van der Waals (vdW) heterostructures based on atomically thin 2D materials have led to a new era in next-generation optoelectronics due to their tailored energy band alignments and ultrathin morphological features, especially in photodetectors. However, these photodetectors often show an inevitable compromise between photodetectivity and photoresponsivity with one high and the other low. Herein, a highly sensitive WSe /SnS photodiode is constructed on BN thin film by exfoliating each material and manually stacking them. The WSe /SnS vdW heterostructure shows ultralow dark currents resulting from the depletion region at the junction and high direct tunneling current when illuminated, which is confirmed by the energy band structures and electrical characteristics fitted with direct tunneling. Thus, the distinctive WSe /SnS vdW heterostructure exhibits both ultrahigh photodetectivity of 1.29 × 10 Jones (I /I ratio of ≈10 ) and photoresponsivity of 244 A W at a reverse bias under the illumination of 550 nm light (3.77 mW cm ).

Van der Waals heterostructures (vdWHs) based on 2D layered materials with selectable materials properties pave the way to integration at the atomic scale, which may give rise to fresh heterostructures exhibiting absolutely novel physics and versatility. This feature article reviews the state-of-the-art research activities that focus on the 2D vdWHs and their optoelectronic applications. First, the preparation methods such as mechanical transfer and chemical vapor deposition growth are comprehensively outlined. Then, unique energy band alignments generated in 2D vdWHs are introduced. Furthermore, this feature article focuses on the applications in light-emitting diodes, photodetectors, and optical modulators based on 2D vdWHs with novel constructions and mechanisms. The recently reported novel constructions of the devices are introduced in three primary aspects: light-emitting diodes (such as single defect light-emitting diodes, circularly polarized light emission arising from valley polarization), photodetectors (such as photo-thermionic, tunneling, electrolyte-gated, and broadband photodetectors), and optical modulators (such as graphene integrated with silicon technology and graphene/hexagonal boron nitride (hBN) heterostructure), which show promising applications in the nextgeneration optoelectronics. Finally, the article provides some conclusions and an outlook on the future development in the field.WSe 2 /MoS 2 along the [001] zone axis in Figure 1c demonstrates that in this specific hetero-bilayer structure, the two hexagonal reciprocal lattices are rotated by 12.5° with respect to each layer without obvious lattice strain, resulting in moiré fringes with a spatial periodicity on the order of four to six times the lattice constants of each layer. [76] The atomically sharp interface of the heterostructure can be obtained by this stacking process confirmed by the high-resolution cross-sectional scanning transmission electron microscope image of the heterostructure (Figure 1d). Furthermore, the complex 2D vdWHs with more stacking layers can be realized by this mechanical transfer process.Mechanical transfer process provides a lot of flexibility in constructing diverse 2D vdWHs with various materials which may give rise to fresh physical properties, it is not scalable, which is imperative for further practical applications in electronics and optoelectronics. Alternatively, the bottom-up method such as direct CVD synthesis of 2D vdWHs has been successful in synthesizing graphene-or transition metal dichalcogenide (TMD)-based vdWHs, [77][78][79] which shows promising applications in scalable production. CVD GrowthCVD growth has shown booming development in the last decades such as the CVD growth of graphene [80,81] and TMDs, [82,83] and has been employed for synthesizing 2D vdWHs recently. [84] The most used method for CVD growth of 2D vdWHs is that evaporating the target sources such as WS 2 , WSe 2 , MoS 2 , MoSe 2 . Xu and co-workers [65] employed the mixture of WSe 2 and MoSe 2 powder as sources and obtained ...

Abstract2D GeSe possesses black phosphorous‐analog‐layered structure and shows excellent environmental stability, as well as highly anisotropic in‐plane properties. Additionally, its high absorption efficiency in the visible range and high charge carrier mobility render it promising for applications in optoelectronics. However, most reported GeSe‐based photodetectors show frustrating performance especially in photoresponsivity. Herein, a 2D GeSe‐based phototransistor with an ultrahigh photoresponsivity is demonstrated. Its optimized photoresponsivity can be up to ≈1.6 × 105 A W−1. This high responsivity can be attributed to the highly efficient light absorption and the enhanced photoconductive gain due to the existence of trap states. The exfoliated GeSe nanosheet is confirmed to be along the [001] (armchair direction) and [010] (zigzag direction) using transmission electron microscopy and anisotropic Raman characterizations. The angle‐dependent electric and photoresponsive performance is systematically explored. Notably, the GeSe‐based phototransistor shows strong polarization‐dependent photoresponse with a peak/valley ratio of 1.3. Furthermore, the charge carrier mobility along the armchair direction is measured to be 1.85 times larger than that along the zigzag direction.

