Copper Doping Blueshifts the Lowest-Energy Optical Transition of PbS Nanocrystals

Doping can tailor the electronic, optical, and magnetic properties of colloidal nanocrystals for optimal performance in devices. We present a novel synthesis of copper (Cu)-doped PbS colloidal nanocrystals based on Cu-to-Pb cation exchange. These nanocrystals exhibit both a blueshift of the energy of the lowest exciton peak, relative to undoped PbS nanocrystals, and photoluminescence. This blueshift of the lowest exciton energy is unique, as only redshifts are observed in Cu-doped II–VI and III–V nanocrystals. Furthermore, retention of the strong infrared photoluminescence is noteworthy because the photoluminescence in other doped PbS nanocrystals is quenched. Three possible reasons for these results are discussed. Cu-doping is confirmed using inductively coupled plasma optical emission spectroscopy (ICP-OES), in addition to high-angle annular dark-field imaging scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS). Coupled with photoluminescence quantum yields (PLQYs) comparable to undoped PbS, the shorter photoluminescence lifetimes measured from time-correlated single photon counting (TCSPC) of the Cu-doped PbS nanocrystals show an increase in the radiative and nonradiative rates by 3–4× as compared to pure oleate-capped PbS nanocrystals. These improved radiative rates could lead to brighter nanocrystal infrared-emitting devices such as single-photon emitters.