The fabrication of flip-covered plasmonic nanostructure surfaces with enhanced wear resistance

Abstract

Exposed nanostructure surfaces often suffer from external dynamic wear, particularly when used in human interaction, resulting in surface defects and the degradation of plasmonic resonance properties particularly in terms of transmittance extinction rate and peak-to-valley slope. In this work, a method for the fabrication of flip-covered silver nanostructure-arrayed surfaces is shown to enhance wear resistance. Selectively transferred silver dot and silver webbed-trench exposed reference samples were fabricated by metal nanoimprint, and flip-covered samples were created by flipping and bonding reference samples onto a PET film coated with an adhesive layer. The samples' spectral transmittance was measured before and after a dynamic wear test. Some spectral shift was observed due to the change in refractive index of the surrounding media, but this was not as significant as the effects of the other chosen geometry factors. It was found that dynamic wear had a greater effect on the plasmonic resonance behavior of the exposed samples than in those that had been flip-covered. This suggests that flip-covering may be an effective strategy for the protection of plasmonic resonators against dynamic wear. It is expected that the slight variations in spectral transmittance could be compensated through proper tuning of the sample geometry.

Introduction

Periodic nanostructure arrays have been shown to be effective for various modes of electromagnetic manipulation such as refractive index gradients, waveguide gratings, diffractive-coupling, and plasmonic resonance. Through the design of periodic nanostructure arrays, various unique nanophotonic properties have been assigned to films, windows, lenses, and various nanophotonic devices [1], [2], [3]. In particular, plasmonic metal nanostructures can be used to control light–matter interactions at nanoscale through resonant matching of the radiating photons, resulting in resonant extinction, localized field confinements and out-coupling to near-field optical modes, and enhanced radiative decay rates [4], [5], [6]. These effects have been reported to be useful for improving the efficiency of sensing, light-emitting and optoelectronics [7], [8], [9], [10], [11].

Nanoimprinting is a simple, inexpensive, and high-quality nanolithographic technique, commonly used in the fabrication of periodic metal nanostructure arrays. Additional process steps require deposition [12], [13], [14], [15], lift-off [16], [17], direct nanoimprinting [18], [19], metal etching [20], [21], or transfer [22], [23], [24], [25] processes to be performed on the predefined nanoimprint template patterns. These nanostructures are often contained by additional layer deposition in multilayer devices [26], [27], [28] or exposed by a window or flexible film. In the latter case, wear resistance should be guaranteed, particularly when exposed to environmentally harsh conditions, or human interaction [29], [30], [31]. Therefore, attempts have been made to identify harder and more robust nanostructure materials, particularly for use in antireflective windows [32], [33], [34]. Robust plasmonic substrates are more demaded to applications such as tip-enhanced Raman spectroscopy, surface-enhanced spectroscopy, and enhanced optical and optoelectronic nanosensors. Hence, protective films such as diamond-like carbon [35], [36], ultrathin (3 nm) SiO_x [37], and ultrathin (3 nm) AlO_x [38], have been investigated to enhance surface hardness and wear resistance in working environments.

Unprotected nanostructures have a limited resistance to repetitive external abrasion, the evaluation of which requires dynamic wear testing, rather than static hardness tests. According to the dynamic wear test scheme described in [36], [39], the contact force is moved across the sample surface with a fixed normal force and speed. More effective protection strategy of the nanostructured surfaces is required against repetitive dynamic forces. Planarization with coating materials and physical bonding are methods that could be used to provide protection. Planarized nanostructures have been shown, using various filling techniques [40], [41], [42], [43], [44], to reduce corrugation-drive current leakage in multilayer optoelectronic devices. Alternatively, physical bonds form air-gaps between the nanostructure surfaces and bonding substrates, reportedly optimizing light extraction efficiency [45], [46], [47]. This is achieved through anodic bonding of the patterned surface to the device substrate [45], [46] using thermal degradation-induced calcination [47] to decrease the dielectric constant in a nanochannel-formed poly-methyl-silsesquioxane (PMSSQ) layer. The basic idea of these techniques can contribute to optimizing the dynamic wear stability of the metal nanostructure surfaces, given that the protection methods used do not significantly alter the plasmonic resonance properties of the exposed nanostructure surface.

In this work, a method for the fabrication of flip-covered flexible plasmonic films is developed, based on previously reported metal nanoimprint transfer techniques [48], enhancing dynamic wear resistance with respect to exposed reference samples. Spectral transmittance profiles were measured, to investigate the wavelength shift and profile variations between the exposed and flip-covered samples, before and after the dynamic wear tests. The damage caused by the wear testing was evaluated qualitatively and quantitatively by scanning electron microscopy (SEM) analysis and measurement of the spectral transmittance and its variations. The results suggest that flip-covering may be an effective technique by which plasmonic resonators can be protected against dynamic wear, increasing the number of potential applications.