Solution-Processable Nanocrystal-Based Broadband Fabry–Perot

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Solution-Processable Nanocrystal-Based Broadband Fabry−Perot Absorber for Reflective Vivid Color Generation Soo-Jung Kim,† Hyun-Kyung Choi,‡ Heon Lee,*,† and Sung-Hoon Hong*,‡ †

Department of Materials Science and Engineering, Korea University, Anam-dong 5-1, Sungbuk-gu, Seoul 136-701, Republic of Korea ‡ ICT Materials & Components Research Laboratory, ETRI, 218 Gajeong-ro, Yuseong-gu, Daejeon 305-700, Republic of Korea

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S Supporting Information *

ABSTRACT: Structural reflective colors based on Fabry− Perot (F−P) cavity resonances have attracted tremendous interest for diverse applications, such as color decoration and printing, display, and imaging devices. However, the asymmetric F−P cavity-based reflective colors proposed to date have low color purity and have difficulty to realize a desired vivid color because of a narrow absorption band characteristic in the visible light region. Here, a solutionprocessed, F−P ultra-broadband light absorber is newly proposed using a high lossy nanoporous material for vivid color generation. An asymmetric metal−insulator−metal structure consists of a high lossy nanoporous metallic film with coupled silver nanocrystals (Ag NCs) as the top layer. The absorbers not only increase the maximum absorption intensity up to ∼98% but also widen the bandwidth by 300 nm, resulting in high color purity in micrometer-scale pixels. Furthermore, the solution-based absorber shows potential to realize a high-resolution display pixel and anticounterfeiting devices having mechanical flexibility using the inkjet printing technology. KEYWORDS: Fabry−Perot, broadband absorber, nanocrystal, reflective color, silver nanocrystal, optical loss

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INTRODUCTION

F−P absorbers have an asymmetric triple structure thin metal−dielectric cavity-opaque mirror metal. This simple structure can be fabricated on a large area to achieve practical applications through a lithographic-free manufacturing. In addition, the reflective color can be easily controlled according to the thickness of the dielectric layer. However, as Aydin et al. suggested,32 asymmetric F−P cavities composed of metal thin films exhibit an absorption spectra that has quite narrow bandwidth (8−15 nm) despite perfect absorption (as high as 97%). It is difficult to achieve pure color and enhance the color reproducibility in reflective color generation. To widen the absorption bandwidth in visible wavelength, the ultrathin Cr,33 Ni,34 and Ge35 lossy metallic films have been applied to the F− P absorber instead of Ag and Au in previous other studies. However, there are at least two major drawbacks in using a general metal deposition process such as sputtering or the vacuum evaporation. First, the available metals are limited, and the physical properties are hardly controlled, making it difficult to change the characteristics of the absorber. Second, it is difficult to fabricate a large area on various substrates because the current processes are complex, of high cost, and time consuming.

The structural colors based on specific structure−light interaction, such as the plasmonic effect and interference effect, have been studied extensively as strong candidates for diverse applications of environment-friendly color printing,1−5 high-resolution display,6−10 sustainable color decorations,11−13 and security colors.14−18 These color elements have great merit in that the specific nanosurface can produce colors without the need for dye or pigments, and colors can be preserved from ultraviolet light and harmful environments, unlike dye or pigments, because the interaction with light is just controlled by the geometrical structure. An interesting color implementation using the plasmonic effect based on metal metasurfaces19−23 has shown full colors with ultrasmall pixels that exceeded the limitations of the existing display technology. However, in case of plasmonic color generation, the costly and complicated lithographic procedures are required to fabricate periodic metal nanostructures that exhibit satisfactory absorption wavelength. The reflectance spectra obtained with plasmonic nanostructures exhibit a relatively wide full width at half-maximum (FWHM), and the differences between the maximum and minimum reflection peaks are relatively small, reducing the saturation and purity of colors.24−27 To overcome the critical limitation, a Fabry−Perot (F−P) cavity resonance-based absorber has been employed to realize vivid colors in many recent studies.28−31 © XXXX American Chemical Society

Received: November 1, 2018 Accepted: January 30, 2019

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DOI: 10.1021/acsami.8b19157 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Three types of Ag metallic films: SEM images, refractive index, and permittivity of (a,d,g) deposited Ag thin film, (b,e,h) sintered Ag NC film, and (c,f,i) coupled Ag NC film by SCN ligands, respectively.

