Tuning the Light Absorption of Cu_1.97S Nanocrystals in Supercrystal Structures

We report on tuning the light absorption properties of Cu_1.97S nanocrystals upon organization in ordered supercrystal structures. In particular, we show that the weak absorption profile of Cu_1.97S nanocrystals can be tuned toward the red part of the visible spectrum. We demonstrate that the controlled addition of ligands to the supercrystals can be used to trigger nanocrystal deassembly, leading to the recovery of the optical properties of the isolated nanocrystals and thus, to a lower absorption in the red part of the spectrum. Supported by structural characterization via electron microscopy and X-ray diffraction our results suggest that the tuning is primarily a consequence of nanocrystal close-packing. Our results highlight an effective approach for extending the light absorption characteristics of Cu_1.97S nanocrystals toward the visible that may be relevant for their application in nanocrystal-based photovoltaics.