Antireflective Coatings for Glass and Transparent Polymers - American

Invited Feature Article pubs.acs.org/Langmuir

Antireflective Coatings for Glass and Transparent Polymers Pascal Buskens,*,†,‡ Marieke Burghoorn,† Maurice Christian Danho Mourad,† and Zeger Vroon†,§ †

The Netherlands Organisation for Applied Scientific Research (TNO), De Rondom 1, 5612 AP Eindhoven, The Netherlands DWI − Leibniz Institute for Interactive Materials e.V., Forckenbeckstrasse 50, 52056 Aachen, Germany § Zuyd University of Applied Sciences, Nieuw Eyckholt 300, 6419 DJ Heerlen, The Netherlands ‡

ABSTRACT: Antireflective coatings (ARCs) are applied to reduce surface reflections. We review coatings that reduce the reflection of the surface of the transparent substrates float glass, polyethylene terephthalate, poly(methyl methacrylate), and polycarbonate. Three main coating concepts exist to lower the reflection at the interface of a transparent substrate and air: multilayer interference coatings, graded index coatings, and quarter-wave coatings. We introduce and discuss these three concepts, and zoom in on porous quarter-wave coatings comprising colloidal particles. We extensively discuss the four routes for introducing porosity in quarter-wave coatings through the use of colloidal particles, which have the highest potential for application: (1) packing of dense nanospheres, (2) integration of voids through hollow nanospheres, (3) integration of voids through sacrificial particle templates, and (4) packing of nonspherical nanoparticles. Finally, we address the remaining challenges in the field of ARCs, and elaborate on potential strategies for future research in this area.

1. INTRODUCTION Optical coatings that reduce the surface reflection of substrates are of importance for a wide range of applications. Such socalled antireflective coatings (ARCs) lower the reflection at an interface between two media with different refractive indices. Here, we address coatings that lower the reflection in the visible and near-infrared at the interface of air and transparent substrates, more specifically glass, polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), and polycarbonate (PC). The transparent substrates addressed in this review all have a refractive index of about 1.5:1.52 for float glass, 1.57 for PET, 1.49 for PMMA and 1.58 for PC. Note: these values are measured at the Na D doublet, with a wavelength of 589 nm. The refractive index of these substrates, and the ARCs discussed in this manuscript, are wavelength dependent. This dispersion is taken into account in the design of ARCs. All refractive indices mentioned throughout this feature article are measured in the midrange of the visible spectrum at specific wavelength of 532 or 589 nm. The reflection of light at an interface between two media with different refractive indices is expressed by Fresnel’s Equation.1 The reflection of light from the surface of a transparent substrate with a refractive index nS in contact with air is given by

Rs =

cos θi − nS cos θt cos θi + nS cos θt

2

( 1−(

cos θi − nS 1 −

1.00 nS

cos θi + nS

1.00 nS

=

2

Rp =

cos θt − nS cos θi cos θt + nS cos θi

( 1−( 1−

2

=

2

2

) sin θ )

1.00 nS

sin θi

1.00 nS

i

− nS cos θi

2

+ nS cos θi

where Rs and Rp are the reflection of s- and p-polarized light, respectively, and θi and θt are the angles of incidence and

(

transmission. For nonpolarized light R =

Rs + Rp 2

) and normal

incidence (θi = 0), this equation simplifies to R=

1.00 − nS 1.00 + nS

2

where nS is the refractive index of the substrate in contact with air. The corresponding reflection of a float glass surface in contact with air is 0.043 (= 4.3%), resulting in a total reflection from both glass−air interfaces of about 8.6%. ARCs typically reduce the reflection per interface to 1% or less, and increase the transmission through a double-side coated glass pane correspondingly by maximum 8.6%. ARCs are either applied for aesthetic purposes, or to improve the functionality of devices. In the case of picture and art glazing, the ARC reduces reflections of sunlight or artificial lighting and improves the view of art works. This is similar for show cases and shop windows. In the case of eye glasses, the ARC reduces reflections that negatively affect the view of the

2

) sin θ ) sin θi

© XXXX American Chemical Society

Received: February 3, 2016 Revised: May 3, 2016

2

i

A

DOI: 10.1021/acs.langmuir.6b00428 Langmuir XXXX, XXX, XXX−XXX

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Invited Feature Article

Figure 1. Schematic representation of (a) multilayer interference coatings (three layer system as example), (b) graded index coating, and (c) quarterwave coating.

