Unveiling the Role of ZnCl₂ in Enhancing the Photoluminescence Efficiency of Amino-As-Based InAs@ZnSe Quantum Dots

We investigated how ZnCl₂, employed as an additive in the amino-As-based synthesis of indium arsenide (InAs) quantum dots (QDs), considerably improves the photoluminescence quantum yield (PLQY) of the resulting InAs@ZnSe core@shell QDs. We achieved this by synthesizing and comparing three distinct InAs QD samples and their corresponding core@shell structures: (1) In(Zn)As QDs (synthesized with ZnCl₂); (2) standard InAs QDs (std-InAs, made without additives); and (3) std-InAs QDs postsynthesis treated with ZnCl₂ (Zn-InAs). High PLQY values (∼70%) were attained only with In(Zn)As@ZnSe QDs, while std-InAs@ZnSe and Zn-InAs@ZnSe samples exhibited much lower PL efficiencies (10-20%). We also demonstrated that (i) the high PLQY in In(Zn)As@ZnSe QDs could not be attributed solely to the presence of an In-Zn-Se interlayer, as this was present in all three samples; (ii) the specific ZnSe shelling procedure had only a minor impact on the final PLQY; and (iii) the PL efficiency was significantly improved only when high amounts of ZnCl₂ additive (specifically with ZnCl₂:InCl₃ precursor ratios over 10:1) were used during the InAs QDs synthesis. These findings were rationalized through density functional theory (DFT) calculations coupled with X-ray absorption spectroscopy measurements. DFT models suggested that std-InAs QDs feature surface trap states, mainly located on the (-1-1-1) facets, thus low PL efficiency even after ZnSe shelling. The use of ZnCl₂ in the InAs synthesis led to surface Zn incorporation, particularly on the (100) and (-1-1-1) facets, effectively passivating surface traps and, consequently, yielding highly emissive In(Zn)As@ZnSe QD systems. In contrast, ZnCl₂ employed in the postsynthesis treatment of std-InAs QDs resulted only in a limited surface Zn incorporation and in ZnCl₂ adsorption on the (-1-1-1) facets (i.e., ZnCl₂ acting as a Z-type ligand), leading to poor passivation of surface traps. Overall, our study demonstrates the critical role of ZnCl₂ as a synthesis additive in delivering highly emissive amino-As-based InAs@ZnSe QDs.