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Switchable photonic crystals using one-dimensional confined liquid crystals for photonic device application Seong Ho Ryu, Min-Jun Gim, Wonsuk Lee, Suk-Won Choi, and Dong Ki Yoon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15361 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on January 3, 2017

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ACS Applied Materials & Interfaces

Switchable Photonic Crystals Using OneDimensional Confined Liquid Crystals for Photonic Device Application Seong Ho Ryu,† Min-Jun Gim,† Wonsuk Lee,† Suk-Won Choi,‡ and Dong Ki Yoon*† † Graduate School of Nanoscience and Technology and KINC, KAIST, Daejeon 34141, Republic of Korea ‡ Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee University, Yongin 17104, Republic of Korea

ACS Paragon Plus Environment

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ABSTRACT: Photonic crystals (PCs) have recently attracted considerable attention, with much effort devoted to photonic bandgap (PBG) control for varying the reflected color. Here, fabrication of a modulated one-dimensional (1D) anodic aluminum oxide (AAO) PC with a periodic porous structure is reported. The PBG of the fabricated PC can be reversibly changed by switching the ultraviolet (UV) light on/off. The AAO nanopores contain a mixture of photoresponsive liquid crystals (LCs) with irradiation-activated cis/trans photo-isomerizable azobenzene. The resultant mixture of LCs in the porous AAO film exhibits a reversible PBG, depending on the cis/trans configuration of azobenzene molecules. The PBG switching is reliable over many cycles, suggesting that the fabricated device can be used in optical and photonic applications such as light modulators, smart windows, and sensors.

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

1. INTRODUCTION Photonic crystals (PCs), defined as periodic structures of dielectric materials with different refractive indices,1 have attracted considerable attention in the fundamental sciences and for use in color displays, sensors, color printing, lasers, and many other photonic applications.2-8 The gamut of colors in PCs is obtained utilizing the forbidden propagation of light in a certain range of wavelengths, known as a photonic bandgap (PBG). The PBG strongly depends on the spatial arrangement of constituent materials;9 thus, numerous experimental approaches have been undertaken for fabricating PCs. A simple one-dimensional (1D) PC can be made in a form of a


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multilayer by alternately depositing different dielectric materials.10-13 More complex PC structures have been obtained for two-dimensional (2D) and three-dimensional (3D) lattices using lithographic techniques,14-16 colloidal particles,17-19 and self-assembled liquid crystal (LC) phases.20-22 Many studies addressed the question of PBG control by combining PCs with other functional materials. Among these, low molecular weight LCs are the most susceptible to electric and magnetic fields, as well as to electromagnetic waves.23-25 This characteristic becomes very important for switchable PC applications as well as for maximizing the anisotropy of LC materials. A typical LC molecule is anisotropic, with refractive indices depending on the molecular orientation. In the LC phase, there are two principal refractive indices: 1) ordinary and 2) extraordinary indices, no and ne, which can be measured when linearly polarized light propagates perpendicular and parallel to the optic axis of the LC molecule, respectively.26 Simple heating and cooling can change the refractive indices by phase transition between LC and isotropic phases; for example, by heating, the refractive index can be changed from no (or ne) to niso, here niso is the refractive index in isotropic phase ~ (2no+ne)/3. Up to this date, LCs have been incorporated into 3D opal or inverse opal structures for PBG control. In this approach, the LC refractive index can be controlled by varying the temperature, light irradiation, and by applying an electric field.27-32 However, achieving fine control of the LC refractive index remains challenging, because LC molecules should follow the spherical shape of colloids, implying that the LC molecules in a 3D structure are not uniformly oriented, even in the presence of an external field. Uniform arrangement of LC molecules is easily achieved in 1D structures,33-39 which exhibits only one principal refractive index. Therefore, a nanoporous anodic aluminum oxide (AAO)-based 1D PC is ideal for implementing this approach. A


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surprising finding was that modulated nanopores of AAO films can exhibit Bragg reflection. Since Wang et al. synthesized modulated AAO films using anodization under periodic voltage,40 many attempts have been made to control the color of the reflected light by modifying the experimental conditions such as the applied voltage, current, time, and temperature, which ensure that the PBG of the AAO PCs covers a very wide range of wavelengths, from ultraviolet (UV) to near infrared.41-48 In this study, we fabricated modulated 1D AAO PCs by varying the applied electrical current, and attempted to control the PBG using nematic (N) LCs doped with photo-responsive azo-LCs. In this approach, azo-LCs triggered the phase transition between the isotropic and N phases via cis-trans isomerization, which changed the refractive index of the LCs, enabling the PBG control. The resultant photo-responsive switching device is important to realize photonic applications such as smart window and color electronic book. We conducted direct investigations using polarized optical microscopy (POM) and a spectrometer, and monitored in-situ changes in the PBG under UV on/off conditions.

