Negative Transconductance Heterojunction Organic Transistors and their Application to Full‐Swing Ternary Circuits

Multivalued logic (MVL) computing could provide bit density beyond that of Boolean logic. Unlike conventional transistors, heterojunction transistors (H‐TRs) exhibit negative transconductance (NTC) regions. Using the NTC characteristics of H‐TRs, ternary inverters have recently been demonstrated. However, they have shown incomplete inverter characteristics; the output voltage (V_OUT) does not fully swing from V_DD to G_ND. A new H‐TR device structure that consists of a dinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐b]thiophene (DNTT) layer stacked on a PTCDI‐C13 layer is presented. Due to the continuous DNTT layer from source to drain, the proposed device exhibits novel switching behavior: p‐type off/p‐type subthreshold region /NTC/ p‐type on. As a result, it has a very high on/off current ratio (≈10⁵) and exhibits NTC behavior. It is also demonstrated that an array of 36 of these H‐TRs have 100% yield, a uniform on/off current ratio, and uniform NTC characteristics. Furthermore, the proposed ternary inverter exhibits full V_DD‐to‐G_ND swing of V_OUT with three distinct logic states. The proposed transistors and inverters exhibit hysteresis‐free operation due to the use of a hydrophobic gate dielectric and encapsulating layers. Based on this, the transient operation of a ternary inverter circuit is demonstrated for the first time.     

Modulating the Function of GeAs/ReS2 van der Waals Heterojunction with its Potential Application for Short-Wave Infrared and Polarization-Sensitive Photodetection

Van der Waals heterojunction (vdWs) of 2D materials with integrated or extended superior characteristics, opening up new opportunities in functional electronic and optoelectric device applications. Exploring methods to achieve multifunctional vdWs heterojunction devices is one of the most promising prospects in this area. Herein, a diverse function of forward rectifying diode, Zener tunneling diode, and backward rectifying diodes are realized in GeAs/ReS₂ heterojunction by modulating the doping level of GeAs. The tunneling diode presents an interesting trend forward negative differential resistance (NDR) behavior which may facilitate the application of multi-value logic. More importantly, the GeAs/ReS₂ forward rectifying diode exhibits highly sensitive photodetection in the wide-spectrum range up to 1550 nm corresponding to a short-wave infrared (SWIR) region. In addition, as two strong anisotropic 2D materials of GeAs and ReS₂, the heterojunction exhibits strong polarization-sensitive photodetection behavior with a dichroic photocurrent ratio of 1.7. This work provides an effective strategy to achieve multifunctional 2D vdW heterojunction devices and develops more possibilities to broaden their functionalities and applications.     

Efficient Self‐Driven Photodetectors Featuring a Mixed‐Dimensional van der Waals Heterojunction Formed from a CdS Nanowire and a MoTe2 Flake

Heterojunctions formed from low‐dimensional materials can result in photovoltaic and photodetection devices displaying exceptional physical properties and excellent performance. Herein, a mixed‐dimensional van der Waals (vdW) heterojunction comprising a 1D n‐type Ga‐doped CdS nanowire and a 2D p‐type MoTe₂ flake is demonstrated; the corresponding photovoltaic device exhibits an outstanding conversion efficiency of 15.01% under illumination with white light at 650 µW cm^?2. A potential difference of 80 meV measured, using Kelvin probe force microscopy, at the CdS–MoTe₂ interface confirms the separation and accumulation of photoexcited carriers upon illumination. Moreover, the photodetection characteristics of the vdW heterojunction device at zero bias reveal a rapid response time (<50 ms) and a photoresponsivity that are linearly proportional to the power density of the light. Interestingly, the response of the vdW heterojunction device is negligible when illuminated at 580 nm; this exceptional behavior is presumably due to the rapid rate of recombination of the photoexcited carriers of MoTe₂. Such mixed‐dimensional vdW heterojunctions appear to be novel design elements for efficient photovoltaic and self‐driven photodetection devices.     

