Epitaxial Growth Method Research Articles

Study of hydrogen absorption in a novel three-dimensional graphene structure: Towards hydrogen storage applications

The use of a novel three-dimensional graphene structure allows circumventing the limitations of the two-dimensional nature of graphene and its application in hydrogen absorption. Here we investigate hydrogen-bonding on monolayer graphene conformally grown via the epitaxial growth method on the (0001) face of a porousified 4H-SiC wafer. Hydrogen absorption is studied via Thermal Desorption Spectroscopy (TDS), exposing the samples to either atomic (D) or molecular (D2) deuterium. The graphene growth temperature, hydrogen exposure temperature, and the morphology of the structure are investigated and related to their effect on hydrogen absorption. The three-dimensional graphene structures chemically bind atomic deuterium when exposed to D2. This is the first report of such an event in unfunctionalized graphene-based materials and implies the presence of a catalytic splitting mechanism. It is further shown that the three-dimensional dendritic structure of the porous material temporarily retains the desorbed molecules and causes delayed emission. The capability of chemisorbing atoms after a catalytic splitting of hydrogen, coupled to its large surface-to-volume ratio, make these structures a promising substrate for hydrogen storage devices.

Synthesis of component-controllable monolayer MoxW(1-x)S2ySe2(1-y) alloys with continuously tunable band gap and carrier type.

Alloying can effectively modify electronic and optical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs). However, efficient and simple methods to synthesize atomically thin TMD alloys need to be further developed. In this study, we synthesized 25 monolayer MoxW(1-x)S2ySe2(1-y) alloys by using a new liquid phase edge epitaxy (LPEE) growth method with high controllability. This straightforward approach can be used to obtain monolayer materials and operates on a self-limiting growth mechanism. The process allows the liquid solution to come into contact with the two-dimensional grains only at their edges, resulting in epitaxy confined only along the in-plane direction, which produces exclusively monolayer epitaxy. By controlling the weight ratio of MoS2/WSe2 (MoSe2/WS2), 25 monolayer MoxW(1-x)S2ySe2(1-y) alloys with different atomic ratios can be obtained on sapphire substrates, with band gap ranging from WS2 (1.55 eV) to MoSe2 (1.99 eV) and a continuously broad spectrum ranging from 623 nm to 800 nm. By adjusting the alloy composition, the carrier type and carrier mobility of alloy-based field-effect transistors can be modulated. In particular, the adjustable conductivity of MoxW(1-x)S2ySe2(1-y) alloys from n-type to bipolar type is achieved for the first time. This general synthetic strategy provides a foundation for the development of monolayer TMD alloys with multiple components and various 2D materials.

Photo-controllable heterostructured crystals of metal-organic frameworks via reversible photocycloaddition.

Metal-organic framework (MOF)-based heterostructures are attractive because they can provide versatile platforms for various applications but are limited by complex liquid epitaxial growth methods. Here, we employ photolithography to fabricate and control MOF-based heterostructured crystals via [4 + 4] photocycloaddition. A layered dysprosium-dianthracene framework, [Dy(NO3)3(depma2)1.5]·(depma2)0.5 (2) [depma2 = pre-photodimerized 9-diethylphosphonomethylanthracene (depma)] underwent a single-crystal-to-single-crystal transition at 140 °C to form [Dy(NO3)3(depma)(depma2)]·(depma2)0.5 (3). The dissociated anthracene moieties are face-to-face π-π interacted allowing a reversible photocycloaddition between 2 and 3. This structural transformation causes a luminescence switch between blue and yellow-green and thus can be used to fabricate erasable 2 + 3 heterostructured crystals for rewritable photonic barcodes. The internal strain at the heterostructure interface leads to photobending and straightening of the crystal, a photomechanical response that is fast, reversible and durable, even operating at 140 °C, making it promising for photoactuation. This work may inspire the development of intelligent MOF-based heterostructures for photonic applications.

