Epitaxial Crystal Growth Research Articles

Enabling Interfacial Lattice Matching by Selective Epitaxial Growth of CuS Crystals on Bi2WO6 Nanosheets for Efficient CO2 Photoreduction into Solar Fuels

Photocatalytic CO2 reduction serves as an important technology for value‐added solar fuel production; however, it is generally limited by interfacial charge transport. To address this limitation, a two‐dimensional/two‐dimensional (2D/2D) p‐n heterojunction CuS‐Bi2WO6 (CS‐BWO) with highly connected and matched interfacial lattices was designed via a two‐step hydrothermal tandem synthesis strategy. The integration of CuS with BWO created a robust interface electric field and provided fast charge transfer channels due to the work function difference, as well as highly connected and matched interfacial lattices. The p‐n heterojunction promoted electron transfer from the Cu to Bi sites, leading to coordination of Bi sites with high electronic density and low oxidation state. The Bi sites in BWO nanosheets facilitated the adsorption and activation of CO2, and generation of high‐coverage key intermediate b‐CO32‐, while broad light‐harvesting CuS (CS) provide abundant photoinduced electrons that were injected into the conduction band of BWO for CO2 photoreduction reaction. Remarkably, the p‐n heterojunction CS‐BWO exhibited CO and CH4 yields of 135.7 and 62.5 µmol g‐1, respectively, which were significantly higher than those of CS, BWO, and physical mixture CS‐BWO nanosheets. This work provided an innovative design strategy for developing high‐activity heterojunction photocatalyst for converting CO2 into value‐added solar fuels.

Enabling Interfacial Lattice Matching by Selective Epitaxial Growth of CuS Crystals on Bi2WO6 Nanosheets for Efficient CO2 Photoreduction into Solar Fuels.

Photocatalytic CO2 reduction serves as an important technology for value-added solar fuel production; however, it is generally limited by interfacial charge transport. To address this limitation, a two-dimensional/two-dimensional (2D/2D) p-n heterojunction CuS-Bi2WO6 (CS-BWO) with highly connected and matched interfacial lattices was designed via a two-step hydrothermal tandem synthesis strategy. The integration of CuS with BWO created a robust interface electric field and provided fast charge transfer channels due to the work function difference, as well as highly connected and matched interfacial lattices. The p-n heterojunctionpromotedelectron transfer from the Cu to Bi sites, leading to coordination of Bi sites with high electronic density and low oxidation state. The Bi sites in BWO nanosheets facilitated the adsorption and activation of CO2, and generation of high-coverage key intermediate b-CO32-, while broad light-harvesting CuS (CS)provide abundant photoinduced electrons that were injected into the conduction band of BWO for CO2 photoreduction reaction. Remarkably, the p-n heterojunction CS-BWO exhibited CO and CH4 yields of 135.7 and 62.5 µmol g-1, respectively, which were significantly higher than those of CS, BWO, and physical mixture CS-BWO nanosheets. This work provided an innovative design strategy for developing high-activity heterojunction photocatalyst for converting CO2into value-added solar fuels.

Host-guest charge transfer for scalable single crystal epitaxy of a metal-organic framework

Methods to grow large crystals provide the foundation for material science and technology. Here we demonstrate single crystal homoepitaxy of a metal-organic framework (MOF) built of zinc, acetate and terephthalate ions, that encapsulate arrays of octahedral zinc dimethyl sulfoxide (DMSO) complex cations within its one-dimensional (1D) channels. The three-dimensional framework is built of two-dimensional Zn-terephthalate square lattices interconnected by anionic acetate pillars through diatomic zinc nodes. The charge of the anionic framework is neutralized by the 1D arrays of Zn(DMSO)62+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\\rm{Zn}}}{({{\\rm{DMSO}}})}_{6}^{2+}$$\\end{document} cations that fill every second 1D channel of the framework. It is demonstrated that the repeatable and scalable epitaxy allows square cuboids of this charge-transfer MOF to grow stepwise to sizes in the centimeter range. The continuous growth with no size limits can be attributed to the ionic nature of the anionic framework with cationic 1D molecular fillers. These findings pave the way for epitaxial growth of bulk crystals of MOFs.

