Facile fabrication of stretchable photonic Ag nanostructures by soft-contact patterning of ionic Ag solution coatings

1 Introduction

The silver (Ag) nanostructures have unique optical tunability and superior applicability to both laboratory and industry, and therefore have been widely used in photonic devices such as color filters [1, 2], surface-enhanced Raman spectroscopy [3], [4], [5], [6], [7], and plasmonic sensors [8], [9], [10], [11]. However, common nanofabrication protocols to create Ag nanostructures generally involve high-vacuum-assisted deposition and etching processes, which are expensive and slow, and therefore obstruct scalable and practical fabrication of the photonic and other functional devices that use Ag nanostructures [12], [13], [14]. To overcome this problem, mechanical nanopatterning processes that centrally include nanoimprint lithography (NIL) have been adopted for the facile fabrication of various Ag nanostructures [15], [16], [17], [18], [19]. NIL can readily replicate high-resolution nanopatterns by pressing the mold into a polymer resin [20], [21], [22]. Moreover, the transferred nanostructures can be tiled to cover large areas and can also be used as another mold or template [23, 24], so processes that use NIL are very favorable for high-throughput, highly reproducible, and scalable fabrication of nanostructures without resorting to high-vacuum processing and optical lithography [25], [26], [27].

An alternative strategy to fabricate Ag nanostructures uses a simple coating of solutions that include ionic Ag or dispersed Ag nanoparticles (NPs), then applies mild annealing [28], [29], [30], which is facile and scalable. Also, resin with embedded NPs has been developed for the NIL-based fabrication of photonic devices with controlled optical properties; the refractive index n and the extinction coefficient k of the NIL resins can be modulated by controlling the types and concentrations of NPs mixed with the polymeric bases [31], [32], [33], [34]. The resin that includes Ag NPs can also be engineered to form functional nanostructures by using continuous mechanical inscribing techniques [28, 35, 36]. However, the production of resin that includes well-dispersed NPs is a challenging task, and the resolution of the NIL-shaped structures obtained by Ag NPs in the resin can be limited by the volume of NPs and NP agglomerates.

Here, we demonstrate shape-stretchable Ag nanostructures that can be obtained using soft-contact patterning of a layer of ionic Ag ink. The layer of ink can be either printed or imprinted by using the polydimethylsiloxane (PDMS) molds, then readily converted to Ag nanostructures by mild thermal annealing. The initial state of the Ag-containing ink is an ionic fluid, so the Ag nanostructure can be readily printed or imprinted with controllable residual layers and in high resolutions that are not limited by the size of NPs or their agglomeration. We systematically investigate the optimal processes for patterning the ionic Ag nanostructures from resin that includes Ag NPs by controlling the initial ink concentration and its coating, printing, imprinting, and annealing conditions. The controlled stretching of flexible elastomer molds enables facile fabrication of shape-stretchable Ag nanostructures with tunable photonic characteristics. As an example, we demonstrate a polarization-sensitive reflective color filter, in which the shape-stretchable Ag nanodot pattern induces optical reflectance at a specific wavelength that shifts depending on the strain applied to the mold during the soft contact of the NIL process.

2 Experimental section

3 Results and discussion

The proposed soft-contact mechanical patterning processes (Figure 1) use a flexible PDMS mold and an ionic Ag ink in two main steps: transfer printing and soft NIL. During transfer printing, the Ag ink concentration and the transfer contact pressure strongly influence the structural morphology and optical properties of the final Ag nanostructure. During soft NIL, the initial conditions (i.e., thickness, viscosity) of the soft-baked Ag layer have a strong effect on the subsequent NIL result, which can be adjusted by controlling Ag concentration and the coating speed of and soft-baking time (SBT) of Ag ink.

Figure 1:

Schemes of the procedures of soft-contact patterning of Ag nanostructures in two main routes. Transfer printing of Ag nanostructures proceeds in the order of (a1) ionic Ag ink coating, (b1) wetting the PDMS mold, and (c1) contacting with the target substrate and Ag-inked PDMS mold. Soft NIL process of Ag nanostructures consists of (a2) ionic Ag ink coating on the target substrate, (b2) soft baking of the ionic Ag ink layer, and (c2) uniform contacting of the PDMS mold by using a roller. Both processes follow (d) hard baking with gentle pressure and cooling to complete (e) final Ag nanopatterns.

