Single Crystalline Domains Research Articles

Structural characterization of SiC nanoparticles**Project supported by the National Natural Science Foundation of China (No. 11505273) and the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA02000000).

The structure and size of SiC nanoparticles were studied by different characterization methods including small angle X-ray scattering (SAXS), transmission electron microscope (TEM), and X-ray diffraction (XRD). The results showed that particle size distributions determined respectively from SAXS and TEM are comparable and follow the log-normal function. The size distribution of the particles is between 10 to 100 nm with most of them being in the range of 20–50 nm. The average particle size is around 42 nm. XRD identifies the phase of the SiC nanoparticles and suggests the average size of the single crystalline domain to be around 21 nm. The combined results from XRD and SAXS suggest the existence of many polycrystals, which is confirmed by the HRTEM observation of particles with twins and stacking faults. The material synthesis methods leading to various particle sizes are also discussed.

Van Der Waals Hybrid Perovskite of High Optical Quality by Chemical Vapor Deposition

Abstract2D hybrid perovksite (RNH3)2PbX4 materials not only serve as ideal platforms to study fundamental physics such as polariton dynamics but also show promise for optoelectronic and electro‐optic applications. However, for the preparation of high optical quality crystals, mechanical exfoliation has to be applied in the past. In this work, the vapor phase growth of single crystalline (C4H9NH3)2PbI4 flakes with high optical quality is reported. Individual single crystalline domains show lateral size about 5–10 µm with defined rectangular shape. Spectroscopic studies show room temperature photoluminescence full width at half maximum of 70 meV and decay lifetime of several nanoseconds, indicating comparably high quality with mechanically exfoliated counterparts. Exciton binding energy 279 ± 46 meV and electron–phonon coupling strength around 20 meV are revealed. This vacuum‐based method may provide a solution for integrating layered perovskites into optoelectronic devices and systems.

Epitaxial Growth of C60 on Rubrene Single Crystals for a Highly Ordered Organic Donor/Acceptor Interface

The highly ordered epitaxial growth of C60 films on rubrene single crystals was demonstrated. The C60 crystals growth commensurate with rubrene (001) surface lattice was confirmed by reflection high energy electron diffraction and X-ray diffraction and grazing incident wide-angle X-ray scattering. Depending on growth conditions, several surface morphologies (rounded, hexagonal, and triangular grains) of C60 grains were observed at the initial growth process. Large grains with layer-by-layer step and terrace structure of C60 were observed at the terrace region of rubrene (001) surface. The growth of high-crystallinity C60 films was achieved at high substrate temperature and slow deposition rate. Long migration length on the substrate which was given by enough substrate temperature enabled the formation of large single crystalline domains.

Suppressing Nucleation in Metal–Organic Chemical Vapor Deposition of MoS2 Monolayers by Alkali Metal Halides

Toward the large-area deposition of MoS2 layers, we employ metal-organic precursors of Mo and S for a facile and reproducible van der Waals epitaxy on c-plane sapphire. Exposing c-sapphire substrates to alkali metal halide salts such as KI or NaCl together with the Mo precursor prior to the start of the growth process results in increasing the lateral dimensions of single crystalline domains by more than 2 orders of magnitude. The MoS2 grown this way exhibits high crystallinity and optoelectronic quality comparable to single-crystal MoS2 produced by conventional chemical vapor deposition methods. The presence of alkali metal halides suppresses the nucleation and enhances enlargement of domains while resulting in chemically pure MoS2 after transfer. Field-effect measurements in polymer electrolyte-gated devices result in promising electron mobility values close to 100 cm2 V-1 s-1 at cryogenic temperatures.

Understanding the Reaction Kinetics to Optimize Graphene Growth on Cu by Chemical Vapor Deposition

AbstractUnderstanding and controlling the growth kinetics of graphene is a prerequisite to synthesize this highly wanted material by chemical vapor deposition on Cu, e.g. for the construction of ultra‐stable electron transparent membranes. It is reviewed that Cu foils contain a considerable amount of carbon in the bulk which significantly exceeds the expected amount of thermally equilibrated dissolved carbon in Cu and that this carbon must be removed before any high quality graphene may be grown. Starting with such conditioned Cu foils, systematic studies of the graphene growth kinetics in a reactive CH4/H2 atmosphere allow to extract the following meaningful data: prediction of the equilibrium constant of the graphene formation reaction within a precision of a factor of two, the confirmation that the graphene growth proceeds from a C(ad)‐phase on Cu which is in thermal equilibrium with the reactive gas phase, its apparent activation barrier and finally the prediction of the achievable growth velocity of the growing graphene flakes during chemical vapor deposition. As a result of the performed study, growth parameters are identified for the synthesis of high quality monolayer graphene with single crystalline domains of 100–1000 μm in diameter within a reasonable growth time.

