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“Graphene on hexagonal lattice substrate : stress and pseudo-magnetic field”. Neek-Amal M, Peeters FM, Applied physics letters 104, 173106 (2014). http://doi.org/10.1063/1.4873342
Abstract: Moire patterns in the pseudo-magnetic field and in the strain profile of graphene (GE) when put on top of a hexagonal lattice substrate are predicted from elasticity theory. The van der Waals interaction between GE and the substrate induces out-of-plane deformations in graphene which results in a strain field, and consequently in a pseudo-magnetic field. When the misorientation angle is about 0.5 degrees, a three-fold symmetric strain field is realized that results in a pseudo-magnetic field very similar to the one proposed by F. Guinea, M. I. Katsnelson, and A. K. Geim [Nature Phys. 6, 30 (2010)]. Our results show that the periodicity and length of the pseudo-magnetic field can be tuned in GE by changing the misorientation angle and substrate adhesion parameters and a considerable energy gap (23 meV) can be obtained due to out-of-plane deformation of graphene which is in the range of recent experimental measurements (20-30 meV). (C) 2014 AIP Publishing LLC.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.411
Times cited: 14
DOI: 10.1063/1.4873342
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“Graphene on boron-nitride : Moiré, pattern in the van der Waals energy”. Neek-Amal M, Peeters FM, Applied physics letters 104, 041909 (2014). http://doi.org/10.1063/1.4863661
Abstract: The spatial dependence of the van der Waals (vdW) energy between graphene and hexagonal boron-nitride (h-BN) is investigated using atomistic simulations. The van der Waals energy between graphene and h-BN shows a hexagonal superlattice structure identical to the observed Moire pattern in the local density of states, which depends on the lattice mismatch and misorientation angle between graphene and h-BN. Our results provide atomistic features of the weak van der Waals interaction between graphene and BN which are in agreement with experiment and provide an analytical expression for the size of the spatial variation of the weak van der Waals interaction. We also found that the A-B-lattice symmetry of graphene is broken along the armchair direction. (C) 2014 AIP Publishing LLC.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.411
Times cited: 61
DOI: 10.1063/1.4863661
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“Electric-field-induced structural changes in water confined between two graphene layers”. Fernandez MS, Peeters FM, Neek-Amal M, Physical review B 94, 045436 (2016). http://doi.org/10.1103/PHYSREVB.94.045436
Abstract: An external electric field changes the physical properties of polar liquids due to the reorientation of their permanent dipoles. Using molecular dynamics simulations, we predict that an in-plane electric field applied parallel to the channel polarizes water molecules which are confined between two graphene layers, resulting in distinct ferroelectricity and electrical hysteresis. We found that electric fields alter the in-plane order of the hydrogen bonds: Reversing the electric field does not restore the system to the nonpolar initial state, instead a residual dipole moment remains in the system. The square-rhombic structure of 2D ice is transformed into two rhombic-rhombic structures. Our study provides insights into the ferroelectric state of water when confined in nanochannels and shows how this can be tuned by an electric field.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 31
DOI: 10.1103/PHYSREVB.94.045436
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“Strain-engineered graphene through a nanostructured substrate : 2 : pseudomagnetic fields”. Neek-Amal M, Peeters FM, Physical review : B : condensed matter and materials physics 85, 195446 (2012). http://doi.org/10.1103/PhysRevB.85.195446
Abstract: The strain-induced pseudomagnetic field in supported graphene deposited on top of a nanostructured substrate is investigated by using atomistic simulations. A step, an elongated trench, a one-dimensional barrier, a spherical bubble, a Gaussian bump, and a Gaussian depression are considered as support structures for graphene. From the obtained optimum configurations we found very strong induced pseudomagnetic fields which can reach up to similar to 1000 T due to the strain-induced deformations in the supported graphene. Different magnetic confinements with controllable geometries are found by tuning the pattern of the substrate. The resulting induced magnetic fields for graphene on top of a step, barrier, and trench are calculated. In contrast to the step and trench the middle part of graphene on top of a barrier has zero pseudomagnetic field. This study provides a theoretical background for designing magnetic structures in graphene by nanostructuring substrates. We found that altering the radial symmetry of the deformation changes the sixfold symmetry of the induced pseudomagnetic field.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 31
