One-Pot Fabrication of Nanocomposites Composed of Carbon Nanotubes and Alumina Powder Using a Rotatable Chemical Vapor Deposition System.

The fabrication of multi-dimensional nanocomposites has been extensively attempted to achieve synergistic performance through the uniform mixing of functional constituents. Herein, we report a one-pot fabrication of nanocomposites composed of carbon nanotubes (CNTs) and Al2O3 powder. Our strategy involves a synthesis of CNTs on the entire Al2O3 surface using a rotatable chemical vapor deposition system (RCVD). Ehylene and ferritin-induced nanoparticles were used as the carbon source and wet catalyst, respectively. The RCVD was composed of a quartz reaction tube, 5.08 cm in diameter and 150 cm in length, with a rotation speed controller. Ferritin dissolved in deionized water was uniformly dispersed on the Al2O3 surface and calcinated to obtain iron nanoparticles. The synthesis temperature, time, and rotation speed of the chamber were the main parameters used to investigate the growth behavior of CNTs. We found that the CNTs can be grown at least around 600 °C, and the number of tubes increases with increasing growth time. A faster rotation of the chamber allows for the uniform growth of CNT by the tip-growth mechanism. Our results are preliminary at present but show that the RCVD process is sufficient for the fabrication of powder-based nanocomposites.

Low-Power Complementary Inverter Based on Graphene/Carbon-Nanotube and Graphene/MoS2 Barristors.

The recent report of a p-type graphene(Gr)/carbon-nanotube(CNT) barristor facilitates the application of graphene barristors in the fabrication of complementary logic devices. Here, a complementary inverter is presented that combines a p-type Gr/CNT barristor with a n-type Gr/MoS2 barristor, and its characteristics are reported. A sub-nW (~0.2 nW) low-power inverter is demonstrated with a moderate gain of 2.5 at an equivalent oxide thickness (EOT) of ~15 nm. Compared to inverters based on field-effect transistors, the sub-nW power consumption was achieved at a much larger EOT, which was attributed to the excellent switching characteristics of Gr barristors.

Single-Walled Carbon Nanotube Synthesis Yield Variation in a Horizontal Chemical Vapor Deposition Reactor.

The controlled synthesis of single-walled carbon nanotubes (SWNTs) is essential for their industrial application. This study investigates the synthesis yield of SWNTs, which depends on the positions of the samples on a horizontal chemical vapor deposition (CVD) system. Methane and Fe thin films were used as the feedstock and catalyst for SWNTs synthesis, respectively. A high-resolution scanning electron microscope was used to examine the synthesis yield variation of the SWNTs along the axial distance of the reactor. The morphology and crystallinity of the fabricated SWNTs were evaluated by atomic force microscopy and Raman spectroscopy, respectively. We observed that the highest synthesis yield of the SWNTs was obtained in the rear region of the horizontal reactor, and not the central region. These results can be applied to the synthesis of various low-dimensional nanomaterials, such as semiconducting nanowires and transition metal dichalcogenides, especially when a horizontal CVD chamber is used.

Investigation of Early Stage of Carbon Nanotube Growth on Plasma-Pretreated Inconel Plates and Comparison with Other Superalloys as Substrates.

We investigate the early stage of carbon nanotube (CNTs) growth on Inconel 600 to address the effect of pretreatments such as annealing and plasma pretreatment on growth behavior. In addition, we compare the growth results to other Ni-based superalloys including Invar 42 and Hastelloy C276. The growth substrates were prepared using mechanical polish, thermal annealing and plasma pretreatment. The air annealing was performed at 725 °C for 10 min and plasma pretreatment was subsequently undergone with 10.5 W at 500 °C for 30 min. The annealed and plasma-pretreated substrates exhibited different surface morphologies on the surface and enhanced growth behavior of CNT was observed from the region of particulate surface. The optimized growth temperature, which produces the highest CNT height, was determined at 525 °C for Ni and Inconel 600 and 625 °C for Invar 42 and Hastelloy C276 substrates. The difference of optimal growth temperature is expected to the existence of high temperature elements such as Mn or Mo in the alloys. X-ray diffraction spectroscopy revealed that the formation of roughened oxide layers caused by the pretreatments would promote the nucleation and growth of the CNTs.

Synthesis of ZnO Nanomaterials Using Low-Cost Compressed Air as Microwave Plasma Gas at Atmospheric Pressure.

Zinc oxide (ZnO) nanomaterials were efficiently synthesized using a microwave plasma torch system at atmospheric pressure. The Zn powder was passed through a microwave plasma region, in which it melted and vaporized. Tetrapod-type ZnO nanomaterials with a diameter of 29.8 ± 8.0 nm were synthesized using a high-purity O2/N2 mixed gas. In particular, ZnO nanowires with a diameter of 109.5 ± 8.0 nm and a length of 5-6 µm were produced using an inexpensive compressed air as a microwave plasma gas. It was confirmed that the nanowires synthesized using the compressed air showed higher light absorption in the visible region than the tetrapod-type ZnO. In addition, the redshifts in the absorption peak and photoluminescence peak were observed from 370.6 to 375.2 nm and 380 to 390 nm, respectively. The obtained results can be explained by the change of energy levels due to the defects in the ZnO nanowires such as vacancies and interstitials of Zn and oxygen. Finally, we can conclude that cost-effective compressed air is appropriate not only for the synthesis of ZnO nanowire, but also the enlargement of optical absorption and emission range.

Size engineering of metal nanoparticles to diameter-specified growth of single-walled carbon nanotubes with horizontal alignment on quartz.

