Formation of a stable p-n junction in a liquid-gated MoS2 ambipolar transistor

Drain-Induced Multifunctional Ambipolar Electronics Based on Junctionless MoS2

Applying a drain bias to a strongly gate-coupled semiconductor influences the carrier density of the channel. However, practical applications of this drain-bias-induced effect in the advancement of switching electronics have remained elusive due to the limited capabilities of its current modulation known to date. Here, we show strategies to largely control the current by utilizing drain-bias-induced carrier type switching in an ambipolar molybdenum disulfide (MoS2) field-effect transistor with Pt bottom contacts. Our CMOS-compatible device architecture, incorporating a partially gate-coupled p-n junction, achieves multifunctionality. The ambipolar MoS2 device operates as an ambipolar transistor (on/off ratios exceeding 107 for both NMOS and PMOS), a rectifier (rectification ratio of ∼3 × 106), a reversible negative breakdown diode with an adjustable breakdown voltage (on/off ratio exceeding 109 with a maximum current as high as 10-4 A), and a photodetector. Finally, we demonstrate a complementary inverter (gain of ∼24 at Vdd = 1.5 V), which is highly facile to fabricate without the need for complex heterostructures and doping processes. Our study provides strategies to achieve high-performance ambipolar MoS2 devices and to effectively utilize drain bias for electrical switching.

Revisiting the Hetero-Interface of Electrolyte/2D Materials in an Electric Double Layer Device

Electric double layer (EDL) devices based on 2D materials have made great achievements for versatile electronic and opto-electronic applications; however, the ion dynamics and electric field distribution of the EDL at the electrolyte/2D material interface and their influence on the physical properties of 2D materials have not been clearly clarified. In this work, by using Kelvin probe force microscope and steady/transient optical techniques, the character of the EDL and its influence on the optical properties of monolayer transition metal dichalcogenides (TMDs) are probed. The potential drop, unscreened EDL potential distribution, and accumulated carriers at the electrolyte/TMD interface are revealed, which can be explained by nonlinear Thomas-Fermi theory. By monitoring the potential distribution along the channel, the evolution of the electric field-induced lateral junction in the TMD EDL transistor is accessed, giving rise to the better exploration of EDL device physics. More importantly, EDL gate-dependent carrier recombination and exciton-exciton annihilation in monolayer TMDs on lithium-ion solid state electrolyte (Li2 Al2 SiP2 TiO13 ) are evaluated for the first time, benefiting from the understanding of the interaction between ions, carriers, and excitons. The work will deepen the understanding of the EDL for the exploitation of functional device applications.

Highly Stable InSe-FET Biosensor for Ultra-Sensitive Detection of Breast Cancer Biomarker CA125

Two-dimensional materials-based field-effect transistors (FETs) are promising biosensors because of their outstanding electrical properties, tunable band gap, high specific surface area, label-free detection, and potential miniaturization for portable diagnostic products. However, it is crucial for FET biosensors to have a high electrical performance and stability degradation in liquid environments for their practical application. Here, a high-performance InSe-FET biosensor is developed and demonstrated for the detection of the CA125 biomarker in clinical samples. The InSe-FET is integrated with a homemade microfluidic channel, exhibiting good electrical stability during the liquid channel process because of the passivation effect on the InSe channel. The InSe-FET biosensor is capable of the quantitative detection of the CA125 biomarker in breast cancer in the range of 0.01-1000 U/mL, with a detection time of 20 min. This work provides a universal detection tool for protein biomarker sensing. The detection results of the clinical samples demonstrate its promising application in early screenings of major diseases.

High-Speed Transition-Metal Dichalcogenides Based Schottky Photodiodes for Visible and Infrared Light Communication

Due to their atomically ultrathin thickness, the development of high-performance transition-metal dichalcogenides (TMDCs) based photodetectors demands device designs distinct from architectures adopted in conventional bulk semiconductor devices. Here, we demonstrate a field-induced Schottky barrier photodiode with three different TMDC materials, WSe₂, MoTe₂, and WS₂. Owing to the high gate efficiency of a high-κ dielectric film, the Schottky barrier at metal contacts is effectively modulated by external bias, giving rise to a strong diode-like rectifying characteristic with high current on/off ratio. The WSe₂ photodiode shows a linear dynamic range of 112 dB, a responsivity of 0.17 A/W, and response time of 8 ns. When this fast WSe₂ device is employed for visible light communication data linking, a maximum real-time data transmission rate of 110 Mbps is achieved. Meanwhile, infrared light communication was also realized with a maximum data rate of 30 Mbps using a field-induced MoTe₂ Schottky barrier photodiode as a light sensor. This work provides a general CMOS-compatible and controllable fabrication strategy for TMDC-based photodetectors.

P-Type 2D Semiconductors for Future Electronics

2D semiconductors represent one of the best candidates to extend Moore's law for their superiorities, such as keeping high carrier mobility and remarkable gate-control capability at atomic thickness. Complementary transistors and van der Waals junctions are critical in realizing 2D semiconductors-based integrated circuits suitable for future electronics. N-type 2D semiconductors have been reported predominantly for the strong electron doping caused by interfacial charge impurities and internal structural defects. By contrast, superior and reliable p-type 2D semiconductors with holes as majority carriers are still scarce. Not only that, but some critical issues have not been adequately addressed, including their controlled synthesis in wafer size and high quality, defect and carrier modulation, optimization of interface and contact, and application in high-speed and low-power integrated devices. Here the material toolkit, synthesis strategies, device basics, and digital electronics closely related to p-type 2D semiconductors are reviewed. Their opportunities, challenges, and prospects for future electronic applications are also discussed, which would be promising or even shining in the post-Moore era.

Deterministic Light-to-Voltage Conversion with a Tunable Two-Dimensional Diode

Heterojunctions accompanied by energy barriers are of significant importance in two-dimensional materials-based electronics and optoelectronics. They provide more functional device performance, compared with their counterparts with uniform channels. Multimodal optoelectronic devices could be accomplished by elaborately designing band diagrams and architectures of the two-dimensional junctions. Here, we demonstrate deterministic light-to-voltage conversion based on strong dielectric screening effect in a tunable two-dimensional Schottky diode based on semiconductor/metal heterostructure, where the resultant photovoltage is dependent on the intensity of light input but independent of gate voltage. The converted photovoltage across the diode is independent of gate voltage under both monochromatic laser and white light illumination. In addition, the Fermi level of two-dimensional semiconductor area on dielectric SiO₂ is highly gate-dependent, leading to the tunable rectifying effect of this heterostructure, which corporates a vertical Schottky junction and a lateral homojunction. As a result, a constant open-circuit voltage of ∼0.44 V and a hybrid "photovoltaic + photoconduction" photoresponse behavior are observed under 1 μW illumination of 403 nm laser, in addition to an electrical rectification ratio up to nearly 10⁴. The scanning photocurrent mappings under different bias voltages indicate that the switchable operation mode (photovoltaic, photoconduction, or hybrid) depends on the bias-dependent effective energy barrier at the two-dimensional semiconductor-metal interface. This approach provides a facile and reliable solution for deterministic on-chip light-to-voltage conversion and optical-to-electrical interconnects.