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RESULTS AND DISCUSSION To understand the effective optical property of various Ag films, the three types of Ag films were prepared by different methods: electron-beam (e-beam) evaporation, thermally sintering Ag NCs, and chemically coupling Ag NCs. Bulk Ag thin film with 50 nm thickness was deposited on the silicon substrate by means of conventional e-beam evaporation. In addition, a dispersion of as-synthesized 10 nm diameter Ag NCs in octane was used to fabricate the nanoporous films. The Ag NC films were prepared by spin-coating of 20 wt % of dispersion with different NC connecting methods onto silicon substrates. For preparing the densely agglomerated Ag NC film, the heat was applied to the Ag NC at 280 °C on the hotplate. Typically, each NC is capped with long-chain hydrocarbon surfactant oleyamine (OLA), and the coated NCs do not interact electrically with each other, being separated into air spacing. To fabricate the porous metallic film made of the chemically coupled NCs, the capped long ligand of NCs were replaced to short ligands in the NC solid state.39,40 A ligand exchange reaction was performed by immersing the OLAcapped NC film samples in 1% ammonium thiocyanate (NH4SCN) solution, replacing the OLA ligands to SCN ligands. As a result, the Ag NC film coupled with conductive SCN ligands, which have short lengths of 0.2 nm, was continuously connected including the air layers. The contraction of the gaps between Ag NCs with ligands can be clearly seen from a transmission electron microscopy (TEM) image (Figure S1 in Supporting Information). The scanning electron microscopy (SEM) image in Figure 1a depicts the deposited bulk Ag as a highly continuous and conductive film without pores. The thermally linked Ag NC film has very low porosity as shown in Figure 1b, whereas the

To engineer the properties of the material, the nanoporous materials have been widely investigated in the visible/nearinfrared (NIR) spectral range because of tailoring the optical properties of materials using the effective medium theory.36−38 These heterogeneous mixtures of two or more components do not exhibit the optical properties of the pure metals or semiconductors because the nanopores which are significantly smaller than a wavelength of light are included in pure materials. Porosity and versatile pore geometry can cause a variety of complex refractive indices and permittivities. In contrast to conventional deposition methods, the solution processing has many benefits to drastically lower the fabrication cost and to simply obtain the functional device in large areas. In particular, inkjet printing as one of the promising solution processing method is suitable for the production of a mechanically flexible devices, which include display, sensors, and memory devices, because of unnecessary of high temperatures and high vacuum. In this work, we proposed a new configuration of an F−P cavity absorber with a lossy metal−dielectric homogenized nanoporous material through the solution process. The nanoporous material was made of chemically coupled silver nanocrystals (Ag NCs) with a conductive ligand, and this metal−dielectric composite film was applied to the top layer of the absorber to achieve a broadband absorption. Through finite-difference time-domain (FDTD) simulations and systematic experiments, we demonstrated that this absorber provides enhanced color purity and an extended color gamut. By realizing a flexible reflective color image using the NC solution process, the applicability to various color applications was confirmed. B

DOI: 10.1021/acsami.8b19157 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. Schematic configurations, absorption, and reflectance spectra with various Ag-based F−P absorbers: (a) diagrams of asymmetric metal− insulator−metal structures composed of an Ag thin film, sintered Ag NCs, and coupled Ag NCs by SCN ligands. (b) Measured absorption spectra and (c) reflectance spectra with SiO2 cavity thickness (including the photograph of the fabricated absorbers on right).

