Petal-Inspired Diffractive Grating on a Wavy Surface: Deterministic

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Petal-inspired diffractive grating on a wavy surface: Deterministic fabrications and applications to colorizations and LED devices Kyung Jin Park, Jae Hoon Park, Ji-Hyeok Huh, Chan Ho Kim, Dong Hae Ho, Gwan H Choi, Pil J. Yoo, Sung Min Cho, Jeong Ho Cho, and Seungwoo Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15536 • Publication Date (Web): 22 Feb 2017 Downloaded from http://pubs.acs.org on March 3, 2017

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ACS Applied Materials & Interfaces

Petal-inspired diffractive grating on a wavy surface: Deterministic fabrications and applications to colorizations and LED devices Kyung Jin Park,†,+ Jae Hoon Park,†,+ Ji-Hyeok Huh,†,+ Chan Ho Kim,‡ Dong Hae Ho,† Gwan H. Choi,‡ Pil J. Yoo,†,‡ Sung Min Cho,†,‡ Jeong Ho Cho,*†,‡ and Seungwoo Lee,*†,‡

†

SKKU Advanced Institute of Nanotechnology (SAINT), Suwon 16419, Republic of Korea

‡

School of Chemical Engineering Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea

+

These authors contributed equally

Corresponding Authors *E-mail (Prof. S. Lee): [email protected] *E-mail (Prof. J. H. Cho): [email protected]

KEYWORDS: Petal, diffractive grating, structural colorizations, light emitting devices, optoelectronics

ACS Paragon Plus Environment

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ABSTRACT:

Interestingly, the petals of flowering plants display unique hierarchical structures, in which surface relief gratings (SRGs) are conformably coated on a curved surface with a large radius of curvature (hereafter referred to as wavy surface). However, systematic studies on the interplay between the diffractive modes and the wavy surface have not yet been reported, due to the absence of deterministic nanofabrication methods capable of generating combinatorially diverse SRGs on a wavy surface. Here, by taking advantage of the recently developed nanofabrication composed of evaporative assembly and photofluidic holography inscription, we were able to achieve (i) combinatorially diverse petal-inspired SRGs with controlled curvatures, periodicities, and dimensionalities, and (ii) systematic optical studies of the relevant diffraction modes. Furthermore, the unique diffraction modes of the petal-inspired SRGs were found to be useful for the enhancement of the outcoupling efficiency of an organic light emitting diode (OLED). Thus, our systematic analysis of the interplay between the diffractive modes and the petalinspired SRGs provides a basis for making more informed decisions in the design of petalinspired diffractive grating and its applications to optoelectronics.


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INTRODUCTION Diffractive gratings split broadband light into a spatially distributed color sequence and have been implemented in a variety of photonic devices, including holographic imaging, spectroscopy, light emitting diode (LED), and solar cell.1-6 Natural organisms have also used diffractive gratings for vivid structural colorization, for example, in the scales of a butterfly wing or the petals of certain flowering plants.7-15 Interestingly, natural gratings include unique structural features that provide tremendous structural complexity through random processes across multiple scales. For example, the scales of a Morpho Didius butterfly wing consist of randomly distributed diffractive stacks that produce diffusional colorization (non-iridescence). Globally curled gratings are found in the scales of Pierella luna butterfly wings, which display reverse color diffraction effects. These structures and diffraction effects are not available in diffractive gratings on a flat surface.7-15 Natural diffractive systems have recently inspired advances in the fabrication of artificial diffractive gratings with controlled structural complexity, such as diffusional color reflectors and dispersive diffractive optical elements.11,14 These methods have expanded the degrees of freedom available to diffractive spectral splitting; efforts to develop optoelectronic devices including solar cells and LED have made considerable progress over the last decade.3,6 Flowering petal surface relief gratings (SRGs) on surfaces with a large radius of curvature present another important biological motif that has not been extensively explored. As summarized in Figure 1a–c, the petals of flowering plants (e.g., Tulipa) have structurally hierarchical surfaces in which 1D SRGs with a periodicity of 1 µm are conformably coated onto a curved surface with a large radius of curvature (1D wavy surface with tens of µm in width and several hundreds of nm in height). The pioneering work reported by Whitney et al. revealed that