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.

Electronic structure of polycrystalline CVD-graphene revealed by Nano-ARPES

The ability to explore electronic structure and their role in determining material’s macroscopic behaviour is essential to explain and engineer functions of material and device. Since its debut in 2004, graphene has attracted global research interest due to its unique properties. Chemical vapor deposition (CVD) has emerged as an important method for the massive preparation and production of graphene for various applications. Here by employing angle-resolved photoemission spectroscopy with nanoscale spatial resolution ∼ 100 nm (Nano-ARPES), we describe the approach to measure the electronic structure of polycrystalline graphene on copper foils, demonstrating the power of Nano-ARPES to detect the electronic structure of microscopic single crystalline domains, being fully compatible with conventional ARPES. Similar analysis could be employed to other microscopic materials

Initial stage of hexagonal boron nitride growth in diffusion and precipitation method

This study investigated the initial stage of hexagonal boron nitride (h-BN) formation on a Ni plate by the diffusion and precipitation method. Regular triangle-shaped domains with two orientations were observed on several Ni faces, as commonly observed in the chemical vapor deposition method. On (213) surfaces, strained triangle-shaped domains with unique orientation were observed, suggesting the possible formation of large single-crystalline domains. Moreover, stripe-shaped h-BN with lengths comparable to Ni grain size (∼100 µm) was formed along the direction on (111) surfaces. Our results show that boron stripes are first formed and the following nitridation converts them into h-BN stripes.

Molecular beam epitaxy of 2D-layered gallium selenide on GaN substrates

Large area epitaxy of two-dimensional (2D) layered materials with high material quality is a crucial step in realizing novel device applications based on 2D materials. In this work, we report high-quality, crystalline, large-area gallium selenide (GaSe) films grown on bulk substrates such as c-plane sapphire and gallium nitride (GaN) using a valved cracker source for Se. (002)-Oriented GaSe with random in-plane orientation of domains was grown on sapphire and GaN substrates at a substrate temperature of 350–450 °C with complete surface coverage. Higher growth temperature (575 °C) resulted in the formation of single-crystalline ε-GaSe triangular domains with six-fold symmetry confirmed by in-situ reflection high electron energy diffraction and off-axis x-ray diffraction. A two-step growth method involving high temperature nucleation of single crystalline domains and low temperature growth to enhance coalescence was adopted to obtain continuous (002)-oriented GaSe with an epitaxial relationship with the substrate. While six-fold symmetry was maintained in the two step growth, β-GaSe phase was observed in addition to the dominant ε-GaSe in cross-sectional scanning transmission electron microscopy images. This work demonstrates the potential of growing high quality 2D-layered materials using molecular beam epitaxy and can be extended to the growth of other transition metal chalcogenides.

Shape-dependent ordering of gold nanocrystals into large-scale superlattices

Self-assembly of individual building blocks into highly ordered structures, analogous to spontaneous growth of crystals from atoms, is a promising approach to realize the collective properties of nanocrystals. Yet the ability to reliably produce macroscopic assemblies is unavailable and key factors determining assembly quality/yield are not understood. Here we report the formation of highly ordered superlattice films, with single crystalline domains of up to half a millimetre in two dimensions and thickness of up to several microns from nanocrystals with tens of nanometres in diameter. Combining experimental and computational results for gold nanocrystals in the shapes of spheres, cubes, octahedra and rhombic dodecahedra, we investigate the entire self-assembly process from disordered suspensions to large-scale ordered superlattices induced by nanocrystal sedimentation and eventual solvent evaporation. Our findings reveal that the ultimate coherence length of superlattices strongly depends on nanocrystal shape. Factors inhibiting the formation of high-quality large-scale superlattices are explored in detail.

Visualizing fast growth of large single-crystalline graphene by tunable isotopic carbon source

The fast growth of large single-crystalline graphene by chemical vapor deposition on Cu foil remains a challenge for industrial-scale applications. To achieve the fast growth of large single-crystalline graphene, understanding the detailed dynamics governing the entire growth process—including nucleation, growth, and coalescence—is important; however, these remain unexplored. In this study, by using a pulsed carbon isotope labeling technique in conjunction with micro-Raman spectroscopy identification, we visualized the growth dynamics, such as nucleation, growth, and coalescence, during the fast growth of large single-crystalline graphene domains. By tuning the supply of the carbon source, a growth rate of 320 μm/min and the growth of centimeter-sized graphene single crystals were achieved on Cu foil.

Carrier Transport Mechanism in Single Crystalline Organic Semiconductor Thin Film Elucidated by Visualized Carrier Motion.

