Facile fabrication of stretchable photonic Ag nanostructures by soft-contact patterning of ionic Ag solution coatings
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Nanophotonics 2022; 11(11): 2693–2700
Research Article
Minwook Kim, Dong Kyo Oh, Jeong Dae Kim, Minsu Jeong, Hongyoon Kim, Chunghwan Jung,
Jungkeun Song, Wonjun Lee, Junsuk Rho* and Jong G. Ok*
Facile fabrication of stretchable photonic Ag
nanostructures by soft-contact patterning of ionic
Ag solution coatings
https://doi.org/10.1515/nanoph-2021-0812 The ionic liquid-phase Ag coating is easily obtained by spin-
Received December 25, 2021; accepted March 4, 2022; coating ionic Ag ink that has appropriate Ag concentration
published online March 17, 2022 and can be either printed or imprinted on the desired sub-
strate by using a soft elastomer patterning mold, then
Abstract: We describe a rapid and simple method to create
reduced to the Ag nanostructure by subsequent thermal
Ag nanostructures by using direct mechanical patterning of
annealing. More specifically, we present two methods:
ionic Ag ink coating under gentle pressure, then thermal
transfer printing and soft nanoimprinting. In transfer print-
annealing to reduce the ionic Ag ink to a metallic Ag layer.
ing, the ionic Ag ink is first inked onto the elastomer mold
which then contacts the target substrate to transfer the Ag
nanopattern. In soft nanoimprinting, the elastomer mold
Minwook Kim, Dong Kyo Oh and Jeong Dae Kim contributed equally to
conducts soft imprinting to engineer the ionic Ag ink coating
this work.
to the Ag nanostructure. We systematically investigate the
*Corresponding authors: Junsuk Rho, Department of Mechanical optimal patterning conditions by controlling the initial Ag
Engineering, Pohang University of Science and Technology ink concentration and the coating, printing, imprinting, and
(POSTECH), Pohang 37673, Republic of Korea; Department of annealing conditions, to derive Ag architecture that has
Chemical Engineering, Pohang University of Science and Technology
tunable photonic functionality. As an example, we demon-
(POSTECH), Pohang 37673, Republic of Korea; POSCO-POSTECH-RIST
strate polarization-sensitive reflective color filters that exploit
Convergence Research Center for Flat Optics and Metaphotonics,
Pohang 37673, Republic of Korea; and National Institute of shape-tunable Ag nanostructures fabricated by soft nano-
Nanomaterials Technology (NINT), Pohang 37673, Republic of Korea, imprinting using a controllably-stretched elastomer mold.
E-mail: jsrho@postech.ac.kr. https://orcid.org/0000-0002-2179-
2890; and Jong G. Ok, Department of Mechanical and Automotive Keywords: ionic Ag ink; nanoimprint lithography; polariza-
Engineering, Seoul National University of Science and Technology, tion-sensitive color filter; soft contact lithography; stretch-
Seoul 01811, Republic of Korea, E-mail: jgok@seoultech.ac.kr. able nanostructure; transfer printing.
https://orcid.org/0000-0002-8703-2915
Minwook Kim, Jungkeun Song and Wonjun Lee, Department of
Mechanical and Automotive Engineering, Seoul National University of
Science and Technology, Seoul 01811, Republic of Korea,
1 Introduction
E-mail: fortune_leaf@seoultech.ac.kr (M. Kim). https://orcid.org/
0000-0002-8979-9976 (M. Kim) The silver (Ag) nanostructures have unique optical
Dong Kyo Oh, Minsu Jeong and Hongyoon Kim, Department of tunability and superior applicability to both laboratory and
Mechanical Engineering, Pohang University of Science and industry, and therefore have been widely used in photonic
Technology (POSTECH), Pohang 37673, Republic of Korea, devices such as color filters [1, 2], surface-enhanced Raman
E-mail: dkoh93@postech.ac.kr (D.K. Oh).
