Nano Focus: Step-by-step synthesis approach leads to complex hybrid nanoparticles
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hoton upconversion is used to convert long-wavelength sources into shortwavelength emission, and has potential applications in compact solid-state lasers, optical data storage, biological imaging, and solar energy conversion. As reported in the December 2011 issue of Nature Materials (DOI: 10.1038/NMAT3149; p. 968), a team of researchers led by X. Liu from the National University of Singapore, Y. Han from the King Abdullah University of Science and Technology (Saudi Arabia), and X. Chen from the Chinese Academy of Science have developed a system which displays efficient upconversion emission. This was
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Tuning upconversion through energy migration in core–shell nanoparticles. (a) Schematic design of a lanthanide-doped NaGdF4@NaGdF4 core–shell nanoparticle for energy migrationmediated upconversion (X: activator ion). (b) Proposed energy transfer mechanisms in the core–shell nanoparticles. (c) Luminescence in core–shell NaGdF4:Yb/Tm@NaGdF4 (blue), NaGdF4:Yb/Tm@NaGdF4:Tb (green), NaGdF4:Yb/Tm@NaGdF4:Eu (red), and NaGdF4:Yb/Tm@NaGdF4:Tb+NaGdF4:Yb/ Tm@NaGdF4:Eu (yellow) nanoparticles attached to polystyrene bead. Reproduced with permission from Nature Mater. 10 (12) (2011), DOI: 10.1038/NMAT3149; p. 968. © 2011 Macmillan Publishers Ltd.
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Nano Focus Step-by-step synthesis approach leads to complex hybrid nanoparticles
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olloidal, hybrid nanoparticles are single particles comprising several active domains, for example, of metallic, semiconducting, or magnetic materials,
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based on photon upconversion in core– shell nanoparticles doped with a series of lanthanide ions. In particular, tunable emission was achieved by controlling gadolinium sublattice-mediated energy migration in NaGdF4 nanoparticles with well-defined core–shell structures. The researchers used a sensitizer (Yb3+) that harvests pump photons and subsequently promotes a neighboring accumulator ion (Tm3+) to excited states. This energy is then transferred to a migrator (Gd3+) from high-lying energy states of the accumulator, and finally, by random energy hopping through the migratory ion sublattice and trapping of the migrating energy by an activation ion (Tb3+, Eu3+, Dy3+, and Sm3+). Importantly, the researchers achieved efficient upconversion emission at room temperature and moderate excitation densities. To regulate the energy exchange inter-
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and have potential applications in a wide variety of fields including solar energy conversion, catalysis, medical therapies, and electronics. The domain boundaries allow for direct electronic and magnetic communication between the component materials, and this intimate contact and selective arrangement of domains provides a unique pathway to tuning the
VOLUME 37 • JANUARY 2012 • www.mrs.org/bulletin
action between the accumulator and the activator, the researchers confined the sensitizer and the accumulator in the core level of the nanoparticles, while the activator was confined in the shell. The presence of Gd3+ in both core and shell levels created an array of migratory ions that bridg
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