Ensemble Monte Carlo Simulations Research Articles

Lithium decoration of boron-doped hybrid fullerenes and nanotubes as a novel 3D architecture for enhanced hydrogen storage: A DFT study

A three dimensional (3D) dumbbell-like nanostructure composed by interconnected fullerenes and nanotubes with Lithium decoration and boron-doping (37Li@C139B31) has been proposed in virtue of density functional theory (DFT) and first-principles molecular dynamics (MD) simulations which shows excellent geometric and thermal stability. First-principles calculations are performed to investigate the hydrogen adsorption onto the 37Li@C139B31. The results indicate that B substitution can improve the metal binding and the average binding energy of 37 adsorbed Li atoms on the C139B31 (2.79 eV) is higher than the cohesive energy of bulk Li (1.63 eV) suppressing the clustering. Meanwhile, the H2 storage gravimetric density of 178H2@37Li@C139B31 reaches up to 15.9 wt% higher than the year 2020 target from the US department of energy (DOE). The average adsorption energy of H2 molecules falls in a desirable range of 0.18–0.27 eV. Moreover, grand canonical ensemble Monte Carlo (GCMC) simulations reveal that at room temperature the hydrogen gravimetric density (HGD) adsorbed on 37Li@C139B31 reaches up to 11.6 wt% at 100 bars higher than the DOE 2020 target. Our multiscale simulations indicate that our proposed nanostructure provides a promising medium for hydrogen storage.

Monte Carlo Simulations of Electron Transport Characteristics of Ternary Carbide Al4SiC4

Electron transport characteristics of a novel wide band gap ternary carbide, Al4SiC4, to be used for efficient power and optoelectronic applications, are predicted using ensemble Monte Carlo (MC) simulations. The MC simulations use a mixture of material parameters obtained from density functional theory (DFT) calculations and experiment, with a preference for the experimental data if they are known. The DFT calculations predict a band gap of 2.48 eV, while the experimental measurements give a band gap between 2.78 and 2.8 eV. We have found that the electron effective mass in the two lowest valleys (M and K) is highly anisotropic; in the K valley, mt* = 0.5678 me and ml* = 0.6952 me, and for the M valley, mt* = 0.9360 me and ml* = 1.0569 me. We simulate electron drift velocity and electron mobility as a function of applied electric field as well as electron mobility as a function of doping concentration in Al4SiC4. We predict a peak electron drift velocity of 1.35 × 107 cm s–1 at an electric field of 1400 ...

Screening metal-organic frameworks for capturing radioactive gas Rn in indoor air.

The radioactive gas Rn in indoor air shows severe harm to the human body and even induces the occurence of lung cancer. Finding an excellent candidate that can efficiently and selectively adsorb Rn, is still a great challenge. In this work, we use the grand canonical ensemble Monte Carlo simulation to systematically investigate the selectivities of 23 different kinds of representative metal organic framework (MOFs) for Rn separation from the Rn/N2 and Rn/O2 mixtures at 298 K. Results indicate that ZIF-12, HKUST-1, IRMOF-62 and ZIF-11 are four kinds of excellent candidates for capturing Rn. In particular, the selectivities of ZIF-12 for Rn/N2 and Rn/O2 mixtures with Rn molar fraction XRn = 0.001 reach ∼2500 and 1200 at 1 atm and 298 K, respectively. Moreover, when the Rn concentration becomes smaller, the selectivity of ZIF-12 for Rn would increase significantly. In short, we found that ZIF-12 is indeed a very promising candidate for trapping radioactive Rn in indoor air.

The magnetization of dense aggregated dipolar fluids

The magnetization for dipolar fluids is studied treated separately at low and high external magnetic field. Canonical ensemble Monte Carlo simulations have been performed in dipolar hard sphere fluid in order to test these theoretical results. New expressions are introduced at low and high external field and ultimately the synthesized formula is given. The main difference in the structure of dipolar liquids at different external field is the average orientation of the formed chains. In case of infinitesimal external magnetic field there is no a well-specified direction, while at high enough external field actually the chains are parallel to this field. The present theory yields good result in the intermediate region as well where the most conspicuous failures are provided by the former theories.

