Moth-Eye-Inspired Biophotonic Surfaces with Antireflective and

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Moth-eye Inspired Biophotonic Surfaces with Antireflective and Hydrophobic Characteristics Wen-Kai Kuo, Jyun-Jheng Hsu, Chih-Kai Nien, and Hsin Her Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10960 • Publication Date (Web): 27 Oct 2016 Downloaded from http://pubs.acs.org on October 29, 2016

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ACS Applied Materials & Interfaces

Moth-eye Inspired Biophotonic Surfaces with Antireflective and Hydrophobic Characteristics Wen-Kai Kuo 1, Jyun-Jheng Hsu 1, Chih-Kai Nien 1 and Hsin Her Yu 2,*

1. Graduate Institute of Electro-Optical and Materials Science, National Formosa University, 64 Wunhua Road, Huwei, Yunlin 63208, Taiwan; E-Mails: [email protected] (W.-K.K.); [email protected] (J.-J.H.); [email protected] (C.-K.N.)

2. Department of Biotechnology, National Formosa University, 64 Wunhua Road, Huwei, Yunlin 63208, Taiwan; E-Mail: [email protected] (H.H.Y.)

* Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +886-5-6315490; Fax: +886-5-6315502. Keywords: Antireflective; hydrophobicity; subwavelength structure; Langmuir–Blodgett (LB) deposition; nanoimprinting lithography. Abstract: In nature, in order to prevent attention from predators, the eyes of night-flying moths have evolutionarily developed an antireflective ability. The surfaces of their eyes are covered with a layer of a subwavelength structure that eliminates reflections of visible light. This layer allows the eyes of moths to escape detection in darkness, without reflections that could reveal the position of the moths to potential predators. In this study, we proposed a novel procedure for manufacturing a non-close-packed polystyrene (PS) nanosphere monolayer by combining the Langmuir–Blodgett (LB) deposition technique and oxygen plasma treatment. An antireflective structure was replicated from the subwavelength structure of moth eyes onto the surface of a glass substrate by nanoimprinting 1 ACS Paragon Plus Environment


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lithography; the structure also displayed hydrophobic properties. The Fresnel reflection of the replicated subwavelength structure is near the theoretical prediction from the effective medium theory (EMT) model. The biomimetic moth-eye structure can be applied to solar cells, monitors, LEDs, and other optical devices in the future.

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1. Introduction Antireflective (AR) coatings are typically used to reduce surface glare from eyeglasses,1 camera lenses,2 solar cells,3 TV screens,4 and LED devices.5 Until now, two main approaches have been followed to fabricate efficient AR coatings: multilayer coating6 and graded index coating.7 The former approach is based on the preparation of inorganic layered thin films, which provide a reflection reduction by inducing the destructive interference of light reflected at the different interfaces within the film.8 The limitation of this approach is that high transmittance can be achieved only in narrow ranges of wavelengths and incidence angles, which are defined by the layer thicknesses and refractive indices of the materials in the film. To widen these ranges, designs of greater complexity may be employed, but these require optimization by numerical algorithms to obtain the precise thickness of each layer.6 Furthermore, such coatings must be fabricated by physical vapor deposition, which is difficult to perform on very large-size optical devices. The latter approach is the manufacturing of graded index coatings, inspired by moth eyes, to reduce surface reflection.9 The coating forms a region with a graded refractive index at the interface between the air and the glass substrate, substantially reducing the amount of light reflected by the interface.7 The natural eye structure of the Philosamia cynthia ricini silkmoth contains so-called subwavelength structures of 250-nm-high protuberances (termed corneal nipples) with a period of 300 nm.10 These structures are arranged in a highly ordered hexagonal array, as shown in Figure 1.

(a)

(b)

(c)

600 µm

100 µm

(d)

(e)

4 µm 3 ACS Paragon Plus Environment

1 µm


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Figure 1. (a) The photograph of the adult Philosamia cynthia ricini was obtained from the Miaoli District Agricultural Research and Extension Station, Council of Agricultural, Taiwan. SEM images of the Philosamia cynthia ricini moth-eye structure at different magnifications: (b) the complete compound eyes, (c) the hexagonal arrangement of ommatidia, (d) the details of the ommatidia, and (e) the corneal nipple arrays in one ommatidium.