DOI: 10.1103/PhysRevB.85.195446
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“Lattice thermal properties of graphane : thermal contraction, roughness, and heat capacity”. Neek-Amal M, Peeters FM, Physical review : B : condensed matter and materials physics 83, 235437 (2011). http://doi.org/10.1103/PhysRevB.83.235437
Abstract: Using atomistic simulations, we determine the roughness and the thermal properties of a suspended graphane sheet. As compared to graphene, we found that (i) hydrogenated graphene has a larger thermal contraction, (ii) the roughness exponent at room temperature is smaller, i.e., ≃ 1.0 versus ≃ 1.2 for graphene, (iii) the wavelengths of the induced ripples in graphane cover a wide range corresponding to length scales in the range 30125 Å at room temperature, and (iv) the heat capacity of graphane is estimated to be 29.32±0.23 J/mol K, which is 14.8% larger than that for graphene, i.e., 24.98±0.14 J/mol K. Above 1500 K, we found that graphane buckles when its edges are supported in the x-y plane.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 42
DOI: 10.1103/PhysRevB.83.235437
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“Buckled circular monolayer graphene : a graphene nano-bowl”. Neek-Amal M, Peeters FM, Journal of physics : condensed matter 23, 045002 (2011). http://doi.org/10.1088/0953-8984/23/4/045002
Abstract: 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.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 2.649
Times cited: 27
DOI: 10.1088/0953-8984/23/4/045002
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“Effect of grain boundary on the buckling of graphene nanoribbons”. Neek-Amal M, Peeters FM, Applied physics letters 100, 101905 (2012). http://doi.org/10.1063/1.3692573
Abstract: The buckling of graphene nano-ribbons containing a grain boundary is studied using atomistic simulations where free and supported boundary conditions are invoked. We consider the buckling transition of two kinds of grain boundaries with special symmetry. When graphene contains a large angle grain boundary with theta = 21.8 degrees, the buckling strains are larger than those of perfect graphene when the ribbons with free (supported) boundary condition are subjected to compressive tension parallel (perpendicular) to the grain boundary. This is opposite for the results of theta = 32.2 degrees. The shape of the deformations of the buckled graphene nanoribbons depends on the boundary conditions, the presence of the particular used grain boundaries, and the direction of applied in-plane compressive tension. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3692573]
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.411
Times cited: 18
DOI: 10.1063/1.3692573
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“Graphene nanoribbons subjected to axial stress”. Neek-Amal M, Peeters FM, Physical review : B : condensed matter and materials physics 82, 085432 (2010). http://doi.org/10.1103/PhysRevB.82.085432
Abstract: Atomistic simulations are used to study the bending of rectangular graphene nanoribbons subjected to axial stress both for free boundary and supported boundary conditions. The shapes of the deformations of the buckled graphene nanoribbons, for small values of the stress, are sine waves where the number of nodal lines depend on the longitudinal size of the system and the applied boundary condition. The buckling strain for the supported boundary condition is found to be independent of the longitudinal size and estimated to be 0.86%. From a calculation of the free energy at finite temperature we find that the equilibrium projected two-dimensional area of the graphene nanoribbon is less than the area of a flat sheet. At the optimum length the boundary strain for the supported boundary condition is 0.48%.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 92
DOI: 10.1103/PhysRevB.82.085432
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“Diffusion of fluorine on and between graphene layers”. Sadeghi A, Neek-Amal M, Berdiyorov GR, Peeters FM, Physical review : B : condensed matter and materials physics 91, 014304 (2015). http://doi.org/10.1103/PhysRevB.91.014304
