Catalyst-free Chemical Vapor Deposition Method Research Articles

Versatile graphene-alumina nanofibers for microwave absorption and EMI shielding

Carbon-based hybrid nanostructures have shown to be promising candidates as cost-efficient and effective fillers for electromagnetic interference shielding and RF absorption. In this work, hybrids of graphene-augmented inorganic (alumina) nanofibers (GAIN) have been incorporated in epoxy resin to fabricate a multilayer structure with tunable absorption. Highly aligned graphene augmented alumina nanofibers of 10 ± 2 nm in diameter were produced with the help of a hot wall one-step catalyst-free chemical vapor deposition method at 1000 °C. Effective medium approximation has been used to calculate the intrinsic dielectric properties of GAIN nanofibers. The highest loss tangent of 0.4 has been achieved in a 5 mm thick composite containing 1 vol% of randomly oriented nanofibers. Furthermore, aligned graphene augmented nanofibers were embedded in an epoxy resin matrix to examine the effect of fiber alignment on the dielectric properties of the composite. Based on the obtained dielectric data, a superposed three-layer structure has been fabricated, offering an absorption of >90% in the entire X-band and an absorption peak of −25 dB at ∼11 GHz. Several multilayer designs based on finite element method coupled with Monte Carlo simulations have been proposed to tune the absorption characteristics. This work demonstrates the potential of the hybrid nanofibers with a dual loss function for versatile design options in the area of RF absorption.

Nanocrystalline graphite nanopores for DNA sensing

Free-standing nanocrystalline graphite (NCG) membranes directly grew on SiNx substrates with the aid of the catalyst-free chemical vapor deposition method. A new type of NCG nanopore was formed in the NCG membrane via controlled dielectric breakdown in KCl electrolyte. The electrical properties, especially the ionic conductance of the NCG nanoporewere examined under different physical parameters (pore size, ion strength, and pH condition). Due to their impressive transport properties, we first realized DNA sensing using the NCG nanopores.

A new approach to fabricate superhydrophobic and antibacterial low density isotropic pyrocarbon by using catalyst free chemical vapor deposition

Low density isotropic pyrocarbon (LDIP) of a novel “litchi” hierarchical structure with a high degree of crystallinity and graphitization was prepared by a simple, catalyst-free chemical vapor deposition method. The fabricated LDIP coating surface exhibits highly durable and robust superhydrophobic, self-cleaning, water and corrosive liquids repelling and oil-water separating properties. Moreover, the antifouling/antibacterial properties and biocompatibility of LDIP were investigated. The LDIP coated surface can not only effective prevent protein and bacterial (Gram-negative E. coli) adhesion but also has good biocompatibility. This new type of carbon material has great promise for potential applications in waste water treatment and biomedical fields.

Large-energy passively Q-switched Er-doped fiber laser based on CVD-Bi2Se3 as saturable absorber

In our work, Bi2Se3 was successfully used for demonstrating a large-energy passively Q-switched erbium-doped fiber laser. Bi2Se3 nanosheets were fabricated by a catalyst-free chemical vapor deposition method. Based on a pyrolysis tape transfer method, the Bi2Se3 thin film on the SiO2 substrate was transferred to the end of the optical connector for constructing the fiber-integrated saturable absorber. The saturation intensity and modulation depth of the Bi2Se3 saturable absorber were 81.1 MW/cm2 and 15.7%, respectively. A stable Q-switched operation at 1549.99 nm with a maximum average output power of 23.61 mW was achieved. The minimum pulse duration and the largest pulse energy were 1.34 μs and 224.5 nJ, respectively. In comparison with previous works, the single pulse energy (224.5 nJ) obtained in our experiment was improved significantly. Our experimental results fully proved that CVD-Bi2Se3 has good performance in obtaining large energy pulse operations and will promote the applications of 2D CVD-materials in the field of pulse laser.

A multilayer-graphene nanosheet film deposited on a ceramic substrate without a catalyst for constructing an electrochemiluminescence imaging platform.

Chemically and electrochemically stable conducting films are very desirable in the electrochemical industry and electrochemical sensing. In this work, ethanol was used as the carbon source to synthesize a multilayer-graphene nanosheet (MLGNS) film on ceramic substrates by a catalyst-free chemical vapor deposition (CVD) method at 900 °C and under ambient pressure. The developed CVD method is simple, economical and safe and avoids damage to the graphene nanosheet film during its transfer from the metal substrate to the non-metal substrate. The synthesized MLGNS film was well characterized by various techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Raman spectroscopy. The prepared MLGNS film has good chemical and electrochemical stability and satisfactory electrical conductivity thus can be used as a new type of electrode material. The MLGNS film on the ceramic substrate has been fabricated into an electrochemiluminescence (ECL) imaging platform to investigate the oxygen reduction reaction (ORR) and evaluate the activities of ORR catalysts, such as PtNPs. The established MLGNS film-based ECL imaging platform may have promising applications in the study of catalysts for fuel cells, high throughput immunoassay in the clinic, and fast screening of anti-cancer drugs.

