Ultrathin Ruddlesden–Popper Perovskite Heterojunction for Sensitive Photodetection

Thanks to their unique optical and electric properties, 2D materials have attracted a lot of interest for optoelectronic applications. Here, the emerging 2D materials, organic–inorganic hybrid perovskites with van der Waals interlayer interaction (Ruddlesden–Popper perovskites), are synthesized and characterized. Photodetectors based on the few‐layer Ruddlesden–Popper perovskite show good photoresponsivity as well as good detectivity. In order to further improve the photoresponse performance, 2D MoS₂ is chosen to construct the perovskite–MoS₂ heterojunction. The performance of the hybrid photodetector is largely improved with 6 and 2 orders of magnitude enhancement for photoresponsivity (10⁴ A W^?1) and detectivity (4 × 10¹⁰ Jones), respectively, which demonstrates the facile charge separation at the interface between perovskite and MoS₂. Furthermore, the contribution of back gate tuning is proved with a greatly reduced dark current. The results demonstrated here will open up a new field for the investigation of 2D perovskites for optoelectronic applications.     

High Detectivity and Transparent Few‐Layer MoS2/Glassy‐Graphene Heterostructure Photodetectors

Layered van der Waals heterostructures have attracted considerable attention recently, due to their unique properties both inherited from individual two‐dimensional (2D) components and imparted from their interactions. Here, a novel few‐layer MoS₂/glassy‐graphene heterostructure, synthesized by a layer‐by‐layer transfer technique, and its application as transparent photodetectors are reported for the first time. Instead of a traditional Schottky junction, coherent ohmic contact is formed at the interface between the MoS₂ and the glassy‐graphene nanosheets. The device exhibits pronounced wavelength selectivity as illuminated by monochromatic lights. A responsivity of 12.3 mA W^?1 and detectivity of 1.8 × 10¹⁰ Jones are obtained from the photodetector under 532 nm light illumination. Density functional theory calculations reveal the impact of specific carbon atomic arrangement in the glassy‐graphene on the electronic band structure. It is demonstrated that the band alignment of the layered heterostructures can be manipulated by lattice engineering of 2D nanosheets to enhance optoelectronic performance.     

High Detectivity Graphene‐Silicon Heterojunction Photodetector

A graphene/n‐type silicon (n‐Si) heterojunction has been demonstrated to exhibit strong rectifying behavior and high photoresponsivity, which can be utilized for the development of high‐performance photodetectors. However, graphene/n‐Si heterojunction photodetectors reported previously suffer from relatively low specific detectivity due to large dark current. Here, by introducing a thin interfacial oxide layer, the dark current of graphene/n‐Si heterojunction has been reduced by two orders of magnitude at zero bias. At room temperature, the graphene/n‐Si photodetector with interfacial oxide exhibits a specific detectivity up to 5.77 × 10¹³ cm Hz^1/2 W^‐1 at the peak wavelength of 890 nm in vacuum, which is highest reported detectivity at room temperature for planar graphene/Si heterojunction photodetectors. In addition, the improved graphene/n‐Si heterojunction photodetectors possess high responsivity of 0.73 A W^?1 and high photo‐to‐dark current ratio of ≈10⁷. The current noise spectral density of the graphene/n‐Si photodetector has been characterized under ambient and vacuum conditions, which shows that the dark current can be further suppressed in vacuum. These results demonstrate that graphene/Si heterojunction with interfacial oxide is promising for the development of high detectivity photodetectors.     

