Micro-Raman spectroscopy has been used to study lattice dynamics associated with the ferroelectric domains of a BiFeO 3 single crystal at low temperature. The phonon assignment shows a large frequency splitting between the transverse and longitudinal components of the A 1 phonon mode related to the Bi-O bonds in contrast with thin films where the splitting is negligible. Applying an external electric field induces frequency shifts of the low energy modes related to the Bi-O bonds. These softenings are due to a tensile stress via the piezoelectric effect. We give estimates of the phonon deformation potentials.

Electrochemical capacitors, also called supercapacitors, are high power devices that exhibit moderate energy density. One interesting strategy to improve this latter parameter is the use of additional redox reactions that can enhance the double layer capacity of carbon electrodes while maintaining high rate capability. Diazonium chemistry is a powerful tool for synthesizing carbonaceous materials with active redox sites over the surface. Grafting of 2-aminoanthraquinone on Kynol active carbon fibers was successfully achieved by reduction of the corresponding diazoquinone. The diazotization reactions were fast and efficient, and the attachment of the quinone groups resulted in a 2.5-fold increase in capacity of the modified carbonaceous material -65 vs. 25 mAh/g for the unmodified carbon cloth. This significant increase in capacity reflects the contribution of the redox reaction of the grafted quinone molecules. Nitrogen gas adsorption measurements showed that the attachment of anthraquinone molecules significantly reduced the specific surface area of the carbon, mainly affecting the micro-porosity of the carbon powder. The electrochemical performance of the modified carbon electrodes was assessed by prolonged cycling experiments, during which capacity fading was observed. The modified carbon electrodes clearly showed very high cycling ability compared to other grafted carbons reported in the literature.