L. Lozzi

Sensors for sub-ppm NO2 gas detection based on carbon nanotube thin films

Carbon nanotubes (CNTs) deposited by plasma-enhanced chemical vapor deposition on Si3N4/Si substrates have been investigated as resistive gas sensors for NO2. Upon exposure to NO2, the electrical resistance of the CNTs was found to decrease. The maximum variation of resistance to NO2 was found at an operating temperature of around 165 °C. The sensor exhibited high sensitivity to NO2 gas at concentrations as low as 10 ppb, fast response time, and good selectivity. A thermal treatment method, based on repeated heating and cooling of the films, adjusted the resistance of the sensor film and optimized the sensor response to NO2.

NO2 and CO gas adsorption on carbon nanotubes: Experiment and theory

In this work a combined experimental and theoretical study on carbon nanotube (CNT) based system for gas sensing applications is reported. Carbon nanotubes thin films have been deposited by plasma-enhanced chemical vapor deposition on Si3N4/Si substrates provided with Pt electrodes. Microstructural features as determined by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy highlight the growth of defective tubular carbon structures. The electrical resistivity of the CNT film shows a semiconductinglike temperature dependence and a p-type response with decreasing electrical resistance upon exposure to NO2 gas (100 ppb). No response has been found by exposing the film to CO gas in the temperature range between 25 and 250 °C. In order to obtain a theoretical validation of the experimental results, the equilibrium position, charge transfer, and density of states are calculated from first principles for the CNT+CO and CNT+NO2 systems. Our spin-unrestricted density functional...

NO2 sensitivity of WO3 thin film obtained by high vacuum thermal evaporation

Abstract The gas sensitivity, selectivity and stability properties of WO 3 thin films for the detection of NO 2 gas in the concentration range 0.2–5 ppm, have been evaluated and discussed in the light of the preparation conditions and working temperature. Thin films were obtained by evaporating high purity WO 3 powder by an electrically heated crucible at about 5 × 10 −4 Pa on sapphire substrates provided with Pt interdigital type sputtered electrodes and annealed for 1 h at 400, 500 and 600°C. The film morphology, crystalline phase and chemical composition were characterised through AFM, low angle XRD and XPS. The electrical response was measured by means of DC current mode. The annealed films showed crystallographic orientation belonging to the triclinic structure of WO 3 , while the as-deposited films were found to be amorphous. The binding energies of O 1s and W 4f confirmed the existence of the WO 3 phase, with a stoichiometric ratio close to the theoretical one. All the films showed the highest sensitivity to NO 2 at a working temperature of 200°C. The 500°C annealed film was found to be the most sensitive to NO 2 gas, compared to those annealed at 400 and 600°C. No cross sensitivity effects were found by exposing the sensors to CO, CH 4 . WO 3 films showed strong sensitivity to C 2 H 5 OH and H 2 O. Long term stability test at a working temperature of 350°C, performed by cycling the films in dry air and 5 ppm NO 2 revealed no substantial change in the electrical properties in terms of drift and sensitivity.

NO2 gas sensitivity of carbon nanotubes obtained by plasma enhanced chemical vapor deposition

Carbon nanotubes (CNTs) thin films deposited by plasma enhanced chemical vapor deposition on Si/Si3N4 substrates provided with platinum interdigital electrodes have been investigated as resistive gas sensors towards NO2 oxidizing gas.The electrical response has been measured exposing the films to sub-ppm NO2 concentrations (10‐100 ppb in dry air) at different operating temperatures ranging between 25 and 215 8C. The response to NO2 has been found to be at maximum at around 165 8C. Upon exposure to NO2 the electrical resistance of randomly oriented CNTs is found to decrease. Gas sensitivity, response time and reproducibility of the electrical response resulted to be dependant from the preparation conditions and film thickness. The prepared films show fast dynamic of the electrical response and high reproducibility of the electrical properties. The resistance decrease of the CNTs when exposed to NO2 gas and the sensor response to concentrations as low as 10 ppb NO2, suggest the possibility to utilize CNTs as new sensors for air-quality monitoring. # 2003 Elsevier Science B.V. All rights reserved.

XPS studies on SiOx thin films

Abstract The surface stoichiometry of SiO x thin films ( x = 1–2) has been studied by means of X-ray photoelectron spectroscopy. The presence of three Si oxidation states (SiO 2 , SiO, Si 2 O 3 ) has been observed through an analysis of the Si2p line shape and the intensity variation of these different silicon oxide signals, as a function of the oxygen content, has been followed. The calculated stoichiometry has been compared with that obtained using the modified Auger parameter method. The good agreement between these results supports the validity of the modified Auger parameter as an easy and fast method to know the surface stoichiometry of SiO x films.

