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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 Au25(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 Au25 are determined to couple with the staple-shell phonon (average energy ∼350 cm–1) in the solution state, but in the solid state these electronic transitions couple with the core phonon (average energy ∼180 cm–1). The suppression of the staple-shell phonon in the solid state is attributed to the intracluster and cluster–matrix interactions.
Liu et al. (Tue,) studied this question.