Catalyst-free Chemical Vapor Deposition Research Articles

One-step facile growth of nitrogen-doped graphene nanowalls by catalyst-free thermal chemical vapor deposition

We report an effective technique for the growth of graphene nanowalls (GNWs) and nitrogen-doped graphene nanowalls (N-GNWs) using a thermal chemical vapor deposition (TCVD) on the catalyst-free substrate. Ethanol and mixed ethanol-imidazole were used as carbon and carbon-nitrogen sources for the growth of GNWs and N-GNWs, respectively. The effect of imidazole concentration on the morphology, nanostructure, crystal structure, crystallinity, chemical bonding and chemical structure of GNWs and N-GNWs were investigated by scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy. Focused ion beam combined with TEM was utilized to analyze the growth mechanism. The results show that GNWs and N-GNWs are vertically oriented graphene nanosheets on in-plane-oriented graphitic layer/carbon nanolayer. An increase in imidazole concentration from 2.5 to 5.0 wt% increases the nitrogen content in the N-GNWs from 1.24 to 2.61 at%. The incorporated nitrogen atoms are mainly graphitic-N. One-step facile growth of N-GNWs by TCVD method without the need for a metal catalyst and plasma system was achieved.

Direct CVD-growth and optoelectronic characterizations of monolayer ribbon-like MoS2 flakes

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) materials provide a perspective development for optoelectronic, catalytic and quantum devices. However, relative approach to modify corresponding physical performance via explicit synthetic control of edge structure and dimensionality is still an enormous challenge. Here, we elucidate the successful fabrication of high-yield monolayer ribbon-like MoS2 crystals via a catalyst-free chemical vapor deposition (CVD) strategy. These ribbon-like MoS2 flakes exhibit a monolayer feature with tunable width in the range of 0.6–4.2 µm and a changed optical/electronic performance depending on the flake width. Furthermore, the proposed growth mechanism of such unique structures is analyzed at the basis of the change of Mo/S atom ratio. Our result emphasizes the potential to enable the successful preparation of special quasi one-dimensional (1D) TMDC structures for next-generation micro-nano optoelectronic devices.

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.

UV-durable superhydrophobic ZnO/SiO2 nanorod arrays on an aluminum substrate using catalyst-free chemical vapor deposition and their corrosion performance

Dense ZnO nanorod arrays were obtained directly on Al substrate for the first time using catalyst-free chemical vapor deposition. The influence of temperature on the surface topography of ZnO nanostructures was analyzed. Then the ZnO nanorod arrays surface was coated with SiO2 films via pulsed laser deposition. After modifying with stearic acid, the Al/ZnO/SiO2 nanorod arrays showed superhydrophobic properties with a water contact angle of up to 157.9°. Notably, these Al/ZnO/SiO2 nanorod arrays exhibited UV resistance, and the water contact angle remained almost constant under UV light exposure for a long time. The electrochemical measurements showed that ZnO/SiO2 nanorod arrays had an excellent protective effect on metal Al. These results provided a basis for a feasible method for the corrosion protection of metals.

Kinetics of Catalyst-Free and Position-Controlled Low-Pressure Chemical Vapor Deposition Growth of VO2 Nanowire Arrays on Nanoimprinted Si Substrates.

In the present article, the position-controlled and catalytic-free synthesis of vanadium dioxide (VO2) nanowires (NWs) grown by the chemical vapor deposition (CVD) on nanoimprinted silicon substrates in the form of nanopillar arrays was analyzed. The NW growth on silicon nanopillars with different cross-sectional areas was studied, and it has been shown that the NWs' height decreases with an increase in their cross-sectional area. The X-ray diffraction technique, scanning electron microscopy, and X-ray photoelectron spectroscopy showed the high quality of the grown VO2 NWs. A qualitative description of the growth rate of vertical NWs based on the material balance equation is given. The dependence of the growth rate of vertical and horizontal NWs on the precursor concentration in the gas phase and on the growth time was investigated. It was found that the height of vertical VO2 NWs along the [100] direction exhibited a linear dependence on time and increased with an increase in the precursor concentration. For horizontal VO2 NWs, the height along the direction [011] varied little with the growth time and precursor concentration. These results suggest that the high-aspect ratio vertical VO2 NWs formed due to different growth modes of their crystal faces forming the top of the growing VO2 crystals and their lateral crystal faces related to the difference between the free energies of these crystal faces and implemented experimental conditions. The results obtained permit a better insight into the growth of high-aspect ratio VO2 NWs and into the formation of large VO2 NW arrays with a controlled composition and properties.

