Antonio Tricoli

Semiconductor Gas Sensors: Dry Synthesis and Application

Since the development of the first chemoresistive metal oxide based gas sensors, transducers with innovative properties have been prepared by a variety of wet- and dry-deposition methods. Among these, direct assembly of nanostructured films from the gas phase promises simple fabrication and control and with the appropriate synthesis and deposition methods nm to μm thick films, can be prepared. Dense structures are achieved by tuning chemical or vapor deposition methods whereas particulate films are obtained by deposition of airborne, mono- or polydisperse, aggregated or agglomerated nanoparticles. Innovative materials in non-equilibrium or sub-stoichiometric states are captured by rapid cooling during their synthesis. This Review presents some of the most common chemical and vapor-deposition methods for the synthesis of semiconductor metal oxide based detectors for chemical gas sensors. In addition, the synthesis of highly porous films by novel aerosol methods is discussed. A direct comparison of structural and chemical properties with sensing performance is given.

Si:WO3 Sensors for Highly Selective Detection of Acetone for Easy Diagnosis of Diabetes by Breath Analysis

Acetone in the human breath is an important marker for noninvasive diagnosis of diabetes. Here, novel chemo-resistive detectors have been developed that allow rapid measurement of ultralow acetone concentrations (down to 20 ppb) with high signal-to-noise ratio in ideal (dry air) and realistic (up to 90% RH) conditions. The detector films consist of (highly sensitive) pure and Si-doped WO(3) nanoparticles (10-13 nm in diameter) made in the gas phase and directly deposited onto interdigitated electrodes. Their sensing properties (selectivity, limit of detection, response, and recovery times) have been investigated as a function of operating temperature (325-500 degrees C), relative humidity (RH), and interfering analyte (ethanol or water vapor) concentration. It was found that Si-doping increases and stabilizes the acetone-selective epsilon-WO(3) phase while increasing its thermal stability and, thus, results in superior sensing performance with an optimum at about 10 mol % Si content. Furthermore, increasing the operation temperature decreased the detector response to water vapor, and above 400 degrees C, it was (<or=0.7) always below the threshold (10.6) for fake diabetes detection in ideal conditions. At this temperature and at 90% RH, healthy humans (<or=900 ppb acetone) and diabetes patients (>or=1800 ppb) can be clearly distinguished by a remarkable gap (40%) in sensor response. As a result, these solid state detectors may offer a portable and cost-effective alternative to more bulky systems for noninvasive diabetes detection by human breath analysis.

Anti-Fogging Nanofibrous SiO2 and Nanostructured SiO2-TiO2 Films Made by Rapid Flame Deposition and In Situ Annealing

Transparent, pure SiO(2), TiO(2), and mixed silica-titania films were (stochastically) deposited directly onto glass substrates by flame spray pyrolysis of organometallic solutions (hexamethyldisiloxane or tetraethyl orthosilicate and/or titanium tetra isopropoxide in xylene) and stabilized by in situ flame annealing. Silicon dioxide films consisted of a network of interwoven nanofibers or nanowires several hundred nm long and 10-15 nm thick, as determined by microscopy. These nanowire or nanofibrous films were formed by chemical vapor deposition (surface growth) on bare glass substrates during scalable combustion of precursor solutions at ambient conditions, for the first time to our knowledge, as determined by thermophoretic sampling of the flame aerosol and microscopy. In contrast, titanium dioxide films consisted of nanoparticles 3-5 nm in diameter that were formed in the flame and deposited onto the glass substrate, resulting in highly porous, lace-like nanostructures. Mixed SiO(2)-TiO(2) films (40 mol % SiO(2)) had similar morphology to pure TiO(2) films. Under normal solar radiation, all such films having a minimal thickness of about 300 nm completely prevented fogging of the glass substrates. These anti-fogging properties were attributed to inhibition of water droplet formation by such super-hydrophilic coatings as determined by wetting angle measurements. Deactivated (without UV radiation) pure TiO(2) coatings lost their super-hydrophilicity and anti-fogging properties even though their wetting angle was reduced by their nanowicking. In contrast, SiO(2)-TiO(2) coatings exhibited the best anti-fogging performance at all conditions taking advantage of the high surface coverage by TiO(2) nanoparticles and the super-hydrophilic properties of SiO(2) on their surface.

Breath acetone monitoring by portable Si:WO3 gas sensors

Breath analysis has the potential for early stage detection and monitoring of illnesses to drastically reduce the corresponding medical diagnostic costs and improve the quality of life of patients suffering from chronic illnesses. In particular, the detection of acetone in the human breath is promising for non-invasive diagnosis and painless monitoring of diabetes (no finger pricking). Here, a portable acetone sensor consisting of flame-deposited and in situ annealed, Si-doped epsilon-WO(3) nanostructured films was developed. The chamber volume was miniaturized while reaction-limited and transport-limited gas flow rates were identified and sensing temperatures were optimized resulting in a low detection limit of acetone (∼20ppb) with short response (10-15s) and recovery times (35-70s). Furthermore, the sensor signal (response) was robust against variations of the exhaled breath flow rate facilitating application of these sensors at realistic relative humidities (80-90%) as in the human breath. The acetone content in the breath of test persons was monitored continuously and compared to that of state-of-the-art proton transfer reaction mass spectrometry (PTR-MS). Such portable devices can accurately track breath acetone concentration to become an alternative to more elaborate breath analysis techniques.