Light Confinement Effect Induced Highly Sensitive,Self‐Driven Near‐Infrared Photodetector and Image Sensor Based on Multilayer PdSe2/Pyramid Si Heterojunction

In this study, a highly sensitive and self‐driven near‐infrared (NIR) light photodetector based on PdSe₂/pyramid Si heterojunction arrays, which are fabricated through simple selenization of predeposited Pd nanofilm on black Si, is demonstrated. The as‐fabricated hybrid device exhibits excellent photoresponse performance in terms of a large on/off ratio of 1.6 × 10⁵, a responsivity of 456 mA W^?1, and a high specific detectivity of up to 9.97 × 10¹³ Jones under 980 nm illumination at zero bias. Such a relatively high sensitivity can be ascribed to the light trapping effect of the pyramid microstructure, which is confirmed by numerical modeling based on finite‐difference time domain. On the other hand, thanks to the broad optical absorption properties of PdSe₂, the as‐fabricated device also exhibits obvious sensitivity to other NIR illuminations with wavelengths of 1300, 1550, and 1650 nm, which is beyond the photoresponse range of Si‐based devices. It is also found that the PdSe₂/pyramid Si heterojunction device can also function as an NIR light sensor, which can readily record both “tree” and “house” images produced by 980 and 1300 nm illumination, respectively.     

Asymmetrically Encapsulated Vertical ITO/MoS2/Cu2O Photodetector with Ultrahigh Sensitivity

Strong light absorption, coupled with moderate carrier transport properties, makes 2D layered transition metal dichalcogenide semiconductors promising candidates for low intensity photodetection applications. However, the performance of these devices is severely bottlenecked by slow response with persistent photocurrent due to long lived charge trapping, and nonreliable characteristics due to undesirable ambience and substrate effects. Here ultrahigh specific detectivity (D*) of 3.2 × 10¹⁴ Jones and responsivity (R) of 5.77 × 10⁴ A W^?1 are demonstrated at an optical power density (P_op) of 0.26 W m^?2 and external bias (V_ext) of ?0.5 V in an indium tin oxide/MoS₂/copper oxide/Au vertical multi‐heterojunction photodetector exhibiting small carrier transit time. The active MoS₂ layer being encapsulated by carrier collection layers allows us to achieve repeatable characteristics over large number of cycles with negligible trap assisted persistent photocurrent. A large D* > 10¹⁴ Jones at zero external bias is also achieved due to the built‐in field of the asymmetric photodetector. Benchmarking the performance against existing reports in literature shows a viable pathway for achieving reliable and highly sensitive photodetectors for ultralow intensity photodetection applications.     

MoS2–HgTe Quantum Dot Hybrid Photodetectors beyond 2 µm

Mercury telluride (HgTe) colloidal quantum dots (CQDs) have been developed as promising materials for the short and mid‐wave infrared photodetection applications because of their low cost, solution processing, and size tunable absorption in the short wave and mid‐infrared spectrum. However, the low mobility and poor photogain have limited the responsivity of HgTe CQD‐based photodetectors to only tens of mA W^?1. Here, HgTe CQDs are integrated on a TiO₂ encapsulated MoS₂ transistor channel to form hybrid phototransistors with high responsivity of ≈10⁶ A W^?1, the highest reported to date for HgTe QDs. By operating the phototransistor in the depletion regime enabled by the gate modulated current of MoS₂, the noise current is significantly suppressed, leading to an experimentally measured specific detectivity D* of ≈10¹² Jones at a wavelength of 2 µm. This work demonstrates for the first time the potential of the hybrid 2D/QD detector technology in reaching out to wavelengths beyond 2 µm with compelling sensitivity.     

High‐Performance Photovoltaic Detector Based on MoTe2/MoS2 Van der Waals Heterostructure

Van der Waals heterostructures based on 2D layered materials have received wide attention for their multiple applications in optoelectronic devices, such as solar cells, light‐emitting devices, and photodiodes. In this work, high‐performance photovoltaic photodetectors based on MoTe₂/MoS₂ vertical heterojunctions are demonstrated by exfoliating‐restacking approach. The fundamental electric properties and band structures of the junction are revealed and analyzed. It is shown that this kind of photodetectors can operate under zero bias with high on/off ratio (>10⁵) and ultralow dark current (≈3 pA). Moreover, a fast response time of 60 µs and high photoresponsivity of 46 mA W^?1 are also attained at room temperature. The junctions based on 2D materials are expected to constitute the ultimate functional elements of nanoscale electronic and optoelectronic applications.     

