Evolution of the flat band and the role of lattice relaxations in twisted bilayer graphene.

Magic-angle twisted bilayer graphene exhibits correlated phenomena such as superconductivity and Mott insulating states related to the weakly dispersing flat band near the Fermi energy. Such a flat band is expected to be sensitive to both the moiré period and lattice relaxations. Thus, clarifying the evolution of the electronic structure with the twist angle is critical for understanding the physics of magic-angle twisted bilayer graphene. Here we combine nano-spot angle-resolved photoemission spectroscopy and atomic force microscopy to resolve the fine electronic structure of the flat band and remote bands, as well as their evolution with twist angle from 1.07° to 2.60°. Near the magic angle, the dispersion is characterized by a flat band near the Fermi energy with a strongly reduced band width. Moreover, we observe a spectral weight transfer between remote bands at higher binding energy, which allows to extract the modulated interlayer spacing near the magic angle. Our work provides direct spectroscopic information on flat band physics and highlights the important role of lattice relaxations.

Deducing the internal interfaces of twisted multilayer graphene via moiré-regulated surface conductivity.

The stacking state of atomic layers critically determines the physical properties of twisted van der Waals materials. Unfortunately, precise characterization of the stacked interfaces remains a great challenge as they are buried internally. With conductive atomic force microscopy, we show that the moiré superlattice structure formed at the embedded interfaces of small-angle twisted multilayer graphene (tMLG) can noticeably regulate surface conductivity even when the twisted interfaces are 10 atomic layers beneath the surface. Assisted by molecular dynamics (MD) simulations, a theoretical model is proposed to correlate surface conductivity with the sequential stacking state of the graphene layers of tMLG. The theoretical model is then employed to extract the complex structure of a tMLG sample with crystalline defects. Probing and visualizing the internal stacking structures of twisted layered materials is essential for understanding their unique physical properties, and our work offers a powerful tool for this via simple surface conductivity mapping.

Modularized batch production of 12-inch transition metal dichalcogenides by local element supply.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are regarded as pivotal semiconductor candidates for next-generation devices due to their atomic-scale thickness, high carrier mobility and ultrafast charge transfer. In analog to the traditional semiconductor industry, batch production of wafer-scale TMDs is the prerequisite to proceeding with their integrated circuits evolution. However, the production capacity of TMD wafers is typically constrained to a single and small piece per batch (mainly ranging from 2 to 4 inches), due to the stringent conditions required for effective mass transport of multiple precursors during growth. Here we developed a modularized growth strategy for batch production of wafer-scale TMDs, enabling the fabrication of 2-inch wafers (15 pieces per batch) up to a record-large size 12-inch wafers (3 pieces per batch). Each module, comprising a self-sufficient local precursor supply unit for robust individual TMD wafer growth, is vertically stacked with others to form an integrated array and thus a batch growth. Comprehensive characterization techniques, including optical spectroscopy, electron microscopy, and transport measurements unambiguously illustrate the high-crystallinity and the large-area uniformity of as-prepared monolayer films. Furthermore, these modularized units demonstrate versatility by enabling the conversion of as-produced wafer-scale MoS2 into various structures, such as Janus structures of MoSSe, alloy compounds of MoS2(1-x)Se2x, and in-plane heterostructures of MoS2-MoSe2. This methodology showcases high-quality and high-yield wafer output and potentially enables the seamless transition from lab-scale to industrial-scale 2D semiconductor complementary to silicon technology.

Inorganic-Organic Silica/PDMS Nanocomposite Antiadhesive Coating with Ultrahigh Hardness and Thermal Stability.

Antiadhesive surfaces have been gaining continuous attention, because of the scientific and industrial significance. Slippery surfaces and antismudge coatings with antiadhesive behavior have been readily designed and prepared. However, improving robustness of the surfaces, especially the simultaneous demonstration of features of high hardness, excellent adhesion to different substrates, and high thermal stability, is constantly challenging. Herein, we present a silica/polydimethylsiloxane (PDMS) nanocomposite coating (SPNC), wherein silica acts as a consecutive phase and nanophased PDMS is covalently embedded. The nanoconfined PDMS phase exhibits enhanced thermal stability and endows SPNC with slippery behavior; meanwhile, enrichment of PDMS on the surface renders a gradient composition of the coating. Accordingly, the inorganic-organic SPNC simultaneously displays a high nanoindentation hardness of 3.07 GPa and a pencil hardness over 9H, outstanding thermal stability of the slippery performance up to 400 °C, and excellent adhesion strength to different substrates. Additionally, SPNC exhibits high optical transparency, flexibility, resistance to bacterial clone, and chemical corrosion. With the scalable fabrication process, it can be envisioned that the antiadhesive coating with unprecedented comprehensive merits in this work has significant potentials for large-area applications, especially under severe service environments.

