Postsynthesis of h‐BN/Graphene Heterostructures Inside a STEM

Combinations of 2D materials with different physical properties can form heterostructures with modified electrical, mechanical, magnetic, and optical properties. The direct observation of a lateral heterostructure synthesis is reported by epitaxial in‐plane graphene growth from the step‐edge of hexagonal BN (h‐BN) within a scanning transmission electron microscope chamber. Residual hydrocarbon in the chamber is the carbon source. The growth interface between h‐BN and graphene is atomically identified as largely N–C bonds. This postgrowth method can form graphene nanoribbons connecting two h‐BN domains with different twisting angles, as well as isolated carbon islands with arbitrary shapes embedded in the h‐BN layer. The electronic properties of the vertically stacked h‐BN/graphene heterostructures are investigated by electron energy‐loss spectroscopy (EELS). Low‐loss EELS analysis of the dielectric response suggests a robust coupling effect between the graphene and h‐BN layers.     

Low‐Temperature and Rapid Growth of Large Single‐Crystalline Graphene with Ethane

Future applications of graphene rely highly on the production of large‐area high‐quality graphene, especially large single‐crystalline graphene, due to the reduction of defects caused by grain boundaries. However, current large single‐crystalline graphene growing methodologies are suffering from low growth rate and as a result, industrial graphene production is always confronted by high energy consumption, which is primarily caused by high growth temperature and long growth time. Herein, a new growth condition achieved via ethane being the carbon feedstock to achieve low‐temperature yet rapid growth of large single‐crystalline graphene is reported. Ethane condition gives a growth rate about four times faster than methane, achieving about 420 µm min^?1 for the growth of sub‐centimeter graphene single crystals at temperature about 1000 °C. In addition, the temperature threshold to obtain graphene using ethane can be reduced to 750 °C, lower than the general growth temperature threshold (about 1000 °C) with methane on copper foil. Meanwhile ethane always keeps higher graphene growth rate than methane under the same growth temperature. This study demonstrates that ethane is indeed a potential carbon source for efficient growth of large single‐crystalline graphene, thus paves the way for graphene in high‐end electronical and optoelectronical applications.     

Aligned Growth of Millimeter‐Size Hexagonal Boron Nitride Single‐Crystal Domains on Epitaxial Nickel Thin Film

Atomically thin hexagonal boron nitride (h‐BN) is gaining significant attention for many applications such as a dielectric layer or substrate for graphene‐based devices. For these applications, synthesis of high‐quality and large‐area h‐BN layers with few defects is strongly desirable. In this work, the aligned growth of millimeter‐size single‐crystal h‐BN domains on epitaxial Ni (111)/sapphire substrates by ion beam sputtering deposition is demonstrated. Under the optimized growth conditions, single‐crystal h‐BN domains up to 0.6 mm in edge length are obtained, the largest reported to date. The formation of large‐size h‐BN domains results mainly from the reduced Ni‐grain boundaries and the improved crystallinity of Ni film. Furthermore, the h‐BN domains show well‐aligned orientation and excellent dielectric properties. In addition, the sapphire substrates can be repeatedly used with almost no limit. This work provides an effective approach for synthesizing large‐scale high‐quality h‐BN layers for electronic applications.     

High‐Brightness Blue Light‐Emitting Diodes Enabled by a Directly Grown Graphene Buffer Layer

Single‐crystalline GaN‐based light‐emitting diodes (LEDs) with high efficiency and long lifetime are the most promising solid‐state lighting source compared with conventional incandescent and fluorescent lamps. However, the lattice and thermal mismatch between GaN and sapphire substrate always induces high stress and high density of dislocations and thus degrades the performance of LEDs. Here, the growth of high‐quality GaN with low stress and a low density of dislocations on graphene (Gr) buffered sapphire substrate is reported for high‐brightness blue LEDs. Gr films are directly grown on sapphire substrate to avoid the tedious transfer process and GaN is grown by metal–organic chemical vapor deposition (MOCVD). The introduced Gr buffer layer greatly releases biaxial stress and reduces the density of dislocations in GaN film and In_xGa_1?xN/GaN multiple quantum well structures. The as‐fabricated LED devices therefore deliver much higher light output power compared to that on a bare sapphire substrate, which even outperforms the mature process derived counterpart. The GaN growth on Gr buffered sapphire only requires one‐step growth, which largely shortens the MOCVD growth time. This facile strategy may pave a new way for applications of Gr films and bring several disruptive technologies for epitaxial growth of GaN film and its applications in high‐brightness LEDs.     