In the case of quarter-wave coatings, one single coating is applied on the surface of a substrate, such that the reflection from the air-coating and substrate-coating interfaces undergo destructive interference.8 To achieve this and minimize the reflection, the optical thickness of this coating should be equal to a quarter of the wavelength of the incident light and its refractive index equal to the square root of the refractive index of the substrate. In the design of ARCs with optical properties tailored for an application of choice, and in understanding the optics of such coatings in detail, optical simulations are of high importance. Suitable optical models for the design of multilayer interference systems have been reviewed by MacLeod et al.8 Han et al. recently reviewed the progress of mathematical modeling of ARCs comprising subwavelength structures.12

user. In the case of photovoltaics, lighting modules, and greenhouses, the performance is improved through use of ARCs. For photovoltaic modules, the application of an ARC on the top side of the float glass cover can increase light transmission by up to 4.3%, typically leading to a corresponding increase in efficiency, e.g., from 18.0% for typical wafer-based modules to about 18.8%.2,3 For lighting modules, the application of ARCs on the cover plate results in an increase of light emission of up to 4.3% or 8.6%, depending on the type of module and whether the coating is applied on one or both sides of the cover glass.4,5 For greenhouses, the use of ARCs leads to up to 8.6% more light transmission in the photosynthesis active region from 400 to 700 nm. This can result in a significant increase in crop yield.6,7 Both specular and diffuse reflection contribute to the total reflection of a coated substrate. Some of the applications mentioned above may benefit from light diffusion. This article, however, will highlight ARCs that do not scatter light in the visible or near-infrared, i.e., ARCs for which the contribution of diffuse reflection in the total reflection is negligible. Three main coating concepts exist to lower the reflection at the interface of a transparent substrate and air: (1) multilayer interference coatings,8 (2) graded index coatings,9 and (3) quarter-wave coatings (Figure 1).8 Multilayer stacks with alternating layers of high and low refractive index can be used to create an interference filter (Figure 1a).8 The principle is based on destructive interference of the reflected light at the interfaces of the thin layers in the multilayer stack. Depending on the optical requirements of the ARC, designs of two and more layers are reported. The second way of reducing the surface reflection of transparent substrates is through use of graded index coatings (Figure 1b).9 Such coatings display a gradient in refractive index normal to the substrate surface. This graded refractive index increases from the refractive index of air to that of the substrate. Moth eye textured coatings are the most common example for this technology (Figure 2).10,11 They are inspired by the surface texture on the cornea of night-flying moths.

2. ANTIREFLECTIVE COATING CONCEPTS 2.1. Multi-Layer Interference Coatings. Multilayer interference coatings consist of a stack of nonabsorbing layers of high and low refractive index. The simplest multilayer ARC is the so-called quarter−quarter stack, which consists of two coating layers that each have a quarter-wave optical thickness.8 In this coating stack, the reflection at the top surface is reduced through destructive interference with the two weaker out-ofphase reflections from the underlying coating−coating and coating-substrate interfaces. The outer layer consists of a material with a refractive index lower than the refractive index of the substrate, the inner coating layer consists of a high refractive index material. Quarter-quarter stacks display a Vshaped reflection curve with a distinct reflection minimum. For a substrate coated with a quarter−quarter stack in contact with air, which is optimized for one wavelength at normal incidence, the required refractive indices of the outer, low refractive index and inner, high refractive index coating are given by nL2 nH2

