Laser-Induced Dewetting of Metal Thin Films for Template-Free

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Functional Nanostructured Materials (including low-D carbon)

Laser-Induced Dewetting of Metal Thin Films for Template-Free Plasmonic Color Printing Harim Oh, Jeeyoung Lee, Minseok Seo, In Uk Baek, Ji Young Byun, and Myeongkyu Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13675 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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ACS Applied Materials & Interfaces

Laser-Induced Dewetting of Metal Thin Films for Template-Free Plasmonic Color Printing Harim Oh,# Jeeyoung Lee,# Minseok Seo,# In Uk Baek,& Ji Young Byun,& and Myeongkyu Lee*,#

#Department

of Materials Science and Engineering, Yonsei University, Seoul 120-749,

Korea &Materials Architecture Research Center, Korea Institute of Science and Technology, Seoul 02792, Korea

ABSTRACT: Plasmonic color laser printing has several advantages over pigment-based technology, including the absence of ink and toner and the production of non-fading colors. However, the current printing method requires a template that should be prepared via nano-fabrication processes, making it impractical for large-area color images. In this study, we show that laser-induced dewetting of metal thin films by a nanosecond pulsed laser can be effectively utilized for plasmonic color printing. Ag, Au, and their complex films deposited on a glass substrate were dewetted into different surface structures such

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as droplets, rods, and ripples, depending on the incident laser energy. The resulting morphological evolutions could be explained by Rayleigh and capillary instabilities. For a bimetallic film comprising Ag nanowires coated on a Au layer, a few different plasmonic colors were generated from a single sample simply by changing the laser fluence. This provides a possible method for implementing plasmonic color laser printing without using a pre-patterned template.

Keywords: Dewetting, Metal thin film, Plasmonics, Laser, Color printing. 1. INTRODUCTION

Plasmonic colors are structural colors that emerge from resonant interactions between light and metallic nanostructures.1-3 The engineering of plasmonic colors is a rapidly developing, promising research field that has a wide variety of applications, including solar cells,4,5 plasmonic sensors,6 filters for color imaging,7-9 flat-panel displays,10-12 and product identification and anti-counterfeiting.13 Surfaces decorated with structural colors are also receiving tremendous attention owing to their widespread use. Metal nanostructure-based coloration employs the spectral tunability of localized surface


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plasmon resonance (LSPR), thus, it is highly sensitive to the shape, size, and material of the nanostructure.14-16 This means that different shapes or sizes are required to produce different colors. Au, Ag, and Al are the most common plasmonic materials. Subwavelength-scale plasmonic color printings have been extensively investigated.17-20 A fundamental problem with these subwavelength plasmonic printings is that the color patterns must be carefully designed and fabricated via electron-beam (e-beam) lithography or focused ion beams. While both processes allow the fabrication of nanoscale features with high precision, they are neither scalable nor economical. Recently, a method of laser printing on nanoimprinted plasmonic surfaces has been reported,21,22 where laser pulses melt and reshape the imprinted nanostructures. The presented color printing technology has several advantages over pigment-based technology, including the absence of ink and toner and the production of non-fading colors. Additionally, it is possible to save data invisible to the naked eye. However, this template-based printing may not be applicable for the production of large-area color images; the template is prepared via e-beam lithography and nanoimprinting. From a practical viewpoint, a printing scheme that does not require a pre-patterned template should be developed. A


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lithography-free laser printing method has also recently been suggested,23 which is based on the direct local ablative modification of a Au film with tightly focused femtosecond laser pulses. According to the method, the laser pulse locally melts a circular microscale area on the film, which detaches from the underlying glass substrate via acoustic relaxation and resolidifies in the form of a parabolic cupola. These parabolic nanovoids support different types of LSPR and produce different colors depending on their size. However, the colors produced by this approach were not only polarizationdependent but also poor in uniformity and purity. In addition, the implemented printing area was as small as 0.01 mm2. A more scalable and economical method, that still produce diverse colors of good uniformity, is highly desired. Nanosecond lasers are widely used, cost-effective laser sources that occupy less space than femtosecond lasers.

