Porous Silicon Matrix Research Articles

Study of nanocomposite iron/porous silicon material

Porous silicon (PS) is a nanostructured material based on silicon, fabricated using an electrochemical process that can be used for a broad spectrum of applications, from optoelectronic to biomedicine. In this paper there are presented the fabrication processes of microelement monolithic reservoirs using PS matrix; two impregnation processes of porous structure with iron ions or iron oxides have been studied: iron electrochemical deposition and iron salts deposition and melting, both followed by thermal treatments. The test structures were analyzed from morphological and compositional point of view and the measurements reveal that the iron compounds are distributed both inside the pores network and in the Si skeleton.

Highly Sensitive Porous Silicon Sensor: Detection of Organic Vapours Using Photoluminescence Quenching Technique

Abstract Porous silicon based sensors were tested in the presence of various linear aliphatic alcohols (methanol to n-hexanol) and water in the range of 10-100 ppm by photoluminescence quenching technique. An increasing trend in the degree of quenching was observed with the chain length of alcohols while minimum response was given to water. Sensitivity as high as 80-90% and nearly instant response time has proven the sensors to be highly efficient. Photoluminescence quenching phenomena is discussed on the basis of charge transfer mechanism between the host and the vapour-induced surface states, but the degree of quenching and anomalous response as a function of chain length suggests no unique quenching theory for estimating the sensitivity for the set of alcohols tested. From methanol to butanol, the sensitivity was dependent on the effective concentration of analytes in the porous silicon matrix, while for pentanol and hexanol having high boiling point, the sensitivity was linked to dielectric quenching mechanism due to the condensation of vapours inside the pores.

Ferromagnetic nanostructure arrays self-assembled in mesoporous silicon

The investigated nanocomposite system is composed of a porosified silicon wafer and embedded ferromagnetic nanostructures. Porous silicon achieved by anodization of a highly doped (100) silicon wafer offers straight pores grown in (001)-direction which are clearly separated from each other. Within the pores of the porous silicon matrix ferromagnetic nanostructures are precipitated. The obtained self-assembled hybrid system combines the electronic properties of silicon with the magnetic properties of the incorporated ferromagnetic metal. Such a magnetic system can be achieved by electrochemical deposition of a transition metal from a metal salt solution into the pores of the nanostructured silicon skeleton. The porous silicon/metal nanocomposites are characterized by electron microscopy (SEM, TEM) to get information about the interior interface and SQUID-magnetometry. Furthermore the structural analyzed regions are investigated by MFM to gain more local information about the magnetic state of the embedded nanostructures.

Manipulation of Spontaneous Emission Dynamics of Organic Dyes in the Porous Silicon Matrix

The control of the spontaneous emission (SE) rate of dye molecules (4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) and Coumarin 523 (C523)) embedded in the Porous Silicon (PS) matrix has been studied using picosecond resolved fluorescence decay and polarization studies. We have shown that the SE rates of the two organic dyes embedded in the PS matrix depend on the relative positions of the emission maxima of the dyes with respect to electronic band gap energy of the PS matrix. We have also explored that the electronic band gap of the host PS matrix can easily be tuned by partial oxidation of the PS and the nature of SE of the embedded dyes can be tuned accordingly. The demonstrated retardation or enhancement of the spontaneous photon emission may enable the application of fluorescent organic molecules in PS matrix in several quantum optical devices including the realization of single photon sources.

The interior interfaces of a semiconductor/metal nanocomposite and their influence on its physical properties

AbstractA semiconductor/metal hybrid system with silicon as base material and embedded metal nanostructures is fabricated in two steps by anodizing the silicon substrate in an aqueous hydrofluoric acid solution and a subsequent electrodeposition process. The achieved self‐assembled nanoscopic hybrid system consists of a quasi‐regular 3‐dimensional arrangement of metal nanostructures embedded within the pores of the silicon matrix. The used metals are Ni, Co and their alloys, whereas mainly Ni is considered in the following. In such a hybrid system the interface between metal and silicon nanostructures is of interest in correlation with the properties of the material and can be influenced by different fabrication conditions. If the matrices are aged in ambient air before filling with a metal the inner surface of the porous silicon template offers a native SiOx layer due to the oxidation of the hydrogen terminated surface of the pore‐walls. Using as‐etched samples for deposition of the metal into the pores leads to the formation of a more complex interface during the cathodic deposition procedure which is observed by FTIR‐spectroscopy. Knowledge about the composition and the formation of the interior surface of the porous silicon matrix as well as the interface between semiconductor and metal is necessary on the one hand for interpretation of the gained experimental results and on the other hand to figure out possible applications of the fabricated hybrid system. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical performance in iron-passivated porous silicon film

