Migration Of Dimers Research Articles

The Role of Stable and Mobile Carbon Adspecies in Copper-Promoted Graphene Growth

Early stages of carbon monolayer nucleation on the copper (111) surface are systematically studied using density-functional theory calculations in the context of chemical vapor deposition and irradiation-mediated growth of graphene. By analyzing the kinetics of carbon atoms during their agglomeration, including surface, subsurface, and surface-to-bulk migration as well as dimer formation and diffusion, we draw a qualitative picture of the first stages of graphene growth on copper. The formation and migration of dimers and graphitic fragments happens at a much faster rate than the other competing processes, such as carbon migration into copper bulk and the dissociation of dimers into carbon monomers. To explain this tendency, which is an important factor in making copper such an effective graphene catalyst, we analyze in detail the electronic structure of dimers on surfaces and suggest that dimer stabilization and mobility stem from a delicate interplay between the carbon dimer σp bonding orbitals and copper d and s electrons. Our results emphasize the role of mobile carbon dimer intermediates during the growth of graphene on Cu, Ag, and Au surfaces by chemical vapor deposition and irradiation-mediated methods, in which carbon atoms are implanted into copper foils beyond the solubility limit.

Adatom and dimer migration in heteroepitaxy: Co/Pt(1 1 1)

By means of tight-binding molecular-dynamics simulations, Co adatom and dimer migration on a Pt(1 1 1) surface is investigated. Combining static and dynamic calculations, activation energies associated to these processes are determined. Since the size mismatch between Co and Pt is large, the presented simulations provide an illustration of the way in which growth can be affected by size effects in heteroepitaxy. In particular an increase of the mobility is found for Co dimers (heteroepitaxy) relatively to Pt ones (homoepitaxy).

W dimer diffusion on W(110) and (211) surfaces

Tungsten (W) dimer self-diffusion on the W(110) and (211) surfaces is investigated using a fourth moment approximation to tight-binding theory. For the (110) surface, we find the optimal diffusion mechanism is a concerted jump, i.e., adatom pairs hopping simultaneously on the surface. The linear displacement of a W dimer in the 〈111〉 direction is energetically favored over orientation changes. On the (211) surface, there are two different diffusion mechanisms. For a dimer in one channel along the 〈111〉 direction, the mechanism is a concerted jump with a high activation energy. For adatoms located in adjacent channels, dimers diffuse mainly via individual adatom jumps, and such dimer diffusion is highly correlated. For the present calculations, the activation energies for W dimer migration are similar to those for monomer diffusion, in good agreement with field ion microscopy (FIM) observations. The diffusion mechanism and diffusion anisotropy can be understood with a simple bond coordination model.

The relationship between chemical and tracer diffusion coefficients of aliovalent ions in an ionic lattice

The theoretical model developed by Lidiard was extended to describe the relationship between the chemical and tracer diffusion coefficients of aliovalent ions in an ionic lattice. It is shown that the relationship between the chemical diffusion coefficient, D, and the tracer diffusion coefficient, D ∗, is D = 2 D ∗ if the migration of dimers is the principal mechanism of transport and for the migration of trimers D = 3 D ∗ if the concentration of impurity ion is relatively small. These relationships are valid regardless of the charge of the aliovalent or lattice ions. The chemical diffusion coefficients of Cr 3+ in Cr-doped MgO were determined for three different temperatures, 1656, 1717 and 1768K, and for the concentration region 2.5×10 −2−2.8×10 −1 mole% Cr 2O 3. Using previously determined values for the tracer diffusion coefficient of 51Cr in Cr-doped MgO it was found that for the temperature and concentration region investigated D = (2.00±0.17) D ∗ which indicates that diffusion proceeds primarily by the migration of dimers.