High-Performance Solution-Processed 2D P-Type WSe2 Transistors and Circuits through Molecular Doping

Phenylselenol Healing Selenium Vacancies of Solution-Processed Tungsten Diselenide Enables High-Performance P-Type Thin-Film Transistors

Solution-processed semiconducting 2D single-crystal nanosheets with dangling-bond-free surfaces are tremendously promising in cost-effective and scalable printed (opto)electronic devices. Nevertheless, the scarcity of solution-processed p-type 2D semiconducting materials with high mobility remains a significant impediment to the advancement of thin-film transistors (TFTs) and, by extension, the development of complementary integrated circuits. In this work, we introduce a molecular approach to enhance the electrical performance of p-type tungsten diselenide van der Waals thin-film devices by employing phenylselenol modification. First, phenylselenol can effectively heal selenium vacancies in WSe2 nanosheets, thereby enhancing their crystallinity and, consequently, improving the overall quality of WSe2 thin film. In addition, the interface contact and energy-level structure between the WSe2 thin film and the metal electrode are optimized, thereby reducing the Schottky barrier height for hole transportation. As a result, the WSe2 TFTs exhibit a significant enhancement in hole mobility, reaching 35.22 cm2 V-1 s-1. This simple yet effective strategy for improving the electrical performance of WSe2 TFTs opens avenues for the development of advanced large-area logic technologies based on transition-metal dichalcogenide (TMD) semiconductors.

Efficient n- and p-Type Molecular Dopings in Large-Scale Monolayer Dichalcogenides for High-Performance Field-Effect Transistors

Doping engineering has been actively investigated for two-dimensional (2D) transition metal dichalcogenides (TMDs) to enhance their electrical behavior, particularly for use in field-effect transistors (FETs). Here, we propose unprecedented redox-active n-type and p-type dopants, naphthalene and WCl6, respectively, for large-scale monolayer MoS2 films synthesized via low-pressure chemical vapor deposition using a Na2S promoter. These molecular dopants were selected based on their high redox potentials versus the reference ferrocene, which facilitated the ionization of the dopants via charge transfer. Along with the suppression effect of sulfur vacancies in the monolayer, the electronic transport behavior exhibits an ultrahigh electron mobility of 331.7 cm2 V-1 s-1 for the n-doped MoS2 FET and an excellent hole mobility of 31.8 cm2 V-1 s-1 with a high on/off ratio of ∼107 for the p-type FET, all of which are record-setting values among those reported for large-scale monolayer MoS2 and chemically doped TMD-based FETs. The modulation in the dopant concentration and its correlation with the transistor performance are mainly demonstrated, along with the adjusted band structures as the potential origin of the exceptional outcomes. The extended exploration of multiple FET devices within a single large-scale monolayer film demonstrated uniform electrical characteristics.

Radio frequency switching devices based on two-dimensional materials for high-speed communication applications

Two-dimensional (2D) materials, with their atomic-scale thickness, high carrier mobility, tunable wide bandgap, and excellent electrical and mechanical properties, have demonstrated vast application prospects in research on radio frequency (RF) switch devices. This review summarizes the recent advances in 2D materials for RF switch applications, focusing on the performance and mechanisms of 2D material-based RF switch devices at high frequencies, wide bandwidths, and high transmission rates. The analysis includes the design and optimization of devices based on graphene, transition metal dichalcogenides, hexagonal boron nitride, and their heterojunctions. By comparing the key performance parameters such as insertion loss, isolation, and cutoff frequency of the switches, this review reveals the influence of material selection, structural design, and defect control on device performance. Furthermore, it discusses the challenges of 2D material-based RF switches in practical applications, including material defect control, reduction of contact resistance, and the technical bottlenecks of large-scale industrial production. Finally, this review envisions future research directions, proposing potential pathways for improving device performance through heterojunction structure design, multifunctional integration, and process optimization. This study is of great significance for advancing the development of high-performance RF switches and the application of communication technologies in 6G and higher frequency bands.

