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Growth kinetics of individual carbon nanotubes studied by in situ optical microscopy

Abstract : One very desirable application of carbon nanotubes (CNTs) is their use as building blocks for electronic and optical devices. In order to realize this, it is first necessary to develop methods to produce nanotubes with the required structure and properties. One of the most promising methods for this is the development of directed synthesis methods based on kinetic selectivity. This work is devoted to experimentally addressing questions considered as key in this area, such as the kinetics of CNT growth and how they depend on the CNT structure and on synthesis parameters.For this purpose, an original in-situ optical setup was used to image the growth process of individual carbon nanotubes on a substrate. A method of differential video treatment was developed, which both improved the video quality and made it possible to observe the structural and kinetic changes during the growth of a CNT. The speed and quality of growth kinetics extraction from videos was further increased by developing an automated analysis system based on a segmentation neural network.As a result of this work, it was found that the growth rate of a CNT is constant during growth for about half of cases, but that they can stochastically change to faster or slower ones, as well as switch between growth and shrinkage in the other half of cases. All these transitions can occur either instantaneously or with a pause in between. An important result is the discovery of a rate proportionality factor with an average value of 1.7 for both acceleration and deceleration events, a value which displays no or little dependence on temperature, precursor pressure, or the catalyst used. Interestingly, the same 1.7 factor was recently reported in the literature for CNT growth before and after a water vapor exposure treatment.Moreover, we found an inverse relationship between lifetime and growth rate. This inverse relationship is still observed when temperature and pressure are changed: when either temperature or pressure is increased, the growth rate is observed to increase and the lifetime to decrease. Analysis of the precursor pressure dependences made it possible to determine a reaction order of 0.3-0.4, which suggests the same reaction mechanism whatever the type of kinetics (linear or broken-line) including a pre-equilibrium regime for the decomposition step. Analysis of the Arrhenius plot also allowed us to establish that the activation energies for broken-line growth are the same for high and low growth rates, while linear kinetics appears as an intermediate case between them. Comparison of the kinetic data with the results of Raman analysis showed that even at the level of individual chirality the distribution of growth rates is not monomodal, which contradicts existing models. We also proposed possible explanations in terms of fluctuations at the interface between the nanotube egde and the catalyst particle for the dynamic instabilities revealed by our observations,The analysis of Raman spectra of horizontally aligned CNTs grown on quartz allowed us to develop a method for internal calibration of the spectra, as well as an approach for discriminating individual SWCNTs from other cases. Both approaches exploit the peculiarities of the spectra due to the strong interaction between horizontally aligned CNTs and the quartz substrate. To determine the CNT structure, the RBM frequencies of two sets of spectra were analyzed: i) measured on the catalyst and ii) measured along individual aligned CNTs. This made it possible to determine the most appropriate environment constants in the relation between diameter and RBM frequency for each case. An unexpected result was that two different environment constants were found for the aligned CNTs.
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Submitted on : Monday, March 21, 2022 - 5:39:24 PM
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  • HAL Id : tel-03615720, version 1

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Vladimir Pimonov. Growth kinetics of individual carbon nanotubes studied by in situ optical microscopy. Electronics. Université Montpellier, 2021. English. ⟨NNT : 2021MONTS101⟩. ⟨tel-03615720⟩

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