2. RESULTS AND DISCUSSION


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Figure 1. Pulse anodization and the 1D AAO PC. (a) Periodic constant currents during the pulse anodization and schematic illustration of the 1D AAO PC. Periodic porous structures that were observed using FESEM (b) before and (c) after the pore widening, and (d) the corresponding transmittance spectra. Fabrication of the 1D AAO PC was successfully completed using pulse anodization with alternating electrical currents (with jhigh and jlow of ~11 mA/cm2 and ~2.8 mA/cm2, respectively) that were applied to the sample for 24 s and 96 s, respectively. This process was repeated 200 times, to obtain 44-µm-long nanopores (Figure S1 in Supporting Information). In general, straight pores were obtained in the high current (jhigh) regime, while branching nanopores were obtained in the low current (jlow) regime at which the anodization voltage (V) gradually decreased from Vhigh (Figure 1a). Pore branching was initiated at V =

1 2

Vhigh .49 The resultant

AAO PC exhibited a periodically modulated porous structure (Figures 1b and S1). After the fabrication process, the pores widened, increasing the air volume (Figure 1c). Although the fabricated AAO film was composed of one material, aluminum oxide (Al2O3), the effective


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refractive indices in the straight and branched parts were different, owing to the different air volume ratios. The AAO film exhibited a repetitive change in the effective refractive indices along the long axis of the nanopores; thus, the film could be considered as a 1D PC with processing-dependent PBG.47 As mentioned above, the selective reflection of light from a PC can be explained in terms of the constructive interference of light at the interfaces between two different porous structures; thus,

the

PBG

of

the

1D

AAO

PC

is

governed

by

Bragg’s

equation,

2 mλ = 2(lStraight (nStraight − sinθ ) + l branched (n 2branced − sinθ )) , where λ is the reflected wavelength,

m is the diffraction order, and nstraight and nbranched are the effective refractive indices of the straight and branched pores, respectively. The parameters lstraight and lbranched are the lengths of the nanopores with nstraight and nbranched, respectively, and θ is the angle of incident white light.50 If light is normally incident onto the 1D AAO PC, Bragg’s equation can be simplified to mλ = 2(l straight n straight + l branched n branched ) . Here, lstraight and lbranched are fixed because the same type

of the AAO PC was used in all experiments. The optical properties of the modulated 1D AAO PC before and after the pore widening were compared in terms of the transmittance spectra (Figure 1d). The minimum of the PC transmittance before the pore widening was at 638 nm; thus, the film reflected the red color. After the pore widening, the transmittance minimum blueshifted to 550–610 nm, which corresponds to the yellow-green color. Because the proportion of air in the AAO film increased after the pore widening, nstraight and nbranched decreased. From Bragg’s equation, the PBG can be expected to shift toward shorter wavelengths with decreasing refractive index, which is in good agreement with our experimental result.51 The quantities for calculating the effective refractive index are summarized in Table 1. The values of l and pore diameter (Dp) were measured directly from the FESEM images of the AAO


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PC, and the value of interpore distance (Dint) was predicted from the empirical equation, ~ kV (k ~ 2.5 nm V-1),52 where Vstraight and Vbranched were 55 and 31 V in the cycle, respectively. Based on the measured and calculated values, we obtained the values of porosity (P) from 2

 π  D p   . The values of nstraight and nbranched were calculated using the Bruggeman  P =   D  2 3  int  effective medium approximation,53

(1 - P)

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

2

3

3

(nAl2O3 ~ 1.7, nair ~ 1, and ε = n2). Table 1. Length of nanopores (l), pore diameter (Dp), interpore distance (Dint), porosity (P), and calculated refractive index (n) values. l (nm)

Dp (nm)

Dint (nm)

P

neff

Straight

132

62

137.5

0.18

1.570

Branched

56

37

75

0.23

1.534

The transmittance minimum calculated from the simplified Bragg’s equation was ~ 586 nm, similar to the experimental result.