The transfer printing route (Figure 1a1–c1) can create Ag nanohole structures by transfer printing of an ionic Ag ink on the desired substrate and subsequent thermal annealing, by using the PDMS nanohole mold (hole diameter 800 nm, period 1.2 µm). As described in the Experimental section, during the transfer printing process, the Ag ink that coated the top surface of the mold was transferred to the substrate under adequate contact pressure (∼2 kPa), so essentially no residual layer remained (Figure 2a and b). Here, the ionic Ag ink concentration determines the thickness, morphology, and pattern quality of the resulting Ag layer. If the concentration of the ionic Ag ink is too low, the transferred Ag structure has insufficient Ag ions and is too porous and thin to form a continuous Ag layer that has a clean pattern profile. In contrast, if the concentration of the ionic Ag inks is too high, the excessive Ag material leaks into and fills the opening area (Figure 2c). The optimal condition to create a faithful Ag nanohole pattern with a clean opening profile was 20 vol% ionic Ag ink and contact pressure of 2 kPa (Figure 2b). This Ag nanostructure that has been transferred by soft contact can provide tunable optical transmittance depending on the ionic ink concentration (Figure 2d). The appropriate contact pressure depends on the pattern shape and scale; for instance, a 200-nm-wide Ag nanograting pattern that has a 500-nm period can be created with the most distinct profile and clean openings at 5 kPa (Figure S2 in Supplementary Material). Studies to increase the diversity of patterns will follow in the future.

Figure 2:

Ag nanohole patterns fabricated by transfer printing of ionic Ag ink. SEM images of Ag nanostructures fabricated by transfer printing of ionic Ag ink diluted with (a) 10, (b) 20, (c) 30 vol% in isopropyl alcohol (IPA). (d) Optical transmittance of the Ag nanohole patterns fabricated by using the ionic Ag inks with various concentrations. (e) Optical photograph of the Ag nanohole structure transfer-printed on the glass substrate by using a 20 vol% ionic Ag ink. All scale bars: 2 µm.

We also used soft NIL to fabricate Ag nanostructures (Figure 1a2–c2), by using the ionic Ag ink as an imprinting resin. In the soft NIL process, the PDMS nanopillar mold (pillar diameter 800 nm, pillar height 650 nm, period 1.2 µm) was applied to the soft-baked Ag layer (Experimental section). Here, SBT is a crucial factor for precise control of residual layer thickness as well as overall patterning quality. The use of different SBTs and coating speeds (all with 50 vol% ionic Ag ink) yielded different results. Slow spin-coating (i.e., 2000 rpm) and short SBT (i.e., approaching 0 s) are generally most favorable for the patterning using NIL (Figure 3). The definition of the nanoporous morphology in the resulting Ag nanostructure increases as SBT increases. This trend is consistent with a previous investigation, which showed that the soft-baking process initiates the reductive texturing of an ionic Ag ink layer into a nanoporous structure [29], and that the resolution of this texturing increases as the thickness of the Ag layer decreases. The residual layer thickness also increases as SBT increases (Figure S3 in the Supplementary Material). We used these findings to design an Ag nanostructure with specific photonic functionality by controlling the coating and annealing processes of the ionic Ag coating layer and by modulating the flexible mold geometry and soft NIL process condition.

Figure 3:

SEM images of Ag nanohole patterns fabricated by performing soft NIL onto the 50 vol% ionic Ag ink-coated substrates. The coating speeds and soft baking times for preparing the ionic Ag ink coatings were varied: 2000, 3000, and 4000 rpm, and 0, 1, 2, 3, and 4 min, respectively. All scale bars: 1 µm.

Here, we demonstrate one practical application: shape-stretchable Ag nanodot structures can be easily fabricated by performing soft NIL using a stretched PDMS nanohole mold (either 600-nm-period hexagonal array (600H) or 300-nm-period square array (300S), both with a 1:1 ratio of hole diameter to hole interspacing). Despite the scaling down to the nanoscale resolution, the soft NIL of the ionic Ag ink by using the modified processing conditions (Experimental Section) can achieve highly uniform and faithful fabrication of Ag nanostructures (Figure 4). Moving forward, the use of flexible elastomeric PDMS mold enables the fabrication of shape-tunable Ag nanostructure without the use of multiple master molds [39].

