Scalable high performance radio frequency electronics based on large domain bilayer MoS2

Interfacial epitaxy of multilayer rhombohedral transition-metal dichalcogenide single crystals

Rhombohedral-stacked transition-metal dichalcogenides (3R-TMDs), which are distinct from their hexagonal counterparts, exhibit higher carrier mobility, sliding ferroelectricity, and coherently enhanced nonlinear optical responses. However, surface epitaxial growth of large multilayer 3R-TMD single crystals is difficult. We report an interfacial epitaxy methodology for their growth of several compositions, including molybdenum disulfide (MoS2), molybdenum diselenide, tungsten disulfide, tungsten diselenide, niobium disulfide, niobium diselenide, and molybdenum sulfoselenide. Feeding of metals and chalcogens continuously to the interface between a single-crystal Ni substrate and grown layers ensured consistent 3R stacking sequence and controlled thickness from a few to 15,000 layers. Comprehensive characterizations confirmed the large-scale uniformity, high crystallinity, and phase purity of these films. The as-grown 3R-MoS2 exhibited room-temperature mobilities up to 155 and 190 square centimeters per volt second for bi- and trilayers, respectively. Optical difference frequency generation with thick 3R-MoS2 showed markedly enhanced nonlinear response under a quasi-phase matching condition (five orders of magnitude greater than monolayers).

Remote epitaxy of single-crystal rhombohedral WS2 bilayers

Compared to transition metal dichalcogenide (TMD) monolayers, rhombohedral-stacked (R-stacked) TMD bilayers exhibit remarkable electrical performance, enhanced nonlinear optical response, giant piezo-photovoltaic effect and intrinsic interfacial ferroelectricity. However, from a thermodynamics perspective, the formation energies of R-stacked and hexagonal-stacked (H-stacked) TMD bilayers are nearly identical, leading to mixed stacking of both H- and R-stacked bilayers in epitaxial films. Here, we report the remote epitaxy of centimetre-scale single-crystal R-stacked WS2 bilayer films on sapphire substrates. The bilayer growth is realized by a high flux feeding of the tungsten source at high temperature on substrates. The R-stacked configuration is achieved by the symmetry breaking in a-plane sapphire, where the influence of atomic steps passes through the lower TMD layer and controls the R-stacking of the upper layer. The as-grown R-stacked bilayers show up-to-30-fold enhancements in carrier mobility (34 cm2V-1s-1), nearly doubled circular helicity (61%) and interfacial ferroelectricity, in contrast to monolayer films. Our work reveals a growth mechanism to obtain stacking-controlled bilayer TMD single crystals, and promotes large-scale applications of R-stacked TMD.

Ku-Band Mixers Based on Random-Oriented Carbon Nanotube Films

Carbon nanotubes (CNTs) are a type of nanomaterial that have excellent electrical properties such as high carrier mobility, high saturation velocity, and small inherent capacitance, showing great promise in radio frequency (RF) applications. Decades of development have been made mainly on cut-off frequency and amplification; however, frequency conversion for RF transceivers, such as CNT-based mixers, has been rarely reported. In this work, based on randomly oriented carbon nanotube films, we focused on exploring the frequency conversion capability of CNT-based RF mixers. CNT-based RF transistors were designed and fabricated with a gate length of 50 nm and gate width of 100 μm to obtain nearly 30 mA of total current and 34 mS of transconductance. The Champion RF transistor has demonstrated cut-off frequencies of 78 GHz and 60 GHz for fT and fmax, respectively. CNT-based mixers achieve high conversion gain from -11.4 dB to -17.5 dB at 10 to 15 GHz in the X and Ku bands. Additionally, linearity is achieved with an input third intercept (IIP3) of 18 dBm. It is worth noting that the results from this work have no matching technology or tuning instrument assistance, which lay the foundations for the application of Ku band transceivers integrated with CNT amplifiers.

The Roadmap of 2D Materials and Devices Toward Chips

Due to the constraints imposed by physical effects and performance degradation, silicon-based chip technology is facing certain limitations in sustaining the advancement of Moore's law. Two-dimensional (2D) materials have emerged as highly promising candidates for the post-Moore era, offering significant potential in domains such as integrated circuits and next-generation computing. Here, in this review, the progress of 2D semiconductors in process engineering and various electronic applications are summarized. A careful introduction of material synthesis, transistor engineering focused on device configuration, dielectric engineering, contact engineering, and material integration are given first. Then 2D transistors for certain electronic applications including digital and analog circuits, heterogeneous integration chips, and sensing circuits are discussed. Moreover, several promising applications (artificial intelligence chips and quantum chips) based on specific mechanism devices are introduced. Finally, the challenges for 2D materials encountered in achieving circuit-level or system-level applications are analyzed, and potential development pathways or roadmaps are further speculated and outlooked.

Reconfiguring nucleation for CVD growth of twisted bilayer MoS2 with a wide range of twist angles

Twisted bilayer (TB) transition metal dichalcogenides (TMDCs) beyond TB-graphene are considered an ideal platform for investigating condensed matter physics, due to the moiré superlattices-related peculiar band structures and distinct electronic properties. The growth of large-area and high-quality TB-TMDCs with wide twist angles would be significant for exploring twist angle-dependent physics and applications, but remains challenging to implement. Here, we propose a reconfiguring nucleation chemical vapor deposition (CVD) strategy for directly synthesizing TB-MoS2 with twist angles from 0° to 120°. The twist angles-dependent Moiré periodicity can be clearly observed, and the interlayer coupling shows a strong relationship to the twist angles. Moreover, the yield of TB-MoS2 in bilayer MoS2 and density of TB-MoS2 are significantly improved to 17.2% and 28.9 pieces/mm2 by tailoring gas flow rate and molar ratio of NaCl to MoO3. The proposed reconfiguring nucleation approach opens an avenue for the precise growth of TB-TMDCs for both fundamental research and practical applications.

Physical vapor deposition-free scalable high-efficiency electrical contacts to MoS2

Fermi-level pinning caused by the kinetic damage during metallization has been recognized as one of the major reasons for the non-ideal behavior of electrical contacts, forbidding reaching the Schottky-Mott limit. In this manuscript, we present a scalable technique wherein Indium, a low-work-function metal, is diffused to contact a few-layered MoS2flake. The technique exploits a smooth outflow of Indium over gold electrodes to make edge contacts to pre-transferred MoS2flakes. We compare the performance of three pairs of contacts made onto the same MoS2flake, the bottom-gold, top-gold, and Indium contacts, and find that the Indium contacts are superior to other contacts. The Indium contacts maintain linearI-Vcharacteristics down to cryogenic temperatures with an extracted Schottky barrier height of ∼2.1 meV. First-principle calculations show the induced in-gap states close to the Fermi level, and the damage-free contact interface could be the reason for the nearly Ohmic behavior of the Indium/MoS2interface.

