Nickelocene‐Precursor‐Facilitated Fast Growth of Graphene/h‐BN Vertical Heterostructures and Its Applications in OLEDs

The direct growth of high‐quality, large‐area, uniform, vertically stacked Gr/h‐BN heterostructures is of vital importance for applications in electronics and optoelectronics. However, the main challenge lies in the catalytically inert nature of the hexagonal boron nitride (h‐BN) substrates, which usually afford a rather low decomposition rate of carbon precursors, and thus relatively low growth rate of graphene. Herein, a nickelocene‐precursor‐facilitated route is developed for the fast growth of Gr/h‐BN vertical heterostructures on Cu foils, which shows much improved synthesis efficiency (8–10 times faster) and crystalline quality of graphene (large single‐crystalline domain up to ≈20 µm). The key advantage of our synthetic route is the utilization of nickel atoms that are decomposed from nickelocene molecules as the gaseous catalyst, which can decrease the energy barrier for graphene growth and facilitate the decomposition of carbon sources, according to our density functional theory calculations. The high‐quality Gr/h‐BN stacks are proved to be perfect anode/protecting layers for high‐performance organic light‐emitting diode devices. In this regard, this work offers a brand‐new route for the fast growth of Gr/h‐BN heterostructures with practical scalability and high crystalline quality, thus should propel its wide applications in transparent electrodes, high‐performance electronic devices, and energy harvesting/transition directions.     

Precisely Aligned Monolayer MoS2 Epitaxially Grown on h‐BN basal Plane

Control of the precise lattice alignment of monolayer molybdenum disulfide (MoS₂) on hexagonal boron nitride (h‐BN) is important for both fundamental and applied studies of this heterostructure but remains elusive. The growth of precisely aligned MoS₂ domains on the basal plane of h‐BN by a low‐pressure chemical vapor deposition technique is reported. Only relative rotation angles of 0° or 60° between MoS₂ and h‐BN basal plane are present. Domains with same orientation stitch and form single‐crystal, domains with different orientations stitch and from mirror grain boundaries. In this way, the grain boundary is minimized and a continuous film stitched by these two types of domains with only mirror grain boundaries is obtained. This growth strategy is also applicable to other 2D materials growth.     

Tubular 3D Resistive Random Access Memory Based on Rolled‐Up h‐BN Tube

Due to their advantages compared with planar structures, rolled‐up tubes have been applied in many fields, such as field‐effect transistors, compact capacitors, inductors, and integrative sensors. On the other hand, because of its perfect insulating nature, ultrahigh mechanical strength and atomic thickness property, 2D hexagonal boron nitride (h‐BN) is a very suitable material for rolled‐up memory applications. In this work, a tubular 3D resistive random access memory (RRAM) device based on rolled‐up h‐BN tube is realized, which is achieved by self‐rolled‐up technology. The tubular RRAM device exhibits bipolar resistive switching behavior, nonvolatile data storage ability, and satisfactorily low programming current compared with other 2D material‐based RRAM devices. Moreover, by releasing from the substrate, the footprint area of the tubular device is reduced by six times. This tubular RRAM device has great potential for increasing the data storage density, lowering the power consumption, and may be applied in the fields of rolled‐up systems and sensing‐storage integration.     

Large Photothermal Effect in Sub‐40 nm h‐BN Nanostructures Patterned Via High‐Resolution Ion Beam

The controlled nanoscale patterning of 2D materials is a promising approach for engineering the optoelectronic, thermal, and mechanical properties of these materials to achieve novel functionalities and devices. Herein, high‐resolution patterning of hexagonal boron nitride (h‐BN) is demonstrated via both helium and neon ion beams and an optimal dosage range for both ions that serve as a baseline for insulating 2D materials is identified. Through this nanofabrication approach, a grating with a 35 nm pitch, individual structure sizes down to 20 nm, and additional nanostructures created by patterning crystal step edges are demonstrated. Raman spectroscopy is used to study the defects induced by the ion beam patterning and is correlated to scanning probe microscopy. Photothermal and scanning near‐field optical microscopy measure the resulting near‐field absorption and scattering of the nanostructures. These measurements reveal a large photothermal expansion of nanostructured h‐BN that is dependent on the height to width aspect ratio of the nanostructures. This effect is attributed to the large anisotropy of the thermal expansion coefficients of h‐BN and the nanostructuring implemented. The photothermal expansion should be present in other van der Waals materials with large anisotropy and can lead to applications such as nanomechanical switches driven by light.     

