Piezoresistive pressure sensing by porous silicon membrane

In this paper, the piezoresistive pressure-sensing property of porous silicon has been reported. The pressure sensitivity of a porous silicon membrane of 63% porosity and 20-/spl mu/m thickness has been observed to be about three times more than that of a conventional bulk silicon membrane of the same dimensions. The increased sensitivity is attributed to the improvement in piezoresistance due to quantum confinement in the porous silicon nanostructure. The piezoresistive coefficient of porous silicon is estimated for the first time and is observed to be about 50% larger than that of monocrystalline silicon for a 63% porosity porous silicon membrane. The response time has also been studied and observed to be significantly shorter. Power dissipation of the porous silicon pressure sensor is also much less compared to that of commercial bulk silicon piezoresistive pressure sensors.     

Multi-walled carbon nanotube arrays for gas sensing applications

Vertically aligned multi-walled carbon nanotube (MWCNT) arrays fabricated by xylene pyrolysis in anodized aluminum oxide (AAO) templates without the use of a catalyst were integrated into a resistive sensor design. Steady state sensitivities as high as 5% and 10% for 100?ppm of NH(3) and NO(2), respectively, at a flow rate of 750?sccm were observed. A thin layer of amorphous carbon (5-50?nm), formed on both sides of the template during xylene pyrolysis, was part of the sensor design. The thickness of the conducting amorphous carbon layers was found to play a crucial role in determining the sensitivity of the resistive sensor. A study was undertaken to elucidate (i) the dependence of sensitivity on the thickness of amorphous carbon layers, (ii) the effect of UV light on gas desorption characteristics and (iii) the dependence of room temperature sensitivity on different NH(3) flow rates. Variations in sensor resistance with exposure to oxidizing and reducing gases are explained on the basis of charge transfer between the analytes and the CNTs which were modeled as p-type semiconductors.     

The potential of porous silicon gas sensors

Recent developments in porous silicon gas sensors have been reviewed. Monitored species detection levels, and the mechanisms of sensing for different sensor designs are also discussed. Porous silicon surface modification methods have been employed for detecting different gas molecules; H₂O, ethanol, methanol, isopropanol, CO_x, NO_x, NH₃, O₂, H₂, HCl, SO₂, H₂S and PH₃.     

Optical sensing of flammable substances using porous silicon microcavities

A porous silicon multilayer, constituted by a Fabry–Pèrot cavity between two distributed Bragg reflectors, is exposed to vapor of several organic species. Different resonant peak shifts in the reflectivity spectra, ascribed to capillary condensation of the vapor in the silicon pores, have been observed. Starting from experimental data, the layer liquid volume fractions condensed in the sensing stack have been numerically estimated. Values ranging between 0.27 (for ethanol) and 0.33 (for iso-propanol) have been found. Time-resolved measurements show that the solvent identification occurs in less then 10 s.     

Sputter deposition of tungsten trioxide for gas sensing applications

Tungsten trioxide thin films were grown by reactively sputtering a circular WO3 target in different Ar–O2 atmospheres. After deposition, data on the structural, optical and electrical properties were obtained by using transmission electron microscopy (TEM) to examine the structure and the morphology of the films, UV-VIS spectrophotometry to determine optical absorption edge characteristics, and Hall effect measurements to determine the change carrier mobility and the film resistivity. In addition, the film resistance variations in controlled atmospheres were examined and the gas sensing properties of films grown under different conditions were compared. The aim of the study was to examine the effect of O2 concentration in the sputtering atmosphere on structural, optical, electrical and sensing properties. © 1998 Kluwer Academic Publishers     

Large-scale integration of nanoelectromechanical systems for gas sensing applications

We have developed arrays of nanomechanical systems (NEMS) by large-scale integration, comprising thousands of individual nanoresonators with densities of up to 6 million NEMS per square centimeter. The individual NEMS devices are electrically coupled using a combined series-parallel configuration that is extremely robust with respect to lithographical defects and mechanical or electrostatic-discharge damage. Given the large number of connected nanoresonators, the arrays are able to handle extremely high input powers (>1 W per array, corresponding to <1 mW per nanoresonator) without excessive heating or deterioration of resonance response. We demonstrate the utility of integrated NEMS arrays as high-performance chemical vapor sensors, detecting a part-per-billion concentration of a chemical warfare simulant within only a 2 s exposure period.     

