Engineering Metal Adhesion Layers That Do Not Deteriorate Plasmon Resonances

Abstract

G reat amounts of numerical and experimental investigations have been devoted to boosting the signal of plasmonic sensors fabricated from noble metal nanoantennas with sharp edges or with gaps separated by a few nanometers. 1À8 The field enhancement depends mainly on the ability to couple incident photons to localized surface plasmon modes. Such resonant modes are prone to damping by scattering and absorption in the metal and its surrounding materials, thus, limiting the achievable enhancement. 9 By interference with subradiant modes, as in Fano resonant systems, radiative losses can be reduced, but not suppressed completely. 10,11 In any case, the field enhancement is often severely reduced by the presence of adhesion layers. These layers are required to pin the noble metals to the substrate, particularly when the fabrication requires lift-off and sonication or when the applications demand high structural robustness. 12 Thanks to their broad availability and process compatibility with the evaporation of the noble metals, Cr and Ti are the most commonly used adhesion materials, with typical thicknesses ranging between 1 and 10 nm. 13À15 Although the existence of the adhesion layer is an important part of the plasmonic structure, its influence on the near and far field of the plasmonic modes is often neglected. 9,16 The perturbation of the plasmon resonance can be derived from the real and imaginary parts of the adhesion materials dielectric function which affects the refractive index locally and introduces absorption. Under such circumstances, surface and localized surface plasmon resonances (SPR and LSPR) are red-shifted and broadened, leading to reduced signal sensitivities, caused by the reduced quality factor Q of the resonant mode. 17À21 As a consequence, processes where high near-field amplitudes are needed, such as surface enhanced Raman scattering (SERS) and fluorescence, 16 ABSTRACT Adhesion layers, required to stabilize metallic nanostructures, dramatically deteriorate the performances of plasmonic sensors, by severely damping the plasmon modes. In this article, we show that these detrimental effects critically depend on the overlap of the electromagnetic near-field of the resonant plasmon mode with the adhesion layer and can be minimized by careful engineering of the latter. We study the dependence of the geometrical parameters such as layer thickness and shape on the near-field of localized plasmon resonances for traditional adhesion layers such as Cr, Ti, and TiO 2 . Our experiments and simulations reveal a strong dependence of the damping on the layer thickness, in agreement with the exponential decay of the plasmon near-field. We developed a method to minimize the damping by selective deposition of thin adhesion layers (<1 nm) in a manner that prevents the layer to overlap with the hotspots of the plasmonic structure. Such a designed structure enables the use of standard Cr and Ti adhesion materials to fabricate robust plasmonic sensors without deteriorating their sensitivity.

Fabrication and analysis of metallic nanoslit structures: advancements in the nanomasking method

“…Previous work studying the effects of a Ti adhesion layer on the plasmonic response of an Au nanostructure to incident light illumination has shown that the optical enhancement produced by the structures decreases with increasing Ti layer thickness. [64][65][66][67][68] Therefore, for optical applications, ideally no adhesion layer would be used, but in the case in which it is required, the smallest possible adhesion layer should be used to preserve the optical characteristics of the patterned structures. The thickness of the Au layer has also been found to significantly affect the optical response of a patterned nanostructure, with and without nanoslits.…”

Gap Plasmons and Near-Field Enhancement in Closely Packed Sub-10 nm Gap Resonators

“…17,28 A 1-nm-thick Cr layer evaporated under normal incidence forms an effective adhesion layer which does not affect the plasmonic performance of the system. 29 The cross-section and top-view SEM images of the obtained sub-10 nm gap arrays are shown in Figure 1a and b.…”

Modeling and optimization of Au-GaAs plasmonic nanoslit array structures for enhanced near-infrared photodetector applications

“…Commonly, thin layers (nanometers) of Ti, Cr, or other metals are used as the adhesive. 49 Nanofabrication techniques usually deposit a semiuniform layer of the adhesion material prior to deposition of the desired metal. Although usually necessary, it has been shown that this layer produces "damping" effects of the plasmonic enhancement that occurs in the near-field of the structure.…”

“…Although usually necessary, it has been shown that this layer produces "damping" effects of the plasmonic enhancement that occurs in the near-field of the structure. 49 In the nanomasking process utilized by the Herzog group, a Ti adhesion layer is most commonly used because a sacrificial Cr layer is required for the fabrication process. 47 Thus, the current work studies the effects of varying the thickness of a Ti adhesion layer on the local plasmonic field enhancement.…”