spectroscopy [3–7], and plasmonic sensors [8–11]. However,
https://orcid.org/0000-0001-7025-6720 (D.K. Oh)
Jeong Dae Kim, Department of Mechanical and Automotive common nanofabrication protocols to create Ag nano-
Engineering, Seoul National University of Science and Technology, structures generally involve high-vacuum-assisted deposition
Seoul 01811, Republic of Korea; and Etch Team, SEMES Co., Ltd., and etching processes, which are expensive and slow, and
Cheonan, Chungcheongnam-do 31040, Republic of Korea, therefore obstruct scalable and practical fabrication of the
E-mail: 17510100@seoultech.ac.kr. https://orcid.org/0000-0002-
photonic and other functional devices that use Ag nano-
3644-1698
Chunghwan Jung, Department of Chemical Engineering, Pohang
structures [12–14]. To overcome this problem, mechanical
University of Science and Technology (POSTECH), Pohang 37673, nanopatterning processes that centrally include nanoimprint
Republic of Korea lithography (NIL) have been adopted for the facile fabrication
Open Access. © 2022 Minwook Kim et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0
International License.
2694 M. Kim et al.: Stretchable Ag nanostructures by soft-contact patterning of ionic Ag solution
of various Ag nanostructures [15–19]. NIL can readily replicate transferring hexagonal or square pillar arrays (EULITHA) in SiO2 sub-
high-resolution nanopatterns by pressing the mold into a strate onto PDMS (Sylgard 184, Dow Corning) layers, then curing for 1 h
at 70 °C. The surface of the PDMS nanohole mold was treated with an
polymer resin [20–22]. Moreover, the transferred nano-
anti-sticking agent ((tridecafluoro-1,1,2,2-tetrahydrooctyl)tri-
structures can be tiled to cover large areas and can also be used chlorosilane, Gelest) (Figure S1 in Supplementary Material), then the
as another mold or template [23, 24], so processes that use NIL PDMS ‘nanopillar’ mold was prepared by transferring the nanohole
are very favorable for high-throughput, highly reproducible, pattern to another PDMS layer under the identical condition. The PDMS
and scalable fabrication of nanostructures without resorting to mold thickness was controlled to be 3–4 mm to ensure uniform printing
and imprinting, and reliable stretching. An additional degassing pro-
high-vacuum processing and optical lithography [25–27].
cess with a low vacuum might help to minimize defects (e.g., air bub-
An alternative strategy to fabricate Ag nanostructures
bles) of the PDMS pattern mold.
uses a simple coating of solutions that include ionic Ag or Ionic Ag ink (TEC-CO-011, InkTec Co., Ltd.) was diluted in iso-
dispersed Ag nanoparticles (NPs), then applies mild propyl alcohol (IPA, 99.5%, Daejung Chemicals & Materials Co., Ltd.) at
annealing [28–30], which is facile and scalable. Also, resin controlled volume ratios (10–50 vol% of Ag ink in IPA) by sonication for
with embedded NPs has been developed for the NIL-based 10 min, then filtered through a PTFE membrane (0.2 µm pore size,
Advantec).
fabrication of photonic devices with controlled optical
properties; the refractive index n and the extinction coef-
2.2 Pre-treatment of interfaces for successful transfer of
ficient k of the NIL resins can be modulated by controlling
Ag nanopatterns
the types and concentrations of NPs mixed with the poly-
meric bases [31–34]. The resin that includes Ag NPs can
The work of adhesion (W a ) between two interfaces can be calculated
also be engineered to form functional nanostructures by
by harmonic-mean equation [37, 38], given by:
using continuous mechanical inscribing techniques [28,
35, 36]. However, the production of resin that includes well- γd γd γp γp
W a = 4( d s1 s2d + p s1 s2p ), (1)
γs1 + γs2 γs1 + γs2
dispersed NPs is a challenging task, and the resolution of
the NIL-shaped structures obtained by Ag NPs in the resin where the subscript s is for the interface of solid and the superscripts
can be limited by the volume of NPs and NP agglomerates. d and p are for the dispersive and polar components of the surface
Here, we demonstrate shape-stretchable Ag nano- tension γ, respectively. At this point, surface tension is determined by
the geometric-mean equation [37, 38], given by:
structures that can be obtained using soft-contact patterning
1 1
of a layer of ionic Ag ink. The layer of ink can be either printed (1 + cos θl )γl = 2(γds γdl )2 + 2(γps γpl )2 , (2)
or imprinted by using the polydimethylsiloxane (PDMS)
where θ is for the contact angle of the liquid on the solid and subscript
molds, then readily converted to Ag nanostructures by mild
l and s are for the liquid and solid components of the γ, respectively.
thermal annealing. The initial state of the Ag-containing ink By solving the simultaneous equations about the two different liquids
is an ionic fluid, so the Ag nanostructure can be readily of which know the dispersive and polar surface tensions, typically
printed or imprinted with controllable residual layers and in deionized (DI) water and diiodomethane, we can decide dispersive
high resolutions that are not limited by the size of NPs or and polar surface tensions of various interfaces and calculate the W a
their agglomeration. We systematically investigate the between different interfaces.
