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1.
J Phys Chem Lett ; 13(1): 66-74, 2022 Jan 13.
Artículo en Inglés | MEDLINE | ID: mdl-34958221

RESUMEN

A multiscale modeling and simulation approach, including first-principles calculations, ab initio molecular dynamics simulations, and a tight binding approach, is employed to study band flattening of the electronic band structure of oxidized monolayer graphene. The width of flat bands can be tuned by strain, the external electric field, and the density of functional groups and their distribution. A transition to a conducting state is found for monolayer graphene with impurities when it is subjected to an electric field of ∼1.0 V/Å. Several parallel impurity-induced flat bands appear in the low-energy spectrum of monolayer graphene when the number of epoxy groups is changed. The width of the flat band decreases with an increase in tensile strain but is independent of the electric field strength. Here an alternative and easy route for obtaining band flattening in thermodynamically stable functionalized monolayer graphene is introduced. Our work discloses a new avenue for research on band flattening in monolayer graphene.

2.
Phys Rev E ; 102(2-1): 022803, 2020 Aug.
Artículo en Inglés | MEDLINE | ID: mdl-32942431

RESUMEN

The dielectric properties of confined water is of fundamental interest and is still controversial. For water confined in channels with height smaller than h=8Å, we found a commensurability effect and an extraordinary decrease in the out-of-plane dielectric constant down to the limit of the dielectric constant of optical water. Spatial resolved polarization density data obtained from molecular dynamics simulations are found to be antisymmetric across the channel and are used as input in a mean-field model for the dielectric constant as a function of the height of the channel for h>15Å. Our results are in excellent agreement with a recent experiment [L. Fumagalli et al., Science 360, 1339 (2018)SCIEAS0036-807510.1126/science.aat4191].

3.
Phys Chem Chem Phys ; 21(18): 9285-9295, 2019 May 08.
Artículo en Inglés | MEDLINE | ID: mdl-30931451

RESUMEN

When confined between graphene layers, water behaves differently from the bulk and exhibits unusual properties such as fast water flow and ordering into a crystal. The hydrogen-bonded network is affected by the limited space and by the characteristics of the confining walls. The presence of an extraordinary number of hydronium and hydroxide ions in narrow channels has the following effects: (i) they affect water permeation through the channel, (ii) they may interact with functional groups on the graphene oxide surface and on the edges, and (iii) they change the thermochemistry of water, which are fundamentally important to understand, especially when confined water is subjected to an external electric field. Here we study the physical properties of water when confined between two graphene sheets and containing hydronium and hydroxide. We found that: (i) there is a disruption in the solvation structure of the ions, which is also affected by the layered structure of confined water, (ii) hydronium and hydroxide occupy specific regions inside the nanochannel, with a prevalence of hydronium (hydroxide) ions at the edges (interior), and (iii) ions recombine more slowly in confined systems than in bulk water, with the recombination process depending on the channel height and commensurability between the size of the molecules and the nanochannel height - a decay of 20% (40%) in the number of ions in 8 ps is observed for a channel height of h = 7 Å (bulk water). Our work reveals distinctive properties of water confined in a nanocapillary in the presence of additional hydronium and hydroxide ions.

4.
Nature ; 559(7713): 236-240, 2018 07.
Artículo en Inglés | MEDLINE | ID: mdl-29995867

RESUMEN

Controlled transport of water molecules through membranes and capillaries is important in areas as diverse as water purification and healthcare technologies1-7. Previous attempts to control water permeation through membranes (mainly polymeric ones) have concentrated on modulating the structure of the membrane and the physicochemical properties of its surface by varying the pH, temperature or ionic strength3,8. Electrical control over water transport is an attractive alternative; however, theory and simulations9-14 have often yielded conflicting results, from freezing of water molecules to melting of ice14-16 under an applied electric field. Here we report electrically controlled water permeation through micrometre-thick graphene oxide membranes17-21. Such membranes have previously been shown to exhibit ultrafast permeation of water17,22 and molecular sieving properties18,21, with the potential for industrial-scale production. To achieve electrical control over water permeation, we create conductive filaments in the graphene oxide membranes via controllable electrical breakdown. The electric field that concentrates around these current-carrying filaments ionizes water molecules inside graphene capillaries within the graphene oxide membranes, which impedes water transport. We thus demonstrate precise control of water permeation, from ultrafast permeation to complete blocking. Our work opens up an avenue for developing smart membrane technologies for artificial biological systems, tissue engineering and filtration.

