14 research outputs found
Molecular Valves for Controlling Gas Phase Transport Made from Discrete Angstrom-Sized Pores in Graphene
An ability to precisely regulate the quantity and location of molecular flux
is of value in applications such as nanoscale 3D printing, catalysis, and
sensor design. Barrier materials containing pores with molecular dimensions
have previously been used to manipulate molecular compositions in the gas
phase, but have so far been unable to offer controlled gas transport through
individual pores. Here, we show that gas flux through discrete angstrom-sized
pores in monolayer graphene can be detected and then controlled using
nanometer-sized gold clusters, which are formed on the surface of the graphene
and can migrate and partially block a pore. In samples without gold clusters,
we observe stochastic switching of the magnitude of the gas permeance, which we
attribute to molecular rearrangements of the pore. Our molecular valves could
be used, for example, to develop unique approaches to molecular synthesis that
are based on the controllable switching of a molecular gas flux, reminiscent of
ion channels in biological cell membranes and solid state nanopores.Comment: to appear in Nature Nanotechnolog
Fundamental scaling laws for the direct-write chemical vapor deposition of nanoscale features: modeling mass transport around a translating nanonozzle
Ā© 2019 The Royal Society of Chemistry. The nanometer placement of nanomaterials, such as nanoribbons and nanotubes, at a specific pitch and orientation on a surface, remains an unsolved fundamental problem in nanotechnology. In this work, we introduce and analyze the concept of a direct-write chemical vapor deposition (CVD) system that enables the in-place synthesis of such structures with control over orientation and characteristic features. A nanometer scale pore or conduit, called the nanonozzle, delivers precursor gases for CVD locally on a substrate, with spatial translation of either the nozzle or the substrate to enable a novel direct write (DW) tool. We analyze the nozzle under conditions where it delivers reactants to a substrate while translating at a constant velocity over the surface at a fixed reaction temperature. We formulate and solve a multi-phase three-dimensional reaction and diffusion model of the direct-write operation, and evaluate specific analytically-solvable limits to determine the allowable operating conditions, including pore dimensions, reactant flow rates, and nozzle translation speed. A Buckingham Ī analysis identifies six dimensionless quantities crucial for the design and operation of the direct-write synthesis process. Importantly, we derive and validate what we call the ribbon extension inequality that brackets the allowable nozzle velocity relative to the CVD growth rate-a key constraint to enabling direct-write operation. Lastly, we include a practical analysis using attainable values towards the experimental design of such a system, building the nozzle around a commercially available near-field scanning optical microscopy (NSOM) tip as a feasible example
Analysis of Time-Varying, Stochastic Gas Transport through Graphene Membranes
Molecular transport
measurements through isolated nanopores can
greatly inform our understanding of how such systems can select for
molecular size and shape. In this work, we present a detailed analysis
of experimental gas permeation data through single layer graphene
membranes under batch depletion conditions parametric in starting
pressure for He, H<sub>2</sub>, Ne, and CO<sub>2</sub> between 100
and 670 kPa. We show mathematically that the observed intersections
of the membrane deflection curves parametric in starting pressure
are indicative of a time dependent membrane permeance (pressure normalized
molecular flow). Analyzing these time dependent permeance data for
He, Ne, H<sub>2</sub>, and CO<sub>2</sub> shows remarkably that the
latter three gases exhibit discretized permeance values that are temporally
repeated. Such quantized fluctuations (called āgatingā
for liquid phase nanopore and ion channel systems) are a hallmark
of isolated nanopores, since small, but rapid changes in the transport
pathway necessarily influence a single detectable flux. We analyze
the fluctuations using a Hidden Markov model to fit to discrete states
and estimate the activation barrier for switching at 1.0 eV. This
barrier is and the relative fluxes are consistent with a chemical
bond rearrangement of an 8ā10 atom vacancy pore. Furthermore,
we use the relations between the states given by the Markov network
for few pores to determine that three pores, each exhibiting two state
switching, are responsible for the observed fluctuations; and we compare
simulated control data sets with and without the Markov network for
comparison and to establish confidence in our evaluation of the limited
experimental data set
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Low Dimensional Carbon Materials for Applications in Mass and Energy Transport
Understanding and Analyzing Freezing-Point Transitions of Confined Fluids within Nanopores
Understanding phase transitions of
fluids confined within nanopores
is important for a wide variety of technological applications. It
is well known that fluids confined in nanopores typically demonstrate
freezing-point depressions, Ī<i><i>T</i></i><sub>f</sub>, described by the GibbsāThomson (GT) equation.
Herein, we highlight and correct several thermodynamic inconsistencies
in the conventional use of the GT equation, including the fact that
the enthalpy of melting, Ī<i><i>H</i></i><sub>m</sub>, and the solidāliquid surface energy, Ī³<sub>SL</sub>, are functions of pore diameter, complicating their prediction.
We propose a theoretical analysis that employs the Turnbull coefficient,
originally derived from metal nucleation theory, and show its consistency
as a more reliable quantity for the prediction of Ī<i><i>T</i></i><sub>f</sub>. This analysis provides a straightforward
method to estimate Ī<i><i>T</i></i><sub>f</sub> of nanoconfined organic fluids. As an example, we apply this
technique to ibuprofen, an active pharmaceutical ingredient (API),
and show that this theory fits well to the experimental Ī<i><i>T</i></i><sub>f</sub> of nanoconfined ibuprofen
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