Figure 3. Transmission, reflection, and absorption spectral comparison of differently integrated structures. Measured spectra for (a) nanoporous Ag NC layer, (b) SiO2 cavity layer, (c) combined Ag NC−SiO2 stack, (d) combined SiO2−Ag reflective layer stack and (e) Ag NC−SiO2−Ag triple structure.

while including dielectric air pores in the conductive film using the coupled Ag NC film. Asymmetric F−P cavity absorbers consist of the thin Ag film as a top layer, SiO2 dielectric cavity, and the opaque reflective Ag as a bottom layer. To observe the absorption and reflection characteristics of the Ag/SiO2/Ag F−P absorbers according to the top Ag film types (bulk Ag, sintered Ag NC, and SCNcapped Ag NC film), the absorbers were fabricated to have four different SiO2 thicknesses (100, 145, 205, and 240 nm), respectively, as shown in the illustrations of Figure 2a. The visible reflectance was measured using Varian’s Cary 5000 UV−vis−NIR spectrometer. The absorbance (A) was simply calculated by a formula A = 1 − R − T, where R and T represent the measured reflectance and transmittance, respectively. In comparison with bulk Ag-based absorbers, which have a narrow absorption band, the sintered Ag NCbased absorber shows the broader absorption band in certain wavelengths. Furthermore, the coupled Ag NC-based absorber has excellent absorptive spectral selectivity with broad (∼300 nm) and high intensity (∼98%) as shown in Figure 2b. Through the measured reflectance spectra as plotted in Figure 2c, the bulk Ag-based absorber has difficulty reflecting only

chemically coupled Ag NC film is more porous because of narrow NC spacing in Figure 1c. The optical properties of those from the deposited, densely packed Ag film to nanoporous Ag NC films were extracted using spectroscopic ellipsometry. An effective medium approximation was used to derive an average, macroscopic Ag materials, which is a homogeneous mixture of metal and air. Figure 1d−f shows a metallic behavior of various Ag-based films considering that the value of κ(λ) is larger than that of that of n(λ) in the entire wavelength range between 400 and 1000 nm. When the porosity is increased from bulk Ag film to nanoporous SCN-capped NCs film, n(λ) tends to be higher and κ(λ) become lower. The optical dielectric functions are shown in Figure 1g−i. The real parts of the complex dielectric function (ε′) are everywhere negative and the imaginary parts of the function (ε″), which means the optical loss, were positive from 400 nm onward in the three types of Ag-based films. As the porosity of the film increases, ε′ was slightly less negative and less steeply sloped and ε″ was larger than that of the deposited bulk Ag as a complete metal. Importantly, the optical loss could be controlled to desire and highly optimized, C

DOI: 10.1021/acsami.8b19157 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 4. Reflection characteristics in terms of top metallic layer-based SiO2 cavity fixed 340 nm (a) reflection spectra and (b) corresponding 1931 CIE color coordinates for green with thickness of top layer.

Figure 5. SiO2 cavity thickness-dependent reflection spectra and corresponding color responses: (a) calculated and measured reflection spectra depending on cavity thickness ranging from 100 to 240 nm for a fixed top layer and (b) chromaticity coordinates in the CIE 1931 chromaticity diagram corresponding to the spectra as a function of the cavity thickness.

Ag film and does not selectively absorb or reflect wavelengths as shown in Figure 3d. In these cases in Figure 3a−d, the calculated absorption does not exceed 30% in entire wavelength and could not selectively absorb or reflect a particular range, which illustrates that the filtering reflective color is unachievable from individual or incomplete structure. On the other hand, in the combined Ag NC−SiO2−Ag stack of Figure 3e, the electric field is highly confined in the SiO2 layer between both Ag layers, leading to an enhancement. In a resonance cavity, the incident light with resonance wavelength is continuously reflected and loss some power by the Ag NC top layer and Ag bottom layer. Particularly, the broadband absorption is caused by the upper Ag NC film, which has the high optical loss. The nanoporous Ag NC film, an effective medium consisting of a metal and a dielectric, was applied to a top metallic layer of the F−P absorber, and the geometry of the structure was optimized for color generation. The location of the resonance wavelength is more sensitive to the SiO2 thickness, whereas the magnitude of reflectance is more sensitive to the top metallic layer thickness. Therefore, the reflectance according to the thickness of the top layer was measured to set an appropriate thickness where we fix the thickness of the SiO2 layer to be 340 nm. The thickness of the continuous porous Ag NC film should be approximately the light penetration depth to enable

