Laser induced effects in carbon nanotubes implications for Raman characterization of functionalized systems

Abstract: Carbon nanotubes (CNTs) have attracted attention over the past decade because of their outstanding mechanical and electronic properties. These tiny tubular shells made of carbon atoms can be metallic or semiconducting and while having diameters of only about one nanometer (10-9 m), they can be up to centimeters in length, making them quasi one-dimensional (molecular wires). Along with a Young's modulus several times that of steel, CNTs are close to perfect (ballistic) electric conductors and conduct heat better than diamond. This makes them candidates for a variety of applications from nanoscale electronics and composites reinforced with CNTs on the molecular level to nano-actuators and high performance flatscreen displays. Beside electron microscopy, no other experimental method has been employed more in research on  carbon nanotubes than Raman spectroscopy since it can noninvasively probe single CNTs and provide direct information about their molecular properties, for example, diameter and chirality. That is possible because in the case of CNTs Raman scattering is resonantly enhanced, giving an increase in signal by a factor of 106. Due to their high surface energy and the van der Waals inter-tube interactions, carbon nanotubes naturally form bundles of up to hundreds of tubes. Heat dissipation in CNT bundles is inefficient and, as a result, their exposure to high incident laser power causes overheating and results in several thermal effects dominating the Raman spectrum. The high cost of CNT production has strongly impeded design of "pure nanotube" functional materials, thus shifting the focus of CNT materials research to creation of CNT-based composites. Such new multifunctional materials, based on the outstanding physical properties of nanotubes, are created by mixing relatively small amounts of CNTs with polymers or metals (matrix). This is still a big challenge because of poor dispersion of CNT bundles in the matrix and weak bonding of the nanotubes to the surrounding host (matrix) molecules. One proposedsolution to solve the latter problem is to establish bonding of CNTs to the matrix via functional groups covalently attached to the CNT surface, i.e., to use so-called "functionalized" CNTs in composites. The aim of this work is to explore the possibilities of using Raman spectroscopy for expressive characterization of functionalized CNTs, the source material for synthesis of CNT-based composites. CNTs produced by two synthesis techniques, with different diameter distributions, were probed using several laser excitations. Evaluating the efficiency of the functionalization process requires first determination of the intrinsic spectroscopic properties of the pristine (non-functionalized) CNTs. Because functionalization is carried out on bundled CNTs, a detailed investigation of whether the incident laser irradiation causes thermal effects in the sample during Raman experiments was performed in order to find experimental protocols for recording the intrinsic (unperturbed by heating) spectrum of the CNT bundles. From this study a set of "reference conditions" defining laser irradiance levels that do not result in overheating of the CNT bundles was established. Exceeding these laser irradiation levels (thresholds) first results in reversible changes of the Raman spectrum due to heating, while further increase of the laser irradiation leads to irreversible changes in the spectra and, hence, destruction of the CNTs in the sample. Evaluation of this destruction demonstrates its dependence on CNT diameter and high sensitivity to photon energy. Additional experiments show that in some cases a similar instability of the Raman spectra and irreversible destruction of the CNTs occur at laser irradiation levels below those that increase sample temperature. Finally, we used the "reference" laser irradiation regimes to characterize the effects of CNT sidewall functionalization. Specifically, HiPCO-produced, single-walled CNTs functionalized by methoxypenyl functional groups were studied in detail and the influence on the three main vibrational bands investigated. Results from analysis of the radial breathing mode band show that the functionalization process is selective and depends on both nanotube diameter and type.