Conformal hexagonal-boron nitride dielectric interface for tungsten diselenide devices with improved mobility and thermal dissipation

Amorphous boron nitride: synthesis, properties and device application

Amorphous boron nitride (a-BN) exhibits remarkable electrical, optical, and chemical properties, alongside robust mechanical stability, making it a compelling material for advanced applications in nanoelectronics and photonics. This review comprehensively examines the unique characteristics of a-BN, emphasizing its electrical and optical attributes, state-of-the-art synthesis techniques, and device applications. Key advancements in low-temperature growth methods for a-BN are highlighted, offering insights into their potential for integration into scalable, CMOS-compatible platforms. Additionally, the review discusses the emerging role of a-BN as a dielectric material in electronic and photonic devices, serving as substrates, encapsulation layers, and gate insulators. Finally, perspectives on future challenges, including defect control, interface engineering, and scalability, are presented, providing a roadmap for realizing the full potential of a-BN in next-generation device technologies.

Van der Waals Epitaxy of High-Quality Transition Metal Dichalcogenides on Single-Crystal Hexagonal Boron Nitride

Van der Waals (vdW) heterostructures comprising of transition metal dichalcogenides (TMDs) and hexagonal boron nitride (h-BN) are promising building blocks for novel 2D devices. The vdW epitaxy provides a straightforward integration method for fabricating high-quality TMDs/h-BN vertical heterostructures. In this work, the vdW epitaxy of high-quality single-crystal HfSe2 on epitaxial h-BN/sapphire substrates by chemical vapor deposition is demonstrated. The epitaxial HfSe2 layers exhibit a uniform and atomically sharp interface with the underlying h-BN template, and the epitaxial relationship between HfSe2 and h-BN/sapphire is determined to HfSe2 (0001)[12 ¯ ${\mathrm{\bar{2}}}$ 10]//h-BN (0001)[11 ¯ ${\mathrm{\bar{1}}}$ 00]//sapphire (0001)[11 ¯ ${\mathrm{\bar{1}}}$ 00]. Impressively, the full width at half maximum of the rocking curve for the epitaxial HfSe2 layer on single-crystal h-BN is as narrow as 9.6 arcmin, indicating an extremely high degree of out-plane orientation and high crystallinity. Benefitting from the high crystalline quality of HfSe2 epilayers and the weak interfacial scattering of HfSe2/h-BN, the photodetector fabricated from the vdW epitaxial HfSe2 on single-crystal h-BN shows the best performance with an on/off ratio of 1 × 104 and a responsivity up to 43 mA W-1. Furthermore, the vdW epitaxy of other TMDs such as HfS2, ZrS2, and ZrSe2 is also experimentally demonstrated on single-crystal h-BN, suggesting the broad applicability of the h-BN template for the vdW epitaxy.

Interfacial Self-Assembly Nanostructures: Constructions and Applications

Interfacial self-assembly nanoarrays refer to the spontaneously organized nanostructures at interfaces, relying on the intrinsic properties of involved materials, such as surface energy, molecular structure, and interactions. In recent years, the exponential growth of self-assembly nanotechnology has substantially expanded the utility of nanomaterials. Particularly, non-covalent interactions-based interfacial self-assembly represents a viable and promising approach for the synthesis of novel nanostructure. This review introduces the significance and current development status of interfacial self-assembly technology, focusing on the driving mode, application, and prospects of interfacial self-assembly nanoarrays over the past few years.

Enhancing Thermal Management of Graphene Devices by Self-Assembled Monolayers

Two-dimensional graphene has emerged as a promising competitor to silicon in the post-Moore era due to its superior electrical, optical, and thermal properties. However, graphene undergoes a strong degradation in its in-plane thermal conductivity when it is coupled to an amorphous substrate. Meanwhile, the weak van der Waals interaction between graphene and the dielectric substrate leads to high interfacial thermal resistance. Severe challenges in the device's heat dissipation rise, resulting in elevated hotspot and deteriorated electrical performance. Here, we applied self-assembled monolayers (SAMs) to modify the interface between graphene and the oxide substrate and mitigate the thermal issues in the device. The -NH2 terminated SAM demonstrates enhanced interfacial coupling strength between graphene and substrate, increasing the interfacial thermal conductance. The -CH3 terminated SAM effectively suppresses the substrate phonon scattering, preserving the high in-plane thermal conductivity of graphene. Particularly, the -NH2 terminated SAM significantly enhances the heat dissipation efficacy of graphene field-effect transistors and alleviates the self-heating issues. Enhancements of 28.1% and 48.2% were observed in the devices' current-carrying capacity and maximum power density, respectively. Our research provides a highly attractive platform for incorporating SAMs to improve thermal management in two-dimensional electronic devices.

