ZnO Structures with Surface Nanoscale Interfaces Formed by Au, Fe2O3, or Cu2O Modifier Nanoparticles: Characterization and Gas Sensing Properties.

Zinc oxide rod structures are synthetized and subsequently modified with Au, Fe2O3, or Cu2O to form nanoscale interfaces at the rod surface. X-ray photoelectron spectroscopy corroborates the presence of Fe in the form of oxide-Fe2O3; Cu in the form of two oxides-CuO and Cu2O, with the major presence of Cu2O; and Au in three oxidation states-Au3+, Au+, and Au0, with the content of metallic Au being the highest among the other states. These structures are tested towards nitrogen dioxide, ethanol, acetone, carbon monoxide, and toluene, finding a remarkable increase in the response and sensitivity of the Au-modified ZnO films, especially towards nitrogen dioxide and ethanol. The results for the Au-modified ZnO films report about 47 times higher response to 10 ppm of nitrogen dioxide as compared to the non-modified structures with a sensitivity of 39.96% ppm-1 and a limit of detection of 26 ppb to this gas. These results are attributed to the cumulative effects of several factors, such as the presence of oxygen vacancies, the gas-sensing mechanism influenced by the nano-interfaces formed between ZnO and Au, and the catalytic nature of the Au nanoparticles.

Love-wave sensors combined with microfluidics for fast detection of biological warfare agents.

The following paper examines a time-efficient method for detecting biological warfare agents (BWAs). The method is based on a system of a Love-wave immunosensor combined with a microfluidic chip which detects BWA samples in a dynamic mode. In this way a continuous flow-through of the sample is created, promoting the reaction between antigen and antibody and allowing a fast detection of the BWAs. In order to prove this method, static and dynamic modes have been simulated and different concentrations of BWA simulants have been tested with two immunoreactions: phage M13 has been detected using the mouse monoclonal antibody anti-M13 (AM13), and the rabbit immunoglobulin (Rabbit IgG) has been detected using the polyclonal antibody goat anti-rabbit (GAR). Finally, different concentrations of each BWA simulants have been detected with a fast response time and a desirable level of discrimination among them has been achieved.

Nanoscale, surface-confined phase separation by electron beam induced oxidation.

Electron-assisted oxidation of Co-Si-based focused electron beam induced deposition (FEBID) materials is shown to form a 2-4 nm metal oxide surface layer on top of an electrically insulating silicon oxide layer less than 10 nm thick. Differences between thermal and electron-induced oxidation on the resulting microstructure are illustrated.

Gas-Phase Synthesis of Iron Silicide Nanostructures Using a Single-Source Precursor: Comparing Direct-Write Processing and Thermal Conversion.

The investigation of precursor classes for the fabrication of nanostructures is of specific interest for maskless fabrication and direct nanoprinting. In this study, the differences in material composition depending on the employed process are illustrated for focused-ion-beam- and focused-electron-beam-induced deposition (FIBID/FEBID) and compared to the thermal decomposition in chemical vapor deposition (CVD). This article reports on specific differences in the deposit composition and microstructure when the (H3Si)2Fe(CO)4 precursor is converted into an inorganic material. Maximum metal/metalloid contents of up to 90 at. % are obtained in FIBID deposits and higher than 90 at. % in CVD films, while FEBID with the same precursor provides material containing less than 45 at. % total metal/metalloid content. Moreover, the Fe:Si ratio is retained well in FEBID and CVD processes, but FIBID using Ga+ ions liberates more than 50% of the initial Si provided by the precursor. This suggests that precursors for FIBID processes targeting binary materials should include multiple bonding such as bridging positions for nonmetals. In addition, an in situ method for investigations of supporting thermal effects of precursor fragmentation during the direct-writing processes is presented, and the applicability of the precursor for nanoscale 3D FEBID writing is demonstrated.

Residual stress of free-standing membranes of yttria-stabilized zirconia for micro solid oxide fuel cell applications.

The present study is devoted to analyze the compatibility of yttria-stabilized zirconia thin films prepared by pulsed laser deposition and metalorganic chemical vapor deposition techniques, with microfabrication processes based on silicon technologies for micro solid oxide fuel cells applications. Deposition of yttria-stabilized zirconia on Si/SiO2/Si3N4 substrates was optimized for both techniques in order to obtain high density and homogeneity, as well as a good crystallinity for film thicknesses ranging from 60 to 240 nm. In addition, stabilized zirconia free-standing membranes were fabricated from the deposited films with surface areas between 50 x 50 microm2 and 820 x 820 microm2. Particular emphasis was made on the analysis of the effect of the nature of the deposition technique and the different design and fabrication parameters (membrane area, thickness and substrate deposition temperature) on the residual stress of the membranes in order to control their thermomechanical stability for application as electrolyte in micro solid oxide fuel cells.

A Parts Per Billion (ppb) Sensor for NO2 with Microwatt (µW) Power Requirements Based on Micro Light Plates.

