Epitaxial Growth of a Single-Crystal Hybridized Boron Nitride and Graphene Layer on a Wide-Band Gap Semiconductor

A Review on Carrier Mobilities of Epitaxial Graphene on Silicon Carbide

Graphene growth by thermal decomposition of silicon carbide (SiC) is a technique that produces wafer-scale, single-orientation graphene on an insulating substrate. It is often referred to as epigraphene, and has been thought to be suitable for electronics applications. In particular, high-frequency devices for communication technology or large quantum Hall plateau for metrology applications using epigraphene are expected, which require high carrier mobility. However, the carrier mobility of as-grown epigraphene exhibit the relatively low values of about 1000 cm2/Vs. Fortunately, we can hope to improve this situation by controlling the electronic state of epigraphene by modifying the surface and interface structures. In this paper, the mobility of epigraphene and the factors that govern it will be described, followed by a discussion of attempts that have been made to improve mobility in this field. These understandings are of great importance for next-generation high-speed electronics using graphene.

Non-Linear Optics at Twist Interfaces in h-BN/SiC Heterostructures

Understanding the emergent electronic structure in twisted atomically thin layers has led to the exciting field of twistronics. However, practical applications of such systems are challenging since the specific angular correlations between the layers must be precisely controlled and the layers have to be single crystalline with uniform atomic ordering. Here, an alternative, simple, and scalable approach is suggested, where nanocrystallinetwo-dimensional (2D) film on 3D substrates yields twisted-interface-dependent properties. Ultrawide-bandgap hexagonal boron nitride (h-BN) thin films are directly grown on high in-plane lattice mismatched wide-bandgap silicon carbide (4H-SiC) substrates to explore the twist-dependent structure-property correlations. Concurrently, nanocrystalline h-BN thin film shows strong non-linear second-harmonic generation and ultra-low cross-plane thermal conductivity at room temperature, which are attributed to the twisted domain edges between van der Waals stacked nanocrystals with random in-plane orientations. First-principles calculations based on time-dependent density functional theory manifest strong even-order optical nonlinearity in twisted h-BN layers. This work unveils that directly deposited 2D nanocrystalline thin film on 3D substrates could provide easily accessible twist-interfaces, therefore enabling a simple and scalable approach to utilize the 2D-twistronics integrated in 3D material devices for next-generation nanotechnology.

Recent progress of two-dimensional heterostructures for thermoelectric applications

The rapid development of synthesis and fabrication techniques has opened up a research upsurge in two-dimensional (2D) material heterostructures, which have received extensive attention due to their superior physical and chemical properties. Currently, thermoelectric energy conversion is an effective means to deal with the energy crisis and increasingly serious environmental pollution. Therefore, an in-depth understanding of thermoelectric transport properties in 2D heterostructures is crucial for the development of micro-nano energy devices. In this review, the recent progress of 2D heterostructures for thermoelectric applications is summarized in detail. Firstly, we systematically introduce diverse theoretical simulations and experimental measurements of the thermoelectric properties of 2D heterostructures. Then, the thermoelectric applications and performance regulation of several common 2D materials, as well as in-plane heterostructures and van der Waals heterostructures, are also discussed. Finally, the challenges of improving the thermoelectric performance of 2D heterostructures materials are summarized, and related prospects are described.

Broad background in electron diffraction of 2D materials as a signature of their superior quality

An unusually broad bell-shaped component (BSC) has been previously observed in surface electron diffraction on different types of 2D systems. It was suggested to be an indicator of uniformity of epitaxial graphene (Gr) and hexagonal boron nitride (hBN). In the current study we use low-energy electron microscopy and micro-diffraction to directly relate the BSC to the crystal quality of the diffracting 2D material. Specially designed lateral heterostructures were used to map the spatial evolution of the diffraction profile across different 2D materials, namely pure hBN, BCN alloy and pure Gr, where the alloy region exhibits deteriorated structural coherency. The presented results show that the BSC intensity has a minimum in the alloyed region, consequently showing that BSC is sensitive to the lateral domain size and homogeneity of the material under examination. This is further confirmed by the presence of a larger number of sharp moiré spots when the BSC is most pronounced in the pure hBN and Gr regions. Consequently, it is proposed that the BSC can be used as a diagnostic tool for determining the quality of the 2D materials.