Synthesis and Characterization of Boron Nitride Thin Films Deposited by High-Power Impulse Reactive Magnetron Sputtering

In the present research, hexagonal boron nitride (h-BN) films were deposited by reactive high-power impulse magnetron sputtering (HiPIMS) of the pure boron target. Nitrogen was used as both a sputtering gas and a reactive gas. It was shown that, using only nitrogen gas, hexagonal-boron-phase thin films were synthesized successfully. The deposition temperature, time, and nitrogen gas flow effects were studied. It was found that an increase in deposition temperature resulted in hydrogen desorption, less intensive hydrogen-bond-related luminescence features in the Raman spectra of the films, and increased h-BN crystallite size. Increases in deposition time affect crystallites, which form larger conglomerates, with size decreases. The conglomerates' size and surface roughness increase with increases in both time and temperature. An increase in the nitrogen flow was beneficial for a significant reduction in the carbon amount in the h-BN films and the appearance of the h-BN-related features in the lateral force microscopy images.

Crucial role of interfacial thermal dissipation in the operational stability of organic field-effect transistors

The operational stability becomes a key issue affecting the commercialization for organic field-effect transistors (OFETs). It is widely recognized to be closely related to the defects and traps at the interface between dielectric and organic semiconductors, but this understanding does not always effectively address operational instability, implying that the factors influencing the operational stability have not been fully understood. Here, we reveal that the self-heating effect is another crucial factor in operational stability. By using hexagonal boron nitride (hBN) to assist interfacial thermal dissipation, the dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) FETs exhibit high mobility of 14.18 cm2 V-1 s-1 and saturated power density up to 1.8 × 104 W cm-2. The OFET can operate at a power density of 1.06 × 104 W cm-2 for 30,000 s with negligible performance degradation, showing excellent operational stability under high power density. This work deepens the understanding on operational stability and develops an effective way for ultrahigh stable devices.

Performance Limits and Advancements in Single 2D Transition Metal Dichalcogenide Transistor

Two-dimensional (2D) transition metal dichalcogenides (TMDs) allow for atomic-scale manipulation, challenging the conventional limitations of semiconductor materials. This capability may overcome the short-channel effect, sparking significant advancements in electronic devices that utilize 2D TMDs. Exploring the dimension and performance limits of transistors based on 2D TMDs has gained substantial importance. This review provides a comprehensive investigation into these limits of the single 2D-TMD transistor. It delves into the impacts of miniaturization, including the reduction of channel length, gate length, source/drain contact length, and dielectric thickness on transistor operation and performance. In addition, this review provides a detailed analysis of performance parameters such as source/drain contact resistance, subthreshold swing, hysteresis loop, carrier mobility, on/off ratio, and the development of p-type and single logic transistors. This review details the two logical expressions of the single 2D-TMD logic transistor, including current and voltage. It also emphasizes the role of 2D TMD-based transistors as memory devices, focusing on enhancing memory operation speed, endurance, data retention, and extinction ratio, as well as reducing energy consumption in memory devices functioning as artificial synapses. This review demonstrates the two calculating methods for dynamic energy consumption of 2D synaptic devices. This review not only summarizes the current state of the art in this field but also highlights potential future research directions and applications. It underscores the anticipated challenges, opportunities, and potential solutions in navigating the dimension and performance boundaries of 2D transistors.

Recent Advances on Pulsed Laser Deposition of Large-Scale Thin Films

2D thin films, possessing atomically thin thickness, are emerging as promising candidates for next-generation electronic devices, due to their novel properties and high performance. In the early years, a wide variety of 2D materials are prepared using several methods (mechanical/liquid exfoliation, chemical vapor deposition, etc.). However, the limited size of 2D flakes hinders their fundamental research and device applications, and hence the effective large-scale preparation of 2D films is still challenging. Recently, pulsed laser deposition (PLD) has appeared to be an impactful method for wafer-scale growth of 2D films, owing to target-maintained stoichiometry, high growth rate, and efficiency. In this review, the recent advances on the PLD preparation of 2D films are summarized, including the growth mechanisms, strategies, and materials classification. First, efficacious strategies of PLD growth are highlighted. Then, the growth, characterization, and device applications of various 2D films are presented, such as graphene, h-BN, MoS2, BP, oxide, perovskite, semi-metal, etc. Finally, the potential challenges and further research directions of PLD technique is envisioned.

Large-Area Synthesis and Fabrication of Few-Layer hBN/Monolayer RGO Heterostructures for Enhanced Contact Surface Potential

Hexagonal boron nitride (hBN) has a property similar to that of graphene, and it has become one of the most popular materials due to its flexible physical and chemical properties for a variety of applications, especially in nanoelectronics. Enhanced properties of hBN-based heterostructures are crucial for future electronic devices. In this work, a sheet-like hBN crystal was synthesized and transferred onto SiO2/Si substrate and reduced graphene oxide (RGO)/SiO2/Si substrate. Accordingly, the hBN and hBN/RGO films are investigated by optical microscopy, X-ray diffraction, high-resolution transmission electron microscopy, Raman spectroscopy, and atomic force microscopy. The thickness of a single hBN layer is approximately 0.4 nm. A few layers of hBN stacked in large areas are mostly observed in both hBN and the hBN/RGO films. By using Kelvin probe force microscopy, it was found that the hBN/RGO heterostructure has a contact surface potential higher than that of the hBN layer. The large-scale synthesis and fabrication of hBN/RGO films could be extended to fabricate other van der Waals heterostructures.

