167 research outputs found
Melting temperature of graphene
We present an approach to the melting of graphene based on nucleation theory
for a first order phase transition from the 2D solid to the 3D liquid via an
intermediate quasi-2D liquid.
The applicability of nucleation theory, supported by the results of
systematic atomistic Monte Carlo simulations, provides an intrinsic definition
of the melting temperature of graphene, , and allows us to determine it.
We find K, about 250 K higher than that of graphite using the
same interatomic interaction model. The found melting temperature is shown to
be in good agreement with the asymptotic results of melting simulations for
finite disks and ribbons of graphene. Our results strongly suggest that
graphene is the most refractory of all known materials
Self-Consistent Screening Approximation for Flexible Membranes: Application to Graphene
Crystalline membranes at finite temperatures have an anomalous behavior of
the bending rigidity that makes them more rigid in the long wavelength limit.
This issue is particularly relevant for applications of graphene in nano- and
micro-electromechanical systems. We calculate numerically the height-height
correlation function of crystalline two-dimensional membranes,
determining the renormalized bending rigidity, in the range of wavevectors
from \AA till 10 \AA in the self-consistent screening
approximation (SCSA). For parameters appropriate to graphene, the calculated
correlation function agrees reasonably with the results of atomistic Monte
Carlo simulations for this material within the range of from
\AA till 1 \AA. In the limit our data for the
exponent of the renormalized bending rigidity is compatible with the previously known analytical results for the
SCSA . However, this limit appears to be reached only for
\AA whereas at intermediate the behavior of
cannot be described by a single exponent.Comment: 5 pages, 4 figure
Atomistic simulations of structural and thermodynamic properties of bilayer graphene
We study the structural and thermodynamic properties of bilayer graphene, a
prototype two-layer membrane, by means of Monte Carlo simulations based on the
empirical bond order potential LCBOPII. We present the temperature dependence
of lattice parameter, bending rigidity and high temperature heat capacity as
well as the correlation function of out-of-plane atomic displacements. The
thermal expansion coefficient changes sign from negative to positive above
K, which is lower than previously found for single layer graphene
and close to the experimental value of bulk graphite. The bending rigidity is
twice as large than for single layer graphene, making the out-of-plane
fluctuations smaller. The crossover from correlated to uncorrelated
out-of-plane fluctuations of the two carbon planes occurs for wavevectors
shorter than nmComment: 6 pages, 7 figures
Thermomechanical properties of graphene: valence force field model approach
Using the valence force field model of Perebeinos and Tersoff [Phys. Rev. B
{\bf79}, 241409(R) (2009)], 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 () the total energy is symmetrical in the strain, while it
behaves completely different beyond this threshold; ii) the important energy
contributions in stretching experiments are stretching, angle bending,
out-of-plane term and a term that provides repulsion against
misalignment; iii) in compressing experiments the two latter terms increase
rapidly and beyond the buckling transition stretching and bending energies are
found to be constant; iv) from stretching-compressing simulations we calculated
the Young modulus at room temperature 350\,N/m, which is in good
agreement with experimental results (340\,N/m) and with ab-initio
results [322-353]\,N/m; v) molar heat capacity is estimated to be
24.64\,J/molK which is comparable with the Dulong-Petit value,
i.e. 24.94\,J/molK and is almost independent of the strain; vi)
non-linear scaling properties are obtained from height-height correlations at
finite temperature; vii) the used valence force field model results in a
temperature independent bending modulus for graphene, and viii) the Gruneisen
parameter is estimated to be 0.64.Comment: 8 pages, 5 figures. To appear in J. Phys.: Condens. Matte
Self-folding nano- and micropatterned hydrogel tissue engineering scaffolds by single step photolithographic process
Current progress in tissue engineering is focused on the creation of environments in which cultures of relevant cells can adhere, grow and form functional tissue. We propose a method for controlled chemical and topographical cues through surface patterning of self-folding hydrogel films. This provides a conversion of 2D patterning techniques into a viable method of manufacturing a 3D scaffold. While similar bilayers have previously been demonstrated, here we present a faster and high throughput process for fabricating self-folding hydrogel devices incorporating controllable surface nanotopographies by serial hot embossing of sacrificial layers and photolithography
Scaling Properties of Flexible Membranes from Atomistic Simulations: Application to Graphene
Structure and thermodynamics of crystalline membranes are characterized by
the long wavelength behavior of the normal-normal correlation function G(q). We
calculate G(q) by Monte Carlo and Molecular Dynamics simulations for a
quasi-harmonic model potential and for a realistic potential for graphene. To
access the long wavelength limit for finite-size systems (up to 40000 atoms) we
introduce a Monte Carlo sampling based on collective atomic moves (wave moves).