Bis(2-hydroxyethyl) Terephthalate-Modified Ti3C2Tx/Graphene Nanohybrids as Three-Dimensional Functional Chain Extenders for Polyurethane Composite Films with Strain-Sensing and Conductive Properties.

Incorporation of functional nanofillers can unlock the potential of polymers as advanced materials. Herein, single-layered and three-dimensional reduced graphene oxide (rGO)/Ti3C2Tx (B-rGO@Ti3C2Tx) nanohybrids were constructed using bis(2-hydroxyethyl) terephthalate (BHET) as a coupling agent between rGO and Ti3C2Tx through covalent and hydrogen bonds. It is found that BHET can not only resist the weak oxidization of Ti3C2Tx to some degree but also prevent the self-stacking of Ti3C2Tx and rGO sheets. Then, B-rGO@Ti3C2Tx was used as a functional nanofiller and three-dimensional chain extender for preparing the waterborne polyurethane (WPU) nanocomposite through in situ polymerization. Compared with WPU nanocomposites with an equivalent amount of Ti3C2Tx/rGO@Ti3C2Tx, although containing an equivalent amount of BHET, WPU/B-rGO@Ti3C2Tx nanocomposites show significantly improved performance. For example, 5.66 wt % of B-rGO@Ti3C2Tx endows WPU with a high tensile strength of 36.0 MPa (improved by 380%), thermal conductivity of 0.697 W·m-1·K-1, electrical conductivity of 1.69 × 10-2 S/m (enhanced by 39 times), good strain-sensing behavior, electromagnetic interference (EMI)-shielding performance of 49.5 dB in the X-band, and excellent thermal stability. Therefore, the construction of rGO@Ti3C2Tx nanohybrids with the aid of chain extenders may unlock new possibilities of polyurethane as smart materials.

Continuous epitaxy of single-crystal graphite films by isothermal carbon diffusion through nickel.

Multilayer van der Waals (vdW) film materials have attracted extensive interest from the perspective of both fundamental research1-3 and technology4-7. However, the synthesis of large, thick, single-crystal vdW materials remains a great challenge because the lack of out-of-plane chemical bonds weakens the epitaxial relationship between neighbouring layers8-31. Here we report the continuous epitaxial growth of single-crystal graphite films with thickness up to 100,000 layers on high-index, single-crystal nickel (Ni) foils. Our epitaxial graphite films demonstrate high single crystallinity, including an ultra-flat surface, centimetre-size single-crystal domains and a perfect AB-stacking structure. The exfoliated graphene shows excellent physical properties, such as a high thermal conductivity of ~2,880 W m-1 K-1, intrinsic Young's modulus of ~1.0 TPa and low doping density of ~2.2 × 1010 cm-2. The growth of each single-crystal graphene layer is realized by step edge-guided epitaxy on a high-index Ni surface, and continuous growth is enabled by the isothermal dissolution-diffusion-precipitation of carbon atoms driven by a chemical potential gradient between the two Ni surfaces. The isothermal growth enables the layers to grow at optimal conditions, without stacking disorders or stress gradients in the final graphite. Our findings provide a facile and scalable avenue for the synthesis of high-quality, thick vdW films for various applications.

Revisiting Frictional Characteristics of Graphene: Effect of In-Plane Straining.