Epitaxial growth of excitonic single crystals and heterostructures: Oxides and nitrides

Epitaxial growth of excitonic single crystals and heterostructures: Oxides and nitrides

Interfacial epitaxy of multilayer rhombohedral transition-metal dichalcogenide single crystals.

Rhombohedral-stacked transition-metal dichalcogenides (3R-TMDs), which are distinct from their hexagonal counterparts, exhibit higher carrier mobility, sliding ferroelectricity, and coherently enhanced nonlinear optical responses. However, surface epitaxial growth of large multilayer 3R-TMD single crystals is difficult. We report an interfacial epitaxy methodology for their growth of several compositions, including molybdenum disulfide (MoS2), molybdenum diselenide, tungsten disulfide, tungsten diselenide, niobium disulfide, niobium diselenide, and molybdenum sulfoselenide. Feeding of metals and chalcogens continuously to the interface between a single-crystal Ni substrate and grown layers ensured consistent 3R stacking sequence and controlled thickness from a few to 15,000 layers. Comprehensive characterizations confirmed the large-scale uniformity, high crystallinity, and phase purity of these films. The as-grown 3R-MoS2 exhibited room-temperature mobilities up to 155 and 190 square centimeters per volt second for bi- and trilayers, respectively. Optical difference frequency generation with thick 3R-MoS2 showed markedly enhanced nonlinear response under a quasi-phase matching condition (five orders of magnitude greater than monolayers).

Mixed‐Dimensional All‐Organic Polymer Heterostructures with Enhanced Piezoelectricity

AbstractEnhancing the piezoelectricity of polymers while maintaining all components organic is still challenging but significantly important for developing flexible wearable energy harvesters and self‐powered devices. Here, a novel and versatile strategy is introduced to construct mixed‐dimensional all‐organic polymer heterostructures (MPHs) for enhanced piezoelectricity. By combining all‐polymer 1D nanofibers (NFs) with 2D crystals through epitaxial crystallization‐driven assembly (ECA), MPHs are engineered to capitalize on the synergistic effects of both dimensional nanostructures. The intrinsic piezoelectric activity of 1D NFs is amplified by the growth of 2D crystals, enhancing force‐sensitivity and overall piezoelectricity. The mixed‐dimensional assembly not only enables controlled in‐situ growth of MPHs, but also simultaneously induces preferential formation of electroactive phases through solvent‐induced phase transition. By modulating the epitaxial growth of 2D crystals on 1D NFs, effective tuning of MPH growth amount and morphology is achieved, resulting in significant improvements in deformability, dipole polarization, and durability. MPHs exhibit remarkable piezoelectric improvements, achieving higher output under lower‐level forces with a record‐high sensitivity of ≈670 mV kPa−1. Their superior responsivity enables the development of self‐powered wireless wearable motion‐monitoring systems for real‐time physiological movement detection and analysis. This work inspires the development of all‐organic piezoelectric devices for innovative flexible energy‐harvesting and sensing applications.

Research on the effect of remelting on the epitaxial growth process of single crystal superalloy

In this paper, the remelting process is applied to Laser Directed Energy Deposition (L-DED) to repair single crystal superalloys. The main formation modes of stray grains in single crystal epitaxial growth were analyzed, and the inhibitory effect of remelting process on stray grains in single crystal epitaxial growth using L-DED was studied. Comparative experiments between remelting and non-remelting processes have been conducted. Three process parameters in the remelting process: power, z-axis lift and cladding-remelting interval time were studied. The impact of different remelting process parameters on single crystal epitaxial growth and the formation of stray grains has been studied. Remelting has been shown to effectively prevent the formation of stray grains during the solidification of the melt pool, which are otherwise caused by the presence of unmelted powder during the powder feeding phase of the L-DED process. A cladding layer without stray grains on both sides is obtained. After remelting, the epitaxial growth structure can achieve larger cellular and columnar dendritic zones and smaller primary dendrite arms. A favorable epitaxial growth structure was obtained in this paper, under the optimized remelting process window.

