On/Off Switchable Interfacial Thermal Resistance in Graphene/Fullerene/Graphene Heterostructures
Y. Xue, H.S. Park and J-W Jiang
International Journal of Heat and Mass Transfer 2023; 212:124222
Abstract
Switchable thermal devices are attracting significant research interest as a basic thermal management component. In this work, taking
graphene/fullerene/graphene sandwiches as an example, we demonstrate that the interfacial thermal resistance can show a switchable,
step-like change by varying the number of fullerenes in the sandwich structure. Changing the number of fullerenes causes a structural
transition between the graphene layers from adhered to separated, resulting in an enhancement of about a factor of two in the interfacial
thermal resistance during this on/off switchable phenomenon, which we analyze using analytic expressions based on the thermal transport
theory. We further illustrate that the switchable phenomenon can also be realized by applying mechanical strain or by varying the
temperature of the sandwich structure. This work demonstrates that the sandwich-liked nanoscale heterostructures can exhibit hysteretic
changes in heat transport, and thus have promise for potential applications in switchable thermal devices.
This paper is available in PDF form
.
Atomistic Configurational Forces in Crystalline Fracture
S. Elmira Birang O., H.S. Park, A-S Smith and P. Steinmann
Forces in Mechanics 2021; 4:100044
Abstract
Configurational atomistic forces contribute to the configurational mechanics (i.e. non- equilibrium) problem that determines the release of
total potential energy of an atomistic system upon variation of the atomistic positions relative to the initial atomic configuration. These forces
drive energetically favorable irreversible re-organizations of the material body, and thus characterize the tendency of crystalline defects to
propagate. In this work, we provide new expressions for the atomistic configurational forces for two realistic interatomic potentials, i.e. the
embedded atom potential (EAM) for metals, and second generation reactive bond order (REBO-II) potential for hydrocarbons. We present a range of
numerical examples involving quasistatic fracture for both FCC metals and mono and bi- layer graphene at zero Kelvin that demonstrate the ability to
predict defect nucleation and evolution using the proposed atomistic configurational mechanics approach. Furthermore, we provide the contributions
for each potential including two-body stretching, three-body mixed-mode stretching-bending, and four-body mixed-mode stretching-bending-twisting terms
that make towards defect nucleation and propagation.
This paper is available in PDF form
.
Graphene Origami with Highly Tunable Coefficient of Thermal Expansion
D.T. Ho, H.S Park, S.Y. Kim and U. Schwingenschlogl
ACS Nano 2020; 14:8969-8974
Abstract
The coefficient of thermal expansion, which measures the
change in length, area, or volume of a material upon heating, is a fundamental
parameter with great relevance for many applications. While there are various
routes to design materials with targeted coefficient of thermal expansion
at the macroscale, no approaches exist to achieve a wide range of values
in graphene-based structures. Here, we use molecular dynamics simulations to
show that graphene origami structures obtained through pattern-based surface
functionalization provide tunable coefficients of thermal expansion from large
negative to large positive. We show that the mechanisms giving rise to this
property are exclusive to graphene origami structures, emerging from a
combination of surface functionalization, large out-of-plane thermal
fluctuations, and the three-dimensional geometry of origami structures.
This paper is available in PDF form
.
Multiscale Computational Understanding and Growth of 2D Materials: A Review
K. Momeni, Y. Ji, Y. Wang, S. Paul, S. Neshani, D.E. Yilmaz, Y. K. Shin, D. Zhang, J-W Jiang, H.S. Park, S. Sinnott, A. van Duin, V. Crespi and L-Q Chen
NPJ Computational Materials 2020; 6:22
Abstract
The successful discovery and isolation of graphene in 2004, and the subsequent synthesis of layered semiconductors and heterostructures beyond
graphene have led to the exploding field of two-dimensional (2D) materials that explore their growth, new atomic-scale physics, and potential
device applications. This review aims to provide an overview of theoretical, computational, and machine learning methods and tools at multiple
length and time scales, and discuss how they can be utilized to assist/guide the design and synthesis of 2D materials beyond graphene. We focus on
three methods at different length and time scales as follows: (i) nanoscale atomistic simulations including density functional theory (DFT) calculations
and molecular dynamics simulations employing empirical and reactive interatomic potentials; (ii) mesoscale methods such as phase-field method; and
(iii) macroscale continuum approaches by coupling thermal and chemical transport equations. We discuss how machine learning can be combined with
computation and experiments to understand the correlations between structures and properties of 2D materials, and to guide the discovery of new 2D
materials. We will also provide an outlook for the applications of computational approaches to 2D materials synthesis and growth in general.
This paper is available in PDF form
.
Strain-Induced Gauge and Rashba Fields in Two-Dimensional Ferroelectric Rashba Lead Chalcogenide PbX (X=S, Se, Te) Monolayers
P.Z. Hanakata, A.S. Rodin, H.S. Park, D.K. Campbell and A.H. Castro Neto
Physical Review B 2018; 97:235312
Abstract
One of the exciting features of two-dimensional (2D) materials is
their electronic and optical tunability through strain engineering.
Previously we found a new class of 2D ferroelectric Rashba
semiconductors PbX (X=S, Se, Te) with tunable spin-orbital
properties. In this work, based on our previous tight-binding (TB)
results, we derive an effective low-energy Hamiltonian around the
symmetry points that captures the effects of strain on the
electronic properties of PbX. We find that strains induce gauge
fields which shift the Rashba point and modify the Rashba
parameter. This effect is equivalent to the application of in-plane
magnetic fields. The out-of-plane strain, which is roughly
proportional to the electric polarization, is also shown to modify
the Rashba parameter. Overall, our theory connects strain and
spin-splitting in ferroelectric Rashba materials, which will be
important to understand the strain-induced variations in local
Rashba parameters that will occur in practical applications.
This paper is available in PDF form
.
Intrinsic Rippling Enhances Static Non-Reciprocity in a Graphene Metamaterial
D.T. Ho, H.S. Park and S.Y. Kim
Nanoscale 2018; 10:1207-1214
Abstract
In mechanical systems, Maxwell-Betti reciprocity means that the displacement at point B in response to a force at point A is the same
as the displacement at point A in response to the same force applied at point B. Because the notion of reciprocity is general, fundamental,
and is operant for other physical systems like electromagnetics, acoustics, and optics, there is significant interest in understanding
systems that are not reciprocal, or exhibit non-reciprocity. However, most studies of non-reciprocity have occurred in bulk-scale structures
for dynamic problems involving time reversal symmetry. As a result, little is known about the mechanisms governing static non-reciprocal
responses, particularly in atomically-thin two-dimensional materials like graphene. Here, we use classical atomistic simulations to demonstrate
that out of plane ripples, which are intrinsic to graphene, enable significant, multiple orders of magnitude enhancements in the statically
non-reciprocal response of graphene metamaterials. Specifically, we find that a striking interplay between the ripples and the stress fields
that are induced in the metamaterials due to their geometry impact the displacements that are transmitted by the metamaterial, thus leading
to a significantly enhanced static non-reciprocal response. This study thus demonstrates the potential of two-dimensional mechanical
metamaterials for symmetry-breaking applications.