In this article, we examine the morphological evolutions of metal thin films under nanosecond laser irradiation. The aim of the study is to exploit the resulting morphological change for spectrally tuning the LSPR and thus generating different


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plasmonic colors. It is well known that heated metal thin films can be dewetted into droplets or other nanostructures as a result of Rayleigh instability and/or capillary instability.24-30 This instability-induced morphological change can develop into a viable processing route if it is controllable.31-36 The dewetting of Ag, Au, and their complex films by a nanosecond-pulsed laser on glass substrate was investigated. The surface structures of these films changed into droplets, ripples, and broken ripples, as the laser fluence increased. The resulting morphological evolutions (the formation of these structures) could be explained either by the Rayleigh/capillary instabilities or the interference of the incident laser with the propagating surface plasmon polaritons (SPPs). For the bimetallic film comprising Ag nanowires (AgNWs) and a Au layer, a few different plasmonic colors were generated from a single sample simply by changing the laser fluence. This finding indicates potential for template-free plasmonic color laser printing.

2. EXPERIMENTAL


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Four different types of metal thin films were investigated: Ag thin film, Au thin film, Ag/Au bilayer film, and AgNWs/Au bimetallic film. The Ag and Au layers were deposited onto a glass substrate (Marienfeld-Superior, microscope slides of 1 mm thickness, 2.5 cm  2.5 cm wide) using a thermal evaporator. The monolayer film had thicknesses of 5 and 10 nm. For the Ag/Au bilayer film, each layer was 5 nm thick. The deposition chamber was evacuated to less than 10-5 Torr using a vacuum pump and the deposition rate was maintained at 1 Å/s. The thicknesses of the layers were controlled by a controller equipped with the evaporator and were checked by a surface profiler. A 1 wt% solution of AgNWs dispersed in ethanol (diameter = 32 ± 5 nm, length = 25 ± 5 μm) was supplied by NANOPYXIS Inc. After being diluted to 0.3 wt%, the solution was sonicated for 5 min. A 200 μL drop of the AgNW solution was then spin-coated onto a 5 nm Au layer-deposited glass substrate at 1,000 rpm for 1 min, followed by drying for 5 min at 100oC. The sonication step helped to obtain more uniformly coated AgNWs by reducing their average length to half the original length. A nanosecond-pulsed ultraviolet (UV) laser (Coherent AVIA 355-5; wavelength = 355 nm, pulse width < 20 ns, repetition rate = 30 kHz, maximum output power = 5.0 W, output beam diameter = 2.85 mm) was


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employed as the laser source. The output laser beam was made incident onto a galvanometric scanner. An F-theta lens (focal length = 205 mm) was combined with the galvanometric scanner to maintain a uniform spot size on the film surface. The sample was placed on a z-translation stage and the laser spot size on the sample surface was varied by vertically displacing the stage from the focal plane. The laser beam was automatically steered by computer software that controlled the gavanometric system. To observe the effect of dielectric environments, a dielectric layer of TiO2 or SU-8 was coated onto the laser-irradiated sample. The TiO2 layer was deposited using a directcurrent (DC) magnetron sputtering system by sputtering a Ti target reactively under the flow of 5 sccm Ar and 5 sccm O2. The SU-8 layer was spin-coated at 6,000 rpm for 1 min. Its thickness was controlled by varying the SU-8 solution concentration and measured by a surface profiler. Surface treatment of the sample was conducted using O2 plasma, at a DC power of 140 W for 40 s under an O2 flow rate of 52 sccm. Transmission spectra were measured using a UV-visible spectrophotometer (Model: Jasco V-530). Structural analysis was performed using a field-emission scanning electron microscope (Model: JSM-7001F, JEOL Inc. 15 kV) and a spherical aberration-


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corrected transmission electron microscope (Model: JEM-ARM 200F, JEOL Inc.). For transmission electron microscopy (TEM) analysis, nanostructure pieces were collected using a razor blade. Color images of the samples were captured by a digital camera without using a polarizer. They were taken at near-normal angles under a fluorescent lamp in the laboratory.