Channel-Length-Modulated Avalanche Multiplication in Ambipolar WSe2 Field-Effect Transistors

Recently there has been growing interest in avalanche multiplication in two-dimensional (2D) materials and device applications such as avalanche photodetectors and transistors. Previous studies have mainly utilized unipolar semiconductors as the active material and focused on developing high-performance devices. However, fundamental analysis of the multiplication process, particularly in ambipolar materials, is required to establish high-performance electronic devices and emerging architectures. Although ambipolar 2D materials have the advantage of facile carrier-type tuning through electrostatic gating, simultaneously allowing both carrier types in a single channel poses an inherent difficulty in analyzing their individual contributions to avalanche multiplication. In ambipolar field-effect transistors (FETs), two phenomena of ambipolar transport and avalanche multiplication can occur, and both exhibit secondary rise of output current at high lateral voltage. We distinguished these two competing phenomena using the method of channel length modulation and successfully analyzed the properties of electron- and hole-initiated multiplication in ambipolar WSe₂ FETs. Our study provides a simple and robust method to examine carrier multiplication in ambipolar materials and will foster the development of high-performance atomically thin electronic devices utilizing avalanche multiplication.

Analysis of Low-Frequency 1/f Noise Characteristics for MoTe2 Ambipolar Field-Effect Transistors

Low-frequency electronic noise is an important parameter used for the electronic and sensing applications of transistors. Here, we performed a systematic study on the low-frequency noise mechanism for both p-channel and n-channel MoTe2 field-effect transistors (FET) at different temperatures, finding that low-frequency noise for both p-type and n-type conduction in MoTe2 devices come from the variable range hopping (VRH) transport process where carrier number fluctuations (CNF) occur. This process results in the broad distribution of the waiting time of the carriers between successive hops, causing the noise to increase as the temperature decreases. Moreover, we found the noise magnitude for p-type MoTe2 FET hardly changed after exposure to the ambient conditions, whereas for n-FET, the magnitude increased by nearly one order. These noise characteristics may provide useful guidelines for developing high-performance electronics based on the emerging transition metal dichalcogenides.

Gradual Edge Contact between Mo and MoS2 Formed by Graphene-Masked Sulfurization for High-Performance Field-Effect Transistors

Two-dimensional materials have attracted great attention for their outstanding electronic properties. In particular, molybdenum disulfide (MoS₂) shows great potential as a next-generation semiconductor due to its tunable direct bandgap with a high on-off ratio and extraordinary stability. However, the performance of MoS₂ synthesized by physical vapor deposition has been limited by contact resistance between an electrode and MoS₂, which determines overall device characteristics. Here, in order to reduce the contact resistance, we use in situ sulfurization of Mo by H₂S gas treatment masked by a patterned graphene gas barrier, so that the Mo channel area can be selectively formed, resulting in a gradual edge contact between Mo and MoS₂. Compared with field-effect transistors with a top contact between the Au/Ti electrode and the MoS₂ channel, a gradual edge contact between the Mo electrode and the MoS₂ channel provides a considerably enhanced electrical performance.

Design and tailoring of two-dimensional Schottky, PN and tunnelling junctions for electronics and optoelectronics

Owing to their superior carrier mobility, strong light-matter interactions, and flexibility at the atomically thin thickness, two-dimensional (2D) materials are attracting wide interest for application in electronic and optoelectronic devices, including rectifying diodes, transistors, memory, photodetectors, and light-emitting diodes. At the heart of these devices, Schottky, PN, and tunneling junctions are playing an essential role in defining device function. Intriguingly, the ultrathin thickness and unique van der Waals (vdW) interlayer coupling in 2D materials has rendered enormous opportunities for the design and tailoring of various 2D junctions, e.g. using Lego-like hetero-stacking, surface decoration, and field-effect modulation methods. Such flexibility has led to marvelous breakthroughs during the exploration of 2D electronics and optoelectronic devices. To advance further, it is imperative to provide an overview of existing strategies for the engineering of various 2D junctions for their integration in the future. Thus, in this review, we provide a comprehensive survey of previous efforts toward 2D Schottky, PN, and tunneling junctions, and the functional devices built from them. Though these junctions exhibit similar configurations, distinct strategies have been developed for their optimal figures of merit based on their working principles and functional purposes.

2D Homojunctions for Electronics and Optoelectronics

In the post-Moore era, 2D materials with rich physical properties have attracted widespread attention from the scientific and industrial communities. Among 2D materials, the 2D homojunctions are of great promise in designing novel electronic and optoelectronic devices due to their unique geometries and properties such as homogeneous components, perfect lattice matching, and efficient charge transfer at the interface. In this article, a pioneering review focusing on the structural design and device application of 2D homojunctions such as p-n homojunctions, heterophase homojunctions, and layer-engineered homojunctions is provided. The preparation strategies to construct 2D homojunctions including vapor-phase deposition, lithium intercalation, laser irradiation, chemical doping, electrostatic doping, and photodoping are summarized in detail. Specifically, a careful review on the applications of the 2D homojunctions in electronics (e.g., field-effect transistors, rectifiers, and inverters) and optoelectronics (e.g., light-emitting diodes, photovoltaics, and photodetectors) is provided. Eventually, the current challenges and future perspectives are commented for promoting the rapid development of 2D homojunctions.

Understanding ambipolar transport in MoS2 field effect transistors: the substrate is the key

2D materials offer a pathway for further scaling of CMOS technology. However, for this to become a reality, both n-MOS and p-MOS should be realized, ideally with the same (standard) material. In the specific case of MoS2 field effect transistors (FETs), ambipolar transport is seldom reported, primarily due to the phenomenon of Fermi level pinning (FLP). In this study we identify the possible sources of FLP in MoS2 FETs and resolve them individually. A novel contact transfer technique is used to transfer contacts on top of MoS2 flake devices that results in a significant increase in the hole branch of the transfer characteristics as compared to conventionally fabricated contacts. We hypothesize that the pinning not only comes from the contact-MoS2 interface, but also from the MoS2-substrate interface. We confirm this by shifting to an hBN substrate which leads to a 10 fold increase in the hole current compared to the SiO2 substrate. Furthermore, we analyse MoS2 FETs of different channel thickness on three different substrates, SiO2, hBN and Al2O3, by correlating the p-branch I ON/I OFF to the position of oxide defect band in these substrates. FLP from the oxide is reduced in the case of Al2O3 which enables us to observe ambipolar transport in a bilayer MoS2 FET. These results highlight that MoS2 is indeed an ambipolar material, and the absence of ambipolar transport in MoS2 FETs is strongly correlated to its dielectric environment and processing conditions.