The surface of PDMS molds was treated with an anti-sticking agent
optimal processes for patterning the ionic Ag nanostructures
((tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane, Gelest) to
from resin that includes Ag NPs by controlling the initial ink reduce W a to the Ag layer. In addition, O2 plasma treatment was
concentration and its coating, printing, imprinting, and applied to the surface of the target substrate to strengthen the adhesion
annealing conditions. The controlled stretching of flexible to the Ag layer (Figure S1 in Supplementary Material).
elastomer molds enables facile fabrication of shape-
stretchable Ag nanostructures with tunable photonic char- 2.3 Fabrication of Ag nanostructures by soft contact-
acteristics. As an example, we demonstrate a polarization- based transfer printing
sensitive reflective color filter, in which the shape-stretchable
Ag nanodot pattern induces optical reflectance at a specific The ionic Ag inks with various concentrations (i.e., 10, 20, and 30 vol%)
wavelength that shifts depending on the strain applied to the were spin-coated on pre-cleaned general-purpose slide glass
mold during the soft contact of the NIL process. (HSU-1000412, Paul Marienfeld GmbH & Co. KG; ‘glass’ hereafter) at
100 rpm for 20 s, and then at 300 rpm for 10 s. The PDMS nanohole mold
was then gently attached to the substrate that had been coated with ionic
2 Experimental section Ag ink, for the selective adsorption of the ionic Ag ink on the top surface
of the nanohole mold. The ionic Ag-inked PDMS mold was carefully
2.1 Materials detached from the glass substrate, then brought into contact with the
target substrate. The soft contact between the substrate and the PDMS
The flexible molds that bore micro- or nanoscale hexagonal or square mold was maintained for 20 s at 90 °C, under contact pressures of
hole arrays (nanohole molds) with various periods were fabricated by 0 (‘put’), 2, 5, or 10 kPa. Specifically, the pressurized force was precisely
M. Kim et al.: Stretchable Ag nanostructures by soft-contact patterning of ionic Ag solution 2695
controlled on the balance and contact pressures were calculated by Figure 5 for the coordinate), using n = 1.462 for the SiO2 substrate.
dividing the applied force by the uniform contacting area of soft mold. Contact angles on various substrates and the Ag layer were precisely
Then the mold was carefully detached from the Ag pattern-printed measured by the contact angle meter (Smart drop, Femtofab, Korea).
substrate, which was then annealed for 300 s at 180 °C to reduce the
metallic Ag nanohole nanostructure.
3 Results and discussion
2.4 Fabrication of Ag nanostructures by soft NIL
The proposed soft-contact mechanical patterning processes
First, 50 vol% ionic Ag ink was spin-coated on the glass substrate at (Figure 1) use a flexible PDMS mold and an ionic Ag ink in two
coating speeds of 2000, 3000, or 4000 rpm. The Ag ink-coated sub-
main steps: transfer printing and soft NIL. During transfer
strate was then soft-baked at 70 °C for 0, 1, 2, 3, or 4 min, then cooled to
room temperature (RT). The PDMS nanopillar mold and the soft-baked printing, the Ag ink concentration and the transfer contact
Ag-ink-coated substrate were first adhered to each other by gently pressure strongly influence the structural morphology and
rubbing by using a roller, then subjected to NIL at gentle pressure optical properties of the final Ag nanostructure. During soft
(∼94 Pa) during baking for 5 min at 180 °C. The imprinted Ag nanohole NIL, the initial conditions (i.e., thickness, viscosity) of the
structure was completed by demolding and cooling to RT.