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underlying wavy surface makes the grating diffraction of 1D SRGs more dispersive.15 Nevertheless, this field would benefit further from an improved understanding of the interplay between the periodicity of an SRG and underlying wavy surface, as this relationship plays a key role in structural colorization and dispersive diffraction gratings. A lack of systematic studies in this area mainly results from the absence of deterministic fabrication methods capable of generating diverse SRGs on wavy surface. Here, we address this challenge by taking advantage of a recently developed nanofabrication composed of evaporative assembly and photofluidic SRG inscription.15-17 In particular, we used the evaporative assembly of azobenzene polymers (azopolymer, polydispersed orange 3 (PDO 3)) to develop a large-area pristine surface consisting of a 1D wavy surface with a relatively large radius of curvature. We then conformably textured this curved surface with diverse SRGs; this was achieved by the directional photofluidic movement of PDO 3 along the polarization, which was periodically varied within the interference pattern. In this way, we achieved (i) a series of petal-inspired SRGs with controlled curvature, periodicity, and dimensionality, and (ii) systematic optical studies of their reflective diffraction (i.e., structural colorization) and transmissive diffraction (i.e., enhancement of OLED outcoupling efficiency).

RESULTS AND DISCUSSION Deterministic Fabrication of petal-inspired SRGs. Figure 1d presents a schematic for the fabrication process: (i) evaporative assembly19-21 of the azopolymeric 1D wavy structure to form a 1D wavy surface, and (ii) holographic texturing22-26 of SRGs. In this work, the PDO 3 was used as a representative azopolymer, as found to be highly efficient for photofluidic movement along the light polarization.18 As shown in Figure 1d, the PDO 3 solution, dissolved in 1-4 dioxane,


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filled the spaces between the plastic blade and the target substrate. Thus, a meniscus, this is, the three-phase contact line (i.e., the atmosphere–PDO 3 solution–solid interface), was formed at the edge of the plastic blade. The evaporation of 1-4 dioxane occurred preferably at this meniscus, so as to form a solidified PDO 3 layer below the edge of the blade. Then, the stick–slip motions of this meniscus, which was controlled by adjusting the movement of the substrate, allowed us to assemble PDO 3 into a periodic wavy surface, as summarized in Figure 1e–f. Several experimental variables, including the surface chemistry, stick–slip interval, wetting properties, and blade running speed, were additionally optimized to obtain 1D wavy surface. Then, by the control of PDO 3 solution concentration (from 2 wt% to 5 wt%), the average width of the 1D wavy structure was tuned between 25 µm to 50 µm; the height was tuned between 600 nm to 1.45 µm. The averaged radius of a curvature across all 1D wavy structures was about 1.2 mm; indicating that the achieved curvature was quite similar to that of a Tulipa petal. The characteristics of the developed 1D wavy pattern are presented in greater detail in Figure S1. Also, it is noteworthy that the cross-sectional height profile was not symmetric (referred to hereafter as a head-to-tail profile) due to imbalances between the velocity of the plastic blade motion and the evaporation rate of PDO 3 solution at the meniscus.20,21 The inscription of the SRGs onto the pristine 1D wavy surface (i.e., holographic photofluidization) completed the development of the petal-inspired diffractive gratings. In general, even below the glass transition temperature (e.g., room temperature), the azobenzene materials can be soften, when irradiated by light with appropriate wavelength (referred to as athermal photofluidization). It is known that this athermal photofluidization results from the repetitive isomerization of azobenzene molecules under light irradiation. More interestingly, this photo-softening of azobenzene materials can be contrasted with isotropic thermal-softening, as


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migrating directionally only along the incident light polarization (i.e., directional photofluidization).16,17,23,27-33 Even if not yet fully understood, directional photofluidization of azobenzene materials is believed to be related with the alignment of azobenzene molecules in a direction perpendicular to the light polarization.18 Because the polarization was varied periodically within the interference pattern,23,25 SRGs can be conformably inscribed onto the surfaces of the 1D wavy structure via holographic irradiation without any chemical wet etching processes. Circularly polarized light having opposite rotations was used to generate interference patterns (i.e., a polarization interference pattern resulting from the combination of left-handed and right-handed circularly polarized light (LCP + RCP)), since leading to the most efficient photofluidic migration among other polarization combinations.23 The grating vector of the 1D SRGs was designed to be parallel to that of the 1D wavy pattern, as similar to that of Tulipa petals. The periodicity of the SRGs was controlled to be ranged from 500 nm to 2200 nm by adjusting the incident angle of the two beams. The optical set-up and irradiation conditions are described in Figure S2. Atomic force microscopy (AFM, Figure 1g–i) and scanning electron microscopy (SEM, Figure 1j) measurements unveiled the periodicity and height of the SRGs, inscribed onto three different 1D wavy patterns developed using 2 wt%, 3 wt%, and 5 wt% solutions (see the characterizations presented in Figure S3–5). It turned out that the 1D SRGs were conformably coated onto a 1D wavy surface. The grating vector and periodicity of these SRGs were nearly same compared with those of the incident interference pattern. Meanwhile, the inscribed SRGs became less pronounced as the height of the 1D wavy surface decreased (see both ends of the 1D wavy structure in Figure 1j). This is because the height of the 1D SRGs was generally proportional to the amount of movable PDO 3.18


