Ultrafast Light-Controlled Growth of Silver Nanoparticles for Direct

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Ultrafast Light-Controlled Growth of Silver Nanoparticles for Direct Plasmonic Color Printing Yangxi Zhang, Qiang Zhang, Xia Ouyang, Dangyuan Lei, A. Ping Zhang, and Hwa Yaw Tam ACS Nano, Just Accepted Manuscript • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 29, 2018

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Ultrafast Light-Controlled Growth of Silver Nanoparticles for Direct Plasmonic Color Printing Yangxi Zhang†, Qiang Zhang‡, Xia Ouyang†, Dang Yuan Lei‡, A. Ping Zhang*† and Hwa-Yaw Tam† †

Photonics Research Center, Department of Electrical Engineering, The Hong Kong Polytechnic

University, Hong Kong SAR, China. ‡

Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong SAR,

China. *

Email: [email protected].

ABSTRACT: A precision photoreduction technology for the ultrafast high-precision lightcontrolled growth of silver nanoparticles for printing plasmonic color images is presented. Ultraviolet (UV) patterns with about a million pixels are generated to temporally and spatially regulate the photoreduction of silver salts to precisely create around a million clusters of distinct silver nanoparticles on a titanium dioxide (TiO2)-capped quartz substrate. The silver nanoparticle-TiO2-quartz structure exhibits a Fano-like reflection spectrum, whose spectral dip can be tuned by the dimension of the silver nanoparticles for structural color generation. This technology allows the one-step production of multiscale engineered large-area plasmonic substrates without the use of either nanostructured templates or additional nanofabrication processes and thus offers an approach to plasmonic engineering for a myriad of applications ranging from structural color decoration to plasmonic microdevices and biosensors. KEYWORDS: silver nanoparticles, plasmonics, Fano resonance, photoreduction, color printing

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Color generation using metallic nanostructures has a very long history that can be traced back to Roman times when craftsmen added metal nanoparticles to glass to make the famous colorful Lycurgus Cup.

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Recently, such dye-free structural colors have again attracted much

attention with our increasing understanding of complex plasmonic structures, such as plasmonic metasurfaces and metamaterials.

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The fabrication of such sophisticated plasmonic

nanostructures requires advanced nanofabrication approaches to craft user-defined geometry at the nanoscale level. E-beam lithography,

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focused ion beam milling,

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and direct

femtosecond laser writing 17 have been demonstrated to fabricate various kinds of designed nanopatterns for plasmonic color generation. However, all of these methods are costly and have low throughput, mainly due to their single-spot scanning nature. The laser post-writing method proposed by Zhu et al. improved the fabrication efficiency of some content because it reshapes the previously prepared metallic nanostructures for different plasmonic resonances via laser pulse-induced melting. 18 However, a special substrate must still be fabricated with aluminum nanostructures before the post-writing process. A similar drawback exists in the chromotropic plasmonic printing approach demonstrated by Xue et al. 19 In this work, a specialized template of anodic alumina oxide with nanopores must be prepared prior to the writing process. Moreover, the method cannot directly create metallic microstructures; direct-writing photolithography and two additional etching processes are required before the deposition of aluminum. Here, we present an optical nanofabrication technology, called precision photoreduction, to precisely regulate the photoreduction of silver ions in both the time and spatial domains to rapidly fabricate plasmonic substrates with multiscale customized structures (i.e., nanometerscale dimension-controlled silver nanoparticles and micrometer-scale user-defined patterns) for direct color printing. A schematic representation of the method is shown in Fig. 1A. An in-house


