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One of chemistry's biggest contribution to science and society is the Mendeleev's periodic table of elements. Created in 1869 before the discovery of the electron and the knowledge of quantum mechanics, the periodic table accounts for the chemistry of elements in terms of their valence electrons. At present, the periodic table contains 118 elements, 94 of which occur in nature. Atoms of these naturally occurring elements are the building blocks of all matter. For ages, alchemists have tried to alter the chemistry of elements with the hope of transforming, for example, base metals to gold, but to no avail. With the knowledge we have gained over more than a century, we now ask the question: Can we achieve an alchemist's dream? Progress in nanoscience over the past few decades has provided some hope. As matter is reduced to nanometer or even sub‐nanometer length scale, surface atoms dominate over bulk atoms. Combined with quantum confinement and reduced coordination, nanomaterials possess properties not seen in their bulk phase.

Atomic clusters, composed of a few to a few hundred atoms, are the ultimate nanoparticles where every atom counts. Research over the past 50 years has shown that properties of clusters are size‐ and composition‐specific and can be tailored such that they mimic the chemistry of elements in the periodic table, even when none of their constituents belong to that group. Such clusters, termed as superatoms, can be used to build a three‐dimensional periodic table where superatoms constitute the third dimension. Because there are unlimited ways of designing these superatoms, the three‐dimensional periodic table will have no limits, at least in principle. Can properties of matter be fundamentally altered if we change their building blocks from individual atoms to these superatomic clusters? For example, can a superatomic cluster containing only nonmagnetic atoms be magnetic? Can a cluster composed of metal atoms behave like a semiconductor? Can noble gas atoms and inert molecules form chemical bonds with tailored superatoms? Can a cluster of gold atoms be reactive? This book examines these possibilities.

Although the field of atomic clusters can be traced to 1930s, it became attractive to the physics and chemistry community in 1970s when clusters of atoms and molecules with specific size and composition could be produced in the gas phase and studied both experimentally and theoretically. The motivation for studying clusters at the dawn of this field was to understand how properties of matter evolve, one atom at a time. It was soon realized that the structure and properties of atomic clusters not only are very different from their bulk matter but also evolve non‐monotonically, often varying unpredictably and abruptly with the addition of a single atom. In contrast to conventional materials composed of individual atoms, new matter with tailored properties can be created by using these superatoms as the building blocks, primarily because of their unique size, shape, composition, and electronic structure. For example, lattice constants of cluster‐assembled crystals would be larger than those of conventional crystals. The energy bands of cluster‐assembled crystals formed by the overlap of superatomic orbitals would be different from the conventional energy band structure formed by the overlap of atomic orbitals. The nonspherical geometry of clusters would introduce novel features as their rotational degree of freedom can affect the energy landscape. Similarly, intra‐cluster and inter‐cluster vibrations would further affect the electron–phonon interaction. This paradigm shift in materials design and synthesis would open new doors for focused discovery of materials with unprecedented properties.

This book contains 13 chapters covering the recent advances in our understanding of superatoms and their assemblies. A group of international experts discuss the design of superatoms, study properties that make these unique, and explore their potential as building blocks of a new class of materials with applications in devices and energy‐related technologies. Chapter 1 highlights some seminal papers and reviews on atomic clusters that made cluster science an emerging field bridging physics, chemistry, and materials science. Chapter 2 shows how superatoms mimicking the properties of group 1, 15, 16, and 17 elements can be rationally designed using electron‐counting rules. Also discussed in this chapter is how different electron‐counting rules can be used simultaneously to design multiply charged superatoms that neither fragment nor spontaneously eject the added electrons. Such superatoms can be used to promote unusual reactions enabling noble gas atoms such as argon to form chemical bonds at room temperature. Chapter 3 provides a comprehensive review of superhalogens that mimic the chemistry of halogens but possess electron affinities far exceeding those of halogen atoms. The design, stability, and structure‐property relationships of endohedral clusters where metal atoms are encapsulated inside cage clusters composed of Si, Ge, Sn, and coinage metal atoms are discussed in Chapter 4. Also discussed in this chapter are assemblies of these metal encapsulated clusters. Chapter 5 shows how magnetic superatoms that mimic the properties of transition and rare earth metal atoms can be created by tailoring the role of s and p electrons that account for bonding while d and f electrons carry the magnetic moment. Chapters 6 and 7 cover the experimental and theoretical aspects of cluster‐assembled materials focusing on atomically precise ligand‐protected clusters composed of noble metal atoms. Two‐dimensional materials created with superatoms as the building blocks are presented in Chapter 8. Potential applications of superatom‐based materials in ferroelectrics, solar cells, Li‐ion batteries, hydrogen storage, and capture and conversion of CO₂ are covered in Chapters 9–12. The challenges and opportunities that lie ahead are summarized in the concluding chapter. Cluster science in general and superatoms in particular are expected to remain a vibrant field for years to come, leading the way to new developments in materials design, synthesis, and applications.

Puru Jena

Qiang Sun