Cu2-xS/PbS Core/Shell Nanocrystals with Improved Chemical Stability

Nanocrystals of doped semiconductors, such as Cu2-xS, hold much promise for near infrared active devices because, unlike in noble metals, their tunable infrared plasmon can exist in nanocrystals with diameters <40 nm. Combining infrared plasmonic Cu2-xS nanocrystals with infrared excitonic PbS nanocrystals has the potential to improve the optical properties of PbS nanocrystals via exciton-plasmon coupling. To explore this potential, we have developed the first synthetic method to deposit a PbS shell onto Cu2-xS nanocrystals and confirmed the structure to have a mostly Cu2-xS core with a patchy PbS shell using high-angle annular dark-field imaging scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy spectrum images. Addition of the PbS shell changes the crystallographic phase of the Cu2-xS cores and blueshifts and enhances their infrared plasmonic resonance. The shell also provides chemical stability to the Cu2-xS nanocrystals such that they no longer completely chemically quench the photoluminescence of neighboring excitonic PbS nanocrystals in assembled films; consequently, these Cu2-xS/PbS core/shell nanocrystals will enable future studies of infrared exciton-plasmon coupling at the nanoscale. This work demonstrates the importance of understanding the chemical interactions between nanocrystals of different materials when characterizing their optoelectronic properties in mixtures and composite materials.