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Figure 2. POM images of the photo-responsive LC mixture. (a) The molecular structures of E7 and BMAB, the weight ratio of the photo-responsive LC mixture, and its thermal phase transition. The optical textures of the LCs in a sandwich cell made by glasses were observed (b) in the N phase after cooling from the isotropic phase and with UV irradiation on (c) and off (d), at 35 °C. To demonstrate the dynamic control of the PBG, photo-responsive azo-compound 4-butyl-4′methoxyazobenzene (BMAB) was mixed with a common rod-type LC mixture, E7 (Figure 2a). The azobenzene moiety in BMAB can induce cis and trans isomers that are reversibly switched by photo-isomerization when UV irradiation is on and off, respectively.54-56 This reversible transition was observed with the photo-responsive mixture of LCs prepared in a sandwich cell made of two glass substrates with a ~3-µm-wide cell gap. The optical textures were then examined using POM. Upon cooling from the isotropic temperature, typical schlieren textures were observed in the N phase (Figure 2b), in which BMAB molecules were well-mixed with E7 because BMAM was in the trans form that is similar to a rod-shaped LC molecule (Figure 2a). UV irradiation, however, induced dramatic changes, yielding a totally dark phase. This can be explained by the trans-to-cis conversion of BMAB molecules, and by the fact that a bent-like conformation of BMAB induced the isotropic arrangement of the LC mixture (Figure 2c). This dark image was reversibly changed to schlieren textures without UV irradiation (Figure 2d). This observation confirms that the N-to-isotropic phase transition of the photo-responsive LC mixture can be easily driven by light irradiation.


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Figure 3. Schematic illustration of the optical setup for measuring transmittance spectra.

Figure 4. Transmittance spectra during cooling from 70 °C (isotropic phase) to 35 °C (N phase). Before investigating the PBG of the sample under light irradiation, we estimated the temperature dependence of the sample’s optical properties. It is well-known that rod-type LC molecules are aligned parallel to the long axis of nanopores when the molecules are filled in an untreated AAO film.37-39 This can be explained in terms of the surface anchoring condition. In our system, the planar arrangement of the N phase LC molecules in the nanopores implied that


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the optic axis of the LC molecules was parallel to the observation direction; thus, we measured no. At temperatures sufficiently high for transforming the LC molecules into the isotropic phase, the penetrating light experienced niso, which was higher than no (niso ~ 1.61, no ~ 1.547, ne ~ 1.742 at λ ~ 500 nm),57 resulting in a smaller refractive index difference. Temperature-dependent transmittance spectra were examined upon cooling from 70 °C (isotropic phase) to 35 °C (N phase) in the optical setup illustrated in Figure 3, and the results are shown in Figure 4. In the isotropic phase, the PBG of the sample was at ~702 nm, which was red-shifted compared with the vacant AAO PC sample, because the effective refractive index was increased. The phase transition was observed at 65 °C on cooling, and this critical temperature slightly differed from that for the bulk sample (~58 °C), likely owing to the nano-confinement effect or difference in stability of N phase that is usually observed in the mixture.58-60 Upon further cooling from 60 °C to 35 °C, the PBG blue-shifted to 693 nm, because no was lower than niso, which reduced nstraight and nbranched. The overall transmittance was also reduced, owing to the opaqueness of the LCs. This preliminary result can be compared with the results of the following dynamic photoresponse experiment.


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Figure 5. Transmittance spectra in response to switching UV on (blue) and off (red), at 35 °C. The optical properties of light-driven switching of the sample were measured by shining light in the N phase at 35 °C, at which cis-trans isomerization could be achieved. During this process, the refractive index of the LCs confined in the pores was changed from no to niso, and vice versa, with and without shining the UV light, respectively. Figure 5 shows the irradiation-induced changes in the transmittance spectra of the sample. When the UV light intensity increased from 0 to 20.8 mW/cm2, the transmittance of the sample increased and finally the PBG red-shifted to 702 nm (Figure S2), consistent with the transmittance spectrum in the isotropic phase (blue line in Figure 4). In this condition, bent BMAB molecules in the cis configuration disorganize the LC molecules in the N phase, inducing the isotropic phase in the entire photo-responsive LC mixture. Linear rod-like BMAB molecules were obtained after turning the UV light off, inducing the N phase again to exhibit a blue shift to 695 nm (red line in Figure 4). Although photoisomerization of azobenzene from cis to trans configuration was fast, the relaxation of the LC molecules could be hindered in the confined geometry owing to a strong surface anchoring condition.44 This might explain why the transmittance measured after the cis-to-trans transformation was not identical to the bulk transmittance (695 nm). This light-driven switching was also successful via photoisomerization in the smectic A (SmA) phase (Figure S3).