Vapor–liquid equilibrium and molecular simulation data for carbon dioxide (CO2) + trans-1,3,3,3-tetrafluoroprop-1-ene (R-1234ze(E)) mixture at temperatures from 283.32 to 353.02 K and pressures up to 7.6 MPa

Isothermal Vapor–Liquid Equilibrium (VLE) data for the binary mixture of CO2 + R-1234ze(E)(trans-1,3,3,3-tetrafluoroprop-1-ene) were measured using a static-analytic method apparatus at seven temperatures between 283.32 and 353.02 K and pressures up to 7.6 MPa. For temperatures over the critical temperature of pure CO2, the critical compositions and pressures of binary mixtures were approximated applying power laws with asymptotic behavior at critical point. The data were well correlated using the Peng–Robinson equation of state incorporating the generalized alpha function, with the Wong–Sandler mixing rules involving NRTL activity coefficient model. The experimental and correlated phase compositions were compared with Gibbs Ensemble Monte Carlo simulation data obtained at 293.15 K and 353.02 K. Though the molecular simulation predictions at low temperature (293.15 K) properly match experimental data, a slight shift appeared at high temperature (353.02 K), and the expected convergence of phase compositions near critical point was not fully observed.

The Influence of Bond Angle on Thermophysical Properties of Three-Center Lennard-Jones Fluids: Computer Simulation and Theory

Abstract We carry out extensive computer simulations to study the phase equilibrium, thermodynamics, and diffusion coefficient of three-center Lennard-Jones (3CLJ) fluids with an emphasis on the effects of bond angle on these properties. We take into account several bond angles ranging from θ 0 = 60 to 180 degrees and two equilibrium bond elongations le = 1.0 and 0.5 (in Lennard-Jones length unit). Moreover, we study the fully flexible (FF) 3CLJ fluids for molecules with bond length 1.0. Gibbs ensemble Monte Carlo (MC) simulations are performed to compute the densities of the vapor-liquid coexisting phases and the vapor pressure, and direct three-phase (vapor-liquid-vapor) molecular dynamics (MD) simulations are carried out to calculate the surface tension. We then apply constant NVT MC simulations to obtain the internal energy, the pressure, and the pair correlation function, and utilize equilibrium MD simulations to compute the diffusion coefficient of systems with le = 1.0. In addition to MD simulations, the modified Cohen-Turnbull (mCT) theory is used to to compute the diffusion coefficient and the mean free volume appearing in the mCT relation is provided by the results of the Generic van der Waals (GvdW) theory. We show that the mCT theory is capable of reproducing the MD simulation values quite well over a wide range of density with slight overestimations at medium range. The angle dependence of different thermophysical properties are analyzed and discussed in details.

Monte Carlo investigation of noise and high-order harmonic extraction in graphene

High-order harmonic generation is an interesting option to reach the THz range for generation and detection applications. In this work the time-domain and spectral behavior of carrier velocity fluctuations in free-standing graphene under the action of a high frequency alternating electric field are studied by means of an ensemble Monte Carlo simulator. The distortion of the velocity response is explained in terms of the carrier wavevector distribution, average energies and scattering activity. The intensity of the harmonics is analyzed for several frequencies and electric field amplitudes. Noise and regular contributions to the power spectral density of the velocity response are discerned, and the harmonic modes available for several bandwidths are discussed. Results show that it is possible to reach the THz range with conditions comparable to those found for III–V materials, at the expense of narrow bandwidths.

Source-to-Drain Tunneling Analysis in FDSOI, DGSOI, and FinFET Devices by Means of Multisubband Ensemble Monte Carlo

The inclusion of quantum effects in the transport direction plays an important role in the extensive research of ultrascaled electronic devices. In this context, it is necessary to study how these phenomena affect different technological architectures in order to conclude which one can be the best candidate to replace standard technology. This paper presents the implementation of direct source-to-drain tunneling effect in a multisubband ensemble Monte Carlo simulator showing its influence in different structures such as fully depleted silicon-on-insulator, double-gate silicon-on-insulator, and FinFET devices. The differences in the potential profile and the electron distribution in the subbands for these architectures modify the number of electrons affected by this quantum mechanism and, therefore, their short-channel behavior.

Impact of the hot phonon effect on electronic transport in monolayer silicene

The hot phonon effect is a non-equilibrium phenomenon that has an important role in high-field electronic transport in two-dimensional electron gases and in 2D materials like graphene. In this work, the influence of this effect on the electronic transport properties of silicene is investigated by means on an ensemble Monte Carlo simulator that accounts self-consistently for the out-of-equilibrium phonon population and the scattering events under high electric field conditions. In this way, the observed reduction in the carrier drift velocity is analyzed in terms of the microscopic scattering events, wavevector distributions, net phonon emission and phonon temperatures. A comparison with the case of graphene is also carried out, showing that despite the similarities concerning the linear bandstructure near the Dirac points, the hot phonon effect is less dominant in silicene due to the differences in the phonon branches and electron–phonon couplings. Even so, neglecting the role of an out-of-equilibrium phonon population may overestimate the high-field drift velocity in silicene up to 25% at moderate carrier concentrations.

Using molecular simulations to probe pore structures and polymer partitioning in size exclusion chromatography

Molecular simulations have been extensively utilized to understand and predict the polymer partitioning in size-exclusion chromatography (SEC). However, idealized pore models (e.g., cylindrical, spherical, and slit pores) were often used to represent the porous media in an SEC column, which leads to significant deviations in describing the geometry and the size of the pores. In this work, several complex pore models were derived from body-centered cubic, random, and gel packing of monodisperse spherical sol particles using simulation methodology. The mechanical stabilities of these structures were determined based on particle coordination numbers. Pore size distributions of these porous structures were compared to a commercially available, wide-pore superficially porous particle. Then, Gibbs ensemble Monte Carlo simulations were performed to compute the pore-to-bulk partitioning coefficient KSEC of a polymer chain with complex pore models. The effects of particle size, packing structure, and porosity on KSEC were explored. In addition, structural analysis provides insight into the conformation of polymers in the pores and its effect on the partitioning behavior. This study promotes the understanding of pore structures in SEC columns and enables more accurate predictions of KSEC with less ambiguity in pore geometry.

Electron mobility in β-Ga2O3: an ensemble Monte Carlo study

Numerical simulations are performed to evaluate electron mobility in β-Ga2O3. Following scattering mechanisms were found to be important: acoustic deformation potential, ionized impurity, and polar optical phonons. In β-Ga2O3, a large primitive unit cell (containing ten atoms) leads to multiple phonon modes, which complicate the mobility calculation. Here, by restructuring an in-house ensemble Monte Carlo simulator, we were able to include and examine the effects of all relevant longitudinal optical phonon modes. For low electrical fields at 300 K, we report an electron mobility of 110 cm2/V s. Also, in the range of 150‒500 K, our simulation results match very well with the reported Hall mobility measurement data.

Investigating adsorption- and diffusion selectivity of CO2 and CH4 from air on zeolitic imidazolate Framework-78 using molecular simulations

Abstract The adsorption and diffusion of N2, O2 representing air, in ternary mixtures with CO2 or CH4, on Zeolitic Imidazolate Framework-78 are investigated by Gibbs ensemble Monte Carlo (GEMC) simulations for adsorption and by molecular dynamics (MD) simulations for diffusion. The adsorption and diffusion data are used for determining the membrane selectivities. N2/O2 ratios are found to be nearly the same in the gas and the adsorbed phase. At 228 K and low concentrations the CO2/N2 adsorption selectivity is found to attain values of as large as 140, further increasing with decreasing CO2 concentration.

Capture of CO2 in carbon nanotube bundles supported with room-temperature ionic liquids: A molecular simulation study

Novel gas adsorption properties by room-temperature ionic liquids (ILs) confined in carbon nanotube (CNT) bundles were ascertained using grand canonical ensemble Monte Carlo (GCMC) simulations in which the adsorption and separation of CO2 from CO2/N2 mixtures. The introduction of ILs into the CNT channels significantly enhances the adsorption of CO2, while it is not sensitive to N2 adsorption. This is due to the strong interaction between specific ILs in the CNTs and CO2 molecules. The presence of water showed a significant impact on the adsorption capacity of CO2 as well.

