Tailoring Cu ⁺ for Ga ³⁺ Cation Exchange in Cu _{2– x} S and CuInS ₂ Nanocrystals by Controlling the Ga Precursor Chemistry

Abstract

Nanoscale cation exchange (CE) has resulted in colloidal nanomaterials that are unattainable by direct synthesis methods. Aliovalent CE is complex and synthetically challenging because the exchange of an unequal number of host and guest cations is required to maintain charge balance. An approach to control aliovalent CE reactions is the use of a single reactant to both supply the guest cation and extract the host cation. Here, we study the application of GaCl ₃–L complexes [L = trioctylphosphine (TOP), triphenylphosphite (TPP), diphenylphosphine (DPP)] as reactants in the exchange of Cu ⁺ for Ga ³⁺ in Cu _{2– x} S nanocrystals. We find that noncomplexed GaCl ₃ etches the nanocrystals by S ^2– extraction, whereas GaCl ₃–TOP is unreactive. Successful exchange of Cu ⁺ for Ga ³⁺ is only possible when GaCl ₃ is complexed with either TPP or DPP. This is attributed to the pivotal role of the Cu _{2– x} S–GaCl ₃–L activated complex that forms at the surface of the nanocrystal at the onset of the CE reaction, which must be such that simultaneous Ga ³⁺ insertion and Cu ⁺ extraction can occur. This requisite is only met if GaCl ₃ is bound to a phosphine ligand, with a moderate bond strength, to allow facile dissociation of the complex at the nanocrystal surface. The general validity of this mechanism is demonstrated by using GaCl ₃–DPP to convert CuInS ₂ into (Cu,Ga,In)S ₂ nanocrystals, which increases the photoluminescence quantum yield 10-fold, while blue-shifting the photoluminescence into the NIR biological window. This highlights the general applicability of the mechanistic insights provided by our work.