Analysis of large-scale high-quality graphene production and applications

Graphene, a single-atom-thick layer of carbon atoms arranged in a hexagonal lattice, is the thinnest and strongest material known to mankind. It has excellent electrical, thermal, and mechanical properties, making it a promising material for a wide range of applications in electronics, optoelectronics, energy storage, and more. With the increasing demand for graphene in various applications, large-scale and high-quality graphene production has become a significant challenge. While early methods of graphene production involved mechanical exfoliation, this method is limited in terms of scalability and yield. To meet the increasing demand for large-scale production of graphene, various methods have been developed in recent years, including chemical vapor precipitation, epitaxial crystal growth, graphene oxide reduction and solvent exfoliation and so on. This study aims to introduce several existing methods for the mass production of graphene with high quality and analyzes the advantages and disadvantage involve thereof. The findings in this paper may provide a valuable reference for the industrial-scale production of graphene.

Elastic and breaking properties of epitaxial face‐centered crystals in neutron star crusts and white dwarf cores

AbstractCrystallization of dense matter in neutron star crusts and white dwarf cores may be similar to epitaxial crystal growth in terrestrial laboratories. However, in stellar crystals, the spacing between horizontal planes has to gradually increase with the outward movement of the crystallization front, tracing decrease of the electron density. This process produces Coulomb crystals with stretched rather than cubic elementary cells. We extend the analysis of the elastic and breaking properties of such crystals to the face‐centered (fc) lattice. Shear deformations orthogonal to the stretch direction have been studied for 22 crystallographic shear planes. A common property for all these planes is the reduction and eventual nulling of the breaking shear strain with deviation from the unstretched configuration. The effective shear moduli for deformations orthogonal to the stretch direction have been calculated. It is possible that the epitaxial crystallization in compact stars results in a formation of large‐scale crystallites or, at least, in growth of the whole crystallization front perpendicular to particular crystallographic planes. For fc structure growth orthogonal to the planes, we expect that, at any density, () of crystallite height is occupied by layers one (two) orders of magnitude weaker than the bulk of the crystallite. This may be important for realistic modeling of crustquakes on neutron stars.

Self‐assembly of supramolecular monomers into 2D twinned crystals on mica surface

AbstractDespite the widespread use of microscopy techniques to characterize supramolecular polymers, the role of surfaces in the formation of these structures is unclear. Here we describe the epitaxial growth of intriguing 2D crystals of alkyl benzene‐1,3,5‐triscarboxamides formed by competitive attractive interactions with the mica surface. In contrast to the fibrous structure observed in apolar diluted solution, five‐fold twinned crystals or ribbon‐like structures are obtained on the mica surface. Moreover, these structures are still dynamic on the surface, being able to reorganize during the atomic force microscopy measurements. This work demonstrates that surface‐monomer interactions can be exploited to modulate the structure of 1D supramolecular polymers into 2D crystals.

