- Email: [email protected]
Fabrication and characterization of moth-eye mimicking nanostructured convex lens
Fabrication and characterization of moth-eye mimicking nanostructured convex lens Seung-Chul Park, Namsu Kim, Seungmuk Ji, Hyuneui Lim PII: DOI: Reference:
S0167-9317(16)30117-4 doi: 10.1016/j.mee.2016.03.011 MEE 10194
To appear in: Received date: Revised date: Accepted date:
8 November 2015 21 February 2016 3 March 2016
Please cite this article as: Seung-Chul Park, Namsu Kim, Seungmuk Ji, Hyuneui Lim, Fabrication and characterization of moth-eye mimicking nanostructured convex lens, (2016), doi: 10.1016/j.mee.2016.03.011
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
PT
Fabrication and Characterization of Moth-eye Mimicking
SC
RI
Nanostructured Convex Lens
a
NU
Seung-Chul Parka, §, Namsu Kima,c, §, Seungmuk Jia,b, Hyuneui Lima,d,*
Department of Nature-Inspired Nanoconvergence Systems, Korea Institute of Machinery
Yonsei Institute of Convergence Technology, Yonsei University, Incheon 406-840, Repub
D
b
MA
and Materials, Daejeon 305-343, Republic of Korea
TE
lic of Korea
School of Mechanical Engineering, Hanyang University, Seoul 133-791, Republic of Korea
d
Department of Nanobiotechnology, University of Science & Technology, Daejeon 305-350,
AC CE P
c
Republic of Korea
Keywords: Moth-eye, Nanostructure, Antireflection, Convex lens, Self-cleaning
§
Seung-Chul Park and Namsu Kim were equally contributed as a first author.
*To whom correspondence should be addressed. Tel : +82-72-868-7106 Fax : +82-72-868-7933 Email : [email protected]
1
ACCEPTED MANUSCRIPT Abstract In this work, we demonstrate a simple fabrication method for the periodic nanopillar structures
PT
on both sides of convex lens and evaluation of their optical characteristics. The periodic
RI
nanopillar structures on convex lens are inspired from the moth-eye which can provide
SC
antireflective property. The fabrication is composed with the coating of colloidal nanoparticles on a convex lens and the plasma etching of it. Because the optical behavior depends on the
NU
surface morphology, the structural dimensions of nanopillars should be modulated by the
MA
various experimental conditions such as the processing times of two steps, which reduce the size of self-assembled particles and perform the pattern transfer into the lens material.
D
Nanopillar structured convex lens exhibits the antireflective effect showing the increase in
TE
transmittance and the decrease in reflectance without any deformation of lens property such as chromatic and spherical aberration. Furthermore, the antireflective property of nanostructured
AC CE P
convex lens can make the photography high quality without flare and ghost phenomena, even though the pictures are taken under the strong external light environment such as sunshine, sunset, and concentrated external light. The wettability of convex lens surface is also controlled to express the self-cleaning effect which is inspired from a lotus leaf by the low surface tension chemical coating on a nanostructured surface. This moth-eye mimicking nanostructured convex lens will be easily applicable to optical industry with easy and cost-effective fabrication for the advanced optical and wetting property.
2
ACCEPTED MANUSCRIPT 1. Introduction The bioinspired functional surface can be applied as a key component for the advanced
PT
functional devices or advanced materials due to the unique functionality such as self-cleaning
RI
of Lotus leaf, structural color in Morpho-butterfly, low friction surface from the snake skin, adhesive surface of gecko foot, and water harvesting from Namib beetle back [1-4]. Especially,
SC
the optically advanced functional structure exhibits the amazing properties based on their
NU
adaptive hierarchical structures from macrosize to nanosize. The hierarchical structures in nature can provide the various unique optical properties such as structural color, multi-view
MA
phenomenon, and antireflection in broadband based on the periodic nanostructures on surface [1, 5-8]. Because these properties are based on the interaction between the incident light and the
TE
D
periodic structures in nanoscale, one of the central issues in such new area concerns how to construct uniform surface nanostructure with specific structural morphology such as height, size,
AC CE P
shape, and periodicity for practical applications. The tailored hierarchical structures have been made by taking either top-down methods such as a photolithography and e-beam lithography or bottom-up methods which include a soft lithography and self-assembly [9-12]. Among them, self-assembly method is promising due to the merits of an easy large area processing, low cost, high throughput, and high resolution [13]. For the construction of periodic nanostructures on surfaces using the self-assembled colloidal particles, there are several methods to make monolayered particles such as the convective method, floating method, template assisted method, and spin coating method [14-17]. Particularly, among them, the floating method has a great potential in generating self-assembled monolayer of particles on the flat surface as well as the curved surface owing to the flexibility in process. Generally, it is not easy to make nanostructures on curved surface because the non-flat shape hinders the
3
ACCEPTED MANUSCRIPT formation of nanostructures on substrate uniformly. However, the curved surface with nanostructures is the critical component for the practical applications in advanced optical devices
PT
with the controllability of the light pathway such as focusing or defocusing the light [18, 19].
RI
The transparent curved substrate with nanostructures on the surface can provide not only the conventional lens effect but also the antireflective property which is inspired from moth-eye.
SC
These combinations of lens with nanostructures can help photo images clear by removing the
NU
ghost and flare phenomena which are caused by the internal reflection of incident light in lens [20]. Therefore, the construction of nanostructures on curved surface with a controlled height,
MA
size, shape, and periodicity is the essential and important issue for practical applications. Here, we demonstrate the simple and promising method for constructing periodic nanopillar
TE
D
structures on both sides of convex lens. Self-assembled colloidal particles on convex lens which act as the mask layer are made by the floating method and then those convex lenses are etched to
AC CE P
form the nanopatterns via the following plasma etching process. The fabrication conditions such as size reduction of colloidal particles and etching process of lens are modulated for the constructing nanopillar structures that can induce the antireflective property. The nanostructured convex lens with low reflectance is applied to the optical camera lens that can remove the ghost and flare phenomena. The surface modification is also performed to get the superhydrophobicity on nanostructured lens surface.