Morphotaxial Halogenation of Solution-Processed Two-Dimensional Indium Selenide

Morphotaxy, a process by which a 2D material is chemically modified while retaining its original physical dimensions, is an emerging strategy for synthesizing unconventional materials at the atomically thin limit. Morphotaxy is typically implemented by vapor-phase reactions on mechanically exfoliated or vapor-deposited 2D van der Waals (vdW) materials. Here we report a method for converting solution-processed films of 2D InSe into InI2 and InBr2 using dilute I2 and Br2 solutions, respectively. The converted materials retain the physical dimensions of the original 2D flakes, providing access to non-vdW indium halides in ultrathin form. Liquid-phase exfoliation directly enables this morphotaxial reaction by producing nanosheets with high surface areas and introducing residual polyvinylpyrrolidone that stabilizes the flake morphology and slows the reactivity of I2 and Br2. Overall, this work presents a versatile strategy for achieving atomically thin metal halides and offers mechanistic insights relevant to the morphotaxial halogenation of other solution-processed 2D materials.

Recent advances in CMOS-compatible synthesis and integration of 2D materials

The upcoming generation of functional electronics in the era of artificial intelligence, and IoT requires extensive data storage and processing, necessitating further device miniaturization. Conventional Si CMOS technology is struggling to enhance integration density beyond a certain limit to uphold Moore's law, primarily due to performance degradation at smaller dimensions caused by various physical effects, including surface scattering, quantum tunneling, and other short-channel effects. The two-dimensional materials have emerged as highly promising alternatives, which exhibit excellent electrical and mechanical properties at atomically thin thicknesses and show exceptional potential for future CMOS technology. This review article presents the chronological progress made in the development of two-dimensional materials-based CMOS devices with comprehensively discussing the advancements made in material production, device development, associated challenges, and the strategies to address these issues. The future prospects for the use of two-dimensional materials in functional CMOS circuitry are outlooked, highlighting key opportunities and challenges toward industrial adaptation.

Fluidized Electrochemical Exfoliation of Layered Transition Metal Dichalcogenides toward Fast Production of High-Quality Nanosheets in the Aqueous Phase

The transformation of bulk transition-metal dichalcogenide (TMD) particles into ultrathin nanosheets with both an acceptable yield and preserved crystalline integrity presents a substantial challenge in electrochemical exfoliation. This challenge arises from the continuous potential stress that the materials experience in traditional exfoliation setups. Herein, we propose a new fluidized electrochemical exfoliation (FEE) method to efficiently transform TMD powders into high-quality, few-layered TMD nanosheets in the aqueous phase. This approach builds upon the concept of single particle impact electrochemistry (SPIE), where Faradaic or non-Faradaic reaction processes can occur at individual particles upon their collisions with a potentiostatted electrode. The distinct advantages of this method, including enhanced mass transport and a recycling reaction mode, contribute to more efficient charging and ion intercalation during the electrochemical exfoliation of powdery materials, while also preserving the material's crystalline integrity. This work suggests an alternative and efficient approach for the exfoliation of two-dimensional materials, in general.

Using Electrical Impedance Spectroscopy to Separately Quantify the Effect of Strain on Nanosheet and Junction Resistance in Printed Nanosheet Networks

Many printed electronic applications require strain-independent electrical properties to ensure deformation-independent performance. Thus, developing printed, flexible devices using 2D and other nanomaterials will require an understanding of the effect of strain on the electrical properties of nano-networks. Here, novel AC electrical techniques are introduced to fully characterize the effect of strain on the resistance of high-mobility printed networks, fabricated from of electrochemically exfoliated MoS2 nanosheets. These devices are initially characterized using DC piezoresistance measurements and show good cyclability and a linear strain response, consistent with a low gauge factor of G ≈ 3. However, AC impedance spectroscopy measurements, performed as a function of strain, allow the measurement of the effects of strain on both the nanosheets and the inter-nanosheet junctions separately. The junction resistance is found to increase linearly with strain, while the nanosheet resistance remains constant. This response is consistent with strain-induced sliding of the highly-aligned nanosheets past one another, without any strain being transferred to the sheets themselves. The approach allows for the individual estimation of the contributions of dimensional factors (G ≈ 1.4) and material factors (G ≈ 1.9) to the total gauge factor. This novel technique may provide insights into other piezoresistive systems.

