MoS2/Rubrene van der Waals Heterostructure: Toward Ambipolar Field‐Effect Transistors and Inverter Circuits

2D transition metal dichalcogenides are promising channel materials for the next‐generation electronic device. Here, vertically 2D heterostructures, so called van der Waals solids, are constructed using inorganic molybdenum sulfide (MoS₂) few layers and organic crystal – 5,6,11,12‐tetraphenylnaphthacene (rubrene). In this work, ambipolar field‐effect transistors are successfully achieved based on MoS₂ and rubrene crystals with the well balanced electron and hole mobilities of 1.27 and 0.36 cm² V^?1 s^?1, respectively. The ambipolar behavior is explained based on the band alignment of MoS₂ and rubrene. Furthermore, being a building block, the MoS₂/rubrene ambipolar transistors are used to fabricate CMOS (complementary metal oxide semiconductor) inverters that show good performance with a gain of 2.3 at a switching threshold voltage of ?26 V. This work paves a way to the novel organic/inorganic ultrathin heterostructure based flexible electronics and optoelectronic devices.     

High‐Performance Photodiode Based on Atomically Thin WSe2/MoS2 Nanoscroll Integration

Self‐assembled structures of 2D materials with novel physical and chemical properties, such as the good electrical and optoelectrical performance in nanoscrolls, have attracted a lot of attention. However, high photoresponse speed as well as high responsivity cannot be achieved simultaneously in the nanoscrolls. Here, a photodiode consisting of single MoS₂ nanoscrolls and a p‐type WSe₂ is demonstrated and shows excellent photovoltaic characteristics with a large open‐circuit voltage of 0.18 V and high current intensity. Benefiting from the heterostructure, the dark current is suppressed resulting in an increased ratio of photocurrent to dark current (two orders of magnitude higher than the single MoS₂ nanoscroll device). Furthermore, it yields high responsivity of 0.3 A W^?1 (corresponding high external quantum efficiency of ≈75%) and fast response time of 5 ms, simultaneously. The response speed is increased by three orders of magnitude over the single MoS₂ nanoscroll device. In addition, broadband photoresponse up to near‐infrared could be achieved. This atomically thin WSe₂/MoS₂ nanoscroll integration not only overcomes the disadvantage of MoS₂ nanoscrolls, but also demonstrates a single nanoscroll‐based heterostructure with high performance, promising its potential in the future optoelectronic applications.     

Graphene‐Contacted Ultrashort Channel Monolayer MoS2 Transistors

2D semiconductors are promising channel materials for field‐effect transistors (FETs) with potentially strong immunity to short‐channel effects (SCEs). In this paper, a grain boundary widening technique is developed to fabricate graphene electrodes for contacting monolayer MoS₂. FETs with channel lengths scaling down to ≈4 nm can be realized reliably. These graphene‐contacted ultrashort channel MoS₂ FETs exhibit superior performances including the nearly Ohmic contacts and excellent immunity to SCEs. This work provides a facile route toward the fabrication of various 2D material‐based devices for ultrascaled electronics.     

SnSe/MoS2 van der Waals Heterostructure Junction Field‐Effect Transistors with Nearly Ideal Subthreshold Slope

The minimization of the subthreshold swing (SS) in transistors is essential for low‐voltage operation and lower power consumption, both critical for mobile devices and internet of things (IoT) devices. The conventional metal‐oxide‐semiconductor field‐effect transistor requires sophisticated dielectric engineering to achieve nearly ideal SS (60 mV dec^?1 at room temperature). However, another type of transistor, the junction field‐effect transistor (JFET) is free of dielectric layer and can reach the theoretical SS limit without complicated dielectric engineering. The construction of a 2D SnSe/MoS₂ van der Waals (vdW) heterostructure‐based JFET with nearly ideal SS is reported. It is shown that the SnSe/MoS₂ vdW heterostructure exhibits excellent p–n diode rectifying characteristics with low saturate current. Using the SnSe as the gate and MoS₂ as the channel, the SnSe/MoS₂ vdW heterostructure exhibit well‐behavioured n‐channel JFET characteristics with a small pinch‐off voltage V_P of ?0.25 V, nearly ideal subthreshold swing SS of 60.3 mV dec^?1 and high ON/OFF ratio over 10⁶, demonstrating excellent electronic performance especially in the subthreshold regime.     