8nm基区宽度负阻双异质结晶体管的设计与研制

本文利用分子束外延(MBE)技术能精确控制外延层厚度的特点,与选择性腐蚀技术相结合,实现了纳米级超薄基区宽度.利用集电极电压VCE调制中性基区宽度可以改变基极电阻,从而产生微分负电阻(NDR),根据这一物理机制,设计并研制成功性能优良的8 nm基区n-InGaP/P+-GaAs/n-InGaP负阻双异质结晶体管(NDRDHBT).该器件显示出基极电压VBE调制的"∧"型负阻集电极电流IC-集电极电压VCE特性,电流峰谷比(PVCR)趋于无穷大;表征基极电压调制电流能力的峰值电流跨导ΔIP/ΔVBE高达11.2 ms;击穿电压达到12 V,可用于高频振荡调制和高速数字电路.     

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.     

Metal Thio‐ and Selenophosphates as Multifunctional van der Waals Layered Materials

Since the discovery of Dirac physics in graphene, research in 2D materials has exploded with the aim of finding new materials and harnessing their unique and tunable electronic and optical properties. The follow‐on work on 2D dielectrics and semiconductors has led to the emergence and development of hexagonal boron nitride, black phosphorus, and transition metal disulfides. However, the spectrum of good insulating materials is still very narrow. Likewise, 2D materials exhibiting correlated phenomena such as superconductivity, magnetism, and ferroelectricity have yet to be developed or discovered. These properties will significantly enrich the spectrum of functional 2D materials, particularly in the case of high phase‐transition temperatures. They will also advance a fascinating fundamental frontier of size and proximity effects on correlated ground states. Here, a broad family of layered metal thio(seleno)phosphate materials that are moderate‐ to wide‐bandgap semiconductors with incipient ionic conductivity and a host of ferroic properties are reviewed. It is argued that this material class has the potential to merge the sought‐after properties of complex oxides with electronic functions of 2D and quasi‐2D electronic materials, as well as to create new avenues for both applied and fundamental materials research in structural and magnetic correlations.     

Band‐Gap Engineering at a Semiconductor–Crystalline Oxide Interface

The epitaxial growth of crystalline oxides on semiconductors provides a pathway to introduce new functionalities to semiconductor devices. Key to electrically coupling crystalline oxides with semiconductors to realize functional behavior is to control the manner in which their bands align at interfaces. Here, principles of band‐gap engineering traditionally used at heterojunctions between conventional semiconductors are applied to control the band offset between a single crystalline oxide and a semiconductor. Reactive molecular beam epitaxy is used to realize atomically abrupt and structurally coherent interfaces between SrZr_xTi_1−xO₃ and Ge, in which the band‐gap of the former is enhanced with Zr content x. Structural and electrical characterization of SrZr_xTi_1−xO₃‐Ge heterojunctions for x = 0.2 to 0.75 are presented and it is demonstrated that the band offset can be tuned from type‐II to type‐I, with the latter being verified using photoemission measurements. The type‐I band offset provides a platform to integrate the dielectric, ferroelectric, and ferromagnetic functionalities of oxides with semiconducting devices.     

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.     

Metal‐Free 2D/2D Phosphorene/g‐C3N4 Van der Waals Heterojunction for Highly Enhanced Visible‐Light Photocatalytic H2 Production

The generation of green hydrogen (H₂) energy using sunlight is of great significance to solve the worldwide energy and environmental issues. Particularly, photocatalytic H₂ production is a highly promising strategy for solar‐to‐H₂ conversion. Recently, various heterostructured photocatalysts with high efficiency and good stability have been fabricated. Among them, 2D/2D van der Waals (VDW) heterojunctions have received tremendous attention, since this architecture can promote the interfacial charge separation and transfer and provide massive reactive centers. On the other hand, currently, most photocatalysts are composed of metal elements with high cost, limited reserves, and hazardous environmental impact. Hence, the development of metal‐free photocatalysts is desirable. Here, a novel 2D/2D VDW heterostructure of metal‐free phosphorene/graphitic carbon nitride (g‐C₃N₄) is fabricated. The phosphorene/g‐C₃N₄ nanocomposite shows an enhanced visible‐light photocatalytic H₂ production activity of 571 µmol h^?1 g^?1 in 18 v% lactic acid aqueous solution. This improved performance arises from the intimate electronic coupling at the 2D/2D interface, corroborated by the advanced characterizations techniques, e.g., synchrotron‐based X‐ray absorption near‐edge structure, and theoretical calculations. This work not only reports a new metal‐free phosphorene/g‐C₃N₄ photocatalyst but also sheds lights on the design and fabrication of 2D/2D VDW heterojunction for applications in catalysis, electronics, and optoelectronics.     

