Hybrid porous silicon, consisting of a microporous framework whose walls are covered with a thin nanoporous layer, proves to be an inexpensive and robust platform for fabrication of rapid, reversible, and sensitive semiconductor sensors. An extremely high surface area provides a mechanism for the detection of ppm and ppb levels of a range of gases including ammonia, NOx, and CO. A general method of coating electroless metal onto the surface provides a basis for nanostructure induced selectivity between these gases. Further, the introduction of FFT analysis to the rapidly reversible and linear response of the porous silicon gas sensor allows the gas response to be acquired and filtered on a drifting baseline.

Tin oxide nanoparticles for gas sensing application have been synthesized with an aerosol method. The particles were manufactured with the versatile Flame spray Pyrolysis (FSP) method producing highly crystalline powders with closely controlled a primary particle and crystallite size of 10 nm and 17 nm. The single crystalline particles were only slightly aggregated and directly used for thick film sensor deposition by drop coating and screen printing.The flame made SnO2 nanoparticles showed high and rapidresponse to reducing gases such as propanal and CO.

The vertically aligned carbon nanotubes (CNTs) deposited by microwave plasma-enhanced chemical vapor deposition (MPCVD) were utilized as resistive gas sensors.

The carbon nanotubes were annealed between 200 to 800°C under N2 flow (500 sccm) for 15 minute, respectively. After that, the carbon nanotubes were exposed to an N2 filling and pumping environment. Upon exposure to N2 the electrical resistance of vertically aligned carbon nanotubes was found to increase. It was found that the N2 absorption of unannealed carbon nanotubes was reversible, whereas which of annealing ones was not. However, the sensitivity of the N2 absorption on carbon nanotubes was improved after annealing. From the Raman spectra, the ID/IG ratio of carbon nanotubes also decreased after annealing, indicating that more graphenes were formed by the annealing process. Furthermore, from X-ray photoelectron spectroscopy (XPS), it was observed that the ratio of the oxygen to carbon (O/C) signal intensity increased from 0.094 to 3.943 as the annealing temperature increased. As a consequence, it was suggested that the surface of carbon nanotubes was oxygenated and the absorption of N2 changed from physisorption to chemisorption.

We have demonstrated a template-free, site-specific, and scalable electrochemical method for the fabrication of individually addressable conducting polymer nanoframework electrode junctions in a parallel-oriented array. These conducting polymer nanoframeworks, which are composed of numerous intercrossing conducting polymer nanowires that have uniform diameters ranging from 40 to 150 nm, can be used for the chemical sensing of HCl and NH3 gases and organic vapors and for sensing pH values of aqueous solutions.

We present the gas sensing properties of InOx thin films deposited by dc sputtering and ZnOx thin films deposited both by dc sputtering and PLD. The sensitivity of the films towards ozone is correlated with the deposition parameters like film thickness, substrate deposition temperature and growth rate. Secondary Ion Mass Spectrometry (SIMS) analysis showed a noticeable deficit in oxygen in the top 5 nm for films in the “conducting” state, i.e., after UV exposure. Analysis of the sensing response for alumina-based transducers of InOx thin films revealed high sensitivity (less than 25 ppb) with fast and stable response towards ozone while ultimate sensitivity levels down to 10 ppb for ozone and 50 ppb for NO2 were achieved. Surface topography study of ZnOx films utilizing optical, AFM and SEM analyses has shown a distinct surface morphology variation correlated to the growth technique It is demonstrated that PLD leads to very rough surfaces with characteristic non-coordinated columnar features in contrast with the rather smooth surfaces obtained by sputtering.

Sr(Tix, Fe1-x)O3-δ solid solutions were found to change their temperature coefficient of resistance (TCR) from negative to positive as iron increasingly substitutes for titanium, with the TCR tending towards zero at × = 0.35. This composition, Sr(Ti0.65Fe0.35)O3-δ thus shows temperature independent characteristic.

For the development of a planar type sensing element for automotive applications, Sr(Ti, Fe)O3-δ has to be applied as a thick film. To confirm the sensor characteristic temperature independence (at T = 750…950 °C, p O2 = 10−5…1 bar) and fast response times (t90 = 6.5 ms at 900 °C), both key issues of Sr(Ti, Fe)O3-δ, thick film sensors have to be maintained over the entire lifetime. In this work, the structural and electrical properties of the sensor are investigated with regard to the chemical stability of the sensing element.

We report direct, real-time electrical detection of single virus particles with high selectivity using nanowire field effect transistors. Measurements made with nanowire arrays modified with antibodies for influenza A showed discrete conductance changes characteristic of binding and unbinding in the presence of influenza A but not paramyxovirus or adenovirus. Moreover, simultaneous electrical and optical measurements using fluorescently-labelled influenza A demonstrate conclusively that the conductance changes correspond to binding/unbinding of single viruses at the surface of nanowire devices. In addition, studies of nanowire devices modified with antibodies specific for either influenza or adenovirus show that multiple viruses can be selectively detected in parallel. The possibility of large scale integration of these nanowire devices suggests potential for simultaneous detection of a large number of distinct viral threats at the single virus level.