Ultrafast Non-Volatile Floating-Gate Memory Based on All-2D Materials

The explosive growth of massive-data storage and the demand for ultrafast data processing require innovative memory devices with exceptional performance. 2D materials and their van der Waal heterostructures with atomically sharp interfaces hold great promise for innovations in memory devices. Here, this work presents non-volatile, floating-gate memory devices with all functional layers made of 2D materials, achieving ultrafast programming/erasing speeds (20 ns), high extinction ratios (up to 108), and multi-bit storage capability. These devices also exhibit long-term data retention exceeding 10 years, facilitated by a high gate-coupling ratio (GCR) and atomically sharp interfaces between functional layers. Additionally, this work demonstrates the realization of an "OR" logic gate on a single-device unit by synergistic electrical and optical operations. The present results provide a solid foundation for next-generation ultrahigh-speed, ultralong lifespan, non-volatile memory devices, with a potential for scale-up manufacturing and flexible electronics applications.

Low-Temperature Vapor-Phase Growth of 2D Metal Chalcogenides

2D metal chalcogenides (MCs) have garnered significant attention from both scientific and industrial communities due to their potential in developing next-generation functional devices. Vapor-phase deposition methods have proven highly effective in fabricating high-quality 2D MCs. Nevertheless, the conventionally high thermal budgets required for synthesizing 2D MCs pose limitations, particularly in the integration of multiple components and in specialized applications (such as flexible electronics). To overcome these challenges, it is desirable to reduce the thermal energy requirements, thus facilitating the growth of various 2D MCs at lower temperatures. Numerous endeavors have been undertaken to develop low-temperature vapor-phase growth techniques for 2D MCs, and this review aims to provide an overview of the latest advances in low-temperature vapor-phase growth of 2D MCs. Initially, the review highlights the latest progress in achieving high-quality 2D MCs through various low-temperature vapor-phase techniques, including chemical vapor deposition (CVD), metal-organic CVD, plasma-enhanced CVD, atomic layer deposition (ALD), etc. The strengths and current limitations of these methods are also evaluated. Subsequently, the review consolidates the diverse applications of 2D MCs grown at low temperatures, covering fields such as electronics, optoelectronics, flexible devices, and catalysis. Finally, current challenges and future research directions are briefly discussed, considering the most recent progress in the field.

Identifying optical microscope images of CVD-grown two-dimensional MoS2 by convolutional neural networks and transfer learning

Background

In Complementary Metal-Oxide Semiconductor (CMOS) technology, scaling down has been a key strategy to improve chip performance and reduce power losses. However, challenges such as sub-threshold leakage and gate leakage, resulting from short-channel effects, contribute to an increase in distributed static power. Two-dimensional transition metal dichalcogenides (2D TMDs) emerge as potential solutions, serving as channel materials with steep sub-threshold swings and lower power consumption. However, the production and development of these 2-dimensional materials require some time-consuming tasks. In order to employ them in different fields, including chip technology, it is crucial to ensure that their production meets the required standards of quality and uniformity; in this context, deep learning techniques show significant potential.

Methods

This research introduces a transfer learning-based deep convolutional neural network (CNN) to classify chemical vapor deposition (CVD) grown molybdenum disulfide (MoS2) flakes based on their uniformity or the occurrence of defects affecting electronic properties. Acquiring and labeling a sufficient number of microscope images for CNN training may not be realistic. To address this challenge, artificial images were generated using Fresnel equations to pre-train the CNN. Subsequently, accuracy was improved through fine-tuning with a limited set of real images.

Results

The proposed transfer learning-based CNN method significantly improved all measurement metrics with respect to the ordinary CNNs. The initial CNN, trained with limited data and without transfer learning, achieved 68% average accuracy for binary classification. Through transfer learning and artificial images, the same CNN achieved 85% average accuracy, demonstrating an average increase of approximately 17%. While this study specifically focuses on MoS2 structures, the same methodology can be extended to other 2-dimensional materials by simply incorporating their specific parameters when generating artificial images.

Plasma Processing and Treatment of 2D Transition Metal Dichalcogenides: Tuning Properties and Defect Engineering

Two-dimensional transition metal dichalcogenides (TMDs) offer fascinating opportunities for fundamental nanoscale science and various technological applications. They are a promising platform for next generation optoelectronics and energy harvesting devices due to their exceptional characteristics at the nanoscale, such as tunable bandgap and strong light-matter interactions. The performance of TMD-based devices is mainly governed by the structure, composition, size, defects, and the state of their interfaces. Many properties of TMDs are influenced by the method of synthesis so numerous studies have focused on processing high-quality TMDs with controlled physicochemical properties. Plasma-based methods are cost-effective, well controllable, and scalable techniques that have recently attracted researchers' interest in the synthesis and modification of 2D TMDs. TMDs' reactivity toward plasma offers numerous opportunities to modify the surface of TMDs, including functionalization, defect engineering, doping, oxidation, phase engineering, etching, healing, morphological changes, and altering the surface energy. Here we comprehensively review all roles of plasma in the realm of TMDs. The fundamental science behind plasma processing and modification of TMDs and their applications in different fields are presented and discussed. Future perspectives and challenges are highlighted to demonstrate the prominence of TMDs and the importance of surface engineering in next-generation optoelectronic applications.