Time-resolved microscopic second harmonic generation (TRM-SHG) measurement was conducted to evaluate temperature dependence of the anisotropic carrier transport process in 6,13-Bis(triisopropylsilylethynyl) (TIPS) pentacene single crystalline domains for two orthogonal directions. Enhancement of the electric field induced SHG (EFI-SHG) signal at the electrode edge at low temperature suggests the presence of potential drop in the injection process. We directly evaluated temperature dependence of the carrier mobility by taking into account the potential drop, and concluded that the Marcus theory is appropriate to interpret the carrier transport in anisotropic TIPS pentacene thin film. TRM-SHG method is a facile and effective way to directly visualize transport process in anisotropic materials and to evaluate injection and transport processes simultaneously.

Structural, Vibrational, and Thermal Properties of Nanocrystalline Graphene in Atomistic Simulations

Two different methods have been applied to create differently structured nanocrystalline graphene samples used in molecular dynamics simulations. In the first method, graphene sheets are generated by grain growth from individual nucleation seeds. The second method applies Voronoi tessellation to define single crystalline domains in the simulation cells. The differently generated nanocrystalline graphene sheets show significant variations in the grain size distribution and the shape of the crystalline domains. Furthermore, out-of-plane corrugation is found to be more pronounced in samples generated by the Voronoi method, in particular for small grain sizes (≤14 nm). Marginal differences are observed in the distribution of polygonal rings in the grain boundaries which might result from the geometrical shape of the grain boundaries. Thermal conductivity has been determined using the approach-to-equilibrium molecular dynamics formalism. A lower thermal conductivity is observed in Voronoi samples for grain siz...

Mesoscopic crystallographic textures on shells of a hyaline radial foraminifer Ammonia beccarii

Most foraminifera, simple unicellular protists, create calcareous shells with precisely controlled architectures. We investigated the crystal fabric of porous shells of a hyaline radial foraminifer, Ammonia beccarii. The complex structures of the calcitic shell include micrometric single-crystalline domains around a straight pore ∼2 μm in diameter which are divided by strained intermediate regions. The c axes of calcite in the domains were almost perpendicular to the shell surface. The mesoscopic textures, such as periodic lateral lines with a ∼200–450 nm interval and vertical cracks with a ∼150 nm interval, were observed throughout the single-crystalline domains in the shell wall. Specific mechanical properties, such as a lower Young's modulus and a higher hardness of the shells than those of Iceland spar, are ascribed to the presence of the specific mesoscopic textures.

Atomically precise semiconductor--graphene and hBN interfaces by Ge intercalation.

The full exploration of the potential, which graphene offers to nanoelectronics requires its integration into semiconductor technology. So far the real-world applications are limited by the ability to concomitantly achieve large single-crystalline domains on dielectrics and semiconductors and to tailor the interfaces between them. Here we show a new direct bottom-up method for the fabrication of high-quality atomically precise interfaces between 2D materials, like graphene and hexagonal boron nitride (hBN), and classical semiconductor via Ge intercalation. Using angle-resolved photoemission spectroscopy and complementary DFT modelling we observed for the first time that epitaxially grown graphene with the Ge monolayer underneath demonstrates Dirac Fermions unaffected by the substrate as well as an unperturbed electronic band structure of hBN. This approach provides the intrinsic relativistic 2D electron gas towards integration in semiconductor technology. Hence, these new interfaces are a promising path for the integration of graphene and hBN into state-of-the-art semiconductor technology.

Fast growth of inch-sized single-crystalline graphene from a controlled single nucleus on Cu-Ni alloys.

Wafer-scale single-crystalline graphene monolayers are highly sought after as an ideal platform for electronic and other applications. At present, state-of-the-art growth methods based on chemical vapour deposition allow the synthesis of one-centimetre-sized single-crystalline graphene domains in ∼12 h, by suppressing nucleation events on the growth substrate. Here we demonstrate an efficient strategy for achieving large-area single-crystalline graphene by letting a single nucleus evolve into a monolayer at a fast rate. By locally feeding carbon precursors to a desired position of a substrate composed of an optimized Cu-Ni alloy, we synthesized an ∼1.5-inch-large graphene monolayer in 2.5 h. Localized feeding induces the formation of a single nucleus on the entire substrate, and the optimized alloy activates an isothermal segregation mechanism that greatly expedites the growth rate. This approach may also prove effective for the synthesis of wafer-scale single-crystalline monolayers of other two-dimensional materials.

Single Crystalline Film of Hexagonal Boron Nitride Atomic Monolayer by Controlling Nucleation Seeds and Domains.

