Synthesis of Cu_2-xS/PbS Core/Shell and Cu_xPb_yS Alloy Nanocrystals for Optoelectronics

Excitonic PbS nanoparticles have shown excellent infrared photosensitivity, but their limited diffusion lengths of photoexcited charge carriers result in poor device efficiencies. They also have slow radiative recombination rates that limit the performance of light-emitting diodes. To address these limitations, we work to couple PbS excitons with localized surface plasmons on the nanoscale in order to increase the radiative rate and absorption cross section. Combining infrared plasmonic Cu_2-xS nanocrystals with infrared excitonic PbS nanocrystals has the potential to improve the optical properties of PbS nanocrystals via exciton-plasmon coupling. We develop the first synthetic method to deposit a PbS shell onto Cu_2-xS nanocrystals.¹ This structure is confirmed to have a Cu_2-xS core with a patchy PbS shell using high-angle annular dark-field imaging scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy spectrum (EDS). Addition of the PbS shell changes the crystallographic phase of the Cu_2-xS cores and blueshifts and enhances their infrared plasmonic resonance. The shell also provides chemical stability to the Cu_2-xS nanocrystals such that they no longer completely chemically quench the photoluminescence of neighboring excitonic PbS nanocrystals in solution; consequently, these Cu_2-xS/PbS core/shell nanocrystals will enable future studies of infrared exciton-plasmon coupling at the nanoscale.<br/><br/>Increasing the PbS precursor concentration in the reaction used to deposit PbS shells onto the Cu_2-xS cores leads to cation exchange of some of the Cu_2-xS cores into an alloyed Cu_xPb_yS nanocrystals that have a new excitonic signature. Time-resolved photoluminescence spectroscopy shows a shorter photoluminescence lifetime relative to PbS nanocrystals but a comparable photoluminescence quantum yield, indicating faster radiative decay rates. This is particularly attractive for infrared single-photon emission. Initial HAADF-STEM EDS maps show two populations of nanocrystals: Cu_2-xS/PbS core/shell nanocrystals and Cu_xPb_yS alloyed nanocrystals. The Cu_2-xS/PbS core/shell nanocrystal population can be controlled and even removed when smaller Cu_2-xS cores are used. With decreasing initial Cu_2-xS core size in a given PbS shell reaction, the plasmonic signature decreases and the exciton energy begins to blueshift. These new Cu_xPb_yS alloyed nanocrystals lie on a sizing curve (1^st exciton transition energy vs nanocrystal diameter) distinct from that of pure PbS nanocrystals, while also showing shorter photoluminescence lifetimes and faster radiative decay rates. Thus, these Cu_xPb_yS alloyed nanocrystals are promising as a new material for infrared single-photon emitters.<br/><br/>1. Yee, P. Y.; Brittman, S.; Mahadik, N. A.; Tischler, J.G.; Stroud, R.M.; Efros, A. L.; Sercerl, P.C.; and Boercker, J. E. Cu_2-xS/PbS Core/Shell Nanocrystals with Improved Chemical Stability. <i>Chem. Mater. </i><b>2021</b>, <i>33</i>, 17, 6685-6691.