Impact of the Metallic Crystalline Structure on the Properties of Nanocrystals and Their Mesoscopic Assemblies

The spontaneous assembly of uniform-sized globular entities into ordered arrays is a universal phenomenon observed for objects with diameters spanning a broad range of length scales. These extend from the atomic scale (10-8 cm), through molecular and macromolecular scales with proteins, synthetic low polymers, and colloidal crystals (∼10-6 cm), to the wavelength of visible light (∼10-5 cm). The associated concepts of sphere packing have had an influence in diverse fields ranging from pure geometrical analysis to architectural models or ideals. Self-assembly of atoms, supramolecules, or nanocrystals into ordered functional superstructures is a universal process and prevalent topic in science. About five billion years ago in the early solar system, highly uniform magnetite particles of a few hundred nanometers in size were assembled in 3D arrays.1 Thirty million years ago, silicate particles with submicrometer size were self-organized in the form of opal.2 Opal is colorless when composed of disordered silicate microparticles whereas it shows specific reflectivity when particles order in arrays. Nowadays, nanocrystals, characterized by a narrow size distribution and coated with alkyl chains to maintain their integrity, self-assemble to form crystallographic orders called supracrystals. Nanocrystals and supracrystals are arrangements of highly ordered atoms and nanocrystals, respectively. The morphologies of nanocrystals, supracrystals, and minerals are similar at various scales from nanometer to millimeter scale.3,4 Such suprastructures, which enable the design of novel materials, are expected to become one of the main driving forces in material research for the 21st century.5,6 Nanocrystals vibrate coherently in a supracrystal as atoms in a nanocrystal. Longitudinal acoustic phonons are detected in supracrystals as with atomic crystals, where longitudinal acoustic phonons propagate through coherent movements of atoms of the lattice out of their equilibrium positions. These vibrational properties show a full analogy with atomic crystals: In supracrystals, atoms are replaced by (uncompressible) nanocrystals and atomic bonds by coating agents (carbon chains), which act like mechanical springs holding together the nanocrystals. Electronic properties of very thick (more than a few micrometers) supracrystals reveal homogeneous conductance with the fingerprint of the isolated nanocrystal. Triangular single crystals formed by heat-induced (50 °C) coalescence of thin supracrystals deposited on a substrate as epitaxial growth of metal particles on a substrate with specific orientation produced by ultrahigh vacuum (UHV). Here we demonstrate that marked changes can occur in the chemical and physical properties of nanocrystals differing by their nanocrystallinity, that is, their crystalline structure. Furthermore, the properties (mechanical, growth processes) of supracrystals also change with the nanocrystallinity of the nanoparticles used as building blocks.