Surface Modification of Monolayer MoS2 by Baking for Biomedical Applications

Molybdenum disulfide (MoS₂), a transition metal dichalcogenide material, possesses great potential in biomedical applications such as chemical/biological sensing, drug/gene delivery, bioimaging, phototherapy, and so on. In particular, monolayer MoS₂ has more extensive applications because of its superior physical and chemical properties; for example, it has an ultra-high surface area, is easily modified, and has high biodegradability. It is important to prepare advanced monolayer MoS₂ with enhanced energy exchange efficiency (EEE) for the development of MoS₂-based nanodevices and therapeutic strategies. In this work, a monolayer MoS₂ film was first synthesized through a chemical vapor deposition method, and the surface of MoS₂ was further modified via a baking process to develop p-type doping of monolayer MoS₂ with high EEE, followed by confirmation by X-ray photoelectron spectroscopy and Raman spectroscopy analysis. The morphology, surface roughness, and layer thickness of monolayer MoS₂ before and after baking were thoroughly investigated using atomic force microscopy. The results showed that the surface roughness and layer thickness of monolayer MoS₂ modified by baking were obviously increased in comparison with MoS₂ without baking, indicating that the surface topography of the monolayer MoS₂ film was obviously influenced. Moreover, a photoluminescence spectrum study revealed that p-type doping of monolayer MoS₂ displayed much greater photoluminescence ability, which was taken as evidence of higher photothermal conversion efficiency. This study not only developed a novel MoS₂ with high EEE for future biomedical applications but also demonstrated that a baking process is a promising way to modify the surface of monolayer MoS₂.

Two-dimensional electronic devices modulated by the activation of donor-like states in boron nitride

A two-dimensional (2D) layered material-based p-n diode is an essential element in the modern semiconductor industry for facilitating the miniaturization and structural flexibility of devices with high efficiency for future optoelectronic and electronic applications. Planar devices constructed previously required a complicated device structure using a photoresist, as they needed to consider non-abrupt interfaces. Here, we demonstrated a WSe₂ based lateral homojunction diode obtained by applying a photo-induced effect in BN/WSe₂ heterostructures upon illumination via visible and deep UV light, which represents a stable and flexible charge doping technique. We have discovered that with this technique, a field-effect transistor (FET) based on p-type WSe₂ is inverted to n-WSe₂ so that a high electron mobility is maintained in the h-BN/n-WSe₂ heterostructures. To confirm this hypothesis, we deduced the work function values of p-WSe₂ and n-WSe₂ FETs by conducting Kelvin probe force microscopy (KPFM) measurements, which revealed the decline of the Fermi level from 5.07 (p-WSe₂) to 4.21 eV (n-WSe₂). The contact potential difference (CPD) between doped and undoped junctions was found to be 165 meV. We employed ohmic metal contacts for the planar homojunction diode by utilizing an ionic liquid gate to achieve a diode rectification ratio up to ∼10⁵ with n = 1. An exceptional photovoltaic performance is also observed. The presence of a built-in potential in our devices leads to an open-circuit voltage (V_oc) and short-circuit current (I_sc) without an external electric field. This effective doping technique is promising to advance the concept of preparing future functional devices.

WSe2 Homojunction p-n Diode Formed by Photoinduced Activation of Mid-Gap Defect States in Boron Nitride

A single nanoflake lateral p-n diode (in-plane) based on a two-dimensional material can facilitate electronic architecture miniaturization. Here, a novel lateral homojunction p-n diode of a single WSe₂ nanoflake is fabricated by photoinduced doping via optical excitation of defect states in an h-BN nanoflake upon illumination. This lateral diode is fabricated using a mechanical exfoliation technique by stacking the WSe₂ nanoflake partially on the h-BN and Si substrates. The carrier type in the part of the WSe₂ film on the h-BN substrate is inverted and a built-in potential difference is formed, ranging from 5.0 to 4.50 eV, which is measured by Kelvin probe force microscopy. The contact potential difference across the junction of p-WSe₂ and n-WSe₂ is found to be ∼492 mV. The lateral diode shows an excellent rectification ratio, up to ∼3.9 × 10⁴, with an ideality factor of ∼1.1. A typical self-biased photovoltaic behavior is observed at the p-n junction upon the illumination of incident light, that is, a positive open-circuit voltage (V_oc) is generated, that is, voltage obtained (at I_ds = 0 V), and also a negative short-circuit current (I_sc) is generated, that is, current obtained (at V_ds = 0 V). The presence of built-in potential in the proposed homojunction diode establishes I_sc and V_oc upon illumination, which can be implemented for a self-powered photovoltaic system in future electronics. The proposed doping technique can be effectively applied to form planar homojunction devices without a photoresist for future electronic and optoelectronic applications.

Electric-double-layer p-i-n junctions in WSe2

While p-n homojunctions in two-dimensional transition metal dichalcogenide materials have been widely reported, few show an ideality factor that is constant over more than a decade in current. In this paper, electric double layer p-i-n junctions in WSe2 are shown with substantially constant ideality factors (2-3) over more than 3 orders of magnitude in current. These lateral junctions use the solid polymer, polyethylene oxide: cesium perchlorate (PEO:CsClO4), to induce degenerate electron and hole carrier densities at the device contacts to form the junction. These high carrier densities aid in reducing the contact resistance and enable the exponential current dependence on voltage to be measured at higher currents than prior reports. Transport measurements of these WSe2 p-i-n homojunctions in combination with COMSOL multiphysics simulations are used to quantify the ion distributions, the semiconductor charge distributions, and the simulated band diagram of these junctions, to allow applications to be more clearly considered.

Multifunctional MoS2 Transistors with Electrolyte Gel Gating

MoS₂ , one of the most valued 2D materials beyond graphene, shows potential for future applications in postsilicon digital electronics and optoelectronics. However, achieving hole transport in MoS₂ , which is dominated by electron transport, is always a challenge. Here, MoS₂ transistors gated by electrolyte gel exhibit the characteristics of hole and electron transport, a high on/off ratio over 10⁵ , and a low subthreshold swing below 50 mV per decade. Due to the electrolyte gel, the density of electrons and holes in the MoS₂ channel reaches ≈9 × 10¹³ and 8.85 × 10¹³ cm^-2 , respectively. The electrolyte gel-assisted MoS₂ phototransistor exhibits adjustable positive and negative photoconductive effects. Additionally, the MoS₂ p-n homojunction diode affected by electrolyte gel shows high performance and a rectification ratio over 10⁷ . These results demonstrate that modifying the conductance of MoS₂ through electrolyte gel has great potential in highly integrated electronics and optoelectronic photodetectors.