In Situ Fabrication of Vertical Multilayered MoS2/Si Homotype Heterojunction for High‐Speed Visible–Near‐Infrared Photodetectors

c2D transition metal dichalcogenides (TMDCs)‐based heterostructures have been demonstrated to achieve superior light absorption and photovoltaic effects theoretically and experimentally, making them extremely attractive for realizing optoelectronic devices. In this work, a vertical multilayered n‐MoS₂/n‐silicon homotype heterojunction is fabricated, which takes advantage of multilayered MoS₂ grown in situ directly on plane silicon. Electrical characterization reveals that the resultant device exhibits high sensitivity to visible–near‐infrared light with responsivity up to 11.9 A W^–1. Notably, the photodetector shows high‐speed response time of ≈30.5 µs/71.6 µs and capability to work under higher pulsed light irradiation approaching 100 kHz. The high response speed could be attributed to a good quality of the multilayer MoS₂, as well as in situ device fabrication process. These findings suggest that the multilayered MoS₂/Si homotype heterojunction have great potential application in the field of visible–near‐infrared detection and might be used as elements for construction of high‐speed integrated optoelectronic sensor circuitry.     

Growth of Ultraflat PbI2 Nanoflakes by Solvent Evaporation Suppression for High‐Performance UV Photodetectors

2D lead iodide (PbI₂) is attracting great interest due to its great potential in the application of UV photodetectors. In this work, a facile solution‐based method is developed to synthesize ultraflat PbI₂ nanoflakes for high‐performance UV photodetectors. By maintaining at proximate room temperature and adding an evaporation suppression solvent for slow‐rate crystal growth, high‐quality PbI₂ nanoflakes with an ultraflat surface are obtained. The UV photodetectors based on 2D PbI₂ nanoflakes exhibit a high photoresponsivity of 0.51 A W^?1, a high detectivity of 4.0 × 10¹⁰ Jones, a high external quantum efficiency (EQE) of 168.9%, and a rapid response speed including a rise time of 14.1 ms and a decay time of 31.0 ms. The balanced and excellent photodetector performance of these devices paves the road for practical UV photodetection based on 2D PbI₂ nanoflakes.     

Atomically Thin Oxyhalide Solar‐Blind Photodetectors

2D wide‐bandgap semiconductors demonstrate great potential in fabricating solar‐blind ultraviolet (SBUV) photodetectors. However, the low responsivity of 2D solar‐blind photodetectors still limits their practical applications. Here, high‐responsivity solar‐blind photodetectors are achieved based on 2D bismuth oxychloride (BiOCl) flakes. The 2D BiOCl photodetectors exhibit a responsivity up to 35.7 A W^?1 and a specific detectivity of 2.2 × 10¹⁰ Jones under 250 nm illumination with 17.8 µW cm^?2 power density. In particular, the enhanced photodetective performances are demonstrated in BiOCl photodetectors with increasing ambient temperature. Surprisingly, their responsivity can reach 2060 A W^?1 at 450 K under solar‐blind light illumination, maybe owing to the formation of defective BiOCl grains evidenced by in situ transmission electron microscopy. The high responsivity throughout the solar‐blind range indicates that 2D BiOCl is a promising candidate for SBUV detection.     

Gap‐Mode Surface‐Plasmon‐Enhanced Photoluminescence and Photoresponse of MoS2

2D materials hold great potential for designing novel electronic and optoelectronic devices. However, 2D material can only absorb limited incident light. As a representative 2D semiconductor, monolayer MoS₂ can only absorb up to 10% of the incident light in the visible, which is not sufficient to achieve a high optical‐to‐electrical conversion efficiency. To overcome this shortcoming, a “gap‐mode” plasmon‐enhanced monolayer MoS₂ fluorescent emitter and photodetector is designed by squeezing the light‐field into Ag shell‐isolated nanoparticles–Au film gap, where the confined electromagnetic field can interact with monolayer MoS₂. With this gap‐mode plasmon‐enhanced configuration, a 110‐fold enhancement of photoluminescence intensity is achieved, exceeding values reached by other plasmon‐enhanced MoS₂ fluorescent emitters. In addition, a gap‐mode plasmon‐enhanced monolayer MoS₂ photodetector with an 880% enhancement in photocurrent and a responsivity of 287.5 A W^?1 is demonstrated, exceeding previously reported plasmon‐enhanced monolayer MoS₂ photodetectors.     