Abstract: Using first-principles calculations and reactive force field molecular dynamics simulations, we study the structural properties and dynamics of a fluorine (F) atom, either adsorbed on the surface of single layer graphene (F/GE) or between the layers of AB stacked bilayer graphene (F@ bilayer graphene). It is found that the diffusion of the F atom is very different in those cases, and that the mobility of the F atom increases by about an order of magnitude when inserted between two graphene layers. The obtained diffusion constant for F/GE is twice larger than that experimentally found for gold adatom and theoretically found for C-60 molecule on graphene. Our study provides important physical insights into the dynamics of fluorine atoms between and on graphene layers and explains the mechanism behind the separation of graphite layers due to intercalation of F atoms.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 15
DOI: 10.1103/PhysRevB.91.014304
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“Nanoengineered nonuniform strain in graphene using nanopillars”. Neek-Amal M, Covaci L, Peeters FM, Physical review : B : condensed matter and materials physics 86, 041405 (2012). http://doi.org/10.1103/PhysRevB.86.041405
Abstract: Recent experiments showed that nonuniform strain can be produced by depositing graphene over pillars. We employed atomistic calculations to study the nonuniform strain and the induced pseudomagnetic field in graphene on top of nanopillars. By decreasing the distance between the nanopillars a complex distribution for the pseudomagnetic field can be generated. Furthermore, we performed tight-binding calculations of the local density of states (LDOS) by using the relaxed graphene configuration obtained from atomistic calculations. We find that the quasiparticle LDOS are strongly modified near the pillars, both at low energies showing sublattice polarization and at high energies showing shifts of the van Hove singularity. Our study shows that changing the specific pattern of the nanopillars allows us to create a desired shape of the pseudomagnetic field profile while the LDOS maps provide an input for experimental verification by scanning tunneling microscopy.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 51
DOI: 10.1103/PhysRevB.86.041405
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“Electronic structure of a hexagonal graphene flake subjected to triaxial stress”. Neek-Amal M, Covaci L, Shakouri K, Peeters FM, Physical review : B : condensed matter and materials physics 88, 115428 (2013). http://doi.org/10.1103/PhysRevB.88.115428
Abstract: The electronic properties of a triaxially strained hexagonal graphene flake with either armchair or zigzag edges are investigated using molecular dynamics simulations and tight-binding calculations. We found that (i) the pseudomagnetic field in strained graphene flakes is not uniform neither in the center nor at the edge of zigzag terminated flakes, (ii) the pseudomagnetic field is almost zero in the center of armchair terminated flakes but increases dramatically near the edges, (iii) the pseudomagnetic field increases linearly with strain, for strains lower than 15% but increases nonlinearly beyond it, (iv) the local density of states in the center of the zigzag hexagon exhibits pseudo-Landau levels with broken sublattice symmetry in the zeroth pseudo-Landau level, and in addition there is a shift in the Dirac cone due to strain induced scalar potentials, and (v) there is size effect in pseudomagnetic field. This study provides a realistic model of the electronic properties of inhomogeneously strained graphene where the relaxation of the atomic positions is correctly included together with strain induced modifications of the hopping terms up to next-nearest neighbors.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 46
DOI: 10.1103/PhysRevB.88.115428
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“Electronic properties of graphene nano-flakes : energy gap, permanent dipole, termination effect, and Raman spectroscopy”. Singh SK, Neek-Amal M, Peeters FM, The journal of chemical physics 140, 074304 (2014). http://doi.org/10.1063/1.4865414
Abstract: 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-Nc X-Nx (X = F or H). We studied GNFs with 10 < N-c < 56 and limit ourselves to the lowest energy configurations. We found that: (i) the energy difference Delta between the highest occupied molecular orbital and the lowest unoccupied molecular orbital decreases with N-c, (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. (C) 2014 AIP Publishing LLC.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 2.965
Times cited: 30
DOI: 10.1063/1.4865414
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“Melting of graphene clusters”. Singh SK, Neek-Amal M, Peeters FM, Physical review : B : condensed matter and materials physics 87, 134103 (2013). http://doi.org/10.1103/PhysRevB.87.134103
