had been developed. DBR PSi samples were prepared by electrochemical etch of heavily doped <TEX>$p^{++}-type$</TEX> silicon wafers (boron doped, polished on the <100> face, resistivity of <TEX>$0.8-1.2m{\Omega}-cm$</TEX>, Siltronix, Inc.). Free-standing DBR PSi films were treated with PMMA (polymethyl methacrylate) to produce flexible, stable composite materials in which the PSi matrix is covered with PMMA containing photoluminescent polysiloles. Optical characteristics of DBR PSi/polysilole-impregnated PMMA composite materials exhibit both their photonic reflectivity at 565 nm and photoluminescence at 510 nm, simultaneously. A possible application of this materials will be discussed.

Composite structures of porous silicon and polyaniline for optoelectronic applications

AbstractComposite structures of p‐type meso‐porous silicon (PS) and polyaniline (PAN) have been fabricated by electrochemical polymerization process. The composite structure provides a unique way to control the conduction either through the porous silicon matrix or via the polyaniline, by changing the polarity of the applied bias voltage. In addition, photoluminescence (PL) experiments revealed enhanced PL emission at the green wavelengths that are originated from the polyaniline network, rather than the porous silicon. The above composite structure can be used as a building block for improved electroluminescence and light emitting devices in a wide range of applications. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Multiple Bit Encodings of Multilayer Porous Silicon

Photonic crystals containing the rugate structure result in a mirror with high reflectivity in a specific narrow spectral region and are prepared by applying a computer-generated pseudo-sinusoidal current waveform. Strategies to encode multiple rugate structures have been investigated. Multiple rugate structures can be etched on a silicon wafer and placed in the same physical location, showing that many sharp spectral lines can be obtained in the optical reflectivity spectrum. The method used to generate multiple rugate structures containing all the encoding information displays five rugate peaks corresponding to each of the sine components from 0.16 to 0.24 Hz, with a spacing of 0.02 Hz between each sine component. The complete deletion of a peak has been achieved and demonstrates the capacity of this method to create binary codes. The free-standing rugate porous silicon has been treated with the solution PMMA to produce composite materials which are highly flexible at room temperature. They display significantly improved mechanical stability and no apparent degradation in their optical reflectivity properties upon flexing. The spectral feature is shifted to longer wavelengths, indicating that the polymer has infiltrated the pores of the porous silicon matrix. The composites are stable in aqueous HF or alkaline (3 M KOH) solutions for several days without apparent degradation.

Polymerization of Supramolecular Diacetylenic Monomer Embedded in Porous Silicon Matrix

We have used a porous silicon matrix filled with polydiacetylenic derivatives to obtain new nanocomposites. The polymerization, known as a topochemical process, requires optimal packing of the diacetylenic segments to allow polymerization propagation across the assembly to give a conjugated backbone. We have explored the use of intermolecular H-bond interactions between diacetylenic diacid monomers to control the polymer chain organization after the polymerization step. Resonance Raman and IRTF spectroscopy have been used to directly probe the diacetylenic segment of the monomer and the pendent side-chain arrangement. Diacetylenic derivatives have been successfully incorporated inside the pores after a specific chemical treatment to improve the filling rate of the monomer. Polarized Raman scattering indicated that the monomers have a preferential orientation along the pore axis, probably because of the confinement effect and hydrogen bonding of the self-associated carboxylic dimeric form. After the polymerization process, resonance Raman spectroscopy shows a highly oriented ene−yne backbone characterized by a blue phase.

비고정화 된 일차원 광결정의 DBR 다공성 실리콘을 이용한 센서와 Drug Delivery로의 응용

Free-standing multilayer distributed Bragg reflectors (DBR) porous silicon dielectric mirrors, prepared by electrochemical etching of crystalline silicon using square wave currents are treated with polystyrene to produce flexible, stable composite materials in which the porous silicon matrix is covered with caffeine-impregnated polystyrene. Optically encoded DBR PSi/polystyrene composite films retain the optical reflectivity. Optical characteristics of DBR PSi/polystyrene composite films are stable and robust for 2 hrs in a pH=7 aqueous buffer solution. The appearance of caffeine and change of DBR peak were simultaneously measured by UV-vis spectrometer and Ocean optics 2000 spectrometer, respectively.

