146 research outputs found
Electronic and Vibrational Properties of PbI 2 : From Bulk to Monolayer
Using first-principles calculations, we study the dependence of the
electronic and vibrational properties of multi-layered PbI 2 crystals on the
number of layers and focus on the electronic-band structure and the Raman
spectrum. Electronic-band structure calculations reveal that the direct or
indirect semiconducting behavior of PbI 2 is strongly influenced by the number
of layers. We find that at 3L-thickness there is a direct-to-indirect band gap
transition (from bulk-to-monolayer). It is shown that in the Raman spectrum two
prominent peaks, A 1g and E g , exhibit phonon hardening with increasing number
of layers due to the inter-layer van der Waals interaction. Moreover, the Raman
activity of the A 1g mode significantly increases with increasing number of
layers due to the enhanced out-of-plane dielectric constant in the few-layer
case. We further characterize rigid-layer vibrations of low-frequency
inter-layer shear (C) and breathing (LB) modes in few-layer PbI 2 . A reduced
mono-atomic (linear) chain model (LCM) provides a fairly accurate picture of
the number of layers dependence of the low-frequency modes and it is shown also
to be a powerful tool to study the inter-layer coupling strength in layered PbI
2 .Comment: To appear in Phys. Rev.
Wien effect in interfacial water dissociation through proton-permeable graphene electrodes
Strong electric fields can accelerate molecular dissociation reactions. The
phenomenon known as the Wien effect was previously observed using high-voltage
electrolysis cells that produced fields of about 10^7 V m-1, sufficient to
accelerate the dissociation of weakly bound molecules (e.g., organics and weak
electrolytes). The observation of the Wien effect for the common case of water
dissociation (H2O = H+ + OH-) has remained elusive. Here we study the
dissociation of interfacial water adjacent to proton-permeable graphene
electrodes and observe strong acceleration of the reaction in fields reaching
above 10^8 V m-1. The use of graphene electrodes allow measuring the proton
currents arising exclusively from the dissociation of interfacial water, while
the electric field driving the reaction is monitored through the carrier
density induced in graphene by the same field. The observed exponential
increase in proton currents is in quantitative agreement with Onsager's theory.
Our results also demonstrate that graphene electrodes can be valuable for the
investigation of various interfacial phenomena involving proton transport
Exponentially selective molecular sieving through angstrom pores
Two-dimensional crystals with angstrom-scale pores are widely considered as
candidates for a next generation of molecular separation technologies aiming to
provide extreme, exponentially large selectivity combined with high flow rates.
No such pores have been demonstrated experimentally. Here we study gas
transport through individual graphene pores created by low intensity exposure
to low kV electrons. Helium and hydrogen permeate easily through these pores
whereas larger species such as xenon and methane are practically blocked.
Permeating gases experience activation barriers that increase quadratically
with molecules' kinetic diameter, and the effective diameter of the created
pores is estimated as ~2 angstroms, about one missing carbon ring. Our work
reveals stringent conditions for achieving the long sought-after exponential
selectivity using porous two-dimensional membranes and suggests limits on their
possible performance
Exponentially selective molecular sieving through angstrom pores
From Springer Nature via Jisc Publications RouterHistory: received 2021-09-18, accepted 2021-11-12, registration 2021-11-17, collection 2021-12, pub-electronic 2021-12-09, online 2021-12-09Publication status: PublishedFunder: National Natural Science Foundation of China (National Science Foundation of China); doi: https://doi.org/10.13039/501100001809; Grant(s): 51920105002Funder: RCUK | Engineering and Physical Sciences Research Council (EPSRC); doi: https://doi.org/10.13039/501100000266; Grant(s): EP/S030719Funder: EC | EU Framework Programme for Research and Innovation H2020 | H2020 Priority Excellent Science | H2020 European Research Council (H2020 Excellent Science - European Research Council); doi: https://doi.org/10.13039/100010663; Grant(s): 786532 VANDERFunder: Lloyd’s Register Foundation (LRF); doi: https://doi.org/10.13039/100008885Abstract: Two-dimensional crystals with angstrom-scale pores are widely considered as candidates for a next generation of molecular separation technologies aiming to provide extreme, exponentially large selectivity combined with high flow rates. No such pores have been demonstrated experimentally. Here we study gas transport through individual graphene pores created by low intensity exposure to low kV electrons. Helium and hydrogen permeate easily through these pores whereas larger species such as xenon and methane are practically blocked. Permeating gases experience activation barriers that increase quadratically with molecules’ kinetic diameter, and the effective diameter of the created pores is estimated as ∼2 angstroms, about one missing carbon ring. Our work reveals stringent conditions for achieving the long sought-after exponential selectivity using porous two-dimensional membranes and suggests limits on their possible performance
Proton transport through nanoscale corrugations in two-dimensional crystals
Defect-free graphene is impermeable to all atoms1,2,3,4,5 and ions6,7 under ambient conditions. Experiments that can resolve gas flows of a few atoms per hour through micrometre-sized membranes found that monocrystalline graphene is completely impermeable to helium, the smallest atom2,5. Such membranes were also shown to be impermeable to all ions, including the smallest one, lithium6,7. By contrast, graphene was reported to be highly permeable to protons, nuclei of hydrogen atoms8,9. There is no consensus, however, either on the mechanism behind the unexpectedly high proton permeability10,11,12,13,14 or even on whether it requires defects in graphene’s crystal lattice6,8,15,16,17. Here, using high-resolution scanning electrochemical cell microscopy, we show that, although proton permeation through mechanically exfoliated monolayers of graphene and hexagonal boron nitride cannot be attributed to any structural defects, nanoscale non-flatness of two-dimensional membranes greatly facilitates proton transport. The spatial distribution of proton currents visualized by scanning electrochemical cell microscopy reveals marked inhomogeneities that are strongly correlated with nanoscale wrinkles and other features where strain is accumulated. Our results highlight nanoscale morphology as an important parameter enabling proton transport through two-dimensional crystals, mostly considered and modelled as flat, and indicate that strain and curvature can be used as additional degrees of freedom to control the proton permeability of two-dimensional materials
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