Ligand−Surface Interactions and Band Structure Engineering in Cd-Doped MAPbI3 Quantum Dots for Photodetector Applications

Abstract

Cadmium-doped methylammonium lead iodide (MAPbI3) quantum dots (QDs) offer a promising route to engineer defect states, enhance structural stability, and tune optoelectronic performance, yet their fundamental photophysics, surface chemistry, and ligand interactions remain poorly understood. Here, MAPb1−xCdxI3 QDs (x = 0−0.25) were synthesized at room temperature in a nonpolar solvent using a ligand-mediated transport (LMT) strategy, enabling systematic Cd2+ substitution at the B-site. The resulting nanocrystals are highly uniform (∼4 nm) and exhibit absolute photoluminescence quantum yields (PLQY) of up to ∼75%. X-ray diffraction with Rietveld refinement, HRTEM, and XPS confirm substitutional incorporation of Cd2+ into the tetragonal (I4/mcm) lattice, with progressive lattice contraction and subtle octahedral distortion. UV−visible absorption, PL emission, and CIE chromaticity analyses reveal a Cd-induced widening of the optical gap near 5 mol % Cd, consistent with trap-state passivation. Temperature-dependent PL between 20 and 70 °C further shows that low-level Cd-substitution suppresses thermal quenching and modifies exciton−phonon coupling, as captured by Varshni analysis. Complementary first-principles calculations establish (i) the binding modes and energetics of oleate and octylammonium ligands on MAPbI3 surfaces via natural energy decomposition analysis (NEDA), and (ii) the evolution of band dispersion and projected density of states across MAPb1−xCdxI3 from relativistic DFT calculations, including a direct-to-indirect band−gap transition near 20% Cd. Together, experiment and theory provide a coherent mechanistic picture of how Cd-substitution and surface ligand chemistry govern the structural, electronic, and emissive properties of MAPbI3 QDs, offering design guidelines for compositional engineering of hybrid perovskite nanocrystals for photodetector and related visible-to-near-infrared optoelectronic applications.