Two-step Chemical Vapor Deposition Method Research Articles

Controlled Fabrication of Quality ZnO NWs/CNTs and ZnO NWs/Gr Heterostructures via Direct Two-Step CVD Method.

A novel and advanced approach of growing zinc oxide nanowires (ZnO NWs) directly on single-walled carbon nanotubes (SWCNTs) and graphene (Gr) surfaces has been demonstrated through the successful formation of 1D–1D and 1D–2D heterostructure interfaces. The direct two-step chemical vapor deposition (CVD) method was utilized to ensure high-quality materials’ synthesis and scalable production of different architectures. Iron-based universal compound molecular ink was used as a catalyst in both processes (a) to form a monolayer of horizontally defined networks of SWCNTs interfaced with vertically oriented ZnO NWs and (b) to grow densely packed ZnO NWs directly on a graphene surface. We show here that our universal compound molecular ink is efficient and selective in the direct synthesis of ZnO NWs/CNTs and ZnO NWs/Gr heterostructures. Heterostructures were also selectively patterned through different fabrication techniques and grown in predefined locations, demonstrating an ability to control materials’ placement and morphology. Several characterization tools were employed to interrogate the prepared heterostructures. ZnO NWs were shown to grow uniformly over the network of SWCNTs, and much denser packed vertically oriented ZnO NWs were produced on graphene thin films. Such heterostructures can be used widely in many potential applications, such as photocatalysts, supercapacitors, solar cells, piezoelectric or thermal actuators, as well as chemical or biological sensors.

Chemical Vapor Deposited Mixed Metal Halide Perovskite Thin Films.

In this article, we used a two-step chemical vapor deposition (CVD) method to synthesize methylammonium lead-tin triiodide perovskite films, MAPb1−xSnxI3, with x varying from 0 to 1. We successfully controlled the concentration of Sn in the perovskite films and used Rutherford backscattering spectroscopy (RBS) to quantify the composition of the precursor films for conversion into perovskite films. According to the RBS results, increasing the SnCl2 source amount in the reaction chamber translate into an increase in Sn concentration in the films. The crystal structure and the optical properties of perovskite films were examined by X-ray diffraction (XRD) and UV-Vis spectrometry. All the perovskite films depicted similar XRD patterns corresponding to a tetragonal structure with I4cm space group despite the precursor films having different crystal structures. The increasing concentration of Sn in the perovskite films linearly decreased the unit volume from about 988.4 Å3 for MAPbI3 to about 983.3 Å3 for MAPb0.39Sn0.61I3, which consequently influenced the optical properties of the films manifested by the decrease in energy bandgap (Eg) and an increase in the disorder in the band gap. The SEM micrographs depicted improvements in the grain size (0.3–1 µm) and surface coverage of the perovskite films compared with the precursor films.

Silicon/carbon composites based on natural microcrystalline graphite as anode for lithium-ion batteries

Silicon/carbon composite is considered as one of the most potential anode materials to be commercialized because of silicon’s high specific capacity and carbon’s stability. Natural microcrystalline graphite features abundant resources, low price, and high stability. In consideration of these, it is an ingenious strategy to combine natural microcrystalline graphite with silicon as the anode material of lithium-ion batteries. In this paper, silicon/natural microcrystalline graphite@ carbon layer composites were fabricated by a two-step chemical vapor deposition (CVD) method. In this structure, natural microcrystalline graphite serves as the carbon matrix; silicon grows evenly on the surface and then pyrolytic carbon provides a tight coating. Therefore, the prepared composites deliver a high specific capacity of 560 mA h g−1 after 200 cycles at a current density of 100 mA g−1. This paper provides a potential method for large-scale production and is expected to solve the high-level application problem of natural microcrystalline graphite.

