Optical spectra of mercuric oxide (Hg2O) quantum dots

Mercuric oxide (Hg2O) quantum dots (QDs) exhibit a remarkable evolution in their optical properties with decreasing size, transitioning from semiconductor-like behavior in bulk to quasi-metallic character in ultrasmall clusters. Using first-principles hybrid functional calculations, we investigate electronic structures, optical conductivity, dielectric responses, and extinction spectra of bulk Hg2O and three quantum-confined counterparts (15.18 Å, 18.51 Å, and 21.92 Å diameter). The results reveal that quantum confinement breakdown, strong Hg22+ dimer bonding, and relativistic effects (due to mercury's high atomic number) collaboratively induce broadband, featureless optical conductivity, and a reduced or near-zero bandgap in the smallest QD. Notably, QD (3) supports plasmon-like oscillations with a defined plasma frequency (~1.8 eV) and a high-quality factor (Q≈8.2), rivaling those found in noble metal nanoparticles. These findings demonstrate the tunability of optical responses via quantum size modulation and highlight Hg2O QDs as multifunctional nanomaterials for applications in NIR photodetectors, plasmonic waveguides, and broadband photovoltaics.