Direct Evidence for Root Growth of Vertically Aligned Single-Walled Carbon Nanotubes by Microwave Plasma Chemical Vapor Deposition

Reversible diameter modulation of single-walled carbon nanotubes by acetonitrile-containing feedstock

Changing the carbon feedstock from pure ethanol to a 5 vol % mixture of acetonitrile in ethanol during the growth of vertically aligned single-walled carbon nanotubes (SWNTs) reduces the mean diameter of the emerging SWNTs from approximately 2 to 1 nm. We show this feedstock-dependent change is reversible and repeatable, as demonstrated by multilayered vertically aligned SWNT structures. The reversibility of this process and lack of necessity for catalyst modification provides insight into the role of nitrogen in reducing the SWNT diameter.

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.

Population growth dynamics of carbon nanotubes

Understanding the population growth behavior of filamentary nanostructures, such as carbon nanotubes (CNTs), is hampered by the lack of characterization techniques capable of probing statistical variations with high spatial resolution. We present a comprehensive methodology for studying the population growth dynamics of vertically aligned CNT forests, utilizing high-resolution spatial mapping of synchrotron X-ray scattering and attenuation, along with real-time height kinetics. We map the CNT alignment and dimensions within CNT forests, revealing broadening and focusing of size distributions during different stages of the process. Then, we calculate the number density and mass density of the CNT population versus time, which are true measures of the reaction kinetics. We find that the mass-based kinetics of a CNT population is accurately represented by the S-shaped Gompertz model of population growth, although the forest height and CNT length kinetics are essentially linear. Competition between catalyst activation and deactivation govern the rapid initial acceleration and slow decay of the CNT number density. The maximum CNT density (i.e., the overall catalyst activity) is limited by gas-phase reactions and catalyst-surface interactions, which collectively exhibit autocatalytic behavior. Thus, we propose a comprehensive picture of CNT population growth which combines both chemical and mechanical cooperation. Our findings are relevant to both bulk and substrate-based CNT synthesis methods and provide general insights into the self-assembly and collective growth of filamentary nanostructures.

Synthesis of carbon nanotubes by plasma-enhanced chemical vapor deposition in an atmospheric-pressure microwave torch

There are many different techniques for the synthesis of carbon nanotubes (CNTs), and plasma technologies experience a significant competitor in thermal chemical vapor deposition (CVD) processes. A particular process is, therefore, selected according to the specific requirements of an application, which clearly differ for the development of composites as compared to nanoelectronics, field emission, displays, sensors, and the like. This paper discusses the method for the synthesis of CNTs using an atmospheric-pressure microwave (MW) torch. It was successfully applied in the fast deposition of multiwalled nanotubes (MWNTs) on a substrate without the necessity of any vacuum or heating equipment. Dense straight-standing nanotubes were prepared on Si substrates with and also without barrier SiOx layer. Therefore, it was possible to produce CNTs directly on conductive Si and to use them as an electron-emitting electrode of the gas pressure sensor. The CNTs grown in MW torch were also used to create a gas sensor based on the changes of electrical resistance measured between two planar electrodes connected by the CNTs.

Vertically aligned single-walled carbon nanotubes by chemical assembly--methodology, properties, and applications

Single-walled carbon nanotubes (SWNTs), as one of the most promising one-dimension nanomaterials due to its unique structure, peculiar chemical, mechanical, thermal, and electronic properties, have long been considered as an important building block to construct ordered alignments. Vertically aligned SWNTs (v-SWNTs) have been successfully prepared by using direct growth and chemical assembly strategies. In this review, we focus explicitly on the v-SWNTs fabricated via chemical assembly strategy. We provide the readers with a full and systematic summary covering the advances in all aspects of this area, including various approaches for the preparation of v-SWNTs using chemical assembly techniques, characterization, assembly kinetics, and electrochemical properties of v-SWNTs. We also review the applications of v-SWNTs in electrochemical and bioelectrochemical sensors, photoelectric conversion, and scanning probe microscopy.