Substrate Engineering for Chemical Vapor Deposition Growth of Large-Scale 2D Transition Metal Dichalcogenides

The large-scale production of 2D transition metal dichalcogenides (TMDs) is essential to realize their industrial applications. Chemical vapor deposition (CVD) has been considered as a promising method for the controlled growth of high-quality and large-scale 2D TMDs. During a CVD process, the substrate plays a crucial role in anchoring the source materials, promoting the nucleation and stimulating the epitaxial growth. It thus significantly affects the thickness, microstructure, and crystal quality of the products, which are particularly important for obtaining 2D TMDs with expected morphology and size. Here, an insightful review is provided by focusing on the recent development associated with the substrate engineering strategies for CVD preparation of large-scale 2D TMDs. First, the interaction between 2D TMDs and substrates, a key factor for the growth of high-quality materials, is systematically discussed by combining the latest theoretical calculations. Based on this, the effect of various substrate engineering approaches on the growth of large-area 2D TMDs is summarized in detail. Finally, the opportunities and challenges of substrate engineering for the future development of 2D TMDs are discussed. This review might provide deep insight into the controllable growth of high-quality 2D TMDs toward their industrial-scale practical applications.

Approaching Ohmic Contacts for Ideal Monolayer MoS2 Transistors Through Sulfur-Vacancy Engineering

Field-effect transistors (FETs) made of monolayer 2D semiconductors (e.g., MoS2 ) are among the basis of the future modern wafer chip industry. However, unusually high contact resistances at the metal-semiconductor interfaces have seriously limited the improvement of monolayer 2D semiconductor FETs so far. Here, a high-scale processable strategy is reported to achieve ohmic contact between the metal and monolayer MoS2 with a large number of sulfur vacancies (SVs) by using simple sulfur-vacancy engineering. Due to the successful doping of the contact regions by introducing SVs, the contact resistance of monolayer MoS2 FET is as low as 1.7 kΩ·µm. This low contact resistance enables high-performance MoS2 FETs with ultrahigh carrier mobility of 153 cm2 V-1 s-1 , a large on/off ratio of 4 × 109 , and high saturation current of 342 µA µm-1 . With the comprehensive investigation of different SV concentrations by adjusting the plasma duration, it is also demonstrated that the SV-increased electron doping, with its resulting reduced Schottky barrier, is the dominant factor driving enhanced electrical performance. The work provides a simple method to promote the development of industrialized atomically thin integrated circuits.

Toward High-Performance p-Type Two-Dimensional Field Effect Transistors: Contact Engineering, Scaling, and Doping

n-type field effect transistors (FETs) based on two-dimensional (2D) transition-metal dichalcogenides (TMDs) such as MoS2 and WS2 have come close to meeting the requirements set forth in the International Roadmap for Devices and Systems (IRDS). However, p-type 2D FETs are dramatically lagging behind in meeting performance standards. Here, we adopt a three-pronged approach that includes contact engineering, channel length (Lch) scaling, and monolayer doping to achieve high performance p-type FETs based on synthetic WSe2. Using electrical measurements backed by atomistic imaging and rigorous analysis, Pd was identified as the favorable contact metal for WSe2 owing to better epitaxy, larger grain size, and higher compressive strain, leading to a lower Schottky barrier height. While the ON-state performance of Pd-contacted WSe2 FETs was improved by ∼10× by aggressively scaling Lch from 1 μm down to ∼20 nm, ultrascaled FETs were found to be contact limited. To reduce the contact resistance, monolayer tungsten oxyselenide (WOxSey) obtained using self-limiting oxidation of bilayer WSe2 was used as a p-type dopant. This led to ∼5× improvement in the ON-state performance and ∼9× reduction in the contact resistance. We were able to achieve a median ON-state current as high as ∼10 μA/μm for ultrascaled and doped p-type WSe2 FETs with Pd contacts. We also show the applicability of our monolayer doping strategy to other 2D materials such as MoS2, MoTe2, and MoSe2.

Polarizability characteristics of twisted bilayer graphene quantum dots in the absence of periodic moiré potential

Recent studies have documented a rich phenomenology in twisted bilayer graphene (TBG), which is significantly relevant to interlayer electronic coupling, in particular to the cases under an applied electric field. While polarizability measures the response of electrons against applied fields, this work adopts a unique strategy of decomposing global polarizability into distributional contributions to access the interlayer polarization in TBG, as a function of varying twisting angles (θ). Through the construction of a model of twisted graphene quantum dots, we assess distributional polarizability at the first-principles level. Our findings demonstrate that the polarizability perpendicular to the graphene plates can be decomposed into intralayer dipoles and interlayer charge-transfer (CT) components, the latter of which provides an explicit measurement of the interlayer coupling strength and charge transfer potential. Our analysis further reveals that interlayer polarizability dominates the polarizability variation during twisting. Intriguingly, the largest interlayer polarizability and CT driven by an external field occur in the misaligned structures with a size-dependent small angle corresponding to the first appearance of AB stacking, rather than the well-recognized Bernal structures. A derived equation is then employed to address the size dependence on the angle corresponding to the largest values in interlayer polarizability and CT. Our investigation not only characterizes the CT features in the interlayer polarizability of TBG quantum dots, but also sheds light on the existence of the strongest interlayer coupling and charge transfer at small twist angles in the presence of an external electric field, thereby providing a comprehensive understanding of the novel properties of graphene-based nanomaterials.

Observation of Rich Defect Dynamics in Monolayer MoS2

Defects play a pivotal role in limiting the performance and reliability of nanoscale devices. Field-effect transistors (FETs) based on atomically thin two-dimensional (2D) semiconductors such as monolayer MoS₂ are no exception. Probing defect dynamics in 2D FETs is therefore of significant interest. Here, we present a comprehensive insight into various defect dynamics observed in monolayer MoS₂ FETs at varying gate biases and temperatures. The measured source-to-drain currents exhibit random telegraph signals (RTS) owing to the transfer of charges between the semiconducting channel and individual defects. Based on the modeled temperature and gate bias dependence, oxygen vacancies or aluminum interstitials are probable defect candidates. Several types of RTSs are observed including anomalous RTS and giant RTS indicating local current crowding effects and rich defect dynamics in monolayer MoS₂ FETs. This study explores defect dynamics in large area-grown monolayer MoS₂ with ALD-grown Al₂O₃ as the gate dielectric.

Aligned Carbon Nanotubes-Based Radiofrequency Transistors for Amplitude Amplification and Frequency Conversion at Millimeter Wave Band

Aligned carbon nanotubes (ACNTs) have been considered as a promising candidate semiconductor with great potential in radiofrequency (RF) electronics due to their high carrier mobility/saturation velocity and small intrinsic capacitance. However, almost all of previously reported works focused on only the cutoff frequency, which is far from enough for practical RF application. In this work, given the speed advantage of ACNTs, we further explore amplitude amplification and frequency conversion capability of ACNTs based RF devices simultaneously, which are two basic functions in RF electronics. Considering there is no de-embedding process for amplification/conversion and reduction power loss, multifinger configuration RF transistors (still having current density around 1 mA/μm) were fabricated with cutoff frequency and maximum oscillation frequency exceeding 150 and 130 GHz, respectively. Based on dedicated ACNTs based RF FETs, we demonstrate almost 7 dB power gain (S₂₁) with over 40 GHz 3-dB bandwidth for amplification and from -12.7 to -17 dB of conversion gain with over 25 dBm IIP3 (input third-order intercept point) of linearity for conversion simultaneously operating at 30 GHz in millimeter wave (mmWave) band both without any tuning instruments and matching technology assistance. The performance achieved here is the best among all the nanomaterials at the mmWave band.

