66 research outputs found
Engineering Phonon Polaritons in van der Waals Heterostructures to Enhance In-Plane Optical Anisotropy
Van der Waals heterostructures assembled from layers of 2D materials have
attracted considerable interest due to their novel optical and electrical
properties. Here we report a scattering-type scanning near field optical
microscopy study of hexagonal boron nitride on black phosphorous (h-BN/BP)
heterostructures, demonstrating the first direct observation of in-plane
anisotropic phonon polariton modes in vdW heterostructures. Strikingly, the
measured in-plane optical anisotropy along armchair and zigzag crystal axes
exceeds the ratio of refractive indices of BP in the x-y plane. We explain that
this enhancement is due to the high confinement of the phonon polaritons in
h-BN. We observe a maximum in-plane optical anisotropy of {\alpha}_max=1.25 in
the 1405-1440 cm-1 frequency spectrum. These results provide new insights on
the behavior of polaritons in vdW heterostructures, and the observed anisotropy
enhancement paves the way to novel nanophotonic devices and to a new way to
characterize optical anisotropy in thin films
Modulating the electrochemical intercalation of graphene interfaces with -RuCl as a solid-state electron acceptor
Intercalation reactions modify the charge density in van der Waals (vdW)
materials through coupled electronic-ionic charge accumulation, and are
susceptible to modulation by interlayer hybridization in vdW heterostructures.
Here, we demonstrate that charge transfer between graphene and
-RuCl, which dopes the graphene positively, greatly favors the
intercalation of lithium ions into graphene-based vdW heterostructures. We
systematically tune this effect on Li ion intercalation, modulating the
intercalation potential, by using varying thicknesses of hexagonal boron
nitride (hBN) as spacer layers between graphene and -RuCl. Confocal
Raman spectroscopy and electronic transport measurements are used to monitor
electrochemical intercalation and density functional theory computations help
quantify charge transfer to both -RuCl and graphene upon Li
intercalation. This work demonstrates a versatile approach for systematically
modulating the electrochemical intercalation behavior of two-dimensional layers
akin to electron donating/withdrawing substituent effects used to tune
molecular redox potentials
Atomic engineering of interfacial polarization switching in van der Waals multilayers
In conventional ferroelectric materials, polarization is an intrinsic
property limited by bulk crystallographic structure and symmetry. Recently, it
has been demonstrated that polar order can also be accessed using inherently
non-polar van der Waals materials through layer-by-layer assembly into
heterostructures, wherein interfacial interactions can generate spontaneous,
switchable polarization. Here, we show that introducing interlayer rotations in
multilayer vdW heterostructures modulates both the spatial ordering and
switching dynamics of polar domains, engendering unique tunability that is
unparalleled in conventional bulk ferroelectrics or polar bilayers. Using
operando transmission electron microscopy we show how changing the relative
rotations of three WSe2 layers produces structural polytypes with distinct
arrangements of polar domains, leading to either a global or localized
switching response. Introducing uniaxial strain generates structural anisotropy
that yields a range of switching behaviors, coercivities, and even tunable
biased responses. We also provide evidence of physical coupling between the two
interfaces of the trilayer, a key consideration for controlling switching
dynamics in polar multilayer structures more broadly.Comment: 22 pages, 5 figure
Local atomic stacking and symmetry in twisted graphene trilayers
Moir\'e superlattices formed from twisting trilayers of graphene are an ideal
model for studying electronic correlation, and offer several advantages over
bilayer analogues, including more robust and tunable superconductivity and a
wide range of twist angles associated with flat band formation. Atomic
reconstruction, which strongly impacts the electronic structure of twisted
graphene structures, has been suggested to play a major role in the relative
versatility of superconductivity in trilayers. Here, we exploit an
inteferometric 4D-STEM approach to image a wide range of trilayer graphene
structures. Our results unveil a considerably different model for moir\'e
lattice relaxation in trilayers than that proposed from previous measurements,
informing a thorough understanding of how reconstruction modulates the atomic
stacking symmetries crucial for establishing superconductivity and other
correlated phases in twisted graphene trilayers.Comment: 18 pages, 5 figure
Strain fields in twisted bilayer graphene
Van der Waals heteroepitaxy allows deterministic control over lattice
mismatch or azimuthal orientation between atomic layers to produce long
wavelength superlattices. The resulting electronic phases depend critically on
the superlattice periodicity as well as localized structural deformations that
introduce disorder and strain. Here, we introduce Bragg interferometry, based
on four-dimensional scanning transmission electron microscopy, to capture
atomic displacement fields in twisted bilayer graphene with twist angles <
2{\deg}. Nanoscale spatial fluctuations in twist angle and uniaxial
heterostrain are statistically evaluated, revealing the prevalence of
short-range disorder in this class of materials. By quantitatively mapping
strain tensor fields we uncover two distinct regimes of structural relaxation
-- in contrast to previous models depicting a single continuous process -- and
we disentangle the electronic contributions of the rotation modes that comprise
this relaxation. Further, we find that applied heterostrain accumulates
anisotropically in saddle point regions to generate distinctive striped shear
strain phases. Our results thus establish the reconstruction mechanics
underpinning the twist angle dependent electronic behaviour of twisted bilayer
graphene, and provide a new framework for directly visualizing structural
relaxation, disorder, and strain in any moir\'e material.Comment: 29 pages, 6 figures plus supporting information (42 pages, 28
figures
Rotational and Dilational Reconstruction in Transition Metal Dichalcogenide Moir\'e Bilayers
Lattice reconstruction and corresponding strain accumulation play a key role
in defining the electronic structure of two-dimensional moir\'e superlattices,
including those of transition metal dichalcogenides (TMDs). Imaging of TMD
moir\'es has so far provided a qualitative understanding of this relaxation
process in terms of interlayer stacking energy, while models of the underlying
deformation mechanisms have relied on simulations. Here, we use interferometric
four-dimensional scanning transmission electron microscopy to quantitatively
map the mechanical deformations through which reconstruction occurs in
small-angle twisted bilayer MoS2 and WSe2/MoS2 heterobilayers. We provide
direct evidence that local rotations govern relaxation for twisted
homobilayers, while local dilations are prominent in heterobilayers possessing
a sufficiently large lattice mismatch. Encapsulation of the moir\'e layers in
hBN further localizes and enhances these in-plane reconstruction pathways,
suppressing out-of-plane corrugation. We also find that extrinsic uniaxial
heterostrain, which introduces a lattice constant difference in twisted
homobilayers, leads to accumulation and redistribution of reconstruction
strain, demonstrating another route to modify the moir\'e potential.Comment: 27 pages, 5 figure
Water Oxidation Catalysis by Co(II) Impurities in Co(III)4O4 Cubanes
The observed water oxidation activity of the compound class Co4O4(OAc)4(Py–X)4 emanates from a Co(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co(II) impurity as the major source of water oxidation activity that has been reported for Co4O4 molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis
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