Metallic or nonmetallic state of gold nanoparticles

As mentioned previously, the study of the evolution from molecular to metallic states is of great significance for exploring the origins of metal bonds and the birth of SPR [167, 170]. Ligand-protected gold nanoclusters are an excellent choice for studying this evolution due to their precise atomic structure. However, due to the difficulty of synthesis, the ultrasmall size of the transition zone from the nonmetallic to the metallic state has only recently been explored [86- 89]. Knappenberger et al. studied the electron energy relaxation of Aui44(SC6Ha3)604 by femtosecond time-resolved transient absorption spectroscopy and found that Au144(SC6H13)6o exhibited the characteristics of metals [171]. However, in a subsequent study, Jin et al. explored the property transitions of a series of gold nanoclusters and found that the electron dynamics of Au144 do not depend on the pump power, revealing its nonmetallic nature [167]. The difference between the findings of these two research groups on the nature of Au144 was that the intense overlap of the probe light at 525 nm with the ESA led to an underestimation of the decay time. Interestingly, Hakkinen et al. conducted both low-temperature optical absorption spectroscopy and density functional theory calculations, which revealed that Au144(SC12)60 has a molecular-like electronic structure and that the critical point of the metal-to-molecule transition is between Au144 and Au187 [76]. Whetten et al. transmitted the information of the quantum state of Au144 by theoretical and experimental means [170]. Ramakrishna also observed no power dependence of Au144 when studying the exciton dynamics of gold clusters [172]. Based on these studies, Au144 should behave as a quantum state. It is worth noting, however, that the crystal structure of the thiol ligand-protected Au144 was not determined until 2018 when Jin and Wu et al. reported it [83].

In addition, other brief discussions about the metallic or nonmetallic nature of gold clusters have also been conducted. In

2015, Jin et al. used femtosecond broadband transient absorption spectroscopy to confirm that Au133 has similar molecular properties [81]. In 2016, Jin et al. reported the crystal structure of large-sized Au246 clusters [85], and subsequent kinetic analysis indicated that it still has molecular properties [86]. In 2017, along with the determination of the crystal structure of another large-sized Au279 cluster, Dass and Jin et al. conducted electron dynamics studies on the cluster [87, 88]. The single SPR peak in the steady-state absorption spectrum and the power-dependent lifetime in the ТА spectrum revealed the metal-like properties of Au279 in fee configuration. Even so, the long-life phonon-phonon coupling process and the broad excited-state absorption in the near-infrared region of Au279 indicate its nascent plasma properties [88, 89].

Recently, Jin et al. observed interesting electrokinetic characteristics in Au333(SR)79 nanoparticles [168]. It is worth noting that early research indicated that Au333(SR]79 (R: CH2CH2Ph) is a metallic nanomolecule [173]. The steady-state absorption spectrum of Au333(SRJ79 protected by three different SR groups exhibited a single absorption peak at room temperature. Interestingly, the low-temperature spectrum showed a splitting of the absorption peak. Further analysis indicated that the splitting of the SPR peak was derived from the oblate core of Au333(SR)79. In addition, the femtosecond ТА spectra of Au333(SRJ79 presented two distinct GSB signals, which are consistent with the steady-state absorption peaks at cryogenic temperatures. Global fitting of the spectral data reveals that the electrodynamic process of Au333(SR)79 includes electron-phonon coupling (~1 ps], normal phonon-phonon coupling (>100 ps), and an unusual 4-5 ps relaxation process (a fast phonon-phonon relaxation process). These two unique features (the phonon-phonon coupling process of 4-5 ps and the two GSB signals in the ТА spectrum) are independent of the protecting groups and solvents, indicating that they are derived from the core structure of Aii333(SR)79.

As a result, this work considers that the splitting of plasmon and quantum effects occurs in the core of Au333. Note that the atomic structure of Au333 has not yet been resolved. The origin mechanism of plasmon resonance and the critical point of transition from the metallic to the nonmetallic state still remain to be further explored.

Electron and energy transfer

It is well known that ligand-protected gold clusters are composed of a gold core and outer protective ligands [10, 34, 51]. The photophysical properties of gold clusters can be adjusted by their composition, size, and ligands [174]. In the cluster structure, the ligand plays a role in regulating and modifying the surface function and helps to stabilize these gold clusters [175-177]. Thus, the study of the interaction between surface ligands and metal cores assists in the profound understanding of the important role of ligands in the structure-property relationships of gold clusters. The different chemical potential between the ligand and the gold core may result in electron transfer between them [178]. Electron transfer has an important influence on the physicochemical properties of gold clusters [179, 180]. For example, by changing the structure of the cluster to affect its electron transfer process, the exciton radiation relaxation process can be promoted and luminescence enhanced.

Johansson and Gong et al. predicted the structure of the bare Au32 cage with high stability [181, 182] and believed that the structure of the Au32 cage should be very similar to that of C60 [183, 184]; for this reason, Au32 was named "golden fullerene” [181]. The source of stability of Au32 is attributed to its aurophilicity and aromatic nature. In a subsequent study, Wang et al. synthesized a “golden fullerene" Au32 cluster by using pyridinamine and triphenylphosphine as protective agents [185]. Single-crystal X-ray structure analysis revealed that the [Au32(Ph3P)8(dpa)6](SbF6]2 (dpa: 2,2'-dipyridylamine) cluster has the point group symmetry of S6 and that the kernel is the Au328+ in the /h configuration. Quantum chemistry studies have demonstrated that the particular stability of the structure of the cluster is derived from metal-ligand interactions. Zhou et al. found significant charge transfer and solvation kinetics in Au20(SR)16 and phosphine-protected Au20 nanoclusters [186, 187]. Solvent-dependent oscillations in the kinetic traces confirmed charge transfer between the surface ligand and the metal core.

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