Abstract: Density-functional tight-binding and classical molecular dynamics simulations are used to investigate the structural deformations and melting of planar carbon nanoclusters C-N with N = 2-55. The minimum-energy configurations for different clusters are used as starting configurations for the study of the temperature effects on the bond breaking and rotation in carbon lines (N < 6), carbon rings (5 < N < 19), and graphene nanoflakes. The larger the rings (graphene nanoflakes) the higher the transition temperature (melting point) with ring-to-line (perfect-to-defective) transition structures. The melting point was obtained by using the bond energy, the Lindemann criteria, and the specific heat. We found that hydrogen-passivated graphene nanoflakes (CNHM) have a larger melting temperature with a much smaller dependence on size. The edges in the graphene nanoflakes exhibit several different metastable configurations (isomers) during heating before melting occurs. DOI: 10.1103/PhysRevB.87.134103
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 28
DOI: 10.1103/PhysRevB.87.134103
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“Melting of partially fluorinated graphene : from detachment of fluorine atoms to large defects and random coils”. Singh SK, Costamagna S, Neek-Amal M, Peeters FM, The journal of physical chemistry: C : nanomaterials and interfaces 118, 4460 (2014). http://doi.org/10.1021/JP4109333
Abstract: The melting of fluorographene is very unusual and depends strongly on the degree of fluorination. For temperatures below 1000 K, fully fluorinated graphene (FFG) is thermomechanically more stable than graphene but at T-m approximate to 2800 K FFG transits to random coils which is almost 2 times lower than the melting temperature of graphene, i.e., 5300 K. For fluorinated graphene up to 30% ripples causes detachment of individual F-atoms around 2000 K, while for 40%-60% fluorination large defects are formed beyond 1500 K and beyond 60% of fluorination F-atoms remain bonded to graphene until melting. The results agree with recent experiments on the dependence of the reversibility of the fluorination process on the percentage of fluorination.
Keywords: A1 Journal article; Engineering sciences. Technology; Condensed Matter Theory (CMT)
Impact Factor: 4.536
Times cited: 16
DOI: 10.1021/JP4109333
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“Boron nitride mono layer : a strain-tunable nanosensor”. Neek-Amal M, Beheshtian J, Sadeghi A, Michel KH, Peeters FM, The journal of physical chemistry: C : nanomaterials and interfaces 117, 13261 (2013). http://doi.org/10.1021/jp402122c
Abstract: The influence of triaxial in-plane strain on the electronic properties of a hexagonal boron-nitride sheet is investigated using density functional theory. Different from graphene, the triaxial strain localizes the molecular orbitals of the boron-nitride flake in its center depending on the direction of the applied strain. The proposed technique for localizing the molecular orbitals that are close to the Fermi level in the center of boron nitride flakes can be used to actualize engineered nanosensors, for instance, to selectively detect gas molecules. We show that the central part of the strained flake adsorbs polar molecules more strongly as compared with an unstrained sheet.
Keywords: A1 Journal article; Engineering sciences. Technology; Condensed Matter Theory (CMT)
Impact Factor: 4.536
Times cited: 38
DOI: 10.1021/jp402122c
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“Doping effect on the adsorption of NH3 molecule onto graphene quantum dot : from the physisorption to the chemisorption”. Seyed-Talebi SM, Beheshtian J, Neek-Amal M, Journal of applied physics 114, 124307 (2013). http://doi.org/10.1063/1.4822165
Abstract: The adsorption of ammonia molecule onto a graphene hexagonal flake, aluminum (Al) and boron (B) doped graphene flakes (graphene quantum dots, GQDs) are investigated using density functional theory. We found that NH3 molecule is absorbed to the hollow site through the physisorption mechanism without altering the electronic properties of GQD. However, the adsorption energy of NH3 molecule onto the Al- and B-doped GQDs increases with respect GQD resulting chemisorption. The adsorption of NH3 onto the Al-doped and B-doped GQDs makes graphene locally buckled, i.e., B-doped and Al-doped GQDs are not planar. The adsorption mechanism onto a GQD is different than that of graphene. This study reveals important features of the edge passivation and doping effects of the adsorption mechanism of external molecules onto the graphene quantum dots. (C) 2013 AIP Publishing LLC.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 2.068
Times cited: 10
DOI: 10.1063/1.4822165
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“AA-stacked bilayer square ice between graphene layers”. Sobrino Fernandez MM, Neek-Amal M, Peeters FM, Physical review : B : condensed matter and materials physics 92, 245428 (2015). http://doi.org/10.1103/PhysRevB.92.245428