Graphene-coated alumina nano/microfibers as filler for composites

The present work is focused on the preparation and characterization of composite nano/microfibers based on alumina applicable as fillers in ceramic-matrix composites. The graphene-coated Al2O3 microfibers have been prepared in the form of continuous alumina/graphene fibers from electrospun precursors using needle-less electrospinning, calcination, and catalyst-free chemical vapor deposition of graphene layers on the surface of the microfibers. The phase composition and morphology of the composite fibers were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning, and transmission electron microscopy (SEM and TEM). The porosity of the final fibers was evaluated by low-temperature nitrogen adsorption. The surface microstructures of composite fibers were rough and their diameters were in the range from 300 nm to 2 μm. The phase transformation at 1100 °C led to the formation of a mixture of γ-, δ-, and α-Al2O3. TEM results confirmed that the alumina fibers were covered by the 3–4 layers of graphene-based structure. The specific surface area of pure alumina fibers after carbon deposition slightly increased from the initial 40 m2/g to 45 m2/g, due to a slightly wavy surface of graphene layers. Raman spectroscopy and XPS analysis results showed that the surface layer of fibers is formed as graphene oxide. Between the graphene oxide and alumina grains a chemical bonding C–O–Al, C–O–O–Al was discovered as the nucleation of the graphene oxide sheet. The growth mechanism of graphene oxide layers on the surface edges of Al2O3 grains was described.

Optical microcavity based on a single ZnO microwire grown on Si(111) substrate by catalyst-free mist chemical vapor deposition

Optical microcavity based on a single ZnO microwire grown on Si(111) substrate by catalyst-free mist chemical vapor deposition

Spectral Engineering of InSe Nanobelts for Full-Color Imaging by Tailoring the Thickness.

In this work, we report on the synthesis of InSe nanobelts through a catalyst-free chemical vapor deposition (CVD) growth approach. A remarkable blue shift of the peak photoresponse was observed when the thickness of the InSe nanobelt decreases from 562 to 165 nm. Silvaco Technology Computer Aided Design (TCAD) simulation reveals that such a shift in spectral response should be ascribed to the wavelength-dependent absorption coefficient of InSe, for which incident light with shorter wavelengths will be absorbed near the surface, while light with longer wavelengths will have a greater penetration depth, leading to a red shift of the absorption edge for thicker nanobelt devices. Considering the above theory, three kinds of photodetectors sensitive to blue (450 nm), green (530 nm), and red (660 nm) incident light were achieved by tailoring the thickness of the nanobelts, which can enable the spectral reconstruction of a purple "H" pattern, suggesting the potential application of 2D layered semiconductors in full-color imaging.

Significant enhancement in field emission and photoluminescence properties of vertically aligned tellurium nanorods by plasma treatment

Significant improvement in electron field emission (FE) properties of high density vertically aligned tellurium nanorods (TNRs) arrays has been observed by plasma treatment. At first the TNRs are grown by catalyst free chemical vapor deposition (CVD) method and are systematically characterized by FESEM, EDS, TEM, HRTEM, XRD and XPS. Later on TNRs were treated with plasma (Ar and N) and their effects on morphology, optical and FE properties are investigated comprehensively. The FE of TNRs containing 40–50 nm size have been enhanced from 6.22 Vμm−1 to 3.25 Vμm−1 which is more than 52%. This research reveals that plasma treatment has effectively modified the surface morphology (confirmed by TEM) and created sharp tips on TNRs which act as extra source of field emitters believed to contribute the enhancement of field emission. Plasma treated TNRs not only exhibited excellent FE properties (3.25 Vμm−1), these aligned TNRs also portrayed the tremendous stability with insignificant FE current fluctuation (<4%) for 60 min. Crucially, the reported FE turn-on value is low enough for their potential applications in device fabrications. So these findings offer the improvement in the field emission of TNRs by plasma treatment proposing their potential utilization in devices.

