Elucidation of the structure of a highly efficient blue emitting lithium boron 2-(2-hydroxyphenyl) benzoxazole

We have studied the new blue electroluminescent material (compound 1) synthesized from the reaction of 2-(2-hydroxyphenyl)benzoxazole with LiBH₄ according to Tao's procedure [J. Am. Chem. Soc. 121 (1999) 9447; Appl. Phys. Lett. 75 (1999) 1665] Compound 1 showed the UV and the PL peak at 366 and 432 nm, respectively. The manufactured device (ITO/compound 1(600 Å)/Al) showed the EL peak at 412 nm. However, the structure of compound 1 was not fully identified as examined by experiment. In order to disclose the structure of compound 1, we proposed several possible compounds and calculated each compound, using the following methods. Hartree–Fock with the 3-21G(d) basis set was used for energy calculation and geometry optimizations of those compounds. Excitation energies were obtained from time-dependent density functional theory (TD-DFT) using the B3LYP functional with the 3-21G(d) basis set. As a result, [(box)(boxH)Li]₂ was found to be one of the possible compounds which had the most stable geometry structure. Its optical properties showed remarkably good agreement with the characterization data of compound 1. In addition, the structure of [(box)(boxH)Li]₂ was also compared well with that of LiBq₄ which was reported by the groups of Tao and was argumentatively asserted by Radu and coworkers [J. Am. Chem. Soc. 125 (2003) 880].     

CBE growth of GaAs/GaAlAs HBTs using the new DEAlH-NMe3 precursor and all-gaseous dopants

This paper describes the first reported use of diethylaluminium hydride-trimethylamine adduct (DEAlH-NMe₃) for the growth of GaAs/GaAlAs power heterojunction bipolar transistors (HBTs) by chemical beam epitaxy (CBE). This precursor possesses a significantly higher vapour pressure than the more conventionally used triethylaluminium (TEA), and leads to much less stringent requirements for bubbler and gas-line heating, and also much-improved GaAs/GaAlAs heterojunction definition when no carrier gas is employed. The use of all-gaseous n- and p-type dopants offers significant technological advantages in CBE, and the current paper also provides the first report of the use of hydrogen sulphide for n-type doping of CBE-grown GaAlAs HBT emitter regions. In conclusion, DC and RF data obtained from the heterojunction bipolar transistors fabricated to date are described. A DC gain of 40 has already been measured and encouraging early data obtained from RF-probed devices are also presented.     

High-performance liquid chromatography systems for the separation of local anaesthetic drugs with applicability to the analysis of illicit cocaine samples

Two high-performance liquid chromatography (HPLC) systems are presented which are suitable for the separation of local anaesthetic drugs. An octadecyl-silica column is used with aqueous methanolic eluents (15 and 50% respectively) containing n-hexylamine-orthophosphoric acid buffers and data for 36 compounds are given. The HPLC systems have application for the identification of unknown drugs in this class while the first eluent (15% methanol, pH 2.5) is particularly useful for the examination of illicit cocaine samples because it gives good separation of common adulterants and impurities.     

Vanadium (III), oxovanadium (IV) and oxovanadium (V) trifluoro-methanesulfonates

V(SO₃CF₃)₃, VO(SO₃CF₃)₂ and VO(SO₃CF₃)₃ have been prepared by reacting V(O₂CCF₃)₃, VO(O₂CCF₃)₂ and VOC1₃ with HSO₃CF₃. The i.r. data suggest a bridging bidentate nature for SO₃CF₃ groups. The diffuse reflectance spectrum of V(SO₃CF₃)₃ suggests hexacoordination of vanadium, whilst that of VO(SO₃CF₃)₂ is comparable to either five or six coordinated oxovanadium (IV) systems. The magnetic moments of V(SO₃CF₃)₃ and VO(SO₃CF₃)₂ are slightly lower than the spin-only values. Thermal decomposition of these triflates is simple. All the three triflates form coordination complexes with pyridine, 2, 2′-bipyridyl and triphenylphosphine oxide.     

Menus of kuratowski subgraphs

Motivated by heuristic embedding algorithms, this paper is concerned with the organization of potentially large lists of Kuratowski subgraphs of an arbitrary nonpianar graph. A graphical structure called a "nearly Hamiltonian" graph is defined. It is shown that lists of Kuralowski subgraphs can be lexicographically organized in such structures. It is shown that any nonpianar graph contains such structures and at least one such structure with a nonempty list of Kuratowski subgraphs can be located in linear time in ihe edges of the graph.     

