Passivation of Electron Trap States in InP Quantum Dots with Benzoic Acid Ligands
Low toxicity indium phosphide (InP) quantum dots represent an attractive alternative to heavy-metal-based quantum dots (QDs) in numerous applications including lighting, displays, and photosensitization. However, low photoluminescence quantum yields (PLQYs) resulting from a high density of surface trap states ultimately limit the applications of as-synthesized InP QDs in commercial products. Postsynthetic treatment with Z-type ligands (two-electron acceptors) is often used to passivate the surface traps of the InP QDs, while approaches using X-type ligands (one-electron donors) are less prevalent due to the lack of understanding of how the X-type ligands function to passivate the surface traps. In this study, we report a postsynthetic surface passivation utilizing benzoic acid (BZA) as an X-type surface ligand. To understand how BZA impacts their electronic structure, we conducted spectroscopic studies on InP QDs with various surface modifications, including in situ fluorination and postsynthetic BZA treatment. A comparison of a variety of time-resolved spectroscopic techniques reveals that BZA can selectively remove electron trap states in InP QDs by passivating unsaturated indium atoms at the QD surface. When the BZA treatment is used in combination with a well-established fluoride treatment, the PLQYs of these unshelled InP QDs exceeds 20%. This research advances our understanding of the function of X-type ligands at the InP QD surface and their role in the passivation of unsaturated indium atoms to reduce electron trap states.