2. Experiment Fabrication process Fig. 1 shows the fabrication process of nanopillar structures on convex lens. We used a solution of commercial polystyrene colloidal nanoparticles (NPs, Polyscience Inc.) to prepare the monolayer of hexagonally packed NPs as the mask layer for etching process. The
4
ACCEPTED MANUSCRIPT size of NPs determines the periodicity of nanopillars on surface. The average size of used NPs is about 200 nm in diameter and the size distribution is ± 10 nm. First, the hexagonally packed NPs
PT
monolayer on water surface was formed using the self-assembly property of colloidal particles,
RI
as shown in Fig. 1(a) and (b). After the assembling of NPs on water surface, some area of the well packed NPs assembly on water was lifted up with convex lens (PCX 25mm Dia, Edmund
SC
optics), which was already treated with UV-Ozone for 15 mins, by the simple floating method,
NU
as shown in Fig. 1(c). The drying of residual water between the NPs was followed in air at room temperature and the convex lens was etched with NPs as a mask. The plasma etching process has
MA
two steps: size reduction process of the NPs with O2 plasma treatment; and etching process with CF4 and H2 plasma treatment for the regulating of shape and height of nanopillar, as shown in
TE
D
Fig. 1(d) - (f). The size reduction process provides the open area between the NPs, as depicted in Fig. 1(e). When the close-packed NPs cover the whole area of surface without the size reduction
AC CE P
process, the attacks of plasma into convex lens cannot be performed easily. Moreover, the modulation of open area between NPs generates the various shapes of nanopillars such as pointy shape or cylindrical shape, as we reported before [21]. To obtain the optimum condition of antireflective property of convex lens, we applied the size reduction process and etching process for 20, 45, 60 secs and 600, 900, and 1200 secs, respectively. The same processes were repeated of Fig. 1(c) - (f) on back side of convex lens to get the both sides nanostructured convex lens, as shown in Fig. 1(g). Finally, the both sides nanostructured convex lens in Fig. 1(h) was obtained after removing the residual polystyrene NPs and deposited carbon wastes on surface by piranha cleaning process (H2SO4 : H2O2 = 3: 1) for 30 mins and rinsing with deionized water.
5
ACCEPTED MANUSCRIPT Chemical modification The nanopillar structure on convex lens was modified with 0.1 % of Perfluoropolyether (PFPE, Nicca Korea Co., Ltd) in Novec 7200 solution (3M) via a dipping
PT
method for 30 mins, to get the superhydrophobicity of the lens.
RI
Characterization The surface morphology was observed using a field-emission scanning electron microscope (FE-SEM, Nano Nova 200, FEI). The optical properties; total transmittance
SC
and total reflectance were measured by using a UV-Vis-NIR spectrophotometer (Carry 5000,
NU
Agilent Technologies) equipped with integrating sphere for a range of 350 nm to 800 nm. When the reflectance was measured, the angle of incident light was fixed with 6° which is the lowest
MA
angle can measure the regular reflectance by the spectrometer. To test the deformation of lens such as chromatic aberration and spherical aberration, the effective focal lengths (EFL) of
TE
D
lens with various wavelength of laser such as 650 nm (Red), 550 nm (Green), and 450 nm (Blue) were measured using the photodetector that the maximum penetrated light intensity can be
AC CE P
detected. Furthermore, to confirm the antireflective property in the view of removing the ghost and flare phenomena in real usage, the photo image was captured with optical camera (SD700IS, Canon) using the convex lens with and without nanopillar structure under the certain external light environment.
The contact angles of the nanopillar structured convex lens with and without fluorinated polymer coating were measured to observe the wettability of the lens surface, by using a contact angle goniometer (Smart Drop, Femtofab Co., Ltd.) with 5 µL of DI water droplet at room temperature.
3. Results and Discussion The effects of structural morphology on optical property
6
ACCEPTED MANUSCRIPT The reflectance can be changed by the interaction between the surface nanopillar structures and the incident light. The structural morphology of surface nanopillars should be controlled to
PT
reduce the reflection of convex lens for improving the optical property. The proposed
RI
fabrication method in this paper can control the structural morphology such as height, shape, size, and periodicity by modulating the diameter of NPs and etching conditions via the time of size
SC
reduction process and etching process. Based on our previous study [21], the diameter of NPs is
NU
fixed in 200 nm to obtain the ordered nanopillar periodicity in self-assembly. While the larger size of NPs than 200 nm deteriorates the optical property in short wavelength range, the smaller
MA
size of NPs than 200 nm shows the difficulty in hexagonal packing because of the large distribution in particle diameter. Therefore, the size and periodicity are fixed with 200 nm. The
TE
D
effect of height and shape of nanopillar structures on convex lens are studied with size reduction process and etching process. The shape of nanopillar structure is changed depending on the size
AC CE P
reduction time. When the time of size reduction is increased, the uncovered area of convex lens by NPs is increased. The increased gap between the NPs can provide the enough lateral dimension of etching process thus the shape of nanopillar structure becomes sharper. Sharp nanopillar structures exhibit the gradual changes of refractive index to the incident light with the mixed layer of nanostructures and air at the interface. As the nanopillar structure is getting sharper, the reflectance is getting lower. The time of size reduction process was performed for 20, 45, 60 secs with the following 900 secs etching process, as shown in Fig 2(a). As time of size reduction process is increasing, the reflectance is decreasing. The obtained reflectance values are the total values including the sum of specular and diffuse reflectance. When the time of size reduction is more than 60 secs, the shape of nanopillar structure is crushed showing that
7
ACCEPTED MANUSCRIPT the reflectance is getting increasing due to the scattering of light. Based on this result, the critical time of size reduction process is fixed with 60 secs for the top side of the convex lens.
PT
The time of etching process was also determined. Under the fixed ratio of CF4 and H2
RI
concentration as 2:3 [21], the etching process should be applied for a certain time with CF4 and H2 plasma. Since the thickness and shape of nanopillar structures depend on the time of etching
SC
process, the time of etching process is proceeded with 600, 900, and 1200 secs, as shown in Fig.
NU
2(b). For the low reflectance, the height of nanopillar structures should be high enough to generate gradual change of refractive index at the interface between air and convex lens.
MA
However, the important point of increasing the etching time is the durability of polystyrene NPs as a mask layer during the etching process. If the size of NPs would be big enough, the etching
TE
D
process can be applied for the long time because the NPs still last as a mask on convex lens surface during the etching process. However, when the NPs are etched completely at the long
AC CE P
etching time, the nanopillar structures should be over-etched without mask under the long etching process. Therefore, the critical etching time depends on the size of NPs as the mask layer. As shown in Fig. 2(b), the critical etching condition with 200 nm polystyrene particles is 60 secs for size reduction and 900 secs for etching time to make the low reflectance on the top surface of convex lens. The over-etching behavior happens for the 1200 secs etching showing that the heights of nanopillar structures are shorter than the results of 900 secs etching process. The reflection of convex lens also exists on the bottom surface of lens. To improve the antireflective property of the convex lens exceedingly high, the bottom surface of the lens was also nanostructured with the same process. Because the incident light shows the different aspects according to the height of the nanopillars in each wavelength, the combinations of the top and bottom surface structuring with different heights are need to maximize the optical properties [21].
