Structure Affects Optical Limiting in Aqueous Suspensions of CNTs
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nanoparticles of organic fluorescent semiconductor materials. The research team said that these techniques have previously been applied to isolated, single-polymer chains (molecules) of conjugated polymers. The investigation of single morphological domains of nanoparticles of organic dyes are of particular interest, as these affect their fluorescent properties, which may find wide application in promising devices such as organic light-emitting diodes (OLEDs). The researchers reported the characterization of N,N’-bis(2,5-di-tertbutylphenyl)-3,4,9,10-perylenedicarboximide (DBPI) nanoparticles by atomic force microscopy (AFM), bulk optical spectroscopy, and single-molecule/particle optical spectroscopy. The researchers said that DBPI nanoparticles were prepared using the reprecipitation method. The optical emission spectra of the nanoparticles showed a mixture of monomer-like (blue, 538 nm and 578 nm) and excimer-like (red, 637 nm) emission. The nanoparticle size and shape were characterized by tapping-mode AFM. They said that a broad size distribution of platelike nanoparticles was observed, varying in height from 1–13 nm and in width from 10–45 nm. All the samples were mixtures of blue and red nanoparticles. The researchers were able to separate blue nanoparticles from red nanoparticles by centrifugation. The density and size of the red particles was such that the centrifugation process removed red particles from fractions with lower sucrose gradients, leaving only blue particles. However, the researchers could not obtain a fraction that contained only red particles. According to the researchers, the two types of particles in the samples probably possess different crystal morphology, designated α and β forms. The α-perylene has a dimeric crystal structure with red emission spectra, and β-perylene has a monomeric crystal structure with blue emission spectra. Alternatively, the morphological difference could find its origin in the size of the nanoparticles, said the researchers; small amorphous particles could show blue emission and the larger crystalline particles could show red emission. Three types of fluorescence time trajectories were found for the nanoparticles: trajectories with constant intensity, trajectories with intensity fluctuations, and trajectories that showed blinking. Time transients of red particles showed only constant intensity or intensity fluctuations. Blue particles, however, showed all three types of behavior. Blinking is caused by reversible oxidation/reduction events at special sites in the polymer backbone to which the excitation 576
energy is efficiently funneled, said the researchers. DBPI nanoparticles could develop low-energy trap sites, which may result in fluorescence blinking. Intensity fluctuations, however, are a typical observation for multichromophoric systems in which the chromophores are not coupled. The researchers said that a significant fraction of the nanoparticles (25%) exhibited fluorescence intermittence (i.e., “blinking”). According to the researchers, blinking in a mul
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