Synthesis of high porosity and high adsorption performance BNNRs aerogels with long straight and their application for water cleaning

In this paper, a novel low dimensional boron nitride nanoribbons (BNNRs) aerogels were successfully developed by a cost-effective, catalyst-free chemical vapor deposition using Boric acid and Melamine as the B and N sources. The obtained BNNRs aerogels were possessed porous structure (average pore diameter: 2 nm) and the width of a single nanoribbon was approximately 200 nm and large aspect ratio more than 50 times and the BET surface area of 530 m2/g approximately. Attractively, BNNRs aerogels exhibited an excellent adsorption capacity for both PEDOT: PSS (3, 4-ethylenedioxyethiophene: polystyrene sulfonate) (6020 mg/g) and mustard seed oil (5010 mg/g). The adsorbed BNNRs aerogels can be cleaned and reused by burning them in the air and they create favorable conditions for recycling. After comparison, our experimental study found that after two test cycles, the adsorption ratio of BNNRs aerogels to mustard seed oil was 261.28%, which was only 6.97% less than that of the first adsorption with 280.87%. After the 4 combustion-adsorption cycles, the aerogels also maintained a 105.53% adsorption ratio, which confirmed the ability of BNNRs aerogels to adsorb oil. Therefore, BNNRs aerogels material is a promising nanomaterial for environmental remediation and selective water purification and treatment.

Two-Step Dry Synthesis of Binderless 3D Low Pt-Loading Electrocatalysts for Direct Alkaline Methanol Fuel Cell Anodes

Three-dimensional (3D) porous electrocatalysts of low platinum loading were synthesized by gas-phase pulsed laser ablation (PLA) and deposited on multiwalled carbon nanotubes (MWCNTs) directly grown on a stainless steel mesh current collector by catalyst-free chemical vapor deposition (Pt/MWCNT/SS). The performance of the developed electrocatalysts was evaluated for the methanol oxidation reaction (MOR). The PLA chamber pressure and ablation time were optimized to achieve the highest methanol oxidation current per Pt loading and electrochemically active surface area while maintaining structural stability. The most robust electrocatalyst, obtained with PLA at 10–5 Torr for 5 min, exhibited a 50% higher methanol oxidation current compared to the commercial Pt/C with similar Pt loading. In addition, it showed the lowest loss of active sites after stability tests while maintaining its structural integrity. Increasing the electrolyte temperature from 0 to 80 °C significantly improved the methanol oxidation current on Pt/MWCNT/SS by 13 times compared to 5.8 times for Pt/C. Electrochemical impedance spectroscopy confirmed the faster kinetics of MOR and accelerated oxidation of adsorbed CO on the surface of the Pt/MWCNT/SS relative to the Pt/C.

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.

Nanoscale structural and electrical properties of graphene grown on AlGaN by catalyst-free chemical vapor deposition

The integration of graphene (Gr) with nitride semiconductors is highly interesting for applications in high-power/high-frequency electronics and optoelectronics. In this work, we demonstrated the direct growth of Gr on Al0.5Ga0.5N/sapphire templates by propane (C3H8) chemical vapor deposition at a temperature of 1350 °C. After optimization of the C3H8 flow rate, a uniform and conformal Gr coverage was achieved, which proved beneficial to prevent degradation of AlGaN morphology. X-ray photoemission spectroscopy revealed Ga loss and partial oxidation of Al in the near-surface AlGaN region. Such chemical modification of a ∼2 nm thick AlGaN surface region was confirmed by cross-sectional scanning transmission electron microscopy combined with electron energy loss spectroscopy, which also showed the presence of a bilayer of Gr with partial sp2/sp3 hybridization. Raman spectra indicated that the deposited Gr is nanocrystalline (with domain size ∼7 nm) and compressively strained. A Gr sheet resistance of ∼15.8 kΩ sq−1 was evaluated by four-point-probe measurements, consistently with the nanocrystalline nature of these films. Furthermore, nanoscale resolution current mapping by conductive atomic force microscopy indicated local variations of the Gr carrier density at a mesoscopic scale, which can be ascribed to changes in the charge transfer from the substrate due to local oxidation of AlGaN or to the presence of Gr wrinkles.

Self-confined vapour-liquid-solid growth of SnS/SiOx core/shell nanowires

Tin sulfide/silicon oxide (SnS/SiOx) core/shell nanowires (NWs) were synthesized by a catalyst-free chemical vapor deposition (CVD) technique using SnS powder as the source material. The morphology and microstructure of the as-prepared NWs were characterized, and it was found that the NWs have core/shell structures composed of crystalline orthorhombic SnS cores and amorphous SiOx shells. Interestingly, there were holes encapsulated in the core/shell NWs to separate the SnS cores into segments. Based on the examinations on the morphology evolution of the NWs, we propose a self-confined vapour-liquid-solid (VLS) growth mechanism, in which silicon oxide shells confined the lateral growth of SnS cores resulting in the formation of core/shell NWs.

