Effect of lanthanide doping on crystal phase and near-infrared to near-infrared upconversion emission of Tm3+ doped KF-YbF3 nanocrystals

Tm3+ doped KF-YbF3 nanocrystals were synthesized by a hydrothermal method using oleic acid as a stabilizing agent at 190 °C. The influence of Gd3+ and Sm3+ content on the phase structure and upconversion (UC) emission of the final products was investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and UC spectra. XRD analyses and TEM observations evidence that the phase and size of the as prepared Tm3+ doped KF-YbF3 nanocrystals are closely related to the Gd3+ doping content. Without Gd3+ impurity, the undoped nanocrystals crystallize in orthorhombic KYb2F7 with an average diameter of 42 nm. When the Gd3+ doping is below 10 mol%, the orthorhombic KYb2F7 nanocrystals grow up. However, with Gd3+ addition beyond about 30 mol%, the complete phase transformation from orthorhombic KYb2F7 to cubic KGdF4 occurs in the final products. Under the excitation of a 980 nm laser diode, the as prepared Tm3+ doped nanocrystals exhibit strong near-infrared UC emission at 800 nm. Particularly, the intensity of high energy UV and blue UC emissions of Tm3+ ions in Tm3+ doped KYb2F7 nanocrystals are selectively reduced compared to the NIR emission at 800 nm by co-doping a small amount of Sm3+ ions into the host matrix. Possible dynamic processes for UC emissions in Tm3+ doped nanocrystals are discussed in detail.