Although FE resonance in STM has been extensively studied, researchers from Academia Sinica in Taiwan now find that two effects may endow it with the capability for nano-scale imaging under a long tip-sample distance. The first is field enhancement factor. They find that the tip-sample distance and the number of FE resonances increase with the increase of the field enhancement factor. The other is self-focus function. This effect can depress the divergence of the electron beam emitted from the tip apex with the increase of the tip-sample distance.

Jumping the gap

The group demonstrate that the number of FE resonances can reach 11 at 9 V, corresponding to a tip-sample distance of 60 Å, as the field enhancement factor is high. Due to the self-focus function, surface reconstruction on the Au(111) surface can still be clearly mapped across a long distance through the intensity variation of the highest energy FE resonance. The self-focus function can be realized by calculating the electron trajectories in the STM gap. However, all electrons emitting from the emission area should coherently couple into the same standing-wave state. The researchers exploit the quantization rule to illustrate how electrons travelling along different trajectories may stay in the same standing-wave state.

The combination of FE resonance with the high enhancement factor and the self-focus function has potential applications in nano-scale imaging. For example, it is difficult to observe suspended graphene by STM operating in the normal mode because of its flexibility. However the researchers expect that the technique shown here is insensitive to the deformation of graphene, and can be applied to map the structures of materials grown on suspended graphene.

More information about the research can be found in the journal Nanotechnology 27 175705.