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Devices based on 2DMs van der Waals (vdW) heterostructures always compose of multiple contacts. Due to the instability of nanoscale 2DMs and interfaces, these contacts can be affected by the operation-induced photo or thermal effect. They can trigger the evolution of junctions and rearrange the junctions across a device, which are detrimental for applications. Herein, vdW heterostructure of indium selenide (InSe) and black phosphorus (BP) on Au electrodes are investigated to reveal the contact evolution and its relation to device performance. During operation, light irradiation changes the I-V characteristics from symmetry to strong rectification. Photocurrent mapping and Kelvin-probe force microscopy (KPFM) reveal triple junctions in this heterostructure, i.e., Au-InSe junction, InSe homojunction, and InSe-BP heterojunction. The variation of I-V characteristics of vdW heterostructure is ascribed to the evolution of Au-InSe junction from quasi-ohmic junction with a near-zero work function difference (Δφ) to a strong Schottky junction (Δφ = ≈0.27 eV). The stabilized device demonstrates distinguished time-domain response at individual junctions and overall device, indicating the evolution of contacts and the consequent opposite junction directions degrade the overall device performance. This research emphasizes the importance of dealing with heterogeneous contacts and junction directions in designing vdW heterostructure photodetectors. [ABSTRACT FROM AUTHOR]

The unique morphology of 2D van der Waals materials enables them to withstand large deformations and significant nonuniform strain, potentially inducing a strong flexoelectric effect. Despite the size‐dependent flexoelectric effect showing potential for modulating the optoelectronic performance of 2D van der Waals materials, it is far from being fully exploited owing to various challenges. Herein, the use of nanowires with different diameters as the bending media to fabricate 2D α‐In2Se3/β‐InSe heterojunctions with large curvatures of 0.1–1 µm−1 is proposed. The significant band alignment modulation in α‐In2Se3 resulting from the bending‐induced flexoelectric effect is verified through Kelvin probe force microscopy. The strain‐induced piezoelectric effect can be negated because of the weak vdW forces at the interface. The flexoelectric polarization in β‐InSe is screened via the accumulated electrons in the unilateral depleted heterojunction. Compared to the flat heterojunction, the curved heterojunction with an average curvature of 0.9 µm−1 shows 2.48‐fold and 7.62‐fold increases in open‐circuit voltage and zero‐biased responsivity, respectively. This study demonstrates the first successful modulation of photodetection in 2D heterojunctions by exploiting the flexoelectric effect, providing a new perspective for high‐performance 2D vdW optoelectronic devices. [ABSTRACT FROM AUTHOR]

Monolayer transition metal dichalcogenides (TMDs) have drawn significant attention for their potential applications in electronics and optoelectronics. To achieve consistent electronic properties and high device yield, uniform large monolayer crystals are crucial. In this report, we describe the growth of high-quality and uniform monolayer WSe2 film using chemical vapor deposition on polycrystalline Au substrates. This method allows for the fabrication of continuous large-area WSe2 film with large-size domains. Additionally, a novel transfer-free method is used to fabricate field-effect transistors (FETs) based on the as-grown WSe2. The exceptional metal/semiconductor interfaces achieved through this fabrication method result in monolayer WSe2 FETs with extraordinary electrical performance comparable to those with thermal deposition electrodes, with a high mobility of up to ≈62.95 cm2 V−1 s−1 at room temperature. In addition, the as-fabricated transfer-free devices can maintain their original performance after weeks without obvious device decay. The transfer-free WSe2-based photodetectors exhibit prominent photoresponse with a high photoresponsivity of ~1.7 × 104 A W−1 at Vds = 1 V and Vg = −60 V and a maximum detectivity value of ~1.2 × 1013 Jones. Our study presents a robust pathway for the growth of high-quality monolayer TMDs thin films and large-scale device fabrication. [ABSTRACT FROM AUTHOR]