(479b) Exploiting Equilibrium Properties for Assembly of Core/Shell-Like Compound Semiconductor Nanocrystals

Typical synthesis of core/shell nanostructures involves a rigorous two-step protocol. It is common to synthesize the semiconducting core first, followed by the overgrowth of a thin shell of a different band-gap material. The few attempted single-step and one-pot synthesis approaches have been limited in their use and specific to the type of precursors involved. The core/shell nanocrystals prepared by over-coating methods enclose interfaces of lattice-mismatched core and shell regions that lead to undesirable misfit strain and defect-induced states in the band gap. Motivated by the need for single-step synthesis routes to design minimally strained core/shell-like nanostructures, we study the equilibrium properties of a prototypical ternary compound semiconductor nanocrystal system, emphasizing on equilibrium surface segregation, as a potential means to self-assemble core/shell-like semiconductor quantum dots.

In this presentation, we report results of equilibrium surface segregation in the prototypical system, namely, ZnSe_1-xTe_x nanocrystals based on a computational analysis of coupled compositional, structural, and volume relaxation of the nanocrystals. The analysis employs Monte Carlo and conjugate-gradient methods according to a description of interatomic interactions that has been parameterized based on first-principles density functional theory (DFT) calculations of surface segregation energies. We have determined the equilibrium concentration distribution as a function of nanocrystal size and composition for nanocrystal morphologies that include faceted equilibrium crystal shapes and spherical nanocrystals. The results identify the nanoparticle size and composition ranges that allow for self-assembly of core/shell-like nanocrystal structures that are characterized by Te-deficient core and Te-rich compositionally graded shell-like regions. In addition, core/shell ZnSe/ZnTe nanocrystals and the equilibrium configurations generated from the equilibrium segregation analysis in ZnSe_1-xTe_x nanocrystals are compared and contrasted within the DFT framework. Results for core/shell-like structure formation due to equilibrium surface segregation also are presented for ternary compound semiconductor nanocrystals other than the prototypical system, including III-V compounds such as In_xGa_1-xAs.