8
ACCEPTED MANUSCRIPT Fig 3 shows the total reflectance and total transmittance values of the nanostructured of convex lens on both sides. Among several combinations of asymmetrical heights of nanostructures on
PT
top and bottom surfaces, the best antireflective property is achieved with the height of around
RI
590 nm on the top and around 230 nm on the bottom, respectively. The size reductions for the top and bottom surface are performed with the duration of 60 secs and 30 secs, respectively. The
SC
etching times are applied as 900 secs for the top surface and 300 secs for the bottom surface,
NU
respectively. The both sides nanostructured convex lens prepared with these conditions shows the reflectance of below 1 % at 550 nm wavelength, whereas the total reflectance of bare lens
MA
and flat quartz substrate which are about 8.4 % and 7.0 %, respectively in Fig 3(a). The difference between bare lens and flat quartz substrate might be caused by the different material
TE
D
composition and convex shape. Nanopillars on the convex lens also provide the higher transmittance values comparing with ones of bare lens and flat quartz substrate, as shown in Fig.
AC CE P
3(b). The optical properties such as the low reflectance and high transmittance of nanopillar structured convex lens are originated from the moth-eye mimicking antireflective property. Fig. 3(c) demonstrates the antireflective effect of nanopillar structured convex lens well. While the bare lens glitters with the external light, the nano lens which is the nanopillar structured convex lens exhibits the low reflectance appearing the letters under the lens. These antireflective properties are based on the nanopillars fabricated on both sides of convex lens which are shown in SEM images.
The applications of antireflective convex lens on photography Besides the prevention of the glaring and clear visibility of the nanostructured convex lens, antireflective effect can provide the reduction of the internal reflection in lens for elimination of
9
ACCEPTED MANUSCRIPT flare and ghost phenomena when we take a picture or image by the optical camera under the strong external light. The test of the anti-ghost and anti-flare phenomena is performed by taking
PT
pictures with bare and nanopillar structured convex lens under the outdoor and indoor conditions,
RI
as shown in Fig. 4. Under the strong sunlight environment, the picture shows the blurred image caused by the ghost image of sun as well as the flared image caused by a path of sunlight in the
SC
bare lens, while it is hard to see such phenomena in the nano lens. In addition, the ghost image Even under the indoor
NU
was observed only with the bare lens under the sunset situation.
environment, picture deteriorates with the ghost image which is made by the internal reflection
MA
of external strong light through bare lens. Therefore, antireflective nano lens can be applied as the key component for advanced optical devices having the anti-ghost and anti-flare properties.
TE
D
The test to check the deformation of lens property is also performed by measuring the focal length with various wavelengths, 650 nm (Red), 550 nm (Green), and 450 nm (Blue). The EFLs
AC CE P
of bare and nano lens show the same focal lengths. The EFLs of Red, Green, and Blue light are 125.3, 124.5, and 123.2 mm in both bare and nano lens, respectively. The differences of EFL with various wavelengths only depend on the bulk structural dimension of convex lens, such as curvature, size, and shape of the lens. This result shows that the nanofabrication process cannot make any deformation of lens but rather improve the lens property. These results suggest the promising usage of nanopillar structured convex lens in optical imaging systems.
The superhydrophobicity and self-cleaning effect of nanopillar structured convex lens The antireflective effect is demonstrated in a surface of nanopillar structured convex lens which is the subwavelength scale. Introducing the nanopillar structures into the glass lens surface can provide supehydrophilic property that the contact angle is lower than 10°. Supehydrophilic
10
ACCEPTED MANUSCRIPT property of the nanostructured surface is changed into supehydrophobic property by a simple low surface tension chemicals coating. Here, the fluorinated polymer, PFPE is used to cover the
PT
surface by a dipping method for 30 mins. After the PFPE coating on nanopillar structured convex
RI
lens, the contact angle is changed higher than 150 °. Fig. 5 shows the contact angles of the bare lens having the value of 30 °, supehydrophilic nano lens, and supehydrophobic nano lens after
SC
PFPE coating. Superhydrophobic surface generates the particular behavior of self-cleaning
NU
effect which is the biomimic of the lotus leaf [22]. In real application, self-cleaning effect is more important than Superhydrophobicity. The nanopillar structured convex lens represents the
MA
self-cleaning effect that the water droplets are rolling up and removed with sands, while the water droplet on the bare lens surface is spread on the surface showing that grain of sand is hard
TE
D
to be removed from the surface. This self-cleaning effect with nanopillar structures and simple chemical coating is a very fascinating property for the sustainable and practical application of
AC CE P
lens.
4. Conclusions
The nanopillar structuring on convex lens is demonstrated via a simple fabrication process which is the combination of self-assembling the NPs and plasma etching of the lens. The nanopillar structures induce the antireflective property which is based on the moth-eye structures. Optical properties are examined with various nanopillar morphologies according to the size reduction time of NPs and etching process time to control the shape and height of nanopillars. The both sides nanostructured convex lens shows the low reflectance and high transmittance based on the gradual change of the refractive index at the air-nanostructure interface. The convex lens having sharp shape with 590 nm height on top surface and 230 nm height on bottom surface shows the reflectance of below 1 % at 550 nm wavelength. Such antireflective property of convex lens is 11
ACCEPTED MANUSCRIPT also applied to optical camera lens demonstrating that the nano lens prevents the ghost and flare phenomena originated from the internal reflection of lens under strong light without any
PT
deformation in original lens property. In addition, the surface modification by PFPE coating on
RI
nanopillar structured lens generates self-cleaning effect which is the biomimic of the lotus leaf. The antireflective and self-cleaning convex lens inspired from nature can provide the advance
NU
SC
optical applications.
Acknowledgement
MA
This research was supported by the Korea Institute of Machinery and Materials as a part of the
AC CE P
TE
D
project "Development of nature-inspired multi optic functional element.”
12
ACCEPTED MANUSCRIPT References (1) K. Chung, S. Yu, C. Heo, J. W. Shim, S. Yang, M. G. Han, H. Lee, Y. Jin, S. Y. Lee, N.
PT
Park, and J. H. Shin, Adv. Mater., 24(2012) 2375-2379.
RI
(2) J. Hazel, M. Stone, M.S. Grace, V.V. Tsukruk, Journal of Biomechanics, 32(1999) 477484.
SC
(3) K. Autumn, Y. A. Liang, S. T. Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R. Fearing, and
NU
R. J. Full, Nature, 405(2000), 681-685.
(4) R. D. Narhe and D. A. Beysens, Europhys. Lett., 75(2006) 98–104.