Controlled Synthesis and Accurate Doping of Wafer-Scale 2D Semiconducting Transition Metal Dichalcogenides

2D semiconducting transition metal dichalcogenide (TMDCs) possess atomically thin thickness, a dangling-bond-free surface, flexible band structure, and silicon-compatible feature, making them one of the most promising channels for constructing state-of-the-art field-effect transistors in the post-Moore's era. However, the existing 2D semiconducting TMDCs fall short of meeting the industry criteria for practical applications in electronics due to their small domain size and the lack of an effective approach to modulate intrinsic physical properties. Therefore, it is crucial to prepare and dope 2D semiconducting TMDCs single crystals with wafer size. In this review, the up-to-date progress regarding the wafer-scale growth of 2D semiconducting TMDC polycrystalline and single-crystal films is systematically summarized. The domain orientation control of 2D TMDCs and the seamless stitching of unidirectionally aligned 2D islands by means of substrate design are proposed. In addition, the accurate and uniform doping of 2D semiconducting TMDCs and the effect on electronic device performances are also discussed. Finally, the dominating challenges pertaining to the enhancement of the electronic device performances of TMDCs are emphasized, and further development directions are put forward. This review provides a systematic and in-depth summary of high-performance device applications of 2D semiconducting TMDCs.

Bridging Synthesis and Controllable Doping of Monolayer 4 in. Length Transition-Metal Dichalcogenides Single Crystals with High Electron Mobility

Epitaxial growth and controllable doping of wafer-scale atomically thin semiconductor single crystals are two central tasks to tackle the scaling challenge of transistors. Despite considerable efforts are devoted, addressing such crucial issues simultaneously under 2D confinement is yet to be realized. Here, an ingenious strategy to synthesize record-breaking 4 in. length Fe-doped transition-metal dichalcogenides (TMDCs) single crystals on industry-compatible c-plane sapphire without special miscut angle is designed. Atomically thin transistors with high electron mobility (≈146 cm² V^-1 s^-1 ) and remarkable on/off current ratio (≈10⁹ ) are fabricated based on 4 in. length Fe-MoS₂ single crystals, due to the ultralow contact resistance (≈489 Ω µm). In-depth characterizations and theoretical calculations reveal that the introduction of Fe significantly decreases the formation energy of parallel steps on sapphire surfaces and contributes to the edge-nucleation of unidirectional alignment TMDCs domains (>99%). This work represents a substantial leap in terms of bridging synthesis and doping of wafer-scale 2D semiconductor single crystals, which should promote the further device downscaling and extension of Moore's law.

Liquid Phase Edge Epitaxy of Transition-Metal Dichalcogenide Monolayers

Precise monolayer epitaxy is important for two-dimensional (2D) semiconductors toward future electronics. Here, we report a new self-limited epitaxy approach, liquid phase edge epitaxy (LPEE), for precise-monolayer epitaxy of transition-metal dichalcogenides. In this method, the liquid solution contacts 2D grains only at the edges, which confines the epitaxy only at the grain edges and then precise monolayer epitaxy can be achieved. High-temperature in situ imaging of the epitaxy progress directly supports this edge-contact epitaxy mechanism. Typical transition-metal dichalcogenide monolayers (MX₂, M = Mo, W, and Re; X = S or Se) have been obtained by LPEE with a proper choice of molten alkali halide solvents (AL, A = Li, Na, K, and Cs; L = Cl, Br, or I). Furthermore, alloying and magnetic-element doping have also been realized by taking advantage of the liquid phase epitaxy approach. This LPEE method provides a precise and highly versatile approach for 2D monolayer epitaxy and can revolutionize the growth of 2D materials toward electronic applications.

2D fin field-effect transistors integrated with epitaxial high-k gate oxide

Precise integration of two-dimensional (2D) semiconductors and high-dielectric-constant (k) gate oxides into three-dimensional (3D) vertical-architecture arrays holds promise for developing ultrascaled transistors1-5, but has proved challenging. Here we report the epitaxial synthesis of vertically aligned arrays of 2D fin-oxide heterostructures, a new class of 3D architecture in which high-mobility 2D semiconductor fin Bi2O2Se and single-crystal high-k gate oxide Bi2SeO5 are epitaxially integrated. These 2D fin-oxide epitaxial heterostructures have atomically flat interfaces and ultrathin fin thickness down to one unit cell (1.2 nm), achieving wafer-scale, site-specific and high-density growth of mono-oriented arrays. The as-fabricated 2D fin field-effect transistors (FinFETs) based on Bi2O2Se/Bi2SeO5 epitaxial heterostructures exhibit high electron mobility (μ) up to 270 cm2 V-1 s-1, ultralow off-state current (IOFF) down to about 1 pA μm-1, high on/off current ratios (ION/IOFF) up to 108 and high on-state current (ION) up to 830 μA μm-1 at 400-nm channel length, which meet the low-power specifications projected by the International Roadmap for Devices and Systems (IRDS)6. The 2D fin-oxide epitaxial heterostructures open up new avenues for the further extension of Moore's law.

Heterogeneous Integration of Atomically-Thin Indium Tungsten Oxide Transistors for Low-Power 3D Monolithic Complementary Inverter

In this work, the authors demonstrate a novel vertically-stacked thin film transistor (TFT) architecture for heterogeneously complementary inverter applications, composed of p-channel polycrystalline silicon (poly-Si) and n-channel amorphous indium tungsten oxide (a-IWO), with a low footprint than planar structure. The a-IWO TFT with channel thickness of approximately 3-4 atomic layers exhibits high mobility of 24 cm² V^-1 s^-1 , near ideally subthreshold swing of 63 mV dec^-1 , low leakage current below 10^-13 A, high on/off current ratio of larger than 10⁹ , extremely small hysteresis of 0 mV, low contact resistance of 0.44 kΩ-µm, and high stability after encapsulating a passivation layer. The electrical characteristics of n-channel a-IWO TFT are well-matched with p-channel poly-Si TFT for superior complementary metal-oxide-semiconductor technology applications. The inverter can exhibit a high voltage gain of 152 V V^-1 at low supply voltage of 1.5 V. The noise margin can be up to 80% of supply voltage and perform the symmetrical window. The pico-watt static power consumption inverter is achieved by the wide energy bandgap of a-IWO channel and atomically-thin channel. The vertically-stacked complementary field-effect transistors (CFET) with high energy-efficiency can increase the circuit density in a chip to conform the development of next-generation semiconductor technology.

Rhombohedral-stacked bilayer transition metal dichalcogenides for high-performance atomically thin CMOS devices

Van der Waals coupling with different stacking configurations is emerging as a powerful method to tune the optical and electronic properties of atomically thin two-dimensional materials. Here, we investigate 3R-stacked transition-metal dichalcogenides as a possible option for high-performance atomically thin field-effect transistors (FETs). We report that the effective mobility of 3R bilayer WS₂ (WSe₂) is 65% (50%) higher than that of 2H WS₂ (WSe₂). The 3R bilayer WS₂ n-type FET exhibits a high on-state current of 480 μA/μm at V_ds = 1 V and an ultralow on-state resistance of 1 kilohm·μm. Our observations, together with multiscale simulations, reveal that these improvements originate from the strong interlayer coupling in the 3R stacking, which is reflected in a higher conductance compared to the 2H stacking. Our method provides a general and scalable route toward advanced channel materials in future electronic devices for ultimate scaling, especially for complementary metal oxide semiconductor applications.