Numerical simulation of temperature gradient effects on gallium nitride crystal growth by sodium-flux method

During the growth of gallium nitride single crystals by sodium-flux method, temperature significantly impacts crystal quality. In this study, the mechanism of the effect of different temperature gradients on crystal growth is analyzed in depth using a combination of numerical simulation and experiment. The experimental results show that epitaxial growth of crystals occurs under positive temperature gradient conditions, while there is dissolution of seed crystals under negative temperature gradient conditions. The temperature, flow, and concentration data of the melted material during crystal growth were calculated using numerical simulation. The simulation findings reveal that the distribution of solution supersaturation varies according to temperature. High supersaturation at the bottom of the melt is favorable for crystal epitaxial growth on the surface of seed crystals under a positive temperature gradient. Meanwhile, low supersaturation at the top of the melt suppresses the hard polycrystalline layer here. Under negative temperature gradient conditions, low supersaturation at the bottom of the melt may lead to remelting of seed crystals, which is consistent with the experimental phenomenon. Furthermore, we propose an optimized heat source profile. This profile manages high supersaturation near seed crystals, aiding continuous growth. Finally, we have applied the curve in an applied way by proposing a multi-stage heating device, based on which the desired arbitrary temperature profile can be modulated. This research has broad applications in a variety of crystal growth experiments using fluid as the mother phase.

Biomimetic Self-Maturation Mineralization System for Enamel Repair.

Enamel repair is crucial for restoring tooth function and halting dental caries. However, contemporary research often overlooks the retention of organic residues within the repair layer, which hinders the growth of dense crystals and compromises the properties of the repaired enamel. During the maturation of natural enamel, the organic matrix undergoes enzymatic processing to facilitate further crystal growth, resulting in a highly mineralized tissue. Inspired by this process, a biomimetic self-maturation mineralization system is developed, comprising ribonucleic acid-stabilized amorphous calcium phosphate (RNA-ACP) and ribonuclease (RNase). The RNA-ACP induces initial mineralization in the form of epitaxial crystal growth, while the RNase present in saliva automatically triggers a biomimetic self-maturation process. The mechanistic study further indicates that RNA degradation prompts conformational rearrangement of the RNA-ACP, effectively excluding the organic matter introduced earlier. This exclusion process promotes lateral crystal growth, resulting in the generation of denser enamel-like apatite crystals that are devoid of organic residues. This strategy of eliminating organic residues from enamel crystals enhances the mechanical and physiochemical properties of the repaired enamel. The present study introduces a conceptual biomimetic mineralization strategy for effective enamel repair in clinical practice and offers potential insights into the mechanisms of biomineral formation.

Liquid-phase epitaxy of neutron star crusts and white dwarf cores

ABSTRACT Near-equilibrium bottom-up crystallization of fully ionized neutron star crusts or white dwarf cores is considered. We argue that this process is similar to liquid-phase epitaxial (i.e. preserving order of previous layers) crystal growth or crystal pulling from melt in Earth laboratories whereby lateral positions of newly crystallizing ions are anchored by already solidified layers. Their vertical positions are set by charge neutrality. Consequently, interplane spacing of a growing crystal either gradually increases, tracing ne decrease, as the crystallization front moves away from the stellar centre, or decreases, tracing decrease of 〈Z〉, when the crystallization front crosses a boundary between layers of different compositions. This results in the formation of stretched Coulomb crystals, in contrast to the standard assumption of cubic crystal formation, which is based on energetics arguments but does not take into account growth kinetics. Overstretched crystals break, which limits the vertical sizes of growing crystallites. We study breaking shear strain and effective shear modulus of stretched matter and discuss possibility of macrocrystallite formation. The latter has interesting astrophysical implications, for instance, appearance of weak crustal layers, whose strength may increase by a few orders of magnitude upon breaking and refreezing at a late-time event. We also analyse interaction of adjacent Coulomb crystals, having different ion compositions, and estimate the strength of such interfaces.