The electrical properties in iron-passivated porous silicon (PS:Fe) with stable optical properties were studied. Iron is incorporated in porous silicon (PS) by impregnation method using Fe(NO 3) 3 aqueous solution. Energy dispersive X-ray analysis and Raman scattering revealed the presence of iron atoms over most of the PS layer surface. We discussed the current–voltage ( I– V) of PS and PS:Fe structures under both forward and reverse bias conditions. The I– V measurements obtained on PS:Fe showed a rectifying effect and an improvement of both the threshold voltage and current intensity as compared with those relative to PS. By using the Richardson–Schottky diode equation and taking into account a series resistance ( R S), all the parameters of the structures are determined and discussed. The presence of the iron in the PS matrix reduces the value of ideality factor n (close to 2) and the series resistance. At low voltage, the PS:Fe produces an important change in the transport mechanism of the PS; from ohmic to space charge limited conduction (SCLC) mechanism. The obtained results indicate the presence of the shallow acceptor levels in the gap due to the iron, allowing us to propose a band-gap diagram of Ag/PS:Fe/c-Si.

Cu-Si Nanocomposites Based on Porous Silicon Matrix

Cu-Si nanocomposites formed by an immersion displacement deposition of Cu into porous silicon (PS) matrix have been experimentally studied. SEM and AES were used to investigate the structure and elemental composition of Cu-Si samples. The top part of the Cu-PS samples is shown to demonstrate the following structure: large faceted Cu grains at the top, a porous fine-grained copper film underneath the large grains, and the copper pointed rods extended from the surface into the PS layer. The top part of the silicon skeleton of the PS layer is converted into the copper by the etching followed by Cu displacement deposition. The porosity of the porous layer and displacement deposition times are found to form Cu-Si nanocomposites of various structures and various Cu-Si contents because of various extent of the silicon skeleton transformation into copper.

Magnetite nanoparticles embedded in biodegradable porous silicon

Magnetite nanoparticles, which are coated with oleic acid in a hexane solution and exhibit an average diameter of 7.7 nm, were embedded in a porous silicon (PS) matrix by immersion under defined parameters (e.g. concentration, temperature, time). The porous silicon matrix is prepared by anodization of a highly n-doped silicon wafer in an aqueous HF-solution. Magnetic characterization of the samples has been performed by SQUID-magnetometry. The superparamagnetic behaviour of the magnetite nanoparticles is represented by temperature-dependent magnetization measurements. Zero field (ZFC)/field cooled (FC) experiments indicate magnetic interactions between the particles. For the infiltration into the PS-templates different concentrations of the magnetite nanoparticles are used and magnetization measurements are performed in respect with magnetic interactions between the particles. The achieved porous silicon/magnetite specimens are not only interesting due to their transition between superparamagnetic and ferromagnetic behaviour, and thus for magnetic applications but also because of the non-toxicity of both materials giving the opportunity to employ the system in medical applications as drug delivery or in medical diagnostics.

An alternative approach to understand the photoluminescence and the photoluminescence peak shift with excitation in porous silicon

There have been many different models proposed for the luminescence in porous silicon (PS), yet it is believed that the quantum confinement effect persists at the absorption. However, from our investigation on both constant and pulsed electrochemically etched silicon (PS), the absence of quantum confinement effect at the absorption has been identified from the close correspondence of photoluminescence excitation (PLE) spectra of PS to the simulated absorption spectrum of an ultrathin silicon film with the bulk optical constants. In the simulation of absorption spectrum, the spectral dependence of reflectivity of the solid, which had been omitted in the traditional analysis of PLE, is considered. Further, although nanocrystallites of silicon are present in the PS matrix, the absence of quantum confinement is explained on the basis of structural characteristics of PS. Following that, many common observations in the luminescence of PS are attributed to the surface states. The blueshift of the PL peak with the increase in excitation energy is explained with the idea of quasithermal equilibrium and the probability of occupation of the carriers at the surface states.