Freeze-casting production of thermal insulating and fire-retardant lightweight aerogels based on nanocellulose and boron nitride

Cellulose aerogels exhibit biocompatibility and biodegradability, rendering them promising candidate for application in building energy conservation and insulation materials. However, the intrinsic inflammability of pristine cellulose aerogel causes unneglectable safety concerns, hindering their application in energy-efficient buildings. Herein, a thermal insulating, fire-retardant, strong, and lightweight aerogel was produced via freeze-casting suspensions of cellulose nanofibril (CNF) and l-glutamine functionalized boron nitride nanosheets (BNNS-g). The aerogel with a BNNS-g:CNF concentration ratio of 15:5 exhibited outstanding mechanical strength owing to the strong interaction between BNNS-g and CNF as well as satisfactory thermal insulating performance (0.052 W/m·K). Particularly, this aerogel showed excellent fire-retardant and self-extinguishing capabilities in the vertical burning test, which remained unscathed after over 60 s of burning in a butane flame. Further, the limit oxygen index (LOI) of this aerogel was 36.0 %, which was better than the LOIs of traditional petrochemical-based insulating materials. This study provides a promising strategy for producing aerogels with excellent properties using cellulose and other inorganic nano-fillers.

Stamped production of single-crystal hexagonal boron nitride monolayers on various insulating substrates

Controllable growth of two-dimensional (2D) single crystals on insulating substrates is the ultimate pursuit for realizing high-end applications in electronics and optoelectronics. However, for the most typical 2D insulator, hexagonal boron nitride (hBN), the production of a single-crystal monolayer on insulating substrates remains challenging. Here, we propose a methodology to realize the facile production of inch-sized single-crystal hBN monolayers on various insulating substrates by an atomic-scale stamp-like technique. The single-crystal Cu foils grown with hBN films can stick tightly (within 0.35 nm) to the insulating substrate at sub-melting temperature of Cu and extrude the hBN grown on the metallic surface onto the insulating substrate. Single-crystal hBN films can then be obtained by removing the Cu foil similar to the stamp process, regardless of the type or crystallinity of the insulating substrates. Our work will likely promote the manufacturing process of fully single-crystal 2D material-based devices and their applications.

Two-Step Flux Synthesis of Ultrapure Transition-Metal Dichalcogenides

Two-dimensional transition-metal dichalcogenides (TMDs) have attracted tremendous interest due to the unusual electronic and optoelectronic properties of isolated monolayers and the ability to assemble diverse monolayers into complex heterostructures. To understand the intrinsic properties of TMDs and fully realize their potential in applications and fundamental studies, high-purity materials are required. Here, we describe the synthesis of TMD crystals using a two-step flux growth method that eliminates a major potential source of contamination. Detailed characterization of TMDs grown by this two-step method reveals charged and isovalent defects with densities an order of magnitude lower than those in TMDs grown by a single-step flux technique. For WSe2, we show that increasing the Se/W ratio during growth reduces point defect density, with crystals grown at 100:1 ratio achieving charged and isovalent defect densities below 1010 and 1011 cm-2, respectively. Initial temperature-dependent electrical transport measurements of monolayer WSe2 yield room-temperature hole mobility above 840 cm2/(V s) and low-temperature disorder-limited mobility above 44,000 cm2/(V s). Electrical transport measurements of graphene-WSe2 heterostructures fabricated from the two-step flux grown WSe2 also show superior performance: higher graphene mobility, lower charged impurity density, and well-resolved integer quantum Hall states. Finally, we demonstrate that the two-step flux technique can be used to synthesize other TMDs with similar defect densities, including semiconducting 2H-MoSe2 and 2H-MoTe2 and semimetallic Td-WTe2 and 1T'-MoTe2.

Gate Dielectrics Integration for 2D Electronics: Challenges, Advances, and Outlook

2D semiconductors have emerged both as an ideal platform for fundamental studies and as promising channel materials in beyond-silicon field-effect-transistors due to their outstanding electrical properties and exceptional tunability via external field. However, the lack of proper dielectrics for 2D semiconductors has become a major roadblock for their further development toward practical applications. The prominent issues between conventional 3D dielectrics and 2D semiconductors arise from the integration and interface quality, where defect states and imperfections lead to dramatic deterioration of device performance. In this review article, the root causes of such issues are briefly analyzed and recent advances on some possible solutions, including various approaches of adapting conventional dielectrics to 2D semiconductors, and the development of novel dielectrics with van der Waals surface toward high-performance 2D electronics are summarized. Then, in the perspective, the requirements of ideal dielectrics for state-of-the-art 2D devices are outlined and an outlook for their future development is provided.

