Stokes shift and fine structure splitting in composition-tunable Zn_xCd_1−xSe nanocrystals: Atomistic tight-binding theory

I report on the atomistic correlation of the structural properties and excitonic splitting of ternary alloy Zn_xCd_1−xSe wurtzite nanocrystals using the sp³s* empirical tight-binding method with the description of the first nearest neighbouring interaction and bowing effect. Based on a successful model, the computations are presented under various Zn compositions (x) and diameters of alloy Zn_xCd_1−xSe nanocrystals with the experimentally synthesized compositions and sizes. With increasing Zn contents (x), the optical band gaps and electron-hole coulomb energies are improved, while ground electron-hole wave function overlaps, electron-hole exchange energies, stokes shift and fine structure splitting are reduced. A composition-tunable emission from blue to yellow wavelength is obviously demonstrated. The optical band gaps, ground electron-hole wave function overlaps, electron-hole interactions, stokes shift and fine structure splitting are progressively decreased with the increasing diameters. Alloy Zn_xCd_1−xSe nanocrystal with Zn rich and large diameter is the best candidate to optimistically be used as a source of entangled photon pairs. The agreement with the experimental data is remarkable. Finally, the present systematic study on the structural properties and excitonic splitting predominantly opens a new perspective to understand the size- and composition-dependent properties of Zn_xCd_1−xSe nanocrystals with a comprehensive strategy to design the optoelectronic devices.