Dynamic surface structural engineering via alternating epitaxy for narrow-bandgap PbS quantum dots

Pb (acac) 2 suppresses water-induced defects, accelerating growth kinetics. • Alternating epitaxy enables purification-free, atomic-level tuning of PbS CQDs. • Periodic absorption shifts synchronized with Pb/S-rich surface cycling. Narrow-bandgap lead sulfide (PbS) colloidal quantum dots (CQDs) are promising for high-performance short-wave infrared (SWIR) optoelectronics, yet achieving concurrent control over their size, bandgap, and uniformity remains challenging. Here, we present an alternating epitaxial growth strategy grounded in dynamic surface structural engineering. Using lead acetylacetonate (Pb(acac) 2 ) as a highly reactive precursor, this approach enables atomic-level tuning of PbS CQD size and bandgap via periodic precursor injections, eliminating intermediate purification steps. We observed in situ cyclic shifts in the absorption peak position, strictly synchronized with the injection cycles. Combined structural and theoretical analyses reveal that these shifts stem from dynamic surface lattice strain, triggering surface reconstruction. This mechanism originates from cyclic transitions between Pb-rich and S-rich surface chemistries. By leveraging it, our strategy redshifts the absorption edge of PbS CQDs to ∼1971 nm while yielding an exceptionally low Urbach energy of 15 meV, reflecting superior narrow-bandgap characteristics and minimal energetic disorder. This work provides a robust method for fabricating high-quality PbS CQDs and introduces a new in situ paradigm for probing colloidal nanocrystal growth, offering a pathway to advanced infrared devices. We construct a dynamic Pb-rich/S-rich surface environment via periodic precursor injection, and the cycles drive atomic-scale size expansion of the quantum dots, with the absorption edge ultimately redshifting to ∼1971 nm. This strategy enables precise regulation of narrow-gap PbS quantum dots, centered on the dynamic coupling between surface chemistry and lattice strain. Download: Download high-res image (118KB) Download: Download full-size image