Materials research has driven the development of modern nano-electronic devices. In particular, research in magnetic thin films has revolutionized the development of spintronic devices1,2 because identifying new magnetic materials is key to better device performance and design. Van der Waals crystals retain their chemical stability and structural integrity down to the monolayer and, being atomically thin, are readily tuned by various kinds of gate modulation3,4. Recent experiments have demonstrated that it is possible to obtain two-dimensional ferromagnetic order in insulating Cr2Ge2Te6 (ref. 5) and CrI3 (ref. 6) at low temperatures. Here we develop a device fabrication technique and isolate monolayers from the layered metallic magnet Fe3GeTe2 to study magnetotransport. We find that the itinerant ferromagnetism persists in Fe3GeTe2 down to the monolayer with an out-of-plane magnetocrystalline anisotropy. The ferromagnetic transition temperature, Tc, is suppressed relative to the bulk Tc of 205 kelvin in pristine Fe3GeTe2 thin flakes. An ionic gate, however, raises Tc to room temperature, much higher than the bulk Tc. The gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2 opens up opportunities for potential voltage-controlled magnetoelectronics7-11 based on atomically thin van der Waals crystals.

Gate-tunable Room-temperature Ferromagnetism in Two-dimensional Fe 3 GeTe 2

Broadband photodetectors are of great importance for numerous optoelectronic applications. Two-dimensional (2D) tungsten disulfide (WS2), an important family member of transition-metal dichalcogenides (TMDs), has shown great potential for high-sensitivity photodetection due to its extraordinary properties. However, the inherent large bandgap of WS2 and the strong interface recombination impede the actualization of high-sensitivity broadband photodetectors. Here, we demonstrate the fabrication of an ultrabroadband WS2/Ge heterojunction photodetector through defect engineering and interface passivation. Thanks to the narrowed bandgap of WS2 induced by the vacancy defects, the effective surface modification with an ultrathin AlOx layer, and the well-designed vertical n-n heterojunction structure, the WS2/AlOx/Ge photodetector exhibits an excellent device performance in terms of a high responsivity of 634.5 mA/W, a large specific detectivity up to 4.3 × 1011 Jones, and an ultrafast response speed. Significantly, the device possesses an ultrawide spectral response spanning from deep ultraviolet (200 nm) to mid-wave infrared (MWIR) of 4.6 μm, along with a superior MWIR imaging capability at room temperature. The detection range has surpassed the WS2-based photodetectors in previous reports and is among the broadest for TMD-based photodetectors. Our work provides a strategy for the fabrication of high-performance ultrabroadband photodetectors based on 2D TMD materials.

Ultrabroadband and High-Detectivity Photodetector Based on WS2/Ge Heterojunction through Defect Engineering and Interface Passivation.

Two-dimensional crystals have emerged as a class of materials that may impact future electronic technologies. Experimentally identifying and characterizing new functional two-dimensional materials is challenging, but also potentially rewarding. Here, we fabricate field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometres. Reliable transistor performance is achieved at room temperature in samples thinner than 7.5 nm, with drain current modulation on the order of 10(5) and well-developed current saturation in the I-V characteristics. The charge-carrier mobility is found to be thickness-dependent, with the highest values up to ∼ 1,000 cm(2) V(-1) s(-1) obtained for a thickness of ∼ 10 nm. Our results demonstrate the potential of black phosphorus thin crystals as a new two-dimensional material for applications in nanoelectronic devices.

http://absimage.aps.org/image/MAR14/MWS_MAR14-2013-005031.pdf

Black Phosphorus Field-effect Transistors

Atomically thin transition metal dichalcogenides have emerged as promising candidates for sensitive photodetection. Here, we report a photoconductivity study of biased mono- and bilayer molybdenum disulfide field-effect transistors. We identify photovoltaic and photoconductive effects, which both show strong photogain. The photovoltaic effect is described as a shift in transistor threshold voltage due to charge transfer from the channel to nearby molecules, including SiO2 surface-bound water. The photoconductive effect is attributed to the trapping of carriers in band tail states in the molybdenum disulfide itself. A simple model is presented that reproduces our experimental observations, such as the dependence on incident optical power and gate voltage. Our findings offer design and engineering strategies for atomically thin molybdenum disulfide photodetectors, and we anticipate that the results are generalizable to other transition metal dichalcogenides as well.