The electronic, physical and optical properties of single-walled carbon nanotubes (SWNTs) are governed by their diameter and chirality, and thus much research has been focused on controlling the diameter and chirality of SWNTs. To date, control of the catalyst particle size has been thought to be one of the most promising approaches to control the diameter or chirality of SWNTs owing to the correlation between catalyst particle size and tube diameter.In this study, we demonstrate the size engineering of catalytic nanoparticles for the controlled growth of diameter-specified and horizontally aligned SWNTs on quartz substrates. Uniformly sized iron nanoparticles derived from ferritin molecules were used as a catalyst, and their size was intentionally decreased via thermal heat treatment at 900 °C under atmospheric Ar ambient. ST-cut quartz wafers were used as growth substrates in order to elucidate the effect of the size of the nanoparticles on the tube diameter and the effect of catalyst size on the degree of parallel alignment on the quartz substrates. SWNTs grown by chemical vapor deposition using methane as feedstock exhibited a high degree of horizontal alignment when the particle density was low enough to produce individual SWNTs without bundling. Annealing for 60 min at 900 °C produced a reduction of nanoparticle diameter from 2.6 to 1.8 nm and a decrease in the mean tube diameter from 1.2 to 0.8 nm, respectively. Raman spectroscopy results corroborated the observation that prolonged heat treatment of nanoparticles yields thinner tubes with narrower size distributions. The results of this work suggest that straightforward thermal annealing can be a facile way to obtain uniform-sized SWNTs as well as catalytic nanoparticles.

Thermal oxidation of synthesized graphenes and their optical property characterization.

The results of the thermal oxidation of synthesized graphenes and their optical property characterization using Raman spectroscopy are reported. Graphene was synthesized via thermal-chemical vapor deposition on Ni catalytic thin films deposited by electron beam deposition, and was successfully transferred onto three-dimensional trench substrates to obtain a suspended structure, which is the most appropriate template for use in probing the changes of physical properties of graphene by ignoring the substrate effects. The thermal oxidation was performed in a tube furnace at an elevated temperature of 500 degrees C under air, and Raman analysis was repeatedly carried out to investigate the oxidation effects. A drastic structural change of graphene was anticipated from the based on the dramatic changes in the Raman spectra. It is expected that controlled oxidation will help systematically decrease in the number of graphene layers, which will contribute to the successful development of graphene-based devices that are capable of operating under oxidative environments.

Controlled Synthesis of Monolayer Graphene Toward Transparent Flexible Conductive Film Application.

We demonstrate the synthesis of monolayer graphene using thermal chemical vapor deposition and successive transfer onto arbitrary substrates toward transparent flexible conductive film application. We used electron-beam-deposited Ni thin film as a synthetic catalyst and introduced a gas mixture consisting of methane and hydrogen. To optimize the synthesis condition, we investigated the effects of synthetic temperature and cooling rate in the ranges of 850-1,000°C and 2-8°C/min, respectively. It was found that a cooling rate of 4°C/min after 1,000°C synthesis is the most effective condition for monolayer graphene production. We also successfully transferred as-synthesized graphene films to arbitrary substrates such as silicon-dioxide-coated wafers, glass, and polyethylene terephthalate sheets to develop transparent, flexible, and conductive film application.

Synthesis and characterization of Au-attached single-walled carbon nanotube bundles.

We synthesized suspended single-walled carbon nanotubes (SWNTs) by the thermal chemical vapor deposition method and functionalized them with Au nanoparticles (NPs). We used 3-(aminopropyl)triethoxysilane as a linker and controlled the Au NP density on the SWNT surface by changing the reaction time. In the Raman scattering spectra of the Au-functionalized SWNTs, an enhanced peak frequency and peak intensity were observed in the non-resonant region. A significant enhancement of the metallic character in the high frequency region was also observed, especially when we used a 633 nm laser. By measuring the electric properties using a standard field effect transistor configuration, we found that charge transfer occurred during the functionalization processes. It is expected that the charge transfer related optical enhancement may affect the observed change in the Raman profiles.

In situ raman measurements of suspended individual single-walled carbon nanotubes under strain.

We present a technique for in situ Raman measurements of suspended individual single-walled carbon nanotubes (SWNTs) under strain. We observe a strong change in the radial breathing mode intensity with increasing strain as the nanotube moves out of (or into) resonance, and for strain greater than approximately 2%, there is a clear irreversible upshift in the G-mode frequencies accompanied by an increase in intensity of a broad peak at a position associated with the D mode. For lower strain, the G-mode peaks (A1, E1, and E2) do not change significantly in position but change in relative intensity.

Cobalt-filled apoferritin for suspended single-walled carbon nanotube growth with narrow diameter distribution.

Cobalt-filled apoferritin (Co-ferritin) was, for the first time, used as a wet catalyst for the synthesis of single-walled carbon nanotubes (SWNTs) with narrow diameter distribution. Co-ferritins were spin-coated and converted to cobalt nanoparticles by calcination. Using chemical vapor deposition, suspended networks of SWNTs were formed on pillar-structured substrates. The suspended SWNTs show narrow tube diameter distribution with a relatively good graphite structure. By virtue of the low diffusion coefficient of cobalt, Co-ferritin might be more useful for narrow diameter SWNTs growth than ferritins, which encase iron particles.

Formation and structural observation of cesium encapsulated single-walled carbon nanotubes.

Cesium encapsulation inside single-walled carbon nanotubes (SWNTs) is for the first time realized by ion irradiation of SWNTs immersed in a magnetized alkali-metal plasma, the configuration of which is confirmed to comprise three varieties by field emission type transmission electron microscopy (FE-TEM) and scanning TEM (STEM) observation.