Abstract: Water confined between two graphene layers with a separation of a few A forms a layered two-dimensional ice structure. Using large scale molecular dynamics simulations with the adoptable ReaxFF interatomic potential we found that flat monolayer ice with a rhombic-square structure nucleates between the graphene layers which is nonpolar and nonferroelectric. We provide different energetic considerations and H-bonding results that explain the interlayer and intralayer properties of two-dimensional ice. The controversial AA stacking found experimentally [Algara-Siller et al., Nature (London) 519, 443 (2015)] is consistent with our minimum-energy crystal structure of bilayer ice. Furthermore, we predict that an odd number of layers of ice has the same lattice structure as monolayer ice, while an even number of ice layers exhibits the square ice AA stacking of bilayer ice.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 40
DOI: 10.1103/PhysRevB.92.245428
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“N-doped graphene : polarization effects and structural properties”. Ghorbanfekr-Kalashami H, Neek-Amal M, Peeters FM, Physical review B 93, 174112 (2016). http://doi.org/10.1103/PhysRevB.93.174112
Abstract: The structural and mechanical properties of N-doped graphene (NG) are investigated using reactive force field (ReaxFF) potentials in large-scale molecular dynamics simulations. We found that ripples, which are induced by the dopants, change the roughness of NG, which depends on the number of dopants and their local arrangement. For any doping ratio N/C, the NG becomes ferroelectric with a net dipole moment. The formation energy increases nonlinearly with N/C ratio, while the Young's modulus, tensile strength, and intrinsic strain decrease with the number of dopants. Our results for the structural deformation and the thermoelectricity of the NG sheet are in good agreement with recent experiments and ab initio calculations.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 15
DOI: 10.1103/PhysRevB.93.174112
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“The formation of Cr2O3 nanoclusters over graphene sheet and carbon nanotubes”. Dabaghmanesh S, Neek-Amal M, Partoens B, Neyts EC, Chemical physics letters 687, 188 (2017). http://doi.org/10.1016/J.CPLETT.2017.09.005
Keywords: A1 Journal article; Condensed Matter Theory (CMT); Plasma Lab for Applications in Sustainability and Medicine – Antwerp (PLASMANT)
Impact Factor: 1.815
Times cited: 2
DOI: 10.1016/J.CPLETT.2017.09.005
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“Unusual ultra-low-frequency fluctuations in freestanding graphene”. Xu P, Neek-Amal M, Barber SD, Schoelz JK, Ackerman ML, Thibado PM, Sadeghi A, Peeters FM, Nature communications 5, 3720 (2014). http://doi.org/10.1038/ncomms4720
Abstract: 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.
Keywords: A1 Journal article; Engineering sciences. Technology; Condensed Matter Theory (CMT)
Impact Factor: 12.124
Times cited: 62
DOI: 10.1038/ncomms4720
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“Rippling, buckling, and melting of single- and multilayer MoS2”. Singh SK, Neek-Amal M, Costamagna S, Peeters FM, Physical Review B 91, 014101 (2015). http://doi.org/10.1103/PhysRevB.91.014101
Abstract: Large-scale atomistic simulations using the reactive empirical bond order force field approach is implemented to investigate thermal and mechanical properties of single-layer (SL) and multilayer (ML) molybdenum disulfide (MoS2). The amplitude of the intrinsic ripples of SL MoS2 are found to be smaller than those exhibited by graphene (GE). Furthermore, because of the van der Waals interaction between layers, the out-of-plane thermal fluctuations of ML MoS2 decreases rapidly with increasing number of layers. This trend is confirmed by the buckling transition due to uniaxial stress which occurs for a significantly larger applied tension as compared to graphene. For SL MoS2, the melting temperature is estimated to be 3700 K which occurs through dimerization followed by the formation of small molecules consisting of two to five atoms. When different types of vacancies are inserted in the SL MoS2 it results in a decrease of both the melting temperature as well as the stiffness.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 40
DOI: 10.1103/PhysRevB.91.014101
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“Anomalous dynamical behavior of freestanding graphene membranes”. Ackerman ML, Kumar P, Neek-Amal M, Thibado PM, Peeters FM, Singh S, Physical review letters 117, 126801 (2016). http://doi.org/10.1103/PHYSREVLETT.117.126801
Abstract: 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.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 8.462
Times cited: 46
DOI: 10.1103/PHYSREVLETT.117.126801
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“Static flexural modes and piezoelectricity in 2D and layered crystals”. Michel KH, Neek-Amal M, Peeters FM, Physica status solidi: B: basic research 253, 2311 (2016). http://doi.org/10.1002/PSSB.201600226