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Structural colorization via reflection diffraction from petal-inspired SRGs. We next characterized the optical responses of these petal-inspired SRGs, as summarized in Figures 2–3. The role of wavy surface in the diffraction mode was examined by comparing the diffractive structural colorizations obtained from the petal-inspired 1D SRGs and a flat 1D SRGs. The optical set-up and the spectrum of the white light source are presented in Figures S6 and S7. The incident angle of the broadband light sources was ±18°, while the collection angle (2θc), defined by the numerical aperture (NA) of the objective lens, was ranged from +17.5° to -17.5° (NA of 0.3): the collectable half maximum angle of the reflection cone (θc) with respect to the normal direction is given by θc = sin−1(NA). Figures 2a and 2b respectively show the dark-field optical microscopy (DFOM) images of flat PDO 3 films before and after the inscription of 1D SRGs (with a 500 nm period). In both cases, no distinct colors were collected: defective protrusions were only observed as a result of the resonant light scattering (see the white arrows in Figure 2b). Also the pristine 1D wavy surface (developed using a 2wt% PDO 3 solution) looked dark, even if its edges were visible as a result of light scattering (see Figure 2c). A higher NA (0.8) (see the inset of Figure 2c) enabling a wider range of collection angles (+53° to -53°) captured more scattered light from the tail part of the 1D wavy surface. After the 1D SRG inscription, the darkened 1D wavy structure (imaged using an NA of 0.3) were converted into a bluish color due to the diffraction and collection processes (Figure 2d). The time over which the interference pattern was irradiated was optimized to maximize the intensity of the 1st order diffraction (see Figure S8). The spectrum of this structural colorization, which was selectively collected using an aperture matched to the width of the 1D wavy surface, is shown in Figure 2e. It turned out that diffracted light with a 480 nm wavelength (bluish color)


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was predominantly collected. A relatively small amount of light at 535 nm and 595 nm wavelengths was also collected, mainly resulting from light scattering at the structural defects, such as rough surfaces. It should be noted that the single-hue structural colorization (monochromatic) was contrasted starkly with the polychromatic structural colorization, which was obtained from the 1D or 2D SRGs fabricated on surfaces with a relatively small radius of curvature (e.g., a radius of curvature of 50 µm, see Figure S9).34,35 Given that the periodicity and modulation height of the petal-inspired 1D SRG were same with those of the 1D SRGs on a flat film, we concluded that wavy surface played a key role in producing monochromatic structural colorization. In general, the grating splits the different colors into different directions according to the diffraction law: mλ = d(sinθi + sinθd), where m, λ, d, θi, and θd, indicate the diffraction order, the wavelength of diffracted light, the grating period, the incident angle, and the diffraction angle, respectively. Then, the diffracted light can be collected, if θd is smaller than θc (±17.5°) (see Figure 2f). Under normal white light irradiation conditions (θi of 0°), for example, bluish light with a wavelength of 480 nm, diffracted from the 500 nm periodic 1D SRGs prepared on a flat surface, yield a θd of ±74° (for ±1st order diffraction), far exceeding θc. A longer wavelength of light (e.g., greenish or reddish light) yielded a value of θd that higher than 74°. Therefore, under θi of 0°, normally reflected light can be only collected, as shown in Figure S10. The dark-field illumination of a flat 1D SRGs produced θd of +40.6° (for θi of +18°, +1st order) and -40.6° (for θi of -18°, -1st order); consequently, showing a darkened image (Figure 2b). The monochromatic colorization of petal-inspired 1D SRGs can also be rationalized analytically. The peak of the 1D wavy surface was relatively flat, as its radius of curvature was quite large (see Figure 1j); thus, 1D SRGs on this top area of wavy surface cannot contribute to


