Plasmon-Mediated Synthesis of Periodic Arrays of Gold Nanoplates Using Substrate-Immobilized Seeds Lined with Planar Defects

The seed-mediated growth of noble metal nanostructures with planar geometries requires the use of seeds lined with parallel stacking faults so as to provide a break in symmetry in an otherwise isotropic metal. Although such seeds are now routinely synthesized using colloidal pathways, equivalent pathways have not yet been reported for the fabrication of substrate-based seeds with the same internal defect structures. The challenge is not merely to form seeds with planar defects but to do so in a deterministic manner so as to have stacking faults that only run parallel to the substrate surface while still allowing for the lithographic processes needed to regulate the placement of seeds. Here, we demonstrate substrate-imposed epitaxy as a viable synthetic control able to induce planar defects in Au seeds while simultaneously dictating nanostructure in-plane alignment and crystallographic orientation. The seeds, which are formed in periodic arrays using nanoimprint lithography in combination with a vapor-phase assembly process, are subjected to a liquid-phase plasmon-mediated synthesis that uses light as an external stimuli to drive a reaction yielding periodic arrays of hexagonal Au nanoplates. These achievements not only represent the first of their kind demonstrations but also advance the possibility of integrating wafer-based technologies with a rich and exciting nanoplate colloidal chemistry. P lasmon-mediated syntheses represent a distinct branch of photochemistry in which short-lived plasmonic excitations act as the primary driver for a redox chemistry yielding highly faceted nanostructures. 1−4 Mirkin and co-workers 5 were the first to demonstrate that citrate-protected colloidal Ag nanospheres, when illuminated with a fluorescent light source, are transformed into a monodisperse population of triangular nanoprisms. The discovery not only opened the door to a new photochemistry but represented a significant advance in that it was the first synthesis to realize plasmonic nanoplates in high yield. This demonstration was followed by a series of reports that forwarded a mechanistic understanding of the growth mode. 6−11 One of the quandaries faced was that the plasmon-mediated growth seemed limited to Ag. This elemental exclusivity was, however, recently overcome by Wei and co-workers 12 who demonstrated the synthesis of single-crystal Au nanoplates with triangular and hexagonal geometries using a plasmon-mediated chemistry. With the vast majority of plasmonically driven growth modes being seed-mediated, it has become apparent that the decisive factor in determining the nanostructure shape is the internal defect structure of the seed. 1,2 While singly twinned and multitwinned (e.g., icosahedral, decahedral) seeds have been used to generate some of the most intricate nanostructure architectures, 13−15 it is seeds lined with parallel planar defects that are most closely connected to the nanoplate geometry. 1,2,13 Planar defects, which in face-centered cubic (fcc) metals manifest themselves as deviations from the ABCABC··· stacking order along ⟨111⟩-axes, can take the form of an added or missing row or a twin defect where the stacking order is reversed at a mirror plane (i.e., ABC|A|CBA where |A| is the mirror plane). The roles played by such defects are to (i) provide a symmetry-breaking structure with a two-dimensional character in an otherwise isotropic metal, (ii) create high surface energy sites conducive to the reduction of metal ions, and (iii) facilitate the selective adsorption of suitably chosen capping agents. These mechanistic roles are not limited to plasmon-driven syntheses, as they apply equally to thermally driven growth modes yielding planar architectures. 16−20 Although the synthesis and application of plasmonic nanoplates as colloids has been an unmitigated success, it has proven quite challenging to form organized surfaces of such structures on planar substrates using the same solution-based chemistry. Obtaining close packed arrangements of nanoplates over large areas using self-assembly is inherently difficult and although progress has been made, 21,22 long-range order has not yet been achieved. Although numerous examples exist where colloidal seeds have been dispersed onto substrates