or cubic array of Fe 50 Pt 50 NPs can be controlled by the length of alkyl ligands. [27] Assembling two different types of NPs into binary superlattices is a versatile way to design nanomaterials in which two components are blended into periodic arrangements, and dozens of binary superlattices of structures of fcc, orthorhombic, cuboctahedral, etc., have been reported. [28][29][30] To sum up, by size/shape control and surface ligand engineering of gold NPs, programmable assembly of NPs into superlattices (e.g., primitive cubic, body-centered cubic (bcc), fcc, and hcp) has been well developed (Scheme 1, left).With the realization of programmable assembly of NPs, it would be highly desirable to achieve similarly exquisite control over the NP itself, such as the structure and functionality. For gold, the atoms typically adopt the fcc packing in both the bulk and nanoparticle forms. [31] In the past decade, significant efforts have been made in achieving atomically precise, ultrasmall gold NPs (often called nanoclusters, NCs, with core sizes of 1-3 nm). More importantly, their total structures have been determined by single-crystal X-ray diffraction (SCXRD). The current atomically precise nanochemistry is indeed moving toward programmable synthesis of NCs with control over the structure, such as the bcc, fcc, hcp, decahedra (D h ), icosahedra (I h ), multi-tetrahedral (T d ) network, etc., (Scheme 1, right). Such successes provide an excellent opportunity to correlate the structure of NCs with their properties such as optical absorption. Besides, recent progress in controlling NCs with atomic precision has opened new horizons, as the quantum-sized NCs exhibit fundamentally different properties from their plasmonic counterparts (5-100 nm). [32,33] Specifically, the attained atomic level insights reveal that the kernel structure plays decisive roles in the optical properties, [34] while the organic-inorganic interface is decisive to chirality and catalysis, [35,36] and the surface ligand shell is critical to assembly and photoluminescence. [37,38] We foresee that the advances in nanochemistry will be capable of producing atomically precise NC materials that rival, and to some extent exceed, what conventional molecular chemistry has achieved at the molecular scale.Among the appealing physicochemical properties of NPs, the optical properties have long been one of the major topics in nanoscience. [1][2][3][4][5][6][39][40][41] Gold NPs account for the red color of stained glass in church windows, whereas silver NPs are typically yellow. Scientific studies on the optical properties of Au NPs date back to Faraday's time. Later, Mie solved the Maxwell's equation for spherical particles and successfully simulated the extinction (i.e., scattering + absorption) spectra. [42][43][44] With the recent establishment of atomically precise nanochemistry, capabilities toward programmable control over the nanoparticle size and structure are being developed. Advances in the synthesis of atomically precise nanoclusters (NCs, 1-3 nm) have...