Improved sensitivity gas detection by spontaneous Raman scattering

Accurate, real-time measurement of the dilute constituents of a gaseous mixture poses a significant challenge usually relegated to mass spectrometry. Here, spontaneous Raman backscattering is used to detect low pressure molecular gases. Rapid detection of gases in the ∼100 parts in 106 (ppm) range is described. Improved sensitivity is brought about by use of a hollow-core, photonic bandgap fiber gas cell in the back-scattering configuration to increase collection efficiency and an image-plane aperture to greatly reduce silica-Raman background noise. Spatial and spectral properties of the silica noise were examined with a two-dimensional CCD detector array. © 2009 Optical Society of America

Diffusion in Model Networks as Studied by NMR and Fluorescence Correlation Spectroscopy

We have studied the diffusion of small solvent molecules (octane) and larger hydrophobic dye probes in octane-swollen poly(dimethyl siloxane) linear-chain solutions and end-linked model networks, using pulsed-gradient nuclear magnetic resonance (NMR) and fluorescence correlation spectroscopy (FCS), respectively, focusing on diffusion in the bulk polymer up to the equilibrium degree of swelling of the networks, that is, 4.8 at most. The combination of these results allows for new conclusions on the feasibility of different theories describing probe diffusion in concentrated polymer systems. While octane diffusion shows no cross-link dependence, the larger dyes are increasingly restricted by fixed chemical meshes. The simple Fujita free-volume theory proved most feasible to describe probe diffusion in linear long-chain solutions with realistic parameters, while better fits were obtained assuming a stretched exponential dependence on concentration. Importantly, we have analyzed the cross-link specific effect on probe diffusion independently of any specific model by comparing the best-fit interpolation of the solution data with the diffusion in the networks. The most reasonable description is obtained by assuming that the cross-link effect is additive in the effective friction coefficient of the probes. The concentration dependences as well as the data compared at the equilibrium degrees of swelling indicate that swelling heterogeneities and diffusant shape have a substantial influence on small-molecule diffusion in networks.

Identification and quantification of explosives in nanolitre solution volumes by Raman spectroscopy in suspended core optical fibers

A novel approach for identifying explosive species is reported, using Raman spectroscopy in suspended core optical fibers. Numerical simulations are presented that predict the strength of the observed signal as a function of fiber geometry, with the calculated trends verified experimentally and used to optimize the sensors. This technique is used to identify hydrogen peroxide in water solutions at volumes less than 60 nL and to quantify microgram amounts of material using the solvent’s Raman signature as an internal calibration standard. The same system, without further modifications, is also used to detect 1,4-dinitrobenzene, a model molecule for nitrobenzene-based explosives such as 2,4,6-trinitrotoluene (TNT).Georgios Tsiminis, Fenghong Chu, Stephen C. Warren-Smith, Nigel A. Spooner and Tanya M. Monr

Optical and Electrical Properties of Percolated Graphene Networks from Liquid Exfoliation of Graphite

Abstract not Available.</jats:p

Optical and Electrical Properties of Graphene Percolated Networks from Liquid Exfoliation of Graphite

Thin films obtained from liquid phase exfoliation of graphite exhibit the conductivity and transparency properties of percolated networks having an interconnectivity distance of about 2 micrometers to 3 micrometers. The mean radius of the platelets in solution determined by dynamic light scattering measurements was found to be in the range of about 3 micrometers. Representative transmittance electron microscope images show evidence of individual graphene sheets. The platelets have an irregular shape with apparent folding and bending, which contributes to disorder and imperfections of the films as revealed by the Raman spectra. The scaling universal exponents describing the percolation transition from an insulating, 100 % transparent state, to a transparent, conducting state are consistent with the two-dimensional (2D) percolation model. These findings provide a framework for engineering the optical and electrical properties of graphene networks for technological applications where flexibility, transparency, and conductivity are required.</jats:p

Appendix B. Stability of the stationary state given by Eqs. 16–18.

Stability of the stationary state given by Eqs. 16–18