Mechanisms of photoconductivity in atomically thin MoS2

Self-Driven WSe2/Bi2O2Se Van der Waals Heterostructure Photodetectors with High Light On/Off Ratio and Fast Response

Infrared (IR) photodetectors are finding diverse applications in imaging, information communication, military, etc. 2D metal chalcogenides (2DMCs) have attracted increasing interest in view of their unique structures and extraordinary physical properties. They have demonstrated outstanding IR detection performance including high responsivity and detectivity, high on/off ratio, fast response rate, stable room temperature operability, and good mechanical flexibility, which has opened up a new prospect in next-generation IR photodetectors. This Review presents a comprehensive summary of recent progress in advanced IR photodetectors based on 2DMCs. The rationale of the photodetectors containing photocurrent generation mechanisms and performance parameters are briefly introduced. The device performances of 2DMCs-based IR photodetectors are also systematically summarized, and some representative achievements are highlighted as well. Finally, conclusions and outlooks are delivered as a guideline for this thriving field.

2D Metal Chalcogenides for IR Photodetection.

2D GeP as a Novel Broadband Nonlinear Optical Material for Ultrafast Photonics

Tunneling heterostructures are emerging as a versatile architecture for photodetection due to their advanced optical sensitivity, tailorable detection band, and well- balanced photoelectric performances However, the existing tunneling heterostructures are mainly operated in the visible wavelengths and have been rarely investigated for the near-infrared detection Herein, we report the design and realization of a novel broken-gap tunneling heterostructure by combining WSe2 and Bi2Se3, which is able to realize the simultaneous visible and near-infrared detection because of the complementary bandgaps of WSe2 and Bi2Se3 (146 and 03 eV, respectively) Thanks to the realigned band structure, the heterostructure shows an ultralow dark current below pico-ampere and a high tunneling-dominated photocurrent The photodetector based on our tunneling heterostructure exhibits a superior specific detectivity of 79× 1012 Jones for a visible incident of 532 nm and 22× 1010 Jones for a 1456 nm near-infrared illumination Our study demonstrates a new band structure engineering avenue for the construction of van der Waals tunneling heterostructures for high-performance wide band photodetection

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Band structure engineered tunneling heterostructures for high-performance visible and near-infrared photodetection

The emerging 2D layered transition metal trihalides (MX3 ) have attracted extremely high interest given their exceptional structural and physical properties. Continuing to extend the library of 2D MX3 is essential for exploring new physical phenomena and enabling new functionality. Herein, the optical and electrical properties and the photodetection behavior of atomically thin RhI3 flakes exfoliated from bulk crystals are reported. This compound exhibits superior air and thermal stability, as well as thickness-dependent bandgap from 1.1 (18L) to 1.4 eV (2L). Field-effect transistors based on the few-layer RhI3 flakes display n-type semiconducting behavior with competitive mobility of 2.5 cm2 V-1 s-1 and ON/OFF current ratio of 4 × 104 . Importantly, the outstanding responsivity of 11.5 A W-1 and high specific detectivity of 2 × 1010 Jones are recorded from the RhI3 photodetectors under 980 nm illumination at room temperature in air. These findings indicate a variety of potential applications of atomically thin RhI3 flakes in future 2D-material-based electronic and optoelectronic devices.

Honeycomb RhI3 Flakes with High Environmental Stability for Optoelectronics.

Owing to their attractive high carrier mobility, ambient stability, superior mechanical flexibility, large band gap and excellent optical properties, group IV–V compounds, as a new kind of 2D materials, show a promising potential application for optoelectronic devices. Herein, 2D GeP nanosheets were exfoliated by a facile LPE method and a photoelectrochemical (PEC)-type photodetector employing a doctor blade deposited 2D GeP nanosheet electrode on an ITO-coated glass was fabricated. Ultrafast carrier dynamics was carefully probed by transient absorption spectroscopy, and the parameters of the photodetector, such as the voltage and electrolyte concentration, were highly optimized. It shows remarkable performance with a responsivity of 187.5 μA W−1, a detectivity of 2.14 × 1012 Jones and an EQE of 61.3% at an ultraviolet wavelength of 380 nm. The proposed strategy avoids complicated material preparation and device fabrication and facilitates large-area photodetection.

Yue Zhang

Papers

2D Metal Chalcogenides for IR Photodetection.

2D GeP as a Novel Broadband Nonlinear Optical Material for Ultrafast Photonics

Band structure engineered tunneling heterostructures for high-performance visible and near-infrared photodetection

Honeycomb RhI3 Flakes with High Environmental Stability for Optoelectronics.

Facile liquid-phase exfoliated few-layer GeP nanosheets and their optoelectronic device applications