Such subwavelength structures also occur on the eyes of other species of moths and certain butterflies,11 and on the transparent sections of cicada wings.12 The structured eye surface allows the moth to see clearly in darkness and decreases the reflection of light from its compound eyes to avoid detection by nocturnal predators. These moth-eye structures effectively act as continuous refractive index gradients between the air and the medium of the eye, ensuring that incident light does not encounter a sudden change in refractive index, which would cause some proportion of the light wave to be reflected.13 This characteristic makes the moth-eye pattern one of the most effective AR coatings in nature. Rigorous numerical studies have accurately predicted the broadband reflection of subwavelength structures.14 The ideal structure for an AR coating component is parabolic in shape with dimensions smaller than the wavelengths of visible light (400–700 nm), based on theoretical calculations using a rigorous coupled wave analysis (RCWA) method.15 When light interacts with structures of dimensions significantly below this range, the structures behave as an effective medium with a refractive index gradient, as shown in Figure 2. This is the basis of the approach known as effective medium theory (EMT).16,17 Consequently, a surface textured with ridges smaller than the wavelength of incident light interacts with the light as if it is a single-layer AR coating with a refractive index governed by the ratio between the ridges and channels (Figure 2a). Likewise, a parabola-shaped profile behaves as an infinite stack of infinitesimally thin layers, introducing a gradual change in the refractive index from one medium to the other (Figure 2b).18,19 The effective refractive index 4 ACS Paragon Plus Environment

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depends on the relative amounts of each medium per layer, which is determined from the profile of the graded index region.

(a)

(b)

Figure 2. Schematics of the subwavelength structures and their analogous refractive index profiles, as experienced by incident light: (a) ridged profile, (b) parabola-shaped profile.

In this study, monodispersed polystyrene (PS) nanospheres were synthesized by an emulsifier-free emulsion polymerization method and then arranged into a periodic close-packed monolayer by the Langmuir–Blodgett (LB) deposition technique. A non-close-packed moth eye-inspired structure was arranged by oxygen plasma etching to produce AR properties. Finally, a subwavelength structure was replicated onto the surface of a glass substrate by nanoimprinting lithography. This subwavelengthstructured glass possessed AR and hydrophobic behavior. The Fresnel reflection of the replicated subwavelength structure is near the theoretical prediction from the EMT model. In addition, the cost for fabricating a film with the AR and hydrophobic properties by the method proposed is much lower than that for semiconductor coating and etching procedures. Therefore, the proposed fabrication method for a biomimetic moth-eye structure could be potentially favorable for applications in the fields of solar cells, monitors, LEDs, and other optical devices in the future.



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2. Experimental Section 2.1 Materials Sodium hydroxide (NaOH) and styrene (St) monomers were purchased from Sigma-Aldrich. Potassium persulfate (KPS) and ethylene glycol (EG) were purchased from J.T.Baker. 4Styrenesulfonic acid sodium salt (NaSS) was purchased from Alfa Aesar. Methanol was purchased from Merck in the highest available purity (>99.9%). Ultrapure water (18.2 MΩ·cm) was used directly from a Millipore A-10 water purification system. Glass slides were used as substrates. A kit for a prepolymer of Sylgard 184 polydimethylsiloxane (PDMS, Dow Corning Corporation) was used for replica molding. Norland Optical Adhesive 63 (NOA 63, Norland Products Inc.) was used for preparing the AR coatings. Methanol and ultrapure water were used as the spreading solvent and subphase for the LB experiments, respectively, and EG was used for adjusting the specific gravity of the water subphase. 2.2 Synthesis of the PS nanospheres The PS nanospheres were synthesized by an emulsifier-free emulsion polymerization method. First, 5 mg NaSS was added to 90 mL ultrapure water and stirred until completely dissolved. The solution was transferred into a 250-mL four-mouth flask, and 10 mL St monomer was added to the flask. The mixture was stirred and reflowed under the protection of nitrogen gas. When the solution was heated to 70°C, 87 mg KPS was added to the solution. After this addition, the system was obturated and the solution was stirred for 24 h. The as-synthesized PS nanospheres were centrifuged and washed with water to remove the residual St. After freeze-drying, the PS nanospheres were diluted with methanol 2.3 Preparation of the hydrophilic substrate Each glass slide was cleaned with acetone, ethanol, and ultrapure water in an ultrasonic bath for 5 min each. Then, the glass substrate was immersed in a 0.25-M NaOH aqueous solution for 1 h to


