Unexpected optical blue shift in large colloidal quantum dots elucidated by XAS

M. MIZRAHI; M. ACEBRÓN; F.C. HERRERA.; F.G. REQUEJO; B. HERNANDEZ JUAREZ

Congreso; 29th Annual Users? Meeting of LNLS; 2019

CNPEM

Semiconductor Nanocrystals (NCs) are the focus of very active research in the quest for materials with tunable optical and electronic properties.[1,2] Despite their current use in nowadays technology including new screens (QLED) or lamps acting as energy-efficient light sources, they are still the motivation of numerous fundamental physico-chemical studies.[1,3] Although a huge effort has been performed by the scientific community to dominate particle size, shape, composition and crystallinity, the diversity of synthetic procedures and suitable chemical precursors have led to a broad variety of NCs. In first place we study an alternative approach to grow inorganic shells on CdSe NCs, which implies the removal of the original long insulating ligands to a large extent, simultaneously with the fabrication of core?shell structures under friendly working conditions (room temperature and ambient atmosphere). In the next step, the compositional changes taking place during the synthesis of alloyed CdSeZnS nanocrystals are studied. While size and composition are the two main factors affecting the optical properties, the spatial distribution of the elements and the nature of the interfaces between the different regions composing the NCs also influence the final optical response. In this case, the traditional red shift concomitant to the increasing NCs? size competes with compositional changes taking place during the reaction. The upshot of this competition is mainly controlled by temperature, which defines the evolution of the NCs? structures and optical properties. Notably, in this work, the changes in composition, responsible for the intriguing blueshifted optical response as the NCs grow larger, have been elucidated through X-Ray Absorption Spectroscopy (XAS) measurements at the L3-Cd, K-Se, K-Zn and K-S edges. The results are understood in the frame of ion exchange and migration mechanisms taking place not after post-treatments but in situ during the synthesis of the NCs, which provides an extra degree of freedom and increases the versatility of the hotinjection method. Further, it demonstrates an additional route to tune the composition of QDs and how to modulate the emission wavelength through band gap engineering.