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Controlling the localization of mobile charges in solids enables the discovery of correlated physical phenomena, but applying it for the development of next-generation electronics requires achieving such control under practical conditions. In this study, we report room-temperature, switchable charge localization in high-quality bilayer transistors that comprise a monolayer of molecular crystal on top of a monolayer semiconductor. By using an ion gate, we selectively populated either localized molecular states or semiconductor band states, achieving complete localization from mobile charges at densities up to 3 × 10¹³per square centimeter. This transition was energetically stabilized by the formation of coupled electron-ion dipoles, which could be tuned through Coulomb engineering. These properties further enabled single-band ambipolar transistor operation without substitutional dopants, demonstrating the potential of electron-ion correlations for practical electronic applications. 

ABSTRACT The ability to tune electronic structure in twisted stacks of layered, two‐dimensional (2D) materials has motivated the exploration of similar moiré physics with stacks of twisted oxide membranes. Due to the intrinsic three‐dimensional nature of bonding in many oxides, achieving atomic‐level coupling is significantly more challenging than in 2D materials. Although clean interfaces with atomic‐level proximity have been demonstrated in bulk ceramic bicrystals using high‐temperature and high‐pressure processing to facilitate atomic diffusion that flattens rough interfaces, such conditions are not readily accessible when bonding oxide membranes. This study shows how topographic mismatch from surface roughness of the membranes restricts atomic‐scale proximity at the interface to isolated patches even after contaminants and amorphous interlayers are eliminated. The reduced ability of 2D materials to conform to a membrane's step‐terrace topography also limits atomic‐scale contact. In all these material systems, the interface morphology is best characterized using cross‐sectional imaging and is necessary to corroborate investigations of interlayer coupling. When imaging the stacked membranes in projection, conventional through‐focal imaging is found to be insensitive to the buried interface, whereas electron ptychography reliably resolves structural variations on the order of a nanometer. These findings highlight interface roughness as a key challenge for oxide twistronics. 

The efficient, large-scale generation and control of photonic modes guided by van der Waals materials remains as a challenge despite their potential for on-chip photonic circuitry. We report three-atom-thick waveguides—δ waveguides—based on wafer-scale molybdenum disulfide (MoS₂) monolayers that can guide visible and near-infrared light over millimeter-scale distances with low loss and an efficient in-coupling. The extreme thinness provides a light-trapping mechanism analogous to a δ-potential well in quantum mechanics and enables the guided waves that are essentially a plane wave freely propagating along the in-plane, but confined along the out-of-plane, direction of the waveguide. We further demonstrate key functionalities essential for two-dimensional photonics, including refraction, focusing, grating, interconnection, and intensity modulation, by integrating thin-film optical components with δ waveguides using microfabricated dielectric, metal, or patterned MoS₂. 

Abstract Twisted 2D materials form complex moiré structures that spontaneously reduce symmetry through picoscale deformation within a mesoscale lattice. We show twisted 2D materials contain a torsional displacement field comprised of three transverse periodic lattice distortions (PLD). The torsional PLD amplitude provides a single order parameter that concisely describes the structural complexity of twisted bilayer moirés. Moreover, the structure and amplitude of a torsional periodic lattice distortion is quantifiable using rudimentary electron diffraction methods sensitive to reciprocal space. In twisted bilayer graphene, the torsional PLD begins to form at angles below 3.89° and the amplitude reaches 8 pm around the magic angle of 1. 1°. At extremely low twist angles (e.g. below 0.25°) the amplitude increases and additional PLD harmonics arise to expand Bernal stacked domains separated by well defined solitonic boundaries. The torsional distortion field in twisted bilayer graphene is analytically described and has an upper bound of 22.6 pm. Similar torsional distortions are observed in twisted WS 2 , CrI 3 , and WSe 2 /MoSe 2 .