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We present a technology to fabricate large-area gapped plasmonic structures deterministically with atomic precision, high throughput and high reliability at low cost. The technology is based on collapsible nano-fingers fabricated using nanoimprint lithography and ALD. A pair of metallic nanoparticles is placed on top of two nano-fingers in flexible polymer with high aspect ratio. ALD is then used to coat a thin conformal dielectric layer. By collapsing the pair of nano-fingers, two metallic nanoparticles with dielectric coating contact each other. Therefore, the gap size between two metallic nanoparticles is well defined by twice the thickness of the ALD-coated dielectric layers.
As metallic nanoparticles are known to dramatically modify the spontaneous emission of nearby fluorescent molecules and materials, here we examine the role of the gap plasmon resonance on the molecular fluorescence enhancement. Considering quenching effect, the distance between fluorescent molecules and gold nanoparticles should not be too small in order to obtain strongest enhancement. In that sense, to fully exploit plasmonic enhancement on the fluorescent molecules, an appropriate gap size should be kept between the molecule and each metallic nanoparticle, which separates molecules away from the metal to avoid quenching effect. The ALD-defined gap plasmonic nano-finger structure facilitate direct and precise control on the gap size between the molecule and metallic nanoparticle by simply changing ALD film thickness that has atomic precision. This makes collapsible nano-fingers the ideal structure for the optimization of molecular fluorescence enhancement. With the optimally engineered collapsible nano-fingers plasmonic structure, field enhancement and fluorescence quenching at hot spots can be studied in detail, which paves the way for optimal design on strongest plasmonic enhancement of molecular fluorescence.
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