First-principles calculations of lattice-strained core-shell nanocrystals

We have examined the properties of CdS-ZnS and ZnS-CdS core-shell nanocrystals over a range of shell thicknesses using a real-space pseudopotential-density functional theory approach. The effect of structural relaxation was shown to be important as it leads to significant changes in the band-gap and frontier orbital localizations. It was also predicted that strains at the core-shell interface are only affected by addition of the first few shell layers, with subsequent layers producing small changes in the strain configuration. This strain saturation gives rise to a “thin” shell regime in which both confinement and strain effects contribute to the evolution of the band gap and a “thick” shell regime in which band-gap variations from bulk values are strongly dependent on confinement effects but approximately constant with respect to strain.

Figure 1

Ball-and-stick models of CdS-ZnS core-shell nanocrystals containing one to three shell layers around a 1.2-nm CdS core. Cd (green), Zn (red), S (yellow), and H (white).

Figure 2

Radially PDOS of fully relaxed CdS-ZnS nanocrystals (2a to 2c) and nonrelaxed CdS-ZnS nanocrystal having two shell layers (2d). Dashed lines indicate the radial position of the core-shell interface. Panels are cross-sectional HOMO and LUMO density plots through nanocrystal center, and red circle denotes core-shell interface.

Figure 3

Radially PDOS of fully relaxed ZnS-CdS nanocrystals (3a to 3c) and nonrelaxed ZnS-CdS nanocrystal having two shell layers (3d). Dashed lines indicate the radial position of the core-shell interface. Panels are cross-sectional HOMO and LUMO density plots through nanocrystal center, and red circle denotes core-shell interface.

Figure 4

Bond lengths as a function of distance from nanocrystal center for (a) CdS-ZnS and (b) ZnS-CdS nanocrystals having different shell thicknesses. Dashed lines indicate bulk DFT bond-length values.

Figure 5

Radial distribution of modulus wave function squared for HOMO and LUMO of CdS-ZnS nanocrystals. Dashed line demarcates boundary between core and shell regions.