Gallium Sulfide Quantum Dots with Zinc Sulfide and Alumina Shells Showing Efficient Deep Blue Emission

Solution-processed quantum dot (QD) based blue emitters are of paramount importance in the field of optoelectronics. Despite large research efforts, examples of efficient deep blue/near UV-emitting QDs remain rare due to lack of luminescent wide band gap materials and high defect densities in the existing ones. Here, we introduce a novel type of QDs based on heavy metal free gallium sulfide (Ga2S3) and their core/shell heterostructures Ga2S3/ZnS as well as Ga2S3/ZnS/Al2O3. The photoluminescence (PL) properties of core Ga2S3 QDs exhibit various decay pathways due to intrinsic defects, resulting in a broad overall PL spectrum. We show that the overgrowth of the Ga2S3 core QDs with a ZnS shell results in the suppression of the intrinsic defect-mediated states leading to efficient deep-blue emission at 400 nm. Passivation of the core/shell structure with amorphous alumina yields a further enhancement of the PL quantum yield approaching 50 % and leads to an excellent optical and colloidal stability. Finally, we develop a strategy for the aqueous phase transfer of the obtained QDs retaining 80 % of the initial fluorescence intensity. Ga2S3 nanocrystals constitute a novel type of quantum dots exempt from toxic heavy metals whose fluorescence can be tuned from broad trap-state to narrow band-edge emission in the deep blue range by coating their surface with a ZnS shell. An additional alumina shell enhances the photoluminescence quantum yield close to 50 % while maintaining a short lifetime around 1 ns.image