Supreme mechanical performance and tribological properties render graphene a promising candidate as a surface friction modifier. Recently, it has been demonstrated that applying in-plane strain can effectively tune friction of suspended graphene in a reversible manner. However, since graphene is deposited on solid surfaces in most tribological applications, whether such operation will result in a similar modulation effect becomes a critical question to be answered. Herein, by depositing graphene onto a stretchable substrate, the frictional characteristics of supported graphene under a wide range of strain are examined with an in situ tensile loading platform. The experimental results show that friction of supported graphene decreases with increasing graphene strain, similar to the suspended system. However, depending on the adherence state of the graphene/substrate interface, the system exhibits two distinct friction regimes with significantly different strain dependences. Assisted by detailed atomic force microscopy imaging, we attribute the unique behavior to the transition between two friction modulation modes, i.e., contact-quality-dominated friction and puckering-dominated friction. This work provides a more comprehensive view of the influence of strain on surface friction of graphene, which is beneficial for active modulation of graphene friction through strain engineering.

Dual-Scale Stick-Slip Friction on Graphene/h-BN Moiré Superlattice Structure.

Using atomic force microscopy, we have shown that friction on graphene/h-BN superlattice structures may exhibit unusual moiré-scale stick slip in addition to the regular ones observed at the atomic scale. Such dual-scale slip instability will lead to unique length-scale dependent energy dissipation when the different slip mechanisms are sequentially activated. Assisted by an improved theoretical model and comparative experiments, we find that accumulation and unstable release of the in-plane strain of the graphene layer is the key mechanism underlying the moiré-scale behavior. This work highlights the distinct role of the internal state of the van der Waals interfaces in determining the rich dynamics and energy dissipation of layer-structured materials.

Visualizing the Anomalous Catalysis in Two-Dimensional Confined Space.

Confined nanospaces provide a new platform to promote catalytic reactions. However, the mechanism of catalytic enhancement in the nanospace still requires insightful exploration due to the lack of direct visualization. Here, we report operando investigations on the etching and growth of graphene in a two-dimensional (2D) confined space between graphene and a Cu substrate. We observed that the graphene layer between the Cu and top graphene layer was surprisingly very active in etching (more than 10 times faster than the etching of the top graphene layer). More strikingly, at a relatively low temperature (∼530 °C), the etched carbon radicals dissociated from the bottom layer, in turn feeding the growth of the top graphene layer with a very high efficiency. Our findings reveal the in situ dynamics of the anomalous confined catalytic processes in 2D confined spaces and thus pave the way for the design of high-efficiency catalysts.

Domino-like stacking order switching in twisted monolayer-multilayer graphene.

Atomic reconstruction has been widely observed in two-dimensional van der Waals structures with small twist angles1-7. This unusual behaviour leads to many novel phenomena, including strong electronic correlation, spontaneous ferromagnetism and topologically protected states1,5,8-14. Nevertheless, atomic reconstruction typically occurs spontaneously, exhibiting only one single stable state. Using conductive atomic force microscopy, here we show that, for small-angle twisted monolayer-multilayer graphene, there exist two metastable reconstruction states with distinct stacking orders and strain soliton structures. More importantly, we demonstrate that these two reconstruction states can be reversibly switched, and the switching can propagate spontaneously in an unusual domino-like fashion. Assisted by lattice-resolved conductive atomic force microscopy imaging and atomistic simulations, the detailed structure of the strain soliton networks has been identified and the associated propagation mechanism is attributed to the strong mechanical coupling among solitons. The fine structure of the bistable states is critical for understanding the unique properties of van der Waals structures with tiny twists, and the switching mechanism offers a viable means for manipulating their stacking states.

Harnessing Friction in Intertwined Structures for High-Capacity Reusable Energy-Absorbing Architected Materials.

Energy-absorbing materials with both high absorption capacity and high reusability are ideal candidates for impact protection. Despite great demands, the current designs either exhibit limited energy-absorption capacities or perform well only for one-time usage. Here a new kind of energy-absorbing architected materials is created with both high absorption capacity and superior reusability, reaching 10 kJ kg-1 per cycle for more than 200 cycles, that is, unprecedentedly 2000 kJ kg-1 per lifetime. The extraordinary performance is achieved by exploiting the rate-dependent frictional dissipation between prestressed stiff cores and a porous soft elastomer, which is reinforced by an intertwined stiff porous frame. The vast interfaces between the cores and elastomer enable high energy dissipation, while the magnitude of the friction force can adapt passively with the loading rate. The intertwined structure prevents stress concentration and ensures no damage and reusability of the constituents after hundreds of loading cycles. The behaviors of the architected materials, such as self-recoverability, force magnitude, and working stroke, are further tailored by tuning their structure and geometry. This design strategy opens an avenue for developing high-performance reusable energy-absorbing materials that enable novel designs of machines or structures.

