Quantum-sized metal nanoclusters

The recent years have witnessed rapidly growing research interest in quantum-sized metal nanoclusters. We thank the opportunity provided by Nanoscale for us to organize this Themed Issue on metal clusters. This Special Issue offers a snapshot of the very diverse research work being carried out on metal clusters, including the synthesis and isolation of monodisperse clusters, studies on the structure and physicochemical properties, and the development of various applications, as well as the theoretical work on computing cluster structures and properties.

Metal clusters composed of a specific number of atoms are of fundamental importance for investigating the evolution of the atom packing structure and physicochemical properties from the atomic state to the metallic state. For gas phase clusters, much work has been done in the past few decades. Specific sizes of gas phase clusters can be readily obtained by mass selection, but the atom packing structures can only be probed indirectly, rather than being directly solved by X-ray crystallography. On the other hand, solution phase clusters can be obtained in large quantities and crystallized; their structures can be unambiguously solved by single-crystal X-ray diffraction, albeit crystallization is still quite challenging. Based upon the crystal structures, the electronic and optical properties as well as the size-dependent evolution can be ultimately understood. A paradigm system is the gold–thiolate cluster system, referred to as Au_n(SR)_m, where SR = thiolate. Recent advances have permitted the synthesis of a series of gold nanoclusters ranging from about a dozen to a few hundred gold atoms (equivalent diameter of metal core from subnanometer to ∼2 nm). Significantly, some of these clusters have been crystallographically characterized, such as [Au₂₅(SR)₁₈]^q (q = −1, 0), [Au₃₈(SR)₂₄]⁰, and [Au₁₀₂(SR)₄₄]⁰. Such nanoclusters exhibit many interesting properties due primarily to quantum size effects and hold promise in a wide range of applications, such as catalysis, sensing, fluorescence labeling, and so forth.

In this Themed Issue, the contributions from over 30 research groups have shed some exciting light on the fundamental science of metal clusters, mainly on Au but also including Ag, Pt, Pd, Cu, Ru, etc. Several review articles serve as entries by summarizing the recent research progress in the field. For solution phase metal clusters, various types of ligands have been used to protect clusters, such as thiolate, phosphine, selenolate, carbonyl, alkyne, DNA, protein cages, and so on. The effect of thiolate ligands on the overall stability of Au_n(SR)_m nanoclusters has been discussed. The halogen analogs of thiolated Au clusters revealed some interesting electronic effects. Bimetallic clusters have received considerable research interest; for example, doping gold nanoclusters with silver has been found to induce distinct changes to the optical absorption and enhance fluorescence. The fluorescence (or luminescence) properties of quantum-sized metal nanoclusters (e.g. Au, Ag, Pt), including poly-grained plasmonic Au nanoparticles, have long been of major interest in the field, and are also reflected in many reports in this Special Issue. Research works on the structure and bonding, chirality, ligand symmetry equivalence, electron transfer catalysis, and photoelectrochemistry of nanoclusters have revealed some particularly interesting aspects of their properties. There has been sufficient evidence that ligand-protected metal clusters are far more complicated than gas phase bare clusters in terms of electronic structure, magnetic properties, electron dynamics, and many other properties.

On the application side, several reports have demonstrated the utility of gold clusters in sensing chemicals (e.g. dopamine) and ions (e.g. Ag⁺, Hg²⁺, I⁻). With well defined clusters, some important insights into the sensing mechanisms have been gained, which will facilitate the future design of highly sensitive and selective sensors. The catalytic application of nanoclusters has received increasing interest and will offer some opportunities for understanding the structure–reactivity relationship at the atomic level. Luminescent nanoclusters have also been extensively explored as biological labels and utilized for fluorescence-based cellular imaging.

Finally, we are grateful to all the contributors, including those researchers who could not deliver their works within the tight timeframe. We hope this Themed Issue will boost future research on metal clusters.

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