Thinnest Nonvolatile Memory Based on Monolayer h‐BN

2D materials have attracted much interest over the past decade in nanoelectronics. However, it was believed that the atomically thin layered materials are not able to show memristive effect in vertically stacked structure, until the recent discovery of monolayer transition metal dichalcogenide (TMD) atomristors, overcoming the scaling limit to sub‐nanometer. Herein, the nonvolatile resistance switching (NVRS) phenomenon in monolayer hexagonal boron nitride (h‐BN), a typical 2D insulator, is reported. The h‐BN atomristors are studied using different electrodes and structures, featuring forming‐free switching in both unipolar and bipolar operations, with large on/off ratio (up to 10⁷). Moreover, fast switching speed (<15 ns) is demonstrated via pulse operation. Compared with monolayer TMDs, the one‐atom‐thin h‐BN sheet reduces the vertical scaling to ≈0.33 nm, representing a record thickness for memory materials. Simulation results based on ab‐initio method reveal that substitution of metal ions into h‐BN vacancies during electrical switching is a likely mechanism. The existence of NVRS in monolayer h‐BN indicates fruitful interactions between defects, metal ions and interfaces, and can advance emerging applications on ultrathin flexible memory, printed electronics, neuromorphic computing, and radio frequency switches.     

Fractal‐Theory‐Based Control of the Shape and Quality of CVD‐Grown 2D Materials

The precise control of the shape and quality of 2D materials during chemical vapor deposition (CVD) processes remains a challenging task, due to a lack of understanding of their underlying growth mechanisms. The existence of a fractal‐growth‐based mechanism in the CVD synthesis of several 2D materials is revealed, to which a modified traditional fractal theory is applied in order to build a 2D diffusion‐limited aggregation (2D‐DLA) model based on an atomic‐scale growth mechanism. The strength of this model is validated by the perfect correlation between theoretically simulated data, predicted by 2D‐DLA, and experimental results obtained from the CVD synthesis of graphene, hexagonal boron nitride, and transition metal dichalcogenides. By applying the 2D‐DLA model, it is also discovered that the single‐domain net growth rate (SD‐NGR) plays a crucial factor in governing the shape and quality of 2D‐material crystals. By carefully tuning SD‐NGR, various fractal‐morphology high‐quality single‐crystal 2D materials are synthesized, achieving, for the first time, the precise control of 2D‐material CVD growth. This work lays the theoretical foundation for the precise adjustment of the morphologies and physical properties of 2D materials, which is essential to the use of fractal‐shaped nanomaterials for the fabrication of new‐generation neural‐network nanodevices.     

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.     

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.     

Highly Crystalline C8‐BTBT Thin‐Film Transistors by Lateral Homo‐Epitaxial Growth on Printed Templates

Highly crystalline thin films of organic semiconductors offer great potential for fundamental material studies as well as for realizing high‐performance, low‐cost flexible electronics. The fabrication of these films directly on inert substrates is typically done by meniscus‐guided coating techniques. The resulting layers show morphological defects that hinder charge transport and induce large device‐to‐device variability. Here, a double‐step method for organic semiconductor layers combining a solution‐processed templating layer and a lateral homo‐epitaxial growth by a thermal evaporation step is reported. The epitaxial regrowth repairs most of the morphological defects inherent to meniscus‐guided coatings. The resulting film is highly crystalline and features a mobility increased by a factor of three and a relative spread in device characteristics improved by almost half an order of magnitude. This method is easily adaptable to other coating techniques and offers a route toward the fabrication of high‐performance, large‐area electronics based on highly crystalline thin films of organic semiconductors.     

