Robin Nicholas

Abstract:

Energy relaxation of hot Dirac fermions in bilayer epitaxial graphene is experimentally investigated by magnetotransport measurements on Shubnikov-de Haas oscillations and weak localization. The hot-electron energy loss rate is found to follow the predicted Bloch-Grüneisen power-law behaviour of T(4) at carrier temperatures from 1.4 K up to ∼100 K, due to electron-acoustic phonon interactions with a deformation potential coupling constant of 22 eV. A carrier density dependence n(e)(-1.5) in the scaling of the T(4) power law is observed in bilayer graphene, in contrast to the n(e)(-0.5) dependence in monolayer graphene, leading to a crossover in the energy loss rate as a function of carrier density between these two systems. The electron-phonon relaxation time in bilayer graphene is also shown to be strongly carrier density dependent, while it remains constant for a wide range of carrier densities in monolayer graphene. Our results and comparisons between the bilayer and monolayer exhibit a more comprehensive picture of hot carrier dynamics in graphene systems.

Abstract:

We report the synthesis of four new cationic dipolar push–pull dyes, together with an evaluation of their photophysical and photobiological characteristics pertinent to imaging membranes by fluorescence and second harmonic generation (SHG). All four dyes consist of an N,N-diethylaniline electron-donor conjugated to a pyridinium electron-acceptor via a thiophene bridge, with either vinylene (–CH=CH–) or ethynylene (–C≡C–) linking groups, and with either singly-charged or doubly-charged pyridinium terminals. The absorption and fluorescence behavior of these dyes were compared to a commercially available fluorescent membrane stain, the styryl dye FM4-64. The hyperpolarizabilities of all dyes were compared using hyper-Rayleigh scattering at 800 nm. Cellular uptake, localization, toxicity and phototoxicity were evaluated using tissue cell cultures (HeLa, SK-OV-3 and MDA-231). Replacing the central alkene bridge of FM4-64 with a thiophene does not substantially change the absorption, fluorescence or hyperpolarizability, whereas changing the vinylene-links to ethynylenes shifts the absorption and fluorescence to shorter wavelengths, and reduces the hyperpolarizability by about a factor of two. SHG and fluorescence imaging experiments in live cells showed that the doubly-charged thiophene dyes localize in plasma membranes, and exhibit lower internalization rates compared to FM4-64, resulting in less signal from the cell cytosol. At a typical imaging concentration of 1 μM, the doubly-charged dyes showed no significant light or dark toxicity, whereas the singly-charged dyes are phototoxic even at 0.5 μM. The doubly-charged dyes showed phototoxicity at concentrations greater than 10 μM, although they do not generate singlet oxygen, indicating that the phototoxicity is type I rather than type II. The doubly-charged thiophene dyes are more effective than FM4-64 as SHG dyes for live cells.

Abstract:

Organic and hybrid organic-inorganic systems are promising candidates for low cost photovoltaics. Recently, perovskite-based systems have been attracting a large amount of research attention, where the highest performing devices employ a small molecule (2,2',7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene) (Spiro-OMeTAD) hole transporter. Here, we demonstrate the production of single-walled carbon nanotube (SWNT)/single molecule nanostructures using a simple solution processing technique for effective and strong binding of Spiro-OMeTAD to individual polymer-wrapped SWNTs. These small molecules bind very strongly, which causes both large mechanical strain of the nanotubes and also improves the separation of individual SWNTs, thus improving the nanotube photoluminescence quantum efficiency by 1 order of magnitude compared to simple polymer-nanotube nanohybrids. Using absorption and photoluminescence measurements, we show that there is a dramatic variation in the electronic properties of the polymer-NT nanocomposites due to the band alignment formed with Spiro-OMeTAD. These self-assembled nanocomposites offer the potential for integration into high performance optoelectronic such as photovoltaic cells and light emission devices.

Abstract:

Here, we report the use of polymer-wrapped carbon nanotubes as a means to enhance charge extraction through undoped spiro-OMeTAD. With this approach a good solar cell performance is achieved without the implementation of conventional doping methods. We demonstrate that a stratified two-layer architecture of sequentially deposited layers of carbon nanotubes and spiro-OMeTAD, outperforms a conventional blend of the hole-conductor and the carbon nanotubes. We also provide insights into the mechanism of the rapid hole extraction observed in the two-layer approach.