Two-Dimensional Materials for Brain-Inspired Computing Hardware

Recent breakthroughs in brain-inspired computing promise to address a wide range of problems from security to healthcare. However, the current strategy of implementing artificial intelligence algorithms using conventional silicon hardware is leading to unsustainable energy consumption. Neuromorphic hardware based on electronic devices mimicking biological systems is emerging as a low-energy alternative, although further progress requires materials that can mimic biological function while maintaining scalability and speed. As a result of their diverse unique properties, atomically thin two-dimensional (2D) materials are promising building blocks for next-generation electronics including nonvolatile memory, in-memory and neuromorphic computing, and flexible edge-computing systems. Furthermore, 2D materials achieve biorealistic synaptic and neuronal responses that extend beyond conventional logic and memory systems. Here, we provide a comprehensive review of the growth, fabrication, and integration of 2D materials and van der Waals heterojunctions for neuromorphic electronic and optoelectronic devices, circuits, and systems. For each case, the relationship between physical properties and device responses is emphasized followed by a critical comparison of technologies for different applications. We conclude with a forward-looking perspective on the key remaining challenges and opportunities for neuromorphic applications that leverage the fundamental properties of 2D materials and heterojunctions.

Ultra-Low Power Consumption Artificial Photoelectric Synapses Based on Lewis Acid Doped WSe2 for Neuromorphic Computing

Capitalizing on the extensive spectral capacity and minimal crosstalk properties inherent in optical signals, photoelectric synapses are poised to assume a pivotal stance in the realm of neuromorphic computation. Herein, a photoelectric synapse based on Lewis acid-doped semiconducting tungsten diselenide (WSe2) is introduced, exhibiting tunable short-term and long-term plasticity. The device consumes a mere 0.1 fJ per synaptic operation, which is lower than the energy required by a single synaptic event observed in the human brain. Furthermore, these devices demonstrate high-pass filtering capabilities, highlighting their potential in image-sharpening applications. In particular, by synergistically modulating the photoconductivity and electrical gate bias, versatile logic capabilities are demonstrated within a single device, enabling it to flexibly perform both Boolean AND and OR gate operations. This work demonstrates a viable approach for Lewis acid-treated TMDs to realize multifunctional photoelectric synapses for neuromorphic computing.

Intralayer/Interlayer Codoping Stabilizes Polarity Modulation in 2D Semiconductors for Scalable Electronics

2D semiconductors show promise as a competitive candidate for developing future integrated circuits due to their immunity to short-channel effects and high carrier mobility at atomic layer thicknesses. The inherent defects and Fermi level pinning effect lead to n-type transport characteristics in most 2D semiconductors, while unstable and unsustainable p-type doping by various strategies hinders their application in many areas, such as complementary metal-oxide-semiconductor (CMOS) devices. In this study, an intralayer/interlayer codoping strategy is introduced that stabilizes p-type doping in 2D semiconductors. By incorporating oppositely charged ions (F and Li) with the intralayer/interlayer of 2D semiconductors, remarkable p-type doping in WSe2 and MoTe2 with air stability up to 9 months is achieved. Notably, the hole mobility presents a 100-fold enhancement (0.7 to 92 cm2 V-1 s-1) with the codoping procedure. Structural and elemental characterizations, combined with theoretical calculations validate the codoping mechanism. Moreover, a CMOS inverter and more complex logic functions such as NOR and XNOR, as well as large-area device arrays are demonstrated to showcase its applications and scalability. These findings suggest that stable and straightforward intralayer/interlayer codoping strategy with charge-space synergy holds the key to unlocking the potential of 2D semiconductors in complex and scalable device applications.

