All-Dielectric Full-Color Printing with TiO2 ... - ACS Publications

All-Dielectric Full-Color Printing with TiO2 Metasurfaces Shang Sun, Zhenxing Zhou, Chen Zhang, Yisheng Gao, Zonghui Duan, Shumin Xiao,* and Qinghai Song* State Key Laboratory on Tunable laser Technology, Ministry of Industry and Information Technology Key Lab of Micro-Nano Optoelectronic Information System, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China S Supporting Information *

ABSTRACT: Recently, color generation in resonant nanostructures have been intensively studied. Despite of their exciting progresses, the structural colors are usually generated by the plasmonic resonances of metallic nanoparticles. Due to the inherent plasmon damping, such plasmonic nanostructures are usually hard to create very distinct color impressions. Here we utilize the concept of metasurfaces to produce all-dielectric, low-loss, and highresolution structural colors. We have fabricated TiO2 metasurfaces with electron-beam lithography and a very simple lift-off process. The optical characterizations showed that the TiO2 metasurfaces with different unit sizes could generate high reflection peaks at designed wavelengths. The maximal reflectance was as high as 64% with full width at half-maximum (fwhm) around 30 nm. Consequently, distinct colors have been observed in bright field and the generated colors covered the entire visible spectral range. The detailed numerical analysis shows that the distinct colors were generated by the electric resonance and magnetic resonances in TiO2 metasurfaces. Based on the unique properties of magnetic resonances, distinct colors have been observed in bright field when the metasurfaces were reduced to a 4 × 4 array, giving a spatial resolution around 16000 dpi. Considering the cost, stability, and CMOS-compatibility, this research will be important for the structural colors to reach real-world industrial applications. KEYWORDS: all dielectric, metasurface, structural color, high reflection, full-color The structural color was first studied by Load Rayleigh to explain the blue sky with light scattering by nanoparticles.4 Later, Gustov Mie had extended the scattering model and successfully explained the colors of colloidal gold nanoparticles.5 Since then, the colors of metallic nanoparticles have been closely related to their structural information and thus the possibility of controlling the resonant properties has been opened up.6−9 Consequently, the recent decade has witnessed the growing interest in the structural colors of metallic nanostructures.10−14 Plasmon resonances in metal films with periodic subwavelength holes were initially used in macroscopic color hologram,15−18 full color filters,19−21 and polarizers.22 In past few years, the periodic holes have been replaced by small isolated metal nanoparticles (tens of nanometers to a few hundred nanometers) or subunits to produce different colors.23−25 Compared with typical dye or

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olors are playing very important roles in human’s daily lives due to their abilities in carrying information for both of the natural world and virtual world.1 In principle, there are three dominant mechanisms to generate colors, (i) absorbing partial light and reflecting the others; (ii) separating different color with scattering, diffraction, and material dispersion; and (iii) emitting light at particular wavelengths. The third method is the most efficient way to generate bright and pure colors. But it relies on the external power supply and cannot be used in passive devices. The first approach is the most popular mechanism within our colorful world to produce the colors of flowers, grasses, paints and so on. However, due to the material absorption and relatively low reflection, the generated colors are usually very dime and not pure. Sometimes the generation of painting colors can even cause environmental pollution or induce cancer.2 Consequently, the second technique, which directly generates colors with nanostructures, has recently been one of the most interesting scientific and engineering topics.1,3 © 2017 American Chemical Society

Received: January 19, 2017 Accepted: March 19, 2017 Published: March 19, 2017 4445

DOI: 10.1021/acsnano.7b00415 ACS Nano 2017, 11, 4445−4452

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Figure 1. Simulated reflection spectra from TiO2 based Mie resonances. (a) The tilt-view and parameters of the TiO2-based metasurface. (b) The reflection spectra versus different bottom square size length w of the block with periodic p fixed to 300 nm and thickness t fixed to 200 nm. The dashed lines indicate the ED and MD resonances. (c) The reflection spectra of TiO2 metasurfaces with w = 200, 230, 250 nm. The inset shows the detail of the resonances. (d) The electric and magnetic fields of the cross-section at the ED and MD resonances respectively in the TiO2 particles with w = 230 nm, p = 300 nm. (e) The angle-dependent reflection spectra for s-polarized incident. The inset shows the angle and polarization of the incident light. The white circles in the figure indicate the reflection peak positions for different incident angles in the experiment.