Mater., 25(2013) 2239-2245.
MA
(5) M. Kolle, A. Lethbridge, M. Kreysing, J. J. Baumberg, J. Aizenberg, and P. Vukusic, Adv.
607–610.
TE
D
(6) Y. Verma, K.D. Rao, M.K. Suresh, H.S. Patel, and P.K. Gupta, Appl. Phys. B 87(2007)
AC CE P
(7) C. C. Striemera and P. M. Fauchet, Appl. Phys. Lett., 81(2002) 2980-2982. (8) S. Ji, K. Song, T. B. Nguyen, N. Kim, and H. Lim, Acs Appl. Mater. Interfaces, 5(2013) 10731-10737.
(9) A. Revzin, R. J. Russell, V. K. Yadavalli, W. Koh, C. Deister, D. D. Hile, M. B. Mellott, and M. V. Pishko, Langmuir 17(2001) 5440-5447. (10) C. Vieu, F. Carcenac, A. Pepin, Y. Chen, M. Mejias, A. Lebib, L. Manin-Ferlazzo, L. Couraud, and H. Launois, Applied Surface Science, 164(2000) 111–117. (11) Y. Xia and G. M. Whitesides, Annu. Rev. Mater. Sci, 28(1998) 153-184. (12) D. Choi, S. Kim, S. Jang, S. Yang, J. Jeong, and S. Shin, Chem. Mater., 16(2004) 42084211. (13) S. Zhang, Nat. Biotech., 21(2003) 1171 - 1178.
13
ACCEPTED MANUSCRIPT (14) B. G. Prevo, Y. Hwang, and O. D. Velev, Chem. Mater., 17(2005) 3642-3651. (15) Y. Liu, S. Wang, J. W. Lee, and N. A. Kotov, Chem. Mater., 17(2005) 4918-4924.
PT
(16) L. Malaquin, T. Kraus, H. Schmid, E. Delamarche, and H. Wolf, Langmuir, 23(2007)
RI
11513-11521.
(17) P. Jiang and M. J. McFarland, J. Am. Chem. Soc., 126(2004) 13778-13786.
SC
(18) H. Ren, Y. Fan, S. Gauza, and Shin-Tson Wu, Appl. Phys. Lett., 84(2004) 4789-4791.
NU
(19) Z. Liu, J. M. Steele, H. Lee, and X. Zhang, Appl. Phys. Lett., 88(2006) 171108-1-3. (20) C. A. Mack, Proc. SPIE 5040, Optical Microlithography XVI, 151(2003).
MA
(21) S. Ji, J. Park, H. Lim, Nanoscale, 4(2012) 4603-4610.
AC CE P
TE
D
(22) L. Jiang, Y. Zhao, and J. Zhai, Angew. Chem., 116(2004) 4438 –4441.
14
ACCEPTED MANUSCRIPT Figure 1. Schematic diagram of the fabricating processes for the nanopillar structure on convex lens; (a) filing the water in water bath and dropping the NPs solution on air-water interface, (b) self-assembling the NPs on water surface spontaneously, (c) lifting up the convex lens with self-
PT
assembled NPs layer, (d) monolayer of NPs on convex lens, (e) size reduced NPs on convex lens after size reduction process, (f) nanopillar structures on convex lens after etching process, (g)
RI
both sides nanostructured convex lens after repeating the same process, and (h) nanopillar
SC
structured on both sides of convex lens after whole fabrication.
Figure 2. The reflectance and SEM images of nanopillars structured top surfaces of convex lens
NU
according to (a) the time of size reduction process and (b) the time of etching process. The scale
MA
bars in insets are 500 nm with magnification of 80,000 times. Figure 3. (a) Total reflectance of nano lens, bare lens, and flat quartz, (b) total transmittance of nano lens, bare lens, and flat quartz, and (c) antireflective behavior of nano lens with SEM
TE
with magnification of 80,000 times.
D
images of nanopillar structures on top and bottom surface. The scale bars in insets are 500 nm
AC CE P
Figure 4. The comparisons of anti-ghost and anti-flare behaviors of bare lens and nano lens in real photograph under sunlight, sunset, and strong external light. Figure 5. (a) The contact angles of bare lens, nano lens without PFPE coating, and nano lens with PFPE coating, and (b) the images of movie when water droplet is placed on the surface of bare lens and superhydrophobic nano lens. Self-cleaning effect is demonstrated well in nano lens.
15
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Figure 1. Schematic diagram of the fabricating process for the nanopillar structure on convex lens; (a) filing the water in water bath and dropping the NPs solution on air-water interface, (b) self-assembling the NPs on water surface spontaneously, (c) lifting up the convex lens with selfassembled NPs layer, (d) monolayer of NPs on convex lens, (e) size reduced NPs on convex lens after size reduction process, (f) nanopillar structures on convex lens after etching process, (g) both sides nanostructured convex lens after repeating the same process, and (h) nanopillar structured on both sides of convex lens after whole fabrication. 16
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
MA
Figure 2. The reflectance and SEM images of nanopillars structured top surface of convex lens according to (a) the time of size reduction process and (b) the time of etching process. The scale
AC CE P
TE
D
bars in insets are 500 nm with magnification of 80,000 times.
17
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Figure 3. (a) Total reflectance of nano lens, bare lens, and flat quartz, (b) total transmittance of
AC CE P
nano lens, bare lens, and flat quartz, and (c) antireflective behavior of nano lens with SEM images of nanopillar structures on top and bottom surface. The scale bars in insets are 500 nm with magnification of 80,000 times.
18
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Figure 4. The comparisons of anti-ghost and anti-flare behaviors of bare lens and nano lens in
AC CE P
TE
D
real photograph under sunlight, sunset, and strong external light.
19
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
AC CE P
Figure 5. (a) The contact angles of bare lens, nano lens without PFPE coating, and nano lens with PFPE coating, and (b) the images of movie when water droplet is placed on the surface of bare lens and superhydrophobic nano lens. Self-cleaning effect is demonstrated well in nano lens.
20
ACCEPTED MANUSCRIPT Graphical abstract
AC CE P
TE
D
MA
NU
SC
RI
PT
Moth-eye mimicking nanostructured convex lens is demonstrated with anti-flare and anti-ghost photograph based on antireflective property.
21
ACCEPTED MANUSCRIPT Highlights
AC CE P
TE
D
MA
NU
SC
RI
PT
Nanofabrication is reported for nanopillar structuring on both sides of convex lens. Moth-eye mimicking convex lens shows reflectance below 1 % at 550 nm wavelength. Anti-flare & anti-ghost photograph is demonstrated based on antireflective property. Superhydrophobicity & self-cleaning effect are made by PFPE coating on nanostructured lens.
