Trap Passivation in Indium-based Quantum Dots through Surface Fluorination: Mechanism and Applications
Journal
ACS nano
Date
2018.10.16
Abstract
Indium based, cadmium free quantum dots (QDs) with high luminescence efficiency are of great interest both for practical and fundamental studies. Common practice for getting bright III-V semiconducting QDs demand the growth of uniform shell of a wider band gap semiconductor for the passivation of surface states. Hydrofluoric acid (HF) treatment of InP colloidal QDs was previously introduced as an effective pathway to improve photoluminescence quantum yield (PLQY) without growing a shell. The accepted mechanism for this process is dissolution of fluorinated phosphorus and subsequent passivation of indium on the reconstructed surface by excess ligands. However, here, we find the evidence supporting an alternative process, in which the main role of the fluoride ions is direct passivation of the surface indium dangling bonds in the form of atomic ligands. PLQY enhancement is accompanied by the red-shifts of emission and absorption peaks along with the increased Stokes shift rather than the blue shifts caused by etching. Density functional theory suggests the observed red-shifts and the increased Stokes shift might be due to the exciton delocalization over InP QDs and the alteration of the optical oscillator strength induced by the fluoride layer with large electronegativity. The passivation of surface indium with fluorides can be generalized also to other indium based QDs, i.e. PLQY of InAs QDs can increases by an order of magnitude through the fluorination. We fabricated the fluorinated InAs QD-based devices which demonstrate better switching property and higher mobility than those of 1,2-ethanedithiol cross-linked QD devices. The effective surface passivation results in removal of the persistent photoconductivity usually found in InAs QD-based solid films, enabling advances of solution processable QD-based photoelectric applications.