Engineering 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 deliberate interlayer rotations in multilayer van der Waals heterostructures modulate both the spatial ordering and switching dynamics of polar domains. The engendered tunability is unparalleled in conventional bulk ferroelectrics or polar bilayers. By means of operando transmission electron microscopy we show how alterations of the relative rotations of three WSe2 layers produce structural polytypes with distinct arrangements of polar domains with either a global or localized switching response. Furthermore, the presence of uniaxial strain generates structural anisotropy that yields a range of switching behaviours, coercivities and even tunable biased responses. We also provide evidence of mechanical coupling between the two interfaces of the trilayer, a key consideration for the control of switching dynamics in polar multilayer structures more broadly.

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.

Modulating the Electrochemical Intercalation of Graphene Interfaces with α-RuCl3 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 α-RuCl3, which hole-dopes the graphene, 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 α-RuCl3. Confocal Raman spectroscopy and electronic transport measurements are used to monitor electrochemical intercalation, and density functional theory computations help quantify charge transfer to both α-RuCl3 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.

Consequences and Control of Multiscale Order/Disorder in Chiral Magnetic Textures.

Transition metal intercalated transition metal dichalcogenides (TMDs) are promising platforms for next-generation spintronic devices based on their wide range of electronic and magnetic phases, which can be tuned by varying the host lattice or intercalant's identity, stoichiometry, or spatial order. Some of these compounds host a chiral magnetic phase in which the helical winding of magnetic moments propagates along a high-symmetry crystalline axis. Previous studies have demonstrated that variation in intercalant concentrations can have a dramatic effect on the formation of chiral domains and ensemble magnetic properties. However, a systematic and comprehensive study of how atomic-scale order and disorder impact these chiral magnetic textures is so far lacking. Here, we leverage a combination of imaging modes in the (scanning) transmission electron microscope (S/TEM) to directly probe (dis)order across multiple length scales and show how subtle changes in the atomic lattice can tune the mesoscale spin textures and bulk magnetic response in Cr1/3NbS2, with direct implications for the fundamental understanding and technological implementation of such compounds.

Encoding multistate charge order and chirality in endotaxial heterostructures.

High-density phase change memory (PCM) storage is proposed for materials with multiple intermediate resistance states, which have been observed in 1T-TaS2 due to charge density wave (CDW) phase transitions. However, the metastability responsible for this behavior makes the presence of multistate switching unpredictable in TaS2 devices. Here, we demonstrate the fabrication of nanothick verti-lateral H-TaS2/1T-TaS2 heterostructures in which the number of endotaxial metallic H-TaS2 monolayers dictates the number of resistance transitions in 1T-TaS2 lamellae near room temperature. Further, we also observe optically active heterochirality in the CDW superlattice structure, which is modulated in concert with the resistivity steps, and we show how strain engineering can be used to nucleate these polytype conversions. This work positions the principle of endotaxial heterostructures as a promising conceptual framework for reliable, non-volatile, and multi-level switching of structure, chirality, and resistance.

Comparative Electronic Structures of the Chiral Helimagnets Cr1/3NbS2 and Cr1/3TaS2.

Magnetic materials with noncollinear spin textures are promising for spintronic applications. To realize practical devices, control over the length and energy scales of such spin textures is imperative. The chiral helimagnets Cr1/3NbS2 and Cr1/3TaS2 exhibit analogous magnetic-phase diagrams with different real-space periodicities and field dependence, positioning them as model systems for studying the relative strengths of the microscopic mechanisms giving rise to exotic spin textures. Although the electronic structure of the Nb analogue has been experimentally investigated, the Ta analogue has received far less attention. Here, we present a comprehensive suite of electronic structure studies on both Cr1/3NbS2 and Cr1/3TaS2 using angle-resolved photoemission spectroscopy and density functional theory. We show that bands in Cr1/3TaS2 are more dispersive than their counterparts in Cr1/3NbS2, resulting in markedly different Fermi wavevectors. The fact that their qualitative magnetic phase diagrams are nevertheless identical shows that hybridization between the intercalant and host lattice mediates the magnetic exchange interactions in both of these materials. We ultimately find that ferromagnetic coupling is stronger in Cr1/3TaS2, but larger spin-orbit coupling (and a stronger Dzyaloshinskii-Moriya interaction) from the heavier host lattice ultimately gives rise to shorter spin textures.

Near Room-Temperature Intrinsic Exchange Bias in an Fe Intercalated ZrSe2 Spin Glass.