Two‐Terminal Multibit Optical Memory via van der Waals Heterostructure

2D van der Waals (vdWs) heterostructures exhibit intriguing optoelectronic properties in photodetectors, solar cells, and light‐emitting diodes. In addition, these materials have the potential to be further extended to optical memories with promising broadband applications for image sensing, logic gates, and synaptic devices for neuromorphic computing. In particular, high programming voltage, high off‐power consumption, and circuital complexity in integration are primary concerns in the development of three‐terminal optical memory devices. This study describes a multilevel nonvolatile optical memory device with a two‐terminal floating‐gate field‐effect transistor with a MoS₂/hexagonal boron nitride/graphene heterostructure. The device exhibits an extremely low off‐current of ≈10^?14 A and high optical switching on/off current ratio of over ≈10⁶, allowing 18 distinct current levels corresponding to more than four‐bit information storage. Furthermore, it demonstrates an extended endurance of over ≈10⁴ program–erase cycles and a long retention time exceeding 3.6 × 10⁴ s with a low programming voltage of ?10 V. This device paves the way for miniaturization and high‐density integration of future optical memories with vdWs heterostructures.     

Graphene‐Based Mixed‐Dimensional van der Waals Heterostructures for Advanced Optoelectronics

Although the library of 2D atomic crystals has greatly expanded over the past years, research into graphene is still one of the focuses for both academia and business communities. Due to its unique electronic structure, graphene offers a powerful platform for exploration of novel 2D physics, and has significantly impacted a wide range of fields including energy, electronics, and photonics. Moreover, the versatility of combining graphene with other functional components provides a powerful strategy to design artificial van der Waals (vdWs) heterostructures. Aside from the stacked 2D–2D vdWs heterostructure, in a broad sense graphene can hybridize with other non‐2D materials through vdWs interactions. Such mixed‐dimensional vdWs (MDWs) structures allow considerable freedom in material selection and help to harness the synergistic advantage of different dimensionalities, which may compensate for graphene's intrinsic shortcomings. A succinct overview of representative advances in graphene‐based MDWs heterostructures is presented, ranging from assembly strategies to applications in optoelectronics. The scientific merit and application advantages of these hybrid structures are particularly emphasized. Moreover, considering possible breakthroughs in new physics and application potential on an industrial scale, the challenges and future prospects in this active research field are highlighted.     

Misorientation‐Angle‐Dependent Phase Transformation in van der Waals Multilayers via Electron‐Beam Irradiation

Misorientation‐angle dependence on layer thickness is an intriguing feature of van der Waals materials, which causes stark optical gain and electrical transport modulation. However, the influence of misorientation angle on phase transformation is not determined yet. Herein, this phenomenon in a MoS₂ multilayer via in situ electron‐beam irradiation is reported. An AA′‐stacked MoS₂ bilayer undergoes structural transformation from the 2H semiconducting phase to the 1T′ metallic phase, similar to a MoS₂ monolayer, which is confirmed via in situ transmission electron microscopy. Moreover, non‐AA′ stacking, which has no local AA′ stacking order in the Moiré pattern, does not reveal such a phase transformation. While a collective sliding motion of chalcogen atoms easily occurs during the transformation in AA′ stacking, in non‐AA′ stacking it is suppressed by the weak van der Waals strength and by the chalcogen atoms interlocked at different orientations, which disfavor their kinetics by the increased entropy of mixing.     