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render the surface hydrophilic. Finally, the hydrophilic glass was rinsed with ultrapure water and dried under a nitrogen gas flow. 2.4 π-A isotherms of the PS nanosphere monolayer in the LB experiment Before use in the LB experiment, the PS nanospheres were diluted with methanol to obtain the required solids concentration of 5 mg/mL. Surface pressure versus surface area (π-A) isotherms were obtained by applying 200 µL of the diluted PS nanospheres to a subphase comprising a 1:1 mixture of water and EG in an LB deposition trough (KN 2002, KSV NIMA). The system was allowed to equilibrate for 15 min, during which the spreading solvent partially evaporated but mostly mixed with the subphase, while the PS nanospheres became redistributed at the air-water interface. The PS nanosphere monolayer was then compressed at a constant rate of 1 cm2/min using a movable Teflon barrier to decrease the available surface area. During this compression, the surface pressure was continuously recorded by a floating barrier connected to a highly sensitive film balance, which produced a curve of surface pressure versus surface area. In order to test the stability of the PS nanosphere monolayer, the monolayer was compressed to a constant surface pressure. This deposition of the monolayer was performed by dipping a vertically aligned glass substrate through the monolayer and into the subphase as quickly as possible, followed by withdrawing the substrate at a constant rate of ~1 mm/min while the surface pressure was kept constant, as shown in Figure 3.



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Figure 3. Schematic of the LB deposition process for depositing PS nanosphere monolayer on the hydrophilic glass substrate.

2.5 Fabrication of the subwavelength structured surface


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Figure 4. Schematic of the experimental procedures for fabricating of (a) the hexagonally arranged PS nanosphere monolayer array, (b) the PDMS stamp, and (c) the single-side subwavelengthstructured NOA 63-coated antireflective glass.

Schematics for the fabrication procedures are presented in Figure 4. By lifting the substrate from the air-water interface, a highly ordered PS monolayer was transferred onto the hydrophilic substrate. The close-packed nanosphere monolayer was further tailored through a 300-W oxygen plasma etching treatment (Plasma Cleaner, PCD 150, All Real Tech.) to a non-close-packed geometry. The non-close-packed or close-packed PS nanosphere monolayer on the glass substrate was employed as a template for a PDMS stamp by a molding technique. The resulting PDMS stamp was used for UV-nanoimprinting lithography (UV-NIL). To achieve the subwavelength-structured surface, a NOA 63 layer was prepared by casting on glass slide. A UV-NIL process was performed using UV



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light (Philips Actinic BL TL 8W, 365 nm, 5 mW/cm2) for 20 min with the PDMS stamp. In the UV-NIL process, a pressure of 15 kPa was employed for 2 min at room temperature. 2.6. Samples characterizations The particle size distribution and zeta potential were measured by a Zetasizer (3000 HS, Malvern Instruments). The surface morphology was characterized by field-emission scanning electron microscopy (FE-SEM, JSM-7800F, JEOL) and atomic force microscopy (AFM, DI 3100, Digital Instruments). The transmittance were measured and averaged from three different samples using an ultraviolet–visible (UV-Vis) spectrophotometer (Cary 50, Varian) with an integrating sphere at normal incidence. The angle-dependent reflectance spectra were observed by a variable-angle multifunctional optical characteristic measuring system. The apparatus was equipped with a halogen light source (MFS-630, Hong-Ming Technology Co., Ltd., New Taipei City, Taiwan) and a barium sulfate coated standard integrating sphere was placed inside the detector module. The spectral data were recorded by an optical fiber spectrometer (Ocean Optics USB4000, Dunedin, FL, USA).20 A contact angle measurement instrument (FTA125, Pentad Scientific Corporation) was used to evaluate the hydrophobic performance of the sample surface.


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3. Results and Discussion 3.1 The particle size distribution and zeta potential of the synthesized PS nanospheres The average particle size of the synthesized PS nanospheres was approximately 340 nm, as shown in Figure 5.