Some magnetic systems display a shift in the center of their magnetic hysteresis loop away from zero field, a phenomenon termed exchange bias. Despite the extensive use of the exchange bias effect, particularly in magnetic multilayers, for the design of spin-based memory/electronics devices, a comprehensive mechanistic understanding of this effect remains a longstanding problem. Recent work has shown that disorder-induced spin frustration might play a key role in exchange bias, suggesting new materials design approaches for spin-based electronic devices that harness this effect. Here, we design a spin glass with strong spin frustration induced by magnetic disorder by exploiting the distinctive structure of Fe intercalated ZrSe2, where Fe(II) centers are shown to occupy both octahedral and tetrahedral interstitial sites and to distribute between ZrSe2 layers without long-range structural order. Notably, we observe behavior consistent with a magnetically frustrated and multidegenerate ground state in these Fe0.17ZrSe2 single crystals, which persists above room temperature. Moreover, this magnetic frustration leads to a robust and tunable exchange bias up to 250 K. These results not only offer important insights into the effects of magnetic disorder and frustration in magnetic materials generally, but also highlight as design strategy the idea that a large exchange bias can arise from an inhomogeneous microscopic environment without discernible long-range magnetic order. In addition, these results show that intercalated TMDs like Fe0.17ZrSe2 hold potential for spintronic technologies that can achieve room temperature applications.

Anomalous Interfacial Electron-Transfer Kinetics in Twisted Trilayer Graphene Caused by Layer-Specific Localization.

Interfacial electron-transfer (ET) reactions underpin the interconversion of electrical and chemical energy. It is known that the electronic state of electrodes strongly influences ET rates because of differences in the electronic density of states (DOS) across metals, semimetals, and semiconductors. Here, by controlling interlayer twists in well-defined trilayer graphene moirés, we show that ET rates are strikingly dependent on electronic localization in each atomic layer and not the overall DOS. The large degree of tunability inherent to moiré electrodes leads to local ET kinetics that range over 3 orders of magnitude across different constructions of only three atomic layers, even exceeding rates at bulk metals. Our results demonstrate that beyond the ensemble DOS, electronic localization is critical in facilitating interfacial ET, with implications for understanding the origin of high interfacial reactivity typically exhibited by defects at electrode-electrolyte interfaces.

Decoupling Effects of Electrostatic Gating on Electronic Transport and Interfacial Charge-Transfer Kinetics at Few-Layer Molybdenum Disulfide.

The electronic properties of electrode materials play a crucial role in defining their electrochemical behavior in energy conversion and storage devices. The assembly of van der Waals heterostructures and fabrication into mesoscopic devices enable the dependence of an electrochemical response on electronic properties to be systematically interrogated. Here, we evaluate the effect of charge carrier concentration on heterogeneous electron transfer at few-layer MoS2 electrodes by combining spatially resolved electrochemical measurements with field-effect electrostatic manipulation of band alignment. Steady-state cyclic voltammograms and finite-element simulations reveal a strong modulation of the measured electrochemical response for outer-sphere charge transfer at the electrostatic gate voltage. In addition, spatially resolved voltammetric responses, obtained at a series of locations at the surface of few-layer MoS2, reveal the governing role of in-plane charge transport on the electrochemical behavior of 2D electrodes, especially under conditions of low carrier densities.

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.

Bridging Structure, Magnetism, and Disorder in Iron-Intercalated Niobium Diselenide, FexNbSe2, below x = 0.25.

Transition-metal dichalcogenides (TMDs) intercalated with magnetic ions serve as a promising materials platform for developing next-generation, spin-based electronic technologies. In these materials, one can access a rich magnetic phase space depending on the choice of intercalant, host lattice, and relative stoichiometry. The distribution of these intercalant ions across given crystals, however, is less well defined-particularly away from ideal packing stoichiometries-and a convenient probe to assess potential longer-range ordering of intercalants is lacking. Here, we demonstrate that confocal Raman spectroscopy is a powerful tool for mapping the onset of intercalant superlattice formation in Fe-intercalated NbSe2 (FexNbSe2) for 0.14 ≤ x < 0.25. We use single-crystal X-ray diffraction to confirm the presence of longer-range intercalant superstructure and employ polarization-, temperature-, and magnetic field-dependent Raman measurements to examine both the symmetry of emergent phonon modes in the intercalated material and potential magnetoelastic coupling. Magnetometry measurements further indicate a correlation between the onset of magnetic ordering and the relative degree of intercalant superlattice formation. These results show Raman spectroscopy to be an expedient, local probe for mapping intercalant ordering in this class of magnetic materials.