Tunneling Photocurrent Assisted by Interlayer Excitons in Staggered van der Waals Hetero‐Bilayers

Vertically stacked van der Waals (vdW) heterostructures have been suggested as a robust platform for studying interfacial phenomena and related electric/optoelectronic devices. While the interlayer Coulomb interaction mediated by the vdW coupling has been extensively studied for carrier recombination processes in a diode transport, its correlation with the interlayer tunneling transport has not been elucidated. Here, a contrast is reported between tunneling and drift photocurrents tailored by the interlayer coupling strength in MoSe₂/MoS₂ hetero‐bilayers (HBs). The interfacial coupling modulated by thermal annealing is identified by the interlayer phonon coupling in Raman spectra and the emerging interlayer exciton peak in photoluminescence spectra. In strongly coupled HBs, positive photocurrents are observed owing to the inelastic band‐to‐band tunneling assisted by interlayer excitons that prevail over exciton recombinations. By contrast, weakly coupled HBs exhibit a negative photovoltaic diode behavior, manifested as a drift current without interlayer excitonic emissions. This study sheds light on tailoring the tunneling transport for numerous optoelectronic HB devices.     

In‐Plane Isotropic/Anisotropic 2D van der Waals Heterostructures for Future Devices

Mono‐ to few‐layers of 2D semiconducting materials have uniquely inherent optical, electronic, and magnetic properties that make them ideal for probing fundamental scientific phenomena up to the 2D quantum limit and exploring their emerging technological applications. This Review focuses on the fundamental optoelectronic studies and potential applications of in‐plane isotropic/anisotropic 2D semiconducting heterostructures. Strong light–matter interaction, reduced dimensionality, and dielectric screening in mono‐ to few‐layers of 2D semiconducting materials result in strong many‐body interactions, leading to the formation of robust quasiparticles such as excitons, trions, and biexcitons. An in‐plane isotropic nature leads to the quasi‐2D particles, whereas, an anisotropic nature leads to quasi‐1D particles. Hence, in‐plane isotropic/anisotropic 2D heterostructures lead to the formation of quasi‐1D/2D particle systems allowing for the manipulation of high binding energy quasi‐1D particle populations for use in a wide variety of applications. This Review emphasizes an exciting 1D–2D particles dynamic in such heterostructures and their potential for high‐performance photoemitters and exciton–polariton lasers. Moreover, their scopes are also broadened in thermoelectricity, piezoelectricity, photostriction, energy storage, hydrogen evolution reactions, and chemical sensor fields. The unique in‐plane isotropic/anisotropic 2D heterostructures may open the possibility of engineering smart devices in the nanodomain with complex opto‐electromechanical functions.     

Site‐Selective and van der Waals Epitaxial Growth of Rhenium Disulfide on Graphene

The surface property of growth substrate imposes significant influence in the growth behaviors of 2D materials. Rhenium disulfide (ReS₂) is a new family of 2D transition metal dichalcogenides with unique distorted 1T crystal structure and thickness‐independent direct bandgap. The role of growth substrate is more critical for ReS₂ owing to its weak interlayer coupling property, which leads to preferred growth along the out‐of‐plane direction while suppressing the uniform in‐plane growth. Herein, graphene is introduced as the growth substrate for ReS₂ and the synthesis of graphene/ReS₂ vertical heterostructure is demonstrated via chemical vapor deposition. Compared with the rough surface of SiO₂/Si substrate with dangling bonds which hinders the uniform growth of ReS₂, the inert and smooth surface nature of graphene sheet provides a lower energy barrier for migration of the adatoms, thereby promoting the growth of ReS₂ on the graphene surface along the in‐plane direction. Furthermore, patterning of the graphene/ReS₂ heterostructure is achieved by the selective growth of ReS₂, which is attributed to the strong binding energy between sulfur atoms and graphene surface. The fundamental studies in the role of graphene as the growth template in the formation of van der Waals heterostructures provide better insights into the synthesis of 2D heterostructures.