Tatsuya Tsukuda focused on the fact that stabilized gold clusters possess hierarchical electronic structures analogous to those of atoms and positioned them as "gold superatoms." He developed methods for the precise control of diverse structural factors and systematically elucidated their correlations with electronic structures and physical properties. Furthermore, he established techniques for synthesizing and evaluating pseudo-molecules and assemblies of superatoms as building blocks, proposed their bonding modes, and achieved the modulation of physical properties through bonding and integration. His primary achievements are introduced below.

1. Development of Atomically-Precise Synthesis Methods for Gold Superatoms and Elucidation of Structure-Property Correlations
Tsukuda focused on size (number of constituent atoms), chemical composition, shape, and surface modification as structural factors governing the physical properties of superatoms and developed methods to control these individually with atomic and molecular precision. Furthermore, he evaluated their geometric and electronic structures by fully utilizing single-crystal X-ray diffraction, X-ray absorption spectroscopy, mass-selected photoelectron spectroscopy using a custom-built apparatus, high-resolution transmission electron microscopy observations, and density functional theory calculations. He clarified the correlations between the obtained structural information and their optical properties or catalytic performance and explained the origins of these properties from the perspective of modulating the electronic structure of the gold superatoms. Representative achievements for each primary structural factor (size, chemical composition, and surface modification) are introduced below.

First, he established a method for systematically controlling the size - the most important structural factor characterizing superatoms - and elucidated the sequential structural transitions during the initial growth stages of gold superatoms, as well as the critical size at which the metal-ro-nonmetal transition occurs. He also succeeded in synthesizing novel magic number gold superatoms, such as Au₂₄ and Au₃₈ stabilized by polyvinylpyrrolidone (PVP), by controlling the growth rate using a micromixer, and clarified the size effect on catalytic performance using the aerobic oxidation of benzyl alcohol as a model reaction. 

Next, he developed a systematic method for synthesizing alloy superatoms in which a single gold atom is replaced by a different element while maintaining the size and geometric structure of the gold superatom, demonstrating that single-atom doping can dramatically modulate physical properties. Using gas-phase photoelectron spectroscopy, he discovered that the icosahedral alloy superatom Au@M₁₂ (M = Au, Ag) exhibits spin-orbit splitting and elucidated its structural origin through theoretical calculations.
Furthermore, he found that the optical properties of alloy superatoms M@Au₁₂ (M = Ir, Pt, Pd, etc.) depend significantly on the type of M; notably, for M = Ir and Pt, the photoluminescence quantum yield and photocatalytic activity are drastically enhanced compared to Au₁₃. He also prepared alloy model catalysts such as PVP-protected PdAu₂₃ and layered double hydroxide-supported MAu₁₂ (M = Rh, Ir, Pd, Pt), finding that doping with a single Pd atom significantly improves catalytic performance for alcohol oxidation, and proposed its mechanism. Additionally, he revealed that hydride acts as a dopant for gold superatoms, influencing their electronic structure and reactivity. For instance, by leveraging the nucleophilic reactivity of the hydride-doped superatom HM@Au₈, he achieved regioselective introduction of Au, Ag, and Cu atoms, as well as highly efficient conversion into ligand-protected alloy superatoms M@Au₁₂.

Furthermore, he demonstrated for the first time that organic ligands, such as terminal alkynyls and N-heterocyclic carbenes (NHCs), are effective protecting agents for gold superatoms. By allowing a portion of the ligands to remain during the calcination of ligand-protected, size-defined gold superatoms on carbon supports, he succeeded in creating superatom model catalysts that possess robustness derived from multi-point non-bonding interactions between the support and the residual ligands. Other significant achievements include the manifestation of circular dichroism and circularly polarized luminescence through the use of chiral diphosphines as protecting agents, the elucidation of how ligand polarity and charge influence the electron affinity of superatoms, and the observation of accelerated inter-superatom electron transfer mediated by long-chain alkyl layers.

2. Exploration of Novel Materials Based on Gold Superatoms and Investigation of their Properties
Tsukuda regarded superatoms as nanoscale artificial atoms and developed methods for creating new classes of materials by bonding or assembling them, subsequently exploring the properties that emerge due to inter-superatom interactions. 

First, he developed a targeted synthesis method for structures in which multiple superatoms are fused while retaining their partial structures (superatomic molecules). For example, by nucleophilic reactions of HM@Au₈ (M = Au, Pd, Pt) with M@Au₁₂, he obtained the bi-icosahedral superatomic molecule MMAu₂₁. Furthermore, he clarified that the electronic orbitals of the MMAu₂₁ superatomic molecule can be constructed by the superposition of the superatomic orbitals of the corresponding MAu₁₂ units. 

Next, he discovered that heating the superatoms Au@Au₁₂ or Au₈ in solution promotes fusion, generating a series of Au₁(Au₃)_2+4nAu₁ (n = 1, 2, 3, ...), which correspond to superatom oligomers. This series, named gold quantum needles, possesses a characteristic structure where triangular Au₃ units are stacked one-dimensionally; they exhibit strong absorption and emission in the near-infrared region associated with HOMO-LUMO transitions depending on their length. He demonstrated that these electronic structures and optical properties can be qualitatively explained by a one-dimensional particle-in-a-box model and applied them in photon upconversion. 

Furthermore, he obtained ultrathin gold nanorods (diameter approximately 1.8 nm) consisting of cuboctahedra equivalent to Au147 linked one-dimensionally. He demonstrated that these rods exhibit a surface plasmon resonance (SPR) absorption band along the longitudinal axis in the near-infrared to infrared region, and that the absorption wavelength can be controlled by adjusting the length or coating the surface atomic layer with silver.

Furthermore, he developed molecular techniques for the self-assembly of a specific number of superatoms while controlling distance, symmetry, and dimension. First, he developed Ir@Au₁₂　superatoms with a defined number of exposed sites for bridging, to serve as assembly components. Next, he constructed various assemblies using these superatoms and bridging ligands and successfully visualized their structures. Based on the observation of the optical properties inherent to these assemblies, he demonstrated the potential for new functions to emerge through the cooperative action of superatoms. 

As described above, Tsukuda pioneered atomic-precision nanochemistry and established the fundamental principles of superatoms. These achievements constitute the foundation of a new material chemistry that positions superatoms as nanoscale artificial atoms and were recognized as being worthy of The Chemical Society of Japan Award.