H2O‐Etchant‐Promoted Synthesis of High‐Quality Graphene on Glass and Its Application in See‐Through Thermochromic Displays

Direct growth of graphene on glass can bring an innovative revolution by coupling the complementary properties of traditional glass and modern graphene (such as transparency and conductivity), offering brand new daily‐life related applications. However, preparation of high‐quality graphene on nonmetallic glass is still challenging. Herein, the direct route of low sheet resistance graphene on glass is reported by using in situ‐introduced water as a mild etchant and methane as a carbon precursor via chemical vapor deposition. The derived graphene features with large domain sizes and few amorphous carbon impurities. Intriguingly, the sheet resistance of graphene on glass is dramatically lowered down to ≈1170 Ω sq^?1 at the optical transmittance ≈93%, ≈20% of that derived without the water etchant. Based on the highly conductive and optical transparent graphene on glass, a see‐through thermochromic display is thus fabricated with transparent graphene glass as a heater. This work can motivate further investigations of the direct synthesis of high‐quality graphene on functional glass and its versatile applications in transparent electronic devices or displays.     

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

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

Improved Epitaxy of AlN Film for Deep‐Ultraviolet Light‐Emitting Diodes Enabled by Graphene

The growth of single‐crystal III‐nitride films with a low stress and dislocation density is crucial for the semiconductor industry. In particular, AlN‐derived deep‐ultraviolet light‐emitting diodes (DUV‐LEDs) have important applications in microelectronic technologies and environmental sciences but are still limited by large lattice and thermal mismatches between the epilayer and substrate. Here, the quasi‐van der Waals epitaxial (QvdWE) growth of high‐quality AlN films on graphene/sapphire substrates is reported and their application in high‐performance DUV‐LEDs is demonstrated. Guided by density functional theory calculations, it is found that pyrrolic nitrogen in graphene introduced by a plasma treatment greatly facilitates the AlN nucleation and enables fast growth of a mirror‐smooth single‐crystal film in a very short time of ≈0.5 h (≈50% decrease compared with the conventional process), thus leading to a largely reduced cost. Additionally, graphene effectively releases the biaxial stress (0.11 GPa) and reduces the dislocation density in the epilayer. The as‐fabricated DUV‐LED shows a low turn‐on voltage, good reliability, and high output power. This study may provide a revolutionary technology for the epitaxial growth of AlN films and provide opportunities for scalable applications of graphene films.     

Hexagonal Boron Nitride Growth on Cu‐Si Alloy: Morphologies and Large Domains

Controllable synthesis of high‐quality hexagonal boron nitride (h‐BN) is desired toward the industrial application of 2D devices based on van der Waals heterostructures. Substantial efforts are devoted to synthesize h‐BN on copper through chemical vapor deposition, which has been successfully applied to grow graphene. However, the progress in synthesizing h‐BN has been significantly retarded, and it is still challenging to realize millimeter‐scale domains and control their morphologies reliably. Here, the nucleation density of h‐BN on Cu is successfully reduced by over two orders of magnitude by simply introducing a small amount of silicon, giving rise to large triangular domains with maximum 0.25 mm lateral size. Moreover, the domain morphologies can be modified from needles, tree patterns, and leaf darts to triangles through controlling the growth temperature. The presence of silicon alters the growth mechanism from attachment‐limited mode to diffusion‐limited mode, leading to dendrite domains that are rarely observed on pure Cu. A phase‐field model is utilized to reveal the growing dynamics regarding B‐N diffusion, desorption, flux, and reactivity variables, and explain the morphology evolution. The work sheds lights on the h‐BN growth toward large single crystals and morphology probabilities.     

Fabrication of Millimeter‐Scale,Single‐Crystal One‐Third‐Hydrogenated Graphene with Anisotropic Electronic Properties

Periodically hydrogenated graphene is predicted to form new kinds of crystalline 2D materials such as graphane, graphone, and 2D C_xH_y, which exhibit unique electronic properties. Controlled synthesis of periodically hydrogenated graphene is needed for fundamental research and possible electronic applications. Only small patches of such materials have been grown so far, while the experimental fabrication of large‐scale, periodically hydrogenated graphene has remained challenging. In the present work, large‐scale, periodically hydrogenated graphene is fabricated on Ru(0001). The as‐fabricated hydrogenated graphene is highly ordered, with a √3 × √3/R30° period relative to the pristine graphene. As the ratio of hydrogen and carbon is 1:3, the periodically hydrogenated graphene is named “one‐third‐hydrogenated graphene” (OTHG). The area of OTHG is up to 16 mm². Density functional theory calculations demonstrate that the OTHG has two deformed Dirac cones along one high‐symmetry direction and a finite energy gap along the other directions at the Fermi energy, indicating strong anisotropic electrical properties. An efficient method is thus provided to produce large‐scale crystalline functionalized graphene with specially desired properties.     

