Flexible Plasmonic Color Filters Fabricated via Nanotransfer Printing

Letter www.acsami.org

Flexible Plasmonic Color Filters Fabricated via Nanotransfer Printing with Nanoimprint-Based Planarization Boyeon Hwang,†,‡ Sang-Ho Shin,†,‡ Soon-Hyoung Hwang,§ Joo-Yun Jung,‡ Jun-Hyuk Choi,‡ Byeong-Kwon Ju,*,† and Jun-Ho Jeong*,‡ †

School of Electrical Engineering, Collage of Engineering, Korea University, Seoul 02841, Republic of Korea Nanomechanical Systems Research Division, Korea Institute of Machinery and Materials (KIMM), Daejeon 34103, Republic of Korea § Research Institute of Advanced Materials (RIAM) Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Republic of Korea ‡

S Supporting Information *

ABSTRACT: We investigated the preparation and performance of largearea transmission-type flexible plasmonic color filters (PCFs). These large-area PCFs were fabricated based on a nanotransfer printing (nTP) process that involves nanoimprint-based planarization. This process is a simple surface treatment for easy transfer of a metal to a flexible plastic substrate and formation of patterned aluminum nanodots and nanoholes on a substrate surface with poor roughness. Rabbit-ear structures can form during the nTP process, and this phenomenon was analyzed by numerical simulation. As defects were not detected in a 10 000-round bending test, the PCFs fabricated using this nTP process have excellent mechanical properties. KEYWORDS: surface plasmon resonance, flexible plasmonic color filter, nanotranster printing, large-area nanopatterning, surface planarization

S

nanoimprint process,9 laser interference lithography,10 the use of block copolymers,11 and nanosphere lithography,12 have drawn more recent attention; however, these processes suffer from plasma or gas damage because of the metal etching process and require the application of several additional chemical treatments. In our study, we used the nanotransfer printing (nTP) process instead, and nanostructures were easily fabricated by our method. The nTP process relies on chemical bonding interactions between the substrate and ink, which improve covalent bonding between the substrate and metallic film, thereby minimizing adhesion of the stamp to the metal.13 Moreover, since a master can be fabricated, multiple copies can be reproduced either by using a UV-curing resin on an inexpensive substrate, such as polyethylene terephthalate (PET) or poly(methyl methacrylate (PMMA), or by using an elastomer, such as polydimethylsiloxane (PDMS). Similarly, in template stripping, a Si substrate with an ultraflat surface is used as a template, such that the metal formed on the substrate also has an ultraflat surface.14 However, although template stripping is limited to Si substrates, nTP process can be extended to various template substrates, regardless of roughness. As the nTP process is an outstanding and simple fabrication process, it

urface plasmon resonance (SPR) is related to the electron oscillations that occur when light of a constant wavelength is incident on a nanoscale metal thin film and dielectric surface at a specific angle. Many universities and research institutions have been involved in the development of SPR technology in recent years. To date, SPR technology has been applied to various fields such as biosensors and display applications, including organic light-emitting diodes (OLEDs), photovoltaics,1−3 and optical security applications.4 In particular, this technology can be utilized for complementary metal− oxide−semiconductor (CMOS) image sensors in plasmonic color filters (PCFs).5 Metals are used in PCFs to generate SPR, but gold and silver have the disadvantage of being more expensive than other metals. Moreover, optical loss in gold is caused by interband transitions below 500 nm, and silver is readily oxidized.6 Therefore, gold and silver are disadvantageous for the fabrication of PCFs with SPR in the visible region. On the other hand, aluminum, which is less expensive than other metals, has the advantage of being able to generate SPR from the visible region to the UV region. The focused ion beam (FIB) method7 and electron-beam (e-beam) lithography8 are the most widely used nanopatterning methods in the manufacture of plasmonic devices. However, as these methods are limited to a small patterning area and the processing time is long, they are quite expensive and unfavorable for mass production. Therefore, alternative techniques, such as the © XXXX American Chemical Society

Received: May 4, 2017 Accepted: August 8, 2017 Published: August 8, 2017 A

DOI: 10.1021/acsami.7b06228 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic of a dot-type PCF (left) and a hole-type PCF (right). (b) Optical image of a dot-type replica deposited on Al (thickness: 30 nm) with pattern period of 300 nm (area: 10 cm × 10 cm). Images of (c) hole-type PCF (period: 300 nm), (d) dot-type PCF (period: 350 nm), and (e) dot-type PCF (period: 400 nm). (f) Schematic of the fabrication of a dot-type PCF by using the nTP process. For the fabrication of a hole-type PCF, a replica was fabricated from the dot-type PCF (Figure S1).

