First-Principles Insights into Surface and Ligand Effects in Stoichiometric HgTe Quantum Dots.

HgTe quantum dots are promising colloidal infrared semiconductors for photodetection, imaging, and optoelectronics because their electronic structure can be tuned from the short-wave to mid-infrared. Despite this technological promise, how the electronic structure of ultrasmall HgTe quantum dots emerges from confined semimetallic HgTe and how surface coordination governs frontier-state localization remain unresolved. Here, first-principles calculations on stoichiometric HgTe nanoclusters spanning ∼0.86-1.85 nm reveal a transition from confinement-dominated to surface-controlled electronic structure. Small self-passivated clusters exhibit clean HOMO-LUMO gaps and delocalized frontier states, whereas larger nanoclusters develop localized surface-associated near-edge states driven by coordination and bond-length heterogeneity. At intermediate sizes, the HOMO and LUMO become spatially separated without forming deep-gap states. Neutral ligands suppress localized states by restoring surface coordination while tuning frontier-state energetics through ligand-surface hybridization. These findings establish an atomistic framework for controlling surface-driven electronic structure in infrared quantum dots.