Metal nanoclusters, serving as a mesoscopic bridge between discrete atoms and bulk materials, have emerged as a frontier in the convergence of coordination chemistry and material science [1-16]. Organic ligand-protected metal nanoclusters, with well-defined atomic compositions, unique geometric topologies, and tunable properties, show great potential in catalysis, sensing, and energy conversion [17-33]. Platonic and Archimedean polyhedra-based geometric analysis reveals structure-property relationships in these clusters [34-45]. Early breakthroughs focused on the construction of fullerene-mimetic cage clusters. For instance, Burns et al. successfully synthesized the U₆₀O_x30 cage cluster with buckminsterfullerene (C₆₀)-like topology by bridging uranyl hexagonal bipyramids with oxalate ligands [45]. However, research on polynuclear metal nanoclusters remains relatively limited, primarily due to the intrinsic instability of non-carbon-based fullerenes. The long-predicted “golden fullerene” Au₃₂, first theorized in 2004, [46] was not realized until 2019 when Wang’s team achieved its synthesis using a dual-ligand protection strategy [47]. Subsequently, Schnepf’s group expanded the structural diversity of the Au₃₂ family through a NaBH₄ reduction approach [48]. Notably, Zintl-type triple-shell clusters, such as [K@Au₁₂Sb₂₀]⁵⁻[49]and [Sb@Pd₁₂@Sb₂₀]ⁿ⁻ [50], exhibit near-spherical icosahedral frameworks but suffer from structural collapse due to the lack of ligand stabilization. These advancements highlight a key challenge: balancing geometric perfection and chemical stability hinders atomic-level assembly of metal fullerene nanoclusters. In a groundbreaking collaboration, the research teams led by Professor Shuxin Wang and Professor Zhennan Wu have recently published their latest findings in Nature Synthesis [51]. Leveraging the dynamic coordination of flexible ligands combined with the synergistic effects of halide ligands, the researchers successfully synthesized the largest Ag-Cu alloy nanocluster to date — Ag₁₃₅Cu₆₀. Comprehensive characterization using methods like UV-vis, EPR, DPV, and fs-TA revealed unique hybrid molecular-metallic properties, paving the way for in-depth exploration of its structural and functional correlations. Ag₁₃₅Cu₆₀ nanoclusters were synthesized by the one-pot method, and by optimizing the reaction conditions, such as reactant ratio, solvent selection, and stirring time, a yield of about 10.6% was obtained. This yield is considered intermediate in the synthesis of large core-shell metal nanoclusters. The choice of flexible thiol ligands and synergistic effect of ligands is critical for the formation of target clusters, as they provide more flexibility and adaptability. Flexible thiol ligands have more flexibility and adaptability, which can adapt to different surface geometry and size during the synthesis process to regulate the growth pattern and morphology of clusters. This adaptability is also beneficial for halogen access to the metal surface, promoting the stability and growth of large-sized nanoclusters. In addition, flexible ligands can mitigate static disorder and promote crystallization. The stability of the newly synthesized Ag₁₃₅Cu₆₀ nanoclusters is superior to that of the previously reported Au₃₂ and K@Au₁₂Sb₂₀ nanoclusters. Ag₁₃₅Cu₆₀ nanoclusters can exist stably for more than 60 days at room temperature, while Au₃₂ can only maintain for 40 days, [47] and K@Au₁₂Sb₂₀ nanoclusters decompose directly [49]. The Ag₁₃₅Cu₆₀ nanocluster has a smaller energy gap than the other two nanoclusters. However, in the process of synthesizing Ag₁₃₅Cu₆₀, its yield is only 10.6%, while the yields of the other two nanoclusters can reach 12% or even 25%. Single-crystal X-ray diffraction (SC-XRD) analysis reveals that the Ag₁₃₅Cu₆₀ nanocluster exhibits a distinctive five-layered core-shell architecture: a central Ag₁₃@Ag₄₂@Ag₈₀ sandwich core, an intermediate Cl₁₂ ligand layer, and an outer Cu₆₀(S(CH₂)₂Ph)₆₀Cl₃₀ shell (Ag₁₃@Ag₄₂@Ag₈₀@Cl₁₂@Cu₆₀(S(CH₂)₂Ph)₆₀Cl₃₀). This “Russian doll” -like hierarchical assembly not only endows the cluster with an axial dimension of 1.88 nm but also enables precise modulation of electron delocalization through graded variations in metal-metal bond lengths, laying a foundation for tailored property optimization (Figures 1a-1h). Further structural and electronic characterization confirms that Ag₁₃₅Cu₆₀ adopts a neutral open-shell configuration with 93 free electrons, consistent with theoretical calculations and validated by electron paramagnetic resonance (EPR) spectroscopy (Figure 1i). The UV-visible absorption spectrum exhibits multiple peaks, notably a prominent plasmon-like absorption band at 500 nm, distinct from the typical molecular-state transitions observed in conventional clusters (Figure 1j). Electrochemical measurements reveal a remarkably small HOMO-LUMO gap (~0.01 eV), which further supports its possible hybrid molecular-metallic nature (Figure 1k).