Ion-Locking in Solid Polymer Electrolytes for Reconfigurable Gateless Lateral Graphene p-n Junctions

A gateless lateral p-n junction with reconfigurability is demonstrated on graphene by ion-locking using solid polymer electrolytes. Ions in the electrolytes are used to configure electric-double-layers (EDLs) that induce p- and n-type regions in graphene. These EDLs are locked in place by two different electrolytes with distinct mechanisms: (1) a polyethylene oxide (PEO)-based electrolyte, PEO:CsClO₄, is locked by thermal quenching (i.e., operating temperature < T_g (glass transition temperature)), and (2) a custom-synthesized, doubly-polymerizable ionic liquid (DPIL) is locked by thermally triggered polymerization that enables room temperature operation. Both approaches are gateless because only the source/drain terminals are required to create the junction, and both show two current minima in the backgated transfer measurements, which is a signature of a graphene p-n junction. The PEO:CsClO₄ gated p-n junction is reconfigured to n-p by resetting the device at room temperature, reprogramming, and cooling to T < T_g. These results show an alternate approach to locking EDLs on 2D devices and suggest a path forward to reconfigurable, gateless lateral p-n junctions with potential applications in polymorphic logic circuits.

Band offsets engineering in asymmetric Janus bilayer transition-metal dichalcogenides

Using the first-principles calculation, we systematically studied the electronic properties of the bilayer transition metal dichalcogenides (TMDs) MX₂ (M  =  Mo, W; X  =  S, Se, Te) with replacing one, two, three or four layers of X atoms as Y atoms (X  ≠  Y  =  S, Se, Te). By comparison, it is found that when the inner two layers of chalcogenide atoms are different, the system has both valence band offset (VBO) and conduction band offset (CBO). Among them, values of the band offsets reach maxima when the inner one layer of X atoms is replaced by Y atoms, namely forming the asymmetric Janus bilayer XMX/YMX. We take SMoS/SeMoS as an example to analyze the formation of the band offsets and the improvement of optoelectronic properties. Importantly, it is also found that both external electric field and biaxial strain can regulate electronic structures of asymmetric Janus bilayer TMDs with noticeable modulation of the values of band offsets. When the external electric field changes from negative to positive continually, CBO decreases and VBO increases. While when the biaxial strain changes from compression to stretch continually, CBO increases and VBO decreases. These findings enrich the study of bilayer TMDs that can be used as optoelectronic, nanoelectronic and valleytronic devices.

Defect-Engineered Atomically Thin MoS2 Homogeneous Electronics for Logic Inverters

Ultrathin molybdenum disulfide (MoS₂ ) presents ideal properties for building next-generation atomically thin circuitry. However, it is difficult to construct logic units of MoS₂ monolayer using traditional silicon-based doping schemes, such as atomic substitution and ion implantation, as they cause lattice disruption and doping instability. An accurate and feasible electronic structure modulation strategy from defect engineering is proposed to construct homogeneous electronics for MoS₂ monolayer logic inverters. By utilizing the energy-matched electron induction of the solution process, numerous pure and lattice-stable monosulfur vacancies (V_monos ) are introduced to modulate the electronic structure of monolayer MoS₂ via a shallow trapping effect. The resulting modulation effectively reduces the electronic concentration of MoS₂ and improves the work function by 100 meV. Under modulation of V_monos , an atomically thin homogenous monolayer MoS₂ logic inverter with a voltage gain of 4 is successfully constructed. A brand-new and practical design route of defect modulation for 2D-based circuit development is provided.

A tunable floating-base bipolar transistor based on a 2D material homojunction realized using a solid ionic dielectric material

Floating-base bipolar transistors are widely used semiconductor devices because they could both amplify signal current and suppress noise. Employing two-dimensional (2D) materials of ultrahigh photoelectric properties could further improve the device performance. Due to the difficulty in doping, homojunctions are usually not realizable for many 2D materials. Instead, a heterojunction of various 2D materials of different Fermi levels is usually needed. However, the material interface of a heterojunction deteriorates device performance and makes the fabrication process difficult. Here, the doping difficulties have been solved by utilizing a solid ionic dielectric material (LiTaO3) and a floating-base bipolar transistor based on a 2D material (monolayer MoS2 here) homojunction is realized. The transistor shows tunable ambipolar transport characteristics. Particularly, under illumination, the amplification coefficient of a phototransistor can be optimized by changing the gate voltage. The optimized photoresponsivity of the device could reach up to 7.9 A W-1 with an ultrahigh detectivity of 3.39 × 1011 Jones. The overall fabrication processing is compatible to conventional processing. This design can effectively extend the application of 2D materials.

Quest for p-Type Two-Dimensional Semiconductors

Two-dimensional (2D) semiconductors have demonstrated great potential in modern nanotechnologies across a variety of research fields, including (opto-)electronics, spintronics, and electro-/photocatalysis. Interestingly, the vast majority of 2D semiconductors, such as the widely explored transition-metal dichalcogenides, are n-type or ambipolar. The search for p-type 2D semiconductors in the past decade has succeeded in identifying only a few promising candidate materials. In this Perspective, we discuss various strategies to obtain p-type conduction in normally n-type or ambipolar 2D semiconductors and, more importantly, the direct synthesis of p-type 2D semiconductors such as black phosphorus, 2D tellurium, and, most recently, α-MnS.

Reconfigurable two-dimensional optoelectronic devices enabled by local ferroelectric polarization

Ferroelectric engineered pn doping in two-dimensional (2D) semiconductors hold essential promise in realizing customized functional devices in a reconfigurable manner. Here, we report the successful pn doping in molybdenum disulfide (MoS2) optoelectronic device by local patterned ferroelectric polarization, and its configuration into lateral diode and npn bipolar phototransistors for photodetection from such a versatile playground. The lateral pn diode formed in this way manifests efficient self-powered detection by separating ~12% photo-generated electrons and holes. When polarized as bipolar phototransistor, the device is customized with a gain ~1000 by its transistor action, reaching the responsivity ~12 A W-1 and detectivity over 1013 Jones while keeping a fast response speed within 20 μs. A promising pathway toward high performance optoelectronics is thus opened up based on local ferroelectric polarization coupled 2D semiconductors.

MoTe2 van der Waals homojunction p-n diode with low resistance metal contacts

Although many studies have focused on transition metal dichalcogenide heterojunction p-n diodes, homojunction p-n diodes still require more extensive study. We present a van der Waals p-MoTe2/n-MoTe2 homojunction p-n diode with low resistance metal contacts. Such two-dimensional homojunction devices with low contact resistance can be used in various applications in the electronics industry. The device structure consists of stacked nanoflakes of p-MoTe2 and n-MoTe2. In this investigation, we implement a deep ultraviolet light-driven doping technique in a N2 gas environment to modulate the carrier concentration in a multilayered p-MoTe2 flake, which is consequently inverted to n-MoTe2. The deep ultraviolet light-driven doping provides environmental stability in the treated devices. We use ohmic metal contacts for the homojunction p-n diode and achieve excellent gate-dependent rectifying behavior with a rectification ratio of up to 104. Contrary to heterojunctions, the ideality factor is found to be 1.05 for the zero gate bias, indicative of good interface quality at the p-MoTe2/n-MoTe2 junction, owing to low charge trapping sites at the homojunction interface. In addition, low-temperature measurements are performed to determine the barrier height for different gate biases. This study contributes to research on van der Waals homojunction p-n diodes, which show much potential for nanoelectronic and optoelectronic devices.