selective colors of interest because of forming very flat and wide reflective peaks as demonstrated in actual fabricated absorbers. As porosity increases from bulk Ag to SCN-capped Ag NCs, the optical loss of the top layer increases and the color purity increases. The optimized vivid color of the SCN-capped Ag NC-based absorber showed blue, sky blue, yellow, and orange colors, which have high color saturation and brightness, with increasing thickness of the SiO2. By analyzing the spectral response of the Ag NC−SiO2−Ag triple structure individually, the respective role of each layer could be closely examined. The transmission (T), reflection (R), and absorption (A) spectra for the nanoporous Ag NC film (50 nm thickness), dielectric SiO2 (100 nm thickness), and thick Ag film (100 nm thickness) were, respectively, measured as shown in Figure 3. The Ag NC metallic layer reveals a high transparency of ∼70% transmission with ∼15% reflection drawn in Figure 3a, meaning that the sufficient light penetrates into the opposite side. The SiO2 cavity layer is transparent with ∼90% transmission and does not specially change its optical properties when combined with the Ag NC layer (Figure 3b,c). The thick Ag film usually could block entire visible light transmission and has reflectivity of 95−99% even into the infrared, but reflectance decreased slightly toward the short visible wavelength (>90%). Therefore, the lossless SiO2 covered Ag film exhibits a spectrum similar to that of the D

DOI: 10.1021/acsami.8b19157 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 6. (a) Optical images and (b) reflection spectra of microscopic patterns of a zebra image. (c) Photography of color zebra image fabricated on flexible PET substrate. (d) Various microscale square pixels for blue, red, and green colors based on an F−P absorber.

In the CIE 1931 chromaticity space, the color trends are visualized from two circles (represented by dashed black and white lines with arrows) that represent the change in the order of blue−green−red colors, as shown in Figure 5b. According to the clockwise trace of the plotted chromaticity coordinates from the white dashed line including white circle dots, it is shown that the first reflection peak can obtain a full-color palette when the cavity thickness is from 100 to 240 nm. As the cavity thickness increases from 260 to 460 nm, the trace curve (represented by black lines and triangle dots) covers a larger portion of the CIE color space, demonstrating the increase in color purity, which is generated from the second reflection peak. The second reflection spectra shape is sharper than the first peak with narrow FWHM, proving that the color purity and saturation can be enhanced in the same color, as shown in Figure S4 in the Supporting Information. At a wavelength of 542 nm representing green color, FWHM of the second peak is 90 nm smaller than that of the first peak. The overall color change position of the experimental spectrums match well with the calculated ones. The proposed porous metal-based absorber exhibits a high color reproducibility of more than 33.8%, which was beyond the limitation of reflective color generation until today. To highlight the wide range of color and pixel sizes that could be widely applicable to display imaging in the proposed F−P absorbers, an attempt was made to demonstrate the microscale multicolored images and pixel design. Figure 6a shows the printed zebra microscope image with total dimensions of 2.5 cm × 2.5 cm, which has six different cavity thicknesses. The microscopic zebra image was fabricated by a photolithography process, and six SiO2 cavities were separately deposited through shadow masks. Without a porous Ag film coating, the image shows only an ambiguous outline and no clear color. Following uniform coating of porous Ag film on the cavities, the vivid multicolors were generated simultaneously. Figure 6b shows the higher magnification image of the selected

