Switchable Plasmonic Film Using Nanoconfined Liquid Crystals - ACS

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Switchable Plasmonic Film Using Nanoconfined Liquid Crystals Seong Ho Ryu, and Dong Ki Yoon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07693 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 6, 2017

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ACS Applied Materials & Interfaces

Switchable Plasmonic Film Using Nanoconfined Liquid Crystals Seong Ho Ryu,† and Dong Ki Yoon*† † Graduate School of Nanoscience and Technology and KINC, KAIST, Daejeon 34141, Republic of Korea

ACS Paragon Plus Environment

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ABSTRACT: Structural coloration using plasmonic particles has received substantial attention due to its robust, permanent, and scalable characteristics across a full color range. In this study, a plasmonic structure based on a porous anodic aluminum oxide (AAO) film coated with a metallic film was fabricated. Colors were varied by changing the refractive index, which was achieved with a convolution with nanopores of AAO film and an infiltrated liquid crystal (LC) material. LC molecules confined in the porous AAO film were uniformly aligned, and they exhibited pore-size-dependent colors because of the specific refractive index. The thermal phase transition of the LC material in the nanopores changed the effective refractive index, switching the reflected colors and the LC-infiltrated AAO remained stable over a month. We believe LC materials can extend the use of rigid conventional plasmonic structures from simple sensor applications to multifunctional uses such as color printing, writing pens, and displays.

KEYWORDS: Plasmonic, Anodic aluminum oxide (AAO), Confinement, Liquid crystals, Switchable

1. INTRODUCTION Unusual optical properties in metal nanoparticles (NPs) are responsible for electron oscillation on the surfaces of the NPs when they are excited by light, a phenomenon known as localized surface plasmon resonance (LSPR), thus revealing specific colors.1-3 This light absorption phenomenon resulting from LSPR is highly sensitive to changes in the local refractive index of the surrounding media, which can be used in practical bio-chemical sensing applications.4,5


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Particularly, anisotropic plasmonic NPs can be self-aligned via liquid crystal (LC) self-assembly and the corresponding optical properties are tunable as the LC orientation is changed.6,7 Furthermore, helical structures fabricated by self-assembled LCs itself strongly interacts with light, producing specific colors.8-10 More interesting optical characteristics are observed when LSPR occurs in a two-dimensional (2D) periodic arrangement of metal nanoholes, which results in wavelength-dependent light absorption, so-called “extraordinary optical transmission”.11,12 Since these pioneering works, many attempts have been made to obtain vivid colors using 2D metal nanopatterns as color filters. These nanopatterns can be integrated into thin dielectric/metal structures to fabricate plasmonic structures, which were very recently realized.13-17 The hybrid interplay between constructive interference at the dielectric layer and the extraordinary optical transmission at the metal nanopatterns gives rise to the reflection of light with a narrow bandwidth, showing a distinctive color. And the appearance of colors is theoretically permanent unlike the pigment and dye-based chemical colors, where the specific wavelength-selective absorption occurs. To vary the colors, the geometrical parameters of the plasmonic metal nanopattern, such as its periodicity, size, height, and shape, should be controlled.13,18-21 The methods used to obtain such various nanopatterns tend to be very expensive nanofabrication techniques such as electron-beam lithography, nanoimprinting, and focused ion beam etching,18-21 which are also time-consuming and difficult to scale up for commercialization. Therefore, recent works have focused on easy and cost-effective fabrication methods, including block copolymer lithography, colloidal assembly, and manipulation of porous anodic aluminum oxide (AAO) film to obtain nanostructures with a high areal density and low polydispersity in size.22-30 Among these, porous AAO films are good candidates for plasmonic structures because they spontaneously form


