Quantitative evaluation of bundling effect on single walled carbon nanotubes by resonance Raman spectra
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HH13.14.1
Quantitative evaluation of bundling effect on single walled carbon nanotubes by resonance Raman spectra Zhengtang Luo, Rongfu Li, Sang Nyon Kim, Fotios Papadimitrakopoulos* Nanomaterials Optoelectronics Laboratory, Department of Chemistry, Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3136
To whom correspondence should be addressed, email: [email protected] ABSTRACT
The radial breathing mode (RBM) region of the resonance Raman spectra of HiPco single walled carbon nanotubes (SWNTs) was investigated as a function of aggregation. This was modeled using an energetic deviation term (∆E), imparted to the optical transitions (Eii(n,m)) by bundling effect. Eii(n,m) values obtained from photoluminescence (PL) measurements were used to reconstruct these RBM profiles. The simulation revealed that the PL-determined Eii(n,m) set provided a good fit in terms of peak position. Providing an accurate set of Eii(n,m) values becomes available, the RBM profile reconstruction methodology discussed herein could greatly enhance our ability to model a range of physicochemical changes to the immediate environment of SWNTs. I. INTRODUCTION
The theoretically predicted conductivity-modulated properties of single walled carbon nanotubes (SWNTs), along with other extraordinary properties such as high Young’s modulus while retaining their high strength, have attracted considerable interest in the scientific community.[1] At present all SWNT samples available contain a broad distribution of chiral angles and diameters. Moreover, nanotubes tend to form bundles, in which several tubes are held together by van der Waals interactions. Such diversity of SWNT structures and sample morphology creates enormous diversity in their electronic properties and presents a major challenge to their basic understanding and their device implementation. A number of reports indicate that the RBM region varies upon aggregation of SWNTs.[2, 3] This renders the qualitative and quantitative comparison of the abundance of the specific (n,m) SWNT or group of SWNTs to the RBM band of their Raman spectra problematic. Various efforts have been reported for simulating the RBM band, with moderate degree of success.[2, 4, 5] Heller et al.[2] were able to model the RBM band as a function of sample morphology using a limited set of six (n,m) (where n and m are integers that define the wrapping vectors associated with tube geometry) SWNTs. However, as will be shown in this contribution, when all possible (n,m) values are included, such fitting becomes problematic. In the present study, we focused on the 785 nm excitation laser of HiPco SWNTs and reconstructed the RRS RBM profiles using a complete set of Eii(n,m) values and a single adjustable parameter ∆E that accounts for their energetic deviation caused by the change in the SWNT aggregation states. Here, Eii(n,m) is the energy separation between the ith pair of density of states (DOS) singularities in the valence (v) and conduction (c) band, where for
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