A. Romano-Rodriguez

The complete Raman spectrum of nanometric SnO2 particles

14 and space group P4 2 /mnm. The unit cell consists of two metal atoms and four oxygen atoms. Each metal atom is situated amidst six oxygen atoms which approximately form the corners of a regular octahedron. Oxygen atoms are surrounded by three tin atoms which approximate the corners of an equilateral triangle. The lattice parameters are a5b 54.737 A, and c53.186 A. The ionic radii for O 22 and Sn 41 are 1.40 and 0.71 A, respectively. 1 The 6 unit cell atoms give a total of 18 branches for the vibrational modes in the first Brillouin zone. The mechanical representation of the normal vibration modes at the center of the Brillouin zone is given by 2,3 G5G 1 ~ A1g!1G 2 ~ A2g!1G 3 ~ B1g!1G 4 ~ B2g! 1G 5 ~ Eg!12G 1 ~ A2u!12G 4 ~ B1u!14G 5 ~ Eu!, ~1! using the Koster notation with the commonly used symmetry designations listed in parenthesis. The latter will be used throughout this article. Of these 18 modes, 2 are active in infrared ~the single A2u and the triply degenerate Eu), 4 are Raman active ~three nondegenerated modes, A1g , B1g , B2g , and a doubly degenerate Eg), and two are silent ( A2g , and B1u). One A2u and two Eu modes are acoustic. In the Raman active modes oxygen atoms vibrate while Sn atoms are at rest ~see Fig. 1 in Ref. 4!. The nondegenerate mode, A1g , B1g , and B2g , vibrate in the plane perpendicular to the c axis while the doubly degenerated E g mode vibrates in the direction of the c axis. The B 1g mode consists of rotation of the oxygen atoms around the c axis, with all six oxygen atoms of the octahedra participating in the vibration. In the A2g infrared active mode, Sn and oxygen atoms vibrate in the c axis direction, and in the Eu mode both Sn and O atoms vibrate in the plane perpendicular to the c axis. The silent modes correspond to vibrations of the Sn and O atoms in the direction of the c axis (B1u) or in the plane perpendicular to this direction ( A2g). According to the literature, the corresponding calculated or observed frequencies of the optical modes are presented in Table I. When the size of the SnO2 crystal is reduced, the infrared spectrum is modified because the interaction between electromagnetic radiation and the particles depends on the crystal’s size, shape, and state of aggregation. 8‐1 0 Experiments using Raman spectroscopy have also reported spectrum modification, at least partially. Low frequency bands have been observed previously in SnO2, 11 and several authors have reported the existence of bands not observed in single-crystal or polycrystalline SnO 2 which have been found to be closely related to grain size. 12‐15 However, some of these reports do not adequately explain the origin of the abnormal spectrum. The aim of this article is to present a complete Raman spectrum of SnO2 nanoparticles. The analysis comprises ~i! modification of the normal vibration modes active in Raman when the spectra are obtained from nanocrystals of SnO2 ~‘‘classical modes’’ !, ~ii! the disorder activated surface modes in the region around 475‐775 cm 21 , and ~iii! the appearance of the acoustic modes in the low-frequency region of the spectra.

Morphological analysis of nanocrystalline SnO2 for gas sensor applications

Abstract Structural and morphological analysis of nanocrystalline SnO2 for gas sensor applications were performed at different annealing conditions by using nanopowders and thin nanocrystalline layers. The evolution of the grain size and the morphology of Pt doped tin dioxide nanoparticles with increase of annealing temperature from 450 to 1000°C were analyzed by means of transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) and micro-Raman spectroscopies. TEM shows that the average particle size increases, the size distribution becomes more spread out, and the grain faceting, as a mechanism of energy minimization, is more evident with increasing temperature. Furthermore, the shape of the particles changes with the annealing temperature, which explains the results of the FTIR spectra using the Theory of the Average Dielectric Constant (TADC). As temperature increases, the Raman spectra are modified in agreement with a reduction of the crystalline defect concentration and a grain size increase. The the tin nanocrystalline SnO2 layers, deposited on α-Al2O3 or on thermally oxidezed Si substrates, have been annealed at 700° C for 8 h under different atmospheres, such as oxygen or synthetic air. TEM proves that the annealing atmosphere has a strong influence on the size and size distribution of the nanoparticles in the thin layer. The main differences are found near the layer-substrate interface and are dependent on the annealing atmosphere as well as the nature of the substrate.

Influence of the catalytic introduction procedure on the nano-SnO2 gas sensor performances: Where and how stay the catalytic atoms?☆

Abstract The role and activity of catalytic additives on solid-state gas sensors are determined by the additive chemical state, aggregation form and interaction with the semiconductor oxide. All these parameters depend on the technological steps involved in the element introduction and the treatments applied to the sensor material. The aim of this work is to analyse the influence of the additive introduction procedure on the gas sensor performance. In order to achieve this objective, two sets of different palladium, platinum or gold modified tin oxide materials have been prepared. In a first set of samples, additives were introduced by impregnation of the previously thermally stabilised oxide. In the second set, catalyst addition was carried out before any thermal treatment was applied. The study of the catalytically modified materials, calcined at different treatment temperatures between 250 and 1000°C, has been performed by means of HRTEM, XRD, XPS, and Raman spectroscopy. The influence of both processes on additive surface concentration, chemical state, nanoparticle growth and resistivity values are presented and discussed. Moreover, electrical characterisation of the sensors prepared from these materials has been carried out.

