Fig. 6. Evolution of the plasmon resonance and near-field heating as a function of time at a 2nd location.

a Waterfall plot of a series of spectra of a second location on the sample, 3 s after the disappearance of the signal in the first series (Fig. 2a). b Plasmon band and near-field temperature quantified from a as a function of time. The blue-shift of the plasmon is associated with partial oxidation of the tip, probably accelerated by near-field heating and/or larger scale morphology changes of the tip apex than that represented in Fig. 5