| AVS 63rd International Symposium & Exhibition | |
| Nanometer-scale Science and Technology | Monday Sessions |
| Session NS-MoA |
| Session: | Nanophotonics, Plasmonics, and Energy |
| Presenter: | Boxiang Song, University of Southern California |
| Authors: | B. Song, University of Southern California W. Wu, University of Southern California |
| Correspondent: | Click to Email |
Plasmonic nanostructures are of great interests recently due to their ability to concentrate light to small volume. They have many potential applications in optical communication, disease diagnosis, and chemical sensing. Therefore it is extremely important to investigate the plasmonic hot spots both theoretically and experimentally. While it is theoretically predicted that the optimal hot spot is a sub-5 nm gap between two metallic particles , due to the difficulties in fabrication of sub-5 nm structures, most of the studies on hot spot behaviors at that scale are theoretical only. Therefore, it is essential to find a way to fabricate hot spots with sub-5 nm gap sizes deterministically and reliably as the experimental platform to probe and utilize those hot spots.
Recently, we have successfully fabricated gap plasmonic structure with precisely controlled nano-gap by using collapsible nano-fingers. First, a nano-finger array in flexible polymer (i.e. nanoimprint resist) is fabricated using nanoimprint lithography (NIL), and metallic caps, such as gold disks, are deposited on the top of each finger using electron-beam evaporation. Second, atomic-layer deposition (ALD) is used to coat a thin conformal dielectric layer. Finally, the nano-finger sample is dipped into Ethanol (water works too) and air-dried. When the Ethanol dries up, the capillary force makes the nano-fingers close together. The ALD-coated dielectric layer serves as the spacer to define the gaps between the metallic particles. If we use TiO2 as an example, each atomic layer of TiO2 is only about 1Å thick, which means the gap between the metallic particles can be precisely controlled with an accuracy of 2 Å and as small as 2 Å. For the first time, we can reliably achieve such small gaps deterministically and precisely. It is the ideal experimental platform to probe the rich sciences at the gap plasmonic hot spots.
As the polarized light shone on the dimer-like structure, it will trigger dipole-like charge distribution inside gold nanoparticle. Based on classical electromagnetic theory, field at gap center increases as the gap gets smaller. However, as gap size reduces, for sub-5 nm gap structure, electron tunneling between two gold nanoparticles becomes significant, which would cancel part of the charge in opposite sides and hence reduce the field. The competing factors result in an optimal gap size for the strongest optical field enhancement. But such a small gap structure has not been fabricated reliably until recently we demonstrated how to define and scale sub-5nm gaps by using collapsible nano-fingers