soft-baked Ag layer have a strong effect on the subsequent
To fabricate the stretchable Ag nanodot array with increased
thickness as photonic structures, the PDMS nanohole mold was
NIL result, which can be adjusted by controlling Ag con-
stretched by applying controlled strain, then fixed using a heat- centration and the coating speed of and soft-baking time
resistant polyimide tape on a dummy wafer. Then 0.2 ml of undiluted (SBT) of Ag ink.
ionic Ag ink was drop-cast on the glass substrate, then the stretched The transfer printing route (Figure 1a1–c1) can create
mold was brought into contact with the ink layer. With the contact Ag nanohole structures by transfer printing of an ionic Ag
maintained, the mold-substrate assembly was put in a low-vacuum
ink on the desired substrate and subsequent thermal
chamber (∼0.7 bar) for 5 min to facilitate infiltration of the Ag ink into
the nanohole openings in the mold, and to help minimize the residual annealing, by using the PDMS nanohole mold (hole
layer in the contacted region, then subjected to NIL at 1 MPa at 110 °C diameter 800 nm, period 1.2 µm). As described in the
for 15 min, then demolded and cooled to RT. Experimental section, during the transfer printing process,
the Ag ink that coated the top surface of the mold was
2.5 Characterization transferred to the substrate under adequate contact pres-
sure (∼2 kPa), so essentially no residual layer remained
The scanning electron microscopy (SEM) images were taken using a (Figure 2a and b). Here, the ionic Ag ink concentration
JEOL JSM-6700F field-emission SEM (typical operation voltage: determines the thickness, morphology, and pattern quality
10 kV) after sputtering a thin Pt film with 2–3 nm thickness to prevent of the resulting Ag layer. If the concentration of the ionic Ag
electron charging. Optical transmittance and reflectance measure- ink is too low, the transferred Ag structure has insufficient
ments were obtained using a UV-VIS Spectrophotometer (UV-2600-
Ag ions and is too porous and thin to form a continuous Ag
Plus, Shimadzu, Japan), using bare glass as a reference. Reflection
spectra were simulated using a commercial FDTD software (Lumer-
layer that has a clean pattern profile. In contrast, if the
ical FDTD), with periodic boundary conditions along the x and y axes concentration of the ionic Ag inks is too high, the excessive
and a perfectly-matched layer along the z-axis (see the markup in Ag material leaks into and fills the opening area (Figure 2c).
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.
2696 M. Kim et al.: Stretchable Ag nanostructures by soft-contact patterning of ionic Ag solution
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 con-
tact 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.
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 (Experi-
mental 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
Figure 2: Ag nanohole patterns fabricated by transfer printing of nanoporous morphology in the resulting Ag nanostructure
ionic Ag ink. SEM images of Ag nanostructures fabricated by transfer increases as SBT increases. This trend is consistent with a
printing of ionic Ag ink diluted with (a) 10, (b) 20, (c) 30 vol% in previous investigation, which showed that the soft-baking
isopropyl alcohol (IPA). (d) Optical transmittance of the Ag nanohole
process initiates the reductive texturing of an ionic Ag ink
patterns fabricated by using the ionic Ag inks with various
concentrations. (e) Optical photograph of the Ag nanohole structure
layer into a nanoporous structure [29], and that the reso-
transfer-printed on the glass substrate by using a 20 vol% ionic Ag lution of this texturing increases as the thickness of the Ag
ink. All scale bars: 2 µm. layer decreases. The residual layer thickness also increases
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.
M. Kim et al.: Stretchable Ag nanostructures by soft-contact patterning of ionic Ag solution 2697
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.
as SBT increases (Figure S3 in the Supplementary Material). (Experimental Section) can achieve highly uniform and
We used these findings to design an Ag nanostructure with faithful fabrication of Ag nanostructures (Figure 4). Moving
specific photonic functionality by controlling the coating forward, the use of flexible elastomeric PDMS mold enables
and annealing processes of the ionic Ag coating layer and the fabrication of shape-tunable Ag nanostructure without
by modulating the flexible mold geometry and soft NIL the use of multiple master molds [39].
