20 research outputs found
Highly Stretchable MoS Kirigami
We report the results of classical molecular dynamics simulations focused on
studying the mechanical properties of MoS 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 finding is
significant enhancements in tensile yield (by a factor of four) and fracture
strains (by a factor of six) as compared to pristine MoS nanoribbons.
These results in conjunction with recent results on graphene suggest that the
kirigami approach may be a generally useful one for enhancing the ductility of
two-dimensional nanomaterials
Polarization and valley switching in monolayer group-IV monochalcogenides
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 are dictated
by the puckering orientation, can be excited selectively using linearly
polarized light, and this provides an 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
Accelerated search and design of stretchable graphene kirigami using machine learning
Making kirigami-inspired cuts into a sheet has been shown to be an effective way of designing stretchable materials with metamorphic properties where the 2D shape can transform into complex 3D shapes. However, finding the optimal solutions is not straightforward as the number of possible cutting patterns grows exponentially with system size. Here, we report on how machine learning (ML) can be used to approximate the target properties, such as yield stress and yield strain, as a function of cutting pattern. Our approach enables the rapid discovery of kirigami designs that yield extreme stretchability as verified by molecular dynamics (MD) simulations. We find that convolutional neural networks, commonly used for classification in vision tasks, can be applied for regression to achieve an accuracy close to the precision of the MD simulations. This approach can then be used to search for optimal designs that maximize elastic stretchability with only 1000 training samples in a large design space of ∼4×106 candidate designs. This example demonstrates the power and potential of ML in finding optimal kirigami designs at a fraction of iterations that would be required of a purely MD or experiment-based approach, where no prior knowledge of the governing physics is known or available.P. Z. H. developed the codes, performed the simulations and data analysis, and wrote the manuscript with input from all authors. P. Z. H. and E. D. C. developed the machine learning methods. P. Z. H., D. K. C. and H. S. P. acknowledge the Hariri Institute Research Incubation Grant No. 2018-02-002 and the Boston University High Performance Shared Computing Cluster. P. Z. H. is grateful for the Hariri Graduate Fellowship. P. Z. H. thank Grace Gu and Adrian Yi for helpful discussions. (2018-02-002 - Hariri Graduate Fellowship)Published versio
Curvature as an external field in mechanical antiferromagnets
A puckered sheet is a freestanding crystalline membrane with an embedded
array of bistable buckled units. Recent work has shown that the bistable units
behave like spins in a two-dimensional compressible Ising antiferromagnet with,
however, a coupling to flexural phonons. At finite temperature, this purely
mechanical system displays Ising-like phase transitions, which drive anomalous
thermal expansion. Here, we show that geometry can be used to control phase
behavior: curvature produces a radius-dependent "external field" that
encourages alignment between neighboring "spins," disrupting the ordered
checkerboard ground state of anti-aligned neighbors. The effective field
strength scales as the inverse of the radius of curvature. We identify this
effective field theoretically with both a discrete real space model and a
nonlinear continuum elastic model. We then present molecular dynamics
simulations of puckered sheets in cylindrical geometries at zero and finite
temperature, probing the influence of curvature on the stability of
configurations and phase transitions. Our work demonstrates how curvature and
temperature can be used to design and operate a responsive and tunable
metamaterial at either the macroscale or nanoscale.Comment: 19 pages, 10 figure
Strain-induced gauge and Rashba fields in ferroelectric Rashba lead chalcogenide PbX monolayers (X = S, Se, Te)
One of the exciting features of two-dimensional (2D) materials is their
electronic and optical tunability through strain engineering. Previously, we
found a 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 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
Two-dimensional square buckled Rashba lead chalcogenides
We propose the lead sulphide (PbS) monolayer as a two-dimensional 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 large effective Rashba parameters of λ∼5 eV Å and λ∼1 eV Å, 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 strain to vary the spin texture as well as the Rashba parameter 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.P.Z.H., H.S.P., and D.K.C. acknowledge the support of the Physics and Mechanical Engineering Department at Boston University. P.Z.H. is grateful for the hospitality of the NUS Centre for Advanced 2D Materials and Graphene Research Centre where this work was initiated. D.K.C. acknowledges the hospitality of the Aspen Center for Physics, which is supported by the US National Science Foundation Grant No. PHY-1607611. A.S.R., A.C.,and A.H.C.N. acknowledge support by the National Research Foundation, Prime Minister Office, Singapore, under its Medium Sized Centre Programme and CRP award "Novel 2D materials with tailored properties: Beyond graphene" (Grant No. R-144-000295-281). (Physics and Mechanical Engineering Department at Boston University; PHY-1607611 - US National Science Foundation; R-144-000295-281 - National Research Foundation, Prime Minister Office, Singapore, under its Medium Sized Centre Programme and CRP award "Novel 2D materials with tailored properties: Beyond graphene")Published versio
Spin-Orbit Dirac Fermions in 2D Systems
We propose a novel model for including spin-orbit interactions in buckled two
dimensional systems. Our results show that in such systems, intrinsic
spin-orbit coupling leads to a formation of Dirac cones, similar to Rashba
model. We explore the microscopic origins of this behaviour and confirm our
results using DFT calculations
Kirigami Actuators
Thin elastic sheets bend easily and, if they are patterned with cuts, can
deform in sophisticated ways. Here we show that carefully tuning the location
and arrangement of cuts within thin sheets enables the design of mechanical
actuators that scale down to atomically-thin 2D materials. We first show that
by understanding the mechanics of a single, non-propagating crack in a sheet we
can generate four fundamental forms of linear actuation: roll, pitch, yaw, and
lift. Our analytical model shows that these deformations are only weakly
dependent on thickness, which we confirm with experiments at centimeter scale
objects and molecular dynamics simulations of graphene and MoS nanoscale
sheets. We show how the interactions between non-propagating cracks can enable
either lift or rotation, and we use a combination of experiments, theory,
continuum computational analysis, and molecular dynamics simulations to provide
mechanistic insights into the geometric and topological design of kirigami
actuators.Comment: Soft Matter, 201