Abstract: Piezo- and flexoelectricity are manifestations of electromechanical coupling in solids with potential applications in nanoscale materials. Naumov etal. [Phys. Rev. Lett. 102, 217601 (2009)] have shown by first principles calculations that a monolayer BN sheet becomes macroscopically polarized in-plane when in a corrugated state. Here, we investigate the interplay of layer corrugation and in-plane polarization by atomistic lattice dynamics. We treat the coupling between static flexural modes and in-plane atomic ion displacements as an anharmonic effect, similar to the membrane effect that is at the origin of negative thermal expansion in layered crystals. We have derived analytical expressions for the corrugation-induced static in-plane strains and the optical displacements with the resulting polarization response functions. Beyond h-BN, the theory applies to transition metal dichalcogenides and dioxides. Numerical calculations show that the effects are considerably stronger for 2D h-BN than for 2H-MoS2.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 1.674
Times cited: 5
DOI: 10.1002/PSSB.201600226
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“Transport of hydrogen isotopes through interlayer spacing in van der Waals crystals”. Hu S, Gopinadhan K, Rakowski A, Neek-Amal M, Heine T, Grigorieva IV, Haigh SJ, Peeters FM, Geim AK, Lozada-Hidalgo M, Nature nanotechnology 13, 468 (2018). http://doi.org/10.1038/S41565-018-0088-0
Abstract: 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 angstrom even for the lightest atom, hydrogen. This restricts quantum-confinement phenomena for atomic species to the realm of very low temperatures(1-5). Here, we show that van der Waals gaps between atomic planes of layered crystals provide angstrom-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 angstrom-size channels can be exploited for further studies of atomistic quantum confinement and, if the technology can be scaled up, for sieving hydrogen isotopes.
Keywords: A1 Journal article; Engineering sciences. Technology; Condensed Matter Theory (CMT)
Impact Factor: 38.986
Times cited: 32
DOI: 10.1038/S41565-018-0088-0
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“Multilayer graphene, Moire patterns, grain boundaries and defects identified by scanning tunneling microscopy on the m-plane, non-polar surface of SiC”. Xu P, Qi D, Schoelz JK, Thompson J, Thibado PM, Wheeler VD, Nyakiti LO, Myers-Ward RL, Eddy CR, Gaskill DK, Neek-Amal M, Peeters FM;, Carbon 80, 75 (2014). http://doi.org/10.1016/j.carbon.2014.08.028
Abstract: Epitaxial graphene is grown on a non-polar n(+) 6H-SiC m-plane substrate and studied using atomic scale scanning tunneling microscopy. Multilayer graphene is found throughout the surface and exhibits rotational disorder. Moire patterns of different spatial periodicities are found, and we found that as the wavelength increases, so does the amplitude of the modulations. This relationship reveals information about the interplay between the energy required to bend graphene and the interaction energy, i.e. van der Waals energy, with the graphene layer below. Our experiments are supported by theoretical calculations which predict that the membrane topographical amplitude scales with the Moire pattern wavelength, L as L-1 + alpha L-2. (C) 2014 Elsevier Ltd. All rights reserved.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 6.337
Times cited: 14
DOI: 10.1016/j.carbon.2014.08.028
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“Commensurability Effects in Viscosity of Nanoconfined Water”. Neek-Amal M, Peeters FM, Grigorieva IV, Geim AK, ACS nano 10, 3685 (2016). http://doi.org/10.1021/acsnano.6b00187
Abstract: The rate of water flow through hydrophobic nanocapillaries is greatly enhanced as compared to that expected from macroscopic hydrodynamics. This phenomenon is usually described in terms of a relatively large slip length, which is in turn defined by such microscopic properties as the friction between water and capillary surfaces and the viscosity of water. We show that the viscosity of water and, therefore, its flow rate are profoundly affected by the layered structure of confined water if the capillary size becomes less than 2 nm. To this end, we study the structure and dynamics of water confined between two parallel graphene layers using equilibrium molecular dynamics simulations. We find that the shear viscosity is not only greatly enhanced for subnanometer capillaries, but also exhibits large oscillations that originate from commensurability between the capillary size and the size of water molecules. Such oscillating behavior of viscosity and, consequently, the slip length should be taken into account in designing and studying graphene-based and similar membranes for desalination and filtration.