Surfactant-Mediated Epitaxial Growth of Single-Layer Graphene in an Unconventional Orientation on SiC

We report the use of a surfactant molecule during the epitaxy of graphene on SiC(0001) that leads to the growth in an unconventional orientation, namely R0° rotation with respect to the SiC lattice. It yields a very high-quality single-layer graphene with a uniform orientation with respect to the substrate, on the wafer scale. We find an increased quality and homogeneity compared to the approach based on the use of a preoriented template to induce the unconventional orientation. Using spot profile analysis low-energy electron diffraction, angle-resolved photoelectron spectroscopy, and the normal incidence x-ray standing wave technique, we assess the crystalline quality and coverage of the graphene layer. Combined with the presence of a covalently bound graphene layer in the conventional orientation underneath, our surfactant-mediated growth offers an ideal platform to prepare epitaxial twisted bilayer graphene via intercalation.

Wet-Chemical Synthesis and Applications of Semiconductor Nanomaterial-Based Epitaxial Heterostructures

Semiconductor nanomaterial-based epitaxial heterostructures with precisely controlled compositions and morphologies are of great importance for various applications in optoelectronics, thermoelectrics, and catalysis. Until now, various kinds of epitaxial heterostructures have been constructed. In this minireview, we will first introduce the synthesis of semiconductor nanomaterial-based epitaxial heterostructures by wet-chemical methods. Various architectures based on different kinds of seeds or templates are illustrated, and their growth mechanisms are discussed in detail. Then, the applications of epitaxial heterostructures in optoelectronics, catalysis, and thermoelectrics are described. Finally, we provide some challenges and personal perspectives for the future research directions of semiconductor nanomaterial-based epitaxial heterostructures.

Millimeter-Scale Growth of Single-Oriented Graphene on a Palladium Silicide Amorphous Film

It is widely accepted in condensed matter physics and material science communities that a single-oriented overlayer cannot be grown on an amorphous substrate because the disordered substrate randomizes the orientation of the seeds, leading to polycrystalline grains. In the case of two-dimensional materials such as graphene, the large-scale growth of single-oriented materials on an amorphous substrate has remained unsolved. Here, we demonstrate experimentally that the presence of uniformly oriented graphene seeds facilitates the growth of millimeter-scale single-oriented graphene with 3 × 4 mm² on palladium silicide, which is an amorphous thin film, where the uniformly oriented graphene seeds were epitaxially grown. The amorphous palladium silicide film promotes the growth of the single-oriented growth of graphene by causing carbon atoms to be diffusive and mobile within and on the substrate. In contrast to these results, without the uniformly oriented seeds, the amorphous substrate leads to the growth of polycrystalline graphene grains. This millimeter-scale single-oriented growth from uniformly oriented seeds can be applied to other amorphous substrates.

Recent progress in synthesis, properties, and applications of hexagonal boron nitride-based heterostructures

Featuring an absence of dangling bonds, large band gap, low dielectric constant, and excellent chemical inertness, atomically thin hexagonal boron nitride (h-BN) is considered an ideal candidate for integration with graphene and other 2D materials. During the past years, great efforts have been devoted to the research of h-BN-based heterostructures, from fundamental study to practical applications. In this review we summarize the recent progress in the synthesis, novel properties, and potential applications of h-BN-based heterostructures, especially the synthesis technique. Firstly, various approaches to the preparation of both in-plane and vertically stacked h-BN-based heterostructures are introduced in detail, including top-down strategies associated with exfoliation transfer processes and bottom-up strategies such as chemical vapor deposition (CVD)-based growth. Secondly, we discuss some novel properties arising in these heterostructures. Several promising applications in electronic and optoelectronic devices are also reviewed. Finally, we discuss the main challenges and possible research directions in this field.

