https://orcid.org/0000-0003-3847-4781
The nanometre-sized metal atom clusters (nanoclusters) or superatoms (<2 nm), which consist of less than a few dozens of metal atoms, could be defined as a link between atom and nanoparticle. Various dissimilar metal nanoclusters (Cu, Nb, Mo, Ag, Ta, W, Au…) are used and could find applications in catalysis, biotechnology, energy conversion, and storage. By experience, there are several scientific communities working on nanoclusters and superatoms with limited transversal communication. The first idea of this focus issue was to try to overcome this issue and to cover recent advances in the emerging fields of functional nanocomposite materials based on these nanometallic objects. This focus issue collected five papers (one regular article and four review papers) and we focused on new results or reviews involving metal atom clusters or superatoms for optical, nanobiotechnology, catalytic, photonic, energy, and/or environmental applications.
Three review articles were published on this focus issue on fundamental properties or characterizations of nanoclusters and superatoms and the fascinating properties of these ultrasmall metallic particles are very well described. One of these reviews described the strong efficiency of the synthesis and geometric structures of anion-templated Ag nanoclusters. The paper clearly discussed the influences of the central anions on the geometric structure of anion-templated Ag nanoclusters by comparing representative anion-templated Ag nanoclusters. Another review overviewed the main results obtained on free metal clusters produced in the gas phase including mainly electronic properties, the giant atom concept, the optical properties, the role of the metal atom and finally the atomic structure of clusters. It was clearly demonstrated that the addition of a protective monolayer stabilizes the exceptional properties of metal clusters. As a result, atomically precise nanoparticles can be produced at the gram scale and can be used to get a deep understanding of light matter interactions in finite systems while protected clusters, in parallel to new concepts, allow to take advantage of finite size effects in applications. The last one summarized recent advances in the photoluminescent (PL) mechanism of nanoclusters. Different mechanisms were presented and a model that structural water molecules dominated p band intermediate state (PBIS) was proposed to give a unified understanding of the PL mechanism.
In parallel, the last review was dedicated to functional nanoarchitectonic composite materials based on octahedral molecular metal atom clusters (Nb6, Mo6, Ta6, W6, Re6). Functional nanocomposites represent a particular class of nanoarchitectured materials (0D, 1D, 2D …). These heterogeneous composite nanostructured materials are composed by definition of multi-(nano)components, each tailored to address different requirements. Different morphologies were reported like powder and film nanocomposites with two-dimensional, one-dimensional, and zero-dimensional morphologies, as well as film matrices from organic polymers to inorganic layered oxides. The main objective of this review was to provide significant guiding for the design of new high-performance nanocomposites based on transition metal atom clusters. The high potential and synergetic effects of these nanocomposites for biotechnology applications, photovoltaic, solar control, catalytic, photonic, and sensor applications are very well demonstrated. This review also provides a basic level of understanding how nanocomposites are characterized and processed using different techniques and methods. As a new strategy, the electrophoretic deposition process appears to be very efficient to fabricate highly transparent, homogeneous, and functional nanocomposite thin films and coatings.
A regular article focused on high performance {Nb5TaX12}@PVP (X = Cl, Br) cluster-based nanocomposites coatings for solar glazing applications. Indeed, in the general context of energy saving due to the increase in energy consumption and serious environmental problems induced by global warming, the realization of low-cost selective films as an ultraviolet and near-infrared barrier with a highly visible transparency is a field of research of growing interest. The simple, low cost and low toxicity process presented in this article preserved the optimal optical properties of the film up to 100°C and up to 1 year. A significant improvement of 30% in the figure of merit was obtained compared to the previous results on nanocomposite coatings based on metal atom clusters.
Of course, this field of research is quite young and new challenges and opportunities using transition metal clusters as building blocks for multifunctional nanocomposites are numerous. Nevertheless, this focus issue largely reveals the strong potential of metal nanoclusters and associated nanocomposite materials to provide some answers to the problems we are currently facing. This field can be extended to the use of heterometallic systems transition metal clusters. This family of nanoclusters is extremely rich and could be even probably enriched by using machine learning and calculation methods. Controlled self-assembly of nanoclusters or their mixing with semiconductor nanocrystals could play a key role in customizing advanced functional materials. Moreover, to increase the dimensionality of the metal atom clusters could be very interesting in terms of thermal stability for instance, which is still a weak point for the molecular nanoclusters. The last but not least, these transition metal atom clusters can be used as precursors for the discovery of new compounds and nanocomposites like nitrides, carbides, borides, sulfides, or alloys. The field of possibilities is open to everyone and the adventure of the nanocomposites based on metal atom clusters has only just begun.
Finally, we would like to extend our deepest gratitude to all contributors.
Disclosure statement
No potential conflict of interest was reported by the author(s).