High-Fidelity Transfer of 2D Semiconductors and Electrodes for van der Waals Devices

As traditional silicon-based materials almost reach their limits in the post-Moore era, two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been regarded as next-generation semiconductors for high-performance electrical and optical devices. Chemical vapor deposition (CVD) is a widely used technique for preparing large-area and high-quality TMDCs. Yet, it suffers from the challenge of transfer due to the strong interaction between 2D materials and substrates. The traditional PMMA-assisted wet etching method tends to induce damage, wrinkles, and inevitable polymer residues. In this work, we propose an etch-free and clean transfer method via a water intercalation strategy for TMDCs, ensuring a high-fidelity, wrinkle-free, and crack-free transfer with negligible residues. Furthermore, metal electrodes can also be transferred via this method and back-gate field-effect transistors (FETs) based on CVD-grown monolayer WSe2 with van der Waals (vdW) metal/semiconductor contacts are fabricated. Compared to the PMMA-assisted transfer method (∼1.2 cm2 V-1 s-1 hole mobility with ∼2 × 106 ON/OFF ratio), our high-fidelity transfer method significantly enhances the electrical performance of WSe2 FET over one order of magnitude, achieving a hole mobility of ∼43 cm2 V-1 s-1 and a high ON/OFF ratio of ∼5 × 107 in air at room temperature.

Design of stimuli-responsive transition metal dichalcogenides

Stimuli-responsive systems are an emerging class of materials in fields as diverse as electronics, optoelectronics, cancer detection, drug delivery, or sensing. Especially focusing on nanomaterials, 2D transition metal dichalcogenides have recently attracted the scientific community's attention due to their remarkable intrinsic stimuli-responsive behaviour upon external stimuli such as pH, light, voltage, or certain pathogens. This significant response can be further enhanced by forming mixed-dimensional heterostructures and by molecular functionalization, capitalizing on chemistry to manipulate and boost their intrinsic stimuli-responsive properties. Furthermore, thanks to the endless possibilities of chemistry, a new class of smart materials based on the combination of stimuli-responsive molecular systems with transition metal dichalcogenides has recently been synthesized. In these materials, the physical properties of the 2D layers are reversibly modified by the switchable molecules, not only enhancing their stimuli-responsive behaviour but also providing memory to the hybrid. Therefore, this review explores the recent breakthroughs in the chemical design of smart transition metal dichalcogenides with built-in responsiveness.

Electrical Polarity Modulation in V-Doped Monolayer WS2 for Homogeneous CMOS Inverters

As demand for higher integration density and smaller devices grows, silicon-based complementary metal-oxide-semiconductor (CMOS) devices will soon reach their ultimate limits. 2D transition metal dichalcogenides (TMDs) semiconductors, known for excellent electrical performance and stable atomic structure, are seen as promising materials for future integrated circuits. However, controlled and reliable doping of 2D TMDs, a key step for creating homogeneous CMOS logic components, remains a challenge. In this study, a continuous electrical polarity modulation of monolayer WS2 from intrinsic n-type to ambipolar, then to p-type, and ultimately to a quasi-metallic state is achieved simply by introducing controllable amounts of vanadium (V) atoms into the WS2 lattice as p-type dopants during chemical vapor deposition (CVD). The achievement of purely p-type field-effect transistors (FETs) is particularly noteworthy based on the 4.7 at% V-doped monolayer WS2, demonstrating a remarkable on/off current ratio of 105. Expanding on this triumph, the first initial prototype of ultrathin homogeneous CMOS inverters based on monolayer WS2 is being constructed. These outcomes validate the feasibility of constructing homogeneous CMOS devices through the atomic doping process of 2D materials, marking a significant milestone for the future development of integrated circuits.

