David Grumbine

Monomer adsorption on equilateral triangular lattices with attractive first-neighbor interactions.

We have recently studied a model of monomer adsorption on infinitely long equilateral triangular lattices with terraces of finite width M and nonperiodic boundaries. This study was restricted to the case of repulsive adsorbate-adsorbate first-neighbor interactions but included attractive, repulsive, and negligible second-neighbor interactions. The present work extends this study to the case of attractive first-neighbors, and the phases are determined, as before, with a confidence exceeding 10 significant figures. Phase diagrams are included for terrace widths M < or =11. Most of the occupational characteristics of the phases fit exact analytic expressions in M. The infinite-M limit of these expressions, combined with other analyses, provide the complete phase diagram for the infinite two-dimensional lattice. In addition to the empty and full coverage phases, there are three phases exhibiting stripe and cluster features that were not observed in the case of repulsive first-neighbors.

Phenomenological study of monomer adsorption on fcc (335) surfaces with application to CO, O, and N(2) adsorption on Pt(335).

We extend our recent study of adsorption on fcc (112) to fcc (335) surfaces, still considering only first- and second-neighbor interactions with repulsive first-neighbors. We consider the adsorbate-substrate interaction on the step sites of one of the two edges of the infinitely long terraces to be different from that on the remaining sites. The adsorption features on fcc (335) surfaces are richer than those on fcc (112), which can be attributed to the fact that the equilateral triangular terraces are now four-atoms wide rather than three. Our approach is independent of the chemical composition of the substrate and adsorbates and consequently may be applied to a variety of adsorption systems on fcc (335) surfaces which satisfy the limitations of our model. The basic question that our phenomenological approach intends to answer is: what are the constraints that can be obtained on the interaction energies from the experimental observation of one or more phases? This question is answered in the cases of CO, O, and N(2) adsorbed on Pt(335).

Low temperature energy phase diagram for adsorption on Fcc(112) stepped surfaces with attractive first neighbor interactions.

The transfer matrix method developed for the study of monomer adsorption on terraces and nanotubes is applied to on top adsorption on fcc(112) stepped surfaces. The effect due to the step is taken into account by considering adsorbate-substrate interaction on step sites to be different from that on the other bulk sites. We also consider first- and second- neighbor adsorbate-adsorbate interactions, with attractive first-neighbors, thus completing the work published three years ago on repulsive first-neighbors. In the three-dimensional, low temperature energy phase diagram, other than empty and full coverage, there are eight phases: three cluster formations consisting of 4, 5, and 6 adatoms, and five stripe patterns parallel to the steps. Of the thirty-seven phases reported in the repulsive case, none exhibits clusters, and only four of them have stripes that match the ones found in the attractive case. These and other selection rules allow one to predict whether first-neighbors are attractive or repulsive, while the energy phase diagram can be used as a guide to obtain additional information on the remaining interaction energies.

Adsorption on Nanotubes Having Equilateral Triangular Geometry with First- and Second-Neighbor Interactions: Attractive First Neighbors

The recently published general method of studying adsorption on terraces and nanotubes is applied to adsorption on nanotubes in hollows or on-tops, which form a zigzag equilateral triangular adsorbate lattice. We consider adsorbate-adsorbate first- and second-neighbor interactions, with attractive first neighbors. In addition to empty and full coverage, there are three phases of occupational characteristics which are independent of the diameter of the nanotube. The low temperature energy phase diagram is the same for nanotubes of increasing diameters and, thus, is also valid in the infinite-diameter limit. As expected, this diagram is the same as that of the infinite-width limit of an equilateral triangular terrace, on which we have recently reported. The current study also includes temperature effects on the phases and the transition between phases.

Hollow adsorption on zigzag single-walled carbon nanotubes: repulsive first-neighbor interactions.

This article completes the study of adsorption at the centers of the hexagons (hollows) making up zigzag (M, 0) single-walled carbon nanotubes, with first- and second-neighbor adsorbate-adsorbate interactions, using our established transfer matrix method. We have previously determined that the low temperature energy phase diagrams for attractive first-neighbors are the same for all M. We now show that this is not the case for repulsive first-neighbors. Therefore, our model predicts whether first-neighbors are repulsive or attractive, based solely on whether or not experimental results are M-dependent. The numerical computations are carried out for values of M < or = 18. The progressions with increasing M of the structures of the occupational configurations of the phases, and of the low temperature phase diagrams, are compared with the results known in the infinite-M limit. They strongly suggest six families of low temperature phase diagrams with M = 4, 6, 8, 10, 12, and 14, modulo 12. This can be experimentally verified, and can then be used to make further predictions on hollow adsorption on any zigzag (M, 0) nanotube.