3. RESULTS AND DISCUSSION

Fig. 1(a) schematically illustrates the concept of our template-free color laser printing. A printed image is shown in Fig. 1(b) along with its target image. Our approach is to create different dewetting morphologies by controlling the laser pulse energy. The different morphologies can support different plasmonic resonances, ultimately leading to different color appearances. Dewetting refers to the decomposition of a film into droplets or other structures on an inert substrate. It is driven by the minimization of the total energy of the free surfaces and interface of the film and substrate. Dewetting can be divided into solid-state dewetting and liquid-phase dewetting. While the solid-state


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dewetting35 is typically performed inside a furnace or on a hot plate at temperatures lower than the melting temperature, energetic sources like lasers are necessary for the liquid-phase dewetting. The use of a laser beam allows for position-dependent, selective dewetting. Laser-induced dewetting using nanosecond laser pulses has been extensively investigated.24-27,30,36 Multiple laser pulses are often required for complete dewetting (i.e., to completely transform a continuous film into droplets) when the liquidphase has a lifetime much smaller than the dewetting time.25 It has recently been demonstrated that Ag thin films can be completely transformed into nanoparticles by a single laser pulse with high energy on a glass substrate.30 A very thin film is hardly dewettable because laser energy absorption is too low. When the film has a thickness larger than a certain critical value, it can be delaminated before dewetting.30 The initial film thickness is also a crucial factor that determines the mean particle size.24,25,30 We paid attention to the fact that the dewetted surface structure may be significantly influenced by the total energy absorbed by the film. The current work shows that several different surface structures can be produced from a single thin-film system once the laser energy impinging on the film is controlled. Of course, this does not necessarily


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guarantee the production of many different plasmonic colors, because dewetted structures are generally random rather than regular periodic structures. Therefore, different structures may exhibit similar colors. That is, the generation of multiple surface morphologies is a required condition for template-free color laser printing, not a sufficient condition. Here, we define the laser fluence as the energy per unit area incident onto the film. Although less energy is actually absorbed by the film, owing to surface reflection, the absorbed energy is proportional to the laser fluence. For a fixed spot size, two experimental parameters determining the fluence are the laser power and exposure time. As the laser fluence increased, the film exhibited gradual morphological transformations into droplets, rods, ripples, broken ripples, and large particles.

As an experimental result, Fig 2 shows how the surface morphology of a 10 nm Ag film changed with the increase of the laser power. A laser spot with a diameter of 275 m was scanned over the film at a fixed scan rate of 2 mm/s. At a relatively low power, the film broke up into droplets (Fig. 2(b)). This morphological evolution can be explained by capillary-driven instabilities.27,30,36,37 Considering the irradiation condition, the


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dewetting of the film should occur in the liquid phase. In solid-state dewetting by heat treatment, mass transport is primarily determined by surface and bulk diffusion and generally exhibits much slower time scales compared to transport in the liquid phase. Thus, the surface and volume diffusion is excluded from the mechanism. In general, a continuous film breaks up into droplets via one of three mechanisms: 1) homogeneous nucleation caused by a small thermal density fluctuation, 2) heterogeneous nucleation initiating from a defect, and 3) spinodal dewetting that occurs by the amplification of film thickness fluctuation (i.e., capillary wave). When the molten phase of a thin film poorly wets the substrate, it can be dewetted into droplets. The formation of droplets usually requires large fluctuations, and these fluctuations may occur at grain boundaries. As shown in the inset of Fig. 2(a), thermally evaporated films generally have grooves at the grain boundaries. Dewetting can initiate at these grooves by forming tiny voids there, as observed previously.27,30 The material ejected from the void accumulates around the periphery, which leads to the thickening of the film edge. The thickened edge of the retracting film can produce discrete droplets owing to capillary instabilities. The sizes of dewetted particles were measured using the SEM image of Fig. 2(b). The particles


