Resonance Raman study of polyynes encapsulated in single-wall carbon nanotubes with different diameters L.G. Moura 1 , L. M. Malard 1 , M. A. Carneiro 2 , P. Venezuela 2 , Rodrigo B. Capaz 3 , D. Nishide 4 , Y.Achiba 5 , H. Shinohara 4 , and M. A. Pimenta 1 1 Departamento de F´ ısica, Universidade Federal de Minas Gerais, 30123-970 Belo Horizonte, Brazil 2 Instituto de F´ ısica, Universidade Federal Fluminense, 24210-346, Niter´oi, RJ, Brazil 3 Instituto de F´ ısica, Universidade Federal do Rio de Janeiro, Caixa Postal 68528, Rio de Janeiro, RJ 21941-972, Brazil 4 Department of Chemistry, Nagoya University, Nagoya 464-8602, Japan 5 Department of Chemistry, Tokyo Metropolitan University, Hachioji 192-0397, Japan (Dated: January 28, 2009) In this work, we present a resonance Raman study of C 10 H 2 and C 12 H 2 polyynes encapsulated in single-wall carbon nanotubes of different diameters. We show that polyyne resonance energies decrease with increasing diameter, as a consequence of changes in the the environment screening. Strong changes in the shape of the G band of the metallic nanotubes are observed when they are encapsulating the carbon linear chains, reflecting charge transfers between these two systems. From ab initio calculations, we find that such electronic transfers are most likely occurring from the nanotubes to the polyyne molecules. PACS numbers: Valid PACS appear here I. INTRODUCTION Polyynes are the simplest linear carbon chains and their study opens the way to understand sp-hydrized car- bon structures and to study electronic correlation effects in 1D systems with potential applications in nanoelec- tronics. These linear carbon chains exhibit an alterna- tion of single and triple bonds and they are terminated by other kind of atoms or chemical groups. Polyynes have been synthesized by plants and microor- ganisms and they are found in the interstellar medium 1,2 . They are considered as an intermediate structure in the mechanism of thermal decomposition of hydrocarbons and can also be produced artificially. However, polyyne chains are very unstable, and the high reactivity between unsaturated sp chains tends to cause an evolution from sp to sp 2 structures 2 . The hydrogen terminated polyyne chain (C 2n H 2 ,n ≥ 1) shown in Fig.1 is the simplest polyyne molecule. These chains have been synthesized by laser ablation of graphite, coal or C 60 in organic solvents or by electric arc between graphite electrodes submerged in hydrocarbons or alcohols solvents 2 . They have been well studied in gas phase, in organic solution and in vacuum 3,4 . Carbon nanotubes can encapsulate many different structures such as organic compounds and metal com- plexes forming the hybrid structures called peapods 3,5,6 . These hydrid structures open new opportunities for basic research and nanomaterial science. Because of their reactivity, experimental investigation of isolated polyyne molecules is a hard task. However, they can be trapped inside single-wall carbon nanotubes (SWNTs) and, in this case, they become stable even at high temperatures 1,3 . Carbon nanowire structures involving more than 100 carbon atoms have also been observed inside a multiwalled carbon nanotube 7 . FIG. 1: Molecular structure of C2nH2 with n = 3, showing the alternation between simple and triple bonds between sp- hybridized carbon atoms Malard et al 8 performed a resonance Raman study of two kinds of polyynes (C 10 H 2 and C 12 H 2 ) trapped in- side single-wall carbon nanotubes (SWNTs) using many different laser energy in the visible range, and observed that the intensity of the Raman band associated with the stretching modes of the polyynes inside SWNTs is strongly enhanced for laser energies around 2.10 eV. The energy of this resonance is much lower than the energy of the absorption peaks of C 10 H 2 and C 12 H 2 polyynes in organic solvents (4.9 and 4.5 eV, respectively) or in gas phase (5.4 and 5.0 eV, respectively). Therefore, this was assigned to dipole-forbidden transitions becoming ”bright” due to symmetry breaking. When trapped in- side the nanotubes, the polyynes molecules are displaced from the axis of the tube, thus lowering the symmetry from D ∞h to C 2v . DFT calculations estimated that the energy of these ”dark” states are between 2 and 3 eV, in agreement with the Raman measurements 8 . In this paper, we present a resonance Raman study of C 10 H 2 and C 12 H 2 polyynes trapped inside single-wall carbon nanotubes of different diameters, using many dif- ferent laser energies in the visible range. We show that