Room-Temperature Surface Engineering of CuInS₂ Nanocrystals: Synergistic Effects of Ionic Salts and Dodecanethiol for Enhanced Optical Properties

Abstract

Copper indium sulfide (CIS) quantum dots are promising heavy-metal-free NIR emitters, but their practical use is limited by low photoluminescence quantum yield caused by surface defects and trap-assisted recombination, while conventional core/shell approaches increase size and heterogeneity. Herein, we report a rapid, room-temperature post-synthetic treatment that overcomes these long-standing bottlenecks. Exposure of sub-2 nm CIS nanocrystals to Zn²⁺ or Cd²⁺ salts in a biphasic system, followed by thiol-based surface passivation, yields a remarkable enhancement in PLQY and excited-state carrier lifetime while minimally altering the intrinsic emission linewidth and core size. The strategy relies on controlled defect-passivation rather than shell growth, effectively suppressing nonradiative pathways without inducing alloy-driven spectral shifts or particle coarsening. Notably, both chalcopyrite and wurtzite CIS nanocrystals respond favourably to this treatment across the visible-to-NIR window, demonstrating its broad applicability. While Cd²⁺ delivers the highest optical improvement, Zn²⁺ offers a nontoxic and sustainable alternative suitable for biological use. This work establishes a scalable defect-engineering route that decouples quantum yield enhancement from particle size growth, unlocking a new design framework for ultrasmall, bright, NIR-emitting CIS nanocrystals. The resulting materials may serve as compact emitters for bioimaging applications where renal clearance is desirable.