=

1.00 nS

where nL and nH are the refractive indices of the low- and highrefractive index coatings, respectively, and nS is the refractive index of the substrate.8 Assuming a quarter−quarter stack on float glass with MgF2 as outer layer (nL = 1.38), the inner coating should have a refractive index of nH = 1.70 to minimize the reflection. This refractive index can be realized, for example, by applying a coating layer comprising a mixture of silica and titanium dioxide. Since many optical systems use polychromatic light, it is of importance to develop multilayer interference filters that

Figure 2. Schematic representation of a moth-eye textured ARC. B


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Figure 3. (a) Helium ion microscopy image of nanotextured ARC prepared using UV NIL. (b) Reflection and transmission of uncoated polycarbonate (PC) (red) and PC with moth eye-structured Ormocomp ARC (blue); dashed lines = reflection, solid lines = transmission, reported by Buskens and co-workers. Reproduced with permission from ref 31. Copyright 2013, MDPI.

index of the substrate (ns ≈ 1.5) to the refractive index of air, i.e., 1.00, are not optimized for one specific wavelength and angle of incidence, but display omnidirectional broad-band antireflective properties.9 Although graded-index coatings comprising a porosity gradient in the z-direction have been reported,23,24 in most cases the required gradient in refractive index is achieved through application of a surface nanotexture, inspired by the surface texture on the cornea of a night-flying moth (Figure 2).10,11 When the period of the surface nanotexture is small compared to the wavelength of incident light, the so-called effective medium theory can be used to determine the effective refractive index of the nanotextured coating.9 At periods larger than one tenth of the wavelength, the light waves are in the Bragg regime, and (multiple) diffraction orders need to be considered. At periods much larger than the wavelength of interest, geometrical optics is used, assuming an abrupt change of the refractive index at the textured interface. Although multiple approaches have been reported to generate the desired surface nanotexture, ranging from nanosphere lithography and subsequent plasma etching25 to self-assembly of porous templates26 and colloidal particles,27,28 nanoimprint lithography (NIL) seems to be the only technology reported to date that has the potential to scale up to high volumes (multimillion m2 p.a.) of large substrates (>1 m2) within the cost restrictions given by markets such as photovoltaic covers, greenhouses, and lighting covers. NIL was developed in the mid-1990s as a technology for producing micro- and nanotextured surfaces on wafers in a wafer-by-wafer process.29,30 In this process, a nanostructured mold is used and either pressed into a softened polymeric surface (thermal NIL) or a wet UV-curable resist layer (UV NIL). In the case of thermal NIL, the substrate is typically heated to about 50 °C above its glass transition temperature, the mold is pressed into the heated substrate surface and, after cooling, the mold is removed, leaving the polymer surface with a negative copy of the structure of the mold. In the case of UV NIL, a nanostructured mold is pressed into a wet, UV-curable resist. Then, the coating is cured through exposure to UV light, and after demolding, the negative copy of the mold’s structure is left in the UV-cured coating layer. Most commercially available coating materialsimprint resistsare designed for subtractive lithography processes in which the resist is used for structuring silicon wafers, and does not end up in the final product. In 2013, our group reported a