Optical properties of porphyrin molecules immobilized in nano-porous silicon

The paper aims at studying optical properties of porous silicon powders and thin films which were impregnated with different porphyrin molecules. It has been shown that introducing porphyrins into porous silicon matrix results in quenching of luminescence from porous silicon, while luminescence of porphyrins survives, though its structure changes. At the same time, porphyrins in porous silicon matrix which was preliminarily oxidized does not alter luminescence from porphyrins. Generation of singlet oxygen by illuminated porphyrin/porous silicon composite is confirmed by additional oxidation of porous silicon and by the observation of characteristic 1270 nm luminescence band.

Structure and relaxation processes of an anisotropic molecular fluid confined into 1D nanochannels

Structural order parameters of a smectic liquid crystal confined into the columnar form of porous silicon are studied using neutron scattering and optical spectroscopic techniques. It is shown that both the translational and orientational anisotropic properties of the confined phase strongly couple to the one-dimensional character of the porous silicon matrix. The influence of this confinement induced anisotropic local structure on the molecular reorientations occurring in the picosecond timescale is discussed.

In situ monitoring of protein adsorption on functionalized porous Si surfaces

In situ monitoring of the photoluminescence (PL) of undecylenic acid functionalized porous silicon (PS) was carried out in bovine serum albumin (BSA) solution. Two factors dictated the PL behavior during the protein incubation process. A PL redshift was observed within a few hours of PS incubation in BSA solution followed by an overall gradual blueshift of the PL energy. Correlating this result with Fourier transform infrared (FTIR) spectroscopy performed before and after BSA incubation as well as with the PL and FTIR studies performed in parallel on planar Si, the redshift was attributed to the adsorption (or incorporation) of protein in the porous matrix while the blueshift was assigned to surface oxidation due to the erosion of Si nanocrystallites in the PS matrix. Understanding the protein incorporation in the PS matrix has important implications in utilizing the PL as an optical transduction method in a PS-based biosensor device.

Resonant cavity enhanced optical microsensor for molecular interactions based on porous silicon

AbstractThe molecular binding between the glutamine binding‐protein (GlnBP) from Escherichia coli and L‐glutamine (Gln) is detected by means of an optical biosensor based on porous silicon technology. The binding event is optically transduced in the wavelength shift of the porous silicon optical microcavity (PSMC) reflectivity spectrum. The hydrophobic interaction links the GlnBP, which acts as a molecular probe for Gln, to the hydrogenated porous silicon surface area. We can thus avoid any preliminary surface functionalization process. The protein infiltrated PSMC results stable to oxidation at least for few cycles of wet measurements. The penetration of the proteins into the pores of the porous silicon matrix has been optimized: a strong base post‐etch process increases the pore size and removes any nanostructure on top and inside the porous silicon multilayer while does not degrade the optical response and the quality of the microcavity. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Fabrication of porous silicon using a gas etching method

A gas-based etching method for the fabrication of porous silicon was developed. It consists of exposing silicon samples to a mixture of oxygen (O2) and nitrogen dioxide (NO2) gases, and hydrogen fluoride (HF) and water vapors. The morphology, structure, porosity, photoluminescence properties, refractive index, and extinction coefficient of the porous layers, for various O2:NO2 flow rate ratios were investigated. The porous layers were characterized by island-like structures, attributed to the condensation of etchant-laden drops. Sections of porous silicon, with an average porosity and thickness of 88% and 1230 nm, respectively, were dispersed within a porous silicon matrix characterized by an average porosity and thickness of 79% and 301 nm, respectively. In addition to the striking difference in thickness between the islands and matrix, the morphology of the porous layer surface was different between the two areas. The size and density of the islands were found to strongly depend on the O2:NO2 flow rate ratio. Furthermore, the porous layers demonstrated a strong photoluminescence emission at an average peak wavelength of 658 nm.

