Catalyst-assisted Chemical Vapor Deposition Research Articles

Gaseous Catalyst Assisted Growth of Graphene on Silicon Carbide for Quantum Hall Resistance Standard Device

AbstractDirect growth of graphene on silicon carbide (SiC) is a very promising method for preparing high‐quality graphene. However, high quality single crystal epitaxial graphene films on SiC always form at a temperature higher than 1800 °C. Here, the synthesis of graphene on the silicon surface (0001) of SiC at ≈1300 °C by gaseous catalyst‐assisted chemical vapor deposition (CVD) method is reported. As the step height of terraces on SiC surface can influence the performance of graphene Hall devices, low‐temperature growth of graphene benefits for keeping the steps of the SiC surface at a small height, which can be achieved by a pre‐treatment before growth. A graphene quantum Hall resistance standard (G‐QHRS) device fabricated on the SiC surface with a small step height of ≈0.5 nm exhibits an accuracy of 1.15 × 10−8 and a reproducibility of 3.6 × 10−9 within 7 days in the measurement of quantum resistance at B = 6 T and T = 4.5 K. The gaseous catalyst‐assisted CVD on SiC is an effective scalable growth approach that produces high‐quality graphene for the resistance metrology, and it represents a promising step toward a low magnetic field QHRS setting the basis of low‐cost and transportable QHRS in a near future.

Field emission characteristics of ZnO nanowires grown by catalyst-assisted MOCVD on free-standing inorganic nanomembranes

ZnO field emitters on freely suspended inorganic nanomembranes (NMs) were synthesized by catalyst-assisted metal organic chemical vapor deposition (MOCVD) using the precursors zinc acetylacetonate hydrate and oxygen. The morphology and the possibly involved growth mechanisms of the randomly distributed ZnO nanostructures were investigated by scanning electron microscopy and by energy-dispersive x-ray spectroscopy. The findings indicate a growth process that involves the vapor–liquid–solid as well as the vapor–solid–solid mechanism. The field emission (FE) properties of such ZnO nanowire (NW) samples showed to be highly reproducible and independent of the investigated Si-based substrate types. Herein, a new type of flexible substrate for the MOCVD process has been introduced that allows for the direct growth of ZnO NWs on free-standing membranes for potential FE-based sensor applications. A turn-on field of 4.1 V µm−1 was detected for a macroscopic emission current density of 10 µA cm−2 and the stability test revealed fluctuations of only 9% around the mean emission current over a duration of 3 h proving a reliable and stable operation of such devices. Moreover, approaches were identified to further enhance the FE characteristics of the ZnO NWs by variation of the synthesis parameters and by enlargement of the NM area.

Strategies for Scalable Gas-Phase Preparation of Free-Standing Graphene

The preparation of graphene with high quality serves as a prerequisite for its usage. Traditional methods of graphene production, represented by liquid-phase exfoliation and chemical vapor depositi...

Stress induced modification of electronic band structure and enhanced optical emission in 1-D GaN nanostructures

1-dimensional GaN nanowires (NW) and nanorods (NR) grown using catalyst assisted Chemical Vapour Deposition are investigated for their structural and optical properties, and their electronic band structure. X-ray Diffraction (XRD) studies reveal that biaxial strain increases with aspect ratio, which is corroborated by Raman studies wherein phonon related confinement effects are observed for NR due to the crystal strain. Photoluminescence studies show 3 and 6 times enhancement in the intensity of band edge emission of NW and NR, respectively, signifying the presence of a large number of band tail states. A small red shift in NW (3.43 eV) and large red-shift in NR (3.28eV) is reported, which are attributed to the change in the band gap and band structure of these morphologies. Valence band studies by X-ray Photoelectron Spectroscopy reveal the presence of band tail states for NR and surface bond contraction for NW. The effect of crystal strain on the electronic band structure on optical properties of 1-D GaN nanostructures with different aspect ratios is elucidated.

