5 research outputs found
Ab Initio Molecular Dynamics and Lattice Dynamics-Based Force Field for Modeling Hexagonal Boron Nitride in Mechanical and Interfacial Applications
Hexagonal boron nitride
(hBN) is an up-and-coming two-dimensional
material, with applications in electronic devices, tribology, and
separation membranes. Herein, we utilize density-functional-theory-based
ab initio molecular dynamics (MD) simulations and lattice dynamics
calculations to develop a classical force field (FF) for modeling
hBN. The FF predicts the crystal structure, elastic constants, and
phonon dispersion relation of hBN with good accuracy and exhibits
remarkable agreement with the interlayer binding energy predicted
by random phase approximation calculations. We demonstrate the importance
of including Coulombic interactions but excluding 1ā4 intrasheet
interactions to obtain the correct phonon dispersion relation. We
find that improper dihedrals do not modify the bulk mechanical properties
and the extent of thermal vibrations in hBN, although they impact
its flexural rigidity. Combining the FF with the accurate TIP4P/Ice
water model yields excellent agreement with interaction energies predicted
by quantum Monte Carlo calculations. Our FF should enable an accurate
description of hBN interfaces in classical MD simulations
Generalized Mechanistic Model for the Chemical Vapor Deposition of 2D Transition Metal Dichalcogenide Monolayers
Transition metal dichalcogenides
(TMDs) like molybdenum disulfide
(MoS<sub>2</sub>) and tungsten disulfide (WS<sub>2</sub>) are layered
materials capable of growth to one monolayer thickness <i>via</i> chemical vapor deposition (CVD). Such CVD methods, while powerful,
are notoriously difficult to extend across different reactor types
and conditions, with subtle variations often confounding reproducibility,
particularly for 2D TMD growth. In this work, we formulate the first
generalized TMD synthetic theory by constructing a thermodynamic and
kinetic growth mechanism linked to CVD reactor parameters that is
predictive of specific geometric shape, size, and aspect ratio from
triangular to hexagonal growth, depending on specific CVD reactor
conditions. We validate our model using experimental data from Wang <i>et al.</i> (<i>Chem. Mater.</i> <b>2014</b>, <i>26</i>, 6371ā6379) that demonstrate the systemic evolution
of MoS<sub>2</sub> morphology down the length of a flow CVD reactor
where variations in gas phase concentrations can be accurately estimated
using a transport model (<i>C</i><sub>Sulfur</sub> = 9ā965
Ī¼mol/m<sup>3</sup>; <i>C</i><sub>MoO3</sub> = 15ā16
mmol/m<sup>3</sup>) under otherwise isothermal conditions (700 Ā°C).
A stochastic model which utilizes a site-dependent activation energy
barrier based on the intrinsic TMD bond energies and a series of EvansāPolanyi
relations leads to remarkable, quantitative agreement with both shape
and size evolution along the reactor. The model is shown to extend
to the growth of WS<sub>2</sub> at 800 Ā°C and MoS<sub>2</sub> under varied process conditions. Finally, a simplified theory is
developed to translate the model into a ākinetic phase diagramā
of the growth process. The predictive capability of this model and
its extension to other TMD systems promise to significantly increase
the controlled synthesis of such materials
Quantitative Modeling of MoS<sub>2</sub>āSolvent Interfaces: Predicting Contact Angles and Exfoliation Performance using Molecular Dynamics
The
large-scale synthesis of molybdenum disulfide (MoS<sub>2</sub>) using
liquid-phase exfoliation, as well as several of its intended
applications, including desalination membranes and biosensors, involve
liquids coming into intimate contact with MoS<sub>2</sub> surfaces.