Progress in the Application of Biomimetic Mineralization for Tooth Repair

The tooth, including enamel and dentin, is a prominent biomineral that is produced by the biomineralization of living organisms. Although the mechanical performance of the tooth is outstanding, caries easily develop in a complex oral environment. The analysis of the chemical composition and the relationship between the mechanical properties and the structure is of great importance in solving caries. In this review, the multilevel structure and mechanical properties of enamel and dentin are briefly introduced, along with caries formation and the limitations of clinical dental restoration. Furthermore, the progress of the application of a wide range of biomimetic strategies for tooth remineralization is highlighted, including the use of calcium phosphate ionic clusters to construct the mineralization front, ensuring the oriented epitaxial growth of enamel crystals and replicating the complex structure of the enamel. Moreover, compared with the current clinical treatment, in which the resin composite and glass ionomer cement are the main repair materials and the high incidence of secondary caries leads to imperfect restorations, the remineralization tactics could achieve excellent repair effectiveness in reconstructing the complicated structure, restoring mechanical strength and gaining permanent repair. A basic understanding of enamel and dentin, their potential for restoration, and hopeful prospects for tooth repair that can be applied in the clinical setting, not just in the laboratory, is provided by this review.

Permeability evolution in open fractures during precipitation and dissolution - A phase-field study

In dilated fractures in the Earth’s crust fluid flow in combination with precipitation or dissolution processes can occur, which in turn influences the mechanical and transport properties of the rock system. We use a phase-field modeling framework to investigate these processes of epitaxial crystal growth and dissolution on the fracture walls of crystalline rock systems on microscale. Fluid flow simulations are performed and analyzed during intermediate crystallization and dissolution stages and the obtained hydraulic properties of the partly open fractures are compared to existing literature. The systematic simulation studies show how the rock properties are affected by factors as mineral type with different crystal morphologies, fracture type (inter- vs. transgranular), aperture of the open fracture, and presence of accessory minerals. The results indicate that within the considered parameter space the flow paths remain open until late stages of fracture sealing. Moreover, the long-term permeability and porosity evolution is strongly affected by fracture surface heterogeneities and initial fracture apertures. The simulations enable insights into the evolution of microstructural and fluid flow characteristics and can lay the basis for applications in fractured porous media as groundwater protection, geothermal and hydrocarbon reservoir prediction, water recovery, or storing H2 or CO2 in the subsurface.

A wide ultraviolet spectra response photodetector based on epitaxial growth of highly-oriented ε-Ga2O3 crystal on diamond substrate

In recent years, diamond has shown great potential in solar-blind ultraviolet (UV) photodetection due to its ultrawide bandgap (∼ 5.5 eV) and other superior semiconductor properties. However, the response region of diamond photodetector is usually smaller than 230 nm, which cannot cover the whole solar-blind region from 200 to 280 nm. In this work, an ε-Ga2O3/diamond photodetector with wide spectra responsivity from 210 (or even lower) to 260 nm was fabricated. High-quality ε-Ga2O3 film with columnar crystal was epitaxially grown on single crystalline CVD diamond substrate by pulse laser deposition (PLD). TEM characterization revealed that the ε-Ga2O3 film grew along the <001> orientation on diamond (100) substrate. The deep ultraviolet (DUV) photodetector based on the ε-Ga2O3/diamond structure showed a high light-to-dark ratio over 5.7 × 104 and good linear response to the incident light power density from 10 to 400 mW/cm2. Moreover, compared to other photodetectors, the fabricated ε-Ga2O3/diamond photodetector achieved high responsivity and wide spectra response region from 210 to 260 nm, with high solar-blind rejection ratio of 104 (R 240/R 280) and 165 (R 210/R 280), respectively. The extension of spectra region with high responsivity of the ε-Ga2O3/diamond photodetector can be attributed to the thin thickness of ε-Ga2O3 film (around 200 nm) and parts of the DUV light were absorbed by diamond. The high responsivity and wide spectra response region indicate the fabricated ε-Ga2O3/diamond photodetector can be used for the detection of ultraviolet in the most of the DUV region.

Epitaxial growth of hexagonal pillar structure InN single crystals on sapphire (11−20) substrate by halide CVD method under atmospheric pressure

Epitaxial growth of hexagonal pillar structure InN single crystals on sapphire (11−20) substrate by halide CVD method under atmospheric pressure

Robust, Fully Biodegradable Films of Polyesters Realized by In Situ Formation of an Interconnected Multi-Nanolayer Structure under Extensional Flow.