Ultraporous Electron-Depleted ZnO Nanoparticle Networks for Highly Sensitive Portable Visible-Blind UV Photodetectors

A hierarchical nano- and microstructured morphology for visible-blind UV photo-detectors is developed, which provides record-high milliampere photocurrents, nanoampere dark currents, and excellent selectivity to ultralow UV light intensities. This is a significant step toward the integration of high-performance UV photodetectors in wearable devices.

Dispersed nanoelectrode devices

The enhanced performance and reduced scale that nanoparticles can bring to a device are frequently compromised by the poor electrical conductivity of nanoparticle structures or assemblies. Here, we demonstrate a unique nanoscale electrode assembly in which conduction is carried out by one set of nanoparticles, and other device functions by another set. Using a scalable process, nanoparticles with tailored conductivity are stochastically deposited above or below a functional nanoparticle film, and serve as extensions of the bulk electrodes, greatly reducing the total film resistance. We apply this approach to solid-state gas sensors and achieve controlled device resistance with an exceptionally high sensitivity to ethanol of 20 ppb. This approach can be extended to other classes of devices such as actuators, batteries, and fuel and solar cells.

Minimal cross-sensitivity to humidity during ethanol detection by SnO2-TiO2 solid solutions.

A nanocomposite material is presented that optimally combines the excellent gas sensitivity of SnO2 and the selectivity of TiO2. Nanostructured, rutile titanium-tin oxide solid solutions up to 81.5% Ti, as determined by x-ray diffraction, are made by scalable spray combustion (flame spray pyrolysis) of organometallic precursor solutions, directly deposited and in situ annealed onto sensing electrodes in one step. Above that content, segregation of anatase TiO2 takes place. It was discovered that at low titanium contents (less than 5 Ti%), these materials exhibit higher sensitivity to ethanol vapor than pure SnO2 and, in particular, limited cross-sensitivity to relative humidity, a long standing challenge for metal oxide gas sensors. These solid solutions are aggregated nanoparticles with an enhanced presence of Ti on their surface as indicated by Raman and IR-spectroscopy. The presence of such low Ti-content in the SnO2 lattice drastically reduces the band gap of these solid solutions, as determined by UV-vis absorption, almost to that of pure TiO2. Furthermore, titania reduces the number of rooted and terminal OH species (that are correlated to the cross-sensitivity of tin oxide to water) on the particle surface as determined by IR-spectroscopy. The present material represents a new class of sensors where detection of gases and organic vapors can be accomplished without pre-treatment of the gas mixture, avoiding other semiconducting components that require more heating power and that add bulkiness to a sensing device. This is attractive in developing miniaturized sensors especially for microelectronics and medical diagnostics.

Toward portable breath acetone analysis for diabetes detection

Diabetes is a lifelong condition that may cause death and seriously affects the quality of life of a rapidly growing number of individuals. Acetone is a selective breath marker for diabetes that may contribute to the monitoring of related metabolic disorder and thus simplify the management of this illness. Here, the overall performance of Si-doped WO(3) nanoparticles, made by flame spray pyrolysis, as portable acetone detectors is critically reviewed focusing on the requirements for medical diagnostics. The effect of flow rate, chamber volume and acetone dissociation within the measuring chamber is discussed with respect to the calibration of the sensor response. The challenges for the fabrication of portable breath acetone sensors based on chemo-resistive detectors are underlined indicating possible solutions and novel research directions.

Ultra-Durable and Transparent Self-Cleaning Surfaces by Large-Scale Self-Assembly of Hierarchical Interpenetrated Polymer Networks

In nature, durable self-cleaning surfaces such as the Lotus leaf rely on the multiscale architecture and cohesive regenerative properties of organic tissue. Real-world impact of synthetic replicas has been limited by the poor mechanical and chemical stability of the ultrafine hierarchical textures required for attaining a highly dewetting superhydrophobic state. Here, we present the low-cost synthesis of large-scale ultradurable superhydrophobic coatings by rapid template-free micronano texturing of interpenetrated polymer networks (IPNs). A highly transparent texture of soft yielding marshmallow-like pillars with an ultralow surface energy is obtained by sequential spraying of a novel polyurethane-acrylic colloidal suspension and a superhydrophobic nanoparticle solution. The resulting coatings demonstrate outstanding antiabrasion resistance, maintaining superhydrophobic water contact angles and a pristine lotus effect with sliding angles of below 10° for up to 120 continuous abrasion cycles. Furthermore, they also have excellent chemical- and photostability, preserving the initial performance upon more than 50 h exposure to intense UVC light (254 nm, 3.3 mW cm(-2)), 24 h of oil contamination, and highly acidic conditions (1 M HCl). This sprayable polyurethane-acrylic colloidal suspension and surface texture provide a rapid and low-cost approach for the substrate-independent fabrication of ultradurable transparent self-cleaning surfaces with superior abrasion, chemical, and UV-resistance.

Wafer-level flame-spray-pyrolysis deposition of gas-sensitive layers on microsensors

This paper presents a CMOS-compatible wafer-level fabrication process for monolithic CMOS/MEMS sensor systems coated with sensitive layers directly deposited by means of flame spray pyrolysis (FSP). Microhotplate (?HP)-based devices, featuring an FSP directly deposited SnO2/Pt layer, have successfully been realized on a wafer level. The thermal characterization evidenced a thermal resistance of 10.6 ?C mW?1; moreover, gas test measurements with ethanol have been performed. Microhotplate membrane deformations during device operation have been investigated and have been reduced by adjustment of the intrinsic stress of a deposited silicon nitride layer.