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Graphene oxide frameworks (GOFs) are interesting adsorbent materials with well-defined slit-shaped pores of almost monodisperse separation of ∼1 nm between graphene-like layers; however, the exact nature of the structure has remained undetermined. Recently, GOFs were observed to swell monotonically upon the adsorption of methane and xenon under supercritical conditions. Here, we present the results of molecular dynamics simulations of the adsorption of methane and xenon for various proposed GOF structures based upon force fields based on

The development of high-performance materials for hydrogen storage by adsorption requires detailed understanding of the adsorbate-adsorbent interactions, e.g., the enthalpy of adsorption ΔH, which measures the interaction strength. The determination of ΔH for a weakly adsorbing gas such as hydrogen in a carbonaceous porous material is difficult experimentally, normally requiring measuring two cryogenic adsorption isotherms. Here we demonstrate a calculation of ΔH based on ca. room temperature adsorption isotherms at 273 K and 296 K using the Clausius-Clapeyron equation. This requires an estimation of the volume of the adsorbed film (~40%, ~12% of the total pore volume at 77 K, 296 K, respectively) obtained from fits of the excess adsorption isotherms to an Ono-Kondo model with the auxiliary use of a fixed point corresponding to the saturation film density (estimated as 100 ± 20 g/L) which appears to be remarkably sample and temperature independent, i.e., a property of the adsorbate. The calculated room temperature enthalpy of adsorption ΔH = 8.3 ± 0.4 kJ/mol is in excellent agreement with the low-coverage cryogenic determination of ΔH. The methodology hereby proposed facilitates reliable calculations of the enthalpy of adsorption at room temperatures for weakly-adsorbing gases.

Natural gas (NG) is an interesting primary fuel; its larger-scale use is hindered by the difficulties of storing it under high pressures or low temperatures; a viable alternative is its storage via physisorption in porous materials. Most NG adsorption studies have focused on adsorption of pure methane, its primary component. Here we investigate the influence of heavier alkanes commonly found in NG (propane, ethane) on the adsorption process. We present the results of extensive molecular dynamics simulations of mixtures of methane-propane and methane-ethane at T = 300 and 400 K and P = 0-1500 bar in slit-shaped pores with interlayer spacings H = 8-20 A. We observed that heavier hydrocarbons adsorb preferentially but remain mobile, which is promising for the intended application. We also solved a common problem with simulations of molecules with high adsorption affinity: the difficulty to determine their partial pressure. We developed an Arrhenius-type relationship allowing the calculation of these partial pressures from relationships between energy distributions of the different molecules in the simulations in conditions where a direct determination of these is impractical or impossible.

We have investigated adsorption-induced deformation in graphene oxide framework materials (GOFs) using neutron diffraction at sample pressures up to 140 bar. GOFs, made by the solvothermal reaction of graphite oxide and benzene-1,4-diboronic acid, are a suitable candidate for deformation studies due to their narrow (∼1 nm), monodispersed, slit-shaped pores whose width can be measured by diffraction techniques. We have observed, in situ, a monotonic expansion of the slit width with increasing pressure upon adsorption of xenon, methane, and hydrogen under supercritical conditions. The expansion of ∼4% observed for xenon at a pressure of 48 bar is the largest deformation yet reported for supercritical adsorption on a carbonaceous material. We find that the expansion of the three gases can be mapped onto a common curve based solely on their Lennard-Jones parameters, in a manner similar to a law of corresponding states.

An exciting development in the field of correlated systems is the possibility of realizing two-dimensional (2D) phases of quantum matter. For a systems of bosons, an example of strong correlations manifesting themselves in a 2D environment is provided by helium adsorbed on graphene. We construct the effective Bose-Hubbard model for this system which involves hard-core bosons $(U\approx\infty)$, repulsive nearest-neighbor $(V>0)$ and small attractive $(V', 22 pages, 18 figures. Reorganized text and extended an appendix. For associated data and code repository see: https://github.com/DelMaestroGroup/papers-code-BoseHubbardModelHeAdsorptionGraphene

Due to their unique geometry complex, self-assembled nanoporous 2D molecular crystals offer a broad landscape of potential applications, ranging from adsorption and catalysis to optoelectronics, substrate processes, and future nanomachine applications. Here we report and discuss the results of extensive all-atom Molecular Dynamics (MD) investigations of self-assembled organic monolayers (SAOM) of interdigitated 1,3,5-tristyrilbenzene (TSB) molecules terminated by alkoxy peripheral chains C

