Lead sulfide (PbS) nanocrystals have been used as the active material in high performance, solution-processed, room temperature devices, such as photodetectors, light-emitting diodes, and solar cells. The addition of a zinc sulfide (ZnS) shell to PbS nanocrystals could be advantageous for these devices because it could lead to higher/more stable photoluminescence quantum yields and reduced non-radiative recombination from electron-phonon coupling. However, while ZnS shells have been successfully added to several nanocrystals such as CdS and CdSe it has never been added directly (without a spacer layer) to PbS nanocrystals. This is because it is difficult to add shells to Pb chalcogenide nanocrystals due to their tendency to Ostwald ripen at even moderate temperatures. We have overcome this roadblock and are the first to demonstrate the synthesis of PbS/ZnS core/shell nanocrystals using a “flash” type synthesis with Zn oleate and thioacetamide as the ZnS precursors. We have found that the reaction is self-limiting and deposits a single monolayer of ZnS per shell reaction without causing the PbS nanocrystals to Ostwald ripen. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) verified the presence of the ZnS shell. Furthermore, the absorbance and photoluminescence peak energies were found to redshift upon adding the ZnS shell due to the relaxation of a ligand-induced tensile strain, as well as wave function leakage into the ZnS shell.
While multiple exciton generation (MEG) is known to occur more efficiently in semiconductor nanocrystals than in the
bulk, the required energy threshold prevents visible photons from being utilized. We report two-color pump-probe
measurements demonstrating a two-fold increase in the MEG efficiency of solution samples of PbSe quasi onedimensional
nanorods over zero-dimensional nanocrystals to a value of 0.78, where 1 is the largest efficiency possible.
This improvement is accompanied by a reduction of the MEG threshold energy to 2.28Eg, which allows visible photons
to participate in MEG. This approaches the theoretical limit for the threshold energy of 2Eg imposed by energy
conservation. Detailed balance calculations show that, unlike nanocrystals, photovoltaic cells based on PbSe nanorods
can use MEG to improve power conversion efficiencies, particularly when used in conjunction with solar concentrators.