Eight Inch Wafer-Scale Flexible Polarization-Dependent Color Filters

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Article

8 inch-wafer scale flexible polarization-dependent color filters with Ag-TiO composite nanowires 2

Zhi-Jun Zhao, Minpyo Lee, Hyeokjung Kang, SoonHyoung Hwang, Sohee Jeon, Namkyoo Park, Sanghu Park, and Jun-Ho Jeong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02128 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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ACS Applied Materials & Interfaces

8 inch-wafer scale flexible polarization-dependent color filters with Ag-TiO2 composite nanowires

Zhi-Jun Zhaoa,b#, Minpyo Leec#, Hyeokjung Kangb, SoonHyoung Hwangb,d, Sohee Jeonb, Namkyoo Parkc, Sang-Hu Parka* and Jun-Ho Jeongb* a

School of Mechanical Engineering, Pusan National University, Busandaehak-ro 63beon-gil,

Geumjeong-gu, Busan 609-735, Republic of Korea b

Department of Nano Manufacturing Technology, Korea Institute of Machinery and

Materials, Daejeon 305-343, South Korea c

Department of Electrical and Computer Engineering, Seoul National University, Daehak-

Dong, Gwanak-Gu, Seoul 151-744, Korea d

Research Institute of Advanced Materials (RIAM), Department of Materials Science and

Engineering, Seoul National University, Daehak-Dong, Gwanak-Gu, Seoul 151-744, Korea # Equal contribution * Corresponding author: Prof. Sang-Hu Park Tel.: +82-51-510-1011; Fax: +82-51-514-0685 E-mail: [email protected] Dr. Jun-Ho Jeong Tel.: +82-42-868-7604; Fax: +82-42-868-7123 E-mail: [email protected]

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Abstract In this study, 8 inch-wafer scale flexible polarization-dependent color filters with Ag-TiO2 composite nanowires have been fabricated using nanoimprint and E-beam evaporation. The filters change their color via a simple rotation of the polarizer. In addition, the color of the filter can be controlled by altering the thickness of the Ag and TiO2 nanowires deposited on the polymer patterns. Polarization-dependent color filters were realized by selective inhibition

of

the

transmission

spectra

using

plasmonic

resonance

at

the

insulator/metal/insulator nanostructure interface, which occurs at particular wavelengths for the transverse magnetic polarizations. Special colors, including purple, blue, green, yellow, and pink, could be obtained with high transmission beyond 65% by varying the thickness of the deposited Ag and TiO2 nanowires on the periodic polymer pattern under transverse magnetic polarization. In addition, a continuous color change was achieved by varying the polarization angle. Lastly, numerical simulations were implemented in comparison with the experimental results, and the mechanism was explained. We believe that this simple and costeffective method can be applied to processes such as anti-counterfeiting and holographic imaging, as well as to color displays. Keywords: polarization-dependent color filter, 8 inch-wafer scale, composite nanowires, FDTD simulation, insulator/metal/insulator.


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Introduction Nanostructured color filters have consistently been the focus of investigation because of their applications in digital displays, 1-3 high-resolution color printing, 4-5 anti-counterfeiting, and information encoding.

6-7

Various types of nanostructures based on plasmonic materials

such as Ag, Au, and Al have been studied in order to achieve diverse spectral filters.8-12 With the development of nanofabrication technologies, color generation has been achieved by the use of diverse methods, including surface plasmons and the interaction of light with metallic nanostructures, metal nanoresonators,

13-15

19-21

subwavelength holes arrays in metal films,

16-18

and plasmonic nanostructure of various designs.

metal insulator 22-26

Numerous

studies have demonstrated the improvement of transmission through thick metal films by the use of arrays with subwavelength size holes, such as nanopatches, nanoparticle arrays, and nanodisks.

27-29

So far, researchers have mainly focused on the development of one fixed

color for the formatting of color filters. These designs are based on the use of incident white light-based nanopatterning and are limited to the fabrication of hard and micro-sized substrates. Large, flexible polarization-dependent color filters that can be implemented for a variety of continuous color generations via the rotation of a polarizer laid on the incident light may be applied in various fields, such as ultra-fast, large-area flexible color displays, and dynamic holographic imaging.