25 20

Intensity (%)

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15 10 5 0 100

200

300

400

500

600

700

800

Size (nm)

Figure 5. The size distribution of PS nanospheres synthesized by emulsifier-free emulsion polymerization.

The polydispersity index (PDI) was 0.002, indicating that the PS nanospheres were uniform in size. The zeta potential value of the synthesized PS nanospheres suspended in methanol is −38.46 mV, as shown in Table 1.



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Table 1. Zeta-potential of the PS nanospheres suspended in different organic solvents and the properties of the solvents used.

Boiling point (°C)

Solubility parameter

Dielectric constant

Zeta potential

(MPa1/2)21

(mV)

Methanol

65

29.7

32.7

-38.46

Ethanol

78

26.4

24.5

-31.10

Isopropyl alcohol

82

23.5

17.9

-18.43

This indicates the formation of a stable homogeneous dispersion of synthesized PS nanospheres through repulsion forces from the negative charge on the surface.22 The selection of the spreading solvent in LB experiments is guided by three important requirements:23 immiscibility with water, to prevent the material under investigation from dispersing in the subphase; volatility to induce rapid elimination from the resulting monolayer; and inability to destabilize, dissolve, or swell the PS nanospheres.21 In practice, this third demand implies that commonly used spreading solvents, like chloroform or hexane, cannot be used for PS nanospheres. The former will dissolve the polymer, while the latter, as a result of its polar character, will cause early flocculation and precipitation of the electrostatically stabilized PS nanospheres. Therefore, the selected spreading solvent used for LB experiments with PS nanospheres is methanol in this study. Methanol is not a solvent for PS, so the nanospheres will not be dissolved, while its polarity character will ensure a reasonably stable suspension. 12 ACS Paragon Plus Environment

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3.2. π-A isotherm diagram of the PS nanosphere monolayer of the LB experiment

Figure 6. The π-A isotherm of the PS nanosphere monolayer.

Figure 6 shows a typical π-A isotherm diagram of the LB experiment. It can be seen that the slope of the isotherm curve is very steep, indicating that a solid condensed phase with low compressibility is formed on the air-water interface. The optimal surface pressure for deposition was ~0.6 mN/m. This can be deduced from the compression isotherms, obtained by measuring the surface pressure of the interfacial film as a function of the mean molecular area of the PS nanospheres at the airwater interface. Under compression, the PS nanosphere monolayer adopts different physical states of gas, liquid-expanded (LE) (π = ~0.25–0.30 mN/m), liquid-condensed (LC) (π = ~0.30–0.55 mN/m), and solid (π = ~0.55–0.75 mN/m). The state is related to the presence of PS nanosphere interactions within the monolayer. The three distinct regions of the isotherm can be associated with different levels of monolayer order, as shown schematically on the right side of Figure 6. Because of the presence of the PS monolayer film covering the air-water interface, the PS layer moves toward a 13 ACS Paragon Plus Environment


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collapse phase and instability as the surface pressure increases to 0.75 mN/m. Collapse occurs when the PS monolayer is essentially incompressible in two dimensions; further compression forces the PS nanospheres out of the interface or into the subphase to form a three-dimensional entity. Thus, an inhomogeneous LB monolayer was obtained when it was deposited on the substrate.24 3.3. SΕΜ images of the PS nanosphere monolayer-structured templates Figure 7a shows a typical SEM image of the close-packed PS nanosphere monolayer template fabricated by LB deposition.

(a)

(b)

500 nm

500 nm

(c)

(d)

500 nm

500 nm

Figure 7. SEM images of PS nanosphere monolayer after oxygen plasma treatment at 300 W with a duration of: (a) 0 min, (b) 2.5 min, (c) 5 min, and (d) 10 min.