resonance inside the cavity. The thickness was proportionally adjusted by spin-coating with the varying Ag NC solution concentration (Figure S2 in the Supporting Information). Figure 4a shows the dependence of the reflective spectral response on the properties of the top metallic film, in terms of the thickness. When the thickness changed from 42 to 101 nm, the intensity of the peak was significantly susceptible to the thickness and reduced. The depth of the dip tends to deviate significantly from the zero value depending on the thickness. The corresponding color coordinates are presented in the 1931 International Commission on Illumination (CIE) color diagram, as shown in Figure 4b, where appropriate thickness was acquired for enhanced color saturation. The green color maintained a high purity when the thickness was between 42 and 61 nm but drastically degraded to the near white color when the thickness exceeded 61 nm. For the optimized reflective performance, the thickness of the top metallic film was chosen to be 53 nm, and the three-layered absorber was fabricated as shown in the SEM image of Figure S3 in the Supporting Information. Figure 5a shows the calculated and measured reflection spectra as the dielectric cavity thickness increases from 100 to 240 nm for a constant thickness of top layer of 50 nm. In these spectra, a single peak was observed between two deep dips at distinct wavelengths that can show high color purity. The measured reflection peaks, traced by a red dashed line, appear to gradually red shift from wavelength 410 nm to 780 nm with an increasing cavity thickness, which well matched the calculated results. The reflection spectra for porous metallic film-based F−P absorbers are calculated by performing electromagnetic full-wave FDTD simulations using the commercial software package Lumerical. The complex refractive index of homogeneous porous SCN-capped Ag film analyzed from ellipsometry was used, and SiO2 and a bottom thick Ag layer were applied from the data of Palik. E

DOI: 10.1021/acsami.8b19157 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

SiO2 with various thicknesses and coupling NC with short ligands, the reflective colors can be implemented simultaneously. Chemically Coupled NC-Based Nanoporous Metallic Films by Ligand Exchange Method. Before coating the Ag NCs on the substrates, the substrates were immersed in a mixture of 5 mM 3aminopropyltriethoxysilane (APTES) in hexane for 1 h to improve the adhesion between the Ag NCs and the substrate. It was then rinsed with hexane to remove APTES molecules that were weakly attached to the surface. To fabricate nanoporous metallic Ag film, a 7 wt % Ag NC octane dispersion was uniformly spin-coated on the substrate. The NC layer was finally prepared by heat treatment at 90 °C for 3 min for stabilization. To couple the Ag NCs via a short ligand, NC-coated samples were immersed in 1% NH4SCN acetone solution for 1 min to replace ligand from OLA to SCN. Subsequently, the samples were transferred to a pure acetone to wash the extra ligands, and the heat treatment was performed at 90 °C for 1 min. Optical Characterization. Spectroscopic ellipsometry (SE) (M2000D ellipsometer, J.A. Woollam Co.) was used to obtain the refractive index n(λ) and extinction coefficient κ(λ) of the various Ag films (deposited Ag film, sintered Ag NC, coupled Ag NC) in the wavelength range of 192−1654 nm. The measurements were carried out at room temperature with an angle of incidence of 75°. Ellipsometer measures the ratio of the complex Fresnel coefficients, confirming the changed polarization of the reflected light, which relates to sample properties. The ratio is expressed as ρ = rp/rs = tan(ψ) eiΔ. The physical quantities were extracted by comparing the measured SE data (ψ, Δ) with the theoretically calculated counterpart using the CompleteEASE software package (J.A. Woollam Co.). In addition, we used the B-spline model. To build a model for calculation, the two multilayer structure was set to analyze the metallic Ag materials without considering the silicon wafer substrate. In this model, the first and second layers represent the surface roughness, which is intermixed with air, and dense metallic Ag materials, respectively. The reflectance spectra of the F−P cavity absorbers were measured with a Cary 5000 UV−vis−NIR spectrophotometer without polarization, at a 1 nm spectral bandpass between 350 and 1000 nm wavelength. These were reported based on the 100% reflectivity of the deposited 200 nm thick Ag mirror. For the measurement of the reflectance spectrum of sample areas smaller than a few hundred micrometers, we used the UV−vis microspectrometer (CRAIC).

