Correlation of the Madelung constant and I—M—I bonding angle with cohesive energy contributions in layered metal diiodides (MI2) with CdI2 (2H polytype) structure

This study uses theoretically methods to investigate, for metal diiodides MI2 (M = Mg, Ca, Mn, Fe, Cd, Pb) with CdI2 (2H polytype) structure, the mutual correlation between the structure-characterizing parameters (the flatness parameter of monolayers f, the Madelung constant A, and bonding angle I—M—I) and correlation of these parameters with contributions of the Coulomb and covalent energies to cohesive energy. The energy contributions to cohesive energy are determined with the use of empirical atomic potentials. It is demonstrated that the parameters f and A, and the bonding angle I—M—I are strictly correlated and increase in the same order: FeI2 < PbI2 < MnI2 < CdI2 < MgI2 < CaI2. It is found that with an increase of parameter A and bonding angle I—M—I the relative contribution of the Coulomb energy to cohesive energy increases, whereas the relative contribution of the covalent energy decreases. For a hypothetical MX 2 layered compound with the CdI2 (2H polytype) structure, composed of regular MX 6 octahedra (angle X—M—X = 90°), the flatness parameter and the Madelung constant are found to be f reg = 2.449 and A reg = 2.183, respectively. Correlation of the covalent energy with the type of distortion of MI6 octahedra (elongation or compression) with respect to regular configuration (angle I—M—I = 90°) is also analyzed.

Formation Conditions for Epitaxial Graphene on Diamond (111) Surfaces

The phase transformation from a non-terminated diamond (111) surface to graphene has in the present study been simulated by using ab initio MD calculations at different temperatures and under various reaction conditions. For strict vacuum conditions, the graphitization process was observed to start at about 800 K, with a final graphene-like ad layer obtained at 2500 K. The C-C bonds across the interface were found to be broken gradually with an increase in temperature. The resulting graphene-like ad layer at 2500 K was observed to chemisorb to the underlying diamond surface with 33% of the initial C-C bonds, and with a C-C covalent energy value of 3.4 eV. The corresponding DOS spectra showed a p-doped character, as compared with graphene. When introducing H radicals during the annealing process, a graphene-like ad layer started to be formed at a much lower temperature; 500K.The completeness of the diamond-to-graphene process was found to strongly depend on the concentration of H radicals. When introducing a larger concentration of H radicals into the lattice in the initial part of the annealing process, the formation of a free-standing graphene layer was observed to take place at an even lower H concentration and temperature (1000 K).

Array-based detection of persistent organic pollutants via cyclodextrin promoted energy transfer

Reported herein is the selective detection of 30 different persistent organic pollutants using cyclodextrin-promoted non-covalent energy transfer for array-based detection.

Multi-Scale Modelling of Low Index Rutile TiO2 Surfaces with a Tight-Binding Variable-Charge Model and Ab Initio Calculations

Developing large scale modelling of oxide surfaces as well as of interfaces formed upon various deposits (oxide, metal, nanoclusters) is of great importance to get prediction on functional material properties under different conditions and in interpreting experiments. We recently developed a new variable-charge model in which the covalent energy is described in the framework of the second-moment approximation of the tight-binding scheme. This model is applied here to the study of the low index rutile TiO2 surfaces. The surface energies, the atomic relaxations and the charge transfer at the (110), (100) and (001) surfaces are calculated and the results compared well, for the first time, with DFT calculations, performed both with GGA and hybrid B3LYP functional.