Designing superatomic clusters
Superatoms are groups of connected atoms acting collectively like an atom in the periodic table that can serve as building blocks for the design of new materials. For example, a cluster composed of thirteen aluminum atoms, Al13, behaves like a chlorine atom in terms of its propensity to attach an electron and form a stable anion. Similarly, tungsten carbide clusters are much less expensive analogs of platinum in terms of their ability to catalyze reactions.
The reason superatoms have their unique atom-like characteristics is because they pool their electrons into electronic layers around a central core and such layers determine their chemical properties. However, unlike their elemental analogs, the electron configuration and molecular orbitals of superatoms may be finely adjusted through the atomic-level engineering of their composition to attain a desired property. This is exactly the focus of my Ph.D. research. I have applied the atom-by-atom substitution approach to [Co6S8(PEt3)6]+ which is a well-known superatom with Co6S8 core attached to six surface ligands (triethyl phosphine, PEt3) and could substitute one of the central cobalt atoms in the core with Ni and Fe atom. Using high resolution mass spectrometry, I identified novel [Co5NiS8(PEt3)6]+ and [Co5FeS8(PEt3)6]+ clusters and studied their stability. The synthesized [Co5NiS8(PEt3)6]+ cluster contains one more electron and [Co5FeS8(PEt3)6]+ has one less electron compared to [Co6S8(PEt3)6]+. I use this series of clusters to understand how the properties of the superatom change when an electron is added or removed from the core. I found that replacing a single atom in the cluster has a pronounced effect on the overall optical, magnetic, and electrochemical properties of the cluster which gives us great freedom for the rational design of materials at the atomic scale. Additionally, these clusters are stable both in solution and on surfaces, which indicates their potential for being used as building blocks of functional materials. Interestingly, we found out that [Co5NiS8(PEt3)6] is readily oxidized and its ionization energy is similar to that of Rubidium (Rb) atom. Therefore, this cluster may be viewed as a superatomic analog of Rb. Our results demonstrate that atom-by-atom substitution strategy is a promising approach for designing superatoms with tailored electronic, magnetic, and optical properties.
Overall, generating a group of connected atoms acting collectively like an atom in the periodic table is becoming a reality and a three-dimensional periodic table populated with superatomic clusters is on the horizon. Further efforts are still required to build up the knowledge and develop the technologies in this realm to discover other fascinating materials with exceptional properties, which is a research direction in the Prof. Julia Laskin`s lab. In this regard, we have developed a mass spectrometry-based deposition method called ion soft-landing, which will allow us to prepare novel superatom-based thin films for applications in energy storage, molecular, electronics, and catalysis.