Simple Chemical Vapor Deposition Research Articles

(Invited) Diluted Magnetic Monolayer MoS2 Synthesis, Characterization and Device Applications

Keywords: Two dimensional; Monolayer; CVD; Spintronics; Spin injectionRecently, monolayer MoS2 has attracted intensive interests because of its special properties. MoS2-based field effect transistor exhibits a carrier mobility of 900 cm2 V-1 s-1 and an on/off ratio larger than 108.[1] However, its non-magnetic property has retarded its applications in the quantum devices. Diluted magnetic semiconductors have been extensively studied, as it bridges the ferromagnetic and semiconductor properties.[2] It shows a variety of new physical phenomena and functionalities.[3] There is few reports on diluted magnetic monolayer MoS2 semiconductors. In this paper, we design a rational synthesis of diluted magnetic monolayer MoS2 by a simple chemical vapor deposition method. By characterizing the structural, chemical composition, magnetic property, magnetic transport, we confirm the as-synthesized sample is single phase of diluted magnetic monolayer MoS2 with promising ferromagnetic property. We probe the mechanism of the ferromagnetic behavior observed in the as-grown diluted magnetic monolayer MoS2. The spin-injection efficiency is investigated by studying the injected relaxation and lift time. Our results would open up a new path way to develop the future flexible quantum devices and quantum computers.

Anharmonicity effects in Cu-doped ZnO nanocombs by temperature-dependent Raman scattering

Micro-Raman spectra of E2(high) phonon mode in Cu-doped ZnO nanocombs have been presented in detail with different Cu compositions under the temperature ranging from 83 to 443 K grown by a simple catalyst-free chemical vapor deposition method. The alloy disorder effect has been investigated by analyzing the asymmetric broadening of E2(high) phonon mode and Cu-induced localized vibration mode at room temperature. In addition, we resort to a theory model including the lattice thermal expansion and anharmonic phonon–phonon interaction, which can well describe the temperature dependence of Raman shift and linewidth of E2(high) phonon. In combining with the theory model, we have revealed an increasing anharmonic effect on the Raman shift and linewidth behaviors with increasing Cu composition. Furthermore, it is found that the lifetime of E2(high) phonon mode shortens with enhancing the anharmonicity.

Synchronous Generation of Nano- and Microscaled Hierarchical Porous Polyelectrolyte Multilayers for Superwettable Surfaces.

We created both a superhydrophilic polymer surface and a superhydrophobic surface by using the poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) multilayers with the synchronously generated hierarchical porous surface structures. The formation of surface and pore structures induced at acidic pH values is subject to the composition, distribution, and molecular weights of polyelectrolytes in the layer-by-layer (LbL) assembled film, leading to a variety of unique surface topographies and porous structures located on different scales. During the porous induction at pH 2.0, both nano- and microscaled features synchronously developed on the surface as a result of the unique combination of high-molecular-weight PAH (900K g/mol) and low molecular weight PAA (15K g/mol), along with a much reduced deposition time of 1 min. Although thermally cross-linked, the porous surface with hierarchical structure could achieve superhydrophilicity due to the remaining free amine and carboxylate groups on the porous structures. A complete switch from the superhydrophilic to the superhydrophobic surface was achieved via a simple chemical vapor deposition of trichloro(1H,1H,2H,2H-perfluoro-octyl)silane. In this work, the effects of molecular weight of polyelectrolytes (15K-900K g/mol), deposition time (10-900 s) during the LbL assembly, and pH (1.8 to 2.4) for the porous induction on the surface topography, pore structures, and wetting behavior were investigated in detail. A variety of unique porous surface structures on different length scales were systematically studied by controlling the above parameters.

