| The interest in lanthanide (Ln3+)-doped nanocrystals have renewed by their unique optical properties at the end of the 20th century. These nanomaterials have sharp absorption and emission lines arising from the intra 4f transitions with high quantum yields, long lifetimes, and superior photostability, due to the effective shielding of the 4f orbitals by the higher lying 5s and 5p orbitals, thereby minimizing the effect of the outer ligand field. In particular, owing to the ladder-like arranged energy levels of Ln3+ ions, high efficiency of the photon upconversion (UC) that absorb one or more low-energy near-infrared (NIR) photons and subsequently convert them to high-energy emissions can be obtained under the excitation of infrared lasers with moderate excitation densities. Therefore, the Ln3+ doped nanocrystals are envisioned to have extensively potential applications in solid-state lasers, infrared detection, multicolor three dimensional displays,and especially biological fluorescent labels. In recent years, obtaining highly efficient and strong blue, purple, ultraviolet (UV) and multicolor (including white light) nanocrystals become a research focus. The former can especially be used to produce singlet oxygen for photodynamic therapies in the field of biomedicine and develop solution-based scintillator materials besides tunable UV laser, while the latter can especially serve as lighting sources in nano-optics devices and fluorescent biolabels offering more simultaneous detection channels or allowing molecular fluorescence detection independent of the color of solution. On the basis of our group's previous studies, we achieved some important conclusions on blue, purple, UV and multicolor UC luminescence by changing matrix and rare-earth ions. The major contents of our study as follows:1. Yb3+/Tm3+ codoped hexagonal NaYF4 nanocrystals were fabricated via changing dosage of oleic acid and sodium hydroxide using a facile hydrothermal method. Intense infrared-to-visible UC emissions were obtained in these nanocrystals under excitation at 980 nm. Especially, luminescent switching among different UC emissions wavelengths at 800, 475 and 450 nm were observed by adjusting excitation powers.2. Deep UV UC emissions in the region of 270~330 nm of Gd3+ under the excitation of 980 nm laser diode in hexagonal Yb3+/Tm3+/Gd3+ triply doped NaYF4 nanorods synthesized using hydrothermal method were studied. Spectral analyses indicate that the UV UC emissions originate from highly efficient energy-transfer from Yb3+ to Tm3+, then to Gd3+ ions, and the intensity of the emission as well as the ratios of the emission peaks are strongly dependent on the doping concentrations and pump power. XRD results indicate that crystallite size can be controlled by changing concentration of Gd3+.3. Ln3+ doped cubic KGdF4 (Ln= Yb, Er, Ho, Tm) nanocrystals, approximately 12 nm in diameter, synthesized via a hydrothermal method with multicolor UC emissions including red, yellow, blue and white, under the excitation of a 980 nm diode laser was studied. The transmission electron microscopy (TEM),X-ray diffraction (XRD) and UC spectra results indicated that the UC fluorescence intensity is remarkably enhanced after low-temperature heat treatment, but the gain size dose not increased obviously. The calculated color coordinate demonstrated that white UC emission including nearly standard white light (CIE-X=0.351, CIE-Y=0.346) can be tuned through controlling the intensities of red, green and blue emissions by adjusting the dopant concentration in Yb3+/Ho3+/Tm3+ triply doped nanocrystals. Keeping in mind that the Gd3+ ion is a paramagnetic relaxation agent extensively used in magnetic resonance imaging, our results suggest that these Ln3+ doped KGdF4 nanocrystals have promising applications in optical and magnetic dual modal nanoprobes for biomedicine besides anti-counterfeiting, color displays and back light sources.
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