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We report the demonstration of defect-assisted tunneling spectroscopy of the electronic band structure in twisted bilayer graphene (tBLG)/hexagonal boron nitride ( h-BN) moiré superlattices in which the moiré period between the two graphene layers is close to that between the graphene and h-BN layers. We measured both the in-plane and vertical carrier transport in the tBLG/ h-BN van der Waals (vdW) tunneling device. The moiré periods were determined from the in-plane carrier transport measurements. The observed vertical tunneling transport characteristics indicated that resonant tunneling occurs from the graphite electrode to tBLG through localized defect states in the h-BN tunnel barrier. We observed multiple defect-assisted resonant tunneling trajectories, from which we derived the density of states (DOS) for tBLG. The obtained DOS has broad flatband features, in qualitative agreement with the theoretical predictions. Furthermore, we obtained three types of DOS, suggesting that we probed local band structures corresponding to AA, AB/BA, and domain wall sites in tBLG. Thus, defect-assisted tunneling spectroscopy has potential as a tool to determine the local band structures in twisted 2D vdW materials.

We demonstrate twist-controlled resonant tunneling in a monolayer graphene (MLG)/hexagonal boron nitride ( h-BN)/ABA-stacked trilayer graphene (TLG) van der Waals (vdW) junction, in which MLG and TLG flakes are aligned with a small twist angle θ of ∼1.05° between their crystallographic orientations through a thin h-BN barrier. Owing to the small interlayer twist, resonant tunneling attributed to the conservation of momentum and energy was observed between the single linear band of MLG and multiple subbands of TLG. We show that different subbands of TLG—bilayer-graphene-like subbands and a MLG-like subband—exhibit distinctly different resonant tunneling behaviors. Therefore, we demonstrate subband-resolved resonant tunneling. This technique provides a method to determine band parameters (Slonczewski–Weiss–McClure parameters) and probes the band dispersion of different two-dimensional materials by utilizing a MLG electrode.

Semiconducting group-VI-B transition metal dichalcogenides (TMDs) with the triangular prism crystal structure exhibit unique features. A particularly striking recent example is the observation of an Ising superconducting transition at high carrier densities induced through the field effect in an electric double layer transistor (EDLT). So far, this phenomenon has been accounted for by assuming that the original triangular prismatic bulk crystal structure is unaffected by the heavy doping. However, this assumption has not been verified so far, even though other crystal phases of these layered materials were shown to host Ising superconductivity as well. Here, we provide experimental evidence that the initial triangular prismatic crystal structure of the MoS2, which serves here as a representative member of the TMD family, is not altered during operation of the EDLT even when the carrier density is large enough to induce the superconducting transition. The latter is monitored via transport measurements, while the crystal structure is simultaneously examined in situ with micro-Raman spectroscopy at low temperature.

The coupling between the valley degree of freedom and the optical helicity is one of the unique phenomena in transition metal dichalcogenides. The significant valley polarization evaluated from circularly polarized photoluminescence (PL) has been reported in many transition metal dichalcogenides, except in MoSe2. This compound is an anomalous material showing ultra-fast relaxation of the valley polarized states, which causes negligible polarization in the PL. Meanwhile, circularly polarized electroluminescence (EL) has been recently reported in a WSe2 light-emitting transistor, providing another method for using the valley degree of freedom. Here, we report the EL properties of MoSe2, demonstrating electrical switching of the optical helicity. Importantly, we observed high circular polarization reaching 66%. The results imply that the dominant mechanism of circularly polarized EL is robust against intervalley scattering, in marked contrast to the PL.

We fabricated stretchable molybdenum disulfide thin-film transistors (MoS2 TFTs) on poly(dimethylsiloxane) substrates using ion gels as elastic gate dielectrics. The TFTs exhibited an electron mobility of 1.40 cm2/(V·s) and an on/off current ratio of 104 with a notably low threshold voltage (∼1 V). Furthermore, our MoS2 TFTs operated at a mechanical strain of 5% without significant degradation of their electrical properties. These results demonstrate the potential for using MoS2 films for stretchable electronics.