The construction of rare-earth doped micro/nano core-shell structure is not only beneficial to enhance the upconversion emission intensity, but also can realize the fine control of luminescence color through the spatial separation of ions. In this work, a series of NaYF<sub>4</sub>@NaYF<sub>4</sub> core-shell (CS) microcrystals doped with different ion concentrations were constructed by means of epitaxial growth technology. The structure and morphology of the prepared microcrystals were characterized by X-ray diffractometer (XRD) and scanning electron microscope (SEM). The experimental results show that the prepared CS structures had a pure hexagonal-phase crystal structure, and exhibited a disk-like shape. Under the excitation of 980 nm laser, the energy transfer characteristics of doped ions in single CS microcrystals were carefully studied by using a confocal microscope spectroscopy test system and changing the excitation position. The study shows that the ions doped in different regions of the CS microdisks exhibit different spectral characteristics when the excitation position is changed, which is mainly due to the different directions of excitation energy transfer in the CS structure. Based on the emission spectra of different positions and power variation spectra, it is proved that the excitation energy of the micron CS is mainly transmitted from outside to inside. Meanwhile, the colorful emission pattern of the CS microdisk is revealed by the corresponding optical waveguide model, which is mainly due to the Optical waveguide effect. Therefore, by constructing different micron core-shell structures, the luminescence characteristics of microcrystals can be controlled and adjusted, and the application of microcrystals in optoelectronic devices, optical coding and multicolor display can be provided with important experimental reference.
White upconversion (UC) luminescent materials have shown incomparable advantages over other light sources in the fields of solid-state lighting, liquid crystal display and bioimaging, which has attracted extensive attention from researchers. In this work, a series of microcrystals doped with different ion concentrations were synthesized by hydrothermal method, such as NaYF<sub>4</sub>: Yb<sup>3+</sup>/Ho<sup>3+</sup>/Tm<sup>3+</sup> and NaYF<sub>4</sub>: Yb<sup>3+</sup>/Ho<sup>3+</sup>/Tm<sup>3+</sup>, and their corresponding micron core-shell (CS) structures were constructed based on epitaxial growth technology. The structure and morphology of the prepared microcrystals were characterised by X-ray diffractometer (XRD) and scanning electron microscope (SEM), which showed that the microcrystals had a pure hexagonal-phase crystal structure with a rod-like shape. Under the excitation of 980 nm near infrared laser, the white UC luminescence characteristics of Ho<sup>3+</sup>/Tm<sup>3+</sup> and Er<sup>3+</sup>/Tm<sup>3+</sup> co-doped single-particle NaYF<sub>4</sub> microcrystals were systematically studied by modulating the concentration of the doping ions. The study shows that in Ho<sup>3+</sup>/Tm<sup>3+</sup> co-doped NaYF<sub>4</sub> microcrystals, white UC luminescence can be easily achieved by modulating the concentration of Yb<sup>3+</sup> ions, while in the Er<sup>3+</sup>/Tm<sup>3+</sup> co-doped NaYF<sub>4</sub> microcrystal, white UC luminescence can be effectively achieved by modulating the concentration of Er<sup>3+</sup>ion. According to the luminescence characteristics of the microncrystals in different doping systems, the physical mechanism of white light emission regulation is revealed, which is mainly due to the interaction between the doped ions, including cross relaxation (CR) processes and energy back transfer (EBT) processes. Meanwhile, an effective enhancement of the white UC luminescence on CS microrod was achieved by coating the NaYF<sub>4</sub> inert shell. Therefore, ion doping technique and the construction of CS structure can not only realize the white UC luminescence of microrods, but also provide important experimental reference for further enhancing the luminescence characteristics of microrods, and expand the applications of microcrystals in the fields of display, optoelectronics and anti-counterfeiting.
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