MoS2 Negative‐Capacitance Field‐Effect Transistors with Subthreshold Swing below the Physics Limit

The Boltzmann distribution of electrons induced fundamental barrier prevents subthreshold swing (SS) from less than 60 mV dec^‐1 at room temperature, leading to high energy consumption of MOSFETs. Herein, it is demonstrated that an aggressive introduction of the negative capacitance (NC) effect of ferroelectrics can decisively break the fundamental limit governed by the “Boltzmann tyranny”. Such MoS₂ negative‐capacitance field‐effect transistors (NC‐FETs) with self‐aligned top‐gated geometry demonstrated here pull down the SS value to 42.5 mV dec^‐1, and simultaneously achieve superior performance of a transconductance of 45.5 μS μm and an on/off ratio of 4 × 10⁶ with channel length less than 100 nm. Furthermore, the inserted HfO₂ layer not only realizes a stable NC gate stack structure, but also prevents the ferroelectric P(VDF‐TrFE) from fatigue with robust stability. Notably, the fabricated MoS₂ NC‐FETs are distinctly different from traditional MOSFETs. The on‐state current increases as the temperature decreases even down to 20 K, and the SS values exhibit nonlinear dependence with temperature due to the implementation of the ferroelectric gate stack. The NC‐FETs enable fundamental applications through overcoming the Boltzmann limit in nanoelectronics and open up an avenue to low‐power transistors needed for many exciting long‐endurance portable consumer products.     

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 Mobility 2D Palladium Diselenide Field‐Effect Transistors with Tunable Ambipolar Characteristics

Due to the intriguing optical and electronic properties, 2D materials have attracted a lot of interest for the electronic and optoelectronic applications. Identifying new promising 2D materials will be rewarding toward the development of next generation 2D electronics. Here, palladium diselenide (PdSe₂), a noble‐transition metal dichalcogenide (TMDC), is introduced as a promising high mobility 2D material into the fast growing 2D community. Field‐effect transistors (FETs) based on ultrathin PdSe₂ show intrinsic ambipolar characteristic. The polarity of the FET can be tuned. After vacuum annealing, the authors find PdSe₂ to exhibit electron‐dominated transport with high mobility (µ _{e (max)} = 216 cm² V^?1 s^?1) and on/off ratio up to 10³. Hole‐dominated‐transport PdSe₂ can be obtained by molecular doping using F₄‐TCNQ. This pioneer work on PdSe₂ will spark interests in the less explored regime of noble‐TMDCs.     

Carrier Transport and Photoresponse in GeSe/MoS2 Heterojunction p–n Diodes

Simple stacking of thin van der Waals 2D materials with different physical properties enables one to create heterojunctions (HJs) with novel functionalities and new potential applications. Here, a 2D material p–n HJ of GeSe/MoS₂ is fabricated and its vertical and horizontal carrier transport and photoresponse properties are studied. Substantial rectification with a very high contrast (>10⁴) through the potential barrier in the vertical‐direction tunneling of HJs is observed. The negative differential transconductance with high peak‐to‐valley ratio (>10⁵) due to the series resistance change of GeSe, MoS₂, and HJs at different gate voltages is observed. Moreover, strong and broad‐band photoresponse via the photoconductive effect are also demonstrated. The explored multifunctional properties of the GeSe/MoS₂ HJs are expected to be important for understanding the carrier transport and photoresponse of 2D‐material HJs for achieving their use in various new applications in the electronics and optoelectronics fields.     

Contact‐Engineered Electrical Properties of MoS2 Field‐Effect Transistors via Selectively Deposited Thiol‐Molecules

Although 2D molybdenum disulfide (MoS₂) has gained much attention due to its unique electrical and optical properties, the limited electrical contact to 2D semiconductors still impedes the realization of high‐performance 2D MoS₂‐based devices. In this regard, many studies have been conducted to improve the carrier‐injection properties by inserting functional paths, such as graphene or hexagonal boron nitride, between the electrodes and 2D semiconductors. The reported strategies, however, require relatively time‐consuming and low‐yield transfer processes on sub‐micrometer MoS₂ flakes. Here, a simple contact‐engineering method is suggested, introducing chemically adsorbed thiol‐molecules as thin tunneling barriers between the metal electrodes and MoS₂ channels. The selectively deposited thiol‐molecules via the vapor‐deposition process provide additional tunneling paths at the contact regions, improving the carrier‐injection properties with lower activation energies in MoS₂ field‐effect transistors. Additionally, by inserting thiol‐molecules at the only one contact region, asymmetric carrier‐injection is feasible depending on the temperature and gate bias.     

