Distinct Infrared Spectral Signatures of the 1,2- and 1,4-Fluorinated Single-Walled Carbon Nanotubes: A Molecular Dynamics Study

Fluorinated single-walled carbon nanotubes (F-SWNTs) form important intermediates in SWNT sidewall functionalization, leading to a variety of materials and biological applications. By simulating the infrared (IR) signals for the 1,2and 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)-C(sp)-] collective vibrational peak at 400 cm is optically active only in the 1,2-isomer, while the 1300 cm band arising due to the F-C(sp) stretchingmotion coupled with the neighboring C(sp) 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 explosionof studies focusing onunderstanding and controlling CNT atomic and electronic properties, as motivated by a variety of electronics, biological, materials, energy, and other applications. Chemical functionalization of CNT sidewalls provides one of the most efficient routes to the desired property control. Examples are abundant. Addition of fluorinatedolefins and chlorine atoms represents an effective approach toward converting commercial mixtures of metallic and semiconducting CNTs into high-mobility semiconducting tubes. Functionalization with carboxylic acid, nitroso, and maleic anhydride groups allows one to control CNTcharging. TheCNToptical properties can be selectively modified by fluorination. Functionalization and bioconjugation of CNTs have led to multiple protocols for biomedical applications, including biological imaging, labeling, sensing, and drug delivery. Key biological advantages of functionalized carbon nanotubes include their excellent ability to translocate through membranes while retaining low toxicity. Fluoride atoms and other substituents on the CNT surface can be used to transform CNT films between the superhydrophobic and nearly hydrophilic states. Control of surface adsorption properties by covalent and noncovalent CNT functionalization leads to superior CNT-polymer composites, materials with improved friction properties, and strongly interconnectedCNT blocks. Fluorination and defluorination reactions form the basis for CNT applications in hydrogen storage and Li ion batteries. Finally, complexation of CNTs with organic sensitizers leads to promising photovoltaic materials. 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 physicalchemical properties of carbon materials. Using technology developed for the fluorination of graphite, Mickelson et al. produced fluorinated single-walled carbon nanotubes (F-SWNTs), which serve as a staging point of chemical modification for a wide variety of sidewall functionalizations. The structures of F-SWNT were investigated by various methods involving infrared (IR) and Raman spectroscopies, nuclear magnetic resonance (NMR), transmission electronmicroscopy (TEM), scanning tunneling microscopy (STM), electron energy loss spectroscopy (EELS), and X-ray photoemission spectroscopy (XPS). Despite strong synthetic efforts aswell as extensive experimental and computational characterization of the fluorinated structures, the most favorable pattern of fluorine atom addition remains controversial. Both 1,2-addition and 1,4-addition 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. While the former pattern was predicted to be more stable in the semiempirical calculation, the latter pattern was Received Date: February 2, 2010 Accepted Date: March 30, 2010