First-principle study on the effect of S/Se/Te doping and VZn-Hi coexistence on ZnO electrical conductivity

Abstract In a vacuum environment, when ZnO is prepared using the chemical vapor deposition method and the molecular beam epitaxial growth method, H-gap impurities inevitably remain in the ZnO system, which is often ignored. The study of Zn vacancies under experimental conditions poses a challenge. Second, as an n-type semiconductor, ZnO is characterized by a self-compensation of natural donor defects and poor stability, which severely limit the acquisition of p-type ZnO. Based on the above problems, the conductive properties of S/Se/Te doped and VZn-Hi coexisting ZnO were investigated by first principle to acquire high-stability and high-quality p-ZnO. The study found that Zn35SO35, Zn35SeO35, and Zn35SHiO35 all have good p-type conductivity, which can effectively improve hole mobility and electrical conductivity. Among them, Zn35SO35 has the largest hole concentration at 2.80×1021 cm−3, as well as the best conductivity. The choice of Zn35SO35 provides a reference for obtaining new high-quality p-type ZnO semiconductors.

Efficient preparation and characterization of graphene based on ball milling

Graphene has a unique two-dimensional crystal structure and excellent physical and chemical properties. It is one of the new strategic materials developed by countries around the world in the 21st century. In recent years, the method of preparing graphene by mechanical stripping is becoming mature, including oxidation reduction method, mechanical stripping method, SiC epitaxial growth method, chemical vapor deposition method, etc. Based on the ball milling method, graphene is prepared in this study using a uniform design test method, scaled graphite as the raw material, and metallic aluminum as the grinding aid while considering the influence of ball ratio, grinding time, and rotation rate. A mathematical model was established based on partial least squares regression analysis and stepwise regression analysis. The best experimental scheme was obtained through the selection and optimization of the mathematical model, and the process of efficiently preparing graphene in the laboratory was explored. The prepared products were characterized by X-ray diffractometer, automatic specific surface area and porosity analyzer, and laser confocal Raman spectrometer. The results indicate that using aluminum powder as a grinding aid can significantly improve the efficiency of ball milling, the specific surface area of graphene produced is 542.598 m2/g, with the number of layers ranging from 2 to 5 layers. It is proved that the proposed graphene preparation method can obtain graphene with better quality. The study's findings serve as a guide for the production of graphene.

Development of the Composite Thermoluminescent Detectors Based on the Single Crystalline Films and Crystals of Perovskite Compounds.

This work is dedicated to the development of new types of composite thermoluminescent detectors based on the single crystalline films of Ce-doped GdAlO3 perovskite and Mn-doped YAlO3 and (Lu0.8Y0.2)AlO3:Mn perovskites as well as Ce and Pr-doped YAlO3 single crystal substrates. These detectors were obtained using the Liquid Phase Epitaxy growth method from the melt solution based on the PbO-B2O3 fluxes. Such composite detectors can by applied for the simultaneous registration of different components of mixed ionization fluxes using the differences between the thermoluminescent glow curves, recorded from the film and crystal parts of epitaxial structures. For creation of the new composite detectors, we considered using, for the film and crystal components of epitaxial structures (i) the different perovskite matrixes doped with the same type of activator or (ii) the same perovskite host with various types of activators. The thermoluminescent properties of the different types of epitaxial structures based on the abovementioned films and crystal substrates were examined in the conditions of β-particles and X-ray excitation with aim of determination of the optimal combination of perovskites for composite detectors. It was shown that, among the structures with all the studied compositions, the best properties for the simultaneous thermoluminescent detection of α- and X-rays were the GdAlO3:Ce film/YAlO3:Ce crystal epitaxial structure.

Constructing Microcavity for Perovskite Laser Power Converter: A Theoretical Study

Laser power converters (LPCs) used in wireless energy transmission can realize noncontact power supply under extreme conditions and provide continuous energy support to unmanned probes, which is believed to play an essential role in the exploration of deep ocean. However, commercially available LPCs usually employ III–V semiconductors as light absorbers due to their industrial maturity. In order to match the underwater 450∼540 nm laser source, it requires an epitaxial growth method to tune the bandgap of those materials, which is highly costly that severely restricts the industrial preparation of LPCs on a large scale. Herein, the potential of perovskite in the application of underwater LPCs is presented theoretically by constructing MAPbBr3 LPC and its theoretical power conversion efficiency (PCE) of 48.55% is demonstrated. Moreover, a microcavity (MC) constructing strategy to further improve the device performance is proposed, in which the transfer matrix method and an admittance‐based antireflection condition approach are combined for the optical simulation. By modulating the MC condition, the significant optical resonance inside the device while avoiding the obvious light reflection is realized. As a result, MC LPCs can obtain a simulated PCE of 64.27%, which is highly competitive with the existing UV–vis LPCs.

