We report an electron transport study of lithographically fabricated graphene nanoribbons (GNRs) of various widths and lengths. At the charge neutrality point, a length-independent transport gap forms whose size is inversely proportional to the GNR width. In this gap, electrons are localized, and charge transport exhibits a transition between thermally activated behavior at higher temperatures and variable range hopping at lower temperatures. By varying the geometric capacitance, we find that charging effects constitute a significant portion of the activation energy.

The electronic structure of bilayer graphene is investigated from a resonant Raman study using different laser excitation energies. The values of the parameters of the Slonczewski-Weiss-McClure model for graphite are measured experimentally and some of them differ significantly from those reported previously for graphite, specially that associated with the difference of the effective mass of electrons and holes. The splitting of the two TO phonon branches in bilayer graphene is also obtained from the experimental data. Our results have implications for bilayer graphene electronic devices.

We present a fast method to fabricate high quality heterostructure devices by picking up crystals of arbitrary sizes. Bilayer graphene is encapsulated with hexagonal boron nitride to demonstrate this approach, showing good electronic quality with mobilities ranging from 17 000 cm 2 V −1 s −1 at room temperature to 49 000 cm 2 V −1 s −1 at 4.2 K, and entering the quantum Hall regime below 0.5 T. This method provides a strong and useful tool for the fabrication of future high quality layered crystal devices.A critical step for high mobility graphene device fabrication and the rising field of van der Waals heterostructures[1] is marked by the development of polymer based dry transfer methods for two-dimensional (2D) crystals. [2][3][4][5] With these methods, high quality graphene devices on hexagonal boron nitride (h-BN) and more complicated stacks have become generally accessible, but the early methods face a major setback. The method of stacking the crystals one by one typically leaves each transferred crystal contaminated by polymer. To obtain a high quality device, thorough cleaning is required before proceeding with device fabrication or measurement. This cleaning step typically involves several hours of annealing [3][4][5] or it may go as far as nanobrooming the entire graphene flake using contact mode atomic force microscopy (AFM). [6,7] Altogether this makes the fabrication of a multilayer heterostructure not only very time consuming, but also risky as each step may again introduce contaminants to the stack. This issue has recently been overcome by L. Wang et al. [8], introducing a method that allows for polymer free assembly of layered materials based on van der Waals force. Instead of depositing a 2D crystal, e.g. h-BN, directly on top of another crystal, e.g. graphene, one can use the h-BN to pick up the graphene from the substrate. This can be done because the van der Waals force between the atomically flat h-BN and graphene is stronger than between the graphene or the h-BN and the rough SiO 2 substrate. The power of this method lies in the fact that now the interface between the two crystals has not been contaminated by polymer, and one can directly pick up the next crystal. This way the materials inside the stack not only remain much cleaner, but a stack can also be fabricated considerably faster. One problem when using this method is the reduced capability to pick up graphene flakes larger than the used top h-BN crystal. Therefore one has to etch through the stack before making one-dimensional (1D) contacts to the graphene. While resulting in very high quality devices with electron mean free paths up to 21 µm and good electrical * pj.zomer@gmail.com contact, [8] this limitation can be problematic for certain device types for which 1D contacts are not desirable, e.g. spintronic devices that include tunnel barriers at the contact interface. [9] In this letter we present a method which allows for fabrication of high quality graphene devices encapsulated in h-BN by successively picking up crystals. The a...