International audience; Due to their unique geometry complex, self-assembled nanoporous 2D molecular crystals offer a broad landscape ofpotential applications, ranging from adsorption and catalysis to optoelectronics, substrate processes, and future nanomachineapplications. Here we report and discuss the results of extensive all-atom Molecular Dynamics (MD) investigations of self-assembledorganic monolayers (SAOM) of interdigitated 1,3,5-tristyrilbenzene (TSB) molecules terminated by alkoxy peripheral chains Cncontaining n carbon atoms (TSB3,5-Cn) deposited onto highly ordered pyrolytic graphite (HOPG). In vacuo structural andelectronic properties of the TSB3,5-Cn molecules were initially determined using ab initio second order Møller−Plesset (MP2)calculations. The MD simulations were then used to analyze the behavior of the self-assembled superlattices, including relaxed latticegeometry (in good agreement with experimental results) and stability at ambient temperatures. We show that the intermoleculardisordering of the TSB3,5-Cn monolayers arises from competition between decreased rigidity of the alkoxy chains (loss ofintramolecular order) and increased stabilization with increasing chain length (afforded by interdigitation). We show that theinclusion of guest organic molecules (e.g., benzene, pyrene, coronene, hexabenzocoronene) into the nanopores (voids formed byinterdigitated alkoxy chains) of the TSB3,5-Cn superlattices stabilizes the superstructure, and we highlight the importance of alkoxychain mobility and available pore space in the dynamics of the systems and their potential application in selective adsorption.

International audience; Nano-porous two-dimensional molecular crystals, self-assembled on atomically flat host surfaces offer abroad range of possible applications, from molecular electronics to future nano-machines. Computerassisteddesigning of such complex structures requires numerically intensive modeling methods. Here wepresent the results of extensive, fully atomistic simulations of self-assembled monolayers of interdigitatedmolecules of 1,3,5-tristyrilbenzene substituted by C6 alkoxy peripheral chains (TSB3,5-C6), depositedonto highly-ordered pyrolytic graphite. Structural and electronic properties of the TSB3,5-C6 moleculeswere determined from ab initio calculations, then used in Molecular Dynamics simulations to analyze themechanism of formation, epitaxy, and stability of the TSB3,5-C6 nanoporous superlattice. We show thatthe monolayer disordering results from the competition between flexibility of the C6 chains and theirstabilization by interdigitation. The inclusion of guest molecules (benzene and pyrene) into superlatticenanopores stabilizes the monolayer. The alkoxy chain mobility and available pore space defines thesystems dynamics, essential for potential application.

International audience; Phase properties of gases adsorbed in small nanopores are mainly determined by the pore size and shape as well as the structural heterogeneity of the adsorbate. Here we analyze the evolution of the melting mechanism that occurs in pores

Inelastic neutron scattering spectra have been collected over a wide range of momentum transfer from H 2 adsorbed in several high-porosity carbon substrates. We show theoretical spectra which consider the relationship between rotational and translational transitions in the highly anisotropic adsorption environment, proving that different rotational excitations contain different amount of recoil broadening and motivating a new analysis method which considers both types of transitions at once. Spectra for most of the samples, including two activated carbons, are very similar to one another, supporting models of nanoporous carbons which are quite similar on the sub-nanometer scale. The exception is the low-energy side of the rotational peak, indicating important differences in the initial distribution of motion. We also find more subtle differences in the spectra which may be linked to differences in sample heterogeneity and surface rugosity. One sample does have a very different spectrum, which is not explained by standard models of this system. We also observe a significantly reduced effective mass in the spectrum of recoil transitions and evidence of coupling of rotational and translational motion resulting from periodic variations in orientation of the rotational states.

