Enhanced Air Stability, Charge Separation, and Photocurrent in CdSe/CdTe Heterojunction Nanorods by Thiols

CdSe/CdTe heterojunction nanorods with type II staggered band offset can allow directional and efficient separation of photogenerated charge carriers. However, CdTe nanocrystals can often be easily oxidized even with postsynthesis processing in air, which can then lead to charge traps that negate the benefits of the type II band offset. Here, we introduce a simple ligand exchange method to replace the native ligands on CdSe/CdTe heterojunction nanorods with 1-octanethiol resulting in improved photoluminescence and good stability in air. Transient absorption measurements reveal that electron transfer from CdTe to CdSe remains efficient/ fast (similar to 400 fs) despite the hole trapping nature of thiol ligands for CdSe. Absorption bleach arising from CdTe-to-CdSe electron transfer can be observed out to 1 mu s even after days of storage in air, an order of magnitude longer than heterojunction nanorods with native ligands that are processed with anhydrous solvents under air-free conditions and kept air-free. This improved stability/robustness that preserves efficient charge separation translates to enhanced photocurrent generation especially with respect to contribution from photoexcitation of CdTe transitions.