22
S0167-9317(16)30117-4 doi: 10.1016/j.mee.2016.03.011 MEE 10194
To appear in: Received date: Revised date: Accepted date:
8 November 2015 21 February 2016 3 March 2016
Please cite this article as: Seung-Chul Park, Namsu Kim, Seungmuk Ji, Hyuneui Lim, Fabrication and characterization of moth-eye mimicking nanostructured convex lens, (2016), doi: 10.1016/j.mee.2016.03.011
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
PT
Fabrication and Characterization of Moth-eye Mimicking
SC
RI
Nanostructured Convex Lens
a
NU
Seung-Chul Parka, §, Namsu Kima,c, §, Seungmuk Jia,b, Hyuneui Lima,d,*
Department of Nature-Inspired Nanoconvergence Systems, Korea Institute of Machinery
Yonsei Institute of Convergence Technology, Yonsei University, Incheon 406-840, Repub
D
b
MA
and Materials, Daejeon 305-343, Republic of Korea
TE
lic of Korea
School of Mechanical Engineering, Hanyang University, Seoul 133-791, Republic of Korea
d
Department of Nanobiotechnology, University of Science & Technology, Daejeon 305-350,
AC CE P
c
Republic of Korea
Keywords: Moth-eye, Nanostructure, Antireflection, Convex lens, Self-cleaning
§
Seung-Chul Park and Namsu Kim were equally contributed as a first author.
*To whom correspondence should be addressed. Tel : +82-72-868-7106 Fax : +82-72-868-7933 Email : [email protected]
1
ACCEPTED MANUSCRIPT Abstract In this work, we demonstrate a simple fabrication method for the periodic nanopillar structures
PT
on both sides of convex lens and evaluation of their optical characteristics. The periodic
RI
nanopillar structures on convex lens are inspired from the moth-eye which can provide
SC
antireflective property. The fabrication is composed with the coating of colloidal nanoparticles on a convex lens and the plasma etching of it. Because the optical behavior depends on the
NU
surface morphology, the structural dimensions of nanopillars should be modulated by the
MA
various experimental conditions such as the processing times of two steps, which reduce the size of self-assembled particles and perform the pattern transfer into the lens material.
D
Nanopillar structured convex lens exhibits the antireflective effect showing the increase in
TE
transmittance and the decrease in reflectance without any deformation of lens property such as chromatic and spherical aberration. Furthermore, the antireflective property of nanostructured
AC CE P
convex lens can make the photography high quality without flare and ghost phenomena, even though the pictures are taken under the strong external light environment such as sunshine, sunset, and concentrated external light. The wettability of convex lens surface is also controlled to express the self-cleaning effect which is inspired from a lotus leaf by the low surface tension chemical coating on a nanostructured surface. This moth-eye mimicking nanostructured convex lens will be easily applicable to optical industry with easy and cost-effective fabrication for the advanced optical and wetting property.
2
ACCEPTED MANUSCRIPT 1. Introduction The bioinspired functional surface can be applied as a key component for the advanced
PT
functional devices or advanced materials due to the unique functionality such as self-cleaning
RI
of Lotus leaf, structural color in Morpho-butterfly, low friction surface from the snake skin, adhesive surface of gecko foot, and water harvesting from Namib beetle back [1-4]. Especially,
SC
the optically advanced functional structure exhibits the amazing properties based on their
NU
adaptive hierarchical structures from macrosize to nanosize. The hierarchical structures in nature can provide the various unique optical properties such as structural color, multi-view
MA
phenomenon, and antireflection in broadband based on the periodic nanostructures on surface [1, 5-8]. Because these properties are based on the interaction between the incident light and the
TE
D
periodic structures in nanoscale, one of the central issues in such new area concerns how to construct uniform surface nanostructure with specific structural morphology such as height, size,
AC CE P
shape, and periodicity for practical applications. The tailored hierarchical structures have been made by taking either top-down methods such as a photolithography and e-beam lithography or bottom-up methods which include a soft lithography and self-assembly [9-12]. Among them, self-assembly method is promising due to the merits of an easy large area processing, low cost, high throughput, and high resolution [13]. For the construction of periodic nanostructures on surfaces using the self-assembled colloidal particles, there are several methods to make monolayered particles such as the convective method, floating method, template assisted method, and spin coating method [14-17]. Particularly, among them, the floating method has a great potential in generating self-assembled monolayer of particles on the flat surface as well as the curved surface owing to the flexibility in process. Generally, it is not easy to make nanostructures on curved surface because the non-flat shape hinders the
3
ACCEPTED MANUSCRIPT formation of nanostructures on substrate uniformly. However, the curved surface with nanostructures is the critical component for the practical applications in advanced optical devices
PT
with the controllability of the light pathway such as focusing or defocusing the light [18, 19].
RI
The transparent curved substrate with nanostructures on the surface can provide not only the conventional lens effect but also the antireflective property which is inspired from moth-eye.
SC
These combinations of lens with nanostructures can help photo images clear by removing the
NU
ghost and flare phenomena which are caused by the internal reflection of incident light in lens [20]. Therefore, the construction of nanostructures on curved surface with a controlled height,
MA
size, shape, and periodicity is the essential and important issue for practical applications. Here, we demonstrate the simple and promising method for constructing periodic nanopillar
TE
D
structures on both sides of convex lens. Self-assembled colloidal particles on convex lens which act as the mask layer are made by the floating method and then those convex lenses are etched to
AC CE P
form the nanopatterns via the following plasma etching process. The fabrication conditions such as size reduction of colloidal particles and etching process of lens are modulated for the constructing nanopillar structures that can induce the antireflective property. The nanostructured convex lens with low reflectance is applied to the optical camera lens that can remove the ghost and flare phenomena. The surface modification is also performed to get the superhydrophobicity on nanostructured lens surface.
