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

Abstract

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.

Figure 1

Bowtie nanoantennas fabricated on single-crystal and polycrystalline Au films. (a) Cartoon of the FIB milling of a single-crystal gold Au(100) film with an incident beam of Ga³⁺ ions (red) to realize a monocrystalline bowtie nanoantenna. (b) Top-view SEM images of bowtie antennas fabricated on monocrystalline (left) and polycrystalline (right) Au films, respectively. (c) Fabrication steps for the FIB milling of bowtie nanoantennas. For the structures fabricated here, L = 1560 nm, g = 20 nm.

Figure 2

Yield and functionality of bowtie nanoantenna arrays. Two-photon photoluminescence (2PPL) intensity maps (left) and 2PPL cross-sectional gap intensity from the dashed regions of the centre row of antennas (right) fabricated from (a) single-crystal Au, and (b) polycrystalline Au films.

Figure 3

The effect of polarization on the activity of bowtie nanoantennas. FDTD modeled antenna response for (a) vertically and (b) horizontally polarized 780 nm excitation. Two photon photoluminescence intensity maps of a single-crystal bowtie nanoantenna (c) and (d) and polycrystalline bowtie nanoantenna (e) and (f) for vertically and horizontally polarized 780 nm excitation, respectively. 2PPL image intensities are normalized to the maximum intensity of each map.

Figure 4

Surface enhanced Raman scattering (SERS) of benzoic acid (BA). In (a) a monocrystalline Au bowtie nanoantenna and (b) a polycrystalline Au bowtie nanoantenna are depicted. (c) The SERS spectra of BA from single-crystal (black) and polycrystalline (red) bowtie nanoantennas obtained with 785 nm excitation, illustrating enhanced SERS activity from single-crystal bowties. See text for vibrational assignments.