Field emission spectra of single-atom tips with thermodynamically stable structures

Energy spectra of field emitted electrons from well-characterized tungsten nanoemitters covered with different metals have been measured in detail while changing the electric field and the topmost atomic structure. At very high electric fields, additional humps appear in the spectra of a single-atom tip. Their energy positions depend on both the coated material and structure termination but not on the electric field. On the other hand, their intensities increase with increasing field. The current spectra did not include either peculiar features attributable to resonant tunneling or electric-field penetration, or significantly narrow FWHM, but are rather analogous to those of the conventional metallic field emitters. The spectral features along with a recent ab-initio theory indicate a large reduction in the tunneling barrier height in front of the single-atom electron source.