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The influence of aggregates and solvent aromaticity on the photophysics and fluorescence dynamics of two conjugated polymers is studied. The two polymers are derivatives of poly(p-phenylene vinylene) (PPV) containing different kinked moieties along the main chain. The polymers contain 2,6-diphenylpyridine and m-terphenyl kinked moieties and they are abbreviated as PN and PC, respectively. The insertion of kinked segments along the main chain shifts the emission spectrum from the yellow-orange spectral region, common to PPV derivatives, to the blue-green spectral region. The results show that in dilute solutions the polymers decay monoexponentially, while in concentrated ones the fluorescence decays biexponentially, indicating fluorescence quenching. This is attributed to an energy transfer process from polymer chains to aggregates that occurs within a few tens of picoseconds. By comparing the photophysics and fluorescence dynamics of polymer PN in a nonaromatic and an aromatic solvent, we conclude that the polymer conformation adopted in the aromatic solvent leads to a higher fluorescence quantum yield and a longer fluorescence lifetime. Furthermore, the fluorescence quenching of PN because of aggregates is faster and more efficient in the aromatic than in the nonaromatic solvent. These results can be explained through a more extended chain conformation of PN in the aromatic solvent.
Single- and bi-layer MoS2 are two-dimensional semiconductors able to withstand very large deformations before failure, standing out as suitable templates for strain engineering applications and flexible electronics. It is imperative, for the proper integration of this material in practical applications, that the relationship between material property and strain is well understood. Two dimensional MoS2 crystals fabricated by chemical vapor deposition or micromechanical exfoliation are transferred onto flexible substrates and subjected to biaxial tension on a carefully designed and assessed loading stage with high accuracy and control. The successful stress transfer from substrate to the overlying 2D crystal is identified by in-situ monitoring of the strain-induced phonon frequency and photoluminescence peak shifts. Reliable values for the mode Grüneisen parameters and exciton deformation potentials were obtained by studying a significant number of crystals. The experimental results are backed by density functional theory calculations and are in good agreement with the experiments. This work highlights the potential of these materials in strain engineering applications and gives accurate values for single- and bi-layer MoS2 thermomechanical parameters.
To cite this article: Antonios Michail et al 2018 2D Mater. 5 035035 View the article online for updates and enhancements. Related content Improved luminescence properties of MoS2 monolayers grown via MOCVD: role of pre-treatment and growth parameters D Andrzejewski, M Marx, A Grundmann et al.-Growth mechanism of largescale MoS2 monolayer by sulfurization of MoO3 film Payam Taheri, Jieqiong Wang, Hui Xing et al.-Effect of Mo concentration on shape and size of monolayer MoS2 crystals by chemical vapor deposition Wenzhao Wang, Xiangbin Zeng, Shaoxiong Wu et al.
A recently synthesized cationic water-soluble poly(fluorenevinylene-co-phenylenevinylene) was studied by means of steady state and femtosecond time resolved upconversion spectroscopy in aqueous and EtOH solutions. Steady state spectroscopic measurements showed that the polymer emits at the blue-green spectral region and that aggregates are formed in concentrated polymer solutions. The fluorescence dynamics of the polymer in concentrated solutions, studied at a range of emission wavelengths, exhibited a wavelength dependent and multiexponential decay, indicating the existence of various decay mechanisms. Specifically, a rapid decay at short emission wavelengths and a slow rise at long wavelengths were observed. Both features reveal an energy transfer process from isolated to aggregated chains. The contribution of the energy transfer process as well as of the isolated chains and the aggregates on the overall fluorescence decay of the polymer was determined. The dependence of the energy transfer rate and efficiency on polymer concentration was also examined.
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