Shohei Chiashi

Low-temperature synthesis of high-purity single-walled carbon nanotubes from alcohol

Abstract By using alcohol as the carbon source, a new simple catalytic chemical vapor deposition technique to synthesize high-purity single-walled carbon nanotubes at low temperature is demonstrated. Because of the etching effect of decomposed OH radical attacking carbon atoms with a dangling bond, impurities such as amorphous carbon, multi-walled carbon nanotubes, metal particles and carbon nanoparticles are completely suppressed even at relatively low reaction temperature such as 700–800 °C. By using methanol, generation of SWNTs even at 550 °C is demonstrated. The high-purity synthesis at low temperature promises large scale production at low cost and the direct growth of SWNTs on conventional semiconductor devices already patterned with aluminum.

Direct synthesis of high-quality single-walled carbon nanotubes on silicon and quartz substrates

Abstract A new technique of synthesizing high-quality single-walled carbon nanotubes (SWNTs) directly on the surface of silicon and quartz substrates has been developed by means of the low-temperature catalytic CVD method using ethanol. The proposed method does not employ conventional deposition/sputtering for the mounting of catalytic metals on the substrates, but it adopts an easy and costless liquid-based dip-coat approach without need of support/underlayer materials that were often used in previous studies. The substrate surface is blackened with a uniform layer of SWNTs after the CVD at an optimum condition. The optical absorption of ‘as-grown’ SWNTs has first been measured using thereby prepared SWNT-synthesized quartz substrate.

Characterization of single-walled carbon nanotubes catalytically synthesized from alcohol

High purity single-walled carbon nanotubes (SWNTs) were synthesized from ethanol by catalytic CVD method. The yield of SWNTs was determined based on TGA complemented by Raman and TEM analyses. The effects of CVD reaction time and pre-reduction of catalytic metal on the yield and quality of synthesized SWNTs were investigated. The SWNT yield of more than 40% was achieved over the weight of zeolite support powder with Fe/Co catalyst, which corresponded to more than 800% yield over the weight of the catalytic metal, within the CVD reaction time of 120 min assuring as-grown high quality.

Growth of Single-Walled Carbon Nanotubes from Ceramic Particles by Alcohol Chemical Vapor Deposition

Al2O3 ceramic nano-particles, which were regarded as an inactive catalyst in the growth of carbon nanotubes in the past, have been prepared as the catalyst for single-walled carbon nanotube (SWCNT) growth using an alcohol chemical vapor deposition method. Dense SWCNTs have been successfully synthesized, indicating that Al2O3 serves as an efficient catalyst. Moreover, it was found that many SWCNTs were also grown from irregular large Al2O3 particles ranging from several tens of nanometers to hundreds of nanometers, which has never been observed in the case of metallic catalyst particles. These results give more insights into the role of catalyst in SWCNT growth.

Exciton Diffusion in Air-Suspended Single-Walled Carbon Nanotubes

Direct measurements of the diffusion length of excitons in air-suspended single-walled carbon nanotubes are reported. Photoluminescence microscopy is used to identify individual nanotubes and to determine their lengths and chiral indices. Exciton diffusion length is obtained by comparing the dependence of photoluminescence intensity on the nanotube length to numerical solutions of diffusion equations. We find that the diffusion length in these clean, as-grown nanotubes is significantly longer than those reported for micelle-encapsulated nanotubes.

Growth of Vertically Aligned Single-Walled Carbon Nanotubes on Alumina and Sapphire Substrates

The synthesis of vertically aligned carbon nanotubes with submillimeter-order heights was performed using ethanol chemical vapor deposition with Co catalysts supported on Al2O3 substrates. The effects of Al2O3 in the form of amorphous alumina and single-crystalline sapphire were investigated through a characterization of the Co catalyst particles on the substrates. An important effect of Al2O3 was found to be the production of highly dense and nanosized Co particles, owing to a low surface diffusivity.

Enhancement of carbon nanotube photoluminescence by photonic crystal nanocavities

Photonic crystal nanocavities are used to enhance photoluminescence from single-walled carbon nanotubes. Micelle-encapsulated nanotubes are deposited on nanocavities within Si photonic crystal slabs and confocal microscopy is used to characterize the devices. Photoluminescence spectra and images reveal nanotube emission coupled to nanocavity modes. The cavity modes can be tuned throughout the emission wavelengths of carbon nanotubes, demonstrating the ability to enhance photoluminescence from a variety of chiralities.

Diameter Modulation of Vertically Aligned Single-Walled Carbon Nanotubes

We demonstrate wide-range diameter modulation of vertically aligned single-walled carbon nanotubes (SWNTs) using a wet chemistry prepared catalyst. In order to ensure compatibility to electronic applications, the current minimum mean diameter of 2 nm for vertically aligned SWNTs is challenged. The mean diameter is decreased to about 1.4 nm by reducing Co catalyst concentrations to 1/100 or by increasing Mo catalyst concentrations by five times. We also propose a novel spectral analysis method that allows one to distinguish absorbance contributions from the upper, middle, and lower parts of a nanotube array. We use this method to quantitatively characterize the slight diameter change observed along the array height. On the basis of further investigation of the array and catalyst particles, we conclude that catalyst aggregation-rather than Ostwald ripening-dominates the growth of metal particles.

Temperature Dependence of Raman Scattering from Single-Walled Carbon Nanotubes : Undefined Radial Breathing Mode Peaks at High Temperatures

We measured the Raman scatterings from various single-walled carbon nanotube (SWNT) samples over a wide temperature range (4–1000 K). The G-band peaks showed a clear temperature dependence, namely, decrease in the Raman shift frequency and broadening of the peak width as temperature increases. The main G-band peak (G+ peak) showed universal temperature dependence in Raman shift for the various SWNT samples and for the three excitation laser wavelengths (488.0, 514.5, and 632.8 nm), although the G-band features changed depending on the SWNT samples and excitation laser wavelengths owing to the resonance Raman effects. The Raman shift, peak width, and intensity of the radial breathing mode (RBM) peaks also exhibited temperature dependence. At high temperatures, undefined RBM peaks appeared, which could not be assigned to specific chiralities, and showed opposite temperature dependence in their intensities. The intensities of the RBM peaks are strongly enhanced by the resonance Raman effects, and the temperature may change the resonance Raman conditions.

Optical characterization of single-walled carbon nanotubes synthesized by catalytic decomposition of alcohol

Single-walled carbon nanotubes (SWNTs), synthesized by a catalytic decomposition of alcohol (alcohol CVD method, ACCVD), are compared with high-pressure CO (HiPco) SWNT samples through optical spectroscopic measurements such as resonant Raman scattering, optical absorption and near infrared fluorescence. By the ACCVD method, SWNTs were synthesized either on zeolite catalyst-support particles or directly on the surface of a quartz substrate; in the latter case, a simple dip-coat technique was employed for mounting the metal catalyst. Specific morphological characteristics of as-grown SWNTs generated on zeolite support are presented using SEM and TEM, revealing that the SWNTs produced by the proposed method possess the significant quality of being almost free from amorphous carbons or metal particle impurities. The quality and diameter distribution of SWNTs were investigated and discussed through the results of Raman scattering and optical absorption. The average diameter was slightly smaller for SWNTs grown on zeolite particles than for HiPco SWNTs. Finally, fluorescent emission spectra from isolated SWNTs in an aqueous surfactant suspension were measured for various excitation wavelengths to determine the structural (n,m) distribution of the SWNTs. The narrower chirality distribution for ACCVD SWNTs grown on zeolite compared with HiPco SWNTs was demonstrated.