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diffractive structural colorization. By contrast, the head and tail parts of the 1D wavy surface were continuously inclined (slanted angle (θs) was gradually varied as shown in Figure 2f). As such, the actual θi in the diffraction equation for petal-inspired 1D SRGs should be replaced by θi-θs. In the case of the 2 wt% 1D wavy surface, θs for the head and tail parts was ranged from 8.1° to 0° and from 0° to -5.2° (Figure S11). As θi was ±18.0°, the range of θi-θs can be summarized as presented in table 1:

Table 1: The range of θi-θs θi of -18°

θi of +18°

Head part

9.9° ~ 18°

-26.1° ~ -18°

Tail part

18° ~ 23.2°

-18° ~ -12.8°

Thus, the total range of θi-θs to be considered was 9.9° to 23.2° and -26.1° to -12.8° (see a more detailed analysis, presented in Figure S12). Given these geometrical constraints, the dispersion of θd as a function of λ and θi-θs (i.e., mλ = d(sin(θi-θs) + sinθd)) was mapped, as presented in Figure 2g; it turned out that bluish -1st order diffraction (460 – 490 nm wavelength) can be predominantly collected (i.e., monochromatic structural colorization), whereas other colors can not be collected. This analytic interpretation agreed well with the experimental observations (Figure 2d–e). The monochromatic color hue of the diffracted light can be also confirmed with the macroscopic view as well as mesoscopic view. Once θi was fixed to 0°, the view angle (θc) was precisely varied by adjusting the tilt angle of the sample. In this case, θc is a discretized value. Figure S13 shows a series of macroscopic structural colorizations obtained at different tilt angles


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of the sample. The pristine 1D wavy structure appeared dark reddish due to the strong absorption properties of PDO 3 (Figure S13a);36,37 however, after inscribing the 1D SRGs, one vivid color became activated at a specific tilt angle of the sample (Figure S13b–d). To isolate the role of a 1D wavy surface in the monochromatic colorization, we removed the effects of the intrinsic PDO 3 properties by the replication of the as-prepared structure with an optically transparent PDMS counterpart (i.e., rigiflex lithography, see Figure S14).38 The structural features were successfully replicated (Figure S15). Figure S13e presents a DFOM image of the PDMS-based, petal-inspired 1D SRGs; a distinct bluish color was clearly observed. The spectrum of this structural colorization also resembled that of the PDO 3 master’s diffractive color (Figure S13f). However, compared with PDO 3-based counterpart, PDMS-based petalinspired SRGs showed lower intensity of bluish color. This is due to the fact that the refractive index of PDMS (~1.45), similar to that of flowering petal (an average of 1.425), is much smaller than that of PDO 3 (~1.8) at the wavelength of interest. In principle, the diffraction efficiency is enhanced with increasing refractive index difference between surrounding materials (air) and SRGs. Therefore, regardless of the fundamental material properties, such monochromatic colorization originated from the unique multi-scale features of the petal-inspired 1D SRGs. By taking advantages of holographic texturing, the structural features of the petal-inspired SRGs can be deterministically controlled. For instance, the incident angle of the two beams was adjusted to precisely control the period of the 1D SRGs.35 In addition to the 500 nm periodic structure, 1000 nm and 2200 nm periodic 1D SRGs were inscribed onto the 1D wavy surface (2 wt%) (see Figure S16). As with the results presented in Figures 2d–e, the irradiation time was optimized to maximize the diffraction efficiency (30 – 40 min) (see Figure S17). According to the period of the 1D SRGs, the dispersion of θd as function of λ and θi-θs was tuned. Thus, for a


