Crystallization and surface morphology of Au/SiO2 thin films following furnace and flame annealing

A crystallization and surface evolution study of Au thin film on SiO₂ substrates following annealing at different temperatures above the eutectic point of the Au/Si system are reported. Samples were prepared by conventional evaporation of gold in a high vacuum (10⁻⁷ mbar) environment on substrates at room temperature. Thermal treatments were performed by both furnace and flame annealing techniques. Au thin films can be crystallized on SiO₂ substrates by both furnace and flame annealing. Annealing arranges the Au crystallites in the (1 1 1) plane direction and changes the morphology of the surface. Both, slow and rapid annealing result in a good background in the XRD spectra and hence clean and complete crystallization which depends more on the temperature than on the time of annealing. The epitaxial temperature for the Au/SiO₂ system decreases in the range of 350-400 °C. Furnace and flame annealing also form crystallized gold islands over the Au/SiO₂ surface. Relaxation at high temperatures of the strained Au layer, obtained after deposition, should be responsible for the initial stages of clusters formation. Gold nucleation sites may be formed at disordered points on the surface and they become islands when the temperature and time of annealing are increased. The growth rate of crystallites is highest around 360 °C. Above this temperature, the layer melts and gold diffuses from the substrate to the nucleation sites to increase the distance between islands and modify their shapes. Well above the eutectic temperature, the relaxed islands have hexagonally shaped borders. The mean crystallite diameters grow up to a maximum mean size of around 90 nm. The free activation energy for grain boundary migration above 360 °C is 0.2 eV. Therefore the type of the silicon substrate changes the mechanism of diffusion and growth of crystallites during annealing of the Au/Si system. Epitaxial Au(1 1 1) layers without formation of islands can be prepared by furnace annealing in the range of 300-310 °C and by flame annealing of a few seconds and up to 0.5 min.     

Dielectric investigations in Sr<Subscript>0.75</Subscript>Ba<Subscript>0.25</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> relaxor ferroelectric thin films

The dielectric properties of Sr_0.75Ba_0.25Nb₂O₆ relaxor ferroelectric thin films were carefully analyzed. In contrast to bulk samples which present three distinct dielectric relaxation phenomena Sr_0.75Ba_0.25Nb₂O₆ thin films present only two of them. The suppression of the third anomaly can be mainly attributed to the narrow grain size distribution of nanograins and weak tensile strains imposed to the film from the substrate. The whole set of results point to the interpretation of a dielectric response characteristic of mesoscopic structure, which is composed of clusters and nanodomains.     

Influence of Sequential Nitrogen Substitution on the Redox Properties of Bis(thiazolylidene)hydrazine and on the Charge Transfer of TCNQ Complexes

Various extended π-donor molecules incorporating an azino spacer between two heterocyclic moieties, either dithiole or thiazole rings, are described. As evidenced by cyclic voltammetric studies, the donor ability of these derivatives can be modulated depending on the nature of the two heterocycles. Three similar crystalline complexes were obtained with TCNQ and the donors 4, 7 and 8. Crystal structures and magnetic susceptibility data of these complexes are presented and the degree of charge transfer within these complexes has been correlated with the redox properties.     

Development and evaluation of magnetic resonance technologies, particularly NMR, for detection of explosives

Beginning in 1972 nuclear magnetic resonance (NMR) technologies for detecting explosives and illegal contraband were developed and evaluated at the Southwest Research Institute. Fullscale systems on the basis of hydrogen transient NMR were developed and evaluated in laboratory and field tests with generally favorable results but with some limitations. These included (1) an experimental, mobile system for detecting buried, nonmetallic land mines; (2) an instrument for inspection of letters and small parcels for small quantities of explosives or illegal drugs; (3) a system for inspection of checked airline baggage and air cargo for concealed explosives and illegal drugs; and (4) a system for rapid inspection of quantities of mail for illegal drugs.¹H NMR offers high sensitivity and detects high-energy explosives such as RDX, TNT, and PETN, as well as nitroglycerine and ammonium-nitrate-based explosives and illegal drugs. Challenges in both physics and engineering were successfully addressed to achieve the goals of rapid inspection with low false-alarm and high detection probability. Electron paramagnetic resonance was found suitable for detecting black powder in laboratory tests as was nuclear quadrupole resonance for a few high-energy explosives. Low-field¹H NMR was also explored in the laboratory to make it practical for explosives detection and found to have potential, but numerous implementation problems must be overcome.     

Chemi-ionization in alkali-methylhalogen collisions

Chemi-ionization has been observed in collisions between the alkali atoms K, Na and Li and the methylhalogens CH₃] and CH₃Br. Total cross sections were measured as a function of the translational energy and the internal energy of methylhalogens. The molecular electron affinities were determined from the thresholds of these reactions. The thresholds indicate that these negative ions are formed by vertical or near-vertical transitions, in contrast to most experiments on chemi-ionization, which yield the adiabatic electron affinities. In the threshold region, the cross sections are strongly dependent upon the internal energy of the molecules. At very high temperature the methyliodide dissociates, and so chemi-ionization was observed with iodine atoms.     

Use of an internal reference in 13C chemical shift measurements

The use of the nominal value of the chemical shift of the solvent to calibrate the spectrometer in ¹³C NMR spectroscopy was found to introduce errors due to the effect which the solute has on the solvent. In addition, hexamethyldisiloxane (HM) is proposed as an internal standard; owing to its high boiling point it is easier to manipulate than tetramethylsilane (TMS) and it is therefore possible to prepare solutions of known concentrations. In order to convert the data obtained with this standard to the TMS scale, the chemical shift of HM was determined in 16 solvents using cyclohexane as external standard in a spherical cell (5% w/w concentration of HM) as a function of the solvent factor g². Comparing these results with a similar plot obtained previously for TMS by other workers, it is possible to convert one type of data to the other by a simple linear expression.     

Differential cross sections for elastic scattering and electron detachment in collisions of halogen anions with noble gas atoms

Electron detachment in collision of halogen anions and noble gas atoms was investigated in the collision energy range of 500-3000 eV. Differential cross sections were determined both for elastic scattering and for the electron detachment process in the angular range of 0-3°. It was found that direct electron detachment, due to the interaction of the discrete ion-atom state with a continuum of states increases with increasing collisional energy. Furthermore, the experimental evidence indicates that direct detachment primarily occurs in a limited range of impact parameters. A model, based on angular coupling, is described which qualitatively explains these two experimentally observed features.     

The well depths of XeF− and XeCl− from differential scattering measurements

Elastic differential cross section measurements were performed for F^? + Xe and Cl^? + Xe in an energy range of 250–500 eV and in an angular range of 0–6 mrad. In contrast to the case of C1^? on Xe, the rainbow structure in the cross section for F^? on Xe was well resolved. From the measurements the well depth of XeF^? has been determined to be 0.28 ± 0.04 eV, while for XeC1^? the well depth should be less than 0.14 eV.     

A time-dependent quantal analysis of vibronic excitation via charge transfer in ion-molecule collisions

A semi-classical multiple state model describing charge transfer in ion-molecule system is presented. Analytical expressions for the state-to-state transition probabilities are given for both the weak-coupling and the high-velocity limits. The expressions are compared to a two-state model describing charge transfer in ion-atom systems. Some numerical calculations are presented to illustrate the various phenomena which can occur in multiple state charge transfer processes. These numerical calculations will be based upon a simple system derived from the system (Ar + N₂)⁺.