Catalyst‐Selective Growth of Single‐Orientation Hexagonal Boron Nitride toward High‐Performance Atomically Thin Electric Barriers

The ability to control the crystal orientation of 2D van der Waals (vdW) layered materials grown on large‐scale substrates is crucial for tailoring their electrical properties, as well as for integration of functional 2D devices. In general, multiple orientations, i.e., two or four orientations, appear through the crystal rotational symmetry matching between the material and its substrate. Here, it is reported that hexagonal boron nitride (h‐BN), an ideal electric barrier in the family of 2D materials, has a single orientation on inclined Cu (1 0 1) surfaces, where the Cu planes are tilted from the (1 0 1) facet around specific in‐plane axes. Density functional theory (DFT) calculation indicates that this is a manifestation of only one favored h‐BN orientation with the minimum vdW energy on the inclined Cu (1 0 1) surface. Moreover, thanks to the high interfacial strength with the underlying Cu, the single‐orientation h‐BN is free of thermal wrinkles, and exhibits a spatially homogeneous morphology and tunnel conductance. The findings point to a feasible approach to direct growth of single‐orientation, wrinkle‐free h‐BN thin film for high‐performance 2D electrical devices, and will be of benefit for controllable synthesis of other vdW materials.     

Ultrahigh‐Detectivity Photodetectors with Van der Waals Epitaxial CdTe Single‐Crystalline Films

Van der Waals epitaxy (vdWE) is crucial for heteroepitaxy of covalence‐bonded semiconductors on 2D layered materials because it is not subject to strict substrate requirements and the epitaxial materials can be transferred onto various substrates. However, planar film growth in covalence‐bonded semiconductors remains a critical challenge of vdWE because of the extremely low surface energy of 2D materials. In this study, direct growth of wafer‐scale single‐crystalline cadmium telluride (CdTe) films is achieved on 2D layered transparent mica through molecular beam epitaxy. The vdWE CdTe films exhibit a flat surface resulting from the 2D growth regime, and high crystal quality as evidenced by a low full width at half maximum of 0.05° for 120 nm thick films. A perfect lattice fringe appears at the interfaces, implying a fully relaxed state of the epitaxial CdTe films correlated closely to the unique nature of vdWE. Moreover, the vdWE CdTe photodetectors demonstrate not only ultrasensitive optoelectronic response with optimal responsivity of 834 A W^?1 and ultrahigh detectivity of 2.4 × 10¹⁴ Jones but also excellent mechanical flexibility and durability, indicating great potential in flexible and wearable devices.     

Recent Advances in Growth of Large‐Sized 2D Single Crystals on Cu Substrates

Large‐scale and high‐quality 2D materials have been an emerging and promising choice for use in modern chemistry and physics owing to their fascinating property profile. The past few years have witnessed inspiringly progressing development in controlled fabrication of large‐sized and single‐crystal 2D materials. Among those production methods, chemical vapor deposition (CVD) has drawn the most attention because of its fine control over size and quality of 2D materials by modulating the growth conditions. Meanwhile, Cu has been widely accepted as the most popular catalyst due to its significant merit in growing monolayer 2D materials in the CVD process. Herein, very recent advances in preparing large‐sized 2D single crystals on Cu substrates by CVD are presented. First, the unique features of Cu will be given in terms of ultralow precursor solubility and feasible surface engineering. Then, scaled growth of graphene and hexagonal boron nitride (h‐BN) crystals on Cu substrates is demonstrated, wherein different kinds of Cu surfaces have been employed. Furthermore, the growth mechanism for the growth of 2D single crystals is exhibited, offering a guideline to elucidate the in‐depth growth dynamics and kinetics. Finally, relevant issues for industrial‐scale mass production of 2D single crystals are discussed and a promising future is expected.     