Porous Silicon/Metal Hybrid System With Ferro and Paramagnetic Behavior

Fabricated ferromagnetic hybrid systems consisting of a porous silicon (PS) matrix with incorporated metal nanostructures exhibit magnetic properties which can be distinguished in two ranges. A first one is related to magnetic fields below the saturation magnetization of the deposited metal and a second one to higher magnetic fields. Considering the low field region the samples show magnetic characteristics due to the metal-precipitations (geometry, distribution, kind of metal) tunable by the deposition parameters. At higher fields an additional novel non-saturating term is observed which is dependent on the temperature and differs in strength between Ni and Co. In the frame of this work this additional paramagnetic term is experimentally investigated.

Silicon/Metal Hybrid Nanosystem with Tailored Magnetic Properties in Two Characteristic Field Regions

Electrodeposition of ferromagnetic metal-nanostructures into a porous silicon (PS) matrix merges the electronic properties of a semiconductor and the ferromagnetic ones of the precipitated metal on one material level. The deposition is performed using a metal-salt solution as electrolyte and an applied pulsed current under cathodic conditions. The metal precipitation within a PS-template exhibiting unconnected mesopores with aspect ratios of about 1000 leads to nanostructures with a maximum elongation of a few microns (aspect ratio 100). The achieved hybrid system exhibits magnetic characteristics which drastically differ from the corresponding bulk materials. Magnetization measurements of this material show two characteristics, one at magnetic fields below the saturation magnetization MS of the deposited metals due to the spin-magnetism (e.g. MS (Ni)=0.62 T, MS (Co)=1.82 T) and another one at higher fields above MS which is assigned to a novel, recently intensively investigated magnetic contribution likely caused by spin-orbit interaction.

Catalytic Activity of Hybrid Metal/Silicon and Metal/Silicon Dioxide Nanocomposites

Here we demonstrate that H-terminated porous silicon (PSi) powders and layers can be utilized as highly efficient reductive templates for noble metal salts. The metal salt reduction results in a metal nanoparticle (NP) formation in the porous matrix of PSi. Gold NP formation has been monitored in-situ by measuring the plasmon resonance response. Pt NPs, formed in the pores of PSi, were investigated by transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) analysis. Furthermore, hybrid Pt/PSi nanocomposites exhibit a high catalytic activity for CO oxidation. Some of the samples were oxidized to obtain Pt/PSiOx nanocomposites for comparison of catalytic activity.

Conductivity-Based Catechol Sensor Using Tyrosinase Immobilized in Porous Silicon

A conductivity-based catechol biosensor was developed using porous silicon as an immobilization matrix for enzyme tyrosinase. The enzyme was extracted from plant source Amorphophallus companulatus and immobilized in an electrochemically etched surface of p-type silicon. The presence of enzyme in a porous structure and the retention of enzyme activity were confirmed by scanning electron microscopy and spectrophotometric studies, respectively. The principle of the sensor is based on the change in the conductivity of the tyrosinase-entrapped porous silicon matrix. When the entrapped tyrosinase interacted with catechol, the change in the current voltage (I-V) characteristics was obtained, which was proportional to analyte concentration. The analytical characteristics of the sensor including response time, linearity range, lower detection limit, reusability, and storage stability were studied.

On‐line cell lysis and DNA extraction on a microfluidic biochip fabricated by microelectromechanical system technology

Integrating cell lysis and DNA purification process into a micrototal analytical system (microTAS) is one critical step for the analysis of nucleic acids. On-chip cell lysis based on a chemical method is realized by sufficient blend of blood sample and the lyzing reagent. In this paper two mixing models, T-type mixing model and sandwich-type mixing model, are proposed and simulation of those models is conducted. Result of simulation shows that the sandwich-type mixing model with coiled channel performs best and this model is further used to construct the microfluidic biochip for on-line cell lysis and DNA extraction. The result of simulation is further verified by experiments. It asserts that more than 80% mixing of blood sample and lyzing reagent which guarantees that completed cell lysis can be achieved near the inlet location when the cell/buffer velocity ratio is less than 1:5. After cell lysis, DNA extraction by means of a solid-phase method is implemented by using porous silicon matrix which is integrated in the biochip. During continuous flow process in the microchip, rapid cell lysis and PCR-amplifiable genomic DNA purification can be achieved within 20 min. The potential of this microfluidic biochip is illustrated by pretreating a whole blood sample, which shows the possibility of integration of sample preparation, PCR, and separation on a single device to work as portable point-of-care medical diagnostic system.