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given range of θc (i.e., from +17.5° to -17.5°), the wavelength of diffracted light collected can be changed, as summarized in the DFOM images and spectral results (Figure 3). For the case of 1000 nm periodic 1D SRGs on the 1D wavy surface, both the + 1st and - 1st ordered diffractions can be collected, as presented in Figure 3a. It is important to note that the region over which θc ranged from -17.5° to -8.5° and from 14.5° to 17.5° collected broadband light resulting from crosstalk between the reddish, greenish, and bluish diffractions (highlighted by orange box). The greenish and bluish light, which were diffracted at the +1st and -1st order cannot be overlapped with reddish diffraction (outside of orange box); however, the greenish area was larger than the bluish area. Indeed, the collected diffraction light was found to be greenish, together with a white background, as shown in the DFOM image (Figure 3b). This spectrum (Figure 3c) confirmed the dominant collection of greenish wavelengths (around 550 nm) together with a white background. Thus, the structural colorization of petal-inspired, 1000 nm periodic 1D SRGs was polychromatic rather than monochromatic. When the period was increased further to 2200 nm, analysis of the θd dispersion as a function of the λ and θi-θs became more complicated. This is due to the fact that the higher-order diffractions (± 2nd and ±3rd orders) were included within the range of θc (Figure 3d). However, the intensities of ± 2nd and ± 3rd order diffractions are much lower than that of ± 1st order diffractions; thus, the experimentally measured diffraction shown in Figure 3e–f was mainly governed by ± 1st order. Interestingly, the reddish colors were predominantly observed (Figure 3e). This observation agreed well with the analytical analysis. As shown in Figure 3d, reddish area is the largest in the regime of non-crosstalk, highlighted by orange box. Spectral analysis (Figure 3f) revealed that the intensity of the reddish region was indeed much higher than the


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intensities of the greenish and bluish regions. The structural colorization behavior with respect to the periodicity of SRGs is summarized in Table 2.

Table 2: Summary of diffraction-enabled structural colorization as function of SRG periodicity Period of SRGs

Color hue

Diffraction order

Chromatic level

500 nm

Bluish

-1st order

Monochromatic

1000 nm

Greenish

± 1st order

Polychromatic

2200 nm

Reddish

± 1st/± 2nd/± 3rd

Polychromatic

orders

In addition to varying the period, the grating vectors and dimensions of the petal-inspired SRGs can be controlled in a versatile way. For example, rotation of the sample through 90° makes a grating vector of the 1D SRGs perpendicular to that of the 1D wavy structure (Figure 4a). The resultant structural color was accordingly reconfigured (Figure 4b). Also, repeated 1D holographic irradiation with a 90 ° sample rotation converted the already inscribed 1D SRGs into 2D SRGs (Figure 4c). These steps further changed the structural color (Figure 4d). As such, various diffractive modes can be deterministically encoded by holographic irradiation and decoded with respect to the wavevector and incident angle of the light source. The control over the pristine 1D wavy pattern offered additional flexibility for tuning the structural colorization. As mentioned in Figures 1f–i, both the width and height of the 1D wavy surface were tuned by adjusting the solution concentration. The colorization areas can thereby be reconfigured. Diffractive colorizations from the 500 nm, 1000 nm, and 2200 nm periodic 1D SRGs prepared on the taller and wider 1D wavy structure (5wt%) are presented in Figures 4e–j,


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respectively. The grating vector was still parallel to that of the periodic 1D wavy surfaces. As expected, increasing the width of the 1D wavy structure enhanced the areal factor of the structural colorization; however, the dominant wavelength of the diffracted light remained unchanged because an increase in the PDO 3 solution concentration did not significantly change θs of the 1D wavy surfaces. Experimental verifications of the structural colorization obtained from the 1D SRGs prepared on a 3wt% 1D wavy surfaces are summarized in Figure S18. Enhancement of OLED outcoupling efficiency via transmission diffraction. Finally, we implemented petal-inspired, 1D SRGs in an organic LED (OLED) device to enhance the outcoupling efficiency and the resultant power efficiency. Existing flat OLED devices consisting of multi-stacked layers suffer from several loss mechanisms, including (i) total internal reflection (TIR) at the interface between the top glass substrate and air (i.e., glass–mode propagation), (ii) the confined waveguide mode between the anode (ITO) and the organic emitting layer (waveguide modes), and (iii) plasmonic dissipation at the back of the metallic electrode.39-42 Addressing these optical limitations and enhancing the outcoupling efficiency remain intriguing goals in the field of OLED devices. First let us consider the loss of outcoupling of the emitted light mainly due to the underlying mechanisms associated with (i) and (ii). Numerical electromagnetic simulations (the finite element method) provided a quantitative rationalization of the light extraction behavior from conventional OLED devices, as shown in Figure 5a–d. The numerical simulation and structural information regarding the OLED device used in this work are summarized in Figures S19–21. A 480 nm light source was employed because blue-emitting OLEDs were used in this work (see the emission spectrum of the OLED presented in Figure S22). Light emission from an organic fluorophore can be approximated as dipolar radiation.43 The light emitted from dipolar