This paper is available in PDF form
.
Self-Cleaning by Harnessing Wrinkles in Two-Dimensional Layered Crystals
J-S Sun, J-W Jiang, H.S. Park and S. Zhang
Nanoscale 2018; 10:312-318
Abstract
Two-dimensional (2D) layered crystals are prone to bending and folding owing to their ultra-low bending stiffness. Folds are traditionally
viewed as defects that degrade the material performance. Here, we demonstrate that folds and cohesive forces in 2D layered crystals like
graphene and MoS2 can be exploited to collect and clean up interlayer impurities, wherein multiple separated impurities agglomerate
into a single, large cluster. We combine classical molecular dynamics simulations and an analytic model to elucidate the competing roles
of membrane bending and impurity-membrane cohesive energies in the self-cleaning process. Our findings shed light on the mechanisms by
which the forces that are present in 2D layered crystals can positively impact, through the possibility of intrinsic cleaning and defect
engineering, the synthesis of van der Waals homo- and heterostructures with improved reliability and functionalities.
This paper is available in PDF form
.
Irreversible Crumpling of Graphene from Hydrostatic and Biaxial Compression
J. Wan, J-W Jiang and H.S. Park
Journal of Physics D: Applied Physics 2018; 51:015302
Abstract
We perform molecular dynamics simulations to investigate the irreversibility of crumpled graphene obtained by hydrostatic or biaxial
compression. Our results show that there is a critical degree of crumpling, above which the crumpling is irreversible after the external
force is removed. The critical degree of irreversible crumpling is closely related to the self-adhesion phenomenon of graphene, which
leads to a step-like jump or decrease in the adhesion energy. We find the critical degree of crumpling is about 0.5 or 0.55 for hydrostatic
or biaxial compression, which matches analytic predictions based on a competition between adhesive and bending energies in folded graphene.
This paper is available in PDF form
.
Negative In-Plane Poisson's Ratio of Single Layer Black Phosphorus: An Atomistic Simulation Study
D.T. Ho, V.H. Ho, H.S. Park and S.Y. Kim
Physica Status Solidi (b) 2017; 254:1700285
Abstract
We utilized molecular statics (MS) simulations to investigate the auxeticity of single layer black phosphorus (SLBP). Our simulation results
show that the SLBP has a negative in-plane Poisson's ratio in the zigzag direction when the applied strain along the armchair direction exceeds 0.018.
We show that the interplay between bond stretching and bond rotating modes determines the in-plane Poisson's ratio behavior. While the bond
stretching mode always tends to increase the in-plane auxeticity, the bond rotating mode might increase or decrease the in-plane auxeticity.
Furthermore, we show that graphite also exhibits an in-plane negative Poisson's ratio at finite strains due to a similar mechanism.
This paper is available in PDF form
.
Rashba-Like Dispersion in Buckled Square Lattices
A.S. Rodin, P.Z. Hanakata, A. Carvalho, H.S. Park, D.K. Campbell and A.H. Castro Neto
Physical Review B 2017; 96:115450
Abstract
Band structure of a general class of buckled square materials is investigated using ab initio calculations along with tight-binding modelling.
We show that buckling and spin-orbit interaction give rise to a large quasi-Rashba splitting in the absence of an external electric field.
The generality and the robustness of the effect make this class of make this class of materials a candidate for spintronic applications.
This paper is available in PDF form
.
Two-Dimensional Square Buckled Rashba Lead Chalcogenides
P.Z. Hanakata, A.S. Rodin, A. Carvalho, H.S. Park, D.K. Campbell and A.H. Castro Neto
Physical Review B 2017; 96:161401(R)
Abstract
We propose the lead sulphide (PbS) monolayer as a 2D semiconductor with a large Rashba-like spin-orbit effect
controlled by the out-of-plane buckling. The buckled PbS conduction band is found
to possess Rashba-like dispersion and spin texture at the M and
Γ points, with a large effective Rashba parameter of
λ~5 eVÅ and λ~1 eVÅ, respectively. Using
a tight-binding formalism, we show that the Rashba effect originates
from the very large spin-orbit interaction and the hopping term that
mixes the in-plane and out-of-plane p orbitals of Pb and S
atoms. The latter, which depends on the buckling angle, can be
controlled by applying compressive strain to vary the spin
texture as well as the Rashba paramater at Γ and M. Our
density functional theory results together with tight-binding
formalism provide a unifying framework for designing Rashba
monolayers and for manipulating their spin properties.
This paper is available in PDF form
.
A Perspective on Auxetic Nanomaterials
H.S. Park and S.Y. Kim
Nanoconvergence 2017; 4:10
Abstract
Nanomaterials have recently been found to exhibit auxetic behavior, or a negative Poisson's ratio, whereby the lateral dimensions of
the material expand, rather than shrink, in response to applied tensile loading. In this brief review, we use the form of question-answer
to highlight key points and outstanding issues related to the field of auxetic nanomaterials.
This paper is available in PDF form
.
Negative Poisson's Ratio in Graphene Oxide
J. Wan, J-W Jiang, and H.S. Park
Nanoscale 2017; 9:4007-4012
Abstract
We perform molecular dynamics simulations to investigate the Poisson's ratio of graphene oxide. We find that the
Poisson's ratio can be effectively tuned by increasing the degree of oxidation of graphene oxide. More specifically,
the Poisson's ratio decreases linearly from positive to negative with increasing oxidation, turning negative at room
temperature for a degree of oxidation of 0.27, and reaching a value of -0.567 for fully oxidized graphene. The oxidation
dependence of the Poisson's ratio is attributed to the tension-induced suppression of the ripples resulted from the
oxidation, whose amplitude increases with increasing oxidation. Finally, we also demonstrate the temperature dependence
for the Poisson's ratio in the graphene oxide.
This paper is available in PDF form
.
A Review on Mechanics and Mechanical Properties of 2D Materials - Graphene and Beyond
D. Akinwande, C. Brennan, J.S. Bunch, P. Egberts, J. Felts, H. Gao, R. Huang, J. Kim, T. Li, Y. Li, K.M. Liechti, N. Lu, H.S. Park, E. Reed,
B.I. Yakobson, T. Zhang, Y-W Zhang, Y. Zhou and Y. Zhu
Extreme Mechanics Letters 2017; 13:42-72
Abstract
Since the first successful synthesis of graphene just over a decade ago, a variety of two-dimensional (2D) materials
(e.g., transition metal-dichalcogenides, hexagonal boron-nitride, etc.) have been discovered. Among the many unique and
attractive properties of 2D materials, mechanical properties play important roles in manufacturing, integration and
performance for their potential applications. Mechanics is indispensable in the study of mechanical properties, both
experimentally and theoretically. The coupling between the mechanical and other physical properties (thermal, electronic, optical)
is also of great interest in exploring novel applications, where mechanics has to be combined with condensed matter physics
to establish a scalable theoretical framework. Moreover, mechanical interactions between 2D materials and various substrate
materials are essential for integrated device applications of 2D materials, for which the mechanics of interfaces
(adhesion and friction) has to be developed for the 2D materials. Here we review recent theoretical and experimental works
related to mechanics and mechanical properties of 2D materials. While graphene is the most studied 2D material to date,
we expect continual growth of interest in the mechanics of other 2D materials beyond graphene.
