The interaction of a water molecule with the (111) surfaces of stoichiometric and reduced ceria is investigated using first principle density functional theory with the inclusion of the on-site Coulomb interaction (DFT+U). It is found that on the stoichiometric ceria(111) surface, the water molecule is adsorbed spontaneously through single hydrogen bond configuration. In contrast, on the lightly reduced ceria(111), there exist both molecular adsorption (no-H-bond configuration) and dissociative adsorption (surface hydroxyl) modes. It is obvious that oxygen vacancies can enhance the interaction of water with the substrate. Phase diagrams for stoichiometric and reduced ceria(111) surfaces in equilibrium with water vapor in the complete range of experimentally accessible gas phase condition are calculated and discussed combining the DFT results and thermodynamics data using the ab initio atomistic thermodynamic method. We present a detailed analysis of the stability of the water-ceria system as a function of the ambient conditions, and focus on two important surface processes for water adsorption on the stoichiometric and on the lightly reduced surfaces, respectively.

Based on density functional theory, the characteristics of n- and p-type impurities are investigated firstly by means of group V and VII atoms substituting sulfur atoms in the SnS2 monolayer nanosheets. Numerical results show that the formation energy and transition levels depend highly on the atomic size and electronegativity of the impurity atom. The formation energies increase with the increasing impurity atom size for each considered doping case. For group V atom-doped SnS2 monolayer nanosheet systems, the calculations of the transition level indicate that N, P or As doping is not effective for p-type conductivity. However, for group VII atom doping cases, F, Cl, Br and I impurities can offer effective n-type carriers in the SnS2 monolayer nanosheets.

Within the framework of the effective-mass approximation, exciton states confined in wurtzite InxGa1−xN∕GaN strained coupled quantum dots (QDs) are investigated by means of a variational approach, including three-dimensional confinement of the electrons and holes in the QDs and strong built-in electric field effects caused by the piezoelectricity and spontaneous polarization. The relationship between exciton states and structural parameters of coupled QDs is studied in detail. We find that the strong built-in electric field in the InxGa1−xN∕GaN strained coupled QDs gives rise to a marked reduction of the effective band gap of InxGa1−xN QDs and leads to a remarkable increasing of the emission wavelengths. Both the sizes and alloy fluctuations of QDs have a significant influence on the exciton states and interband optical transitions in coupled QDs. Moreover, the barrier thickness between the two coupled InxGa1−xN QDs has a considerable influence on the exciton states and optical properties. When the barrier thickness is increased, the exciton binding energy is reduced, the emission wavelength is increased, and the electron-hole recombination rate is obviously reduced. Our theoretical results are in good agreement with the experimental measurements.

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The characteristics are investigated in the p-type Mg-doped GaS and GaSe nanosheets by means of first-principles calculations, showing that with increasing Mg doping concentration, the formation energy increases while the transition level decreases. Moreover, Mg impurity can create a shallower acceptor level in the GaSe nanosheet than in the GaS nanosheet. In particular, the transition level is 39.3 meV when Mg doping concentration is 0.056 in the GaSe nanosheets, which indicates that Mg impurity can offer effective p-type carriers in the GaSe nanosheets.

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The interaction of water molecules with the Cu-CeO(2)(111) catalyst (Cu/CeO(2) and Cu(0.08)Ce(0.92)O(2)) is studied systematically by using the DFT+U method. Although both molecular and dissociative adsorption states of water are observed on all the considered Cu-CeO(2)(111) systems, the dissociation is preferable thermodynamically. Furthermore, the dissociation of water molecule relates to the geometric structure (e.g. whether or not there are oxygen vacancies; whether or not the reduced substrate retains a fluorite structure) and the electronic structure (e.g. whether or not there is reduced cerium, Ce(3+)) of the substrate.In addition, the adsorption of water molecules induces variations of the electronic structure of the substrate, especially for Cu/CeO(2-x)(111)-B (a Cu atom adsorbed symmetrically above the vacancy of the reduced ceria) and highly reduced Cu(0.08)Ce(0.92)O(2)(111), i.e. the Cu(0.08)Ce(0.92)O(2-x)(111)-h. The variations of electronic structure promote the dissociation of water for the highly reduced system Cu(0.08)Ce(0.92)O(2-x)(111)-h. More importantly, the improvement of WGS reaction by Cu-ceria is expected to be by the associative route through different intermediates.

The p-type impurity properties are investigated in the Mg-doped AlN nanosheet by means of first-principles calculations. Numerical results show that the transition energy levels reduce monotonously with the increase in Mg doping concentration in the Mg-doped AlN nanosheet systems, and are lower than that of the Mg-doped bulk AlN case for the cases with larger doping concentration. Moreover, Mg substituting Al atom is energy favorably under N-rich growth experimental conditions. These results are new and interesting to further improve p-type doping efficiency in the AlN nanostructures.