Photoluminescent carbon nitride films grown by vapor transport of carbon nitride powders

The thermal vapor transport of nitrogen-rich carbon nitride powders produces carbon nitride films on substrates that retain significant nitrogen content, have conjugated bond character, and show blue photoluminescent emission near 450 nm.     

Atomic force algorithms in density functional theory electronic-structure techniques based on local orbitals

Electronic structure methods based on density-functional theory, pseudopotentials, and local-orbital basis sets offer a hierarchy of techniques for modeling complex condensed-matter systems with a wide range of precisions and computational speeds. We analyze the relationships between the algorithms for atomic forces in this hierarchy of techniques, going from empirical tight-binding through ab initio tight-binding to full ab initio. The analysis gives a unified overview of the force algorithms as applied within techniques based either on diagonalization or on linear-scaling approaches. The use of these force algorithms is illustrated by practical calculations with the CONQUEST code, in which different techniques in the hierarchy are applied in a concerted manner.     

Methods for calculating the desorption rate of an isolated molecule from a surface: Water on MgO(0 0 1)

We present two types of Molecular Dynamics (MD) simulation for calculating the desorption rate of molecules from a surface. In the first, the molecules move freely between two surfaces, and the desorption rate is obtained either by counting the number of desorption events in a given time, or by looking at the average density of the molecules as a function of distance from the surface and then applying transition state theory (TST). In the second, the potential of mean force (PMF) for a molecule is determined as a function of distance from the surface and the desorption rate is obtained by means of TST. The methods are applied to water on the MgO(0 0 1) surface at low coverage. Classical potentials are used so that long simulations can be performed, to minimise statistical errors. The two sets of MD simulations agree well at high temperatures. The PMF method reproduces the 0 K adsorption energy of the molecule to within 5 meV, and finds that the well depth of the PMF is not linear with temperature. This implies the prefactor frequency f in the Polanyi-Wigner equation is a function of temperature, increasing at lower temperatures due to the reduction of the available configuration space associated with an adsorbed molecule compared with a free molecule.     

Ca-intercalated graphite as a hydrogen storage material: Stability against decomposition into CaH2 and graphite

We have used calculations based on density functional theory to investigate the energetics of hydrogen absorption in calcium-intercalated graphites. We focus particularly on the absorption energy and the stability of the hydrogenated material with respect to decomposition into graphite and calcium hydride, which is essential if this material is to be used for practical H₂ storage. The calculations are performed with two commonly used approximations for the exchange-correlation energies. Our calculations confirm earlier predictions that the absorption energy is approximately −0.2 to −0.4 eV, which is favourable for practical use of Ca-intercalated graphite as a hydrogen storage medium. However, we find that the hydrogenated material is strongly unstable against decomposition. Our results therefore explain recent experiments which show that H₂ does not remain stable in CaC₆ but instead forms a hydride plus graphite.     

Bulk diffusion in the undamped Frenkel-Kontorova model

We present new simulation results for the undamped Frenkel-Kontorova model. These support the existence of the bulk diffusion coefficient D, which has been called into question by earlier simulation work of Schneider and Stoll. We point out that D can be studied by three independent routes, and we show that these all provide evidence for the existence of D and yield the same numerical value for it in the particular thermodynamic state examined.     

Methods for calculating the desorption rate of molecules from a surface at non-zero coverage: Water on MgO(0 0 1)

We present calculations of the desorption rate of water molecules from MgO(0 0 1) at a range of coverages θ and temperatures T. Our aim is to demonstrate that this can be done without making uncontrollable statistical mechanical approximations, and we achieve this by using the potential of mean force method reported previously. As in our earlier work on desorption of isolated molecules, we use a classical interaction model. We find that correlations between adsorbed molecules greatly increase the simulation time needed to obtain good statistical accuracy, compared with the isolated molecule. The activation energy for desorption varies significantly with coverage. The calculations also yield the chemical potential of adsorbed molecules as a function of θ and T, from which we can deduce the critical temperature and coverage for phase separation of adsorbed molecules.