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Figure 1. Schematics of plasmonic color printing via the multiscale engineering of silver nanoparticles on a TiO2-capped quartz substrate. (A) Schematic illustration of precision photoreduction for plasmonic color printing. (B) Schematics of the light-controlled growth of silver nanoparticles and the FSEM cross-sectional views of the printed silver nanoparticles. (C) SEM images of the directly printed micropatterns. digital ultraviolet (UV) lithography system consisting of a digital micromirror device (DMD) with a million pixels or so dynamically generates predefined UV patterns. 20, 21 The projection optics demagnifies the UV patterns and projects them on the titanium dioxide (TiO2) photocatalyst layer under a quartz substrate to initiate and regulate the reduction of silver salts. Each pixel of the UV pattern individually irradiates the TiO2 to generate photoexcited electrons, and the silver ions in the surrounding solution are reduced to elemental silver nanoparticles on the TiO2 surface. High-speed DMD (up to 20 kHz) allows rapid generation and switching of UV


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patterns with a predefined time sequence, which enables the precise control of irradiation dose for regulating the growth of silver nanoparticles toward the target dimension for color generation. Figure 1B shows the mechanism of light-controlled growth of silver nanoparticles toward different dimensions, as revealed by corresponding scanning-electron microscopic (SEM) images. The substrates were prepared by spin-coating of acidulated titanate/isopropyl alcohol (IPA) solutions on quartz plates (see substrate preparation section in Materials and Methods). From the cross-sectional SEM images shown in Fig. 1B, it can be seen that a single layer of silver nanoparticles was fabricated on the substrate, and the dimension of silver nanoparticles gradually increases with the exposure dose. One can thus intuitively infer a heterogeneous process, which is very different from the liquid-phase synthesis of nanoparticles, 22-25 for the light-controlled growth of nanoparticles on the substrate. The TiO2 layer absorbs UV light to generate photoelectrons, which initiate the reduction of silver ions into elemental nanoparticles; when the UV light continues to illuminate the TiO2 layer, the newly generated electrons move across the previously generated silver nanoparticles to the outside surface and reduce more silver ions to create bigger silver nanoparticles. UV irradiation is switched off to stop the growth process when the silver nanoparticles grow to a target size. In addition to precise dimensional control, the technology can rapidly pattern silver nanoparticles into micron-scale structures simultaneously. Fig. 1C (i) and (ii) show the fabricated periodic arrays of micro-rods and split-rings, respectively. It can be seen that nanoparticles were well arranged to form predefined patterns. Notably, the resolution of the printing technology is associated with the expected AgNP size. Fig. S1 shows the AgNP patterns of different expected sizes produced by using the same 0.88-µm wide line-shape optical pattern. It can be seen that the


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linewidths of the printed AgNP patterns increase from 1.1 µm to 2.0 µm with the increase of average particle size from 17.0 to 54.3 nm. The phenomenon was caused by the proximity effect of the printing process, which makes AgNPs grow outside of the area under optical projection. Compared to optical proximity effect (i.e., non-zero optical exposure dose outside of the target area) in lithography process, it is more prominent here because of three aspects: 1) The patterns of relatively larger AgNPs need longer exposure time than those of small AgNPs, which thus magnifies the optical proximity effect because of incremental process; 2) The generated AgNPs scatter the illumination light, which further aggrandizes the optical proximity effect; 3) Photogenerated electron-hole pairs randomly diffuse in TiO2 layer and might react with silver ions outside of optical pattern to create nanoparticles. The diffusion effect is noticeable when a thick TiO2 layer is used in printing process. To study the color-generation ability of the samples, five groups of substrates, labeled S1, S2, S3, S4, and S5, with different thicknesses and effective indices (see Table S1) were prepared, and a pattern of Hong Kong bauhinia flower with five petals was used for printing tests. The exposure time of each petal was set to different values given by n×T0, where T0 is the base exposure time and n is an integer. For each group of substrates, different exposure doses ranging from 0.37 J/mm2 (corresponding to T0) to 7.37 J/mm2 (corresponding to 20T0) were applied to fabricate the five petals of the bauhinia pattern. The color images of fabricated Hong Kong bauhinia flowers using the substrates S1 – S5 are shown in Fig. 2A. It can be seen that only light yellow and yellow colors were obtained when the patterns were fabricated on substrates S1 and S2. The thickness of the TiO2 layer of substrate S2 is 28.7 nm. When the substrates S4 and S5 with a TiO2 layer thicknesses of 58.9 nm and 73.3