reflectivity.37,38 However, due to the nature of photonic crystals and the small refractive index contrast, these dielectric structures require large number of periods to form the bandgaps, significantly hampering the spatial resolution and the potential applications in painting, textiles, and passive displays.39,40 Therefore, it is highly important and desirable to develop a mechanism to produce bright and high-contrast colors with reasonable spatial resolutions. In this article, we demonstrate the attempt by utilizing the electric and magnetic resonances in TiO2 metasurfaces.

pigment based colors, such kind of plasmonic colors can have spatial resolution around 100 000 dpi and are immune to photo bleaching.26−28 However, since there is an inherent loss of metals in the visible spectral range, the reflection peaks from plasmonic nanostructures are typically broad and less intensive.1,29,30 To reduce the material absorption, Proust et al. replaced plasmonic nanoparticle or subunits with isolated Si Mie resonators and achieved an all-dielectric colored metasurfaces with spatial resolution around 25400 dpi.31 While the refractive index of silicon is around 3.5, the corresponding Mie scattering cross sections are very small. The generated structural colors have only been observed in dark field.32 Meanwhile, silicon has very strong absorption in the visible light range (when the wavelength λ is smaller than 450 nm, the imaginary part of permittivity εSi′ quickly increases with the decrease of λ). Thus, the silicon Mie scatters also face severe challenge in producing distinct color impression, especially for the blue and purple colors. Naively, the silicon particles can be replaced with transparent materials, such as SiO2, TiO2, polymer et al. However, due to their relatively smaller refractive indices, these transparent materials are even hard to produce structural color with isolated nanoparticle in dark field, let alone the bright field. Considering the practical applications, the high spatial resolution is not the only key parameter. The brightness and color contrast are equally important. For dielectric nanostructures, the natural world has demonstrated bright and highly contrast structural colors in many systems, e.g., the photonic crystals in peacock features,33 butterfly wings,34 opals,35 and the pseudogap in bird’s features.36 Compared with the colors of plasmonic devices and single dielectric nanoparticle, the structural colors of periodic dielectric nanostructures are usually much brighter and sharper because of their high

RESULTS AND DISCUSSION Prior to the structure design, it is necessary to determine the material for the system. The conventional high refractive index materials, such as Si and GaAs, usually have strong material loss in the visible frequency range.41,42 Meanwhile, the transparent materials, such as silica and polymer, have very low refractive indices and are hard to be compatible with CMOS technology. In this sense, TiO2 has been widely accepted as a good candidate for designing and fabricating all-dielectric metamaterials in visible range. In past few years, high quality TiO2 films have been successfully formed by either atomic layer deposition43 or electron-beam (E-beam) evaporation.44 The corresponding optical measurements show that the real part of refractive index is around 2.2 at 632 nm and the imaginary part of refractive index is negligibly small.45 The TiO2 based metasurface is schematically depicted in Figure 1a. It is a square lattice of TiO2 blocks on an ITO (15 nm in thickness) coated glass substrate. The lattice size is p and the whole system is surrounded by air. Based on the experiment tests (see below), the cross-section of each subunit is a trapezoid instead of a rectangle in the vertical direction. While the top and bottom surfaces are both squares, the side length of 4446


