Xiaobo Lu

Tunable Piezoresistivity of Nanographene Films for Strain Sensing

Graphene-based strain sensors have attracted much attention recently. Usually, there is a trade-off between the sensitivity and resistance of such devices, while larger resistance devices have higher energy consumption. In this paper, we report a tuning of both sensitivity and resistance of graphene strain sensing devices by tailoring graphene nanostructures. For a typical piezoresistive nanographene film with a sheet resistance of ∼100 KΩ/□, a gauge factor of more than 600 can be achieved, which is 50× larger than those in previous studies. These films with high sensitivity and low resistivity were also transferred on flexible substrates for device integration for force mapping. Each device shows a high gauge factor of more than 500, a long lifetime of more than 10(4) cycles, and a fast response time of less than 4 ms, suggesting a great potential in electronic skin applications.

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.

Observation of an intrinsic bandgap and Landau level renormalization in graphene/boron-nitride heterostructures

Van der Waals heterostructures formed by assembling different two-dimensional atomic crystals into stacks can lead to many new phenomena and device functionalities. In particular, graphene/boron-nitride heterostructures have emerged as a very promising system for band engineering of graphene. However, the intrinsic value and origin of the bandgap in such heterostructures remain unresolved. Here we report the observation of an intrinsic bandgap in epitaxial graphene/boron-nitride heterostructures with zero crystallographic alignment angle. Magneto-optical spectroscopy provides a direct probe of the Landau level transitions in this system and reveals a bandgap of ~38 meV (440 K). Moreover, the Landau level transitions are characterized by effective Fermi velocities with a critical dependence on specific transitions and magnetic field. These findings highlight the important role of many-body interactions in determining the fundamental properties of graphene heterostructures.

Gaps induced by inversion symmetry breaking and second-generation Dirac cones in graphene/hexagonal boron nitride

Graphene/h-BN has emerged as a model van der Waals heterostructure [1], and the band structure engineering by the superlattice potential has led to various novel quantum phenomena including the self-similar Hofstadter butterfly states [2–5]. Although newly generated second generation Dirac cones (SDCs) are believed to be crucial for understanding such intriguing phenomena, so far fundamental knowledge of SDCs in such heterostructure, e.g. locations and dispersion of SDCs, the effect of inversion symmetry breaking on the gap opening, still remains highly debated due to the lack of direct experimental results. Here we report first direct experimental results on the dispersion of SDCs in 0◦ aligned graphene/h-BN heterostructure using angle-resolved photoemission spectroscopy. Our data reveal unambiguously SDCs at the corners of the superlattice Brillouin zone, and at only one of the two superlattice valleys. Moreover, gaps of ≈ 100 meV and ≈ 160 meV are observed at the SDCs and the original graphene Dirac cone respectively. Our work highlights the important role of a strong inversion symmetry breaking perturbation potential in the physics of graphene/h-BN, and fills critical knowledge gaps in the band structure engineering of Dirac fermions by a superlattice potential. ∗These authors contribute equally to this work. †Correspondence should be sent to syzhou@mail.tsinghua.edu.cn

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-dependent Pseudospin Mixing in Graphene/boron Nitride Moire Superlattices

Electrons in graphene have a pseudospin, but controlling this degree of freedom is challenging. Evidence now suggests that the moire superlattices arising in two-dimensional heterostructures can be used to electrically manipulate pseudospins.

Argon Plasma Induced Phase Transition in Monolayer MoS2

In this work, we report a facile, clean, controllable and scalable phase engineering technique for monolayer MoS2. We found that weak Ar-plasma bombardment can locally induce 2H→1T phase transition in monolayer MoS2 to form mosaic structures. These 2H→1T phase transitions are stabilized by point defects (single S-vacancies) and the sizes of induced 1T domains are typically a few nanometers, as revealed by scanning tunneling microscopy measurements. On the basis of a selected-area phase patterning process, we fabricated MoS2 FETs inducing 1T phase transition within the metal contact areas, which exhibit substantially improved device performances. Our results open up a new route for phase engineering in monolayer MoS2 and other transition metal dichalcogenide (TMD) materials.

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.

Graphene‐Contacted Ultrashort Channel Monolayer MoS2 Transistors

2D semiconductors are promising channel materials for field-effect transistors (FETs) with potentially strong immunity to short-channel effects (SCEs). In this paper, a grain boundary widening technique is developed to fabricate graphene electrodes for contacting monolayer MoS2 . FETs with channel lengths scaling down to ≈4 nm can be realized reliably. These graphene-contacted ultrashort channel MoS2 FETs exhibit superior performances including the nearly Ohmic contacts and excellent immunity to SCEs. This work provides a facile route toward the fabrication of various 2D material-based devices for ultrascaled electronics.

Switchable friction enabled by nanoscale self-assembly on graphene

Graphene monolayers are known to display domains of anisotropic friction with twofold symmetry and anisotropy exceeding 200%. This anisotropy has been thought to originate from periodic nanoscale ripples in the graphene sheet, which enhance puckering around a sliding asperity to a degree determined by the sliding direction. Here we demonstrate that these frictional domains derive not from structural features in the graphene but from self-assembly of environmental adsorbates into a highly regular superlattice of stripes with period 4–6u2009nm. The stripes and resulting frictional domains appear on monolayer and multilayer graphene on a variety of substrates, as well as on exfoliated flakes of hexagonal boron nitride. We show that the stripe-superlattices can be reproducibly and reversibly manipulated with submicrometre precision using a scanning probe microscope, allowing us to create arbitrary arrangements of frictional domains within a single flake. Our results suggest a revised understanding of the anisotropic friction observed on graphene and bulk graphite in terms of adsorbates.