Enhancing the upconversion luminescence properties of Er3+–Yb3+ doped yttrium molybdate through Mg2+ incorporation: effect of laser excitation power on temperature sensing and heat generation

Abstract

Upconversion nanomaterials possess various unique properties, but they suffer from low emission efficiency, making them challenging for optical and security applications. In this work, Mg²⁺ ion enhanced upconversion emission of rod-like YMoO₄:Er³⁺/Yb³⁺ nanocrystals synthesized via a hydrothermal method is reported. The influence of rare earth (Er³⁺) as well as non-rare earth (Mg²⁺) ion concentration on the upconversion emission of YMoO₄:Er³⁺/Yb³⁺ was investigated at various laser pump powers and external temperatures upon light excitation at 980 nm. The codoping of Mg²⁺ ions into YMoO₄:Er³⁺/Yb³⁺ has shown efficient enhancement in the green and red upconversion luminescence intensities which has been explained based on the structural and optical properties of the material. The emission-optimized materials were further investigated to understand the dependence of excitation power density on optical thermometry and nano-heating behaviour using the fluorescence intensity ratio technique from the two thermally coupled energy levels – ²H_11/2 and ⁴S_3/2 of Er³⁺ ions. Strikingly, the sensor sensitivity of the material was noticed to decrease upon increasing the excitation power from 7 to 66 W cm⁻² and a maximum thermal sensitivity (0.0137 K⁻¹ at 308 K) was achieved at a low excitation power (19 W cm⁻²). Moreover, the laser-induced optical heating effect in the material was determined with different excitation power densities and the maximum temperature of the sample particles was calculated to be 470 K at 66 W cm⁻². The obtained results indicate the potential use of the Mg²⁺ co-doped YMoO₄:Er³⁺/Yb³⁺ phosphor in color displays, temperature sensing and optical heaters, and provide insights into understanding the effect of laser excitation power on its upconversion luminescence and optical sensing properties.