(435b) Toward Optimal Design of Compound Semiconductor Nanocrystals: First-Principles Analysis, Monte Carlo Simulations, and Experimental Synthesis and Characterization

Crystalline
semiconductor nanocrystals with dimensions over the range from 2 to 10 nm
(quantum dots) exhibit size-dependent luminescence due to quantum confinement
of excitons. This leads to unprecedented tunability in band gap, which can be controlled
by varying the composition, morphology, and size of the nanocrystals. Based on
optimization of band gap by such controlled variables, nanocrystals can be
designed for pertinent photovoltaic applications, light-emitting devices, and
as luminescent biological labels. Synthesis of core/shell quantum-dot
structures produces materials that are more stable against photo-oxidation and
have improved photoluminescence (PL) efficiencies. Various colloidal synthesis
routes to core/shell structures exist, including the coating of a
wider-band-gap semiconductor core with a shell of a narrower-band-gap material
in a two-step process.  However, one-step processes for synthesizing such
core/shell structures remain elusive.

In
this presentation, we report theoretical and experimental results toward
understanding the underlying physics that governs such core/shell structure
formation and can lead to one-step synthesis strategies. Specifically, we
address the problem of equilibrium surface segregation in ternary II-VI and
III-V semiconductor nanocrystals using Monte Carlo (MC) and conjugate gradient
(CG) methods based on classical force fields in conjunction with
first-principles density functional theory (DFT) calculations.  We
have conducted MC/CG simulations for structural and compositional relaxation of
In_xGa_1-xAs and ZnSe_1-xS_x slab
supercells exposing two free surfaces with either [001] or [110]
crystallographic orientation. For the simulations, we have used properly
modified/extended parameterizations of the valence-force-field (VFF)
description of directional bond stretching and bending. The parameterizations
were validated by comparisons with DFT calculations of energy differences for
various configurations of In_xGa_1-xAs and ZnSe_1-xS_x
unreconstructed slabs at different values of the compositional parameter x. The
VFF models were then used to analyze surface segregation phenomena and
determine the resulting equilibrium concentration profiles in ternary
crystalline nanoparticles with well-defined facets and diameters d > 1.7 nm.
Our relaxation method consists of a multi-stage sequence that includes MC
sweeps employing exchanges between In and Ga atoms in In_xGa_1-xAs
structures and between Se and S atoms in ZnSe_1-xS_x
structures, followed by many continuous-space MC sweeps over all atoms for
structural relaxation with an MC step for strain/volume relaxation of the
slabs/particles after each such sweep; in all of the above stages, trials are
accepted or rejected according to the Metropolis criterion. The MC simulation is
preceded and followed by energy minimization according to a CG scheme to
account for local structural relaxation. Our DFT calculations were carried out
within the generalized gradient approximation (GGA) and the local density
approximation (LDA) and employed the projector-augmented-wave method and slab
supercell models. We calculated surface energies, model
surfactant binding energies, and segregation tendencies for the passivated and
unpassivated In_xGa_1-xAs and ZnSe_1-xS_x
slabs with
[001] and [110] crystallographic orientations. In our experiments,
we synthesized nanocrystals using injection of organometallic precursors into
hot coordinating solvents. The nanocrystal synthesis wascarried out
under vacuum and in the presence of argon to avoid reaction with oxygen. The
synthesized nanocrystals were then dispersed in butanol.

We
present simulation results for the equilibrium concentration distributions in
slabs of In_xGa_1-xAs and ZnSe_1-xS_x
as a function of composition, x, slab thickness, and slab surface crystallographic
orientation, as well as in In_xGa_1-xAs and ZnSe_1-xS_x
nanocrystals with well-defined [001] and [110] surface facets as a function of
x and nanocrystal size. The results identify the parameter range for the
assembly of core/shell structures, such as an In-deficient core surrounded by
an In-rich shell in In_xGa_1-xAs nanocrystals. We also
report PL spectra, transmission-electron-microscopy (TEM) images, and X-ray
photoelectron spectra (XPS) of our experimentally synthesized nanocrystals. The
underlying surface segregation is analyzed in particle-size and composition
space and its impact on the experimental one-step synthesis of core/shell
nanocrystal structures is discussed.