30-33

In previous studies, the color filters exhibited a constant

color, and were fabricated via e-beam lithography, focused ion beam milling, etching, and other lithography techniques.34-42 The Ju group suggested a fabrication process involving a laser interference lithography step, and fabricated a large-area (25 mm × 25 mm) color filter on a glass substrate.43 A highly conductive and flexible color filter electrode by fabricating a multilayer film using an e-beam evaporator was deplovped.44-45 Recently, polarization-tuned dynamic color filters and four-fold color filters were demonstrated by Choi et al. Novtny et al.

46

and

47

, respectively. The two types of color filters were fabricated via e-beam

lithography to produce various colors depending on the polarization of the incident light. The continuum-color generation was solved by rotating a polarizer set on the source of the incident light. Therefore, to achieve large, flexible, and dynamic displays, the production of color filters is now considered important. i) it is difficult to fabricate color filter with a large square area using e-beam lithography (previous studies: 40 µm x 40 µm). ii) Flexible devices cannot be fabricated using e-beam lithography (previous studies have used glass substrates). iii) The use of e-beam lithography is expensive, and therefore, its application for mass



production is unlikely. In order to address these issues, a simple method is proposed for the fabrication of Ag-TiO2 composite nanowires as flexible polarization-dependent color filters via nanoimprint lithography and vertical evaporation. The proposed filters can achieve a continuous change in color by the rotation of the polarizer under a vertical incident white light, which is based on the selective inhibition of the transmission spectra by plasmonic resonance at the insulator/metal/insulator nanostructure interface, which occurs at special wavelengths for transverse-magnetic polarizations. In addition, different colors were fabricated by control of the thickness of Ag and TiO2 in the same incident polarization. A continuum of colors could be produced by the formatted filters and represented as a single line on a chromaticity diagram. This denotes the output of the customized color based on the polarization of the incident light. In general, conventional color filters produce only single colors, which are represented by points on the CIE 1931 chromaticity diagram. To compare with the experimental results and explain the mechanism behind the polarization of the incident light, FDTD simulations were implemented. 8 inch-wafer scale filters were fabricated on flexible substrates. The proposed method has the following advantages: i) it enables the fabrication of flexible devices with a large area; ii) nanoimprinting and e-beam evaporation can achieve high-throughput, and convenient post-processing; iii) it is cost-effective; iv) mass production of filters is relatively easy. The proposed method can be effectively applied to information encoding, color displays, steganography, and anti-counterfeiting processes.

Experimental procedure Fabrication of flexible polarization-dependent color filters Flexible polarization-dependent color filters composed of Ag-TiO2 composite nanowires were fabricated by nanoimprint lithography and e-beam evaporation. The following sequence was followed. First, a nanoimprint resin (Ormostamp, Micro Resist Technology, Germany) was coated on silicon stamps with a 100 nm width/100 nm space line pattern (Figure. 1a-b). (The reason for choosing single polymer patterns with 100 nm width × 100 nm space × 150 nm depth has been explained in the Supporting Information) Second, the stamp was covered with a polymethyl methacrylate (PMMA) film and uniformly pressed with a roller (Figure.


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1b). Third, UV-curing (Figure. 1c) for 90 s was implemented twice to achieve full polymerization of the nanoimprint resin. Then, the PMMA films complete with line patterns were detached from the silicon stamp (Figure. 1d). Finally, Ag and TiO2 were deposited on the polymer patterns, respectively (Figure. 1e-f). The transmission spectra of the fabricated samples were measured by using a spectrometer (QE Pro 6000, Ocean Optics, USA) and a rotatable polarizer set in the top of the halogen light source was used to change the polarization of the incident white light. The color images of the fabricated filters were captured by an optical microscope (Eclipse LV 100, Nikon Instruments, Inc., USA). In order to better understand the mechanism of the fabricated filters, numerical simulations were implemented using a finite difference time domain (FDTD) method (FDTD Solutions, Lumerical, Canada). The refractive index (n) and extinction coefficient (k) of Ag and TiO2 were measured as a reference for the simulations by using a spectroscopic reflectometer (ST5000_Auto200, K-MAC CO., LTD). The measured data are provided in the supporting information (Figure. S1). In this work, simulations were implemented by using a plane wave under normal TM and TE polarization. A fine mesh with the size of 4 nm was divided.

