Metal–Solvent\nComplex Formation at the Surface\nof InP Colloidal Quantum Dots

The surface chemistry\nof colloidal semiconductor nanocrystals\n(QDs)\nprofoundly influences their physical and chemical attributes. The\ninsulating organic shell ensuring colloidal stability impedes charge\ntransfer, thus limiting optoelectronic applications. Exchanging these\nligands with shorter inorganic ones enhances charge mobility and stability,\nwhich is pivotal for using these materials as active layers for LEDs,\nphotodetectors, and transistors. Among those, InP QDs also serve as\na model for surface chemistry investigations. This study focuses on\ngroup III metal salts as inorganic ligands for InP QDs. We explored\nthe ligand exchange mechanism when metal halide, nitrate, and perchlorate\nsalts of group III (Al, In Ga), common Lewis acids, are used as ligands\nfor the conductive inks. Moreover, we compared the exchange mechanism\nfor two starting model systems: InP QDs capped with myristate and\noleylamine as X- and L-type native organic ligands, respectively.\nWe found that all metal halide, nitrate, and perchlorate salts dissolved\nin polar solvents (such as <i>n</i>-methylformamide, dimethylformamide,\ndimethyl sulfoxide, H₂O) with various polarity formed metal–solvent\ncomplex cations [M­(Solvent)₆]³⁺ (e.g., [Al­(MFA)₆]³⁺, [Ga­(MFA)₆]³⁺, [In­(MFA)₆]³⁺), which passivated the surface of InP QDs after\nthe removal of the initial organic ligand. All metal halide capped\nInP/[M­(Solvent)₆]³⁺ QDs show excellent colloidal\nstability in polar solvents with high dielectric constant even after\n6 months in concentrations up to 74 mg/mL. Our findings demonstrate\nthe dominance of dissociation–complexation mechanisms in polar\nsolvents, ensuring colloidal stability. This comprehensive understanding\nof InP QD surface chemistry paves the way for exploring more complex\nQD systems such as InAs and InSb QDs.