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Figure 2. Printed plasmonic color images and color palette. (A) Color images of Hong Kong bauhinia flowers printed on various substrates. (B) Color palette illustrates all colors generated at various fabrication conditions. The UV irradiation intensity for the printing process is 3397 mW/cm2, and the base exposure time T0 is 10.8 seconds. nm, respectively, were used in the experiments, more colors, such as magenta and dark blue, were created under varying exposure doses. Figure 2B summarizes the colors generated on the five substrates. As see, a robust chromatogram can be generated using the printing process. In particular, color can be tuned from


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light yellow, yellow, brown, magenta, dark blue, to royal blue when different exposure doses were applied to print the silver nanoparticles on substrate S5. Moreover, the experimental results revealed that the surface protector polyvinylpyrrolidone (PVP; K30) played an important role in the enhancement of color clearing of the samples. The molar ratio of PVP to AgNO3 is 5:1 in the printing of the color patterns in Fig. 2. If PVP was removed from the mixture, the colors of the printed samples became less vivid, as shown in Fig. S2. The reflection spectra of the silver nanoparticles printed on substrate S5 were measured as shown in Fig. 3A. A reflection dip appears in the short wavelength region (i.e., 400 – 500 nm) of

Figure 3. Measured reflection spectra of eight samples and their corresponding FSEM images of silver nanoparticles. (A) Reflection spectra of the samples. (B) FSEM images of the samples. The exposure doses of the samples increase from 1.11 J/mm2 to 5.53 J/mm2.


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visible light when the exposure dose is relatively low. With increasing exposure doses, the reflection dip shifts toward the middle band (i.e., 500 – 600 nm) of visible light, which leads to more reflection of red and blue light and consequently the appearance of purple and red purple. When the exposure dose was further increased, the reflection dip moves to the long wavelength region (i.e., > 600 nm) of visible light, resulting in stronger reflection of blue light. Compared with the reflection spectra of the samples fabricated without using PVP, as shown in Fig. S3A, the spectral dips of Fig. S3A (i) – (iv) are redshifted and broader than those using PVP, as shown in Fig. 3A (i) – (iv). Field-emission scanning-electron microscopic (FSEM) images of the samples are shown in Fig. 3B. It can be clearly seen that silver nanoparticles grew with increasing exposure dose. From the statistical analysis results shown in Fig. S4, the expected particle size increased from 33.1 to 106.7 nm when the exposure dose increased from 1.11 to 5.53 J/mm2. Meanwhile, the standard deviation of the size distributions increased from 8.85 nm to 60.61 nm, which is detrimental to the sharpness and uniformity of color. Notably, some silver nanoplates were observed when the particles grew bigger. However, the silver nanoparticles fabricated without PVP showed rounder corners with polyhedral structures, as shown in Fig. S3B. This observation is consistent with results for the liquid-phase synthesis of silver nanoparticles demonstrated by Jiang et al. 26 and Washio et al.,

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in which PVP played an important role in controlling the growth and

morphology of silver nanoparticles through adsorption on the surface and slowing of the reduction. To understand the optical responses of the fabricated silver nanoparticle-TiO2-quartz structures, we numerically calculated the reflection spectra of the structures by assuming that the particles are periodic arrays in a square lattice on the surface of a TiO2 layer deposited on a


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quartz substrate. In Fig. 4A (i), small nanoparticles are represented by spheres of diameter D, and the face-to-face inter-particle distance g was not larger than 60 nm and 50 nm, respectively. Larger nanoparticles are represented by mushroom-like structures with spherical bases and elliptical hats, as shown in Fig. 4A (ii). The vertical offset h between the bases and hats is assumed to be particle-size dependent, as h = D/3. The thickness and refractive index of the TiO2 layer are obtained from the measurement using an ellipsometer. The permittivity of the silver is

