The impact of plasmonic heating with a 532 nm laser source on the morphology and structure of the P25-Au sample is presented in Figure . The P25-Au film on silicon substrate was irradiated by scanning a 20 × 20 µm 2 area with the laser beam with a 500 ms exposure. The Raman map of the area was simultaneously collected during the scan. The SEM image (Figure ) of the P25-Au surface after laser irradiation with various increasing laser power shows that the heat generated during the scan causes dramatic changes in the morphology of the scan areas. At lower power (6 mW), cracks formed on the surface (Figure ). As the laser power increased, the size and depth of the cracks became more extensive (Figure ), and the formation of holes/melted pools and molten balls of TiO2 was also observed (Figures ). The Raman spectrum (Figure ) at 6 mW power showed that the P25 remained almost unchanged (black curve Figure ). The P25 nanocrystal structure gradually transformed from mainly anatase to mainly rutile phase as more rutile peaks were observed (Figure ). In addition to the ART phase transition, agglomerations of melted TiO2 were seen at higher power (> ~12 mW, Figures ). The exposed area of the Si surface also increased as the melted TiO2 and Au NPs aggregates have higher pack density than that of the TiO2/Au NP porous film (Figure ). The results suggest that as high as the melting temperature (~1830°C) of the TiO2 NPs was reached due to the dissipation of the plasmonic heating as the laser power increased. More details on these effects of plasmonic heating are explained in the later section. For comparison, the sintering of P25 and P25-Au films on Si substrate was carried out separately in a muffle oven. No indication of melting was observed after annealing to 1080 °C (Figure ), which is expected from the melting temperature of TiO2 NPs. XRD showed that anatase-rutile transition occurred in P25-Au films between 600 o C to 700 o C (Figure ), consistent with the reported P25 phase transition. In order to check whether the structural change of TiO2 was because of the presence of LSPR, two different laser wavelengths that were on-and off-resonance with the LSPR of the Aunanoparticle were used to irradiate the same P25-Au sample. When an on-resonance 532 nm laser scanned the P25-Au film, the surface morphology was changed, as shown above. Cracks and clumps of melted TiO2 particles were seen in the SEM image (Figure ) after irradiated at 12 mW laser power. In comparison, when an off-resonance 638 nm laser at 18 mW power (Figure ) was used, the surface only had a small crack in the scan area due to the laser heat. The drastic differences of the on-and off-resonance laser irradiation were also observed in an optical microscope image (Figure ). The effects can be more clearly understood after studying the Raman spectra, which were simultaneously obtained during the scans. At the center area (green spot) with a 532 nm scan, the cluster area had converted to a rutile phase (green curve in Figure ) due to intense plasmonic heat generated by the Au NPs. At the edge area (red spot), the P25 was partially converted to rutile as both anatase/rutile were observed, and rutile peak intensity was weaker when compared with the center spot (red curve in Figure ). For the off-resonance scan, the P25 Raman spectra showed no change. Both the center (green curve) and the edge (red curve) were comparable to the reference P25 spectrum (black curve Figure ), which was dominated by the anatase phase. Thus confirming the critical role of LSPR resonance in the heating of the sample.