We find a power-law behaviour with the same exponent
for both potentials. This finding supports, from the
microscopic side, the adequacy of the scaling theory of membranes in the
continuum medium approach, even for an extremely rigid material like graphene
НАУКОВО-МЕТОДИЧНЕ СПРЯМУВАННЯ ОРГАНІЗАЦІЇ ТА КОНТРОЛЮ САМОСТІЙНОЇ РОБОТИ СТУДЕНТІВ У НМУ ІМЕНІ О. О. БОГОМОЛЬЦЯ
The article analyzes the quality level of organisation and methodological basis of self learning, namely, among students of clinical disciplines at the Medical Faculty of Bohomolets National Medical University as an essential component of effective training of specialists. It covers the basic aspects of University internal management system that deals with quality of education as regards the implementation of scientifically grounded approaches to improve the organization of self learning among students.У статті висвітлено результати аналізу якості організації самостійної роботи студентів та її методичного забезпечення з клінічних дисциплін на медичних факультетах Національного медичного університету імені О. О. Богомольця як важливого компонента у системі ефективного управління підготовкою фахівців. Описано основні аспекти функціонування внутрішньовузівської системи управління якістю освіти у визначенні та реалізації науково обґрунтованих підходів до удосконалення організації самостійної роботи студентів.
Determination of the Bending Rigidity of Graphene via Electrostatic Actuation of Buckled Membranes
The small mass and atomic-scale thickness of graphene membranes make them
highly suitable for nanoelectromechanical devices such as e.g. mass sensors,
high frequency resonators or memory elements. Although only atomically thick,
many of the mechanical properties of graphene membranes can be described by
classical continuum mechanics. An important parameter for predicting the
performance and linearity of graphene nanoelectromechanical devices as well as
for describing ripple formation and other properties such as electron
scattering mechanisms, is the bending rigidity, {\kappa}. In spite of the
importance of this parameter it has so far only been estimated indirectly for
monolayer graphene from the phonon spectrum of graphite, estimated from AFM
measurements or predicted from ab initio calculations or bond-order potential
models. Here, we employ a new approach to the experimental determination of
{\kappa} by exploiting the snap-through instability in pre-buckled graphene
membranes. We demonstrate the reproducible fabrication of convex buckled
graphene membranes by controlling the thermal stress during the fabrication
procedure and show the abrupt switching from convex to concave geometry that
occurs when electrostatic pressure is applied via an underlying gate electrode.
The bending rigidity of bilayer graphene membranes under ambient conditions was
determined to be eV. Monolayers have significantly lower
{\kappa} than bilayers
Finite temperature lattice properties of graphene beyond the quasiharmonic approximation
The thermal and mechanical stability of graphene is important for many
potential applications in nanotechnology. We calculate the temperature
dependence of lattice parameter, elastic properties and heat capacity by means
of atomistic Monte Carlo simulations that allow to go beyond the quasiharmonic
approximation. We predict an unusual, non-monotonic, behavior of the lattice
parameter with minimum at temperature about 900 K and of the shear modulus with
maximum at the same temperature. The Poisson ratio in graphene is found to be
small ~0.1 in a broad temperature interval.Comment: 4 pages, 5 figure
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