The fabrication of flip-covered plasmonic nanostructure surfaces with enhanced wear resistance
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) SiOx [37], and ultrathin (3 nm) AlOx [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.
Section snippets
Experiments
The metal imprint transfer process starts with the preparation of nanoimprint molds replicated in polyethylene terephthalate (PET), as shown in Fig. 1. Two designs of the hexagonal hole array patterns were used in this work, 150 nm diameter holes with a 300 nm period, and 200 nm diameter holes with a 400 nm period. These master patterns were fabricated by KrF optical lithography, followed by a well-established anisotropic etching procedure, at the National Nanofab Center in Daejeon, Korea. The
Results and discussion
As shown in Fig. 1, the silver deposited on the nanoimprinted mold pattern was selectively transferred to the imprinted trenches of the adhesive layer on the receiver film, while silver remained in the donor trenches. Images of the donor and receiver with exposed nanostructure surfaces are shown in Fig. 2(a) and (b), respectively. Different diffraction colors appeared due to different silver array and structural configurations in addition to viewing angle dependence. The master pattern had 200
Conclusion
A novel fabrication method was shown to protect exposed nanostructure surfaces from dynamic wear. Reference samples with exposed nanostructure surfaces were obtained by metal nanoimprint transfer following metal deposition. The samples were flip-covered onto a protective adhesive film to protect the exposed nanostructure. Through measurement of the spectral transmittance, it was shown that the flip-covered samples differed slightly from the exposed samples, but the effect was less significant
Acknowledgments
This research was supported by Grants from the Basic Science Research Program (2011-0028585), and STEAM Program (2014M3C1A3052569), funded by the National Research Foundation of Korea (NRF) under the Ministry of Education, Science, and Technology. It was also funded by the Institute Project (NK196B) of KIMM (Korea Institute of Machinery & Materials).
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