Laser heating effects in bundled single wall carbon nanotubes

Carbon nanotubes (CNTs) have outstanding physical properties (several times stronger than steel, better thermal conductor than diamond, etc.) therefore they will play an important role in future technologies. Raman spectroscopy is a powerful tool in the analysis of CNTs but require the use of lasers which may damage the sample if the laser fluence/power density (LPD) is too high. This work is aimed at experimental study of laser heating effects on bundled single wall carbon nanotubes (SWNTs). Specifically, we want to understand the process of CNT destruction under high-power laser irradiation, to establish safe LPD regimes for CNTs characterization and to learn how to identify the effects of laser heating when working outside those regimes. This Master Thesis project was conducted in the High Pressure Spectroscopy Laboratory, Materials Physics group at Luleå University of Technology. The equipment used was a confocal Raman imaging system CRM-200 from WiTec with two lasers: 532 nm (2.33 eV) and 633 nm (1.96 eV) and a gas flow cell. Also, two different CNT samples were studied - one produced using arc-discharge and the other - a high pressure carbon monoxide disproportionation process (HiPCO) method respectively. The HiPCO sample is comprised of high-purity CNTs having small average diameter with fairly wide distribution. The arc- discharge sample is less pure and contains CNTs narrowly distributed in size around of larger average diameter. Oxidation is believed to be of importance in the CNT destruction caused by high laser irradiation thus its role was investigated through comparison of the laser heating effects on SWNTs in air with the SWNTs in an argon environment under similar conditions. Argon protects the sample from oxidation and thus CNT destruction was expected to be reduced compared to the same experiment in air. A reference regime (REF) for the laser power density where no CNT destruction could be observed was established. The results for the HiPCO sample show that the CNT destruction is much greater in air than in an argon environment which clearly indicates that the CNTs are destroyed through oxidation. However, the analogous experiments on the arc-discharge CNTs revealed the behaviour opposite to that observed in HiPCO material. We demonstrate that there are two factors that are important in the oxidation process - the CNT diameter and the sample purity. Small diameter CNTs are more prone to oxidation than larger ones which explain why the HiPCO CNTs are more easily destroyed in air than the arc-discharge produced CNTs. The reason for the increase in CNT destruction in argon environment for the arc-discharge CNTs is believed to be related to sample purity but can not be fully ascertained at the moment and, therefore, requires further investigation. From the use of different lasers we can conclude that that the higher energy photons cause more damage to the CNTs at the same power density.Validerat; 20101217 (root