Direct Chemical Vapor Deposition Growth Research Articles

Polarity Effect on the Heteroepitaxial Growth of B<sub>x</sub>C on 4H-SiC by CVD

The chemical vapor deposition (CVD) growth of boron carbide (BxC) layers on 4H-SiC, 4°off substrates was studied. Depending on the polarity of the substrate, different results were obtained. On Si face, the direct CVD growth at 1600°C under a mixture of BCl3+C3H8 systematically led to polycrystalline BxC films, whatever the C/B ratio in the gas phase. On the C face, heteroepitaxial growth was obtained for C/B ratios = 12 or higher with a step bunched morphology. If a boridation step (10 min at 1200°C under BCl3 flow) was used before the CVD growth, then heteroepitaxy was successful on both substrate polarities. To explain these results, a mechanism is proposed which involves the nature of the chemical bonds at the early stage of nucleation. It is suggested that a full B coverage of the SiC surface should favor the nucleation of the B-rich (0001) plane of BxC, promoting thus the heteroepitaxial growth along this direction.

Direct CVD Growth of Transferable 3D Graphene for Sensitive and Flexible SERS Sensor

Three-dimensional (3D) graphene (Gr) has been successfully grown on a patterned sapphire substrate (PSS) with very low mismatch between Gr and the sapphire nanostructure through metal-catalyst-assisted chemical vapor deposition (CVD). However, the transfer of the 3D Gr film without compromising the structural integrity of Gr is challenging because of the low etching rate of PSS. For easy and high-quality transfer of 3D Gr, we propose to coat a transfer-support layer (TSL) on PSS before direct CVD growth of 3D Gr. The TSL is directly deposited on PSS by atomic layer deposition without causing any structural changes in the substrate, as verified through atomic force microscopy (AFM). Few-layer 3D Gr is conformally produced along the surface of the TSL/PSS and successfully transferred onto a flexible substrate through wet-etching transfer, as confirmed by scanning electron microscopy, AFM, and Raman spectroscopy studies. We also present the fabrication of a sensitive and flexible surface-enhanced Raman scattering sensor based on 3D Gr on PMMA with high detection performance for low concentrations of R6G (10-9 M). The proposed transfer method with TSL is expected to broaden the use of 3D graphene in next-generation device applications.

Mosaic Nanocrystalline Graphene Skin Empowers Highly Reversible Zn Metal Anodes.

Constructing a conductive carbon-based artificial interphase layer (AIL) to inhibit dendritic formation and side reaction plays a pivotal role in achieving longevous Zn anodes. Distinct from the previously reported carbonaceous overlayers with singular dopants and thick foreign coatings, a new type of N/O co-doped carbon skin with ultrathin feature (i.e., 20nm thickness) is developed via the direct chemical vapor deposition growth over Zn foil. Throughout fine-tuning the growth conditions, mosaic nanocrystalline graphene can be obtained, which is proven crucial to enable the orientational deposition along Zn (002), thereby inducing a planar Zn texture. Moreover, the abundant heteroatoms help reduce the solvation energy and accelerate the reaction kinetics. As a result, dendrite growth, hydrogen evolution, and side reactions are concurrently mitigated. Symmetric cell harvests durable electrochemical cycling of 3040h at 1.0mA cm-2 /1.0 mAh cm-2 and 136h at 30.0mA cm-2 /30.0 mAh cm-2 . Assembled full battery further realizes elongated lifespans under stringent conditions of fast charging, bending operation, and low N/P ratio. This strategy opens up a new avenue for the in situ construction of conductive AIL toward pragmatic Zn anode.

A dual-functional micro-focus X-ray source based on carbon nanotube field emission

A micro-focus X-ray source with miniature pressure sensor was developed based on the multi-walled carbon nanotube (MWNT) field emission. The field emitters for both X-ray cathode and the pressure sensor were synthesized by direct chemical vapor deposition growth of MWNTs on catalytic substrates. MWNTs exhibited excellent field emission properties with the threshold field at 10 mA/cm2 of 2.30 V/μm, and the high current density of 10 A/cm2 could be reached at the field of 4.36 V/μm. The field emission electron gun was constructed with the aid of electrostatic simulations on the electron beam trajectories and the focal spot size (FSS) on the target with the demagnification factor of 5. The practical objects, including the semiconductor chip and the ceramic heating element, were tested by the X-ray source with the spatial resolution of less than 35 μm, and the fracture of the ceramic element inside metal case could be clearly detected. The pressure sensor monitored the in-situ pressures inside the source during the processing and after the sealing-off. The pressures in the sealed sources were generally in 10−4 Pa level and could rise to 10−2 Pa level during the operation.