Low-Temperature Direct Synthesis of Multilayered h-BN without Catalysts by Inductively Coupled Plasma-Enhanced Chemical Vapor Deposition

Low-temperature direct synthesis of thick multilayered hexagonal-boron nitride (h-BN) on semiconducting and insulating substrates is required to produce high-performance electronic devices based on two-dimensional (2D) materials. In this study, multilayered h-BN with a thickness exceeding 5 nm was directly synthesized on quartz and Si at low temperatures, between 400 and 500 °C, by inductively coupled plasma-enhanced chemical vapor deposition using borazine as the precursor material. The quality and thickness of the h-BN crystals were investigated with respect to synthesis parameters, namely, temperature, radio frequency power, N₂ flow rate, and H₂ flow rate. Introducing N₂ and H₂ carrier gases critically affected the deposition rate, and increasing the carrier gas flow rate enhanced the h-BN crystal quality. The typical optical band gap of synthesized h-BN was approximately 5.8 eV, consistent with that of previous studies. The full width at half-maximum of the h-BN Raman peak was 32-33 cm^-1, comparable to that of commercially available multilayered h-BN on Cu foil. These results are expected to facilitate the development of 2D materials for electronics applications.

Efficient modulation of thermal transport in two-dimensional materials for thermal management in device applications

With the development of chip technology, the density of transistors on integrated circuits is increasing and the size is gradually shrinking to the micro-/nanoscale, with the consequent problem of heat dissipation on chips becoming increasingly serious. For device applications, efficient heat dissipation and thermal management play a key role in ensuring device operation reliability. In this review, we summarize the thermal management applications based on 2D materials from both theoretical and experimental perspectives. The regulation approaches of thermal transport can be divided into two main types: intrinsic structure engineering (acting on the intrinsic structure) and non-structure engineering (applying external fields). On one hand, the thermal transport properties of 2D materials can be modulated by defects and disorders, size effect (including length, width, and the number of layers), heterostructures, structure regulation, doping, alloy, functionalizing, and isotope purity. On the other hand, strain engineering, electric field, and substrate can also modulate thermal transport efficiently without changing the intrinsic structure of the materials. Furthermore, we propose a perspective on the topic of using magnetism and light field to modulate the thermal transport properties of 2D materials. In short, we comprehensively review the existing thermal management modulation applications as well as the latest research progress, and conclude with a discussion and perspective on the applications of 2D materials in thermal management, which will be of great significance to the development of next-generation nanoelectronic devices.

Hexagonal Boron Nitride for Next-Generation Photonics and Electronics

Hexagonal boron nitride (h-BN), an insulating 2D layered material, has recently attracted tremendous interest motivated by the extraordinary properties it shows across the fields of optoelectronics, quantum optics, and electronics, being exotic material platforms for various applications. At an early stage of h-BN research, it is explored as an ideal substrate and insulating layers for other 2D materials due to its atomically flat surface that is free of dangling bonds and charged impurities, and its high thermal conductivity. Recent discoveries of structural and optical properties of h-BN have expanded potential applications into emerging electronics and photonics fields. h-BN shows a very efficient deep-ultraviolet band-edge emission despite its indirect-bandgap nature, as well as stable room-temperature single-photon emission over a wide wavelength range, showing a great potential for next-generation photonics. In addition, h-BN is extensively being adopted as active media for low-energy electronics, including nonvolatile resistive switching memory, radio-frequency devices, and low-dielectric-constant materials for next-generation electronics.

Interfacial thermal conductance between atomically thin boron nitride and graphene

Atomically thin hexagonal boron nitride (BN) is a promising dielectric substrate for graphene and other two-dimensional (2D) materials for performance enhancement and heat dissipation. However, the interfacial heat conductance between atomically thin BN and graphene has not been experimentally studied yet. Here, we report that the interfacial thermal conductance between high-quality graphene and trilayer BN is 9.64 ± 2.12 MW m^-2 K^-1 in the temperature range of 293-393 K, indicating that the interfacial thermal conductance is depressed when the heterostructure thickness is smaller than the wavelength of the low-frequency phonons, e.g. ZA in BN.

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.

Direct growth of h-BN multilayers with controlled thickness on non-crystalline dielectric substrates without metal catalysts

We report an rGO-assisted CVD approach that enables the direct growth of high-quality single crystalline h-BN films with adjustable thickness and layered order on amorphous quartz and SiO₂/Si substrates at relatively low temperatures. This work demonstrates a viable prototype for growing continuous ultrathin h-BN films on desired substrates without the requirement of lattice orientation, thus offering a great opportunity for their appealing applications.