AlxGa1-xN -GaN core-shell nanorods: Synthesis and field emission properties

Quasi-aligned AlxGa1-xN-GaN core-shell nanorods have been synthesized on Si (100) substrate by a one-step and catalyst-free chemical vapor deposition method. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis, Raman spectroscopy, and cathodoluminescence analyses reveal that the core/shell nanorods consist of AlxGa1-xN core and GaN shell. Field-emission measurements show that the AlxGa1-xN-GaN core-shell nanorod arrays present a turn-on field of ∼3.6 V/μm, threshold field of ∼4.8 V/μm, and a field-enhancement factor of ∼1197, which is much better than most AlN or GaN nanostructures. The excellent field-emission characteristics are originated from the low electron affinity of AlxGa1-xN and narrow band gap of GaN, resulting in good electron transport from AlxGa1-xN to GaN as evident from the band diagram of the core-shell material. Such AlxGa1-xN-GaN core-shell nanorods are envisaged to be one of decent candidates for advanced electronic and optoelectronic nanodevices.

Enhanced local photoluminescence of a multilayer MoS_2 nanodot stacked on monolayer MoS_2 flakes

Atomically thin two-dimensional (2D) layered materials, including graphene, boron nitride, and transition metal dichalcogenides (TMDCs), can exhibit novel phenomena distinct from their bulk counterparts. In this work, we report the growth of 2D MoS2 with the configuration of a multilayer MoS2 nanodot stacked on the center of the monolayer MoS2 flake by a single-step and catalyst-free chemical vapor deposition method. The local photoluminescence of the multilayer MoS2 nanodot is significantly enhanced in comparison to that of the basal monolayer MoS2. The reason of this novel phenomenon is owing to the existence of oxygen-related defects, which resulted from the tapered morphology of MoS2 nanodots with many edge states and adsorbed O molecules. The results presented here may open a pathway to modulate the optical properties of future optoelectronic devices using MoS2 flakes with special configuration.

Na-doped ZnO nanorods fabricated by chemical vapor deposition and their optoelectrical properties

Na-doped ZnO (ZnO:Na) nanorods have been grown on Si substrates by a catalyst-free chemical vapor deposition method. Element analysis reveals that the doping concentration of Na is in the range of 0.5–3.7 at.%. Morphological and electrical properties of the ZnO nanorods were found to be highly dependent on the Na concentration. Hall measurement indicates the realization of p-ZnO:Na nanorods with hole concentrations in the order of 1015 cm−3. Temperature dependent photoluminescence measurement down to 10 K confirms the shallow acceptor level, which is ∼132 meV. Desirable rectifying behavior has also been observed from the I-V characteristic of the p-ZnO:Na nanorods/ZnO/n-Si structure and the turn on voltage is ∼2.5 V.

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.

Growth and composition-dependent optoelectronic properties of AlxGa1−xN alloy nanowires arrays across the entire composition range on carbon cloth

Composition-tunable ternary AlxGa1−xN (0 ≤ x ≤ 1) alloy nanowires have been grown on flexible carbon cloth in large scale through a one-step and catalyst-free chemical vapor deposition method. Field emission scanning electron microscopy, x-ray diffraction analysis, transmission electron microscopy and x-ray energy-dispersive spectroscopy were employed to characterize the morphology, crystal structure, composition properties. All AlxGa1−xN samples exhibit hexagonal wurtzite single-phase structure and a preferred orientation along [0001] direction. The progressively evolutions in structural, Raman and field emission properties of AlxGa1−xN nanowires demonstrate the continuous composition tunability across the entire composition range. The enhancement in the field emission characteristic of AlxGa1−xN nanowires on carbon cloth is ascribed to the combined effect of the nanostructures and the excellent electron transport path of the conductive carbon cloth. The successful synthesis of AlxGa1−xN nanowires on carbon cloth provides the new possibility for the further development of flexible nano- and opto-electronic devices.

Mechanical Properties of Crystalline Silicon Carbide Nanowires.

In this paper, the mechanical properties of crystalline silicon carbide nanowires, synthesized with a catalyst-free chemical vapor deposition method, were characterized with nanoscale tensile testing and mechanical resonance testing methods inside a scanning electron microscope. Tensile testing of individual silicon carbide nanowire was performed to determine the tensile properties of the material including the tensile strength, failure strain and Young's modulus. The silicon carbide nanowires were also excited to mechanical resonance in the scanning electron microscope vacuum chamber using mechanical excitation and electrical excitation methods, and the corresponding resonance frequencies were used to determine the Young's modulus of the material according to the simple beam theory. The Young's modulus values from tensile tests were in good agreement with the ones obtained from the mechanical resonance tests.

