Titanium Dioxide Engineered for Near-dispersionless High Terahertz Permittivity and Ultra-low-loss.

Realising engineering ceramics to serve as substrate materials in high-performance terahertz(THz) that are low-cost, have low dielectric loss and near-dispersionless broadband, high permittivity, is exceedingly demanding. Such substrates are deployed in, for example, integrated circuits for synthesizing and converting nonplanar and 3D structures into planar forms. The Rutile form of titanium dioxide (TiO2) has been widely accepted as commercially economical candidate substrate that meets demands for both low-loss and high permittivities at sub-THz bands. However, the relationship between its mechanisms of dielectric response to the microstructure have never been systematically investigated in order to engineer ultra-low dielectric-loss and high value, dispersionless permittivities. Here we show TiO2 THz dielectrics with high permittivity (ca. 102.30) and ultra-low loss (ca. 0.0042). These were prepared by insight gleaned from a broad use of materials characterisation methods to successfully engineer porosities, second phase, crystallography shear-planes and oxygen vacancies during sintering. The dielectric loss achieved here is not only with negligible dispersion over 0.2-0.8 THz, but also has the lowest value measured for known high-permittivity dielectrics. We expect the insight afforded by this study will underpin the development of subwavelength-scale, planar integrated circuits, compact high Q-resonators and broadband, slow-light devices in the THz band.

On the influence of DC electric fields on the aerosol assisted chemical vapor deposition growth of photoactive titanium dioxide thin films.

Titanium dioxide thin films were deposited on fluorine doped tin oxide glass substrate from the electric field assisted aerosol chemical vapor deposition (EACVD) reaction of titanium isopropoxide (TTIP, Ti(OC3H7)4) in toluene on glass substrates at a temperature of 450 °C. DC electric fields were generated by applying a potential difference between the electrodes of the transparent coated oxide coated glass substrates during the deposition. The deposited films were characterized using scanning electron microscopy, X-ray diffraction, atomic force microscopy, Raman spectroscopy, and UV-vis spectroscopy. The photoactivity and hydrophilicity of the deposited films were also analyzed using a dye-ink test and water-contact angle measurements. The characterization work revealed that the incorporation of DC electric fields produced significant reproducible changes in the film microstructure, preferred crystallographic orientation, roughness, and film thickness. Photocatalytic activity was calculated from the half-time (t1/2) or time taken to degrade 50% of the initial resazurin dye concentration. A large improvement in photocatalytic activity was observed for films deposited using an electric field with a strong orientation in the (004) direction (t1/2 17 min) as compared to a film deposited with no electric field (t1/2 40 min).

Highly sensitive ZnO nanorod- and nanoprism-based NO2 gas sensors: size and shape control using a continuous hydrothermal pilot plant.

Continuous hydrothermal flow synthesis of crystalline ZnO nanorods and prisms is reported via a new pilot-scale continuous hydrothermal reactor (at nominal production rates of up to 1.2 g/h). Different size and shape particles of ZnO (wurtsite structure) were obtained via altering reaction conditions such as the concentration of either additive H2O2 or metal salt. Selected ZnO samples (used as prepared) were evaluated as solid oxide gas sensors, showing excellent sensitivity toward NO2 gas. It was found that both the working temperature and gas concentration significantly affected the NO2 gas response at concentrations as low as 1 ppm.

The spatial distribution of postblast RDX residue: forensic implications.

Locating exactly where trace explosive residue samples should be sought during sample collection at bomb scenes is not specified in the published literature or guidelines; in this area, it is generally acknowledged that forensic practices are based on tradition rather than evidence. This study investigated patterns in the spatial distribution of postblast 1,3,5-trinitro-1,3,5-triazocyclohexane residue from a series of unconfined detonations, over a range of sampling sites, and at two different detonation heights. The amount of residue recovered from the sites decreased as a function of distance from the center of the explosion. [Correction added after online publication 27 December 2012: In the preceding sentence, "increased" was corrected to "decreased" to agree with the conclusion of the article.] As the height of the detonations increased, more residues were found from all sampling sites. The findings of this empirical study have a number of important practical implications including determining where residue samples are best sought at crime scenes.

Gas sensing with nano-indium oxides (In2O3) prepared via continuous hydrothermal flow synthesis.

