Bipolaron formation in the presence of magnetic impurities

The formation of bipolarons in the presence of magnetic impurities is studied theoretically. We use the extended Hubbard Su–Schrieffer–Heeger (SSH) model with Brazovskii–Kirova and Kondo interaction terms. Parameters are chosen suitably for cis-polyacetylene. A Quantum Monte Carlo (QMC) algorithm is used to study the equilibrium lattice structure and charge distribution as a function of doping level and Kondo exchange integral. The magnetic impurities can have a destructive effect on bipolaron stability, instead favouring the two polaron configuration. However by suitably adjusting the doping level, bipolarons can be stabilized over a wide range of impurity strength.     

Hafnium oxide thin films deposited from a filtered cathodic vacuum arc

The electrical and structural characteristics of hafnium oxide thin films reactively deposited from a filtered cathodic vacuum arc have been investigated. X-ray photoelectron spectroscopy was used to determine the deposition conditions (Ar/O₂ ratio) which produced stoichiometric HfO₂ films. Cross-sectional transmission electron microscopy showed that the micro-structure of the films was highly disordered with electron-diffraction analysis providing evidence for the presence of sub-nano-metre crystallites of the monoclinic HfO₂ (P2₁/c) phase. Further evidence for the presence of this phase was provided by measuring the O k-edge using electron energy loss spectroscopy and comparing it with calculations performed using FEFF8.2, a multiple scattering code. Surface imaging revealed that local film damage occurred in films deposited with substrate bias voltages exceeding −200 V. The current-leakage characteristics of the HfO₂ films deposited with a bias of approximately −100 V suggest that device grade HfO₂ films can be produced from a filtered cathodic vacuum arc.     

Quantum cascade semiconductor infrared and far-infrared lasers: from trace gas sensing to non-linear optics

The Quantum cascade (QC) laser is an entirely new type of semiconductor device in which the laser wavelength depends on the band-gap engineering. It can be made to operate over a much larger range than lead salt lasers, covering significant parts of both the infrared and submillimetre regions, and with higher output power. In this tutorial review we survey some of the applications of these new lasers, which range from trace gas detection for atmospheric or medical purposes to sub-Doppler and time dependent non-linear spectroscopy.     

Hexyl-substituted oligothiophenes with a central tetrafluorophenylene unit: crystal engineering of planar structures for p-type organic semiconductors

Rigidification has been achieved in thiophene-tetrafluorophenylene architectures through strong S...F and H...F intramolecular interactions; the resulting materials are promising candidates for p-type organic field effect transistors.     

Towards a synthesis of naphthalene derived natural products

Dieckmann-type cyclization reactions have been employed in the synthesis of the alkyl substituted naphthoquinone 11 and the naphthalenes 10 and 12. Various conditions for the benzylic oxidation of these compounds have been investigated with a view towards the synthesis of some naphthalene based natural products.     

