Selective acetone electrical detection using functionalized nano-porous silicon

Abstract: The red shift and the broadening of the Raman signal from microcrystalline silicon films is described in terms of a relaxation in the q-vector selection rule for the excitation of the Raman active optical phonons. The relationship between width and shift calculated from the known dispersion relation in c-Si is in good agreement with available data. An increase in the decay rate of the optical phonons predicted on the basis of the same model is confirmed experimentally.

Abstract: A miniaturized prototype sensor based on TiO2 nanotubes/porous silicon (PS) heterojunction is developed for selective ethanol sensing in sub-ppm range. Titanium (Ti) of thickness ∼ 200 nm was deposited on PS using RF sputtering technique. Both silicon and Ti were sequentially anodized to form PS and nanotubes respectively. Electrical contacts for testing of resistive sensors were fabricated using lift off process. The sensor was packaged onto a 12-pin header and tested in presence of different VOCs with concentration ranging from 0.5 to 100 ppm. The selective ethanol sensing at around 150 °C stems from the formation of TiO2 nanotubes/PS heterojunction. The sensitivity of such a sensor, improved manifold in comparison to the response of pure PS and pure TiO2 based sensors. The formation of heterojunction, selective response to ethanol, sub-ppm level sensing at comparatively low operating temperature is discussed. The study unfolds the collective properties of TiO2/PS heterojunction and demonstrates the potential of wafer scale integrated repeatable ethanol sensor tested at sub-ppm level.

Abstract: Finding a non-invasive, painless and simple method for monitoring of health parameters and early detection of physiological disorders is of utmost importance in today’s world. Breath analysis has been attracted tremendous attention as a potentially powerful tool for studying the medical diagnosis diseases because of its noninvasive nature and capability of real-time monitoring. For metabolic monitoring, several candidates are attractive. As an example, acetone has been considered as a main breath biomarker for diabetes. In addition to diabetes, fat burn monitoring is another application for breath acetone. This article provides a comprehensive study on the state of the art research activities in gas sensor field. It mainly focuses on the metal oxide semiconductor based acetone sensors such as WO3, ZnO, SnO2, Fe2O3, In2O3, TiO2, Co2O3, NiO and etc. Several different type of acetone sensor technologies such as polymer based chemiresistors, mixed potential, optical and mass sensitive devices are presented as well. The effect of nano structures and morphology on response of sensors are discussed. The advantages and disadvantages of different techniques used for metal oxide semiconductor synthesis are also discussed. Finally, sensor arrays with different receptor materials and transducers for breath analyzing are discussed.

Abstract: Synthesis of orthorhombic (α) MoO3 nano-flakes by dry oxidation of RF sputtered Mo thin film is presented. The influence of Mo thickness variation, oxidation temperature and time on the crystallographic structure, surface morphology and roughness of MoO3 thin films was studied using SEM, AFM, XRD and Raman spectroscopy. A structural study shows that MoO3 is polycrystalline in nature with an α phase. It was noticed that oxidation temperature plays an important role in the formation of nano-flakes. The synthesis technique proposed is simple and suitable for large scale productions. The synthesis parameters were optimized for the fabrication of sensors. Chrome gold-based IDE (interdigitated electrodes) structures were patterned for the electrical detection of organic vapors. Sensors were exposed to wide range 5–100 ppm of organic vapors like ethanol, acetone, IPA (isopropanol alcohol) and water vapors. α-MoO3 nano-flakes have demonstrated selective sensing to acetone in the range of 10–100 ppm at 150 °C. The morphology of such nanostructures has potential in applications such as sensor devices due to their high surface area and thermal stability.

Abstract: This paper highlights the surface treatment of porous silicon (PSi) for enhancing the sensitivity of water vapors at room temperature. A simple and low cost technique was used for fabrication and functionalization of PSi. Spin coated polyvinyl alcohol (PVA) was used for functionalizing PSi surface. Morphological and structural studies were conducted to analyze samples using SEM and XRD/Raman spectroscopy respectively. Contact angle measurements were performed for assessing the wettability of the surfaces. PSi and functionalized PSi samples were tested as sensors in presence of different analytes like ethanol, acetone, isopropyl alcohol (IPA) and water vapors in the range of 50–500 ppm. Electrical measurements were taken from parallel aluminium electrodes fabricated on the functionalized surface, using metal mask and thermal evaporation. Functionalized PSi sensors in comparison to non-functionalized sensors depicted selective and enhanced response to water vapor at room temperature. The results portray an efficient and selective water vapor detection at room temperature.

Abstract: The red shift and the broadening of the Raman signal from microcrystalline silicon films is described in terms of a relaxation in the q-vector selection rule for the excitation of the Raman active optical phonons. The relationship between width and shift calculated from the known dispersion relation in c-Si is in good agreement with available data. An increase in the decay rate of the optical phonons predicted on the basis of the same model is confirmed experimentally.

