Hierarchical self-assembly of atomically precise Au₆ nanoclusters into fibrillar superstructures with collective...

Self-assembly, a key approach in material science, enables modulation of nanostructures to achieve distinct materials properties. Atomically precise metal nanoclusters (NCs), consisting of a few metal atoms, exhibit distinctive optical and electronic properties due to their "molecule-like" discrete energy levels. Such NCs offer advantages over conventional metal nanoparticles by avoiding dispersity in size and uncontrolled aggregation. Here we demonstrate a study on crystalline aggregates of Au₆ NCs, formed under varying conditions. Notably, the controlled aggregation of these Au₆ NCs, synchronising by protonation or employing specific hydrogen bonding, yields self-assembled nanoribbons and percolated networks of nanofibers that further produce extended 3D fibrillar architectures. X-ray scattering and electron microscopy reveal two distinct packing modes: a monoclinic 2D oblique lattice with sparse NC arrangement, and a nematic 2D hexagonal packing, resembling liquid-crystalline rod-like assemblies. The resulting superstructures exhibit enhanced optical responses, retaining the photoluminescence of their constituents, and manifesting third-harmonic generation, slow photoluminescence decay driven by charge-migration effects, and polarization effects influenced by the ordered structure with the periodicity of the NCs. Overall, this work emphasizes the potential of NCs as versatile building blocks for tunable and responsive optoelectronic materials, providing insights into the mechanisms of self-assembly.