Durable Broadband and Omnidirectional Ultra-antireflective Surfaces

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Applications of Polymer, Composite, and Coating Materials

Durable Broadband and Omnidirectional Ultra-antireflective Surfaces Zhigang Li, Jianjian Lin, Zhengqi Liu, Shangshen Feng, Yanping Liu, Caifen Wang, Yue Liu, and Shikuan Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15537 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on October 31, 2018

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ACS Applied Materials & Interfaces

Durable Broadband and Omnidirectional Ultraantireflective Surfaces Zhigang Li†,*, Jianjian Lin‡, Zhengqi Liuǁ, Shangshen Feng†, Yanping Liu†, Caifen Wang†, Yue Liu§, Shikuan Yang§,*

†Department

of Physics & Electronic Engineering, Taizhou University, Taizhou 318000,

China ‡Shandong

Key Laboratory of Biochemical Analysis; College of Chemistry and Molecular

Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China ǁInstitute

of Optoelectronic Materials and Technology, College of Physics and

Communication Electronics, Jiangxi Normal University, Nanchang 330022, China §Insititute

for Composites Science Innovation, School of Materials Science and

Engineering, Zhejiang University, Hangzhou 310027, China KEYWORDS: colloidal lithography, monolayer colloidal crystal, omnidirectional antireflection surface, nanocap array, coating

ACS Paragon Plus Environment

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ABSTRACT

Light reflection from surfaces is ubiquitous in nature. Diverse optoelectronic devices need durable omnidirectional transparent ultra-antireflective surfaces. Here, we engineered antireflective transparent surfaces composed of silica nanocaps through a simple thermal treatment of the silica-coated monolayer colloidal crystal template. The relationship between the structure and the antireflective performance of the silica nanocaps was systematically studied both experimentally and numerically. Based on the understanding of the structure-antireflection relationships, ultra-antireflection coatings with a transmittance of ~ 98.75  0.15% in the visible wavelength range were prepared through fabricating two different sized silica nanocaps. More importantly, the antireflection of the coatings formed by two different sized nanocaps demonstrated a poor dependence on the angle of the incident light (i.e., omnidirectionality). The reflection is < 2.5% even at an incident angle of 60o. Such prepared ultra-antireflective silica nanocap coatings outperform state-of-the-art transparent antireflective coatings regarding the antireflection performance, the wavelength range, and the omnidirectionality. The silica nanoshells


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were welded together with the underneath fused silica. Therefore, they could suffer from common mechanical friction and scratching, demonstrating an extraordinary mechanical durability as verified by the sand abrasion tests. Further, the silanized silica nanoshells were turned to be hydrophobic with an outstanding self-cleaning performance without prominently influencing the transmittance. The durable omnidirectional ultra-antireflective transparent silica nanocaps will have promising applications in solar energy conversion and storage, displays, optical lenses, and a wide range of optoelectronic devices.


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Introduction Light reflection from surfaces is ubiquitous in nature. Suppression of the light reflection is highly desirable to improve the performance of some optical1-4 and optoelectronic devices5-7. Various approaches have been developed to engineer ultra-antireflective surfaces through manipulating the spatial distribution of their dielectric constant8-10. Among which, preparation of multiple layers of films with appropriate dielectric constants has achieved great success in generating high-performance antireflective coatings11, which has already been commercialized in many industrial fields (e.g., optical lens, etc.). However, such prepared antireflective coatings suffer from prominent incident-angledependent antireflective properties9. Particularly, the reflection turns to be strong at glancing incident angles. The incident-angle-dependent reflection leads to many serious problems in practical applications12,13, for example, the ghost image for display and limited solar application efficiency for solar energy conversion/storage devices14. In recent years, inspired by nature represented by the wide-angle ultra-antireflective compound eyes of insects15,16, and fueled by the rapid development of surface nanopatterning techniques, synthetic nanopatterns on a surface have demonstrated great


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potentials in creating high-performance omnidirectional ultra-antireflective surfaces16-20. These nanopatterns are usually prepared by top-down etching of the surfaces, sometimes under the protection of nanotemplates. One of the most famous examples is the black silicon with the state-of-the-art antireflection performance composed of a long nanowire array synthesized by reactive ion etching (RIE)

21.