Abstract: Small physical dimensions of the scattering crystals lead to a downshift and broadening of the first order Raman line through a relaxation of the q = 0 selection rule. We consider the effect of the exact shape of the microcrystal and show that there are significant differences between spherical, columnar and thin slab microcrystals. The relationship between the width, shift and asymmetry of the Raman line is calculated and is in good agreement with available experimental data.

Abstract: Porous silicon has been the focus of much research activity in recent years, in view of its luminescence, which may enable light emitting devices to be integrated with silicon chips. This work comprises over 50 commissioned essays on the subject from scientists in Europe, America and Japan .

Abstract: All manifestations of pores in silicon are reviewed and discussed with respect to possible applications. Particular emphasis is put on macropores, which are classified in detail and reviewed in the context of pore formation models. Applications of macro-, meso-, and micropores are discussed separately together with some consideration of specific experimental topics. A brief discussion of a stochastic model of Si electrochemistry that was found useful in guiding experimental design for specific pore formation concludes the paper.

Abstract: Preface FUNDAMENTALS OF POROUS SILICON PREPARATION Introduction Chemical Reactions Governing the Dissolution of Silicon Experimental Set-up and Terminology for Electrochemical Etching of Porous Silicon Electrochemical Reactions in the Silicon System Density, Porosity, and Pore Size Definitions Mechanisms of Electrochemical Dissolution and Pore Formation Resume of the Properties of Crystalline Silicon Choosing, Characterizing, and Preparing a Silicon Wafer PREPARATION OF MICRO-, MESO-, AND MACRO-POROUS SILICON LAYERS Etch Cell: Materials and Construction Power Supply Other Supplies Safety Precautions and Handling of Waste Preparing HF Electrolyte Solutions Cleaning Wafers Prior to Etching Preparation of Microporous Silicon from a p-Type Wafer Preparation of Mesoporous Silicon from a p++-Type Wafer Preparation of Macroporous, Luminescent Porous Silicon from an n-Type Wafer (Frontside Illumination) Preparation of Macroporous, Luminescent Porous Silicon from an n-Type Wafer (Back Side Illumination) Preparation of Porous Silicon by Stain Etching Preparation of Silicon Nanowire Arrays by Metal-Assisted Etching PREPARATION OF SPATIALLY MODULATED POROUS SILICON LAYERS Time-Programmable Current Source Pore Modulation in the z-Direction: Double Layer Pore Modulation in the z-Direction: Rugate Filter More Complicated Photonic Devices: Bragg Stacks, Microcavities, and Multi-Line Spectral Filters Lateral Pore Gradients (in the x-y Plane) Patterning in the x-y Plane Using Physical or Virtual Masks Other Patterning Methods FREESTANDING POROUS SILICON FILMS AND PARTICLES Freestanding Films of Porous Silicon-"Lift-offs" Micron-Scale Particles of Porous Silicon by Ultrasonication of Lift-off Films Core-Shell (Si/SiO2) Nanoparticles of Luminescent Porous Silicon by Ultrasonication CHARACTERIZATION OF POROUS SILICON Gravimetric Determination of Porosity and Thickness Electron Microscopy and Scanned Probe Imaging Methods Optical Reflectance Measurements Porosity, Pore Size, and Pore Size Distribution by Nitrogen Adsorption Analysis (BET, BJH, and BdB Methods) Measurement of Steady-State Photoluminescence Spectra Time-Resolved Photoluminescence Spectra Infrared Spectroscopy of Porous Silicon CHEMISTRY OF POROUS SILICON Oxide-Forming Reactions of Porous Silicon Biological Implications of the Aqueous Chemistry of Porous Silicon Formation of Silicon-Carbon Bonds Thermal Carbonization Reactions Conjugation of Biomolecules to Modified Porous Silicon Chemical Modification in Tandem with Etching Metallization Reactions of Porous Silicon APPENDIX A1. ETCH CELL ENGINEERING DIAGRAMS AND SCHEMATICS Standard or Small Etch Cell-Complete Standard Etch Cell Top Piece Small Etch Cell Top Piece Etch Cell Base (for Either Standard or Small Etch Cell) Large Etch Cell-Complete Large Etch Cell Top Piece Large Etch Cell Base APPENDIX A2. SAFETY PRECAUTIONS WHEN WORKING WITH HYDROFLUORIC ACID Hydrofluoric Acid Hazards First Aid Measures for HF Contact Note to Physician HF Antidote Gel APPENDIX A3. GAS DOSING CELL ENGINEERING DIAGRAMS AND SCHEMATICS Gas Dosing Cell Top Piece Gas Dosing Cell Middle Piece Gas Dosing Cell Bottom Piece