Space-Confined One-Step Growth of 2D MoO2 /MoS2 Vertical Heterostructures for Superior Hydrogen Evolution in Alkaline Electrolytes

2D material-based heterostructures are constructed by stacking or spicing individual 2D layers to create an interface between them, which have exotic properties. Here, a new strategy for the in situ growth of large numbers of 2D heterostructures on the centimeter-scale substrate is developed. In the method, large numbers of 2D MoS₂ , MoO₂ , or their heterostructures of MoO₂ /MoS₂ are controllably grown in the same setup by simply tuning the gap distance between metal precursor and growth substrate, which changes the concentration of metal precursors feed. A lateral force microscope is used first to identify the locations of each material in the heterostructures, which have MoO₂ on the top of MoS₂ . Noteworthy, the creation of a clean interface between atomic thin MoO₂ (metallic) and MoS₂ (semiconducting) results in a different electronic structure compared with pure MoO₂ and MoS₂ . Theoretical calculations show that the charge redistribution at such an interface results in an improved HER performance on the MoO₂ /MoS₂ heterostructures, showing an overpotential of 60 mV at 10 mA cm^-2 and a Tafel slope of 47 mV dec^-1 . This work reports a new strategy for the in situ growth of heterostructures on large-scale substrates and provides platforms to exploit their applications.

Low-Temperature Synthesis of Boron Nitride as a Large-Scale Passivation and Protection Layer for Two-Dimensional Materials and High-Performance Devices

Two-dimensional materials (2DMs) with extraordinary electronic and optical properties have attracted great interest in optoelectronic applications. Due to their atomically thin feature, 2DM-based devices are generally sensitive to oxygen and moisture in ambient air, and thus, practical application of durable 2DM-based devices remains challenging. Here, we report a novel strategy to directly synthesize amorphous BN film on various 2DMs and field-effect transistor (FET) devices at low temperatures by conventional chemical vapor deposition. The wafer-scale BN film with controllable thickness serves as a passivation and heat dissipation layer, further improving the long-term stability, the resistance to laser irradiation, and the antioxidation performance of the underneath 2DMs. In particular, the BN capping layer could be deposited directly on a WSe₂ FET at low temperature to achieve a clean and conformal interface. The high performance of the BN-capped WSe₂ device is realized with suppressed current fluctuations and 10-fold enhanced carrier mobility. The transfer-free amorphous BN synthesis technique is simple and applicable to various 2DMs grown on arbitrary substrates, which shows great potential for applications in future two-dimensional electronics.

Epitaxial single-crystal hexagonal boron nitride multilayers on Ni (111)

Large-area single-crystal monolayers of two-dimensional (2D) materials such as graphene^1-3, hexagonal boron nitride (hBN)^4-6 and transition metal dichalcogenides^7,8 have been grown. hBN is considered to be the 'ideal' dielectric for 2D-materials-based field-effect transistors (FETs), offering the potential for extending Moore's law^9,10. Although hBN thicker than a monolayer is more desirable as substrate for 2D semiconductors^11,12, highly uniform and single-crystal multilayer hBN growth has yet to be demonstrated. Here we report the epitaxial growth of wafer-scale single-crystal trilayer hBN by a chemical vapour deposition (CVD) method. Uniformly aligned hBN islands are found to grow on single-crystal Ni (111) at early stage and finally to coalesce into a single-crystal film. Cross-sectional transmission electron microscopy (TEM) results show that a Ni₂₃B₆ interlayer is formed (during cooling) between the single-crystal hBN film and Ni substrate by boron dissolution in Ni. There are epitaxial relationships between hBN and Ni₂₃B₆ and between Ni₂₃B₆ and Ni. We also find that the hBN film acts as a protective layer that remains intact during catalytic evolution of hydrogen, suggesting continuous single-crystal hBN. This hBN transferred onto the SiO₂ (300 nm)/Si wafer acts as a dielectric layer to reduce electron doping from the SiO₂ substrate in MoS₂ FETs. Our results demonstrate high-quality single-crystal  multilayered hBN over large areas, which should open up new pathways for making it a ubiquitous substrate for 2D semiconductors.

Two-Dimensional Field-Effect Transistor Sensors: The Road toward Commercialization

The evolutionary success in information technology has been sustained by the rapid growth of sensor technology. Recently, advances in sensor technology have promoted the ambitious requirement to build intelligent systems that can be controlled by external stimuli along with independent operation, adaptivity, and low energy expenditure. Among various sensing techniques, field-effect transistors (FETs) with channels made of two-dimensional (2D) materials attract increasing attention for advantages such as label-free detection, fast response, easy operation, and capability of integration. With atomic thickness, 2D materials restrict the carrier flow within the material surface and expose it directly to the external environment, leading to efficient signal acquisition and conversion. This review summarizes the latest advances of 2D-materials-based FET (2D FET) sensors in a comprehensive manner that contains the material, operating principles, fabrication technologies, proof-of-concept applications, and prototypes. First, a brief description of the background and fundamentals is provided. The subsequent contents summarize physical, chemical, and biological 2D FET sensors and their applications. Then, we highlight the challenges of their commercialization and discuss corresponding solution techniques. The following section presents a systematic survey of recent progress in developing commercial prototypes. Lastly, we summarize the long-standing efforts and prospective future development of 2D FET-based sensing systems toward commercialization.

