Hydrogen chemisorption and thermal desorption on the diamond C(111) surface

Nitridation of diamond(111) surface by density functional theory

Density functional theory was employed to examine the adsorption and thermal evolution of nitrogen species on diamond(111) impacted by microwave N2 plasma. On bare domains of diamond, as represented by the models of C(111)-2 × 1 and graphite-like C(111), N2(ad) is identified as the major surface species; the desorption of N2(ad) proceeds on both models via a concerted process of breaking two C-N bonds. By contrast, there is evidence of the formation of (NH)2(ad) via the insertion reaction of microwave N2 plasma on hydrogenated domains of diamond, as represented by the models of C(111)-2 × 1-H and C(111)-1 × 1-H. Interestingly, contrasting dynamics of desorption of (NH)2(ad) are presented on these two models, that is, via sequential breaking of two C-N bonds on C(111)-2 × 1-H and via concerted breaking of both C-N bonds on C(111)-1 × 1-H. Our results demonstrate that the observed diversity of surface nitrogen species in composition, bonding, vibration, and desorption in prior experiments is linked to domains of a variety of surface terminations and reconstructions on diamond(111).

Ab Initio Thermochemistry of the Hydrogenation of Hydrocarbon Radicals Using Silicon-, Germanium-, Tin-, and Lead-Substituted Methane and Isobutane

A series of reactions of the type Y. + XH(4) --> YH + .XH(3) and Y'. + HX(CH(3))(3) --> Y'H + .X(CH(3))(3), where Y = H, CH(3); Y' = CH(3), C(CH(3))(3); and X = Si, Ge, Sn, Pb are studied using state-of-the-art ab initio electronic structure methods. Second-order Møller-Plesset perturbation theory; the coupled-cluster singles, doubles, and perturbative triples method; and density functional theory are used with correlation-consistent basis sets (cc-pVNZ, where N = D, T, Q) and their pseudopotential analogs (cc-pVNZ-PP) to determine the transition-state geometries, activation barriers, and thermodynamic properties of these reactions. Trends in the barrier heights as a function of the group IVA atom (Si, Ge, Sn, and Pb) are examined. With respect to kinetics and thermodynamics, the use of a hydrogen attached to a group IVA element as a possible hydrogen donation tool in the mechanosynthesis of diamondoids appears feasible.

Clustering of chemisorbed H(D) atoms on the graphite (0001) surface due to preferential sticking

We present scanning tunneling microscopy experiments and density functional theory calculations which reveal a unique mechanism for the formation of hydrogen adsorbate clusters on graphite surfaces. Our results show that diffusion of hydrogen atoms is largely inactive and that clustering is a consequence of preferential sticking into specific adsorbate structures. These surprising findings are caused by reduced or even vanishing adsorption barriers for hydrogen in the vicinity of already adsorbed H atoms on the surface and point to a possible novel route to interstellar H2 formation.

High translational energy release in H2 (D2) associative desorption from H (D) chemisorbed on C(0001)

Highly energetic translational energy distributions are reported for hydrogen and deuterium molecules desorbing associatively from the atomic chemisorption states on highly oriented pyrolytic graphite (HOPG). Laser assisted associative desorption is used to measure the time of flight of molecules desorbing from a hydrogen (deuterium) saturated HOPG surface produced by atomic exposure from a thermal atom source at around 2100 K. The translational energy distributions normal to the surface are very broad, from approximately 0.5 to approximately 3 eV, with a peak at approximately 1.3 eV. The highest translational energy measured is close to the theoretically predicted barrier height. The angular distribution of the desorbing molecules is sharply peaked along the surface normal and is consistent with thermal broadening contributing to energy release parallel to the surface. All results are in qualitative agreement with recent density functional theory calculations suggesting a lowest energy para-type dimer recombination path.