5 research outputs found
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 âgatingâ
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 8â10 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
Graphene Blisters with Switchable Shapes Controlled by Pressure and Adhesion
We created graphene blisters that
cover and seal an annular cylinder-shaped
microcavity in a SiO<sub>2</sub> substrate filled with a gas. By controlling
the pressure difference between the gas inside and outside of the
microcavity, we switch the graphene membrane between multiple stable
equilibrium configurations. We carried out experiments starting from
the situation where the pressure of the gas inside and outside of
the microcavity is set equal to a prescribed charging pressure, <i>p</i><sub>0</sub> and the graphene membrane covers the cavity
like an annular drum, adhered to the central post and the surrounding
substrate due to van der Waals forces. We decrease the outside pressure
to a value, <i>p</i><sub>e</sub> which causes it to bulge
into an annular blister. We systematically increase the charging pressure
by repeating this procedure causing the annular blister to continue
to bulge until a critical charging pressure <i>p</i><sub>c</sub><sup>i</sup> is reached. At
this point the graphene membrane delaminates from the post in an unstable
manner, resulting in a switch of graphene membrane shape from an annular
to a spherical blister. Continued increase of the charging pressure
results in the spherical blister growing with its height increasing,
but maintaining a constant radius until a second critical charging
pressure <i>p</i><sub>c</sub><sup>o</sup> is reached at which point the blister begins
to delaminate from the periphery of the cavity in a stable manner.
Here, we report a series of experiments as well as a mechanics and
thermodynamic model that demonstrate how the interplay among system
parameters (geometry, graphene stiffness (number of layers), pressure,
and adhesion energy) results in the ability to controllably switch
graphene blisters among different shapes. Arrays of these blisters
can be envisioned to create pressure-switchable surface properties
where the difference between patterns of annular versus spherical
blisters will impact functionalities such as wettability, friction,
adhesion, and surface wave characteristics
Adhesion, Stiffness, and Instability in Atomically Thin MoS<sub>2</sub> Bubbles
We
measured the work of separation of single and few-layer MoS<sub>2</sub> membranes from a SiO<sub><i>x</i></sub> substrate
using a mechanical blister test and found a value of 220 ± 35
mJ/m<sup>2</sup>. Our measurements were also used to determine the
2D Youngâs modulus (<i>E</i><sub>2D</sub>) of a single
MoS<sub>2</sub> layer to be 160 ± 40 N/m. We then studied the
delamination mechanics of pressurized MoS<sub>2</sub> bubbles, demonstrating
both stable and unstable transitions between the bubblesâ laminated
and delaminated states as the bubbles were inflated. When they were
deflated, we observed edge pinning and a snap-in transition that are
not accounted for by the previously reported models. We attribute
this result to adhesion hysteresis and use our results to estimate
the work of adhesion of our membranes to be 42 ± 20 mJ/m<sup>2</sup>
Ultrathin Oxide Films by Atomic Layer Deposition on Graphene
In this paper, a method is presented to create and characterize
mechanically robust, free-standing, ultrathin, oxide films with controlled,
nanometer-scale thickness using atomic layer deposition (ALD) on graphene.
Aluminum oxide films were deposited onto suspended graphene membranes
using ALD. Subsequent etching of the graphene left pure aluminum oxide
films only a few atoms in thickness. A pressurized blister test was
used to determine that these ultrathin films have a Youngâs
modulus of 154 ± 13 GPa. This Youngâs modulus is comparable
to much thicker alumina ALD films. This behavior indicates that these
ultrathin two-dimensional films have excellent mechanical integrity.
The films are also impermeable to standard gases suggesting they are
pinhole-free. These continuous ultrathin films are expected to enable
new applications in fields such as thin film coatings, membranes,
and flexible electronics
Observation of Pull-In Instability in Graphene Membranes under Interfacial Forces
We
present a unique experimental configuration that allows us to determine
the interfacial forces on nearly parallel plates made from the thinnest
possible mechanical structures, single and few layer graphene membranes.
Our approach consists of using a pressure difference across a graphene
membrane to bring the membrane to within âŒ10â20 nm above
a circular post covered with SiO<sub><i>x</i></sub> or Au
until a critical point is reached whereby the membrane snaps into
adhesive contact with the post. Continuous measurements of the deforming
membrane with an AFM coupled with a theoretical model allow us to
deduce the magnitude of the interfacial forces between graphene and
SiO<sub><i>x</i></sub> and graphene and Au. The nature of
the interfacial forces at âŒ10â20 nm separation is consistent
with an inverse fourth power distance dependence, implying that the
interfacial forces are dominated by van der Waals interactions. Furthermore,
the strength of the interactions is found to increase linearly with
the number of graphene layers. The experimental approach can be used
to measure the strength of the interfacial forces for other atomically
thin two-dimensional materials and help guide the development of nanomechanical
devices such as switches, resonators, and sensors