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showed a monomodal size distribution (Figure S1, Supporting Information). The average particle size (D) was ~155 nm and the mean spacing (S) between particles was ~175 nm. According to the hydrodynamic spinodal dewetting theory,25 D and S increase in proportion to h5/3 and h2, respectively, where h is the film thickness. In the current plasmonic printing study, the film thickness was fixed at 10 nm. A quantitative analysis of the theory-experiment consistency needs more experiments with films of varying thickness. As the laser power increased, the surface structure gradually changed into rods, ripples, broken ripples, and large particles. Fig. 2(c) shows the state in which periodic ripples are about to decompose into droplets. In Fig. 2(d), these droplets are agglomerated to form larger particles. The breakup of the ripple structure can be explained by the Plateau-Rayleigh instability,38-40 which is simply called the Rayleigh instability. Plateau found that a falling stream of water breaks up into droplets when its length-to-diameter ratio exceeds a certain value.41 Rayleigh developed a theory on the stability of liquid jets.42 Rayleigh’s theory was extended to solids by Nichols and Mullins.43 One-dimensional (1D) structures fluidized by laser irradiation will not maintain


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their initial shapes, owing to the Rayleigh instability, and ultimately break up into droplets.

Fig. 3 shows a periodic ripple structure formed from a 5 nm-thick Ag thin film. The direction of the ripples was perpendicular to the polarization direction of the laser. Such a regular 1D structure was achievable only in a very narrow fluence range. The inset of Fig. 3 shows that the formed ripple structure has diameter perturbations along the ripple length. As a result, the structure readily broke up into individual droplets even when the laser power increased slightly. Consequently, well-aligned periodic ripple structures were only statistically reproducible. Laser-induced periodic surface structures44-54 have been observed on a variety of materials including metals, semiconductors, and dielectrics. It is already known that linearly-polarized pulsed laser beams can lead to subwavelength, aperiodic/periodic ripples. A widely accepted mechanism for the formation of such subwavelength ripples is the interference of the incident laser and SPPs.46,49,50,51 When irradiated by ultrashort laser pulses, semiconductors or dielectric materials form a thin surface layer with a high density of free electrons. The existence of


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abundant hot electrons makes the material behave as a metal, regardless of whether it is intrinsically a semiconductor or dielectric. It has been shown that on a metallic surface, SPPs can be efficiently excited by sub-wavelength apertures under laser irradiation.55-58 A focused pulsed laser beam may play a similar role to the aperture. It has also been proposed that SPPs can be launched by a sufficiently rough surface or surface defects.46 SPPs can be excited when the following dispersion relation is satisfied.15,16

1/ 2

   d  s    m   m d 

(1)

Here,  and s are the wavelengths of the incident laser and SPPs, respectively. m is the real part of the dielectric function of the metal, and d is the dielectric constant of the surrounding medium (d = 1 for air). When the laser beam is incident at an angle  with respect to the surface normal, it interferes with the propagating SPPs, forming a fringe whose grating vector G is given by G = Ki – Ks. The magnitudes of the wave vectors Ki and Ks are 2 /  and 2 /  s , respectively, and the grating vector G has a magnitude of


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2 /  f , where  f is the fringe period. Considering the components along the surface, the ripples formed by the laser-SPP interference have a spatial period,  , expressed as follows.



   s  sin 

(2)

For normal incidence ( = 0o), the period is  = s. A literature value (m = -2.0435 at  = 355 nm)59 for the real dielectric constant of Ag leads to  = 254 nm, which is close to the experimental period of 230 nm (see the inset of Fig. 3). The ripple orientation, which is orthogonal to the laser-beam polarization, supports the laser-SPP interference mechanism. Increasing the exposure time (i.e., the number of pulses) resulted in similar morphological evolutions (Figure S2), while the laser power had a more profound effect on the surface morphology. For a low-repetition rate laser, each pulse is thermally independent because the inter-pulse interval is relatively long. Therefore, heating and cooling are repeated by multiple pulses. The laser used in this work had a high repetition rate of 30 kHz. Thus, the pulses may have cumulatively increased the temperature of the material.