Cross sensitivity and stability of NO2 sensors from WO3 thin film

The H2O, C2H5OH, CO, CH4, NO and SO2 cross sensitivity to NO2 gas, as well as the long term stability of the electrical response of WO3 thin films have been evaluated and discussed in the light of different preparation conditions and working temperatures. Thin films have been obtained by evaporating high purity WO3 powder at 5 × 10−4 Pa on sapphire substrates provided with Pt interdigital sputtered electrodes and annealed at 500°C for 6, 12 and 24 h. The film morphology, crystalline phase and chemical composition have been characterized through AFM, glancing angle XRD and XPS. The as-deposited film is amorphous with WO3 stoichiometry on the surface, after annealing at 500°C the films are well crystallised but with preferential orientation of WO3 along the (200) plane. The increasing of the annealing time shows a positive effect on the crystallite and grain size of the film, while the mean roughness and surface area difference slightly decrease. The binding energies of the annealed films are close to that of WO3 and small downshifts from the characteristic binding energy of W 4f72 reflects the formation of oxygen vacancies on the longer time annealed films. All the films show the highest sensitivity to NO2 gas (0.7–5 ppm concentration range), at 250°C working temperature. At this temperature and 1.7 ppm NO2 the calculated sensitivities yield S = 12, S = 43 and S = 45 for 6, 12 and 24 h annealed films, respectively. No cross sensitivity has been found by exposing the WO3 films to CO and CH4. Negligible H2O cross to NO2 has resulted for the 24 h annealed film in the 40–80% relative humidity range, as well as to 300 ppm SO2 and 10 ppm NO. Only 1000 ppm C2H5OH has resulted in a significant cross to the NO2 measure. The increase in the annealing time had positive effects on the sensitivity, cross sensitivity and long term stability properties. The 45-fold increase in the resistance of the 24 h annealed on exposure to 1.7 ppm of NO2, as well as the good long term stability properties of its electrical response, suggest the possibility of utilising the sensor for air-quality monitoring.

Thin and ultra-thin films of nickel phthalocyanine grown on highly oriented pyrolitic graphite : an XPS, UHV-AFM and air tapping-mode AFM study

Abstract Thin and ultra-thin films of nickel phthalocyanine have been deposited in ultra-high vacuum on highly oriented pyrolitic graphite. The growth mode and the interaction with the substrate have been studied in situ by X-ray photoelectron spectroscopy, scanning tunneling microscopy and atomic force microscopy, and ex situ by means of tapping-mode atomic force microscopy. We present photoelectron spectroscopy measurements at various film thicknesses which show no detectable interaction of the adsorbed organic layers with the substrate, but, varying the deposited film thickness by in-situ annealing the substrate, we show desorption effects semiquantitatively described. The XPS spectrum of the C 1s multiplet structure of NiPC obtained using a monochromatized source is also presented. The scanning probe images presented address morphological issues like the growth mode at initial stages of deposition and at higher coverages. Moreover, upon annealing of the PC films, we show high-resolution measurements consistent with the low-size stable α crystalline phase of phthalocyanine molecules. The potential of tapping-mode atomic force microscopy in imaging soft adlayers deposited over soft substrates is addressed throughout the paper.

Microstructural effect on NO2 sensitivity of WO3 thin film gas sensors Part 1. Thin film devices, sensors and actuators

Abstract Microstructures of thermally evaporated WO3 thin films on sapphire substrates are investigated by wide-angle X-ray diffraction, atomic force microscope and X-ray photoelectron spectrum. The as-deposited film is amorphous and crack-free with WO3 stoichiometry on the surface. After annealing at above 400 °C, the film is crystallized. Compared to the monoclinic phase of the starting WO3 powder, fewer peaks are evident at room temperature for the annealed film. This highlights that the film grown on the sapphire has a preferential orientation of WO3 (200), probably because of the atomic arrangement similar to the sapphire. The crystallite sizes are estimated from the major peak to be 17.5–23.9 nm according to the Scherrer equation. The increasing of annealing temperature exhibits positive effects on the surface roughness or fractal dimension, surface area and grain size of the film. However, abnormal increments in the topographical parameters occur in case of annealing at 600 °C. The binding energy of the annealed film is close to that of WO3, and a small downshift of 0.1 eV reflecs the formation of oxygen vacancies on the surface. The heterogeneity parameter of the film is introduced into the Schottky barrier equation. The highest sensitivity of the 500 °C-annealed sensor is explained in terms of the annealing temperature effect on the geometrical and chemical heterogeneities.

Ozone adsorption on carbon nanotubes: The role of Stone–Wales defects

First-principles calculations within the density functional theory have been performed in order to investigate ozone adsorption on carbon nanotubes. Particular emphasis is placed on the effects of Stone-Wales-like defects on the structural and electronic properties of (i) ideal tubes and (ii) tubes in the presence of ozone. Our results show that structural deformations induced on the pure carbon nanotubes by Stone-Wales defects are similar, as expected, to those induced on graphite; for the (10,0) tube, the semiconducting character is kept, though with a small reduction of the band gap. As for the ozone adsorption, the process on ideal nanotubes is most likely physisorption, though slightly stronger if compared to other previously studied molecules and consistent with the strong oxydizing nature of O(3). However, when ozone adsorbs on Stone-Wales defects, a strong chemisorption occurs, leading to relevant structural relaxations and to the formation of a CO covalent bond; this is consistent with experimental observations of CO functional groups, as well as of the liberation of CO gas phase and of the formation of C vacancies, thus explaining the consumption of the nanotube film upon ozone exposure.

Structural characterization of bulk ZnWO4 prepared by solid state method

Uniform crystals of ZnWO4 have been synthesised from the equimolar mixtures of ZnO and WO3 by conventional solid state method. For the first time the sample has been characterised detailedly to confirm the formation of pure single phase of perovskite ZnWO4. The formation of ZnWO4 has been confirmed by sintering the mixtures of ZnO and WO3 at two different temperatures one at 900 °C and other at 1000 °C. It is observed that the sample sintered at 1000 °C for 24 h shows complete formation of the single phase of ZnWO4. The crystallinity and the phase formation has been confirmed by X-ray diffraction technique. X-ray photoelectron spectroscopy measurements have been carried out for the bulk ZnWO4 sintered at 1000 °C for 24 h, showing 16% of Zn, 16% of W and 68% of O indicating stoichiometric ZnWO4. Surface morphology studies by scanning electron microscopy showed uniform crystals of ZnWO4. The purity of the compound has also been checked in depth by Energy Dispersive X-ray method indicating the absence of foreign ions apart from that, the ratio of Zn : W has been calculated and found to be 1 : 1 confirming the stoichiometric ZnWO4 inside the crystals.