Self-assembled hemispherical nanowell arrays for superhydrophobic antireflection coatings

doi:10.1016/j.jcis.2016.11.064

Journal of Colloid and Interface Science

Volume 490, 15 March 2017, Pages 174-180

https://doi.org/10.1016/j.jcis.2016.11.064 Get rights and content

Abstract

The present study reports an inexpensive and simple bottom-up technology for fabricating superhydrophobic antireflection coatings with highly ordered hemispherical nanowell structures, which are assembled by a scalable Langmuir-Blodgett technology. The subwavelength gratings lead to a gradual change in the effective refractive index, substantially suppressing specular reflection over the entire visible wavelength range. After surface modification with fluorinated silane, the resulting nanowell arrays exhibit superhydrophobic surface with high static water contact angle (154°) and low water contact angle hysteresis (7°). The experimental results on the wettability can be qualitatively interpreted by adopting the Cassie’s dewetting model. Moreover, the effect of the nanowell shape on the antireflective and superhydrophobic properties has also been investigated in the study.

Graphical abstract

Introduction

In most optical applications, optical-grade polymers have replaced traditional optical glass owning to low cost of materials and high production volume with fast repeatability as well as high precision. The ever growing demand of polymer optics in areas as diverse as optical devices to consumer electronics has led to the search for ways to minimize the Fresnel’s reflection at air/polymer interface, which could impair the legibility of displays and pose safety hazards [1], [2], [3]. To suppress the reflection, single quarter-wavelength antireflection coatings, multilayer antireflection coatings, and porous antireflection coatings are commonly applied to increase optical transmission [4], [5], [6], [7]. However, the coating procedures usually require high cost and complex fabrication processes [8].

Periodic nanostructures in biological systems have inspired scientists to biomimic natural structures for practical applications [9], [10]. A prominent case is the fabrication of subwavelength-structured moth-eye antireflection coatings, which consist of non-close-packed conical nipples [11]. The graded transition of effective refractive index between air and cornea leads to a superior broadband antireflection performance than single layer or multilayer coatings [12]. In addition, the gratings are more durable than multilayer antireflection coatings owning to no foreign material is involved. Moreover, inspired by the cicada wing, the resulting structures can be surface modified to achieve superhydrophobic property [13], [14].

Currently, numerous top-down technologies, including photolithography, nanoimprint lithography, E-beam lithography, and interference lithography are applied to develop the subwavelength nipple arrays [15], [16], [17], [18]. However, the technologies suffer from high cost of fabrication and low resolution of features. On the other hand, self-assembly of colloidal particles is a simple and inexpensive technology for creating highly ordered colloidal crystals, which can be used as templates to produce nipple arrays [19], [20], [21]. Nevertheless, the fabrication procedures involve either infiltration of matrix-forming materials or lithography-based technologies, which are still difficult to implement. Moreover, most of the bottom-up technologies involve multiple steps, and are favorable for low-volume laboratory-scale production only [22], [23], [24]. Recently, a one-pot evaporation-induced self-assembly of colloidal silica nanoparticles in the presence of monomer is applied to fabricate three-dimensional inverse opal structures [25]. Although three-dimensionally ordered porous structures can be attained using this method, the construction of large-area two-dimensional periodic structures has remained a substantial challenge.

Inspired by this methodology, we demonstrate scalable Langmuir Blodgett (LB) technology can be utilized for assembling monolayer silica colloidal crystals, partially embedded in polymer matrices, on polymer substrates directly in a signal step. The silica spheres can be removed to create periodic hemispherical nanowell arrays for antireflection coatings. The method is easily optimized for a large variety of substrates, including plastic plates, glass microslides, and silicon wafers [26]. Further advantage is that the resulting gratings exhibit superhydrophobic and self-cleaning through a surface modification procedure. This prevents scattering losses due to accumulation of water droplets.

Section snippets

Materials and substrates

The regents used for silica sphere synthesis, including tetraethyl orthosilicate (TEOS) (98%) and ammonium hydroxide (NH₄OH) (28%), are obtained from Sigma-Aldrich. Absolute ethanol (99.9%) is provided by Echo Chemicals. Deionized water (18.2 MΩ cm) is used directly from a Millipore A-10 water purification system. The UV-curable ethoxylated trimethylolpropane triacrylate monomer (ETPTA, SR 454) and photoinitiator, 2-hydroxy-2-methyl-1-phenyl-1-propanone (Darocur 1173), are purchased from Sartomer

Results and discussion

Periodic polymer nanowell-structured antireflection coatings were created according to the self-assembly procedures presented in Fig. 1. In this LB deposition process, the ETPTA monomer-covered 155 nm silica spheres self-organized into close-packed ordering by tuning the surface tension of water with ETPTA monomer, instead of physical compression using mobile arms [27]. Monolayer of ETPTA monomer-covered silica colloidal crystals floating at the air/water interface were transferred onto the

Conclusions

In summary, we have developed a non-lithography-based approach that utilizes the self-organized and highly ordered silica spheres as templates to scalable fabricate periodic hemispherical nanowell structures in a single step. The resulting coatings exhibit broadband antireflection and superhydrophoic properties after a surface functionalization procedure, which are promising for creating self-cleaning antireflection coatings in a variety of important technological application in optical devices.

Acknowledgment

Acknowledgment is made to National Science Council – Taiwan (Grant No. MOST 104-2221-E-005-086) for support of this research.

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Regular ArticleSelf-assembled hemispherical nanowell arrays for superhydrophobic antireflection coatings

Abstract

Graphical abstract

Introduction

Section snippets

Materials and substrates

Results and discussion

Conclusions

Acknowledgment

Prog. Mater. Sci.

J. Colloid Interface Sci.

J. Colloid Interface Sci.

Nature

Optics

Thin-Film Optical Filters

Nat. Mater.

Appl. Opt.

ACS Nano

Nanotechnology

Nature

Chem. Rev.

Adv. Mater.

Crit. Rev. Solid State Mater. Sci.

Annu. Rev. Fluid Mech.

ACS Appl. Mater. Interface

Regular Article
Self-assembled hemispherical nanowell arrays for superhydrophobic antireflection coatings