Bioinspired and Low-Power 2D Machine Vision with Adaptive Machine Learning and Forgetting

Natural intelligence has many dimensions, with some of its most important manifestations being tied to learning about the environment and making behavioral changes. In primates, vision plays a critical role in learning. The underlying biological neural networks contain specialized neurons and synapses which not only sense and process visual stimuli but also learn and adapt with remarkable energy efficiency. Forgetting also plays an active role in learning. Mimicking the adaptive neurobiological mechanisms for seeing, learning, and forgetting can, therefore, accelerate the development of artificial intelligence (AI) and bridge the massive energy gap that exists between AI and biological intelligence. Here, we demonstrate a bioinspired machine vision system based on a 2D phototransistor array fabricated from large-area monolayer molybdenum disulfide (MoS₂) and integrated with an analog, nonvolatile, and programmable memory gate-stack; this architecture not only enables dynamic learning and relearning from visual stimuli but also offers learning adaptability under noisy illumination conditions at miniscule energy expenditure. In short, our demonstrated "all-in-one" hardware vision platform combines "sensing", "computing", and "storage" to not only overcome the von Neumann bottleneck of conventional complementary metal-oxide-semiconductor (CMOS) technology but also to eliminate the need for peripheral circuits and sensors.

An All-in-One Bioinspired Neural Network

In spite of recent advancements in artificial neural networks (ANNs), the energy efficiency, multifunctionality, adaptability, and integrated nature of biological neural networks remain largely unimitated by hardware neuromorphic computing systems. Here, we exploit optoelectronic, computing, and programmable memory devices based on emerging two-dimensional (2D) layered materials such as MoS₂ to demonstrate a monolithically integrated, multipixel, and "all-in-one" bioinspired neural network (BNN) capable of sensing, encoding, learning, forgetting, and inferring at minuscule energy expenditure. We also demonstrate learning adaptability and simulate learning challenges under specific synaptic conditions to mimic biological learning. Our findings highlight the potential of in-memory computing and sensing based on emerging 2D materials, devices, and integrated circuits to not only overcome the bottleneck of von Neumann computing in conventional CMOS designs but also to aid in eliminating the peripheral components necessary for competing technologies such as memristors.

Ultrashort 15-nm flexible radio frequency ITO transistors enduring mechanical and temperature stress

Flexible radio frequency (RF) transistors play an important role in the fast-growing wearable smart sensors for data communication. However, the scaling capability and high-speed performance of the flexible transistor are far below the counterparts on rigid substrates, impeding the gigahertz high-speed applications. Here, we address the scaling and performance bottlenecks in flexible transistors by demonstrating natively flexible RF indium tin oxide transistors with deeply scaled 15-nm-long channel, capable of operating in the 10-GHz frequency range. The record-high cutoff frequency of 11.8 GHz and maximum oscillation frequency of 15 GHz can rival those on rigid substrates. Furthermore, the robustness of flexible RF transistors was examined, capable of enduring heavy-duty 10,000 bending cycles at 1-mm radius and extreme thermal stress from cryogenic temperature of 4.3 K and high temperature of 380 K.

Two dimensional semiconducting materials for ultimately scaled transistors

Two dimensional (2D) semiconductors have been established as promising candidates to break through the short channel effect that existed in Si metal-oxide-semiconductor field-effect-transistor (MOSFET), owing to their unique atomically layered structure and dangling-bond-free surface. The last decade has witnessed the significant progress in the size scaling of 2D transistors by various approaches, in which the physical gate length of the transistors has shrank from micrometer to sub-one nanometer with superior performance, illustrating their potential as a replacement technology for Si MOSFETs. Here, we review state-of-the-art techniques to achieve ultra-scaled 2D transistors with novel configurations through the scaling of channel, gate, and contact length. We provide comprehensive views of the merits and drawbacks of the ultra-scaled 2D transistors by summarizing the relevant fabrication processes with the corresponding critical parameters achieved. Finally, we identify the key opportunities and challenges for integrating ultra-scaled 2D transistors in the next-generation heterogeneous circuitry.

Evolution Application of Two-Dimensional MoS2-Based Field-Effect Transistors

High-performance and low-power field-effect transistors (FETs) are the basis of integrated circuit fields, which undoubtedly require researchers to find better film channel layer materials and improve device structure technology. MoS₂ has recently shown a special two-dimensional (2D) structure and superior photoelectric performance, and it has shown new potential for next-generation electronics. However, the natural atomic layer thickness and large specific surface area of MoS₂ make the contact interface and dielectric interface have a great influence on the performance of MoS₂ FET. Thus, we focus on its main performance improvement strategies, including optimizing the contact behavior, regulating the conductive channel, and rationalizing the dielectric layer. On this basis, we summarize the applications of 2D MoS₂ FETs in key and emerging fields, specifically involving logic, RF circuits, optoelectronic devices, biosensors, piezoelectric devices, and synaptic transistors. As a whole, we discuss the state-of-the-art, key merits, and limitations of each of these 2D MoS₂-based FET systems, and prospects in the future.

NaCl-Assisted Chemical Vapor Deposition of Large-Domain Bilayer MoS2 on Soda-Lime Glass

In recent years, two-dimensional molybdenum disulfide (MoS₂) has attracted extensive attention in the application field of next-generation electronics. Compared with single-layer MoS₂, bilayer MoS₂ has higher carrier mobility and has more promising applications for future novel electronic devices. Nevertheless, the large-scale low-cost synthesis of high-quality bilayer MoS₂ still has much room for exploration, requiring further research. In this study, bilayer MoS₂ crystals grown on soda-lime glass substrate by sodium chloride (NaCl)-assisted chemical vapor deposition (CVD) were reported, the growth mechanism of NaCl in CVD of bilayer MoS₂ was analyzed, and the effects of molybdenum trioxide (Mo) mass and growth pressure on the growth of bilayer MoS₂ under the assistance of NaCl were further explored. Through characterization with an optical microscope, atomic force microscopy and Raman analyzer, the domain size of bilayer MoS₂ prepared by NaCl-assisted CVD was shown to reach 214 μm, which is a 4.2X improvement of the domain size of bilayer MoS₂ prepared without NaCl-assisted CVD. Moreover, the bilayer structure accounted for about 85%, which is a 2.1X improvement of bilayer MoS₂ prepared without NaCl-assisted CVD. This study provides a meaningful method for the growth of high-quality bilayer MoS₂, and promotes the large-scale and low-cost applications of CVD MoS₂.

Synthesis of a Selectively Nb-Doped WS2-MoS2 Lateral Heterostructure for a High-Detectivity PN Photodiode

In this study, selective Nb doping (P-type) at the WS₂ layer in a WS₂-MoS₂ lateral heterostructure via a chemical vapor deposition (CVD) method using a solution-phase precursor containing W, Mo, and Nb atoms is proposed. The different chemical activity reactivity (MoO₃ > WO₃ > Nb₂O₅) enable the separation of the growth temperature of intrinsic MoS₂ to 700 °C (first grown inner layer) and Nb-doped WS₂ to 800 °C (second grown outer layer). By controlling the Nb/(W+Nb) molar ratio in the solution precursor, the hole carrier density in the p-type WS₂ layer is selectively controlled from approximately 1.87 × 10⁷/cm² at 1.5 at.% Nb to approximately 1.16 × 10¹³/cm² at 8.1 at.% Nb, while the electron carrier density in n-type MoS₂ shows negligible change with variation of the Nb molar ratio. As a result, the electrical behavior of the WS₂-MoS₂ heterostructure transforms from the N-N junction (0 at.% Nb) to the P-N junction (4.5 at.% Nb) and the P-N tunnel junction (8.1 at.% Nb). The band-to-band tunneling at the P-N tunnel junction (8.1 at.% Nb) is eliminated by applying negative gate bias, resulting in a maximum rectification ratio (10⁵) and a minimum channel resistance (10⁸ Ω). With this optimized photodiode (8.1 at.% Nb at V_g = -30 V), an I_photo/I_dark ratio of 6000 and a detectivity of 1.1 × 10¹⁴ Jones are achieved, which are approximately 20 and 3 times higher, respectively, than the previously reported highest values for CVD-grown transition-metal dichalcogenide P-N junctions.

