High Conductivity and Excitation-Power Sensitivity of Upconversion Emission in Silica Decoration of Regular Hexagonal Yb and Er Codoped ZnO Core–Shell Particles

Abstract

The structure of upconversion materials has an important influence on their photoelectric properties. Herein, we show that the quadrilateral-like structure of Yb/Er/ZnO micronanoparticles grew to regular hexagonal-like structure by increasing the reaction time from 6 to 24 h. Yb/Er/ZnO samples were prepared via a hydrothermal method with polyethylene glycol (PEG) as the dispersant. The structural evolution originated from the addition of PEG dispersant. The Yb/Er/ZnO particles with hexagonal-like structure had increased carrier concentration, carrier mobility rate, Hall coefficient, and resistivity. In addition, a silica shell coating on Yb/Er/ZnO micronanoparticles with hexagonal-like structure (Yb/Er/ZnO@SiO2) was achieved by the Stöber method and improved the electrical conductivity. The migration rate of Yb/Er/ZnO@SiO2 increased to 163.6 cm V s–1. The core–shell interactions decreased the band gap of Yb/Er/ZnO and the energy of defect transition in ZnO host, which significantly improved host sensitization. With increasing power density from 2.04 to 9.04 W/cm2, the intensity of the red upconversion emission was clearly enhanced. Power-dependent multicolor emission excitation was achieved, and the color changed from red to yellow and to green. Moreover, color-correlated temperature (CCT) values changed between the cool region and the warm CCT region, indicating that tricolor emission was controlled via adjusting the excitation power. The regular hexagonal-like structure of Yb/Er/ZnO@SiO2 can be potentially extended to optoelectronic device applications.