Shuang Wu

Observation of Strong Interlayer Coupling in MoS2/WS2 Heterostructures

Epitaxial growth of A-A and A-B stacking MoS2 on WS2 via a two-step chemical vapor deposition method is reported. These epitaxial heterostructures show an atomic clean interface and a strong interlayer coupling, as evidenced by systematic characterization. Low-frequency Raman breathing and shear modes are observed in commensurate stacking bilayers for the first time; these can serve as persuasive fingerprints for interfacial quality and stacking configurations.

Thermally Induced Graphene Rotation on Hexagonal Boron Nitride.

In this Letter, we report the observation of thermally induced rotation of graphene on hexagonal boron nitride (h-BN). After the rotation, two thermally stable configurations of graphene on h-BN with a relative lattice twisting angle of 0° (most stable) and 30° (metastable), respectively, were found. Graphene on h-BN with a twisting angle below (above) a critical angle of ∼12±2° tends to rotate towards 0° (30°) at a temperature of >100u2009°C, which is in line with our theoretical simulations. In addition, by manipulating the annealing temperature and the flake sizes of graphene, moiré superlattices with large spatial periods of graphene on h-BN are achieved. Our studies provide a detailed understanding of the thermodynamic properties of graphene on h-BN and a feasible approach to obtaining van der Waals heterostructures with aligned lattices.

Gate tunable MoS2–black phosphorus heterojunction devices

Heterojunctions are essential building blocks for modern electronic and optoelectronic devices. The recent discovery of two-dimensional semiconductors offers an opportunity to build these heterojunctions with atomic sharp interfaces by van der Waals interaction. Here we fabricated MoS2–black phosphorus (BP) heterojunction devices. Due to the narrow band-gap and unpinned Fermi level of BP, this heterojunction could be tuned to either p–n or n–n by the electrostatic gating. The current rectification behaviors were observed in both p–n and n–n junctions. The current rectification of the MoS2–BP n–n junction was attributed to the energy barrier formed at the interface of wide band-gap MoS2 and narrow band-gap BP. The gate dependence of forward current, reverse current and current rectification properties of the heterojunction at different thickness scale were systematically studied, suggesting the electrical properties of the heterojunction could be controlled by designing the thickness of MoS2 and BP flake.

A General Route Towards Defect and Pore Engineering in Graphene

Defect engineering in graphene is important for tailoring graphenes properties thus applicable in various applications such as porous membranes and ultra-capacitors. In this paper, we report a general route towards defect- and pore- engineering in graphene through remote plasma treatments. Oxygen plasma irradiation was employed to create homogenous defects in graphene with controllable density from a few to ≈10(3) (μm(-2)). The created defects can be further enlarged into nanopores by hydrogen plasma anisotropic etching with well-defined pore size of a few nm or above. The achieved smallest nanopores are ≈2 nm in size, showing the potential for ultra-small graphene nanopores fabrication.

Hofstadter Butterfly and Many-Body Effects in Epitaxial Graphene Superlattice

Graphene placed on hexagonal boron nitride (h-BN) has received a wide range of interest due to the improved electrical performance and rich physics from the interface, especially the emergence of superlattice Dirac points as well as Hofstadter butterfly in high magnetic field. Instead of transferring graphene onto h-BN, epitaxial growth of graphene directly on a single-crystal h-BN provides an alternative and promising way to study these interesting superlattice effects due to their precise lattice alignment. Here we report an electrical transport study on epitaxial graphene superlattice on h-BN with a period of ∼15.6 nm. The epitaxial graphene superlattice is clean, intrinsic, and of high quality with a carrier mobility of ∼27u202f000 cm(2) V(-1) s(-1), which enables the observation of Hofstadter butterfly features originated from the superlattice at a magnetic field as low as 6.4 T. A metal-insulator transition and magnetic field dependent Fermi velocity were also observed, suggesting prominent electron-electron interaction-induced many-body effects.