Results and discussion For a better understanding of the principle of the formatted filters and the effect of the thickness of Ag and TiO2 nanowire, 8 inch wafer-scale flexible polarization-dependent color filters composed of Ag-TiO2 composite nanowires were fabricated (Figure. 2). The schematic of the proposed filters is illustrated in Figure. 2a, which consists of polymer patterns with widths (100 nm) x space (100 nm) x high (150 nm), Ag nanowires with the thickness of a (20~50 nm) and TiO2 nanowires of thickness of b (10~60 nm), and a flexible substrate (PMMA film). The angle between the xz-planes of the fabricated filter (θ) is altered via rotation of the polarizer, which determines the polarization of the incident light. The transmission of light through the color filter can be selectively impeded at specific wavelengths depending on the value of θ in order to produce various colors. Figure. 2b displays scanning electron microscope (SEM) images of the fabricated filters and the insets display the colors produced with Ag and TiO2 nanowires of thickness 30 and 10 nm at a θ of 0o, 45o and 90o, respectively. The generated color images were observed by using an optical microscope fitted with a polarizer. By rotating the polarizer from 0o to 90o, colors of dark



gray and red were obtained as shown in Figure. 2b(i) and 2b(iii), respectively. The morphology and cross-section of the Ag-TiO2 nanowire composites were observed with a high resolution SEM-FIB (FIB; Helios Nanolab, FEI Netherlands). Figure. 2c shows the cross-sectional images of the prepared samples with Ag and TiO2 nanowires of varying thickness. The layered structures of Ag and TiO2 are shown in Figure 2c. In addition, the top layer is a Pt coating layer, to obtain better FIB cross-sectional images. The generated colors were established for the two orthogonal polarizations of TM (90o) and TE (0o). Dark gray and purple are shown in Figure. 2c(i), dark gray and light blue are shown in Figure. 2c(ii), and finally yellowish and green are shown in Figure. 2c(iii). In order to show the capabilities of the large area color filter, a display fitted with a polarizer was used to check the generated colors of the TM and TE polarization. A color filter of large area (120 mm x120 mm) and with a thickness of 20 and 30 nm for the Ag and TiO2 nanowires respectively is shown in Figure. 2d. For the TM polarization, a blue color is displayed in the Figure. 2d(i) and for the TE polarization, high transmission is observed in Figure. 2d(ii). Figure. 2d(iii) reveals the morphology of the nanowires (the scale is 5 µm). An image of the fabricated samples are provided in Figure. 2d(iv) showing their flexibility. It is expected that the flexible color filters will be applicable in flexible color displays, steganography, and anti-counterfeiting processes. The thickness of TiO2 is maintained between 10 and 60 nm at intervals of 10 nm. The measured transmission spectra for various thicknesses of the wires under different polarizations as function of θ from 0o to 90o (in steps of 15o) are provided in Figure. 3a(i) to Figure. 3a(vi). Our structure shows clear fluctuation with transmission spectra for the incident angles. This polarization-dependent color filtering is caused by the inhibition of transmission by plasmonic resonance at the TiO2-Ag-Resin nanostructure in the TM case. However, there is no plasmonic resonance in the TE case; therefore, there are arbitrary fluctuations with incident angles. For the TM polarization where θ = 90o, the filters with TiO2 at all thicknesses analyzed exhibited broad dips in their transmission and were located at λ = (a) ~520, (b) 550, (c) 570, (d) 600, (e) 625, and (f) 630 nm, respectively. We obtained results for the reflection and absorption of our fabricated sample by FDTD simulation. This result has been included in Figure S5 in the Supporting Information for reference. A red shift is observed for all nanowire thicknesses. When the thickness of the TiO2 nanowires is >40nm, a low transmission feature appears at λ = 400-500 nm, but transmission >70% does not appear until λ = 500-700 nm. The mechanism behind the broad dip features is explained in the following


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section (see Fig. 7). For TE (θ = 0o) filters composed of TiO2 with thickness of 10, 20, 30, 40, 50, and 60 nm all exhibit a broad transmission. From Figure. 3a(i) and 3a(vi), it is shown that a change in color generation was observed. This is because the effective index increases as the thickness of the TiO2 nanowires increases. In addition, at a thickness of 40, 50, and 60 nm, a peak appears at λ = 450-550 nm due to the increase in the effective index. For comparison with the results of the simulation, fabricated samples with different thickness of Ag (20 nm) and TiO2 nanowires (20 nm, 40 nm, and 60 nm) were chosen. The FDTD simulation was implemented under the same experimental conditions with polarizations varying from 0o to 90o in steps of 15o. The results of the simulations are provided in the supporting information Figure. S2(a)-i to iii. In order to easily observe the color response of the filters under different polarizations, the chromaticity coordinates of the measured transmission spectra are illustrated in Fig. 3b. Figure. 3b shows a standard CIE (International Commission on Illumination) 1931 chromaticity diagram. The chromaticity coordinates for the measured spectra of the fabricated filters for TiO2 thickness of 10 to 60 nm under different polarization angles (from θ = 0o to 90o in steps of 15o) are depicted in Figure. 3b(i) to (vi), respectively. The coordinates of the corresponding simulated spectra are displayed in the supporting information Figure. S2(b)-i to iii. The variations in color for the filters in the chromaticity coordinates are marked by the black arrows and represent the different polarizations from θ = 0o to 90o in steps of 15o. For the filter fabricated with TiO2 nanowires with a thickness of 10 nm, the customized color can be converted from yellowish toward pink when θ is changed from 0o to 90o, and these are marked by the coordinates in the diagram. Similarly, when θ is changed from 0o to 90o, the output color for filters composed of 20, 30, 40, 50 and 60-nmthick TiO2 nanowires can be changed from dark yellow to purple, yellowish to blue, yellowish to light blue, yellowish to green, and yellowish to light green respectively. The simulated spectra can provide a good reference for the measured spectra. In experimental studies factors such as fabrication defects, morphologies of Ag-TiO2 composite nanowires, deviations in the intended thickness of the nanowires deposited by the e-beam evaporator, and the use of non-parallel incident light sources are inevitable issues. Simulated experiments do not include such errors; therefore, any subtle differences between the simulated and experimental results may explained by the absence of such errors. In order to better evaluate the effect TiO2 nanowire thickness on the functionality of the polarization-dependent color filter, the transmission spectra of the filters using TM



polarization was measured and simulated by using a spectrometer and FDTD analysis. Figure. 4a shows the measured and simulated spectra of the Ag-TiO2 nanocomposite wires for the TM polarizations with the Ag nanowire thickness of 20 nm and TiO2 nanowire thickness ranging from 10 to 60 nm in increments of 10 nm. The measured and simulated spectra for the TM polarizations are shown in Figure. 4a(i) and (ii). The resonance red shift from left to right was obtained as the thickness of TiO2 nanowires increased from 10 to 60 nm. This is likely due to a subsequent increase in the effective index. Figure. 4a(ii) shows that the results from the measured and simulated spectra are in agreement. However, there are subtle differences found at the sharp transmission peak. These could be explained by the following: 1) shrinking and rounding of the nanoimprint resin caused by the e-beam evaporation during the fabrication process. To directly observe the color variation in the fabricated filters in relation to the variation in the TiO2 thickness, the chromaticity coordinates based on the measured and simulated spectra under TM polarization incidences are shown in a standard CIE 1931 chromaticity diagram, as illustrated in Figure. 4a(iii-iv). The profile of the chromaticity coordinates with different TiO2 nanowire thickness (T = 10~60nm) is indicated by the curved black arrow. The thickness of the TiO2 nanowires has an important effect on the customization of the color input. In order to understand the role of the thickness of the Ag nanowires plays, filters with different thicknesses of Ag nanowires from 30 to 50 nm in steps of 10 nm were analyzed. Figure. 4b shows the measured and simulated spectra of the proposed Ag-TiO2 nanocomposite wires for the TM polarizations, at a thickness of 30 nm for the Ag nanowires and 10 to 60 nm for the TiO2 nanowires. In general, by comparison to Figure. 4a, all transmission spectra exhibited a red shift for the TM polarization. The measured and simulated spectra for filters with Ag nanowires of thicknesses 40 and 50 nm and 10 to 60 nm for TiO2 nanowires are shown in Figure. S3. Optical images of the filters during the rotation of the polarizer from 0o to 90o for TM and TE polarizations are provided in Figure. 5.The columns are arranged in order of increasing thickness of the Ag nanowires from a range of 20 to 50 nm. The rows are arranged in order of increasing thickness of the TiO2 nanowires from 10 to 60 nm. In order to observe the continuum color of the filters Ag nanowires of thickness 20 nm and TiO2 nanowires of thickness 10 to 60nm were chosen to capture the color by a rotation of the polarizer. Figure. S4 shows the color output of the filter based on the polarization angle, the columns are arranged in order of increasing angle of the polarizer from 0 to 90o and the rows are arranged


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in order of increasing thickness of the TiO2 nanowires from 10 to 60 nm. Various high contrast colors could be adjusted by the control of the TiO2 nanowire thickness and the polarization angle (θ). Thence, our proposed flexible polarization-dependent color filters are anticipated to be applicable in the flexible displays and multi-colors optics. Figure. 6 shows prints with varying colors which are varied by changing the thickness of the TiO2 nanowires and the polarization angle (θ). The shadow mask designed in the form of letters was used to produce letters of color under the incident polarization. The letters were filled with Ag and TiO2 nanowires via e-beam evaporator. In order to observe different colored letters on the flexible substrate, TiO2 nanowires of thickness 10, 30, and 60 nm were deposited with 20nm-thick of Ag nanowires. Figure. 6a (i-ii) shows the different colors of the prints under the different polarization angles (TM = 90o and TE = 0o), respectively. Figure. 6b shows the transmission spectrum of the different prints. In the graph, three kinds of colors are displayed. The different colors were generated by using the display as a polarizer and rotating the fabricated device. Figure. 6c shows the flexible substrate of the imprint. It is proposed that such designs can be applied in multi-spectra, flexible displays, and information encoding. Finally, in an attempt to better understand the phenomenon which generates the various colors and the mechanism behind of the selective spectral dips of the transmission of the TM polarizations, finite element method (FEM) was used. Figure. 7a and 7b shows the magnitude of the electric and magnetic field via cross-sectional of a unit cell for various thickness of TiO2 at the transmission dip for the TM polarization. As shown in the figure, the local field is concentrated at the bottom part of the Ag nanowire where the wave income, which demonstrates that the inhibition of transmission was induced by resonance at the asymmetric IMI (TiO2-Ag-Resion) structure. Figure 7c shows the z-component of the magnetic field and power flows, which indicates the fundamental odd mode LSP resonance IMI structure. When the thickness of the upper TiO2 layer is below 40 nm, the modal volume is larger than that of the upper layer; hence, the effective medium should be considered. In order to validate that the LSP resonance plays an essential role in inhibiting the transmission, the LSP resonance wavelength was analytically derived. The dispersion relation of the asymmetric IMI structure for the TM wave is given by 48 =

/ + / + / , / − / − /


= −


Where , , are dielectric constants of metal, insulators, respectively; is the thickness of metallic core; and and are wavenumbers of vacuum and the LSP mode, respectively. From the dispersion relation and the width of the silver nanowire , the resonance wavelength is derived as follows: , =

2 , ( )

=

Figure. 7d shows the dispersion relation of asymmetric IMI structure with different refractive indices of the topside. Inset shows the change in resonance wavelength for the effective index of the topside. As the thickness of TiO2 increases, the effective index of the upper layer increases so that the resonance wavelength becomes longer. When the thickness of the TiO2 layer is larger than the modal volume, resonance wavelength is converged as shown in Figure. 4a.

Conclusion In summary, we have fabricated 8 inch wafer-scale flexible polarization-dependent color filters via nanoimprint lithography and e-beam evaporation, which can generate the continuum color by a rotation of the polarizer set on the incident light. This was realized by the selective inhibition of the transmission spectra by plasmonic resonance at the insulator/metal/insulator nanostructure interface, occurring at their special wavelengths for transverse magnetic polarizations, respectively. Diverse colors were obtained by controlling the thickness of the Ag and TiO2 nanowires and the polarization angle. The transmission spectrum and simulations were measured and evaluated by a spectrometer and FDTD tool, respectively. The experimental and simulated results were consistent. The large, flexible color filter with dimensions of 120 mm x 120 mm were fabricated and evaluated. This method can be employed to mass produce large, flexible filters for use in flexible color displays, information encoding, and holographic imaging.


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Supporting information. Refractive index and extinction coefficient of Ag and TiO2 thin films. Measured and simulated transmission spectra for TM and TE polarizations of Ag and TiO2 nanowires as a function of thickness. Optical image summarizing the various color outputs at different thickness of the Ag-TiO2 wire composites as a function of the angle of the polarized light for the fabricated filters. Acknowledgements This work was supported by the Center for Advanced Meta-Materials (CAMM), which is funded by the Ministry of Science, ICT and Future Planning, Korea, through the Global Frontier Project (CAMM-No. 2014M3A6B3063707).

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Figure. 1 Fabrication of large, flexible polarization-dependent color filters with composite nanowires. (a-b) nanoimprint process; (c-d) UV-curing and detachment; (e-f) deposition of Ag and TiO2.


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Figure. 2. Large, flexible polarization-dependent color filters with Ag-TiO2 composite nanowires. (a). Schematic diagram of the formatted color filters comprising of polymer patters with width (100nm) x space (100nm) x depth (150nm), Ag nanowires with thickness a,



TiO2 nanowires with thickness b, and flexible substrate (PMMA films). (b) SEM images of the fabricated filters with 30nm and 10 nm-thick Ag and TiO2 nanowires, respectively, including generated color images in the inset for different polarizations (θ). (c) Crosssectional images of the fabricated filter with 20 nm-thick Ag nanowires and 10, 30, and 60 nm thick TiO2 nanowires, including generated color images in the inset for the TM and TE polarizations. (d) The fabricated color filters with a large area (120 mm x120 mm).


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Figure. 3 Measured polarization-dependent transmission spectra and corresponding color responses. (a) Measured transmission spectra of the formatted filters with Ag-TiO2 composite nanowires of (i) b = 10 nm, (ii) b = 20 nm, (iii) b = 30 nm, (iv) b = 40 nm, (v) b = 50 nm, and (vi) b = 60 nm, respectively, for different polarization angles from θ = 0o to 90o in steps of 15o (inset images represent the change of colors for the TE and TM polarizations. (b) Chromaticity coordinates corresponding to the measured spectra for the filters from (i) to (vi).



Figure. 4 Thickness of TiO2-dependent measured and simulated transmission spectra and the corresponding color responses for the TM incident polarizations. (a) Measured and simulated transmission spectra and the corresponding color responses of a filter with 20 nm thick Ag nanowires for the TM incident polarizations. (a-i and ii) Measured and simulated


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transmission spectra (a-iii and iv) the measured and simulated color responses (b) Measured and simulated transmission spectra and the corresponding color responses of a filter with 30 nm thick of Ag nanowires for the TM incident polarizations. (b-i and ii) Measured and simulated transmission spectra. (b-iii and iv) Measured and simulated color responses.

Figure. 5 Optical micro-photographs in accordance with the different thickness of the Ag and TiO2 nanowires for the TE and TM polarizations.



Figure. 6 Color images of the imprint design depending on the different thickness of the TiO2 nanowires for TE and TM polarizations. (a) Different color images of the prints due to the different thicknesses of the TiO2 nanowires. (a-i) the measured color images under TM polarization; (a-ii) the measured color images under TE polarization. (b) Transmission spectrum of the different samples with the different TiO2 thickness of 10 nm, 30 nm, 60 nm, respectively. (c) Image of the fabricated sample (scale: 120 mm x120 mm).


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Figure. 7 Mechanism underlying the generation of various colors for different thicknesses of the TiO2 nanowires. (a) magnitude of the electric field and (b) magnetic field for a crosssection of a unit cell for various thicknesses of TiO2 nanowires at a transmission dip for TM polarization (c) z-component of the magnetic field and power flows near the TiO2-Ag-Resin structure (d) dispersion relations of the asymmetric IMI structure with different refractive indices of the upper dielectric layer, and change in resonance wavelength for effective index of the topside (inset).



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Eight Inch Wafer-Scale Flexible Polarization-Dependent Color Filters

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