Halogen ion doping mediated exciton state modulation in MoS2 quantum dots for fluorescence tuning and optical anti-counterfeiting

Abstract

Developing an ideal luminescent material with exceptional performance in various aspects, such as affordability, low toxicity, high photostability and security, is vital for advanced optical anti-counterfeiting. Here, halogen-doping-dependent luminescence MoS₂ quantum dots were synthesized via a simple hydrothermal method for optical anti-counterfeiting ink. The results show that F⁻, Cl⁻ and Br⁻ doping results in notable fluorescence blueshifts of MoS₂ quantum dots and significant enhancements in the photoluminescence quantum yield (PLQY) of up to 5.7, 5.0, and 3.2 times, respectively. However, I⁻ incorporation causes a fluorescence redshift and a 25% decrease of PLQY. This mechanism is elucidated using density functional theory (DFT) and experiments. Specifically, F⁻, Cl⁻ and Br⁻ doping localizes surface electrons and blocks sulfur vacancies in MoS₂ quantum dots, enhancing the release of neutral excitons from trions and defect-bound excitons. Conversely, I⁻ doping increases the surface charge and sulfur vacancies, favouring the conversion of neutral excitons into trions and defect-bound excitons. The halogen-doped MoS₂ quantum dots are engineered as security ink, demonstrating high concealment, excellent photostability and easy fabrication. The study offers a novel way of tailoring fluorescence of MoS₂ quantum dots, potentially extending to other transition metal dichalcogenide quantum dots and their optical anti-counterfeiting implementation.