Strain engineering is a promising method to manipulate the electronic and optical properties of two-dimensional (2D) materials. However, with weak van der Waals interaction, severe slippage between 2D material and substrate could dominate the bending or stretching processes, leading to inefficiency strain transfer. To overcome this limitation, we report a simple strain engineering method by encapsulating the monolayer 2D material in the flexible PVA substrate through spin-coating approach. The strong interaction force between spin-coated PVA and 2D material ensures the mechanical strain can be effectively transferred with negligible slippage or decoupling. By applying uniaxial strain to monolayer MoS 2 , we observe a higher bandgap modulation up to~300 meV and a highest modulation rate of~136 meV/%, which is approximate two times improvement compared to previous results achieved. Moreover, this simple strategy could be well extended to other 2D materials such as WS 2 or WSe 2 , leading to enhanced bandgap modulation.

Two-dimensional (2D) semiconductors have attracted considerable attention for the development of ultra-thin body transistors. However, the polarity control of 2D transistors and the achievement of complementary logic functions remain critical challenges. Here, we report a doping-free strategy to modulate the polarity of WSe 2 transistors using same contact metal but different integration methods. By applying low-energy van der Waals integration of Au electrodes, we observed robust and optimized p-type transistor behavior, which is in great contrast to the transistors fabricated on the same WSe 2 flake using conventional deposited Au contacts with pronounced n-type characteristics. With the ability to switch majority carrier type and to achieve optimized contact for both electrons and holes, a doping-free logic inverter is demonstrated with higher voltage gain of 340, at the bias voltage of 5.5 V. Furthermore, the simple polarity control strategy is extended for realizing more complex logic functions such as NAND and NOR.

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Abstract2D metals have attracted considerable recent attention for their special physical properties, such as charge density waves, magnetism, and superconductivity. However, despite some recent efforts, the synthesis of ultrathin 2D metals nanosheets down to monolayer thickness remains a significant challenge. Herein, by using atomically flat 2D WSe2 or WS2 as the growth substrate, the synthesis of atomically thin 2D metallic MTe2 (M = V, Nb, Ta) single crystals with the thickness down to the monolayer regime and the creation of atomically thin MTe2/WSe2 (WS2) vertical heterojunctions is reported. Comparison with the growth on the SiO2/Si substrate under the same conditions reveals that the utilization of the dangling‐bond‐free WSe2 or WS2 as the van der Waals epitaxy substrates is crucial for the successful realization of atomically thin MTe2 (M = V, Nb, Ta) nanosheets. It is further shown that the epitaxial grown 2D metals can function as van der Waals contacts for 2D semiconductors with little interface damage and improved electronic performance. This study defines a robust van der Waals epitaxy pathway to ultrathin 2D metals, which is essential for fundamental studies and potential technological applications of this new class of materials at the 2D limit.

Two-dimensional (2D) Ruddlesden-Popper perovskites are currently drawing significant attention as highly-stable photoactive materials for optoelectronic applications. However, the insulating nature of organic ammonium layers in 2D perovskites results in poor charge transport and limited performance. Here, we demonstrate that Al 2 O 3 /2D perovskite heterostructure can be utilized as photoactive dielectric for high-performance MoS 2 phototransistors. The type-II band alignment in 2D perovskites facilitates effective spatial separation of photo-generated carriers, thus achieving ultrahigh photoresponsivity of >10 8 A/ W at 457 nm and >10 6 A/W at 1064 nm. Meanwhile, the hysteresis loops induced by ionic migration in perovskite and charge trapping in Al 2 O 3 can neutralize with each other, leading to low-voltage phototransistors with negligible hysteresis and improved bias stress stability. More importantly, the recombination of photo-generated carriers in 2D perovskites depends on the external biasing field. With an appropriate gate bias, the devices exhibit wavelengthdependent constant photoresponsivity of 10 3-10 8 A/W regardless of incident light intensity.

Van der Waals heterostructures (vdWHs) have attracted tremendous interest owing to the ability to assemble diverse building blocks without the constraints of lattice matching and processing compatibility. However, once assembled, the fabricated vdWHs can hardly be separated into individual building blocks for further manipulation, mainly due to technical difficulties in the disassembling process. Here, we show a method to disassemble the as-fabricated vdWHs into individual building blocks, which can be further reassembled into new vdWHs with different device functionalities. With this technique, we demonstrate reconfigurable transistors from n-type to p-type and back-gate to dual-gate structures through re-stacking. Furthermore, reconfigurable device behaviors from floating gate memory to Schottky diode and reconfigurable anisotropic Raman behaviors have been obtained through layer re-sequencing and re-twisting, respectively. Our results could lead to a reverse engineering concept of disassembled vdWHs electronics in parallel with state-of-the-art vdWHs electronics, offering a general method for multi-functional pluggable electronics and optoelectronics with limited material building blocks.

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Numerous efforts have been made to synthesize 2D atomic semiconductor materials and their heterojunctions because of the diverse novel properties and potential applications in constructing next‐generation highly compact electronic and optoelectronic devices. However, intrinsic 2D p‐type semiconductor materials are still scarce. Herein, to enrich the p‐type 2D semiconductor family, epitaxial growth of a large‐area, ultrathin 2D nonlayered p‐type semiconductor α‐MnSe on mica with the thickness down to one unit crystal cell (0.9 nm) is reported. Moreover, the thickness of the α‐MnSe nanosheets can be systematically tailored from over 150 to 0.9 nm by increasing the flow rate of the carrier gas. X‐ray‐diffraction, transmission electron microscopy, and electron diffraction studies confirm that the resulting 2D nanosheets are high‐quality single crystals. The photodetector based on the p‐type α‐MnSe nanosheet shows a fast response time of 4 ms. Furthermore, α‐MnSe/WS2 heterojunctions are synthesized and a diode based on p‐type α‐MnSe and n‐type WS2 displays outstanding photodetectivity (1.00 × 1013 Jones), high photoresponsivity (49.1 A W−1), and an obvious rectification ratio (283). Together, the synthesis of α‐MnSe and the α‐MnSe/WS2 p–n heterojunction provides opportunities for next‐generation electronics and optoelectronics.