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

Abstract

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 ∼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.

Tuning photoelectron dynamic behavior of thiolate-protected MAu24 nanoclusters via heteroatom substitution

“…3b, where the spectral feature below 200 cm À1 evidently changes with dopant. Since the low-frequency vibrational modes are mainly related to the MAu 12 core, 48 the difference in Raman spectra can be attributed to the heteroatom substitution from two aspects: and the peaks at 500-600 cm À1 come solely from the R groups (simplified by H in this work). Note that the vibrations around 150 cm À1 are assigned to M-Au stretching modes for PdAu 24 and PtAu 24 and to Au-Au and M-Au stretching modes for CdAu 24 and HgAu 24 , respectively.…”

“…For electron transfer from HE states to LUMO+1, the dominant phonon peaks of the four nanoclusters emerge in the frequency range of 33-280 cm À1 (see Table S1, ESI †), which correspond to the low-frequency vibrational modes of MAu 12 cores, the staple Au-S bond-stretching and wagging vibrations. 48 Besides, the weak phonon modes at high frequency, i.e., 400 and 550 cm À1 of HgAu 24 and 517 cm À1 of CdAu 24 (see Table S1, ESI †), are attributed to the vibration of ligands. 48,54 The process A of type II nanoclusters is coupled with stronger low-frequency and high-frequency phonon modes compared to those of type I nanoclusters, leading to faster electron migration from HE states to LUMO+1.…”

“…48 Besides, the weak phonon modes at high frequency, i.e., 400 and 550 cm À1 of HgAu 24 and 517 cm À1 of CdAu 24 (see Table S1, ESI †), are attributed to the vibration of ligands. 48,54 The process A of type II nanoclusters is coupled with stronger low-frequency and high-frequency phonon modes compared to those of type I nanoclusters, leading to faster electron migration from HE states to LUMO+1.…”

Understanding nascent plasmons and metallic bonding in atomically precise gold nanoclusters

“…3A–C . For nonmetallic Au 144 (PET) 60 , the bands at 517 nm and 700 nm clearly show a blueshift and become sharpened as the temperature decreases from r.t. to 2.5 K, and the peak shift fitting 49,50 reveals a 15 meV vibrational mode coupled with the 700 nm peak, indicating a significant contribution of electron-phonon interaction. This phenomenon disagrees with Kubo and Kreibig's earlier prediction, i.e.…”

“…The absorption spectra of the three film samples from r.t. down to 2.5 K are shown in Figure 3A-C. For the non-metallic Au 144 (PET) 60 , the bands at 517 nm and 700 nm clearly show a blueshift and become sharpened as the temperature decreases from r.t. to 2.5 K, and the peak shift fitting 49,50 reveals a 15 meV vibrational mode coupled with the 700 nm peak, indicating a significant contribution of electron-phonon interaction. This phenomenon disagrees with Kubo and Kreibig's earlier prediction, i.e., they believed that the electron-phonon scattering should be suppressed in small-sized NPs (e.g.…”