We aim to understand how the van der Waals force between neutral adatoms and a graphene layer is modified by uniaxial strain and electron correlation effects. A detailed analysis is presented for three atoms (He, H, and Na) and graphene strain ranging from weak to moderately strong. We show that the van der Waals potential can be significantly enhanced by strain, and present applications of our results to the problem of elastic scattering of atoms from graphene. In particular we find that quantum reflection can be significantly suppressed by strain, meaning that dissipative inelastic effects near the surface become of increased importance. Furthermore we introduce a method to independently estimate the Lennard-Jones parameters used in an effective model of He interacting with graphene, and determine how they depend on strain. At short distances, we find that strain tends to reduce the interaction strength by pushing the location of the adsorption potential minima to higher distances above the deformed graphene sheet. This opens up the exciting possibility of mechanically engineering an adsorption potential, with implications for the formation and observation of anisotropic low dimensional superfluid phases., 15 pages, 14 figures

International audience; We present the results of extensive fully atomistic molecular dynamics (MD) simulations of tetracosane (C24H50) bilayer and trilayer systems adsorbed onto the basal plane of graphite. At low temperature, both layers of the bilayer exist in well-defined solid phases. With increasing temperature, the system exhibits separated smectic phases that eventually lead to melting. During this process, we observed a strong interlayer translational correlation and mobility between layers; however, the upper layer presents more intra- (chain) and intermolecular disorder because of a lack of confinement and a greater distance to the graphite substrate. Simulations of the perpendicular trilayer patch show that gauche defects provide the main mechanism for spreading of the bottom and outer perimeter of the patch in the solid, leading to the ultimate collapse of the patch with increasing temperature and formation of a flat (parallel) trilayer that melts at a higher temperature than the bilayer structure. The wide variety of structural order parameters, thermodynamic functions, and probability distributions we employed provide a clear picture of the roles of gauche defects, confinement, and interlayer correlation in the phases and phase transitions exhibited by these confined organic layers.

International audience; ! abstract The mechanisms of methane adsorption in (i) homogeneous carbon slit pores of widths between 1 nm and 2 nm and (ii) heterogeneous MOF pores of similar unit cell sizes have been compared. We discuss the mechanism of layering transition in subcritical conditions, for temperatures between 80 K and 180 K. The layer formation is strongly temperature-dependent. In slit pores it varies from a sharp adsorption at low temperatures to a more continuous uptake at higher temperatures. The pore size defines the number of adsorbed layers: the 1 nm pore allows adsorption of 2 layers while the 2 nm pore allows adsorption of 5 layers of methane molecules. We compare this behavior with the mechanism of adsorption in two MOFs, IRMOF-1 and IRMOF-16, with strongly heterogeneous walls (both structurally and energetically). This comparison allows us to discuss separately the influence of wall topology and intermolecular interactions on the mechanism of layering.

We present results of molecular dynamics computer simulations of hexane (C6H14 or C6) and pentane (C5H12 or C5) adlayers physisorbed onto a graphite substrate, for various submonolayer coverages. The hexane and pentane molecules incorporate explicit hydrogens and the graphite is modelled as a six-layer all-atom structure. Even though C6 and C5 have different structures at monolayer completion, both systems generally behave similarly in the submonolayer regime and results are in reasonable agreement with experiment for both systems. Specifically, there are four distinct topological regimes involving empty space: at densities closest to full coverage, there are large domains with individual vacancies, then with decreasing density, large vacancy patches appear first, followed by the formation of connected networks of smaller domains with multiple orientations that ultimately separate into individual patches. The energetics and melting behaviour of all systems are readily understood within the framework of the topology presented at various densities.

Since 1999, experiments have shown a plethora of surprising results in the low-temperature magnetotransport in intermediate regions between quantum Hall (QH) plateaus: the extreme anisotropies observed for half-filling, or the re-entrant integer QH effects at quarter filling of high Landau levels (LL); or even an apparent melting of a Wigner Crystal (WC) at filling factor ν = 1/7 of the lowest LL. A large body of seemingly distinct experimental evidence has been successfully interpreted in terms of liquid crystalline phases in the two-dimensional electron system (2DES). In this paper, we present a review of the physics of liquid crystalline states for strongly correlated two-dimensional electronic systems in the QH regime. We describe a semi-quantitative theory for the formation of QH smectics (stripes), their zero-temperature melting onto nematic phases and ultimate anisotropic-isotropic transition via the Kosterlitz–Thouless (KT) mechanism. We also describe theories for QH-like states with various liquid crystalline orders and their excitation spectrum. We argue that resulting picture of liquid crystalline states in partially filled LL-s is a valuable starting point to understand the present experimental findings, and to suggest new experiments that will lead to further elucidation of this intriguing system.