5.
Nat Nanotechnol ; 13(6): 468-472, 2018 06.
Artículo en Inglés | MEDLINE | ID: mdl-29556005

RESUMEN

Atoms start behaving as waves rather than classical particles if confined in spaces commensurate with their de Broglie wavelength. At room temperature this length is only about one ångström even for the lightest atom, hydrogen. This restricts quantum-confinement phenomena for atomic species to the realm of very low temperatures1-5. Here, we show that van der Waals gaps between atomic planes of layered crystals provide ångström-size channels that make quantum confinement of protons apparent even at room temperature. Our transport measurements show that thermal protons experience a notably higher barrier than deuterons when entering van der Waals gaps in hexagonal boron nitride and molybdenum disulfide. This is attributed to the difference in the de Broglie wavelengths of the isotopes. Once inside the crystals, transport of both isotopes can be described by classical diffusion, albeit with unexpectedly fast rates comparable to that of protons in water. The demonstrated ångström-size channels can be exploited for further studies of atomistic quantum confinement and, if the technology can be scaled up, for sieving hydrogen isotopes.

6.
Nat Commun ; 8: 15844, 2017 06 16.
Artículo en Inglés | MEDLINE | ID: mdl-28621311

RESUMEN

Van der Waals (vdW) interaction between two-dimensional crystals (2D) can trap substances in high pressurized (of order 1 GPa) on nanobubbles. Increasing the adhesion between the 2D crystals further enhances the pressure and can lead to a phase transition of the trapped material. We found that the shape of the nanobubble can depend critically on the properties of the trapped substance. In the absence of any residual strain in the top 2D crystal, flat nanobubbles can be formed by trapped long hydrocarbons (that is, hexadecane). For large nanobubbles with radius 130 nm, our atomic force microscopy measurements show nanobubbles filled with hydrocarbons (water) have a cylindrical symmetry (asymmetric) shape which is in good agreement with our molecular dynamics simulations. This study provides insights into the effects of the specific material and the vdW pressure on the microscopic details of graphene bubbles.

7.
J Mol Graph Model ; 74: 100-104, 2017 06.
Artículo en Inglés | MEDLINE | ID: mdl-28384497

RESUMEN

Graphene doped with nitrogen exhibits unique properties different than perfect graphene. The temperature distribution in nitrogen-doped graphene (N-graphene) and in the graphene with grain boundary is investigated using molecular dynamics simulations. The temperature distribution in nitrogen-doped graphene nanoribbon, containing two types of grain boundaries, was found to be sensitive to the number of dopants and grain boundary. We also found that there is a remarkable temperature gap in the temperature profile of N-graphene nanoribbon-containing a grain boundary. For any doping ratio N/C we found that the nitrogen atoms enhance roughness of N-graphene and decrease thermal conductivity.


Asunto(s)
Grafito/química , Nitrógeno/química , Simulación de Dinámica Molecular , Nanoestructuras/química , Temperatura , Conductividad Térmica
8.
Phys Rev Lett ; 117(12): 126801, 2016 Sep 16.
Artículo en Inglés | MEDLINE | ID: mdl-27689288

RESUMEN

We report subnanometer, high-bandwidth measurements of the out-of-plane (vertical) motion of atoms in freestanding graphene using scanning tunneling microscopy. By tracking the vertical position over a long time period, a 1000-fold increase in the ability to measure space-time dynamics of atomically thin membranes is achieved over the current state-of-the-art imaging technologies. We observe that the vertical motion of a graphene membrane exhibits rare long-scale excursions characterized by both anomalous mean-squared displacements and Cauchy-Lorentz power law jump distributions.

9.
Nat Commun ; 7: 12168, 2016 07 07.
Artículo en Inglés | MEDLINE | ID: mdl-27385262

RESUMEN

Van der Waals assembly of two-dimensional crystals continue attract intense interest due to the prospect of designing novel materials with on-demand properties. One of the unique features of this technology is the possibility of trapping molecules between two-dimensional crystals. The trapped molecules are predicted to experience pressures as high as 1 GPa. Here we report measurements of this interfacial pressure by capturing pressure-sensitive molecules and studying their structural and conformational changes. Pressures of 1.2±0.3 GPa are found using Raman spectrometry for molecular layers of 1-nm in thickness. We further show that this pressure can induce chemical reactions, and several trapped salts are found to react with water at room temperature, leading to two-dimensional crystals of the corresponding oxides. This pressure and its effect should be taken into account in studies of van der Waals heterostructures and can also be exploited to modify materials confined at the atomic interfaces.