Flexible Gallium Nitride for High‐Performance,Strainable Radio‐Frequency Devices

Flexible gallium nitride (GaN) thin films can enable future strainable and conformal devices for transmission of radio‐frequency (RF) signals over large distances for more efficient wireless communication. For the first time, strainable high‐frequency RF GaN devices are demonstrated, whose exceptional performance is enabled by epitaxial growth on 2D boron nitride for chemical‐free transfer to a soft, flexible substrate. The AlGaN/GaN heterostructures transferred to flexible substrates are uniaxially strained up to 0.85% and reveal near state‐of‐the‐art values for electrical performance, with electron mobility exceeding 2000 cm² V^?1 s^?1 and sheet carrier density above 1.07 × 10¹³ cm^?2. The influence of strain on the RF performance of flexible GaN high‐electron‐mobility transistor (HEMT) devices is evaluated, demonstrating cutoff frequencies and maximum oscillation frequencies greater than 42 and 74 GHz, respectively, at up to 0.43% strain, representing a significant advancement toward conformal, highly integrated electronic materials for RF applications.     

Controlled Synthesis of One‐Dimensional Inorganic Nanostructures Using Pre‐Existing One‐Dimensional Nanostructures as Templates

Template‐directed strategy has become one of the most popular methods for the fabrication of one‐dimensional (1D) nanostructures with uniform size and controllable physical dimensions in recent years. This Review article describes the recent progress in the synthesis of 1D inorganic nanostructures by using suitable templates. A brief survey on the templating method based on the organic templates and porous membrane is firstly given. Then, the article is focused on recent emerging synthetic strategies by templating against the pre‐existing 1D nanostructures using different physical and chemical transformation techniques, including epitaxial growth, nonepitaxial growth, direct chemical transformation, solid‐state interfacial diffusion reaction, and so on. The important reactivity role of the 1D nanostructures will be emphasized in such transformation process. Finally, we conclude this paper with some perspectives and outlook on this research topic.     

Wafer-Scale Single Crystal Hexagonal Boron Nitride Layers Grown by Submicron-Spacing Vapor Deposition

The direct growth of wafer-scale single crystal two-dimensional (2D) hexagonal boron nitride (h-BN) layer with a controllable thickness is highly desirable for 2D-material-based device applications. Here, for the first time, a facile submicron-spacing vapor deposition (SSVD) method is reported to achieve 2-inch single crystal h-BN layers with controllable thickness from monolayer to tens of nanometers on the dielectric sapphire substrates using a boron film as the solid source. In the SSVD growth, the boron film is fully covered by the same-sized sapphire substrate with a submicron spacing, leading to an efficient vapor diffusion transport. The epitaxial h-BN layer exhibits extremely high crystalline quality, as demonstrated by both a sharp Raman E_2g vibration mode (12 cm⁻¹) and a narrow X-ray rocking curve (0.10°). Furthermore, a deep ultraviolet photodetector and a ZrS₂/h-BN heterostructure fabricated from the h-BN layer demonstrate its fascinating properties and potential applications. This facile method to synthesize wafer-scale single crystal h-BN layers with controllable thickness paves the way to future 2D semiconductor-based electronics and optoelectronics.     

High‐Quality Hexagonal Nonlayered CdS Nanoplatelets for Low‐Threshold Whispering‐Gallery‐Mode Lasing

Low threshold micro/nanolasers have attracted extensive attention for wide applications in high‐density storage and optical communication. However, constrained by quantum efficiency and crystalline quality, conventional semiconductor small‐sized lasers are still subjected to a high lasing threshold. In this work, a low‐threshold planar laser based on high‐quality single‐crystalline hexagonal CdS nanoplatelets (NPLs) using a self‐limited epitaxial growth method is demonstrated. The as‐grown CdS NPLs show multiple whispering‐gallery‐mode lasing at room temperature with a threshold of ≈0.6 µJ cm^?2, which is the lowest value among reported CdS‐based lasers. Through power‐dependent lasing studies at 77 K, the lasing action is demonstrated to originate from a exciton–exciton scattering process. Furthermore, the edge length‐ and thickness‐dependent lasing threshold studies reveal that the threshold is inversely proportional to the second power of lateral edge length while partially affected by vertical thickness, and the lasing modes can be sustained in NPLs as thin as 60 nm. The lowest threshold emerges with the thickness of ≈110 nm due to stronger energy confinement in the vertical Fabry–Pérot cavity. The results not only open up a new avenue to fabricate nonlayered material‐based coherent light sources, but also advocate the promise of nonlayered semiconductor materials for the development of novel optoelectronic devices.     

