WSe2 Homojunction Devices: Electrostatically Configurable as Diodes,MOSFETs, and Tunnel FETs for Reconfigurable Computing

In this paper, electrostatically configurable 2D tungsten diselenide (WSe₂) electronic devices are demonstrated. Utilizing a novel triple‐gate design, a WSe₂ device is able to operate as a tunneling field‐effect transistor (TFET), a metal–oxide–semiconductor field‐effect transistor (MOSFET) as well as a diode, by electrostatically tuning the channel doping to the desired profile. The implementation of scaled gate dielectric and gate electrode spacing enables higher band‐to‐band tunneling transmission with the best observed subthreshold swing (SS) among all reported homojunction TFETs on 2D materials. Self‐consistent full‐band atomistic quantum transport simulations quantitatively agree with electrical measurements of both the MOSFET and TFET and suggest that scaling gate oxide below 3 nm is necessary to achieve sub‐60 mV dec^?1 SS, while further improvement can be obtained by optimizing the spacers. Diode operation is also demonstrated with the best ideality factor of 1.5, owing to the enhanced electrostatic control compared to previous reports. This research sheds light on the potential of utilizing electrostatic doping scheme for low‐power electronics and opens a path toward novel designs of field programmable mixed analog/digital circuitry for reconfigurable computing.     

Temperature‐Triggered Sulfur Vacancy Evolution in Monolayer MoS2/Graphene Heterostructures

The existence of defects in 2D semiconductors has been predicted to generate unique physical properties and markedly influence their electronic and optoelectronic properties. In this work, it is found that the monolayer MoS₂ prepared by chemical vapor deposition is nearly defect‐free after annealing under ultrahigh vacuum conditions at ≈400 K, as evidenced by scanning tunneling microscopy observations. However, after thermal annealing process at ≈900 K, the existence of dominant single sulfur vacancies and relatively rare vacancy chains (2S, 3S, and 4S) is convinced in monolayer MoS₂ as‐grown on Au foils. Of particular significance is the revelation that the versatile vacancies can modulate the band structure of the monolayer MoS₂, leading to a decrease of the bandgap and an obvious n‐doping effect. These results are confirmed by scanning tunneling spectroscopy data as well as first‐principles theoretical simulations of the related morphologies and the electronic properties of the various defect types. Briefly, this work should pave a novel route for defect engineering and hence the electronic property modulation of three‐atom‐thin 2D layered semiconductors.     

Tunable SnSe2/WSe2 Heterostructure Tunneling Field Effect Transistor

The burgeoning 2D semiconductors can maintain excellent device electrostatics with an ultranarrow channel length and can realize tunneling by electrostatic gating to avoid deprivation of band‐edge sharpness resulting from chemical doping, which make them perfect candidates for tunneling field effect transistors. Here this study presents SnSe₂/WSe₂ van der Waals heterostructures with SnSe₂ as the p‐layer and WSe₂ as the n‐layer. The energy band alignment changes from a staggered gap band offset (type‐II) to a broken gap (type‐III) when changing the negative back‐gate voltage to positive, resulting in the device operating as a rectifier diode (rectification ratio ~10⁴) or an n‐type tunneling field effect transistor, respectively. A steep average subthreshold swing of 80 mV dec^?1 for exceeding two decades of drain current with a minimum of 37 mV dec^?1 at room temperature is observed, and an evident trend toward negative differential resistance is also accomplished for the tunneling field effect transistor due to the high gate efficiency of 0.36 for single gate devices. The I _ON/I _OFF ratio of the transfer characteristics is >10⁶, accompanying a high ON current >10^?5 A. This work presents original phenomena of multilayer 2D van der Waals heterostructures which can be applied to low‐power consumption devices.     

ZnO Nanorods on a LaAlO3–SrTiO3 Interface: Hybrid 1D–2D Diodes with Engineered Electronic Properties

Integrating nanomaterials with different dimensionalities and properties is a versatile approach toward realizing new functionalities in advanced devices. Here, a novel diode‐type heterostructure is reported consisting of 1D semiconducting ZnO nanorods and 2D metallic LaAlO₃–SrTiO₃ interface. Tunable insulator‐to‐metal transitions, absent in the individual components, are observed as a result of the competing temperature‐dependent conduction mechanisms. Detailed transport analysis reveals direct tunneling at low bias, Fowler–Nordheim tunneling at high forward bias, and Zener breakdown at high reverse bias. Our results highlight the rich electronic properties of such artificial diodes with hybrid dimensionalities, and the design principle may be generalized to other nanomaterials.     