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Figure 2. Fabrication of the TiO2 metasurfaces. (a) The fabrication process of the metasurface with lift-off technology. (1) Pattern the photoresist. (2) E-beam evaporation of TiO2 film. (3) Remove the photoresist. (b) The tilt-view scanning electron microscope (SEM) image of the metasurfaces. (c) The corresponding high-resolution SEM image of one sample. (d−f) Three top-view SEM images of TiO2 metasurfaces with different unit sizes. The inset in each graph shows the corresponding bright-field images of each sample. The scale bars in (b) and (c) are 100 μm and 500 nm, respectively. The scale bars in (d) to (f) are all 1 μm.

resonance. Due to the efforts of periodic structure and low loss, the value of reflection peak can be as high as 100% with fwhm’s around 25 nm. In Figure 1c, there is no resonances on the red side of two peaks because the magnetic resonances are the fundamental resonant mode. Meanwhile, the periodic structure forms a band gap that effectively suppresses the higher order modes at shorter wavelength. In this sense, the narrow reflection peaks contain more than 70% energy of the whole spectra. Therefore, the TiO2 metasurfaces can have a series of intrinsic advantages in generating structural colors, e.g., brightness, color purity, and contrast. Another important advantage of TiO2 metasurfaces is the angular dependence. In principle, the locations of reflection peaks strictly follow the magnetic dipole resonant modes inside the metasurface. Under certain condition, the reflection peak can be designed to have very weak angular dependence.50 One example is illustrated in Figure 1e. We can see that the reflection peak is strictly limited within 450−470 nm when the incident angle was increased from 0 degree to 60 degree, clearly demonstrating the great potentials of TiO2 metasurface in color generation. Moreover, the magnetic dipole resonance is generated by one particle, and the photonic crystals effect can also be realized with a few periods. In this sense, distinct structural colors can still be generated in bright field when the periodicity is significantly reduced (see below). Therefore, the TiO2 metasurface might be an alternative approach for the applications of color printer in visible region. As mentioned by Devlin et al., new materials and fabrication techniques must be developed for dielectric metasurfaces at visible wavelengths.51 Based on above analysis and previous reports, we have fabricated TiO2 metasurfaces and studied their optical properties. In previous reports, the fabrication of TiO2 nanostructures are strongly dependent on reactive ion etching52 and atomic layer deposition.51 While these techniques can fabricate high quality TiO2 nanostructures, they usually cost extremely long time to finish one process. Here we fabricated high quality TiO2 metasurfaces with a much simpler approach. As depicted in Figure 2a, the fabrication technique relies on the typical lift-off process. Basically, periodic nanostructures were generated within electron-beam (E-beam) resist by E-beam lithography. Then 200 nm TiO2 films were deposited onto the nanostructures via E-beam evaporation. The detail of the film preparation procedure was described in the Methods part,

top-square is around 130 nm smaller than the one of bottom square, giving a trapezoidal corner around 72°. The thickness t of the metasurface is fixed at 200 nm. In principle, each TiO2 block can support a Mie electric dipole resonance and a Mie magnetic dipole resonance. However, due to the low refractive index of TiO2, these resonances are too weak to be distinguished from the reflection of the substrate. Thus, the structural color is hard to be observed under the bright field microscope. This situation can be changed by utilizing multiple Mie scatters. When two Mie scatters are placed closely, the proximity resonance can happen and effectively enhance the magnitude of resonant modes. This kind of enhancement can be further improved with the increase of scatter numbers. Consequently, arranging TiO2 scatters into periodic metasurface (see Figure 1a) can be a possible way to generate structural color in bright field. By adjusting the geometry of the scatter and the distance between them, the coupling effect between Mie electric dipole (magnetic dipole) resonances with the photonic crystal radiation46,47 will greatly increase the reflection efficiency and reduce the fwhm to around 30 nm. Such kind of enhancement in reflection efficiency is more important when small period is applied for high spatial resolution. In addition, the periodic arrays of TiO2 blocks can even form the wellknown photonic band gap, which shall suppress the other high order resonant modes and significantly improve the distinct color impressions. Recently, Magnusson et al. have also utilized the mode coupling in Si3N4 grating to achieve high and narrow reflection peaks.48 However, the one-dimensional structure restricts it to linear polarization with electric field and it is quite sensitive to the incident angle. Based on above analysis, we numerically calculated the resonances of designed TiO2 metasurfaces as a function of block size (w) with commercially available finite-element software, known as COMSOL Multiphysics.49 All the results are summarized in Figure 1b. When w is 200 nm, the maximal reflection peak appears at 450 nm. With the increase of w, the resonant modes continuously shift to longer wavelengths and reach 500 nm at w = 300 nm. Figure 1c shows the reflection spectra of TiO2 metasurfaces with different w and Figure 1d depicts examples of field patterns at the resonant peaks with w = 230 nm, p = 300 nm. From the field patterns, it is easy to see that the mode at shorter wavelength is the electric mode, whereas the long wavelength mode is the well-known magnetic 4447


