Fig. 3. Optical response of individual Cu nanoparticles during oxidation.

a Scattering spectra from a Cu nanoparticle during oxidation in 0.1% O₂ in Ar carrier gas at 200 °C. Four stages of oxidation are highlighted: (i) metallic, (ii, iii) partly oxidized, (iv) bulk oxidized. The arrows indicate how the spectra evolve when going from stage i to ii (right arrow) and from stage iii to iv (left arrow). b Total scattering intensity integrated across the entire wavelength range of 500–850 nm for the same particle as in (a) during oxidation in O₂ (shaded area). The three colored arrows correspond to the time of the spectral snapshots presented in (a). c, d Dark-field scattering microscopy image of a 100 Cu nanoparticle array before (c) and after (d) oxidation. e, f FDTD simulated intensity profiles of a 100 Cu nanoparticle array with all particles simulated as metallic Cu (e) or Cu₂O (f). The radius of the simulated Cu nanoparticles was randomized (±5 nm) to illustrate the effect of a small size difference on the intensity profiles. The images in (c–f) have been normalized to the same scattering intensity levels.