Albert Cornet

Effects of Nb doping on the TiO2 anatase-to-rutile phase transition

We study the influence of Nb doping on the TiO2 anatase-to-rutile phase transition, using combined transmission electron microscopy, Raman spectroscopy, x-ray diffraction and selected area electron diffraction analysis. This approach enabled anatase-to-rutile phase transition hindering to be clearly observed for low Nb-doped TiO2 samples. Moreover, there was clear grain growth inhibition in the samples containing Nb. The use of high resolution transmission electron microscopy with our samples provides an innovative perspective compared with previous research on this issue. Our analysis shows that niobium is segregated from the anatase structure before and during the phase transformation, leading to the formation of NbO nanoclusters on the surface of the TiO2 rutile nanoparticles.

Cr-doped TiO2 gas sensor for exhaust NO2 monitoring

Abstract A set of Cr-highly doped TiO 2 samples with Cr contents ranging from 5 to 30xa0at.% were prepared in a sol–gel route and calcined at a temperature between 600 and 900xa0°C. X-ray diffraction (XRD) analyses revealed the persistence of anatase phase up to the calcination temperature of 700xa0°C in all samples, above which rutile phase became dominant. The segregation of Cr 2 O 3 remained modest, only detectable by surface-sensitive technique like X-ray photoelectron spectra (XPS), for the 5 and 10xa0at.% Cr-doped samples calcined at 600 or 700xa0°C, suggesting incorporation of major part of doped Cr in the lattice of TiO 2 . Higher calcination temperatures or higher Cr contents lead to marked segregation of Cr 2 O 3 . XPS spectra in the valence band region of the samples calcined at 600xa0°C revealed a shift of the binding energy (BE) at the band edge to the lower energy side with increasing Cr contents, suggesting a tendency for the electronic conduction to alter from n- to p-type. As tested preliminarily, the thick and thin film devices prepared with these samples exhibited p-type conduction, and, particularly, a thin film device using 10xa0at.% Cr-doped sample calcined at 600xa0°C proved promising performances in the detection of dilute NO 2 in air at 500xa0°C.

Use of zeolite films to improve the selectivity of reactive gas sensors

Abstract Semiconductor (Pd-doped SnO2) gas sensors covered with zeolitic films (MFI or LTA) have been developed and used for gas phase sensing of different species (methane, propane, and ethanol) at different humidity levels. The dynamic responses obtained with these sensors were compared with the response of a reference sensor without a zeolitic layer. The results clearly indicate that a suitable zeolite layer strongly reduces, and in some cases suppresses, the response of the sensor to paraffins, thereby increasing the sensor selectivity to the alcohol, while the reference sensor could not discriminate between these molecules. This clearly shows the potential of zeolite-based sensors to achieve a higher selectivity/sensitivity in gas sensing applications.

Structural and gas-sensing properties of WO/sub 3/ nanocrystalline powders obtained by a sol-gel method from tungstic acid

WO/sub 3/ nanocrystalline powders were obtained from tungstic acid following a sol-gel process. Evolution of structural properties with annealing temperature was studied by X-ray diffraction and Raman spectroscopy. These structural properties were compared with those of WO/sub 3/ nanopowders obtained by the most common process of pyrolysis of ammonium paratungstate, usually used in gas sensors applications. Sol-gel WO/sub 3/ showed a high sensor response to NO/sub 2/ and low response to CO and CH/sub 4/. The response of these sensor devices was compared with that of WO/sub 3/ obtained from pyrolysis, showing the latter a worse sensor response to NO/sub 2/. Influence of operating temperature, humidity, and film thickness on NO/sub 2/ detection was studied in order to improve the sensing conditions to this gas.

Flexible sensor based on carbon nanofibers with multifunctional sensing features

Herein, we present the fabrication and characterization of a flexible gas sensor based on carbon nanofibers. The sensing device is composed of interdigitated silver electrodes deposited by inkjet printing on Kapton substrates, subsequently coated with carbon nanofibers as sensing element. Gas sensing response to CO, NH3 and humidity has been characterized in detail. Thermal, mechanical and electromagnetic radiation effects have also been studied and discussed from the point of view of the cross-sensitivity. The obtained results open the door for a new generation of flexible sensors with multifunctional sensing features, which are producible with scalable techniques based on low cost nanomaterials.

