Atomistically Tailored Surface states of Indium Phosphide Quantum Dots for Shell-Less Light-Emitter

The surface complexity of III-V compound QDs has been progressively elucidated through surface etching and passivation using various surface ligands. Regardless of the synthetic routes, the surfaces of highly covalent InP QDs are prone to the formation of oxidative species due to their high oxygen affinity. These surface traps act as local energy minima that hinder or delay electron-hole recombination, consequently reducing luminous efficiency. Ideally, if the surface oxides of InP QDs can be effectively removed and the exposed DBs can be passivated with additional ligands, InP cores without a pristine shell could potentially exhibit sufficient luminescent efficiency.<br/>As part of this research endeavor, the utilization of Lewis acidic Z-type ligands for partially coupling surface atoms has shed light on the significance of surface stoichiometry and undercoordinated surface sites in the luminescence of QDs. Z-type ligand passivation has proven effective in treating the surfaces of various InP QDs, yielding consistent outcomes irrespective of their types. Recently proposed combinational ligand passivation strategies offer safer chemical routes than HF and have shown high photoluminescence quantum yield (PLQY) for InP cores without any shells, achieving 85% PLQY with the combination of ZnCl₂ and InF₃, and 60% PLQY with the combination of ZnCl₂-TOP and Zn-oleate. These approaches address the challenge of eliminating the remaining undercoordination despite the effective passivation of a significant number of surface DBs using Z-type ligands. Additionally, the introduction of gallium ions in a post-synthetic process of InP QDs has provided further insights into tackling surface complexity. While the enhancement of InP core luminescence has been reported through GaP shell formation, Ga cation exchange, In-Ga alloying, and additional coordination at the InP/ZnSe interface, the specific role of Ga-based ligands in altering the surface chemical state remains unclear. Overall, achieving defect-free and DB-free InP QD surfaces without shell formation while maintaining excellent luminescent properties for applications like electroluminescent (EL) devices remains an unexplored possibility. Although matching factors such as surface coordination of cations and anions and charge neutralization hold promise for eliminating surface defects in compound semiconductor QDs, further research is needed to realize the full potential of defect-free InP QDs with pristine surfaces.<br/>To address this challenge, our novel approach focuses on controlling InP QD surface stoichiometry and coordination without the conventional growth of a complete shell. Through atomic-level modification involving proportional surface passivation using gallium chloride and zinc carboxyl groups, we successfully reduce the density of surface defect states, resulting in remarkably stable luminescence with near-unity PLQY (up to 95%). Theoretical models have offered guiding principles for identifying the chemical composition and quantity of surface traps, which can be verified through spectroscopic analysis. Remarkably, our core-only InP QDs, even without a shell, demonstrate sufficient electroluminescence in conventional LED architectures for the first time. Our findings provide valuable insights into the role of surface chemistry in controlling luminescence and present a novel strategy for developing bright and stable QDs without the necessity of a complete shell.