This paper is available in PDF form
.
Self-Assembly of Water Molecules Using Graphene Nanoresonators
C-X Wang, C. Zhang, J-W Jiang, N. Wei, H.S. Park and T. Rabczuk
RSC Advances 2016; 6:110466-110470
Abstract
Inspired by macroscale self-assembly using the higher order resonant modes of Chladni plates, we use classical molecular
dynamics to investigate the self-assembly of water molecules using graphene nanoresonators. We find that water molecules
can assemble into water chains and that the location of the assembled water chain can be controlled through the resonant
frequency. More specifically, water molecules assemble at the location of maximum amplitude if the resonant frequency
is lower than a critical value. Otherwise, the assembly occurs near the nodes of the resonator provided the resonant
frequency is higher than the critical value. We provide an analytic formula for the critical resonant frequency based
on the interaction between water molecules and graphene. Furthermore, we demonstrate that the water chains assembled
by the graphene nanoresonators have some universal properties including a stable value for the number of hydrogen bonds.
This paper is available in PDF form
.
Auxetic Nanomaterials: Recent Progress and Future Development
J-W Jiang, S-Y Kim and H.S. Park
Applied Physics Reviews 2016; 3:041101
Abstract
Auxetic materials (materials with negative Poisson's ratio) and nanomaterials have independently been for many years
two of the most active research fields in material science. Recently, these formerly independent fields have begun
to intersect in new and interesting ways due to the recent discovery of auxeticity in nanomaterials like graphene,
metal nanoplates, black phosphorus, and others. Here we review the research emerging at the intersection of
auxeticity and nanomaterials. We first survey the atomistic mechanisms, both intrinsic and extrinsic, that have been
found, primarily through atomistic simulations, to cause auxeticity in nanomaterials. We then outline the available
experimental evidence for auxetic nanomaterials. In order to lay the groundwork for future work in this exciting area,
we close by discussing several future prospects as well as the current challenges in this field.
This paper is available in PDF form
.
The Effects of Free Edge Interaction-Induced Knotting on the Buckling of Monolayer Graphene
H-Y Zhang, J-W Jiang, T. Chong, X. Guo and H.S. Park
International Journal of Solids and Structures 2016; 100-101:446-455
Abstract
Edge effects play an important role for many properties of graphene. While most works have focused on the
effects from isolated free edges, we present a novel knotting phenomenon induced by the interactions between
a pair of free edges in graphene, and investigate its effect on the buckling of monolayer graphene. Upon
compression, the buckling of graphene starts gradually in the form of two buckling waves from the warped edges.
The collision of these two buckling waves results in the creation of a knot structure in graphene. The knot
structure enables the buckled graphene to exhibit two unique post-buckling characteristics. First, it induces
a five-fold increase in graphene's mechanical stiffness during the buckling process. Second, the knotted
structure enables graphene to exhibit a mechanically stable post-buckling regime over a large (3%) compressive
strain regime, which is significantly larger than the critical buckling strain of about 0.5%. The combination
of these two effects enables graphene to exhibit an unexpected post-buckling stability that has previously
not been reported. We predict that numerical simulations or experiments should observe two distinct stress
strain relations for the buckling of identical graphene samples, due to the characteristic randomness in
the formation process of the knot structure.
This paper is available in PDF form
.
Intrinsic Negative Poisson's Ratio for Single-Layer Graphene
J-W Jiang, T. Chang, X. Guo and H.S. Park
Nano Letters 2016; 16:5286-5290
Abstract
Negative Poisson's ratio (NPR) materials have drawn significant interest because the enhanced toughness,
shear resistance and vibration absorption that typically are seen in auxetic materials may enable a range of
novel applications. In this work, we report that single-layer graphene exhibits an intrinsic NPR, which is
robust and independent of its size and temperature. The NPR arises due to the interplay between two intrinsic
deformation pathways (one with positive Poisson's ratio, the other with NPR), which correspond to the bond
stretching and angle bending interactions in graphene. We propose an energy-based deformation pathway criteria,
which predicts that the pathway with NPR has lower energy and thus becomes the dominant deformation mode when
graphene is stretched by a strain above 6%, resulting in the NPR phenomenon.
This paper is available in PDF form
.
Polarization and Valley Switching in Monolayer Group-IV Monochalcogenides
P.Z. Hanakata, A. Carvalho, D.K. Campbell and H.S. Park
Physical Review B 2016; 94:035304
Abstract
Group-IV monochalcogenides are a family of two-dimensional puckered
materials with an orthorhombic structure that is comprised of polar
layers. In this article, we use first principles calculations to
show the multistability of monolayer SnS and GeSe, two prototype
materials where the direction of the puckering can be switched by
application of tensile stress or electric field. Furthermore, the two
inequivalent valleys in momentum space, which dictated by the
puckering orientation, can be excited selectively using linearly
polarized light, and this provides additional tool to identify the
polarization direction. Our findings suggest
that SnS and GeSe monolayers may have observable ferroelectricity and multistability,
with potential applications in information storage.
This paper is available in PDF form
.
Graphene Kirigami as a Platform for Stretchable and Tunable Quantum Dot Arrays
D.A. Bahamon, Z.N. Qi, H.S. Park, V.M. Pereira and D.K. Campbell
Physical Review B 2016; 93:235408
Abstract
The quantum transport properties of a graphene kirigami similar to those studied in recent experiments are calculated
in the regime of elastic, reversible deformations. Our results show that, at low electronic densities, the conductance
profile of such structures replicates that of a system of coupled quantum dots, characterized by a sequence of minibands
and stop-gaps. The conductance and I-V curves have different characteristics in the distinct stages of deformation that
characterize the elongation of these structures. Notably, the effective coupling between localized states is strongly
reduced in the small elongation stage but revived at large elongations that allow the reestablishment of resonant tunneling
across the kirigami. This provides an interesting example of interplay between geometry, strain, spatial confinement and
electronic transport. The alternating miniband and stop-gap structure in the transmission leads to I-V characteristics with
negative differential conductance in well defined energy/doping ranges. These effects should be stable in a realistic
scenario that includes edge roughness and Coulomb interactions, as these are expected to further promote localization of
states at low energies in narrow segments of graphene nanostructures.
This paper is available in PDF form
.
Negative Poisson's Ratio in Single-Layer Graphene Ribbons
J-W Jiang and H.S. Park
Nano Letters 2016; 16:2657-2662
Abstract
The Poisson's ratio characterizes the resultant strain in the lateral direction for a material under longitudinal
deformation. Though negative Poisson's ratios (NPR) are theoretically possible within continuum elasticity, they
are most frequently observed in engineered materials and structures, as they are not intrinsic to many materials.