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oscillation can radially propagate along a substrate (see Figure S21). In particular, some fraction of the emitted light is strongly confined within the 150 nm thick ITO. This confinement mainly arose from a relatively high contrast of the refractive indices between ITO (2.00) and the surrounding layers, including the glass substrate (1.50) and the organic layers (1.67). Some of the radiated light can escape from the ITO and go into the glass. This light was then reflected back to the ITO via reflection at the interface between the glass and air, so as to be further coupled to the confined mode within ITO. Indeed, reflection at the surface of the glass substrate can be visualized by using numerical simulations. Because the glass substrate thickness (1 mm) is far much larger than the wavelength of interest, we assumed that the radially emitted light from the organic layer becomes a plane wave near at the surface of the glass substrate. For the clarity of this loss mechanism, this plane wave was irradiated with a slant angle of 40° (see Figure 5a). Significant reflections were actually observed. Also, if larger than the critical angle of total internal reflection, other incident angles showed very similar insights. The simple attachment of the PDMS-based, petal-inspired SRGs onto an OLED device effectively mitigated such loss as follows. Attachement of a PDMS layer with a refractive index of 1.4 onto the surface of OLED device can reduce the impedance mismatch between the glass and air. Additionally, a gradual change in the cross-sectional profile of the 1D wavy surface can further reduce the impedance mismatch. Indeed, as shown in Figure 5b, the 1D PDMS wavy surface (without 1D SRGs) on the glass substrate allowed more of the emitted light to escape from the devices. Integration of the 1D SRGs (500 nm period) onto a 1D wavy surface (i.e., petal-inspired SRGs) further enhanced the outcoupling of the emitted light through a transmissive diffraction (Figure 5c).


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Importantly, this enhancement was expected across a broadband wavelength range (see Figure 5d, which was calculated for an incident angle of 40°). Figures 5e–f present transmission enhancement of (i) flat glass and (ii) flat “ITO” glass before and after attachment of 1D PDMS wavy surface and petal-inspired PDMS 1D SRGs. As expected, transmission through both the bare glass and the ITO-coated glass was significantly enhanced upon the implementation of the petal-inspired 1D SRGs. The enhancement of transmission was up to about 7 % (for glass) and 15 % (for ITO-coated glass) at the wavelength of interest (440–480 nm, corresponding to the OLED emission peak). This enhanced light transmission was visualized macroscopically, as shown in Figure 5g. Compared with the 2wt% structure, the 1D PDMS wavy patterns and petalinspired SRGs, based on 3 wt% and 5 wt% PDO 3 master structures, were less efficient in terms of transmission enhancement. This is mainly because the areal factor of a relatively flat area was increased by enhancing the PDO 3 solution concentration. In line with this, the attachment of petal-inspired PDMS SRGs to the outside of an OLED device (Figure 5h) indeed enhanced the outcoupling efficiency and the resulting power efficiency (Figures 5i–j). The diffraction-enabled enhancement in outcoupling from the petalinspired OLEDs can be further evidenced by the increased angular intensity across all angles compared with that of reference OLED (Figure 5k). Additional analyses of the petal-inspired OLED devices, such as the external quantum efficiency (EQE), are included in Figure S23.

CONCLUSIONS Our experimental approaches combining evaporative assembly and holographic texturing can expand the ability to design and fabricate petal-inspired SRGs. This hybridized fabrication approach contains multiple degrees of freedom that can be independently controlled (i.e., the


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structural features of the pristine 1D wavy structure and the SRGs). Therefore, this unprecedented versatility, in turn, enabled a systematic study of the relevant optical modes and provided a quantitative basis for the rational design of complex diffractive gratings. Although this work focused on applications of petal-inspired SRGs to the structural colorization and LED, the obtained insights into the roles of the multi-scaled structural features in the diffractive modes are expected to facilitate other applications, including solar cells and phototransistors.44-46