Controlled growth of SnSe/MoS2 vertical p–n heterojunction for optoelectronic applications

Two-dimensional (2D) heterostructures have been widely studied in recent years and are envisioned to play a key role in future integrated electronics and optoelectronics. The thus-involved van der Waals integration technique provides a feasible way to integrate different 2D materials even with diverse crystal structures into heterostructures, providing a promising platform to explore new artificial materials with new properties. Here, for the first time, we have successfully realized the combination of orthogonal selenide (SnSe) with hexagonal MoS2 into p–n heterojunctions though a two-step chemical vapor deposition method. High resolution transmission electron microscopy characterization shows that multilayer SnSe nanosheet is vertically stacked on MoS2 nanosheet with high crystallinity. The precise spatial modulation of SnSe/MoS2 heterostructures is verified by Raman diagrams. At the same time, the electrical and optoelectronic properties are probed though designing SnSe/MoS2 p–n junction devices. Typical current rectification behaviors are obviously observed in dark condition. While under light illumination, obvious photovoltaic behavior is observed. Maximum short-circuit current (I sc) and photon-electron conversion efficiency (η) are measured to be 67 nA and 1.8%, respectively. The results also indicate that the heterostructure can be employed for reliable ultra-sensitive photodetection, where maximum photoresponsivity is measured to be 384 A W−1. The direct vapor growth of 2D p–n junctions with different lattice symmetries may expand the platform for the realization of new 2D electronic and optoelectronic devices.

Optical fibres with embedded two-dimensional materials for ultrahigh nonlinearity

Nonlinear optical fibres have been employed for a vast number of applications, including optical frequency conversion, ultrafast laser and optical communication1-4. In current manufacturing technologies, nonlinearity is realized by the injection of nonlinear materials into fibres5-7 or the fabrication of microstructured fibres8-10. Both strategies, however, suffer from either low optical nonlinearity or poor design flexibility. Here, we report the direct growth of MoS2, a highly nonlinear two-dimensional material11, onto the internal walls of a SiO2 optical fibre. This growth is realized via a two-step chemical vapour deposition method, where a solid precursor is pre-deposited to guarantee a homogeneous feedstock before achieving uniform two-dimensional material growth along the entire fibre walls. By using the as-fabricated 25-cm-long fibre, both second- and third-harmonic generation could be enhanced by ~300 times compared with monolayer MoS2/silica. Propagation losses remain at ~0.1 dB cm-1 for a wide frequency range. In addition, we demonstrate an all-fibre mode-locked laser (~6 mW output, ~500 fs pulse width and ~41 MHz repetition rate) by integrating the two-dimensional-material-embedded optical fibre as a saturable absorber. Initial tests show that our fabrication strategy is amenable to other transition metal dichalcogenides, making these embedded fibres versatile for several all-fibre nonlinear optics and optoelectronics applications.

Strong coupling and pressure engineering in WSe2–MoSe2 heterobilayers

Two-dimensional materials offer an exciting platform that enables the creation of van der Waals heterostructures with rich functions and intriguing physical properties that stem from different band alignments and diverse interlayer interactions. However, further exploration of two-dimensional van der Waals heterostructures is hindered by the limited coupling strength and lack of efficient methods for tuning the interlayer interactions. Here, by using a two-step chemical vapour deposition method, we realize high-quality 2H-stacked WSe2–MoSe2 heterostructures with strong interlayer coupling, and effective tuning of their interlayer interaction by hydrostatic pressure. We unambiguously establish the strong coupling nature in these WSe2–MoSe2 heterostructures through the existence of exclusive interlayer excitons instead of the typical intralayer excitons. We further demonstrate efficient tuning of the interlayer coupling by using pressure engineering, and observe a clear evolution and transition of interlayer excitons in WSe2–MoSe2 heterostructures with a pressure-induced band changeover, which is further confirmed by first-principles calculations. Our findings provide new opportunities for producing, exploring and tuning van der Waals heterostructures with strong interlayer coupling that can lead towards the realization of future excitonic devices based on tailor-made, atomically thin, two-dimensional stacks. High-quality WSe2–MoSe2 heterostructures support strong coupling between the two layers, which is associated with tight hybridization and effective charge separation. In these structures, the bands of the interlayer excitons can be pressure-engineered.

Efficient Energy Funnelling by Engineering the Bandgap of a Perovskite: Förster Resonance Energy Transfer or Charge Transfer?