Nanosized Nb-TiO/sub 2/ gas sensors derived from alkoxides hydrolization

Nanocrystalline TiO/sub 2/ modified with Nb has been produced through the sol-gel technique. Nanopowders have been obtained by means of the hydrolysis of pure alkoxides with deionized water and peptization of the resulting hydrolysate with diluted acid nitric at 100/spl deg/C. The addition of Nb stabilizes the anatase phase to higher temperatures. XRD spectra of the undoped and the Nb-doped samples show that the undoped sample has been almost totally converted to rutile at 600/spl deg/C, meanwhile the doped samples present still a low percentage of rutile phase. Nanocrystalline powders stabilized at 600/spl deg/C with grain sizes of about 17 nm have successfully been synthesized by the addition of Nb with a concentration of 2% at., which appears to be an adequate additive concentration to improve the gas sensor performances, such as it is suggested by the catalytic conversion efficiency experiments performed from FTIR measurements. FTIR absorbance spectra show that catalytic conversion of CO occurs at lower temperatures when niobium is introduced. The electrical response of the films to different concentrations of CO and ethanol has been monitored in dry and wet environments in order to test the influence of humidity in the sensor response. The addition of Nb decreases the working temperature and increases the stability of the layers. Also, large enhancement of the response time is obtained even with lower working temperatures. Moreover, humidity effects on the gas sensor response toward CO and ethanol are less important in Nb-doped samples than in the undoped ones.

Competitive evolution of the fine contrast modulation and CuPt ordering in InGaP/GaAs layers

We use transmission electron microscopy to characterize the morphology of InGaP epitaxial layers grown by metal‐organic vapor‐phase epitaxy over misoriented GaAs (001) substrates, with a cutoff angle in a range from 0° to 25°. The occurrence of phase separation and CuPt‐type ordered superstructures has been observed. The most ordered configuration has been found to appear in layers grown on 2° off substrates, and the strength of order decreases with increasing the misorientation angle beyond α=2°. Conversely, whereas the phase separation is less evident in the layer grown at 2°, the sample grown with a misorientation of 25° exhibits the most phase separated configuration. The completion between these two phenomena is discussed depending on the misorientation angle.

Transport in quantum dot stacks using the transfer Hamiltonian method in self-consistent field regime

The non-coherent rate equation approach to the electrical transport in a serial quantum dot system is presented. The charge density in each quantum dot is obtained using the transfer Hamiltonian formalism for the current expressions. The interactions between the quantum dots and between the quantum dots and the electrodes are introduced by transition rates and capacitive couplings. Within this framework analytical expressions for the current and the charge in each quantum dot are presented. The effects of the local potential are computed within the self-consistent field regime. Despite the simplicity of the model, well-known effects are satisfactorily explained and reproduced. We also show how this approach can be extended into a more general case.

Determination of the direct band-gap energy of InAlAs matched to InP by photoluminescence excitation spectroscopy

A series of InxAl1−xAs samples (0.51<x<0.55) coherently grown on InP was studied in order to measure the band-gap energy of the lattice matched composition. As the substrate is opaque to the relevant photon energies, a method is developed to calculate the optical absorption coefficient from the photoluminescence excitation spectra. The effect of strain on the band-gap energy has been taken into account. For x=0.532, at 14 K we have obtained Eg0=1549±6u2009meV.

Raman scattering and photoluminescence analysis of silicon on insulator structures obtained by single and multiple oxygen implants

An analysis of silicon on insulator structures obtained by single and multiple implants by means of Raman scattering and photoluminescence spectroscopy is reported. The Raman spectra obtained with different excitation powers and wavelengths indicate the presence of a tensile strain in the top silicon layer of the structures. The comparison between the spectra measured in both kinds of samples points out the existence in the multiple implant material of a lower strain for a penetration depth about 300 nm and a higher strain for higher penetration depths. These results have been correlated with transmission electron microscopy observations, which have allowed to associate the higher strain to the presence of SiO2 precipitates in the top silicon layer, close to the buried oxide. The found lower strain is in agreement with the better quality expected for this material, which is corroborated by the photoluminescence data.