Lanthanide-doped upconversion materials, capable of converting low-density (< 1000 W cm À2 ) near-infrared (NIR) excitation to ultraviolet (UV) and visible emissions, have generated a large amount of interests in the areas of information technology, biotechnology, energy, and photonics. [1] Significantly, recent developments in the synthetic and multicolor tuning methods have allowed easy access to upconversion nanoparticles with well-defined phase and size, core-shell structure, optical emission, and surface properties. [2][3][4][5] The technological advances provide promising applications in sensitive biodetection and advanced bioimaging without many of the constraints associated with conventional optical biolabels. [6] Despite the attractions, further progress in using upconversion processes has been largely hindered because upconversion nanoparticles are typically sensitized by Yb 3+ ions that only respond to narrowband NIR excitation centered at 980 nm. The absorption of 980 nm light by the water component in biological samples usually limits deep tissue imaging and induces potential thermal damages to cells and tissues. [7] Excitation of conventional upconversion nanoparticles at other wavelengths has been proposed to minimize the effect of water absorption. [8] But the use of this technique is limited mainly by the largely sacrificed excitation efficiency. Efforts have also been devoted to tuning the NIR response of photon upconversion through integration of various sensitizers such as metal ions (e.g.; Nd 3+ , V 3+ or Cr 5+ ) and organic dyes. [9] The progress has resulted in visible emission by NIR excitation in the 700-900 nm range where the transparency of biological samples is maximal. [9e-h] However, upconversion emission across a broad range of spectra in these systems have not been demonstrated largely owing to the uncontrollable nonradiative processes. Herein, we describe a novel design, based on nanostructural engineering to separate unwanted electronic transitions for constructing a new class of materials displaying tunable upconversion emissions spanning from UV to the visible spectral region by single wavelength excitation at 808 nm. We also show that these nanoparticles can surpass the constraints associated with conventional upconversion nanoparticles for biological studies.The nanostructure design for management of energy transitions is depicted in Figure 1. A core-shell-shell nanoparticle platform is used to host light-harvesting, upconverting, and optical tuning processes at separate layers through doping of appropriate lanthanide ions. Interlayer energy exchange interactions are mediated by arrays of lanthanide migrator ions that can bridge efficient energy transfer across the core-shell interface while filtering unwanted crossrelaxations. As a result, incompatible optical processes can be rationally combined to achieve flexible and efficient photon energy conversions.As a proof-of-concept experiment, we employed a NaYbF 4 @Na(Yb,Gd)F 4 @NaGdF 4 core-shell-shell nanoparticle host....

Blue emission at NIR excitation: A strategy, based on energy management in nanostructured materials, is reported for photon upconversion of near‐infrared light. Several optical processes can be integrated into a single nanoparticle (see picture). The effect offers upconversion emissions spanning from ultraviolet to the visible spectral region by excitation at 808 nm.

No abstract

The emission spectra of the tripositive lanthanide ion Eu(3+) have often been employed to probe its environment in the solid state, and the intensity ratio of magnetic dipole ((5)D(0)-->(7)F(1)) and forced electric dipole ((5)D(0)-->(7)F(2)) transitions has been used to estimate the "degree of asymmetry" of a crystal site. From the site-selective, low temperature emission spectra of Eu(3+) doped into a glass, a new empirical relation has been found between the width of spectral features and the relative intensity of the (5)D(0)-->(7)F(0) zero phonon line. In order to explain the observations from experiments with excitation at different wavelengths, a generic quantitative relation has been developed from basic theory and validated from our experimental results. This work gives a deeper insight and understanding of the spectral characteristics of Eu(3+) electronic spectra in the visible region.

No abstract