Penne‐Like MoS2/Carbon Nanocomposite as Anode for Sodium‐Ion‐Based Dual‐Ion Battery

The dual‐ion battery (DIB) system has attracted great attention owing to its merits of low cost, high energy, and environmental friendliness. However, the DIBs based on sodium‐ion electrolytes are seldom reported due to the lack of appropriate anode materials for reversible Na⁺ insertion/extraction. Herein, a new sodium‐ion based DIB named as MoS₂/C‐G DIB using penne‐like MoS₂/C nanotube as anode and expanded graphite as cathode is constructed and optimized for the first time. The hierarchical MoS₂/C nanotube provides expanded (002) interlayer spacing of 2H‐MoS₂, which facilitates fast Na⁺ insertion/extraction reaction kinetics, thus contributing to improved DIB performance. The MoS₂/C‐G DIB delivers a reversible capacity of 65 mA h g^?1 at 2 C in the voltage window of 1.0–4.0 V, with good cycling performance for 200 cycles and 85% capacity retention, indicating the feasibility of potential applications for sodium‐ion based DIBs.     

Tunneling Diode Based on WSe2/SnS2 Heterostructure Incorporating High Detectivity and Responsivity

van der Waals (vdW) heterostructures based on atomically thin 2D materials have led to a new era in next‐generation optoelectronics due to their tailored energy band alignments and ultrathin morphological features, especially in photodetectors. However, these photodetectors often show an inevitable compromise between photodetectivity and photoresponsivity with one high and the other low. Herein, a highly sensitive WSe₂/SnS₂ photodiode is constructed on BN thin film by exfoliating each material and manually stacking them. The WSe₂/SnS₂ vdW heterostructure shows ultralow dark currents resulting from the depletion region at the junction and high direct tunneling current when illuminated, which is confirmed by the energy band structures and electrical characteristics fitted with direct tunneling. Thus, the distinctive WSe₂/SnS₂ vdW heterostructure exhibits both ultrahigh photodetectivity of 1.29 × 10¹³ Jones (I_ph/I_dark ratio of ≈10⁶) and photoresponsivity of 244 A W^?1 at a reverse bias under the illumination of 550 nm light (3.77 mW cm^?2).     

Tunable Polarity Behavior and Self‐Driven Photoswitching in p‐WSe2/n‐WS2 Heterojunctions

Van der Waals (vdW) p–n heterojunctions consisting of various 2D layer compounds are fascinating new artificial materials that can possess novel physics and functionalities enabling the next‐generation of electronics and optoelectronics devices. Here, it is reported that the WSe₂/WS₂ p–n heterojunctions perform novel electrical transport properties such as distinct rectifying, ambipolar, and hysteresis characteristics. Intriguingly, the novel tunable polarity transition along a route of n‐“anti‐bipolar”–p‐ambipolar is observed in the WSe₂/WS₂ heterojunctions owing to the successive work of conducting channels of junctions, p‐WSe₂ and n‐WS₂ on the electrical transport of the whole systems. The type‐II band alignment obtained from first principle calculations and built‐in potential in this vdW heterojunction can also facilitate the efficient electron–hole separation, thus enabling the significant photovoltaic effect and a much enhanced self‐driven photoswitching response in this system.     

尺寸效应对MoS2/WSe2范德华异质结构层间与俄歇复合的界面调控

为探索界面工程对二维材料范德华异质结构中载流子复合率的影响, 本工作基于界面键弛豫理论和费米黄金定则, 建立了范德华异质结俄歇和层间复合率与各结构组元尺寸之间的理论模型。结果表明, MoS₂/WSe₂异质结的俄歇复合寿命随着组元尺寸的增大而增加, 且异质结的俄歇复合率远小于相应的单组元体系。在MoS₂/WSe₂双层异质结中引入薄h-BN插层后, 体系的层间复合率和俄歇复合率随h-BN厚度的增加而分别呈现减小和增大的趋势; 在组元处于单层MoS₂和WSe₂情况下, 当界面插层h-BN厚度达到9.1 nm时, 俄歇复合率将趋于5.3 ns ^-1。该研究结果为二维过渡金属硫族化合物基异质结光电器件的优化设计提供了一种理论依据。     

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.