Selective Synthesis of Subnanometer Diameter Semiconducting Single-Walled Carbon Nanotubes Codruta Zoican Loebick, † Ramakrishna Podila, ‡ Jason Reppert, ‡ Joel Chudow, † Fang Ren, † Gary L. Haller, † Apparao M. Rao, ‡ and Lisa D. Pfefferle* ,† Department of Chemical Engineering, Yale UniVersity, New HaVen, Connecticut 06513, and Department of Physics and Astronomy and Center for Optical Material Science and Engineering Technologies, Clemson UniVersity, Clemson, South Carolina 29634 Received March 9, 2010; E-mail: lisa.pfefferle@yale.edu Abstract: Subnanometer single-walled carbon nanotubes (sub-nm SWNTs) were synthesized at different temperatures (600, 700, and 800 °C) using CoMn bimetallic catalysts supported on MCM-41 silica templates. The state of the catalyst was investigated using X-ray absorption, and the (n,m) indices of the sub-nm SWNTs were determined from Raman spectroscopy and photoluminescence measurements. We find that the size of the metallic particles that seed the growth of sub-nm SWNTs (diameter ∼0.5-1.0 nm) is highly sensitive to the reaction temperature. Low reaction temperature (600 °C) favors the growth of semiconducting tubes whose diameters range from 0.5 to 0.7 nm. These results were also confirmed by electrical transport measurements. Interestingly, dominant intermediate frequency modes on the same intensity scale as the Raman breathing modes were observed. An unusual “S-like” dispersion of the G-band was present in the Raman spectra of sub-nm SWNTs with diameters <0.7 nm. 1. Introduction SWNTs are one-dimensional systems with exceptional chemi- cal and electronic properties and a vast number of emerging applications. 1-3 To date, much research effort has been dedi- cated to producing SWNT materials with a narrow diameter distribution and specific (n,m) indices. These indices are related to the chiral vector C h ) na 1 + ma 2 , where (a 1 ,a 2 ) are the unit vectors of graphene. 4-7 Synthesis of nanotubes with specific (n,m) indices is important for the advancement of SWNTs in electronic applications. The general mechanism by which SWNTs are produced consists of exposing catalytic transition metal particles (especially Fe, Co, and Ni) to a carbon feedstock (CO, ethane, acetylene, etc.) at high temperatures (between 500 and 1000 °C) and varying pressures. When the catalyst particles become saturated with carbon, the growth of SWNTs is initiated by the formation of a stable carbon cap at the surface of the particle, followed by addition of carbon atoms at the growing end of the cap to form a nanotube. The diameter and symmetry of the carbon cap should closely match those of the metal particle. 8,9 Previously, Ding et al. 10 showed that the interaction between the end atoms of a growing SWNT and the catalyst metal atoms has a high bond energy, and the enthalpy of SWNT formation is substantially reduced when the number of carbon-metal bonds is maximized. A key factor in diameter-selective synthesis of SWNTs is therefore the size of the catalyst particles. 9,10 We have previously shown that diameter-selective growth of SWNTs can be achieved using MCM-41 mesoporous silica templates isomorphously substituted with Co. 11,12 Here, we demonstrate an improved synthesis method in which a second transitional metal (Mn) is added to the aforementioned Co- MCM-41 monometallic system. The second metal component does not form metallic particles during synthesis due to its high stability against reduction. Instead, Mn ions highly dispersed in the silica substrate act as anchoring sites for small Co particles, preventing them from sintering into large, inert particles. 13 Similar results were obtained with a CoCr-MCM- 41 bimetallic system. 14,15 A combined photoluminescence and Raman study showed that the CoMn-MCM-41 catalyst with a † Yale University. ‡ Clemson University. (1) Saito, R.; Dresselahus, G.; Dresselahus, M. S. Phys. ReV.B 2000, 61, 2981. (2) Zheng, B.; Lu, C.; Gu, G.; Makarovski, A.; Finkelstein, G.; Liu, J. Nano Lett. 2002, 2, 895. (3) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (4) Herrera, J. E.; Balzano, L.; Borgona, A.; Alvarez, W. E.; Resasco, D. E. J. Catal. 2001, 204, 129. (5) Herrera, J. E.; Resasco, D. E. J. Phys. Chem. B 2003, 107, 3738. (6) Miyauchi, Y.; Chiashi, S.; Marukami, Y.; Hayashida, Y.; Maruyama, S. Chem. Phys. Lett. 2004, 387, 198. (7) Ago, H.; Imamura, S.; Okazaki, T.; Saito, T.; Yumura, M.; Tsuji, M. J. Phys. Chem. 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