Hybrid organic-inorganic perovskite materials have received substantial research attention due to their impressively high performance in photovoltaic devices. As one of the oldest functional materials, it is intriguing to explore the optoelectronic properties in perovskite after reducing it into a few atomic layers in which two-dimensional (2D) confinement may get involved. In this work, we report a combined solution process and vapor-phase conversion method to synthesize 2D hybrid organic-inorganic perovskite (i.e., CH3NH3PbI3) nanocrystals as thin as a single unit cell (∼1.3 nm). High-quality 2D perovskite crystals have triangle and hexagonal shapes, exhibiting tunable photoluminescence while the thickness or composition is changed. Due to the high quantum efficiency and excellent photoelectric properties in 2D perovskites, a high-performance photodetector was demonstrated, in which the current can be enhanced significantly by shining 405 and 532 nm lasers, showing photoresponsivities of 22 and 12 AW(-1) with a voltage bias of 1 V, respectively. The excellent optoelectronic properties make 2D perovskites building blocks to construct 2D heterostructures for wider optoelectronic applications.

Vertical heterojunctions of two two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted considerable attention recently. A variety of heterojunctions can be constructed by stacking different TMDs to form fundamental building blocks in different optoelectronic devices such as photodetectors, solar cells, and light-emitting diodes. However, these applications are significantly hampered by the challenges of large-scale production of van der Waals stacks of atomically thin materials. Here, we demonstrate scalable production of periodic patterns of few-layer WS2, MoS2, and their vertical heterojunction arrays by a thermal reduction sulfurization process. In this method, a two-step chemical vapor deposition approach was developed to effectively prevent the phase mixing of TMDs in an unpredicted manner, thus affording a well-defined interface between WS2 and MoS2 in the vertical dimension. As a result, large-scale, periodic arrays of few-layer WS2, MoS2, and their vertical heterojunctions can be produced with desired size and density. Photodetectors based on the as-produced MoS2/WS2 vertical heterojunction arrays were fabricated, and a high photoresponsivity of 2.3 A·W(-1) at an excitation wavelength of 450 nm was demonstrated. Flexible photodetector devices using MoS2/WS2 heterojunction arrays were also demonstrated with reasonable signal/noise ratio. The approach in this work is also applicable to other TMD materials and can open up the possibilities of producing a variety of vertical van der Waals heterojunctions in a large scale toward optoelectronic applications.

Recently, research on graphene based photodetectors has drawn substantial attention due to ultrafast and broadband photoresponse of graphene. However, they usually have low responsivity and low photoconductive gain induced by the gapless nature of graphene, which greatly limit their applications. The synergetic integration of graphene with other two-dimensional (2D) materials to form van der Waals heterostructure is a very promising approach to overcome these shortcomings. Here we report the growth of graphene-Bi2Te3 heterostructure where Bi2Te3 is a small bandgap material from topological insulator family with a similar hexagonal symmetry to graphene. Because of the effective photocarrier generation and transfer at the interface between graphene and Bi2Te3, the device photocurrent can be effectively enhanced without sacrificing the detecting spectral width. Our results show that the graphene-Bi2Te3 photodetector has much higher photoresponsivity (35 AW(-1) at a wavelength of 532 nm) and higher sensitivity (photoconductive gain up to 83), as compared to the pure monolayer graphene-based devices. More interestingly, the detection wavelength range of our device is further expanded to near-infrared (980 nm) and telecommunication band (1550 nm), which is not observed on the devices based on heterostructures of graphene and transition metal dichalcogenides.

A zeolitic‐imidazolate‐framework (ZIF) nanocrystal layer‐protected carbonization route is developed to prepare N‐doped nanoporous carbon/graphene nano‐sandwiches. The ZIF/graphene oxide/ZIF sandwich‐like structure with ultrasmall ZIF nanocrystals (i.e., ≈20 nm) fully covering the graphene oxide (GO) is prepared via a homogenous nucleation followed by a uniform deposition and confined growth process. The uniform coating of ZIF nanocrystals on the GO layer can effectively inhibit the agglomeration of GO during high‐temperature treatment (800 °C). After carbonization and acid etching, N‐doped nanoporous carbon/graphene nanosheets are formed, with a high specific surface area (1170 m2 g−1). These N‐doped nanoporous carbon/graphene nanosheets are used as the nonprecious metal electrocatalysts for oxygen reduction and exhibit a high onset potential (0.92 V vs reversible hydrogen electrode; RHE) and a large limiting current density (5.2 mA cm−2 at 0.60 V). To further increase the oxygen reduction performance, nanoporous Co‐Nx/carbon nanosheets are also prepared by using cobalt nitrate and zinc nitrate as cometal sources, which reveal higher onset potential (0.96 V) than both commercial Pt/C (0.94 V) and N‐doped nanoporous carbon/graphene nanosheets. Such nanoporous Co‐Nx/carbon nanosheets also exhibit good performance such as high activity, stability, and methanol tolerance in acidic media.

material. Graphene has been demonstrated to be an effective channel material for phototransistor because of its broadband light absorption, fast response time, and ultrahigh carrier mobility. [1][2][3] However, the relatively low absorption cross-section, fast recombination rate and the absence of gain mechanism that can generate multiple charge carriers from one incident photon have limited the responsivity of pure graphene-based phototransistor [ 4,5 ] to ≈10 −2 A W −1 which is much lower than that of commercial Si photodiode. [ 6 ] So far, the rapid development of graphene-based photodetection has focused on enhancement of the light absorption in graphene by variant approaches such as plasmonic coupling [ 7 ] and microcavity confi nement. [8][9][10] Nevertheless, a key to ultrasensitive graphene-based photodetection is the implementation of photoconductive gain which could afford the ability to generate multiple electrical carriers per single incident photon.Until now, the photoconductive gain for improved sensitivity has not been observed in pure grapehene-based photodetector. Alternatively, the hybridization of graphene with a gain material or the formation of a heterostructure has been proved to be an effective approach to enhance the photodetection performance. For example, the mixtures of graphene with TiO 2 [ 11 ] or quantum dots [ 12 ] have shown greatly improved photoconductive gain but the synthesis of gain material needs complicated processes. The formation of vertical heterostructure of graphene and layered transition metal dichalcogenides (TMDs) such as MoS 2 , [ 13,14 ] WS 2 , [ 15 ] and WSe 2 [ 16,17 ] can achieve very high quantum effi ciency upon light illumination due to effective photoexcited carrier separation at the interface. However, the fabrication of these devices is expensive and lack of scalability as it demands delicately controlled sample transfer technique which has low-yield and multiple lithography procedures.Recently, mixed organic-inorganic halide perovskites have emerged as a new class of light harvesting material for highly effi cient solar cells with confi rmed effi ciency of 19.2%. [ 18 ] This family of perovskite materials take the form of ABX 3 (A = CH 3 NH 3 + ; B = Pb 2+ ; X = Cl − /I − /Br − ) and show large absorption cross-section, long photocarrier diffusion length, and high charge carrier mobility. [ 19 ] These unique photoelectrical properties enable many photonic and optoelectronic applications such as random lasing, [ 20 ] light emitting diode, [ 21 ] and Graphene is an attractive optoelectronic material for light detection because of its broadband light absorption and fast response time. However, the relatively low absorption cross-section, fast recombination rate, and the absence of gain mechanism have limited the responsivity of pure graphene-based phototransistor to ≈10 −2 A W −1 . In this work, a photoconductive gain of ≈10 9 electrons per photon and a responsivity of ≈6.0 × 10 5 A W −1 are demonstrated in a hybrid photodetector that consists of monolayer g...