Angle-dispersive powder diffraction at high pressure using an image-plate area detector

Abstract

An imaging-plate system designed for the full Rietveld refinement of crystal structures at high pressure is described. Emphasis is given to techniques that have been developed to obtain data free from contaminating diffraction peaks and to a general method of processing the diffraction data. The advantages of using pressure cells that allow full diffraction patterns to be collected are also described.

Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for High Pressure Crystallography’, Daresbury Laboratory 20-21 July 1991     

Comparison of D.C. and A.C. electro-analytical methods for measuring diode ideality factors and series resistances of silicon solar cells

The diode ideality factor (m) and the series resistance (R_s) of a Si solar cell represent two critical performance-indicator parameters of the device. Since both m and R_s are functions of voltage (V) and temperature (T), simultaneous electrical measurements of these parameters under variable conditions of V and T can often be difficult with traditional direct current (D.C.) techniques. Using the electro-analytical method of linear sweep voltammetry (LSV) and a commonly available Si solar cell, we explore these specific confines of such D.C. measurements. The results are compared with those obtained from a parallel set of alternating current (A.C.) measurements using impedance spectroscopy (IS). LSV provides the main D.C. parameters (open circuit voltage, short circuit current, fill factor, and efficiency) of the cell, but is limited in terms of independently measuring m and R_s beyond strong forward biased conditions. The IS approach is free of the latter experimental constraints, and at the same time can provide several other important electrical parameters of the solar cell. Specifically, IS detects the presence of a low-high (p–p⁺) junction at the back surface of the cell, and serves as an efficient probe of certain electrical characteristics of this junction.     

Improved silicon nitride surfaces for next-generation microarrays

This work reports how the use of a standard integrated circuit (IC) fabrication process can improve the potential of silicon nitride layers as substrates for microarray technology. It has been shown that chemical mechanical polishing (CMP) substantially improves the fluorescent intensity of positive control gene and test gene microarray spots on both low-pressure chemical vapor deposition (LPCVD) and plasma-enhanced chemical vapor deposition (PECVD) silicon nitride films, while maintaining a low fluorescent background. This results in the improved discrimination of low expressing genes. The results for the PECVD silicon nitride, which has been previously reported as unsuitable for microarray spotting, are particularly significant for future devices that hope to incorporate microelectronic control and analysis circuitry, due to the film's use as a final passivating layer.     

Time-Resolved FRET and FLIM of Four-way DNA Junctions

Conformational transitions in a 4-way DNA junction when titrated with ionic solutions are studied using time-resolved fluorescence resonance energy transfer. Parameters characterising the transition in terms of critical ion concentration (c _1/2) and the Hill coefficient for ion binding are obtained by fitting a simple two-state model using steady-state spectra. Data obtained from a fluorescence lifetime plate reader and analysed by fitting a single exponential to donor fluorescence lifetime decays are shown to be in good agreement with the parameters obtained from steady-state measurements. Fluorescence lifetimes, however, offer advantages, particularly in being independent of fluorophore concentration, output intensity, inhomogeneity in the excitation source and output wavelength. We demonstrate preliminary FRET-FLIM images of DNA junction solutions obtained using a picosecond gated CCD which are in agreement with results from a fluorescence lifetime plate reader. The results suggest that time-resolved FRET-FLIM is sensitive to subtle structural changes and may be useful in assays based on 4-way DNA junctions.     

Helium states II. Perturbation treatment of the ground state using the coordinates r< and r>

In an attempt to improve upon the convergence properties of the Hylleraas-Scherr-Knight-Midtdal perturbation expansion for the ground-state energies and eigenfunctions of the helium isoelectronic sequence, the term r is included in the zeroth-order Hamil-tonian. This term dominates the usual perturbation r for the ground state of these systems, and by removing it from H⁽¹⁾ we substantially reduce, in some sense, its size. In order to find the exact eigenfunction of the resulting zeroth-order Hamiltonian it was found necessary to include in H⁽⁰⁾ two additional terms involving the delta function δ(r₁ ? r₂) = δ(r_< ? r_>) and one such term in H⁽¹⁾. Approximate first- and second-order eigenfunctions are calculated variationally giving the energies to fifth order. The results are disappointing. The errors in the energies to fifth order for He, Li⁺, and Be²⁺, although quite small, are significantly larger than the corresponding errors in the more conventional perturbation treatment. Reasons for the failure to improve upon the earlier results are discussed. A “paradox” noted some time ago by Snyder and Parr is examined in an Appendix.     

Probing the Liquid-State Structure and Dynamics of Aqueous Solutions by Fluorescence Spectroscopy

Time-resolved fluorescence spectroscopy of the solvent-sensitive molecule 1,8-anilinonaphthalene sulfonate (ANS) is used to probe the structure and dynamics of an aqueous methanol solution (mole fraction = 0.5). The intensity decay of ANS in the mixed solvent displays single exponential kinetics under ambient conditions. At low temperature, a simple two-state solvent relaxation model describes the fluorescence decay for ANS in both methanol and the mixed solvent. The temperature dependence of ANS fluorescence in the mixed solvent is attributed to the onset of glassy dynamics in the aqueous component at higher temperature, implying a partial demixing of the water and methanol due to self-association. We discuss the absence of more complicated fluorescence decays in such a heterogeneous solvent system.