However, the black silicon is not

transparent in visible wavelength range. RIE has also been employed to prepare antireflective silica transparent in the visible wavelength range22. However, the antireflective performance of the silica nanofilms, particularly at glancing angles, still cannot meet applications in many fields. Moreover, the silica nanofilm usually has a poor mechanical connection with and tends to be detached from the substrate. Further, fluorine-contained gases are indispensable during RIE of silica, which are highly poisonous and environmentally harmful. Taken together, synthesis of durable omnidirectional ultra-antireflective transparent surfaces is still challenging for the existing techniques. Here, a simple thermal deformation of the sputtered silica nanoshells has been developed to prepare durable, omnidirectional transparent ultra-antireflective surfaces.


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Such prepared ultra-antireflective transparent surfaces outperform state-of-the-art ones in durability, antireflectivity, omnidirectionality, and the applicable wavelength range23,24. The fabrication process mainly composes two steps. First, a thin layer of silica film was evaporated onto a monolayer colloidal crystal (MCC) template formed by hexagonally close-packed polystyrene (PS) spheres25-30. Then, thermal treatment of the silica-covered MCC template was performed to deform the silica nanoshells to silica nanocaps and to simultaneously enhance their adhesion to the substrate. The thickness, the deformation degree controllable by the annealing conditions31, and the size of the silica nanocaps have close relationships to the antireflective performance of the silica naonpatterns. Based on the systematic experimental and numerical simulation results, transparent surfaces composed of well-arranged silica nanocaps with an outstanding antireflective performance were constructed. The transmittance could be further improved by preparation of antireflective coatings on the two sides of the substrate. Moreover, the transmittance and the angle-dependence of the transmittance could be further manipulated by synthesizing silica nanocaps of two different sizes. The optimized transmittance can reach > 98% on average in the visible and near-infrared region (e.g.,


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from 350 nm to 2000 nm) with a poor incident angle dependence, which outperforms state-of-the-art antireflective surfaces in many aspects. The antireflective coatings were durable and could sustain common mechanical frictions verified by the sand abrasion measurement. The silanized silica nanocaps were hydrophobic with a contact angle of 143o, demonstrated outstanding self-cleaning performance. This study provides a simple method to prepare durable, omnidirectional transparent ultra-antireflective surfaces capable of working in the wavelength range of 350 nm to 2000 nm, which have promising applications in optical lenses, optical display, and solar energy conversion/harvesting fields. Results and Discussion


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Figure 1. Fabrication process of the silica nanocaps. a, Schematic of the fabrication process of the silica nanocap arrays. Process I. Sputtering a thin layer of silica onto the MCC template. Process II. Thermal treatment removed the PS spheres and welded the silica nanocaps with the underneath fused silica substrate together. b, SEM image of the MCC template. Inset: a photo of a large-area MCC template. c, Silica nanocaps prepared after heating at 400 oC for 1 h. Inset: side-view observation. d, Side-view image of the silica nanocaps prepared after heating treatment at 800 oC for 1 h. Inset: Magnified image. The red arrows pointed the connection area between the silica nanocaps and the underneath fused silica substrate. The fabrication process of the silica nanocaps was schematically illustrated in Figure 1. An MCC template composed of hexagonally arranged PS spheres was first prepared by a self-assembly process at the air/water interface (Figure S1)25-30. The MCC template could be prepared over a large area (e.g., > 50 cm2), which is theoretically only


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constrained by the area of the container (Figure 1b and Figure S2). A thin silica nanoshell was grown onto the PS spheres after radio frequency (RF) sputtering (Process I in Figure 1 and Figure 1c). Subsequent heating treatments removed the PS spheres and meantime deformed the silica nanoshells into nanocaps, as well as seamlessly welded the silica nanocaps and the underneath fused silica substrate together (Process II in Figure 1a and Figure 1d). The profile of the silica nanocaps was highly dependent on the annealing conditions. As the heating temperature increased, the morphology evolved from hemispheres to nanocaps (Figure 2a-c).


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Figure 2. Structural and transmittance evolution of the silica nanocaps along with the variation of the heating temperatures. a-c, SEM images of the silica nanocaps prepared after annealing treatments at 400, 600, and 800 oC for 1 h, respectively. The red arrow pointed the welded interface between the silica nanocaps and the underneath fused silica substrate. d, The corresponding evolution of the transmittance spectrum of the silica nanocaps.