Resonant Tunneling Spectroscopy to Probe the Giant Stark Effect in Atomically Thin Materials

Each atomic layer in van der Waals heterostructures possesses a distinct electronic band structure that can be manipulated for unique device operations. In the precise device architecture, the subtle but critical band splits by the giant Stark effect between atomic layers, varied by the momentum of electrons and external electric fields in device operation, has not yet been demonstrated or applied to design original devices with the full potential of atomically thin materials. Here, resonant tunneling spectroscopy based on the negligible quantum capacitance of 2D semiconductors in resonant tunneling transistors is reported. The bandgaps and sub-band structures of various channel materials could be demonstrated by the new conceptual spectroscopy at the device scale without debatable quasiparticle effects. Moreover, the band splits by the giant Stark effect in the channel materials could be probed, overcoming the limitations of conventional optical, photoemission, and tunneling spectroscopy. The resonant tunneling spectroscopy reveals essential and practical information for novel device applications.     

Giant Thickness‐Tunable Bandgap and Robust Air Stability of 2D Palladium Diselenide

Uncovering the thickness‐dependent electronic property and environmental stability for 2D materials are crucial issues for promoting their applications in high‐performance electronic and optoelectronic devices. Herein, the extrahigh air stability and giant tunable electronic bandgap of chemical vapor deposition (CVD)–derived few‐layer PdSe₂ on Au foils, by using scanning tunneling microscope/spectroscopy (STM/STS), are reported. The robust stability of 2D PdSe₂ is uncovered by the observation of nearly defect/adsorption‐free atomic lattices on long‐time air‐exposed samples. A one‐to‐one correspondence between the electronic bandgap (from ≈1.15 to ≈0 eV) and thickness of PdSe₂/Au (from bilayer to bulk) is established. It is also revealed that few‐layer semiconducting PdSe₂ flakes present zero‐gap edges, induced by hybridization of Pd 4d and Se 4p orbitals. This work hereby provides straightforward evidence for the thickness‐tunable electronic property and air stability of 2D semiconductors, thus shedding light on their applications in next‐generation electronic devices.     

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.     

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.     

Programmable Organic‐Free Negative Differential Resistance Memristor Based on Plasmonic Tunnel Junction

A novel negative differential resistance (NDR) phenomenon is reported herein based on planar plasmonic tunnel junction, resulting from plasmon‐assisted long‐range electron tunneling (P‐tunneling) and electronic caching effect of Au@SiO₂ nanoparticles. The tunnel junction is made of shell‐insulated Au@SiO₂ nanoparticle nanomembrane, in which SiO₂ shells act as a tunable tunneling barrier, while the Au core not only support the plasmonic effect to enable P‐tunneling, but also act as electronic caches to render NDR responses. The NDR peak voltage and current can be programmably controlled by varying the thickness of SiO₂ shell and the size of Au core to tune barrier level for electron transport. In addition, light induced plasmonic effect can be further managed to regulate the NDR behavior by fine‐tuning P‐tunneling. The phenomenon is exploited for robust use as memristors. The work provides a new mechanism for the generation of NDR effect and may open a way for the development of robust and new conceptual nanoelectronic devices.     

Anisotropic Growth and Scanning Tunneling Microscopy Identification of Ultrathin Even‐Layered PdSe2 Ribbons

Palladium diselenide (PdSe₂) is an emerging 2D layered material with anisotropic optical/electrical properties, extra‐high carrier mobility, excellent air stability, etc. So far, ultrathin PdSe₂ is mainly achieved via mechanical exfoliation from its bulk counterpart, and the direct synthesis is still challenging. Herein, the synthesis of ultrathin 2D PdSe₂ on conductive Au foil substrates via a facile chemical vapor deposition route is reported. Intriguingly, an anisotropic growth behavior is detected from the evolution of ribboned flakes with large length/width ratios, which is well explained from the orthorhombic symmetry of PdSe₂. A unique even‐layered growth mode from 2 to 20 layers is also confirmed by the perfect combination of onsite scanning tunneling microscopy characterizations, through deliberately scratching the flake edge to expose both even and odd layers. This even‐layered, ribboned 2D material is expected to serve as a perfect platform for exploring unique physical properties, and for developing high‐performance electronic and optoelectronic devices.     