Long-wavelength vertical-cavity surface-emitting lasers for high-speed applications and gas sensing

Long-wavelength InGaAlAs-InP vertical-cavity surface-emitting lasers (LW-VCSELs) covering the wavelength range from 1.3 to 2.3 mum are presented. Furthermore, these lasers can be fabricated in a novel high-speed design-reducing parasitics to enable bandwidths in excess of 11 GHz at 1.55 mum. To the best of the authors' knowledge, this is the fastest 1.55 mum VCSEL ever presented. As a proof-of-concept one- and two-dimensional arrays have been fabricated with high yield. All devices use a buried tunnel junction for current confinement and a dielectric backside reflector with integrated electroplated gold-heatsink. This concept enables CW operation at room temperature with typical single-mode output powers above 1 mW. Both, wavelength range and modulation performance, together with VCSEL features such as operation voltage around IV and power consumption as low as 10-20 mW enable applications in tunable diode laser spectroscopy (TDLS) and optical data transmission. Error-free data transmission at 10 Gbit/s over 22 km which can be readily applied in uncooled coarse wavelength division multiplex passive optical networks is presented. A laser hygrometer using a 1.84 mum VCSEL demonstrates the functionality of TDLS systems with VCSELs.     

Sol-gel cobalt oxide-silica nanocomposite thin films for gas sensing applications

Various metal oxide-silica nanocomposite films have been recently proposed as gas-sensitive materials. This paper presents results on cobalt oxide-SiO₂ mesoporous nanocomposite thin films templated by a cationic surfactant. The films were deposited on glass substrate by dip-coating process, using [Co(CH₃COO)₂]·4H₂O and tetraethoxysilane (TEOS) as starting materials. The effect of withdrawal speed, number of layers and thermal treatment on the crystalline structure, morphology, Co-doping states, optical, electrical and gas sensing properties of the thin films has been investigated using X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, optical transmittance and room temperature photoreduction-oxidation data.     

Improvement of the durability of porous silicon through functionalisation for biomedical applications

The durability of porous silicon (PS) in solutions was improved by grafting a molecule, 2,4,6,8-tetramethyl-2,4,6,8-tetravinyl-1,3,5,7,2,4,6,8-tetraoxatetrasilocane (TE), with four terminal vinyl groups. With a native PS sample as control, we compared the long-term durability of three modified PS samples: TE-, undec-10-enoic acid (UA)-, and TE/UA(TE first and UA followed)-grafted PS, in a weak organic base of dimethyl sulfoxide, an aqueous mineral solution of CuBr₂, and phosphate buffered saline respectively. Results indicate that TE-grafting is a straightforward and impactful approach to protect PS from oxidation and degradation. Further we used the TE-grafted PS to fabricate a prototype protein microarray by post-grafting UA and subsequently converting UA to nitrilotriacetic acid/Ni²⁺ for binding histidine-tagged proteins.     

Erbium-doped silicon and porous silicon for optoelectronics

Silicon forms the backbone of the microelectronics industry, and possibly, of the optoelectronics industry, hitherto dominated by III/V materials. One of the remaining goals is to build an optical source in silicon. Erbium exhibits luminescent 1.54 μm intra-4f transitions in both silicon and porous silicon. This paper reviews the work which has been carried out in this field and discusses some possible additional applications of erbium-doped silicon in optoelectronics, such as a novel on-chip temperature sensor.     

Temperature dependence of gas conductance through porous silicon carbide plugs

A simple technique for gas flow stabilization in vacuum systems is described. The gas conductance through porous silicon carbide plugs is shown to fall by a factor of 2.5 for the gases He and Ne as the plug temperature is increased from 77K to 800 K. The conductance falls by a factor of 2 for Ar, Kr, N₂ and CO₂ between room temperature and 800 K. The negative temperature coefficient of gas conductance through the plugs is satisfactorily explained by a simple analytical expression. It is suggested that the presence of clay binder material used in the manufacture of the plugs is a major factor in determining the conductance variations.     

Determination of sensoric parameters of porous silicon in sensing of organic vapors

Photoluminescence (PL) properties of as-prepared and surface derivatized porous silicon (PS) in the presence of organic compounds in gas phase were studied. Surface derivatization, aimed at increasing stability of porous silicon properties, was performed by Lewis acid mediated hydrosilylation with methyl 10-undecenoate. We have systematically measured changes in photoluminescence intensity for a set of alcohols from C₁ to C₆. From the variation of the photoluminescence quenching response as a function of alcohol concentration, we determine the sensitivities and detection limits of our porous silicon sensors and these correlate with physical and chemical properties of studied species. For methyl 10-undecenoate derivatized PS samples, we have observed a remarkable enhancement of the selectivity for C₄–C₆ alcohols as compared with C₁–C₃ alcohols.     