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Figure 3. Reflection spectra and structural colors. (a) The simulated and experiment reflection spectra of one TiO2 metasurface. The dashed line illustrates the spectra of the halogen lamp. The top inset and the bottom inset are the calculated color from simulated reflection spectra considering the emission spectra of the light source and the directly recorded color by a CCD camera. (b) The directly calculated (black dots) and corrected (red squares) structural colors from the simulated reflection spectra of metasurfaces with varying dimensions in CIE 1931 color map. (c,d) The colors calculated based on R and R*Lightsource from the simulations with varying periodic p and the gap g.

Figure 4. Experiment reflection spectra and corresponding colors. (a) Experiment and simulation spectra of several rainbow colors from different samples. The calculated colors based on experiment based R and R*Lightsource are shown as left and middle insets in each panel associated with their R, G, B values. The right inset in each panel is the corresponding directly recorded microscope image under bright-field. (b) The colors based on R (black dots) and R*Lightsource (red squares) of total 38 samples in CIE 1931 color map.

ing high-resolution SEM image of one sample is shown in Figure 2c. Based on the design in Figure 1, the TiO2 nanostructures are periodically arranged in square lattice. Figure 2d−f illustrates three top-view SEM images of TiO2 metasurfaces with unit size w = 235, 280, 345 nm and lattice size p = 310, 350, 400 nm, respectively. Following the studies in Figure 1, such metasurfaces have dominant reflection peaks and generate a very bright structural color. This information was then confirmed by taking photographs of the metasurfaces under an optical microscope (ZEISS, Axio Scope AI). The inset in Figure 2f shows the recorded color image of the metasurface with w = 345 nm and p = 400 nm. While the sample was

which will be briefly summarized below. We deposited the TiO2 film on the patterned substrate held at 24 °C in a vacuum chamber after evacuating to a pressure of 2 × 10−7 Torr. The deposition rate is kept at 0.8 Å/s in order to decrease the surface roughness. The root-mean square roughness (RMS) was calculated from the AFM data over a 17 μm × 17 μm area and the value is 0.66 nm, indicating an ultrasmooth surface has been obtained. After removing the E-beam resist, the reversed nanostructures were well transferred to TiO2 film and TiO2 metasurfaces were finally formed. Figure 2b shows the tilt-view scanning electron microscope (SEM) image of the fabricated metasurfaces. The correspond4448


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Figure 5. Structural colors of each pixel. (a) The bright-field images of metasurfaces. This graph demonstrates three different structure colors (purple, green, and red) from 8 × 8 to 4 × 4 TiO2 metasurfaces. (b) The three panels illustrate the bright-field images for the 8 × 8 red metasurface by objective lens with different magnifications and NAs. As the numerical aperture relates to the incident angles, the results demonstrate the angular independence of the structural color. (c) The top-view SEM images of red 7 × 7 to 4 × 4 metasurfaces. The insets demonstrate the corresponding bright-field images. The scale bars in (c) are all 3 μm.