process condition. The process schemes obtained a range of NIL results
Here, we demonstrate one practical application: shape- (Figure 5) by using the 600H mold stretched from 0 to 20%
stretchable Ag nanodot structures can be easily fabricated strain. Using the unstretched 600H mold (Figure 5a), a cir-
by performing soft NIL using a stretched PDMS nanohole cular Ag nanodot pattern was cleanly formed with a negli-
mold (either 600-nm-period hexagonal array (600H) or gible residual layer (Figure 5c). On the contrary, by using the
300-nm-period square array (300S), both with a 1:1 ratio of 600H mold stretched to 20% strain during the soft NIL
hole diameter to hole interspacing). Despite the scaling process (Figure 5b), the Ag nanodot array can be readily
down to the nanoscale resolution, the soft NIL of the ionic turned into elliptical Ag patterns (Figure 5d). This practical
Ag ink by using the modified processing conditions 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 polari-
zation 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 Figure 6: Optical reflectance of Ag nanodots fabricated by soft NIL
corresponding structures. Scale bars: (c) and (d) 2 µm, (inset) by using the 300S PDMS nanohole mold stretched with various
400 nm. strains, in X- or Y-polarized light.
2698 M. Kim et al.: Stretchable Ag nanostructures by soft-contact patterning of ionic Ag solution
A simulation of the 300S pattern (Figure S4) indicates limited to the display components, transparent electrodes,
that the stretchable Ag nanodot structures can function as metasurfaces, and plasmonic sensors [42].
polarization-sensitive reflective color filters, depending on
the stretch degree. Optical reflectance measurements were
Acknowledgments: We thank Hyunsoo Chun, Mingyu Kim,
obtained from unstretched 300S Ag nanodot arrays and the
Min Cheol Kim, Yongju Lee, and Wonseok Lee for their
same arrays after stretching to 20% strain, for incident light
assistance with the prior experiment and additional char-
that had been polarized to 0° and 90° with respect to the
acterization processes. D. K. O. acknowledges the Hyundai
stretch axis (Figure 6). Notably, the reflectance peak for the
Motor Chung Mong-Koo fellowship.
0° linearly-polarized light was red-shifted when the Ag
Author contributions: J. G. O., J. D. K., D. K. O., and M. K.
nanodot was stretched. On the contrary, the 90° linear
conceived the principle and designed the research, with
polarized light generated no measurable shift of the peak
useful advice from J. R. for simulation and characterization.
position, because of the almost invariant period in the
M. K., D. K. O, and J. D. K. conducted most of the experiments
y-direction. Although this strategy has not been fully
and characterizations. M. J., H. K., and C. J. contributed to
optimized here, it provides a pragmatic insight into appli-
the device design and optical simulation. J. S. and W. L.
cations of strain-dependent fabrication of stretchable
contributed to the process development and parametric
Ag nanostructures that have tunable optical properties
experiment. M. K., D. K. O., and J. G. O. wrote the manuscript
without the use of multiple mold prefabrication and use of
with all authors’ assistance with figure preparation and
high-vacuum and etching processes [41].
consistent discussion. All authors read and approved the
final manuscript.
Research funding: This work was supported by grants from
the National Research Foundation of Korea (NRF) funded
4 Conclusions
by the Korean Government (No. 2020R1F1A1073760
(Ministry of Science and ICT (MSIT)). J.R. acknowledges
We have demonstrated two practical methods to fabricate
the NRF grant (NRF-2021M3H4A1A04086554) funded by
Ag nanostructures by using soft-contact mechanical
the MSIT of the Korean government, and the POSTECH-
patterning of ionic Ag ink coating. Transfer printing can
Samsung Electronics Research Center program (IO201215-
directly replicate nanopatterns with no residual layer, but
08187-01) funded by Samsung Electronics. M. K.
contact pressure is critical to creating clean shapes of
acknowledges the financial support from Korea Institute
nanopatterns. In the transfer printing method, the Ag
for Advancement of Technology (KIAT) grant funded by the
nanostructure could be transferred onto the desired sub-
Korea government (MOTIE) (P0002092, The Competency
strate by depositing ionic Ag ink on the PDMS mold,
Development Program for Industry Specialist). H.K.
ensuring conformal contact and adequate pressure of the
acknowledges the POSTECH Alchemist fellowship.
mold to the substrate, then thermally annealing the ionic
Conflict of interest statement: The authors have no
Ag layer. On the other hand, soft NIL can control residual
competing interests.
layer thicknesses according to additional processing con-
ditions, which induces variable optical properties of
nanopatterns. In the subsequent soft NIL process, the References
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