Chemical vapor deposited WS2/MoS2 heterostructure photodetector with enhanced photoresponsivity

Two-dimensional transition metal dichalcogenides (TMDCs) have attracted great interest due to their unique semiconductor properties. Among all TMDC materials, MoS2 and WS2 are promising for composing heterostructures. However, traditional TMDC heterostructure fabrication depends on transfer process, with drawbacks of interface impurity and small size. In this work, a two-step chemical vapor deposition (CVD) process was applied to synthesize large-scale WS2/MoS2 heterostructure. Surface morphology and crystal structure characterizations demonstrate the high-quality WS2/MoS2 heterostructure. The WS2/MoS2 heterostructure photodetector fabricated by photolithography exhibits an enhanced photoresponsivity up to 370 A W−1 in comparison with single WS2 or MoS2 devices. This study suggests a direct CVD growth of large-scale TMDC heterostructure films with clean interface. The built-in electric field at interface contributes to the separation of photo-generated electron–hole pairs, leading to enhanced photocurrent and responsivity, and showing promising potentials in photo-electric applications.

Direct CVD growth of MoS2 on chemically and thermally reduced graphene oxide nanosheets for improved photoresponse

The direct chemical vapor deposition (CVD) of transition metal dichalcogenides (TMDCs) on graphene or graphene oxide (GO) allows for the scalable production of van der Waals heterostructures (vdWHs). Herein, considering that the CVD growth of TMDCs depends largely on the surface property of the substrate, we compared chemically reduced GO (CrGO) and thermally reduced GO (TrGO) as substrates to induce the CVD growth of MoS2 without a seeding promoter. On monolayer (1L) to three-layer CrGO and TrGO nanosheets, more triangular MoS2 flakes were found on CrGO than on TrGO. This is because CrGO with less surface defects could promote the growth of MoS2 as compared to TrGO, which normally carries more defects. Such a difference was not obvious when the thickness of CrGO and TrGO nanosheets exceeded five layer. As a demonstration, the obtained MoS2@1L-CrGO and MoS2@1L-TrGO vdWH nanosheets showed much higher photosensitivities as compared to the 1L CrGO and TrGO nanosheets under both the blue and red laser irradiation. In particular, MoS2@1L-CrGO with a higher density of MoS2 showed larger photosensitivity than MoS2@1L-TrGO. In addition, rolling up the MoS2@1L-CrGO and MoS2@1L-TrGO vdWH nanosheets into MoS2@CrGO and MoS2@TrGO vdWH nanoscrolls further enhanced their photosensitivities, largely due to the presence of multiple vdWH interfaces in the scrolled structure. Our work demonstrates that the reduction in surface defects of chemically modified graphene oxide promotes the deposition of MoS2 to form vdWHs and related scroll structures, which are promising for optoelectronics and flexible electronics.

Direct Growth of Graphene over Insulators by Gaseous‐Promotor‐Assisted CVD: Progress and Prospects

AbstractUtilizing a direct chemical vapor deposition (CVD) approach to allow transfer‐free synthesis of graphene on insulating substrates is appealing. Nevertheless, the quality and uniformity of thus‐grown graphene without the aid of any catalyst stay normally inferior to that of graphene formed on metals by far. This mainly stems from the catalytic inertness of the insulating surface with regard to the decomposition of carbon feedstock. In this respect, delicate methodologies have been devised to date to boost the quality of directly‐grown graphene. In this Minireview, recent advances in the direct CVD growth of graphene on insulators by introducing various gaseous promotors is covered. The effects and mechanisms of different types of promotors on the direct growth are discussed. At the end, existing challenges and future perspectives in this field are further highlighted.

Primary Nucleation-Dominated Chemical Vapor Deposition Growth for Uniform Graphene Monolayers on Dielectric Substrate

Direct chemical vapor deposition growth of high quality graphene on dielectric substrates holds great promise for practical applications in electronics and optoelectronics. However, graphene growth on dielectrics always suffers from the issues of inhomogeneity and/or poor quality. Here, we first reveal that a novel precursor-modification strategy can successfully suppress the secondary nucleation of graphene to evolve ultrauniform graphene monolayer film on dielectric substrates. A mechanistic study indicates that the hydroxylation of silica substrate weakens the binding between graphene edges and substrate, thus realizing the primary nucleation-dominated growth. Field-effect transistors based on the graphene films show exceptional electrical performance with the charge carrier mobility up to 3800 cm2 V-1 s-1 in air, which is much higher than those reported results of graphene films grown on dielectrics.