2. Experiment Fabrication process Fig. 1 shows the fabrication process of nanopillar structures on convex lens. We used a solution of commercial polystyrene colloidal nanoparticles (NPs, Polyscience Inc.) to prepare the monolayer of hexagonally packed NPs as the mask layer for etching process. The
4
ACCEPTED MANUSCRIPT size of NPs determines the periodicity of nanopillars on surface. The average size of used NPs is about 200 nm in diameter and the size distribution is ± 10 nm. First, the hexagonally packed NPs
PT
monolayer on water surface was formed using the self-assembly property of colloidal particles,
RI
as shown in Fig. 1(a) and (b). After the assembling of NPs on water surface, some area of the well packed NPs assembly on water was lifted up with convex lens (PCX 25mm Dia, Edmund
SC
optics), which was already treated with UV-Ozone for 15 mins, by the simple floating method,
NU
as shown in Fig. 1(c). The drying of residual water between the NPs was followed in air at room temperature and the convex lens was etched with NPs as a mask. The plasma etching process has
MA
two steps: size reduction process of the NPs with O2 plasma treatment; and etching process with CF4 and H2 plasma treatment for the regulating of shape and height of nanopillar, as shown in
TE
D
Fig. 1(d) - (f). The size reduction process provides the open area between the NPs, as depicted in Fig. 1(e). When the close-packed NPs cover the whole area of surface without the size reduction
AC CE P
process, the attacks of plasma into convex lens cannot be performed easily. Moreover, the modulation of open area between NPs generates the various shapes of nanopillars such as pointy shape or cylindrical shape, as we reported before [21]. To obtain the optimum condition of antireflective property of convex lens, we applied the size reduction process and etching process for 20, 45, 60 secs and 600, 900, and 1200 secs, respectively. The same processes were repeated of Fig. 1(c) - (f) on back side of convex lens to get the both sides nanostructured convex lens, as shown in Fig. 1(g). Finally, the both sides nanostructured convex lens in Fig. 1(h) was obtained after removing the residual polystyrene NPs and deposited carbon wastes on surface by piranha cleaning process (H2SO4 : H2O2 = 3: 1) for 30 mins and rinsing with deionized water.
5
ACCEPTED MANUSCRIPT Chemical modification The nanopillar structure on convex lens was modified with 0.1 % of Perfluoropolyether (PFPE, Nicca Korea Co., Ltd) in Novec 7200 solution (3M) via a dipping
PT
method for 30 mins, to get the superhydrophobicity of the lens.
RI
Characterization The surface morphology was observed using a field-emission scanning electron microscope (FE-SEM, Nano Nova 200, FEI). The optical properties; total transmittance
SC
and total reflectance were measured by using a UV-Vis-NIR spectrophotometer (Carry 5000,
NU
Agilent Technologies) equipped with integrating sphere for a range of 350 nm to 800 nm. When the reflectance was measured, the angle of incident light was fixed with 6° which is the lowest
MA
angle can measure the regular reflectance by the spectrometer. To test the deformation of lens such as chromatic aberration and spherical aberration, the effective focal lengths (EFL) of
TE
D
lens with various wavelength of laser such as 650 nm (Red), 550 nm (Green), and 450 nm (Blue) were measured using the photodetector that the maximum penetrated light intensity can be
AC CE P
detected. Furthermore, to confirm the antireflective property in the view of removing the ghost and flare phenomena in real usage, the photo image was captured with optical camera (SD700IS, Canon) using the convex lens with and without nanopillar structure under the certain external light environment.
The contact angles of the nanopillar structured convex lens with and without fluorinated polymer coating were measured to observe the wettability of the lens surface, by using a contact angle goniometer (Smart Drop, Femtofab Co., Ltd.) with 5 µL of DI water droplet at room temperature.
3. Results and Discussion The effects of structural morphology on optical property
6
ACCEPTED MANUSCRIPT The reflectance can be changed by the interaction between the surface nanopillar structures and the incident light. The structural morphology of surface nanopillars should be controlled to
PT
reduce the reflection of convex lens for improving the optical property. The proposed
RI
fabrication method in this paper can control the structural morphology such as height, shape, size, and periodicity by modulating the diameter of NPs and etching conditions via the time of size
SC
reduction process and etching process. Based on our previous study [21], the diameter of NPs is
NU
fixed in 200 nm to obtain the ordered nanopillar periodicity in self-assembly. While the larger size of NPs than 200 nm deteriorates the optical property in short wavelength range, the smaller
MA
size of NPs than 200 nm shows the difficulty in hexagonal packing because of the large distribution in particle diameter. Therefore, the size and periodicity are fixed with 200 nm. The
TE
D
effect of height and shape of nanopillar structures on convex lens are studied with size reduction process and etching process. The shape of nanopillar structure is changed depending on the size
AC CE P
reduction time. When the time of size reduction is increased, the uncovered area of convex lens by NPs is increased. The increased gap between the NPs can provide the enough lateral dimension of etching process thus the shape of nanopillar structure becomes sharper. Sharp nanopillar structures exhibit the gradual changes of refractive index to the incident light with the mixed layer of nanostructures and air at the interface. As the nanopillar structure is getting sharper, the reflectance is getting lower. The time of size reduction process was performed for 20, 45, 60 secs with the following 900 secs etching process, as shown in Fig 2(a). As time of size reduction process is increasing, the reflectance is decreasing. The obtained reflectance values are the total values including the sum of specular and diffuse reflectance. When the time of size reduction is more than 60 secs, the shape of nanopillar structure is crushed showing that
7
ACCEPTED MANUSCRIPT the reflectance is getting increasing due to the scattering of light. Based on this result, the critical time of size reduction process is fixed with 60 secs for the top side of the convex lens.
PT
The time of etching process was also determined. Under the fixed ratio of CF4 and H2
RI
concentration as 2:3 [21], the etching process should be applied for a certain time with CF4 and H2 plasma. Since the thickness and shape of nanopillar structures depend on the time of etching
SC
process, the time of etching process is proceeded with 600, 900, and 1200 secs, as shown in Fig.
NU
2(b). For the low reflectance, the height of nanopillar structures should be high enough to generate gradual change of refractive index at the interface between air and convex lens.
MA
However, the important point of increasing the etching time is the durability of polystyrene NPs as a mask layer during the etching process. If the size of NPs would be big enough, the etching
TE
D
process can be applied for the long time because the NPs still last as a mask on convex lens surface during the etching process. However, when the NPs are etched completely at the long
AC CE P
etching time, the nanopillar structures should be over-etched without mask under the long etching process. Therefore, the critical etching time depends on the size of NPs as the mask layer. As shown in Fig. 2(b), the critical etching condition with 200 nm polystyrene particles is 60 secs for size reduction and 900 secs for etching time to make the low reflectance on the top surface of convex lens. The over-etching behavior happens for the 1200 secs etching showing that the heights of nanopillar structures are shorter than the results of 900 secs etching process. The reflection of convex lens also exists on the bottom surface of lens. To improve the antireflective property of the convex lens exceedingly high, the bottom surface of the lens was also nanostructured with the same process. Because the incident light shows the different aspects according to the height of the nanopillars in each wavelength, the combinations of the top and bottom surface structuring with different heights are need to maximize the optical properties [21].