Chemical Vapor Deposition of Uniform and Large-Domain Molybdenum Disulfide Crystals on Glass/Al2O3 Substrates

Two-dimensional molybdenum disulfide (MoS₂) has attracted significant attention for next-generation electronics, flexible devices, and optical applications. Chemical vapor deposition is the most promising route for the production of large-scale, high-quality MoS₂ films. Recently, the chemical vapor deposition of MoS₂ films on soda-lime glass has attracted great attention due to its low cost, fast growth, and large domain size. Typically, a piece of Mo foil or graphite needs to be used as a buffer layer between the glass substrates and the CVD system to prevent the glass substrates from being fragmented. In this study, a novel method was developed for synthesizing MoS₂ on glass substrates. Inert Al₂O₃ was used as the buffer layer and high-quality, uniform, triangular monolayer MoS₂ crystals with domain sizes larger than 400 μm were obtained. To demonstrate the advantages of glass/Al₂O₃ substrates, a direct comparison of CVD MoS₂ on glass/Mo and glass/Al₂O₃ substrates was performed. When Mo foil was used as the buffer layer, serried small bilayer islands and bright core centers could be observed on the MoS₂ domains at the center and edges of glass substrates. As a control, uniform MoS₂ crystals were obtained when Al₂O₃ was used as the buffer layer, both at the center and the edge of glass substrates. Raman and PL spectra were further characterized to show the merit of glass/Al₂O₃ substrates. In addition, the thickness of MoS₂ domains was confirmed by an atomic force microscope and the uniformity of MoS₂ domains was verified by Raman mapping. This work provides a novel method for CVD MoS₂ growth on soda-lime glass and is helpful in realizing commercial applications of MoS₂.

Heterogeneous Integration of Atomically Thin Semiconductors for Non-von Neumann CMOS

Atomically thin, 2D, and semiconducting transition metal dichalcogenides (TMDs) are seen as potential candidates for complementary metal oxide semiconductor (CMOS) technology in future nodes. While high-performance field effect transistors (FETs), logic gates, and integrated circuits (ICs) made from n-type TMDs such as MoS₂ and WS₂ grown at wafer scale have been demonstrated, realizing CMOS electronics necessitates integration of large area p-type semiconductors. Furthermore, the physical separation of memory and logic is a bottleneck of the existing CMOS technology and must be overcome to reduce the energy burden for computation. In this article, the existing limitations are overcome and for the first time, a heterogeneous integration of large area grown n-type MoS₂ and p-type vanadium doped WSe₂ FETs with non-volatile and analog memory storage capabilities to achieve a non-von Neumann 2D CMOS platform is introduced. This manufacturing process flow allows for precise positioning of n-type and p-type FETs, which is critical for any IC development. Inverters and a simplified 2-input-1-output multiplexers and neuromorphic computing primitives such as Gaussian, sigmoid, and tanh activation functions using this non-von Neumann 2D CMOS platform are also demonstrated. This demonstration shows the feasibility of heterogeneous integration of wafer scale 2D materials.

Layer-by-layer epitaxy of multi-layer MoS2 wafers

The 2D semiconductor of MoS₂ has great potential for advanced electronics technologies beyond silicon. So far, high-quality monolayer MoS₂ wafers have been available and various demonstrations from individual transistors to integrated circuits have also been shown. In addition to the monolayer, multilayers have narrower band gaps but improved carrier mobilities and current capacities over the monolayer. However, achieving high-quality multi-layer MoS₂ wafers remains a challenge. Here we report the growth of high-quality multi-layer MoS₂ 4-inch wafers via the layer-by-layer epitaxy process. The epitaxy leads to well-defined stacking orders between adjacent epitaxial layers and offers a delicate control of layer numbers up to six. Systematic evaluations on the atomic structures and electronic properties were carried out for achieved wafers with different layer numbers. Significant improvements in device performances were found in thicker-layer field-effect transistors (FETs), as expected. For example, the average field-effect mobility (μ_FE) at room temperature (RT) can increase from ∼80 cm²·V^–1·s^–1 for monolayers to ∼110/145 cm²·V^–1·s^–1 for bilayer/trilayer devices. The highest RT μ_FE of 234.7 cm²·V^–1·s^–1 and record-high on-current densities of 1.70 mA·μm^–1 at V_ds = 2 V were also achieved in trilayer MoS₂ FETs with a high on/off ratio of >10⁷. Our work hence moves a step closer to practical applications of 2D MoS₂ in electronics.

Uniform nucleation and epitaxy of bilayer molybdenum disulfide on sapphire

Two-dimensional transition-metal dichalcogenides (TMDs) are of interest for beyond-silicon electronics^1,2. It has been suggested that bilayer TMDs, which combine good electrostatic control, smaller bandgap and higher mobility than monolayers, could potentially provide improvements in the energy-delay product of transistors^3-5. However, despite advances in the growth of monolayer TMDs^6-14, the controlled epitaxial growth of multilayers remains a challenge¹⁵. Here we report the uniform nucleation (>99%) of bilayer molybdenum disulfide (MoS₂) on c-plane sapphire. In particular, we engineer the atomic terrace height on c-plane sapphire to enable an edge-nucleation mechanism and the coalescence of MoS₂ domains into continuous, centimetre-scale films. Fabricated field-effect transistor (FET) devices based on bilayer MoS₂ channels show substantial improvements in mobility (up to 122.6 cm² V^-1 s^-1) and variation compared with FETs based on monolayer films. Furthermore, short-channel FETs exhibit an on-state current of 1.27 mA μm^-1, which exceeds the 2028 roadmap target for high-performance FETs¹⁶.

The Intrinsic Thermodynamic Difficulty and a Step-Guided Mechanism for the Epitaxial Growth of Uniform Multilayer MoS2 with Controllable Thickness

Multilayer MoS₂ shows superior performance over the monolayer MoS₂ for electronic devices while the growth of multilayer MoS₂ with controllable and uniform thickness is still very challenging. It is revealed by calculations that monolayer MoS₂ domains are thermodynamically much more favorable than multilayer ones on epitaxial substrates due to the competition between surface interactions and edge formation, leading accordingly to a layer-by-layer growth pattern and non-continuously distributed multilayer domains with uncontrollable thickness uniformity. The thermodynamics model also suggests that multilayer MoS₂ domains with aligned edges can significantly reduce their free energy and represent a local minimum with very prominent energy advantage on a potential energy surface. However, the nucleation probability of multilayer MoS₂ domains with aligned edges is, if not impossible, extremely rare on flat substrates. Herein, a step-guided mechanism for the growth of uniform multilayer MoS₂ on an epitaxial substrate is theoretically proposed. The steps with proper height on sapphire surface are able to guide the simultaneous nucleation of multilayer MoS₂ with aligned edges and uniform thickness, and promote the continuous growth of multilayer MoS₂ films. The proposed mechanism can be reasonably extended to grow multilayer 2D materials with uniform thickness on epitaxial substrates.

