Single-walled carbon nanotubes (SWNTs) provide a variety of new opportunities for the exploration of one-dimensional (1-D) quantum physics as well as novel device applications. Although their mechanical and electrical properties have been extensively studied during the past decade, there has been only limited success in exploring their optical, magnetic, and magneto-optical properties. In standard production methods, SWNTs appear in the form of bundles (or ‘ropes’) due to their strong van der Waals forces, which results in significant broadening of electronic states, smearing out any chirality-dependent features in optical spectra. However, very recently, a new technique for producing individually suspended SWNTs has been reported by Prof. Smalley’s group at Rice University [Oconnelletal02Science]. These samples have revealed, for the first time, a number of clearly observable peaks in linear absorption and emission spectra, corresponding to interband transitions in different types of tubes. A subsequent photoluminescence excitation spectroscopy study by Prof. Weisman’s group at Rice University successfully provided detailed peak assignments [BachiloetAl02Science].
We are currently carrying out magneto-optical and ultrafast optical studies of such micelle-suspended SWNTs:
Project 1: Magneto-Optical Study of the Aharonov-Bohm Effect in SWNTs
A magnetic field applied parallel to the tube axis is expected to make drastic modifications on the electronic states of SWNTs. This is due to a subtle interplay of the Aharonov-Bohm phase, the Bloch theorem, and the boundary condition along the circumference. Metallic SWNTs are predicted to become semiconducting even in an infinitesimally small magnetic field, and semiconducting SWNTs are predicted to become metallic at ultrahigh magnetic fields. Any influence of the magnetic field is expected to vanish, however, whenever the magnetic flux passing through the tube is an integer multiple of the flux quantum, Φ0 = e/h. Thus, the band gap of a SWNT should oscillate as a function of magnetic field with period Φ0, undergoing a repetition of metal-insulator transitions as the field increases. See the PhysicsWorld article by Jing Kong, Leo Kouwenhoven and Cees Dekker.
We have carried out magneto-absorption and magneto-photoluminescence experiments on micelle-suspended SWNTs in magnetic fields up to 45 T. Chirality-assigned spectral peaks exhibit significant changes with increasing magnetic field, which can be quantitatively explained in terms of the theoretically predicted splittings and redshifts of the band edge due to the Aharonov-Bohm effect combined with the magnetic-field-induced alignment of the nanotubes.
Left figure: J. Kong, L. Kouwenhoven, and C. Dekker, PhysicsWorld, July 2004. Right figure: S. Zaric, G. N. Ostojic, J. Kono, J. Shaver, V. C. Moore, R. H. Hauge, R. E. Smalley, and X. Wei, Nano Letters 4, 2219 (2004). See also S. Zaric et al., Science 304, 1129 (2004).
References:
S. Zaric, G. N. Ostojic, J. Kono, J. Shaver, V. C. Moore, R. H. Hauge, R. E. Smalley, and X. Wei, “Estimation of Magnetic Susceptibility Anisotropy of Carbon Nanotubes Using Magneto-Photoluminescence,” Nano Letters 4, 2219 (2004). (abstract, full text)
S. Zaric, G. N. Ostojic, J. Kono , J. Shaver, V. C. Moore, M. S. Strano, R. H. Hauge, R. E. Smalley, and X. Wei, “Optical Signatures of the Aharonov-Bohm Phase in Single-Walled Carbon Nanotubes,” Science 304, 1129 (2004). (abstract, full text)
Project 2: Ultrafast Carrier Dynamics in SWNTs
Ultrafast laser spectroscopy is one of the best methods for measuring carrier distribution functions and relaxation mechanisms. Using a wide variety of techniques employing femtosecond pulses, one can directly examine dynamical processes after creation of electron-hole pairs across the band gap, i.e., carrier dephasing processes, carrier-carrier scattering, carrier-phonon scattering, exciton formation, and radiative and non-radiative recombination dynamics. There have been several ultrafast optical studies of SWNTs, reporting subpicosecond to a ps lifetimes for semiconducting SWNTs via pump-probe spectroscopy. Although such ultrashort lifetimes can be expected for intraband relaxation towards the band edge, the microscopic origin of the fast interband decay (which is likely to be non-radiative) is still unclear. One important issue is that these studies were performed on bundled SWNTs, which did not show any PL. It is thus desired to carry out ultrafast spectroscopy on SWNT samples that show chirality-assigned absorption and PL peaks.
We have carried out a femtosecond pump-probe study of chirality-assigned SWNTs. The linear absorption and PL spectra of such micelle-suspended SWNT samples show a number of chirality-dependent peaks, and as a result, the pump-probe data sensitively depends on the wavelength used. In the wavelength range corresponding to the second van Hove singularities (VHSs), we observe ultrafast (sub-picosecond) decays, as has been reported previously. We ascribe these ultrafast decays to intraband carrier relaxation. On the other hand, in the wavelength range corresponding to the first VHSs, we observe two distinct regimes in ultrafast carrier relaxation. The slow component (5-20 ps), which has not been observed previously, is resonantly enhanced whenever the pump photon energy coincides with an absorption peak and we attribute it to interband carrier recombination, while we interpret the always-present fast component (0.3-1.2 ps) as intraband carrier relaxation in non-resonantly excited nanotubes. The slow component decreases drastically with decreasing pH (or increasing H + doping), especially in large-diameter tubes. This can be explained as a consequence of the disappearance of absorption peaks at high doping due to the entrance of the Fermi energy into the valence band, i.e., a 1-D manifestation of the Burstein-Moss effect.
References:
G. N. Ostojic, S. Zaric, J. Kono, V. C. Moore, R. H. Hauge, and R. E. Smalley, “Stability of High-Density One-Dimensional Excitons in Carbon Nanotubes under High Laser Excitation,” Physical Review Letters 94, 097401 (2005). (abstract, full text)
G. N. Ostojic, S. Zaric, J. Kono, M. S. Strano, V. C. Moore, R. H. Hauge, and R. E. Smalley, “Interband Recombination Dynamics of Resonantly-Excited Single-Walled Carbon Nanotubes,” Physical Review Letters92, 117402 (2004). (abstract, full text)
J. Kono, G. N. Ostojic, S. Zaric, M. S. Strano, V. C. Moore, J. Shaver, R. H. Hauge, and R. E. Smalley, “Ultrafast Optical Spectroscopy of Micelle-Suspended Single-Walled Carbon Nanotubes,” Applied Physics A 78, 1093 (2004). (full text)
M. F. Chisholm, Y. H. Wang, A. R. Lupini, G. Eres, A. A. Puretzky, B. E. Brinson, A. Melechko, D. B. Geohegan, H. Cui, M. P. Johnson, S. J. Pennycook, D. H. Lowndes, S. Arepalli, C. Kittrell, S. Sivaram, M. Kim, G. Lavin, J. Kono, R. H. Hauge, and R. E. Smalley, “Comment on ‘Single Crystals of Single-Walled Carbon Nanotubes Formed by Self-Assembly’,” Science 300, 1236 (2003). (abstract, full text)
G. A. Khodaparast, G. N. Ostojic, A. Srivastava, J. Wang, and J. Kono, “Mid-infrared ultrafast and nonlinear spectroscopy of semiconductors” (invited paper), in: Proceedings of the 2002 Annual Meeting of the IEEE Lasers and Electro-Optics Society (LEOS 2002), pp. 399-400. (full text)
Collaborators:
Rick Smalley, Department of Chemistry, Rice University
Qimiao Si, Department of Physics and Astronomy, Rice University