Graphene has been actively investigated as an electronic material owing to many excellent physical properties, such as high charge mobility and quantum Hall effect, due to the characteristics of a linear band structure and an ideal two-dimensional electron system. However, the correlations between the transport characteristics and the spin states of charge carriers or atomic vacancies in graphene have not yet been fully elucidated. Here, we show the spin states of single-layer graphene to clarify the correlations using electron spin resonance (ESR) spectroscopy as a function of accumulated charge density using transistor structures. Two different electrically induced ESR signals were observed. One is originated from a Fermi-degenerate two-dimensional electron system, demonstrating the first observation of electrically induced Pauli paramagnetism from a microscopic viewpoint, showing a clear contrast to no ESR observation of Pauli paramagnetism in carbon nanotubes (CNTs) due to a one-dimensional electron system. The other is originated from the electrically induced ambipolar spin vanishments due to atomic vacancies in graphene, showing a universal phenomenon for carbon materials including CNTs. The degenerate electron system with the ambipolar spin vanishments would contribute to high charge mobility due to the decrease in spin scatterings in graphene.

Elucidating hole and electron states in organic semiconductor materials is one of the important issues for both their fundamental science and device applications. However, the detailed charge states, in particular, their spin states, have not yet been fully elucidated from a microscopic viewpoint. Here we show electrically controllable spin states of holes and electrons in typical organic semiconductor materials, a polymer regioregular poly­(3-hexyl­thiophene) (RR-P3HT) and a small molecule pentacene, using electron spin resonance (ESR) spectroscopy. By use of their ambipolar organic semiconductor devices, these states were revealed as a function of accumulated charge density. The spin states of the electrically accumulated electrons in RR-P3HT and pentacene are clarified for the first time. Moreover, the formation of spinless states of electrons in RR-P3HT and holes in pentacene are demonstrated under high charge density, showing a contrast to the spin states under low charge density. This result would be important for further understating hole and electron states in organic semiconductor materials and for improving the performance of organic semiconductor devices from a microscopic viewpoint.