Enhanced Performance of a Self-Powered Au/WSe2/Ta2NiS5/Au Heterojunction by the Interfacial Pyro-phototronic Effect

The growing need for wearable electronics and self-powered electronic devices has driven the successful development of self-powered two-dimensional (2D) photodetectors using the photovoltaic effect of Schottky and p-n junctions. However, there is an urgent need to develop multifunctional photodetectors capable of harvesting energy from different sources to overcome their limitations in efficiency and cost. While the pyro-phototronic effect has been shown to effectively influence optoelectronic processes in heterojunctions, the number of reported two-dimensional heterojunctions exhibiting interfacial pyroelectricity is still limited, and the responsivity and detectivity based on such heterojunctions tend to be low. In this study, a photodetector based on an Au/WSe2/Ta2NiS5/Au heterojunction was designed and fabricated. By harnessing the interfacial pyro-phototronic effect arising from the built-in electric fields at the Au/WSe2 Schottky junction and WSe2/Ta2NiS5 heterojunction, the photodetector exhibits a broadband response range of 405-1064 nm, with approximately 12 times enhancement in output current compared to solely relying on the photovoltaic effect. Under 660 nm light irradiation, the self-powered photodetector exhibits a responsivity of 121 mA/W, an external quantum efficiency of 22.64%, and a specific detectivity of 2 × 1012 Jones. Notably, its pyroelectric coefficient exceeds 8 × 103 μC·m-2·K-1. These findings pave the way for effectively detecting weak light and temperature variation while presenting a new strategy for developing high-performance photodetectors utilizing the interfacial pyro-phototronic effect.

Realization of Extremely High-Gain and Low-Power in nMOS Inverter Based on Monolayer WS2 Transistor Operating in Subthreshold Regime

In this work, we report an n-type metal-oxide-semiconductor (nMOS) inverter using chemical vapor deposition (CVD)-grown monolayer WS2 field-effect transistors (FETs). Our large-area CVD-grown monolayer WS2 FETs exhibit outstanding electrical properties including a high on/off ratio, small subthreshold swing, and excellent drain-induced barrier lowering. These are achieved by n-type doping using AlOx/Al2O3 and a double-gate structure employing high-k dielectric HfO2. Due to the superior subthreshold characteristics, monolayer WS2 FETs show high transconductance and high output resistance in the subthreshold regime, resulting in significantly higher intrinsic gain compared to conventional Si MOSFETs. Therefore, we successfully realize subthreshold operating monolayer WS2 nMOS inverters with extremely high gains of 564 and 2056 at supply voltage (VDD) of 1 and 2 V, respectively, and low power consumption of ∼2.3 pW·μm-1 at VDD = 1 V. In addition, the monolayer WS2 nMOS inverter is further expanded to the demonstration of logic circuits such as AND, OR, NAND, NOR logic gates, and SRAM. These findings suggest the potential of monolayer WS2 for high-gain and low-power logic circuits and validate the practical application in large areas.

A library of 2D electronic material inks synthesized by liquid-metal-assisted intercalation of crystal powders

Solution-processable 2D semiconductor inks based on electrochemical molecular intercalation and exfoliation of bulk layered crystals using organic cations has offered an alternative pathway to low-cost fabrication of large-area flexible and wearable electronic devices. However, the growth of large-piece bulk crystals as starting material relies on costly and prolonged high-temperature process, representing a critical roadblock towards practical and large-scale applications. Here we report a general liquid-metal-assisted approach that enables the electrochemical molecular intercalation of low-cost and readily available crystal powders. The resulted solution-processable MoS2 nanosheets are of comparable quality to those exfoliated from bulk crystals. Furthermore, this method can create a rich library of functional 2D electronic inks ( >50 types), including 2D wide-bandgap semiconductors of low electrical conductivity. Lastly, we demonstrated the all-solution-processable integration of 2D semiconductors with 2D conductors and 2D dielectrics for the fabrication of large-area thin-film transistors and memristors at a greatly reduced cost.

