Observation of Core Phonon in Electron-Phonon Coupling in Au25 Nanoclusters

Temperature-dependent optical properties are of paramount importance for fundamentally understanding the electron-phonon interactions and phonon modes in atomically precise nanocluster materials. In this work, low-temperature optical absorption spectra of the icosahedral [Au-25(SR)(18)](-) nanocluster are measured from room temperature down to liquid helium temperature by adopting a thin-film-based technique. The thin-film measurement is further compared with results from the previous solution-based method. Interestingly, the previously missing core phonon is revealed by a quantitative analysis of the film data through peak deconvolution and fitting of the temperature trend with a theoretical model. The two lowest-energy absorption peaks (at 1.6 and 1.8 eV) of Au-25 are determined to couple with the staple-shell phonon (average energy similar to 350 cm(-1)) in the solution state, but in the solid state these electronic transitions couple with the core phonon (average energy similar to 180 cm(-1)). The suppression of the staple-shell phonon in the solid state is attributed to the intracluster and cluster-matrix interactions.

Automated summary

This study compared thin-film and solution-based measurements to reveal the existence of core phonons in Au-25 nanoclusters, as well as the suppression mechanism of staple-shell phonons in the solid state.