Multiform La2O3: Yb3+/Er3+/Tm3+ submicro-/microcrystals derived by hydrothermal process: Morphology control and tunable upconversion luminescence properties

Abstract

LaCO₃OH:Ln³⁺ (Ln = Yb, Er, Tm) submicro/microcrystals with a variety of novel and well-defined morphologies including triagonal or truncated triagonal-prism/plate, hexagonal prism, pine nut and sphere have been synthesized via a facile hydrothermal route. A series of controlled experiments indicate that the inherent crystal structure and the external experimental conditions such as the pH values in the initial reaction systems, the volume ratio of EG to H₂O, and the dosage of CO(NH₂)₂ were responsible for shape determination of the LaCO₃OH products. The possible formation mechanism for these diverse architectures has been presented. After annealing at a suitable temperature, the as-prepared LaCO₃OH:Ln³⁺ crystals were easily converted to La₂O₃:Ln³⁺ without changing their original morphologies. A systematic study on the upconversion (UC) PL properties of La₂O₃:Yb³⁺/Er³⁺/Tm³⁺ products have been performed in detail. Under 980 nm laser diode excitation, La₂O₃:Yb³⁺, Tm³⁺, La₂O₃:Er³⁺ and La₂O₃:Yb³⁺, Er³⁺ products are mainly dominated by blue, green and red light emissions, respectively. By increasing Er³⁺ concentration, the La₂O₃:Er³⁺ gives yellow emission. Furthermore, the UC white light was successfully obtained in the La₂O₃:Yb³⁺/Er³⁺/Tm³⁺ system by adjusting the relative doping concentration of Yb³⁺, Er³⁺ and Tm³⁺. The obtained UC white light can be tuned at a wide range, and the optimal white-emitting CIE (Commission internationale de l'éclairage [International Commission on Illumination]) chromaticity coordinate is (x = 0.3259, y = 0.3187), which is very close to the standard equal energy white light (x = 0.33, y = 0.33). The strategies for color tuning of upconversion emission proposed in the current system may be helpful for other ones to achieve efficient multicolor (including white light) luminescence under 980 nm laser excitation.