J.P. Zhai

Adsorption of O2 on a (4,2) carbon nanotube

The physisorption and chemisorption of O2 on the outer wall of a (4, 2) carbon nanotube have been studied by density functional calculations. The minimum-energy paths from physisorbed products to chemisorbed products are calculated by the nudged elastic band technique. Our theoretical calculations and experimentally measured Raman spectra both indicate that the (4, 2) tube is less air stable than the (5, 0) tube, which could be used to select a single chirality from a mixture of these 4 A tubes.

Resonant Raman study of confined Se single helix and Se8 rings

Electron-phonon coupling in Se species confined in the nanoporous matrix has been investigated by using resonant Raman spectra performed with several different laser lines from deep blue to near infrared. The spectra strongly depend on the energies of the excitation laser lines. The one-phonon symmetric A1 modes for Se single helix and Se8 rings are enhanced in the vicinity of their absorption bands. Detailed analysis shows that the Raman band in the high-frequency range of 450–550cm−1 is composed of three individual second-order Raman bands for the confined Se species. These two-phonon Raman shifts occur at twice the frequency shift of the first-order Raman lines, and their intensities are also enhanced when the excitation laser energy matches an electronic transition in Se nanospecies.

Raman characterization of 0.4 nm single-walled carbon nanotubes formed in the channels of ALPO(4)-5 zeolite single crystals

In this paper, we review our recent research on ultra-small single-walled carbon nanotubes (SWNTs). Using Raman scattering as a tool, we systematically studied the pyrolysis process of carbon precursors in the channels of AlPO4-5 zeolite single crystals, and studied the formation process of the ultra-small SWNTs in the channels. The thermal expansion behaviour and thermal stability of these ultra-small SWNTs, either confined in the AlPO4-5 channels or in a freestanding environment, were also studied as a function of temperature. The in situ Raman-scattering measurement under 1 × 10−5 mbar showed that the (3, 3) and (4, 2) tubes were totally destroyed at a temperature of about 700 K, while the (5, 0) tube can survive to 790 K. The electronic states of the 0.4 nm SWNTs were modulated by means of lithium doping. The continuous electron charge transfer from lithium atoms to the tubes was traced using Raman scattering. With increasing doping level, the radial breathing modes of these tubes shifted to higher frequency because the vibration perpendicular to the tube axis was depressed, in contrast to the conventional softening and downshift of the tangential G-mode vibrations.

Resonant Raman Scattering of Smallest Single-walled Carbon Nanotubes

Ultra-thin single wall carbon nanotubes (SWNTs) with a diameter of only 0.3 nm were synthesized in the nano-channels of AlPO4-11 porous single crystals. Raman spectra, with excitation wavelengths in the range from 457.9 to 647.1 nm, show excellent agreement with the density functional calculations of the Raman-active vibration modes of the armchair (2,2) SWCNTs. Calculated imaginary part of the dielectric function also displays qualitative agreement with the resonant Raman data. Interestingly, the (2,2) nanotube has two metastable ground state corresponding two slightly different lattice constants in axial direction, one state is metallic and the other is semiconducting. The polarizibility of the Raman modes agrees well with the calculated intensities for non-resonant Raman scattering, although the resonant Raman scattering plays a key role in the process. Both theory and experiment show the free-standing (2,2) SWNTs to be unstable. Confinement of the SWNTs in the nano-channels stablilizes the structure.