United atom forcefield for vapor-liquid equilibrium (VLE) properties of cyclic and polycyclic compounds from Monte Carlo simulations

Gibbs ensemble Monte Carlo and NPT simulations are applied to the prediction of VLE properties for cyclic and polycyclic molecules with internal flexibility. An extension of the TraPPE-UA forcefield is defined, considering the molecules fully flexible with appropriate intramolecular potential terms (bond stretching, bending, torsion, Van der Waals) in cycles as well as in side chains. New United Atom types are proposed to handle the ternary and quaternary carbons involved in naphthenes and aromatics bearing alkyl groups or naphthenoaromatics, and published TraPPE-UA parameters are kept unchanged for other groups. The extended forcefield is tested for accuracy and internal consistency on a set of 34 compounds comprising between 5 and 28 carbon atoms including naphthenic hydrocarbons, aromatic hydrocarbons, naphthenoaromatics, and thiophenic compounds. This set of compounds contains several multi-functional polycyclic molecules which can now be described with the forcefield extensions provided by this work (e.g. 5α cholestane, 1-hexyl tetralin) and that are representative of a much wider range of compounds for which predictions through theoretical methods are difficult to be achieved. Comparison with available experimental data on normal boiling temperature, vapor pressure, and liquid density, reveals average absolute deviations of 6.4 K on normal boiling temperature and 0.91% on standard liquid density, without noticeable increase of deviations with molecular weight. Computational efficiency allows simulations for compounds containing up to 30 carbon atoms.

Impact of crystal orientation and surface scattering on DG-MOSFETs in quasi-ballistic region * * Project supported by the National High-Tech R&D Program (No. 2015AA016501).

The characteristics of nano scale n-type double gate MOSFETs with (100) and (110) surfaces are studied using 3D full band ensemble Monte Carlo simulator. The anisotropic surface scattering mechanism is investigated. The (100) case is sensitive to the gate voltage more than the (110) case. The impact of crystal orientation and surface scattering on transport features mainly reflects in the carrier velocity distribution. The electron transport features with (100) direction are greater than that with (110) direction, but are more likely to be affected by the surface scattering.

Probing Additive Loading in the Lamellar Phase of a Nonionic Surfactant: Gibbs Ensemble Monte Carlo Simulations Using the SDK Force Field.

Understanding solute uptake into soft microstructured materials, such as bilayers and worm-like and spherical micelles, is of interest in the pharmaceutical, agricultural, and personal care industries. To obtain molecular-level insight on the effects of solutes loading into a lamellar phase, we utilize the Shinoda-Devane-Klein (SDK) coarse-grained force field in conjunction with configurational-bias Monte Carlo simulations in the osmotic Gibbs ensemble. The lamellar phase is comprised of a bilayer formed by triethylene glycol mono- n-decyl ether (C10E3) surfactants surrounded by water with a 50:50 surfactant/water weight ratio. We study both the unary adsorption isotherm and the effects on bilayer structure and stability caused by n-nonane, 1-hexanol, and ethyl butyrate at several different reduced reservoir pressures. The nonpolar n-nonane molecules load near the center of the bilayer. In contrast, the polar 1-hexanol and ethyl butyrate molecules both load with their polar bead close to the surfactant head groups. Near the center of the bilayer, none of the solute molecules exhibits a significant orientational preference. Solute molecules adsorbed near the polar groups of the surfactant chains show a preference for orientations perpendicular to the interface, and this alignment with the long axis of the surfactant molecules is most pronounced for 1-hexanol. Loading of n-nonane leads to an increase of the bilayer thickness, but does not affect the surface area per surfactant. Loading of polar additives leads to both lateral and transverse swelling. The reduced Henry's law constants of adsorption (expressed as a molar ratio of additive to surfactant per reduced pressure) are 0.23, 1.4, and 14 for n-nonane, 1-hexanol, and ethyl butyrate, respectively, and it appears that the SDK force field significantly overestimates the ethyl butyrate-surfactant interactions.

