Sweeping Surface Trap States under the Carpet: Cd2+ Inclusion in InAs Quantum Dots

We have investigated the effect of CdCl2 as an additive in the colloidal synthesis of amino-As-based InAs quantum dots (QDs). Our results demonstrate that this metal halide promotes the incorporation of Cd into the InAs QDs, leading to the formation of In(Cd)As alloyed QDs featuring a narrower bandgap and enhanced photoluminescence (PL) compared to their undoped InAs counterparts. By varying the relative amount of CdCl2, it was possible to synthesize, in a one-pot process, QDs with PL tunable from ∼1000 to ∼1430 nm with a quantum yield as high as 17% at ∼1430 nm, without requiring any postsynthesis shell growth. These In(Cd)As QDs feature longer exciton lifetimes compared to InAs QDs, due to suppressed ultrafast nonradiative trap pathways, and a fast biexciton lifetime that follows the expected linear scaling with nanocrystal volume. Density functional theory (DFT) calculations revealed that (i) In(Cd)As alloys, which are thermodynamically unfavorable in the bulk phase, are stabilized by finite-size effects; (ii) the bandgap narrowing induced by Cd incorporation arises primarily from a pronounced upward shift of the valence band maximum (VBM); and (iii) this upward shift of the VBM also buries the surface trap states below the valence band edge, thereby accounting for the observed high PL efficiency in In(Cd)As QDs. Overall, this work provides mechanistic insights into defect passivation in InAs QDs and establishes metal-halide-mediated growth as an effective strategy for infrared emitting InAs-based QDs.