Dirac electrons in a dodecagonal graphene quasicrystal

Quantum states of quasiparticles in solids are dictated by symmetry. We have experimentally demonstrated quantum states of Dirac electrons in a two-dimensional quasicrystal without translational symmetry. A dodecagonal quasicrystalline order was realized by epitaxial growth of twisted bilayer graphene rotated exactly 30°. We grew the graphene quasicrystal up to a millimeter scale on a silicon carbide surface while maintaining the single rotation angle over an entire sample and successfully isolated the quasicrystal from a substrate, demonstrating its structural and chemical stability under ambient conditions. Multiple Dirac cones replicated with the 12-fold rotational symmetry were observed in angle-resolved photoemission spectra, which revealed anomalous strong interlayer coupling with quasi-periodicity. Our study provides a way to explore physical properties of relativistic fermions with controllable quasicrystalline orders.

Synthesis, structure and applications of graphene-based 2D heterostructures

With the profuse amount of two-dimensional (2D) materials discovered and the improvements in their synthesis and handling, the field of 2D heterostructures has gained increased interest in recent years. Such heterostructures not only overcome the inherent limitations of each of the materials, but also allow the realization of novel properties by their proper combination. The physical and mechanical properties of graphene mean it has a prominent place in the area of 2D heterostructures. In this review, we will discuss the evolution and current state in the synthesis and applications of graphene-based 2D heterostructures. In addition to stacked and in-plane heterostructures with other 2D materials and their potential applications, we will also cover heterostructures realized with lower dimensionality materials, along with intercalation in few-layer graphene as a special case of a heterostructure. Finally, graphene heterostructures produced using liquid phase exfoliation techniques and their applications to energy storage will be reviewed.

In-situ Liquid Phase Epitaxy: Another Strategy to Synthesize Heterostructured Core-shell Composites

Core-shell Nb2O5/TiO2 composite with hierarchical heterostructure is successfully synthesized In-situ by a facile template-free and acid-free solvothermal method based on the mechanism of liquid phase epitaxy. The chemical circumstance change induced by the alcoholysis of NbCl5 is utilized tactically to trigger core-shell assembling In-situ. The tentative mechanism for the self-assembling of core-shell structure and hierarchical structure is explored. The microstructure and morphology changes during synthesis process are investigated systematically by using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. The dramatic alcoholysis of NbCl5 has been demonstrated to be the fundamental factor for the formation of the spherical core, which changes the acid circumstance of the solution and induces the co-precipitation of TiO2. The homogeneous co-existence of Nb2O5/TiO2 in the core and the co-existence of Nb/Ti ions in the reaction solution facilitate the In-situ nucleation and epitaxial growth of the crystalline shell with the same composition as the core. In-situ liquid phase epitaxy can offer a different strategy for the core-shell assembling for oxide materials.

Two-dimensional B-C-O alloys: a promising class of 2D materials for electronic devices

Graphene, a superior 2D material with high carrier mobility, has limited application in electronic devices due to zero band gap. In this regard, boron and nitrogen atoms have been integrated into the graphene lattice to fabricate 2D semiconducting heterostructures. It is an intriguing question whether oxygen can, as a replacement of nitrogen, enter the sp2 honeycomb lattice and form stable B-C-O monolayer structures. Here we explore the atomic structures, energetic and thermodynamic stability, and electronic properties of various 2D B-C-O alloys using first-principles calculations. Our results show that oxygen can be stably incorporated into the graphene lattice by bonding with boron. The B and O species favor forming alternate patterns into the chain- or ring-like structures embedded in the pristine graphene regions. These B-C-O hybrid sheets can be either metals or semiconductors depending on the B : O ratio. The semiconducting (B2O)nCm and (B6O3)nCm phases exist under the B- and O-rich conditions, and possess a tunable band gap of 1.0-3.8 eV and high carrier mobility, retaining ∼1000 cm2 V(-1) s(-1) even for half coverage of B and O atoms. These B-C-O alloys form a new class of 2D materials that are promising candidates for high-speed electronic devices.

