Crossover between rigid and reconstructed moiré lattice in h-BN-encapsulated twisted bilayer WSe2 with different twist angles.

A moiré lattice in a twisted-bilayer transition metal dichalcogenide (tBL-TMD) exhibits a complex atomic reconstruction effect when its twist angle is less than a few degrees. The influence of the atomic reconstruction on material properties of the tBL-TMD has been of particular interest. In this study, we performed scanning transmission electron microscopy (STEM) imaging of a moiré lattice in h-BN-encapsulated twisted bilayer WSe2 with various twist angles. Atomic-resolution imaging of the moiré lattice revealed a reconstructed moiré lattice below a crossover twist angle of ∼4° and a rigid moiré lattice above this angle. Our findings indicate that h-BN encapsulation has a considerable influence on lattice reconstruction, as the crossover twist angle was larger in h-BN-encapsulated devices compared to non-encapsulated devices. We believe that this difference is due to the improved flatness and uniformity of the twisted bilayers with h-BN encapsulation. Our results provide a foundation for a deeper understanding of the lattice reconstruction in twisted TMD materials with h-BN encapsulation.

Cyclic Wear Reliability of 2D Monolayers.

Understanding wear, a critical factor impacting the reliability of mechanical systems, is vital for nano-, meso-, and macroscale applications. Due to the complex nature of nanoscale wear, the behavior of nanomaterials such as two-dimensional materials under cyclic wear and their surface damage mechanism is yet unexplored. In this study, we used atomic force microscopy coupled with molecular dynamic simulations to statistically examine the cyclic wear behavior of monolayer graphene, MoS2, and WSe2. We show that graphene displays exceptional durability and lasts over 3000 cycles at 85% of the applied critical normal load before failure, while MoS2 and WSe2 last only 500 cycles on average. Moreover, graphene undergoes catastrophic failure as a result of stress concentration induced by local out-of-plane deformation. In contrast, MoS2 and WSe2 exhibit intermittent failure, characterized by damage initiation at the edge of the wear track and subsequent propagation throughout the entire contact area. In addition to direct implications for MEMS and NEMS industries, this work can also enable the optimization of the use of 2D materials as lubricant additives on a macroscopic level.

All-dry flip-over stacking of van der Waals junctions of 2D materials using polyvinyl chloride.

We demonstrated an all-dry polymer-to-polymer transfer technique for two-dimensional (2D) crystal flakes using a polyvinyl chloride (PVC) layer deposited on a piece of polydimethylsiloxane (PDMS). Unexpectedly, the pickup/release temperatures were modified in wider temperature range simply by changing the thickness of the PVC layer than changing the plasticizer ratio. Utilizing the difference in the pickup/release temperatures depending on the PVC film thickness, 2D flakes were transferred from a thicker PVC film to a thinner one. This polymer-to-polymer transfer technique can be utilized to flip over van der Waals heterostructures. As a demonstration, we fabricated a mountain-like stacked structure of hexagonal boron nitride flakes using the flip-over stacking technique. Finally, we compared the results of thermomechanical analysis with the pickup/release temperatures of the PVC/PDMS stamp. The PVC was revealed to be at the glass transition and in the viscoelastic flow regimes when the 2D flakes were picked up and dry released, respectively. Our polymer-to-polymer transfer method facilitates flip-over van der Waals stacking in an all-dry manner, expanding the possibility of 2D materials device fabrications.

Cyclotron Resonance Study of Monolayer Graphene under Double Moiré Potentials.

We report the first cyclotron resonance study of monolayer graphene under double-moiré potentials in which the crystal axis of graphene is nearly aligned to those of both the top and bottom hexagonal boron nitride (h-BN) layers. Under mid-infrared light irradiation, we observe cyclotron resonance absorption with the following unique features: (1) cyclotron resonance magnetic field BCR is entirely different from that of nonaligned monolayer graphene, (2) BCR exhibits strong electron-hole asymmetry, and (3) splitting of BCR is observed for |ν| < 1, with the split maximum at |ν| = 1, resulting in eyeglass-shaped trajectories. These features are well explained by considering the large bandgap induced by the double moiré potentials, the electron-hole asymmetry in the Fermi velocity, and the Fermi-level-dependent enhancement of spin gaps, which suggests a large electron-electron correlation contribution in this system.