The S-Cu-O bonds boosted efficient photocatalytic degradation of semi-coherent interface Cu2O/Cu7S4 heterojunction

The steric effect caused by the lattice mismatch between the two phases hinders the separation and transfer of photogenerated carriers, which is one of the important factors limiting the activity of heterojunction photocatalysts. In this paper, semi-coherent interface Cu2O/Cu7S4 heterojunction was successfully constructed based on the epitaxy growth method of anion exchange. The degradation of tetracycline by semi-coherent interface Cu2O/Cu7S4-30 heterojunction under simulated visible light reached 95.3 % at 60 min, which is a significant improvement compared with single-phase catalysts and ordinary heterojunction catalysts (Cu2O/Cu7S4-mix). The experimental characterization and theoretical calculations confirmed the existence of unique S-Cu-O chemical bonds in the semi-coherent interface Cu2O/Cu7S4 heterojunction. On the basis of heterojunction energy band bending and the formation of built-in electric fields, the S-Cu-O bonds can act as a specific bridge to further facilitate the separation, transfer and delivery of photoelectrons and holes, which contributes to the generation of more oxidatively active species (mainly superoxide radicals and holes). In addition, based on experimental characterization and theoretical calculations, the intermediates, degradation pathways and photodegradation mechanisms of the degradation process have also been discussed in detail.

Design of Sb2Te3 nanoblades serialized by Te nanowires for a low-temperature near-infrared photodetector.

The dangling bond on the surface of bulk materials makes it difficult for a physically contacted heterojunction to form an ideal contact. Thus, periodic epitaxial junctions based on Sb2Te3 nanoblades serialized by Te nanowires (Sb2Te3/Te) were fabricated using a one-step hydrothermal epitaxial growth method. X-ray diffraction and electron microscopy reveal that the as-prepared product has a good crystal shape and heterojunction construction, which are beneficial for a fast photoresponse due to the efficient separation of photogenerated carriers. When the Sb2Te3/Te composite is denoted as a photodetector, it shows superior light response performance. Electrical analysis showed that the photocurrent of the as-fabricated device declined with temperatures rising from 10K to 300K at 980nm. The responsivity and detectivity were 9.5 × 1011μAW-1 and 1.22 × 1011 Jones at 50K, respectively, which shows better detection performance than those of other Te-based photodetector devices. Results suggest that the as-constructed near-infrared photodetector may exhibit prospective application in low-temperature photodetector devices.

Constructing solubility-diffusion domain in pebax by hybrid-phase MOFs for efficient separation of carbon dioxide and methane

A MIL-88B-on-UIO-66 (Abbreviation for UIO@MIL) hybrid-phase MOFs with rod-to-octahedral structure was obtained by the epitaxial growth method, and was added to Pebax matrix to develop a series of Pebax/UIO@MIL MMMs for CO2/CH4 separation. In UIO@MIL hybrid-phase MOF, CO2 solubility domain is formed in the core of UIO@MIL due to the abundant Zr–OH on the surface of UIO-66. MIL-88B has large pores and special rod-like structure that can entangled with the Pebax chain and adjust the spacing of the Pebax chain to form CO2 diffusion domains in MMMs. The results show that Pebax/UIO@MIL-4 membrane has the best CO2 separation performance, the CO2 permeability is 379.07 ± 10.00Barrer, and the CO2/CH4 selectivity is 38.1 ± 1.33. Therefore, hybrid-phase MOFs fillers provide a novel idea for the design of excellent MMMs.

Gradient Heating Epitaxial Growth Gives Well Lattice-Matched Mo2 C-Mo2 N Heterointerfaces that Boost Both Electrocatalytic Hydrogen Evolution and Water Vapor Splitting.

