Langmuir-Blodgett multilayer films were prepared from N,N′-dioctadecyl rhodamine B (RB18) diluted in dioleoylphosphatidic acid (DOPA) or dipalmitoylphosphatidic acid (DPPA) with various mixing ratios. Surface pressure-area isotherms of monolayers of the two matrices were recorded. Absorption and fluorescence spectra and fluorescence decays of the multilayer samples of RB18 were further examined. The fluorescence decay curves of RB18 in the multilayer assemblies, determined using time-correlated single photon counting, were analyzed in the framework of several models leading to nonexponential decays: (1) multiexponential decay; (2) Gaussian distribution of rate constants; (3) stretched-exponential decay, characteristic of Förster energy transfer in self-similar fractal-like structures with a fractal dimensionality of <2; and (4) Förster energy transfer in a two-dimensional system accompanied by an unquenched decay. Using the global analysis method, only the decay law for Förster energy transfer in two dimensions accompanied by a monoexponential decay yielded acceptable statistical fits for multilayer assemblies of RB18 and DOPA or DPPA in the concentration range from 0.2 to 5 mol %. While global analysis of nonexponential decay profiles gave adequate parameters over the whole concentration range studied, single-curve analysis yielded less accurate values for the decay parameters, which became physically unacceptable at the highest and lowest dye concentrations studied. For the DOPA multilayer systems the recovered decay parameters suggest a homogeneous distribution of the monomer and dimer molecules. On the other hand, for DPPA multilayer films a demixing leading to a two-phase system is suggested. The phase separation during compression of the mixed DPPA monolayer forms regions where almost exclusively H-like dimers are present and regions where monomers and a small amount of quenching H-like dimers coexist.

We report on the effect of rapid thermal annealing (RTA) on the photoluminescence (PL) properties of GaNxAs1−x/GaAs structures. In particular, a blueshift of the PL peak energy is observed when annealing the samples. The results are examined as a consequence of a RTA-induced nitrogen diffusion inside the GaNxAs1−x material rather than diffusion out of the alloy, which homogenizes initial nitrogen composition fluctuations. We propose a simple model that describes the RTA-induced blueshift of the low temperature PL peak energy. This model is in good agreement with experimental results and is consistent with recent studies in which lateral composition fluctuations in the GaNAs alloy were reported.

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We report the observation of spin-to-charge current conversion in strained mercury telluride at room temperature, using spin pumping experiments. We show that a HgCdTe barrier can be used to protect the HgTe from direct contact with the ferromagnet, leading to very high conversion rates, with inverse Edelstein lengths up to 2.0±0.5  nm. The influence of the HgTe layer thickness on the conversion efficiency is found to differ strongly from what is expected in spin Hall effect systems. These measurements, associated with the temperature dependence of the resistivity, suggest that these high conversion rates are due to the spin momentum locking property of HgTe surface states.