(739a) First-Principles Analysis and Monte Carlo Simulations of Surface Segregation In ZnSe1-xSx Nanostructures

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.