A monolayer hexagonal boron nitride (h-BN) film with controllable domain morphology and domain size (varying from less than 1 μm to more than 100 μm) with uniform crystalline orientation was successfully synthesized by chemical vapor deposition (CVD). The key for this extremely large single crystalline domain size of a h-BN monolayer is a decrease in the density of nucleation seeds by increasing the hydrogen gas flow during the h-BN growth. Moreover, the well-defined shape of h-BN flakes can be selectively grown by controlling Cu-annealing time under argon atmosphere prior to h-BN growth, which provides the h-BN shape varies in triangular, trapezoidal, hexagonal and complex shapes. The uniform crystalline orientation of h-BN from different nucleation seeds can be easily confirmed by polarized optical microscopy (POM) with a liquid crystal coating. Furthermore, seamlessly merged h-BN flakes without structural domain boundaries were evidence by a selective hydrogen etching after a full coverage of a h-BN film was achieved. This seamless large-area and atomic monolayer of single crystalline h-BN film can offer as an ideal and practical template of graphene-based devices or alternative two-dimensional materials for industrial applications with scalability.

Surface-Induced Polymorphism as a Tool for Enhanced Dissolution: The Example of Phenytoin.

Polymorphism and morphology can represent key factors tremendously limiting the bioavailability of active pharmaceutical ingredients (API), in particular, due to solubility issues. Within this work, the generation of a yet unknown surface-induced polymorph (SIP) of the model drug, 5,5-diphenylimidazolidin-2,4-dion (phenytoin), is demonstrated in thin films through altering the crystallization kinetics and the solvent type. Atomic force microscopy points toward the presence of large single-crystalline domains of the SIP, which is in contrast to samples comprising solely the bulk phase, where extended dendritic phenytoin networks are observed. Grazing incidence X-ray diffraction reveals unit cell dimensions of the SIP significantly different from those of the known bulk crystal structure of phenytoin. Moreover, the aqueous dissolution performance of the new polymorph is benchmarked against a pure bulk phase reference sample. Our results demonstrate that the SIP exhibits markedly advantageous drug release performance in terms of dissolution time. These findings suggest that thin-film growth of pharmaceutical systems in general should be explored, where poor aqueous dissolution represents a key limiting factor in pharmaceutical applications, and illustrate the experimental pathway for determining the physical properties of a pharmaceutically relevant SIP.

Chemical vapor deposition growth of large-scale hexagonal boron nitride with controllable orientation

Chemical vapor deposition (CVD) synthesis of large-domain hexagonal boron nitride (h-BN) with a uniform thickness is very challenging, mainly due to the extremely high nucleation density of this material. Herein, we report the successful growth of wafer-scale, high-quality h-BN monolayer films that have large single-crystalline domain sizes, up to ~72 µm in edge length, prepared using a folded Cu-foil enclosure. The highly confined growth space and the smooth Cu surface inside the enclosure effectively reduced the precursor feeding rate together and induced a drastic decrease in the nucleation density. The orientation of the as-grown h-BN monolayer was found to be strongly correlated to the crystallographic orientation of the Cu substrate: the Cu (111) face being the best substrate for growing aligned h-BN domains and even single-crystalline monolayers. This is consistent with our density functional theory calculations. The present study offers a practical pathway for growing high-quality h-BN films by deepening our fundamental understanding of the process of their growth by CVD.

Van der Waals Epitaxial Growth of Two-Dimensional Single-Crystalline GaSe Domains on Graphene.

Two-dimensional (2D) van der Waals (vdW) heterostructures are a family of artificially structured materials that promise tunable optoelectronic properties for devices with enhanced functionalities. Compared to transferring, direct epitaxy of vdW heterostructures is ideal for clean interlayer interfaces and scalable device fabrication. Here we report the synthesis and preferred orientations of 2D GaSe atomic layers on graphene (Gr) by vdW epitaxy. GaSe crystals are found to nucleate predominantly on random wrinkles or grain boundaries of graphene, share a preferred lattice orientation with underlying graphene, and grow into large (tens of micrometers) irregularly shaped, single-crystalline domains. The domains are found to propagate with triangular edges that merge into the large single crystals during growth. Electron diffraction reveals that approximately 50% of the GaSe domains are oriented with a 10.5 ± 0.3° interlayer rotation with respect to the underlying graphene. Theoretical investigations of interlayer energetics reveal that a 10.9° interlayer rotation is the most energetically preferred vdW heterostructure. In addition, strong charge transfer in these GaSe/Gr vdW heterostructures is predicted, which agrees with the observed enhancement in the Raman E(2)1g band of monolayer GaSe and highly quenched photoluminescence compared to GaSe/SiO2. Despite the very large lattice mismatch of GaSe/Gr through vdW epitaxy, the predominant orientation control and convergent formation of large single-crystal flakes demonstrated here is promising for the scalable synthesis of large-area vdW heterostructures for the development of new optical and optoelectronic devices.