Fabrication of high-quality all-graphene devices with low contact resistances

All-graphene devices are new class of graphene devices with simple layouts and low contact resistances. Here we report a clean fabrication strategy for all-graphene devices via a defect-assisted anisotropic etching. The as-fabricated graphene is free of contamination and retains the quality of pristine graphene. The contact resistance at room temperature (RT) between a bilayer graphene channel and a multilayer graphene electrode can be as low as ∼5 Ω·μm, the lowest ever achieved experimentally. Our results suggest the feasibility of employing such all-graphene devices in high performance carbon-based integrated circuits.

Patterning monolayer graphene with zigzag edges on hexagonal boron nitride by anisotropic etching

Graphene nanostructures are potential building blocks for nanoelectronic and spintronic devices. However, the production of monolayer graphene nanostructures with well-defined zigzag edges remains a challenge. In this paper, we report the patterning of monolayer graphene nanostructures with zigzag edges on hexagonal boron nitride (h-BN) substrates by an anisotropic etching technique. We found that hydrogen plasma etching of monolayer graphene on h-BN is highly anisotropic due to the inert and ultra-flat nature of the h-BN surface, resulting in zigzag edge formation. The as-fabricated zigzag-edged monolayer graphene nanoribbons (Z-GNRs) with widths below 30u2009nm show high carrier mobility and width-dependent energy gaps at liquid helium temperature. These high quality Z-GNRs are thus ideal structures for exploring their valleytronic or spintronic properties.

Graphene nanoribbons epitaxy on boron nitride

In this letter, we report a pilot study on epitaxy of monolayer graphene nanoribbons (GNRs) on hexagonal boron nitride (h-BN). We found that GNRs grow preferentially from the atomic steps of h-BN, forming in-plane heterostructures. GNRs with well-defined widths ranging from ∼15u2009nm to ∼150u2009nm can be obtained reliably. As-grown GNRs on h-BN have high quality with a carrier mobility of ∼20u2009000u2009cm2 V−1 s−1 for ∼100-nm-wide GNRs at a temperature of 1.7u2009K. Besides, a moire pattern induced quasi-one-dimensional superlattice with a periodicity of ∼15u2009nm for GNR/h-BN was also observed, indicating zero crystallographic twisting angle between GNRs and h-BN substrate. The superlattice induced band structure modification is confirmed by our transport results. These epitaxial GNRs/h-BN with clean surfaces/interfaces and tailored widths provide an ideal platform for high-performance GNR devices.

Defect-enhanced coupling between graphene and SiO2 substrate

Identifying the role of defects that limits graphenes quality is important for various graphene devices on SiO2. In this paper, monolayer graphene samples with defect densities varying from ∼0.04u2009μm−2 to ∼10u2009μm−2 on SiO2 are characterized by both microscopic imaging and electrical transport measurements. We found that the height of graphene on SiO2 is directly related to its defect densities with a reverse correlation, which in turn degrade graphenes quality through a complicated mechanism rather than defects scattering only. We suggest that, at relative high defect density regime, graphene-SiO2 coupling is greatly enhanced causing an increasing charged impurity scattering significantly.

Identification of dominant scattering mechanism in epitaxial graphene on SiC

A scheme of identification of scattering mechanisms in epitaxial graphene (EG) on SiC substrate is developed and applied to three EG samples grown on SiC (0001), (11 (2) over bar0), and (10 (1) over bar0) substrates. Hall measurements combined with defect detection technique enable us to evaluate the individual contributions to the carrier scatterings by defects and by substrates. It is found that the dominant scatterings can be due to either substrate or defects, dependent on the substrate orientations. The EG on SiC (11 (2) over bar0) exhibits a better control over the two major scattering mechanisms and achieves the highest mobility even with a high carrier concentration, promising for high performance graphene-based electronic devices. The method developed here will shed light on major aspects in governing carrier transport in EG to harness it effectively