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.

The fundamental understanding of liquid-phase catalytic reactions is unavoidably complicated when the catalyst is prone to leaching since questions inevitably arise as to the true nature of the catalyst. While the catalytic reduction of 4-nitrophenol by borohydride is widely accepted as a trusted model reaction, it has faced little scrutiny concerning the potential impact of leached species or the appropriateness of assigning catalytic activity to the inserted nanostructures without rigorous experimental verification. Here, we present results from a spectroscopically monitored split test in which supported silver catalysts are physically separated from the reactants midway through the reaction. It is unambiguously demonstrated that the influence of leaching is far from benign, instead acting to extinguish the catalytic activity of the inserted nanostructures while giving rise to an unsupported heterogeneous catalyst that is the true catalytic entity. With only submonolayer quantities of silver leached from the supported structures, the unsupported species must be exceedingly catalytic. Moreover, it is shown that leaching is inherent to aqueous media containing dissolved oxygen, without which the supported nanostructures remain catalytically active. With the same nanomaterial being able to act either as a heterogeneous catalyst or as a reservoir from which leached metal is derived, such influences have undoubtedly compromised prior studies. We, nevertheless, capitalize on the sensitivity of 4-nitrophenol reduction to leached species by using it as a reaction-based indicator able to quantitatively determine the time dependence of the leaching process and enhancements to oxidative etching when silver, copper, palladium, platinum, and gold are exposed to chloride ions.

The functionality of well-tailored nanomaterials can only be retained if they are robust to the environmental factors in which they operate. The inability of Cu to withstand such factors is largely responsible for its current status as a second-tier plasmonic nanomaterial. Herein, it is demonstrated that atomic layer deposition can be used as a pliable technique for the application of oxide coatings to substrate-based Cu nanostructures where suitably protected structures become robust to oxidation, high temperatures, and aqueous, acidic, and alkaline solutions without unduly influencing important plasmonic properties. Moreover, strategies are presented for maximizing plasmonic near-fields and allowing for the transport of hot electrons while maintaining coating integrity. The findings demonstrate that there does not exist a one-solution-fits-all approach but that coating design must follow an application-specific methodology. Within the scope of the investigation, alumina, hafnia, titania, and combinations thereof were all shown to be effective under certain conditions, but where hafnia shows the greatest durability in extreme pH environments and alumina-hafnia multilayers provide Cu with protection from oxidation to temperatures as high as 600 °C. The work advances the use of Cu nanostructures as durable plasmonic materials and provides broad-based strategies for protecting other vulnerable nanomaterials from harsh environments.

No abstract

A method for the nanofabrication of periodic arrays of gold nanostars yielding a unique morphology of relevance to SERS applications.