9 research outputs found
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
Entering Flatland in One Piece: Growth and Electronic Transport Measurements of Single Crystal Two-Dimensional Conductive MOFs.
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Local atomic stacking and symmetry in twisted graphene trilayers
Moiré superlattices formed by twisting trilayers of graphene are a useful model for studying correlated electron behaviour and offer several advantages over their formative bilayer analogues, including a more diverse collection of correlated phases and more robust superconductivity. Spontaneous structural relaxation alters the behaviour of moiré superlattices considerably and has been suggested to play an important role in the relative stability of superconductivity in trilayers. Here we use an interferometric four-dimensional scanning transmission electron microscopy approach to directly probe the local graphene layer alignment over a wide range of trilayer graphene structures. Our results inform a thorough understanding of how reconstruction modulates the local lattice symmetries crucial for establishing correlated phases in twisted graphene trilayers, evincing a relaxed structure that is markedly different from that proposed previously
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Tunable angle-dependent electrochemistry at twisted bilayer graphene with moiré flat bands
Tailoring electron transfer dynamics across solid-liquid interfaces is fundamental to the interconversion of electrical and chemical energy. Stacking atomically thin layers with a small azimuthal misorientation to produce moiré superlattices enables the controlled engineering of electronic band structures and the formation of extremely flat electronic bands. Here, we report a strong twist-angle dependence of heterogeneous charge transfer kinetics at twisted bilayer graphene electrodes with the greatest enhancement observed near the 'magic angle' (~1.1°). This effect is driven by the angle-dependent tuning of moiré-derived flat bands that modulate electron transfer processes with the solution-phase redox couple. Combined experimental and computational analysis reveals that the variation in electrochemical activity with moiré angle is controlled by a structural relaxation of the moiré superlattice at twist angles of <2°, and 'topological defect' AA stacking regions, where flat bands are localized, produce a large anomalous local electrochemical enhancement that cannot be accounted for by the elevated local density of states alone
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Rotational and dilational reconstruction in transition metal dichalcogenide moiré bilayers.
Lattice reconstruction and corresponding strain accumulation plays a key role in defining the electronic structure of two-dimensional moiré superlattices, including those of transition metal dichalcogenides (TMDs). Imaging of TMD moirés 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 moiré layers in hBN further localizes and enhances these in-plane reconstruction pathways by 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 moiré potential