High Photoresponsivity in Ultrathin 2D Lateral Graphene:WS2:Graphene Photodetectors Using Direct CVD Growth.

We show that reducing the degree of van der Waals overlapping in all 2D ultrathin lateral devices composed of graphene:WS2:graphene leads to significant increase in photodetector responsivity. This is achieved by directly growing WS2 using chemical vapor deposition (CVD) in prepatterned graphene gaps to create epitaxial interfaces. Direct-CVD-grown graphene:WS2:graphene lateral photodetecting transistors exhibit high photoresponsivities reaching 121 A/W under 2.7 × 105 mW/cm2 532 nm illumination, which is around 2 orders of magnitude higher than similar devices made by the layer-by-layer transfer method. The photoresponsivity of our direct-CVD-grown device shows negative correlation with illumination power under different gate voltages, which is different from similar devices made by the transfer method. We show that the high photoresponsivity is due to the lowering of effective Schottky barrier height by improving the contact between graphene and WS2. Furthermore, the direct CVD growth reduces overlapping sections of WS2:Gr and leads to more uniform lateral systems. This approach provides insights into scalable manufacturing of high-quality 2D lateral electronic and optoelectronic devices.

Direct CVD Growth of Graphene on Traditional Glass: Methods and Mechanisms.

Chemical vapor deposition (CVD) on catalytic metal surfaces is considered to be the most effective way to obtain large-area, high-quality graphene films. For practical applications, a transfer process from metal catalysts to target substrates (e.g., poly(ethylene terephthalate) (PET), glass, and SiO2 /Si) is unavoidable and severely degrades the quality of graphene. In particular, the direct growth of graphene on glass can avoid the tedious transfer process and endow traditional glass with prominent electrical and thermal conductivities. Such a combination of graphene and glass creates a new type of glass, the so-called "super graphene glass," which has attracted great interest from the viewpoints of both fundamental research and daily-life applications. In the last few years, great progress has been achieved in pursuit of this goal. Here, these growth methods as well as the specific growth mechanisms of graphene on glass surfaces are summarized. The typical techniques developed include direct thermal CVD growth, molten-bed CVD growth, metal-catalyst-assisted growth, and plasma-enhanced growth. Emphasis is placed on the strategy of growth corresponding to the different natures of glass substrates. A comprehensive understanding of graphene growth on nonmetal glass substrates and the latest status of "super graphene glass" production are provided.

Direct CVD Growth of Graphene on Technologically Important Dielectric and Semiconducting Substrates.

To fabricate graphene based electronic and optoelectronic devices, it is highly desirable to develop a variety of metal‐catalyst free chemical vapor deposition (CVD) techniques for direct synthesis of graphene on dielectric and semiconducting substrates. This will help to avoid metallic impurities, high costs, time consuming processes, and defect‐inducing graphene transfer processes. Direct CVD growth of graphene on dielectric substrates is usually difficult to accomplish due to their low surface energy. However, a low‐temperature plasma enhanced CVD technique could help to solve this problem. Here, the recent progress of metal‐catalyst free direct CVD growth of graphene on technologically important dielectric (SiO2, ZrO2, HfO2, h‐BN, Al2O3, Si3N4, quartz, MgO, SrTiO3, TiO2, etc.) and semiconducting (Si, Ge, GaN, and SiC) substrates is reviewed. High and low temperature direct CVD growth of graphene on these substrates including growth mechanism and morphology is discussed. Detailed discussions are also presented for Si and Ge substrates, which are necessary for next generation graphene/Si/Ge based hybrid electronic devices. Finally, the technology development of the metal‐catalyst free direct CVD growth of graphene on these substrates is concluded, with future outlooks.