An optimized approach to producing lattice-matched heterointerfaces for electrocatalytic hydrogen evolution has not yet been reported. Herein, we present the synthesis of lattice-matched Mo2 C-Mo2 N heterostructures using a gradient heating epitaxial growth method. The well lattice-matched heterointerface of Mo2 C-Mo2 N generates near-zero hydrogen-adsorption free energy and facilitates water dissociation in acid and alkaline media. The lattice-matched Mo2 C-Mo2 N heterostructures have low overpotentials of 73 mV and 80 mV at 10 mA cm-2 in acid and alkaline solutions, respectively, comparable to commercial Pt/C. A novel photothermal-electrocatalytic water vapor splitting device using the lattice-matched Mo2 C-Mo2 N heterostructure as a hydrogen evolution electrocatalyst displays a competitive cell voltage for electrocatalytic water splitting.

Gradient Heating Epitaxial Growth Gives Well Lattice‐Matched Mo2C−Mo2N Heterointerfaces that Boost Both Electrocatalytic Hydrogen Evolution and Water Vapor Splitting

AbstractAn optimized approach to producing lattice‐matched heterointerfaces for electrocatalytic hydrogen evolution has not yet been reported. Herein, we present the synthesis of lattice‐matched Mo2C−Mo2N heterostructures using a gradient heating epitaxial growth method. The well lattice‐matched heterointerface of Mo2C−Mo2N generates near‐zero hydrogen‐adsorption free energy and facilitates water dissociation in acid and alkaline media. The lattice‐matched Mo2C−Mo2N heterostructures have low overpotentials of 73 mV and 80 mV at 10 mA cm−2 in acid and alkaline solutions, respectively, comparable to commercial Pt/C. A novel photothermal‐electrocatalytic water vapor splitting device using the lattice‐matched Mo2C−Mo2N heterostructure as a hydrogen evolution electrocatalyst displays a competitive cell voltage for electrocatalytic water splitting.

(Invited) Synthesis, Functionalization, and Three-Dimensional Structuring of Carbon Nanomaterials By Gas-Liquid Interface Plasma

Carbon nanomaterials, such as fullerene, carbon nanotubes (CNT), graphene sheets, and so forth, play indispensable roles in nanotechnology research and applications. Due to their unique self-organized nanostructures and properties, various types of applications using them are expected and have been developed. One of recent hottest topics among them is graphene sheets and their electric device applications. High quality graphene sheets for such the device applications, epitaxial growth or chemical vapor deposition (CVD) methods at high temperature up to 1,000°C, are used as a synthesis method in general. On the other hand, some kinds of applications, such as sensors, batteries, additives to polymers, and so forth, need a large amount of nanographene power. For that purpose, a reduction of graphene oxide (GO) is well known, but the quality of the synthesized graphene is not high enough. Recently, we have established a high-speed synthesis method of nanographene materials with high crystallinity by a plasma discharge at gas-liquid interfaces with alcohol sources. By this method, a synthesis rate of nanographene over 1 mg/min and higher crystallinity of nanographene than the reduced GO have been realized. On the other hand, there is a trade-of relationship between the synthesis rate and crystallinity, when different types of alcohols were used as a feed stock gas. When ethanol,1-propanol, and 1-butanol were used, it was found that the higher synthesis rates were obtained by the higher-molecular weight alcohols, while its crystallinity was lower. In the comparison between hexane (C6H14) and hexanol (C6H13OH), in the case of hexane, the synthesis rate is about twice as high as that in the case of hexanol, but the crystallinity is lowered. These results indicate that this trade-of relationship is attributed to a ratio of carbon (C) and oxygen (O) atoms. O-related radicals (O, OH, etc.) in plasma could have etching effects of amorphous or low-crystallinity carbon components. Actually, according to the results of plasma diagnostic measurements and residual liquid analyses, it was found that crystallinity of nanographene materials degraded with decrease in OH intensity in plasma. Furthermore, small radicals such as C2 and CHx contribute to the synthesis of nanographene rather than by-products with a six-membered ring structure. Furthermore, we have also found functionalization and structural control of nanographene materials by additive agents to alcohol sources at in-liquid plasma processes. Using an iron phthalocyanine with ethanol, size of carbon nanosheets increased up to micrometer. And they showed excellent catalytic characteristics thorough 4-electron reduction pathway. According to the verification results of dependence on synthesis conditions such as the type of additive, such the catalytic activity is induced by pyridinic C-N bonds. In the case of this way, to increase pyridinic C-N bonds and improve catalytic performance, iron phthalocyanine is much better than Hemin, even which also included Fe and N. These knowledges obtained in this study will open the way to the next-generation green energy solutions, such as high-performance catalytic electrode for the fuel cell.

