(21b) First-Principles Theoretical Analysis of Doping in ZnO Nanocrystals

Dilute magnetic semiconductors (DMSs) are compound semiconductors doped with transition metals (TMs). ZnO doped with TMs has been predicted to exhibit room-temperature ferromagnetism with potential applications in spintronic devices. Typically, thin films of doped ZnO are synthesized using vacuum deposition methods, while colloidal chemistry approaches are used to synthesize them in nanocrystalline form. However, successful synthesis of doped nanomaterials with reproducible and uniform properties remains a major challenge. The properties of the synthesized DMSs are strongly dependent upon the preparation methods and processing conditions. For the optimal design and synthesis of doped ZnO nanocrystals, a fundamental understanding of the underlying mechanism of doping is required.

Toward this end, in this presentation, we report results on the adsorption and segregation tendencies of Mn, Co, and Ni dopants on low-Miller-index surfaces [(0001), (0001), (1010), & (1120)] of ZnO. Our analysis is based on first-principles density functional theory (DFT) calculations within the generalized gradient approximation. In our DFT calculations, we have employed slab supercells, plane-wave basis sets, and the projector-augmented wave method. We model each nanocrystal facet as a surface of bulk ZnO given that colloidal nanocrystals have diameters d ~ 5 nm and polyhedral shapes with well-defined facets. The findings of our DFT analysis are discussed in the context of experimental measurements of doping efficiencies in ZnO colloidal nanocrystals.

Our DFT calculations reveal that the species concentration in the growth solution affects the stable structure of ZnO nanocrystal surfaces. The nucleation and growth temperature determines the stability of the adsorbed dopants onto the nanocrystal facets. We find that the Zn-vacancy site on the (0001) surface and the O-vacancy site on the (0001) surface have high dopant binding energies (~3.5-6 eV). These surface vacancies provide viable sites for substitutional incorporation of dopants; such substitutional dopant incorporation is consistent with the experimental reports in the literature. In addition, our DFT calculations indicate that, for all ZnO nanocrystal surface facets, the binding energy for dopant adsorption onto various surface sites increases with increasing dopant surface concentration. This low binding energy at low dopant surface concentration explains the doping difficulties during nanocrystal growth. Furthermore, we have determined the segregation energies of the three dopants and found that these dopants have strong tendencies for surface segregation in ZnO crystals. Significant surface segregation of dopants inhibits their incorporation into the nanocrystals and generates difficulties for efficient ZnO nanocrystal doping.