Synthesis and catalytic activity of hybrid metal/silicon nanocomposites

AbstractIn this Letter we demonstrate that hydrogen‐terminated porous silicon (PSi) layers and powders can serve as highly efficient reductive templates for noble metal salts. The reduction results in metal nanoparticle (NP) formation in the pores of PSi. Gold NP formation has been monitored in‐situ by measuring the plasmon resonance response. Pt NPs, formed in the PSi matrix, were investigated by transmission electron microscopy and energy‐dispersive X‐ray analysis. Furthermore, hybrid Pt/PSi nanocomposites exhibit a high catalytic activity for CO oxidation.magnified imageHRTEM image of a Pt nanoparticle reduced inside the PSi matrix. Platinum lattice fringes d〈111〉 = 2.27 Å can be seen.(© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence enhancement and stabilisation of porous silicon passivated by iron

Iron is incorporated in porous silicon (PS) by impregnation method using Fe(NO 3) 3 aqueous solution. The presence of iron in PS matrix is shown from energy-dispersive X-ray (EDX) analysis and Fourier transform infrared (FTIR) measurements. The optical properties of PS and PS-doped iron are studied by photoluminescence (PL). The iron deposited in PS quenched the silicon dangling bonds then increased the PL intensity. The PL peak intensity of impregnated PS is seven times stronger than that in normal PS. Upon exposing iron-PS sample to ambient air, there is no significant change in peak position but the PL intensity increases during the first 3 weeks and then stabilises. The stability is attributed to passivation of the Si nanocrystallites by iron.

A ferromagnetic (porous silicon/metal)-nanocomposite with an additional paramagnetic behavior

Investigations on a nanostructured metal/semiconductor hybrid system show a novel magnetic behavior with a spin-magnetism at low magnetic fields and an additional paramagnetic term at higher fields. The system is composed of a porous silicon (PS) matrix formed by anodization of an n-type silicon wafer with electrochemically deposited metal-structures. The morphology of the membrane is tuneable by the anodization conditions, but the metal deposition within the channels of the template can also be modified by changing the galvanic parameters of the metal-loading procedure. Both the variation of the template (pore-diameter, pore-distance) and the adjustment of the metal-filling (spatial distribution along the channels, shape of the precipitations) enable selective modification of the magnetic properties of the nanocomposite system. The geometry of the precipitated metal-structures, which vary between spheres, ellipsoids and wires as well as their mutual arrangement, determines the magnetic behavior (coercivity, squareness) at magnetic fields beneath the saturation magnetization of the deposited metal. At higher magnetic fields a subsequent non-saturating paramagnetic term occurs which is due to orbital currents in silicon. Both properties together, the spin-magnetism and the additionally appearing orbital magnetism form a novel magnetic behavior, characteristic for the introduced hybrid nanocomposite.

Color Me Sensitive: Amplification and Discrimination in Photonic Silicon Nanostructures

A new paper in this issue discusses some of the considerations for fabrication of biosensor devices in the porous silicon material system. The results focus on optical detection of protease activity on specific substrates that have been immobilized in a porous silicon matrix. Solutions to important limitations of porous microsensors are presented: stabilizing the sensor against corrosion-induced zero point drift, minimizing the effects of nonspecific protein binding, and enhancing the optical response by incorporation of a catalytic reaction in the sensing scheme.

AC impedance spectroscopy of porous silicon thin films containing metallic cations

Thin porous silicon (PS) films were prepared by HF/HNO 3 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. Al 3+, 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 +, Al 3+ and Li + cations respectively. These kinds of structures are suitable for gas sensing, optical sensing or for the fabrication of fuel cell membranes.

Composite Structures of Polyaniline and Mesoporous Silicon: Electrochemistry, Optical and Transport Properties

Composite structures of p-type mesoporous silicon and polyaniline have been fabricated by a one-step electrochemical polymerization process. The composite structure shows good conduction properties and new, intense photoluminescence emission bands that are assigned to polyaniline that penetrates and fulfills the entire volume of the porous silicon matrix quite uniformly. We have found that at room temperature, the reverse bias conductivity of the composite structure is related to transport through the polyaniline network, giving rise to a 5 orders of magnitude larger current relative to that of a reference porous silicon matrix. At low temperatures, the current at both polarities of the bias voltage is carried in the polyaniline network and is related to variable range hopping in three dimensions.