Ultrahigh‐Sensitive Broadband Photodetectors Based on Dielectric Shielded MoTe2/Graphene/SnS2 p–g–n Junctions

2D atomic sheets of transition metal dichalcogenides (TMDs) have a tremendous potential for next‐generation optoelectronics since they can be stacked layer‐by‐layer to form van der Waals (vdW) heterostructures. This allows not only bypassing difficulties in heteroepitaxy of lattice‐mismatched semiconductors of desired functionalities but also providing a scheme to design new optoelectronics that can surpass the fundamental limitations on their conventional semiconductor counterparts. Herein, a novel 2D h‐BN/p‐MoTe₂/graphene/n‐SnS₂/h‐BN p–g–n junction, fabricated by a layer‐by‐layer dry transfer, demonstrates high‐sensitivity, broadband photodetection at room temperature. The combination of the MoTe₂ and SnS₂ of complementary bandgaps, and the graphene interlayer provides a unique vdW heterostructure with a vertical built‐in electric field for high‐efficiency broadband light absorption, exciton dissociation, and carrier transfer. The graphene interlayer plays a critical role in enhancing sensitivity and broadening the spectral range. An optimized device containing 5?7‐layer graphene has been achieved and shows an extraordinary responsivity exceeding 2600 A W^?1 with fast photoresponse and specific detectivity up to ≈10¹³ Jones in the ultraviolet–visible–near‐infrared spectrum. This result suggests that the vdW p–g–n junctions containing multiple photoactive TMDs can provide a viable approach toward future ultrahigh‐sensitivity and broadband photonic detectors.     

2D In2S3 Nanoflake Coupled with Graphene toward High‐Sensitivity and Fast‐Response Bulk‐Silicon Schottky Photodetector

Silicon‐based electronic devices, especially graphene/Si photodetectors (Gr/Si PDs), have triggered tremendous attention due to their simple structure and flexible integration of the Schottky junction. However, due to the relatively poor light–matter interaction and mobility of silicon, these Gr/Si PDs typically suffer an inevitable compromise between photoresponsivity and response speed. Herein, a novel strategy for coupling 2D In₂S₃ with Gr/Si PDs is demonstrated. The introduction of the double‐heterojunction design not only strengthens the light absorption of graphene/Si but also combines the advantages of the photogating effect and photovoltaic effect, which suppresses the dark current, accelerates the separation of photogenerated carriers, and brings photoconductive gain. As a result, In₂S₃/graphene/Si devices present an ultrahigh photoresponsivity of 4.53 × 10⁴ A W^?1 and fast response speed less than 40 µs, simultaneously. These parameters are an order of magnitude higher than pristine Gr/Si PDs and among the best values compared with reported 2D materials/Si heterojunction PDs. Furthermore, the In₂S₃/graphene/Si PD expresses outstanding long‐term stability, with negligible performance degradation even after 1 month in air or 1000 cycles of operation. These findings highlight a simple and novel strategy for constructing high‐sensitivity and ultrafast Gr/Si PDs for further optoelectronic applications.     

Chemically Integrated Inorganic‐Graphene Two‐Dimensional Hybrid Materials for Flexible Energy Storage Devices

State‐of‐the‐art energy storage devices are capable of delivering reasonably high energy density (lithium ion batteries) or high power density (supercapacitors). There is an increasing need for these power sources with not only superior electrochemical performance, but also exceptional flexibility. Graphene has come on to the scene and advancements are being made in integration of various electrochemically active compounds onto graphene or its derivatives so as to utilize their flexibility. Many innovative synthesis techniques have led to novel graphene‐based hybrid two‐dimensional nanostructures. Here, the chemically integrated inorganic‐graphene hybrid two‐dimensional materials and their applications for energy storage devices are examined. First, the synthesis and characterization of different kinds of inorganic‐graphene hybrid nanostructures are summarized, and then the most relevant applications of inorganic‐graphene hybrid materials in flexible energy storage devices are reviewed. The general design rules of using graphene‐based hybrid 2D materials for energy storage devices and their current limitations and future potential to advance energy storage technologies are also discussed.     