Self‐Confined Growth of Ultrathin 2D Nonlayered Wide‐Bandgap Semiconductor CuBr Flakes

2D planar structures of nonlayered wide‐bandgap semiconductors enable distinguished electronic properties, desirable short wavelength emission, and facile construction of 2D heterojunction without lattice match. However, the growth of ultrathin 2D nonlayered materials is limited by their strong covalent bonded nature. Herein, the synthesis of ultrathin 2D nonlayered CuBr nanosheets with a thickness of about 0.91 nm and an edge size of 45 µm via a controllable self‐confined chemical vapor deposition method is described. The enhanced spin‐triplet exciton (Z_f, 2.98 eV) luminescence and polarization‐enhanced second‐harmonic generation based on the 2D CuBr flakes demonstrate the potential of short‐wavelength luminescent applications. Solar‐blind and self‐driven ultraviolet (UV) photodetectors based on the as‐synthesized 2D CuBr flakes exhibit a high photoresponsivity of 3.17 A W^?1, an external quantum efficiency of 1126%, and a detectivity (D*) of 1.4 × 10¹¹ Jones, accompanied by a fast rise time of 32 ms and a decay time of 48 ms. The unique nonlayered structure and novel optical properties of the 2D CuBr flakes, together with their controllable growth, make them a highly promising candidate for future applications in short‐wavelength light‐emitting devices, nonlinear optical devices, and UV photodetectors.     

A Dual‐Gate MoS2 Photodetector Based on Interface Coupling Effect

2D transition metal dichalcogenides (TMDs) based photodetectors have shown great potential for the next generation optoelectronics. However, most of the reported MoS₂ photodetectors function under the photogating effect originated from the charge‐trap mechanism, which is difficult for quantitative control. Such devices generally suffer from a poor compromise between response speed and responsivity (R) and large dark current. Here, a dual‐gated (DG) MoS₂ phototransistor operating based on the interface coupling effect (ICE) is demonstrated. By simultaneously applying a negative top‐gate voltage (V_TG) and positive back‐gate voltage (V_BG) to the MoS₂ channel, the photogenerated holes can be effectively trapped in the depleted region under TG. An ultrahigh R of ≈10⁵ A W^?1 and detectivity (D*) of ≈10¹⁴ Jones are achieved in several devices with different thickness under P_in of 53 µW cm^?2 at V_TG = ?5 V. Moreover, the response time of the DG phototransistor can also be modulated based on the ICE. Based on these systematic measurements of MoS₂ DG phototransistors, the results show that the ICE plays an important role in the modulation of photoelectric performances. The results also pave the way for the future optoelectrical application of 2D TMDs materials and prompt for further investigation in the DG structured phototransistors.     

Ultrabroadband MoS2 Photodetector with Spectral Response from 445 to 2717 nm

Photodetectors with excellent detecting properties over a broad spectral range have advantages for the application in many optoelectronic devices. Introducing imperfections to the atomic lattices in semiconductors is a significant way for tuning the bandgap and achieving broadband response, but the imperfection may renovate their intrinsic properties far from the desire. Here, by controlling the deviation from the perfection of the atomic lattice, ultrabroadband multilayer MoS₂ photodetectors are originally designed and realized with the detection range over 2000 nm from 445 nm (blue) to 2717 nm (mid‐infrared). Associated with the narrow but nonzero bandgap and large photoresponsivity, the optimized deviation from the perfection of MoS₂ samples is theoretically found and experimentally achieved aiming at the ultrabroadband photoresponse. By the photodetection characterization, the responsivity and detectivity of the present photodetectors are investigated in the wavelength range from 445 to 2717 nm with the maximum values of 50.7 mA W^?1 and 1.55 × 10⁹ Jones, respectively, which represent the most broadband MoS₂ photodetectors. Based on the easy manipulation, low cost, large scale, and broadband photoresponse, this present detector has significant potential for the applications in optoelectronics and electronics in the future.     