Dual-coupling-guided epitaxial growth of wafer-scale single-crystal WS2 monolayer on vicinal a-plane sapphire.

The growth of wafer-scale single-crystal two-dimensional transition metal dichalcogenides (TMDs) on insulating substrates is critically important for a variety of high-end applications1-4. Although the epitaxial growth of wafer-scale graphene and hexagonal boron nitride on metal surfaces has been reported5-8, these techniques are not applicable for growing TMDs on insulating substrates because of substantial differences in growth kinetics. Thus, despite great efforts9-20, the direct growth of wafer-scale single-crystal TMDs on insulating substrates is yet to be realized. Here we report the successful epitaxial growth of two-inch single-crystal WS2 monolayer films on vicinal a-plane sapphire surfaces. In-depth characterizations and theoretical calculations reveal that the epitaxy is driven by a dual-coupling-guided mechanism, where the sapphire plane-WS2 interaction leads to two preferred antiparallel orientations of the WS2 crystal, and sapphire step edge-WS2 interaction breaks the symmetry of the antiparallel orientations. These two interactions result in the unidirectional alignment of nearly all the WS2 islands. The unidirectional alignment and seamless stitching of WS2 islands are illustrated via multiscale characterization techniques; the high quality of WS2 monolayers is further evidenced by a photoluminescent circular helicity of ~55%, comparable to that of exfoliated WS2 flakes. Our findings offer the opportunity to boost the production of wafer-scale single crystals of a broad range of two-dimensional materials on insulators, paving the way to applications in integrated devices.

Elastocapillary cleaning of twisted bilayer graphene interfaces.

Although layered van der Waals (vdW) materials involve vast interface areas that are often subject to contamination, vdW interactions between layers may squeeze interfacial contaminants into nanopockets. More intriguingly, those nanopockets could spontaneously coalesce into larger ones, which are easier to be squeezed out the atomic channels. Such unusual phenomena have been thought of as an Ostwald ripening process that is driven by the capillarity of the confined liquid. The underlying mechanism, however, is unclear as the crucial role played by the sheet's elasticity has not been previously appreciated. Here, we demonstrate the coalescence of separated nanopockets and propose a cleaning mechanism in which both elastic and capillary forces are at play. We elucidate this mechanism in terms of control of the nanopocket morphology and the coalescence of nanopockets via a mechanical stretch. Besides, we demonstrate that bilayer graphene interfaces excel in self-renewal phenomena.

Abnormal Raman Characteristics of Graphene Originating from Contact Interface Inhomogeneity.

The Raman peak position shift rate per strain (RSS) coefficient of graphene is crucial for quantitative strain measurement by Raman spectroscopy. Despite its essential role, the experimentally measured RSS values are found to be highly scattered and many times significantly lower than the theoretical prediction. Here, using in situ Raman spectroscopy with a tensile test system, we resolve this controversy by examining the Raman characteristics of graphene derived from chemical vapor deposition (CVD) transferred on polymer substrates. Our experiments show that the Raman 2D-peak position of CVD graphene can shift nonlinearly with applied strain, in contrast to its intrinsically linear trait. More importantly, the resultant RSS coefficient at the steady state is much lower than the theoretical prediction. By analyzing atomic force microscopy (AFM) phase images and full width at half-maximum (FWHM) of Raman spectra, we attribute the abnormal behavior to nanometer-scale inhomogeneity of the graphene/substrate contact interface. Assisted by a simplified discrete interface slip model, we correlate the evolution of nanometer-scale inhomogeneity with that of the apparent Raman response. The theoretical model provides a useful tool for understanding and optimizing the contact interface behavior of various two-dimensional materials on substrates; the revealed mechanism is critical for correct interpretation of data obtained by Raman or any other spectroscopies based on homogenized laser signals.

Sequential growth and twisted stacking of chemical-vapor-deposited graphene.

