(347d) On the Thermodynamic Stability of ZnSe/ZnS Core/Shell Nanocrystals

Ternary semiconductor nanocrystals exhibit size-dependent excitonic properties and allow for exceptional tunability of their band gaps by modulation of the nanocrystal composition. Engineering of band structures based on such controllable parameters enables the development of nanomaterials suitable for photovoltaic and light-emitting devices and for highly luminescent biological labels. A similar class of hetero-nanocomposite materials with widespread interest is that of core/shell quantum dots. The so-called type-I quantum dots, with wider band-gap shell material such as ZnSe/ZnS, exhibit an increased stability against photodegradation and enhanced quantum yields. Various colloidal synthesis routes to core/shell structures exist, including the coating of a narrower-band-gap semiconductor core with a shell of a wider-band-gap material in a two-step process. However, the stability of core/shell nanostructures synthesized from such processes has not been studied systematically and the resulting equilibrium compositional distributions have not been investigated.

In this presentation, we report theoretical and experimental results toward understanding the underlying physics that governs the thermodynamic stability of such core/shell structures. Specifically, we address the problem of the equilibrium compositional distribution in ternary ZnSe_1-xS_x semiconductor nanocrystals using Monte Carlo (MC) and conjugate gradient (CG) methods according to classical force fields that have been parameterized properly based on first-principles density functional theory (DFT) calculations. Within the DFT framework, we have calculated the equilibrium properties of bulk ZnSe_1-xS_x crystals and crystal surfaces modeled by slab supercells exposing two free surfaces with either [001], [110], or [111]-[111] crystallographic orientations. For the classical simulations, we have used a properly extended parameterization of the valence-force-field (VFF) description of directional bond stretching and bond bending that includes electrostatic interactions explicitly. The extension and parameterization procedures were based on fitting DFT predictions of equilibrium properties of ZnSe_1-xS_x unreconstructed slabs at different values of the compositional parameter x and surface orientations. The extended VFF models were then used to relax fully ZnSe_1-xS_x nanocrystals in order to determine the resulting equilibrium concentration profiles in crystalline nanoparticles with well-defined facets and with spherical morphologies; full relaxation consisted of coupled compositional, structural, and volume relaxation.

We report simulation results for the equilibrium concentration distributions in nanocrystals of ZnSe_1-xS_x as a function of composition and nanocrystal size, using various initial configurations including ZnSe/ZnS core/shell nanocrystals. The results indicate a clear tendency of ZnSe/ZnS nanocrystals to transition into alloyed structures by equilibrating toward a solid solution of group-VI atoms. This computational prediction that the ZnSe/ZnS system favors thermodynamically the random-alloyed phases was subjected to experimental verification. Both ZnSe/ZnS core/shell nanocrystals and alloyed ZnSe_1-xS_x nanocrystals were grown following standard synthesis routes and their optical and chemical properties were determined. The photoluminescence (PL) emission and X-ray photoelectron spectra (XPS) of our experimentally synthesized nanocrystals in conjunction with our atomic-scale computational analysis provide clear insights into the compositional distribution in such structures; moreover, they provide a physical explanation of the underlying kinetic phenomena responsible for this equilibrium distribution and its impact on the optoelectronic properties of the nanocrystals.