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Posted: May 18, 2012

Scientists illustrate an atlas of carbon nanotube optical transitions

(Nanowerk News) Led by Professor Wang Enge at Peking University (PKU) and Professor Wang Feng at the University of California, Berkeley, a joint research team recently reported their major progress on an atlas of carbon nanotube optical transitions, which was published in Nature Nanotechnology ("An atlas of carbon nanotube optical transitions").
Scheme of determining chiral index and optical resonances of the same individual carbon nanotubes
Figure 1. Scheme of determining chiral index and optical resonances of the same individual carbon nanotubes through combined electron diffraction and Rayleigh scattering techniques
Periodic table is one of the most important discoveries ever in science because it presents a systematic structure-property relation for each atom. A similar relation is equally important for nanostructures as the material properties of nanostructures depend sensitively on their structures. Single-walled carbon nanotubes (SWNTs), a model one-dimensional (1D) nanomaterial system, constitute a rich family of structures with distinctly different electrical and optical properties. The diversity of nanotube physical properties, together with their perfect structural integrity, makes SWNTs model systems to probe 1D physics and to promise materials for nanoscale electronics and photonics. However, a long-standing goal in nanotube research is how to establish the structure-property relation for hundreds of different SWNTs species with high accuracy.
The researchers illustrated the first comprehensive and accurate map between the structure and optical transitions in SWNTs through independent determination of chiral indices and optical transitions in over 200 individual nanotubes (Fig. 1). This map, effectively an "atlas" for SWNT optical transitions, has an uncertainty less than 20meV. It provides a valuable reference for nanotube spectroscopic identification, electronic and photonic applications. Once they know the optical resonances of a single-walled nanotube, they can identify its chiral index without any ambiguity, and vice versa.
momentum-resolved optical transition energy dispersion
Figure 2. (a) The momentum-resolved optical transition energy dispersion Ep(k) for p="5" (or S44 transition) in the graphene Brillouin zone. (b) Renormalized effective Fermi velocity vF as a function of transition index p (or mean diameter d in the top panel).
In addition, this atlas opens the door for systematic understanding of fascinating 1D many-body effects in SWNTs of different types and diameters. By systematically investigating the electron-electron interaction induced optical resonance shifts in different nanotubes, they discovered surprisingly that the Fermi velocity renormalization is the same in metallic and semiconducting SWNTs, but increases monotonically with nanotube diameter towards the two-dimensional graphene limit (Fig. 2). This unusual behavior reveals an intriguing perfect cancellation of long-range electron-electron interaction effects and a diameter dependent short-range electron-electron interaction effects.
This study demonstrates the importance of a systematic approach in characterizing the property-structure relation in nanostructures. The atlas provides the prerequisite reference for the future energy-related applications. The revealed distinct behavior of long-range and short-range electron-electron interactions can be of general importance for differing low-dimensional materials.
Source: Peking University
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