The gas-phase pyrolysis of alkyl nitrites. II. s-butyl nitrite

The rate of decomposition of s-butyl nitrite (SBN) has been studied in the absence (130–160°C) and presence (160–200°C) of NO. Under the former conditions, for low concentrations of SBN (6 × 10^?5 ? 10^?4M) and small extents of reaction (～1.5%), the first-order homogeneous rates of acetaldehyde (AcH) formation are a direct measure of reaction (1) since k_3c » k₂(NO): . Unlike t-butyl nitrite (TBN), d(AcH)/dt is independent of added CF₄ (～0.9 atm). Thus k_3c is always » k₂ (NO) over this pressure range. Large amounts of NO (～0.9 atm) (130–160°C) completely suppress AcH formation. k₁ = 10^16.2–40.9/θ sec^?1. Since (E₁ + RT) and ΔH°₁ are identical, within experimental error, both may be equated with D(s-BuO-NO) = 41.5 ± 0.8 kcal/mol and E₂ = 0 ± 0.8 kcal/mol. The thermochemistry leads to the result ΔH°_f (s-\documentclass{article}\pagestyle{empty}\begin{document}${\rm Bu}\mathop {\rm O}\limits^{\rm .}$\end{document}) = ? 16.6 ± 0.8 kcal/mol. From ΔS°₁ and A₁, k₂ is calculated to be 10^10.4 M^?1 · sec^?1, identical to that for TBN. From an independent observation that k₆/k₂ = 0.26 ± 0.01 independent of temperature, \documentclass{article}\pagestyle{empty}\begin{document}${\rm s - Bu}\mathop {\rm O}\limits^{\rm .} + {\rm NO}\mathop \to \limits^{\rm 6} {\rm MEK} + {\rm HNO}$\end{document}, we find E₆ = 0 ± 1 kcal/mol and k₆ = 10^9.8M^?1 · sec^?1. Under the conditions first cited, methyl ethyl ketone (MEK) is also a product of the reaction, the rate of which becomes measurable at extents of conversion >2%. However, this rate is ～0.1 that of AcH formation. Although MEK formation is affected by the ratio S/V for different reaction vessels, in a spherical reaction vessel, this MEK arises as the result of an essentially homogeneous first-order 4-centre elimination of HNO. \documentclass{article}\pagestyle{empty}\begin{document}${\rm SBN}\mathop \to \limits^{\rm 5} {\rm MEK} + {\rm HNO}$\end{document}; k₅ = 10^12.8–35.8/θ sec^?1. Sec-butyl alcohol (SBA), formed at a rate comparable to MEK, is thought to arise via the hydrolysis of SBN, the water being formed from HNO. The rate of disappearance of SBN, that is, d(MEK + SBA + AcH)/dt, is given by k_global = 10^15.7–39.6/θ sec^?1. In NO (～1 atm) the rate of formation of MEK was about twice that in the absence of NO, whereas the SBA was greatly reduced. This reaction was also affected by the ratio S/V of different reaction vessels. It was again concluded that in a spherical reaction vessel, the rate of MEK formation was essentially homogeneous and first order. This rate is given by k_obs = 10^12.9–35.4/θ sec^?1, very similar to k₅. However, although it is clear that the rate of formation of MEK is doubled in the presence of NO, the value for k_obs makes it difficult to associate this extra MEK with the disproportionation of s-\documentclass{article}\pagestyle{empty}\begin{document}${\rm Bu}\mathop {\rm O}\limits^{\rm .}$\end{document} and NO: s-\documentclass{article}\pagestyle{empty}\begin{document}$s{\rm - Bu}\mathop {\rm O}\limits^{\rm .} + {\rm NO}\mathop \to \limits^{\rm 6} {\rm MEK} + {\rm HNO}$\end{document}. NO at temperatures of 130–160°C completely suppresses AcH formation. AcH reappears at higher temperatures (165–200°C), enabling k_3c to be determined. Ignoring reaction (6), d(AcH)/dt = k₁k₃ (SBN )/[k_3c + k₂(NO)]; k_3c = 10^14.8–15.3/θ sec^?1. Inclusion of reaction (6) into the mechanism makes very little difference to the result. Reaction (3c) is expected to be a pressure-dependent process.     

Dark field microscopy for diffraction analysis of amorphous carbon solids

Diffraction methods for determining structure in non-crystalline materials often rely solely on the determination of pair correlation functions, extracted from measurements of the diffracted intensity. A dark field image of a non-crystalline solid taken with a conventional transmission electron microscope contains phase information lost in the measurement of the diffracted intensity which can be accessed by evaluating a variance function. This variance function is defined in terms of spherical averages of the diffracted intensity and the mean square of the diffracted intensity. The latter contains higher order correlation information derived from correlations between two pairs of atoms. We examine the sensitivity of the variance function, to subtle atomic structural differences between carbon network structures. The structures have similar pair correlations, but different levels of diamond like bonding. The variance function is shown to give improved discrimination between the networks.     

X-ray scattering study of thin films of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene)

Poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene), PBTTT, is a semiconducting polymer that forms thin film transistors (TFTs) with high field effect mobility on silicon dioxide dielectrics that are treated with alkyltrichlorosilanes ( approximately 0.2 to 0.5 cm2/V s) but forms TFTs with poor mobility on bare silicon dioxide (<0.005 cm2/V s). The microstructure of spin-coated thin films of PBTTT on these surfaces was studied using synchrotron X-ray diffraction and atomic force microscopy. PBTTT crystallizes with lamellae of pi-stacked polymer chains on both surfaces. The crystalline domains are well-oriented relative to the substrate in the as-spun state and become highly oriented and more ordered with thermal annealing in the liquid crystalline mesophase. Although the X-ray scattering from PBTTT is nearly identical on both surfaces, atomic force microscopy showed that the domain size of the crystalline regions depends on the substrate surface. These results suggest that electrical transport in PBTTT films is strongly affected by the domain size of the crystalline regions and the disordered regions between them.     

Fluorinated NLO polymers with improved optical transparency in the near infrared

The absorption from vibrational overtones in the near infrared and specifically at 1.55 μm, is a major contribution to the optical loss of NLO polymer waveguides. In an effort to reduce this value, the specific molecular vibrations responsible, which had previously been predicted by theoretical means, were identified by solvent spectra analysis. A scheme was proposed to reduce the intrinsic absorption through synthetic modification of the polymer by replacement of appropriate aliphatic C? H bonds with C? F bonds. Specific bonds were chosen for replacement based on a consideration of both contribution to intrinsic absorption as well as ease of synthetic modification. The target polymer was synthesized and its absorption measured by a solution technique. A reduction in optical loss at 1.55 µ from 0.75 to 0.35 dB cm^?1 was achieved. © 1995 John Wiley & Sons, Inc.