Molecular dynamics (MD) simulations offer a robust methodology to
investigate nanomaterial/liquid interactions involving weak van der
Waals forces. However, MD force fields for MoS<sub>2</sub> currently
available in the literature incorrectly describe not only the cohesive
interactions between layers of MoS<sub>2</sub> but also the adhesive
interactions of MoS<sub>2</sub> with liquids such as water. Here,
we develop a set of force-field parameters that reproduce the properties
of bulk 2H-MoS<sub>2</sub>, with special attention to the distinction
between the covalent, intralayer terms and the noncovalent, interlayer
Coulombic and van der Waals interactions. The resulting force field
is compatible with MD force fields for liquids and can correctly describe
interactions at MoS<sub>2</sub>āliquid interfaces, yielding
excellent agreement with experimental contact angles for water (a
polar solvent) and diiodomethane (a nonpolar solvent). In light of
these results, previously published simulations studies on the desalination
potential and biocompatibility of MoS<sub>2</sub> devices need to
be reevaluated. Potential of mean force (PMF) calculations demonstrate
that use of our new force field can explain the current selection
of solvents used in experimental studies of the liquid-phase exfoliation
of MoS<sub>2</sub> flakes, including the colloidal stability of the
resulting dispersion. Our new force field enables an accurate description
of MoS<sub>2</sub> interfaces and will hopefully pave the way for
simulation-aided design in applications including membranes, microfluidic
devices, and sensors
Mechanism and Prediction of Gas Permeation through Sub-Nanometer Graphene Pores: Comparison of Theory and Simulation
Due to its atomic
thickness, porous graphene with sub-nanometer
pore sizes constitutes a promising candidate for gas separation membranes
that exhibit ultrahigh permeances. While graphene pores can greatly
facilitate gas mixture separation, there is currently no validated
analytical framework with which one can predict gas permeation through
a given graphene pore. In this work, we simulate the permeation of
adsorptive gases, such as CO<sub>2</sub> and CH<sub>4</sub>, through
sub-nanometer graphene pores using molecular dynamics simulations.
We show that gas permeation can typically be decoupled into two steps:
(1) adsorption of gas molecules to the pore mouth and (2) translocation
of gas molecules from the pore mouth on one side of the graphene membrane
to the pore mouth on the other side. We find that the translocation
rate coefficient can be expressed using an Arrhenius-type equation,
where the energy barrier and the pre-exponential factor can be theoretically
predicted using the transition state theory for classical barrier
crossing events. We propose a relation between the pre-exponential
factor and the entropy penalty of a gas molecule crossing the pore.
Furthermore, on the basis of the theory, we propose an efficient algorithm
to calculate CO<sub>2</sub> and CH<sub>4</sub> permeances per pore
for sub-nanometer graphene pores of any shape. For the CO<sub>2</sub>/CH<sub>4</sub> mixture, the graphene nanopores exhibit a trade-off
between the CO<sub>2</sub> permeance and the CO<sub>2</sub>/CH<sub>4</sub> separation factor. This upper bound on a Robeson plot of
selectivity <i>versus</i> permeance for a given pore density
is predicted and described by the theory. Pores with CO<sub>2</sub>/CH<sub>4</sub> separation factors higher than 10<sup>2</sup> have
CO<sub>2</sub> permeances per pore lower than 10<sup>ā22</sup> mol s<sup>ā1</sup> Pa<sup>ā1</sup>, and pores with
separation factors of ā¼10 have CO<sub>2</sub> permeances per
pore between 10<sup>ā22</sup> and 10<sup>ā21</sup> mol
s<sup>ā1</sup> Pa<sup>ā1</sup>. Finally, we show that
a pore density of 10<sup>14</sup> m<sup>ā2</sup> is required
for a porous graphene membrane to exceed the permeance-selectivity
upper bound of polymeric materials. Moreover, we show that a higher
pore density can potentially further boost the permeation performance
of a porous graphene membrane above all existing membranes. Our findings
provide insights into the potential and the limitations of porous
graphene membranes for gas separation and provide an efficient methodology
for screening nanopore configurations and sizes for the efficient
separation of desired gas mixtures
Fabrication, Pressure Testing, and Nanopore Formation of Single-Layer Graphene Membranes
Single-layer
graphene (SLG) membranes have great promise as ultrahigh
flux, high selectivity membranes for gas mixture separations due to
their single atom thickness. It remains a central question whether
SLG membranes of a requisite area can exist under an imposed pressure
drop and temperatures needed for industrial gas separation. An additional
challenge is the development of techniques to perforate or otherwise
control the porosity in graphene membranes to impart molecularly sized
pores, the size regime predicted to produce high gas separation factors.
Herein, we report fabrication, pressure testing, temperature cycling,
and gas permeance measurements through free-standing, low defect density
SLG membranes. Our measurements demonstrate the remarkable chemical
and mechanical stability of these 5 Ī¼m diameter suspended SLG
membranes, which remain intact over weeks of testing at pressure differentials
of >0.5 bar, repeated temperature cycling from 25 to 200 Ā°C,
and exposure to 15 mol % ozone for up to 3 min. These membranes act
as molecularly impermeable barriers, with very low or near negligible
background permeance. We also demonstrate a 1077 Ā°C temperature
O<sub>2</sub> etching technique to create nanopores on the order of
ā¼1 nm diameter as imaged by scanning tunneling microscopy,
although transport through such pores has not yet been successfully
measured. Overall, these results represent an important advancement
that will enable graphene gas separation membranes to be fabricated,
tested, and modified <i>in situ</i> while maintaining remarkable
mechanical and thermal stability