Nanocrystals of compound semiconductors with zinc-blende structure, such as the II-VI compounds ZnS, CdSe, and ZnSe, exhibit size-dependent optoelectronic properties and form the basis for a new generation of nanoelectronic and photovoltaic devices, as well as biological labels. Doping in semiconductor nanocrystals could allow for precise control of their optical and electronic properties. However, in spite of its feasibility, doping of semiconductor nanocrystals has been an extremely difficult task and the doping mechanisms remain elusive. Colloidal Mn-doped ZnSe nanocrystals have been typically grown using hot-injection organometallic synthesis. It has been observed that increasing the anion:cation ratio leads to higher doping efficiencies. This has been attributed to incorporation of Mn dopant atoms into ZnSe nanocrystals through their surface facets, in particular the ZnSe(001)-(2x1) surface. In this context, we aim at obtaining a fundamental and quantitative understanding of dopant adsorption and diffusion on surfaces of II-VI semiconductor nanocrystals with zinc-blende structure, which can help elucidate the mechanisms of dopant incorporation into growing nanocrystals.
In this presentation, we report results on dopant adsorption and diffusion on surface facets of ZnSe nanocrystals 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 have also implemented the climbing-image nudged elastic band method to construct fully optimized dopant diffusion pathways. We have made indirect comparisons between our DFT analysis and experimental findings on doping efficiencies.
We have computed the surface energies of low-Miller-index surfaces [(001), (110), (111) & (111)] and examined the stable reconstructions as a function of anion (Se) chemical potential. We have constructed the equilibrium crystal shape (ECS) of ZnSe nanocrystals as a function of Se chemical potential and found that anion-rich reconstructed surfaces are present in the ECS of a ZnSe nanocrystal. We have also found that dopants may induce surface structural transitions in semiconductor nanocrystals, which resemble remarkably the adsorbate-induced surface reconstructions in metal catalysts. In addition, we have computed the binding energies of Mn atoms onto all the nanocrystal facets and found that they depend strongly on the surface structure and nanocrystal shape. We conclude that all of the anion-rich surfaces contribute toward dopant adsorption onto nanocrystal surface facets. Furthermore, we have found that a critical dopant surface coverage is required to induce surface structural transitions. Our DFT calculations indicate that the binding energy for Mn adsorption onto various sites of all surface facets increases with increasing dopant surface concentration. This low binding energy at low dopant surface concentration explains the doping difficulties during nanocrystal growth.
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