Plasmonic Devices Based on Collapsible
Nanofingers
Boxiang Song, Pan Hu, Yunxiang Wang, and Wei Wu*
Ming Hsieh Department of Electrical and Computer Engineering
University of Southern California
Los Angeles, CA USA 90089
E-mail: wu.w@usc.edu

Nano-gap structure has attracted great scientific interests. For example, plasmonicnanostructures with nano-gaps concentrate light to a small volume, which can lead to manypotential applications. While it is theoretically predicted that the optimal plasmonic hot spot isa gap of less than 1 nanometer between two metallic particles, there is still no manufacturing
technology to reliably fabricate it with high-precision and controllability at practical cost. In
this work, we present a novel fabrication technique that combines both top-down and bottomup process to produce large-area nanogap structures with atomic-precision gap size control.
The fabrication approach is described in Figure 1. First, two layers of photoresist and one liftoff
layer were spin-coated on substrates, then Nanoimprint (NIL)[1], Reactive Ion Etching (RIE)and Metal Deposition were utilized to achieve desired metal patterns. Afterwards, uncovered
areas were etched by RIE to form high aspect ratio nanofingers and Atomic Layer Deposition
was used to grow selected thin dielectric layer outside nanofingers. After dripping ethanol onto
samples, high aspect ratio nanofingers would be driven to collapse by capillary force during
ethanol evaporation. By manipulating nanofinger heights and dielectric thicknesses, gap size
down to 0.8 nm can be precisely defined after finger collapsing and such structure can beutilized as an ideal platform to study the rich sciences of plasmon and quantum effect. For
example, we experimentally demonstrated that tunneling barrier height across the gap can be
adjusted by using appropriate gap spacer materials and gap sizes.[2] As a result, plasmonicenhancement can be strongly affected since the electron tunneling mechanism is the mostessential part in determining the plasmonic enhancement factor. It was found that as thetunneling barrier height is decreased, the enhancement of electron tunneling manifests itself ina wider optimal gap, a redshift of the plasmon frequency with increasing gap size was also
observed when the gaps narrowed to sub-5 nm range[2]. Moreover, many photonic applications
have been studied and implemented intensively based on our controllable nanogap structures.
Those include plasmonic enhanced fluorescence (figure 2), single-molecule label-free sensing
by SERS[3] and plasmonic enhanced photocatalysis.

Reference:[1]
S. Y. Chou, P. R. Krauss, P. J. Renstrom, Appl. Phys. Lett. 1995, 67, 3114.
[2]
B. Song, Y. Yao, R. E. Groenewald, Y. Wang, H. Liu, Y. Wang, Y. Li, F. Liu, S. B. Cronin, A. M.
Schwartzberg, S. Cabrini, S. Haas, W. Wu, Acs Nano 2017, 11, 5836.
[3]
F. Liu, B. Song, G. Su, O. Liang, P. Zhan, H. Wang, W. Wu, Y. Xie, Z. Wang, Small 2018, 14,
1801146.

Figure 1. Schematic of nanofinger fabrication process: (a) Two layers of UV-curable resist and one liftoff layer spin-coated on a silicon wafer. (b) Nanoimprinting by a pillar array mold. (c) After etching
residual layer, metal evaporation and lift-off, metal etching mask was left on the bottom UV-curable
resist. (d) Etching uncovered resist. (e) ALD deposition of dielectric coating. (f) Soaking into pureethanol to collapse the nanofingers.

Figure 2. (a) Fluorescence intensity of Nile blue on collapsed nanofingers with different Al2O3 gap
sizes. (b) The measured fluorescence intensity and the measured 592 cm-1 signature Raman intensity ofNile blue for different values of gap size. (c) FDTD calculation of local field enhancement in collapsed
nanofingers with 5 nm Al2O3 gap. (d) Fluorescence intensity of Nile blue on collapsed nanofingers with
5 nm Al2O3 gap, non-collapsible nanofingers with same Al2O3 coating and plain glass.