Low-Temperature Direct Growth of Few-Layer Hexagonal Boron Nitride on Catalyst-Free Sapphire Substrates

Wide-band-gap layered semiconductor hexagonal boron nitride (h-BN) is attracting intense interest due to its unique optoelectronic properties and versatile applications in deep ultraviolet optoelectronic and two-dimensional electronic devices. However, it is still a great challenge to directly grow high-quality h-BN on dielectric substrates, and an extremely high substrate temperature or annealing is usually required. In this work, high-quality few-layer h-BN is directly grown on sapphire substrates via ion beam sputtering deposition at a relatively low temperature of 700 °C by introducing NH₃ into the growth chamber. Such low growth temperature is attributed to the presence of abundant active N species, originating from the decomposition of NH₃ under ion beam irradiation. To further tailor the properties of h-BN, carbon was introduced into the h-BN layer by simultaneously introducing CH₄ and NH₃ during the growth process, indicating the wide applicability of this approach. Moreover, a deep ultraviolet (DUV) photodetector is also fabricated from a C-doped h-BN layer and exhibits superior performance compared with an intrinsic h-BN device.

Structure, Properties and Applications of Two-Dimensional Hexagonal Boron Nitride

Hexagonal boron nitride (h-BN) has emerged as a strong candidate for two-dimensional (2D) material owing to its exciting optoelectrical properties combined with mechanical robustness, thermal stability, and chemical inertness. Super-thin h-BN layers have gained significant attention from the scientific community for many applications, including nanoelectronics, photonics, biomedical, anti-corrosion, and catalysis, among others. This review provides a systematic elaboration of the structural, electrical, mechanical, optical, and thermal properties of h-BN followed by a comprehensive account of state-of-the-art synthesis strategies for 2D h-BN, including chemical exfoliation, chemical, and physical vapor deposition, and other methods that have been successfully developed in recent years. It further elaborates a wide variety of processing routes developed for doping, substitution, functionalization, and combination with other materials to form heterostructures. Based on the extraordinary properties and thermal-mechanical-chemical stability of 2D h-BN, various potential applications of these structures are described.

Thermal Conductivity of VO2 Nanowires at Metal-Insulator Transition Temperature

Vanadium dioxide (VO2) nanowires endowed with a dramatic metal-insulator transition have attracted enormous attention. Here, the thermal conductance of VO2 nanowires with different sizes, measured using the thermal bridge method, is reported. A size-dependent thermal conductivity was observed where the thicker nanowire showed a higher thermal conductivity. Meanwhile, the thermal conductivity jump at metal-insulator transition temperature was measured to be much higher in the thicker samples. The dominant heat carriers were phonons both at the metallic and the insulating regimes in the measured samples, which may result from the coexistence of metal and insulator phases at high temperature. Our results provide a window into exploring the mechanism of the metal-insulator transition of VO2 nanowires.

Plasma-Enhanced Chemical Vapor Deposition of Two-Dimensional Materials for Applications

ConspectusSince the rise of two-dimensional (2D) materials, synthetic methods including mechanical exfoliation, solution synthesis, and chemical vapor deposition (CVD) have been developed. Mechanical exfoliation prepares randomly shaped materials with small size. Solution synthesis introduces impurities that degrade the performances. CVD is the most successful one for low-cost scalable preparation. However, when it comes to practical applications, disadvantages such as high operating temperature (∼1000 °C), probable usage of metal catalysts, contamination, defects, and interstices introduced by postgrowth transfer are not negligible. These are the reasons why plasma-enhanced CVD (PECVD), a method that enables catalyst-free in situ preparation at low temperature, is imperatively desirable.In this Account, we summarize our recent progress on controllable preparation of 2D materials by PECVD and their applications. We found that there was a competition between etching and nucleation and deposition in PECVD, making it highly controllable to obtain desired materials. Under different equilibrium states of the competition, various 2D materials with diverse morphologies and properties were prepared including pristine or nitrogen-doped graphene crystals, graphene quantum dots, graphene nanowalls, hexagonal boron nitride (h-BN), B-C-N ternary materials (BC_xN), etc. We also used mild plasma to modify or treat 2D materials (e.g., WSe₂) for desired properties.PECVD has advantages such as low temperature, transfer-free process, and industrial compatibility, which enable facile, scalable, and low-cost preparation of 2D materials with clean surfaces and interfaces directly on noncatalytic substrates. These merits significantly benefit the as-prepared materials in the applications. Field-effect transistors with high motilities were directly fabricated on graphene and nitrogen-doped graphene. By use of h-BN as the dielectric interfacial layer, both mobilities and saturated power densities of the devices were improved owing to the clean, closely contacted interface and enhanced interfacial thermal dissipation. High-quality materials and interfaces also enabled promising applications of these materials in photodetectors, pressure sensors, biochemical sensors, electronic skins, Raman enhancement, etc. To demonstrate the commercial applications, several prototypical devices were studied such as distributed pressure sensor arrays, touching module on a robot hand for braille recognition, and smart gloves for recording sign language. Finally, we discuss opportunities and challenges of PECVD as a comprehensive preparation methodology of 2D materials for future applications beyond traditional CVD.