The attachment of flux tubes to electrons by a Chern–Simons (CS) singular gauge transformation of the wavefunction opened up the field theoretical description of the fractional quantum Hall effect (FQHE). Nevertheless, in Jain's composite fermion (CF) theory, quasiparticles are believed to be vortices carrying a fractional charge in addition to the winding phase of the CS flux tubes. The different structure of the wavefunction in these two cases directly affects the excitation energy gaps. By using a simple ansatz we were able to evaluate analytically the Coulomb excitation energies for the mean-field level CS wavefunction, thus allowing a direct comparison with corresponding numerical results obtained from Jain's CF picture. The considerable difference between the excitation energies found in these two cases demonstrates in quantitative terms the very different impact that the internal structure of the wavefunction has in these two approaches, often used interchangeably to describe the FQHE.

We investigate liquid crystalline phases with nematic order at (1/3)-filling of the valence Landau level (LL). We generalize Laughlin's fractional quantum Hall (QH) effect wavefunction at ν = (1/3) to include anisotropic nodal distribution by modifying the Jastrow factors. Lengthy Monte Carlo simulations are then used to determine with unprecedented accuracy the (anisotropic) pair distribution function g(r) and static structure factor S(q) for various degrees of anisotropy. The determination of the correlation energies at (1/3)-filling of an arbitrary LL is then performed by using standard mappings of g(r) and S(q) to higher LLs. Our results indicate that while Laughlin's state is stable in the lowest LL, there are regions of instability towards nematic order in higher LLs. Possible connections to the recently discovered QH liquid crystals are discussed.

The Jain's composite fermion wavefunction has proven quite succesful to describe most of the fractional quantum Hall states. Its mathematical foundation lies in the Chern-Simons field theory for the electrons in the lowest Landau level, despite the fact that such wavefunction is different from a typical mean-field level Chern-Simons wavefunction. It is known that the energy excitation gaps for fractional Hall states described by Jain's composite fermion wavefunction cannot be calculated analytically. We note that analytic results for the energy excitation gaps of fractional Hall states described by a fermion Chern-Simons wavefunction are readily obtained by using a technique originating from nuclear matter studies. By adopting this technique to the fractional quantum Hall effect we obtained analytical results for the excitation energy gaps of all fractional Hall states described by a Chern-Simons wavefunction.

We present a theoretical study of the excitations on the edge of a two-dimensional electron system in a perpendicular magnetic field in terms of a contour dynamics formalism. In particular, we focus on edge excitations in the quantum Hall effect. Beyond the usual linear approximation, a nonlinear analysis of the shape deformations of an incompressible droplet yields soliton solutions which correspond to shapes that propagate without distortion. A perturbative analysis is used and the results are compared to analogous systems, like vortex patches in ideal hydrodynamics. Under a local induction approximation, we find that the contour dynamics is described by a nonlinear partial differential equation for the curvature: the modified Korteweg‐de Vries equation. @S0163-1829~99!13339-3#

We give a general review of recent developments in the theory of vortices in superfluids and superconductors, discussing why the dynamics of vortices is important, and why some key results are still controversial. We discuss work that we have done on the dynamics of quantized vortices in a superfluid. Despite the fact that this problem has been recognized as important for forty years, there is still a lot of controversy about the forces on and masses of quantized vortices. We think that one can get unambiguous answers by considering a broken symmetry state that consists of one vortex in an infinite ideal system. We argue for a Magnus force that is proportional to the superfluid density, and we find that the effective mass density of a vortex in a neutral superfluid is divergent at low frequencies. We have generalized some of the results for a neutral superfluid to a charged system.

Strongly correlated electronic systems in two dimensions have constantly been a source of new discoveries. For example, the integer and fractional quantum Hall (QH) effects have emerged when such systems have been subjected to strong perpendicular magnetic fields. Recently, in the transitional regions between QH plateaus, strong magneto-transport anisotropies have been observed below ca. 100 mK. In this paper, we explain the emergence of broken rotational symmetry at half-filled valence Landau levels in terms of quantum liquid crystalline states with nematic order. Quantum Monte Carlo simulations indicate that while isotropic states are stable in the lowest and first excited Landau level, there are regions of instability towards liquid crystalline states in higher Landau levels. A possible connection of the recently discovered magneto-transport anisotropy in low magnetic fields and these liquid crystalline states is discussed.