In this work, we report NPR in single-layer graphene ribbons, which results from the compressive edge stress induced
warping of the edges. The effect is robust, as the NPR is observed for graphene ribbons with widths smaller than
about 10 nm, and for tensile strains smaller than about 0.5%, with NPR values reaching as large as -1.51. The NPR
is explained analytically using an inclined plate model, which is able to predict the Poisson's ratio for graphene
sheets of arbitrary size. The inclined plate model demonstrates that the NPR is governed by the interplay between
the width (a bulk property), and the warping amplitude of the edge (an edge property), which eventually yields a
phase diagram determining the sign of the Poisson's ratio as a function of the graphene geometry.
This paper is available in PDF form
.
Interlayer Breathing and Shear Modes in Few-Layer Black Phosphorus
J-W Jiang, B-S Wang and H.S. Park
Journal of Physics: Condensed Matter 2016; 28:165401
Abstract
The interlayer breathing and shear modes in few-layer black phosphorus are investigated for their symmetry and
lattice dynamical properties. The symmetry groups for the even-layer and odd-layer few-layer black phosphorus are
utilized to determine the irreducible representation and the infrared and Raman activity for the interlayer modes.
The valence force field model is applied to calculate the eigenvectors and frequencies for the interlayer breathing
and shear modes, which are explained using the atomic chain model. The anisotropic puckered configuration for black
phosphorus leads to a highly anisotropic frequency for the two interlayer shear modes. More specifically, the frequency
for the shear mode in the direction perpendicular to the pucker is less than half of the shear mode in the direction
parallel with the pucker. We also report a set of specular interlayer modes having the same frequency for all few-layer
black phosphorus with layer numbers N being a multiple of 3, because these modes manifest themselves as collective
vibrations of atoms in specific layers. The optical activity of the collective modes enables possible experimental
identification for these modes.
This paper is available in PDF form
.
Highly Stretchable MoS2 Kirigami
P.Z. Hanakata, Z.N. Qi, D.K. Campbell and H.S. Park
Nanoscale 2016; 8:458-463
Abstract
We report the results of classical molecular
dynamics simulations focused on studying the mechanical properties
of MoS2 kirigami. Several different kirigami structures were
studied based upon two simple non-dimensional parameters, which
are related to the density of cuts, as well as the ratio of the
overlapping cut length to the nanoribbon length. Our key findings
are significant enhancements in tensile yield (by a factor of four)
and fracture strains (by a factor of six) as compared to pristine
MoS2 nanoribbons. These results, in conjunction with recent
results on graphen, suggest that the kirigami approach may be
generally useful for enhancing the ductility of two-dimensional
nanomaterials.
This paper is available in PDF form
.
Mechanical Strain Effects on Black Phosphorus Nanoresonators
C-X Wang, C. Zhao, J-W Jiang, H.S. Park and T. Rabczuk
Nanoscale 2016; 8:901-905
Abstract
We perform classical molecular dynamics simulations to investigate the effects of mechanical strain on single-layer
black phosphorus nanoresonators at different temperatures. We find that the resonant frequency is highly anisotropic
in black phosphorus due to its intrinsic puckered configuration, and that the quality factor in the armchair direction
is higher than in the zigzag direction at room temperature. The quality factors are also found to be intrinsically
larger than graphene and MoS2 nanoresonators. The quality factors can be increased by more than a factor of
two by applying tensile strain, with uniaxial strain in the armchair direction being most effective. However, there
is an upper bound for the quality factor increase due to nonlinear effects at large strains, after which the quality
factor decreases. The tension induced nonlinear effect is stronger along the zigzag direction, resulting in a smaller
maximum strain for quality factor enhancement.
This paper is available in PDF form
.
Conductance Signatures of Electron Confinement Induced by Strained Nanobubbles in Graphene
D.A. Bahamon, Z.N. Qi, H.S. Park, V.M. Pereira and D.K. Campbell
Nanoscale 2015; 7:15300-15309
Abstract
We investigate the impact of strained nanobubbles on the conductance
characteristics of graphene nanoribbons using a combined molecular
dynamics-tight-binding simulation scheme. We describe in detail how the conductance, density of
states, and current density of zigzag or armchair graphene nanoribbons are
modified by the presence of a nanobubble. In particular, we establish that
low-energy electrons can be confined in the vicinity of or within the nanobubbles
by the delicate interplay among the pseudomagnetic field pattern created by
the shape of the bubble, mode mixing, and substrate interaction. The coupling
between confined evanescent states and propagating modes can be enhanced
under different clamping conditions, which translates into Fano resonances in
the conductance traces.
This paper is available in PDF form
.
Analytic Study of Strain Engineering the Electronic Bandgap in Single-Layer Black Phosphorus
J-W Jiang and H.S. Park
Physical Review B 2015; 91:235118
Abstract
We present an analytic study, based on the tight-binding approximation, of strain effects on the electronic bandgap in
single-layer black phosphorus. We obtain an expression for the variation of the bandgap induced by a general strain type
that includes both tension in and out of the plane and shear, and use this to determine the most efficient strain direction
for different strain types, along which the strongest bandgap manipulation can be achieved. We find that the strain direction
that enables the maximum manipulation of the bandgap is not necessarily in the armchair or zigzag direction. Instead, to
achieve the strongest bandgap modulation, the direction of the applied mechanical strain is dependent on the type of applied
strain.
This paper is available in PDF form
.
Coupling Tension and Shear for Highly Sensitive Graphene-Based Strain Sensors
Z.N. Qi, J. Zhang, G.P. Zhang and H.S. Park
2D Materials 2015; 2:035002
Abstract
We report, based on its variation in electronic transport to coupled tension and shear deformation, a highly sensitive
graphene-based strain sensor consisting of an armchair graphene nanoribbon (AGNR) between metallic contacts. As the nominal
strain at any direction increases from 2.5 to 10%, the conductance decreases, particularly when the system changes from
the electrically neutral region. At finite bias voltage, both the raw conductance and the relative proportion of the
conductance depends smoothly on the gate voltage with negligible fluctuations, which is in contrast to that of pristine
graphene. Specifically, when the nominal strain is 10% and the angle varies from 0° to 90°, the relative
proportion of the conductance changes from 60 to ~90%.
This paper is available in PDF form
.
A Gaussian Treatment for the Friction Issue of Lennard-Jones Potential in Layered Materials: Application to Friction
between Graphene, MoS2 and Black Phosphorus
J-W Jiang and H.S. Park
Journal of Applied Physics 2015; 117:124304
Abstract
The Lennard-Jones potential is widely used to describe the interlayer interactions within layered materials like graphene.
However, it is also widely known that this potential strongly underestimates the frictional properties for layered materials.
Here we propose to supplement the Lennard-Jones potential by a Gaussian-type potential, which enables more accurate
calculations of the frictional properties of two-dimensional layered materials. Furthermore, the Gaussian potential is
computationally simple as it introduces only one additional potential parameter that is determined by the interlayer shear
mode in the layered structure. The resulting Lennard-Jones-Gaussian potential is applied to compute the interlayer cohesive
energy and frictional energy for graphene, MoS2, black phosphorus, and their heterostructures.
This paper is available in PDF form
.