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All samples exhibited nearly the same trend in their morphological transformation. Their surface structures gradually changed into droplets, ripples, and broken ripples, as the laser fluence increased. A slightly different situation was observed for the AgNWs/Au film. Fig. 4 shows how its surface morphology changed with the increasing laser power. A laser beam with a diameter of 275 m was scanned over the film in a line-by-line fashion at a scan rate of 10 mm/s. The scanning area was approximately 10 mm  10 mm. The insets of Fig. 4 show the sample colors captured by a digital camera. The as-coated AgNWs exhibited a typical random network (Fig. 4(a)). Fig. 4(b) shows the morphology and color of the film observed after irradiation at a power of 1.5 W. Interestingly, the AgNWs were decomposed into droplets before the Au layer. As the power increased to 3.0 W, the whole film transformed into a particulate structure consisting of large particles and relatively smaller particles (Fig. 4(c)). The small particles had a higher number density than the large particles. It appears that the AgNWs broke up into large droplets owing to the Rayleigh instability, while the Au layer was dewetted into smaller droplets by the capillary-driven instability. Further increasing the power to 4.5 W resulted in an aperiodic ripple structure, as shown in Fig. 4(d). The


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transmission spectra measured for these surface structures are plotted in Fig. 5. Metals have a very small skin depth in the visible range; it varies from a few nanometers to tens of nanometers, depending on their conductivity and the frequency of incoming light. This accounts for the general opacity of bulk metals. Some metals (e.g., Au and Cu) exhibit characteristic colors because they reflect specific wavelengths more strongly than others. If a metal is made very thin, it can be partially transparent. While the color of a bulk metal is determined by reflected light, the color of a thin metal film is more influenced by transmitted light. Bulk Au appears reddish yellow because the imaginary refractive index of Au increases with the visible wavelength; thus, red wavelengths are strongly reflected. Consequently, a thin Au film appears greenish blue because it predominantly transmits shorter visible wavelengths (see the inset of Fig. 4(a)). The asdeposited AgNWs/Au film showed a broad transmittance dip centered at 650 nm, which is related to the Au layer. A small absorption peak was observed around 350 nm. This is attributed to the transverse plasmonic resonance of the AgNWs.60,61 Because this LSPR peak is located in the UV region, it does not influence the color of the film. However, the resonance peak may act to efficiently absorb the laser energy. This may be responsible


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for the fact that the AgNWs underwent a transformation before the Au layer. When the film was irradiated at a power of 1.5 W, some of the AgNWs transformed into droplets, while the Au layer remained in a nearly continuous state. Thus, the broad transmittance dip slightly blue-shifted, making the film appear more blue. The particulate structure of Fig. 4(c), which was obtained at 3.0 W, produced a broad, asymmetric LSPR peak near 475 nm, simultaneously increasing the transmittance at long visible wavelengths. Because the Au layer was no longer continuous, the surface reflection was significantly suppressed. The structure consisted of Ag-rich large particles and Au-rich small particles. The size distributions of these particles are plotted in Figure S3. The Ag-rich large particles showed a mean size of 62 nm, while the average size of the Au-rich small particles was ~30 nm. The broad LSPR peak and the resulting dark yellowish color may arise from the combined effects of the different particle sizes and compositions. As well known, the LSPR wavelength depends on the material and size of the particles. The LSPR peak of Ag nanoparticles is located at shorter wavelengths than that for Au particles with similar sizes. The resonance peak red-shifts as the particle size increases. Thus if the Ag-rich large particles and Au-rich small particles


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give resonances at wavelengths close to each other, we would have a single broad peak instead of two separate peaks. As the power increased to 4.5 W, the film changed to the aperiodic ripple structure shown in Fig. 4(d). The resonance peak became stronger and shifted to 520 nm, making the sample appear pink. With powers between 1.5 and 4.5 W, intermediate structures and colors were produced (not shown). However, these colors were often not distinct or uniform. Regarding the diversity of available colors, the AgNWs/Au film was the best among the investigated samples. This is attributed to the bimetallic nature of the induced structures. Elemental mapping was thus performed using an energy-dispersive X-ray spectroscopy (EDS) system attached to the scanning electron microscopy (SEM) apparatus. However, strong oxygen signals from the substrate made it difficult to analyze the compositional distribution of the structure. Although elemental analysis via TEM-EDS showed that Au and Ag are mixed within some collected materials (Figure S4), compositional mapping over a large area using TEM is fundamentally limited.