Synthesis of photoluminescent SiC-SiOx nanowires using coal tar pitch as carbon source

SiC-SiOx nanowires (NWs) with core-shell and chain-bead structures were synthesized via a catalyst-free chemical vapor deposition (CVD) route without Ar gas, using silicon and coal tar pitch powders as raw materials. The SiC-SiOx NWs with sharp tips formed by solid-vapor between Si (s) and CO (g) or C (s) and SiO (g), liquid-vapor between Si (l) and CO (g), and vapor-vapor between SiO (g) and CO (g) growth process along the [111] direction. The NWs were several millimeters in length, and the average diameters of the chains and beads were 40–90 nm and 325 nm, respectively. The obtained NWs had good blue-green photoluminescence property owing to the stacking faults and amorphous SiOx. This novel CVD route is simple, low cost and suitable for large-scale production.

Graphene-Assisted Quasi-van der Waals Epitaxy of AlN Film on Nano-Patterned Sapphire Substrate for Ultraviolet Light Emitting Diodes.

This protocol demonstrates a method for graphene-assisted quick growth and coalescence of AlN on nano-pattened sapphire substrate (NPSS). Graphene layers are directly grown on NPSS using catalyst-free atmospheric-pressure chemical vapor deposition (APCVD). By applying nitrogen reactive ion etching (RIE) plasma treatment, defects are introduced into the graphene film to enhance chemical reactivity. During metal-organic chemical vapor deposition (MOCVD) growth of AlN, this N-plasma treated graphene buffer enables AlN quick growth, and coalescence on NPSS is confirmed by cross-sectional scanning electron microscopy (SEM). The high quality of AlN on graphene-NPSS is then evaluated by X-ray rocking curves (XRCs) with narrow (0002) and (10-12) full width at half-maximum (FWHM) as 267.2 arcsec and 503.4 arcsec, respectively. Compared to bare NPSS, AlN growth on graphene-NPSS shows significant reduction of residual stress from 0.87 GPa to 0.25 Gpa, based on Raman measurements. Followed by AlGaN multiple quantum wells (MQWS) growth on graphene-NPSS, AlGaN-based deep ultraviolet light-emitting-diodes (DUV LEDs) are fabricated. The fabricated DUV-LEDs also demonstrate obvious, enhanced luminescence performance. This work provides a new solution for the growth of high quality AlN and fabrication of high performance DUV-LEDs using a shorter process and less costs.

P-p Heterojunction Sensors of p-Cu3Mo2O9 Micro/Nanorods Vertically Grown on p-CuO Layers for Room-Temperature Ultrasensitive and Fast Recoverable Detection of NO2.

High sensitivity, low limit of detection (LOD), and short response and recovery times at room temperature (RT) are critical for gas sensors. For NO2, different binary metal oxide-based sensors were developed to achieve superior performance at elevated temperatures instead of RT. Herein, we report on CuO@CuO and Cu3Mo2O9@CuO sensors with CuO and Cu3Mo2O9 micro/nanorods vertically aligned on the CuO layers, which were directly fabricated using a facile, low-cost, and catalyst-free chemical vapor deposition (CVD) technique. Their sensing performance tests revealed that the Cu3Mo2O9@CuO p-p heterojunction sensors exhibited a high response of 160% to 5 ppm NO2, an excellent sensitivity of 50% ppm-1, a low LOD of 2.30 ppb, a short response time of 49 s, and a rapid recovery of 241 s at RT, obviously better than those for CuO@CuO sensors. The superior performance of Cu3Mo2O9@CuO sensors could be attributed to the Schottky heterojunction formed between p-Cu3Mo2O9 micro/nanorods and p-CuO films, the catalytic effect, and the anisotropic nature of Cu3Mo2O9 micro/nanorods. This study not only provides a simple, low-cost, and batchable fabrication method of homo/heterojunction sensors with micro/nanorods vertically aligned on films but also opens an avenue for sensor design by tuning the Schottky barrier height to enhance RT performance.