Catalyst size effects on the growth of single-walled nanotubes in neutral and plasma systems

The results of large-scale ( approximately 10(9) atoms) numerical simulations of the growth of different-diameter vertically-aligned single-walled carbon nanotubes in plasma systems with different sheath widths and in neutral gases with the same operating parameters are reported. It is shown that the nanotube lengths and growth rates can be effectively controlled by varying the process conditions. The SWCNT growth rates in the plasma can be up to two orders of magnitude higher than in the equivalent neutral gas systems. Under specific process conditions, thin SWCNTs can grow much faster than their thicker counterparts despite the higher energies required for catalyst activation and nanotube nucleation. This selective growth of thin SWCNTs opens new avenues for the solution of the currently intractable problem of simultaneous control of the nanotube chirality and length during the growth stage.

Enhanced field emission properties of vertically aligned double-walled carbon nanotube arrays

Vertically aligned double-walled carbon nanotube (VA-DWCNT) arrays were synthesized by point-arc microwave plasma chemical vapor deposition on Cr/n-Si and SiO(2)/n-Si substrates. The outer tube diameters of VA-DWCNTs are in the range of 2.5-3.8 nm, and the average interlayer spacing is approximately 0.42 nm. The field emission properties of these VA-DWCNTs were studied. It was found that a VA-DWCNT array grown on a Cr/n-Si substrate had better field emission properties as compared with a VA-DWCNT array grown on a SiO(2)/n-Si substrate and randomly oriented DWCNTs, showing a turn-on field of about 0.85 V µm(-1) at the emission current density of 0.1 µA cm(-2) and a threshold field of 1.67 V µm(-1) at the emission current density of 1.0 mA cm(-2). The better field emission performance of the VA-DWCNT array was mainly attributed to the vertical alignment of DWCNTs on the Cr/n-Si substrate and the low contact resistance between CNTs and the Cr/n-Si substrate.

Preferential syntheses of semiconducting vertically aligned single-walled carbon nanotubes for direct use in FETs

We have combined fast heating with plasma enhanced chemical vapor deposition (PECVD) for preferential growth of semiconducting vertically aligned single-walled carbon nanotubes (VA-SWNTs). Raman spectroscopic estimation indicated a high yield of up to 96% semiconducting SWNTs in the VA-SWNT array. The as-synthesized semiconducting SWNTs can be used directly for fabricating FET devices without the need for any postsynthesis purification or separation.

Mechanism analysis of interrupted growth of single-walled carbon nanotube arrays

We investigated the growth mechanism of layered single-walled carbon nanotube (SWNT) mats by a cutting method. Transmission electron microscope observations revealed that new SWNTs grown below first grown SWNTs also have caps at their tips. Raman spectroscopy suggests that the SWNTs in each layer have the same chirality distribution. This growth method might be a way to prove a factor of chirality selection of SWNTs.

Growth Kinetics of 0.5 cm Vertically Aligned Single-Walled Carbon Nanotubes

Half-centimeter-high mats of vertically aligned single-walled carbon nanotubes were grown at 600 degrees C by point-arc microwave plasma chemical vapor deposition. The mats were produced from 0.5 nm of an Fe catalyst layer, thus showing one of the highest catalytic yields of approximately 105 times. The growth process shows a lack of poisoning of the catalyst, in contrast to other reports. The experimental results confirm that the growth rate is ultimately limited by the gas phase diffusion of hydrocarbon radicals.

Efficient Transfer of a VA-SWNT Film by a Flip-Over Technique

A transfer of a VA-SWNT film onto a conductive surface has been achieved using a novel "flip-over" technique. The top surface of a VA-SWNT film was covered by entangled bundles in an as-grown sample. When a VA-SWNT film was flipped over, an optically flat surface consisting of the tips of very well aligned, clean bundles from the bottom of the film are exposed while the top of the film is well contacted to the substrate. Thus, we expect this technique to provide us with means to prepare carbon nanotube electrodes for device applications such as super capacitors, thermo-electric devices, fuel cells, and field emission filaments.

Vertical Array Growth of Small Diameter Single-Walled Carbon Nanotubes

A hot filament chemical vapor deposition method has been developed to grow vertical array single-walled carbon nanotubes (SWNTs). In this study, a hot filament (temperature greater than 2000 degrees C) was used to activate gas mixtures of hydrogen and carbon containing species at sub-atmospheric pressures. Silicon substrates decorated with islands of iron were directly inserted into a preheated furnace in which a hot filament is activating the gas. Vertical arrays of SWNTs are produced with diameters ranging from 0.78 to 1.6 nm. The samples were characterized with Raman and fluorescence spectroscopy and SEM and TEM microscopy.