We report on the demonstration of atomic layer van der Waals (vdW) heterostructure photodiodes operating in the visible regime, enabled by stacking single- to few-layer n-type molybdenum disulfide (MoS2) on top of few-layer p-type gallium selenide (GaSe) crystals. The atomic layer vdW photodiode exhibits an excellent photoresponsivity of ∼3A/W at the wavelength of 532 nm when symmetric few-layer graphene (FLG) contacts with low contact resistance are used. On the other hand, for a GaSe/MoS2 photodiode with asymmetric GaSe/FLG and MoS2/gold (Au) contacts, a very low noise equivalent power of NEP ∼ 10–14 W/ is obtained due to dark current reduction, which demonstrates the feasibility of detecting sub-pW (<10–12 W) level optical illumination. Further, the same photodiode exhibits a large linear dynamic range of DR ≈ 70 dB due to the remarkable photocurrent to dark current ratio. These results show that not only the p–n junction formed at the interface between p-type GaSe and n-type MoS2 but also the metal–semiconductor junction with each 2D material play a pivotal role in determining the diode characteristics and photoresponse of the vdW photodiodes.

Two-dimensional (2D) layered molybdenum ditelluride (MoTe 2 ) crystals, featuring a low energy barrier in the crystalline phase transition and a sizable band gap close to that of silicon, are rapidly emerging with substantial potential and promise for future nanoelectronics. It has been challenging, however, to realize n-type MoTe 2 field-effect transistors (FETs), thus complementary logic, because MoTe 2 FETs mainly exhibit p-type behavior. Here, we report a dopant-free method for controlling polarity of MoTe 2 FETs by modifying Schottky barriers at their MoTe 2 −metal contacts via thermal annealing. Upon annealing, MoTe 2 FETs encapsulated by hexagonal boron nitride (h-BN) are consistently changed from hole to electron conduction, displaying an on/off current ratio of 10 5 or higher. When the MoTe 2 channel is sandwiched between top and bottom h-BN thin layers (h-BN/MoTe 2 /h-BN FETs), higher field-effect mobility is attained, up to 48.1 cm 2 V −1 s −1 (hole) and 52.4 cm 2 V −1 s −1 (electron) before and after thermal annealing, respectively. The thermally controlled FET polarity change further enables high-performance MoTe 2 monolithic complementary inverters with gain as high as 36, suggesting this simple and effectual approach may lead to compelling possibilities of rationally controlling transport polarity, on demand, in atomically thin transistors with metal contacts and their 2D integrated circuits.

We report on the experimental demonstration of atomically thin molybdenum disulfide (MoS2)graphene van der Waals (vdW) heterostructure nanoelectromechanical resonators with ultrawide frequency tuning. With direct electrostatic gate tuning, these vdW resonators exhibit exceptional tunability, in general, Δf/f0 >200%, for continuously tuning the same device and the same mode (e.g., from ~23 to ~107MHz), up to Δf/f0370%, the largest fractional tuning range in such resonators to date. This remarkable electromechanical resonance tuning is investigated by two different analytical models and finite element simulations. Further, we carefully perform clear control experiments and simulations to elucidate the difference in frequency tuning between heterostructure and single-material resonators. At a given initial strain level, the tuning range depends on the two-dimensional (2D) Young's moduli of the constitutive crystals; devices built on materials with lower 2D moduli show wider tuning ranges. This study exemplifies that vdW heterostructure resonators can retain unconventionally broad, continuous tuning, which is promising for voltage-controlled, tunable nanosystems.

Atomic layer semiconducting black phosphorus (P) exfoliated from its bulk crystals offers excellent properties and promises for emerging two-dimensional (2D) electronics, photonics, and transducers. It also possesses unique strong inplane anisotropy among many 2D semiconductors, stemming from its corrugated crystal structure. As an important thermophysical aspect, probing the anisotropic thermal conductivity of black P is essential for device engineering, especially for energy dissipation and thermal management. Here, we report on measurement and analysis of anisotropic in-plane thermal conductivity of black P crystal, in a mechanically suspended device platform, by exploiting a novel opto-thermomechanical resonance spectromicroscopy (OTMRS) technique. With spatially resolved heating effects and thermomechanical resonance motions of suspended structures, anisotropic in-plane thermal conductivity (κ AC and κ ZZ ) is determined for black P crystals of 10−100 nm thick. This study validates a new noninvasive approach to determining anisotropic thermal conductivity without any requirement of preknowledge of crystal orientation or specific configurations of structure and electrodes according to the anisotropy.

Atomically thin semiconductors such as transition metal dichalcogenides have recently enabled diverse devices in the emerging two-dimensional (2D) electronics. While scalable 2D electronics demand monolithic integrated circuits consisting of complementary p-type and n-type transistors, conventional p-type and n-type doping in desired regions, monolithically in the same semiconducting atomic layers, remains elusive or impractical. Here, we report on an agile, high-precision scanning laser annealing approach to realizing 2D monolithic complementary logic circuits on atomically thin MoTe2, by reliably designating p-type and n-type transport polarity in the constituent transistors via localized laser annealing and modification of their Schottky contacts. Pristine p-type field-effect transistors (FETs) transform into n-type ones upon controlled laser annealing on their source/drain gold electrodes, exhibiting a mobility of 96.5 cm2 V–1 s–1 (the highest known to date) and an On/Off ratio of 106. Elucidation and validation of such an on-demand configuration of polarity in MoTe2 FETs further enable the construction and demonstration of essential logic circuits, including both inverter and NOR gates. This dopant-free, spatially precise scanning laser annealing approach to configuring monolithic complementary logic integrated circuits may enable programmable functions in 2D semiconductors, exhibiting potential for additively manufactured, scalable 2D electronics.