Novel forsterite nanostructures with high aspect ratio via catalyst-free route

Forsterite nanostructures with high aspect ratio were successfully synthesized via catalyst-free chemical vapor deposition. The composition and morphology of the structures were determined by microscopic and spectroscopic techniques. Results show that a pure phase of forsterite nanostructures was obtained by this catalyst-free method. The formation of these structures is attributed to the presence of a reducing atmosphere. The produced forsterite structures are composed of wires, typically ca. 3.5μm in diameter and ca. 2mm long, covered with nanorods whose lengths are calculated to be less than 200nm. Their thickness varies considerably but it is generally uniform down the length of each single nanorod. These results indicate that catalyst-free chemical vapor deposition method is a powerful technique for the production of nanostructured forsterite.

Effect of temperature on growth and ultraviolet photoluminescence of Zn doped AlN nanostructures

Zn-doped AlN nanostructures were synthesized via a catalyst-free chemical vapor deposition method under different growth temperatures. The surface morphologies and size of nanostructures are significantly affected by the growth temperature. The undoped sample shows an ultraviolet emission band at 360nm, which is attributed to the transition from ONtoVAl defect complex to the ON level. With increasing temperature, the doped samples exhibit two new emissions centered at 282 and 320nm. The band at 320nm is related to the nitrogen vacancies, and the band at 282nm can be ascribed to the transition from nitrogen vacancies with three positive charges to neutral Zn acceptors. These results indicate that temperature increase favors the Zn incorporation which strongly affects the growth and luminescence of samples.

Simultaneous Catalyst-Free Growth of Highly Oriented ZnO Nanowires and Microtubes

Through a catalyst-free chemical vapor deposition method, highly oriented ZnO nanowires and microtubes were simultaneously grown on the side and the top surface of Si substrate respectively. The ZnO nanowires and microtubes were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy and selected area electron diffraction. The growth mechanism of the microtubes was discussed and the room temperature photoluminescence properties of individual microtubes were investigated.

Millimeter-sized single crystalline ZnO networks composed of nanostructures

Feather-like ZnO single-crystalline networks are synthesized via a facile one-step catalyst-free chemical vapor deposition method. Their axial size extends to millimeter-scale along their backbone while their construction unit is submicron, naturally constructing a bridge between microcosm and macrocosm. The whole feather-like network is an interconnection of a single crystal ZnO without any phase interface or crystal boundary in the junctions. The millimeter-scale ZnO networks have single-crystal nature, parallel connectivity, plenty of holes and large specific surface area, with which potential applications for carrier transporters, filters, sensors, wave-absorbing materials and interconnected computing units are expected.

Growth and Structure of Pure ZnO Micro/Nanocombs

Comb‐shaped ZnO micro/nanostructures were grown on copper substrate using a highly repeatable catalyst‐free chemical vapor deposition method. The structure of the ZnO micro/nanocombs was analyzed, and the necking‐down phenomena of the comb teeth was explained. The cathodoluminescence (CL) spectrum was measured on an individual ZnO comb, and a strong deep‐level emission centered at about 520 nm was observed.

Novel Type-II Zn3P2/ZnO Core/Shell Nanowires: Synthesis, Characteristic, and Photoluminescence Properties

Novel type-II Zn3P2/ZnO core/shell nanowires were synthesized for the first time. First, Zn3P2 nanowires with high yield were synthesized via a catalyst-free chemical vapor deposition method. Then, a facile low-cost surface oxidization process was used to obtain the ZnO nanocrystalline shells. Field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and high-angle angular darkfield scanning transmission electron microscopy (HAADF-STEM) were used to characterize the morphologies, crystal structures, and element composition of the as-fabricated core/shell nanowires. Room-temperature photoluminescence properties of them were studied. A possible growth mechanism was proposed. The heterostructure fabrication approach developed here may be extended to other functional heterostructure material systems. The experimental results together with the energy band analysis demonstrated that the type-II Zn3P2/ZnO core/shell nanowire heterostructures have substantial potential for future photovoltaic nanodevice applications.

Fabrication and gas sensing properties of In2O3 nanopushpins

The growth of nanopushpin-like In2O3 nanostructures was demonstrated on the silicon substrate by catalyst-free chemical vapor deposition method. Structural analysis revealed single-crystalline nature of the In2O3 nanopushpins with a cubic crystal structure. The hydrogen sensor made from the In2O3 nanopushpins showed swift response and excellent stability. The influence of operation temperature on the hydrogen gas sensing property of In2O3 nanostructures was also investigated. Our results reveal that the sensor response of In2O3 nanopushpins increases with increasing the operation temperature.