PS nanospheres with an average diameter of 335 nm form contacts with other neighboring spheres in a hexagonal packing arrangement. Non-close-packed PS nanosphere monolayer templates are obtained after oxygen plasma etching for 2.5, 5, and 10 min, with corresponding PS 14 ACS Paragon Plus Environment

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nanosphere sizes of 288 nm, 202 nm, and 150 nm, respectively (see Figure 7b to d). These arrays demonstrate that the interval between the nearest neighboring PS nanospheres can be adjusted effectively by changing the oxygen plasma etching time. When the duration of oxygen plasma etching treatment exceeds 5 min, the spherical PS nanoparticles become irregular with jagged exteriors. 3.4. The morphologies and optical performances of the subwavelength-structured NOA 63-coated glass The AFM images of Figures 8a,b show the subwavelength-structured NOA 63-coated glass replicated from the subwavelength-structured template treated with oxygen plasma etching for 0 min and 2.5 min. The former structure is hemispherical with a diameter of 348 nm and a height of 113 nm, and the latter is parabolic with a diameter of 310 nm and a height of 238 nm. The parabola shape yields a nearly linear refractive index gradient according to EMT,25 which is efficient in reducing the surface reflection. However, if the oxygen plasma etching time is too long, then the smooth surface of the original PS nanospheres is destroyed. Figures 8c,d show coarse hemispherical structures with diameters of 194 nm and 145 nm and heights of 95 nm and 70 nm, from the singleside subwavelength-structured templates etched for 5 min and 10 min, respectively.



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Figure 8. AFM images of the subwavelength-structured NOA 63-coated glass fabricated from the PS nanosphere monolayer treated with oxygen plasma for: (a) 0 min (untreated), (b) 2.5 min, (c) 5 min, 16 ACS Paragon Plus Environment

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and (d) 10 min. A typical 3D image (left side), 2D image (middle), and line profile (right side) using AFM.

Figures 9a,b show the optical reflectance and transmittance spectra at normal incidence, respectively, of a flat NOA 63-coated glass, bare glass, and NOA 63-coated glass samples fabricated from the single-side subwavelength-structured template treated with different oxygen plasma etching times.

Figure 9. The (a) reflectance and (b) transmittance spectra of (i) flat NOA 63-coated glass, (ii) the bare glass, and the single-side subwavelength-structured NOA 63-coated glass fabricated from the PS nanosphere monolayer treated with oxygen plasma for: (iii) 0 min, (iv) 2.5 min, (v) 5 min, and (vi) 10 min.

The average reflectance of the single-side subwavelength-structured NOA 63-coated glass is 1.11, 0.86, 2.33, and 3.75%, for etching times of 0, 2.5, 5, and 10 min, respectively (observed from Figure 9a(iii) to (vi)). The single-side subwavelength-structured NOA 63-coated glasses are also less reflective than the bare glass substrates. The average transmittance of the corresponding single-side



subwavelength-structured NOA 63-coated glass specimens are 94.4, 94.9, 92.8, and 91.1%, respectively, as observed from Figure 9b (iii) to (vi). Furthermore, the angle-dependent reflection of the single-side subwavelength-structured NOA 63-coated glass is characterized with results shown in Figure 10. The average reflectance is enhanced as the incident angle is increased, since the average refractive index profile from air to the glass substrate changes too steeply in the case at normal incidence, but the reflectance remains below 4% at 70°.

3.2 3.0 2.8 2.6

Reflectance (%)

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2.4

(e)

2.2 2.0

(d) (c)

1.8 1.6 1.4

(b)

1.2 1.0

(a)

0.8 0.6 400

500

600

700

800

Wavelength (nm) Figure 10. Measured angle-dependent reflectance of the single-side AR film (fabricated from the PS nanosphere monolayer treated with oxygen plasma for 2.5 min) at the incidence angles of (a) 0°, (b) 10°, (c) 30°, (d) 50°, and (e) 70°.

These results show that the light propagating on the single-side subwavelength-structured surface experiences a continuous refractive index gradient between the air and the medium, which decreases the reflected light by effectively removing the air-glass interface, as shown in Figure 11. 18 ACS Paragon Plus Environment

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(a)

(b)

Figure 11. Photographs of (a) the single-side subwavelength structured NOA63 coated glass obtained by the PS nanosphere monolayer treated with oxygen plasma for 2.5 min, and (b) the bare glass exposed to conventional room illumination.

The proposed procedure for preparing subwavelength moth-eye inspired structures has two essential advantages: (A) The LB-assembly technology has high flexibility in the scalable fabrication of uniform particle coatings on many substrates with complex geometries. (B) Combining O2 plasma etching significantly increases the depths of the templated gratings and improves the AR performance and transmittance. The obtained performance can be compared to the technologies of reactive ion etching (RIE)26 and soft lithography27–29 that are most commonly used in mimicking the subwavelength moth-eye nipple array structure; this comparison is shown in Table 2.