EXPERIMENTAL SECTION

Synthesis of polydisperse orange 3 (PDO 3): PDO 3 was synthesized by solid-state polymerization between disperse orange 3 and bisphenol A diglycidyl ether. These chemical reagents were purchased from Sigma Aldrich and used as-received. After homogeneous mixing of disperse orange 3 and bisphenol A diglycidyl ether with 1:1 ratio, which was assisted by solvent dissolution (tetrahydrofuran (THF)), the mixture was heated up to 120 °C for 2 days. Then, the mixture was cooled down to room temperature and dissolved in THF solution. The non-solvent precipitation of PDO 3 solution (in THF) with methanol and subsequent dry in vacuum oven completed the synthesis of PDO 3. Evaporation-induced assembly of PDO 3 1D wavy: Herein, we used our custom-built facility of the evaporative assembly. The blade-coating setup consisted of two main parts: (i) an angled polymer blade attached to the vertical translation stage and (ii) linear translation stage attached to the piezo nanopositioner. A 75 µm thick and 1.2 mm scored polyethylene terephthalate (PET) blade was attached at 40° angled, translation stage. As-synthesized PDO 3 was dissolved in 1,4-dioxane with controlled concentration (i.e., 2 wt%, 3wt%, and 5 wt%). The PDO 3 solution was then injected into the gap between polymeric blade and substrate. Due to the capillary forces, the injected solution was trapped between the scored region of polymeric blade


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and substrate; subsequently forming a three-phase contact line (meniscus between PDO 3 solution, air, and solid substrate). The evaporation of solvent made the PDO 3 solutes to migrate to the meniscus and then the PDO 3 line was formed. Then, the linear translation stage was moved by a step distance (50 µm) and stopped for a given periods (1 s). The meniscus was stretched until the contact angle decreases below the critical angle, at which point capillary forces became larger than the pinning forces. Consequently, the contact line moved a new position and contact angle was recovered to initial value. By repetitive stick-slip motion, 1D PDO 3 wavy patterns were successfully fabricated over the large-area (wafer-scale). Holographic light irradiation: The inscription of SRGs was carried out by using conventional holographic optical setup presented in schematic of Figure S2. High power laser (5W, LightHouse Sprout) with 532 nm wavelength was first expanded by using beam expander. Generally, we expanded the original laser beam to 1 cm diameter. Then, this expanded beam was divided into two beams by polarizing beam splitter (PBS): to make the intensities of two divided beams equal, the linear polarization of the expanded beam was precisely controlled by polarizer, located between beam expander and PBS. Then, by using half and quarter waveplates, the polarization of two beams was controlled to be right-handed and left-handed circular and mixed together to form a polarization interference pattern. The sample holder was located exactly at the position where two beams were mixed. Manipulation of incident angles of two beams allowed us to control the periodicity of SRGs; electronic shutter with 0.001 sec time interval resolution enabled the precise control of irradiation time. Dark-field spectroscopy & transmission measurement: We used homebuilt dark-field spectroscopic system for spectral analysis of structural colorization. Nikon Eclipse Ni series optical microscope, equipped with ×100 (numerical aperture (NA) of 0.9), ×50 (NA of 0.8), and ×30 (NA of 0.3) dark-field objective lenses, was used for DFOM image (see Figure S6a). This optical microscope system was integrated with CCD (PIXIS-400B, Princeton Instruments) and imaging spectrometer (IsoPlane, Princeton Instruments), in that the collected DFOM images


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were spectrally analyzed. The collimated light by a set of proper lens was irradiated with slant angle of ±18°. The collected DFOM images were spatially filtered before reaching to the spectrometer and CCD. The spectrum of light sources used for DFOM imaging and spectral analysis was indicated in Figure S7; all obtained spectra were normalized with respect to the light source spectrum. The white light source (plane wave) was focused and passed through the sample (e.g., petal-inspired PDMS SRGs). Then, the transmitted light was collected with bright-field objective lens (×100 (NA of 1.3)) and spatially filtered. Finally, the spectrum of transmission was analyzed with CCD and spectrometer.

ASSOCIATED CONTENT Supporting Information Additional information associated with 1D wavy pattern fabrication, structural features of 1D wavy pattern and petal-inspired SRGs, holographic optical setup, spectral analysis setup, analytical analysis of diffractive behavior, and OLED experiments is described in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail (Prof. S. Lee): [email protected] *E-mail (Prof. J. H. Cho): [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1402-09.

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Figure 1. (a-b) Macroscopic and (c) microscopic scanning electron microscope (SEM) images of Tulipa flower. (d) Schematic for the fabrication method comprising evaporative assembly and holographic photofluidic texturing of surface relief gratings (SRGs). (e) Macroscopic image of a representative example of 1D polydisperse orange 3 (PDO 3) wavy pattern. Wafer-scale assembly of 1D PDO 3 wavy can be achieved. (f) Cross-sectional profile of 1D wavy pattern assembled using differently concentrated PDO 3 solutions (2 wt%, 3 wt%, and 5 wt%). (g-i) Atomic force microscope (AFM) images of petal-inspired 1D SRGs (500 nm period) together with cross-sectional profile: (g) 2 wt%, (h) 3 wt%, and (i) 5 wt%. (j) A representative SEM image of petal-inspired 1D SRGs (500 nm periodic SRGs on 5 wt% wavy pattern).