Direct synthesis and in situ characterization of monolayer parallelogrammic rhenium diselenide on gold foil

Rhenium diselenide (ReSe2) has recently garnered great research interest due to its distorted 1T structure, anisotropic physical properties, and applications in polarization-sensitive photodetectors. However, ReSe2 synthesized by chemical vapor deposition (CVD) is usually a multilayer/polycrystalline material containing numerous grain boundaries, thereby hindering its further applications. Here we describe the direct CVD growth of high-quality monolayer ReSe2 single crystals with a parallelogram shape arising from its anisotropic structure on a gold foil substrate. In particular, we use low-energy electron microscopy/diffraction combined with scanning tunneling microscopy/spectroscopy to determine the atomic-scale structure, domain orientation/boundaries, and band features of monolayer ReSe2 flakes grown directly on gold foils. This work may open new opportunities for the direct synthesis and in situ characterization of CVD-grown monolayer ReSe2.

Direct CVD growth of graphene on three-dimensionally-shaped dielectric substrates

Large area growth of single- and few-layer graphene has been established, yet its prerequisite of metallic catalysts has impeded direct graphene growth on three-dimensional (3D) dielectric substrates. Here, we report a new strategy for direct chemical-vapor-deposition (CVD) growth of graphene films on 3D patterned sapphire substrates by placing the target surfaces in contact with or close to a Cu catalyst. This approach can yield uniform coatings of few-layer graphene films on complex structures on the centimeter scale, as confirmed by detailed scanning electron microscopy and Raman spectroscopy studies. In addition, we showed that 3D graphene can possess ambipolar field-effect transistor (FET) characteristics by applying the gate voltage through an ion-gel dielectric sheet. The 3D graphene FET device was further employed as a pressure and touch sensing device, where the sensitivity was effectively enhanced by the 3D topology.

CVD growth of large-area and high-quality HfS2 nanoforest on diverse substrates

Two-dimensional layered transition metal dichalcogenides (TMDs) have attracted burgeoning attention due to their various properties and wide potential applications. As a new TMD, hafnium disulfide (HfS2) is theoretically predicted to have better electrical performance than widely studied MoS2. The experimental researches also confirmed the extraordinary feature in electronics and optoelectronics. However, the maximal device performance may not be achieved due to its own limitation of planar structure and challenge of transfer without contamination. Here, through the chemical vapor deposition (CVD) technique, inch-size HfS2 nanoforest has been directly grown on diverse objective substrates covering insulating, semiconducting and conducting substrates. This direct CVD growth without conventional transfer process avoids contamination and degradation in quality, suggesting its promising and wide applications in high-quality and multifarious devices. It is noted that all the HfS2 nanoforests grown on diverse substrates are constructed with vertically aligned few-layered HfS2 nanosheets with high crystalline quality and edge orientation. Moreover, due to its unique structure, the HfS2 nanoforest owns abundant exposed edge sites and large active surface area, which is essential to apply in high-performance catalyst, sensor, and energy storage or field emitter.

Direct Chemical Vapor Deposition Growth of Graphene Nanosheets on Supported Copper Oxide

A facile chemical vapor deposition method was used to grow multi-layer graphene. A copper-based catalyst was homogeneously deposited on magnesium oxide powder surface by impregnation. The synthesis was conducted under atmospheric pressure without a dedicated reduction step prior to the reaction. The mechanism of multi-layer graphene growth was investigated by high-resolution transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. For the first time, we show that copper under its oxide form could catalyze the growth of multi-layer graphene. Such a simple method leading to produce multi-layer graphene of quite high structural quality could help to develop alternative ways for graphene production by chemical vapor deposition.

Fast growth of graphene on SiO2/Si substrates by atmospheric pressure chemical vapor deposition with floating metal catalysts

Graphene on dielectric substrates is essential for its electronic applications. Graphene is typically synthesized on the surface of metal and then transferred to an appropriate substrate for fabricating device applications. This post growth transfer process is detrimental to the quality and performance of the as-grown graphene. Therefore, direct growth of graphene films on dielectric substrates without any transfer process is highly desirable. However, fast growth of graphene on dielectric substrates remains challenging. Here, we demonstrate a transfer-free chemical vapor deposition (CVD) method to directly grow graphene films on dielectric substrates at fast growth rate using Cu as floating catalyst. A large area (centimeter level) graphene can be grown within 15 min using this CVD method, which is increased by 500 times compared to other direct CVD growth on dielectric substrate in the literatures. This research presents a significant progress in transfer-free growth of graphene and graphene device applications.

