Elimination of Hole–Surface Overlap in Graded CdS_xSe_1–x Nanocrystals Revealed by Ultrafast Fluorescence Upconversion Spectroscopy

Interaction of charge carriers with the surface of semiconductor nanocrystals plays an integral role in determining the ultimate fate of the excited state. The surface contains a dynamic ensemble of trap states that can localize excited charges, preventing radiative recombination and reducing fluorescence quantum yields. Here we report quasi-type-II band alignment in graded alloy CdSxSe1-x nanocrystals revealed by femtosecond fluorescence upconversion spectroscopy. Graded alloy CdS(x)Se(1-x) quantum dots are a compositionally inhomogeneous nano-heterostructure designed to decouple the exciton from the nanocrystal surface. The large valence band offset between the CdSe-rich core and CdS-rich shell separates the excited hole from the surface by confining it to the core of the nanocrystal. The small conduction band offset, however, allows the electron to delocalize throughout the entire nanocrystal and maintain overlap with the surface. Indeed, the ultrafast charge carrier dynamics reveal that the fast 1-3 ps hole-trapping process is fully eliminated with increasing sulfur composition and the decay constant for electron trapping (∼ 20-25 ps) shows a slight increase. These findings demonstrate progress toward highly efficient nanocrystal fluorophores that are independent of their surface chemistry to ultimately enable their incorporation into a diverse range of applications without experiencing adverse effects arising from dissimilar environments.