Growth of GaN micro/nanolaser arrays by chemical vapor deposition

Optically pumped ultraviolet lasing at room temperature based on GaN microwire arrays with Fabry–Perot cavities is demonstrated. GaN microwires have been grown perpendicularly on c-GaN/sapphire substrates through simple catalyst-free chemical vapor deposition. The GaN microwires are [0001] oriented single-crystal structures with hexagonal cross sections, each with a diameter of ∼1 μm and a length of ∼15 μm. A possible growth mechanism of the vertical GaN microwire arrays is proposed. Furthermore, we report room-temperature lasing in optically pumped GaN microwire arrays based on the Fabry–Perot cavity. Photoluminescence spectra exhibit lasing typically at 372 nm with an excitation threshold of 410 kW cm−2. The result indicates that these aligned GaN microwire arrays may offer promising prospects for ultraviolet-emitting micro/nanodevices.

Two-Dimensional Atomic-Layered Alloy Junctions for High-Performance Wearable Chemical Sensor.

We first report that two-dimensional (2D) metal (NbSe2)-semiconductor (WSe2)-based flexible, wearable, and launderable gas sensors can be prepared through simple one-step chemical vapor deposition of prepatterned WO3 and Nb2O5. Compared to a control device with a Au/WSe2 junction, gas-sensing performance of the 2D NbSe2/WSe2 device was significantly enhanced, which might have resulted from the formation of a NbxW1-xSe2 transition alloy junction lowering the Schottky barrier height. This would make it easier to collect charges of channels induced by molecule adsorption, improving gas response characteristics toward chemical species including NO2 and NH3. 2D NbSe2/WSe2 devices on a flexible substrate provide gas-sensing properties with excellent durability under harsh bending. Furthermore, the device stitched on a T-shirt still performed well even after conventional cleaning with a laundry machine, enabling wearable and launderable chemical sensors. These results could pave a road toward futuristic gas-sensing platforms based on only 2D materials.

Quadrangular ZnO:Cu nanocombs: mechanism and optical properties

Cu-doped ZnO nanocombs were fabricated on Si substrate by a simple chemical vapor deposition method. Scanning electron microscopy analyses indicate that the single nanocomb tooth has a quadrangle structure. We propose that the synthesis of the Cu-doped ZnO nanocombs includes the formation of Cu/Zn core of square-like and surface oxidation. The structural and optical properties of the ZnO:Cu nanocombs were investigated by X-ray diffraction, photoluminescence, and Raman spectra. The results show that native point defects were introduced with increasing of the Cu composition by the substitution of Cu2+ at Zn2+ lattice sites. Meanwhile, the photoluminescence show that the visible luminescence greatly enhances by the introduction of Cu and the annealing in vacuum (1 × 10−4 Pa). Magnetization measurements show that Cu-doped ZnO nanocombs exhibit room-temperature ferromagnetism (RTFM), and the ferromagnetism increases with the Cu composition. It was suggested that the origin of the enhanced RTFM in ZnO:Cu nanocombs are related to the increased intrinsic lattice defects and vacancies oxygen.

Direct Growth of Graphene Films on 3D Grating Structural Quartz Substrates for High-Performance Pressure-Sensitive Sensors.

Conformal graphene films have directly been synthesized on the surface of grating microstructured quartz substrates by a simple chemical vapor deposition process. The wonderful conformality and relatively high quality of the as-prepared graphene on the three-dimensional substrate have been verified by scanning electron microscopy and Raman spectra. This conformal graphene film possesses excellent electrical and optical properties with a sheet resistance of <2000 Ω·sq(-1) and a transmittance of >80% (at 550 nm), which can be attached with a flat graphene film on a poly(dimethylsiloxane) substrate, and then could work as a pressure-sensitive sensor. This device possesses a high-pressure sensitivity of -6.524 kPa(-1) in a low-pressure range of 0-200 Pa. Meanwhile, this pressure-sensitive sensor exhibits super-reliability (≥5000 cycles) and an ultrafast response time (≤4 ms). Owing to these features, this pressure-sensitive sensor based on 3D conformal graphene is adequately introduced to test wind pressure, expressing higher accuracy and a lower background noise level than a market anemometer.

Growth and Photovoltaic Properties of High-Quality GaAs Nanowires Prepared by the Two-Source CVD Method.

