High-energy exciton transitions in quasi-two-dimensional cadmium chalcogenide nanoplatelets

Semiconductor nanoparticles of cadmium chalcogenides are known to exhibit pronounced thickness-dependent E-0 series of exciton transitions at the Gamma point of the Brillouin zone (BZ). In this work we report an experimental evidence for high-energy series of exciton transitions, which originates from BZ points different from the Gamma point, in the family of cadmium chalcogenide quasi-two-dimensional (2D) nanoplatelets (NPLs). Intensive UV absorption bands demonstrating a pronounced size effect are observed for CdTe, CdSe, and CdS NPLs in addition to the E-0 exciton bands in the visible region. These additional bands are attributed to transitions analogous to the E-1, E-1 + Delta(1), and E-2 series observed in bulk crystals. First-principles DFT calculations of the electronic structure and absorption spectra support this explanation and show that the main contribution to these optical transitions comes from X and M points of the 2D BZ, which originate from L and X points of the 3D BZ. At the same time, the E-0 series of transitions at the Gamma point is well described by the multiband effective-mass model. The observation of the UV exciton bands reveals tunable optical properties of cadmium chalcogenide NPLs in UV spectral region, which may be interesting for practical applications.