Split-Wedge Antennas with Sub-5 nm Gaps for Plasmonic Nanofocusing

Abstract

We present a novel plasmonic antenna structure, a split-wedge antenna, created by splitting an ultrasharp metallic wedge with a nanogap perpendicular to its apex. The nanogap can tightly confine gap plasmons and boost the local optical field intensity in and around these opposing metallic wedge tips. This three-dimensional split-wedge antenna integrates the key features of nanogaps and sharp tips, i.e., tight field confinement and three-dimensional nanofocusing, respectively, into a single platform. We fabricate split-wedge antennas with gaps that are as small as 1 nm in width at the wafer scale by combining silicon V-grooves with template stripping and atomic layer lithography. Computer simulations show that the field enhancement and confinement are stronger at the tip–gap interface compared to what standalone tips or nanogaps produce, with electric field amplitude enhancement factors exceeding 50 when near-infrared light is focused on the tip–gap geometry. The resulting nanometric hotspot volume is on the order of λ3/106. Experimentally, Raman enhancement factors exceeding 107 are observed from a 2 nm gap split-wedge antenna, demonstrating its potential for sensing and spectroscopy applications.

Sub‐5 nm Metal Nanogaps: Physical Properties, Fabrication Methods, and Device Applications

“…To date, many remarkable results of SERS based on sub‐5 nm nanogaps have been reported, reflecting the fact that sub‐5 nm nanogaps with significant enchantment in the local electromagnetic field are ideal candidates for application in ultrasensitive detection of various molecules and even single molecules. As shown in Figure a, a 2 nm gap 3D split‐wedge SERS antenna was fabricated using atomic layer lithography, exhibiting Raman enhancement factors exceeding 10 7 . Furthermore, DNA interior gapped Au core–shell nanoparticles also exhibited the strongest and most reproducible SERS signals with time, and a sensitivity as high as 10 × 10 −15 m concentrations and enhancement factors greater than 1.0 × 10 8 were demonstrated for this type of interior nanogap, as shown in Figure b.…”

“…In addition to ion milling, another low‐cost technique has been employed, using adhesive tape to remove the second layer metal to form nanometer‐sized gaps . As illustrated in the top panel of Figure c, after patterning the vertical sidewall first, nanogaps are exposed by partially peeling off the second metal layer that coated on the ALD layer.…”

“…A periodic metal–dielectric–metal (MDM) nanostructure in which each pair of nanometer gaps was connected using planar ALD layers was massively fabricated using atomic layer lithography . Furthermore, 3D split‐wedge antennas with gaps as small as 1 nm were also produced over an entire wafer by combining template stripping and atomic layer lithography . This peeling‐off strategy is promising for realizing nanopatterns with ALD‐defined gaps over large areas and can increase the fabrication quality and yield.…”

“…SEM images of arrays of split‐wedge antennas with a 5 nm nanogap on a silver wedge tip. Reproduced with permission . Copyright 2016, American Chemical Society.…”

Engineering State‐of‐the‐Art Plasmonic Nanomaterials for SERS‐Based Clinical Liquid Biopsy Applications

“…For example, Ma et al developed a plasmonic silver "nanopore-in-nanogap" hybrid structure (Figure 9). The resulting "nanopore-innanogap" hybrid nanostructures thus possessed a combined nanopore and nanogap plasmon modes, which provided an additional electromagnetic enhancement for the ultrasensitive detection of 4-aminothiophenol down to 0.1 × 10 −15 m. Currently, there are many other hybrid nanostructures in reports, including a split-wedge antenna with 1 nm gaps, [71] vertically coupled complementary antenna, [72] bimetallic 3D Adv. An etching process was then performed to create ultrasmall nanopores of sub 10 nm in diameter on 2D silver nanoparticle supercrystals.…”