Growing high-quality and low-cost GaAs nanowires (NWs) as well as fabricating high-performance NW solar cells by facile means is an important development towards the cost-effective next-generation photovoltaics. In this work, highly crystalline, dense, and long GaAs NWs are successfully synthesized using a two-source method on non-crystalline SiO2 substrates by a simple solid-source chemical vapor deposition method. The high V/III ratio and precursor concentration enabled by this two-source configuration can significantly benefit the NW growth and suppress the crystal defect formation as compared with the conventional one-source system. Since less NW crystal defects would contribute fewer electrons being trapped by the surface oxides, the p-type conductivity is then greatly enhanced as revealed by the electrical characterization of fabricated NW devices. Furthermore, the individual single NW and high-density NW parallel arrays achieved by contact printing can be effectively fabricated into Schottky barrier solar cells simply by employing asymmetric Ni-Al contacts, along with an open circuit voltage of ~0.3 V. All these results indicate the technological promise of these high-quality two-source grown GaAs NWs, especially for the realization of facile Schottky solar cells utilizing the asymmetric Ni-Al contact.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-016-1420-y) contains supplementary material, which is available to authorized users.

Synthesis and Photoluminescence of Single-Crystalline Fe(III)-Doped CdS Nanobelts.

In this paper, we report the synthesis and optical properties of Fe(III) doped CdS nanobelts (NBs) via simple Chemical Vapor Deposition (CVD) technique to explore their potential in nano-optics. The energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) analysis manifested the presence of Fe(III) ions in the NBs subsequently confirmed by the peak shifting to lower phonon energies as recorded by Raman spectra and shorter lifetime in ns. Photoluminescence (PL) spectrum investigations of the single Fe(III)-doped CdS NBs depicted an additional PL peak centered at 573 nm (orange emission) in addition to the bandedge(BE) emission. The redshift and decrease in the BE intensity of the PL peaks, as compared to the bulk CdS, confirmed the quenching of spectra upon Fe doping. The synthesis and orange emission for Fe-doped CdS NBs have been observed for the first time and point out their potential in nanoscale devices.

One‐Step In‐Situ Growth of Core–Shell SiC@Graphene Nanoparticles/Graphene Hybrids by Chemical Vapor Deposition

A one‐step in‐situ route to free standing core–shell silicon carbide in graphene nanoparticles on monolayer graphene is presented. The core–shell SiC@Graphene nanoparticle growth is realized by a simple chemical vapor deposition (CVD) process where carbon and silica precursors are simultaneously introduced into the growth chamber. This process permits the synthesis of a monolayer graphene sheet dressed with silicon carbide nanoparticles in a single CVD step, with the product controlled by growth temperature and the carbon/SiO2 exposure time. Growth of a high density SiC@Graphene distribution on a continuous graphene layer requires long exposure times (&gt;1 h) and high temperature (1000 °C). The growth process proceeds by a carbothermal mechanism. The simultaneous growth of graphene and SiC nanoparticles enables uniform core–shell SiC@Graphene nanoparticle formation rather than SiC/carbon nanofiber growth. As a proof of concept, the functionalization of preformed nanoparticle graphene surface with a diazonium salt is studied, demonstrating an increase in grafting rate with increasing nanoparticle population. This work provides a general procedure for one‐step synthesis, with further investigation required to develop precursors for hybrid core–shell CVD material growth.

Three-dimensional Co3O4/CNTs/CFP composite as binder-free cathode for rechargeable Li-O2 batteries

A novel cobalt oxide/carbon nanotubes/carbon fiber composite for high-performance rechargeable lithium-oxygen batteries oxygen electrode is synthesized. The CNTs are deposited on the surface of CFP via a simple chemical vapor deposition (CVD) and Co3O4 nanosheets are directly coated on CNTs by electrochemical deposition. The unique three-dimensional network structural of the composite can facilitate the wetting of electrolyte and diffusion of oxygen. When applied as a self-supporting, binder-free cathode material for rechargeable nonaqueous lithium-oxygen batteries (LOBs), the novel Co3O4/CNTs/CFP electrode exhibited excellent performance (with a specific capacity of 7196.5mAh/g(CNTs+Co3O4) at a current density of 200mA/g and 25 stable cycles with 1000mAh/g(CNTs+Co3O4)). The superior performance is proposed to be associated with the 3D nanoporous structures and the absence of side reactions related to binders.