Figure 4. Numerical analysis of the silver nanoparticles-TiO2-quartz structures. (A) Models of silver nanoparticle-TiO2-quartz structures. The parameters D and g represent the size of silver nanoparticles and their inter-particle distance, respectively. (B) Simulated reflection spectra (black circles) and Fano-fittings of the eight silver nanoparticle-TiO2-quartz structures. The geometric models of the structures with different parameters (D, g) are given in the insets. (C) Electric near field |E| at the reflection peak (left panel) and the dip (right panel) for the geometric structure (40 nm, 20 nm).


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taken from the empirical data by Johnson and Christy. 28 The numerical analysis of the effects of the TiO2 layer thickness on the spectral responses of the silver nanoparticle-TiO2-quartz structures are shown in Fig. S5. The results revealed that a reflection dip will appear in the visible region and shift towards a longer wavelength with increasing TiO2 thickness, which explains the color change from yellow to violet to blue with the change in substrates from S1 to S5 in Fig. 2 and Fig. S2. Fig. 4B shows the calculated reflection spectra of the eight structures with different geometric parameters (D, g); the value of D ranges from 30 to 110 nm and g from 15 to 60 nm. A dip in reflection is visible in the short wavelength region when the particles are small and shifts toward the long wavelength region with increasing particle size, consistent with the measured spectra shown in Fig. 3A. The numerical analysis revealed that the reflection dips result from the generation of Fano resonances that form between the collective dipolar localized surface plasmon resonances (LSPRs) of the silver nanoparticles and the continuum background. The phase differences between the LSPRs and the background state produce either constructive or destructive interference of the reflected waves, which leads to Fano dips in the visible region and the reflection peaks at wavelengths below 400 nm, as shown in the short wavelength region in Fig. 4B. The simulated electric near-field distributions at the reflection peak (left panel) and the Fano dip (right panel) are shown in Fig. 4C, in which the geometric parameters of the structure are shown (40 nm, 25 nm). The dipolar resonance of the silver nanoparticles manifests as two lobes of plasmonic “hot spots” at the particle surface. To reveal the resonant properties of the silver nanoparticle-TiO2-quartz structures, the simulated reflection spectra near the peak and dip were fitted by using a simplified Fano-like formula incorporating intrinsic loss: 29, 30


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ω − ω0

)2 + q R Γ = , ω − ω0 2 R0 1+ ( ) Γ (p+

(1)

where R0 is the reflection of the background (w/o particles), ω0 and Γ represents the frequency and spectral width of the resonance,

p is an asymmetric factor of the spectrum, and q stands for

the intrinsic loss. The fitting results for the eight structures are also given in Fig. 4B. The fitted spectra agree well with the simulation results, confirming the Fano resonant characteristics of the silver nanoparticle-TiO2-quartz structures. One feature of the proposed printing technology is the ability to rapidly fabricate large-area plasmonic color images. Based on the palette generated above, the color of each pixel in an image can be converted to a specific exposure dose to precisely control the growth of the target silver nanoparticles. Figs. 5A (i) and (ii) show the printing flow and results of an image of the administration building of The Hong Kong Polytechnic University (PolyU). The original photo shown in Fig. 5A (i) was first converted into a grayscale image in Fig. 5A (ii), whose brightness in each pixel represents the exposure time calculated using the mapping relationship defined by the color palette in Fig. 2B. Fig. 5A (iii) shows the fabricated image of the campus building on the substrate S5. The size of the printed sample was 1.76×1.76 mm2, which was stitched by 36 sub-images with a size of 293×293 µm2. The exposure time for each sub-image was 90 seconds, which is associated with a UV irradiation intensity of 3397 mW/cm2. The technology can be scaled up for high-productivity printing if higher-power UV source or/and larger DMD with more pixels can be used in the setup. Further integration with micro-optics and complicated scanning strategy can power the technology toward high-resolution high-throughput color