A Stillinger-Weber Potential for Single-Layer Black Phosphorus, and the Importance of Cross-Pucker Interactions
for Negative Poisson's Ratio and Edge Stress-Induced Bending
J-W Jiang, T. Rabczuk and H.S. Park
Nanoscale 2015; 7:6059-6068
Abstract
The distinguishing structural feature of single-layer black phosphorus is its puckered structure, which leads to many
novel physical properties. In this work, we first present a new parameterization of the Stillinger-Weber potential
for single-layer black phosphorus. In doing so, we reveal the importance of a cross-pucker interaction term in capturing
its unique mechanical properties, such as a negative Poisson's ratio. In particular, we show that the cross-pucker
interaction enables the pucker to act as a re-entrant hinge, which expands in the lateral direction when it is stretched
in the longitudinal direction. As a consequence, single-layer black phosphorus has a negative Poisson's ratio in the
direction perpendicular to the atomic plane. As an additional demonstration of the impact of the cross-pucker interaction,
we show that it is also the key factor that enables capturing the edge stress-induced bending of single-layer black
phosphorus that has been reported in ab initio calculations.
This paper is available in PDF form
.
A Review on the Flexural Mode of Graphene: Lattice Dynamics, Thermal Conduction, Thermal Expansion,
Elasticity, and Nanomechanical Resonance
J-W Jiang, B-S Wang, J-S Wang and H.S. Park
Journal of Physics: Condensed Matter 2015; 27:083001
Abstract
Single-layer graphene is so flexible that its flexural mode (also called the ZA mode, bending mode, or out-of-plane
transverse acoustic mode) is important for its thermal and mechanical properties. Accordingly, this review focuses
on exploring the relationship between the flexural mode and thermal and mechanical properties of graphene. We first
survey the lattice dynamic properties of the flexural mode, where the rigid translational and rotational invariances
play a crucial role. After that, we outline contributions from the flexural mode in four different physical
properties or phenomena of graphene -- its thermal conductivity, thermal expansion, Young's modulus, and nanomechanical
resonance. We explain how graphene's superior thermal conductivity is mainly due to its three acoustic phonon
modes at room temperature, including the flexural mode. Its coefficient of thermal expansion is negative in a wide
temperature range resulting from the particular vibration morphology of the flexural mode. We then describe how
the Young's modulus of graphene can be extracted from its thermal fluctuations, which are dominated by the flexural
mode. Finally, we discuss the effects of the flexural mode on graphene nanomechanical resonators, while also
discussing how the essential properties of the resonators, including mass sensitivity and quality factor, can be enhanced.
This paper is available in PDF form
.
Atomistic Simulations of Tension-Induced Large Deformation and
Stretchability in Graphene Kirigami
Z.N. Qi, D.K. Campbell and H.S. Park
Physical Review B 2014; 90:245437
Abstract
Graphene's exceptional mechanical properties, including its highest-known stiffness (1 TPa) and strength (100 GPa)
have been exploited for various structural applications. However, graphene is also known to be quite brittle,
with experimentally-measured tensile fracture strains that do not exceed a few percent. In this work, we introduce
the notion of graphene kirigami, where concepts that have been used almost exclusively for macroscale structures are
applied to dramatically enhance the stretchability of both zigzag and armchair graphene. Specifically, we show using
classical molecular dynamics simulations that the yield and fracture strains of graphene can be enhanced by about
a factor of three using kirigami as compared to standard monolayer graphene. Finally, we demonstrate that this
enhanced ductility in graphene may open up interesting opportunities in coupling to graphene's electronic behavior.
This paper is available in PDF form
.
Pseudomagnetic Fields in Graphene Nanobubbles of Constrained Geometry: a
Molecular Dynamics Study
Z.N. Qi, A.L. Kitt, H.S. Park, V.M. Pereira, D.K. Campbell and A.H. Castro Neto
Physical Review B 2014; 90:125419
Abstract
Analysis of the strain-induced pseudomagnetic fields generated in graphene
nanobulges under three different substrate scenarios shows that, in addition to
the shape, the graphene-substrate interaction can crucially determine the
overall distribution and magnitude of strain and those fields, in and outside
the bulge region. We utilize a combination of classical molecular dynamics,
continuum mechanics, and tight-binding electronic structure calculations as an
unbiased means of studying pressure-induced deformations and the resulting
pseudomagnetic field distribution in graphene nanobubbles of various geometries.
The geometry is defined by inflating graphene against a rigid aperture of a
specified shape in the substrate. The interplay among substrate aperture
geometry, lattice orientation, internal gas pressure, and substrate type is
analyzed in view of the prospect of using strain-engineered graphene
nanostructures capable of confining and/or guiding electrons at low energies.
Except in highly anisotropic geometries, the magnitude of the pseudomagnetic
field is generally significant only near the boundaries of the aperture and
rapidly decays towards the center of the bubble because under gas pressure at
the scales considered here there is considerable bending at the edges and the
central region of the nanobubble displays nearly isotropic strain. When
the deflection conditions lead to sharp bends at the edges of the bubble,
curvature and the tilting of the pz orbitals cannot be ignored and
contributes substantially to the total field. The strong and localized nature of
the pseudomagnetic field at the boundaries and its polarity-changing profile can
be exploited as a means of trapping electrons inside the bubble region or of
guiding them in channel-like geometries defined by nano-blister edges. However,
we establish that slippage of graphene against the substrate is an important
factor in determining the degree of concentration of PMFs in or around the bulge
since it can lead to considerable softening of the strain gradients there. The
nature of the substrate emerges thus as a decisive factor determining the
effectiveness of nanoscale pseudomagnetic field tailoring in graphene.
This paper is available in PDF form
.
Negative Poisson's Ratio in Single-Layer Black Phosphorus
J-W Jiang and H.S. Park
Nature Communications 2014; 5:4727
Abstract
The Poisson's ratio is a fundamental mechanical property that relates the resulting lateral strain to applied axial strain.
While this value can theoretically be negative, it is positive for nearly all materials, though negative values have been
observed in so-called auxetic structures. However, nearly all auxetic materials are bulk materials whose microstructure has
been specifically engineered to generate a negative Poisson's ratio. Here, we report using first principles calculations the
existence of a negative Poisson's ratio in a single-layer, two-dimensional material, black phosphorus. In contrast to
engineered bulk auxetics, this behavior is intrinsic for single layer black phosphorus, and originates from its puckered
structure, where the pucker can be regarded as a re-entrant structure that is comprised of two coupled orthogonal hinges.
As a result of this atomic structure, a negative Poisson's ratio is observed in the out-of-plane direction under uniaxial
deformation in the direction parallel to the pucker.
This paper is available in PDF form
.
Mechanical Properties of Single-Layer Black Phosphorus
J-W Jiang and H.S. Park
Journal of Physics D: Applied Physics 2014; 47:385304
Abstract
The mechanical properties of single-layer black phosphrous under uniaxial deformation are investigated using first-principles
calculations. Both Young's modulus and the ideal strain are found to be highly anisotropic and nonlinear as a result of
its quasi-two-dimensional puckered structure. Specifically, the in-plane Young's modulus is 41.3 GPa in the direction
perpendicular to the pucker, and 106.4 GPa in the parallel direction. The ideal strain is 0.48 and 0.11 in the
perpendicular and parallel directions, respectively.
