(651a) Toward Design of Semiconductor Ternary Quantum Dots with Optimal Optoelectronic Function

Heterogeneous semiconductor quantum dots (QDs), such as core/shell QDs, are semiconductor nanocrystals with non-uniform compositional distribution that have attracted significant technological interest in optoelectronic and photovoltaic device fabrication.  Compositionally, it is very common for such QDs to be ternary semiconductor nanocrystals; random alloys of such ternary QDs exhibit size-dependent excitonic properties and allow for exceptional tunability of their band gaps by modulation of the nanocrystal composition.  Band structure engineering based on such controllable parameters enables the development of nanomaterials with optimal function for photovoltaic and light-emitting devices. Synthesis of such semiconductor nanocrystals has followed various colloidal chemistry routes.

In this presentation, we investigate the possibility of rigorous single-step synthesis routes to minimally strained core/shell-like QDs with optimal optoelectronic function by studying systematically the distribution of atomic species in ternary QD systems, emphasizing on surface segregation as a potential means for self-assembly of core/shell-like semiconductor QDs.  Toward this end, we have combined atomic-scale computations of full relaxation of semiconductor nanocrystals, first-principles electronic band structure calculations, and continuum mass transport theory and numerical modeling.  The computational results determine the thermodynamic stability of core/shell QDs synthesized by two-step synthesis routes (growth of a shell on a core QD), elucidate the effects of compositional distribution on the electronic band structure of ternary QDs, and provide interpretations to X-ray photoelectron spectra (XPS) and photoluminescence (PL) emission spectra of ternary QDs. 

We have computed the equilibrium concentration profiles in QDs of ZnSe_1-xTe_x, In_xGa_1-xAs, and ZnSe_1-xS_x according to a computational analysis of full nanocrystal relaxation and report results over a broad range of x and QD diameter (from ~1 to ~6 nm).  The computational procedure consists of coupled compositional, structural, and strain relaxation of the semiconductor nanocrystals.  The analysis is based on Monte Carlo (MC) simulations according to extended classical valence-force-field descriptions of interatomic interactions, which have been parameterized according to first-principles DFT calculations of segregation energies using slab supercells.  The equilibrium profiles reflect the different tendencies of the QD constituents to segregate on the QD surface: ZnSe_1-xTe_x and In_xGa_1-xAs form core/shell-like structures with Te- and In-deficient cores, respectively, and Te- and In-rich shells, respectively, characterized by compositionally graded core/shell interfacial regions, while the equilibrium compositional distribution in ZnSe_1-xS_x QDs resembles a random alloy with only a weak tendency for Se surface segregation.  We have determined the electronic structure of ZnSe_1-xTe_x and ZnSe_1-xS_xQDs for various concentration distributions (core/shell, reverse core/shell, and random alloy) over the entire compositional range (0 ≤ x ≤ 1).  We have predicted the dependence of the band gap as a function of x in all cases and demonstrated that, for the materials examined, the thermodynamically stable concentration distributions lead to QDs with tunable band gaps.

In the dilute limit of low x, the computed equilibrium concentration profiles are explained fully by a phenomenological species transport theory that we have developed, which accounts for Fickian diffusion of species in ternary QDs and drift due to the thermodynamic driving force for species surface segregation.  The predictions of the continuum theory provide a powerful tool for the design of novel ternary QDs for applications in optoelectronics and photovoltaics.  We have derived analytical and numerical solutions to the boundary-value problems for species transport in ternary QDs at equilibrium, steady state, and during transient periods.  We have used the predictions for the time dependence of the near-surface-region concentration to fit XPS data for ternary ZnSe_1-xS_x QD near-surface compositional changes during annealing at temperatures ~100 ^oC from ZnSe/ZnS core/shell initial configurations.  The theoretical predictions provide excellent fits to the XPS data and can be used to evaluate diffusion coefficients of species interdiffusion in the QDs and the strength of the thermodynamic driving force for species surface segregation, which are the two fitting parameters used in this analysis.