10.
J Mol Graph Model ; 62: 38-42, 2015 Nov.
Artículo en Inglés | MEDLINE | ID: mdl-26356047

RESUMEN

Temperature profile in graphene, graphene with grain boundary and vacancy defects and hydrogenated graphene with different percentage of H-atoms are determined using molecular dynamics simulation. We also obtained the temperature profile in a graphene nanoribbon containing two types of grain boundaries with different misorientation angles, θ=21.8° and θ=32.2°. We found that a temperature gap appears in the temperature profile of a graphene nanoribbon with a grain boundary at the middle. Moreover, we found that the temperature profile in the partially hydrogenated graphene varies with the percentage of hydrogens, i.e. the C:H ratio. Our results show that a grain boundary line in the graphene sheet can change the thermal transport through the system which might be useful for controlling thermal flow in nanostructured graphene.


Asunto(s)
Grafito/química , Hidrógeno/química , Hidrogenación , Simulación de Dinámica Molecular , Nanoestructuras/química , Oxidación-Reducción , Temperatura
11.
Nat Commun ; 5: 4962, 2014 Sep 17.
Artículo en Inglés | MEDLINE | ID: mdl-25230052

RESUMEN

Knowledge of and control over the curvature of ripples in freestanding graphene are desirable for fabricating and designing flexible electronic devices, and recent progress in these pursuits has been achieved using several advanced techniques such as scanning tunnelling microscopy. The electrostatic forces induced through a bias voltage (or gate voltage) were used to manipulate the interaction of freestanding graphene with a tip (substrate). Such forces can cause large movements and sudden changes in curvature through mirror buckling. Here we explore an alternative mechanism, thermal load, to control the curvature of graphene. We demonstrate thermal mirror buckling of graphene by scanning tunnelling microscopy and large-scale molecular dynamic simulations. The negative thermal expansion coefficient of graphene is an essential ingredient in explaining the observed effects. This new control mechanism represents a fundamental advance in understanding the influence of temperature gradients on the dynamics of freestanding graphene and future applications with electro-thermal-mechanical nanodevices.

12.
Nat Commun ; 5: 3720, 2014 Apr 28.
Artículo en Inglés | MEDLINE | ID: mdl-24770734

RESUMEN

Intrinsic ripples in freestanding graphene have been exceedingly difficult to study. Individual ripple geometry was recently imaged using scanning tunnelling microscopy, but these measurements are limited to static configurations. Thermally-activated flexural phonon modes should generate dynamic changes in curvature. Here we show how to track the vertical movement of a one-square-angstrom region of freestanding graphene using scanning tunnelling microscopy, thereby allowing measurement of the out-of-plane time trajectory and fluctuations over long time periods. We also present a model from elasticity theory to explain the very-low-frequency oscillations. Unexpectedly, we sometimes detect a sudden colossal jump, which we interpret as due to mirror buckling. This innovative technique provides a much needed atomic-scale probe for the time-dependent behaviours of intrinsic ripples. The discovery of this novel progenitor represents a fundamental advance in the use of scanning tunnelling microscopy, which together with the application of a thermal load provides a low-frequency nano-resonator.


Asunto(s)
Grafito/química , Microscopía de Túnel de Rastreo/métodos , Modelos Químicos , Nanotecnología/métodos , Elasticidad , Fonones , Temperatura
13.
J Chem Phys ; 140(7): 074304, 2014 Feb 21.
Artículo en Inglés | MEDLINE | ID: mdl-24559350

RESUMEN

The electronic properties of graphene nano-flakes (GNFs) with different edge passivation are investigated by using density functional theory. Passivation with F and H atoms is considered: C(N(c)) X(N(x)) (X = F or H). We studied GNFs with 10 < Nc < 56 and limit ourselves to the lowest energy configurations. We found that: (i) the energy difference Δ between the highest occupied molecular orbital and the lowest unoccupied molecular orbital decreases with Nc, (ii) topological defects (pentagon and heptagon) break the symmetry of the GNFs and enhance the electric polarization, (iii) the mutual interaction of bilayer GNFs can be understood by dipole-dipole interaction which were found sensitive to the relative orientation of the GNFs, (iv) the permanent dipoles depend on the edge terminated atom, while the energy gap is independent of it, and (v) the presence of heptagon and pentagon defects in the GNFs results in the largest difference between the energy of the spin-up and spin-down electrons which is larger for the H-passivated GNFs as compared to F-passivated GNFs. Our study shows clearly the effect of geometry, size, termination, and bilayer on the electronic properties of small GNFs. This study reveals important features of graphene nano-flakes which can be detected using Raman spectroscopy.