Polarized Raman and photoluminescence studies of a sub-micron sized hexagonal AlGaN crystallite for structural and optical properties

The polarized Raman spectroscopy is capable of giving confirmation regarding the crystalline phase as well as the crystallographic orientation of the sample. In this context, apart from crystallographic x-ray and electron diffraction tools, polarized Raman spectroscopy and corresponding spectral imaging can be a promising crystallographic tool for determining both crystalline phase and orientation. Sub-micron sized hexagonal AlGaN crystallites are grown by a simple atmospheric pressure chemical vapor deposition technique using the self catalytic vapor-solid process under N-rich condition. The crystallites are used for the polarized Raman spectra in different crystalline orientations along with spectral imaging studies. The results obtained from the polarized Raman spectral studies shows single crystalline nature of sub-micron sized hexagonal AlGaN crystallites. Optical properties of the crystallites for different crystalline orientations are also studied using polarized photoluminescence measurements. The influence of internal crystal field to the photoluminescence spectra is proposed to explain the distinctive observation of splitting of emission intensity reported, for the first time, in case of c-plane oriented single crystalline AlGaN crystallite as compared to that of m-plane oriented crystallite.

Growth analysis and high-yield synthesis of aligned-stacked branched nitrogen-doped carbon nanotubes using sesame oil as a natural botanical hydrocarbon precursor

A simple spray pyrolysis-assisted chemical vapor deposition (CVD) method is employed for the growth of aligned-stack of branched nitrogen doped carbon nanotubes (bN-CNTs) using less cost effective sesame oil as natural hydrocarbon precursor. The acetonitrile (CH3CN) and ferrocene (C10H10Fe) are used for the N-doping and catalyst, respectively for the growth of nitrogen doped carbon nanotubes (N-CNTs) in aligned form. The Ar gas is passed through liquid thiophene (C4H4S) which is responsible for the branching in aligned-stacked bN-CNTs structure. The aligned-stacked bN-CNTs have been grown inside the quartz tube and no additional substrate has been used. The inner wall of quartz tubes contains black thick flake type deposition which contains aligned-stacked bN-CNT. The deposited flakes products were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and Fourier transform IR spectroscopy. A suitable mechanism is elucidated for the growth of aligned-stacked bN-CNT. We have found that the initial primary growth of N-CNTs show the base growth, however secondary bN-CNTs show nearly similar diameter and follows tip growth mechanism.

Synthesis of large size monolayer MoS2 with a simple chemical vapor deposition

Monolayer molybdenum disulfide (MoS2) has recently aroused the great interest of researchers due to its direct-gap property and potential applications in electronics, catalysis, photovoltaics, and optoelectronics. Chemical vapor deposition (CVD) has been one of the most practical methods of synthesizing large-area and high-quality monolayer MoS2. However, The process of preparation is complex and cumbersome. Here we report that high-quality monolayer MoS2 can be obtained through using sulfurization of MoO3 by a simple and convenient CVD on sapphire substrates.The substrate cleaning is simplified. Substrates are cleaned in detergent solution, deionized water and acetone without sopropanol or piranha solution (H2SO4/H2O2=3:1) in sequence, avoiding their potential dangers. The MoO3 powder (Alfa Aesar, 99.995%, 0.02 g) is placed in an alumina boat, and a sapphire substrate is faced down and is placed 6 cm away from MoO3 powder in the same boat. The sapphire substrate is placed in the center of the heating zone of the furnace. Another alumina boat containing sulfur powder (Alfa Aesar, 99.999%, 0.2 g) is placed upstream with respect to the gas flow direction in the low temperature area. We adopt an atmospheric pressure chemical vapor deposition method, so it does not require a vacuum process. After 30 min of Ar purging, the furnace temperature is directly increased from room temperature to 800 ℃ in 30 min, reducing the heating steps. After 60 min, the furnace is cooled down naturally to room temperature. Optical microscopy (OM) images, Raman spectra and photoluminescence (PL) are all obtained by confocal Raman microscopic system (LabRAM HR Evolution). From the OM images, we can see that isolated islands (triangles) have edge lengths up to 50 m, which is far larger than that grown by micromechanical exfoliation. The color of the triangles is uniform, which has a strong contrast with the substrate. We can obtain a preliminary result that the sample is a uniform monolayer MoS2. Raman spectra are collected for MoS2 samples on sapphire substrates. Two typical Raman active modes can be found: E2g1 at 386.4 cm-1 and A1g at 406 cm-1 ( =19.6 cm-1), which correspond to single-layered MoS2 sample. Raman mapping shows that the sample is a uniform monolayer MoS2. The PL spectrum of MoS2 shows a pronounced emission peak at 669 nm, which is consistent with other reported results for MoS2 thin sheets obtained from exfoliation methods. When the layer number of MoS2 decreases, with its bandgap transforming from indirect to direct one, the fluorescence efficiency will be significantly enhanced. So the results further prove that the sample is high-quality monolayer MoS2.