six areas in Figure 6a, demonstrating the color consistency at higher resolution and the clear sharpness of the color boundary between where the top metallic layers were and were not applied. In addition, the reflection spectrum of the microscope image was nearly 100% through the measurement using the UV−vis−NIR microspectrometer. To protect these colors from the oxidation of sulfuration of Ag NC, we deposited a SiO2 passivation layer with 25 nm thickness on the absorbers. The protected color absorbers exhibited the high thermal stability over 350 °C, and the reflectance spectra did not change (Figure S5 in Supporting Information). The NC-based solution process using the ligand coupling process in this study are attractive owing to their compatibility with the efficient inkjet printing technology and potential applications in lowcost, low-temperature flexible full-reflective color pixelated display. Therefore, the color absorber image were well implemented on flexible polyethylene terephthalate (PET) substrate as shown in Figure 6c, and the absorber structure have durability from damage even under bending stress after a 1500 times of bending with a bending radius of 6.08 mm (Figure S6 in Supporting Information). Furthermore, the achievement of small-size pixels satisfying high resolution was proved by fabricating and measuring the absorbers with scale areas of 100 μm × 100 μm, 50 μm × 50 μm, 10 μm × 10 μm with SiO2 thicknesses of 100, 240, and 340 nm as shown in Figure 6d. The reflectance spectra corresponding to various pixels show almost the same aspect with a large area. Although this current work shows only microscopic color patterns based on multiple deposition processes, these color pixels based on lossy porous films could be scaled down by combining soft lithography methods or imprinting for full-color printing and painting markets.

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CONCLUSION The F−P absorber based on nanoporous metallic films using a solution process is a newly proposed broadband absorber for realizing a vivid reflective color. With the aid of the coupled Ag NC-based nanoporous films that have high optical loss, the absorption bandwidth of absorber was increased and the reflective color purity was enhanced. Furthermore, it is demonstrated that the solution-processed absorber can be applied to flexible color devices requiring low temperatures and shows the potential to realize a variety of applications such as displays and anticounterfeiting devices by combining the inkjet printing technology.

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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.8b19157.

METHODS

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Fabrication of F−P Cavity Absorbers. For the fabrication of F− P cavity absorbers, the bottom Ag mirror with 100 nm thickness was deposited on a polished silicon substrate using an e-beam evaporator. The SiO2 cavity layer was deposited with sequential thicknesses onto the bottom Ag layer by radio frequency sputtering for different durations of time. Then, the Ag thin film with 30 nm thickness was deposited on the prepared SiO2/Ag substrate by a e-beam evaporator for the bulk Ag-based absorber. In case of the Ag NC-based absorber, a dispersion of as-synthesized 10 nm diameter Ag NCs in octane was spin-coated onto substrates. To fabricate the thermally sintered Ag NC film as a top layer, the heat treatment process was performed at 280 °C in 10 min on the hot-plate. In case of the chemically coupled Ag NC film, a ligand−exchange reaction was performed. (See below for more details.) The microscale images shown in Figure 6 were fabricated through a photolithography process on the bottom Ag film layer, and then SiO2 layers were selectively deposited using a shadow mask for generation of various colors. By coating Ag NC solution on

Additional TEM image of SCN ligand-coupled Ag NCs; extra information of the thickness of Ag NC; additional cross sectional image of absorber; detailed reflectance spectra; results of thermal stability; and bending test of flexible absorber (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.L.). *E-mail: [email protected] (S.-H.H.). ORCID

Soo-Jung Kim: 0000-0001-7075-7891 Sung-Hoon Hong: 0000-0002-3408-2820 Author Contributions

The manuscript was written through contributions of all the authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acsami.8b19157 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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ACKNOWLEDGMENTS This work was financially supported by the Pioneer Research Center Program through the National Research Foundation of Korea (NRF-2014M3A6B3063702), a Global Ph.D. Fellowship (NRF-2016H1A2A1909313) and an Electronics and Telecommunications Research Institute (ETRI) grant funded through the Korean government (18ZB1100, Development of Basic Technologies for 3D Photo-Electronics).

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DOI: 10.1021/acsami.8b19157 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX