# Er3+/Yb3+ co-doped ZnS quantum dots: structure, optical properties and up-conversion luminescence

## TL;DR

This paper studies Er3+/Yb3+ co-doped ZnS quantum dots and their upconversion luminescence properties for potential use in optoelectronic devices.

## Contribution

The study reports the first detailed investigation of upconversion luminescence in Er3+/Yb3+ co-doped ZnS quantum dots.

## Key findings

- Er3+/Yb3+ co-doped ZnS QDs exhibit green and red upconversion bands under 980 nm excitation.
- The UC emission intensity is strongly dependent on Yb3+ concentration.
- The UC process is governed by a two-photon mechanism with efficient energy transfer from Yb3+ to Er3+.

## Abstract

Er3+/Yb3+ co-doped ZnS quantum dots (QDs) were synthesized using a wet chemical approach and studied for their structural and optical properties, as well as upconversion (UC) luminescence. X-ray diffraction (XRD) confirmed the formation of phase cubic ZnS, where rare-earth Er3+ and Yb3+ ions were effectively incorporated into the host lattice without secondary phases. X-ray photoelectron spectroscopy (XPS) analyses further verified the trivalent states of Er and Yb ions in the QDs. For the first time, the UC luminescence phenomenon in Er3+/Yb3+ co-doped ZnS QDs was studied and explained in detail. Under 980 nm excitation, the Er3+/Yb3+ co-doped ZnS QDs exhibited green and red UC bands, dominated by the green emission, whose intensity strongly depended on Yb3+ concentration. Power-dependent and lifetime measurements indicated that the UC process was primarily governed by a two-photon mechanism facilitated by efficient Yb3+ → Er3+ energy transfer. Chromaticity analyses demonstrated a distinct emission color shift from deep-blue (pure ZnS QDs) to stable green-yellow in co-doped QDs. These results highlight the potential of Er3+/Yb3+ co-doped ZnS QDs as efficient UC nanomaterials for applications in photonic and optoelectronic devices.

Upconversion emission spectra of ZnEr1%Yb1%S QDs under 980 nm excitation (4I15/2–4I11/2) with excitation power varying from 1 to 10 mW. The inset shows the dependence of the upconversion emission intensity on the excitation power.