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uniform cylindrical pores and channels with millimeter-scale dimensions.31,32 The pore diameter (Dp) and interpore distance (Dint) of porous AAO films are easily modulated, which is essential for multicolor generation when the metal is deposited on these films.26-30 Indeed, the color and intensity of the reflected light is readily seen when these films are infiltrated with fluids because the refractive index changes from ~1 for air to ~1.35 for various kinds of fluids.27,29 However, this strategy has the limited ability to realize bistable applications because of the high volatility and isotropic characteristic of the solvents. In this study, we used a LC material as a filtration material in a metal-deposited porous AAO film to make a bistable and switchable plasmonic film. It is quite well known that LC materials are generally stable under ambient conditions and are optically anisotropic, and thus LC materials have been widely used in display applications. The molecular orientation of LC materials in pores can be controlled to show one principal LC refractive index in the nemtaic (N) phase, which can be varied by the thermal phase transition,33 revealing a wavelength shift in plasmonic reflection. The changes in reflecting colors with varying pore sizes and temperatures were measured using a spectrometer. A new color painting approach using our platform is introduced, which is quite stable and non-volatile.

2. RESULTS AND DISCUSSION Figure 1a illustrates the fabrication process for the AAO-based switchable plasmonic film using LC material. The porous AAO film was prepared by second anodization, followed by depositing Au on the film to a thickness of 20 nm. The topographic patterns of the Au layer on the AAO film exhibited enhanced plasmonic resonance as compared to the plain film.34 When


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white light was shined on the Au-coated AAO film, vivid structural colors were reflected as a result of the interplay between the interference and plasmonic resonance. To realize the switchable plasmonic structure, a common rod-type LC mixture, E7, was infiltrated into the Aucoated AAO film at isotropic temperature (Fig. 1b) and cooled to the N phase, resulting in a change in the refractive of the sample (Fig. 1a).

Figure 1. Plasmonic reflection. (a) Schematic illustration of fabrication of switchable plasmonic structure. (b) Chemical structure of the LC. The reflected wavelength (λ) of the Au-coated AAO is simply controlled by the thin-film interference theory, where the refractive index (n) and the thickness (l) of the dielectric layer, here, the porous AAO film are the main factors used to determine λ according to the equation mλ = 2nlcos θ , where m is the interference order and θ is the incidence angle of the light to the

sample. The porous AAO film itself was composed of air and Al2O3, which can be considered as a dielectric layer. The value of l was varied by adjusting the anodizing time from 480 to 580 nm as measured in the cross-sectional field emission scanning electron microscope (FESEM) images (Supporting Fig. S1a). The reflectance spectra red-shifted as l increased because of the proportional relationship between λ and l, and thus Au-coated AAOs with different l values produced distinct plasmonic colors (Supporting Fig. S1b).26 The effective n value (neff) of this


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dielectric layer is governed by the volume ratio between air and Al2O3. Therefore, a gradual change in neff can be achieved by varying the pore size during the pore-widening process. AAO films with pore diameters (Dps) from 30 to 93 nm were obtained by adjusting the pore-widening time from 0 to 30 min at l ≈ 580 nm (Fig. 2a and Supporting Fig. S2). Dp was converted to the 2

 π  Dp   , where Dint is the pore volume fraction, porosity (P), to correlate with n, i.e., P =    2 3  Dint  interpore distance. The value of Dint was predicted from the empirical equation ~ kV (V: anodization voltage and k ≈ 2.5 nm V−1).32 Here, Dint ≈ 100 nm because the anodization was carried out at 40 V for all AAO films. The resultant effective refractive index of the nanopores be

calculated

using

the

Bruggeman

effective

medium

approximation,35

(neff)

can

(1 - P)

ε Al O − ε eff ε − ε eff + P air = 0 , where εeff is the effective dielectric constant of the ε Al O + 2ε eff ε air + 2ε eff 2 3

2 3

nanopores (nAl2O3 ≈ 1.7, nair ≈ 1, and ε = n2). The quantities for Dint, Dp, P, and neff are summarized in Table 1. The resultant neff was inversely proportional to P (Supporting Fig. S3). As P increased, the reflectance curves blue-shifted because of the reduction of neff (Fig. 2b).