Metal-Guided Selective Growth of 2D Materials: Demonstration of a Bottom-Up CMOS Inverter

2D transition metal dichalcogenide (TMD) layered materials are promising for future electronic and optoelectronic applications. The realization of large-area electronics and circuits strongly relies on wafer-scale, selective growth of quality 2D TMDs. Here, a scalable method, namely, metal-guided selective growth (MGSG), is reported. The success of control over the transition-metal-precursor vapor pressure, the first concurrent growth of two dissimilar monolayer TMDs, is demonstrated in conjunction with lateral or vertical TMD heterojunctions at precisely desired locations over the entire wafer in a single chemical vapor deposition (VCD) process. Owing to the location selectivity, MGSG allows the growth of p- and n-type TMDs with spatial homogeneity and uniform electrical performance for circuit applications. As a demonstration, the first bottom-up complementary metal-oxide-semiconductor inverter based on p-type WSe₂ and n-type MoSe₂ is achieved, which exhibits a high and reproducible voltage gain of 23 with little dependence on position.

High Optical Response of Niobium-Doped WSe₂-Layered Crystals

The optical properties of WSe₂-layered crystals doped with 0.5% niobium (Nb) grown by the chemical vapor transport method were characterized by piezoreflectance (PzR), photoconductivity (PC) spectroscopy, frequency-dependent photocurrent, and time-resolved photoresponse. With the incorporation of 0.5% Nb, the WSe₂ crystal showed slight blue shifts in the near band edge excitonic transitions and exhibited strongly enhanced photoresponsivity. Frequency-dependent photocurrent and time-resolved photoresponse were measured to explore the kinetic decay processes of carriers. Our results show the potential application of layered crystals for photodetection devices based on Nb-doped WSe₂-layered crystals.

Improved contacts to p-type MoS2 transistors by charge-transfer doping and contact engineering

MoS₂ is known to show stubborn n-type behavior due to its intrinsic band structure and Fermi level pinning. Here, we investigate the combined effects of molecular doping and contact engineering on the transport and contact properties of monolayer (ML) MoS₂ devices. Significant p-type (hole-transport) behavior was only observed for chemically doped MoS₂ devices with high work function palladium (Pd) contacts, while MoS₂ devices with low work function metal contacts made from titanium showed ambipolar behavior with electron transport favored even after prolonged p-doping treatment. ML MoS₂ transistors with Pd contacts exhibit effective hole mobilities of (2.3 ± 0.7) cm² V^-1 S^-1 and an on/off ratio exceeding 10⁶. We also show that p-doping can help to improve electrical contacts in p-type field-effect transistors: relatively low contact resistances of (482 ± 40) kΩ μm and a Schottky barrier height of ≈156 meV were obtained for ML MoS₂ transistors. To demonstrate the potential application of 2D-based complementary electronic devices, a MoS₂ inverter based on pristine (n-type) and p-doped monolayer MoS₂ was fabricated. This work presents a simple and effective route for contact engineering, which enables the exploration and development of high-efficiency 2D-based semiconductor devices.

Wafer-Scale Fabrication of Two-Dimensional PtS2/PtSe2 Heterojunctions for Efficient and Broad band Photodetection

The fabrication of van der Waals heterostructures mainly extends to two-dimensional (2D) materials that are exfoliated from their bulk counterparts, which is greatly limited by high-volume manufacturing. Here, we demonstrate multilayered PtS₂/PtSe₂ heterojunctions covering a large area on the SiO₂/Si substrate with a maximum size of 2″ in diameter, offering throughputs that can meet the practical application demand. Theoretical simulation was carried out to understand the electronic properties of the PtS₂/PtSe₂ heterojunctions. Zero-bias photoresponse in the heterojunctions is observed under laser illumination of different wavelengths (405-2200 nm). The PtS₂/PtSe₂ heterojunctions exhibit broad band photoresponse and high quantum efficiency at infrared wavelengths with lower bounds for the external quantum efficiencies being 1.2% at 1064 nm, 0.2% at 1550 nm, and 0.05% at 2200 nm, and also relatively fast response time at the dozens of millisecond level. The large area, broad band 2D heterojunction photodetector demonstrated in this work further corroborates the great potential of 2D materials in the future low-energy optoelectronics.

Charge carrier injection and transport engineering in two-dimensional transition metal dichalcogenides

Ever since two dimensional-transition (2D) metal dichalcogenides (TMDs) were discovered, their fascinating electronic properties have attracted a great deal of attention for harnessing them as critical components in novel electronic devices. 2D-TMDs endowed with an atomically thin structure, dangling bond-free nature, electrostatic integrity, and tunable wide band gaps enable low power consumption, low leakage, ambipolar transport, high mobility, superconductivity, robustness against short channel effects and tunneling in highly scaled devices. However, the progress of 2D-TMDs has been hampered by severe charge transport issues arising from undesired phenomena occurring at the surfaces and interfaces. Therefore, this review provides three distinct engineering strategies embodied with distinct innovative approaches to optimize both carrier injection and transport. First, contact engineering involves 2D-metal contacts and tunneling interlayers to overcome metal-induced interface states and the Fermi level pinning effect caused by low vacancy energy formation. Second, dielectric engineering covers high-k dielectrics, ionic liquids or 2D-insulators to screen scattering centers caused by carrier traps, imperfections and rough substrates, to finely tune the Fermi level across the band gap, and to provide dangling bond-free media. Third, material engineering focuses on charge transfer via substitutional, chemical and plasma doping to precisely modulate the carrier concentration and to passivate defects while preserving material integrity. Finally, we provide an outlook of the conceptual and technical achievements in 2D-TMDs to give a prospective view of the future development of highly scaled nanoelectronic devices.

High performance self-gating graphene/MoS2 diode enabled by asymmetric contacts

A graphene-MoS₂ (GM) heterostructure based diode is fabricated using asymmetric contacts to MoS₂, as well as an asymmetric top gate (ATG). The GM diode exhibits a rectification ratio of 5 from asymmetric contacts, which is improved to 10⁵ after the incorporation of an ATG. This improvement is attributed to the asymmetric modulation of carrier concentration and effective Schottky barrier height (SBH) by the ATG during forward and reverse bias. This is further confirmed from the temperature dependent measurement, where a difference of 0.22 eV is observed between the effective SBH for forward and reverse bias. Moreover, the rectification ratio also depends on carrier concentration in MoS₂ and can be varied with the change in temperature as well as back gate voltage. Under laser light illumination, the device demonstrates strong opto-electric response with 100 times improvement in the relative photo current, as well as a responsivity of 1.9 A W^-1 and a specific detectivity of 2.4 × 10¹⁰ Jones. These devices can also be implemented using other two dimensional (2D) materials and suggest a promising approach to incorporate diverse 2D materials for future nano-electronics and optoelectronics applications.