Hard Ferromagnetism Down to the Thinnest Limit of Iron-Intercalated Tantalum Disulfide.

Two-dimensional (2D) magnetic crystals hold promise for miniaturized and ultralow power electronic devices that exploit spin manipulation. In these materials, large, controllable magnetocrystalline anisotropy (MCA) is a prerequisite for the stabilization and manipulation of long-range magnetic order. In known 2D magnetic crystals, relatively weak MCA typically results in soft ferromagnetism. Here, we demonstrate that ferromagnetic order persists down to the thinnest limit of FexTaS2 (Fe-intercalated bilayer 2H-TaS2) with giant coercivities up to 3 T. We prepare Fe-intercalated TaS2 by chemical intercalation of van der Waals-layered 2H-TaS2 crystals and perform variable-temperature transport, transmission electron microscopy, and confocal Raman spectroscopy measurements to shed new light on the coupled effects of dimensionality, degree of intercalation, and intercalant order/disorder on the hard ferromagnetic behavior of FexTaS2. More generally, we show that chemical intercalation gives access to a rich synthetic parameter space for low-dimensional magnets, in which magnetic properties can be tailored by the choice of the host material and intercalant identity/amount, in addition to the manifold distinctive degrees of freedom available in atomically thin, van der Waals crystals.

Structure and Magnetism of Iron- and Chromium-Intercalated Niobium and Tantalum Disulfides.

Transition metal dichalcogenides (TMDs) intercalated with spin-bearing transition metal centers are a diverse class of magnetic materials where the spin density and ordering behavior can be varied by the choice of host lattice, intercalant identity, level of intercalation, and intercalant disorder. Each of these degrees of freedom alters the interplay between several key magnetic interactions to produce disparate collective electronic and magnetic phases. The array of magnetic and electronic behavior typified by these systems renders them distinctive platforms for realizing tunable magnetism in solid-state materials and promising candidates for spin-based electronic devices. This Perspective provides an overview of the rich magnetism displayed by transition metal-intercalated TMDs by considering Fe- and Cr-intercalated NbS2 and TaS2. These four exemplars of this large family of materials exhibit a wide range of magnetic properties, including sharp switching of magnetic states, current-driven magnetic switching, and chiral spin textures. An understanding of the fundamental origins of the resultant magnetic/electronic phases in these materials is discussed in the context of composition, bonding, electronic structure, and magnetic anisotropy in each case study.

Chloroaluminate Anion Intercalation in Graphene and Graphite: From Two-Dimensional Devices to Aluminum-Ion Batteries.

A rechargeable aluminum-ion battery based on chloroaluminate electrolytes has received intense attention due to the high abundance and chemical stability of aluminum. However, the fundamental intercalation processes and dynamics in these battery systems remain unresolved. Here, the energetics and dynamics of chloroaluminate ion intercalation in atomically thin single crystal graphite are investigated by fabricating mesoscopic devices for charge transport and operando optical microscopy. These mesoscopic measurements are compared to the high-performance rechargeable Al-based battery consisting of a few-layer graphene-multiwall carbon nanotube composite cathode. These composites exhibit a 60% capacity enhancement over pyrolytic graphite, while an ∼3-fold improvement in overall ion diffusivity is also obtained exhibiting ∼1% of those in atomically thin single crystals. Our results thus establish the distinction between intrinsic and ensemble electrochemical behavior in Al-based batteries and show that engineering ion transport in these devices can yet lead to vast improvements in battery performance.

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.

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 and localized structural deformations that introduce disorder and strain. In this study we used Bragg interferometry to capture atomic displacement fields in twisted bilayer graphene with twist angles <2°. Nanoscale spatial fluctuations in twist angle and uniaxial heterostrain were statistically evaluated, revealing the prevalence of short-range disorder in moiré heterostructures. By quantitatively mapping strain tensor fields, we uncovered two regimes of structural relaxation and disentangled the electronic contributions of constituent rotation modes. Further, we found that applied heterostrain accumulates anisotropically in saddle-point regions, generating distinctive striped strain phases. Our results establish the reconstruction mechanics underpinning the twist-angle-dependent electronic behaviour of twisted bilayer graphene and provide a framework for directly visualizing structural relaxation, disorder and strain in moiré materials.