The total transverse force acting on a quantized vortex in a superfluid is a problem that has eluded a complete understanding for more than three decades. In this letter I propose a remarkably simple argument, somewhat reminiscent of Laughlin's beautiful argument for the quantization of conductance in the quantum Hall effect, to define the superfluid velocity part of the transverse force. This term is found to be $- \rho_s {\kappa}_s \times {v}_s$. Although this result does not seem to be overly controversial, this thermodynamic argument based only on macroscopic properties of the superfluid does offer a robust derivation. A recent publication by Thouless, Ao and Niu has demonstrated that the vortex velocity part of the transverse force in a homogeneous neutral superfluid is given by the usual form $\rho_s {\kappa}_s \times {v}_V$. A combination of these two independent results and the required Galilean invariance yields that there cannot be any transverse force proportional to the normal fluid velocity, in apparent conflict with Iordanskii's theory of the transverse force due to phonon scattering by the vortex., Comment: RevTex, 1 Encapsulated Postscript figure

Breakdown of the Quantum Hall Effect at high values of injected current is explained as a consequence of an abrupt formation of a metallic ``river'' percolating from one edge of the sample to the other. Such river is formed when lakes of compressible liquid, where the long-range disorder potential is screened, get connected with each other due to the strong electric field. Our theory predicts critical currents consistent with experiment values and explains various features of the breakdown., Presented at the 12th conference on High Magnetic Fields in Semiconductor Physics, Wuerzburg, Germany, July/1996

International audience; We report Fourier transform infrared spectroscopy (FTIR) studies of boron-doped activated carbons. The functional groups for hydrogen adsorption in these materials, the boron-related chemical bonds, are studied by comparing the activated carbon materials with and without boron doping. The activated carbon materials are prepared from corncob biomass waste feedstock through KOH activation, yielding adsorbents with a high surface area. Boron atoms are doped into the activated carbon by vapor deposition of decaborane up to a solubility of 6.8 wt.%. Extra boron atoms (2-3 wt.%) are located on the surface of the carbon matrix. Results from conventional FTIR show serious spectral broadenings and band overlaps. To overcome the spectral broadenings and band overlaps, the sample concentration is reduced to a very low weight percent (0.03%) of activated carbon in KBr, and spectra are acquired by using microscopic FTIR. Activated boron carbide is used as a reference material to validate the boron-carbon bond in the nanoporous materials. For activated carbon doped via vapor deposition of decaborane, the substitutions of carbon atoms with boron atoms is confirmed using microscopic FTIR through the appearance of boron-carbon bonds, although it cannot be observed with conventional FTIR.

International audience; The efficient storage and transportation of natural gas is one of the most important enabling technologies for use in energy applications. Adsorption in porous systems, which will allow the transportation of high-density fuel under low pressure, is one of the possible solutions. We present and discuss extensive grand canonical Monte Carlo (GCMC) simulation results of the adsorption of methane into slit-shaped graphitic pores of various widths (between 7 angstrom and 50 angstrom), and at pressures P between 0 bar and 360 bar. Our results shed light on the dependence of film structure on pore width and pressure. For large widths, we observe multi-layer adsorption at supercritical conditions, with excess amounts even at large distances from the pore walls originating from the attractive interaction exerted by a very high-density film in the first layer. We are also able to successfully model the experimental adsorption isotherms of heterogeneous activated carbon samples by means of an ensemble average of the pore widths, based exclusively on the pore-size distributions (PSD) calculated from subcritical nitrogen adsorption isotherms. Finally, we propose a new formula, based on the PSD ensemble averages, to calculate the isosteric heat of adsorption of heterogeneous systems from singlepore-width calculations. The methods proposed here will contribute to the rational design and optimization of future adsorption-based storage tanks.