Gate‐Dependent Tunnelling Current Modulation of Graphene/hBN Vertical Heterostructures

Combining with layered thin crystalline films, graphene has expanded its application scope beyond the regime where a gapless semimetal cannot serve. Here, we report the modulation of tunneling characteristics in graphene/hexagonal boron nitride (hBN) vertical heterostructure at different interlayer hBN thickness. These results signify an upshift in threshold voltages with hBN layer thickness. Furthermore, the gate‐dependent tunneling characteristics of the device has been demonstrated. The back‐gate voltages are used to adjust the fermi level of bottom graphene layer, which in turns tune the threshold voltages and tunneling current through ultrathin hBN layer. Our findings offer an effective tool to modulate the tunneling characteristics of vertical transistors for their potential applications in high frequency logic and tunnel devices.

     

Buckling Patterns of Graphene–Boron Nitride Alloy on Ru(0001)

Buckling nanopatterns of monoatomic layer 2D materials on metal substrates attract significant attention due to their rich interface morphology affecting electronic applications. An experimental–theoretical study of a 2D boron–nitrogen–carbon (B _{x /2}N _{x /2}C_1−x ) alloy on a Ru(0001) surface is conducted and a profound relation between the composition x and the degree of buckling is discovered. Experimentally, real carbon–boron–nitrogen alloys on the Ru(0001) surface are demonstrated and various morphologies of pure and mixed compounds are shown. Density functional theory calculations are further carried out using the supercells of graphene, hexagonal boron nitride (h‐BN), and random BNC on Ru(0001), as well as Monte Carlo simulations for elucidating the kinetics of their growth. The results show that unlike pure compounds (h‐BN or C), the carbon–boron–nitrogen mix on Ru(0001) mostly exists in an uncorrugated form, thus greatly improving the interface contact. The likely cause of the diminished corrugation is a softening of bond angular interactions in the alloy relative to the pure phases.     

Quasi‐Epitaxial Growth of Magnetic Nanostructures on 4H‐Au Nanoribbons

Phase engineering of nanomaterials is an effective strategy to tune the physicochemical properties of nanomaterials for various promising applications. Herein, by using the 4H‐Au nanoribbons as templates, four novel magnetic nanostructures, namely 4H‐Au @ 14H‐Co nanobranches, 4H‐Au @ 14H‐Co nanoribbons, 4H‐Au @ 2H‐Co nanoribbons, and 4H‐Au @ 2H‐Ni nanoribbons, are synthesized based on the quasi‐epitaxial growth. Different from the conventional epitaxial growth of metal nanomaterials, the obtained Co and Ni nanostructures possess different crystal phases from the Au template. Due to the large lattice mismatch between Au and the grown metals (i.e., Co and Ni), ordered misfit dislocations are generated at the Co/Au and Ni/Au interfaces. Notably, a new super‐structure of Co is formed, denoted as 14H. Both 4H‐Au @ 14H‐Co nanobranches and nanoribbons are ferromagnetic at room temperature, showing similar Curie temperature. However, their magnetic behaviors exhibit distinct temperature dependence, resulting from the competition between spin and volume fluctuations as well as the unique geometry. This work paves the way to the templated synthesis of nanomaterials with unconventional crystal phases for the exploration of phase‐dependent properties.     

Flat Boron: A New Cousin of Graphene

The mechanical exfoliation of graphene from graphite provides the cornerstone for the synthesis of other 2D materials with layered bulk structures, such as hexagonal boron nitride, transition metal dichalcogenides, black phosphorus, and so on. However, the experimental production of 2D flat boron is challenging because bulk boron has very complex spatial structures and a rich variety of chemical properties. Therefore, the realization of 2D flat boron marks a milestone for the synthesis of 2D materials without layered bulk structures. The historical efforts in this field, particularly the most recent experimental progress, such as the growth of 2D flat boron on a metal substrate by chemical vapor deposition and molecular beam epitaxy, or liquid exfoliation from bulk boron, are described.