A rapid, clean, and continuous hydrothermal route to the synthesis of ca. 14 nm indium oxide (In(2)O(3)) nanoparticles using a superheated water flow at 400 °C and 24.1 MPa as a crystallizing medium and reagent is described. Powder X-ray diffraction (XRD) of the particles revealed that they were highly crystalline despite their very short time under hydrothermal flow conditions. Gas sensing substrates were prepared from an In(2)O(3) suspension via drop-coating, and their gas sensing properties were tested for response to butane, ethanol, CO, ammonia, and NO(2) gases. The sensors showed excellent selectivity toward ethanol, giving a response of 18-20 ppm.

On the effects of electric fields in aerosol assisted chemical vapour deposition reactions of vanadyl acetylacetonate solutions in ethanol.

Thin films of thermochromic vanadium dioxide have been deposited on glass substrates at 530 degrees C from the aerosol assisted chemical vapour deposition of vanadyl acetylacetonate solutions in ethanol under the influence of electric fields. Electric fields were generated by applying a potential difference between the top plate and the substrate of the reactor. The deposited films were analysed and characterised using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and variable temperature UV/Visible spectroscopy. The application of an electric field led to significant changes in the deposited films microstructure and functional properties. It was found that an increase in electric field strength caused a decrease in crystallite size and in an increase in the change in transmission in the near infrared when compared to films grown without the use of an electric field.

Electric fields in the chemical vapour deposition growth of vanadium dioxide thin films.

Thin films of thermochromic vanadium dioxide have been the subject of intensive research in recent years year due to their postulated use as "intelligent" window coatings. The usefulness of such technology depends on a semi-conducting to metal transition with an associated change in infra-red optical properties. This exact nature of this transition depends on a large number of factors such as doping, crystallite size, strain, crystallographic orientation etc. In this paper we discuss the nature of these factors with a particular focus on how the application of electric fields in the deposition affects crystallite size and film strain with reference to recent results.

Metal oxide semi-conductor gas sensors in environmental monitoring.

Metal oxide semiconductor gas sensors are utilised in a variety of different roles and industries. They are relatively inexpensive compared to other sensing technologies, robust, lightweight, long lasting and benefit from high material sensitivity and quick response times. They have been used extensively to measure and monitor trace amounts of environmentally important gases such as carbon monoxide and nitrogen dioxide. In this review the nature of the gas response and how it is fundamentally linked to surface structure is explored. Synthetic routes to metal oxide semiconductor gas sensors are also discussed and related to their affect on surface structure. An overview of important contributions and recent advances are discussed for the use of metal oxide semiconductor sensors for the detection of a variety of gases--CO, NO(x), NH(3) and the particularly challenging case of CO(2). Finally a description of recent advances in work completed at University College London is presented including the use of selective zeolites layers, new perovskite type materials and an innovative chemical vapour deposition approach to film deposition.

Gallium oxide thin films from the AACVD of [Ga(NMe2)3]2 and donor functionalised alcohols.

Thin films of Ga(2)O(3) have been produced from [Ga(NMe(2))(3)](2) and ROH (R = CH(2)CH(2)NMe(2), CH(CH(2)NMe(2))(2), CH(CH(3))CH(2)NMe(2), CH(2)CH(2)OMe and C(CH(3))(2)CH(2)OMe) by aerosol assisted chemical vapour deposition on glass. Transparent, unreflective films were obtained at a deposition temperature of 550 degrees C using toluene as solvent. The gallium oxide films were analyzed by Scanning electron microscopy (SEM), Raman spectroscopy, wavelength dispersive analysis of X-rays (WDX) and X-ray photoelectron spectroscopy (XPS). The gallium oxide films obtained were X-ray amorphous. Gas-sensing experiments indicated that the films showed an n-type response to ethanol at a variety of temperatures.

The reaction of GeCl4 with primary and secondary phosphines.

Reaction of GeCl4 and CyPH2 leads to a range of products from which crystals of [CyPH3]+[GeCl3]- have been obtained. The major intermediate in this reaction, Cy(H)PGeCl3, can be obtained as the dominant product only when an excess of GeCl4 is used in the preparation. Similarly, crystals of [Ph2PH2]+[GeCl3]- have been obtained from reaction of Ph2PH and GeCl4(1 : 1). The structures of both primary and secondary phosphonium cations are reported. Aerosol-assisted and low-pressure chemical vapour deposition experiments using Cy(H)PGeCl3 as precursor lead to the deposition of thin films containing both germanium and phosphorus, contaminated with large amounts of oxygen. GeP has been identified as components of the film from bandgap measurements.