Thermo-optical responses of nanoparticles: Melting of ice and nanocalorimetry approach

The thermo-optical properties of gold nanoparticles (NPs) embedded in an ice matrix were investigated using photoluminescence and Raman spectroscopy. An intense laser beam alone will not melt ice, but the addition of embedded Au NPs allows for melting with resonant laser light of relatively weak intensity. This is due to the strong absorption of Au NPs in the plasmon resonance regimen. We can determine the threshold melting power, P (melting)(T), where T is the background temperature by recording time-resolved Raman scattering signals of the system. A resultant loss of ice signal indicates melting and an absence of conversion to water implicates an irreversible loss of water molecules to the gas phase due to the location of the Au NP agglomerate at or near the ice/vapor surface. For fully embedded NP agglomerates, the ice/water phase transition can be monitored through Raman spectroscopy and the number of NPs in an agglomerate and their interactions can have a greater effect on localized heat generation. The local temperature inside and around the NP agglomerate depends strongly on its geometry and leads to a large scatter in the measured P (melting) as a function of the reduced temperature for different agglomerates. Immobilized Au NP agglomerates can also be characterized using single-particle spectroscopy, and results show that the plasmon emission of Au NPs scales with the number of NPs in an agglomerate.