Large Area Metal Nanowire Arrays with Tunable Sub-20 nm Nanogaps

Abstract

We report a new top-down nanofabrication technology to realize large area metal nanowire (m-NW) arrays with tunable sub-20 nm separation nanogaps without the use of chemical etching or milling of the metal layer. The m-NW array nanofabrication technology is based on a self-regulating metal deposition process that is facilitated by closely spaced and isolated heterogeneous template surfaces that confine the metal deposition into two dimensions, and therefore, electrically isolated parallel arrays of m-NW can be realized with uniform and controllable nanogaps. Au-NW and Ag-NW arrays are presented with high-density ~10(5) NWs cm(-1), variable NW diameters down to ~50 nm, variable nanogaps down to ~5 nm, and very large nanogap length density ~1 km cm(-2). The m-NW arrays are designed and implemented as interdigitated nanoelectrodes for electrochemical applications and as plasmonic substrates where the coupled-mode localized surface plasmon resonance (LSPR) wavelength in the nanogaps between adjacent m-NW dimers can be precisely tuned to match any excitation source in the range from 500 to 1000 nm, thus providing optimal local electromagnetic field enhancement. A spatially averaged (n = 2500) surface-enhanced Raman scattering (SERS) analytical enhancement factor of (1.2 ± 0.1) × 10(7) is demonstrated from a benzenethiol monolayer chemisorbed on a Au-NW array substrate with LSPR wavelength matched to a He-Ne laser source.

Tailored nanoantennas for directional Raman studies of individual carbon nanotubes

“…We notice that the higher the enhancement factor, the more the angle β approaches γ . This is consistent with the observations on macroscopic arrays of metal nanowires [37].…”

“…The authors also show that the resonating wavelength increases when the gap is decreased, while the resonance width decreases for larger strip width. We notice that for a nominal gap width of 20 nm, the actual one is roughly 30 nm [38], similar to the actual gap of our metal strips. Therefore, we expect to observe in our antennas a strong SPP coupling around λ L = 633 nm.…”

“…The design of the antennas (and in particular the gap between the strips) has been chosen such that a SPP is excited when the array is illuminated with light at λ L = 633 nm. For the choice of the gap and strip width we refer to the work of Le Thi Ngoc et al [37]. In this paper the authors present reflectance spectroscopy measurements on macroscopic arrays of gold strips.…”

“…We assume that both the incident and the scattered light are amplified in the same way by the antennas. For the G mode the energy difference between incident and scattered light is ω G ≈ 0.2 eV, comparable to the width of the SPP resonance [37]. Thus, if the optical antennas are effective for both frequencies, the resulting enhancement will be proportional to the fourth power of the local electric field [7,16].…”

Surface- and Tip-Enhanced Raman Spectroscopy in Catalysis

“…With this technique, structures such as nanogroove, nanopyramid, and nanogap–nanowire arrays have been recently realized with tunable gap spacing and high spatially averaged analytical enhancement factors (AEF), 44 as shown in Figure 2. 45,46 Since in the first two of these approaches LIL is used in combination with anisotropic (100) silicon etching to create nanometer pitches and spacing, no mask is needed and the method is therefore relatively cheap. However, a disadvantage of these anisotropically etched structures is that the SERS hot spots with high signal enhancement occur only at a very small percentage of the surface area because the groove width and spacing between the pyramid sidewalls vary strongly.…”