International audience; The results of extensive molecular dynamics simulations and theoretical considerations of the adsorption of methane-methyl mercaptan mixtures in slit-shaped carbon nanopores are presented. We observe significant mobility of both methane and mercaptan molecules within the pore volume, between pores, and between adsorbed and gas phases for a wide range of temperatures and pressures. Although mercaptans adsorb preferentially relative to methane, the process remains reversible, provided non-oxidizing conditions are maintained. A mercaptan/methane ratio of the order of 200 ppm in the adsorbed phase is sufficient for the gas phase to have a mercaptan concentration above the human threshold for detection. The reversibility of the adsorption process and low concentration of mercaptans makes it unlikely that these would be harmful for adsorbed natural gas storage systems.

When current is forced through a quantum Hall bar it is understood that it flows primarily through edge states. These represent extra charges that in turn produce a more widely distributed current density that falls off rather slowly with distance and then drops rapidly as a result of screening by particles beyond the depletion region. We have solved numerically the self consistent Schro\ifmmode\ddot\else\textasciidieresis\fi{}dinger equation in the Hartree approximation for ideal samples with small widths and sharp boundaries, and combined these results with the Wiener-Hopf technique to obtain an accurate picture of the situation for macroscopically wide samples. Our results indicate that the redistribution of states in the bulk of a quantum Hall bar is a very important effect and that the bulk states contribute to a significant fraction of the total current.

International audience; his paper demonstrates that nanospace engineering of KOH activated carbon is possible by controlling the degree of carbon consumption and metallic potassium intercalation into the carbon lattice during the activation process. High specific surface areas, porosities, sub-nanometer (

International audience; Inelastic neutron scattering spectra are calculated for hydrogen molecules adsorbed on activated carbon. The slit-shaped pore model is used to calculate the adsorption potentials of a hydrogen molecule in pores of variable width. The motion of the hydrogen molecules is quantized perpendicular to the plane of the pore and both rotational and vibrational transition energies are found. The perturbation of adjacent hydrogen molecules on the transition energies is discussed. Form factors and Debye-Waller factors are calculated for momentum transfer parallel and perpendicular to the pore; this anisotropy is particularly pronounced in pores where there is nearly enough room for two adsorbed layers. A spectrum composed of a uniform distribution of pore sizes agrees only qualitatively with experimental results for activated carbon. Reasons for this disparity are discussed, including the possibility that transitions in the translational motion parallel to the adsorption plane contribute significantly to the spectra. In addition, the positioning of the center of gravity of the first rotational transition is discussed, with several factors contributing to shift it from the 14.7 meV of the gas phase. Our results suggest that inelastic neutron scattering can be valuable as a complementary sub-nanometer pore characterization technique.

Carbon-based materials, due to their low cost and weight, have long been considered as suitable physisorption substrates for the reversible storage of hydrogen. Nanoporous carbons can be engineered to achieve exceptional storage capacities: gravimetric excess adsorption of 0.073 ± 0.003 kg H 2/ kg carbon, gravimetric storage capacity of 0.106 ± 0.005 kg H 2/ kg carbon, and volumetric storage capacity of 0.040 ± 0.002 kg H 2/liter carbon, at 80 K and 50 bar. The nanopores generage high storage capacity by having a very high surface are, by generating a high H 2-wall interaction potential, and by allowing multi-layer adsorption of H 2 (at cryogenic temperatures). In this paper we show how the experimental adsorption isotherms can be understood from basic theoretical considerations and computational simulations of the adsorption in a bimodal distribution of narrow and wide pore spaces. We also analyze the possibility of multi-layer adsorption, and the effects of hypothetical larger adsorption energies. Finally, we present the results of coupled ab initio calculations and Monte Carlo simulations showing that partial substitution of carbon atoms in nanoporous matrix with boron results in significant increases of the adsorption energy and storage capacity.

International audience; The interplay between the torsional potential energy and the scaling of the 1-4 van der Waals and Coulomb interactions determines the stiffness of flexible molecules. In this paper we demonstrate for the first time that the precise value of the nonbond scaling factor (SF)--often a value assumed without justification--has a significant effect on the critical properties and mechanisms of systems undergoing a phase transition, and that, for accurate simulations, this scaling factor is highly dependent on the system under consideration. In particular, by analyzing the melting of n-alkanes (hexane C6, dodecane C12, tetracosane C24) on graphite, we show that the SF is not constant over varying alkane chain lengths when the structural correlated transformations are concerned. Instead, monotonic decrease of SF with the molecular length drives a cross-over between two distinct mechanisms for melting in such systems. In a broad sense we show that the choice for SF in any simulation containing adsorbed or correlated long molecules needs to be carefully considered.