Catalyst-assisted growth of InGaN NWs for photoelectrochemical water-splitting applications

In this work, we have successfully grown InGaN nanowires by catalyst-assisted chemical vapour deposition technique with high aspect ratio for solar-driven water splitting applications. The band gap of the InGaN nanowires has been tuned to absorb a wide range of visible parts of electromagnetic spectrum by optimizing the composition of In:Ga. The photoelectrochemical analysis has been carried out for InGaN nanowires and that evidences the significant solar oxygen evolution reaction with a small onset potential of 0.234 V vs. reversible hydrogen electrode. From the analysis, it has been witnessed the maximum applied bias to photo-conversion efficiency of ~ 1% at the applied bias of 0.63 V vs. reversible hydrogen electrode. Moreover, the ultra-long stability of InGaN nanowires has been evidenced by 3000 s with a flat current density of 0.43 mA/cm2 in chronoamperometry analysis.

Reduced holey graphene oxide film and carbon nanotubes sandwich structure as a binder-free electrode material for supercapcitor

A novel carbon nanotubes (CNTs) and reduced holey graphene oxide film (RHGOF) sandwich structure has been fabricated to enhance its electrochemical properties. CNTs are grown by a catalyst assisted chemical vapor deposition technique, interpenetrated between the RHGOF layers. A RHGOF/CNTs hybrid film is used as a binder-free supercapacitor electrode. The grown CNTs in the graphene layers structure act as spacers and bridges to increase the counductivity of RHGOF, while the grown CNTs on the surfaces of the graphene contribute to increase the specific surface area of RHGOF. The results demonstrate that the synthesized porous, flexible and binder free hybrid electrode has advantages of higher ion diffusion rate, longer diffusion length and larger ion accessible surface area as compared to the pristine graphene which results in an extra ordinary galvanostatic charge-discharge specific capacitance of 557 F/g at a current density of 0.5 A/g, with excellent rate capabilities and superior cyclic stabilities.

2.5 D carbon/carbon composites modified by in situ grown hafnium carbide nanowires for enhanced electromagnetic shielding properties and oxidation resistance

Hafnium carbide nanowires (HfCnw) were in-situ grown in 2.5 D needle-punched carbon felts by a catalyst-assisted chemical vapor deposition method. Subsequently, the carbon felts were densified to obtain HfCnw reinforced carbon/carbon (HfCnw-C/C) composites via chemical vapor infiltration. Effects of HfCnw on electrical conductivity, electromagnetic shielding properties and oxidation resistance of C/C composites were investigated for the first time. The results show that the SET of C/C composites with 1.75 wt % HfCnw exceeds 40 dB in the whole frequency range of 8.2–12.4 GHz, implying more than 99.99% of the incident beam was shielded. Moreover, it is found that the relative magnitude relationship of the total shielding effectiveness of composites (SET(S-2h) > SET(S-3h) > SET(S-0h)) is similar with the conductance (σS-2h >σS-3h >σS-0h), which indicates the conductance can effectively predict the relative magnitude of the SET. In addition, the analysis based on oxidation curves demonstrates that HfCnw significantly enhance the oxidation resistance of the composites even with a small amount of HfCnw. The high electromagnetic interference (EMI) shielding effectiveness coupled with relatively low density and good oxidation resistance of HfCnw-C/C composites make itself exhibit remarkable potential as lightweight and high-performance structural-functional integrated materials for future EMI shielding application.

Ultrahigh Hole Mobility of Sn-Catalyzed GaSb Nanowires for High Speed Infrared Photodetectors.

Owing to the relatively low hole mobility, the development of GaSb nanowire (NW) electronic and photoelectronic devices has stagnated in the past decade. During a typical catalyst-assisted chemical vapor deposition (CVD) process, the adopted metallic catalyst can be incorporated into the NW body to act as a slight dopant, thus regulating the electrical properties of the NW. In this work, we demonstrate the use of Sn as a catalyst and dopant for GaSb NWs in the surfactant-assisted CVD growth process. The Sn-catalyzed zinc-blende GaSb NWs are thin, long, and straight with good crystallinity, resulting in a record peak hole mobility of 1028 cm2 V-1 s-1. This high mobility is attributed to the slight doping of Sn atoms from the catalyst tip into the NW body, which is verified by the red-shifted photoluminescence peak of Sn-catalyzed GaSb NWs (0.69 eV) compared with that of Au-catalyzed NWs (0.74 eV). Furthermore, the parallel array NWs also show a high peak hole mobility of 170 cm2 V-1 s-1, a high responsivity of 61 A W-1, and fast rise and decay times of 195.1 and 380.4 μs, respectively, under the illumination of 1550 nm infrared light. All of the results demonstrate that the as-prepared Sn-catalyzed GaSb NWs are promising for application in next-generation electronics and optoelectronics.