Benefiting from the superior electron mobility and good air‐stability, the emerging layered bismuth oxyselenide (Bi2O2Se) nanosheet has received considerable attention with the promising prospects for electronics and optoelectronics applications. However, the high charge carrier concentration and bolometric effect of Bi2O2Se give rise to the high dark current and relatively slow photoresponse, which severely impede further improvement of the performance of Bi2O2Se based photodetectors. Here, a WSe2/Bi2O2Se Van der Waals p‐n heterostructure is reported with a pronounced rectification ratio of 105 and a low reverse dark current of 10−11 A, as well as an enhanced light on/off ratio up to 618 under 532 nm light illumination. The device also exhibits a fast response speed of 2.6 µs and a broadband detection capability from 365 to 2000 nm due to the efficient charge separation and strong interlayer coupling at the interface of the two flakes. Importantly, the built‐in potential in the WSe2/Bi2O2Se heterostructure offers a competitive self‐powered photodetector with the light on/off ratio above 105 and a photovoltaic responsivity of 284 mA W−1. The WSe2/Bi2O2Se heterostructure shows promising potentials for high‐performance self‐driven photodetector applications.

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Van der Waals (vdW) dielectrics such as hBN are widely used to preserve the intrinsic properties of twodimensional (2D) semiconductors and support the fabrication of high-performance 2D devices. This is fundamentally attributed to their dangling-bond-free surface, carrying far lower density of charged scattering sources and trap states with respect to the conventional dielectrics (SiO 2 etc.). However, their wafer-scale fabrication and compatible integration with 2D semiconductors remain cumbersome, giving rise to the di culties in scalable fabrication of high-performance 2D devices. Here we report a high-κ vdW dielectric (ε r =11.5) composed of inorganic molecular crystal (IMC) Sb 2 O 3 , allowing for large-scale fabrication and facile integration via standard thermal evaporation process thanks to its particular crystal structure. Similarly, our vdW dielectric also supports remarkably improved 2D devices with respect to the typical conventional dielectric SiO 2 . The monolayer MoS 2 eld effect transistors (FET) supported by our vdW dielectric exhibits high on/off ratio (10 8 ), greatly enhanced electron mobility (from 20 to 80 cm 2 /Vs) and reduced transfer-curve hysteresis over an order of magnitude. Our results may open a new avenue towards compatible fabrication of vdW dielectrics using IMCs and lead to the scalable fabrication of high-performance 2D devices.

Polarized photodetection based on anisotropic two-dimensional materials display promising prospects for practical application in optical communication and optoelectronic fields. However, most of the reported polarized photodetection are limited by the lack of valid tunable strategy and low linear dichroism ratio. A peculiar noble metal dichalcogenide-PdSe 2 with a puckered pentagonal structure and abnormal linear dichroism conversion-potentially removes these restrictions and is demonstrated in this study. Herein, azimuthdependent reflectance difference microscopy combined with anisotropic electrical transport measurements indicate strong in-plane anisotropic optical and electrical properties of two-dimensional PdSe 2. Remarkably, the typical polarization-resolved photodetection exhibits anisotropic photodetection characteristics with a dichroic ratio up to ≈1.8 at 532 nm and ≈2.2 at 369 nm, and their dominant polarization orientation differs by 90° corresponding to the a-axis and b-axis, respectively. The unique orientation selection behavior in polarizationdependent photodetection can be attributed to the intrinsic linear dichroism conversion. The results make 2D PdSe 2 a promising platform for investigating anisotropic structure-property correlations and integrated optical applications for novel polarization-sensitive photodetection.

As a layered p-type semiconductor with a wide bandgap of 2.7 eV, GeSe 2 can compensate for the rarity of p-type semiconductors, which are desired for the production of high-integration logic circuits with low power consumption. Herein, ultrathin 2D single crystals of β-GeSe 2 are produced using van der Waals epitaxy and halide assistance; each crystalline flake is ≈7 nm thick and shaped as a rhombus. The optical and electrical properties of the flakes are studied systematically, and the temperature-dependent Raman spectra of the flakes reveal that the intensity of the Raman peaks decrease with increasing temperature. Low-temperature electrical measurements suggest that the variable-range hopping model is best for describing the electrical transport at 20-180 K; meanwhile, optical-phonon-assisted hopping can account for the transport behavior at 180-460 K. Impressively, the angle-resolved polarized Raman measurements indicate strong in-plane anisotropy of the rhombic GeSe 2 flake under a parallel polarization configuration, which may result from the low symmetry of the monoclinic crystal structure of GeSe 2 . Furthermore, a photodetector based on a rhombic GeSe 2 flake is constructed and shown to exhibit a high responsivity of 2.5 A W −1 and a fast response of ≈0.2 s. flakes under a parallel polarization configuration. Additionally, a flake is used to produce a photodetector, which exhibits a high responsivity of 2.5 A W −1 and a fast response of ≈0.2 s.