measured under bright field, high contrast red color can be clearly observed. Similarly, blue color (inset in Figure 2d) and green color (inset in Figure 2e) have also been observed. Then the reflection spectra of different TiO2 metasurfaces have been experimentally studied (see Methods Section) with a homemade optical setup. Taking the metasurface in Figure 2e as an example, a reflection peak can be clearly observed at 549 nm (see the solid line in Figure 3a). The peak value is 64% and the fwhm is 30 nm. Since there is no additional peak at longer and shorter wavelength ranges, this reflection peak processes more than 80% energy of the whole spectrum. All of this information is consistent with the distinct green color in bright field well (see inset in Figure 2e). By using the experimental structures, the reflection spectrum has also been numerically calculated. As the dash dotted line shown in Figure 3a, both of the central wavelength and the corresponding fwhm match the experiment results very well. From the reflection spectra, the structural color in CIE1931 color space can also be calculated. The numerically calculated result is depicted as the top inset in Figure 3a. While the numerical color is very close to the experimental color, some slight difference can still be observed. This is because the structure was illuminated by a hungsten lamp in optical microscope. The recorded color (R*Lightsource) is the combination of the spectrum of the lamp (Lightsource, shown as the dash line in Figure 3a) and the reflectance spectrum of the structure R. By varying the structural information (p and g, here g is the gap between two adjacent blocks), we have calculated the structure colors of 88 samples. All the results are shown as blocks in Figure 3c. Vivid rainbow colors are directly presented, indicating the potentials of TiO2 metasurfaces in full-color printing. Figure 3d shows the corresponding structural colors by taking account of the emission spectrum halogen lamp. The detailed color information on Figure 3c,d has been summarized in the CIE map (see Figure 3b). We can see that the calculated colors of TiO2 metasurfaces are well distributed in the CIE map and match the standard RGB colors (white dots in Figure 3b) very well. Based on the simulation results from Figure 3, we have fabricated 38 samples with different sizes and studied their optical characteristics. Figure 4a summarizes 7 samples to

demonstrate the tunability of structural color. By keeping the gap (g) as a constant (∼70 nm), the period p and the square size w are increased from the bottom panel to the top panel. When the period p is 300 nm, the reflection emerges at 466 nm. With the increase of p, the peak position gradually shifts to 632 nm, whereas the peak values, fwhm’s, and the ratios of energy within the main peak are all preserved. The reflection spectra of TiO2 metasurfaces in Figure 4a have also been used to calculate the CIE colors. The calculated colors based on R and R*Lightsource are shown as left and middle insets in each panel associated with their R, G, B values. The right inset in each panel is the corresponding microscope image under bright-field. We can see that the TiO2 metasurfaces can have seven bright colors ranging from purple to blue, cyan, green, yellow, orange, and red. We can see the structure colors that are corrected with the emission of halogen lamp match the experimental results very well. By using the experimental structures, the reflection spectra of these nanostructures have also been numerically calculated. All the results are plotted as dashed lines in Figure 4a. Both of the peak positions and the fwhm’s match the experimental results very well. In some cases, the electric resonance and magnetic resonance can even be experimentally resolved. The only difference is the peak value calculated reflectance is much smaller than the numerical calculation (∼100% in Figure 1). This difference should be attributed to the surface roughness of each TiO2 block, which can introduce extra scattering to the nanoparticle. Interestingly, while the maximal values are different, the trends of whole spectra match very well. The experimental peaks can still hold around 80% energy of the whole spectrum and thus generate very distinct colors. The black and red dots in Figure 4b summarize the calculated colors of total 38 samples in CIE 1931 chromaticity map. It can be seen that all the experimental colors are well distributed in the CIE map, just as the simulation prediction. Most importantly, the experimentally recorded structural colors match the pure red, green, and blue colors (white dots in Figure 4b) very well. Notably, the experimentally achieved color distribution in CIE map is much better than most of the previous reports. In additional to the brightness and contrast, the spatial resolution is also important for the structural colors. As we 4449


