Search for: All records

Metal-organic frameworks (MOFs) with tunable structures and unique host-guest chemistry have emerged as promising candidates for conductive materials. However, the tunability of conductivity and porosity in conductive MOFs and their interrelationship still lack a systematic study. Herein, we report the synthesis of a series of 3D copper MOFs (NU-4000 to NU-4003) using a triphenylene-based hexatopic carboxylate linker. By modulating the ratio of mixed solvents, distinct structural topologies and π-π stacking arrangements were achieved, resulting in electrical conductivity ranging from insulators (˂ 10-6 S/cm) to semiconductors (10-8 ~ 102 S/cm). Among them, NU-4003 features continuous π-π stacking and exhibits a conductivity of 1.7 × 10-6 S/cm. To further enhance conductivity, we encapsulated C60, a strong electron acceptor, within the circular channels of NU-4003, resulting in a remarkable conductivity increase to 140 S/cm with approximately 100% pore occupancy. Even at lower C60 loadings that leave 54% of the pore volume remaining accessible, the conductivity remains exceptionally high at 104 S/cm. This represents an eight-order magnitude enhancement and positions NU-4003-C60 as one of the most conductive 3D MOFs reported to date. This work integrates two charge transport pathways (through-space and electron donor and acceptor) into a single MOF host-guest material, achieving a significant enhancement in conductivity. This study demonstrates the potential of combining host-guest chemistry and π-π stacking to design conductive MOFs with permanent porosity maintained, providing a blueprint for the development of next-generation materials for electronic and energy-related applications. 

Abstract Moiré superlattices of semiconducting transition metal dichalcogenides enable unprecedented spatial control of electron wavefunctions, leading to emerging quantum states. The breaking of translational symmetry further introduces a new degree of freedom: high symmetry moiré sites of energy minima behaving as spatially separated quantum dots. We demonstrate the superposition between two moiré sites by constructing a trilayer WSe₂/monolayer WS₂moiré heterojunction. The two moiré sites in the first layer WSe₂interfacing WS₂allow the formation of two different interlayer excitons, with the hole residing in either moiré site of the first layer WSe₂and the electron in the third layer WSe₂. An electric field can drive the hybridization of either of the interlayer excitons with the intralayer excitons in the third WSe₂layer, realizing the continuous tuning of interlayer exciton hopping between two moiré sites and a superposition of the two interlayer excitons, distinctively different from the natural trilayer WSe₂. 

Abstract A Chern insulator is a two-dimensional material that hosts chiral edge states produced by the combination of topology with time reversal symmetry breaking. Such edge states are perfect one-dimensional conductors, which may exist not only on sample edges, but on any boundary between two materials with distinct topological invariants (or Chern numbers). Engineering of such interfaces is highly desirable due to emerging opportunities of using topological edge states for energy-efficient information transmission. Here, we report a chiral edge-current divider based on Chern insulator junctions formed within the layered topological magnet MnBi 2 Te 4 . We find that in a device containing a boundary between regions of different thickness, topological domains with different Chern numbers can coexist. At the domain boundary, a Chern insulator junction forms, where we identify a chiral edge mode along the junction interface. We use this to construct topological circuits in which the chiral edge current can be split, rerouted, or switched off by controlling the Chern numbers of the individual domains. Our results demonstrate MnBi 2 Te 4 as an emerging platform for topological circuits design. 

Abstract Transition metal dichalcogenide (TMDC) moiré superlattices, owing to the moiré flatbands and strong correlation, can host periodic electron crystals and fascinating correlated physics. The TMDC heterojunctions in the type-II alignment also enable long-lived interlayer excitons that are promising for correlated bosonic states, while the interaction is dictated by the asymmetry of the heterojunction. Here we demonstrate a new excitonic state, quadrupolar exciton, in a symmetric WSe₂-WS₂-WSe₂trilayer moiré superlattice. The quadrupolar excitons exhibit a quadratic dependence on the electric field, distinctively different from the linear Stark shift of the dipolar excitons in heterobilayers. This quadrupolar exciton stems from the hybridization of WSe₂valence moiré flatbands. The same mechanism also gives rise to an interlayer Mott insulator state, in which the two WSe₂layers share one hole laterally confined in one moiré unit cell. In contrast, the hole occupation probability in each layer can be continuously tuned via an out-of-plane electric field, reaching 100% in the top or bottom WSe₂under a large electric field, accompanying the transition from quadrupolar excitons to dipolar excitons. Our work demonstrates a trilayer moiré system as a new exciting playground for realizing novel correlated states and engineering quantum phase transitions. 

Abstract Van der Waals heterostructures offer great versatility to tailor unique interactions at the atomically flat interfaces between dissimilar layered materials and induce novel physical phenomena. By bringing monolayer 1 T’ WTe₂, a two-dimensional quantum spin Hall insulator, and few-layer Cr₂Ge₂Te₆, an insulating ferromagnet, into close proximity in an heterostructure, we introduce a ferromagnetic order in the former via the interfacial exchange interaction. The ferromagnetism in WTe₂manifests in the anomalous Nernst effect, anomalous Hall effect as well as anisotropic magnetoresistance effect. Using local electrodes, we identify separate transport contributions from the metallic edge and insulating bulk. When driven by an AC current, the second harmonic voltage responses closely resemble the anomalous Nernst responses to AC temperature gradient generated by nonlocal heater, which appear as nonreciprocal signals with respect to the induced magnetization orientation. Our results from different electrodes reveal spin-polarized edge states in the magnetized quantum spin Hall insulator.