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

Abstract

A phenomenon that 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 microscope (TEM) technique. During field electron emission with high emission current, FLG's top 5 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 temperature induced by joule heat. The heat induced structure self-transformation optimizes the field electron emission from the graphene 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/E^3/2)~1/E and ln(I/E^3)~1/E^2 in high- and low field regimes respectively has been used to analyze the phenomenon. The graphene's line current density of 2-dimensional structure and its special energy dispersion relation at K state of Dirac point makes the curves of ln(I/E^3/2)~1/E and ln(I/E^3)~1/E^2 to show an up-bending features, which lead 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 structure 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.