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Abstract Precisely controlling shape and crystallinity is central to defining structure-dependent properties in colloidal nanocrystals (NCs), which is particularly important yet challenging for silver (Ag) NCs. Here, we report catalytic regulation that facilitates cluster-mediated growth pathways to enable the scalable synthesis of single-crystalline Ag-NCs with programmable shapes. A copper (Cu)-assisted, one-pot heating-up process produces highly uniform Ag-NCs with tunable diameters (5–11 nm) and morphologies ranging from truncated-octahedral (“mecon”) to spherical geometries, while maintaining gram-scale batch output, air tolerance, and compatibility with continuous flow synthesis. Spectroscopic and mass spectrometric analyses reveal a stepwise growth pathway involving discrete Ag cluster intermediates, whose formation and interconversion are accelerated by trace Cu species acting as catalysts. Density functional theory calculations further show that Cu modulates surface and stacking-fault energetics, promoting single-crystalline growth with well-defined facets. By introducing a quantitative shape descriptor, we establish predictive relationships linking NC morphology to plasmonic response, supported by discrete dipole approximation simulations. These results define a general synthetic principle in which catalytic species and dynamic ligand environments cooperatively govern cluster-phase evolution, enabling precise and scalable control of NC structure and function. 

In nanoscale assemblies, observations of structures in the transition pathways between high-symmetry lattices are rare because of the inherent instability of the intermediate phases. We report that silver nanocrystals with shapes similar to truncated octahedra (mecons) self-assemble into superlattices that are stable and resemble a previously unobserved phase that bridges face-centered and body-centered cubic structures along the Nishiyama-Wassermann martensitic pathway. These superlattices exhibited high structural purity and stability and created a robust and tunable dissymmetric phase landscape. Notably, light-matter coupling emerged in these silver nanocrystal superlattices through plasmon-photon hybridization at the quantum level.