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The resonance properties of plasmonic structures depend not only on their geometry but also on the local dielectric environment: the LSPR peaks shift to longer wavelengths as the refractive index (n) of the surrounding medium increases. In an attempt to produce more different colors, the effect of a dielectric overlayer or surface treatment was investigated. Fig. 6(a) shows the change in the transmittance spectrum of a laserinduced structure when a 50 nm-thick TiO2 layer (n = 2.4 – 2.5) was deposited over it. As expected, this dielectric layer red-shifted the LSPR peak. Fig. 6(b) shows how the colors of three different structures changed with a 50 nm-TiO2 overlayer. A color image printed on an AgNWs/Au sample is depicted in Fig. 7(a). Different colors were printed under different laser fluences. Fig. 7(b) shows an image taken after a 50 nm-thick TiO2 overlayer was deposited. While the corresponding color change is consistent with Fig. 6(b), the constituent colors moved significantly towards red, thus being close to one another. Although this effect was mitigated by depositing a thinner SU-8 layer with a lower refractive index of n = 1.59 (Fig. 7(c)) or introducing a surface oxide layer via O2 plasma treatment (Fig. 7(d)), distinct red/green/blue (R/G/B) colors have not yet been produced. To produce more colorful images, we need to have resonance peaks at


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shorter wavelengths. Plasmonic colors are subtractive colors; thus, green is more difficult to produce than red and blue. It requires two absorption peaks on both sides of the green region. While Ag/Au and AgNWs/Au are both bimetallic, the former film exhibited different resonance behaviors. As shown in Figure S5(a), the induced ripple structure gave two spectrally separated peaks, making the colors of the ripple and particulate structures similar to each other. Consequently, the printed image was almost mono-colored (Figure S5(b)). Thermodynamically, Ag and Au are completely miscible for all compositions. The appearance of the two separated peaks implies the existence of two distinct plasmonic moieties. It also indicates that laser-dewetting processes are not always equilibrium processes and thus cannot be explained simply by thermodynamics for bulk state. A recent theoretical study62 showed that for the nanoscale Au-Ag alloy system, the phase diagram is shifted downward with a “tilting effect” on the solidus-liquidus curves for some particular particle shapes. The segregation calculation revealed the preferential presence of Ag at the surface. It is not yet clear whether these two moieties result from an abrupt compositional change or from other factors. If the resonance peak at the longer wavelength had underwent a


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greater red-shift, green color could have appeared. We also attempted to print images on Ag and Au films. However, diverse colors were not produced (Figure S6). Since dewetted structures are random rather than regular periodic structures, different structures may exhibit similar colors. It is to be noted that the colors shown in Fig. 4, Fig. 6, and Fig. 7 are transmission colors taken by placing a white paper behind the sample. Reflection colors obtained with a black paper placed behind the sample were not so vivid as the transmission colors (Figure S7). Plasmonic resonances can be strongly influenced by surface morphologies. The surface morphologies may depend on laser irradiation conditions (beam size, pulse width and repetition rate, scan rate etc.) as well as structural factors (film thickness, density of AgNWs). In the current study, color printing was carried out using an F-theta lens with a focal length of 205 mm. The use of an objective with different focal lengths may lead to better results. The surface morphology will be finely tuned by adjusting these parameters in our future work. The color gamut is still limited, and further research is thus necessary to generate more diverse colors, including the R/G/B primary colors. It has yet to find out whether the process is applicable to flexible substrates. Nonetheless, the present work introduces a


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route for template-free, scalable plasmonic color laser printing. A similar approach to produce structural colors has been proposed,63 in which silica nanowires are decorated with thermally dewetted metal nanoparticles. In this method, however, the produced color is fixed once the used materials are fixed. As already described, our approach enables the tuning of colors simply by varying the laser fluence. Besides their utilization for color printing, the periodic ripple structures are very effective for structural color filters.64,65 Large-area, reproducible fabrication of such structures is also an important future-research topic.