Influence of polarized bias and porous silicon morphology on the electrical behavior of Au-porous silicon contacts

This paper reports the surface morphology and I-V curves of porous silicon (PS) samples and related devices. The observed fabrics on the PS surface were found to affect the electrical property of PS devices. When the devices were operated under different external bias (10 V or 3 V) for 10 min, their observed obvious differences in electrical properties may be due to the different control mechanisms in the Al/PS interface and PS matrix morphology.

Porous silicon-based optical microsensor for the detection of l-glutamine

The molecular binding between the glutamine-binding protein (GlnBP) from Escherichia coli and L-glutamine (Gln) is optically transduced by means of a biosensor based on porous silicon nano-technology. The sensor operates by the measurement of the interferometric fringes in the reflectivity spectrum of a porous silicon Fabry-Perot layer. The binding event is revealed as a shift in wavelength of the fringes. Due to the hydrophobic interaction with the Si-H terminated surface of the porous silicon, the GlnBP protein, which acts as a molecular probe for Gln, penetrates and links into the pores of the porous silicon matrix. We can thus avoid any preliminary functionalization process of the porous layer surface, which is also prevented from oxidation, at least for few cycles of wet measurements. The binding of Gln to GlnBP has also been investigated at different concentration of GlnBP.

Anisotropy of the magnetic properties of nickel clusters

A technology of polygonal nickel microclusters aligned in the same direction has been developed. The new technology is based on the incorporation of nickel from a salt solution into a porous silicon matrix. Anisotropic magnetic properties of nickel microclusters formed in the silicon matrix are established and studied for the first time.

Cathodoluminescent properties of tin dioxide sol–gel doped with Tb3+ and Eu3+ grown in porous silicon

Abstract Tin dioxide doped with antimony (SnO2:Sb) and sol–gel doped with terbium (Tb3+) were deposited on porous silicon (PS) layers and on monocrystalline Si wafers. Energy dispersive X‐ray (EDX) analysis and Auger electron spectroscopy (AES) show the presence of tin, oxygen and terbium in the PS matrix. Preliminary measurements on cathodoluminescent emission are presented. The mechanism of rare‐earth emission is discussed through a comparative study of the cathodoluminescence (CL) of SnO2:Sb:Tb3+/PS and SnO2:Sb:Tb3+/Si. A strongly enhanced CL efficiency of SnO2:Sb sol–gel doped with Tb3+ and Eu3+ ions deposited in PS is reported. The enhancement is attributed to a process of excitation transfer between rare‐earth ions. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Porous silicon matrix for chemical synthesis and chromatograpy

AbstractThis paper reports the fabrication of a porous silicon matrix with two narrow pore size regimes: an ordered array of macropores and a network of mesopores within the walls of each macropore. Both types of pores are well defined in their length and show a narrow pore diameter distribution, but the difference in pore diameter between the two types may be as large as 103. By free choice of the dimensions for each size regime this material can be tailor‐made according to its specific application; for example as stationary phase in chromatograpy or as immobilisation and reaction site for chemical or biochemical synthesis. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Electrical and thermal modulation of silicon photonic bandgap microcavities containing liquid crystals

Electrical and thermal modulation of porous silicon microcavities is demonstrated based on a change in the refractive index of liquid crystals infiltrated in the porous silicon matrix. Positive and negative anisotropy liquid crystals are investigated, leading to controllable tuning to both longer and shorter wavelengths. Extinction ratios greater than 10 dB have been demonstrated. Larger attenuation can be achieved by increasing the Q-factor of the microcavities.

Quantatitive Assessment of Enzyme Immobilization Capacity in Porous Silicon

Immobilized enzyme systems are important in a broad range of applications, from biological sensing to the industrial-scale biocatalytic synthesis of chiral products. We demonstrate the ability to systematically vary and quantitatively assess the immobilization capacity of porous silicon thin films for the enzyme glutathione-S-transferase in a manner predicted by a simple geometric model of the porous silicon matrix. We find that the immobilization capacity quantatitively correlates with systematic changes in the device thickness. These results are significant since, despite the wide range over which porous silicon morphology and surface area can be varied, few attempts have been made to systematically characterize surface binding capacity. Our findings suggest that porous silicon can be an ideal matrix, where immobilization of a predictable quantity of biological material is desired.