J.F. Ryan

Dynamics of bacteriorhodopsin 2D crystal observed by high-speed atomic force microscopy

We have used high-speed atomic force microscopy to study the dynamics of bacteriorhodopsin (bR) molecules at the free interface of the crystalline phase that occurs naturally in purple membrane. Our results reveal temporal fluctuations at the crystal edges arising from the association and dissociation of bR molecules, most predominantly pre-formed trimers. Analysis of the dissociation kinetics yields an estimate of the inter-trimer single-bond energy of -0.9kcal/mol. Rotational motion of individual bound trimers indicates that the inter-trimer bond involves W10-W12 tryptophan residues.

Experimental and Theoretical Studies of Effects of Anharmonicity and High-Temperature Disorder on Raman Scattering in Fluorite Crystals

Some crystals with the fluorite structure, e.g. CaF2, SrF2, BaF2, SrCl2 and PbF2, are known to exhibit a specific heat anomaly at temperatures Tc well below the melting point. It is generally assumed that this anomaly is associated with the development of extensive disorder in the anion sublattice. To study this disorder we have carried out polarized Raman scattering investigations of these crystals in the temperature range 4-1420 K. Fluorite crystals have one Raman-active phonon with T2g symmetry. We find th at the position and shape of this line below Tc can be explained in detail by using third- and fourth-order anharmonicity. At Tc and above additional scattering develops on the low-energy side of the T2g phonon and this is accounted for by a theory of defect-induced scattering which includes effects of both anion vacancies and interstitials. Both force-constant and polarizability changes are considered in a nearest-neighbour approximation.

Optical time-of-flight measurement of carrier diffusion and trapping in an InGaAs/InP heterostructure

We have measured the diffusion and trapping of photoexcited hot carriers in an InGaAs/InP heterostructure using an optical time‐of‐fight technique with picosecond time resolution. The ambipolar diffusivity is found to decrease by an order of magnitude between 4 K and room temperature, and the efficiency of trapping of carriers into the well increases rapidly in the same temperature range. A mean trapping time of 4 ps is measured for a 50 A well.

Nonlinear excitonic optical absorption in GaInAs/InP quantum wells

We have studied the saturation of optical absorption in GaInAs/InP quantum wells at room temperature. Using optical excitation from a tunable cw Co:MgF2 laser, we find a saturation intensity of 70 W cm−2 when exciting resonantly at the n=1 heavy‐hole exciton, and we deduce values of the nonlinear absorption and refraction coefficients −60 cm W−1 and −0.3 cm2 kW−1, respectively. The saturation intensity in the quantum well is significantly lower than in bulk GaInAs, and also in GaAs quantum wells.

Hot carrier energy loss rates in GaInAs/InP quantum wells

We have measured the energy loss rates of hot carriers in bulk Ga.47In.53As and in Ga.47In.53As/InP quantum wells of widths 154 A and 14 A. We find that the energy loss rates are considerably lower than predicted by the unscreened carrier-LO phonon interaction. We show that the discrepancy may be resolved, at least in part, by invoking the presence of a non-equilibrium phonon distribution, the magnitude of the effect depending on the well width.

Correlation between conjugation length and non-radiative relaxation rate in poly(p-phenylene vinylene): a picosecond photoluminescence study

Time-resolved photoluminescence of poly(p-phenylene vinylene) in the picosecond regime shows that the non-radiative decay rate increases with increasing polymer chain conjugation length and that the luminescence decay is much faster for excitation parallel to the polymer chain than for perpendicular excitation. The results provide evidence for bimolecular recombination processes.

Formation of self-assembled InAs quantum dots on (110) GaAs substrates

The formation of indium arsenide self-assembled quantum dots (QDs) has been observed on (110) GaAs substrates. The quantum dots were grown using solid source molecular-beam epitaxy. Both (110) and (100) GaAs wafers were placed in the growth chamber and grown on simultaneously. QDs were found to form on the (110) GaAs substrate when grown upon a thin AlAs layer. Buried layers of InAs were studied using photoluminescence spectroscopy, while surface layers were studied by atomic force microscopy. While the luminescence spectra for simultaneously grown (110) and (100) QDs are similar in structure, the changes in each sample’s spectrum as a function of both temperature and excitation intensity suggest quantum dot density is over an order of magnitude lower on (110) samples than it is on the (100) samples. This difference is conditionally confirmed by atomic force microscopy studies.

Electrical conductance and breakdown in individual CNx multiwalled nanotubes

Doping of carbon nanotubes with nitrogen during growth strongly modifies their electronic structure through n-type doping. This provides the possibility of producing nanotubes with high conductances, independent of tube chirality. To date, electrical measurements on individual nitrogen-doped multiwalled nanotubes (CNx MWNTs) have reported surprisingly low conductances (∼0.01G0). Here the authors present high conductance (1.0±0.3G0) measurements at low bias for individual CNx MWNTs. Conductance increases linearly with voltage at a rate of 0.7±0.2G0∕V until the threshold for electrical breakdown is reached. Discrete current steps of 20±10μA are then observed.

Raman scattering from electronic excitations in periodically δ-doped GaAs

Abstract We report photoluminescence and Raman scattering measurements of periodically δ-doped Si : GaAs. The spectra of short-period structures are similar to those of uniformly doped material, but new lines appear in the Raman spectra of longer-period structures that arise from inter-subband transitions between confined electron levels in a single δ-layer or between minibands in the δ-doping superlattice.

Optical studies of excitons in Ga0.47In0.53As/InP multiple quantum wells

We report optical absorption and photoluminescence measurements of excitons in Ga0.47In0.53As/InP multiple quantum wells grown by metalorganic chemical vapor deposition. At 4 K the luminescence linewidth for n=1 heavy‐hole excitons is measured to be 7 meV for a 30‐period structure with wells of width 154 A. The absorption spectrum at low temperature shows four peaks which we assign to confined heavy‐hole excitons. A theoretical calculation of the energies of these states indicates that the ratio of the conduction‐band to valence‐band energy discontinuities is 45:55.