The emergence of flat electronic bands and of the recently discovered strongly correlated and superconducting phases in twisted bilayer graphene crucially depends on the interlayer twist angle upon approaching the magic angle ߠ ெ ≈ 1.1°. Although advanced fabrication methods allow alignment of graphene layers with global twist angle control of about 0.1°, little information is currently available on the distribution of the local twist angles in actual magic angle twisted bilayer graphene (MATBG) transport devices. Here we map the local ߠ variations in hBN encapsulated devices with relative precision better than 0.002° and spatial resolution of a few moiré periods. Utilizing a scanning nanoSQUID-on-tip, we attain tomographic imaging of the Landau levels in the quantum Hall state in MATBG, which provides a highly sensitive probe of the charge disorder and of the local band structure determined by the local ߠ. We find a correlation between the degree of twist angle disorder and the quality of the typical MATBG transport characteristics. However, even state-of-the-art transport devices, exhibiting pronounced global MATBG features, such as multiple correlated insulator states, high-quality Landau fan diagrams, and superconductivity, display significant variations in the local ߠ with a span that can be close to 0.1°. Devices may even have substantial areas where no local MATBG behavior is detected, yet still display global MATBG characteristics in transport, highlighting the importance of percolation physics. The derived ߠ maps reveal substantial gradients and a network of jumps. We show that the twist angle gradients generate large unscreened electric fields that drastically change the quantum Hall state by forming edge states in the bulk of the sample, and may also significantly affect the phase diagram of correlated and superconducting states. The findings call for exploration of band structure engineering utilizing twist-angle gradients and gate-tunable built-in planar electric fields for novel correlated phenomena and applications.

The search of a p-type metal contact on MoS2 has remained inconclusive, with high work function metals such as Au, Ni, and Pt showing n-type behavior and mixed reports of n as well as p-type behavior for Pd. In this work, we report quantitative Schottky barrier heights for Au and Pd contacts to MoS2 obtained by analysing low temperature transistor characteristics and contact resistance data obtained using the transfer length method. Both Au and Pd exhibit n-type behavior on multilayer as well as monolayer MoS2 transistors with Schottky barrier heights of 0.126 eV and 0.4 eV, and contact resistances of 42 Ω.mm and 18 × 104 Ω.mm respectively. Scanning photocurrent spectroscopy data is in agreement with the resulting energy band alignment in Au-MoS2-Pd devices further reinforcing the observation that the Fermi-level is pinned in the upper half of MoS2 bandgap.

Rhenium disulfide (ReS2) is an attractive candidate for photodetection applications owing to its thickness-independent direct band gap. Despite various photodetection studies using two-dimensional semiconductors, the trade-off between responsivity and response time under varying measurement conditions has not been studied in detail. This report presents a comprehensive study of the architectural, laser power and gate bias dependence of responsivity and speed in supported and suspended ReS2 phototransistors. Photocurrent scans show uniform photogeneration across the entire channel because of enhanced optical absorption and a direct band gap in multilayer ReS2. A high responsivity of 4 A W–1 (at 50 ms response time) and a low response time of 20 μs (at 4 mA W–1 responsivity) make this one of the fastest reported transition-metal dichalcogenide photodetectors. Occupancy of intrinsic (bulk ReS2) and extrinsic (ReS2/SiO2 interface) traps is modulated using gate bias to demonstrate tunability of the response time (responsivity) over 4 orders (15×) of magnitude, highlighting the versatility of these photodetectors. Differences in the trap distributions of suspended and supported channel architectures, and their occupancy under different gate biases enable switching the dominant operating mechanism between either photogating or photoconduction. Further, a new metric that captures intrinsic photodetector performance by including the trade-off between its responsivity and speed, besides normalizing for the applied bias and geometry, is proposed and benchmarked for this work.

Superlattice in graphene generates extra Dirac points in the band structure and their number depends on the superlattice potential strength. Here, we have created a lateral superlattice in a graphene device with a tunable barrier height using a combination of two gates. In this Letter, we demonstrate the use of lateral superlattice to modify the band structure of graphene leading to the emergence of new Dirac cones. This controlled modification of the band structure persists up to 100 K.

We demonstrate a simple technique to transfer chemical vapour deposited (CVD) graphene from copper and platinum substrates using a soak-and-peel delamination technique utilizing only hot deionized water. The lack of chemical etchants results in cleaner CVD graphene films minimizing unintentional doping, as confirmed by Raman and electrical measurements. The process allows the reuse of substrates and hence can enable the use of oriented substrates for growth of higher quality graphene, and is an inherently inexpensive and scalable process for large-area production.

