29 research outputs found
Strain engineering of graphene
The focus of this thesis is on using mechanical strain to tailor the electronic properties
of graphene. The first half covers the electro-mechanical coupling for graphene
in different configurations, namely a hexagonal Y-junction, various shaped bubbles on
different substrates, and with kirigami cuts. For all of these cases, a novel combination
of tight-binding electronic structure calculations and molecular dynamics is utilized
to demonstrate how mechanical loading and deformation impacts the resulting electronic
structure and transport. For the Y-junction, a quasi-uniform pseudo magnetic
field induced by strain restricts transport to Landau-level and edge-state-assisted resonant tunneling. For the bubbles, the shape and the nature of the substrate emerge
as decisive factors determining the effectiveness of the nanoscale pseudo magnetic
field tailoring in graphene. Finally, for the kirigami, it is shown that the yield and
fracture strains of graphene, a well-known brittle material, can be enhanced by a factor
of more than three using the kirigami structure, while also leading to significant
enhancements in the localized pseudo magnetic fields.
The second part of the thesis focuses on dissipation mechanisms in graphene
nanomechanical resonators. 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, it is shown 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. The relationship between thermalization rate, radius, temperature and prestrain is explored and investigated
Highly Deformable Graphene Kirigami
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. This
enhanced ductility in graphene should open up interesting opportunities not
only mechanically, but also in coupling to graphene's electronic behavior.Comment: 5 pages, 7 figure
FPU physics with nanomechanical graphene resonators: intrinsic relaxation and thermalization from flexural mode coupling
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 to be independent of radius and scaling as
, where and
are effective resonator temperature and prestrain
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
Graphene kirigami as a platform for stretchable and tunable quantum dot arrays
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 elastic deformation that characterize the elongation of these
structures. Notably, the effective coupling between localized states is
strongly reduced in the small elongation stage, whereas in the large elongation
regime the development of strong, localized pseudomagnetic field barriers can
reinforce the coupling and reestablish resonant tunneling across the kirigami.
This provides an interesting example of interplay between geometry and
pseudomagnetic field-induced confinement. The alternating miniband and
stop-gaps in the transmission lead 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.Comment: 10 pages, 10 figure