display broadband antireflective properties. This can be achieved through introduction of a third layer in between the low and high refractive index layer of the quarter−quarter stack, the so-called absentee layer.13 An absentee layer is a layer of a dielectric material that does not change the performance of the original quarter−quarter stack at the wavelength for which the two-layered coating stack was optimized. This is realized by designing the absentee layer in such a way that its optical thickness equals half the wavelength for which the quarter− quarter stack was optimized. In that way, no additional phase shifts are introduced for that specific wavelength. For all other wavelengths, the absentee layer has an effect. Commercial software tools can be applied to design and optimize these three layer systems.8 In addition to two and three layer stacks, interference filters with more than three layers have been reported. In general, the optics allow further tailoring upon use of more layers in the stack. Furthermore, such multilayer filters allow reduction of reflection in a broad wavelength regime and for a wide range of light incidence angles. Multilayer interference filters are applied to glass14,15 and polymeric substrates,16,17 and have been on the market for several decades.18 They can be optimized to yield broadband antireflective properties, and the optics can be fine-tuned for the application of choice. They are produced by vapor deposition technologies, like physical19 or chemical vapor deposition,20 or by wet processing, e.g., using sol−gel chemistry.21,22 Coatings used in multilayer stacks are dense layers of rather hard materials such as silica and metal oxides, e.g., titanium or zirconium dioxide. Ergo, multilayer interference stacks are mechanically robust and durable. The major disadvantage of interference stacks is their high cost: due to the multiple coating andin the case of wet processingcuring steps, they are expensive, and are solely applied in high end markets in which the ARC represents a high added value, e.g., eye wear, architectural glass, displays, show cases, and picture and art glazing. Markets in which the added value of the ARC is low to medium, e.g., photovoltaic covers, greenhouses and lighting covers, require a low-cost ARC that is easily applicable to high volumes (multimillion m2 p.a.) of large substrates (>1 m2). 2.2. Graded Index Coatings. An example of an ARC that is potentially low-cost when compared to a multilayer interference stack is a graded index coating, merely because it consists of one single coating layer. Graded-index coatings, with a refractive index gradient in the z-direction from the refractive C


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Figure 4. Relationship between reflection (R) of a float glass substrate coated with a quarter wave coating and the refractive index of the coating (nC).

⎛ n 2(1.00 − n 2)2 cos 2 k h + (n − n 2)2 sin 2 k h ⎞ S 0 S C 0 ⎟ R = ⎜ C2 2 2 2 2 2 2 ⎝ nC(1.00 + nS ) cos k 0h + (nS + nC) sin k 0h ⎠

study on the validation of the commercial resist Ormocomp for the production of moth-eye ARCs on PET, PMMA, and PC using wafer-by-wafer and roll-to-roll UV NIL (Figure 3).31 The optical performance and durability of the resulting coatings in the damp-heat test, which is one of the accelerated aging tests typically used to predict the outdoor performance of ARCs, were excellent. The scratch resistance, however, was poor and requires further optimization. In spite of a variety of studies performed in the past few years on the development of roll-to-roll and roll-to-plate processes for NIL, such processes are not yet commonly applied in industry.32,33 The technical and commercial feasibility of a selection of such processes is currently under evaluation, i.e., by the research and technology organizations TNO in The Netherlands and VTT in Finland. To the best of our knowledge, to date it has not yet been demonstrated that it is technically feasible to produce nanotextured moth-eye ARCs in high volumes (multimillion m2 p.a.) and on large substrates (>1 m2) of a quality and at a cost level acceptable for technical applications such as photovoltaic covers, greenhouses, and lighting covers. Ergo, currently nanotextured ARCs prepared through NIL are not ready for use in large-scale applications for the reasons mentioned above. Additionally, it is still unclear whether the mechanical robustness required for successful application of surface-textured ARCs can be realized, due to the intrinsic fragility of the surface nanotexture. This texture may also have an effect on pollution and ease of cleaning of the ARCs. Depending on the chemical composition of the coating, the geometry of the surface texture, and the characteristics of the pollutant, the texture may positively or negatively contribute to pollution and ease of cleaning. 2.3. Quarter-Wave Coatings. An example of an ARC that is potentially low-cost when compared to a multilayer interference stack, and does not require surface texturing, is a quarter-wave coating. The reflection of a substrate coated with one coating layer in contact with air can be calculated using Fresnel’s equation:

where nS is the refractive index of the substrate, nC is the refractive index of the coating, k0 is the phase angle of the incoming light and h is the optical thickness of the coating.8 π Under the condition k 0h = 2 , which is equal to the requirement that the coating should have quarter-wave optical thickness, the equation can be further simplified to R=