Chemical reactions and applications of the reductive surface of porous silicon

The chemical reactivity of freshly prepared porous silicon is similar to that of a reducing agent on the surface of the nanocrystallites. Ag+ spontaneously reduces to form Ag0 granular coatings on the surface of porous silicon at the expense of the oxidation of silicon hydride and silicon. Atomic Force Microscopy shows that the thickness and topography of the Ag0 coating depend on the concentration of Ag+ with the porous silicon surface being the limiting reagent. In-situ Raman Spectroscopy shows an Ag layer on the silicon and Si:O layer immediately after etching and exposure to Ag+ and O2 respectively. Ag0 coated on the surface and in the pores of the porous silicon proves to be an excellent material for Surface Enhanced Raman Spectroscopy and the natural low electron affinity on the surface of porous silicon replaces the need for a negative bias to prepare very stable diamond coatings on the surface of silicon.     

Functionalization control of porous silicon optical structures using reflectance spectra modeling for biosensing applications

Modeling and experimental reflectance spectra of porous silicon single layers at different steps of functionalization and protein grafting process are adjusted in order to determine the volume fraction of the biomolecules attached to the internal pore surface. This method is applied in order to control the efficiency of the chemical functionalization process of porous silicon single layers. Using results from single porous silicon layer study, theoretical microcavity is simulated at each step of the functionalization process. The calculated reflectance spectrum is in good agreement to the experimental one. Therefore the single layers study can be applied to multilayer structures and can be adapted for other optical structures such as waveguides, interferometers for biosensing applications.     

NH3 sensing characteristics of nano-WO3 thin films deposited on porous silicon

The NH3 sensing characteristics of nano-tungsten trioxide (WO3) thin films deposited on porous silicon (PS) were investigated in the present study. Porous silicon layer was first prepared by electrochemical etching in an HF-based solution on a p(+)-type silicon substrate. Then, WO3 nano-films were deposited on the porous silicon layer by DC magnetron sputtering. Pt electrodes were deposited on the top surface of the WO3 films to obtain the WO3/PS gas sensor. The WO3 films deposited on PS were characterized by SEM, XRD and XPS. The NH3 sensing characteristics for WO3/PS gas sensor were tested at room temperature and 50 degrees C. The results showed that the NH3 sensing characteristics of WO3/PS were superior to WO3/Al2O3 at room temperature. The sensing mechanism of the nano-WO3 thin films based on PS was also discussed.     

Hydrothermal growth of symmetrical ZnO nanorod arrays on nanosheets for gas sensing applications

The hierarchical ZnO nanostructures with 2-fold symmetrical nanorod arrays on zinc aluminum carbonate (ZnAl-CO₃) nanosheets have been successfully synthesized through a two-step hydrothermal process. The primary nanosheets, which serve as the lattice-matched substrate for the self-assembly nanorod arrays at the second-step of the hydrothermal route, have been synthesized by using a template of anodic aluminum oxide (AAO). The as-prepared samples were characterized by XRD, FESEM, TEM and SAED. The nanorods have a diameter of about 100 nm and a length of about 2 μm. A growth mechanism was proposed according to the experimental results. The gas sensor fabricated from ZnO nanorod arrays showed a high sensitivity to ethanol at 230°C. In addition, the response mechanism of the sensors has also been discussed according to the transient response of the gas sensors.     

Fabrication of silicon/polymer composite nanopost arrays and their sensing applications

A novel technique is reported for fabricating silicon/polymer composite nanopost arrays by combining colloidal lithography and surface-initiated atom-transfer radical polymerization. The composite nanopost arrays possess a core/shell nanoarchitecture, with shells of poly(2-hydroxyethyl methacrylate) and cores of silicon nanoposts. The polymer brush possesses quasi-3D homogeneous nanoarchitectures due to the controllable polymerization process using the surface-initiated atom-transfer radical polymerization technique. The composite nanopost arrays are durable due to the particular nanoarchitectures. The backbone templates of the composites are silicon nanopost arrays directly etched from silicon substrates, and the polymer shell is covalently grafted from the arrays. The composite nanopost arrays exhibit vivid colors. Moreover, the colors of the composite nanopost arrays can be tuned from green to red by changing the thickness of fi lm. Specifically, the composite nanopost arrays can be used as sensors to rapidly detect water vapors with high stability and reproducibility. Many different functional surfaces could be prepared through this technique using other functional monomers.     

Light-emitting porous silicon

Although porous silicon has been known for more than 35 years, only in 1990 was it recognized that porous silicon shows an increased bandgap and efficient room-temperature photoluminescence in the visible. This paper will give an overview of porous silicon research, with special emphasis on the formation mechanism of microporous silicon in terms of a depletion of holes in the porous region due to quantum confinement and the understanding of the origin of the visible luminescence. The status of research on electroluminescent and other devices based on porous silicon will be discussed, as well as results for other luminescent forms of nanocrystalline silicon.