Carbon nanotubes (CNTs) have been extensively studied due to their excellent properties such as ballistic transport. Electrically induced charge carriers in CNTs and the relation between the spin states and the ballistic transport, however, have not yet been microscopically investigated owing to experimental difficulties. Here we review electron spin resonance (ESR) spectroscopy of semiconductor single-walled CNT (SW-CNT) thin films and their transistors. We have investigated the spin states and the electrically induced charge carriers in the SW-CNTs by utilizing a transistor structure under device operation. The electrically induced ESR method is useful for the microscopic investigation into CNTs because it is capable of directly observing the spins in CNTs. We have observed a clear reverse correlation between the ESR intensity and the transistor current under high charge-density conditions. This result directly demonstrates electrically induced ambipolar spin vanishment in CNTs, providing a first clear evidence of antiparallel spin fillings of the electrically induced charges’ spins and the vacancies’ spins in CNTs. The ambipolar spin vanishment is considered to improve the transport properties of CNTs because it seems to greatly reduce carrier scatterings. Similar spin vanishment has been observed in single-layer graphene transistors. Thus, this result suggests that the electrically induced ambipolar spin vanishment is a universal phenomenon for carbon materials.
Keywords: Ambipolar spin vanishment, Anisotropy, Antiparallel spin fillings, Carbon nanotubes (CNTs), Charge carriers, Electron spin resonance (ESR), Gate voltage, Number of spins, Single-walled CNTs (SW-CNTs), SW-CNT thin films, SW-CNT transistor, Spin-orbital interaction, Spin states, Spin susceptibility, Temperature dependence, Transport properties, Thin films, Tomonaga-Luttinge- -liquid (TLL) states.