Despite many decades of research of diodes, which are fundamental components of electronic and photoelectronic devices with p-n or Schottky junctions using bulk or 2D materials, stereotyped architectures and complex technological processing (doping and multiple material operations) have limited future development. Here, a novel rectification device, an orientation-induced diode, assembled using only few-layered black phosphorus (BP) is investigated. The key to its realization is to utilize the remarkable anisotropy of BP in low dimensions and change the charge-transport conditions abruptly along the different crystal orientations. Rectification ratios of 6.8, 22, and 115 can be achieved in cruciform BP, cross-stacked BP junctions, and BP junctions stacked with vertical orientations, respectively. The underlying physical processes and mechanisms can be explained using "orientation barrier" band theory. The theoretical results are experimentally confirmed using localized scanning photocurrent imaging. These orientation-induced optoelectronic devices open possibilities for 2D anisotropic materials with a new degree of freedom to improve modulation in diodes.

Being the first successfully prepared two-dimensional material, graphene has attracted extensive attention from researchers due to its excellent properties and extremely wide range of applications. In particular, graphene and its derivatives have displayed several ideal properties, including broadband light absorption, ability to quench fluorescence, excellent biocompatibility, and strong polarization-dependent effects, thus emerging as one of the most popular platforms for optical sensors. Graphene and its derivatives-based optical sensors have numerous advantages, such as high sensitivity, low-cost, fast response time, and small dimensions. In this review, recent developments in graphene and its derivatives-based optical sensors are summarized, covering aspects related to fluorescence, graphene-based substrates for surface-enhanced Raman scattering (SERS), optical fiber biological sensors, and other kinds of graphene-based optical sensors. Various sensing applications, such as single-cell detection, cancer diagnosis, protein, and DNA sensing, are introduced and discussed systematically. Finally, a summary and roadmap of current and future trends are presented in order to provide a prospect for the development of graphene and its derivatives-based optical sensors.

as diodes, [18,19] photodetectors, [20] and memory devices. [21,22] However, the stacking of many heterojunctions simply focuses on the stacking of different materials and seldom mentions anisotropy of the material itself. Considering the anisotropy of the materials during stacking, a heterojunction with excellent properties may be obtained. A series of works in anisotropic materials has been explored, including photothermal detection, determination of crystalline orientation, and diodes. [23][24][25][26] These results have laid the foundation for using the anisotropy of materials to investigate functional devices. BP is a p-type semiconducting 2D material that can be used as a channel material in field-effect transistors (FETs) with hole mobility up to ≈1000 cm 2 V −1 s −1 . [2] The small and direct bandgap makes BP as an ideal material for broadband photodetection. Further, the BP FETs show a fast photoresponse to excitation wavelengths from the visible region to near-infrared region, indicating BP as a promising material for high-speed broadband photodetector. [27] What is more, BP exhibits an anisotropic optical response due to the existence of strong in-plane anisotropy, [28,29] and thus detection of light polarization can be achieved. ReS 2 is an n-type semiconductor, and also have anisotropic carrier transport and optical property. [30] Thus, it is expected to obtain a stronger polarized photoresponse through the formation of a p-n junction with BP and ReS 2 .In this paper, we designed heterojunctions with the b-axis of ReS 2 parallel to the crystal orientation of BP, due to the stronger optical absorption of the b-axis of ReS 2 . [31] The p-n junction with BP and ReS 2 shows a better polarization-dependent photoresponse than individual BP or ReS 2 . Here, we investigated variations in photocurrent with linear polarized light for these orientation-induced heterojunctions. Two types of heterojunctions were fabricated with the direction of armchair (AC) and zigzag (ZZ) parallel to the b-axis of ReS 2 , and the maximum photocurrent ratio of two polarization directions with parallel to AC and ZZ of BP can reach 31 at gate voltage of 0 V with V ds = −1 V. Based on the strong polarization-dependent characteristic of BP/ReS 2 heterojunction with AC direction parallel to the b-axis of ReS 2 , a photodetector is provided to apply in polarization optoelectronic detection.Van der Waals (vdW) heterojunctions of 2D layered materials possess excellent interface quality without the constraint of lattice mismatch, which enables the application of nanomaterials in electronic and optoelectronic devices. The anisotropy of 2D materials however, also plays an important role in the stacking process of vdW heterojunction. Black phosphorus (BP) and rhenium disulfide (ReS 2 ), as two strong anisotropic 2D materials, have intrinsic in-plane anisotropic properties that can be used in polarization-sensitive photoelectric devices. Herein, two types of BP/ReS 2 heterojunctions are stacked by controlling their crystal orientation, with ...

van der Waals p-n heterostructures based on p-type black phosphorus (BP) integrated with other two-dimensional (2D) layered materials have shown potential applications in electronic and optoelectronic devices, including logic rectifiers and polarization-sensitive photodetectors. However, the engineering of carriers transport anisotropy, which is related to the linear dichroism, have not yet been investigated. Here, we demonstrate a novel van der Waals device of orientation-perpendicular BP homojunction based on the anisotropic band structures between the armchair and zigzag directions. The structure exhibits good gate-tunable diode-like rectification characteristics caused by the barrier between the two perpendicular crystal orientations. Moreover, we demonstrate that the unique mechanisms of the polarization-sensitivity properties of this junction are involved with the linear dichroism and the anisotropic carriers transport engineering. These results were verified by the scanning photocurrent images experiments. This work paves the way for 2D anisotropic layered materials for next-generation electronic and optoelectronic devices.

Abstract Two-dimensional (2D) materials such as graphene, black phosphorus, and transition metal dichalcogenides have attracted significant research attention due to their novel properties and wide range of applications in electronic and optoelectronic devices. In particular, investigation of twist-controlled 2D materials has attracted tremendous attention due to their excellent properties such as smooth heterointerfaces, highly gate-tunable bandgaps, and ultrafast carrier transport. Twist-controlled 2D materials combined with their fascinating electronic structures have also indicated their outstanding potential in electronic and optoelectronic applications. In this review, the recent developments in twisted 2D materials are summarized, covering aspects related to their fabrication, twist angle-dependent optical properties, and optoelectronic applications. The photodetectors and orientation-dependent van der Waals junctions are introduced and discussed systematically. Finally, we deliver a summary and outlook to provide a guideline for the future development of this rapidly growing field.