Computational Analysis of Core/Shell-like Structure Formation through Equilibrium Segregation in Ternary Compound Semiconductor Nanocrystals

We present a computational analysis of equilibrium surface segregation in nanocrystals of In$_{x}$Ga$_{1-x}$As, ZnSe$_{1-x}$Te$_{x}$, and ZnSe$_{1-x}$S$_{x}$. The analysis is based on coupled compositional, structural, and strain relaxation employing Monte Carlo and conjugate-gradient methods according to proper parameterizations within the valence-force-field (VFF) description. The VFF parameterizations are validated by comparisons of their segregation energy predictions with first-principles density functional theory (DFT) calculations. We report results for the equilibrium concentration distributions in the nanocrystals as a function of the compositional parameter x and nanocrystal size; the nanocrystal morphologies are polyhedral with distinct facets of low-index surface orientation as determined from DFT calculations of equilibrium crystal shapes. The results identify the particle-size and composition ranges that allow for assembly of core/shell-like nanocrystal structures with increased band-gap tunability.