The morphological evolution of the silica nanocaps determines that the heating temperature has important influence on their antireflection performance. The transmittance spectra of the silica nanocaps prepared by evaporating 25-nm-thick silica


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onto the 50 nm sized PS spheres after heating at different temperatures for 1 h were measured (Figure 2d). As expected, the heating temperature has prominent impact on the antireflection properties of the silica nanocaps. As the heating temperature was increased from 400 oC to 800 oC, the antireflection performance was prominently improved in the visible wavelength range (Figure 2d). Therefore, thermal treatment at relatively higher temperatures is preferred to prepare ultra-antireflective silica nanocaps. Notably, the silica nanocaps demonstrated much better ultra-antireflective performance than the self-assembled MCC templates (Figure S3). As mentioned above, the silica nanocaps and the underneath fused silica substrate were welded together seamlessly after high temperature thermal treatments, which make the silica nanocaps not only have outstanding antireflection property, but extraordinary durability as discussed later on. Further prolonged heating treatment for 4 h at 800 oC destroyed the silica nanocaps, leading to a worse transmittance (Figure S4). Finite-difference time-domain (FDTD) simulations were carried out to understand the relationship between the antireflection performance and the annealing temperature of the silica nanocaps (Figure S5). We simplified the complex morphology of the silica nanocaps


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obtained at high annealing temperatures into ideal spherical caps with different heights (h). Increased annealing temperatures reduced the h of the spherical caps. As h was decreased from 50 nm to 20 nm, the transmittance in visible region at long wavelengths decreased, while at short wavelengths improved (Figure S5a). The simulation results contradict with the experimental observations, which might be originated from the simplified model or the change of the effective dielectric constant of the silica nanocaps during thermal treatment at high temperatures. The effective dielectric constant of the silica nanocaps will significantly influence their transmittance as verified by the FDTD simulations (Figure S5b). Future efforts will be devoted to determining the exact morphology and the effective dielectric constant of the silica nanocaps at different thermal annealing temperatures. Based on the theoretical understanding, except for the heating temperature, the thickness (t) and the size (D) of the silica nanoshells should have close relationships to their antireflective performance. The influence of the thickness of the silica nanocaps on the antireflection performance was therefore investigated. When the thickness of the silica nanocaps decreased from 40 nm to 30 nm (or 20 nm) on the 50 nm sized PS spheres,


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the transmittance was increased dramatically at short wavelengths (e.g., < 580 nm or 580 nm or > 520 nm) (Figure 3a). FDTD simulation results revealed the same trend as the experimental measurements (Figure 3b). The slightly increased transmittance compared with the experimental measurements may come from the ideal spherical cap model used for FDTD simulations. However, the thin silica nanocaps have a poor mechanical strength. The transmittance turned to be substantially lowered as the thickness of the silica nanocaps was reduced to 10 nm, which arising from the formation of intact silica nanocaps. Depending on the requirements of the mechanical strength and the antireflection performance of the application fields, an appropriate thickness of the silica nanocaps could be determined. Further studies demonstrated that the antireflection performance could be further improved by preparing silica nanocaps on both sides of the fused silica. The transmittance was enhanced from about 95.5% to about 97% on average in the wavelength range of 400 to 2000 nm when both sides of the fused silica were covered by the silica nanocaps (Figure 3c).


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Figure 3. The relationship between the structure and the transmittance of the silica nanocaps. a, The transmittance spectra of the silica nanocaps prepared using 50 nm sized PS spheres with different shell thicknesses. b, FDTD simulation results of the silica nanocaps with different thicknesses prepared using 50 nm sized PS spheres. Inset: Schematic of the silica nanocaps used for FDTD simulations. c, Transmittance spectra of the fused silica with one side and both sides covered by the silica nanocaps using 50 nm


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sized PS spheres. d, The transmittance spectra of the fused silica with one side covered by silica nanocaps prepared using different sized PS spheres. e, The transmittance spectra of the fused silica with both sides covered by silica nanocaps prepared using different sized PS spheres. f, FDTD simulation results of the transmittance spectra of the fused silica with one side covered by silica nanocaps prepared using different sized PS spheres.

