Exzitonen in Kohlenstoffnanotubes

• Antragstellerin: Professorin Dr. Janina Maultzsch
• Fachliche Zuordnung: Experimentelle Physik der kondensierten Materie
• Förderung: Förderung von 2008 bis 2012
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 77821810

• Erstellungsjahr: 2012

Zusammenfassung der Projektergebnisse

We determined the exciton binding energy of the first optical transition in the (13,1) branch of metallic carbon nanotubes. By temperature-dependent resonance Raman spectroscopy, we measured a binding energy of about 50 meV, in agreement with theoretical predictions. This method can be transferred to other metallic nanotubes as well, depending on the available excitation wavelengths. Furthermore, it is a direct measurement, in contrast to the previously reported absorption measurement on an individual nanotube. The exciton binding energy was determined by careful comparison of the absorption lineshape with a simulated lineshape, and the experiment cannot easily be applied to a large variety of nanotubes. Our results in Phys. Rev. B were highlighted as “Editor’s choice”. The temperature-dependent measurements are, however, not applicable to semiconducting tubes because of the high temperatures required. On the other hand, theoretical work of the excitonic effects for a large variety of chiral indices, including metallic nanotubes and higherlying transitions of semiconducting tubes, showed good agreement with our experiments. In this work, increasing exciton binding energy for higher-lying transitions is predicted; the Kataura plot of exciton binding energies gives guidance for future experimental work. The two main results in the second part of the project concern double-resonant Raman processes in graphene and few-layer graphene; they will lead to new investigations on carbon nanotubes as well. First, we showed by Raman scattering on bilayer graphene combined with simulations that phonons from Γ − K direction (so-called inner processes) give the dominant contribution to the double-resonant 2D mode (intervalley scattering). Second, we presented a layer-dependent mode in few-layer graphene around 1500 cm−1 , which is due to intravalley double-resonant scattering. Here, the main contribution comes from phonons along the Γ − M direction. The dominant phonons are mainly determined by the anisotropy of the optical matrix elements in the graphene Brillouin zone. Therefore, future work in carbon nanotubes will consider this anisotropy, which we expect to observe via different optical transitions and/or different families of the carbon nanotubes.

Projektbezogene Publikationen (Auswahl)

• Electronic Properties of Propylamine-Functionalized Single- Walled Carbon Nanotubes, ChemPhysChem, 11 2444-2448 (2010)
  M. Müller, R. Meinke, J. Maultzsch, Z. Syrgiannis, F. Hauke, A. Pekker, K. Kamaras, A. Hirsch, and C. Thomsen
  
• Excitonic absorption spectra of metallic single-walled carbon nanotubes, Phys. Rev. B 82, 035433, (2010)
  E. Malic, J. Maultzsch, S. Reich, and A. Knorr
  
• Observation of excitonic effects in metallic single-walled carbon nanotubes, Phys. Rev. B 82, 195412 (2010)
  P. May, H. Telg, G. Zhong, J. Robertson, C. Thomsen, and J. Maultzsch
  
• Temperature dependent band gap behavior and excitons in metallic carbon nanotubes, phys. stat. sol. (b) 247, 3006-3009, (2010)
  P. May, H. Telg, C. Thomsen, and J. Maultzsch
  
• Electronic and vibrational properties of low-dimensional carbon systems, PhD thesis (Technische Universität Berlin 2012)
  P. May
  
• Layer-number determination in graphene by outof-plane phonons, Phys. Rev. B 2012
  F. Herziger, P. May, and J. Maultzsch