(377e) Interfacial Stability and Compositional Distribution Effects On Electronic Properties of ZnSe/ZnS Core/Shell Nanocrystals

Ternary semiconductor nanocrystals exhibit size-dependent excitonic properties and allow for exceptional tunability of their band gaps by modulation of the nanocrystal composition. Band structure enegineering based on such controllable parameters enables the development of nanomaterials suitable for photovoltaic and light-emitting devices, as well as for highly luminescent biological labels. A similar class of hetero-nanocomposite materials with widespread technological interest is that of core/shell quantum dots (QDs). For example, the so-called type-I QDs, with wider band-gap shell material such as ZnSe/ZnS, exhibit an increased stability against photodegradation and enhanced quantum yields. Synthesis of such core/shell structures has followed various colloidal chemistry routes, including the coating of a narrower-band-gap semiconductor core with a shell of a wider-band-gap material in a two-step process. However, the thermodynamic stability of core/shell nanostructures synthesized from such processes and the resulting equilibrium compositional distributions have not been investigated systematically.

The nm-scale diffusion lengths in nanocrystals introduce an interesting interplay between the kinetic and thermodynamic stability of core/shell interfaces. Based on this concept, in this presentation, we report results of a systematic analysis of the thermodynamic stability of ZnSe/ZnS core/shell nanocrystals. The analysis is based on first-principles density functional theory (DFT) calculations and MC simulations combined with X-ray photoelectron spectroscopy (XPS) and determination of the photoluminescence (PL) spectra of QDs that we have synthesized. In this synergistic procedure, the MC simulations are based on classical valence force fields that we have parameterized based on our DFT calculations of surface segregation energies using slab supercells. Our findings indicate the possibility of degradation of electronic properties over time, which has far reaching implications for the application of such nanostructures in devices. Moreover, our electronic structure calculations elucidate compositional effects on the band gaps of ZnSe_1-xS_x nanoclusters. The electronic structure dependence on the compositional distribution obtained for core/shell nanocrystals at various compositions provide an interpretation for the observed peak shifts in the PL spectra of the QDs.