PN heterojunctions comprising layered van der Waals (vdW) semiconductors have been used to demonstrate current rectifiers, photodetectors, and photovoltaic devices. However, a direct or neardirect bandgap at the heterointerface that can significantly enhance optical generation, for high light absorbing few/multi-layer vdW materials, has not yet been shown. In this work, for the first time, few-layer group-6 transition metal dichalcogenide (TMD) WSe2 is shown to form a sizeable (0.7 eV) near-direct bandgap with type-II band alignment at its interface with the group-7 TMD ReS2 through density functional theory calculations. Further, the type-II alignment and photogeneration across the interlayer bandgap have been experimentally confirmed through micro-photoluminescence and IR 2 photodetection measurements, respectively. High optical absorption in few-layer flakes, large conduction and valence band offsets for efficient electron-hole separation and stacking of light facing, direct bandgap ReS2 on top of gate tunable WSe2 are shown to result in excellent and tunable photodetection as well as photovoltaic performance through flake thickness dependent optoelectronic measurements. Few-layer flakes demonstrate ultrafast response time (5 µs) at high responsivity (3 A/W) and large photocurrent generation and responsivity enhancement at the heterostructure overlap region (10-100×) for 532 nm laser illumination. Large open circuit voltage of 0.64 V and short circuit current of 2.6 µA enables high output electrical power. Finally, long term air-stability and a facile single contact metal fabrication process makes the multi-functional few-layer WSe2/ReS2 heterostructure diode technologically promising for next-generation optoelectronic applications.The semiconducting group-6 TMD WSe2, generally found in trigonal prismatic phase, 17 is an indirect bandgap material in its bulk form. 18,19 However, group-7 TMD e.g. 1T phase of ReS2 is distorted octahedral in structure 17,20 and exhibits direct (or, near-direct) bandgap at (or, close to) the Γ point of the Brillouin zone (BZ). Interestingly, unlike the group-6 TMDs, ReS2 exhibits a unique property owing to its distorted structure and weak interlayer coupling-the direct or near-direct nature of its bandgap remains unchanged from monolayer to bulk. [20][21][22] Closer examination of the bandstructure of group-7 TMDs reveals that the conduction band minimum of ReS2 remains at the Γ point, irrespective of the number of layers. 20,23 But for group-6 TMDs (such as WSe2) the valence band maximum relocates from K to Γ point of the BZ with increasing number of layers. 18,24 It is important to note that the valence band maximum at the Γ point differs in energy only slightly from that at the K point. 18 This gives rise to an increased probability of direct as well as indirect transitions from the Γ and the K valence maxima of WSe2 to the Γ conduction minimum of ReS2 respectively, for a predicted type-II band alignment. The possibility of a direct bandgap transition is not observed in a heter...

The origin of threshold voltage instability with gate voltage in MoS 2 transistors is poorly understood but critical for device reliability and performance. Reversibility of the temperature dependence of hysteresis and its inversion with temperature in MoS 2 transistors has not been demonstrated. In this work, we delineate two independent mechanisms responsible for thermally assisted hysteresis inversion in gate transfer characteristics of contact resistance-independent multilayer MoS 2 transistors. Variable temperature hysteresis measurements were performed on gated four-terminal van der Pauw and two-terminal devices of MoS 2 on SiO 2 . Additional hysteresis measurements on suspended (~100 nm air gap between MoS 2 and SiO 2 ) transistors and under different ambient conditions (vacuum/nitrogen) were used to further isolate the mechanisms. Clockwise hysteresis at room temperature (300 K) that decreases with increasing temperature is shown to result from intrinsic defects/traps in MoS 2 . At higher temperatures a second, independent mechanism of charge trapping and de-trapping between the oxide and p + Si gate leads to hysteresis collapse at~350 K and anti-clockwise hysteresis (inversion) for temperatures >350 K. The intrinsic-oxide trap model has been corroborated through device simulations. Further, pulsed current-voltage (I-V) measurements were carried out to extract the trap time constants at different temperatures. Non-volatile memory and temperature sensor applications exploiting temperature dependent hysteresis inversion and its reversibility in MoS 2 transistors have also been demonstrated. npj 2D Materials and Applications (2017) 1:34 ; doi:10.1038/s41699-017-0038-y INTRODUCTION Among two-dimensional materials, graphene 1,2 was the first to be isolated and studied with respect to electronic applications. Due to lack of an energy bandgap in graphene, other 2D materials such as layered transition metal dichalcogenides (TMDs) comprising a wide selection of materials with different bandstructures, and therefore different electrical and optical properties, have garnered significant attention.3-5 Molybdenum disulfide (MoS 2 ) has emerged as a prospective candidate for transistor applications. The presence of a direct bandgap (~1.8 eV) in monolayer form and an indirect bandgap (~1.2 eV) in multilayer MoS 2 makes it a promising channel material for field effect transistors (FETs).

