Charge transfer and screening effects in polyynes encapsulated inside single-wall carbon nanotubes 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 Física, Universidade Federal de Minas Gerais, 30123-970 Belo Horizonte, MG, Brazil 2 Instituto de Física, Universidade Federal Fluminense, 24210-346 Niterói, RJ, Brazil 3 Instituto de Física, Universidade Federal do Rio de Janeiro, Caixa Postal 68528, 21941-972 Rio de Janeiro, RJ, Brazil 4 Department of Chemistry, Nagoya University, Nagoya 464-8602, Japan 5 Department of Chemistry, Tokyo Metropolitan University, Hachioji 192-0397, Japan Received 21 August 2009; published 2 October 2009 A resonance Raman study of C 10 H 2 and C 12 H 2 polyynes encapsulated in single-wall carbon nanotubes of different diameters is presented. We show that the energy of the optical transitions associated with excited dark states of the polyyne depends on the diameter of the nanotube, as a consequence of different dielectric screenings. 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. DOI: 10.1103/PhysRevB.80.161401 PACS numbers: 73.22.-f, 36.20.Ng, 78.30.Na, 78.67.Ch Polyynes are linear carbon chains terminated by other kinds of atoms or chemical groups, such as the hydrogen terminated polyyne chain C 2n H 2 n  2. 1–3 They are ideal systems to understand sp-hydrized carbon structures and to study electronic correlation effects in one-dimensional sys- tems with potential applications in nanoelectronics. Polyynes are very unstable and the high reactivity between unsaturated sp chains tends to cause an evolution from sp to sp 2 carbon structures. 3 Interestingly, they become stable even at high temperatures when encapsulated inside single-wall carbon nanotubes SWNTs, allowing thus easier experimental works. 1,4 However, the electronic structures of both polyynes and carbon nanotubes are extremely susceptible to environ- mental interactions. We will show in this work that nano- tubes with different diameters provide different dielectric screening for the polyynes. Moreover, electron transfer from the nanotubes to the polyynes is evidenced by the renormal- ization of the nanotube phonon energies due to changes in the Fermi level. In a previous work, Malard et al. 5 performed a resonance Raman study of two kinds of polyynes C 10 H 2 and C 12 H 2  trapped inside SWNTs using many different laser energies in the visible range, and observed that the intensity of the Ra- man band associated with the stretching modes of the polyynes inside SWNTs is strongly enhanced for laser ener- gies 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, respec- tively or in gas phase 5.4 and 5.0 eV, respectively. This observed resonance was assigned to dipole-forbidden “dark” transitions becoming allowed due to symmetry breaking, when the polyynes are displaced from the axis of the tube. 5 DFT calculations estimated that the energy of these “dark” states are between 2 and 3 eV, in agreement with the Raman measurements. In this Rapid Communication, we present a resonance Ra- man study of C 10 H 2 and C 12 H 2 polyynes trapped inside single-wall carbon nanotubes of different diameters, using many different laser energies in the visible range. We show that the polyyne resonance energies decrease with increasing diameter of the nanotubes, as a consequence of changes in the screening from the tubes of different diameters. We also observe a redshift and strong changes in the shape of the G band of the metallic nanotubes when they encapsulate polyynes, reflecting charge transfers between these two sys- tems. These charge transfers are interpreted using ab initio calculations. The preparation of the polyynes inside nanotube samples is described in Refs. 1, 6, and 7. Before the encapsulation treatment, SWNTs were purified by the H 2 O 2 / HCl treatment and thermal oxidation, and the SWNTs/polyyne/hexane solu- tion was degassed by freeze-thaw process. Therefore, no chemical functionalization is expected to occur during the purification processes. The Raman measurements were per- formed in aggregates of nanotube bundles. The diameters of the nanotubes were estimated from the radial breathing modes RBMs Raman spectra 8 using the relation d = A /  RBM - B, where  RBM is the RBM frequency, and A =219 cm -1 nm and B =15 cm -1 . 8 The samples with average large, medium, and small diameters were labeled as Ld = 1.45  0.15 nm, Md=1.35  0.15 nm, and Sd = 1.2  0.1 nm, respectively. Raman-scattering experiments were performed at room temperature using a triple monochromator micro-Raman spectrometer Dilor XY. An Ar-Kr and a dye laser were used and experiments were performed using many lasers lines in the visible range from 1.92 to 2.71 eV 647 to 457.9 nm. The laser power used was around 1 mW with a spot diameter of 1 m using a 80 objective. Ab initio calculations based on density-functional theory 9,10 DFT were used to obtain the structural and elec- tronic properties of pristine and polyyne-encapsulated SWNTs. The calculations were performed by the SIESTA code, 11 which performs self-consistent calculations solving PHYSICAL REVIEW B 80, 161401R2009 RAPID COMMUNICATIONS 1098-0121/2009/8016/1614014 ©2009 The American Physical Society 161401-1