Enhanced Upconversion Luminescence of Hexagonal Nayf4: Yb, Er Core Nanorods Coated with the NiR Light Harvester NaYF4 Shell

Abstract

Photovoltaic technology has made considerable progress in recent decades, but its insensitivity towards the absorption of near-infrared (NIR) light limits its competitiveness. Without changing the current designs and benefits of photovoltaic technology, lanthanide-doped upconversion nanoparticle (UCNP) provides a solution to the harvesting of NIR light by combining two or more NIR light photons into a single visible light photon. However, many researchers used core UCNP for solar cell applications, but the low upconversion luminescence (UCL) and light-harvesting ability of core UCNP still limited their use. Therefore, we designed and fabricated core@shell architecture of upconversion nanorods (UCNR) with high UCL by coating an active light-harvesting NaYbF4 shell onto NaYF4:Yb, Er core nanorods, through a hydrothermal method. Moreover, the effect of increased shell thickness on the crystal architecture and luminescent property of synthesized NaYF4:Yb, Er@NaYbF4 core@shell UCNR were systematically investigated. We demonstrated that shell thickness variation from 5.11 nm to 13.01 nm causes lower UCL due to incomplete formation of shell and being more sensitive to residual surface defects or new defects. On increasing the shell thickness to 15.49 nm, the reduction in surface defects facilitated the combined energy transfer between core and shell, resulting in an enhancement in luminescent properties. This optimum shell thickness was found to be at 15.49 nm. Compared to core UCL emission intensity, our approach provided 1.3-time enhanced red UCL emission intensity and 1.2 times enhanced green UCL emission intensity. On increasing the shell thickness beyond 15.49 nm, a decrease in UCL was observed due to the reduction of NIR absorption by the core. Therefore, enhanced UCL performance was only possible within a small range of 13.04 nm to 19.94 nm shell thickness and maximized at 15.49 nm shell thickness of NaYbF4. The UCL results of optimized NaYF4:Yb, Er@NaYbF4 (core@shell) nanorods demonstrated a promising design approach for photovoltaic technology.

Keywords: Photovoltaic, Upconversion nanoparticles, Upconversion luminescence, Core@shell nanorods