Graphene Coating of Silicon Nanoparticles with CO2‐Enhanced Chemical Vapor Deposition

Understanding the growth of graphene over Si species is becoming ever more important as the huge potential for the combination of these two materials becomes more apparent, not only for device fabrication but also in energy applications, particularly in Li-ion batteries. Thus, the drive for the direct fabrication of graphene over Si is crucial because indirect approaches, by their very nature, require processing steps that, in general, contaminate, damage, and are costly. In this work, the direct chemical vapor deposition growth of few-layer graphene over Si nanoparticles is systematically explored through experiment and theory with the use of a reducer, H2 or the use of a mild oxidant, CO2 combined with CH4 . Unlike the case of CH4 , with the use of CO2 as a mild oxidant in the reaction, the graphene layers form neatly over the surface and encapsulate the Si particles. SiC formation is also prevented. These structures show exceptionally good electrochemical performance as high capacity anodes for lithium-ion batteries. Density functional theory studies show the presence of CO2 not only prevents SiC formation but helps enhance the catalytic activity of the particles by maintaining an SiOx surface. In addition, CO2 can enhance graphitization.

Direct Chemical Vapor Deposition Growth of Graphene on Insulating Substrates

AbstractRecent years have witnessed an increase in attention to the direct synthesis of graphene on a variety of insulators by virtue of chemical vapor deposition (CVD), which has shown great potential in a broad range of applications. In this Focus Review, we summarize the key research efforts on the development of CVD routes to allow direct deposition of graphene on insulating materials, with a special focus upon our recent progress towards graphene synthesis employing hexagonal boron nitride, glass, and SrTiO3 planar substrates, as well as NaCl powders. To begin with, we outline the important features and necessities in the field of graphene synthesis by direct CVD. We then provide an up‐to‐date research summary with respect to the employment of substrate (planar vs. powder; novel vs. conventional), the design of synthetic methods, and the quality/property control of products. Finally, we discuss the potential challenges and opportunities for the direct CVD growth of graphene on insulators.

Efficient Photoelectrochemical Hydrogen Evolution of Amorphous Group VI Metal Chalcogenides on Si Micropyramids

Hydrogen, a clean, storable, and high-energy density energy carrier, is a promising sustainable alternative for meeting the global energy demand and achieving an environmentally friendly fuel economy. Utilizing an integrated photoelectrochemical (PEC) system to directly convert solar to hydrogen via water splitting is one of the most promising approaches. Platinum and other noble metals remain the best catalysts for solar-driven hydrogen evolution reaction (HER), but the high cost and scarcity greatly limit their large scale deployment. In this work, we synthesized efficient yet low-cost HER electrocatalysts, amorphous MoSxCly and MoSexCly, via a facial chemical vapor deposition (CVD) method, and discovered when these catalysts were coupled with light absorber planar p-Si could make efficient and robust photocathodes for solar-driven HER. Moreover, by utilizing a special designed n+p Si micropyramid structure, we were able to further improve the photocurrent density, onset voltage, as well as the overall power conversion efficiency. This superior performance is originated from the significant improvement in light harvesting properties from Si nanostructuring as well as the optimized interface between the catalysts and Si substrate via the direct CVD growth. Outstanding stability was also demonstrated over long-term operation, suggesting these heterostructures are promising earth-abundant alternatives to photocathodes based on noble metal catalysts for solar-driven hydrogen production.

Hierarchical NiAl layered double hydroxide/multiwalled carbon nanotube/nickel foam electrodes with excellent pseudocapacitive properties.

The performances of pseudocapacitors usually depend heavily on their hierarchical architectures and composition. Herein, we report a three-dimensional hierarchical NiAl layered double hydroxide/multiwalled carbon nanotube/nickel foam (NiAl-LDH/MWCNT/NF) electrode prepared by a facile three-step fabrication method: in situ hydrothermal growth of NiAl-LDH film on a Ni foam, followed by direct chemical vapor deposition growth of dense MWCNTs onto the NiAl-LDH film, and finally the growth of NiAl-LDH onto the surface of the MWCNTs via an in situ hydrothermal process in the presence of surfactant sodium dodecyl sulfate. The MWCNT surface was fully covered by NiAl-LDH hexagonal platelets, and this hierarchical architecture led to a much enhanced capacitance. The NiAl-LDH/MWCNT/NF electrode has an areal loading mass of 5.8 mg of LDH per cm(2) of MWCNT/NF surface. It also possesses exceptional areal capacitance (7.5 F cm(-2)), specific capacitance (1293 F g(-1)), and cycling stability (83% of its initial value was preserved after 1000 charge-discharge cycles). The NiAl-LDH/MWCNT/NF material is thus a highly promising electrode with potential applications in electrochemical energy storage.