Catalyst-Free, High-Loading Silicon Nanowires Anodes for Lithium Ion Batteries

We report on the scalable synthesis and characterization of novel architecture three-dimensional high-capacity amorphous Silicon Nanowires (SiNWs)-based anodes, with less than a third of the thickness of common graphite anodes. The SiNWs were grown by a novel, catalyst-free chemical vapor deposition (CVD) process, on a commercial stainless steel (SS) mesh. By using our novel, low-cost CVD procedure we synthesized binder-free anodes with loadings of up to 5mgSi/cm2, that have shown stable cycle life for over 600 cycles and provided capacities of up to 6mAh/cm2, very low (<10%) irreversible capacity and good compatibility with commercial cathodes. With the use of two- and three-electrodes coin cells we focus on studying their electrochemical performance and degradation mechanisms. It was found that the observed capacity loss of the SiNWs-based anodes is mainly caused by the increase in the resistivity of the SEI layer, along with the steady rise of the charge/discharge overpotential detected on both the anode and the cathode. Our anodes have been coupled with commercial cathodes (LFP and NCA), and have the potential to increase the energy density of LIBs by over 40%. In order to prove the scalability of our anodes, they have been successfully incorporated in industrial-size cylindrical cells, proving their potential as a viable candidate for anodes of the next generation lithium ion batteries for portable electronics and electric vehicles. Most recently, these anodes have been successfully coated with solid polymer electrolytes and cycled in all-solid state cells as a proof of concept, paving the way for safer, more durable LIBs.

Temperature and excitation dependent ultraviolet lasing in vertically oriented ZnO nanowires

Vertically oriented ZnO nanowires grown by a catalyst free chemical vapor deposition technique on c-plane of sapphire substrate show band-edge related excitonic lasing features. High resolution X-ray, scanning electron microscopy and photoluminescence analyses are collectively employed to understand the lasing behaviour from the nanowires. The nanowires show high degree of crystallinity, strong c-axis preferred orientation as well as relative in-plane alignment and sixfold hexagonal symmetry of the wurtzite phase. The nanowires are optically pumped using a 325 nm laser source and the emission characteristics are studied in detail by varying temperature and excitation intensity. The nanowires show the onset of lasing characteristics below 150 K which become prominent at 3.4 K with sharp multimodal emission characteristics. The threshold pump power for the onset of lasing is found to be of 14.5 mW at 3.4 K. Even though with increasing optical pumping, the lasing features from the nanowires lost their prominence being suppressed by spontaneous excitonic and defect transitions above 150 K.

WS2 Nanotubes, 2D Nanomeshes, and 2D In-Plane Films through One Single Chemical Vapor Deposition Route.

We demonstrate a versatile, catalyst free chemical vapor deposition process on insulating substrates capable of producing in one single stream one-dimensional (1D) WO3–x suboxides leading to a wide range of substrate-supported 2H-WS2 polymorphs: a tunable class of out-of-plane (of the substrate) nanophases, with 1D nanotubes and a pure WS2, two-dimensional (2D) nanomesh (defined as a network of webbed, micron-size, few-layer 2D sheets) at its extremes; and in-plane (parallel to the substrate) mono- and few-layer 2D domains. This entails a two-stage approach in which the 2WO3 + 7S → 2WS2 + 3SO2 reaction is intentionally decoupled. First, various morphologies of nanowires or nanorods of high stoichiometry, WO2.92/WO2.9 suboxides (belonging to the class of Magnéli phases) were formed, followed by their sulfurization to undergo reduction to the aforementioned WS2 polymorphs. The continuous transition of WS2 from nanotubes to the out-of-plane 2D nanomesh, via intermediary, mixed 1D-2D phases, delivers tunable functional properties, for example, linear and nonlinear optical properties, such as reflectivity (linked to optical excitations in the material), and second harmonic generation (SHG) and onset of saturable absorption. The SHG effect is very strong across the entire tunable class of WS2 nanomaterials, weakest in nanotubes, and strongest in the 2D nanomesh. Furthermore, a mechanism via suboxide (WO3–x) intermediate as a possible path to 2D domain growth is demonstrated. 2D, in-plane WS2 domains grow via “self-seeding and feeding” where short WO2.92/WO2.9 nanorods provide both the nucleation sites and the precursor feedstock. Understanding the reaction path (here, in the W–O–S space) is an emerging approach toward controlling the nucleation, growth, and morphology of 2D domains and films of transition-metal dichalcogenides.