Growth Mechanism of SmB6 Nanowires Synthesized by Chemical Vapor Deposition: Catalyst-Assisted and Catalyst-Free.

SmB6 nanowires, as a prototype of nanostructured topological Kondo insulator, have shown rich novel physical phenomena relating to their surface. Catalyst-assisted chemical vapor deposition (CVD) is a common approach to prepare SmB6 nanowires and Ni is the most popular catalyst used to initiate the growth of SmB6 nanowires. Here, we study the effect of growth mechanism on the surface of SmB6 nanowires synthesized by CVD. Two types of SmB6 nanowires are obtained when using Ni as the catalyst. In addition to pure SmB6 nanowires without Ni impurity, a small amount of Ni is detected on the surface of some SmB6 nanowires by element analysis with transmission electron microscopy. In order to eliminate the possible distribution of Ni on nanowire surface, we synthesize single crystalline SmB6 nanowires by CVD without using catalyst. The difference between catalyst-assisted and catalyst-free growth mechanism is discussed.

Production and functionalization of carbon nanotubes for application in membrane synthesis for natural gas separation

Abstract In this study, carbon nanotubes (CNTs) were produced using catalyst-assisted chemical vapour deposition method. Ferrocene was used as the catalyst. Effects of carbon sources (acetylene and methane) and production conditions (temperature and type of carrier gas) on the quality of as-produced CNTs were investigated using field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), electronic precision balance (EPB) and raman spectroscopy. The best of as-produced CNTs was purified and functionalized using carboxylation protocol. The surface chemistry, thermal stability, textural property and crystallinity of functionalized CNTs (FCNTs) were obtained using Fourier transform infrared spectroscopy (FTIR), Thermo-gravimetric analysis (TGA), N2 physisorption at 77 K and X-ray diffraction (XRD), respectively. CNT produced from methane and argon at 900 °C has the best quality with ID/IG ratio of 0.17 while the CNT produced from acetylene and mixture of argon and hydrogen at 1000 °C has the highest yield of 1.78 mg/s. FTIR confirms successful functionalization of CNTs. The degree of functionalization obtained from TGA is consistent with that of EPB. N2 physisorption at 77 bK indicates increase in pore volume and average pore size of the FCNTs indicating more adsorption sites for the adsorbate. Thereby suggesting that FCNT could be a good filler in membrane synthesis for gas separation.

Gold Catalyst-Assisted Metal Organic Chemical Vapor Deposition of Bi-Te-Ni-Cu-Au Complex Thermoelectric Materials on Anodic Aluminum Oxide Nanoporous Template

Complex materials have unique thermal and electron transport properties. In this work, a novel catalyst-assisted metal organic chemical vapor deposition approach was employed to make Bi-Te-Ni-Cu-Au complex materials on an anodic aluminum oxide nanoporous substrate. Nickel acetate, copper nitrate, bismuth acetate, and tellurium (IV) chloride dissolved in N,N-dimethylformamide (DMF) were used as the metal sources for Ni, Bi, Cu, and Te, respectively. Hydrogen was used as the carrier gas. The anodic aluminum oxide substrate sputter-coated on a thin gold coating and was kept at 500 °C in a quartz tube in the reaction chamber. The chemical vapor deposition time was two hours. Scanning electron microscopy was used to reveal the morphology of the deposited materials. Due to metal catalyst assisted growth, the Bi-Te-Ni-Cu-Au materials were self-assembled into islands distributed fairly uniformly on the substrate. The mechanism for the morphological development of the materials was investigated. It was found that the Au nanoparticles facilitated the formation of the complex Bi-Te-Al-Ni-Cu materials. The prepared nanostructure has the highest absolute Seekbeck coefficient value of 260 µV/K, which is more than twice the value obtained from the bulk material.

Synthesis and properties of SrAl2O4:Eu2+, Dy3+ nanowires

The SrAl2O4:Eu2+, Dy3+ nanowire was successfully synthesized by means of catalyst-assisted thermal chemical vapor deposition method. Their morphology, structure, composition, luminescent properties are explored in way of SEM, TEM, XRD and PL analysis. The nanowires diameter is uniform distributed in 50∼80 nm, but orientation distribution is irregular, with the length varying from 4 μm to 20 μm. When heated up to the temperature of 1200 °C for three hours, the optimum synthesis is achieved with the alumina substrate covered by Al nanoparticles. The emission peak reaches to 517 nm with 365 nm light excitation and the luminous intensity was down to 1/10 of the initial brightness in 20 mins. The dielectric property was investigated at the room temperature, which show stronger dielectric loss ability.

Alignment control and atomically-scaled heteroepitaxial interface study of GaN nanowires.

Well-aligned GaN nanowires are promising candidates for building high-performance optoelectronic nanodevices. In this work, we demonstrate the epitaxial growth of well-aligned GaN nanowires on a [0001]-oriented sapphire substrate in a simple catalyst-assisted chemical vapor deposition process and their alignment control. It is found that the ammonia flux plays a key role in dominating the initial nucleation of GaN nanocrystals and their orientation. Typically, significant improvement of the GaN nanowire alignment can be realized at a low NH3 flow rate. X-ray diffraction and cross-sectional scanning electron microscopy studies further verified the preferential orientation of GaN nanowires along the [0001] direction. The growth mechanism of GaN nanowire arrays is also well studied based on cross-sectional high-resolution transmission electron microscopy (HRTEM) characterization and it is observed that GaN nanowires have good epitaxial growth on the sapphire substrate following the crystallographic relationship between (0001)GaN∥(0001)sapphire and (101[combining macron]0)GaN∥(112[combining macron]0)sapphire. Most importantly, periodic misfit dislocations are also experimentally observed in the interface region due to the large lattice mismatch between the GaN nanowire and the sapphire substrate, and the formation of such dislocations will favor the release of structural strain in GaN nanowires. HRTEM analysis also finds the existence of "type I" stacking faults and voids inside the GaN nanowires. Optical investigation suggests that the GaN nanowire arrays have strong emission in the UV range, suggesting their crystalline nature and chemical purity. The achievement of aligned GaN nanowires will further promote the wide applications of GaN nanostructures toward diverse high-performance optoelectronic nanodevices including nano-LEDs, photovoltaic cells, photodetectors etc.

Structural control of photoconductive CdSe nanostructures through a Bi-assisted VLS process

We report a method to grow structurally-controlled CdSe nanostructures by adopting Bi-mixed CdSe powder source to a catalyst-assisted chemical vapor deposition. Using this method, we synthesized Bi-doped CdSe nanocrystals with different shapes, such as nanocrystal film, nano-bumpy film, nanobelts, and nanowires. We fabricated a photodetector and a field-effect device using Bi-doped CdSe nanowires with high responsivity to white light. These results will offer a wide range of application possibilities in optoelectronic devices using CdSe nanostructures.

Effects of deposition temperature and time on HfC nanowires synthesized by CVD on SiC-coated C/C composites

HfC nanowires were synthesized on SiC-coated carbon/carbon composites via a catalyst-assisted chemical vapor deposition (CVD) process from the HfCl4–CH4–H2 system. The effects of deposition temperature (1273, 1323, 1373 and 1423K) and time (20, 40, 60 and 90min) on the formation and microstructure of HfC nanowires were investigated. The results showed that the diameter of HfC nanowires increased with the deposition temperature increasing; both the density and thickness of HfC nanowire films increased with the deposition time prolonging. The growth of HfC nanowires followed the bottom-type vapor–liquid–solid (VLS) mechanism.

Preparation of Highly Flexible SiC Nanowires by Fluidized Bed Chemical Vapor Deposition

High flexible SiC nanowires with diameters in the range of 10 to 50 nm and lengths of several hundred micrometers to several millimeters are synthesized using a novel catalyst-assisted fluidized bed chemical vapor deposition method. Methyltrichlorosilane (MTS) is used as source material and particles containing cobalt were used as catalyst. The fluidized bed is specially designed to form a perpendicular uneven temperature distribution. Suspended nanosized liquid catalyst droplets are produced at middle high temperature zone and SiC nanowires are formed in situ on the catalyst droplets then accumulated at the upper part of fluidized bed. The nanowires exhibit a single crystal feature with some stacking faults. Transmission electron microscopy (TEM) analysis verifies a growth direction along the <111> axis of the nanowires. A blue shift optical emission band at about 420 nm is detected using photoluminescence spectroscopy. From the high-resolution TEM investigation, a nanoscale smooth plane between the nanowire and the catalyst tip is observed and the two phases exhibit highly epitaxial growth interface. The growth process of the nanowires is discussed and a possible formation mechanism is proposed.