Progress in Contact, Doping and Mobility Engineering of MoS2: An Atomically Thin 2D Semiconductor

Atomically thin molybdenum disulfide (MoS2), a member of the transition metal dichalcogenide (TMDC) family, has emerged as the prototypical two-dimensional (2D) semiconductor with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing inherent advantages over conventional bulk semiconducting materials (such as Si, Ge and III-Vs) in terms of enabling ultra-short channel and, thus, energy efficient field-effect transistors (FETs), the mechanically flexible and transparent nature of MoS2 makes it even more attractive for use in ubiquitous flexible and transparent electronic systems. However, before the fascinating properties of MoS2 can be effectively harnessed and put to good use in practical and commercial applications, several important technological roadblocks pertaining to its contact, doping and mobility (µ) engineering must be overcome. This paper reviews the important technologically relevant properties of semiconducting 2D TMDCs followed by a discussion of the performance projections of, and the major engineering challenges that confront, 2D MoS2-based devices. Finally, this review provides a comprehensive overview of the various engineering solutions employed, thus far, to address the all-important issues of contact resistance (RC), controllable and area-selective doping, and charge carrier mobility enhancement in these devices. Several key experimental and theoretical results are cited to supplement the discussions and provide further insight.

Ionic Liquid Gating Control of Spin Reorientation Transition and Switching of Perpendicular Magnetic Anisotropy

Electric field (E-field) modulation of perpendicular magnetic anisotropy (PMA) switching, in an energy-efficient manner, is of great potential to realize magnetoelectric (ME) memories and other ME devices. Voltage control of the spin-reorientation transition (SRT) that allows the magnetic moment rotating between the out-of-plane and the in-plane direction is thereby crucial. In this work, a remarkable magnetic anisotropy field change up to 1572 Oe is achieved under a small operation voltage of 4 V through ionic liquid (IL) gating control of SRT in Au/[DEME]⁺ [TFSI]^- /Pt/(Co/Pt)₂ /Ta capacitor heterostructures at room temperature, corresponding to a large ME coefficient of 378 Oe V^-1 . As revealed by both ferromagnetic resonance measurements and magnetic domain evolution observation, the magnetization can be switched stably and reversibly between the out-of-plane and in-plane directions via IL gating. The key mechanism, revealed by the first-principles calculation, is that the IL gating process influences the interfacial spin-orbital coupling as well as net Rashba magnetic field between the Co and Pt layers, resulting in the modulation of the SRT and in-plane/out-of-plane magnetization switching. This work demonstrates a unique IL-gated PMA with large ME tunability and paves a way toward IL gating spintronic/electronic devices such as voltage tunable PMA memories.

Monolayer Transition Metal Dichalcogenides as Light Sources

Reducing the dimensions of materials is one of the key approaches to discovering novel optical phenomena. The recent emergence of 2D transition metal dichalcogenides (TMDCs) has provided a promising platform for exploring new optoelectronic device applications, with their tunable electronic properties, structural controllability, and unique spin valley-coupled systems. This progress report provides an overview of recent advances in TMDC-based light-emitting devices discussed from several aspects in terms of device concepts, material designs, device fabrication, and their diverse functionalities. First, the advantages of TMDCs used in light-emitting devices and their possible functionalities are presented. Second, conventional approaches for fabricating TMDC light-emitting devices are emphasized, followed by introducing a newly established, versatile method for generating light emission in TMDCs. Third, current growing technologies for heterostructure fabrication, in which distinct TMDCs are vertically stacked or laterally stitched, are explained as a possible means for designing high-performance light-emitting devices. Finally, utilizing the topological features of TMDCs, the challenges for controlling circularly polarized light emission and its device applications are discussed from both theoretical and experimental points of view.

Atomically thin p-n junctions based on two-dimensional materials

Recent research in two-dimensional (2D) materials has boosted a renovated interest in the p-n junction, one of the oldest electrical components which can be used in electronics and optoelectronics. 2D materials offer remarkable flexibility to design novel p-n junction device architectures, not possible with conventional bulk semiconductors. In this Review we thoroughly describe the different 2D p-n junction geometries studied so far, focusing on vertical (out-of-plane) and lateral (in-plane) 2D junctions and on mixed-dimensional junctions. We discuss the assembly methods developed to fabricate 2D p-n junctions making a distinction between top-down and bottom-up approaches. We also revise the literature studying the different applications of these atomically thin p-n junctions in electronic and optoelectronic devices. We discuss experiments on 2D p-n junctions used as current rectifiers, photodetectors, solar cells and light emitting devices. The important electronics and optoelectronics parameters of the discussed devices are listed in a table to facilitate their comparison. We conclude the Review with a critical discussion about the future outlook and challenges of this incipient research field.

Hole Transport in Exfoliated Monolayer MoS2

Ideal monolayers of common semiconducting transition-metal dichalcogenides (TMDCs) such as MoS₂, WS₂, MoSe₂, and WSe₂ possess many similar electronic properties. As it is the case for all semiconductors, however, the physical response of these systems is strongly determined by defects in a way specific to each individual compound. Here we investigate the ability of exfoliated monolayers of these TMDCs to support high-quality, well-balanced ambipolar conduction, which has been demonstrated for WS₂, MoSe₂, and WSe₂, but not for MoS₂. Using ionic-liquid gated transistors, we show that, contrary to WS₂, MoSe₂, and WSe₂, hole transport in exfoliated MoS₂ monolayers is systematically anomalous, exhibiting a maximum in conductivity at negative gate voltage ( V _G) followed by a suppression of up to 100 times upon further increasing V _G. To understand the origin of this difference, we have performed a series of experiments including the comparison of hole transport in MoS₂ monolayers and thicker multilayers, in exfoliated and CVD-grown monolayers, as well as gate-dependent optical measurements (Raman and photoluminescence) and scanning tunneling imaging and spectroscopy. In agreement with existing ab initio calculations, the results of all these experiments are consistently explained in terms of defects associated with chalcogen vacancies that only in MoS₂ monolayers, but not in thicker MoS₂ multilayers nor in monolayers of the other common semiconducting TMDCs, create in-gap states near the top of the valence band that act as strong hole traps. Our results demonstrate the importance of studying systematically how defects determine the properties of 2D semiconducting materials and of developing methods to control them.

Ionic liquid gating control of RKKY interaction in FeCoB/Ru/FeCoB and (Pt/Co)2/Ru/(Co/Pt)2 multilayers

To overcome the fundamental challenge of the weak natural response of antiferromagnetic materials under a magnetic field, voltage manipulation of antiferromagnetic interaction is developed to realize ultrafast, high-density, and power efficient antiferromagnetic spintronics. Here, we report a low voltage modulation of Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction via ionic liquid gating in synthetic antiferromagnetic multilayers of FeCoB/Ru/FeCoB and (Pt/Co)₂/Ru/(Co/Pt)₂. At room temperature, the distinct voltage control of transition between antiferromagnetic and ferromagnetic ordering is realized and up to 80% of perpendicular magnetic moments manage to switch with a small-applied voltage bias of 2.5 V. We related this ionic liquid gating-induced RKKY interaction modification to the disturbance of itinerant electrons inside synthetic antiferromagnetic heterostructure and the corresponding change of its Fermi level. Voltage tuning of RKKY interaction may enable the next generation of switchable spintronics between antiferromagnetic and ferromagnetic modes with both fundamental and practical perspectives.