Two-Dimensional Tunneling Memtransistor with Thin-Film Heterostructure for Low-Power Logic-in-Memory Complementary Metal-Oxide Semiconductor

With the demand for high-performance and miniaturized semiconductor devices continuously rising, the development of innovative tunneling transistors via efficient stacking methods using two-dimensional (2D) building blocks has paramount importance in the electronic industry. Hence, 2D semiconductors with atomically thin geometries hold significant promise for advancements in electronics. In this study, we introduced tunneling memtransistors with a thin-film heterostructure composed of 2D semiconducting MoS2 and WSe2. Devices with the dual function of tuning and memory operation were realized by the gate-regulated modulation of the barrier height at the heterojunction and manipulation of intrinsic defects within the exfoliated nanoflakes using solution processes. Further, our investigation revealed extensive edge defects and four distinct defect types, namely monoselenium vacancies, diselenium vacancies, tungsten vacancies, and tungsten adatoms, in the interior of electrochemically exfoliated WSe2 nanoflakes. Additionally, we constructed complementary metal-oxide semiconductor-based logic-in-memory devices with a small static power in the range of picowatts using the developed tunneling memtransistors, demonstrating a promising approach for next-generation low-power nanoelectronics.

Solution-Processable and Printable Two-Dimensional Transition Metal Dichalcogenide Inks

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with layered crystal structures have been attracting enormous research interest for their atomic thickness, mechanical flexibility, and excellent electronic/optoelectronic properties for applications in diverse technological areas. Solution-processable 2D TMD inks are promising for large-scale production of functional thin films at an affordable cost, using high-throughput solution-based processing techniques such as printing and roll-to-roll fabrications. This paper provides a comprehensive review of the chemical synthesis of solution-processable and printable 2D TMD ink materials and the subsequent assembly into thin films for diverse applications. We start with the chemical principles and protocols of various synthesis methods for 2D TMD nanosheet crystals in the solution phase. The solution-based techniques for depositing ink materials into solid-state thin films are discussed. Then, we review the applications of these solution-processable thin films in diverse technological areas including electronics, optoelectronics, and others. To conclude, a summary of the key scientific/technical challenges and future research opportunities of solution-processable TMD inks is provided.

Electrochemical exfoliation of 2D materials beyond graphene

After the discovery of graphene in 2004, the field of atomically thin crystals has exploded with the discovery of thousands of 2-dimensional materials (2DMs) with unique electronic and optical properties, by making them very attractive for a broad range of applications, from electronics to energy storage and harvesting, and from sensing to biomedical applications. In order to integrate 2DMs into practical applications, it is crucial to develop mass scalable techniques providing crystals of high quality and in large yield. Electrochemical exfoliation is one of the most promising methods for producing 2DMs, as it enables quick and large-scale production of solution processable nanosheets with a thickness well below 10 layers and lateral size above 1 μm. Originally, this technique was developed for the production of graphene; however, in the last few years, this approach has been successfully extended to other 2DMs, such as transition metal dichalcogenides, black phosphorous, hexagonal boron nitride, MXenes and many other emerging 2D materials. This review first provides an introduction to the fundamentals of electrochemical exfoliation and then it discusses the production of each class of 2DMs, by introducing their properties and giving examples of applications. Finally, a summary and perspective are given to address some of the challenges in this research area.

Interaction between pentacene molecules and monolayer transition metal dichalcogenides

Using first-principles calculations based on density-functional theory, we investigated the adsorption of pentacene molecules on monolayer two-dimensional transition metal dichalcogenides (TMD). We considered the four most popular TMDs, namely, MoS2, MoSe2, WS2 and WSe2, and we examined the structural and electronic properties of pentacene/TMD systems. We discuss how monolayer pentacene interacts with the TMDs, and how this interaction affects the charge transfer and work function of the heterostructure. We also analyse the type of band alignment formed in the heterostructure and how it is affected by molecule-molecule and molecule-substrate interactions. Such analysis is valuable since pentacene/TMD heterostructures are considered to be promising for application in flexible, thin and lightweight photovoltaics and photodetectors.