Semiconductors group

Abstract:

We present a simulation of exciton dynamics in supramolecular assemblies of an oligo-p-phenylenevinylene derivative monofunctionalised with a quadruple hydrogen-bonding group (MOPV). MOPV molecules form helical stacks in dodecane solution through solvophobic and π-π interactions with thermotropic reversibility. We apply a model of incoherent excitation hopping using a Monte Carlo scheme to extract microscopic physical quantities relevant to energy diffusion and bimolecular annihilation processes within isolated nanostructures. We compare the simulation to ultrafast spectroscopic data, namely photoinduced absorption transients at various excitation fluences, their polarization anisotropy, and the dynamic photoluminescence red-shift. We observe that energy diffusion and bimolecular annihilation processes can be described with the same microscopic model based on a Förster-like model that takes into account the spatial extent of the excited state; these two processes are interconnected via the same underlying physics. We extract a high diffusion coefficient (∼0.08 cm2 s-1) over the first few picoseconds following excitation, which plays an important role in dictating the bimolecular annihilation dynamics. © 2007 American Chemical Society.

Abstract:

The time-resolved conductivity of isolated GaAs nanowires is investigated by optical-pump terahertz-probe time-domain spectroscopy. The electronic response exhibits a pronounced surface plasmon mode that forms within 300 fs before decaying within 10 ps as a result of charge trapping at the nanowire surface. The mobility is extracted using the Drude model for a plasmon and found to be remarkably high, being roughly one-third of that typical for bulk GaAs at room temperature. © 2007 American Chemical Society.

Abstract:

The authors present here a simple analysis that explains the apparent strengthening of electron phonon interaction upon aggregation in conjugated polymer materials. The overall scheme is that of an intermolecular Herzberg-Teller effect whereby sidebands of a forbidden transition are activated by oppositely phased vibrations. The authors show that upon aggregation, the 0-0 emission becomes symmetry forbidden and the apparent redshift and remaining vibronic structure are due to sideband (0-1,0-2, etc.) emission. At higher temperatures, the 0-0 peak is due to thermal population in a higher lying even-parity vibronic state rather than direct emission from the odd-paritied lowest intermolecular vibronic state.

Abstract:

We have investigated the effect of interchain interactions on the ultrafast depolarization of the photoluminescence from solid films of a conjugated polymer. Accurate control was exercised over the interchain separation by threading of the conjugated chains with insulating macrocycles or complexation with an inert host polymer. Our measurements indicate that excitation into the higher electronic states of a chain aggregate is followed by a fast (<100 fs) relaxation into lower excited states with an associated rotation of the transition dipole moment. These findings emphasize the need for consideration of initial excitonic delocalization across more than one polymeric chain.

Abstract:

We have investigated the conductivity of equilibrium and photoexcited electrons in nanoporous indium phosphide (InP) of various porosities and of two orientations: (100) and (111). We observed an enhanced transmission through the nanoporous samples compared with bulk InP, resulting from a suppression of the conductivity by the pores. The frequency-dependent conductivity was extracted numerically from the transmission data. We examined the dynamical conductivity of photoexcited carriers using optical-pump THz-probe spectroscopy. After the rapid photoexcitation of electrons, the timeresolved conductivity was observed to decay slowly, with carrier recombination lifetimes exceeding 1 ns for all (100)- and (111)-oriented samples.