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ErxYb(3-x)Q9 (1, x = 1, 2), NdYb2Q9 (2), NdEr2Q9 (3) and NdErYbQ9 (4), have been obtained through a simple molecular strategy, by controlling reactant stoichiometry. In 2-4, the templating effect of the molecular framework allows to control the metal distribution across the coordination sites where the central position is occupied by the larger Nd ion and the terminal ones by the almost vicariants Yb3+ and Er3+, drastically reducing molecular speciation. Remarkably, 4 represents the first example of a discrete molecular entity containing three different Ln ions simultaneously emitting in three different spectral regions in the NIR, upon single visible wavelength excitation. Highly transparent and homogeneous doped silica glasses have been prepared, which show the same optical properties of the incorporated complex in solution.
We report successful tuning of laser wavelength from ∼420 nm to ∼600 nm in epitaxially aligned nanofibers grown by periodic deposition of para-sexiphenyl (p6P) and sexithiophene (6T) on p-6P/muscovite mica templates. The nanofibers were photoexcited by subpicosecond pulses tuned to the lowest p6P absorption band, and the emission of 6T, whose coverage was kept in the submonolayer regime, was efficiently sensitized through resonance energy transfer (RET).
The 6T lasing was achieved at room temperature with threshold fluences as low as 10 μJ/cm2 per pulse. Transient photoluminescence measurements, with picosecond resolution, showed that at these pump fluences the decay dynamics of 6T emission is independent of the excitation density, thereby demonstrating the attainment of room-temperature monomolecular lasing from epitaxially oriented 6T submonolayer aggregates. Main lasing properties remained unaltered upon direct photoexcitation of 6T below the p6P absorption edge.
We report on the development of a plasmonic-photonic coupled platform based on a plasmonic periodic nanostructure and a host matrix for active media in the visible-near infrared range, constituted by a thin film of sol-gel glass. Here, we report on preliminary results about two main tasks of the research work. On one side, we have studied and optimized the surface that supports plasmonic resonances with tunable wavelengths. On the other side, we focused on improving the sol-gel techniques to form and deposit thin films appropriate for covering the previous surface as well as to protect it (i.e. for sensing applications), embed suitable fluorophores (for active device applications) while avoiding metal-induced radiative emission quenching. Besides structural and optical characterization of the considered structures and films, finite-difference time-domain numerical simulations have been performed, in order to give a feedback on the structure features and thereby interpret its optical response.
We devise an experiment, variable pulse rate photoluminescence, to control the accumulation of charges and the activation of charge traps in colloidal nanocrystals. The dynamics of these states is studied, with pulse repetition frequencies ranging from a few hundred hertz to the megahertz regime, by monitoring photoluminescence spectrograms with picosecond temporal resolution. We find that both photocharging and charge trapping contribute to photoluminescence quenching, and both processes can be reversibly induced by light.
CdSe/CdS colloidal nanocrystals are light-emitting nanoparticles with remarkable optical properties such as suppressed fluorescence blinking and enhanced emission from multiexciton states. These properties have been attributed to the suppression of non-radiative Auger recombination. In this work we employ ultrafast spectroscopy techniques to identify optical signatures of neutral and charged excitonic and multiexcitonic states.
We demonstrate that drop cast films of colloidal nanocrystals containing excess surfactants develop aggregation over several weeks after solvent evaporation. Fingered structures, typical of diffusion-limited aggregation, are created because of the residual mobility that nanocrystals retain in the film. We are able to control the aggregation through the concentration of surfactant molecules and the drying temperature of the films.
We characterized the optical nonlinearities of CdSe nanocrystals surrounded by rod-like CdS shells with ultrafast measurements of time-resolved photoluminescence. We measured the exciton-exciton interaction to be, depending on structure details, attractive or repulsive, by as much as 29 meV, due to the unique band alignment in the CdSe/CdS. This feature makes CdSe/CdS dot/rods promising gain media for solution-processable lasers, as it appears combined with 80% photoluminescence quantum yield, narrow size and shape distributions and the antenna effect of the CdS rod shell enhancing optical absorption by more than a factor 50 with respect to bare dots.
The optical properties of substituted and unsubstituted oligothiophenes are analysed with respect to their supramolecular organization in the solid state. The spectra typical of the isolated molecules are obtained by reducing the intermolecular interactions by both lateral chain substitution and by inclusion in a host organic crystal. The photophysical properties of the weakly interacting oligothiophenes are strongly influenced by their backbone conformation and conformational mobility. Oligomers included in the channels of a guest crystal show fast torsional relaxation processes during the photoexcitation. Powders of β-substituted oligomers display optical properties dependent on the conformation of their particular chain packing arrangement.
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