Bi-Assisted CdTe/CdS Hierarchical Nanostructure Growth for Photoconductive Applications.

We developed a method to control the structure of CdTe nanowires by adopting Bi-mixed CdTe powder source to a catalyst-assisted chemical vapor deposition, which allowed us to fabricate CdTe/CdS hierarchical nanostructures. We demonstrated that diverse nanostructures can be grown depending on the combination of the Bi powder and film catalysts. As a proof of concepts, we grew CdTe/CdS branched nanowires for the fabrication of photodetectors. The hierarchical nanostructure-based photodetectors showed an improved photoresponsivity compared to the single CdTe nanowire (NW)-based photodetector. Our strategy can be a simple but powerful method for the development of advanced optoelectronic devices and other practical applications.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-015-1037-6) contains supplementary material, which is available to authorized users.

Investigation of the growth and properties of single-crystalline aluminum nitride nanowires

The growth of high-quality, large-scale wurtzite-structured aluminum nitride (AlN) nanowires on c-Al2O3 substrates using a catalyst-assisted chemical vapor deposition method was investigated. The morphology and structure of the nanowires were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. These uniform nanowires exhibit a smooth surface and are 50nm in diameter and 10–30μm long; they also exhibit a hexagonal single-crystal lattice with parameters of c=0.497nm and a=0.272nm and grow along the [100] direction. A room-temperature near-band edge emission at 6.12eV was observed in their electron excitation spectra. Our approach provides a simple and efficient method to grow high-quality AlN nanowires, and our results indicate that they are promising materials for use in various applications.

Plasmonic Switching in Au-Functionalized GaN Nanowires in the Realm of Surface Plasmon Polariton Propagation: a Single Nanowire Switching Device

Photoresponse of Au nanoparticle functionalized semiconducting GaN (Au-GaN) nanowires is reported for an optical switching using 532 excitation. Wide band gap GaN nanowires are grown by catalyst-assisted chemical vapor deposition technique and functionalized with Au in the chemical route. Au-GaN nanowires show surface plasmon resonance (SPR) mode of Au nanoclusters around 550 nm along with characteristic band for GaN around 365 nm. An optical switching is observed for Au-GaN nanowires with a sub-band gap excitation of 532 nm suggesting possible role of surface plasmon polariton-assisted transport of electron in the system. The role of band conduction is ruled out in the absence of optical switching using 325-nm excitation which is higher in energy that the reported band gap of GaN ∼3.4 eV (365 nm) at room temperature. A finite amount of interband contribution of Au plays an important role along with the interparticle separation. The switching device is also successfully tested for a single GaN nanowire functionalized with Au nanoclusters. A resistivity value of 0.05 Ω-cm is measured for surface plasmon polariton-assisted electrical transport of carrier in the single GaN nanowire.

Catalyst‐Assisted Growth of Single‐Crystalline Hafnium Carbide Nanotubes by Chemical Vapor Deposition

Single‐crystalline hafnium carbide (HfC) nanotubes were synthesized by a one‐step catalyst‐assisted chemical vapor deposition (CVD) method. The typical nanotubes had uniform diameters of ~60 nm and wall thicknesses of ~15 nm and preferentially grew along [201]. From HRTEM/EELS analysis, the growth mechanism based on carbon nanotubes (CNT) tip growth and CNT‐templated reaction was proposed for explaining the formation of HfC nanotubes. According to the mechanism, CNTs were first formed by diffusion of C atoms on the surface of solid Ni catalyst particles. Then, gaseous Hf species reacted with C atoms from CNTs to form HfC nanotubes. During the entire growth process, Hf atoms did not participate in the catalytic reaction. Thus, this process was distinguished from the conventional vapor–liquid–solid process.