Energy funnelling enables directional carrier transfer along cascaded energy levels, which can be employed to significantly improve energy transfer efficiency and photoelectronic performances. However, the exact mechanism is still under intensive debate on whether Förster resonance energy transfer (FRET) or charge transfer (CT) is playing the dominant role, hindering broad practical device design and applications. Herein, a spectroscopic method is developed to unveil the energy funnelling mechanism by comparing and modeling the photoluminescence (PL) spectra excited by pulsed and continuous-wave (CW) lasers. The applicability of this method is verified in a typical energy funnelling system constructed by engineering the bandgap of a perovskite. Composite hexagonal microplates (MPs) with FAPbBr3, FAPb(BrxI1-x)3, and FAPbI3 (formamidinium = FA) at the surface, middle mezzanine, and bottom layers are synthesized by a two-step chemical vapor deposition (CVD) method, which introduces a directional energy funnelling from wide-bandgap FAPbBr3 to narrow-bandgap FAPbI3. By using the spectroscopic method developed in this work, we reveal that charge transfer is the dominant mechanism for energy funnelling in the FAPbBr3/FAPb(BrxI1-x)3/FAPbI3 sandwich MP. This study not only provides novel insights into the energy funnelling in multiple-bandgap perovskite systems but also develops a widely applicable spectroscopic method to explore the energy funnelling mechanism in other graded bandgap systems.

Numerical modeling of sintering and dehydroxylation of porous silica preform for low-hydroxyl silica glass fabrication

Dehydroxylation of porous silica preform plays a key role in two-step chemical vapor deposition (CVD) method for low-hydroxyl silica glass fabrication, the good control of which requires a clear understanding of the heat and mass transport characteristics involved in this process. For this purpose, a coupled numerical model is developed in this paper to simulate the sintering and dehydroxylation of porous silica preform, which considers the main phenomena including dehydroxylation reaction, heat and mass transfer in porous silica preform at high temperatures and shrinkage of porous silica preform. Numerical simulations are performed with this model in the configuration of a practical industrial heating furnace. Model verification is carried out by comparing the simulated hydroxyl concentration with experimental measurements at the end of sintering, which shows reasonable agreements. Based on the simulation results, variations of key parameters during heating, including temperature, porosity, hydroxyl concentration, gas pressure and gas velocity are analyzed in detail. Different transport modes of water vapor in porous silica preform are compared which shows that diffusion is the dominant way for its expelling. According to the sensitivity analysis, activation energy and frequency factor of dehydroxylation reaction have significant influences on the simulated hydroxyl concentration. Heat transfer coefficient and intrinsic permeability has little influences on the variation history of hydroxyl concentration, but has certain influences on the final hydroxyl distribution. Finally, the effects of preform size and heating curve on the dehydroxylation effect are determined, which could provide useful guidance for better controlling of the dehydroxylation of porous silica preform.

Epitaxial synthesis of ultrathin β-In2Se3/MoS2 heterostructures with high visible/near-infrared photoresponse.

van der Waals (vdWs) heterostructures, combining different two-dimensional (2D) layered materials with diverse properties, have been demonstrated to be a very promising platform to explore a new physical phenomenon and realize various potential applications in atomically thin electronic and optoelectronic devices. Here, we report the controlled growth of vertically stacked β-In2Se3/MoS2 vdWs heterostructures (despite the existence of large lattice mismatching ∼29%) through a typical two-step chemical vapor deposition (CVD) method. The crystal structure of the achieved heterostructures is characterized by transmission electron microscopy, where evident Moiré patterns are observed, indicating well-aligned lattice orientation. Strong photoluminescence quenching is obeserved in the heterostructure, revealing effective interlayer charge transfer at the interface. Electrical devices are further constructed based on the achieved heterostructures, which have a high on/off ratio and a typical rectifying behavior. Upon laser irradiation, the devices show excellent photosensing properties. A high responsivity of 4.47 A W-1 and a detectivity of 1.07 × 109 Jones are obtained under 450 nm laser illumination with a bias voltage of 1 V, which are much better than those of heterostructures grown via CVD. Most significantly, the detection range can be extended to near-infrared due to the relatively small bandgap nature of β-In2Se3. With 830 nm laser illumination, the devices also show distinct photoresponses with fast response speed even when operating at room temperature. The high-quality β-In2Se3/MoS2 heterostructures broaden the family of the 2D layered heterostructure system and should have significant potential applications in high-performance broadband photodetectors.

