Nano-optics of carbon nanotubes: measurement of unperturbed optical transition energies

WB1 (Invited) 8:30 - 9:00

Nano-optics of Carbon Nanotubes: Measurement of Unperturbed Optical Transition Energies Bennett Goldberg, Yan Yin, Andrew Walsh, Nicholas Vamivakas, Alexander Stolyarov, Stephen Cronin, Michael Tinkham, Wolfgang Bacsa, and Anna Swan Boston University Physics Department, Boston University Electrical and Computer Engineering, and the Center for Nanoscience and Nanobiotechnology, Boston, MA. Université Paul Sabatier, Toulouse, France and Harvard University, Cambridge, MA. Abstract Carbon nanotubes represent an important model system in nanoscience and nanotechnology. The tubes are one-dimensional, typically 1-2 nm in diameter yet extend to lengths of 1 mm. They have extreme mechanical and thermal properties, and the electronic conductance can vary from insulating to metallic depending on the tube chirality or helicity. Their rich electronic and optical spectra make them ideal candidates for nanoscale spectroscopic studies, since despite a large amount of research carried out on nanotubes, there are several key aspects of the electronic and optical properties that are poorly understood. There is a large discrepancy between theoretical models and the observed emission spectra, and the role of excitonic interactions and the observed sensitivity of nanotubes to their local environment as manifest in light emitting and absorption properties are not yet elucidated. Most current emission data comes from ensemble measurements of tubes surrounded by surfactant and suspended in a solution. While two recent reports demonstrate the importance of exchange and correlations, studying these effects in individual tubes remains a challenging aspect of nano-optics. In this work we present resonant inelastic light scattering (Raman) studies of individual tubes suspended in air. Using high resolution micro-Raman spectroscopy with tunable filters for high throughput we have mapped the resonance Raman profiles for a series of individual SWNTs in the 2n+m = 22 and 29 families and found clear evidence of excitonic interactions from the Raman measurements. Individual suspended SWNTs are grown across etched trenches on a quartz substrate with the chemical vapor deposition technique. For a single suspended tube, the laser excitation energy is tuned to find the electronic resonance, indicated by an intensity maximum of the Raman radial breathing mode (RBM). The Stokes and Anti-Stokes doublepeaks are offset in energy as expected from incoming and outgoing light resonance with the electronic level. It is demonstrated that the double peaks in the resonance window are separated by the RBM phonon energy, determining the width of the resonance profile. Figure 1 displays a typical Raman Excitation Profile (REP) of an individual SWNT, identified as a (9,4) tube. We transitions with energy widths approaching 10meV, observe narrow resonances from compared to ~60meV in ensemble measurements. The resonances show a systematic shift to lower energies by 70-90 meV compared to individual SWNTs in sodium dodecyl sulfate (SDS) solution. We ascribe the systematic down-shift compared to tubes in SDS to the change in the surrounding dielectric medium. All of the atoms in a single-wall nanotube (SWNT) are external, and hence, optical transitions can be strongly influenced by the environment. For example, tubes dispersed with different cationic, anionic and nonionic surfactant molecules show variations in the optical resonances up to ~25 meV. PLE from tubes suspended in air between pillars show a shift in and

compared to tubes wrapped in surfactants. In our case, we unambiguously identify a

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Fig 1: The Resonant Raman profile of the RBM mode at 258cm-1 for a (9,4) SWNT. The Stokes and antiStokes excitation profiles (left): The red line is the Stokes REP, while the blue is the calculated REP using the identical energy and broadening parameters plus a 300K phonon bath, showing excellent agreement. The vertical lines show the incoming (solid) and scattered (dotted) resonances. The inset shows the Raman RBM spectrum for Elaser= 1.65 eV. b) Histogram of the symmetry ratio for the 23 tubes for the measured Stokes peaks. c) Histogram of the 25 measured broadening parameters η.

series of individual tubes in the 22 and 29 families as down-shifted as compared to optical absorption energies measured in solution. Theoretical work predicts that the Coulomb driven exchange interaction gives rise to a large increase of the band gap energy counteracted by a somewhat smaller exciton binding energy shift. The resulting energy levels are predicted to be higher than those expected from tight binding calculations in accordance with PLE measurements. We attribute our reduced energies to the larger screening from the surrounding dielectric medium in solution. Reasons for the blue-shift in energy with increased dielectric constants will be discussed. Additionally, with micro-Raman spectroscopy we measure the spatial dependence of suspended tubes across the 1.5 µm trench and demonstrate a 5-10 times stronger RBM intensity for the suspended part section of the same tube, similar to PL signal enhancements of suspended tubes. This shows a remarkable similarity between resonant photoluminescence and resonant Raman measurement, despite the differences in the optical processes.1 1

The authors acknowledge helpful discussions with Millie Dresselhaus, Antonio Castro-Neto and Francisco Guinea. This work is supported in part by NSF # NIRT ECS-02-0210752 and a Boston University SPRInG grant.

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