Competitive Adsorption Mechanism Study of CHClF2 and CHF3 in FAU Zeolite

Separating and recovering CHF3 and CHClF2 greenhouse gases from exhausted gases is quite important for avoiding adverse impacts to the environment. In synthesizing chlorofluorocarbons substitutes, faujasite (FAU) is proved to be effective in separating the hydrofluorocarbons. Grand canonical ensemble Monte Carlo simulations were used without precedent in analyzing the competitive adsorption mechanism of CHF3/CHClF2 in Na58Al58Si134O384 (58Al), Na88Al88Si104O384 (88Al), and Na96Al96Si96O384 (96Al) FAU zeolite model from infinite dilution to saturation adsorption, so as to explore the overall competitive relationship as the adsorption amount increases. As a result, it has been found that the sodium migration degree is affected by the guest–host effect in the 58A1 model. However, the sodium migration is not discovered in 88Al and 96Al models with diversified CHF3 or CHClF2 loadings. The preferred binding site in all FAU zeolite models involves CHClF2 and CHF3 anchored by cations in sites II and site III′. Ad...

Bicanonical ensemble Monte Carlo simulation of water condensation in the field of crystal lattice defects

The free energy, formation work and entropy dependences for water condensate formed from the vapor over the defect-free and containing surface defects basal face of β-AgI crystal at initial stage of condensation at temperatures of 260 K and 400 K are calculated using the bicanonical statistical ensemble method, with Ewald summation for long-range electrostatic and polarization interactions with the substrate. The effect of surface defects in form of rectangular “towers” as a part of regular structure on the stability of condensed phase embryos is investigated. In contrast to small-scale structures, relatively larger coarse-grained nanostructure of crystal surface demonstrates an unconditional advantage in the ability to stimulate condensation compared to defect-free surface. The formation of condensed phase embryos on the surface with multiple defects begins at vapor pressures 5–6 orders of magnitude lower than that on corresponding defect-free surface, and this effect is resistant to temperature variations. The condensate on the surface of the crystal is thermodynamically stable, both on defect-free and nanostructured surfaces, with the exception of short initial stage of the monomolecular film on the defect-free surface.

Damping of acoustic flexural phonons in silicene: influence on high-field electronic transport

Silicene is a two-dimensional buckled material with broken horizontal mirror symmetry and Dirac-like dispersion. Under such conditions, flexural acoustic (ZA) phonons play a dominant role. Consequently, it is necessary to consider some suppression mechanism for electron–phonon interactions with long wavelengths in order to reach mobilities useful for electronic applications. In this work, we analyze, by means of an ensemble Monte Carlo simulator, the influence of several possibilities for the description of the effect of ZA phonon damping on electronic transport in silicene. The results show that a hard cutoff situation (total suppression for phonons with a wavelength longer than a critical one), as it has been proposed in the literature, does not yield a realistic picture regarding the electronic distribution function, and it artificially induces a negative differential resistance at moderate and high fields. Sub-parabolic dispersions, on the other hand, may provide a more realistic description in terms of the behavior of the electron distribution in the momentum space, but need extremely short cutoff wavelengths to reach functional mobility and drift velocity values.

Understanding the Molecular Weight Dependence of χ and the Effect of Dispersity on Polymer Blend Phase Diagrams

Gibbs ensemble Monte Carlo simulations and cloud point measurements were performed to understand the molecular weight dependence of χ and the effect of dispersity on the phase behavior of polymer mixtures. Oligomeric blends consisting of poly(ethylene-alt-propylene) (PEP) and poly(ethylene oxide) dimethyl ether (PEO) were used as the model systems. First, the molecular weight dependence of χ for PEP/PEO mixtures was studied using simulations and experiments for PEP/PEO mixtures with various molecular weights. An empirical model with a single adjustable parameter kij is used to quantify this molecular weight dependence, and it allows for the accurate prediction of χ of PEP/PEO mixtures with arbitrary molecular weights. Second, the effects of molecular weight distribution (MWD) and dispersity (Đ) of PEO on the PEP/PEO phase diagram were investigated via both simulations and experiments. When PEO is relatively monodisperse (Đ < 1.2), the phase diagram is found to be insensitive to either MWD or Đ, despite di...