Influence of growth temperature on dielectric strength of Al2O3 thin films prepared via atomic layer deposition at low temperature

Thin films grown via atomic layer deposition (ALD) suffer from insufficient growth rate and unreliability for temperature-sensitive electronic substrates. This study aimed to examine the growth characteristics and dielectric strength of ALD Al₂O₃ films grown at low temperatures (≤ 150 °C) for potential application in flexible electronic devices. The growth rate of the Al₂O₃ films increased from 0.9 to 1.1 Å/cycle with increasing temperature and saturated at growth temperatures ≥ 150 °C, which is the critical temperature at which a complete oxidation reaction occurred. The dielectric strength was also improved with increasing growth temperature, and the films grown at 150 °C showed a high breakdown field strength (~ 8.3 MV/cm), attributable to the decrease in the carbon impurities and oxygen defects, as confirmed by X-ray photoelectron spectroscopy. Even at low growth temperatures (≤ 150 °C), ALD Al₂O₃ films showed an overall amorphous structure and extremely smooth surfaces regardless of the growth temperature.

An artificial neural network chip based on two-dimensional semiconductor

Recently, research on two-dimensional (2D) semiconductors has begun to translate from the fundamental investigation into rudimentary functional circuits. In this work, we unveil the first functional MoS₂ artificial neural network (ANN) chip, including multiply-and-accumulate (MAC), memory and activation function circuits. Such MoS₂ ANN chip is realized through fabricating 818 field-effect transistors (FETs) on a wafer-scale and high-homogeneity MoS₂ film, with a gate-last process to realize top gate structured FETs. A 62-level simulation program with integrated circuit emphasis (SPICE) model is utilized to design and optimize our analog ANN circuits. To demonstrate a practical application, a tactile digit sensing recognition was demonstrated based on our ANN circuits. After training, the digit recognition rate exceeds 97%. Our work not only demonstrates the protentional of 2D semiconductors in wafer-scale integrated circuits, but also paves the way for its future application in AI computation.

Few-Layer WS2-WSe2 Lateral Heterostructures: Influence of the Gas Precursor Selenium/Tungsten Ratio on the Number of Layers

Two-dimensional (2D) lateral heterostructures based on transition metal dichalcogenides (TMDCs) attract great interest due to their properties and potential applications in electronics and optoelectronics, such as p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors. However, the studies of 2D lateral heterostructures have mainly focused on monolayer nanosheets despite bilayer heterostructures exhibiting higher performance in many electronic and optoelectronic devices. It remains a great challenge to synthesize lateral heterostructures with few layers. Here, we report the growth of bilayer-bilayer (bl-bl), bilayer-bilayer-monolayer (bl-bl-mo), bilayer-monolayer (bl-mo), monolayer-bilayer (mo-bl), and monolayer-monolayer (mo-mo) tungsten disulfide (WS₂) and tungsten diselenide (WSe₂) lateral heterostructures. The selenium/tungsten (Se/W) ratio of WSe₂ precursor powders and the growth atmosphere can be changed with the extension of annealing time, which influences the layer number of the heterostructures. More bilayer WSe₂ epitaxially grows at the WS₂ edge with short annealing time (high Se/W ratio), and more monolayer WSe₂ grows at the WS₂ edge with long annealing time (low Se/W ratio). The density functional theory (DFT) calculations provide an in-depth understanding of the growth mechanism. This report expands the 2D material lateral heterostructure family, which gives impetus to their applications in electronics and optoelectronics.

Transition Metal Dichalcogenides (TMDC)-Based Nanozymes for Biosensing and Therapeutic Applications

Nanozymes, a type of nanomaterial with enzyme-like properties, are a promising alternative to natural enzymes. In particular, transition metal dichalcogenides (TMDCs, with the general formula MX2, where M represents a transition metal and X is a chalcogen element)-based nanozymes have demonstrated exceptional potential in the healthcare and diagnostic sectors. TMDCs have different enzymatic properties due to their unique nano-architecture, high surface area, and semiconducting properties with tunable band gaps. Furthermore, the compatibility of TMDCs with various chemical or physical modification strategies provide a simple and scalable way to engineer and control their enzymatic activity. Here, we discuss recent advances made with TMDC-based nanozymes for biosensing and therapeutic applications. We also discuss their synthesis strategies, various enzymatic properties, current challenges, and the outlook for future developments in this field.

Wafer-Scale Synthesis of WS2 Films with In Situ Controllable p-Type Doping by Atomic Layer Deposition

Wafer-scale synthesis of p-type TMD films is critical for its commercialization in next-generation electro/optoelectronics. In this work, wafer-scale intrinsic n-type WS₂ films and in situ Nb-doped p-type WS₂ films were synthesized through atomic layer deposition (ALD) on 8-inch α-Al₂O₃/Si wafers, 2-inch sapphire, and 1 cm² GaN substrate pieces. The Nb doping concentration was precisely controlled by altering cycle number of Nb precursor and activated by postannealing. WS₂ n-FETs and Nb-doped p-FETs with different Nb concentrations have been fabricated using CMOS-compatible processes. X-ray photoelectron spectroscopy, Raman spectroscopy, and Hall measurements confirmed the effective substitutional doping with Nb. The on/off ratio and electron mobility of WS₂ n-FET are as high as 10⁵ and 6.85 cm² V⁻¹ s⁻¹, respectively. In WS₂ p-FET with 15-cycle Nb doping, the on/off ratio and hole mobility are 10 and 0.016 cm² V⁻¹ s⁻¹, respectively. The p-n structure based on n- and p- type WS₂ films was proved with a 10⁴ rectifying ratio. The realization of controllable in situ Nb-doped WS₂ films paved a way for fabricating wafer-scale complementary WS₂ FETs.

Millimeter-Scale Continuous Film of MoS2 Synthesized Using a Mo, Na, and Seeding Promoter-Based Coating as a Solid Precursor

While the chemical vapor deposition technique can be used to fabricate 2D materials in a larger area, materials like MoS₂ have limited controllability due to their lack of self-controlling nature. This article presents a new technique for synthesizing a void-free millimeter-scale continuous monolayer MoS₂ film through the diffusion of a well-controlled Mo, Na, and seeding promoter-based coating under a low-pressure N₂ atmosphere. Compared to the conventional method, this technique provides precise control of solid precursors, where MoS₂ grows next to the coating. At 800 °C, the synthesized MoS₂ showed a uniform single-layer MoS₂ film; however, a Na-free coating showed nanoscale voids and poor crystal quality, which are attributed to a higher edge-attachment barrier that slows down the MoS₂ lateral growth. The synthesized MoS₂ with Na-containing solution showed an intense PL peak with a 1.86 eV band gap. Even at the relatively low temperature of 700 °C, compared to the Na-excluded condition, MoS₂ showed almost two times higher area coverage with a comparatively larger crystal size. This finding may assist in the future development of MoS₂-based electronic and optoelectronic devices such as transistors and photodetectors.

