Size-selective growth of double-walled carbon nanotube forests from engineered iron catalysts

Abstract

We have succeeded in synthesizing vertically aligned doubled-walled carbon nanotube (DWNT) forests with heights of up to 2.2 mm by water-assisted chemical vapour deposition (CVD). We achieved 85% selectivity of DWNTs through a semi-empirical analysis of the relationships between the tube type and mean diameter and between the mean diameter and the film thickness of sputtered Fe, which was used here as a catalyst. Accordingly, catalysts were engineered for optimum DWNT selectivity by precisely controlling the Fe film thickness. The high efficiency of water-assisted CVD enabled the synthesis of nearly catalyst-free DWNT forests with a carbon purity of 99.95%, which could be templated into organized structures from lithographically patterned catalyst islands.

Connectivity percolation of polydisperse anisotropic nanofillers

“…Based on the very few available experimental measurements of the length distribution of CNTs, we choose a Gamma distribution and the related Weibull distribution. [39][40][41][42] A limiting case of both of these distributions is the exponential distribution with a probability density function (PDF) f e (L) = η exp(−ηL), 43,44 for which φ p (η)/φ p (η 0 ) = 1/2, with η 0 = η/2, irrespective of the value of the distribution parameter η that takes the form of the inverse mean length. 45 So, an exponential distribution reduces the PT by a factor of 2 (relative to the equivalent monodisperse case).…”

“…However, if the length and width distributions have a positive correlation, then the consequences of such a length distribution are completely different. To estimate the level of correlation, one could argue that the probability of breaking SWCNTs also exhibit a diameter variation 39,50 and for these the scission energy, which is proportional to the number of bonds that have to be broken, presumably scales linearly with the diameter. If D γ = αL i , we find that…”

Carbon nanotube diameter tuning using hydrogen amount and temperature on SiO2/Si substrates

“…It is generally accepted that there is a strong correlation between the size of the catalyst particle and CNT diameter [29,30]; however, it is still under debate what controls the size of the catalyst particles when thin film catalyst is used to synthesize nanotubes. There were reports that the thickness of the film [3,31]; pretreatment conditions [17], including plasma etching, gas treatment time; and temperature [32] play an important role in the size of the catalyst particles. From our experience we know that at high growth temperatures catalyst particles become rather large [33].…”

“…This is mostly due to significant variation in the ideal growth conditions from system to system for effective CNT synthesis. Small, seemingly insignificant factors might change the end results to a large extent, leading to misinterpretation of data and wrong conclusions [3]. Failing to exactly repeat such secondary parameters leads to end results that vary considerably.…”

A super-growth method for single-walled carbon nanotube synthesis

“…As a result of research, a single-walled CNT array could be synthesized only when the thickness of the iron catalyst was between 0.8 nm to 1.3 nm. [12] Therefore, the catalyst film was initially formed using the sputtering method that had excellent control of film thickness. However, from the perspective of industrial mass production, the sputtering method had low productivity and high facility costs, and that meant high overall costs.…”