Ultralow power consumption gas sensors based on self-heated individual nanowires

Dissipated power in metal oxide nanowires (rNW<45 nm) often causes important self-heating effects and as a result, undesired aging and failure of the devices. Nevertheless, this effect can be used to optimize the sensing conditions for the detection of various gaseous species, avoiding the requirement of external heaters. In this letter, the sensing capabilities of self-heated individual SnO2 nanowires toward NO2 are presented. These proof-of-concept systems exhibited responses nearly identical to those obtained with integrated microheaters, demonstrating the feasibility of taking advantage of self-heating in nanowires to develop ultralow power consumption integrated devices.

The effects of electron-hole separation on the photoconductivity of individual metal oxide nanowires

The responses of individual ZnO nanowires to UV light demonstrate that the persistent photoconductivity (PPC) state is directly related to the electron-hole separation near the surface. Our results demonstrate that the electrical transport in these nanomaterials is influenced by the surface in two different ways. On the one hand, the effective mobility and the density of free carriers are determined by recombination mechanisms assisted by the oxidizing molecules in air. This phenomenon can also be blocked by surface passivation. On the other hand, the surface built-in potential separates the photogenerated electron-hole pairs and accumulates holes at the surface. After illumination, the charge separation makes the electron-hole recombination difficult and originates PPC. This effect is quickly reverted after increasing either the probing current (self-heating by Joule dissipation) or the oxygen content in air (favouring the surface recombination mechanisms). The model for PPC in individual nanowires presented here illustrates the intrinsic potential of metal oxide nanowires to develop optoelectronic devices or optochemical sensors with better and new performances.

The aging effect on SnO2-Au thin film sensors : electrical and structural characterization

Abstract There is an increasing demand for semiconducting gas sensors for several monitoring applications that have sensitivity, selectivity and reliability on a long-term scale. In this paper, we have described the preparation of SnO2–Au thin film sensors by the RGTO technique; these sensors have proved to be capable of sensitive and selective detection of CO. The TEM and AES analysis showed that 6 months of sensor aging at 400°C did not produce any valuable rearrangement of gold atoms onto a tin dioxide surface. Most of the sensor resistance variation was observed during the first 20 days, and a limited drift was observed in the remaining period.

Influence on the gas sensor performances of the metal chemical states introduced by impregnation of calcinated SnO2 sol–gel nanocrystals

Abstract The effects of the introduction of Pt and Pd by impregnation in sol–gel fabricated SnO 2 nanoparticles after calcination are reported in this paper. The differences in base resistance and sensitivity of sensors prepared using these powders are presented and explained — taking into account the chemical states of the metal additives and the generated surface states in the band gap of the SnO 2 .

Ion‐beam synthesis of amorphous SiC films: Structural analysis and recrystallization

The analysis of SiC films obtained by carbon ion implantation into amorphous Si (preamorphized by Ge ion implantation) has been performed by infrared and Raman scattering spectroscopies, transmission electron microscopy, Rutherford backscattering, and x‐ray photoelectron spectroscopy (XPS). The data obtained show the formation of an amorphous Si1−xCx layer on top of the amorphous Si one by successive Ge and C implantations. The fitting of the XPS spectra indicates the presence of about 70% of Si–C bonds in addition to the Si–Si and C–C ones in the implanted region, with a composition in the range 0.35<x<0.6. This points out the existence of a partial chemical order in the layer, in between the cases of perfect mixing and complete chemical order. Recrystallization of the layers has been achieved by ion‐beam induced epitaxial crystallization (IBIEC), which gives rise to a nanocrystalline SiC layer. However, recrystallization is not complete, observing still the presence of Si–Si and C–C bonds in an amorphou...

Portable microsensors based on individual SnO2 nanowires

Individual SnO(2) nanowires were integrated in suspended micromembrane-based bottom-up devices. Electrical contacts between the nanowires and the electrodes were achieved with the help of electron- and ion-beam-assisted direct-write nanolithography processes. The stability of these nanomaterials was evaluated as function of time and applied current, showing that stable and reliable devices were obtained. Furthermore, the possibility of modulating their temperature using the integrated microheater placed in the membrane was also demonstrated, enabling these devices to be used in gas sensing procedures. We present a methodology and general strategy for the fabrication and characterization of portable and reliable nanowire-based devices.

Raman microprobe characterization of electrodeposited S-rich CuIn(S,Se)2 for photovoltaic applications: Microstructural analysis

This article reports a detailed Raman scattering and microstructural characterization of S-rich CuIn(S,Se)2 absorbers produced by electrodeposition of nanocrystalline CuInSe2 precursors and subsequent reactive annealing under sulfurizing conditions. Surface and in-depth resolved Raman microprobe measurements have been correlated with the analysis of the layers by optical and scanning electron microscopy, x-ray diffraction, and in-depth Auger electron spectroscopy. This has allowed corroboration of the high crystalline quality of the sulfurized layers. The sulfurizing conditions used also lead to the formation of a relatively thick MoS2 intermediate layer between the absorber and the Mo back contact. The analysis of the absorbers has also allowed identification of the presence of In-rich secondary phases, which are likely related to the coexistence in the electrodeposited precursors of ordered vacancy compound domains with the main chalcopyrite phase, in spite of the Cu-rich conditions used in the growth. ...