Figure 2. Measured transmission characteristics of (a) dot-type PCFs and (d) hole-type PCFs with pattern periods between 280 and 430 nm. SEM images of Al patterns in hexagonal arrays after transfer to the PET substrate for (b) dot-type PCFs and (e) hole-type PCFs; the insets show SEM images of the PCFs tilted at 30°. Microscope images of the colors generated by (c) the dot-type PCF and (f) hole-type PCF with pattern periods between 280 and 430 nm.

surface roughness of plastic flexible substrates such as PET films is higher than that of Si and glass substrates. Without any treatment, the PET substrate had poor adhesion to nanometallic film and low transferable quality. However, we improved the roughness through simple surface treatment of

can be used to manufacture sensors, field-effect transistors (FETs), metamaterials, and other nanoelectonic devices.15−18 In addition, nTP process can provide large-area patterning while maintaining nanoscale resolution, allowing for the easy fabrication of patterns of 2D-multilayer nanostructures.19 The B


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Figure 3. Variation in the calculated transmittance with the length of the rabbit-ear structure for (a) dot-type PCFs and (b) hole-type PCFs, and with the thickness of the rabbit-ear structure for (c) dot-type PCFs and (d) hole-type PCFs. (e) SEM images of the dot-type PCF fabricated by the etch process. (f) Comparison of experimental and calculated transmittance spectra of PCFs fabricated by the nTP process and the etch process.

the plastic substrate and easily fabricated an Al-based flexible PCF using the nTP process on a large-area substrate. In our study, flexible PCFs were fabricated by using the nTP process to transfer a pattern of Al nanodots or nanoholes onto a PET substrate. Figure 1c shows the fabrication procedure of the dot-type PCF. One feature of the dot-type PCF is its drastically reduced transmittance at the resonant frequency owing to the extraordinary low transmission (ELT) phenomenon,20 which is the opposite of the well-known extraordinary optical transmission (EOT) phenomenon. The EOT phenomenon refers to the rapid increase of transmittance at a specific wavelength in a hole, whose diameter is smaller than the wavelength, in a metallic film composed of an array of holes or a lattice structure.21,22 The EOT phenomenon is determined by the wavelength and polarization of the incident light, as well as the aperture size, shape, and array pattern on the metal thin film.23 The ELT phenomenon is similar to the EOT phenomenon, as it occurs when the nanostructure has a constant period that is approximately equal to or less than the skin depth. The suppressed transmission is related to the resonance anomaly (Wood’s anomalies). The transmittance decreases below the optical skin depth because each localized

surface plasmon resonance (LSPR) is caused by the building block of the metal lattice structure.24−26 Consequently, the light is blocked at a specific frequency, resulting in nearly zero transmittance.27 The ELT phenomenon changes with the geometry of the nanostructures, and the transmittance can be arbitrarily adjusted in various wavelength ranges in the visible region.28 We performed a finite-difference time-domain (FDTD) simulation using a software package (FDTD Solutions, Lumerical Solutions Inc., Canada) to predict the optimized transmittance and color change in each period in the nanopattern. Figure S2 shows the transmittance of light by the PCFs in each period and the electric-field distribution at a specific wavelength. All of the nanostructures consisted of an Al layer (thickness: 30 nm) on a PET substrate. The refractive index of the PET film was set at 1.5729 for the simulation, and each nanopattern was arranged in a hexagonal array. We measured the PCFs of dot and hole structures with a total of 16 pattern domains, whose periods varied from 280 to 430 nm in steps of 10 nm. Each filter had a footprint of 200 × 200 μm2. The transmittance and microscope images of the fabricated PCFs are shown in Figure 2. When Al was vertically deposited C


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Figure 4. AFM images of the (a) bare Si wafer, (b) pristine PET substrate, and (c) PET substrate after treatment to reduce its surface roughness.