nS − nC2

2

nS + nC2

where nS is the refractive index of the substrate, and nC is the refractive index of the coating. Quarter-wave coatings are optimized for one single wavelength and one angle of incidence, for which the light reflected at the coating-substrate interface is 180° phase shifted with respect to the light reflected at the coating surface. For all other wavelengths, the phase shift deviates from 180°, which results in higher reflection and, consequently, in a V-shaped reflection curve. In the ideal case, the reflection for the optimum wavelength should be reduced to zero. For float glass, this requires a coating with a refractive index of 1.23. When the refractive index of the coating is higher or lower than 1.23, the reflection increases (Figure 4). The solid material with the lowest refractive index suited for producing coatings is magnesium fluoride. MgF2 has a refractive index of 1.38, resulting in a reflection of 1.26% when applied as a quarter-wave coating on float glass (Figure 4). Since such a high level of rest reflection is not acceptable for most commercial applications, there is a need for the development of quarter-wave coatings with a refractive index lower than 1.38. The only way to achieve such low refractive index is through inclusion of air in the coating layer, resulting in the formation of porous quarter-wave coatings. To avoid Mie scattering of visible light, which would result in a milky white appearance of the coating, the pores in the ARC should be smaller than 80 nm. Although various strategies can be applied D


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for the production of such nanoporous coatings, e.g., HF vapor phase etching of glass resulting in the formation of a nanoporous, low refractive index layer consisting of NaF and CaF2 as reported by Barshilia and co-workers,34,35 we zoom in on quarter-wave wet-chemical coatings comprising colloidal particles. We extensively discuss the state of the art and challenges of the four routes based on colloidal particles, which have the highest potential for application: (1) packing of dense nanospheres, (2) integration of voids through hollow nanospheres, (3) integration of voids through sacrificial particle templates, and (4) packing of nonspherical nanoparticles (Figure 5). Figure 6. Schematic drawing of nanoporous quarter-wave ARC based on packed nanospheres by Moulton. Reproduced with permission from ref 41.

comprising additional coating ingredients such as a binder and surfactants. The solid concentration of such formulations is on the order of 1−5% w/w. The coating is applied to the substrate using conventional wet-chemical coating processes such as spin-, roll-, slot-die-, spray- or dip-coating. After application of the formulation to the substrate using any of the above-mentioned techniques, the solvents evaporate, and, through assembly, the particles stack to form the coating layer. To promote the formation of a network, the coating is cured in a thermal process or through irradiation with UV light, mainly depending on the chemistry of the added binder. Assuming a close packing of equal silica nanospheres and neglecting the presence of additional coating ingredients, the fraction of space occupied by the spheres equals π (= 74%).

Figure 5. Four main routes for introducing porosity in quarter-wave coatings comprising colloidal particles: (a) packing of dense nanospheres, (b) integration of voids through hollow nanospheres, (c) integration of voids through sacrificial particle templates, and (d) packing of nonspherical nanoparticles.

3 2

Using the volume averaging theory, the refractive index of such coating can be calculated

Although a variety of vapor deposition technologies have been reported for the production of porous quarter-wave ARCs, e.g., oblique incident thermal evaporation36 and remote plasma-enhanced chemical vapor deposition,37 these will not be discussed in this article. Additionally, porous coatings formed through phase separation of two components, and subsequent selective removal of one phase to generate pores, are not discussed since the application potential of such coatings is rather limited.38 This is due to the difficulty of precisely controlling all parameters that influence the phase separation in a large-scale production process, and the necessity for one or multiple additional processing steps to induce phase separation and selectively remove one of the phases. Furthermore, systems that require multiple application steps for buildup of a coating of quarter-wave thickness, using, e.g., a layer-by-layer approach, are excluded from the discussion, as these are not expected to yield a cost-effective approach.39,40 2.3.1. Packing of Dense Nanospheres. One of the earliest reports on porous quarter-wave ARCs is a patent filed by H. R. Moulton in 1947.41 Subject of this patent was a coating with an optical thickness equal to a quarter of the wavelength of the incident light. It consisted of spherical nanoparticles (Figure 6). Through packing of these particles, nanopores were introduced to lower the refractive index of the coating. In combination with the quarter-wave thickness, this is essential for achieving antireflective properties. Typically, porous quarter-wave ARCs comprise spherical silica nanoparticles.42,43 Coating formulations are dispersions of silica nanospheres in mixtures of alcohols and water,

