Single-Crystalline Gold Nanowires Synthesized from Light-Driven Oriented Attachment and Plasmon-Mediated Self-Assembly of Gold Nanorods or Nanoparticles

Abstract

Through the light-driven geometrically oriented attachment (OA) and self-assembly of Au nanorods (NRs) or nanoparticles (NPs), single-crystalline Au nanowires (NWs) were synthesized by the irradiation of a linearly-polarized (LP) laser. The process was conducted in a droplet of Au colloid on a glass irradiated by LP near-infrared (e.g. 1064 nm and 785 nm) laser beam of low power at room temperature and atmospheric pressure, without any additive. The FE-SEM images show that the cross sections of NWs are various: tetragonal, pentagonal or hexagonal. The EDS spectrum verifies the composition is Au, and the pattern of X-ray diffraction identifies the crystallinity of NWs with the facets of {111}, {200}, {220} and {311}. We proposed a hypothesis for the mechanism that the primary building units are aligned and coalesced by the plasmon-mediated optical torque and force to form the secondary building units. Subsequently, the secondary building units undergo the next self-assembly, and so forth the tertiary ones. The LP light guides the translational and rotational motions of these building units to perform geometrically OA in the side-by-side, end-to-end and T-shaped manners. Consequently, micron-sized ordered mesocrystals are produced. Additionally, the concomitant plasmonic heating causes the annealing for recrystallizing the mesocrystals in water.The mechanism of natural crystal growth has attracted a lot of attentions for centuries. The famous theory of Ostwald ripening, based on thermodynamics, has been used to explain the crystal growth 1-3 ; nature prefers lower energy resulting in that smaller nanoparticles (solute) tend to dissolve and diffuse toward another larger one in a solution spontaneously. The well-known Ostwald ripening theory can be applied to explain the process of seed growth in ions solution for the formation of nanocrystals including nanoparticles (NPs) and even nanorods (NRs) 4,5 . Beside the theory of Ostwald ripening, recently another important theory of oriented attachment (OA) growth of nanocrystals has been proposed and recognized in the process of colloidal self-assembly, whereby adjacent NPs can combine with one another in a same crystal orientation to perform self-organization. The driving force of OA can be the van der Waals force, Coulombic (electrostatic) force, dipole-dipole interaction and so on 6-10 . This nonclassical crystal growth mechanism can successfully explain the mechanisms of the self-assembly synthesis of NPs, NRs and nanowires (NWs) [11][12][13] . However, because these driving forces are too weak and the distance of influence is too short, the probability of OA is very low. As a result, the natural OA processes always take a long time for nanoclusters to aggregate, align and form a large-sized crystal. Because of that, a few of methods using extreme conditions (high pressure 14 , higher temperature 15 ), different types of irradiation energy (light 16-19 ,

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