The recently predicted topological magnetoelectric effect [1] and the response to an electric charge that mimics an induced mirror magnetic monopole [2] are fundamental attributes of topological states of matter with broken time reversal symmetry. Using a SQUID-on-tip [3], acting simultaneously as a tunable scanning electric charge and as ultrasensitive nanoscale magnetometer, we induce and directly image the microscopic currents generating the magnetic monopole response in a graphene quantum Hall electron system. We find a rich and complex nonlinear behavior governed by coexistence of topological and nontopological equilibrium currents that is not captured by the monopole models [2]. Furthermore, by utilizing a tuning fork that induces nanoscale vibrations of the SQUID-on-tip, we directly image the equilibrium currents of individual quantum Hall edge states for the first time. We reveal that the edge states that are commonly assumed to carry only a chiral downstream current, in fact carry a pair of counterpropagating currents [4], in which the topological downstream current in the incompressible region is always counterbalanced by heretofore unobserved nontopological upstream current flowing in the adjacent compressible region. The intricate patterns of the counterpropagating equilibrium-state orbital currents provide new insights into the microscopic origins of the topological and nontopological charge and energy flow in quantum Hall systems. * Corresponding authors SM1. Device fabricationThree graphene based van der Waals heterostructures were measured (Fig. S1). All devices consisted of an hBN/graphene/hBN stack placed on top of the 300 nm thick SiO 2 layer of a thermally oxidized doped silicon wafer, acting as a backgate. A graphitic layer was placed under part of the stack, serving as an additional backgate. The two backgates allowed to induce an interface of two different filling factors, ߥ and ߥ ோ , at the boundary of the graphitic layer (Fig. 3a). The van der Waals stacking of device A, was carried out with the viscoelastic transfer method as explained in Ref. [32]. Device B and C were created with the ELVACITE based pick-up method reported in Refs. [32,33]. In order to minimize the SOT distance to graphene, we used a relatively thin top hBN layer with a thickness of approximately 8 nm (devices A and C) and 11.5 nm (device B). The bottom hBN layer was 23 nm (device A) and 50 nm (devices B and C). The graphite backgate layer had a thickness of approximately 5 nm. The heterostructures were annealed in an Ar/H 2 forming gas atmosphere at 500°C to remove bubbles and wrinkles prior to further processing. Patterning was performed using electron beam lithography and etching as described in Ref.[34]. Contacts and leads were fabricated by thermal evaporation of a 10 nm thick Cr adhesion layer followed by a 50-70 nm Au layer. The SOT scanning studies require an exceptionally clean surface. To ensure this, extra cleaning steps were carried out. After lift-off, devices were re-annealed at 350°C. Contact mode atomic...

The search of a p-type metal contact on MoS2 has remained inconclusive, with high work-function metals such as Au, Ni and Pt showing n-type behavior [1] and mixed reports of n as well as p-type behavior for Pd. In this work we report for the first time, quantitative band alignment of Pd and Au-MoS2 interfaces using low temperature and scanning photocurrent measurements on MoS2 transistors with varying metal contacts (Au-Au, Pd-Pd and Au-Pd). Our results indicate n-type behavior for Pd contacts on multilayer as well as monolayer MoS2 b) of nearly 0.5 eV, four times that for Au contacts indicating that the MoS2 Fermi-level is pinned in the upper half of MoS2 bandgap. Molybdenum disulphide (MoS2) has emerged as an attractive candidate for future CMOS applications owing to its graphenelike 2D nature but with a bandgap of nearly 1.2 eV (for multilayer MoS2). High on/off current ratios with sub-threshold slope ~60 mV/decade and high mobility (>100 cm 2 /Vs) have been reported [1][2][3]. The n/p type behavior of MoS2 transistors is controlled by the contacts (electron vs hole injection). Till date p-type MoS2 transistors have been difficult to demonstrate due to the n-type behavior of high workm) metals on MoS2 [1]. However some recent reports of Pd contacts have reported n as well as p-type behavior [4-7] ( Table 1). In this work we have fabricated back-gated (p + -Si/SiO2 300 nm) transistors on exfoliated MoS2 flakes consisting of a single as well as ~20 stacked layers of MoS2. An optical image and a schematic of a fabricated device are shown in Figures 1(a) and (b). Source/drain contacts with different metal configurations (Au-Au, Pd-Pd and Au-Pd) were fabricated using e-beam lithography and e-beam evaporation. Figure 1(c) shows the alignment of Au and Pd work-function with MoS2 energy bands in free and in contact mode. Figures 1(d) and (e) show the Raman and XPS spectroscopy signatures of monolayer MoS2 measured on fabricated devices respectively. For multilayer flakes, AFM (not shown) measurements were carried out to estimate the number of device layers (~20). Figures 2 and 3 show the ID-VD characteristics of multi and monolayer MoS2 at fixed Vgs indicating (a) different Schottky barrier heights for Au and Pd due to the asymmetric nature of the Aub for Pd/MoS2 interface vs Au due to the higher resistance of the Pd-Pd device. Figure 4 shows the ID-VG transfer characteristics indicating the n-type behavior of Au and Pd contacts irrespective of the thickness of the MoS2 channel layer. Figures 5 and 6 show variable temperature ID-VG and Arrhenius plots (ID vs 1000/T) for the Au-MoS2-Au device respectively. Similar measurements on Pd devices were used to extract the Pd/MoS2 and Au/MoS2 b GS plots as shown in Figure 7 [1]. The effective b for Pd/MoS2 is 0.5 eV, nearly four times that for the Au/MoS2 interface (0.12 eV). Inspite of the b, Pd makes an n-type contact on MoS2, since the band gap for bulk MoS2 is 1.2 eV. Scanning photocurrent microscopy on Au-Au and Au-Pd devices ( Figure 8) further helps to identify the rela...

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