Transient non-linear phenomena and metastable states of charge-density waves in niobium triselenide

Details are given of the transient response of NbSe₃ at 42 K to currents I(t) of various repetitive pulsed waveforms. Nonlinear conduction, due supposedly to motion of one of the two charge-density waves (CDWs) present, was measured as U(t) = IR₀ ? V, where V(t) is the voltage developed across the specimen and R₀ is its resistance in the Ohmic regime.With unidirectional pulses two threshold currents for nonlinearity were observed. On passing the lower threshold, a gradual rise (time-constant 50 μs) of U towards the d.c. value was seen; this behaviour was shown not to originate in the inertia of the CDW, and probably arose thermally. Only after the second threshold was passed did U appear to rise immediately current was applied. The existence of two thresholds accounts for a discrepancy between pulsed and d.c. measurements of conductivity noted by Brill et al. (1981), and also for a phenomenon previously attributed to “locking” between the two CDWs [6].When the pulses were alternately negative- and positive-going, |U| rose beyond its eventual (d.c.) value, towards which it subsequently decayed. A study of this “overshoot” phenomenon has shown the speciment to be left, after a current pulse, in a long-lived metastable state in which pinning stabilises some distortion of the CDW, presumably similar to that of which recent electron micrographs [11] appear to be evidence.A simple model, associating the overshoot with transitions between metastable states, adequately describes the conditions for its occurrence. However, the origin of the conduction associated with the moving CDW remains uncertain, both the Frohlich mechanism and a single-electron alternative finding some experimental support.     

Hall effect and magnetoresistance of (SN)x films

Hall effect and electrical conductivity have been investigated between 77 K and 300 K and the magnetoresistance at 4.2 K for a number of (SN)_x films deposited at substrate temperatures between — 10 and 50°C. The small magnitude of the Hall mobility (? 1 cm² Vsec^?1 at 300 K) and its activated temperature dependence are interpreted in terms of a heterogenous model for (SN)_x films with thin depletion layers separating highly conductive islands. The hole concentration in these islands (p ≈ 10²¹ cm^?3, the microscopic mobility (μ ≈ 500 cm² Vsec^?1 at 4.2 K) and the temperatures dependence of μ are found to be close to values for (SN)_x crystals.     

Assessment of noise from “quiet” pile drivers

This paper summarizes the results of a study aimed at defining the basic characteristics of noise from a range of pile driving devices which were either adapted or designed specifically to generate noise levels below those normally expected. The parameters studied were noise levels, spectra and waveform shapes. In addition, the noise reduction techniques employed by each manufacturer are described in detail.     

Identification and composition of cuticular hydrocarbons of the major Afrotropical malaria vector Anopheles gambiae s.s. (Diptera: Culicidae): analysis of sexual dimorphism and age-related changes

Forty-eight cuticular hydrocarbons (CHCs) were characterized by gas chromatography-mass spectrometry from the epicuticular surface of the major Afrotropical malaria vector Anopheles gambiae. The hydrocarbons identified were 14 n-alkanes, 16 monomethyl alkanes, 13 dimethyl alkanes, 5 alkenes, with main-chain lengths ranging from C(17) to C(47), and the results are consistent with those from other Culicidae species. Qualitative differences were not observed between laboratory pools of three females and males, between different age-groups (0-16 days) and between single field specimens, whereas quantitative differences in CHC profiles were observed. Differences between sexes were more marked in individuals aged 0-2 days than in older ones. Both sexes undergo strong CHC profile changes with age, and individuals aged 0-2 days differ remarkably from the older ones. The possibility of exploiting these changes for estimating the age of mosquito was explored through multivariate analyses of the relative abundance of the compounds, using either the whole CHC profile or a subset of CHCs. Such a method allows us to assign more than 85% of females and 75% of males to the correct age-group. Although preliminary, these results show that the method is promising, as it has already been shown in Aedes aegypti and An. stephensi. The correct determination of the vector age (particularly in the case of the An. gambiae complex of sibling species) provides valuable information in malaria epidemiology and in evaluation of the effectiveness of vector control strategies. Further efforts will be made to validate this method on single specimens reared in seminatural conditions before being proposed to medical entomologists working in the Afrotropical region.