8
ACCEPTED MANUSCRIPT Fig 3 shows the total reflectance and total transmittance values of the nanostructured of convex lens on both sides. Among several combinations of asymmetrical heights of nanostructures on
PT
top and bottom surfaces, the best antireflective property is achieved with the height of around
RI
590 nm on the top and around 230 nm on the bottom, respectively. The size reductions for the top and bottom surface are performed with the duration of 60 secs and 30 secs, respectively. The
SC
etching times are applied as 900 secs for the top surface and 300 secs for the bottom surface,
NU
respectively. The both sides nanostructured convex lens prepared with these conditions shows the reflectance of below 1 % at 550 nm wavelength, whereas the total reflectance of bare lens
MA
and flat quartz substrate which are about 8.4 % and 7.0 %, respectively in Fig 3(a). The difference between bare lens and flat quartz substrate might be caused by the different material
TE
D
composition and convex shape. Nanopillars on the convex lens also provide the higher transmittance values comparing with ones of bare lens and flat quartz substrate, as shown in Fig.
AC CE P
3(b). The optical properties such as the low reflectance and high transmittance of nanopillar structured convex lens are originated from the moth-eye mimicking antireflective property. Fig. 3(c) demonstrates the antireflective effect of nanopillar structured convex lens well. While the bare lens glitters with the external light, the nano lens which is the nanopillar structured convex lens exhibits the low reflectance appearing the letters under the lens. These antireflective properties are based on the nanopillars fabricated on both sides of convex lens which are shown in SEM images.
The applications of antireflective convex lens on photography Besides the prevention of the glaring and clear visibility of the nanostructured convex lens, antireflective effect can provide the reduction of the internal reflection in lens for elimination of
9
ACCEPTED MANUSCRIPT flare and ghost phenomena when we take a picture or image by the optical camera under the strong external light. The test of the anti-ghost and anti-flare phenomena is performed by taking
PT
pictures with bare and nanopillar structured convex lens under the outdoor and indoor conditions,
RI
as shown in Fig. 4. Under the strong sunlight environment, the picture shows the blurred image caused by the ghost image of sun as well as the flared image caused by a path of sunlight in the
SC
bare lens, while it is hard to see such phenomena in the nano lens. In addition, the ghost image Even under the indoor
NU
was observed only with the bare lens under the sunset situation.
environment, picture deteriorates with the ghost image which is made by the internal reflection
MA
of external strong light through bare lens. Therefore, antireflective nano lens can be applied as the key component for advanced optical devices having the anti-ghost and anti-flare properties.
TE
D
The test to check the deformation of lens property is also performed by measuring the focal length with various wavelengths, 650 nm (Red), 550 nm (Green), and 450 nm (Blue). The EFLs
AC CE P
of bare and nano lens show the same focal lengths. The EFLs of Red, Green, and Blue light are 125.3, 124.5, and 123.2 mm in both bare and nano lens, respectively. The differences of EFL with various wavelengths only depend on the bulk structural dimension of convex lens, such as curvature, size, and shape of the lens. This result shows that the nanofabrication process cannot make any deformation of lens but rather improve the lens property. These results suggest the promising usage of nanopillar structured convex lens in optical imaging systems.
The superhydrophobicity and self-cleaning effect of nanopillar structured convex lens The antireflective effect is demonstrated in a surface of nanopillar structured convex lens which is the subwavelength scale. Introducing the nanopillar structures into the glass lens surface can provide supehydrophilic property that the contact angle is lower than 10°. Supehydrophilic
10
ACCEPTED MANUSCRIPT property of the nanostructured surface is changed into supehydrophobic property by a simple low surface tension chemicals coating. Here, the fluorinated polymer, PFPE is used to cover the
PT
surface by a dipping method for 30 mins. After the PFPE coating on nanopillar structured convex
RI
lens, the contact angle is changed higher than 150 °. Fig. 5 shows the contact angles of the bare lens having the value of 30 °, supehydrophilic nano lens, and supehydrophobic nano lens after
SC
PFPE coating. Superhydrophobic surface generates the particular behavior of self-cleaning
NU
effect which is the biomimic of the lotus leaf [22]. In real application, self-cleaning effect is more important than Superhydrophobicity. The nanopillar structured convex lens represents the
MA
self-cleaning effect that the water droplets are rolling up and removed with sands, while the water droplet on the bare lens surface is spread on the surface showing that grain of sand is hard
TE
D
to be removed from the surface. This self-cleaning effect with nanopillar structures and simple chemical coating is a very fascinating property for the sustainable and practical application of
AC CE P
lens.
4. Conclusions
The nanopillar structuring on convex lens is demonstrated via a simple fabrication process which is the combination of self-assembling the NPs and plasma etching of the lens. The nanopillar structures induce the antireflective property which is based on the moth-eye structures. Optical properties are examined with various nanopillar morphologies according to the size reduction time of NPs and etching process time to control the shape and height of nanopillars. The both sides nanostructured convex lens shows the low reflectance and high transmittance based on the gradual change of the refractive index at the air-nanostructure interface. The convex lens having sharp shape with 590 nm height on top surface and 230 nm height on bottom surface shows the reflectance of below 1 % at 550 nm wavelength. Such antireflective property of convex lens is 11
ACCEPTED MANUSCRIPT also applied to optical camera lens demonstrating that the nano lens prevents the ghost and flare phenomena originated from the internal reflection of lens under strong light without any
PT
deformation in original lens property. In addition, the surface modification by PFPE coating on
RI
nanopillar structured lens generates self-cleaning effect which is the biomimic of the lotus leaf. The antireflective and self-cleaning convex lens inspired from nature can provide the advance
NU
SC
optical applications.
Acknowledgement
MA
This research was supported by the Korea Institute of Machinery and Materials as a part of the
AC CE P
TE
D
project "Development of nature-inspired multi optic functional element.”
12
ACCEPTED MANUSCRIPT References (1) K. Chung, S. Yu, C. Heo, J. W. Shim, S. Yang, M. G. Han, H. Lee, Y. Jin, S. Y. Lee, N.
PT
Park, and J. H. Shin, Adv. Mater., 24(2012) 2375-2379.
RI
(2) J. Hazel, M. Stone, M.S. Grace, V.V. Tsukruk, Journal of Biomechanics, 32(1999) 477484.
SC
(3) K. Autumn, Y. A. Liang, S. T. Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R. Fearing, and
NU
R. J. Full, Nature, 405(2000), 681-685.
(4) R. D. Narhe and D. A. Beysens, Europhys. Lett., 75(2006) 98–104.
Mater., 25(2013) 2239-2245.