Keywords: A1 Journal article; Engineering sciences. Technology; Condensed Matter Theory (CMT)
Impact Factor: 13.942
Times cited: 160
DOI: 10.1021/acsnano.6b00187
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“Self-organized platinum nanoparticles on freestanding graphene”. Xu P, Dong L, Neek-Amal M, Ackerman ML, Yu J, Barber SD, Schoelz JK, Qi D, Xu F, Thibado PM, Peeters FM;, ACS nano 8, 2697 (2014). http://doi.org/10.1021/nn406394f
Abstract: Freestanding graphene membranes were successfully functionalized with platinum nanoparticles (Pt NPs). High-resolution transmission electron microscopy revealed a homogeneous distribution of single-crystal Pt NPs that tend to exhibit a preferred orientation. Unexpectedly, the NPs were also found to be partially exposed to the vacuum with the top Pt surface raised above the graphene substrate, as deduced from atomic-scale scanning tunneling microscopy images and detailed molecular dynamics simulations. Local strain accumulation during the growth process is thought to be the origin of the NP self-organization. These findings are expected to shape future approaches in developing Pt NP catalysts for fuel cells as well as NP-functionalized graphene-based high-performance electronics.
Keywords: A1 Journal article; Engineering sciences. Technology; Condensed Matter Theory (CMT)
Impact Factor: 13.942
Times cited: 38
DOI: 10.1021/nn406394f
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“Thermal properties of fluorinated graphene”. Singh SK, Srinivasan SG, Neek-Amal M, Costamagna S, van Duin ACT, Peeters FM, Physical review : B : condensed matter and materials physics 87, 104114 (2013). http://doi.org/10.1103/PhysRevB.87.104114
Abstract: Large-scale atomistic simulations using the reactive force field approach are implemented to investigate the thermomechanical properties of fluorinated graphene (FG). A set of parameters for the reactive force field potential optimized to reproduce key quantum mechanical properties of relevant carbon-fluorine cluster systems are presented. Molecular dynamics simulations are used to investigate the thermal rippling behavior of FG and its mechanical properties and compare them with graphene, graphane and a sheet of boron nitride. The mean square value of the height fluctuations < h(2)> and the height-height correlation function H(q) for different system sizes and temperatures show that FG is an unrippled system in contrast to the thermal rippling behavior of graphene. The effective Young's modulus of a flake of fluorinated graphene is obtained to be 273 N/m and 250 N/m for a flake of FG under uniaxial strain along armchair and zigzag directions, respectively. DOI: 10.1103/PhysRevB.87.104114
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 80
DOI: 10.1103/PhysRevB.87.104114
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“Linear reduction of stiffness and vibration frequencies in defected circular monolayer graphene”. Neek-Amal M, Peeters FM, Physical review : B : condensed matter and materials physics 81, 11 (2010). http://doi.org/10.1103/PhysRevB.81.235437
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 44
DOI: 10.1103/PhysRevB.81.235437
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“Thermal mirror buckling in freestanding graphene locally controlled by scanning tunnelling microscopy”. Neek-Amal M, Xu P, Schoelz JK, Ackerman ML, Barber SD, Thibado PM, Sadeghi A, Peeters FM, Nature communications 5, 4962 (2014). http://doi.org/10.1038/ncomms5962
Abstract: 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.
Keywords: A1 Journal article; Engineering sciences. Technology; Condensed Matter Theory (CMT)
Impact Factor: 12.124
Times cited: 36
DOI: 10.1038/ncomms5962
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