(184k) Thermodynamic and Electronic Properties of Ternary Compound Semiconductor Quantum Dots

Ternary (II-VI, IV-VI, and III-V) compound semiconductor nanocrystals can allow for exceptional tunability of their band gaps by modulation of the nanocrystal composition in addition to the well-understood size-dependent excitonic properties. Engineering of band structures based on such controllable parameters enables the development of semiconductor quantum dots suitable for photovoltaic and light-emitting devices, as well as for highly luminescent biological labels. A similar class of hetero-nanocomposite materials of general interest is the core/shell quantum dots. Type-I quantum dots, with wider band-gap shell material such as ZnSe/ZnS, exhibit an increased stability against photo-degradation and enhanced quantum yields. Core/shell nanocrystals, typically prepared by two-step 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 needs for single-step synthesis routes to design minimally strained core/shell-like nanostructures and to examine the stability of overgrown core/shell nanostructures, we have studied the equilibrium properties of ZnSe_1-xTe_x, ZnSe_1-xS_x, and In_xGa_1-xAs ternary compound semiconductor nanocrystal systems. Our study emphasizes on the compositional distributions in the ternary semiconductor nanocrystals and their effects on the electronic and optoelectronic properties.

In this presentation, we report computational and experimental results toward understanding the underlying physics that governs the self-assembly and thermodynamic stability of such semiconductor nanostructures. Specifically, we address the problem of the equilibrium compositional distribution in ternary ZnSe_1-xTe_x, ZnSe_1-xS_x, and In_xGa_1-xAs semiconductor nanocrystals using Monte Carlo (MC) and conjugate gradient (CG) methods according to a classical valence-force-field (VFF) description that has been extended and re-parameterized properly based on first-principles density functional theory (DFT) calculations. Within the DFT framework, we have calculated the equilibrium properties of ZnSe_1-xTe_x, ZnSe_1-xS_x, and In_xGa_1-xAs crystals with special emphasis on the determination of surface segregation energies on low-Miller-index surfaces. We also report classical simulation results for the equilibrium concentration distributions in nanocrystals as a function of composition and nanocrystal size, using various initial configurations including core/shell nanocrystals. The results identify the nanoparticle size and composition ranges that allow for self-assembly of core/shell-like nanocrystal structures; these are characterized by Te and In-deficient core and Te and In-rich compositionally graded shell-like regions in the cases of ZnSe_1-xTe_x and In_xGa_1-xAs, respectively. In addition, electronic properties of the core/shell and the fully relaxed equilibrium nanocrystals generated from the thermodynamic analysis are compared and contrasted within the DFT approach. The computational predictions have been subjected to experimental verification. Both core/shell and alloyed nanocrystals were synthesized according to the standard synthesis routes and their optical and chemical properties were determined. The photoluminescence (PL) emission and X-ray photoelectron spectra (XPS) of our experimentally synthesized nanocrystals in conjunction with our multi-scale computational analysis provide clear insights into the compositional distribution in such structures and its impact on the optoelectronic properties of the nanocrystals.