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Figure 2. Au-coated AAOs at varied P values (l ≈ 580 nm). (a) SEM images of top view of AAOs at varied P value and (b) their corresponding reflectance spectra. All insets are corresponding photographs for colors of the Au-coated AAOs. Table 1. Values for interpore distance (Dint), pore diameter (Dp), porosity (P), and effective refractive index of the Au-coated nanopores (neff).

Dint (nm)

Dp (nm)

P

neff

100

30

0.08

1.643

100

50

0.23

1.534

100

75

0.51

1.328

100

93

0.78

1.138

It is well known that rod-like LC molecules in the N phase are aligned parallel to the long axis of the AAO nanochannels because of the strong surface anchoring given by the large surface to volume ratio, known as the nanoconfinement effect.36,37 Either a planar or perpendicular orientation can be obtained by self-assembled monolayer (SAM) treatment, enabling control of the refractive index of the LC-filled nanopores (neff,LC).37 Here, Au was coated on the porous AAO film, and during this process the inner surface of the AAO film was also coated to induce homeotropic anchoring of LC molecules. To confirm this orientation behavior, a sandwich cell was made using two glass slides; one slide was coated completely with Au and the other was partially coated (Supporting Fig. S4). In the N phase, a dark domain was observed between the Au-coated glass slides, whereas schlieren textures were observed in the hybrid cell, meaning the LC molecules preferred to be homeotropically aligned on the Au surface, which was confirmed by the cross patterns in the conoscopic image.38,39 Now, one can imagine that the molecular


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orientation on the inner surface of the Au-coated porous AAO film would be homeotropic, meaning that the refractive index of the infiltrated LC material on the top view of the sample is near ne. This is important to understand for our switchable plasmonic film because the light shining on the sample experiences no and ne depending on the molecular orientation. The additional change in neff,LC occurs after the phase transition to the isotropic phase, which is a key idea to realize a switchable plasmonic film.

Figure 3. Reflectance spectra of the LC-filled Au-coated AAOs at 30 °C and 70 °C corresponding to the N and isotropic phases (a–d) at varied P values (l ≈ 580 nm). Table 2. Values for refractive index of the E7 LC (nLC) and effective refractive index of the LCfilled Au-coated nanopores (neff,LC).

no

niso

ne

nLC

1.547

1.61

1.742

neff,LC

1.257

1.277

1.316


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The optical properties of the fabricated plasmonic films were observed with varying P values from 0.08 to 0.78 at l ≈ 580 nm upon cooling at 70 °C and 30 °C, which correspond to the isotropic and N phases, respectively (Figure 3). At P ≈ 0.08, the reflectance spectrum did not shift much during the phase transition (Fig. 3a). This can be explained by the tiny LC volume in the sample. Although the n value of the LC material (nLC) changed during the phase transition from niso ≈ 1.61 to ne ≈ 1.742 at λ ≈ 500 nm (Table 2),40 the change in the neff,LC value of the sample was too small to influence the plasmonic reflection. On the other hand, the reflectance at a higher P value of 0.78 was different at each phase, revealing an approximately 20 nm red-shift after thermal cooling, meaning that the neff,LC increased as expected.41 The neff,LC values of our switchable plasmonic film were calculated using the Bruggeman effective medium approximation at P ≈ 0.78 (Table 2). The resultant change in the neff,LC value of our switchable plasmonic film during the phase transition was ~0.039 at P ≈ 0.78.

Figure 4. LC painting. (a) Schematic illustration for LC printing. (b) Photograph of written letters on the Au-coated AAO using the LC, and (c) the corresponding reflectance spectra. The l and Dp of the nanopores were ~150 and 80 nm, respectively, and the Au thickness was ~20 nm. To demonstrate the versatility of our method, we used the LC material as an ink to paint letters on the plasmonic film (Figure 4). For this, a canvas made of the Au-coated AAO film ~5 cm