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printing31, 32. More printed images, such as the logo of PolyU and Hong Kong city skyline, are shown in Fig. S6. To demonstrate the plasmonic nature of such Fano resonant structures, a printed image of telephone set was immersed into various solutions to test the spectral response of the structures to the change in the external refractive indexes. Fig. 5B shows the color changes of the printed

Figure 5. Printed plasmonic color images and testing of color images in solutions with different refractive indexes. (A) Printing of a campus building: i. original photo of the administration building of PolyU in bitmap format; ii. exposure dose map; iii. printed plasmonic-color image. (B) Optical images of a printed telephone image in various media: i. air; ii. water; iii. ethylene glycol aqueous solution; iv. limonene. (C) Comparison of the measured (solid curves) and simulated (dashed curves) reflection spectra of a printed color in various media.


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image in air followed by immersion in water (refractive index n=1.33), ethylene glycol (EG) aqueous solution (n=1.40), and limonene (n=1.47), sequentially. With the increase in the refractive index, the image appears brighter, with blue colors. Fig. 5C shows the evolution of the reflection spectrum of a printed color, as shown in the inert, in those surrounding media. As seen, the spectral dip shifted from 472 nm to 511 nm, 527 nm, and 562 nm, sequentially. With the estimated effective refractive indexes of the surrounding media (shown in Fig. S7), the numerical simulation showed a similar spectrum shift, which indicates that the simulation model represents well the fabricated samples, despite deviations caused by variations in particle size and geometry. Notably, the AgNPs-based color images are very stable in air at room temperature, although silver is generally considered to be prone to oxidation. As shown in Fig. S8, the printed colors remained nearly unchanged after stored in unsealed plastic petri dish for fourteen months. Such a good anit-corrosion property is consistent with Zhou et al.’s finding that most of silver nanoparticles prepared by the photoreduction technique are single crystals, 33 which are thus with low corrosion rate because of less crystal defects. Moreover, a thin layer of surface protector PVP might be absorbed on the surface of AgNPs and therefore reduces oxidation and other kinds of corrosion. 34, 35 The TiO2 layers of the substrates used in the aforementioned experiments were prepared by the sol-gel method and thus pertain to the amorphous phase. As the crystalline phase of TiO2 has a significant influence on photocatalytic efficiency, 36, 37 one can expect that the speed of printing could be further accelerated if the crystalline phase of TiO2 layer could be further optimized. To investigate this, we further sintered the TiO2 substrate by heating at 500 °C in air for 2 hours to convert its amorphous phase to the anatase phase, 38 and then repeated the printing experiments


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at the same conditions. Fig. S9 compares the colors of the samples printed using the two substrates. The colors generated under the exposure dose from 0 to 4.41 J/mm2 on the substrates with amorphous titanium-dioxide layer were replicated well using the substrate with an anatase titanium-dioxide layer, while the exposure dose was reduced to one ninth, i.e., from 0 to 0.509 J/mm2. On the other hand, the testing results revealed that one promising pathway to further improve the light-controlled growth of nanoparticles toward more uniform silver nanostructures is to engineer the photocatalytic layer on the quartz substrate. The control over morphology and orientation of semiconductor nanocrystals can provide the initial placement of “seeds” for the nucleation and growth of nanoparticles. Meanwhile, chemical or optical approaches, such as surface protector optimization and multi-wavelength plasmonic excitation, 39, 40 can be exploited to regulate the growth kinetics of silver nanoparticles for better size and shape control. Furthermore, some post-processing techniques, such as Ostwald ripening, reshaping,

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photothermal

are likely to further improve the uniformity or even reshape nanoparticles for

emerging plasmonic devices and applications. In summary, we have demonstrated an optical nanofabrication technology, precision photoreduction, for direct plasmonic color printing. Silver nanoparticles with controlled dimension have been precisely generated on a TiO2-coated quartz substrate to create plasmonic color images. The numerical simulation revealed that the silver nanoparticle-TiO2-quartz structure supports a Fano resonance whose spectral dip in the visible light regime can be tuned via the dimension of the silver nanoparticles for structural color printing. This technology offers a pathway for the rapid fabrication of large-area plasmonic micropatterns with metallic nanostructures, which thus have great potential in a large variety of applications, such as


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plasmonic metasurface devices and surface-enhanced Raman and infrared absorption spectroscopies.