Bilayer MoS2 on silicon for higher terahertz amplitude modulation

The terahertz (THz) amplitude modulation has been experimentally demonstrated by employing bilayer molybdenum disulfide (MoS2) on high-resistivity silicon (Si). The Raman spectroscopy and x-ray photoelectron spectra confirm the formation of bilayer MoS2 film. The THz transmission measurements are carried out using a continuous wave (CW) frequency-domain THz system. This reveals the higher modulation depth covering wide THz spectra of 0.1–1 THz at low optical pumping power. The modulation depth up to 72.3% at 0.1 THz and 62.8% at 0.9 THz under low power optical excitation is achieved. After annealing, the strong built-in electric field is induced at the MoS2–Si interface due to p-type doping in MoS2. This improves modulation depth to 86.4% and 79.7%, respectively. The finite-difference time-domain (FDTD) based numerical simulations match well with the experimental results. The higher modulation depth at low optical power, broadband response, low insertion losses, and simplicity in the design are the key attributes of this THz modulator.

A Roadmap for Disruptive Applications and Heterogeneous Integration Using Two-Dimensional Materials: State-of-the-Art and Technological Challenges

This Mini Review attempts to establish a roadmap for two-dimensional (2D) material-based microelectronic technologies for future/disruptive applications with a vision for the semiconductor industry to enable a universal technology platform for heterogeneous integration. The heterogeneous integration would involve integrating orthogonal capabilities, such as different forms of computing (classical, neuromorphic, and quantum), all forms of sensing, digital and analog memories, energy harvesting, and so forth, all in a single chip using a universal technology platform. We have reviewed the state-of-the-art 2D materials such as graphene, transition metal dichalcogenides, phosphorene and hexagonal boron nitride, and so forth, and how they offer unique possibilities for a range of futuristic/disruptive applications. Besides, we have discussed the technological and fundamental challenges in enabling such a universal technology platform, where the world stands today, and what gaps are required to be filled.

Large-Scale Uniform-Patterned Arrays of Ultrathin All-2D Vertical Stacked Photodetector Devices

The key to unlocking the full potential of two-dimensional (2D) materials in ultrathin opto-electronics is their layer-by-layer integration and the ability to produce them on the wafer scale using traditional industry-compatible technology. Here, we demonstrate a novel stacking method for assembling uniform-patterned periodic 2D arrays into vertical-layered heterostructures. The fabricated heterostructure can serve as photodetectors, with graphene electrodes and transition-metal dichalcogenides as the photo-absorber. All 2D materials used are grown into continuous films with only mono- or bilayer thickness. Each layer is prepatterned into a specific shape on a substrate and then transferred to the device substrate with aligned precision. In order to achieve long-range alignment across the wafer, interlocking marker pairs are used to help guide the lateral accuracy and reduce rotational error. We show hundreds of identical devices produced with 2D periodic spacing on a 1 cm × 1 cm SiO₂/Si substrate, a fundamental prerequisite for future pixelated detectors. Statistics of the photovoltaic performance of the devices are reported, with values that are comparable to devices made by chemical vapor deposition-grown materials. Our work provides pathways for the large-scale fabrication of ultrathin all-2D opto-electronics that form the basis of the future in 2D-pixelated cameras and displays.

Effect of Back-Gate Voltage on the High-Frequency Performance of Dual-Gate MoS2 Transistors

As an atomically thin semiconductor, 2D molybdenum disulfide (MoS₂) has demonstrated great potential in realizing next-generation logic circuits, radio-frequency (RF) devices and flexible electronics. Although various methods have been performed to improve the high-frequency characteristics of MoS₂ RF transistors, the impact of the back-gate bias on dual-gate MoS₂ RF transistors is still unexplored. In this work, we study the effect of back-gate control on the static and RF performance metrics of MoS₂ high-frequency transistors. By using high-quality chemical vapor deposited bilayer MoS₂ as channel material, high-performance top-gate transistors with on/off ratio of 10⁷ and on-current up to 179 μA/μm at room temperature were realized. With the back-gate modulation, the source and drain contact resistances decrease to 1.99 kΩ∙μm at V_bg = 3 V, and the corresponding on-current increases to 278 μA/μm. Furthermore, both cut-off frequency and maximum oscillation frequency improves as the back-gate voltage increases to 3 V. In addition, a maximum intrinsic f_max of 29.7 GHz was achieved, which is as high as 2.1 times the f_max without the back-gate bias. This work provides significant insights into the influence of back-gate voltage on MoS₂ RF transistors and presents the potential of dual-gate MoS₂ RF transistors for future high-frequency applications.

Recent progress on kinetic control of chemical vapor deposition growth of high-quality wafer-scale transition metal dichalcogenides

2D transition metal dichalcogenides (TMDs) have attracted significant attention due to their unique physical properties. Chemical vapor deposition (CVD) is generally a promising method to prepare ideal TMD films with high uniformity, large domain size, good single-crystallinity, etc., at wafer-scale for commercial uses. However, the CVD-grown TMD samples often suffer from poor quality due to the improper control of reaction kinetics and lack of understanding about the phenomenon. In this review, we focus on several key challenges in the controllable CVD fabrication of high-quality wafer-scale TMD films and highlight the importance of the control of precursor concentration, nucleation density, and oriented growth. The remaining difficulties in the field and prospective directions of the related topics are further summarized.

Thickness-modulated lateral MoS2 diodes with sub-terahertz cutoff frequency

Thickness-modulated lateral MoS2 diodes with an extracted benchmark cutoff frequency (fc) of up to 126 GHz are implemented and fully characterised. Fabricated diodes demonstrate an on-off current ratio of more than 600 and a short circuit current responsivity at zero-bias of 7 A/W. The excellent performance achieved in our device is attributed to reduced contact resistance from using In/Au contacts and low junction capacitance due to the lateral device structure. In addition, the use of multilayer MoS2 crystals enabled relatively high current flow. Small- and large-signal models are extracted from DC and RF characterisation of the fabricated diode prototype. Extracted compact models are compared to the measured DC and S-parameters of the diode, demonstrating excellent matching between models and measurements. The presented diode is suitable for switching circuits and high frequency applications.

Multi-scale analysis of radio-frequency performance of 2D-material based field-effect transistors

Two-dimensional materials (2DMs) are a promising alternative to complement and upgrade high-frequency electronics. However, in order to boost their adoption, the availability of numerical tools and physically-based models able to support the experimental activities and to provide them with useful guidelines becomes essential. In this context, we propose a theoretical approach that combines numerical simulations and small-signal modeling to analyze 2DM-based FETs for radio-frequency applications. This multi-scale scheme takes into account non-idealities, such as interface traps, carrier velocity saturation, or short channel effects, by means of self-consistent physics-based numerical calculations that later feed the circuit level via a small-signal model based on the dynamic intrinsic capacitances of the device. At the circuit stage, the possibilities range from the evaluation of the performance of a single device to the design of complex circuits combining multiple transistors. In this work, we validate our scheme against experimental results and exemplify its use and capability assessing the impact of the channel scaling on the performance of MoS₂-based FETs targeting RF applications.

High-Performance CVD Bilayer MoS2 Radio Frequency Transistors and Gigahertz Mixers for Flexible Nanoelectronics

Two-dimensional (2D) MoS₂ have attracted tremendous attention due to their potential applications in future flexible high-frequency electronics. Bilayer MoS₂ exhibits the advantages of carrier mobility when compared with monolayer mobility, thus making the former more suitable for use in future flexible high-frequency electronics. However, there are fewer systematical studies of chemical vapor deposition (CVD) bilayer MoS₂ radiofrequency (RF) transistors on flexible polyimide substrates. In this work, CVD bilayer MoS₂ RF transistors on flexible substrates with different gate lengths and gigahertz flexible frequency mixers were constructed and systematically studied. The extrinsic cutoff frequency (f_T) and maximum oscillation frequency (f_max) increased with reducing gate lengths. From transistors with a gate length of 0.3 μm, we demonstrated an extrinsic f_T of 4 GHz and f_max of 10 GHz. Furthermore, statistical analysis of 14 flexible MoS₂ RF transistors is presented in this work. The study of a flexible mixer demonstrates the dependence of conversion gain versus gate voltage, LO power and input signal frequency. These results present the potential of CVD bilayer MoS₂ for future flexible high-frequency electronics.

