Ostensible Steady-State Molecular Cooling with Plasmonic Gold Nanoparticles

The optical and chemical properties of plasmonic materials have sparked extensive research in exploring their applications in various areas such as photocatalysts, chemical sensors, and photonic devices. However, complicated plasmon-molecule interactions have posed substantial obstacles for the development of plasmonic material-based technologies. Quantifying plasmon-molecule energy transfer processes is a crucial step to understand the complex interplay between plasmonic materials and molecules. Here we report an anomalous steady-state reduction in the anti-Stokes to Stokes surface-enhanced Raman spectroscopy (SERS) scattering intensity ratio of aromatic thiols adsorbed on plasmonic gold nanoparticles under continuous-wave laser irradiation. The observed reduction of the scattering intensity ratio is closely related to the excitation wavelength, the surrounding media, and component of the plasmonic substrates used. Moreover, we observed a similar extent of scattering intensity ratio reduction with a range of aromatic thiols and under different external temperatures. Our discovery implies that there are either unexplained wavelength-dependent SERS outcoupling effects, or some unrecognized plasmon-molecule interactions which lead to a nanoscale plasmon refrigerator for molecules. This effect should be taken into consideration for the design of plasmonic catalysts and plasmonic photonic devices. Moreover, it could be useful for cooling large molecules under ambient conditions.