Tunable Infrared Bragg Reflectors of Entirely Ligand-Engineered Lead Sulfide Quantum Dot Layers

We demonstrate the realization of fully colloidal quantum dot (CQD)-based Bragg reflectors based on ligand-engineered lead sulfide (PbS) CQD layers, achieving high reflectance with only a few pairs and exhibiting tunable reflectance and stopband characteristics. By functionalizing the PbS CQD films with various ligands, we achieved precise control of the refractive-index contrast via ligand exchange. The effective refractive index of each PbS CQD layer was systematically determined by spectroscopic ellipsometry. Using the optical constants extracted from the index-controlled CQD films, we performed transfer matrix method calculations to predict the optical properties of the fully CQD-based Bragg reflectors. For the fabricated PbS-based infrared (IR) Bragg reflector, high-refractive-index contrast combinations achieved over 95% reflectance with only five pairs, whereas lower-index contrast structures exhibited narrower stopbands with comparable peak reflectance. The experimental reflectance spectra closely matched the calculated values, confirming the reliability of the ligand-engineering approach. These findings demonstrate that the PbS CQD films are a promising platform for tunable IR dielectric mirrors, extending the utility of CQD-based materials beyond optoelectronics to photonic and IR mirror applications.