Ultraflat Sub-10 Nanometer Gap Electrodes for Two-Dimensional Optoelectronic Devices

Two-dimensional (2D) materials are promising candidates for building ultrashort-channel devices because their thickness can be reduced down to a single atomic layer. Here, we demonstrate an ultraflat nanogap platform based on atomic layer deposition (ALD) and utilize the structure to fabricate 2D material-based optical and electronic devices. In our method, ultraflat metal surfaces, template-stripped from a Si wafer mold, are separated by an Al₂O₃ ALD layer down to a gap width of 10 nm. Surfaces of both electrodes are vertically aligned without a height difference, and each electrode is ultraflat with a measured root-mean-square roughness as low as 0.315 nm, smaller than the thickness of monolayer graphene. Simply by placing 2D material flakes on top of the platform, short-channel field-effect transistors based on black phosphorus and MoS₂ are fabricated, exhibiting their typical transistor characteristics. Furthermore, we use the same platform to demonstrate photodetectors with a nanoscale photosensitive channel, exhibiting higher photosensitivity compared to microscale gap channels. Our wafer-scale atomic layer lithography method can benefit a diverse range of 2D optical and electronic applications.

Recent advances on TMDCs for medical diagnosis

Transition metal dichalcogenides (TMDCs), such as MoS₂ and WS₂, have attracted much attention in biosensing and bioimaging due to its excellent stability, biocompatibility, high specific surface area, and wide varieties. In this review, we overviewed the application of TMDCs in biosensing and bioimaging. Firstly, the synthesis methods and surface functionalization methods of TMDCs were summarized. Secondly, according to the working mechanism, we classified and gave a detailed account of the latest research progress of TMDC-based biosensing for the detection of the enzyme, DNA, and other biological molecules. Then, we outlined the recent progress of applying TMDCs in bio-imaging, including fluorescence, X-ray computed tomographic, magnetic response imaging, photographic and multimodal imaging, respectively. Finally, we discussed the future challenges and development direction of the application of TMDCs in medical diagnosis. Also, we put forward our view on the opportunity of TMDCs in the big data of modern medical diagnosis.

Fast growth of large-grain and continuous MoS2 films through a self-capping vapor-liquid-solid method

Most chemical vapor deposition methods for transition metal dichalcogenides use an extremely small amount of precursor to render large single-crystal flakes, which usually causes low coverage of the materials on the substrate. In this study, a self-capping vapor-liquid-solid reaction is proposed to fabricate large-grain, continuous MoS₂ films. An intermediate liquid phase-Na₂Mo₂O₇ is formed through a eutectic reaction of MoO₃ and NaF, followed by being sulfurized into MoS₂. The as-formed MoS₂ seeds function as a capping layer that reduces the nucleation density and promotes lateral growth. By tuning the driving force of the reaction, large mono/bilayer (1.1 mm/200 μm) flakes or full-coverage films (with a record-high average grain size of 450 μm) can be grown on centimeter-scale substrates. The field-effect transistors fabricated from the full-coverage films show high mobility (33 and 49 cm² V⁻¹ s⁻¹ for the mono and bilayer regions) and on/off ratio (1 ~ 5 × 10⁸) across a 1.5 cm × 1.5 cm region.

Here, the authors develop a self-capping vapour-liquid-solid reaction to fabricate large-grain continuous MoS₂ films, whereby an intermediate liquid phase-Na₂Mo₂O₇ is formed through a eutectic reaction of MoO₃ and NaF, followed by sulphurisation into MoS₂.

Low-voltage 2D materials-based printed field-effect transistors for integrated digital and analog electronics on paper

Paper is the ideal substrate for the development of flexible and environmentally sustainable ubiquitous electronic systems, which, combined with two-dimensional materials, could be exploited in many Internet-of-Things applications, ranging from wearable electronics to smart packaging. Here we report high-performance MoS2 field-effect transistors on paper fabricated with a "channel array" approach, combining the advantages of two large-area techniques: chemical vapor deposition and inkjet-printing. The first allows the pre-deposition of a pattern of MoS2; the second, the printing of dielectric layers, contacts, and connections to complete transistors and circuits fabrication. Average ION/IOFF of 8 × 103 (up to 5 × 104) and mobility of 5.5 cm2 V-1 s-1 (up to 26 cm2 V-1 s-1) are obtained. Fully functional integrated circuits of digital and analog building blocks, such as logic gates and current mirrors, are demonstrated, highlighting the potential of this approach for ubiquitous electronics on paper.

Understanding Interlayer Contact Conductance in Twisted Bilayer Graphene

Bilayer or few-layer 2D materials showing novel electrical properties in electronic device applications have aroused increasing interest in recent years. Obtaining a comprehensive understanding of interlayer contact conductance still remains a challenge, but is significant for improving the performance of bilayer or few-layer 2D electronic devices. Here, conductive atomic force microscope (C-AFM) experiments are reported to explore the interlayer contact conductance between bilayer graphene (BLG) with various twisted stacking structures fabricated by the chemical vapor deposition (CVD) method. The current maps show that the interlayer contact conductance between BLG strongly depends on the twist angle. The interlayer contact conductance of 0° AB-stacking bilayer graphene (AB-BLG) is ≈4 times as large as that of 30° twisted bilayer graphene (t-BLG), which indicates that the twist angle-dependent interlayer contact conductance originates from the coupling-decoupling transitions. Moreover, the moiré superlattice-level current images of t-BLG show modulations of local interlayer contact conductance. Density functional theory calculations together with a theoretical model reproduce the C-AFM current map and show that the modulation is mainly attributed to the overall contribution of local interfacial carrier density and tunneling barrier.

Bilayer Lateral Heterostructures of Transition-Metal Dichalcogenides and Their Optoelectronic Response

Two-dimensional lateral heterojunctions based on monolayer transition-metal dichalcogenides (TMDs) have received increasing attention given that their direct band gap makes them very attractive for optoelectronic applications. Although bilayer TMDs present an indirect band gap, their electrical properties are expected to be less susceptible to ambient conditions, with higher mobilities and density of states when compared to monolayers. Bilayers and few-layers single domain devices have already demonstrated higher performance in radio frequency and photosensing applications. Despite these advantages, lateral heterostructures based on bilayer domains have been less explored. Here, we report the controlled synthesis of multi-junction bilayer lateral heterostructures based on MoS₂-WS₂ and MoSe₂-WSe₂ monodomains. The heterojunctions are created via sequential lateral edge-epitaxy that happens simultaneously in both the first and the second layers. A phenomenological mechanism is proposed to explain the growth mode with self-limited thickness that happens within a certain window of growth conditions. With respect to their as-grown monolayer counterparts, bilayer lateral heterostructures yield nearly 1 order of magnitude higher rectification currents. They also display a clear photovoltaic response, with short circuit currents ∼10³ times larger than those extracted from the as-grown monolayers, in addition to room-temperature electroluminescence. The improved performance of bilayer heterostructures significantly expands the potential of two-dimensional materials for optoelectronics.