Adlayers have been one of the main concerns for controlled synthesis of graphene by the chemical vapor deposition (CVD) method. Here we investigate the CVD growth of graphene adlayers on copper (Cu) using isotope-labeling-based Raman spectroscopy and high-resolution atomic force microscopy (AFM). The results show that, besides conventional simultaneous growth for all the graphene layers, approximately 37% of the adlayers follow a sequential growth which can occur even hours after the nucleation of the first layer. The proportions of AB (Bernal)- and twisted (t)-stacked bilayer graphene (BLG) stacks formed by the two modes are not significantly different. Moreover, in those stacks with both AB- and t-BLG, evidence at the atomic scale demonstrates that they resulted from misoriented domains in their single-crystal-like top layers. We believe that this new understanding of the growth mechanism for graphene adlayers can help pave the way towards the synthesis of large-scale and high-quality graphene with controllable layer numbers.

Length Scale Effect in Frictional Aging of Silica Contacts.

Friction between two solid surfaces often exhibits strong rate and slip-history dependence, which critically determines the dynamic stability of frictional sliding. Empirically, such an evolutional effect has been captured by the rate-and-state friction (RSF) law based on laboratory-scale experiments; but its applicability for generic sliding interfaces under different length scales remains unclear. In this Letter, frictional aging, the key manifestation of the evolutional behavior, of silica-silica contacts is studied via slide-hold-slide tests with apparent contact size spanning across 3 orders of magnitude. The experimental results demonstrate a clear and strong length scale dependency in frictional aging characteristics. Assisted by a multiasperity RSF model, we attribute the length scale effect to roughness-dependent true contact area evolution as well as scale-dependent friction stress due to nonconcurrent slip.

Universal Statistical Laws for the Velocities of Collective Migrating Cells.

Migratory dynamics of collective cells is central to the morphogenesis of biological tissues. The statistical distribution of cell velocities in 2D confluent monolayers is measured through large-scale and long-term experiments of various cell types lying on different substrates. A linear relation is discovered between the variability and the mean of cell speeds during the jamming process of confluent cell monolayers, suggesting time-invariant distribution profile of cell velocities. It is further found that the probability density function of cell velocities obeys the non-canonical q-Gaussian statistics, regardless of cell types and substrate stiffness. It is the Tsallis entropy, instead of the classical Boltzmann-Gibbs entropy, that dictates the universal statistical laws of collective cell migration. The universal statistical law stems from cell-cell interactions, as demonstrated by the wound healing experiments. This previously unappreciated finding provides a linkage between cell-level heterogeneity and tissue-level ensembles in embryonic development and tumor growth.

Abnormal conductivity in low-angle twisted bilayer graphene.

Controlling the interlayer twist angle offers a powerful means for tuning the electronic properties of two-dimensional (2D) van der Waals materials. Typically, the electrical conductivity would increase monotonically with decreasing twist angle owing to the enhanced coupling between adjacent layers. Here, we report a nonmonotonic angle-dependent vertical conductivity across the interface of bilayer graphene with low twist angles. More specifically, the vertical conductivity enhances gradually with decreasing twist angle up to a crossover angle at θc ≈ 5°, and then it drops notably upon further decrease in the twist angle. Revealed by density functional theory calculations and scanning tunneling microscopy, the abnormal behavior is attributed to the unusual reduction in average carrier density originating from local atomic reconstruction. The impact of atomic reconstruction on vertical conductivity is unique for low-angle twisted 2D van der Waals materials and provides a strategy for designing and optimizing their electronic performance.

Optocapillarity-driven assembly and reconfiguration of liquid crystal polymer actuators.

Realizing programmable assembly and reconfiguration of small objects holds promise for technologically-significant applications in such fields as micromechanical systems, biomedical devices, and metamaterials. Although capillary forces have been successfully explored to assemble objects with specific shapes into ordered structures on the liquid surface, reconfiguring these assembled structures on demand remains a challenge. Here we report a strategy, bioinspired by Anurida maritima, to actively reconfigure assembled structures with well-defined selectivity, directionality, robustness, and restorability. This approach, taking advantage of optocapillarity induced by photodeformation of floating liquid crystal polymer actuators, not only achieves programmable and reconfigurable two-dimensional assembly, but also uniquely enables the formation of three-dimensional structures with tunable architectures and topologies across multiple fluid interfaces. This work demonstrates a versatile approach to tailor capillary interaction by optics, as well as a straightforward bottom-up fabrication platform for a wide range of applications.