Oxygen Vacancy Dynamics at Room Temperature in Oxide Heterostructures

Oxygen vacancy dynamic behavior at room temperature in complex oxides was carefully explored by using a combined approach of ion liquid gating technique and resistance measurements. Heterostructures of PrBaCo₂O_5+δ/Gd₂O₃-doped CeO₂ epitaxial thin films were fabricated on (001) Y₂O₃-stabilized ZrO₂ single crystal substrates for systematically investigating the oxygen redox dynamics. The oxygen dynamic changes as response to the gating voltage and duration were precisely detected by in situ resistance measurements. A reversible and nonvolatile resistive switching dynamics was detected at room temperature under the gating voltage >13.5 V with pulse duration >1 s.

Superconductivity in Pristine 2H_{a}-MoS_{2} at Ultrahigh Pressure

As a follow-up of our previous work on pressure-induced metallization of the 2H_{c}-MoS_{2} [Chi et al., Phys. Rev. Lett. 113, 036802 (2014)PRLTAO0031-900710.1103/PhysRevLett.113.036802], here we extend pressure beyond the megabar range to seek after superconductivity via electrical transport measurements. We found that superconductivity emerges in the 2H_{a}-MoS_{2} with an onset critical temperature T_{c} of ca. 3 K at ca. 90 GPa. Upon further increasing the pressure, T_{c} is rapidly enhanced beyond 10 K and stabilized at ca. 12 K over a wide pressure range up to 220 GPa. Synchrotron x-ray diffraction measurements evidenced no further structural phase transition, decomposition, and amorphization up to 155 GPa, implying an intrinsic superconductivity in the 2H_{a}-MoS_{2}. DFT calculations suggest that the emergence of pressure-induced superconductivity is intimately linked to the emergence of a new flat Fermi pocket in the electronic structure. Our finding represents an alternative strategy for achieving superconductivity in 2H-MoS_{2} in addition to chemical intercalation and electrostatic gating.

Two-dimensional transition metal dichalcogenides: interface and defect engineering

This review summarizes the recent advances in understanding the effects of interface and defect engineering on the electronic and optical properties of TMDCs, as well as their applications in advanced (opto)electronic devices.

Potential Profile of Stabilized Field-Induced Lateral p-n Junction in Transition-Metal Dichalcogenides

Electric field-induced p-n junctions are often used to realize peculiar functionalities in various materials. This method can be applied not only to conventional semiconductors but also to carbon nanotubes, graphene, and organic semiconductors to which the conventional chemical doping method is difficult to apply. Transition-metal dichalcogenides (TMDs) are one of such materials where the field-induced p-n junctions play crucial roles in realizing solar cell and light-emitting diode operations as well as circularly polarized electroluminescence. Although the field-induced p-n junction is a well-established technique, many of its physical properties are left to be understood because their doping mechanism is distinct from that of conventional p-n junctions. Here we report a direct electrical measurement of the potential variation along the field-induced p-n junction using multiple pairs of voltage probes. We detected the position of the junction, estimated the built-in potential, and monitored the effect of the bias voltage. We found that the built-in potential becomes negative under a forward bias voltage range where field-induced TMD p-n junctions have been operated as light-emitting diodes. This feature well reproduced the circularly polarized electroluminescence from the WSe₂ p-n junction, indicating that the present observation provides a useful background for understanding and functionalizing field-induced p-n junctions.

A Hybrid Gate Dielectrics of Ion Gel with Ultra-Thin Passivation Layer for High-Performance Transistors Based on Two-Dimensional Semiconductor Channels

We propose a hybrid gate structure for ion gel dielectrics using an ultra-thin Al₂O₃ passivation layer for realizing high-performance devices based on electric-double-layer capacitors. Electric-double-layer transistors can be applied to practical devices with flexibility and transparency as well as research on the fundamental physical properties of channel materials; however, they suffer from inherent unwanted leakage currents between electrodes, especially for channel materials with low off-currents. Therefore, the Al₂O₃ passivation layer was introduced between the metal electrodes and ion gel film as a leakage current barrier; this simple approach effectively reduced the leakage current without capacitance degradation. In addition, we confirmed that a monolayer MoS₂ transistor fabricated with the proposed hybrid gate dielectric exhibited remarkably enhanced device properties compared to a transistor using a normal ion gel gate dielectric. Our findings on a simple method to improve the leakage current properties of ion gels could be applied extensively to realize high-performance electric-double-layer transistors utilizing various channel materials.

Inducing and Manipulating Heteroelectronic States in a Single MoS_{2} Thin Flake

By dual gating a few-layer MoS_{2} flake, we induce spatially separated electronic states showing superconductivity and Shubnikov-de Haas (SdH) oscillations. While the highly confined superconductivity forms at the K/K^{'} valleys of the topmost layer, the SdH oscillations are contributed by the electrons residing in the Q/Q^{'} valleys of the rest of the bottom layers, which is confirmed by the extracted Landau level degeneracy of 3, electron effective mass of 0.6m_{e}, and carrier density of 5×10^{12}  cm^{-2}. Mimicking conventional heterostructures, the interaction between the heteroelectronic states can be electrically manipulated, which enables "bipolarlike" superconducting transistor operation. The off-on-off switching pattern can be continuously accessed at low temperatures by a field effect depletion of carriers with a negative back gate bias and the proximity effect between the top superconducting layer and the bottom metallic layers that quenches the superconductivity at a positive back gate bias.

Progress on Electronic and Optoelectronic Devices of 2D Layered Semiconducting Materials

2D layered semiconducting materials (2DLSMs) represent the thinnest semiconductors, holding many novel properties, such as the absence of surface dangling bonds, sizable band gaps, high flexibility, and ability of artificial assembly. With the prospect of bringing revolutionary opportunities for electronic and optoelectronic applications, 2DLSMs have prospered over the past twelve years. From materials preparation and property exploration to device applications, 2DLSMs have been extensively investigated and have achieved great progress. However, there are still great challenges for high-performance devices. In this review, we provide a brief overview on the recent breakthroughs in device optimization based on 2DLSMs, particularly focussing on three aspects: device configurations, basic properties of channel materials, and heterostructures. The effects from device configurations, i.e., electrical contacts, dielectric layers, channel length, and substrates, are discussed. After that, the affect of the basic properties of 2DLSMs on device performance is summarized, including crystal defects, crystal symmetry, doping, and thickness. Finally, we focus on heterostructures based on 2DLSMs. Through this review, we try to provide a guide to improve electronic and optoelectronic devices of 2DLSMs for achieving practical device applications in the future.