Femtosecond transient absorption spectroscopy (fs-TA) is a powerful ultrafast optical technique for studying ultrafast dynamics of electronic states and energy transfer processes in materials, including nanoclusters. [52] Electron dynamics refers to the rapid evolution of electronic states in a material in response to external stimuli, such as light absorption or electric fields. Phonon coupling describes the interaction between electrons (or electronic states) and lattice vibrations (phonons) in a material. Phonons are quantized representations of the vibrational modes of atoms in a lattice. The key uses of femtosecond transient absorption spectroscopy in the study of metallic nanomaterials are: (i) to reveal the rapid changes of electronic states (excitons, charge carriers, hot electron formation, and relaxation); (ii) to study the phonon coupling and energy transfer mechanism; (iii) to capture the dynamic behavior of the surface plasmon resonance; (iv) to understand size- and shape-dependent electronic and optical properties; (v) to provide basic data for photocatalysis and energy conversion applications. fs-TA elucidated the excited-state dynamics of the Ag₁₃₅Cu₆₀ nanocluster system (Figure. 1-l). The anomalous pump-power dependence and strong phonon coupling in Ag₁₃₅Cu₆₀ demonstrate that excited-state electron-phonon interactions play a pivotal role in modulating its electronic behavior. These distinctive structural vibrational attributes provide substantive experimental evidence for the dual metallic-molecular state characteristics intrinsic to this material system. This finding enhances our mechanistic understanding of hybrid state coexistence in noble metal nanoclusters. In summary, the successful synthesis of Ag₁₃₅Cu₆₀ nanoclusters represents a significant breakthrough in metallic nanocluster research, addressing the contemporary imperative for functional integration in nanomaterial development. This investigation not only achieves structural replication of non-carbon-based buckminsterfullerene (C₆₀)-like topology but also demonstrates dual molecular-metallic state characteristics, thereby providing experimental validation for atomic-scale plasmonic resonance phenomena.

Building upon Professor Wang Shuxin’s team foundational studies of the Ag₁₃₅Cu₆₀ nanocluster, future research faces both opportunities and critical challenges:

i) Surface Modification and Ligand Exchange: By altering surface ligands or conducting ligand exchange, we can further adjust and optimize the optical and electronic properties of Ag₁₃₅Cu₆₀ to meet varying application requirements. (ii) Effect of alloy: Introducing other metal elements for alloying can explore enhancements in specific applications, such as catalytic activity or electrical conductivity.

(iii) Catalysis: The potential of Ag₁₃₅Cu₆₀ in catalysis merits further exploration, especially in photocatalysis and electrocatalysis, such as electrocatalytic CO₂ reduction.

(iv) Optoelectronics: Given its unique optical absorption properties, Ag₁₃₅Cu₆₀ has enormous potential in optoelectronic devices, such as photodetectors or solar cells. Future research will focus on its integration and performance optimization in these devices.

The challenge of large-scale synthesis

(i) Optimization of Reaction Conditions: In large-scale production, we need to optimize the reaction conditions to ensure uniformity and repeatability.

(ii) Reaction Time and Cost Management: Large-scale production requires efficient reaction time to reduce production costs.

(iii) Product Purity and Consistency: We need to develop reliable purification processes and quality inspection methods to ensure that all batches of products achieve the required purity and consistency. This may involve optimization and standardization of multi-step purification processes such as crystallization, filtration and chromatography.

The challenges of integrating into actual devices

(i) Performance Retention and Compatibility: How to maintain the unique optical and electronic properties of Ag₁₃₅Cu₆₀ while achieving its compatibility with existing devices and materials. This requires us to deeply study the behavior and interface properties of nanoclusters on different substrate materials and consider possible chemical and physical interactions to design suitable encapsulation and integration schemes.

(ii) Environmental Stability: We need to study the stability and lifetime of nanoclusters in the face of more complex environments and may need to develop appropriate protective layers or encapsulation technologies to improve their environmental tolerance and long-term stability.

(iii) Functional Testing and Verification: After being integrated into the actual device, the functionality of Ag₁₃₅Cu₆₀ nanoclusters needs to be fully tested and verified, including its optical response, electrical properties and its performance in specific application scenarios. These tests help identify potential problems and guide further optimization and improvement. Through in-depth study and discussion of these opportunities and challenges, we believe that we will not only enhance our understanding and capabilities of Ag₁₃₅Cu₆₀ nanocluster properties but also drive significant advances in the development of multifunctional nanomaterials, especially for applications in catalysis and optoelectronics.

S.W. acknowledges the financial support provided by the National Natural Science Foundation of China (22171156 and 21803001), the Taishan Scholar Foundation of Shandong Province, and Startup Foundation of Qingdao University of Science and Technology.

The authors declare that they have no conflict of interest.