Single-Crystal Nanostructures with Oxide Claddings for Durable Refractory Plasmonics

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The stability and durability of materials is a subject of fundamental importance that often decides whether a technology is viable, reliable and sustainable. Such considerations become amplified when applications require that materials operate at high temperatures. Plasmonics is a field of study that is powered by the ability to shape- and size-engineer metals at the nanoscale. The tendency for such structures to oxidize and morphologically reconfigure when heated can, however, disrupt or destroy properties that were so carefully engineered in the first place. With high-temperature plasmonics employed in a range of critical applications such as heat-assisted magnetic recording, high-temperature plasmonic sensing, and solar thermophotovoltaics, this deficiency puts at risk potentially disruptive technologies that are needed for data storage to the 'cloud' and the energy infrastructure of the U.S. This grant demonstrates that suitably fabricated plasmonic nanostructures, when encapsulated in an ultrathin oxide layer, maintain functionality while proving robust to high temperatures. Moreover, it shows that such structures can be fabricated over large areas using inexpensive and scalable nanofabrication techniques such as nanoimprint lithography. The availability of durable refractory plasmonics impacts data storage and energy industries and the nation?s economy and prosperity. In this project, undergraduate education is being integrated through research internships and outreach activities place emphasis on the matriculation of women and under-represented minority students into the engineering profession.While numerous high-temperature plasmonic applications exist, no single material has yet been identified that is able to maintain performance for long durations. This research aims to overcome this technological barrier through the use of a hybrid structure that combines the plasmonic properties of single-crystal nanostructures with an ultrathin cladding technology such that the combination yields a high-performance refractory plasmonic material. The projected solution is founded on an underlying hypothesis that single-crystal nanostructures are far more resistant to temperature-induced morphological changes than their polycrystalline counterparts. Moreover, if such nanostructures are encased in a refractory oxide, then the surface diffusion pathways, which are responsible for morphological reconfigurations, become blocked. The studies are divided into four thrust areas: (i) defining a robust and pinhole-free cladding technology, (ii) obtaining a mechanistic understanding of the cladding process and any failure modes and then advancing failure mitigation strategies, (iii) assessing and enhancing the capability of oxide claddings to resist chemical degradation, and (iv) devising processing routes that yield partially clad nanostructures which facilitate applications that require an exposed plasmonic surface. Together, this work provides a fundamental understanding of high-temperature diffusion processes that occur for clad metals and advances the processing science needed for manufacturing refractory plasmonic materials.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.