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Figure 6. Reversibility test as a function of time and response time, with UV light on (blue) and off (red), at 35 °C. Irradiation-induced PBG switching time was determined by measuring the change in the transmittance intensity (Figure 6). UV light (~ 40 mW/cm2) was alternately switched on and off at 35 °C. Because the transmittance in the isotropic phase was higher than that in the N phase as shown in Figure 3, the change in the transmittance provides the evidence of cis-trans isomerization. When the 1D AAO PC was UV-irradiated, the isotropic phase was induced and the transmittance intensity increased (blue lines in Figure 5). The transmittance decreased in the absence of UV irradiation, indicating the transition to the N phase (red lines in Figure 5). This reversible switching was repeated and the intensity change was nearly the same over 100 cycles (Figure S4), demonstrating that the change in the refractive index of the LC molecules in the AAO nanopore is reversible and reliable. The response time of both transitions was evaluated using these data. The rise and fall times were measured as 1.47 s and 1.44 s, respectively. These results demonstrate that our light-driven LC-based 1D AAO PC is reversible and reliable during switching of the PBG.

3. CONCLUSION In summary, we successfully fabricated a photo-responsive azo-moiety-based PC with LCs in modulated AAO nanopores. The inner surface of the nanopores can induce the planar anchoring of the LC molecules in the initial state, which reveals no. The phase transition from the isotropic to the N phase (and vice versa) can be induced by cooling (or heating) the sample and by light irradiation. In this approach, the refractive index of the LCs changes from no to niso or the other way, resulting in the shift of the PBG wavelength. We also demonstrated that this light-driven


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switching of the PBG is reversible with respect to the UV light irradiation though the switching time is relatively long compared with the commercial display applications. As other applications suggest that light-induced transition is the most convenient way to control the optical properties, compared with applying an electric/magnetic field or mechanical shearing, we believe that the platform proposed here can be used for designing switching devices for modulating PBG of PCs for potential optical and photonic applications such as smart window and color electronic book. Furthermore, owing to its reliability and simplicity, the proposed platform is compatible with any photo- or heat-responsive soft materials.

4. EXPERIMENTAL METHODS 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 using 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 chemically etching the first-anodized Al2O3 in a mixture of phosphoric acid (6 wt %) and chromic acid (1.8 wt %) at 60 °C. Second anodization was performed under the same condition for 1 h, to obtain a stem porous structure, and a 1D AAO PC was obtained by pulse anodization with periodic constant currents at 11 mA/cm2 and 2.8 mA/cm2 for 24 s and 96 s, respectively, during 200 cycles. The pore was widened in 0.1 M phosphoric acid at 38 °C for 40 min and the remaining Al substrate was then removed by immersion in a mixture of cooper chloride and hydrochloric acid. The glasses were cleaned by ultra-sonication in acetone and then rinsed with ethanol and deionized water and treated with O2 plasma for 10 min to impose planar anchoring. The


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sandwich cell was fabricated using two glasses with a gap of ~3 µm, using silica beads. The photo-responsive LC mixture, composed of E7 (90.8 wt%) and BMAB (9.2 wt%) (Figure S5 in Supporting Information), was filled in the sandwich cell and the pores of the AAO PC at isotropic temperature via capillary action, and then cooled to room temperature at a rate of 1 °C min-1. 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 cross-sectional images of the 1D AAO PC were taken using fieldemission scanning electron microscopy (FESEM, S4800, Hitachi). The birefringent textures of the photo-responsive LCs were observed using POM (LV100POL, Nikon). The transmittance spectra of the LCs-filled AAO PC were collected using a spectrometer (USB-2000+, Ocean Optics) with a solid-state light source (SPECTRA X, Lumencor) for UV (365 nm) irradiations. Transmittance intensity was recorded as a function of time using a digital storage oscilloscope (DSO-X 2012A, Agilent).

ASSOCIATED CONTENT Supporting Information. FESEM images of the 1D AAO PC; Transmittance spectra as a function of UV light intensity; Transmittance spectra of the 1D AAO PC infiltrated with a SmA LC material; Reversibility test over 100 cycles; Phase diagram of the E7/BMAM mixture; “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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