2.2 Important Spectral and Polarized Properties of Semiconducting SWNT Photoluminescence

Photoluminescence (PL) of single-walled carbon nanotubes (SWNTs) [1] has been intensively studied for the optical characterization of SWNTs. By plotting PL emission intensities as a function of emission and excitation photon energy, Bachilo et al. [2] obtained a two-dimensional map of relative emission intensities. Hereafter, we refer to such a plot of PL excitation and emission spectra as a " PL map. " Figure 1 shows typical PL maps for SWNTs synthesized using different methods. SWNTs were dispersed in D 2 O with surfactant. A major peak in a PL map corresponds to the excitation transition energy of the second subband (E 22) and the photon emission energy of the first subband (E 11) of a specific SWNT structure defined by chiral indices (n, m) [3]. The clear deference between these two PL maps reflects difference of chirality and diameter distribution of SWNTs in different samples. Theoretical studies and recent experiments have clarified that these optical transitions are dominated by strongly correlated electron-hole states in the form of excitons [4-9]. Since the pairs of E 11 and E 22 energies depend on the nanotube structure, we can separately measure a photoluminescence excitation (PLE) spectrum from specific (n, m) SWNTs as a cross section of the PL map at an energy corresponding to the emission of the relevant SWNTs. This is useful to study optical properties of different types of SWNTs separately. In addition, as far as semiconducting SWNTs are concerned, such PL mapping is one of the most promising approaches for the determination of the structure distribution in a bulk SWNT sample if complementally combined with other optical spectroscopy such as optical absorption and Raman spectroscopy. Hence, photoluminescence spectroscopy is a powerful tool not only for investigation of physical properties of SWNTs but also in advancing toward development of complete synthesis or separation methods of SWNTs with only one chiral structure, which is one of the ultimate dreams in nano-science and technology field.