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positions can be predefined by the sizes of TiO2 nanoparticles. We then fabricated the pixels in different areas with different sizes. Therefore, when the sample was illuminated by a white light source under bright field microscope, a colorful image has been successfully recorded by the CCD camera (see Figure 6b) even though the TiO2 material is transparent at visible wavelength. The structural details of the images are very similar to the results in Figure 4. In additional to the reflection colors, the TiO2 metasurfaces can also function as a nice color filter and thus generate structural colors in the transmission mode. One example is shown in Figure 6c, where colorful images have also been generated in the bright field. The transmitted image is almost the reversed image of the one in Figure 6b. Moreover, due to the birefringence of TiO2, the light on resonant will be partially converted to the other polarization. Consequently, a more distinct color image has been generated when the sample was characterized with a polarizing microscope (see Figure 6d).

mentioned above, both of Mie resonance and photonic crystal effort do not require extremely large periodicity. Thus, bright and stable colors can be viewed from TiO2 metasurfaces with relatively smaller periodicity. This is a trade-off between distinct color impression and the spatial resolution. To test the smallest pixel size, we have decreased the periodicity of TiO2 metasurface and studied the corresponding structural color. As shown in Figure 5a, red, green, and blue colors can still be seen in bright field when the periodicity was decreased from 8 × 8 to 4 × 4. Figure 5c shows the top-view SEM images for red metasurface in Figure 5a. The insets are their enlarged structural colors that are recorded by a CCD camera. The total sizes of the 4 × 4 metasurface is around 1.6 μm, giving a resolution around 16000 dpi. This value is only a little bit smaller than the one of isolated Si nanoparticle and is good enough for most of applications in imaging, display, and sensing et al. More than the small pixel size, our experimental results also demonstrated the angular independences of the structural colors. As shown in Figure 5b, the structural colors of TiO2 metasurface are almost the same when it was measured by objective lens with different numerical aperture (incident and collection angles). For a quantitative comparison, we have also fabricated the metasurface designed in Figure 1e and experimentally studied the angular dependence. As the white circles shown in Figure 1e, we can see that the reflection peak changes slightly with the incident angle, clearly demonstrating the robustness of TiO2 metasurface and consistent with the numerical calculation well. At last, we fabricated microscopic images to demonstrate the ability of our TiO2 metasurfaces to create arbitrary colors. The creation of structural color image was demonstrated in a 500 × 500 μm2 image of the logo of our university (Harbin Institute of Technology). Figure 6a shows the top-view SEM image of the fabricated structures. Each pixel is an 8 × 8 periodic TiO2 metasurface (see the high-resolution SEM images in Supporting Information). Based on above studies, the colors at different

CONCLUSION In summary, we have presented an approach for full color printing with TiO2 metasurfaces. By employing the interference between reflected waves and the radiation waves from metasurface, high reflection peaks have been obtained in a wide spectra extend the entire visible region. Associated with high reflection, bright and high contrast colors have been generated by the TiO2 metasurfaces. By carefully tuning the structural sizes, the recorded colors were widely distributed in the CIE map and match the R, G, B colors very well. Based on the properties of Fano resonances, the reflected colors have also been achieved from TiO2 metasurface with much smaller pixel sizes and thus can generate bright, high contrast, and highresolution structure colors and even colorful images. We believe this research will be important for the realization of optical display, imaging, data storage, and color filters with high contrast and large brightness. METHODS Numerical Simulation. In this work, all the numerical analysis was performed using commercially available finite element method (FEM)based software, known as COMSOL Multiphysics.49 The simulation was performed on one unit cell with periodic boundary conditions. The whole structure was composed with TiO2 and settled on top of an 15 nm-ITO glass, with lossless refractive index of nsub = 1.52. The optical constant of TiO 2 is determined from spectroscopic ellipsometry measurements (see Supporting Information S1). Fabrication. The metasurfaces were fabricated with electron beam lithography technique followed by lift-off process. First, we cleaned the 15 nm-ITO glass substrates in ultrasound bath in acetone and isopropyl alcohol (IPA) for 10 min respectively and dried under clean nitrogen flow. Then, 350 nm ZEP520 film was spin-coated onto the ITO-coated glass substrate and baked at 180 °C for an hour. After that the sample is exposed to electron beam in E-beam writer (Raith E-line, 30 kV) and developed in ND510 solution for 60 s at 0 °C to form the ZEP nanostructures. Then the sample was transferred into an E-beam evaporator and directly coated with 200 nm TiO2 films (deposition rate 0.8 Å/s, base vacuum pressure 2 × 10−7 Torr). After immersing the sample in acetone for 8 h, the ZEP was removed and the reversed nanostructures were well transferred to TiO2 film. Exposed to oxygen plasma for 5 min to remove the remaining ZEP around nanostructures, TiO2 metasurfaces were finally formed. Finally the samples were transferred into a tubular furnace, where a thermal annealing at 300 °C was performed for 1 h under oxygen atmosphere to make the surface of the TiO2 nanostructure smoother and decrease the radiation loss from the surface roughness.

