We observed field emission microscopy (FEM) patterns of noble-metal (NM) covered W nano-tips with three different apex structures fabricated by field evaporation. Each of the three tips was terminated with a single atom, three atoms or ten atoms. We investigated the temporal changes in the FEM of these tips to discuss the stabilities in the spatial distributions of the field emission (FE) beams. The single-atom tip showed two characteristics that were superior to the others. First, the beams emitted from the singleatom tip were the most collimated among the three tips (the semi-cone angle of 1.0°, FWHM). Second, adsorption of residual gas had little influence on FE from the single-atom tip, while the other tips were easily contaminated even at ultra high vacuum, resulting in the emission fluctuation.
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