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wide and ~2 cm long was prepared (Fig. 4a). The l and Dp values of the AAO film were ~150 nm and ~80 nm, respectively, showing a blue color. A sharp pen was dipped into the E7 LC and then the letters “KAIST” were written on the color substrate by hand. There is a clear color contrast between the letters and the canvas, where the light was reflected at ~500 nm and 420 nm, respectively (Figs. 4b, c). The non-volatile characteristic of the LC offers a permanent painting or sensing approach, unlike the common organic solvent case (Figure 5). The painted color was not stable because the solvent, here, ethanol, evaporated just after the infiltration (Fig. 5a). On the other hand, the color painted with the LC material on the canvas did not change for a long time (Fig. 5b); it was maintained even after a few months. The LC material painted on the canvas could be easily removed by rinsing with a common organic solvent, meaning our switchable plasmonic film is reusable. As compared to previously reported results based on the expensive lithographic technique, our method is very simple, fast, reliable, and reusable, making it a promising material for coloring displays, printing, and coloring books. Furthermore, the LC/AAO-based plasmonic structure suggested here possibly can be combined with the in-plane electrodes and azobenzene-containing dopant to change structural colors more effectively. Definitely, response of our platform under these stimuli should also be of interest.


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Figure 5. Photographs of the Au-coated AAOs after (a) infiltrating ethanol and (b) E7 LC.

3. CONCLUSION In summary, we successfully fabricated a switchable plasmonic film using LC material infiltrated into an Au-coated porous AAO film. The reflected colors varied by varying the geometrical parameters and the phase transition of the LC material from the isotropic to the N phase, which changed effective refractive index, giving rise to the red-shift of the plasmonic reflection. The non-volatile characteristics of the LC material preserved the color change, enabling us to write letters on our plasmonic canvas, which is reusable without damage. Beyond sensing applications, our platform based on a collaboration between LC material and plasmonic nanostructures has potential for use in many applications, including color painting and display. Most of all, our approach is simple, inexpensive, and robust, which is essential for practical uses.

4. EXPERIMENTAL METHODS


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Sample preparation: High-purity annealed Al foil (99.99%, Alfa Aesar) was cleaned by ultrasonication in acetone, followed by rinsing in ethanol and deionized water. Subsequently, the Al foil was electrochemically polished in a mixture of perchloric acid and ethanol (volume ratio of 1:5) at 20 V and 3 °C, followed by anodization at 40 V in 0.3 M oxalic acid. The pre-patterned Al foil was obtained by chemical etching the first-anodized Al2O3 in a mixture of phosphoric acid (6 wt%) and chromic acid (1.8 wt%) at 60 °C. The second anodization was carried out under the same conditions with different anodizing times to vary the l value. The pores were widened in 0.1 M phosphoric acid at 38 °C for different Dps. Au was coated on the top of the AAOs and the glass using a sputter coater (Emitech K550). The E7 LC material filled the Au-coated nanopores at isotropic temperature via capillary action and then the sample was cooled to room temperature at a rate of 1 °C min−1. The temperature was controlled using a heating stage equipped with a temperature controller (LTS420 and TMS94, Linkam). The excess LC material on top the AAO was removed by scrubbing. Characterization: The porous structures of the AAOs were taken using field-emission scanning electron microscopy (FESEM, S4800, Hitachi). The NLC textures were observed using a polarized optical microscope (LV100POL, Nikon). The reflectance spectra were collected under normal incidence of white light using a spectrometer (USB-2000+, Ocean Optics).

ASSOCIATED CONTENT Supporting Information. Plasmonic reflection at varied l; FESEM images of the AAOs with different Dps; the values of neff as a function of P; The POM images of the E7 LC in the sandwich cell made of glasses coated with Au; “This material is available free of charge via the Internet at http://pubs.acs.org.”


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AUTHOR INFORMATION * E-mail: [email protected] ACKNOWLEDGMENT The authors thank Professor Sang Bok Lee for fruitful discussions. This work was supported by a grant from the National Research Foundation (NRF), funded by the Korean Government (MSIP) (2014S1A2A2027911 and 2014M3C1A3052537).

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ToC figure


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Switchable Plasmonic Film Using Nanoconfined Liquid Crystals - ACS

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