nC =

ϵC =

(1 − ϕ)nP2 + ϕ

where nC is the refractive index of the coating, ϵC is the dielectric constant of the coating, ϕ is the porosity, and nP is the refractive index of the particles.44 The refractive index of a coating consisting of closely packed equal silica nanospheres (nP = 1.45) is 1.36. The reflection of float glass coated with such coating would be reduced from 4.3% to 0.8%. This means that for one wavelength, λmin, the reflection is reduced to 0.8% per side (1.6% for double side coated glass). The reflection curve displays a V-shape, meaning that the reflection left and right from λmin increases rather steeply.45 To reduce the reflection to zero for λmin, the refractive index needs to decrease to 1.23 and the porosity of a silica coating on float glass needs to increase from 0.26 (close packing of equal spheres) to 0.53. In addition to nanoparticles, other coating additives are required. At least a binder is required to form a coating network. Addition of binder and other prototypical ingredients to a base coating consisting of particles only, results in a significant increase in refractive index (at least 0.10), which implies that the base coating, which comprises only particles, should have a refractive index of 1.13 or lower for use in ARCs. This corresponds to a porosity of 0.75, which is difficult to realize through packing of silica nanospheres. In addition to this limitation in optical properties, coatings based on packing of nanospheres display a distinct surface nanotexture, which makes them mechanically fragile (cf. nanotextured graded index coatings). This texture may also have an effect on pollution and E


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Figure 7. Refractive index of a coating (nc) consisting of densely packed hollow silica spheres as a function of the ratio of shell thickness (t) to particle radius (R).

Figure 8. (a) Transmission electron microscopy (TEM) image of core−shell particles, and (b) scanning electron microscopy (SEM) image of ARC produced from these core−shell particles through pyrolysis of the polymer core, as reported by Buskens et al. at DSM. Reproduced with permission from ref 49. Copyright Glass Performance Days (GPD), 2009.

of a multilayer array depends on the number of layers m and the monolayer modulus Emon.

ease of cleaning of the ARCs (cf. nanotextured graded index coatings). Hence, there is a need for the development of a system in which the pores are positioned inside a coating with limited or no surface texture, which has the potential to realize a refractive index of 1.23 for the complete coating system. 2.3.2. Hollow Nanospheres. An approach for the production of base coatings with a refractive index of ≤1.13 is the use of hollow nanospheres. When assuming a close packing of equal hollow silica spheres (nsilica = 1.45), the refractive index of the coating increases with an increasing ratio of shell thickness (t) to radius (R) (Figure 7). To realize a refractive index of ≤1.13, the ratio t should be 0.125 or less. R Hence, starting from a base coating system consisting of hollow silica nanospheres enables the addition of substantial amounts of binder and other coating additives, which are required to create a robust ARC. Based on the large amount of binder that can be added without negatively affecting the optics of the coating, it should be possible to realize a coating with a rather smooth surface as compared to coatings comprising dense silica spheres. This may result in an improved mechanical performance. The collective mechanical behavior of multilayer colloidal arrays of hollow silica nanospheres under spherical nanoindentation has been studied by Boyce and co-workers.46 Through a combination of experimental work and simulations, they demonstrated that the effective indentation modulus Eind

E ind = (0.725m−3/2 + 0.275)Emon

The monolayer modulus Emon depends on radius R and shell thickness t of the hollow silica spheres. Emon ∝