High‐Performance Inorganic Perovskite Quantum Dot–Organic Semiconductor Hybrid Phototransistors

All‐inorganic lead halide perovskite quantum dots (IHP QDs) have great potentials in photodetectors. However, the photoresponsivity is limited by the low charge transport efficiency of the IHP QD layers. High‐performance phototransistors based on IHP QDs hybridized with organic semiconductors (OSCs) are developed. The smooth surface of IHP QD layers ensures ordered packing of the OSC molecules above them. The OSCs significantly improve the transportation of the photoexcited charges, and the gate effect of the transistor structure significantly enhances the photoresponsivity while simultaneously maintaining high I _photo/I _dark ratio. The devices exhibit outstanding optoelectronic properties in terms of photoresponsivity (1.7 × 10⁴ A W^?1), detectivity (2.0 × 10¹⁴ Jones), external quantum efficiency (67000%), I _photo/I _dark ratio (8.1 × 10⁴), and stability (100 d in air). The overall performances of our devices are superior to state‐of‐the‐art IHP photodetectors. The strategy utilized here is general and can be easily applied to many other perovskite photodetectors.     

Large‐Scale Synthesis of Freestanding Layer‐Structured PbI2 and MAPbI3 Nanosheets for High‐Performance Photodetection

Recently, due to the possibility of thinning down to the atomic thickness to achieve exotic properties, layered materials have attracted extensive research attention. In particular, PbI₂, a kind of layered material, and its perovskite derivatives, CH₃NH₃PbI₃ (i.e., MAPbI₃), have demonstrated impressive photoresponsivities for efficient photodetection. Herein, the synthesis of large‐scale, high‐density, and freestanding PbI₂ nanosheets is demonstrated by manipulating the microenvironment during physical vapor deposition. In contrast to conventional two‐dimensional (2D) growth along the substrate surface, the essence here is the effective nucleation of microplanes with different angles relative to the in‐plane direction of underlying rough‐surfaced substrates. When configured into photodetectors, the fabricated device exhibits a photoresponsivity of 410 mA W^?1, a detectivity of 3.1 × 10¹¹ Jones, and a fast response with the rise and decay time constants of 86 and 150 ms, respectively, under a wavelength of 405 nm. These PbI₂ nanosheets can also be completely converted into MAPbI₃ materials via chemical vapor deposition with an improved photoresponsivity up to 40 A W^?1. All these performance parameters are comparable to those of state‐of‐the‐art layered‐material‐based photodetectors, revealing the technological potency of these freestanding nanosheets for next‐generation high‐performance optoelectronics.     

Roll‐to‐Roll Production of Layer‐Controlled Molybdenum Disulfide: A Platform for 2D Semiconductor‐Based Industrial Applications

A facile methodology for the large‐scale production of layer‐controlled MoS₂ layers on an inexpensive substrate involving a simple coating of single source precursor with subsequent roll‐to‐roll‐based thermal decomposition is developed. The resulting 50 cm long MoS₂ layers synthesized on Ni foils possess excellent long‐range uniformity and optimum stoichiometry. Moreover, this methodology is promising because it enables simple control of the number of MoS₂ layers by simply adjusting the concentration of (NH₄)₂MoS₄. Additionally, the capability of the MoS₂ for practical applications in electronic/optoelectronic devices and catalysts for hydrogen evolution reaction is verified. The MoS₂‐based field effect transistors exhibit unipolar n‐channel transistor behavior with electron mobility of 0.6 cm² V^?1 s^?1 and an on‐off ratio of ≈10³. The MoS₂‐based visible‐light photodetectors are fabricated in order to evaluate their photoelectrical properties, obtaining an 100% yield for active devices with significant photocurrents and extracted photoresponsivity of ≈22 mA W^?1. Moreover, the MoS₂ layers on Ni foils exhibit applicable catalytic activity with observed overpotential of ≈165 mV and a Tafel slope of 133 mV dec^?1. Based on these results, it is envisaged that the cost‐effective methodology will trigger actual industrial applications, as well as novel research related to 2D semiconductor‐based multifaceted applications.     