Sol-gel processes were developed to prepare nano porous silica materials. The obtained porous sol-gel silica (PSGS) materials have been used as constituent materials in designing optical fiber chemical sensors. A PSGS membrane coated on the surface of an optical fiber was used as a transducer for sensing humidity level in air. A PSGS membrane doped with an ammonia indicator dye has been coated on an optical fiber to sense ammonia in air. Both of the coating based sensors are reversible and fast response. In the tested range, relative humidity (RH) in air down to 3% can be detected with the PSGS coated fiber optic sensor. The fiber optic ammonia sensor with ammonia indicator doped PSGS coating can be used to sense ammonia in air down to sub-ppm level. PSGS has also been used as a constituent material in preparing porous silica optical fibers. The obtained porous optical fibers have been used to design optical fiber chemical sensors for sensing humidity, ammonia and volatile organic compounds. A CuCl2 doped PSGS fiber has been tested for sensing ammonia in a high temperature gas sample. Ammonia in the high temperature air gas diffuses into the PSGS fiber, reversibly reacts with CuCl2 doped in the PSGS fiber to form a complex. The formed complex was detected with fiber optic spectrometric method. This sensor can detect ammonia in a high temperature (450 °C) air gas stream down 0.3 ppm. Techniques of preparing PSGS, coating PSGS on an optical fiber, making a porous optical fiber with PSGS as a constituent material will be presented. Examples of optical fiber sensors using PSGS coatings and a PSGS fiber as transducers for gas sensing are presented.

The convergence of materials science, printing, and electronics promises to offer low cost and high volume production of devices such as transistors, RFID tags, wearable electronics and other novel applications. Although a number of “soft lithographic” techniques have been used to make these devices, they are slow and have a limited production volume [5], [14-15].

Here high volume printing processes like rotary letterpress, flexography and offset lithography have been investigated for patterning conductive materials [1]. The synthesis and development of conducting inks using electrically functional polymers has been studied. The feasibility of using such inks in high volume printing processes has been studied. An attempt has been made to print conductive interdigitated electrodes using these inks to obtain uniform coating properties and appropriate electrical characteristics. Various process parameters like type of substrate, inking time and speed, printing pressure, printing force and ink formulation have been investigated.

The reactivity of nanosized Ru(Pd,Pt)-doped SnO2, obtained by sol-gel synthesis, towards NO/Ar was investigated by Electron Paramagnetic Resonance (EPR), Mössbauer and Electrical Resistance measurements. A sensing mechanism was proposed that involves (i) the formation of bielectronic oxygen vacancies VO, (ii) their single-ionisation to VO•, which injects electrons to SnO2 conduction band, (iii) the transfer of VO• electrons to the transition metal centers reducing them to lower oxidation states. It was suggested that the electronic exchange between oxide and transition metal is responsible for the enhancement of the reactivity in doped SnO2 with respect to the undoped material.

A new approach is discussed for the rational synthesis and development of optimized multifunctional solid-state gas sensors. Multifunctionality—the incorporation of multiple types of reactivities into a material, such as acid and/or base functionalities, oxidation and/or reduction functionalities, etc.—isa requirement in many gas sensing applications. The front end of many gas sensors contains catalytic layers, so that optimization of catalysts and optimization of gas sensors can be carried out in a synergistic fashion.

Multifunctionality presents unique challenges to rational catalyst and sensor systems development because the overall performance of the material is a convolution of the performance of the various subcomponents, and optimization of these individual subcomponents in isolation does not necessarily lead to optimal, or even acceptable, overall performance. A major obstacle to dealing with these difficulties is the inherent complexity of heterogeneous systems prepared by traditional approaches, which makes it difficult to unambiguously identify the compositions and morphologies of the local active sites and their interactions. Further complicating the problem is the requirement to function in environments that can vary on both short and long time scales. A key to understanding, controlling, and optimizing these materials is the ability to produce and study well-defined sensor materials with well-defined composition and morphology, with the flexibility to vary the composition easily without jeopardizing the structural uniformity.

The development of new or improved materials for gas sensor applications requires a search for novel and innovative approaches to the nano-scale design of these materials. The use of the technology of surface modification by successive ionic layer deposition (SILD) method is such an innovative approach that will be discussed in this paper.

Over the past decade, MEMS microhotplate devices have been developed at the National Institute of Standards and Technology to support semiconductor metal oxide films for use in conductometric gas sensor arrays. In most cases, the materials have been based on compact thin films of SnO2 or TiO2 deposited by single-source precursor chemical vapor deposition. Of particular interest to our group is the enhancement of the sensitivity of the microsensors to trace gas species by inducing nanostructured porosity and large internal surface areas in the films. In this presentation, we discuss the development of nanostructured sensor materials based on porous TiO2 nanoparticle thin films. The preparation and evaluation of pure and Nb-doped TiO2 nanoparticle films are described. The films on the MEMS microhotplate substrates are evaluated as conductometric gas sensors based on the critical performance elements of sensitivity, stability, speed and selectivity. The sensor performance, and specifically the sensitivity, of the novel nanoparticle TiO2 films is compared with that of traditional compact CVD-derived films.