Nanoporous carbon offers promise for reversible storage of hydrogen. In this paper we discuss the influence of various parameters (energy of adsorption, H-2-H-2 interaction, pore geometry, chemical substitution of carbon by boron atom or Li+, quantum effects) on the adsorption capacity. The limits for these parameters have been explored using extensive grand canonical Monte Carlo simulations. We show that multilayer structures of hydrogen can be stabilized at low temperatures in pores larger than 1 nm. Furthermore, chemical substitution of carbon atoms by boron leads to increased adsorption. We also discuss how the mechanism of adsorption changes when the adsorption surface area is increased by drilling holes in graphene sheets. (C) 2010 Elsevier B.V. All rights reserved.

Theoretical limits of the hydrogen adsorption in carbon nanospaces are modeled using Monte Carlo simulations. A detailed analysis of storage capacity of slit pores has been performed as a function of the pore size, gas pressure (up to 100 bars) and temperature of adsorption (77 and 298 K). The H-2-slit wall interaction has been modeled assuming energies of adsorption ranging from 4.5 kJ/mol (pure graphene surface) to 15 kJ/mol (hypothetical chemically modified graphene). The quantum nature of H-2 has been incorporated in the calculations using the Feynman-Hibbs approach. it has been shown that in a hypothetical chemically modified porous carbon, with energy of adsorption of 15 kJ/mol or higher and pore size between 0.8 and 1.1 nm, the gravimetric and volumetric storage capacity can achieve targets required for practical applications. The relation between the energy of adsorption and the effective delivery has been discussed. (C) 2009 Elsevier Ltd. All rights reserved.

Nature uses phase transitions as powerful regulators of processes ranging from climate to the alteration of phase behavior of cell membranes to protect cells from cold, building on the fact that thermodynamic properties of a solid, liquid, or gas are sensitive fingerprints of intermolecular interactions. The only known exceptions from this sensitivity are critical points. At a critical point, two phases become indistinguishable and thermodynamic properties exhibit universal behavior: systems with widely different intermolecular interactions behave identically. Here we report a major counterexample. We show that different members of a family of two-dimensional systems —the discrete p-state clock model— with different Hamiltonians describing different microscopic interactions between molecules or spins, may exhibit identical thermodynamic behavior over a wide range of temperatures. The results generate a comprehensive map of the phase diagram of the model and, by virtue of the discrete rotors behaving like continuous rotors, an emergent symmetry, not present in the Hamiltonian. This symmetry, or many-to-one map of intermolecular interactions onto thermodynamic states, demonstrates previously unknown limits for macroscopic distinguishability of different microscopic interactions.

Activated carbons are one of promising groups of materials for reversible storage of hydrogen by physisorption. However, the heat of hydrogen adsorption in such materials is relatively low, in the range of about 4-8 kJ/mol, which limits the total amount of hydrogen adsorbed at P=100 bar to similar to 2 wt % at room temperature and similar to 8 wt % at 77 K. To improve the sorption characteristics the adsorbing surfaces must be modified either by substitution of some atoms in the all-carbon skeleton by other elements, or by doping/intercalation with other species. In this letter we present ab initio calculations and Monte Carlo simulations showing that substitution of 5%-10% of atoms in a nanoporous carbon by boron atoms results in significant increases in the adsorption energy (up to 10-13.5 kJ/mol) and storage capacity (similar to 5 wt % at 298 K, 100 bar) with a 97% delivery rate. (C) 2009 American Institute of Physics. [doi:10.1063/1.3251788]