4. CONCLUSION

Plasmonic color laser printing is a promising technology for engineering structural colors, having a wide variety of applications ranging from optoelectronic devices to surface decoration. However, the current printing method requires a template that should be prepared via nano-fabrication processes, making it impractical for the production of large-


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area color images. In this study, we present a scalable laser printing method that does not involve the use of a pre-patterned template. Our approach is to dewett metal thin films using a nanosecond pulsed laser and to create multiple dewetting structures by controlling the incident laser energy. This method is fundamentally based on the fact that different surface structures can support different plasmonic resonances, ultimately leading to different color appearances. For the bimetallic film consisting of Ag nanowires coated on a Au layer, a few different plasmonic colors were obtained from a single sample simply by changing the laser fluence. Thus, our results provide a method for realizing plasmonic color laser printing over large areas.


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ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publication website at http://pubs.acs.org. Size distribution of dewetted particles: Fig. S1, SEM images of the sample: Fig. S2, Size distributions of dewetted particles: Fig. S3. EDS analysis by TEM: Fig. S4, Transmission spectra, SEM images, and a color printing image: Fig. S5, Printed color images: Fig. S6 and Fig. S7.

AUTHOR INFORMATION

Corresponding Author * Phone: +82 2 2123-2832; fax: +82 2 312-5375; e-mail: [email protected] (M. Lee).


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Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the R&D convergence program of the National Research Council of Science & Technology of the Republic of Korea (NO. CAP-16-10-KIMS) and National Research Foundation of Korea (NRF) grants funded by the Korea government (NRF-2015R1A2A1A15053000 and 2015R1D1A1A09058787).

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Figure 1 (a) Schematic of template-free color laser printing. (b) Printed image compared with its target image. The image was printed on a bimetallic film consisting of AgNWs coated over a 15 nm-thick Au layer. Printing was performed using a laser spot 635 m in size under two different irradiation conditions (laser power = 3.5 W and exposure time = 1 ms, laser power = 4.5 W and exposure time = 3 ms). The number of dots per inch (DPI) is 2,000. The printed image shown in (b) was obtained at the very early stage of


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research on this topic; it was printed on a film consisting of AgNWs and a 15 nm-thick Au layer, while the results of Fig. 4 through Fig. 7 are from films consisting of AgNWs and a 5 nm-thick Au layer.

Figure 2 SEM images showing the surface morphological change in a 10 nm-thick Ag film. A laser spot 275 m in size was scanned over the film with a scan rate of 2 mm/s at different powers (P). (a) As-deposited state (P = 0), (b) P = 0.5 W, (c) P = 0.8 W, and (d) P = 1.5 W. All images except for the inset have the same magnification.


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Figure 3 Periodic ripple structure formed from a 5 nm-thick Ag film (laser spot size = 275 m, scan rate = 2 mm/s, laser power = 3 W).

Figure 4 SEM images showing the surface morphological change in an AgNWs/Au film with laser power (P). (a) As-deposited state (P = 0), (b) P = 1.5 W, (c) P = 3.0 W, and (d) P = 4.5 W. A laser beam with a diameter of 275 m was scanned over the film in a line-by-line fashion at a scan rate of 10 mm/s. The scanning area was 10 mm  10 mm.


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All SEM images have the same magnification. The insets show sample colors captured by a digital camera. The color images are 0.9 cm  0.9 cm wide.

Figure 5 Transmission spectra measured for different surface structures. The spectra were obtained from the four different surface structures shown in Figure 4.


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Figure 6 Effect of a 50 nm-thick TiO2 overlayer. (a) Transmittance spectrum change for a laser-induced structure. (b) Colors of three different structures measured before and after a 50 nm TiO2 layer was deposited.

Figure 7 (a) Color image printed on an AgNWs/Au sample. The image (700 DPI) was printed using a laser spot 275 m in size under three different irradiation conditions (4.5 W and 1 ms, 3 W and 1 s, 1.5 W and 1 s). (b, c) Images taken after a 50 nm TiO2 layer and a 12 nm SU-8 layer was deposited, respectively. (d) Image captured after the sample shown in “(a)” was surface-treated by O2 plasma.


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Laser-Induced Dewetting of Metal Thin Films for Template-Free

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