Carbon Nanotube Reinforced Structural Composite Supercapacitor.

Carbon nanotubes exhibit mechanical properties ideally suited for reinforced structural composites and surface area and conductivity attractive for electrochemical capacitors. Here we demonstrate the multifunctional synergy between these properties in a composite material exhibiting simultaneous mechanical and energy storage properties. This involves a reinforcing electrode developed using dense, aligned carbon nanotubes grown on stainless steel mesh that is layered in an ion conducting epoxy electrolyte matrix with Kevlar or fiberglass mats. The resulting energy storage composites exhibit elastic modulus over 5 GPa, mechanical strength greater than 85 MPa, and energy density up to 3 mWh/kg for the total combined system including electrodes, current collector, Kevlar or fiberglass, and electrolyte matrix. Furthermore, findings from in-situ mechano-electro-chemical tests indicate simultaneous mechanical and electrochemical functionality with invariant and stable supercapacitor performance maintained throughout the elastic regime.

Strong Light-Matter Coupling and Hybridization of Molecular Vibrations in a Low-Loss Infrared Microcavity.

Hybridized polaritons are generated by simultaneously coupling two vibrational modes of two different organic materials to the resonance of a low-loss infrared optical microcavity. A thin film of poly methyl methacrylate with solvent molecules of dimethylformamide trapped inside provided two spectrally narrow, closely spaced carbonyl stretches with absorption peaks at 1731 and 1678 cm(-1). Situating this film in a microcavity based on Ge/ZnS distributed Bragg reflector mirrors produced three distinct polariton branches in the dispersion relation due to hybridization of the vibrational resonances. Two anticrossings were observed with Rabi splittings of 9.6 and 5.2 meV, between the upper-to-middle and middle-to-lower polariton branches, respectively. This system marks the first demonstration of polariton hybridization between a solid and solvent molecules and can open new paths toward chemical reaction modification and energy transfer studies in the mid-infrared spectral range.

Room temperature fabrication of dielectric Bragg reflectors composed of a CaF2/ZnS multilayered coating.

We describe the design, fabrication, and characterization of mechanically stable, reproducible, and highly reflecting distributed Bragg reflectors (DBR) composed of thermally evaporated thin films of calcium fluoride (CaF2) and zinc sulfide (ZnS). CaF2 and ZnS were chosen as the low and high refractive index components of the multilayer DBR structures, with n = 1.43 and n = 2.38 respectively, because neither material requires substrate heating during the deposition process in order to produce optical quality thin films. DBRs consisting of seven pairs of CaF2 and ZnS layers, were fabricated with thicknesses of 96 and 58 nm, respectively, as characterized by high-resolution scanning electron microscopy (HR-SEM), and exhibited a center wavelength of λc = 550 nm and peak reflectance exceeding 99%. The layers showed good adhesion to each other and to the glass substrate, resulting in mechanically stable DBR coatings. Complete optical microcavities consisting of two such DBR coatings and a CaF2 spacer layer between them could be fabricated in a single deposition run. Optically, these structures exhibited a resonator quality factor of Q > 160. When a CaF2/ZnS DBR was grown, without heating the substrate during deposition, on top of a thin film containing the fluorescent dye Rhodamine 6G, the fluorescence intensity showed no degradation compared to an uncoated film, in contrast to a MgF2/ZnS DBR coating grown with substrate heating which showed a 92% reduction in signal. The ability to fabricate optical quality CaF2/ZnS DBRs without substrate heating, as introduced here, can therefore enable formation of low-loss high-reflectivity coatings on top of more delicate heat-sensitive materials such as organics and other nanostructured emitters, and hence facilitate the development of nanoemitter-based microcavity device applications.

Non-Poissonian formation of multiple excitons in photoexcited CdTe colloidal quantum qots by femtosecond nonresonant two-photon absorption.

Using direct multiexcitonic spectroscopy, we experimentally observe for the first time the non-Poissonian formation of multiple excitons by femtosecond nonresonant two-photon absorption process in semiconductor colloidal quantum dots (QDs). Each of the multiple excitons is individually generated via the absorption of a pair of photons during the femtosecond pulse irradiation. The non-Poissonian distribution of the generated excitons is reflected as a non-quadratic dependence on the pulse intensity of the average number of excitons per QD. This is the main observation of the present work. It is explained by a multiexcitonic formation model that is based on the phenomenon of intrapulse state filling of the few quantum electronic states accessed by the two-photon transitions. The experiments are conducted with 3.9-nm CdTe QDs in room-temperature hexane solution using the femtosecond pump-probe transient absorption technique, where an intense pump pulse generates the excitons and a weak probe pulse measures their number via intraband one-photon absorption.