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 AlPO -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 AlPO -5 channels or in a freestanding environment, were also studied as a function of temperature. The in situ Raman-scattering measurement under 1 × 10 mbar showed that the (3, 3) and (4, 2) tubes were totally destroyed at a temperature of about 700K, while the (5, 0) tube can survive to 790K. The electronic states of the 0.4nm 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. © IOP Publishing Ltd.