Multilayer structures are not only applied to manipulate properties of synthetic polymer materials such as rainbow films and barrier films but also widely discovered in natural materials like nacre. In this work, in situ formation of an interconnected multi-nanolayer (IMN) structure in poly(butylene adipate-co-terephthalate) (PBAT)/poly(butylene succinate) (PBS) cocontinuous blends is designed by an extensional flow field during a "casting-thermal stretching" process, combining the properties of two components to a large extent. Hierarchical structures including phase morphology, crystal structure, and lamellar crystals in IMN films have been revealed, which clearly identifies the crucial role of extensional flow. The oriented PBAT phase in the IMN structure can be beneficial to the epitaxial growth of PBS crystals onto the PBAT nanolayers, thus improving interfacial adhesions. Furthermore, intense extensional stress can also promote crystallinity and thicken the lamellar structure. Given such distinct features in the fully biodegradable films, a simultaneous enhancement in tear strength, tensile strength, and puncture resistance has been achieved. To the best of our knowledge, the tear strength of IMN films about 285.9 kN/m is the highest level in the previous works of this system. Moreover, the proposed fabrication way of the IMN structure is facile and scalable, which is highly expected to be an efficient strategy for development of structured biodegradable polymers with excellent comprehensive properties.

Phase‐Field Simulations of Epitaxial Crystal Growth in Open Fractures With Reactive Lateral Flow

AbstractFluid flow in fracture porosity in the Earth's crust is in general accompanied by crystallization or dissolution depending on the state of saturation. The evolution of the microstructure in turn affects the transport and mechanical properties of the rock, but the understanding of this coupled system is incomplete. Here, we aim to simulate spatio‐temporal observations of laboratory experiments at the grain scale (using potash alumn), where crystals grow in a fracture during reactive flow, and show a varying growth rate along the fracture due to saturation differences. We use a multiphase‐field modeling approach, where reactive fluid flow and crystal growth is computed and couple the chemical driving force for grain growth to the local saturation state of the fluid. The supersaturation of the fluid is characterized by a concentration field which is advected by fluid flow and in turn affects the crystal growth with anisotropic growth kinetics. The simulations exhibit good agreement with the experimental results, providing the basis for upscaling our results to larger scale computations of combined multi‐physical processes in fractured porous media for applications as groundwater protection, geothermal, and hydrocarbon reservoir prediction, water recovery, or storing H2 or CO2 in the subsurface.

Wafer-Scale Epitaxial Growth of Two-dimensional Organic Semiconductor Single Crystals toward High-Performance Transistors.

The success of state-of-the-art electronics and optoelectronics relies heavily on the capability to fabricate semiconductor single-crystal wafers. However, the conventional epitaxial growth strategy for inorganic wafers is invalid for growing organic semiconductor single crystals due to the lack of lattice-matched epitaxial substrates and intricate nucleation behaviors, severely impeding the advancement of organic single-crystal electronics. Here, an anchored crystal-seed epitaxial growth method for wafer-scale growth of 2D organic semiconductor single crystals is developed for the first time. The crystal seed is firmly anchored on the viscous liquid surface, ensuring the steady epitaxial growth of organic single crystals from the crystal seed. The atomically flat liquid surface effectively eliminates the disturbance from substrate defects and greatly enhances the 2D growth of organic crystals. Using this approach, a wafer-scale few-layer bis(triethylsilythynyl)-anthradithphene (Dif-TES-ADT) single crystal is formed, yielding a breakthrough for organic field-effect transistors with a high reliable mobility up to 8.6 cm2 V-1 s-1 and an ultralow mobility variable coefficient of 8.9%. This work opens a new avenue to fabricate organic single-crystal wafers for high-performance organic electronics.