Figure 4:

Various Ag nanodot patterns fabricated by soft NIL of the ionic Ag ink. SEM images of Ag nanodot structures with different scales and arrays: (a) 600-nm-period hexagonal array (600H) and (b) 300-nm-period square array (300S), with the corresponding inset images for enlarged views. All scale bars: 2 µm.

The process schemes obtained a range of NIL results (Figure 5) by using the 600H mold stretched from 0 to 20% strain. Using the unstretched 600H mold (Figure 5a), a circular Ag nanodot pattern was cleanly formed with a negligible residual layer (Figure 5c). On the contrary, by using the 600H mold stretched to 20% strain during the soft NIL process (Figure 5b), the Ag nanodot array can be readily turned into elliptical Ag patterns (Figure 5d). This practical strategy yields photonic Ag nanostructures with periods and lengths that differ between the x-axis and the y-axis (markup in Figure 5) [40]; this difference can produce linear polarization angle-dependent effective optical properties.

Figure 5:

Soft NIL of stretchable Ag nanodots by using the strained PDMS mold. Graphical schematics of soft NIL of the ionic Ag ink performed by using the identical PDMS mold with (a) no strain and (b) 20% strain, respectively. SEM images of the 600H Ag nanodot patterns fabricated accordingly: (c) no strain and (d) 20% strain, respectively. The inset images show the enlarged views of corresponding structures. Scale bars: (c) and (d) 2 µm, (inset) 400 nm.

A simulation of the 300S pattern (Figure S4) indicates that the stretchable Ag nanodot structures can function as polarization-sensitive reflective color filters, depending on the stretch degree. Optical reflectance measurements were obtained from unstretched 300S Ag nanodot arrays and the same arrays after stretching to 20% strain, for incident light that had been polarized to 0° and 90° with respect to the stretch axis (Figure 6). Notably, the reflectance peak for the 0° linearly-polarized light was red-shifted when the Ag nanodot was stretched. On the contrary, the 90° linear polarized light generated no measurable shift of the peak position, because of the almost invariant period in the y-direction. Although this strategy has not been fully optimized here, it provides a pragmatic insight into applications of strain-dependent fabrication of stretchable Ag nanostructures that have tunable optical properties without the use of multiple mold prefabrication and use of high-vacuum and etching processes [41].

Figure 6:

Optical reflectance of Ag nanodots fabricated by soft NIL by using the 300S PDMS nanohole mold stretched with various strains, in X- or Y-polarized light.

4 Conclusions

We have demonstrated two practical methods to fabricate Ag nanostructures by using soft-contact mechanical patterning of ionic Ag ink coating. Transfer printing can directly replicate nanopatterns with no residual layer, but contact pressure is critical to creating clean shapes of nanopatterns. In the transfer printing method, the Ag nanostructure could be transferred onto the desired substrate by depositing ionic Ag ink on the PDMS mold, ensuring conformal contact and adequate pressure of the mold to the substrate, then thermally annealing the ionic Ag layer. On the other hand, soft NIL can control residual layer thicknesses according to additional processing conditions, which induces variable optical properties of nanopatterns. In the subsequent soft NIL process, the PDMS mold could induce the formation of Ag nanostructures by mechanically stamping an ionic Ag coating that had been prepared under appropriate spin-coating and SBT conditions, followed by hard baking. Both transfer printing and soft NIL are complementary to one soft mold which can be stretchable for selective optical devices. To demonstrate possible practical photonic device architecture, we developed a polarization-sensitive reflective color filter by using controllably stretched PDMS mold during soft NIL of the ionic Ag coating layer. Although reusability of the PDMS mold is limited by swelling of the PDMS mold due to IPA in ionic Ag ink, the proposed strategy can be used for easy and scalable fabrication of diverse photonic devices with variable optical properties, including but not limited to the display components, transparent electrodes, metasurfaces, and plasmonic sensors [42].