Origin of nonlinear current-voltage curves for suspended zigzag edge graphene nanoribbon

The structure-dependent electronic properties of suspended graphene nanoribbon (GNR) have still been important issues. However, these properties have been rarely investigated experimentally due to technical difficulties to produce high-quality suspended GNRs and the identification of atomic structures. Here, we report the electronic properties of suspended GNRs by in situ transmission electron microscopy observations to simultaneously obtain the structural information while measuring the current-voltage (I-V) curves. The suspended GNRs could be obtained by convergent electron beam nanosculpting after careful cleaning by current annealing. The I-V curves were measured for GNRs with a mixture of both zigzag and armchair edges (MGNR) with widths of 3.7 to 1.9 nm and also zigzag edge GNRs (ZGNR) with widths of 1.7 to 1.2 nm. The I-V curves for the ZGNRs were different from those of the MGNRs, as follows. (1) The ZGNRs showed a sharp increase at the threshold voltage in differential conductance-voltage curves. (2) The band gaps measured for ZGNRs were smaller than the band gaps calculated using the GW approximation. (3) The threshold voltage increased with the GNR length. These findings support magnetic-insulator and nonmagnetic-metal nonequilibrium phase transitions as theoretically predicted. Narrow and short ZGNR represents the potential for further nanosized switching devices.