Robust Quantum Cutting via Halide-Bearing Ligand Passivation and Gradient Halide Reconstruction for Ultrabroadband Ultraviolet-to-Near-Infrared Photodetection and Imaging.

Soft-lattice nanocrystal photon management hinges on controlling relaxation branching, diverting excitations from interfacial losses into radiative pathways. CsPbCl3:Yb3+ quantum cutting benchmarks this, but is still constrained by surface trapping, suboptimal Yb-pair branching energetics, and limited thermal/environmental stability. Here we implement a (2-bromoethyl)trimethylammonium bromide (BETAB)-enabled passivation strategy with annealing-triggered gradient halide reconstruction. BETAB replaces OA/OAm to reduce surface losses, while mild annealing activates ligand-associated Br- as a local reservoir to drive Br- in-diffusion with Cl- counter-migration, writing a continuous radial Cl/Br gradient (Br-rich interior). The graded halide landscape suppresses interfacial quenching and creates a radial band-edge bias that funnels excitations into the QC pathway, accelerating exciton-to-Yb3+ transfer. Consequently, QC PLQY increases stepwise from 84.7% (pristine) to 125.4% (BETAB-treated) and 155.2% (annealed). Integrated as spectral-conversion films on Si photodetectors, the treated NCs enable 200-1100 nm detection with responsivity up to 0.5 A W-1, EQE of 64.17%, and D* >1.02 × 1012 Jones (300-1100 nm) and 4.8 × 1011 Jones (200-300 nm), delivering clear ultrabroadband imaging in 7 × 7 arrays. The reconstructed NCs further show ATQ-like behavior (134% at ∼333 K) and improved aging stability (86.1% retention after 60 days vs 29.2% for pristine). Overall, ligand-enabled gradient writing reroutes relaxation for robust photon management.