Published on Web Date: April 05, 2010 r2010 American Chemical Society 1307 DOI: 10.1021/jz100143z | J. Phys. Chem. Lett. 2010, 1, 1307–1311 pubs.acs.org/JPCL Distinct Infrared Spectral Signatures of the 1,2- and 1,4-Fluorinated Single-Walled Carbon Nanotubes: A Molecular Dynamics Study Akira Ueta, † Yoshitaka Tanimura, † and Oleg V. Prezhdo* ,‡ † Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyoku, Kyoto 606-8502, Japan, and ‡ Department of Chemistry, University of Washington, Seattle, Washington 98195-1700 ABSTRACT Fluorinated single-walled carbon nanotubes (F-SWNTs) form impor- tant intermediates in SWNT sidewall functionalization, leading to a variety of materials and biological applications. By simulating the infrared (IR) signals for the 1,2- and 1,4-addition structures, in which fluorine atoms are arranged in ortho or para positions, respectively, on the aromatic skeleton of the (10,10) SWNT surface, we identify peaks that are unique to each structure. Our full molecular dynamics simulations show that the [-C(sp 3 )-C(sp 3 )-] collective vibrational peak at 400 cm -1 is optically active only in the 1,2-isomer, while the 1300 cm -1 band arising due to the F-C(sp 3 ) stretching motion coupled with the neighboring C(sp 2 ) atoms is seen in the IR spectrum of only the 1,4-isomer. The reported results suggest simple and clear experimental means for distinguishing between the two fluorinated structures and provide a valuable tool for controlled SWNT sidewall functionalization. SECTION Nanoparticles and Nanostructures T he discovery of carbon nanotubes (CNTs) 1 has led to an explosion of studies focusing on understanding and controlling CNT atomic and electronic properties, as motivated by a variety of electronics, 2-13 biological, 14-17 materials, 18-24 energy, 25-27 and other applications. Chemi- cal functionalization of CNT sidewalls provides one of the most efficient routes to the desired property control. Exam- ples are abundant. Addition of fluorinated olefins and chlorine atoms represents an effective approach toward converting commercial mixtures of metallic and semiconducting CNTs into high-mobility semiconducting tubes. 2,3 Functionalization with carboxylic acid, nitroso, and maleic anhydride groups allows one to control CNT charging. 5,6 The CNToptical proper- ties can be selectively modified by fluorination. 7 Functionali- zation and bioconjugation of CNTs have led to multiple protocols for biomedical applications, including biological imaging, labeling, sensing, and drug delivery. 14-16 Key biolo- gical advantages of functionalized carbon nanotubes include their excellent ability to translocate through membranes while retaining low toxicity. 17 Fluoride atoms and other substituents on the CNT surface can be used to transform CNT films between the superhydrophobic and nearly hydro- philic states. 18,19 Control of surface adsorption properties by covalent and noncovalent CNT functionalization 20 leads to superior CNT-polymer composites, 21 materials with im- proved friction properties, 22 and strongly interconnected CNT blocks. 24 Fluorination and defluorination reactions form the basis for CNT applications in hydrogen storage 26 and Li ion batteries. 25 Finally, complexation of CNTs with organic sensi- tizers leads to promising photovoltaic materials. 27 Generally, CNTs are chemically nonreactive and are hard to functionalize due to the efficient carbon-carbon bonding. The strong reactivity of fluorine atoms makes fluorination one of the most effective methods to modify and control physical- chemical properties of carbon materials. 28,29 Using techno- logy developed for the fluorination of graphite, 30 Mickelson et al. produced fluorinated single-walled carbon nanotubes (F-SWNTs), which serve as a staging point of chemical modi- fication for a wide variety of sidewall functionalizations. 31 The structures of F-SWNT were investigated by various methods involving infrared (IR) and Raman spectro- scopies, 29,32,33 nuclear magnetic resonance (NMR), 33,34 transmission electron microscopy (TEM), 29 scanning tunnel- ing microscopy (STM), 35 electron energy loss spectroscopy (EELS), 36 and X-ray photoemission spectroscopy (XPS). 37,38 Despite strong synthetic efforts as well as extensive experi- mental and computational characterization of the fluorinated structures, the most favorable pattern of fluorine atom addi- tion remains controversial. 28 Both 1,2-addition and 1,4-addi- tion patterns have been proposed. The fluorine atoms (the blue balls) are arranged in ortho positions in the 1,2-isomer, while the 1,4-addition puts the atoms in para positions, as illustrated in Figure 1. 35,39,40 While the former pattern was predicted to be more stable in the semiempirical calculation, 35 the latter pattern was Received Date: February 2, 2010 Accepted Date: March 30, 2010 Downloaded via UNIV OF SOUTHERN CALIFORNIA on November 21, 2019 at 21:01:34 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.