with the thickness was calculated using an FDTD simulation, and the results are shown in Figure S4a. For hole-type PCFs with a pattern period of 300 nm, the transmittance increased as the dielectric thickness increased from 30 to 120 nm. As the thickness of the SiO2 layer increased, the transmittance curve showed a slight red shift and the peak intensity increased when compared with the reference. However, the transmittance began to decrease when the dielectric thickness exceeded 70 nm. This increased transmittance in the 30−70 nm range was attributed to the field enhancement caused by SP matching between the nanometallic film and the SiO2 dielectric layer. Nanoimprint-based planarization was performed before transferring the Al nanopattern to the plastic substrate. In order to improve the surface roughness of the PET substrate, a UV-curable resin was first spread on the PET substrate, and the PET substrate was then thinned and flattened with a roller by resting the former on a material with low surface roughness, such as a Si wafer. After UV treatment, the Si wafer was removed and UV curing was performed again, and the surface topography of the obtained UV-cured resin layer imitated that of the Si wafer. Figure 4a−c shows atomic force microscope (AFM) images of the surface roughness of a PET substrate treated with a UV-cured resin and an untreated PET substrate. The surface roughness of the Si wafer was measured to be about 0.04 nm, whereas the surface roughness of the PET substrate was measured to be about 5 nm. In addition, the PET substrate had many protruding parts in certain regions. However, the surface roughness of the substrate that was treated with the UV-cured resin was improved to about 1 nm, and the surface was very flat, similar to the Si wafer. The thickness of the adhesion layer used in this experiment was about 5−10 nm. As the PET substrate had a larger spot size than the adhesion layer, it was difficult to transfer the metal because the irregular surface was not properly coated. However, when the UV-curable resin was used for surface treatment, the protruding spots were covered with the resin, improving the coating property of the adhesion layer and hence the transfer characteristics. Bending tests were performed to evaluate the mechanical durability of the PCFs by measuring the strain with bending. The bending tester is described in Figure S5. The strain values can be expressed by the following equation32

on the replica, a certain amount of Al accumulated on the sidewall of the pattern. When this replica was transferred to the substrate, the Al deposited on the sidewall was also transferred, retaining the shape acquired during the nanoimprint process and resulting in a so-called “rabbit-ear” structure with a protruding thin film,30,31 as shown in Figure 2b. When the rabbit-ear structure was generated, a shift in the resonance frequency occurred, which affected the transmittance and changed the color of the transmitted light. To determine the characteristics of the rabbit-ear structure, we used the FDTD simulation to calculate the transmittance along the length of the rabbit-ear structure. The simulation results are shown in the Figure 3. To investigate the effects of varying the length of the rabbit-ear phenomenon in the dot-type PCFs, we fabricated these structures using the nTP process and etch process on glass substrates, as shown in Figure 3c. The dot-type PCF with a 30 nm Al layer fabricated using an etch process exhibited a peak near 500 nm, but the PCF with a 50 nm Al layer fabricated using an etch process exhibited a peak near 450 nm, which is similar to the results obtained for a dot-type PCF with a 30 nm Al layer fabricated using the nTP process. The FDTD simulation also closely matched the experimental results. As the length of the rabbit-ear structure increased in the dot-type PCF, the resonant frequency shifted to shorter wavelengths and the peak near 450 nm decreased in intensity. Similarly, the FDTD simulation was performed for changes in transmittance with the thickness of the rabbit-ear structure. However, the dottype PCF showed no significant change in transmittance, even when the rabbit-ear thickness was in the range of 20−50 nm. The rabbit-ear shape is affected by its length and is simply an effect of increasing the thickness of the nanopattern. In other words, the E-field distribution obtained through the rabbit-ear phenomenon is similar to the effect of simply increasing the thickness of the Al nanodots. On the other hand, in hole-type PCFs, as the thickness of the rabbit-ear increased, the transmittance decreased, but the resonance frequency remained constant. The surface plasmon (SP) matching between Al and air is poor, leading to weak LSPR and low transmittance. Figure S2b shows that the E-field distribution between the holes was more weakly concentrated in some regions than the distribution in the absence of the rabbit-ear structure. However, the fabricated PCF did not have uniform rabbit-ear structures. In addition, the length of the rabbit-ear structure was only about twice the thickness of the nanopattern because of the vertical deposition process. The variation in transmittance with the incident angle using dottype and hole-type PCFs with a period of 300 nm is shown in Figure S3. We attempted to optimize the EOT phenomenon by using an additional SiO2 layer. We investigated the change in transmittance when dielectric layers of various thicknesses were deposited on the hole-type PCFs. The change in transmittance

strain (%) =

thickness of PET + thickness of Al 2R c

where Rc is the bending radius of the film. In other words, the change in bending radius is due to the change in strain, with a smaller bending radius corresponding to a larger strain applied to the substrate. First, the bending radius of each dot-type PCF and each hole-type PCF was set to 0.2 cm, and then bending was repeated 1000 times, then 5000 times, 10 000 times, and finally 50 000 times. Next, the bending radius was sequentially D


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Figure 5. Measured transmittance of (a) dot-type PCFs and (b) hole-type PCFs after repeated bending cycles with different bending radii. (c−h) SEM images of the dot-type PCFs after bending tests and the reference dot-type PCF; the pattern period of the PCFs is 300 nm (scale bars: 1 μm).

adjusted to 0.75, 0.5, and 0.2 cm, and the transmittance was measured after the test was repeated 10 000 times. No change in transmittance was found when compared to bending reference device. The transmittance spectra during the bending tests for each PCF are shown in Figure 5a, b. When we looked at the external shape of the PCF, we could see that the substrate was warped as a result of the repeated tests, but defects, such as disintegration of the pattern, were not found. The PCFs were also slightly warped after the repeated tests, but defects, such as tears in the pattern, were not found. Figure 5c− h shows SEM images obtained after the bending test. The dottype PCFs showed resilience to cyclic bending because each unit of the nanopattern is separate. Because of the good ductility and malleability of Al, cracking did not occur, despite the repetitive bending tests. In summary, we demonstrated the successful preparation of large-area flexible color filters simply by using the nTP process. We fabricated 2D PCFs using nanodot and nanodot hole patterning. Line-type 1D structures typically show higher transmittance than 2D structures, but have issues related to polarization.33 Although 1D structures are intentionally used to change color through polarization, we attempted to fabricate polarization-independent PCFs. Compared with PCFs manufactured using conventional nanolithography, our PCFs were fabricated in a large area of ∼5 cm2 or more, which is much bigger than the small areas of previous plasmonic devices. Our plasmonic devices with metal nanopatterning could be obtained through simple processes, such as stamping, spin-coating, and metal evaporation, without complex processes, such as photolithography and dry etching. The fabricated nanopattern arrays showed high precision and high regularity. In addition, the nTP-based PCFs could be produced in a roll-to-roll process, which improves productivity and increases their feasibility for mass production. The surface roughness of the PET film was reduced to that of a Si wafer by applying a UVcuring resin on the PET film with high surface roughness. As a result of the decreased surface roughness, the transfer characteristics of the metallic film were improved. Light

transmittance and the generation of various colors could be realized by adjusting the thickness of the metal thin film and its structure. Despite the generation of a rabbit-ear structure, which is a limitation of the nTP process, the PCFs showed almost the same transmission characteristics as those predicted by simulations. Even though the transmittance varied according to the incident angle, if the angular dependence can be removed by changing the structure,34 PCFs can be fabricated using the same process. Notably, even after repetition of the cyclic bending test 10 000 times, defects were not generated in the metallic film, indicating that the PCFs are mechanically stable. We expect our findings to be useful for the development of flexible displays and full-color hologram devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06228. Detailed fabrication process (Figure S1), calculated transmission characteristics (Figure S2), transmittance spectra of PCFs according to incident angle (Figure S3), transmittance of PCFs with SiO2 layers (Figure S4), photographs of the bending tester (Figure S5) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jun-Ho Jeong: 0000-0003-0671-0225 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Center for Advanced MetaMaterials (CAMM) funded by the Ministry of Science, ICT E


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and Future Planning as a Global Frontier Project (CAMM-No. 2014M3A6B3063707). It was also supported by the Industrial Strategic Technology Development Program (10052641) funded by the Ministry of Trade, Industry & Energy (MI, Korea).

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Flexible Plasmonic Color Filters Fabricated via Nanotransfer Printing

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