MA
(5) M. Kolle, A. Lethbridge, M. Kreysing, J. J. Baumberg, J. Aizenberg, and P. Vukusic, Adv.
607–610.
TE
D
(6) Y. Verma, K.D. Rao, M.K. Suresh, H.S. Patel, and P.K. Gupta, Appl. Phys. B 87(2007)
AC CE P
(7) C. C. Striemera and P. M. Fauchet, Appl. Phys. Lett., 81(2002) 2980-2982. (8) S. Ji, K. Song, T. B. Nguyen, N. Kim, and H. Lim, Acs Appl. Mater. Interfaces, 5(2013) 10731-10737.
(9) A. Revzin, R. J. Russell, V. K. Yadavalli, W. Koh, C. Deister, D. D. Hile, M. B. Mellott, and M. V. Pishko, Langmuir 17(2001) 5440-5447. (10) C. Vieu, F. Carcenac, A. Pepin, Y. Chen, M. Mejias, A. Lebib, L. Manin-Ferlazzo, L. Couraud, and H. Launois, Applied Surface Science, 164(2000) 111–117. (11) Y. Xia and G. M. Whitesides, Annu. Rev. Mater. Sci, 28(1998) 153-184. (12) D. Choi, S. Kim, S. Jang, S. Yang, J. Jeong, and S. Shin, Chem. Mater., 16(2004) 42084211. (13) S. Zhang, Nat. Biotech., 21(2003) 1171 - 1178.
13
ACCEPTED MANUSCRIPT (14) B. G. Prevo, Y. Hwang, and O. D. Velev, Chem. Mater., 17(2005) 3642-3651. (15) Y. Liu, S. Wang, J. W. Lee, and N. A. Kotov, Chem. Mater., 17(2005) 4918-4924.
PT
(16) L. Malaquin, T. Kraus, H. Schmid, E. Delamarche, and H. Wolf, Langmuir, 23(2007)
RI
11513-11521.
(17) P. Jiang and M. J. McFarland, J. Am. Chem. Soc., 126(2004) 13778-13786.
SC
(18) H. Ren, Y. Fan, S. Gauza, and Shin-Tson Wu, Appl. Phys. Lett., 84(2004) 4789-4791.
NU
(19) Z. Liu, J. M. Steele, H. Lee, and X. Zhang, Appl. Phys. Lett., 88(2006) 171108-1-3. (20) C. A. Mack, Proc. SPIE 5040, Optical Microlithography XVI, 151(2003).
MA
(21) S. Ji, J. Park, H. Lim, Nanoscale, 4(2012) 4603-4610.
AC CE P
TE
D
(22) L. Jiang, Y. Zhao, and J. Zhai, Angew. Chem., 116(2004) 4438 –4441.
14
ACCEPTED MANUSCRIPT Figure 1. Schematic diagram of the fabricating processes for the nanopillar structure on convex lens; (a) filing the water in water bath and dropping the NPs solution on air-water interface, (b) self-assembling the NPs on water surface spontaneously, (c) lifting up the convex lens with self-
PT
assembled NPs layer, (d) monolayer of NPs on convex lens, (e) size reduced NPs on convex lens after size reduction process, (f) nanopillar structures on convex lens after etching process, (g)
RI
both sides nanostructured convex lens after repeating the same process, and (h) nanopillar
SC
structured on both sides of convex lens after whole fabrication.
Figure 2. The reflectance and SEM images of nanopillars structured top surfaces of convex lens
NU
according to (a) the time of size reduction process and (b) the time of etching process. The scale
MA
bars in insets are 500 nm with magnification of 80,000 times. Figure 3. (a) Total reflectance of nano lens, bare lens, and flat quartz, (b) total transmittance of nano lens, bare lens, and flat quartz, and (c) antireflective behavior of nano lens with SEM
TE
with magnification of 80,000 times.
D
images of nanopillar structures on top and bottom surface. The scale bars in insets are 500 nm
AC CE P
Figure 4. The comparisons of anti-ghost and anti-flare behaviors of bare lens and nano lens in real photograph under sunlight, sunset, and strong external light. Figure 5. (a) The contact angles of bare lens, nano lens without PFPE coating, and nano lens with PFPE coating, and (b) the images of movie when water droplet is placed on the surface of bare lens and superhydrophobic nano lens. Self-cleaning effect is demonstrated well in nano lens.
15
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Figure 1. Schematic diagram of the fabricating process for the nanopillar structure on convex lens; (a) filing the water in water bath and dropping the NPs solution on air-water interface, (b) self-assembling the NPs on water surface spontaneously, (c) lifting up the convex lens with selfassembled NPs layer, (d) monolayer of NPs on convex lens, (e) size reduced NPs on convex lens after size reduction process, (f) nanopillar structures on convex lens after etching process, (g) both sides nanostructured convex lens after repeating the same process, and (h) nanopillar structured on both sides of convex lens after whole fabrication. 16
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
MA
Figure 2. The reflectance and SEM images of nanopillars structured top surface of convex lens according to (a) the time of size reduction process and (b) the time of etching process. The scale
AC CE P
TE
D
bars in insets are 500 nm with magnification of 80,000 times.
17
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Figure 3. (a) Total reflectance of nano lens, bare lens, and flat quartz, (b) total transmittance of
AC CE P
nano lens, bare lens, and flat quartz, and (c) antireflective behavior of nano lens with SEM images of nanopillar structures on top and bottom surface. The scale bars in insets are 500 nm with magnification of 80,000 times.
18
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Figure 4. The comparisons of anti-ghost and anti-flare behaviors of bare lens and nano lens in
AC CE P
TE
D
real photograph under sunlight, sunset, and strong external light.
19
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
AC CE P
Figure 5. (a) The contact angles of bare lens, nano lens without PFPE coating, and nano lens with PFPE coating, and (b) the images of movie when water droplet is placed on the surface of bare lens and superhydrophobic nano lens. Self-cleaning effect is demonstrated well in nano lens.
20
ACCEPTED MANUSCRIPT Graphical abstract
AC CE P
TE
D
MA
NU
SC
RI
PT
Moth-eye mimicking nanostructured convex lens is demonstrated with anti-flare and anti-ghost photograph based on antireflective property.
21
ACCEPTED MANUSCRIPT Highlights
AC CE P
TE
D
MA
NU
SC
RI
PT
Nanofabrication is reported for nanopillar structuring on both sides of convex lens. Moth-eye mimicking convex lens shows reflectance below 1 % at 550 nm wavelength. Anti-flare & anti-ghost photograph is demonstrated based on antireflective property. Superhydrophobicity & self-cleaning effect are made by PFPE coating on nanostructured lens.
22