Cluster-assembled materials based on polyoxometalates (POMs) process unprecedented structural tunability and polymer-like behaviors because of the sub-nanometer-sized building blocks and their non-covalent linkage. Through the whole journey in their development, exquisite POM cluster assemblies with advanced functions spring up, and the control over their nanostructures tends to be more precise and reach their nature at molecular level. In this paper, recent progress in cluster-assembled materials based on POMs is overviewed, including the assembly of covalently and non-covalently modified POM clusters as well as the co-assembly of POM clusters and inorganic entities. By surfactant encapsulation, POM clusters are compatible in various solvents, and hybrid assemblies with diverse morphologies and unique properties have been achieved. The Janus POM-POSS clusters feature intricate assemblies in mixed solvent systems, which provide instructive understanding for solution behaviors of macro-ions. Notably, the POM clusters can also intervene in the nucleation of nanocrystals, resulting in the formation of cluster-nuclei co-assembled materials with well-organized structures and wide-ranging composition compatibility. Meanwhile, the chemistry of cluster-assembled materials has not been well explored, and there are great potentials as well as challenges that lie ahead.

The assembly of atomically precise clusters into superstructures has tremendous potential in structural tunability and applications. Here, we report a series of single-cluster nanowires, single-cluster nanorings, and three-dimensional superstructure assemblies built by POM clusters. By stepwise tuning of interactions at molecular levels, the configurations can be varied from single-cluster nanowires to nanorings. A series of single-cluster nanostructures in different configurations can be achieved with up to 15 kinds of POM clusters. The single-cluster nanowires and three-dimensional superstructures perform enhanced activity in the catalytic and electrochemical sensing fields, illustrating the universal functionality of single-cluster assemblies.

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Cluster‐assembled materials (CAMs) present delicate structures and tailored properties, which have gained extensive interest in recent years. Due to the noncovalent linkage and size of sub‐1 nm, clusters present polymer‐like solution behaviors distinct from conventional nanocrystals, whereby a variety of CAMs with different constructions have been fabricated. To our knowledge, the morphologies and properties of CAMs are closely connected, which are strongly dependent on the structure and coordination state of cluster building blocks. For the CAMs with superior performances, the specific arrangement and surroundings of clusters at subnanometer scale usually result in well‐organized structures. In this review, we will summarize the recent progress of CAMs with highly ordered structures and advanced properties, where polyoxometalate cluster assemblies as well as cluster–nuclei co‐assemblies are involved. The application potentials of CAMs are then discussed, especially on the cluster arrangements and interactions at subnanometer scale. However, current researches are far from reaching the point, and future efforts are needed before the nature of cluster‐based nanostructures can be sufficiently revealed. We hope this article can offer unique insights and instructive understanding to the CAMs.