No abstract

To meet requirements in air quality monitoring, sensors are required that can measure the concentration of gaseous pollutants at concentrations down to the ppb and ppt levels, while at the same time they exhibiting high sensitivity, selectivity, and short response/recovery times. Among the different sensor types, those employing metal oxide semiconductors (MOSs) offer great promises as they can be manufactured in easy/inexpensive ways, and designed to measure the concentration of  a wide range of target gases. MOS sensors rely on the adsorption of target gas molecules on the surface of the sensing material and the consequent capturing of electrons from the conduction band that in turn affects their conductivity. Despite their simplicity and ease of manufacturing, MOS gas sensors are restricted by high limits of detection (LOD; which are typically in the ppm range) as well as poor sensitivity and selectivity. LOD and sensitivity can in principle be addressed by nanostructuring the MOSs, thereby increasing their porosity and surface-to-volume ratio, whereas selectivity can be tailored through their chemical composition. In this paper we provide a critical review of the available techniques for nanostructuring MOSs using chemiresistive materials, and discuss how these can be used to attribute desired properties to the end gas sensors. We start by describing the operating principles of chemiresistive sensors, and key material properties that define their performance. The main part of the paper focuses on the available methods for synthesizing nanostructured MOSs for use in gas sensors. We close by addressing the current needs and provide perspectives for improving sensor performance in ways that can fulfill requirements for air quality monitoring. Graphical abstract

Tungsten oxides (WOx) thin films are currently used in electro-chromic devices, solar-cells and gas sensors as a result of their versatile and unique characteristics. In this study, we produce nanoparticulate WOx films by spark ablation and focused inertial deposition, and demonstrate their application for NO2 sensing. The primary particles in the as-deposited film samples are amorphous with sizes ranging from 10 to 15 nm. To crystallize the samples, the as-deposited films are annealed at 500 °C in air. This also caused the primary particles to grow to 30-50 nm by sintering. The morphologies and crystal structures of the resulting materials are studied using scanning and transmission electron microscopy and X-ray diffraction, whereas information on composition and oxidation states are determined by X-ray photoemission spectroscopy. The observed sensitivity of the resistance of the annealed films is ∼100 when exposed to 1 ppm of NO2 in air at 200 °C, which provides a considerable margin for employing them in gas sensors for measuring even lower concentrations. The films show a stable and repeatable response pattern. Considering the numerous advantages of spark ablation for fabricating nanoparticulate thin films, the results reported here provide a promising first step toward the production of high sensitivity and high accuracy sensors.

To match the properties of a semiconductor to a desired application, it is crucial to control its crystal structure. Here, we show that, by implementing a corona discharge in a chemical vapor deposition process, the growth morphology of gallium oxide can be adjusted to produce nanowires and layerlike and columnar crystal structures. The three morphologies can be explained by the transition from a classic chemical vapor process to a corona-assisted chemical vapor process with directed transport. Specifically, the excitation of the carrier gas by the corona discharge is exploited as a switch to transition from a vapor–liquid–solid nanowire growth mechanism to a layer by layer growth mechanism. The second switching parameter is the substrate bias which affects the directed precursor transport and enhances the growth rate. While the switching of the morphology has no effect on the crystal phase, the photoluminescence properties of the three morphologies are different. The grown structures are used as optical gas sensors at room temperature. Tin-doped gallium oxide is shown to be capable of detecting trace amounts (200 ppm) of alcohols such as acetone, ethanol, and isopropanol. The layerlike morphology provides the highest response.

We demonstrate the realization of core−shell transformation-imprinted solder bumps to enable low-temperature chip assembly, while providing a route to high-temperature interconnects through transformation. The reported core−shell solder bump uses a lower melting point BiIn-based shell and a higher melting point Sn core in the initial stage. The bumps enable fluidic self-assembly and selfalignment at relatively low temperatures (60−80 °C). The bumps use the high surface free energy of the liquid shell during the self-assembly to capture freely suspended Si dies inside a heated (80 °C) water bath, leading to well-ordered defect-free chip arrays; the molten liquid shell wets the metal contact (binding site) on the chips and yields self-aligned and electrically connected devices. The solid core provides the anchor point to the substrate. After the completion of the assembly, a short reflow raises the melting point, yielding a solid electrical connection. The low melting point liquid diffuses into the high melting point core. The tuning of the material ratios leads to tailored transformation-imprinted solders with high melting points (160−206 °C) in the final structure. KEYWORDS: core−shell transformation-imprinted solder bumps, low-temperature fluidic self-assembly, high melting point interconnects, Pb-free solder bumps, self-alignment