This paper is available in PDF form
.
Mechanical Properties of MoS2/Graphene Heterostructures
J-W Jiang and H.S. Park
Applied Physics Letters 2014; 105:033108
Abstract
We perform classical molecular dynamics simulations to comparatively investigate the mechanical properties of single-layer
MoS2 and a graphene/MoS2/graphene heterostructure under uniaxial tension. We show that the lattice mismatch between
MoS2 and graphene will lead to a spontaneous strain energy in the interface. The Young's modulus of the
heterostructure is significantly larger than that of MoS2. While its stiffness is enhanced, the yield strain of the
heterostructure is considerably smaller than MoS2 due to lateral buckling of the outer graphene layers resulting from
the applied mechanical tension.
This paper is available in PDF form
.
Fermi-Pasta-Ulam Physics With Nanomechanical Graphene Resonators:
Intrinsic Relaxation and Thermalization from Flexural Mode Coupling
D. Midtvedt, A. Croy, A. Isacsson, Z.N. Qi and H.S. Park
Physical Review Letters 2014; 112:145503
Abstract
Thermalization in nonlinear systems is a central concept in statistical mechanics and has been extensively studied
theoretically since the seminal work of Fermi, Pasta and Ulam (FPU). Using molecular dynamics and continuum modeling of a
ring-down setup, we show that thermalization due to nonlinear mode coupling intrinsically limits the quality factor of
nanomechanical graphene drums and turns them into potential test beds for FPU physics. We find the thermalization rate Γ
to be independent of radius and scaling as 
, where 
and 
are effective resonator temperature and prestrain.
This paper is available in PDF form
.
MoS2 Nanoresonators: Intrinsically Better Than Graphene?
J-W Jiang, H.S. Park and T. Rabczuk
Nanoscale 2014; 6:3618-3625
Abstract
We perform classical molecular dynamics simulations to examine the intrinsic energy dissipation in single-layer MoS2
nanoresonators, where a point of emphasis is to compare its dissipation characteristics with those of single-layer graphene.
Our key finding is that MoS2 nanoresonators exhibit significantly lower energy dissipation, and thus higher quality
(Q)-factors by at least a factor of four below room temperature, than graphene. Furthermore, this high Q-factor endows
MoS2 nanoresonators with a higher figure of merit, defined as frequency times Q-factor, despite a resonant frequency
that is 50% smaller than graphene for the same size. By utilizing arguments from phonon-phonon scattering theory, we
show that this reduced energy dissipation is enabled by the large energy gap in the phonon dispersion of MoS2, which
separates the acoustic phonon branches from the optical phonon branches, leading to a preserving mechanism for the resonant
oscillation of MoS2 nanoresonators. We further investigate the effects of tensile mechanical strain and nonlinear
actuation on the Q-factors, where the tensile strain is found to counteract the reductions in Q-factor that occur with
higher actuation amplitudes. Overall, our simulations illustrate the potential utility of MoS2 for high frequency
sensing and actuation applications.
This paper is available in PDF form
.
Adsorbate Migration Effects on Continuous and Discontinuous Temperature-Dependent Transitions in the Quality Factors
of Graphene Nanoresonators
J-W Jiang, B-S Wang, H.S. Park and T. Rabczuk
Nanotechnology 2014; 25:025501
Abstract
We perform classical molecular dynamics simulation to investigate the mechanisms underpinning the unresolved,
experimentally-observed temperature-dependent scaling transition in the quality factors of graphene nanomechanical
resonators (GNMR). Our simulations reveal that the mechanism underlying this temperature scaling phenomenon is the
out-of-plane migration of adsorbates on GNMRs. Specifically, the migrating adsorbate undergoes frequent collisions
with the GNMR, which strongly influences the resulting mechanical oscillation, and thus the quality factors. We also
predict a discontinuous transition in the quality factor at a lower critical temperature, which results from the in-plane
migration of the adsorbate. Overall, our work clearly demonstrates the strong effect of adsorbate migration on the
quality factors of GNMRs.
This paper is available in PDF form
.
Density Functional Theory Calculation of Edge Stresses in Monolayer MoS2
Z.N. Qi, P. Cao and H.S. Park
Journal of Applied Physics 2013; 114:163508
Abstract
We utilize density functional theory to calculate the edge energy and edge stress for monolayer MoS2 nanoribbons.
In contrast to previous reports for graphene, for both armchair and zigzag chiralities, the edge stresses for MoS2
nanoribbons are found to be tensile, indicating that their lowest energy configuration is one of compression in which
Mo-S bond lengths are shorter than those in a bulk, periodic MoS2 monolayer. The edge energy and edge stress is
found to converge for both chiralities for nanoribbon widths larger than about 1 nm.
This paper is available in PDF form
.
Elastic Bending Modulus of Single-Layer Molybdenum Disulphide (MoS2): Finite Thickness Effect
J-W Jiang, Z.N. Qi, H.S. Park and T. Rabczuk
Nanotechnology 2013; 24:435705
Abstract
We derive, from an empirical interaction potential, an analytic formula for the elastic bending modulus of single-layer
MoS2 (SLMoS2). By using this approach, we do not need to define or estimate a thickness value for SLMoS2,
which is important due to the substantial controversy in defining this value for two-dimensional or ultrathin nanostructures
such as graphene and nanotubes. The obtained elastic bending modulus of 9.61 eV in SLMoS2 is significantly higher
than the bending modulus of 1.4 eV in graphene, and is found to be within the range of values that are obtained using thin
shell theory with experimentally obtained values for the elastic constants of SLMoS2. This increase in bending modulus
as compared to monolayer graphene is attributed, through our analytic expression, to the finite thickness of SLMoS2.
Specifically, while each monolayer of S atoms contributes 1.75 eV to the bending modulus, which is similar to the 1.4 eV
bending modulus of monolayer graphene, the additional pairwise and angular interactions between out of plane Mo and S atoms
contribute 5.84 eV to the bending modulus of SLMoS2.
This paper is available in PDF form
.
Molecular Dynamics Simulations of Single-Layer Molybdenum Disulphide (MoS2): Stillinger-Weber Parametrization,
Mechanical Properties, and Thermal Conductivity
J-W Jiang, H.S. Park and T. Rabczuk
Journal of Applied Physics 2013; 114:064307
Abstract
We present a parameterization of the Stillinger-Weber potential to describe the interatomic interactions within single-layer
MoS2 (SLMoS2). The potential parameters are fitted to an experimentally-obtained phonon spectrum, and the resulting
empirical potential provides a good description for the energy gap and the crossover in the phonon spectrum. Using this
potential, we perform classical molecular dynamics simulations to study chirality, size, and strain effects on the Young's
modulus and the thermal conductivity of SLMoS2. We demonstrate the importance of the free edges on the mechanical and
thermal properties of SLMoS2 nanoribbons. Specifically, while edge effects are found to reduce the Young's modulus of
SLMoS2 nanoribbons, the free edges also reduce the thermal stability of SLMoS2 nanoribbons, which may induce
melting well below the bulk melt temperature. Finally, uniaxial strain is found to efficiently manipulate the thermal
conductivity of infinite, periodic SLMoS2.
