We investigate the effect of external stimulus (temperature, magnetic field, and gases adsorptions) on anisotropic magnetoresistance (AMR) in multilayer graphene. The graphene sample shows superlinear magnetoresistance when magnetic field is perpendicular to the plane of graphene. A non-saturated AMR with a value of −33% is found at 10 K under a magnetic field of 7 T. It is surprisingly to observe that a two-fold symmetric AMR at high temperature is changed into a one-fold one at low temperature for a sample with an irregular shape. The anomalous AMR behaviors may be understood by considering the anisotropic scattering of carriers from two asymmetric edges and the boundaries of V+(V-) electrodes which serve as active adsorption sites for gas molecules at low temperature. Our results indicate that AMR in graphene can be optimized by tuning the adsorptions, sample shape and electrode distribution in the future application.
Graphene nanoribbon heterostructures and heterojunctions have attracted interest as next-generation molecular diodes with atomic precision. Their mass production via solution methods and prototypical device integration remains to be explored. Here, the bottom-up solution synthesis and characterization of liquid-phase-processable graphene nanoribbon heterostructures (GNRHs) are demonstrated. Joint photoresponsivity measurements and simulations provide evidence of the structurally defined heterostructure motif acting as a type-I heterojunction. Real-time, time-dependent density functional tight-binding simulations further reveal that the photocurrent polarity can be tuned at different excitation wavelengths. Our results introduce liquid-phase-processable, self-assembled heterojunctions for the development of nanoscale diode circuitry and adaptive hardware.
In this paper, we studied the competition of growth and etching during graphene epitaxial growth in the remote plasma enhanced chemical vapor deposition (r-PECVD) system. Epitaxial growth of graphene on HOPG substrates with a simultaneous etching process was systematically explored at various temperatures. It was found that etching of graphene by hydrogen radicals generated in the r-PECVD system was a critical factor during graphene's growth for controlling the nucleation densities, lateral growth rates, and layer thickness. At temperatures lower than 490 °C, the etching effect is dominant, and there is no graphene nucleation. And at temperatures higher than 490 °C, the etching effect decreases gradually with rising temperature and the growth effect stands out. The optimized epitaxial growth was at 520 °C, and at that temperature a monolayer graphene single crystal was achieved with near perfect lattice structure on HOPG substrates.
Fabrication of graphene nanostructures is of importance for both investigating their intrinsic physical properties and applying them into various functional devices. In this paper, we report a scalable fabrication approach for graphene nanostructures. Compared with conventional lithographic fabrication techniques, this new approach uses graphene edges as the templates or masks and offers advantage in technological simplicity and capability of creating small features below 10 nm scale. Moreover, mask layers used in the fabrication process could be simultaneously used as the dielectric layers for top-gated devices. The as-fabricated graphene nanoribbons (GNRs) are of high quality with the carrier mobility ∼400 cm(2)/(V s) for typical 15 nm wide ribbons. Our technique allows easy and reproducible fabrication of various graphene nanostructures, such as ribbons and rings, and can be potentially extended to other materials and systems by use of their edges or facets as templates.
The electronic structure of a crystalline solid is largely determined by its lattice structure. Recent advances in van der Waals solids, artificial crystals with controlled stacking of two-dimensional (2D) atomic films, have enabled the creation of materials with novel electronic structures. In particular, stacking graphene on hexagonal boron nitride (hBN) introduces moir\'e superlattice that fundamentally modifies graphene's band structure and gives rise to secondary Dirac points (SDPs). Here we find that the formation of a moir\'e superlattice in graphene on hBN yields new, unexpected consequences: a set of tertiary Dirac points (TDPs) emerge, which give rise to additional sets of Landau levels when the sample is subjected to an external magnetic field. Our observations hint at the formation of a hidden Kekul\'e superstructure on top of the moir\'e superlattice under appropriate carrier doping and magnetic fields.
Control of the precise lattice alignment of monolayer molybdenum disulfide (MoS 2 ) on hexagonal boron nitride (h‐BN) is important for both fundamental and applied studies of this heterostructure but remains elusive. The growth of precisely aligned MoS 2 domains on the basal plane of h‐BN by a low‐pressure chemical vapor deposition technique is reported. Only relative rotation angles of 0° or 60° between MoS 2 and h‐BN basal plane are present. Domains with same orientation stitch and form single‐crystal, domains with different orientations stitch and from mirror grain boundaries. In this way, the grain boundary is minimized and a continuous film stitched by these two types of domains with only mirror grain boundaries is obtained. This growth strategy is also applicable to other 2D materials growth.
The determination of the electronic structure by edge geometry is unique to graphene. In theory, an evanescent nonchiral edge state is predicted at the zigzag edges of graphene. Up to now, the approach used to study zigzag-edged graphene has mostly been limited to scanning tunneling microscopy. The transport properties have not been revealed. Recent advances in hydrogen plasma-assisted "top-down" fabrication of zigzag-edged graphene nanoribbons (Z-GNRs) have allowed us to investigate edge-related transport properties. In this Letter, we report the magnetotransport properties of Z-GNRs down to ∼70 nm wide on an h-BN substrate. In the quantum Hall effect regime, a prominent conductance peak is observed at Landau ν=0, which is absent in GNRs with nonzigzag edges. The conductance peak persists under perpendicular magnetic fields and low temperatures. At a zero magnetic field, a nonlocal voltage signal, evidenced by edge conduction, is detected. These prominent transport features are closely related to the observable density of states at the hydrogen-etched zigzag edge of graphene probed by scanning tunneling spectroscopy, which qualitatively matches the theoretically predicted electronic structure for zigzag-edged graphene. Our study gives important insights for the design of new edge-related electronic devices.
The propagation of Dirac fermions in graphene through a long-period periodic potential would result in a band folding together with the emergence of a series of cloned Dirac points (DPs) [C.-H. Park et al., Nat. Phys. 4, 213 (2008); C.-H. Park et al.Phys. Rev. Lett. 101, 126804 (2008)]. In highly aligned graphene/hexagonal boron nitride (G/hBN) heterostructures, the lattice mismatch between the two atomic crystals generates a unique kind of periodic structure known as a moir\'e superlattice. Of particular interest is the emergent phenomena related to the reconstructed band-structure of graphene, such as the Hofstadter butterfly [L. A. Ponomarenko et al., Nature (London) 497, 594 (2013); B. Hunt et al., Science 340, 1427 (2013); C. R. Dean et al., Nature(London) 497, 598 (2013)], topological currents [R. V Gorbachev et al., Science 346, 448 (2014)], gate-dependent pseudospin mixing [Z. Shi et al., Nat. Phys. 10, 743 (2014)], and ballistic miniband conduction [M. Lee et al., Science 353, 1526 (2016)]. However, most studies so far have been limited to the lower-order minibands, e.g., the first and second minibands counted from charge neutrality, and consequently the fundamental nature of the reconstructed higher-order miniband spectra still remains largely unknown. Here we report on probing the higher-order minibands of precisely aligned graphene moir\'e superlattices by transport spectroscopy. Using dual electrostatic gating, the edges of these high-order minibands, i.e., the third and fourth minibands, can be reached. Interestingly, we have observed interband Landau level (LL) crossing inducing gap closures in a multiband magnetotransport regime, which originates from band overlap between the second and third minibands. As observed, high-order minibands and LL reconstruction qualitatively match our simulated results.
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