Graphene-coated pearl-chain-shaped SiC nanowires

In this work, graphene-coated SiC nanowires (SiCnw) were fabricated by SiC epitaxial growth method. Firstly, the centimeter-scale single crystal SiC nanowires were synthesized through carbothermal reduction without any catalysts. Secondly, the epitaxial graphene grows on the surface of the SiCnw in a high vacuum and high-temperature environment to obtain graphene-coated pearl-chain-shaped SiCnw. In addition to the observed graphene layer coated on the surfaces of SiCnw, the formed β-SiC particles are densely and orderly arranged along the SiCnw. The growth of graphene layer coated on the surfaces of SiCnw can be ascribed to the thermal evaporation of Si or SiO2 on the surfaces of SiCnw. The formation mechanism of SiC particles is speculated to be the carbothermic reduction between the SiO and the CO or residual carbon generated by decomposition of SiC nanowires. The graphene-coated pearl-chain-shaped SiCnw have many potential applications in ceramic reinforcement, optoelectronics, mid-infrared and EM wave absorption devices.

High Quality Ge Growth on Si (111) and Si (110) by Using Reduced Pressure Chemical Vapor Deposition

A method for high quality epitaxial growth of Ge on Si (111) and Si (110) is investigated by reduced pressure chemical vapor deposition. Two step Ge epitaxy (low temperature Ge seed and high temperature main Ge growth) with several cycles of annealing by interrupting the Ge growth (cyclic annealing) is performed. In the case of Ge seed layer growth below 350 °C for (111) and 400 °C for (110) orientation, huge surface roughening due to too high dislocation density is observed after the following annealing step. For both crystal orientations, a high crystallinity Ge seed layer is realized by combination of 450 °C growth with 800 °C annealing. Once the high-quality Ge seed layer is deposited, high crystal quality Ge can be grown at 600 °C on the seed layer for both crystal orientations. For the 5 µm thick Ge layer deposited with the cyclic annealing process at 800 °C, a Si diffusion length of ~400 nm from the interface, RMS roughness below 0.5 nm and threading dislocation density of 5×106 cm-2 are achieved for both (111) and (110) substrates.

Epitaxial growth of SiGe films by annealing Al–Ge alloyed pastes on Si substrate

A simple, low-cost, and non-vacuum epitaxial growth method to realize large-area semiconductors on crystalline silicon will become the game-changer for various applications. For example, we can expect the disruptive effect on the cost of large-scale III–V multi-junction solar cells if we could replace the high-cost germanium substrate with silicon–germanium (SiGe) on Si. For SiGe epitaxial growth, we attempted to develop a process using original Al–Ge pastes for screen printing and subsequent annealing. We compare two pastes including Al–Ge alloyed pastes with compositional uniformity in each particle and Al–Ge mixed pastes. We revealed that Al–Ge alloyed paste could form flatter SiGe film with much less residual pastes, supported by in-situ observations. The uniform and sufficient dissolution of the alloyed paste is responsible for these and led to higher average Ge-composition by annealing at 500 °C. The composition in SiGe was vertically graded up to ~ 90% at the topmost surface. These results show that printing and firing of Al–Ge alloyed paste on Si is the desirable, simple, and high-speed process for epitaxial growth of SiGe, which could be potentially used as the lattice-matched virtual substrate with III–V semiconductors.

Synthesis, Sorting, and Applications of Single-Chirality Single-Walled Carbon Nanotubes.