Materials and Methods Materials. Silver nitrate (≥99.0%) and polybutyl titanate (≥97.0%) were purchased from Sigma-Aldrich Inc. Glucose anhydrous was purchased from the Guangzhou Chemical Reagent Factory. Polyvinylpyrrolidone (PVP) (K30) (average molecular weight 40,000) was purchased from the Sinopharm Chemical Reagent Co., Ltd. Nitric acid solution (1 mol/L aqueous solution) was purchased from the Shenzhen Huashi Technology Co., Ltd. Isopropyl alcohol was purchased from Anaqua Chemicals Supply Inc. Ltd. Quartz sheets (thickness: 1 mm) were purchased from Donghai County Zhongzheng Quartz Products Factory. All materials were used as received without further purification. Deionized (DI) water with a resistance of 18 MΩ-cm was used in all experiments. Substrate preparation. Polybutyl titanate was diluted with isopropyl alcohol (IPA) into five solutions, whose mass concentrations are 2%, 4%, 6%, 8%, 10%. Then, 1 mol/L nitric acid aqueous solution was added as a hydrolysis protectant to the solutions (volume ratio 1:100). These acidified polybutyl titanate sol solutions were used to prepare the TiO2 layer on quartz sheets by spin-coating (400 rpm / 12 sec + 3000 rpm / 60 sec, 20 ºC, 60% RH). The substrates were baked at 110 ºC for 10 minutes to remove the solvent and to convert titanate into TiO2 through hydrolysis. Corresponding to different concentrations of polybutyl titanate, five substrates, S1, S2, S3, S4, and S5, were


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prepared. The TiO2 photocatalyst layers were characterized by using a Sopra GES5E ellipsometer and Semilab - Spectroscopic Ellipsometry Analyzer Software. The results are summarized in Table S1. Printing processes. A silver salt solution with 0.125-mol/L silver nitrate (as silver ion source), 0.125-mol/L glucose (as a hole scavenger) and 0.625-mol/L PVP K30 (as a surface protective agent) was used for printing. Glucose and PVP were fully dissolved under stirring in DI water, while silver nitrate was separately dissolved and mixed with other components before printing. Quartz substrates with the TiO2 layer were cleaned by rinsing with IPA and blown dry with nitrogen gun. A glass slide with a glass spacer was used to contain a silver salt solution, upon which quartz substrate was placed upside-down (that is, a photocatalyst layer contacted the silver salt solution). In-house digital ultraviolet (UV) lithography was used to print silver nanoparticle-based color images. UV light from an UV lamp (OmniCure 2000 System, Lumen Dynamic Group Inc.) was collimated to illuminate a digital micromirror device (DMD, DLi6500 0.65" 1080p with 1920×1080 pixels / DLi4120 0.7" XGA with 1024×768 pixels, Texas Instruments). The image data generated by using 3D model slicing software was loaded onto the DMD in a specific time sequence to dynamically generate predefined light patterns. UV patterns were then dynamically projected upon the photocatalysis layer of the quartz substrate for the light-controlled growth of silver nanoparticles through photoreduction. After exposure, the quartz substrate with silver nanoparticles was rinsed by using DI water and IPA sequentially and then blown dry with a nitrogen gun. With configurable reduction projection optics, the optical resolution (i.e., the pixel size of light patterns) of the digital UV lithography setup was set to either 0.3 ~ 0.5 or 2.7 µm for high-resolution and moderate-resolution printing processes, respectively, and the corresponding


