Water-assisted Chemical Vapor Deposition Research Articles

Thermal and electrical conduction properties of vertically aligned carbon nanotubes produced by water-assisted chemical vapor deposition

We present both thermal and electrical conduction properties of vertically aligned carbon nanotubes (VACNTs), synthesized by the water-assisted chemical vapor deposition method using Fe–Ti–O nanoparticles as catalyst. Thermal diffusivity and electrical resistance of VACNTs have been measured by the laser flash method and direct-current four-terminal method, respectively. The VACNTs are found to have thermal diffusivities of the same order as isotropic graphite and the electrical characteristics of semiconductors. The electrical resistance shows a T −1/4 temperature dependence, which implies that the conduction of electrons is dominated by 3D Mott variable range hopping.

Integrated CNTs thin film for MEMS mechanical sensors

This paper reports the top-down fabrication of CNTs thin film on MEMS structure, and characterization of piezoresistive coefficients of aligned single wall carbon nanotube (SWNT) forest film. The film was synthesized by water-assisted chemical vapor deposition (CVD), a process known as “super growth”. CNTs film was condensed, manually maneuvered and conveniently patterned by EB lithography to form desirable shapes. The longitudinal and transverse gauge factors of the CNTs thin film were measured to be 3.75 and 0.67, respectively.

Supergrowth of Aligned Carbon Nanotubes Directly on Carbon Papers and Their Properties as Supercapacitors

We demonstrate that vertically aligned carbon nanotubes can be synthesized directly on conductive carbon papers and used as excellent electrochemical capacitors. The carbon nanotubes were synthesized with use of an Al/Fe catalyst via water-assisted chemical vapor deposition. They grew as fast as ∼100 μm/min and approximately 70% of them had double walls with an average diameter of ∼6 nm. Interestingly, the carbon nanotube forest showed microscale patterns defined by the structure of underlying carbon papers. The nanotubes were attached well to the carbon papers and maintained their adhesion under mild ultrasonication in solution. Owing to the direct integration, naturally patterned structure, and good alignment, the carbon nanotubes showed excellent performance as supercapacitors. In aqueous 1 M H2SO4 solution, specific capacitance, energy, and power measured at the current density of 20 A/g were ca. 200 F/g, 20 Wh/kg, and 40 kW/kg, respectively. A specific energy of >100 Wh/kg was achieved when organic e...

A facile route to synthesize titanium oxide nanowires via water-assisted chemical vapor deposition

Single crystalline rutile titanium oxide nanowires have been synthesized in bulk yield based on commercial metal titanium by a facile water-assisted chemical vapor deposition method. The morphology, crystallinity, and phase structure of the nanowires have been characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and X-ray diffraction (XRD). This growth strategy is applicable for commercial metal titanium substrate with different spatial dimensions, such as powder, network mesh, and flat foil. The as-synthesized nanowires are found to be mainly composed of single crystalline rutile TiO2 nanowires in spiral shape with a small amount of hexagonal Ti2O nanowires with zigzag form. A growth mechanism has been proposed to explain the novel spiral and zigzag types of titanium oxide nanowires under moderate temperature (850 °C). This method promises an alternative way for industrialization of titanium oxide nanowires which may serve as a good candidate for various industrial applications such as optoelectronic, electronic, and electrochemical nanodevices.

Very Black Infrared Detector from Vertically Aligned Carbon Nanotubes and Electric-Field Poling of Lithium Tantalate

Vertically aligned multiwall carbon nanotubes were grown by water-assisted chemical vapor deposition on a large-area lithium tantalate pyroelectric detector. The processing parameters are nominally identical to those by which others have achieved the "world's darkest substance" on a silicon substrate. The pyroelectric detector material, though a good candidate for such a coating, presents additional challenges and outcomes. After coating, a cycle of heating, electric field poling, and cooling was employed to restore the spontaneous polarization perpendicular to the detector electrodes. The detector responsivity is reported along with imaging as well as visible and infrared reflectance measurements of the detector and a silicon witness sample. We find that the detector responsivity is slightly compromised by the heat of processing and the coating properties are substrate dependent. However, it is possible to achieve nearly ideal values of detector reflectance uniformly less than 0.1% from 400 nm to 4 microm and less than 1% from 4 to 14 microm.

