Precursor Coordination Engineering Enables Epitaxial-Level Carrier Densities in HgTe Colloidal Quantum Dots

Surface trap-induced carrier doping currently hinders colloidal quantum dots (CQDs) from approaching the electronic purity of epitaxial semiconductors, particularly in the mid-wave infrared (MWIR) regime, where low dark noise is critical. Here, we demonstrate that the coordination geometry of the metal halide precursor strongly influences passivation efficacy, suppressing background carrier densities to an intrinsic level (∼10¹⁴ cm^-3). Mechanistic investigation reveals that the octahedral geometry of HgBr₂ facilitates the formation of a stable HgBr₂(Olam)₄ complex, as traced by the correlated evolution of chemical shifts in ¹H NMR and diffraction patterns in powder X-ray diffraction (pXRD). This behavior contrasts with the linear lattice of HgCl₂, where the absence of comparable spectral evolution indicates much weaker ligand coordination. Consequently, the stable complex is structurally inherited by the HgTe CQDs, yielding a high surface halide coverage of ∼9 at.%, whereas HgCl₂ results in halide-deficient surfaces (<1 at.%). This robust passivation effectively suppresses non-radiative recombination, consistent with improved carrier decay dynamics in time-resolved photoluminescence (TRPL). Cross-verification via field-effect transistor (FET), capacitance-voltage (C-V), and Hall measurements confirms substantially reduced background doping, highlighting coordination engineering as an effective strategy for achieving electronic purity in solution-processed optoelectronics.