A. Diéguez

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.

Grain size control in nanocrystalline In2O3 semiconductor gas sensors

Abstract In2O3 thin films prepared by sol–gel method make it possible to detect low levels (several hundreds ppb) of nitrogen dioxide in air. The possibility of grain size control in indium oxide-sensing layers has been established by using of two preparation methods—electron beam evaporation (EB) and sol–gel technique (SG). SG-prepared samples show smaller particles sizes (down to 5 nm), higher state of agglomeration, higher sensor resistance in air and higher response to NO2 in comparison to EB samples. Sol–gel technique leads to the preparation of polycrystalline indium oxide with particle sizes of about 5–6 nm after calcination at 400°C and 20 nm after calcination at 700°C. The initial state of particle agglomeration in initial indium hydroxide sol (IHS), stabilized with nitric acid, influences the structure and surface morphology of the resulting indium oxide. While the In2O3 layer prepared by using low agglomerated IHS is smooth and porous, In2O3 layers prepared from highly agglomerated IHS consist of two regions—thin layer and crystallite agglomerates in cubic and rectangular parallelepiped form. The last shows the best results in terms of NO2 sensitivity. Sensor resistance and NO2 sensitivity increase with the decrease of the grain sizes in In2O3.

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.

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 .

Microwave processing for the low cost, mass production of undoped and in situ catalytic doped nanosized SnO2 gas sensor powders

Abstract Nanosized tin oxide powders were obtained for application in thick film gas sensor technology. This requires an innovative technique, involving the use of microwave energy with a wavelength of 2.45 GHz, which produces doped or undoped powder precursors in just a few minutes. Further stabilisation treatments — conventional heating, OH-stimulated microwaves and combined treatments — were also considered. Reproducibility, low cost and suitability for mass production demonstrate the industrial and scientific feasibility of this new procedure. Material structural characterisation and electrical properties after gas exposure of improved sensors of Pt and Pd in situ doped, and undoped SnO 2 are introduced, showing the suitability of the material.

Microstructure and morphology of tin dioxide multilayer thin film gas sensors

Abstract Structural, morphological, and electrical measurements have been carried out on SnO 2 multilayer thin film grown by the rheotaxial growth and thermal oxidation method on Al 2 O 3 substrates. The analysis of X-ray and electron diffraction patterns shows that, in addition to the SnO 2 cassiterite phase, a contribution from another SnO 2 phase is present, which can be related to cassiterite by introducing microtwinning effects. The electrical measurements show that these thin films have a higher sensitivity towards CO with respect to the conventional single layer SnO 2 sensors.

Parameter optimisation in SnO2 gas sensors for NO2 detection with low cross-sensitivity to CO: sol–gel preparation, film preparation, powder calcination, doping and grinding

Abstract The influence of the calcination temperature and doping on the structural and electrical properties of SnO 2 nanopowders for gas sensors is investigated. The effectivity of grinding before and/or after calcination of the oxide will be discussed in terms of both structural modifications and electrical response of gas sensors.

New method to obtain stable small-sized SnO2 powders for gas sensors

Abstract A new method based on the pyrolytic reaction of SnCl4⋅5(H2O) in the range of 400–900°C is reported to produce stable small grains of SnO2 from 6 to 34 nm. The characterisation of these nanocrystallites is discussed, giving important results about oxygen incorporation to the SnO2 lattice and grain growth. This allows the understanding of the crystalline features of the samples. The influence of this new method on grain growth topics (nucleation, Ostwald ripening, coalescence) is discussed. Examples of the CO and NO2 detection are reported, showing its relation with structural parameters.

Evaluation of building technology for mass producible millimetre-sized robots using flexible printed circuit boards

Initial tests of a building technology for a compact three-dimensional mass producible microrobot are presented. The 3.9 × 3.9 × 3.3 mm3 sized prototype robot represents a microsystem with actuators, sensors, energy management and integrated electronics. The weight of a folded robot is 65 mg and the total volume is less than 23 mm3. The design of the interfaces of the different modules in the robot, as well as the building technology, is described. The modules are assembled using conductive adhesive with industrial surface mounting technology on a thin double-sided flexible printed circuit board. The final shape of the microrobots is achieved by folding the flexible printed circuit board twice. Electrical and mechanical studies are performed to evaluate the assembly and it is concluded that the technology can be used for this type of microsystem. Several issues using the presented assembly technique are identified and addressed.