Direct CVD Graphene Growth on Semiconductors and Dielectrics for Transfer‐Free Device Fabrication

Graphene is the most broadly discussed and studied two‐dimensional material because of its preeminent physical, mechanical, optical, and thermal properties. Until now, metal‐catalyzed chemical vapor deposition (CVD) has been widely employed for the scalable production of high‐quality graphene. However, in order to incorporate the graphene into electronic devices, a transfer process from metal substrates to targeted substrates is inevitable. This process usually results in contamination, wrinkling, and breakage of graphene samples ‐ undesirable in graphene‐based technology and not compatible with industrial production. Therefore, direct graphene growth on desired semiconductor and dielectric substrates is considered as an effective alternative. Over the past years, there have been intensive investigations to realize direct graphene growth using CVD methods without the catalytic role of metals. Owing to the low catalytic activity of non‐metal substrates for carbon precursor decomposition and graphene growth, several strategies have been designed to facilitate and engineer graphene fabrication on semiconductors and insulators. Here, those developed strategies for direct CVD graphene growth on semiconductors and dielectrics for transfer‐free fabrication of electronic devices are reviewed. By employing these methods, various graphene‐related structures can be directly prepared on desired substrates and exhibit excellent performance, providing versatile routes for varied graphene‐based materials fabrication.     

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

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

Temperature‐Related Morphological Evolution of MoS2 Domains on Graphene and Electron Transfer within Heterostructures

Other than the well‐known sulfurization of molybdate compound to synthesize molybdenum disulfide (MoS₂) layers, the dynamic process in the whole crystalline growth from nuclei to triangular domains has been rarely experimentally explored. Here, a competing sulfur‐capture principle jointly with strict epitaxial mechanism is first proposed for the initial topography evolution and the final intrinsic highly oriented growth of triangular MoS₂ domains with Mo or S terminations on the graphene (Gr) template. Additionally, potential distributions on MoS₂ domains and bare Gr are presented to be different due to the charge transfer within heterostructures. The findings offer the mechanism of templated growth of 2D transition metal dichalcogenides, and provide general principles in syntheses of vertical 2D heterostructures that can be applied to electronics.     

Caging Nb2O5 Nanowires in PECVD‐Derived Graphene Capsules toward Bendable Sodium‐Ion Hybrid Supercapacitors

Sodium‐ion hybrid supercapacitors (Na‐HSCs) by virtue of synergizing the merits of batteries and supercapacitors have attracted considerable attention for high‐energy and high‐power energy‐storage applications. Orthorhombic Nb₂O₅ (T‐Nb₂O₅) has recently been recognized as a promising anode material for Na‐HSCs due to its typical pseudocapacitive feature, but it suffers from intrinsically low electrical conductivity. Reasonably high electrochemical performance of T‐Nb₂O₅‐based electrodes could merely be gained to date when sufficient carbon content was introduced. In addition, flexible Na‐HSC devices have scarcely been demonstrated by far. Herein, an in situ encapsulation strategy is devised to directly grow ultrathin graphene shells over T‐Nb₂O₅ nanowires (denoted as Gr‐Nb₂O₅ composites) by plasma‐enhanced chemical vapor deposition, targeting a highly conductive anode material for Na‐HSCs. The few‐layered graphene capsules with ample topological defects would enable facile electron and Na⁺ ion transport, guaranteeing rapid pseudocapacitive processes at the Nb₂O₅/electrolyte interface. The Na‐HSC full‐cell comprising a Gr‐Nb₂O₅ anode and an activated carbon cathode delivers high energy/power densities (112.9 Wh kg^?1/80.1 W kg^?1 and 62.2 Wh kg^?1/5330 W kg^?1), outperforming those of recently reported Na‐HSC counterparts. Proof‐of‐concept Na‐HSC devices with favorable mechanical robustness manifest stable electrochemical performances under different bending conditions and after various bending–release cycles.     

