Self-Optimizing Effect of a Few-Layer Graphene's Top-Edge Structure during Field Electron Emission Observed by In Situ TEM.

A phenomenon that a few-layer graphene's (FLG) top transformed to single-layer graphene (SLG) with some bilayer/triple-layer patches on its surface during field electron emission was observed using in situ transmission electron microscopy (TEM). During field electron emission with high emission current, the FLG's top five layers split and finally transformed to SLG with some bilayer/triple-layer patches on its surface with a better crystallinity. It was due to thermal exfoliation and atom recombination at high temperatures induced by joule heat. The heat-induced structural self-transformation optimizes the field electron emission from the graphene's top edge. After transformation, the emission current increased with an order of magnitude at high field region (>307 V/μm). A modified field emission theory of graphene with curves of ln (I/E3/2)∼1/E and ln (I/E3)∼1/E2 in high and low field regimes, respectively, has been used to analyze the phenomenon. The graphene's line current density of two-dimensional (2D) structure and its special energy-dispersion relation at K state of Dirac point makes the curves of ln (I/E3/2)∼1/E and ln (I/E3)∼1/E2 to show up-bending features, which leads to the improvement of the field electron emission tunneling efficiency as the applied electric field increases. These results revealed that the intrinsic field emission characteristics of graphene can be achieved after a structural self-optimizing transformation of FLG during high current field electron emission. It offers an efficient post-treatment method to achieve high performance of graphene field emitter.