14.
J Phys Condens Matter ; 24(17): 175303, 2012 May 02.
Artículo en Inglés | MEDLINE | ID: mdl-22475745

RESUMEN

Using the valence force field model of Perebeinos and Tersoff (2009 Phys. Rev. B 79 241409(R)), different energy modes of suspended graphene subjected to tensile or compressive strain are studied. By carrying out Monte Carlo simulations it is found that: (i) only for small strains (|ε|

Asunto(s)
Grafito/química , Física/métodos , Simulación por Computador , Módulo de Elasticidad , Elasticidad , Modelos Estadísticos , Método de Montecarlo , Reproducibilidad de los Resultados , Temperatura , Termodinámica
15.
Phys Rev E Stat Nonlin Soft Matter Phys ; 83(4 Pt 1): 042601, 2011 Apr.
Artículo en Inglés | MEDLINE | ID: mdl-21599222

RESUMEN

Nonequilibrium molecular dynamics simulations are used to study the motion of a C(60) molecule on a graphene sheet subjected to a temperature gradient. The C(60) molecule is actuated and moves along the system while it just randomly dances along the perpendicular direction. Increasing the temperature gradient increases the directed velocity of C(60). It is found that the free energy decreases as the C(60) molecule moves toward the cold end. The driving mechanism based on the temperature gradient suggests the construction of nanoscale graphene-based motors.

16.
J Phys Condens Matter ; 23(4): 045002, 2011 Feb 02.
Artículo en Inglés | MEDLINE | ID: mdl-21406876

RESUMEN

We investigate the stability of circular monolayer graphene subjected to a radial load using non-equilibrium molecular dynamics simulations. When monolayer graphene is radially stressed, after some small circular strain (∼0.4%) it buckles and bends into a new bowl-like shape. Young's modulus is calculated from the linear relation between stress and strain before the buckling threshold, which is in agreement with experimental results. The prediction of elasticity theory for the buckling threshold of a radially stressed plate is presented and its results are compared to the one of our atomistic simulation. The Jarzynski equality is used to estimate the difference between the free energy of the non-compressed states and the buckled states. From a calculation of the free energy we obtain the optimum radius for which the system feels the minimum boundary stress.

17.
Phys Rev E Stat Nonlin Soft Matter Phys ; 82(5 Pt 1): 051605, 2010 Nov.
Artículo en Inglés | MEDLINE | ID: mdl-21230486

RESUMEN

The motion of a C60 molecule over a graphene sheet at finite temperature is investigated both theoretically and computationally. We show that a graphene sheet generates a van der Waals laterally periodic potential, which directly influences the motion of external objects in its proximity. The translational motion of a C60 molecule near a graphene sheet is found to be diffusive in the lateral directions, while in the perpendicular direction, the motion may be described as diffusion in an effective harmonic potential which is determined from the distribution function of the position of the C60 molecule. We also examine the rotational diffusion of C60 and show that its motion over the graphene sheet is not a rolling motion.

18.
Nanotechnology ; 20(13): 135602, 2009 Apr 01.
Artículo en Inglés | MEDLINE | ID: mdl-19420505

RESUMEN

The formation of atomic nanoclusters on suspended graphene sheets has been investigated by employing a molecular dynamics simulation at finite temperature. Our systematic study is based on temperature-dependent molecular dynamics simulations of some transition and alkali atoms on suspended graphene sheets. We find that the transition atoms aggregate and make various size nanoclusters distributed randomly on graphene surfaces. We also report that most alkali atoms make one atomic layer on graphene sheets. Interestingly, the potassium atoms almost deposit regularly on the surface at low temperature. We expect from this behavior that the electrical conductivity of a suspended graphene doped by potassium atoms would be much higher than in the case doped by the other atoms at low temperature.

19.
Phys Rev E Stat Nonlin Soft Matter Phys ; 78(1 Pt 1): 011912, 2008 Jul.
Artículo en Inglés | MEDLINE | ID: mdl-18763987

RESUMEN

An effective potential for longitudinal interactions between adjacent protofilaments in a microtubule is introduced. Our proposed interaction potential is a periodic and continuous function of the offset between two protofilaments, which also incorporates the bending energy of protofilaments. This potential produces the results of atomistic simulations. Further, using the potential, a Monte Carlo simulation gives results for the skew angles of observed structures that are in good agreement with experiments.


Asunto(s)
Biofisica/métodos , Microtúbulos/química , Animales , Simulación por Computador , Microscopía por Crioelectrón , Dimerización , Análisis de Fourier , Humanos , Modelos Biológicos , Modelos Moleculares , Modelos Estadísticos , Método de Montecarlo , Orgánulos/química , Conformación Proteica , Procesos Estocásticos
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