Diffusion-Limited Kinetics of Isovalent Cation Exchange in III–V Nanocrystals Dispersed in Molten Salt Reaction Media

Abstract

The goal of this work is to determine the kinetic factors that govern isovalent cation exchange in III−V colloidal quantum dots using molten salts as the solvent and cation source. We focus on the reactions of InP + GaI 3 → In 1−x Ga x P and InAs + GaI 3 → In 1−x Ga x As to create technologically important ternary III−V phases. We find that the molten salt reaction medium causes the transformation of nearly spherical InP nanocrystals to tetrahedron-shaped In 1−x Ga x P nanocrystals. Furthermore, we determine that the activation energy for the cation exchange reaction is 0.9 eV for incorporation of Ga into InP and 1.2 eV for incorporation of Ga into InAs, both much lower than the measured values in bulk semiconductors. Next, we use powder XRD simulations to constrain our understanding of the structure of the In 1−x Ga x P nanocrystals. Together our results reveal several important features of molten salt-mediated cation exchange and provide guidance for future development of these materials.

Composition-Defined Optical Properties and the Direct-to-Indirect Transition in Core–Shell In1–xGaxP/ZnS Colloidal Quantum Dots

“…An In-to-Ga cation exchange reaction performed on indium pnictide nanocrystals dispersed in molten salts offers a viable route toward synthesizing the otherwise elusive gallium-containing ternary III–V phases. , We exchange the native long-chain surfactants on the as-synthesized nanocrystals with compact charged inorganic ligands in order to disperse them in molten-salt media. Z-type gallium halide ligands constitute a unique candidate for this role, as they avoid introducing additional heteroatom impurities such as chalcogenides. , Gallium is notoriously oxophilic at elevated temperatures, necessitating that oxygen-containing moieties are avoided at all subsequent steps. We, therefore, introduce an amine-based recovery in this report, allowing us to obtain a colloidal solution of alloyed ternary In 1– x Ga x P nanocrystals without using carboxylate ligands.…”

Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution

“…In these reactions, it has been determined that smaller nanocrystals undergo cation exchange faster than larger nanocrystals, potentially leading to a size-dependent composition within an ensemble upon cation exchange. 184 This potentially could exacerbate the effect of the size distribution if the alloy reaction creates a semiconductor with a larger band gap or counteracts the effect if the reaction creates a narrower band-gap semiconductor. Many discussions around compositional heterogeneity in alloy semiconductors remain qualitative and unproven mostly because direct quantification of the composition variation in nanocrystal ensembles is limited by the insensitivity of standard characterization tools to this parameter (Figure 12B).…”

Design rules for obtaining narrow luminescence from semiconductors made in solution

“…In these reactions, it has been determined smaller nanocrystals undergo cation exchange faster than larger nanocrystals potentially leading to sizedependent composition within an ensemble upon cation exchange. 182 This potentially could exacerbate the effect of size distribution if the alloy reaction creates a semiconductor with a larger band gap or counteract the effect if the reaction creates a narrower band gap semiconductor. Many discussions around compositional heterogeneity in alloy semiconductors remain qualitative and unproven mostly because direct quantification of the composition variation in nanocrystal ensembles is limited by the insensitivity of standard characterization tools to this parameter (Figure 12B).…”