Guided Molecular Assembly on a Locally Reactive 2D Material

Atomically precise engineering of the position of molecular adsorbates on surfaces of 2D materials is key to their development in applications ranging from catalysis to single‐molecule spintronics. Here, stable room‐temperature templating of individual molecules with localized electronic states on the surface of a locally reactive 2D material, silicene grown on ZrB₂, is demonstrated. Using a combination of scanning tunneling microscopy and density functional theory, it is shown that the binding of iron phthalocyanine (FePc) molecules is mediated via the strong chemisorption of the central Fe atom to the sp³‐like dangling bond of Si atoms in the linear silicene domain boundaries. Since the planar Pc ligand couples to the Fe atom mostly through the in‐plane d orbitals, localized electronic states resembling those of the free molecule can be resolved. Furthermore, rotation of the molecule is restrained because of charge rearrangement induced by the bonding. These results highlight how nanoscale changes can induce reactivity in 2D materials, which can provide unique surface interactions for enabling novel forms of guided molecular assembly.     

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.     

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.     

Freestanding Borophene and Its Hybrids

Borophene, an elemental metallic Dirac material is predicted to have unprecedented mechanical and electronic character. Need of substrate and ultrahigh vacuum conditions for deposition of borophene restricts its large‐scale applications and significantly hampers the advancement of research on borophene. Herein, a facile and large‐scale synthesis of freestanding atomic sheets of borophene through a novel liquid‐phase exfoliation and the reduction of borophene oxide is demonstrated. Electron microscopy confirms the presence of β₁₂, X₃, and their intermediate phases of borophene; X‐ray photoelectron spectroscopy, and scanning tunneling microscopy, corroborated with density functional theory band structure calculations, validate the phase purity and the metallic nature. Borophene with excellent anchoring capabilities is used for sensing of light, gas, molecules, and strain. Hybrids of borophene as well as that of reduced borophene oxide with other 2D materials are synthesized, and the predicted superior performance in energy storage is explored. The specific capacity of borophene oxide is observed to be ≈4941 mAh g^?1, which significantly exceeds that of existing 2D materials and their hybrids. These freestanding borophene materials and their hybrids will create a huge breakthrough in the field of 2D materials and could help to develop future generations of devices and emerging applications.     

Organic High Electron Mobility Transistors Realized by 2D Electron Gas

A key breakthrough in inorganic modern electronics is the energy‐band engineering that plays important role to improve device performance or develop novel functional devices. A typical application is high electron mobility transistors (HEMTs), which utilizes 2D electron gas (2DEG) as transport channel and exhibits very high electron mobility over traditional field‐effect transistors (FETs). Recently, organic electronics have made very rapid progress and the band transport model is demonstrated to be more suitable for explaining carrier behavior in high‐mobility crystalline organic materials. Therefore, there emerges a chance for applying energy‐band engineering in organic semiconductors to tailor their optoelectronic properties. Here, the idea of energy‐band engineering is introduced and a novel device configuration is constructed, i.e., using quantum well structures as active layers in organic FETs, to realize organic 2DEG. Under the control of gate voltage, electron carriers are accumulated and confined at quantized energy levels, and show efficient 2D transport. The electron mobility is up to 10 cm² V^?1 s^?1, and the operation mechanisms of organic HEMTs are also argued. Our results demonstrate the validity of tailoring optoelectronic properties of organic semiconductors by energy‐band engineering, offering a promising way for the step forward of organic electronics.     

Developing 1D Sb‐Embedded Carbon Nanorods to Improve Efficiency and Stability of Inverted Planar Perovskite Solar Cells

To overcome the zigzag pathway transport of the electron diffusion process and eliminate the surface trap states of phenyl‐C61‐butyric acid methyl ester (PCBM) nanofilms in inverted perovskite solar cells, novel 1D N‐type doped carbon nanorods (CNRs) are developed by a stibonium (Sb) auxiliary ball milling method and introduced into the PCBM film to prepare the PCBM:Sb‐CNRs hybrid transport layer. In this way, the N‐type doped Sb‐CNRs can extend the built‐in electric field between CH₃NH₃PbI₃ and PCBM to facilitate the separation of electron/hole pairs. The discontinuous band with the built‐in potential in the PCBM/Sb‐CNRs heterojunction can boost interfacial charge redistribution and promote electrons diffusion from PCBM to electrode through 1D Sb‐CNRs network. As a result, the high device efficiency of 19.26% with enhanced air stability and little hysteresis are achieved. This work demonstrates a simple strategy to improve the efficiency and stability of perovskite photovoltaic devices using low‐cost carbon nanomaterials.     