A film of gas sensitive ZnO nanoparticles has been coupled with a low-power micro light plate (µLP) to achieve a NO2-parts-per-billion conductometric gas sensor operating at room temperature. In this µLP configuration, an InGaN-based LED (emitting at 455 nm) is integrated at a few hundred nanometers distance from the sensor material, leading to sensor photoactivation with well controlled, uniform, and high irradiance conditions, and very low electrical power needs. The response curves to different NO2 concentrations as a function of the irradiance displayed a bell-like shape. Responses of 20% to 25 ppb of NO2 were already observed at irradiances of 5 mWatts·cm-2 (applying an electrical power as low as 30 µW). In the optimum illumination conditions (around 60 mWatts·cm-2, or 200 µW of electric power), responses of 94% to 25 ppb were achieved, corresponding to a lower detection limit of 1 ppb of NO2. Higher irradiance values worsened the sensor response in the parts-per-billion range of NO2 concentrations. The responses to other gases such as NH3, CO, and CH4 were much smaller, showing a certain selectivity toward NO2. The effects of humidity on the sensor response are also discussed.

Cerium Oxide-Tungsten Oxide Core-Shell Nanowire-Based Microsensors Sensitive to Acetone.

Gas sensitive cerium oxide-tungsten oxide core-shell nanowires are synthesized and integrated directly into micromachined platforms via aerosol assisted chemical vapor deposition. Tests to various volatile organic compounds (acetone, ethanol, and toluene) involved in early disease diagnosis demonstrate enhanced sensitivity to acetone for the core-shell structures in contrast to the non-modified materials (i.e., only tungsten oxide or cerium oxide). This is attributed to the high density of oxygen vacancy defects at the shell, as well as the formation of heterojunctions at the core-shell interface, which provide the modified nanowires with 'extra' chemical and electronic sensitization as compared to the non-modified materials.

Site-Specific Growth and in Situ Integration of Different Nanowire Material Networks on a Single Chip: Toward a Nanowire-Based Electronic Nose for Gas Detection.

A new method for the site-selective synthesis of nanowires has been developed to enable material growth with defined morphology and, at the same time, different composition on the same chip surface. The chemical vapor deposition approach for the growth of these nanowire-based resistive devices using micromembranes can be easily modified and represents a simple, adjustable fabrication process for the direct integration of nanowire meshes in multifunctional devices. This proof-of-concept study includes the deposition of SnO2, WO3, and Ge nanowires on the same chip. The individual resistors exhibit adequate gas sensing responses toward changing gas concentrations of CO, NO2, and humidity diluted in synthetic air. The data have been processed by principal component analysis with cluster responses that can be easily separated, and thus, the devices described herein are in principle suitable for environmental monitoring.

Aerosol-assisted Chemical Vapor Deposition of Metal Oxide Structures: Zinc Oxide Rods.

Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, their high cost production and/or incompatibility with microfabrication technologies, due to the use of pre-deposited catalyst-seeds and/or high processing temperatures exceeding 900 °C, represent a drawback for a widespread use of these methods. Here, however, we report the synthesis of ZnO rods via a non-catalyzed vapor-solid mechanism enabled by using an aerosol-assisted chemical vapor deposition (CVD) method at 400 °C with zinc chloride (ZnCl2) as the precursor and ethanol as the carrier solvent. This method provides both single-step formation of ZnO rods and the possibility of their direct integration with various substrate types, including silicon, silicon-based micromachined platforms, quartz, or high heat resistant polymers. This potentially facilitates the use of this method at a large-scale, due to its compatibility with state-of-the-art microfabrication processes for device manufacture. This report also describes the properties of these structures (e.g., morphology, crystalline phase, optical band gap, chemical composition, electrical resistance) and validates its gas sensing functionality towards carbon monoxide.

ZnO Rods with Exposed {100} Facets Grown via a Self-Catalyzed Vapor-Solid Mechanism and Their Photocatalytic and Gas Sensing Properties.

We present a new method for vapor deposition of columnar ZnO structures in the form of rods on various substrates without the need for substrate modification with catalyst seed particles and at relatively low temperatures compared to other vapor deposition methods. These structures are used for the photodegradation of stearic acid (C18H36O2) and the photoactivated detection of gases such as carbon monoxide (CO), ethanol (C2H6O), toluene (C7H8), and nitrogen dioxide (NO2) at room temperature, showing improved selectivity compared to tests performed in themoactivated mode.

Nanoscale Heterostructures Based on Fe2O3@WO3-x Nanoneedles and Their Direct Integration into Flexible Transducing Platforms for Toluene Sensing.

Nanoscale heterostructures based on WO3-x nanoneedles functionalized with Fe2O3 nanoparticles are integrated directly into flexible polymer-based transducing platforms via aerosol-assisted chemical vapor deposition. Results demonstrate that the incorporation of Fe2O3 nanoparticles at the surface of WO3-x nanoneedles enhances the electronic and sensing properties of WO3-x, providing a 6-fold increase in sensitivity to toluene and low cross-sensitivity to hydrogen and ethanol. These enhanced-sensing properties are comparable to those obtained via functionalization with precious metal (Pt) nanoparticles, which are commonly used to enhance sensor performance.

Localized growth and in situ integration of nanowires for device applications.

Simultaneous localized growth and device integration of inorganic nanostructures on heated micromembranes is demonstrated for single crystalline germanium and tin oxide nanowires. Fully operating CO gas sensors prove the potential of the presented approach. With this simple CMOS compatible technique, issues of assembly, transfer and contact formation are addressed.