Switching Vertical to Horizontal Graphene Growth Using Faraday Cage‐Assisted PECVD Approach for High‐Performance Transparent Heating Device

Plasma‐enhanced chemical vapor deposition (PECVD) is an applicable route to achieve low‐temperature growth of graphene, typically shaped like vertical nanowalls. However, for transparent electronic applications, the rich exposed edges and high specific surface area of vertical graphene (VG) nanowalls can enhance the carrier scattering and light absorption, resulting in high sheet resistance and low transmittance. Thus, the synthesis of laid‐down graphene (LG) is imperative. Here, a Faraday cage is designed to switch graphene growth in PECVD from the vertical to the horizontal direction by weakening ion bombardment and shielding electric field. Consequently, laid‐down graphene is synthesized on low‐softening‐point soda‐lime glass (6 cm × 10 cm) at ≈580 °C. This is hardly realized through the conventional PECVD or the thermal chemical vapor deposition methods with the necessity of high growth temperature (1000 °C–1600 °C). Laid‐down graphene glass has higher transparency, lower sheet resistance, and much improved macroscopic uniformity when compare to its vertical graphene counterpart and it performs better in transparent heating devices. This will inspire the next‐generation applications in low‐cost transparent electronics.     

Recent Progress in Graphene‐Based Noble‐Metal Nanocomposites for Electrocatalytic Applications

The fast industrialization process has led to global challenges in the energy crisis and environmental pollution, which might be solved with clean and renewable energy. Highly efficient electrochemical systems for clean‐energy collection require high‐performance electrocatalysts, including Au, Pt, Pd, Ru, etc. Graphene, a single‐layer 2D carbon nanosheet, possesses many intriguing properties, and has attracted tremendous research attention. Specifically, graphene and graphene derivatives have been utilized as templates for the synthesis of various noble‐metal nanocomposites, showing excellent performance in electrocatalytic‐energy‐conversion applications, such as the hydrogen evolution reaction and CO₂ reduction. Herein, the recent progress in graphene‐based noble‐metal nanocomposites is summarized, focusing on their synthetic methods and electrocatalytic applications. Furthermore, some personal insights on the challenges and possible future work in this research field are proposed.     

Single‐Carbon‐Nanotube Manipulations and Devices Based on Macroscale Anthracene Flakes

Because of the outstanding mechanical and electrical properties of carbon nanotubes (CNTs), a CNT‐based torsion pendulum is demonstrated to show great potential in nano‐electromechanical systems. It is also expected to achieve various manipulations for further characterization and increase device sensitivity using ultrlong CNTs and macroscale moving parts. However, the reported top‐down method limits the CNT performance and device size by introducing inevitable contamination and destruction, which greatly hinders the development of single‐molecule devices. Here, a bottom‐up method is introduced to fabricate heterostructures of anthracene flakes (AFs) and suspended CNTs, providing a nondamaging CNT mechanical measurement before further applications, especially for the twisting behavior, and providing a controllable and clean transfer method to fabricate CNT‐based electrical devices under ambient conditions. Based on the unique geometry of CNT/AF heterostructures, various complex manipulations of single‐CNT devices are conducted to investigate CNT mechanical properties and prompt novel applications of similar structures in nanotechnology. The AF‐decorated CNTs show high Young's modulus (≈1 TPa) and tensile strength (≈100 GPa), and can be considered as the finest and strongest torsional springs. CNT‐based torsion balance enables to measure fN‐level forces and the torsional spring constant is two orders of magnitude lower than previously reported values.     

Freestanding 3D Graphene–Nickel Encapsulated Nitrogen‐Rich Aligned Bamboo Like Carbon Nanotubes for High‐Performance Supercapacitors with Robust Cycle Stability

3D hierarchical structures are reported based on graphene–nickel encapsulated nitrogen‐rich aligned bamboo like carbon nanotubes, which show not only high‐performance supercapacitance behavior but also a great robust cyclic stability. A facile synthesis route is developed of 2D nickel oxide decorated functionalized graphene nanosheets (2D‐NiO‐f:GNSs) hybrids and 3D nitrogen doped bamboo‐shaped carbon nanotubes (NCNTs) vertically standing on the functionalized graphene nanosheets (3D‐NCNT@f:GNSs) by using a thermal decomposition method. The chemical reduction and morphology‐dependent electrochemical response are investigated. The enhanced specific capacitance of 3D‐NCNT@f:GNSs as compared to that of 2D‐NiO‐f:GNSs suggests the synergistic effects and indicates the importance of energy storage and superior long‐term cycling performance that are achieved. This 3D‐NCNT@f:GNSs hybrid shows a remarkable cycling stability with a maximum power density of 12.32 kW kg⁻¹ and maximum energy density of 109.13 Wh kg⁻¹ due to the good connection of NCNT and f:GNSs. This unique 3D nano network architecture enables the availability of large surface areas of NCNT, thus endowing the nanohybrids with high specific capacitance and excellent reusability.