Diffusion barrier

Abstract: A method and an apparatus are provided for performing growth of compound thin films by alternately repeating separate surface reactions of the substances comprising the compound. A carrier gas affects a diffusion barrier between the surface reaction steps to be separated from each other. The gas phase diffusion barrier is also applied to separate the source regions of different reacting vapors both from each other and from the surface reaction zone.

Abstract: We perform density-functional theory (DFT) calculations of carbon dissolution and diffusion in iron, the latter being a typical example of interstitial diffusion. The Kohn-Sham equations are solved with periodic boundary conditions and within the projector-augmented-wave formalism, using the generalized gradient approximation for electron exchange and correlation. With the solution enthalpy as an indication of cell size convergence, we find a supercell with 128 Fe atoms and one C atom is sufficient for describing dilute concentrations of carbon in bcc Fe. The solution enthalpy of carbon in an octahedral site in ferrite is predicted to be 0.74 eV, i.e., the dissolution of carbon in bcc ferromagnetic (FM) Fe is an endothermic process. Using the Fe128C1 periodic cell, we find that the minimum-energy path (MEP) of carbon diffusion from one octahedral site to another (via a tetrahedral site) has a barrier of 0.86 eV, in excellent agreement with the experimental value of 0.87 eV. This encouraging benchmark result prompted us to investigate carbon diffusion in austenite, whose electronic structure is less well characterized experimentally. Cell size convergence results show that a supercell with 32 Fe atoms and one C atom is sufficient. The calculated solution enthalpy is $\ensuremath{-}0.17$ eV, which indicates that the dissolution of carbon in fcc Fe is exothermic, consistent with the known greater solubility of C in austenite compared to ferrite. The MEP shows that carbon moves linearly from an octahedral site to another, contrary to the common notion of an off-plane diffusion path. The diffusion barrier is calculated to be 0.99 eV. Since we model austenite with the FM high-spin phase, the diffusion barrier we obtain is not directly comparable to the experiments in which austenite is usually paramagnetic. However, this prediction is relevant for C incorporation into Fe thin films, since FM high-spin fcc Fe can be obtained by epitaxial growth of thin Fe films on a Cu substrate.

Abstract: The adsorption and diffusion of hydrogen on the (100) surface of titanium nitride was studied using density-functional theory (DFT) and the generalized gradient approximation (GGA) for the exchange and correlation energy. The adsorption site was found to be on top of the titanium atom with the chemisorption energy of -2.88 eV. The diffusion barrier was determined as 0.73 eV along the path connecting the neighboring titanium atoms. The surface energies and surface relaxations of the three most important surfaces of TiN were studied. The surface energies have the following order: ${S}_{100}l{S}_{110}l{S}_{111}.$ Three different GGA functionals, the Perdew-Wang 1991 (PW91), the Perdew-Burke-Ernzerhof (PBE), and the revised PBE (RPBE) functionals, were tested on crystals, small molecules and TiN surfaces. The RPBE functional when applied to the surface studies of TiN was found to produce slightly lower values of surface energies and of hydrogen adsorption energies than the PW91 functional.

Abstract: As a new member of the MXene group, 2D Mo2 C has attracted considerable interest due to its potential application as electrodes for energy storage and catalysis. The large-area synthesis of Mo2 C film is needed for such applications. Here, the one-step direct synthesis of 2D Mo2 C-on-graphene film by molten copper-catalyzed chemical vapor deposition (CVD) is reported. High-quality and uniform Mo2 C film in the centimeter range can be grown on graphene using a Mo-Cu alloy catalyst. Within the vertical heterostructure, graphene acts as a diffusion barrier to the phase-segregated Mo and allows nanometer-thin Mo2 C to be grown. Graphene-templated growth of Mo2 C produces well-faceted, large-sized single crystals with low defect density, as confirmed by scanning transmission electron microscopy (STEM) measurements. Due to its more efficient graphene-mediated charge-transfer kinetics, the as-grown Mo2 C-on-graphene heterostructure shows a much lower onset voltage for hydrogen evolution reactions as compared to Mo2 C-only electrodes.