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Table 2. The most commonly used technologies to prepare subwavelength moth-eye nipple array structures and the resulting structures’ reflection and transmittance characteristics. Fabrication method

AR material

Substrate

Reflection

Transmittance

Ref.

Self-assembly + RIE

ETPTA #1

Glass

0.47%

-

26

LIL + Soft lithography #2

NOA 63

Glass

5.5%

93.2%

27

Soft lithography

PUA #3

PET

1.53%

95%

28

ICP + Soft lithography #4

Ag/NOA 63

PET

6.8%

45.8%

29

LB+ O2 plasma

NOA 63

Glass

0.86%

94.9%

*

- Unavailable from reference. *

Our research work.

#1

ETPTA = Ethoxylated trimethylolpropane triacrylate

#2

LIL = Laser interference lithography

#3

PUA = Polyurethane acrylate

#4

ICP = Inductively coupled plasma The size of the subwavelength structures is small relative to the wavelength of light, such that light

propagation is governed by the effective refractive index of the subwavelength-structured surface, which can be calculated from EMT by Equation (1):30 /

=

/ /

+ 1 −

(1)

where neff is the effective refractive index, f is the fill factor of the subwavelength structures arranged hexagonally, na is the refractive index of NOA 63 (na = 1.56), and nb is the refractive index of air (nb = 1). 20 ACS Paragon Plus Environment

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AR structures inspired by the corneal surface of the Philosamia cynthia ricini moth eye can modulate the effective refractive index by manipulation of the fill factor, as shown in Equation (2). The relative fill factor of the subwavelength structures can thereby satisfy AR conditions at the airglass interface:31

=

=

22 √3

=

2√3

(2)

where A is the base area of subwavelength structures and Aunit is the area of unit cell. a and ap are the diameters of the subwavelength structures before and after different etching times, obtained from the AFM images shown in Figure 7. Figure 12 schematically shows the change in particle size in a unit cell of close-packed and nonclose-packed hexagonal arrays of the subwavelength structures.

Figure 12. Schematics of the unit area in (a) close-packed and (b) non-close-packed hexagonal arrangements of subwavelength structures.

The average sphere sizes, fill factors, effective refractive indexes, and calculated reflectance values for samples treated with different oxygen plasma etching times are listed in Table 3.



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Table 3. Calculated performance of the subwavelength-structured NOA 63-coated glass with different O2 plasma treatment times.

O2 plasma treated time (min)

d (nm)*

Fill factor (f)

Effective refractive index (neff)

Calculated reflectance (R, %)

0

348

0.9069

1.5045

0.0002

2.5

310

0.7197

1.3948

0.1321

5

194

0.2818

1.1494

1.7513

10

145

0.1574

1.0826

2.6121

* Diameter of subwavelength structures obtained from the AFM images of Figure 8.

The fill factors of 0.9069, 0.7197, 0.2818, and 0.1574 and corresponding effective refractive indices of 1.5045, 1.3948, 1.1494, and 1.0826 are achieved for the subwavelength-structured NOA 63-coated glass specimens treated by oxygen plasma etching for 0, 2.5, 5, and 10 min, respectively. The calculated reflectance values of the samples are 0.0002, 0.1321, 1.7513, and 2.6121%, respectively, as obtained by solving Fresnel’s equations (Equation (3)):


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− " % = + "

(3)

where R is the calculated reflectance values, neff is the effective refractive index, and ng is the refractive index of glass (ng = 1.5). The calculated reflectance of the structure without oxygen plasma etching approaches zero, which differs significantly from the experimental data shown in Table 3 and Figure 9a. This is because the calculated reflectance of the AR structure was deduced theoretically from a plain hexagonal unit cell model, shown in Figure 11, and the height of the AR pattern was not considered in Equation (2). For surface AR properties, the gradual transition of the effective refractive index from air to the bulk material is crucial. The correlation between neff and the height of the subwavelength structures is presented in Figure 13.

