As lead sulfide is an important semiconductor, it is of great interest to identify possible (meta)stable modifications, both at standard and elevated pressures. To predict such structures, global optimizations on the energy landscape of PbS were performed for several pressures, using simulated annealing followed by local optimization of the candidates found. In order to be able to take the effect of the nonbonding valence electrons of Pb into account, ab initio energies were employed for both the global and local optimization. Besides the experimentally known modifications (NaCl structure type at standard pressure and the CsCl structure type at high pressure), several additional promising structures, which might be accessible synthetically, were found; exhibiting, e.g., the TlI type (Cmcm), the FeB type (Pnma), and the α-GeTe type ($R$3$m$).

We report on a new class of ZnO/ZnS nanomaterials based on the wurtzite/sphalerite architecture with improved electronic properties. Semiconducting properties of pristine ZnO and ZnS compounds and mixed ZnO_1−xS_x nanomaterials have been investigated using ab initio methods. In particular, we present the results of our theoretical investigation on the electronic structure of the ZnO_1−xS_x (x = 0.20, 0.25, 0.33, 0.50, 0.60, 0.66, and 0.75) nanocrystalline polytypes (2H, 3C, 4H, 5H, 6H, 8H, 9R, 12R, and 15R) calculated using hybrid PBE0 and HSE06 functionals. The main observations are the possibility of alternative polytypic nanomaterials, the effects of structural features of such polytypic nanostructures on semiconducting properties of ZnO/ZnS nanomaterials, the ability to tune the band gap as a function of sulfur content, as well as the influence of the location of sulfur layers in the structure that can dramatically affect electronic properties. Our study opens new fields of ZnO/ZnS band gap engineering on a multi-scale level with possible applications in photovoltaics, light-emitting diodes, laser diodes, heterojunction solar cells, infrared detectors, thermoelectrics, or/and nanostructured ceramics.

Nanomaterials bridge the gaps between crystalline materials, thin films, and molecules, and are of great importance in the design of new classes of materials, since the existence of many modifications of a nano-object for the same overall composition allows us to tune the properties of the nanomaterial. However, the structural analysis of nano-size systems is often difficult and their structural stability is frequently relatively low. Thus, a study of their energy landscape is needed to determine or predict possible structures, and analyse their stability, via the determination of the minima on the landscape and the generalized barriers separating them. In this contribution, we introduce the major concepts of energy landscapes for chemical systems, and present summaries of four applications to nano-materials: a MgO monolayer on a sapphire substrate, possible quasitwo- dimensional carbon-silicon networks, the ab initio energy landscape of Cu4Ag4-clusters, and the possible arrangements of ethane molecules on an ideally smooth substrate.

The range of feasible ZnO/ZnS polytypes has been explored, predicting alternative structural arrangements compared with previously suggested or observed structural forms of ZnO/ZnS compounds, including bulk crystal structures, various nanostructures, heterostructures and heterojunctions. All calculations were performed ab initio using density functional theory–local density approximation and hybrid Heyd–Scuseria–Ernzerhof functionals. Specifically, pure ZnO and ZnS compounds and mixed ZnO_1–x S _x compounds (x = 0.20, 0.25, 0.33, 0.50, 0.60, 0.66 and 0.75) are investigated and a multitude of possible stable polytypes for ZnO/ZnS compounds creating new possibilities for synthesis of new materials with improved physical and chemical properties are identified.