Mobility Engineering in Vertical Field Effect Transistors Based on Van der Waals Heterostructures

Vertical integration of 2D layered materials to form van der Waals heterostructures (vdWHs) offers new functional electronic and optoelectronic devices. However, the mobility in vertical carrier transport in vdWHs of vertical field‐effect transistor (VFET) is not yet investigated in spite of the importance of mobility for the successful application of VFETs in integrated circuits. Here, the mobility in VFET of vdWHs under different drain biases, gate biases, and metal work functions is first investigated and engineered. The traps in WSe₂ are the main source of scattering, which influences the vertical mobility and three distinct transport mechanisms: Ohmic transport, trap‐limited transport, and space‐charge‐limited transport. The vertical mobility in VFET can be improved by suppressing the trap states by raising the Fermi level of WSe₂. This is achieved by increasing the injected carrier density by applying a high drain voltage, or decreasing the Schottky barrier at the graphene/WSe₂ and metal/WSe₂ junctions by applying a gate bias and reducing the metal work function, respectively. Consequently, the mobility in Mn vdWH at +50 V gate voltage is about 76 times higher than the initial mobility of Au vdWH. This work enables further improvements in the VFET for successful application in integrated circuits.     

Direct Growth of High Mobility and Low‐Noise Lateral MoS2–Graphene Heterostructure Electronics

Reliable fabrication of lateral interfaces between conducting and semiconducting 2D materials is considered a major technological advancement for the next generation of highly packed all‐2D electronic circuitry. This study employs seed‐free consecutive chemical vapor deposition processes to synthesize high‐quality lateral MoS₂–graphene heterostructures and comprehensively investigated their electronic properties through a combination of various experimental techniques and theoretical modeling. These results show that the MoS₂–graphene devices exhibit an order of magnitude higher mobility and lower noise metrics compared to conventional MoS₂–metal devices as a result of energy band rearrangement and smaller Schottky barrier height at the contacts. These findings suggest that MoS₂–graphene in‐plane heterostructures are promising materials for the scale‐up of all‐2D circuitry with superlative electrical performance.     

Reconfigurable Diodes Based on Vertical WSe2 Transistors with van der Waals Bonded Contacts

New device concepts can increase the functionality of scaled electronic devices, with reconfigurable diodes allowing the design of more compact logic gates being one of the examples. In recent years, there has been significant interest in creating reconfigurable diodes based on ultrathin transition metal dichalcogenide crystals due to their unique combination of gate‐tunable charge carriers, high mobility, and sizeable band gap. Thanks to their large surface areas, these devices are constructed under planar geometry and the device characteristics are controlled by electrostatic gating through rather complex two independent local gates or ionic‐liquid gating. In this work, similar reconfigurable diode action is demonstrated in a WSe₂ transistor by only utilizing van der Waals bonded graphene and Co/h‐BN contacts. Toward this, first the charge injection efficiencies into WSe₂ by graphene and Co/h‐BN contacts are characterized. While Co/h‐BN contact results in nearly Schottky‐barrier‐free charge injection, graphene/WSe₂ interface has an average barrier height of ≈80 meV. By taking the advantage of the electrostatic transparency of graphene and the different work‐function values of graphene and Co/h‐BN, vertical devices are constructed where different gate‐tunable diode actions are demonstrated. This architecture reveals the opportunities for exploring new device concepts.     

Anisotropic Magneto‐Coulomb Properties of 2D–0D Heterostructure Single Electron Device

Fabrication and spintronics properties of 2D–0D heterostructures are reported. Devices based on graphene (“Gr”)–aluminium nanoclusters heterostructures show robust and reproducible single‐electron transport features, in addition to spin‐dependent functionality when using a top magnetic electrode. The magnetic orientation of this single ferromagnetic electrode enables the modulation of the environmental charge experienced by the aluminium nanoclusters. This anisotropic magneto‐Coulomb effect, originating from spin–orbit coupling within the ferromagnetic electrode, provides tunable spin valve‐like magnetoresistance signatures without the requirement of spin coherent charge tunneling. These results extend the capability of Gr to act both as electrode and as a platform for the growth of 2D–0D mixed‐dimensional van der Waals heterostructures, providing magnetic functionalities in the Coulomb blockade regime on scalable spintronic devices. These heterostructures pave the way towards novel device architectures at the crossroads of 2D material physics and spin electronics.