Spectroscopy of Single- and Double-Wall Carbon Nanotubes in Different Environments Tobias Hertel,* ,²,‡ Axel Hagen, ‡ Vadim Talalaev, § Katharina Arnold, | Frank Hennrich, | Manfred Kappes, | Sandra Rosenthal, ⊥ James McBride, ⊥ Hendrik Ulbricht, ² and Emmanuel Flahaut # Department of Physics and Astronomy, Vanderbilt UniVersity, NashVille, Tennessee, Department of Physical Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany, Max-Born-Institute, Berlin, Germany Karlsruhe, Institut fu¨r Nanotechnologie, Karlsruhe, Germany, Department of Chemistry, Vanderbilt UniVersity, NashVille, Tennessee, and Forschungszentrum CIRIMAT/LCMIE, UMR CNRS 5085, UniVersite´ Paul Sabatier, Toulouse, France Received January 13, 2005; Revised Manuscript Received January 30, 2005 ABSTRACT Individual single-wall carbon nanotubes (SWNTs) and double-wall carbon nanotubes (DWNTs) were suspended in water for optical studies using sodium-cholate and other surfactants. We used time-resolved photoluminescence (PL) spectroscopy to study the influence of tube chirality and diameter as well as of the environment on nonradiative decay in small diameter tubes. The studies provide evidence for PL from small diameter core tubes in DWNTs and for a correlation of nonradiative decay with tube diameter and exciton red shift as induced by interaction with the environment. The optical properties of carbon nanotubes (CNTs) are studied increasingly for their potential in a variety of optical and optoelectronic applications. 1-3 Light-emitting and pho- tosensitive devices have already been fabricated and call for a more thorough investigation of fundamental optical pro- cesses and dynamics associated with optical excitations in CNTs. The photoluminescence-quantum yield (PL-QY) and branching ratios for optical excitations, for example, are of fundamental importance for the performance and potential of CNTs for optical devices. Here, the ability to control and tailor CNTs with specific optical properties will be crucial to facilitate their implementation in viable technologies. However, apart from their alleged potential for applica- tions, the geometrical and electronic structure of CNTs furnishes these one-dimensional macromolecules with unique optical properties that are also fascinating from a fundamental perspective. 4,5 Optical properties and dynamics of excited states in CNTs are thus studied in growing detail with respect to the competition of radiative with nonradiative decay processes, the influence of magnetic fields, and more. 6-17 Here, we explore optical excitations and the dynamics of optically excited single- and double-wall carbon nanotubes (SWNTs and DWNTs) in different environments. The results provide evidence for PL from cores of DWNTs as well as for a correlation of nonradiative decay rates with tube diameter and exciton red shift. Double-wall carbon nanotubes were prepared by the technique developed by Flahaut and co-workers. 18 A sys- tematic analysis of TEM images such as the one shown in Figure 1A reveals that samples produced by this method contain approximately 77% of DWNTssthe high proportion of DWNTs was also confirmed by electron diffraction 19 s with a small admixture of about 18% single-wall carbon nanotubes (SWNTs), and roughly 5% triple-wall carbon nanotubes, see Figure 1B. 19 Figure 1C shows the diameter distributions of core and shell tubes of DWNTs compared to the relative frequency of SWNTs. Important for the following discussion is that about 90% of the tubes with a diameter of below 1 nm are expected to be core tubes of DWNTs and that only about 10% of the small diameter tubes are residual SWNTs in the material. This assertion is justified by the observation that, to the best of our knowledge, no significant enrichment of tube concentrations by the process- * Corresponding author. E-mail: Tobias.hertel@vanderbilt.edu; phone: +1 (615) 322-2864. ² Department of Physics and Astronomy, Vanderbilt University. ‡ Fritz-Haber-Institut der Max-Planck-Gesellschaft. § Max-Born-Institute. | Institut fu¨r Nanotechnologie. ⊥ Department of Chemistry, Vanderbilt University. # Universite´ Paul Sabatier. NANO LETTERS 2005 Vol. 5, No. 3 511-514 10.1021/nl050069a CCC: $30.25 © 2005 American Chemical Society Published on Web 02/18/2005