To study the relationship between the structure of the silica nanocaps and their antireflective performance, we further investigated the antireflective property of 40-nmthick silica nanocaps with different sizes (e.g., different D values) after annealing treatment at 800 oC for 1 h (Figure 3d). When 350 nm sized PS spheres were used to prepare the silica nanocaps, ultra-antireflective property was observed in long wavelengths (i.e., > 1000 nm) with a transmittance of ~ 96% on average. The transmittance was further improved to ~ 99% on average when both sides of the fused silica were covered by the nanocaps (Figure 3e). However, the antireflection property is poor in the visible wavelength range, even when both sides were covered by the silica nanocaps. When 100 nm sized PS spheres were adopted, the transmittance in the visible wavelength range was significantly improved to be ~ 95.5 % on average. When both sides


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of the fused silica were covered by the nanocaps, the value was further improved to be ~ 97.5%. However, the transmittance in the near infrared range was decreased to be ~ 95 % and ~ 97% for the fused silica with one side and both sides covered by silica nanocaps, respectively. If the size of the PS spheres was further reduced to 50 nm, the transmittance in the blue-green range (from 400 nm to 530 nm) was further improved when compared with 100 nm sized PS spheres, while the transmittance in the near-infrared region kept almost unchanged. The FDTD simulation results also revealed the same relationship between the transmittance and the diameter of the silica nanocaps as the experimental measurements (Figure 3f). Therefore, the size of the PS spheres is important in engineering ultra-antireflective silica nanocaps. Generally, both the experimental and the simulation results conclude that large (small) PS spheres are preferred to synthesize ultra-antireflective silica nanocaps in long (short) wavelengths.


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Figure 4. Omnidirectionality of the antireflection performance of the silica nanocaps. The silica nanocaps were prepared by annealing treatment of 30-nm-thick silica covered PS spheres. a, The transmittance spectra at different incident angles. b, The reflection spectra at different incident angles. c, The schematic of the setup for taking photos of the silica nanocaps and fused silica at different angles. d, The photos taken at different angles when the lamp is off. e, The photos taken at different angles when the lamp is on. The dependence of the transmittance on the incident angles was further investigated. When the incident angle was increased from 0o to 40o, no obvious transmittance change was observed (Figure 4a). As the angle was further increased to 60o, the transmittance was decreased by about 3%, but was still ~ 95% in the wavelength range of 450 nm to 800 nm. The reflectance spectra also revealed the same evolution trend as the incident angle varied (Figure 4b). These results indicated that the antireflection of the silica


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nanocaps demonstrated a good omnidirectionality. The optical photos of the fused silica and the fused silica covered by the silica nanocaps at different observation angles also revealed the omnidirectional antireflection property of the silica nanocaps (Figure 4c-e). As discussed above, ultra-antireflection in long (short) wavelengths needs silica nanocaps with a large (small) size. Ultra-antireflective surfaces in a broad wavelength range are highly desirable in many application fields, for example, in solar energy conversion/storage and display fields32,33. To obtain ultra-antireflective silica nanocaps in a broad wavelength range, mixing two different sized silica nanocaps might be an efficient way. Experimentally, 50 nm sized PS spheres were assembled with 240 nm sized PS spheres, forming a binary colloidal crystal (BCC) template (Figure 5a). Then, silica nanocaps with two different sizes but similar heights were obtained using the BCC template34,35 (Figure 5b). Amazingly, the transmittance curve of the silica nanocaps with two different sizes was between those of the silica nanocaps of a single size (Figure 5c). In detail, the antireflection performance in short wavelengths (e.g., from 300 nm to 530 nm) was improved by introducing small silica nanocaps, while at the expense of the antireflection performance in the long


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wavelengths (e.g., from 530 nm to 800 nm). This means that it is highly possible to engineer silica nanocaps with acceptable antireflection performance in a broad wavelength range through mixing silica nanocaps of different sizes. If both sides of the fused silica were covered by silica nanocaps of two different sizes, the transmittance was amazingly higher than 98.6 % in the wavelength range of 350 nm to 800 nm (Figure 5d). Such a uniform transmittance in the visible wavelength range (98.75  0.15 %) is important to construct high-quality display devices with authentic colors, which outperforms the state-of-the-art transparent antireflective surfaces22-24, 36-43. Similarly, the transmittance of the silica nanoscaps prepared by 100 nm sized PS spheres in the long wavelength range (i.e., from 950 nm to 1800 nm) was improved by introducing silica nanocaps prepared with 350 nm sized PS spheres, while at the expense of the transmittance in the visible wavelength range (Figure S6).


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Figure 5. The morphology and the optical properties of antireflective coatings composed of two different sized silica nanocaps. a, a BCC template composed of 50 nm and 240 nm PS spheres. b, Silica nanocaps with two different sizes fabricated by annealing treatment of the 20-nm-thick silica covered BCC template. c, The transmittance spectra of the silica nanocaps prepared using 50 nm PS spheres, 240 nm PS spheres, and the BCC template composed of 50 nm and 240 nm sized PS spheres. d, The transmittance spectra of the fused silica with both sides covered by the silica nanocaps prepared using the BCC template. e, The reflectance spectra of the fused silica with one side covered by


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the silica nanocaps prepared using the BCC template and with multilayer film at different incident angles.