Toward compositional design of reticular type porous films by mixing and coating titania-based frameworks with silica

A recently developed reticular type porous structure, which can be fabricated as the film through the soft colloidal block copolymer (e.g., PS-b-PEO) templating, is very promising as the porous platform showing high-performance based on its high surface area as well as high diffusivity of targeted organic molecules and effective accommodation of bulky molecules, but the compositional design of oxide frameworks has not been developed so enough to date. Here, I report reliable synthetic methods of the reticular type porous structure toward simple compositional variations. Due to the reproducibility of reticular type porous titania films from titanium alkoxide (e.g., TTIP; titanium tetraisopropoxide), a titania-silica film having similar porous structure was obtained by mixing silicon alkoxide (e.g., tetraethoxysilane) and TTIP followed by their pre-hydrolysis, and the mixing ratio of Ti to Si composition was easily reached to 1.0. For further compositional design, a concept of surface coating was widely applicable; the reticular type porous titania surfaces can be coated with other oxides such as silica. Here, a silica coating was successfully achieved by the simple chemical vapor deposition of silicon alkoxide (e.g., tetramethoxysilane) without water (with water at the humidity level), which was also utilized for pore filling with silica by the similar process with water.

Synthesis of ZnO Nanorod Arrays Structure on Si Substrate

ZnO nanorod arrays were synthesized with simple chemical vapor deposition technique with template without using catalyst by controlling the growth time and condensation growth. The surface morphology of nanostructure were characterized by using field emission scanning electron microscopy (FE-SEM), we found that the ZnO nanorod arrays were uniformly covered on substrate. The extremely strong ZnO (0002) peaks were observed by using X-ray diffraction (XRD), shown the preferred (0001) orientation and high crystalline quality of the ZnO nanostructures. The optical properties were investigated by using photoluminescence (PL). These results showed the contribution of green-yellow emission attributed to the strong inner reflection and scattering. Our results indicating that the uniform ZnO nanorods arrays can be synthesized by using a simplified method. Furthermore, they will be implemented as application for nanodevice fabrication or for gas sensors and solar cells.

Effect of wrinkles on electrochemical performance of multiwalled carbon nanotubes as anode material for Li ion battery

A 1-D monohybrid of multiwalled carbon nanotubes and graphene sheets, graphene wrapped multiwalled carbon nanotubes (gC) structure, synthesized in a template-free simple chemical vapor deposition technique without any chemical functionalization, was employed as efficient anode material for Li ion battery. Graphene nanosheets affixed to the multiwalled carbon nanotubes (MWNTs) surface by van der Waal's attraction gives a wrinkled surface to the final 1-D gC configuration. The protrusions on the surface of the tube enhances the porosity of the system and also acts as defects, enhancing lithium adsorption sites while the inner MWNT core gives high electrical conductivity, resulting enhanced electrochemical performance of 373mAhg−1 at 100mAg−1 current density after 150 cycles.

