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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
.