It is shown how appropriately engineered nanoporous carbons provide materials for reversible hydrogen storage, based on physisorption, with exceptional storage capacities (approximately 80 g H2/kg carbon, approximately 50 g H2/liter carbon, at 50 bar and 77 K). Nanopores generate high storage capacities (a) by having high surface area to volume ratios, and (b) by hosting deep potential wells through overlapping substrate potentials from opposite pore walls, giving rise to a binding energy nearly twice the binding energy in wide pores. Experimental case studies are presented with surface areas as high as 3100 m(2) g(-1), in which 40% of all surface sites reside in pores of width approximately 0.7 nm and binding energy approximately 9 kJ mol(-1), and 60% of sites in pores of width>1.0 nm and binding energy approximately 5 kJ mol(-1). The findings, including the prevalence of just two distinct binding energies, are in excellent agreement with results from molecular dynamics simulations. It is also shown, from statistical mechanical models, that one can experimentally distinguish between the situation in which molecules do (mobile adsorption) and do not (localized adsorption) move parallel to the surface, how such lateral dynamics affects the hydrogen storage capacity, and how the two situations are controlled by the vibrational frequencies of adsorbed hydrogen molecules parallel and perpendicular to the surface: in the samples presented, adsorption is mobile at 293 K, and localized at 77 K. These findings make a strong case for it being possible to significantly increase hydrogen storage capacities in nanoporous carbons by suitable engineering of the nanopore space.

We present the first large-scale molecular dynamics simulations of hexane on graphite that completely reproduces all experimental features of the melting transition. The canonical ensemble simulations required and used the most realistic model of the system: (i) fully atomistic representation of hexane; (ii) explicit site-by-site interaction with carbon atoms in graphite; (iii) CHARMM force field with carefully chosen adjustable parameters of non-bonded interaction; (iv) numerous $\ge$ 100 ns runs, requiring a total computation time of ca. 10 CPU-years. This has allowed us to determine correctly the mechanism of the transition: molecular reorientation within lamellae without perturbation of the overall adsorbed film structure. We observe that the melted phase has a dynamically reorienting domain-type structure whose orientations reflect that of graphite., Comment: 5 pages, submitted to Langmuir

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.

We discuss Molecular Dynamics (MD) computer simulations of tetracosane (C24H50) monolayer physisorbed onto a graphite surface. The alkane molecules are simulated with explicit hydrogens, and the graphite substrate is represented as an all-atom struc-ture having six graphene layers. The tetracosane dynamics modeled in the fully ato-mistic manner agree well with experiment. The low-temperature ordered solid orga-nizes in rectangular centered structure, incommensurate with underlying graphite. Above T = 200 K, as the molecules start to lose their translational and orientational order via gauche defects 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 is included in the model. If the electrostatic charges are set to zero (as it is in UA force field), the melting temperature increases by ~70 K with respect to the experimental value. When the non-bonded 1-4 interaction is not scaled, the melting temperature decreases by ~90 K. If the scaling factor is set to 0.5, the melting occurs at T = 350 K, in very good agreement with experimental data. PACS number: 82.45.Mp, 64.70.dj, Comment: 13 pages, 11 embedded figures, PDF format

We discuss an alternative accurate Monte Carlo method to calculate the ground-state energy and related quantities for Laughlin states of the fractional quantum Hall effect in a disk geometry. This alternative approach allows us to obtain accurate bulk regime (thermodynamic limit) values for various quantities from Monte Carlo simulations with a small number of particles (much smaller than that needed with standard Monte Carlo approaches)., Comment: 13 pages, 6 figures, 2 tables

We present the results of parallel Molecular Dynamics computer simulations of hexane (C6H14) adlayers physisorbed onto a graphite substrate in the density range 0.5 ≤ ρ ≤1 in units of monolayers, with emphasis on monolayer completion (ρ = 1). The hexane molecules are modeled to explicitly include hydrogens and the graphite is modeled as a six – layer all atom structure. In the explicit hydrogen simulations, the herringbone solid loses its orientational order at T1 = 140 °K, fairly consistent with results of UA simulations. However there is almost no nematic meso-phase or negative energy change at the loss of herringbone order. The explicit hydrogen melting temperature is T2 = 160 °K—somewhat lower than seen in experiment and in UA simulations. Generally, results for the all–atom model agree well with experiment, as the molecules remain overall flat on the substrate in the solid phase. At densities below about ρ = 0.875 the system supports a connected network which stabilizes it against thermal fluctuations and yields much more reasonable sub-monolayer- melting behavior. The united atom picture, on the other hand, departs significantly from experiment at most sub-monolayer- densities and gives melting temperatures several decades below what is experimentally observed. The purpose of this work is to compare the results of UA and explicit hydrogen MD simulations of hexane on graphite mainly at ρ = 1, to discuss cursory explorations at sub-monolayer- densities and mention open questions related to the system that are worth pursuing. Various structural and thermodynamic order parameters and distributions are presented in order to outline such differences.