Nanocrystals of compound semiconductors, such as the II-VI compounds ZnS, CdSe, and ZnSe, exhibit size-dependent optoelectronic properties and form the basis for a new generation of highly integrated nanoelectronic and photovoltaic devices, as well as biological labels.  Nanocrystalline structures over the 2-10 nm size range (quantum dots) exhibit size-dependent luminescence due to quantum confinement of optically excited electron-hole pairs (excitons), which leads to an unprecedented tunability in band gap that can be controlled by varying the size of the nanocrystals.  Synthesis of core-shell quantum-dot structures, such as ternary (e.g., ZnSe_1-xS_x) nanocrystals, produces materials with even wider band gap.  Various synthesis routes to core/shell structures exist, including the coating of a wider-band-gap semiconductor core with a shell of narrower-band-gap material.  However, the underlying mechanisms for formation of such core/shell structures remain elusive.

In this presentation, we report theoretical results toward understanding some of the underlying physics that governs such core/shell structure formation.  Specifically, we address the problem of equilibrium surface segregation in ternary II-VI semiconductor structures using Monte Carlo (MC) and conjugate gradient (CG) relaxation methods based on classical force fields in conjunction with first-principles density functional theory (DFT) calculations.  Atomic-scale (MC & CG) simulations of combined structural and compositional relaxation have been conducted for ZnSe_1-xS_x slabs and nanocrystal particles with well-defined facets in order to attain the concentration profile of group VI atoms (S and Se). The slab supercell models expose two (001), (111), or (110) free surfaces.  Our MC method employs a multi-step sequence including one MC sweep for compositional relaxation (exchange between Se and S atoms) followed by many continuous-space MC sweeps over all atoms for structural relaxation and an MC step for strain relaxation after each such sweep; in all the steps, trials are accepted or rejected according to the Metropolis criterion.  The MC simulation is preceded and followed by energy minimization with a CG scheme to account for local structural relaxation.  Our DFT calculations are carried out within the generalized gradient approximation (GGA) and employ plane-wave basis sets, ultra-soft pseudopotentials, and supercell models.  We use the DFT calculations to test thoroughly our MC/CG simulation scheme for bulk ZnSe_1-xS_x and ZnSe_1-xS_x (001)-(2x1) slabs.

We present results for the compositional distribution in ternary ZnSe_1-xS_x(001), (111) and (110) slabs, as well as in ternary ZnSe_1-xS_x faceted nanocrystals.  In both cases, we analyze the underlying surface segregation phenomena and the resulting equilibrium state.  For the ternary slabs, surface segregation is studied as a function of the composition, x, for various slab thicknesses.  For the ternary nanocrystal particles, surface segregation is studied as a function of x and of the size or diameter of the nanocrystal.  The underlying surface segregation phenomena are analyzed and plausible core/shell structure formation mechanisms are discussed.