Over the last decades, the scientific community has accumulated extensive knowledge over colloidal nanocrystals (NCs) of II-VI semiconductors. It is by now well-established that defects in these materials are mostly located at the surface and invariably lead to exciton trapping, which is typically followed by non-radiative decay or inefficient radiative recombination resulting in strongly red-shifted photoluminescence (PL) with low quantum yields (QYs). Consequently, overgrowth of shells of wider band-gap semiconductors effectively blocks exciton trapping, thereby allowing near-unity PLQYs to be achieved.

In contrast, I-III-VI₂ semiconductors (e.g., CuInS₂) have a very rich defect chemistry and, as a result, NCs of these materials have not only surface defects but also a variety of native defects, such as vacancies and anti-site defects. Interestingly, radiative recombination in I-III-VI₂ NCs involves hole localization in intrinsic defects, which can lead to PLQYs as high as 85%, provided the surface is passivated by suitable shells [1]. However, ZnS shelling of I-III-VI₂ NCs has been reported to invariably lead to blue-shifts in both the absorption and PL spectra, ranging from tens to hundreds of meV [2]. These spectral blue-shifts have been attributed to a number of reasons: etching of the core prior to shell overgrowth, shell ingrowth by cation exchange, or alloy formation due to interdiffusion [2].

These observations imply that the outcome of shelling reactions on I-III-VI₂ colloidal NCs results from a complex interplay between several processes taking place in solution, at the surface of and within the core NC. However, a fundamental understanding of the parameters determining the balance between these different reaction pathways is still lacking. In this work, we address this need by investigating the role of the surface chemistry of CuInS₂ NCs. To this end, we use XPS, EDS, and ICP to study the chemical composition of CuInS₂ NCs which are subsequently used as cores in ZnS shelling reactions. The effect of different washing procedures and batch-to-batch variability is also investigated. Our results show that the surface composition of the core NCs is a critical variable that, in combination with the reactivity of the precursors and the reaction temperature, can be used to tune the extent of the post-shelling spectral shifts, yielding for the first time CuInS₂/ZnS core/shell NCs displaying red-shifted absorption and PL spectra.  

[1] Berends et al., J. Phys. Chem. Lett. 2016, 7, 3503.

[2] van der Stam et al., ChemPhysChem, 2016, 17, 559.