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Figure 2. Dark-field optical microscope (DFOM) images of (a) a flat PDO 3 film, (b) 1D SRGs (period of 500 nm) prepared on a flat PDO 3 film, (c) 1D PDO 3 wavy pattern (developed using 2 wt% solution), and (d) 1D SRGs (period of 500 nm) inscribed onto 1D PDO 3 wavy pattern (developed using 2 wt% solution): petal-inspired SRGs. Numerical aperture (NA) of objective lens was 0.3; collection angle (θc) is ranged from +17.5° to -17.5°. (e) Spectrum of DFOM image of (e). (f) Schematic for diffraction conditions of petal-inspired PDO 3 SRGs (presented in (d)). (g) Dispersion of diffraction angle (θd) as function of wavelength (λ) and effective incident angle (i.e., incident angle (θi) – slant angle (θs)): mλ = d(sin(θi - θs) + sinθd).


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Figure 3. (a) Dispersion of θd of 1000 nm periodic, peal-inspired SRGs as function of λ and θi θs: mλ = d(sin(θi - θs) + sinθd). (b) DFOM of petal-inspired 1D SRGs with 1000 nm period. NA of objective lens was 0.3. (c) Spectrum of DFOM image of (b). (d) Dispersion of θd of 2200 nm


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periodic, peal-inspired SRGs as function of λ and θi - θs: mλ = d(sin(θi - θs) + sinθd). (e) DFOM of petal-inspired 1D SRGs with 2200 nm period. NA of objective lens was 0.3. (f) Spectrum of DFOM image of (e).


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Figure 4. (a) DFOM image of petal-inspired 1D SRGs having 2200 nm period (herein, 1D wavy pattern was developed using 2 wt% PDO 3 solution). The grating vector of 1D SRGs was designed to be perpendicular to that of 1D wavy. NA of objective lens was 0.9. (b) DFOM image of the structure presented in (a), which was obtained with NA of 0.3. (c) DFOM image of petalinspired 2D SRGs having 2200 nm period after second inscription of 1D SRGs onto a 90 ° rotated, petal-inspired 1D SRGs. NA of objective lens was 0.9. (d) DFOM image of the structure shown presented in (c), which was obtained with NA of 0.3. (e-g) DFOM images of petalinspired, 1D SRGs with different periods (developed onto 5 wt% 1D wavy): (e) 500 nm, (f) 1000


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nm, and (g) 2200 nm. Those were obtained by NA of 0.3. (h-j) Spectral analysis of petal-inspired 1D SRGs, developed onto 5 wt% 1D wavy: (h) 500 nm period, (i) 1000 nm period, and (j) 2200 nm period.


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Figure 5. Spatial mapping of electric field distributions of (a) flat glass, (b) 1D PDMS wavy, replicated from 1D PDO 3 wavy (2 wt%), and (c) petal-inspired PDMS SRGs (inscribed onto 2 wt% 1D PDO 3 wavy). Herein, the plane wave was assumed to be irradiated with a slant angle of 40°. (d) The numerically simulated, power spectrum of the outcoupled light corresponding to (ac). (e) The measured transmission of (1) flat glass, (2) 1D PDMS wavy on a flat glass, and (3) petal-inspired, PDMS SRGs on a flat glass. (f) The measured transmission of (1) flat ITO glass (150 nm thick ITO), (2) 1D PDMS wavy on a flat ITO glass, and (3) petal-inspired PDMS 1D SRGs on a flat ITO glass. (g) Macroscopic views of broadband light, passing through (1) flat ITO glass, (2) 1D PDMS wavy on a flat ITO glass, and (3) petal-inspired PDMS 1D SRGs on a flat ITO glass (from top to bottom panels). (h) Schematic for the petal-inspired organic light emitting diode (OLED) device used in this work: more detailed information about OLED is described in Figure S19 of the Supporting Information. (i) Macroscopic view of lightning from reference OLED and petal-inspired OLED. (j) Comparisons of reference OLED and petalinspired OLED in terms of power efficiency (lm/W). (k) Angular distributions of reference OLED and petal-inspired OLED.


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Table of Contents (TOC)


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Petal-Inspired Diffractive Grating on a Wavy Surface: Deterministic

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