1.5

Effective refractive index

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(a)

1.4

(b)

1.3 1.2

(c) 1.1

(d)

1.0 0

50

100

150

200

250

Height of AR patterns (nm)

Figure 13. The correlation of the effective refractive index and the height of subwavelengthstructured NOA 63-coated glass fabricated from the PS nanosphere monolayer treated with oxygen plasma for: (a) 0 min, (b) 2.5 min, (c) 5 min, and (d) 10 min.



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The Fresnel reflection of incident light arises from the discontinuity of the refractive index at the interface of two media. By inserting a layer with an intermediate index or a multilayer with a stepped index, this larger discontinuity is broken into smaller steps, resulting in a lower reflectance (Figure 13b). However, the hemispherical structures shown in Figure 13a,c,d are not optimal for achieving a good AR effect. In addition, the poorly packed subwavelength structures (for the lower fill factor arrays) would not produce a continuous variation in the refractive index, because they would not completely eliminate the step at the interface between the dispersed hemispheres and the substrate.32 In this study, the AR glass we prepared is coated on only one side with the NOA 63 subwavelength structures. If the coating were applied to both sides of the glass substrate, then the optical transparency could be further promoted.31 For most commercial applications of AR coatings, the reduction of reflection from a glass or polymer substrate is necessary. In these cases, nanoimprinting lithography can replicate the design onto a soft plastic material.33 The moth-eyestructured surface could potentially be applied to transparent materials in the future. 3.5 The hydrophobic performance of the subwavelength structured NOA 63 coated glass Subwavelength structures with large roughness on a surface can enhance hydrophobicity, which can induce self-cleaning of dust particles or other surface contaminants. These self-cleaning properties are very useful in real outdoor applications. Figure 14 shows photographs of water droplets on the surface of a bare glass, a flat NOA 63-coated glass, and NOA 63-coated glass fabricated from the subwavelength-structured template treated with oxygen plasma etching times for 0, 2.5, 5, and 10 min.


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(a)

(b)

θ = 109.7°

θ = 82.7°

θ = 43.0°

(d)

(c)

(e)

(f)

θ = 98.4°

θ = 117.7°

θ = 86.9°

Figure 14. Water contact angles of (a) the bare glass, (b) the flat NOA 63-coated glass, and the subwavelength structured NOA 63-coated glass obtained by the PS nanosphere monolayer treated with oxygen plasma for: (c) 0 min, (d) 2.5 min, (e) 5 min, and (f) 10 min.

The corresponding contact angles are 43.0°, 82.7°, 109.7°, 117.7°, 98.4°, and 86.9°, respectively, as observed from Figure 14a to f. The resulting AR specimens with parabola-shaped subwavelengthstructured NOA 63 coatings exhibit hydrophobicity. This is attributed to the increased surface roughness caused by the deposition of the parabola-shaped subwavelength structures. With increasing hydrophobicity, the AR coating reduces the attraction between the liquid droplet and the surface, thus increasing the tendency for the droplet to form a spherical shape on the surface. In other words, dirt and dust can be more easily washed off by rain for more hydrophobic surfaces.34 This self-cleaning property can be induced using nanosphere lithography35 based on the PS nanosphere monolayer, which also decreases the reflectance of the substrate by increasing the height of the subwavelength structures.36



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4. Conclusions A close-packed, ordered PS nanosphere monolayer was established by the LB deposition technique. An ordered periodic non-close-packed PS nanosphere monolayer template was further tailored from the close-packed PS microarray through oxygen plasma treatment. This enabled the manipulation of the diameter of the PS nanospheres by altering the oxygen plasma etching conditions. We determined that the height and diameter of the features on a subwavelengthstructured surface can be controlled and the effects of these changes on the optical properties of the surfaces in this study. The optimal subwavelength-structured NOA 63-coated glass was fabricated by nanoimprint lithography with a stamp obtained from a PS nanosphere monolayer etched with oxygen plasma for 2.5 min. The average reflectance of this AR coated glass is lower than 1% in the visible range and the optical transmittance approaches 95% at normal incidence. The moth-eye-mimicking surface structure has the dual functionality of AR and hydrophobicity, which could be extended to future applications in solar cells, monitors, and related AR optical devices.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Ministry of Science and Technology of Taiwan (MOST 104-2221-E-150-038 & 104-2221-E-150-064-MY3).


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Moth-Eye-Inspired Biophotonic Surfaces with Antireflective and

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