Ultrasensitive 2D Bi2O2Se Phototransistors on Silicon Substrates

2D materials are considered as intriguing building blocks for next‐generation optoelectronic devices. However, their photoresponse performance still needs to be improved for practical applications. Here, ultrasensitive 2D phototransistors are reported employing chemical vapor deposition (CVD)‐grown 2D Bi₂O₂Se transferred onto silicon substrates with a noncorrosive transfer method. The as‐transferred Bi₂O₂Se preserves high quality in contrast to the serious quality degradation in hydrofluoric‐acid‐assisted transfer. The phototransistors show a responsivity of 3.5 × 10⁴ A W^?1, a photoconductive gain of more than 10⁴, and a time response in the order of sub‐millisecond. With back gating of the silicon substrate, the dark current can be reduced to several pA. This yields an ultrahigh sensitivity with a specific detectivity of 9.0 × 10¹³ Jones, which is one of the highest values among 2D material photodetectors and two orders of magnitude higher than that of other CVD‐grown 2D materials. The high performance of the phototransistor shown here together with the developed unique transfer technique are promising for the development of novel 2D‐material‐based optoelectronic applications as well as integrating with state‐of‐the‐art silicon photonic and electronic technologies.     

Highly Efficient and Ultrasensitive Large‐Area Flexible Photodetector Based on Perovskite Nanowires

Hybrid organic–inorganic perovskites have shown exceptional semiconducting properties and microstructural versatility for inexpensive, solution‐processable photovoltaic and optoelectronic devices. In this work, an all‐solution‐based technique in ambient environment for highly sensitive and high‐speed flexible photodetectors using high crystal quality perovskite nanowires grown on Kapton substrate is presented. At 10 V, the optimized photodetector exhibits a responsivity as high as 0.62 A W^?1, a maximum specific detectivity of 7.3 × 10¹² cm Hz^1/2 W^?1, and a rise time of 227.2 µs. It also shows remarkable photocurrent stability even beyond 5000 bending cycles. Moreover, a deposition of poly(methyl methacrylate) (PMMA) as a protective layer on the perovskite yields significantly better stability under ambient air operation: the PMMA‐protected devices are stable for over 30 days. This work demonstrates a cost‐effective fabrication technique for high‐performance flexible photodetectors and opens opportunities for research advancements in broadband and large‐scale flexible perovskite‐based optoelectronic devices.     

Air-Stable Violet Phosphorus/MoS2 van der Waals Heterostructure for High-Responsivity and Gate-Tunable Photodetection

Violet phosphorus (VP), a newly emerging elemental 2D semiconductor, with attractive properties such as tunable bandgap, high carrier mobility, and unusual structural anisotropy, offers significant opportunities for designing high-performance electronic and optoelectronic devices. However, the study on fundamental property and device application of 2D VP is seriously hindered by its inherent instability in ambient air. Here, a VP/MoS₂ van der Waals heterostructure is constructed by vertically staking few-layer VP and MoS₂, aiming to utilize the synergistic effect of the two materials to achieve a high-performance 2D photodetector. The strong optical absorption of VP combining with the type-II band alignment of VP/MoS₂ heterostructure make VP play a prominent photogating effect. As a result, the VP/MoS₂ heterostructure photodetector achieves an excellent photoresponse performances with ultrahigh responsivity of 3.82 × 10⁵ A W^–1, high specific detectivity of 9.17 × 10¹³ Jones, large external quantum efficiency of 8.91 × 10⁷ %, and gate tunability, which are much superior to that of individual MoS₂ device or VP device. Moreover, the VP/MoS₂ heterostructure photodetector indicates superior air stability due to the effective protection of VP by MoS₂ encapsulation. This work sheds light on the future study of the fundamental property and optoelectronic device application of VP.