We experimentally study the electronic spin transport in hBN encapsulated single layer graphene nonlocal spin valves. The use of top and bottom gates allows us to control the carrier density and the electric field independently. The spin relaxation times in our devices range up to 2 ns with spin relaxation lengths exceeding 12 µm even at room temperature. We obtain that the ratio of the spin relaxation time for spins pointing out-of-plane to spins in-plane is τ ⊥ /τ || ≈ 0.75 for zero applied perpendicular electric field. By tuning the electric field this anisotropy changes to ≈0.65 at 0.7 V/nm, in agreement with an electric field tunable in-plane Rashba spin-orbit coupling.PACS numbers: 72.80. Vp, 85.75.Hh Keywords: Graphene, spin transport, Rashba spin-orbit interaction, anisotropic spin relaxation, Hanle precession, electric fieldThe generation, manipulation and detection of spin information has been the target of several studies due to the implications for novel spintronic devices [1, 2]. In the recent years graphene has attracted a lot of attention in spintronics due to its theoretically large intrinsic spin relaxation time and length of the order of τ s ≈ 100 ns and λ s ≈ 100 µm respectively [4, 9]. Although experimental results still fall short of these expectations [3][4][5] 7], graphene has already achieved the longest measured nonlocal spin relaxation length [5, 9] and furthest transport of spin information at room temperature [10]. However, the mechanisms for spin relaxation in graphene are still under heavy debate with various theoretical models proposed [4, 8, 9,[11][12][13].To take advantage of the long spin relaxation times in graphene, e.g. for spin logic devices, one requires easy control of the spin information, for example by an applied electric field. Single layer graphene is an ideal system for this purpose, not only because of its high mobilities and low intrinsic spin-orbit fields (SOF), but also due to the simple relation between the carriers' wavevector, the applied perpendicular electric field and the induced Rashba SOF [4, 7, 9,[15][16][17][18][19]. In bilayer graphene a more complicated behavior is expected when spin-orbit coupling is considered [21].Here we report nonlocal spin transport measurements on single layer graphene in which we address both topics specified above. Our devices consist of a single layer graphene flake on hexagonal Boron Nitride (hBN) of which a central region is encapsulated with another hBN flake and hence protected from the environment. The presence of a top and bottom gate give rise to two independent electric fields that are experienced by the graphene: [22], where tg(bg) ≈ 3.9 is the dielectric constant, d tg(bg) is the dielectric thickness and V 0 tg(bg) the position of the charge neutrality point for the top (bottom) gate. Their difference controls the carrier density in the graphene (n = (E bg − E tg ) 0 /e) and their average gives the effective electric field experienced by the graphene (Ē = (E tg + E bg )/2), which breaks the inversion symmetry in t...

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In this work, we clarify the features of the lateral damage of line defects in single layer graphene. The line defects were produced through well-controlled etching of graphene using a Ga(+) focused ion beam. The lateral damage length was obtained from both the integrated intensity of the disorder induced Raman D band and the minimum ion fluence. Also, the line defects were characterized by polarized Raman spectroscopy. It was found that graphene is resilient under the etching conditions since the intensity of the defect induced Raman D peak exhibits a dependence on the direction of the lines relative to the crystalline lattice and also on the direction of the laser polarization relative to the lines. In addition, electrical measurements of the modified graphene were performed. Different ion fluences were used in order to obtain a completely insulating defect line in graphene, which was determined experimentally by means of charge injection and electric force microscopy measurements. These studies demonstrate that a Ga+ ion column combined with Raman spectroscopy is a powerful technique to produce and understand well-defined periodic arrays of defects in graphene, opening possibilities for better control of nanocarbon devices.

Low symmetry 2D materials offer an alternative for the fabrication of optoelectronic devices which are sensitive to light polarization. The investigation of electron–phonon interactions in these materials is essential since they affect the electrical conductivity. Raman scattering probes light–matter and electron–phonon interactions, and their anisotropies are described by the Raman tensor. The tensor elements can have complex values, but the origin of this behavior in 2D materials is not yet well established. In this work, we studied a single-layer triclinic ReSe2 by angle-dependent polarized Raman spectroscopy. The obtained values of the Raman tensor elements for each mode can be understood by considering a new coordinate system, which determines the physical origin of the complex nature of the Raman tensor elements. Our results are explained in terms of anisotropy of the electron–phonon coupling relevant to the engineering of new optoelectronic devices based on low-symmetry 2D materials.

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