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intensities of UV irradiation are 3397 mW/cm2 and 613.9 mW/cm2, respectively. The highresolution configuration was adopted to fabricate the samples shown in Figs. 1B, 1C, 2, 3, 5A, 5B, S1, S2, S3, S4, S6, and S9, while the moderate-resolution configuration was used to fabricate the sample shown in Fig. 5C, S8. Morphology and optical spectral characterization. Color images of the samples were measured by using the optical microscope module of the Keyence VK-X200 3D laser scanning confocal microscope. The samples were placed upsidedown on a holder, and the microscope took photographs from the top in reflection mode (i.e., the light of the microscope passed through the quartz sheet and TiO2 layer before illuminating the silver-nanoparticle patterns). Electron microscope images of silver nanoparticles were taken by using field-emission gun scanning electron microscopy (Hitachi S4800). The samples of the silver nanoparticles were coated by a thin carbon conductive layer before measurement. To take images of the cross sections, the samples were cut in half and mounted in a tilted holder for observation. The optical spectra of the samples were measured by using a UV-VIS spectrometer (USB2000+, Ocean Optics), a high-power tungsten-halogen light source (ASBN-W50, Spectral Products), and a fiber-optic reflection probe bundle. The light from the light source was guided by using optical fiber and collimated to illuminate the sample. The reflected light was collected by using the same collimation objective and was guided to the spectrometer with an optical fiber. Numerical simulations. Commercial finite-element analysis software (COMSOL Multiphysics) was employed to numerically simulate the spectral responses of the silver nanoparticle-TiO2-quartz structures. The


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silver nanoparticles are modeled as periodic arrays in a square lattice, as schematically shown in Fig. 4A. A linearly polarized plane wave was assumed as the excitation light, and the structures were assumed to be illuminated at normal incidence from the side of the quartz substrate.

ASSOCIATED CONTENT Supporting Information Fig. S1. Characterization of printing resolutions with respect to nanoparticles of different expected sizes. Table S1. Ellipsometric measurement results of the titanium dioxide layers on quartz substrates. Fig. S2. Printed color images without the use of PVP. Fig. S3. Spectral responses of the samples printed without the use of PVP and the FSEM images of corresponding silver nanoparticles. Fig. S4. Particle-size distribution of the silver nanoparticles. Fig. S5. Numerical study of the effects of TiO2 thickness on reflection spectra. Fig. S6. Collection of printed plasmonic color images. Fig. S7. Effective refractive indices of the media surrounding silver nanoparticles. Fig. S8. Comparison of the printed color images before and after fourteen months of storage. Fig. S9. Comparison between the two groups of colors printed on substrates with amorphousphase and anatase-phase TiO2 layers. Reference (42)

AUTHOR INFORMATION ORCID Dang Yuan Lei: 0000-0002-8963-0193


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ACS Nano

A. Ping Zhang: 0000-0003-2469-5225 Hwa-Yaw Tam: 0000-0002-8441-4846

Author contributions Y.X. Z. performed the plasmonic color printing and testing experiments. Q.Z. and D.Y.L. performed the numerical simulation and theoretical analysis. X.O. measured the optical reflection spectra. H.T. participated in the analysis of optical response and provided feedback on the experiments; A.P.Z. conceived the experiments and wrote the manuscript. All authors reviewed the manuscript.

Funding Sources This work was partially supported by the PolyU Strategic Development Special Project (Grant No.: 1ZVGB) and NSFC/RGC Joint Research Scheme (Grant No.: N_PolyU517/15). The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors would like to thank the Electron Microscope Unit in the University of Hong Kong for their help in field emission scanning electron microscope (FSEM) observation, and the USTC Center for Micro and Nanoscale Research and Fabrication, for their help in ellipsometry measurement.

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ACS Nano

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Table of Content (TOC) figure:


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Ultrafast Light-Controlled Growth of Silver Nanoparticles for Direct

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