Figure 6. Full color image printing with TiO2 metasurfaces. (Logo printed with permission from Harbin Institute of Technology. Copyright 2016 Harbin Institute of Technology.) (a) The top-view SEM image of the logo of Harbin Institute of Technology. (b) and (c) are the corresponding reflection and transmission colorful images of the university logo under bright field microscope. (d) The polarizing microscopy image of the logo. The scale bar in (a) is 150 μm. 4450


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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/acsnano.7b00415. Refractive index of TiO2, experimental setup, polarization and angle dependence of the reflectance properties, experiment attempt for higher resolution, experiment data for all the 38 samples in Figure 4b, and some high resolution SEM images (PDF)

AUTHOR INFORMATION Corresponding Authors

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

Qinghai Song: 0000-0003-1048-411X Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The author would like to thank the financial support from National Natural Science Foundation of China under the Grant No. 11374078, Shenzhen Fundamental research projects (JCYJ20160301154309393, JCYJ20160505175637639, and JCYJ20160427183259083), public platform for fabrication and detection of micro- and nanosized aerospace devices, and Shenzhen engineering laboratory on organic−inorganic perovskite devices. REFERENCES (1) Kristensen, A.; Yang, J. K. W.; Bozhevolnyi, S. I.; Link, S.; Nordlander, P.; Halas, N. J.; Mortensen, N. A. Plasmonic Colour Generation. Nat. Rev. Mater. 2016, 2, 16088. (2) Lundberg, I.; Milatou-Smith, R. Mortality and Cancer Incidence among Swedish Paint Industry Workers with Long-Term Exposure to Organic Solvents. Scand. J. Work, Environ. Health 1998, 24, 270. (3) Kuznetsov, A. I.; Miroshnichenko, A. E.; Brongersma, M. L.; Kivshar, Y. S.; Luk'yanchuk, B. Optically Resonant Dielectric Nanostructures. Science 2016, 354, aag2472−aag2472. (4) Young, A. T. Rayleigh Scattering. Appl. Opt. 1981, 20, 533−535. (5) Mie, G. Beiträge zur Optik trüber Medien, speziell kolloidaler Metallösungen. Ann. Phys. 1908, 330, 377−445. (6) Kelly, K. L.; Coronado, E.; Zhao, L.L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107, 668−677. (7) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Review of Some Interesting Surface Plasmon Resonance-Enhanced Properties of Noble Metal Nanoparticles and Their Applications to Biosystems. Plasmonics 2007, 2, 107−118. (8) Liz-Marzán, L. M. Tailoring Surface Plasmons through the Morphology and Assembly of Metal Nanoparticles. Langmuir 2006, 22, 32−41. (9) Eustis, S.; El-Sayed, M. A. Why Gold Nanoparticles Are More Precious Than Pretty Gold: Noble Metal Surface Plasmon Resonance and Its Enhancement of the Radiative and Nonradiative Properties of Nanocrystals of Different Shapes. Chem. Soc. Rev. 2006, 35, 209−217. 4451


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