Integration of SWNT film into MEMS for a micro-thermoelectric device

This paper reports the top-down fabrication of carbon nanotube (CNT) thin film onmicroelectrical mechanical systems (MEMS) and the characterization of the thermoelectriccoefficients of aligned single-walled carbon nanotube (SWNT) forest film. The film wassynthesized by water-assisted chemical vapor deposition (CVD), a process known as‘super-growth’. CNT film was then condensed, manually maneuvered, and convenientlypatterned by electron beam (EB) lithography to form desirable shapes. We demonstratethis process by patterning an array of Au–CNT thermocouples; measurement under roomtemperature conditions reveals the Seebeck coefficient of aligned SWNT film to be 18–20 µV C − 1.

Lengthening and thickening of multi-walled carbon nanotube arrays grown by chemical vapor deposition in the presence and absence of water

Multi-walled carbon nanotube (MWCNT) array growth demonstrates lengthening and thickening stages by chemical vapor deposition (CVD) in the presence (water-assisted CVD (WACVD)) and absence (conventional CVD (CCVD)) of water. In the lengthening stage of WACVD, CNT wall number remains constant and catalysts preserve the activity; while in the thickening stage of WACVD, MWCNTs thicken substantially and catalysts deactivate following the previously proposed radioactive decay model. In CCVD, however, the lengthening and thickening were found to be competitive. Mechanisms of thickening in WACVD and CCVD were comparatively analyzed, from which the effect of water was elucidated. These findings provide experimental solutions to the fabrication of MWCNT arrays with controlled wall numbers by WACVD and CCVD.

Calibrating the single-wall carbon nanotube resonance Raman intensity by high resolution transmission electron microscopy for a spectroscopy-based diameter distribution determination

We study a single-wall carbon nanotube (SWNT) sample grown by water-assisted chemical vapor deposition with both resonance Raman scattering (RRS) and high resolution transmission electron microscopy. High resolution transmission electron microscopy measurements of 395 SWNTs determined the diameter distribution of the sample, allowing us to calibrate an RRS radial breathing mode (RBM) map obtained with 51 laser excitation energies from 1.26 to 1.73 eV. Thus, we determined the diameter dependence of the RRS RBM cross-section, which in turn allows the determination of the diameter distribution of any SWNT sample by measuring the RBM Raman signal.

Dry Contact Transfer Printing of Aligned Carbon Nanotube Patterns and Characterization of Their Optical Properties for Diameter Distribution and Alignment

A scalable and facile approach is demonstrated where as-grown patterns of well-aligned structures composed of single-walled carbon nanotubes (SWNT) synthesized via water-assisted chemical vapor deposition (CVD) can be transferred, or printed, to any host surface in a single dry, room-temperature step using the growth substrate as a stamp. We demonstrate compatibility of this process with multiple transfers for large-scale device and specifically tailored pattern fabrication. Utilizing this transfer approach, anisotropic optical properties of the SWNT films are probed via polarized absorption, Raman, and photoluminescence spectroscopies. Using a simple model to describe optical transitions in the large SWNT species present in the aligned samples, polarized absorption data are demonstrated as an effective tool for accurate assignment of the diameter distribution from broad absorption features located in the infrared. This can be performed on either well-aligned samples or unaligned doped samples, allowing simple and rapid feedback of the SWNT diameter distribution that can be challenging and time-consuming to obtain in other optical methods. Furthermore, we discuss challenges in accurately characterizing alignment in structures of long versus short carbon nanotubes through optical techniques, where SWNT length makes a difference in the information obtained in such measurements. This work provides new insight to the efficient transfer and optical properties of an emerging class of long, large diameter SWNT species typically produced in the CVD process.

Synthesis of vertically-aligned carbon nanotubes on stainless steel by water-assisted chemical vapor deposition and characterization of their electrochemical properties

We demonstrate synthesis of carbon nanotubes on stainless steel by water-assisted chemical vapor deposition using an Al/Fe bimetallic catalyst. A forest of vertically-oriented carbon nanotubes with a length of a few hundreds of micrometers were grown on the substrates. Electrical measurement confirmed that the carbon nanotubes were electrically connected to the stainless steel. Owing to the carbon nanotubes, electrochemical properties such as faradaic current and electron transfer were considerably improved. Carbon nanotubes grown on the stainless steel substrates would potentially be useful for various applications including electrochemical energy conversion and storage.

High-Density Short-Height Directly Grown CNT Patterned Emitter on Glass

Vertically aligned, high-density (4× 109 /cm2) carbon nanotubes (CNTs) with an average short height of 1.5 μm were grown directly on a Cr electrode patterned on a glass substrate at a substrate temperature below 550 °C by water-assisted chemical vapor deposition (CVD). Threshold field of the emission of the patterned grown CNT could be tuned with the pattern shape, and it was 1.8 V/μm at 1 μA/cm2 for the emitter with a hole spacing parameter of S = 20 μm. The stability of field emission of the patterned CNT emitter on glass was confirmed with continuously working more than 24 hours at a pulse current density of 70 μA/cm2 with the duty ratio of 7.5 %. [DOI: 10.1380/ejssnt.2010.336]

Evolution, Activity, and Lifetime of Alumina-supported Fe Catalyst During Super Growth of Single-walled Carbon Nanotube Carpets: Influence of the Type of Alumina

AbstractThe influence of the type of alumina used as catalyst support on the evolution, activity, and lifetime of the catalyst during water-assisted CVD growth (or ‘supergrowth') of single-walled carbon nanotube (SWNT) carpets has been studied. The catalyst consisted of a thin Fe film supported on alumina films deposited by different methods: atomic layer deposition (ALD), e-beam, and magnetron sputtering. In order to fully understand the influence of the type of alumina on SWNT carpet growth, crystalline alumina (c-cut sapphire) and annealed alumina deposited by e-beam were also used as catalyst supports. The activity and lifetime of Fe catalyst during SWNT carpet growth showed a strong dependence on the type of alumina used as support. Fe supported on sputtered alumina (sputtered/Fe) showed the highest catalytic activity and lifetime, which was closely followed by e-beam/Fe while Fe supported on sapphire (sapphire/Fe) showed the least catalytic activity and lifetime. AFM, XPS depth profile, variable angle spectroscopic ellipsometry (VASE) studies revealed that the catalyst evolution and the porosity of the different alumina supports correlate with the lifetime and activity of the catalysts.

Improved and Large Area Single-Walled Carbon Nanotube Forest Growth by Controlling the Gas Flow Direction

A gas shower system was introduced to improve the growth of single-walled carbon nanotube (SWNT) forests by controlling the gas flow direction. Delivery of gases from the top of the forest enabled direct and precise supply of ethylene and water vapor to the Fe catalysts. As such, this approach solved one of the limiting factors of water-assisted chemical vapor deposition method (CVD), that is, delivery of the very small optimum water level to the catalysts. Consequently, this approach improved SWNT forests growth stability, uniformity, reproducibility, carbon efficiency (32%), and catalyst lifetime. With this improved growth, we could synthesize a 1 cm tall forest with 1 x 1 cm size. Also we employed this approach to grow an A4 size SWNT forest to highlight the scalability of water-assisted CVD.

Synthesis of Aligned Few-Walled Carbon Nanotubes on Conductive Substrates

We report a robust synthesis of densely aligned few-walled carbon nanotubes on industrially useful conductive substrates such as carbon fibers and carbon papers. This was achieved by water-assisted chemical vapor deposition using an Al/Fe bimetallic catalyst and a thin epoxy-based polymer layer beneath the catalyst. The carbon nanotubes had a mean diameter of 6 nm, mainly double walls, and lengths up to a couple of millimeters. Raman spectroscopy showed high G-band/D-band intensity ratio (>5) indicating high quality of the nanotubes. They were tightly bound and electrically connected to the substrates, as confirmed by ultrasonication test and enhanced cyclic voltammetry signals, respectively. High quality carbon nanotubes synthesized on conductive substrates may find applications in fuel cells, lithium-ion batteries, and field emitters.

Abrasion as a Catalyst Deposition Technique for Carbon Nanotube Growth

Mechanical abrasion of stainless steel (SS) surfaces is demonstrated as an effective technique for the deposition of catalyst to support growth of high density layers of carbon nanotubes (CNTs) in water-assisted catalytic chemical vapor deposition. In all cases of Fe-containing materials abraded on Al(2)O(3) substrates, CNT growth is observed; the 400 series of SS appears to deposit catalyst most efficiently. We demonstrate that this simple abrasion technique enables both micro- and nanoscale accuracy in catalyst patterning as well as large area catalyst deposition for uniform, dense CNT growth. Raman spectroscopy characterization indicates high quality CNTs grown by this approach. This technique provides an inexpensive and simple route for addition of catalyst for Fe-based surface growth of CNTs.

Tunable field emission properties of carbon nanotube arrays by engineering Fe catalysts

High-quality carbon nanotube (CNT) arrays composed of nanotubes with different diameters and wall numbers were synthesized by water-assisted chemical vapor deposition (CVD) from engineered Fe catalysts. Interestingly, the distribution of nanotube diameter and wall number broadened over 2.5 times as the catalytic Fe thickness increased. The mean diameter and wall number of nanotubes increased monotonically with the Fe thickness, while the calculated CNT area density within an array dropped about 32 times. Field emission measurements revealed that the turn-on voltage for CNT arrays decreased from 3.5 to 2.5 V/µm with the increased catalytic Fe thickness. It was believed that the screening effect from the proximity of neighboring nanotubes has a dominant influence than the CNT diameter on the resulting turn-on voltage.

Effect of Oxygen Included in Substrates for Growth of Brushlike Carbon Nanotubes

We investigated the effect of oxygen incorporated in substrates for forming Fe-based catalytic particles and growing carbon nanotubes (CNTs) by water-assisted catalytic chemical vapor deposition. We examined two types of SiO2-covered Si (SiO2/Si) and oxygen-free Si3N4-covered Si (Si3N4/Si) as substrates for supporting Fe films. Well-aligned CNTs were synthesized at a higher growth rate on Si3N4/Si compared with those on SiO2/Si. The compositions of Fe-based catalytic particles that were formed by heating the substrates were examined using X-ray photoelectron spectroscopy (XPS) to determine the differences. Results show that the concentration ratio of Fe to Fe oxide in the catalytic particles strongly affects the alignment and height of synthesized brushlike CNTs and is well optimized in oxygen-free substrates.

Patterned carbon nanotube growth using an electron beam sensitive direct writablecatalyst

This paper reports the use of spin coatable Fe naphthenate (Fe(C11H7O2)2) as an electron beam sensitive direct writable catalyst suitable for growing highquality carbon nanotubes (CNTs). Vertically aligned, densely packed, equal inheight, three- to four-walled and 5–7 nm inner diameter CNTs were grown on thenanoscale stripe patterns by water-assisted chemical vapor deposition. The correlationbetween electron beam dosage and the growth characteristics of CNTs is alsodiscussed.

Thin film transistors using preferentially grown semiconducting single-walledcarbon nanotube networks by water-assisted plasma-enhanced chemical vapordeposition

Nearly perfect semiconducting single-walled carbon nanotube random network thin filmtransistors were fabricated and their reproducible transport properties were investigated.The networked single-walled carbon nanotubes were directly grown by water-assistedplasma-enhanced chemical vapor deposition. Optical analysis confirmed that the nanotubeswere mostly semiconductors without clear metallic resonances in both the Raman and theUV–vis–IR spectroscopy. The transistors made by the nanotube networks whose densitywas much larger than the percolation threshold also showed no metallic paths.Estimation based on the conductance change of semiconducting nanotubes in theSWNT network due to applied gate voltage difference (conductance difference foron and off state) indicated a preferential growth of semiconducting nanotubeswith an advantage of water-assisted PECVD. The nanotube transistors showed10−5 ofon/off ratioand ∼8 cm2 V−1 s−1 of field effect mobility.

In situ growth of SnO 2 nanowires on the surface of Au-coated Sn grains using water-assisted chemical vapor deposition

SnO 2 nanowires (NWs) have been directly synthesized in high-yield on the surface of Au-coated Sn grains using a water-assisted chemical vapor deposition (CVD) method. The Au-coated Sn grains served themselves as both the Sn source, which reacted with H 2O vapor at 700–800 °C under the flow of high purity N 2 carrier, and the substrate for the growth of the SnO 2 NWs. The synthesized SnO 2 NWs were systematically studied using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. The single-crystalline SnO 2 NWs are about 100–150 μm in length and 16–60 nm in diameter, which can be tuned by simply adjusting the CVD temperature. A possible formation mechanism of SnO 2 NWs was proposed and discussed. Gas sensors based on SnO 2 NWs prepared at 800 °C exhibited excellent sensing performance, characterized with excellent selectivity and fast response time to ethanol vapor and H 2S gas operating at 320 °C.