More importantly, the silica nanocaps with two different sizes demonstrated extraordinary omnidirectionality, which is much better than the commercialized antireflective coatings composed of multiple layers of films. The reflectance was only increased from 0.2 % to 2 % when the angle of the incident light was increased from 0o to 60o for the silica nanocaps of two different sizes. In contrast, the reflectance of the commercialized multiple layers of films was increased from about 0.5% to about 5% on average when the angle of the incident light was increased from 0o to 60o. Remarkably, the reflection of the silica nanocaps of two different sizes maintained almost the same over the wavelength range of 350 nm to 800 nm, while that of the commercialized multiple layers of films varied significantly in the same wavelength range. The detailed mechanism of the omnidirectional reflection depression by the coating composed of periodically arranged hollow nanocaps with two different sizes still needs further studies.44


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Since the antireflective silica nanocaps were prepared at high temperatures, the antireflective surface can work under high temperatures. The antireflective silica nanocaps can be subject to corrosive chemical environments, benefiting from the chemical inertness of the silica nanocaps. More importantly, because the silica nanocaps were welded together with the underneath fused silica substrate, the silica nanocaps could sustain harsh mechanical friction, as verified by the sand abrasion test results43 (Figure 6). Sand particles ranging from 0.1 to 0.4 mm were impinged on the surface of the fused silica covered by the silica nanocaps, corresponding to an impinging energy of 1 x 10-8 to 16 x 10-8 J per grain (Figure 6a). The transmittance was still ~ 95% after 60 ml sand abrasion. Even after 100 ml sand abrasion, the transmittance of the silica nanocaps was still better than that of the fused silica. The detachment of most of the silica nanocaps gave rise to a poor transmittance after 200 ml sand abrasion (Figure 6b and c). Moreover, the contact angle of a water droplet on the silica nanocaps is about 37o, whereas increases to 143o after silanization treatment (insets in Figure S7a). The advancing and the receding angle of the water droplet on the silanized silica nanocap coating is 145o and 141o, respectively. The transmittance of the silica nanocaps was slightly influenced


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(decreased by ~ 2 %) by the silanization functionalization (Figure S7a). The silanized hydrophobic

silica

nanocap

surfaces

demonstrated

outstanding

self-cleaning

performance (Figure S7b). The self-cleaning function further enhanced the practical applicability of the antireflective silica nanocap coatings45. Future efforts will devote to fabrication of a monolayer molecular hydrophobic coating onto the silica nanocaps to further weaken the influence of the hydrophobic coating on the transmittance.

Figure 6. Mechanical strength of the silica nanocap coatings with a shell thickness of 20 nm prepared using 100 nm sized PS spheres. a, The schematic of the setup for the sand abrasion test. b, a SEM image of the silica nanocap coatings after sand abrasion test with


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200 ml sand. c, The evolution of the transmittance spectra of the silica nanocaps after sand abrasion tests using different volumes of sands.

Conclusion In summary, durable omnidirectional antireflective surfaces were prepared through a simple thermal treatment of the silica-coated monolayer colloidal crystal templates. The size, thickness, and the annealing temperature of the silica nanoshells have close relationships with the antireflection performance of the silica nanocap coatings. Based on the experimental and the numerical simulation investigations, the relationship between the structure and the optical properties of the silica nanocaps was built. Generally, large (small) PS spheres are preferred to synthesize ultra-antireflective silica nanocaps in long (short) wavelengths. Thin (thick) nanocaps are able to attain good transmittance at short (long) wavelengths. Thermal treatment of the silica nanoshells at 800 oC for 1 h can obtain good antireflective performance, and simultaneously welded the silica nanocaps with the underneath fused silica substrate together. Covering both sides of the fused silica with the silica nanocaps can further improve the antireflective performance. Based on the