Rapid and catalyst-free van der Waals epitaxy of graphene on hexagonal boron nitride

Recently, hexagonal boron nitride (h-BN) has been shown to act as an ideal substrate to graphene by greatly improving the material transport properties thanks to its atomically flat surface, low interlayer electronic coupling and almost perfect reticular matching [1]. Chemical vapour deposition (CVD) is presently considered the most scalable approach to grow graphene directly on h-BN. However, for the catalyst-free approach, poor control over the shape and crystallinity of the graphene grains and low growth rates are typically reported [2–5]. In this work we investigate the crystallinity of differently shaped grains and identify a path towards a real van der Waals epitaxy of graphene on h-BN by adopting a catalyst-free CVD process. We demonstrate the polycrystalline nature of circular-shaped pads and attribute the stemming of different oriented grains to airborne contamination of the h-BN flakes. We show that single-crystal grains with six-fold symmetry can be obtained by adopting high hydrogen partial pressures during growth. Notably, growth rates as high as 100 nm/min are obtained by optimizing growth temperature and pressure. The possibility of synthesizing single-crystal graphene on h-BN with appreciable growth rates by adopting a simple CVD approach is a step towards an increased accessibility of this promising van der Waals heterostructure.

Synthesis of Zinc Oxide Nanowire-Nanowall-Like Hybrid Structures on Graphene

ZnO nanowire-nanowall-like hybrid structure on graphene was synthesized with a simple chemical vapor deposition technique. This was done without a template by utilizing a catalyst to control the growth time and condensation. The surface morphology of nanostructure was characterized by using a field emission scanning electron microscope (FE-SEM). We found that the ZnO nanowire-nanowall-like hybrid structure was uniformly deposited on the graphene. The extremely strong ZnO (0002) peaks were observed by using x-ray diffraction (XRD). This method showed the preferred (0001) orientation and high crystalline quality of the ZnO nanostructures. The optical properties were investigated utilizing photoluminescence (PL). These results showed the strong green-yellow emission attributed to the robust inner reflection and scattering. This simplified method has great potential in synthesizing uniform ZnO nanowire-nanowall-like hybrid structures on graphene. These nanostructures may have important applications in new types of flexible gas sensors or other fields demanding high surface area materials.

Flexible Nitrogen Doped SiC Nanoarray for Ultrafast Capacitive Energy Storage.

The current trend with integrated energy-storage units in portable electronics lies in continuous advancements in nanostructured materials, thin-film manufacture technologies, and device architectures with enhanced functionality and reliability of existing components. Despite this, it is still challenging to provide cost-efficient solution to further improve the energy and power densities and cyclability of supercapacitors (SCs), especially at ultrafast rates while maintaining their environmentally friendly and even well-run at arbitrary harsh environments character. In this contribution, we report the fabrication of quasi-aligned single crystalline 3C-SiC nanowire (3C-SiCNW) array with tailored shapes and nitrogen-doping (N-doping). The resultant large-scale SiCNWs were directly grown on the surface of a flexible carbon fabric via a simple chemical vapor deposition method. We found that the SC performance of SiCNW arrays can be substantially enhanced by nitrogen doping, which could favor a more localized impurity state near the conduction band edge that greatly improves the quantum capacitance and hence increases the bulk capacitance and the high-power capability. The measured areal capacitances are higher with values of 4.8 and 4.7 mF cm(-2), in aqueous and gel electrolytes, respectively. The all-solid-state flexible textile-based SCs (TSCs) made with these electrodes are mechanically robust under bent and twisted states. Further, they show a power density of 72.3 mW cm(-2) that is higher than that of electrolytic capacitors, and an energy density of 1.2 × 10(-4) mW·h cm(-2), in association with superior rate ability, cyclability, and being environmentally friendly. Such SiCNW-TSC devices allow for operations at ultrahigh rate up to 30 V s(-1), 2 orders of magnitude higher than that of conventional supercapacitors. All these data are comparable to the reported results for 1D nanostructure-based carbon nanotubes (CNTs) or graphenes, thus showing the promising application as large-area flexible textile electronics.