In situ fabrication of dendritic tin-based carbon nanostructures for hydrogen evolution reaction

In this work, dendritic tin-based carbon nanostructures with different morphologies were synthesized by a facile two-step carbonization and chemical vapor deposition method and were then evaluated for their performance in hydrogen evolution reaction.

CVD growth of perovskite/graphene films for high-performance flexible image sensor

Hybrid perovskite possesses excellent photoelectric properties, including large light-absorption capacity and high carrier mobility, and is an ideal light-absorbing material for photoelectric devices. The grain size and compactness of hybrid perovskite are key factors affecting the performance of photoelectric devices. The photocurrent and photoresponsivity of these devices are relatively low because of the rapidly recombined photoexcited electron-hole pairs in hybrid perovskite. Herein, we develop a facile two-step chemical vapor deposition (CVD) method to synthesize a high-quality van der Waals (vdWs) MAPbI3/graphene heterostructure for high-performance image sensor. We introduced inorganic sources (PbI2) to vdWs epitaxially grown PbI2 film on a seamless graphene monolayer film template through CVD. Methylammonium iodide (MAI) was then reintroduced to prepare the vdWs MAPbI3/graphene heterostructure. The MAPbI3 layer is composed of densely packed, large-size grains and displays a smooth surface. High photoresponsivity of 107 A/W is achieved in the corresponding photodetector. Inspired by the human visual system, we designed a flexible photodetector array containing (24 × 24) pixels, achieving perfect image recognition and color discrimination. Our study may greatly facilitate the construction of high-performance optoelectronic devices in artificial retina, biomedical imaging, remote sensing, and optical communication.

Inlaid ReS2 Quantum Dots in Monolayer MoS2.

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) are prospective materials for quantum devices owing to their inherent 2D confinements. They also provide a platform to realize even lower-dimensional in-plane electron confinement, for example, 0D quantum dots, for exotic physical properties. However, fabrication of such laterally confined monolayer (1L) nanostructure in 1L crystals remains challenging. Here we report the realization of 1L ReS2 quantum dots epitaxially inlaid in 1L MoS2 by a two-step chemical vapor deposition method combining with plasma treatment. The lateral lattice mismatch between ReS2 and MoS2 leads to size-dependent crystal structure evolution and in-plane straining of the 1L ReS2 quantum dots. Optical spectroscopies reveal the abnormal charge transfer between the 1L ReS2 quantum dots and the MoS2 matrix, resulting from electron trapping in the 1L ReS2 quantum dots. This study may shed light on the development of in-plane quantum-confined devices in 2D materials for potential applications in quantum information.

Sensitivity-Tunable Strain Sensors Based on Carbon Nanotube@Carbon Nanocoil Hybrid Networks.

A novel vanelike nanostructure based on the hybridization of carbon nanotubes and carbon nanocoils has been fabricated by a two-step chemical vapor deposition method. A flexible and sensitive strain sensor is prepared by coupling this hybrid structure with polydimethylsiloxane. By regulating the density and length of carbon nanotubes, the gauge factor and strain range of the sensors are tuned from 4.5 to 70 and 9 to 260%, respectively. These sensors exhibit high reliability and stability in a more than 10 000-cycle test and have a prompt response time of less than 37 ms. Owing to the tunable properties, these sensors show great potential in monitoring both subtle and large-scale displacements, which can meet the diverse demands of human motion monitoring.