This paper is available in PDF form
.
Defecting controllability of bombarding graphene with different energetic atoms via reactive force field model
X.Y. Liu, F.C. Wang, H.S. Park and H.A. Wu
Journal of Applied Physics 2013; 114:054313
Abstract
We study the bombardment of a suspended monolayer graphene sheet via different energetic atoms via classical molecular dynamics
based on the reactive force field (ReaxFF). We find that the probability, quality and controllability of defects are mainly
determined by the impact site, the properties of the incident atom and the incident energy. Through comparison with density
functional theory calculations, we demonstrate that defects and vacancies in graphene form only in regions of sufficiently
high electron density. Furthermore, the quality of defects is influenced by the bond order of the incident atom-carbon bonds,
where a higher bond order leads to lower probability of pristine defects (vacancies) but a higher probability of
direct-substitution. Finally, the incident energy plays an important role on the evolution and final pattern of defects in
graphene. Based on the probability, quality and controllability analysis performed, we depict a full-range energy spectrum
for atomic bombardment, where we demonstrate that desirable defects such as single vacancies and direct-substitution can be
created with the appropriate incident energy.
This paper is available in PDF form
.
Resonant Tunneling in Graphene Pseudomagnetic Quantum Dots
Z.N. Qi, D.A. Bahamon, V.M. Pereira, H.S. Park, D.K. Campbell and A.H. Castro Neto
Nano Letters 2013; 13:2692-2697
Abstract
Realistic relaxed configurations of triaxially strained graphene
quantum dots are obtained from unbiased atomistic mechanical
simulations. The local electronic structure and quantum transport
characteristics of y-junctions based on such dots are studied,
revealing that the quasi-uniform pseudomagnetic field induced by
strain restricts transport to Landau level- and edge state-assisted
resonant tunneling. Valley degeneracy is broken in the presence of an
external field, allowing the selective filtering of the valley and
chirality of the states assisting in the resonant tunneling.
Asymmetric strain conditions can be explored to select the exit
channel of the y-junction.
This paper is available in PDF form
.
How graphene slides: Measurement and theory of strain-dependent frictional forces between graphene and SiO2
A.L. Kitt, Z.N. Qi, S. Remi, H.S. Park, A.K. Swan and B.B. Goldberg
Nano Letters 2013; 13:2605-2610
Abstract
Strain, bending rigidity, and adhesion are interwoven in determining how graphene responds when pulled across a substrate. Using Raman spectroscopy of
circular, graphene-sealed microchambers under variable external pressure, we demonstrate that graphene is not firmly anchored to the substrate when pulled.
Instead, as the suspended graphene is pushed into the chamber under pressure, the supported graphene outside the microchamber is stretched and slides,
pulling in an annulus. Analyzing Raman G band line scans with a continuum model extended to include sliding, we extract the pressure dependent sliding
friction between the SiO2 substrate and mono-, bi-, and tri-layer graphene. The sliding friction for trilayer graphene is directly proportional to the
applied load, but the friction for monolayer and bilayer graphene is inversely proportional to the strain in the graphene, in violation of Amontons' law.
We attribute this behavior to the high surface conformation enabled by the low bending rigidity and strong adhesion of few layer graphene.
This paper is available in PDF form
.
Enhancing the Mass Sensitivity of Graphene Nanoresonators Via Nonlinear Oscillations: The Effective Strain Mechanism
J-W Jiang, H.S. Park and T. Rabczuk
Nanotechnology 2012; 23:475501
Abstract
We perform classical molecular dynamics simulations to investigate the enhancement of the mass sensitivity and resonant frequency of graphene
nanomechanical resonators that is achieved by driving them into the nonlinear oscillation regime. The mass sensitivity as measured by the resonant
frequency shift is found to triple if the actuation energy is about 2.5 times the initial kinetic energy of the nanoresonator. The mechanism
underlying the enhanced mass sensitivity is found to be the effective strain that is induced in the nanoresonator due to the nonlinear oscillations,
where we obtain an analytic relationship between the induced effective strain and the actuation energy that is applied to the graphene nanoresonator.
An important implication of this work is that there is no need for experimentalists to apply tensile strain to the resonators before actuation in order
to enhance the mass sensitivity. Instead, enhanced mass sensitivity can be obtained by the far simpler technique of actuating nonlinear oscillations
of an existing graphene nanoresonator.
This paper is available in PDF form
.
Intrinsic Energy Dissipation in CVD-Grown Graphene Nanoresonators
Z.N. Qi and H.S. Park
Nanoscale 2012; 4:3460-3465
Abstract
We utilize classical molecular dynamics to study the the quality (Q)-factors of monolayer CVD-grown graphene nanoresonators. In particular,
we focus on the effects of intrinsic grain boundaries of different orientations, which result from the CVD growth process, on the Q-factors.
For a range of misorientations orientation angles that are consistent with those seen experimentally in CVD-grown graphene, i.e. 0 to 20 degrees,
we find that the Q-factors for graphene with intrinsic grain boundaries are 1-2 orders of magnitude smaller than that of pristine monolayer graphene.
We find that the Q-factor degradation is strongly influenced by both the symmetry and structure of the 5-7 defect pairs that occur at the grain
boundary. Because of this, we also demonstrate that find the Q-factors CVD-grown graphene can be significantly elevated, and approach that of
pristine graphene, through application of modest (1%) tensile strain.
This paper is available in PDF form
.
On The Effective Plate Thickness of Monolayer Graphene from Flexural Wave Propagation
S.Y. Kim and H.S. Park
Journal of Applied Physics 2011; 110:054324
Abstract
We utilize classical molecular dynamics to study flexural, or transverse wave propagation in monolayer graphene sheets, and compare
the resulting dispersion relationships to those expected from continuum thin plate theory. In doing so, we determine that regardless
of the chirality for monolayer graphene, transverse waves exhibit a dispersion relationship that corresponds to the lowest order
antisymmetric (A0) mode of wave propagation in a thin plate with plate thickness of h=0.104 nm. Finally, we find that the
achievable wave speeds in monolayer graphene are found to exceed those reported previously for single walled carbon nanotubes,
while the frequency of wave propagation in the graphene monolayer is found to reach the terahertz range, similar to that of
carbon nanotubes.
This paper is available in PDF form
.
Size-Dependence of the Nonlinear Elastic Softening of Nanoscale Graphene Monolayers Under Plane-Strain Bulge Tests: A Molecular Dynamics Study
S. Jun, T. Tashi and H.S. Park
Journal of Nanomaterials 2011; 380286.
(Invited paper: Special issue on Low-Dimensional Carbon Nanomaterials: Synthesis, Properties, and Applications)
Abstract
The pressure bulge test is an experimental technique to
characterize the mechanical properties of microscale thin films.