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above understanding, ultra-antireflective coatings with > 98.6% transmittance in the visible wavelength range were engineered composed of silica nanocaps with two different sizes, which outperforms state-of-the-art antireflective coatings. More importantly, the antireflection performance demonstrated poor dependence on the incident angle. Even when the incident angle is 60o, the reflection is still < 2.5% in the visible wavelength range. The silica nanocap coatings are mechanically strongly connected to the underneath fused silica, enabling them to be able to resist common mechanical friction or scratch, as verified by the sand abrasion tests. After silanization functionalization, the silica nanocap coatings exhibited outstanding self-cleaning performance. The outstanding omnidirectional antireflective performance, the durability, as well as the simple preparation process, lowcost, and time-effectiveness make the silica nanocap coatings have promising applications in diverse fields, particularly, in solar energy conversion and storage devices, displays, optical lenses, light emitting diodes and other optoelectronic devices. Experimental Section

Fabrication of the MCC template. The MCC template was prepared by a self-assembly process at the air/water interface (Figure S1). PS sphere dispersions (~ 1 wt %, Duke


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Corporation) in a mixture of ethanol and water at a volume ratio of 1:1 was slowly dripped onto a piece of superhydrophilic glass slide with its surface covered by a thin layer of water. The superhydrophilic glass slides were prepared by ultrasonically cleaning in turn with acetone, ethanol, 98%H2SO4/H2O2 (at a volume ratio of 3:1), H2O/NH3H2O/H2O2 (at a volume ratio of 5:1:1) and distilled water for 1 h, respectively. The PS spheres were trapped at the air/water interface and assembled into an ordered film until the entire water surface was covered by an MCC template. More water was introduced into the container to lift up the MCC template. Then, a piece of cleaned glass slide was used to pick up the MCC template. The MCC template was fixed onto the glass slide by heating at 90 oC for 3 min. The BCC template was prepared using the same method as the MCC template, except for two different sized PS spheres were first mixed together at a ratio of about 1:1 before assembling at the air/water interface.

Fabrication of the silica nanocap array. A thin layer of silica was sputtered onto the MCC template by RF sputtering (DE 500). The base pressure of the sputtering chamber was maintained at 10-5 Pa. The argon (with 5% oxygen) pressure was kept at 1.0 Pa during silica sputtering. The power was 100 W during silica sputtering. The deposition time was


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varied from 8 min to 30 min with a growth speed of ~ 1.5 nm/min. Thermal treatments of the silica-covered MCC template were performed at high temperatures in air to burn the PS spheres. The thermal treatment also deformed the silica nanoshells into nanocaps and mechanically anchered the silica nanocaps onto the underneath fused silica substrate. The silica nanocaps were placed inside a vacuum chamber with a volume of about 2 l for vapor phase silanization using 20 µl heptadecafluoro-1,1,2,2tetrahydrodecyltrichlorosilane for 6 h to achieve the hydrophobic modification.

Characterization of the morphology and optical properties. The morphology of the MCC template and the silica nanoshell arrays was observed by a field-emission scanning electron microscope (FE-SEM) (Hitachi S-4800). The transmittance spectra of the silica nanoshell arrays were recorded on a Hitachi U-4100 UV-Vis-NIR spectrophotometer in the wavelength range of 200 to 2600 nm. The reflection spectra were measured on a Lambda 950 spectrophotometer (Perkin-Elmer) in the wavelength range of 250 to 2000 nm without using an integration sphere. However, considering the flat surface of the silica nanocap coatings, most of (> 99%) the reflected light should have been captured. The incident angle can be varied from 8o to 68o. The contact angle of the water droplet on the


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silanized silica nanocaps was measured using a homemade equipment. The transmittance curves were polished using the Origin software.

FDTD simulations. A commercial software (Lumerical Solutions) was used to simulate the transmittance spectra of the silica nanocap coatings10,12,17,19. Since it is not easy to determine the exact structure of the silica nanocaps, we simplified the experimentally prepared silica nanocaps into ideal silica nanocaps. The light source was a plane wave in the wavelength range of 300 nm to 800 nm. A periodic boundary condition was used in the in-plane condition. A frequency-domain transmission monitor placed blow the silica nanocap model was used to generate the transmittance spectra. The optical constants of silica was supplied by the software. Supporting Information. The SEM images of the MCC template, the FDTD simulation results, the transmittance spectra of the silica nanocaps of two different sizes, the selfcleaning performance of the silica nanocaps are included in the Supporting Information file. This material is available free of charge on the ACS Publications website. AUTHOR INFORMATION

Corresponding Author


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* Email: [email protected]; [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT

This work was financially supported by the Natural Science Foundation of China (Grants No. 51671139 and 51702283), and the National Science Foundation of Zhejiang Province (Grant No. LY15E010002).

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TOC figure:


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Durable Broadband and Omnidirectional Ultra-antireflective Surfaces

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