Fabrication of high-strength carbon nanotube bundles using iron oxides co-assisted chemical vapor deposition

We report the fabrication of centimeter-long, defect-free, and parallel-aligned single-wall carbon nanotubes (SWCNTs) using a laboratory-designed two-step catalytic chemical vapor deposition method. Our in situ mass spectral analysis reveals that the initial stainless-derived iron oxide block decomposes methane into acetylene and ethylene, subsequently promoting the growth of high-purity SWCNTs on the second pigment-derived iron oxide fine powders dispersed on the Si/SiO2 substrate. Raman spectral imaging shows the negligible defect-induced D-band in the resulting sample, indicating the highly crystalline structure of SWCNTs. By using a gas-flow-type mechanical test system, an SWCNT bundle containing 20 components of individual SWCNTs exhibits the tensile strength over 50 GPa. Since the catalysts used in this study are widely available on the industrial scale, our iron oxide coassisted method will lead to the scalable production of high-strength SWCNT bundles.

Graphene nanosheet-grafted double-walled carbon nanotube hybrid nanostructures by two-step chemical vapor deposition and their application for ethanol detection

Here, we present a facile technique for synthesis of graphene nanosheet (GNS)-grafted double-walled carbon nanotube (DWCNT) hybrid carbon nanostructures (here after referred to as G-DWCNTs) by directly growing GNSs along the sidewalls of DWCNTs using a two-step chemical vapor deposition (CVD). DWCNTs were synthesized by floating catalyst CVD at 1300 °C using ferrocene and thiophene dissolved in ethanol. Then, GNSs were grafted onto the synthesized DWCNT bundles by thermal CVD at 1300 °C using ethanol. The sharp-edged petal-like structure of GNSs were grown along the sidewalls of DWCNT bundles while maintaining the one-dimensional structure of DWCNT. Next, DWCNTs and G-DWCNTs were dispersed in ethanol, then deposited on the paper using vacuum filtration method and used for ethanol detection. G-DWCNTs sensor exhibited a 3-fold improvement in the response to ethanol vapor compared to the DWCNTs sensor. The sensing mechanism of DWCNTs and G-DWCNTs can be described in terms of charge transfer between the gas molecules and sensing material. These results demonstrate that the facile technique by two-step CVD method provides a promising approach for simple and low-cost technique to synthesize the hybrid nanostructure of GNSs and DWCNTs. The new hybrid carbon nanostructures are attractive for gas sensing application.

A novel method for accelerating bubble rising in highly viscous molten quartz

A two-step chemical vapor deposition method is recently developed in industries for fabricating low-hydroxyl quartz glass. In this technique, bubbles are often trapped in molten quartz due to its highly viscous feature. Main purpose of this study is to analyze the bubble-group transport behavior in molten quartz and to explore effective methods for accelerating bubble rising and removal using simulation methods. An integrated numerical model is established with considering detailed heat transfer and fluid flow in the melting furnace, as well as discrete bubble tracking in molten quartz. Reliability of the simulation results is qualitatively validated by comparing with the theoretical calculations using classical Stokes and Hadamard-Rybczynski equations. Based on the results, it takes hundreds of hours for bubbles to escape from molten quartz in a typical heating furnace, which is unacceptable in practical fabrication. The temperature influence on bubble rising is studied in detail, which shows that increasing temperature could greatly enhance the fining efficiency through decreasing the melt viscosity. Finally, a novel method for accelerating bubble rising is put forward by using a rotating cone stirrer. Under the stirring effect, bubbles rise rapidly in molten quartz at the early stage, which is helpful for reducing the fining time. The rotating speed and size parameters of cone stirrer are further studied for their influences on the bubble rising behavior. Based on these findings, efficient fining of molten quartz is obtained by proper controlling of the stirrer.

Ultrafast Transition of Nonuniform Graphene to High-Quality Uniform Monolayer Films on Liquid Cu.