Progress of Large-Scale Synthesis and Electronic Device Application of Two-Dimensional Transition Metal Dichalcogenides

The recent exploration of semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDs) with atomic thickness has taken both the scientific and technological communities by storm. Extensively investigated TMD that are accessible by large-scale synthetic methods materials are remarkably stable, such as MoS₂ and WSe₂ . They allow superior gate control due to their 2D nature and favorable electronic transport properties, thus suggesting a bright future for digital and RF electronics. In this review, the latest developments in the controlled synthesis of large scale TMDs are firstly introduced by discussing various approaches. The major obstacles that must be overcome to achieve wafer-scale, uniform, and high-quality TMD films for practical electronic applications are included. Advances in the electronic transport studies of TMDs are presented, such as doping, contact engineering, and mobility improvement, which contribute to overall device performance. A perspective and a look at the future for this field is provided in closing.

Junctionless Diode Enabled by Self-Bias Effect of Ion Gel in Single-Layer MoS2 Device

The self-biasing effects of ion gel from source and drain electrodes on electrical characteristics of single layer and few layer molybdenum disulfide (MoS₂) field-effect transistor (FET) have been studied. The self-biasing effect of ion gel is tested for two different configurations, covered and open, where ion gel is in contact with either one or both, source and drain electrodes, respectively. In open configuration, the linear output characteristics of the pristine device becomes nonlinear and on-off ratio drops by 3 orders of magnitude due to the increase in "off" current for both single and few layer MoS₂ FETs. However, the covered configuration results in a highly asymmetric output characteristics with a rectification of around 10³ and an ideality factor of 1.9. This diode like behavior has been attributed to the reduction of Schottky barrier width by the electric field of self-biased ion gel, which enables an efficient injection of electrons by tunneling at metal-MoS₂ interface. Finally, finite element method based simulations are carried out and the simulated results matches well in principle with the experimental analysis. These self-biased diodes can perform a crucial role in the development of high-frequency optoelectronic and valleytronic devices.

Increasing the Room-Temperature Electric Double Layer Retention Time in Two-Dimensional Crystal FETs

Poly(vinyl alcohol) (PVA) and LiClO₄, a solid polymer electrolyte with a glass transition temperature (T_g) of 80 °C, is used to electrostatically gate graphene field-effect transistors. The ions in PVA:LiClO₄ are drifted into place by field-effect at T > T_g, providing n- or p-type doping, and when the device is cooled to room temperature, the polymer mobility and, hence ion mobility are arrested and the electric double layer (EDL) is "locked" into place in the absence of a gate bias. Unlike other electrolytes used to gate two-dimensional devices for which the T_g, and therefore the "locking" temperature, is well below room temperature, the electrolyte demonstrated in this work provides a route to achieve room-temperature EDL stability. Specifically, a 6 orders of magnitude increase in the room temperature EDL retention time is demonstrated over the commonly used electrolyte, poly(ethylene oxide) (PEO) and LiClO₄. Hall measurements confirm that large sheet carrier densities can be achieved with PVA:LiClO₄ at top gate programming voltages of ±2 V (-6.3 ± 0.03 × 10¹³ cm^-2 for electrons and 1.6 ± 0.3 × 10¹⁴ cm^-2 for holes). Transient drain current measurements show that at least 75% of the EDL is retained after more than 4 h at room temperature. Unlike PEO-based electrolytes, PVA:LiClO₄ is compatible with the chemicals used in standard photolithographic processes enabling the direct deposition of patterned, metal contacts on the surface of the electrolyte. A thermal instability in the electrolyte is detected by both I-V measurements and differential scanning calorimetry, and FTIR measurements suggest that thermally catalyzed cross-linking may be driving phase separation between the polymer and the salt. Nevertheless, this work highlights how the relationship between polymer and ion mobility can be exploited to tune the state retention time and the charge carrier density of a 2D crystal transistor.

Field-Effect Tuned Adsorption Dynamics of VSe2 Nanosheets for Enhanced Hydrogen Evolution Reaction

Transition metal dichalcogenides, such as MoS₂ and VSe₂ have emerged as promising catalysts for the hydrogen evolution reaction (HER). Substantial work has been devoted to optimizing the catalytic performance by constructing materials with specific phases and morphologies. However, the optimization of adsorption/desorption process in HER is rare. Herein, we concentrate on tuning the dynamics of the adsorption process in HER by applying a back gate voltage to the pristine VSe₂ nanosheet. The back gate voltage induces the redistribution of the ions at the electrolyte-VSe₂ nanosheet interface, which realizes the enhanced electron transport process and facilitates the rate-limiting step (discharge process) under HER conditions. A considerable low onset overpotential of 70 mV is achieved in VSe₂ nanosheets without any chemical treatment. Such unexpected improvement is attributed to the field tuned adsorption-dynamics of VSe₂ nanosheet, which is demonstrated by the greatly optimized charge transfer resistance (from 1.03 to 0.15 MΩ) and time constant of the adsorption process (from 2.5 × 10^-3 to 5.0 × 10^-4 s). Our results demonstrate enhanced catalysis performance in the VSe₂ nanosheet by tuning the adsorption dynamics with a back gate, which provides new directions for improving the catalytic activity of non-noble materials.

Endeavor of Iontronics: From Fundamentals to Applications of Ion-Controlled Electronics

Iontronics is a newly emerging interdisciplinary concept which bridges electronics and ionics, covering electrochemistry, solid-state physics, electronic engineering, and biological sciences. The recent developments of electronic devices are highlighted, based on electric double layers formed at the interface between ionic conductors (but electronically insulators) and various electronic conductors including organics and inorganics (oxides, chalcogenide, and carbon-based materials). Particular attention is devoted to electric-double-layer transistors (EDLTs), which are producing a significant impact, particularly in electrical control of phase transitions, including superconductivity, which has been difficult or impossible in conventional all-solid-state electronic devices. Besides that, the current state of the art and the future challenges of iontronics are also reviewed for many applications, including flexible electronics, healthcare-related devices, and energy harvesting.

Highly Oriented Atomically Thin Ambipolar MoSe2 Grown by Molecular Beam Epitaxy

Transition metal dichalcogenides (TMDCs), together with other two-dimensional (2D) materials, have attracted great interest due to the unique optical and electrical properties of atomically thin layers. In order to fulfill their potential, developing large-area growth and understanding the properties of TMDCs have become crucial. Here, we have used molecular beam epitaxy (MBE) to grow atomically thin MoSe₂ on GaAs(111)B. No intermediate compounds were detected at the interface of as-grown films. Careful optimization of the growth temperature can result in the growth of highly aligned films with only two possible crystalline orientations due to broken inversion symmetry. As-grown films can be transferred onto insulating substrates, allowing their optical and electrical properties to be probed. By using polymer electrolyte gating, we have achieved ambipolar transport in MBE-grown MoSe₂. The temperature-dependent transport characteristics can be explained by the 2D variable-range hopping (2D-VRH) model, indicating that the transport is strongly limited by the disorder in the film.