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.