Field enhancement induced by surface defects in two-dimensional ReSe₂ field emitters

The field emission properties of rhenium diselenide (ReSe₂) nanosheets on Si/SiO₂ substrates, obtained through mechanical exfoliation, have been investigated. The n-type conduction was confirmed by using nano-manipulated tungsten probes inside a scanning electrode microscope to directly contact the ReSe₂ flake in back-gated field effect transistor configuration, avoiding any lithographic process. By performing a finite element electrostatic simulation of the electric field, it is demonstrated that the use of a tungsten probe as anode, at a controlled distance from the ReSe₂ emitter surface, allows the collection of emitted electrons from a reduced area that furtherly decreases by reducing the tip-sample distance, i.e. allowing a local characterization of the field emission properties. Experimentally, it is shown that the turn-on voltage can be linearly reduced by reducing the cathode-anode separation distance. By comparing the measured current-voltage characteristics with the numerical simulations, it is also shown that the effective field enhancement on the emitter surface is larger than expected because of surface defects. Finally, it is confirmed that ReSe₂ nanosheets are suitable field emitters with high time stability and low current fluctuations.