In-situ formation and evolution of atomic defects in monolayer WSe2 under electron irradiation

Transition metal dichalcogenide (TMD) monolayers such as MoS₂, MoSe₂, MoTe₂, WS₂ and WSe₂ have attracted significant interest due to their remarkable electronic and optical properties, exhibiting a direct band gap, enabling usability in electronics and optics. Their properties can be altered further by the introduction of lattice defects. In this work, the dynamics of the formation of electron-beam-induced lattice defects in monolayer WSe₂ are investigated by in-situ spherical and chromatic aberration-corrected low-voltage transmission electron microscopy. We show and analyze the electron-dose-limited life of a monolayer WSe₂ from the formation of isolated Se vacancies over extended defects such as vacancy lines, mirror twin boundaries (MTBs) and inversion domains towards the loss of W atoms leading to the formation of holes and finally the destruction of the monolayer. We identify, moreover, a new type of MTB. Our study extends the basic understanding of defect dynamics in monolayer WSe₂, sheds further light on the electron radiation response and suggests new ways for engineering the in-plane architecture of TMDs.

Desolvation-Triggered Versatile Transfer-Printing of Pure BN Films with Thermal-Optical Dual Functionality

Although hexagonal boron nitride (BN) nanostructures have recently received significant attention due to their unique physical and chemical properties, their applications have been limited by a lack of processability and poor film quality. In this study, a versatile method to transfer-print high-quality BN films composed of densely stacked BN nanosheets based on a desolvation-induced adhesion switching (DIAS) mechanism is developed. It is shown that edge functionalization of BN sheets and rational selection of membrane surface energy combined with systematic control of solvation and desolvation status enable extensive tunability of interfacial interactions at BN-BN, BN-membrane, and BN-substrate boundaries. Therefore, without incorporating any additives in the BN film and applying any surface treatment on target substrates, DIAS achieves a near 100% transfer yield of pure BN films on diverse substrates, including substrates containing significant surface irregularities. The printed BNs demonstrate high optical transparency (>90%) and excellent thermal conductivity (>167 W m^-1 K^-1) for few-micrometer-thick films due to their dense and well-ordered microstructures. In addition to outstanding heat dissipation capability, substantial optical enhancement effects are confirmed for light-emitting, photoluminescent, and photovoltaic devices, demonstrating their remarkable promise for next-generation optoelectronic device platforms.

Catalyst-Free Growth of Two-Dimensional BCxN Materials on Dielectrics by Temperature-Dependent Plasma-Enhanced Chemical Vapor Deposition

Traditional methods to prepare two-dimensional (2D) B-C-N ternary materials (BC_xN), such as chemical vapor deposition (CVD), require sophisticated experimental conditions such as high temperature, delicate control of precursors, and postgrowth transfer from catalytic substrates, and the products are generally thick or bulky films without the atomically mixed phase of B-C-N, hampering practical applications of these materials. Here, for the first time, we develop a temperature-dependent plasma-enhanced chemical vapor deposition (PECVD) method to grow 2D BC_xN materials directly on noncatalytic dielectrics at low temperature with high controllability. The C, N, and B compositions can be tuned by simply changing the growth temperature. Thus, the properties of the as-made materials including band gap and conductivity are modulated, which is hardly achieved by other methods. A 2D hybridized BC₂N film with a mixed BC₂N phase is produced, for the first time, with a band gap of about 2.3 eV. The growth temperature is 580-620 °C, much lower than that of traditional catalytic CVD for growing BC_xN. The product has a p-type conducting property and can be directly applied in field-effect transistors and sensors without postgrowth transfer, showing great promise for this method in future applications.

Self-Assembled Monolayers for the Polymer/Semiconductor Interface with Improved Interfacial Thermal Management

Reliability and lifespan of highly miniaturized and integrated devices will be effectively improved if excessive accumulated heat can be quickly transported to heat sinks. In this study, both molecular dynamics (MD) simulations and experiments were performed to demonstrate that self-assembled monolayers (SAMs) have high potential in interfacial thermal management and can enhance thermal transport across the polystyrene (PS)/silicon (Si) interface, modeling the common polymer/semiconductor interfaces in actual devices. The influence of packing density and alkyl-chain length of SAMs is investigated. First, MD simulations show that the interfacial thermal transport efficiency of SAM is higher with high packing density. The interfacial thermal conductance (ITC) between PS and Si can be improved up to 127 ± 9 MW m^-2 K^-1, close to the ITC across the metal and semiconductor interface. At moderate packing density, the SAMs with less than eight carbon atoms in the alkyl chain show superior improvements over those with more carbons because of the assembled structure variation. Second, the time-domain thermoreflectance technique was employed to characterize the ITCs of a bunch of Al/PS/SAM/Si samples. C6-SAM enhances the ITC by fivefolds, from 11 ± 1 to 56 ± 17 MW m^-2 K^-1. The interfacial thermal management efficiency will weaken when the alkyl chain exceeds eight carbon atoms, which agrees with the ITC trend from MD simulations at moderate packing density. The relationship between the SAM morphology and interfacial thermal management efficiency is also discussed in detail. This study demonstrates the feasibility of molecular-level design for interfacial thermal management from both the theoretical calculation and experiment and may provide a new idea for improving the heat dissipation efficiency of microdevices.