It is essentially important to synthesize uniform graphene films with a controlled number of layers because their properties strongly depend on the number of layers. Although chemical vapor deposition (CVD) on Cu has been widely used to synthesize large-area graphene films, the growth on solid and liquid Cu (L-Cu) suffers from poor thickness uniformity with a great number of adlayers and difficulty in forming continuous films even after a long growth time of hours, respectively. Here, we found that nonuniform graphene films initially grown on solid Cu (S-Cu) foil can rapidly transform into continuously uniform monolayer graphene film on L-Cu within 3 min. Moreover, the films obtained show larger grain size, higher quality, better optical and electrical properties, and better performance in organic light-emitting diode applications than the original films grown on S-Cu foil. By using carbon isotope labeling, we revealed that the multilayer-to-monolayer transition of graphene on L-Cu experiences etching-"self-aligning"-coalescence processes. This two-step CVD method not only opens up a new way for the rapid growth of uniform monolayer graphene films but also provides helpful information for the controlled growth of uniform monolayers of other 2D materials such as monolayer h-BN.

Epitaxial Growth of Two-Dimensional Metal-Semiconductor Transition-Metal Dichalcogenide Vertical Stacks (VSe2/MX2) and Their Band Alignments.

Two-dimensional (2D) metal-semiconductor transition-metal dichalcogenide (TMDC) vertical heterostructures play a crucial role in device engineering and contact tuning fields, while their direct integration still challenging. Herein, a robust epitaxial growth method is designed to construct multiple lattice-matched 2D metal-semiconductor TMDC vertical stacks (VSe2/MX2, M: Mo, W; X: S, Se) by a two-step chemical vapor deposition method. Intriguingly, the metallic VSe2 preferred to nucleate and extend from the energy-favorable edge site of the semiconducting MX2 underlayer to form VSe2/MX2 vertical heterostructures. This growth behavior was also confirmed by density functional theory calculations of the initial adsorption of VSe2 adatoms. In particular, the formation of Schottky-diode or Ohmic contact-type band alignments was detected for the stacks between VSe2 and p-type WSe2 or n-type MoSe2, respectively. This work hereby provides insights into the direct integration, band-alignment engineering, and potential applications of such 2D metal-semiconductor stacks in next-generation electronics, optoelectronic devices, and energy-related fields.

Two step floating catalyst chemical vapor deposition including in situ fabrication of catalyst nanoparticles and carbon nanotube forest growth with low impurity level

Carbon nanotube (CNT) forest with very low concentration of metallic impurity particle was grown by a two-step floating-catalyst chemical vapor deposition (CVD) method. Fe catalyst nanoparticles were uniformly formed on the substrate by flowing ferrocene/ethanol mist generated by an ultrasonic nebulizer. Then, the growth of the CNT forest was followed by a thermal CVD process in an identical furnace with acetylene as a carbon source. The separated process of catalyst formation and CNT growth leads to no formation of impurity particles on the CNT surfaces with a lower impurity level of 0.8 wt%. The gas-phase flow of the catalyst solution enables precise control of catalyst formation including diameter, radius of curvature and areal density of catalyst particle for long and dense CNT forest. The CNT forest grown under optimum conditions had a spin capability because of its high areal density greater than 1 × 1011 cm−2.

Efficient Energy Transfer in In2Se3-MoSe2 van der Waals Heterostructures.

We show that bilayer α-phase In2Se3 and monolayer MoSe2 form a type-I band alignment, with both the conduction band minimum and the valence band maximum located in MoSe2. Samples were fabricated by a two-step chemical vapor deposition method. The photoluminescence yield of the heterostructure sample was found to be similar to monolayer MoSe2, indicating the lack of an efficient charge transfer from MoSe2 to In2Se3. This is further confirmed by the observation that the photocarrier lifetime in the heterostructure is similar to monolayer MoSe2, showing the lack of layer separation of the electrons and holes. Efficient energy transfer from In2Se3 to MoSe2 was observed by the sevenfold enhancement of the differential reflection signal in the heterostructure and its ultrashort rising time. Furthermore, we observed significant photoluminescence quenching in heterostructures formed by bulk In2Se3 and monolayer MoSe2, which suggests efficient charge transfer and therefore type-II band alignment. These findings suggest that α-In2Se3 ultrathin layers can be effectively integrated as light-absorbing layers with other transition metal dichalcogenides for novel optoelectronic applications.