Rhenium disulfide (ReS2) is an attractive candidate for photodetection applications owing to its thickness-independent direct band gap. Despite various photodetection studies using two-dimensional semiconductors, the trade-off between responsivity and response time under varying measurement conditions has not been studied in detail. This report presents a comprehensive study of the architectural, laser power and gate bias dependence of responsivity and speed in supported and suspended ReS2 phototransistors. Photocurrent scans show uniform photogeneration across the entire channel because of enhanced optical absorption and a direct band gap in multilayer ReS2. A high responsivity of 4 A W–1 (at 50 ms response time) and a low response time of 20 μs (at 4 mA W–1 responsivity) make this one of the fastest reported transition-metal dichalcogenide photodetectors. Occupancy of intrinsic (bulk ReS2) and extrinsic (ReS2/SiO2 interface) traps is modulated using gate bias to demonstrate tunability of the response time (responsivity) over 4 orders (15×) of magnitude, highlighting the versatility of these photodetectors. Differences in the trap distributions of suspended and supported channel architectures, and their occupancy under different gate biases enable switching the dominant operating mechanism between either photogating or photoconduction. Further, a new metric that captures intrinsic photodetector performance by including the trade-off between its responsivity and speed, besides normalizing for the applied bias and geometry, is proposed and benchmarked for this work.

Transition metal dichalcogenides (TMDCs) are a family of two-dimensional layered materials (2DLMs) with extraordinary optical properties. They present an attractive option for future multifunctional and high-performance optoelectronics. However, much remains to be done to realize a mature technology for commercial applications. In this review article, we describe the progress and scope of TMDC devices in optical and photonic applications. Various photoresponse mechanisms observed in such devices and a brief discussion on measurement and analysis methods are described. Three main types of optoelectronic devices, namely photodetectors, photovoltaics and lightemitting devices are discussed in detail with a focus on device architecture and operation. Examples showing experimental integration of 2DLM-based devices with silicon photonics are also discussed briefly. A wide range of data for key performance metrics is analysed with insights into future directions for device design, processing and characterization that can help overcome present gaps and challenges. While the field is still in its infancy and requires significant efforts toward standardizing various approaches towards a mature technology, it has already shown promising results in broader aspects of compatibility, integration and performance for future electronics. Sensors are increasingly becoming an integrated part of the global electronic eco-system with the rise of data-driven technologies. There is a growing need for networks of cost-effective, robust and reliable sensors and their integration with present technologies. Optoelectronic and photonic devices are of importance for both sensing as well as high-speed optical communication applications. 2DLMs demonstrate significant light-matter interaction that is tunable with external physical and electronic parameters such as pressure, strain, electric and magnetic field [1-6]. Moreover, 2DLMs show strong dependence of their band structure on thickness with a transition to direct bandgap in monolayers in most of the known 2DLMs [7][8][9]. Graphene has been the most studied material since its discovery in 2004 [10][11][12]. Apart from graphene, there are nearly 1500 possible 2DLMs with a wide range of material properties as predicted by first principle simulations [13]. Transition metal dichalcogenides (TMDCs) are a family of 2DLMs in the form of MX 2 compounds, where M is a transition metal (Mo, W, Re) and X is a chalcogen (S, Se, Te). TMDCs are promising for electronic applications due to their semiconducting behaviour as opposed to semi-metallic graphene, and better thermal stability than materials such as black phosphorus and silicene [14][15][16][17][18][19]. Optoelectronic devices based on TMDCsTMDCs have been the most studied 2DLMs, apart from graphene and black phosphorus, for electronic and optoelectronic device applications [20,21]. They have a sizeable bandgap in the visible and near infrared (NIR) range (∼1-2 eV) that is optimal for optoelectronic sensors and light-emitting sources for short-ran...

Layered transition metal dichalcogenides have shown tremendous potential for photodetection due to their non-zero direct bandgaps, high light absorption coefficients and carrier mobilities, and ability to form atomically sharp and defect-free heterointerfaces. A critical and fundamental bottleneck in the realization of high performance detectors is their trap-dependent photoresponse that trades off responsivity with speed. This work demonstrates a facile method of attenuating this trade-off by nearly 2x through integration of a lateral, in-plane, electrostatically tunable p-n homojunction with a conventional WSe2 phototransistor. The tunable p-n junction allows modulation of the photocarrier population and width of the conducting channel independently from the phototransistor. Increased illumination current with the lateral p-n junction helps achieve responsivity enhancement upto 2.4x at nearly the same switching speed (14–16 µs) over a wide range of laser power (300 pW–33 nW). The added benefit of reduced dark current enhances specific detectivity (D*) by nearly 25x to yield a maximum measured flicker noise-limited D* of 1.1×1012 Jones. High responsivity of 170 A/W at 300 pW laser power along with the ability to detect sub-1 pW laser switching are demonstrated.