This paper reports the preliminary results of an investigation of the synthesis of monolayer-capped GaAs nanoparticles using different surface capping molecules. Our approach focuses on the surface encapsulation using alkanethiolates. The organic shell can effectively block the aggregation during nanoparticle synthesis, providing molecular-level control of the core-shell structure. The results have demonstrated the effect of surface alkanethiolate modification on the interparticle spatial properties and particle sizes, which upon further refinement could lead to the ability in controlling the size of GaAs nanoparticles.

High-density and high-quality single walled carbon nanotubes are directly grown on surface by Chemical Vapor Depositions. We found that those SWNT thin films could be used to fabricate thin film field-effect transistors. Those transistors are very sensitive to surface charges changes in aqueous solutions and could be used tobuild chemical sensors.

By alternating the gate voltage polarity of a pentacene thin film transistor, we show that the drain current is stabilized and thus the bias stress effect is overcome. This allows for controlled testing of the device sensitivity to environmental conditions. We find that the conductivity of the device decreases on the time scale of seconds when the device is exposed to water vapor, which is manifested through a decrease in mobility and a shift in the threshold voltage. Simple recombination modeling suggests that trapping is the responsible mechanism. However, the effects of water vapor can be reversed by exposing the device to dry nitrogen flow. The time scale for recovery is on the order of 10s of minutes.

An MIS Hydrogen sensor with a Pd0.96Cr0.04/AlN/Si structure was fabricated, exhibiting the dynamic range considerably wider than that of analogous devices with pure Pd gates. A useful response could be obtained for Hydrogen concentrations as large as 50, 000 ppm. Although the response amplitude was much reduced at the lower concentrations, satisfactory signal to noise down to 50 ppm could be obtained. The saturating magnitude of the electrical response is in the range of 0.1 to 0.5 V, which is the same as that for the pure Pd gated devices, inspite of the 3 orders of magnitude difference in the saturation hydrogen concentration. This result will be discussed in terms of the response mechanism of these devices.

This paper presents result of investigation of the oxygen interaction with a surface of semiconductor metal oxide film. The idea to carry out such research is connected with the key role, which oxygen plays in the processes of impurity gas molecules detection in atmosphere and semiconductor surface sensibilization. Numerical modeling of the interaction was carried out for different temperatures and oxygen partial pressures. Results of modeling are compared with the results of experimental study of oxygen-semiconductor film system.

Using a novel low-temperature process, we demonstrate the facile integration of crack-free nanostructured titania (NST) as sensing elements in microsystems. Unlike conventional sol-gel methods, NST layers of interconnected nano-walls and nano-wires were formed by reacting Ti surfaces with aqueous hydrogen peroxide solution. Cracks were observed in NST layers formed on blanket Ti films but absent on arrays of patterned Ti pads below a threshold dimension. Analyses using TEM, high resolution SEM, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) reveal that NST consists of anatase TiO2 nano-crystals. NST pads were found able to detect oxygen gas of a few ppm. NST pad arrays were integrated on rigid and flexible substrates with potential applications in low cost and wearable sensing systems.

Several concepts can be applied for gas detection using solid ionic conductors. The fundamental characteristics and the essential variables for proper design of electrochemical cells are analyzed in terms of the sensing principle applied. The principal properties for materials as components of electrochemical devices are being illustrated. Employment of dynamic method allows implementation of the Fourier coefficients to generate a complex plane plot representation. This can be particularly useful for extracting information regarding the selectivity of an electrochemical cell to multiple gas species. Moreover, modeling the system response upon a periodical perturbation can be beneficial on understanding the individual processes and optimizing the overall performance.

Pure nanophased TiO2 with controlled microstructure and resistant to thermal induced grain growth has been prepared by hydrothermal treatments. The synthesized nano-TiO2 presents small size and well defined and faceted surface, as shown by Raman spectroscopy, XRD, EELS and HRTEM. Such performances are slightly changed with the posterior temperature treatments up to 700 °C, maintaining anatase phase structure and grain size about 20nm. The extent of stabilization depended on the pH of the treatment, being pH 2 more convenient for stabilizing the size, and pH 3 for the phase. HRTEM and EELS measurements showed the coexistence of rutile big particles (∼100 nm) with anatase small particles (∼40 nm) at pH 3 and calcination at 900°C. Thick-films of precipitated TiO2 and hydrothermally treated TiO2 were tested for the CO and the ethanol response. The hydrothermal treatment allowed obtaining stable sensitive films which exhibited enlarged sensor response and improved transients, specially in the case of the materials treated at pH 3.