Hexagonal Boron Nitride Synthesized at Atmospheric Pressure Using Metal Alloy Solvents: Evaluation as a Substrate for 2D Materials.

Hexagonal boron nitride (h-BN) synthesized under high pressure and high temperature (HPHT) has been used worldwide in two-dimensional (2D) materials research as an essential material for constructing van der Waals heterostructures. Here, we study h-BN synthesized with another method, i.e., via synthesis at atmospheric pressure and high temperature (APHT) using a metal alloy solvent. First, we examine the APHT h-BN in a bulk crystal form using cathodoluminescence and find that it does not have carbon-rich domains that inevitably exist in a core region of all the HPHT h-BN crystals. Next, we statistically compare the size of the crystal flakes exfoliated on a SiO2/Si substrate from APHT and HPHT h-BN crystals by employing our automated 2D material searching system. Finally, we provide direct evidence that APHT h-BN can serve as a high-quality substrate for 2D materials by demonstrating high carrier mobility, ballistic transport, and Hofstadter butterfly in graphene and photoluminescence in WS2.

Electrical Control of Cyclotron Resonance in Dual-Gated Trilayer Graphene.

Landau levels (LLs) of ABA-stacked trilayer graphene (TLG) are described as the combination of monolayer graphene-like LLs and bilayer graphene-like LLs. They are extremely sensitive to the applied perpendicular electric displacement field D. Here, we demonstrate the electrical control of cyclotron resonance (CR) in a dual-gated ABA-stacked TLG. Under the irradiation of mid-infrared light, we observed the photovoltage induced by the CR absorption through the photothermoelectric effect. The resonant magnetic field in CR is changed by applying D while keeping the carrier density constant. Numerical simulations based on the tight-binding model complement the experimental observations. We believe that the present study provides a boost to graphene-based photodetectors and photoemitters with an electrically tunable wavelength in mid-infrared to terahertz spectral ranges.

Carbon-Rich Domain in Hexagonal Boron Nitride: Carrier Mobility Degradation and Anomalous Bending of the Landau Fan Diagram in Adjacent Graphene.

Hexagonal boron nitride (h-BN) crystals grown under ultrahigh pressures and ultrahigh temperatures exhibit a high crystallinity and are used throughout the world as ideal substrates and insulating layers in van der Waals heterostructures. However, in their central region, these crystals have domains which contain a significant density of carbon impurities. In this study, we utilized cathodoluminescence and far-ultraviolet photoluminescence to reveal that the carbon (C)-rich domain can exist even after exfoliation. Then, we studied the carrier transport of graphene in h-BN/graphene/h-BN van der Waals heterostructures, precisely arranging the graphene to straddle the border of the C-rich domain in h-BN. We found that the carrier mobility of graphene on the C-rich h-BN domain was significantly suppressed. In addition, characteristic bending of the Landau fan diagram was observed on the electron-doped side. These results suggest that the C-rich domain in h-BN forms an impurity level and induces extrinsic carrier scattering into adjacent graphene.

Autonomous robotic searching and assembly of two-dimensional crystals to build van der Waals superlattices.

Van der Waals heterostructures are comprised of stacked atomically thin two-dimensional crystals and serve as novel materials providing unprecedented properties. However, the random natures in positions and shapes of exfoliated two-dimensional crystals have required the repetitive manual tasks of optical microscopy-based searching and mechanical transferring, thereby severely limiting the complexity of heterostructures. To solve the problem, here we develop a robotic system that searches exfoliated two-dimensional crystals and assembles them into superlattices inside the glovebox. The system can autonomously detect 400 monolayer graphene flakes per hour with a small error rate (<7%) and stack four cycles of the designated two-dimensional crystals per hour with few minutes of human intervention for each stack cycle. The system enabled fabrication of the superlattice consisting of 29 alternating layers of the graphene and the hexagonal boron nitride. This capacity provides a scalable approach for prototyping a variety of van der Waals superlattices.