Realizing diverse electronic and magnetic properties in hybrid zigzag BNC nanoribbons via hydrogenation

By means of first-principles DFT computations, we systematically investigate the geometries, stabilities, electronic and magnetic properties of fully and partially hydrogenated zigzag BNC nanoribbons (fH-zBNCNRs and pH-zBNCNRs) with interfacial N-C or B-C connections. It is revealed that in the lowest-lying configuration of hybrid fH-zBNCNRs, the constituent C and BN segments can possess respective chair and boat conformations and both of them are connected by the chair mode, independent of the N-C/B-C interface. Changing the ribbon width and the ratio of BN to C can endow these fH-zBNCNR systems with abundant electronic and magnetic properties involving nonmagnetic (NM) semiconductivity, ferromagnetic (FM) metallicity, antiferromagnetic (AFM) metallicity as well as AFM half-metallicity. Besides, manipulating the hydrogenation pattern and ratio can also result in rich electronic and magnetic behaviors in pH-zBNCNRs, where NM semiconductivity, AFM semiconductivity, AFM metallicity and even AFM spin gapless semiconductor are observed. Additionally, the origin of the magnetism in these hydrogenated zBNCNRs is analyzed in detail. Finally, all of these hydrogenated BNC structures can possess a favorable formation energy, large binding energy per hydrogen atom and high thermal stability, indicating the great possibility of their experimental realization by hydrogenating pristine zBNCNRs. These valuable insights can be advantageous for promoting hybrid BNC-based nanomaterials in the applications of spintronics and multifunctional nanodevices.

Boron nitride ceramics from molecular precursors: synthesis, properties and applications

Hexagonal boron nitride (h-BN) attracts considerable interest particularly when it is prepared from borazine-based single-source precursors through chemical routes suitable for the shaping and the nanostructuration of the final ceramic.

Establishing the pivotal role of local aromaticity in the electronic properties of boron-nitride graphene lateral hybrids

Within an attempt to unravel the conundrum of irregular bandgap variations in hybrids of white-graphene (hBN) and graphene (G) observed in both experiment and theory, strong proofs about the decisive role of aromaticity in their electronic properties are brought to light.

An Atomistic Tomographic Study of Oxygen and Hydrogen Atoms and their Molecules in CVD Grown Graphene

The properties and growth processes of graphene are greatly influenced by the elemental distributions of impurity atoms and their functional groups within or on the hexagonal carbon lattice. Oxygen and hydrogen atoms and their functional molecules (OH, CO, and CO2 ) positions' and chemical identities are tomographically mapped in three dimensions in a graphene monolayer film grown on a copper substrate, at the atomic part-per-million (atomic ppm) detection level, employing laser assisted atom-probe tomography. The atomistic plan and cross-sectional views of graphene indicate that oxygen, hydrogen, and their co-functionalities, OH, CO, and CO2 , which are locally clustered under or within the graphene lattice. The experimental 3D atomistic portrait of the chemistry is combined with computational density-functional theory (DFT) calculations to enhance the understanding of the surface state of graphene, the positions of the chemical functional groups, their interactions with the underlying Cu substrate, and their influences on the growth of graphene.

Seamless lamination of a concave-convex architecture with single-layer graphene

Graphene has been used as an electrode and channel material in electronic devices because of its superior physical properties. Recently, electronic devices have changed from a planar to a complicated three-dimensional (3D) geometry to overcome the limitations of planar devices. The evolution of electronic devices requires that graphene be adaptable to a 3D substrate. Here, we demonstrate that chemical-vapor-deposited single-layer graphene can be transferred onto a silicon dioxide substrate with a 3D geometry, such as a concave-convex architecture. A variety of silicon dioxide concave-convex architectures were uniformly and seamlessly laminated with graphene using a thermal treatment. The planar graphene was stretched to cover the concave-convex architecture, and the resulting strain on the curved graphene was spatially resolved by confocal Raman spectroscopy; molecular dynamic simulations were also conducted and supported the observations. Changes in electrical resistivity caused by the spatially varying strain induced as the graphene-silicon dioxide laminate varies dimensionally from 2D to 3D were measured by using a four-point probe. The resistivity measurements suggest that the electrical resistivity can be systematically controlled by the 3D geometry of the graphene-silicon dioxide laminate. This 3D graphene-insulator laminate will broaden the range of graphene applications beyond planar structures to 3D materials.