The diameters of the QDs of 0–3 shells are from 0.6 to 3.0 nm with the total number of atoms from 30 to 744. The QDs are of C 3v symmetry, which is imposed in the computations unless an external field is applied. The c-axis of the wurtzite structure is placed in the z direction, and the b-axis in the y direction. 35 In this work, pseudo hydrogen atoms attached to the surface Cd and Se atoms are assigned to a nuclear charge of 1.5 and 0.5 a.u., respectively. Pseudo-hydrogen possesses a fractional charge of (8 – Z)/4 to satisfy the electron counting rule, where Z is the valence charge of the surface atom attached with pseudo hydrogen. Pseudo hydrogen atoms with fractional charges are employed to passivate the surface Cd and Se dangling bonds. In our computations, the core is constructed with a (CdSe) 6 cluster, and the shells are of 1–3 layers. It has a CdSe core covered by ZnS shells. 1 depicts the structure of which is sliced from bulk wurtzite. 28–31 The QDs have various structures among that the wurtzite ones possess good photoelectric properties, and have been the subject of many studies. The core–shell and QDs in various sizes have been synthesized and characterized in previous studies. 19 studied the dielectric response of colloidal semiconductor QDs and revealed that the dielectric function has a sharp transition at the QD–solvent interface. 18 developed a formula that relates the dielectric response function with the band gaps of QD and its bulk. 16 Based on the Penn model, 17 Franceschetti et al. 15 Compared with core-only CdSe nanoplatelets, the trion-binding energies in ZnS shell-passivated CdSe/ZnS nanoplatelets are reduced by one order of magnitude. By capping the outermost ZnS shells, a maximum decrease of Auger rate of 50% in CdSe/CdS QDs is achieved. 14 found that the dielectric screening of core–shell QDs is geometry dependent, and the changes in the Auger rate of CdSe/CdS QDs are up to ∼1 order of magnitude by tuning the dielectric response. 12 found that the promoted dielectric screening in perovskite weakens the carrier trapping ability of defects and ultimately achieves a power conversion efficiency of 22.3% in photovoltaic devices. 10,11 Dielectric screening reduces the probability of carrier trapping and scattering by defects, playing a positive role in improving carrier transport and photoelectric conversion performance. Great effort has been paid to the study of dielectric responses in low-dimensional materials. Our study proposes a new approach for studying the dielectric properties of core–shell quantum dots, which is effective and extendable for other low-dimensional structures. Moreover, this model gives explicit physical origins of the core dipole polarizability in the core–shell QDs, which is determined by the intra-shell polarization and inter-core-shell charge transfer. The shell thickness dependence on the shell effect is then studied, which is significant for the outermost shell but decays rapidly in the additional shells. Based on the first-principles calculated electron density, the polarizability of the core–shell wurtzite quantum dots is decomposed into the distributional contributions among which the dipole polarizability of the core is proposed to measure the shell effect on the dielectric properties of core–shell quantum dots. Different from 3D bulk materials, the dielectric response is, however, ambiguous for the small-sized 0D dots in which the effect of outer atoms on the inner atoms is usually described qualitatively. The dielectric properties in semiconductor quantum dots are crucial for exciton formation, migration, and recombination.
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