Here we perform constant-temperature molecular dynamics
simulations of the plane-strain cylindrical bulge test of
nano-sized monolayer graphene subjected to high gas pressure
induced by hydrogen molecules. We observe a nonlinear elastic
softening of the graphene with an increase in hydrogen pressure
due to the stretching and weakening of the carbon-carbon bonds; we
further observe that this softening behavior depends upon the size
of the graphene monolayers. Our simulation results suggest that
the traditional microscale bulge formulas, which assume constant
elastic moduli, should be modified to incorporate the size
dependence and elastic softening that occur in nano-sized graphene
bulge tests.
This paper is available in PDF form
.
Boundary Condition and Strain Effects on the Quality Factors of Single Walled Carbon Nanotubes
S.Y. Kim and H.S. Park
Journal of Computational and Theoretical Nanoscience 2011; 8:814-819.
(Invited paper: Special Issue on Multiscale and Multiphysics Simulations for Energy Applications)
Abstract
We utilize classical molecular dynamics to study energy dissipation (the Q-factors) of carbon nanotube-based nanoresonators
undergoing flexural oscillations. Specifically, we have studied the difference in Q-factors of nanotubes with fixed/fixed and
fixed/free boundary conditions. In doing so, we have found that fixed/fixed nanotubes have significantly higher Q-factors,
particularly at low temperatures. Furthermore, we have found that mechanical strain can be utilized to enhance the Q-factors of
fixed/fixed nanotubes by factors of 2-4 across a range of temperatures for tensile strains ranging from 0 to 6%. The results
collectively indicate that fixed/fixed carbon nanotubes should be preferable for NEMS applications at low temperature due to a
combination of inherently higher Q-factors, and the fact that the Q-factors can be further improved through the application of
tensile strain.
This paper is available in PDF form
.
A Molecular Simulation Analysis of Producing Monatomic Carbon Chains by Stretching Ultranarrow Graphene Nanoribbons
Z.N. Qi, F.P. Zhao, X.Z. Zhou, H.S. Park and H.A. Wu
Nanotechnology 2010; 21:265702.
Abstract
Atomistic simulations were utilized to develop fundamental insights regarding the elongation process starting from ultranarrow graphene
nanoribbons (GNRs) and resulting in monatomic carbon chains (MACCs). There are three key findings. First, we demonstrate that complete,
elongated, and stable MACCs with fracture strains exceeding 100% can be formed from both ultranarrow armchair and zigzag GNRs. Second,
we demonstrate that the deformation processes leading to the MACCs have strong chirality dependence. Specifically, armchair GNRs first
form DNA-like chains, then develop into monatomic chains by passing through an intermediate configuration in which monatomic chain
sections are separated by two-atom attachments. In contrast, zigzag GNRs form rope-ladder-like chains through a process in which the
carbon hexagons are first elongated into rectangles; these rectangles eventually coalesce into monatomic chains through a novel
triangle-pentagon deformation structure under further tensile deformation. Finally, we show that the width of GNRs plays an important
role in the formation of MACCs, and that the ultranarrow GNRs facilitate the formation of full MACCs. The present work should be of
considerable interest due to the experimentally demonstrated feasibility of using narrow GNRs to fabricate novel nanoelectronic
components based upon monatomic chains of carbon atoms.
This paper is available in PDF form
.
On the Utility of Vacancies and Tensile Strain-Induced Quality Factor Enhancement for Mass Sensing Using Graphene Monolayers
S. Y. Kim and H. S. Park
Nanotechnology 2010; 21:105710.
Abstract
We have utilized classical molecular dynamics to investigate the mass sensing potential of graphene monolayers, using gold as the
model adsorbed atom. In doing so, we report two key findings. First, we find that while perfect graphene monolayers are effective
mass sensors at very low (T<10K) temperatures, their mass sensing capability is lost at higher temperatures due to diffusion of
the adsorbed atom at elevated temperatures. We demonstrate that even if the quality (Q) factors are significantly elevated through
the application of tensile mechanical strain, the mass sensing resolution is still lost at elevated temperatures, which demonstrates
that high Q-factors alone are insufficient to ensure the mass sensing capability of graphene. Second, we find that while the
introduction of single vacancies into the graphene monolayer prevents the diffusion of the adsorbed atom, the mass sensing resolution
is still lost at higher temperatures, again due to Q-factor degradation. We finally demonstrate that if the Q-factors of the graphene
monolayers with single vacancies are kept acceptably high through the application of tensile strain, that the high Q-factors, in
conjunction with the single atom vacancies to stop the diffusion of the adsorbed atom, enables graphene to maintain its mass sensing
capability across a range of technologically relevant operating temperatures.
This paper is available in PDF form
.
Multilayer Friction and Attachment Effects on Energy Dissipation in Graphene Nanoresonators
S. Y. Kim and H. S. Park
Applied Physics Letters 2009; 94:101918.
(Also selected for publication in the Virtual Journal of Nanoscale Science and Technology, March 30, 2009).
Abstract
We utilize classical molecular dynamics to study the effects of intrinsic, interlayer friction between graphene
monolayers, as well as extrinsic attachment or clamping strength between graphene and a model silicon substrate
on the energy dissipation (Q-factors) of oscillating graphene nanoresonators. Both interlayer friction and attachment
effects are found to significantly degrade the graphene Q-factors, with an increase in energy dissipation with increasing
temperature, while both effects are found to be strongly dependent on the strength of the van der Waal's interactions, either
between adjacent layers of graphene or between graphene and the underlying substrate.
This paper is available in PDF form
.
The Importance of Edge Effects on the Intrinsic Loss Mechanisms of Graphene Nanoresonators
S. Y. Kim and H. S. Park
Nano Letters 2009; 9(3):969-974.
Abstract
We utilize classical molecular dynamics simulations to investigate the intrinsic loss mechanisms of monolayer graphene nanoresonators
undergoing flexural oscillations. We find that spurious edge modes of vibration, which arise not due to externally applied stresses
but intrinsically due to the different vibrational properties of edge atoms, are the dominant intrinsic loss mechanism that reduces
the Q-factors. We additionally find that while hydrogen passivation of the free edges is ineffective in reducing the spurious edge
modes, fixing the free edges is critical to removing the spurious edge-induced vibrational states. Our atomistic simulations also
show that the Q-factor degrades inversely proportional to temperature; however, we also demonstrate that the intrinsic losses can be
reduced significantly across a range of operating temperatures through the application of tensile mechanical strain.
This paper is available in PDF form
.
Action-Derived Ab Initio Molecular Dynamics
S. Jun, S. Pendurti, I.-H. Lee, S. Y. Kim, H. S. Park and Y.-H. Kim
International Journal of Applied Mechanics 2009; 1:469-482.
Abstract
Action-derived molecular dynamics (ADMD) is a numerical method to search for minimum-energy dynamic pathways on potential-energy
surface. The method is based on the Hamilton's least action principle and has been developed for problems of activated processes,
rare events, and long-time simulations. In this paper, ADMD is further extended to incorporate ab initio total-energy calculations,
which enables the detailed electronic analysis of transition states as well as the exploration of energy landscapes. Three numerical
examples are solved to demonstrate the capability of this action-derived ab initio molecular dynamics. The proposed approach is
expected to circumvent the severe time-scale limitation of conventional ab intio molecular dynamics simulations.
This paper is available in PDF form
.