Simulated structural and magnetic behavior of Mn-Ti intercalated dichalcogenide crystals

We present the results of extensive Monte Carlo simulations of intercalated manganese-titanium (Mn-Ti) layered TiS2 crystals. The computational model involves mixtures of Mn and Ti in various percentages placed on a triangular lattice with fixed lattice sites and up to five layers. The range of concentrations of intercalated Mn studied was 5%  ⩽  X Mn  ⩽  33% and for Ti, 0%  ⩽  X Ti  ⩽  15%, where X A denotes the percentage of the total number of lattice sites occupied by species A. The species are allowed to interact spatially through a screened Coulomb potential and magnetically with external and RKKY field terms. Structurally, the pure Mn systems present as disordered at very low densities and evolve through a 2  ×  2 structure (perfect at X Mn  =  25%) up to a [Formula: see text]  ×  [Formula: see text] lattice (perfect at X Mn  =  33%), with variations of the two 'perfect' lattice structures depending on density. Changes in density for pure Mn systems as well as those intercalated with both Mn and Ti dramatically affects the system's structural and magnetic properties, and the magnetic behavior of various morphological features present in the system are discussed. The RKKY interaction is adjusted based on the intercalant compositions and is very sensitive to structural variations in the intercalant layers. The composition ranges studied here encompass and exceed those that are experimentally accessible, which helps place experimentally relevant densities in perspective.

Melting of hexane monolayers adsorbed on graphite: the role of domains and defect formation

We present the first large-scale molecular dynamics simulations of hexane on graphite that completely reproduce all experimental features of the melting transition. The canonical ensemble simulations required and used the most realistic model of the system: (i) a fully atomistic representation of hexane; (ii) an explicit site-by-site interaction with carbon atoms in graphite; (iii) the CHARMM force field with carefully chosen adjustable parameters of nonbonded interaction, and (iv) numerous >or=100 ns runs, requiring a total computation time of ca. 10 CPU years. The exhaustive studies have allowed us to determine the mechanism of the transition: proliferation of small domains through molecular reorientation within lamellae and without perturbation of the overall adsorbed film structure. At temperatures greater than that of melting, the system exhibits dynamically reorienting domains whose orientations reflect the graphite substrate's symmetry and whose size decrease with increasing temperature.

Fully atomistic molecular dynamics simulations of the behavior of a simple model of crude oil confined between graphene planes

We present and discuss the results of molecular dynamics computer simulations of crude oil confined between graphene planes. The crude oil is represented as a mixture of alkanes having 6 </= n </= 30 carbons that contain explicit hydrogen atoms; the confining structure is a floor and ceiling, each comprised of graphene sheets. At low temperature, the system adsorbs completely onto the confining layers, showing an interesting domain structure in its own right. As the temperature increases, various species desorb in order of increasing molecular mass and enter the vapor phase between the confining sheets. Desorption proceeds through a roughening of the adsorbed layers but does not appear to couple to any inter- or intramolecular phase transition on the surfaces for any given species. Allowing the graphene sheets to be flexible affects the rate of adsorption as well as the in-plane order and molecular confirmations of the adsorbate. Cursory simulations with more than one layer show droplet-like adsorption at low temperatures and complicated dynamics, which shift the initial desorption temperatures to lower values than those for the monolayer and cause the desorption temperature and process to be much less defined. The results presented here are suggestive of a method of separating alkane mixtures at temperatures significantly different from those of conventional refining processes.

Structural and phase properties of tetracosane (C24H50) monolayers adsorbed on graphite: an explicit hydrogen molecular dynamics study

We discuss molecular dynamics (MD) computer simulations of a tetracosane (C24H50) monolayer physisorbed onto the basal plane of graphite. The adlayer molecules are simulated with explicit hydrogens, and the graphite substrate is represented as an all-atom structure having six graphene layers. The tetracosane dynamics modeled in the fully atomistic manner agree well with experiment. The low-temperature ordered solid organizes into a rectangularly centered structure that is not commensurate with underlying graphite. Above T=200 K, as the molecules start to lose their translational and orientational order via gauche defect formation a weak smectic mesophase (observed experimentally but never reproduced in united atom (UA) simulations) appears. The phase behavior of the adsorbed layer is critically sensitive to the way the electrostatic interactions are included in the model. If the electrostatic charges are set to zero (as for a UA force field), then the melting temperature increases by approximately 70 K with respect to the experimental value. When the nonbonded 1-4 interaction is not scaled, the melting temperature decreases by approximately 90 K. If the scaling factor is set to 0.5, then melting occurs at T=350 K, in very good agreement with experimental data.

Explicit hydrogen molecular dynamics simulations of hexane deposited onto graphite at various coverages

We present results of molecular dynamics (MD) computer simulations of hexane (C6H14) adlayers physisorbed onto a graphite substrate for coverages in the range 0.5 < or = rho < or = 1 monolayers. The hexane molecules are simulated with explicit hydrogens, and the graphite substrate is modeled as an all-atom structure having six graphene layers. At coverages above about rho congruent with 0.9 the low-temperature herringbone solid loses its orientational order at T(1) = 140 +/- 3 K. At rho = 0.878, the system presents vacancy patches and T(1) decreases to ca. 100 K. As coverage decreases further, the vacancy patches become larger and by rho = 0.614 the solid is a connected network of randomly oriented islands and there is no global herringbone order-disorder transition. In all cases we observe a weak nematic mespohase. The melting temperature for our explicit-hydrogen model is T(2) = 160 +/- 3 K and falls to ca. 145 K by rho = 0.614 (somewhat lower than seen in experiment). The dynamics seen in the fully atomistic model agree well with experiment, as the molecules remain overall flat on the substrate in the solid phase and do not show anomalous tilting behavior at any phase transition observed in earlier simulations in the unified atom (UA) approximation. Energetics and structural parameters also are more reasonable and, collectively, the results from the simulations in this work demonstrate that the explicit-hydrogen model of hexane is substantially more realistic than the UA approximation.