Tailoring Plasmonic Fields with Shape-Controlled Single-Crystal Gold Metasurfaces.

Geometry and crystallinity play a critical role in the wavelength-dependent optical responses and plasmonic local near-field distributions of metallic nanostructures. Nevertheless, the ability to tailor the shape and position of crystalline metal surface nanostructures has remained a challenge that limits control of their enhanced local fields and represents a barrier to harnessing their individual and collective responses. Here, we describe a solution deposition method in the presence of anionic additives, which yields shape-controlled, single-crystal plasmonic gold nanostructures on Ag(100) and Au(100) substrates. Use of SO42- ions yields smooth Au(111)-faceted square pyramids with large plasmonic Raman enhancements. Halide additives produce textured hillocks comprising edge- and screw-type dislocations (Cl-), or platelets with large-area Au(100) terraces and (110) step edges (Br-), while SO42- and Br- additive combinations provide Au(110)-faceted square pyramids. With lithographic patterning, this chemistry yields metal deposition with precise geometry and location control to provide single-crystal, plasmonic gold metasurfaces with tailored optical response. The appropriately designed metasurfaces can then generate large Raman scattering enhancements, far greater than high density gold square pyramids with random surface disposition. Shape-controlled single-crystal plasmonic metasurfaces will thus offer opportunities to tune the characteristics of nanostructures, providing enhanced optical, photocatalytic, and sensory response.

High performance, single crystal gold bowtie nanoantennas fabricated via epitaxial electroless deposition.

Material quality plays a critical role in the performance of nanometer-scale plasmonic structures and represents a significant hurdle to large-scale device integration. Progress has been hindered by the challenges of realizing scalable, high quality, ultrasmooth metal deposition strategies, and by the poor pattern transfer and device fabrication yields characteristic of most metal deposition approaches which yield polycrystalline metal structure. Here we highlight a novel and scalable electrochemical method to deposit ultrasmooth, single-crystal (100) gold and to fabricate a series of bowtie nanoantennas through subtractive nanopatterning. We investigate some of the less well-explored design and performance characteristics of these single-crystal nanoantennas in relation to their polycrystalline counterparts, including pattern transfer and device yield, polarization response, gap-field magnitude, and the ability to model accurately the antenna local field response. Our results underscore the performance advantages of single-crystal nanoscale plasmonic materials and provide insight into their use for large-scale manufacturing of plasmon-based devices. We anticipate that this approach will be broadly useful in applications where local near-fields can enhance light-matter interactions, including for the fabrication of optical sensors, photocatalytic structures, hot carrier-based devices, and nanostructured noble metal architectures targeting nano-attophysics.

Scalable, Green Fabrication of Single-Crystal Noble Metal Films and Nanostructures for Low-Loss Nanotechnology Applications.

The confinement of spatially extended electromagnetic waves to nanometer-scale metal structures can be harnessed for application in information processing, energy harvesting, sensing, and catalysis. Metal nanostructures enable negative refractive index, subwavelength resolution imaging, and patterning through engineered metamaterials and promise technologies that will operate in the quantum plasmonics regime. However, the controlled fabrication of high-definition single-crystal subwavelength metal nanostructures has remained a significant hurdle due to the tendency for polycrystalline metal growth using conventional physical vapor deposition methods and the challenges associated with placing solution-grown nanocrystals in desired orientations and locations on a surface to manufacture functional devices. Here, we introduce a scalable and green wet chemical approach to monocrystalline noble metal thin films and nanostructures. The method enables the fabrication of ultrasmooth, epitaxial, single-crystal films of controllable thickness that are ideal for the subtractive manufacture of nanostructures through ion beam milling and additive crystalline nanostructure via lithographic patterning for large-area, single-crystal metasurfaces and high aspect ratio nanowires. Our single-crystal nanostructures demonstrate improved feature quality, pattern transfer yield, reduced optical and resistive losses, and tailored local fields to yield greater optical response and improved stability compared to those of polycrystalline structures-supporting greater local field enhancements and enabling practical advances at the nanoscale.