The synthesis of high-quality chirality-pure single-walled carbon nanotubes (SWCNTs) is vital for their applications. It is of high importance to modernize the synthesis processes to decrease the synthesis temperature and improve the quality and yield of SWCNTs. This review is dedicated to the chirality-selective synthesis, sorting of SWCNTs, and applications of chirality-pure SWCNTs. The review begins with a description of growth mechanisms of carbon nanotubes. Then, we discuss the synthesis methods of semiconducting and metallic conductivity-type and single-chirality SWCNTs, such as the epitaxial growth method of SWCNT (“cloning”) using nanocarbon seeds, the growth method using nanocarbon segments obtained by organic synthesis, and the catalyst-mediated chemical vapor deposition synthesis. Then, we discuss the separation methods of SWCNTs by conductivity type, such as electrophoresis (dielectrophoresis), density gradient ultracentrifugation (DGC), low-speed DGC, ultrahigh DGC, chromatography, two-phase separation, selective solubilization, and selective reaction methods and techniques for single-chirality separation of SWCNTs, including density gradient centrifugation, two-phase separation, and chromatography methods. Finally, the applications of separated SWCNTs, such as field-effect transistors (FETs), sensors, light emitters and photodetectors, transparent electrodes, photovoltaics (solar cells), batteries, bioimaging, and other applications, are presented.

Novel Core–Shell‐Structured Zeolitic Imidazolate Framework (ZIF)‐90@ZIF‐67 Applied for High‐Performance All‐Solid‐State Asymmetric Supercapacitor

Herein, a core–shell structured zeolitic imidazolate framework (ZIF)‐90@ZIF‐67 is successfully synthesized with higher specific area and more micropores by a seed epitaxial growth method at room temperature, combining both the advantages of ZIFs and core–shell‐structured materials. When it is applied for supercapacitors’ electrode materials, ZIF‐90@ZIF‐67 delivers the best energy storage ability and excellent electrochemical stability. In a three‐electrode system, its specific capacitance reaches 1357.8 F g−1 at 1 A g−1 and maintains 55.4% while increasing the current density from 1 A g−1 to 20 A g−1. In a two‐electrode system, an all‐solid‐state asymmetric supercapacitor is assembled using ZIF‐90@ZIF‐67 and activated carbon (CV) as the positive and negative electrode material. The simple device exhibits a high specific capacitance of 151.6 F g−1 at 0.5 A g−1 and a high energy density of 53.9 Wh kg−1 at 510.2 W kg−1. Surprisingly, the device achieves a specific capacitance retention of 95.8% after 10 000 cycles at 10 A g−1. Finally, two simple all‐solid‐state supercapacitors in series power a blue lighting‐emitting diode for over 13 min, which reveals that the core–shell ZIF‐90@ZIF‐67 has great potential application prospects in the field of energy storage.

Thermal evaporated group IV Ge(Sn)-on-Si terahertz photoconductive antenna.

We have experimentally demonstrated thermal evaporated group IV Ge1-xSnx-on-Si terahertz (THz) photoconductive antennas (PCA) pumped by an Er-doped femtosecond laser for broadband THz generation. The Ge1-xSnx THz PCAs, free from material epitaxial growth methods, can offer comparable material properties in photocarrier generation, transportation, recombination, and the collection as group III-V THz PCAs. At the optical pumping power of 90 mW and a bias voltage of 40V, the Ge1-xSnx THz PCAs have achieved a broadband spectrum over 1.5 THz with a 40 dB signal-to-noise ratio (SNR). This CMOS-compatible group IV THz source can be monolithically integrated on the Si photonic platform, paving the way toward THz system-on-chip (SoC) for many on-site applications in the non-destructive evaluation, biomedical imaging, and industrial inspections.

Dual-color light emission and optical waveguide behavior in core-shell heterostructures based on surface doping of cocrystal with rare earth ions

Organic core-shell structures/heterostructures are burgeoning as active photonic materials, i.e., optical waveguides, controllable multi-emissions, input-output optical logic gates. Here, a new conception that rare-earth (RE) ions as the surface dopants for the construction of organic cocrystal@RE complexes core-shell heterostructures was first proposed. Organic cocrystal Tpy-1,4-DITFB (Tpy = 2,2’:6′,2″-terpyridine; 1,4-DITFB = 1,4-diiodotetrafluorobenzenes) as inner-core has been designed. The RE ions of Eu(Ⅲ) and Tb(Ⅲ) are dopped on the surfaces of Tpy-1,4-DITFB cocrystal via epitaxial growth method. The effect of the compositions and morphologies of core-shell heterostructures on the optical behaviors were investagted via fluorescence spectroscopy. Spatially adjacent dual-color emission in the di-block core-shell heterostructures brings about tunable regulation of optical waveguide modes, offering potential applications for input/output optical logic gate and biomedical.