⎛ t ⎞2 ⎜ ⎟ ⎝R⎠

This study could be used as a starting point for quantification of the mechanical behavior of ARCs comprising hollow spheres, potentially in a comparative study with the other three concepts, to illustrate the balance between optical performance and mechanical behavior of all four concepts for porous quarter-wave coatings. To the best of our knowledge, such study has not yet been performed. To the best of our knowledge, the first report on the use of hollow spheres in ARCs was a joint patent application filed by the group of Buskens and Thies at DSM with Prof. Armes from The University of Sheffield as coinventor in 2007.47 They reported the use of core−shell nanoparticles with a polymer core material and silica shell. They deposited silica from molecular precursors such as tetramethyl orthosilicate on micellar templates consisting of block copolymers to form core−shell particles.48 These were combined with a sol−gel F


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carried out in standard industrial reactors, using only low-cost and commercially available starting materials. Preferably, the resulting dispersion should not contain free surfactant, which can, e.g., be realized through use of a reactive surfactant (surfmer) during the particle synthesis. An example of such synthesis based on an o/w emulsion of phenyl trimethoxysilane in aqueous ammonia has recently been reported by our research group.55,56 We did not yet manage, however, to make spheres smaller than 80 nm, which is required for use in ARCs. To avoid Mie scattering in the visible, it is important to restrict the size of particles and voids in quarter-wave coatings to 80 nm or less. Using larger particle and void sizes results in a milky-white appearance of the ARC. In some cases, even distinct colors were observed. Recently, Retsch et al.57 and the group of Armes58 reported color effects based on Mie scattering from nonordered, individual hollow silica spheres. Retsch et al. observed distinct reflection colors for hollow silica spheres with an outer diameter ranging from 350 to 647 nm, a constant silica shell thickness of about 15 nm and a size distribution less than 0.10. They confirmed that the observed colors were a direct representation of resonant Mie modes, shifting toward the red part of the spectrum with increasing particle size. In the case of hollow silica spheres with thin shells (below 25 nm), the path is large enough (tens to hundreds of micrometers) to allow resonantly scattered light to escape the powder. In the case of thicker shells/dense particles, the light is trapped within the powder sample. As an alternative to hollow spheres, the use of mesoporous spheres as building blocks for ARCs has also been reported.59 This, however, will not be discussed here because of its limited potential for application. The latter is due to the capillary condensation of water in such pores, which results in changes of the refractive index of the ARC with changing moisture content in the air, and complex hysteresis behavior, which strongly limits the practical use. To avoid capillary condensation, additional processing steps such as hydrophobization of the coating would be required, resulting in a significant cost increase. 2.3.3. Voids through Sacrificial Particle Templates. To avoid the preparation of hollow spheres, one could consider to introduce polymer particles directly in an inorganic matrix as sacrificial templates. Guillemot et al. reported a coating system in which a conventional silica binder dispersed in an ethanol− water mixture was mixed with a PMMA latex (particle size between 30 and 80 nm), yielding the coating formulation.60 They deposited the coatings on silicon wafers through spin coating, and heated the resulting coated substrate to 450 °C for 1.5 h to cure the sol−gel network and simultaneously remove the PMMA particles through pyrolysis, yielding porous silica coatings. The size and shape of the pores were directly comparable to those of the PMMA template. The pores were disordered and homogeneously distributed. In this way, Guillemot et al. managed to produce silica films with a refractive index between 1.15 and 1.40, depending on the size and content of the latex particles in the silica sol, which corresponds to a porosity between 10 and 74%. For porosity levels of 40% or higher (corresponding to a refractive index of 1.29), pores started to interconnect. Consequently, above 40% porosity, it is reasonable to expect that the mechanical performance of these coatings will substantially decrease in comparison to coatings comprising hollow silica spheres. In all cases, the surface of the coating was not smooth, but contained pores caused by the removal of PMMA beads sticking out of

binder, and the resulting dispersion in alcohol was used as coating formulation. After application to float glass slides using conventional coating processes such as dip- and spin coating, the coated glass samples were treated at a temperature above 400 °C for curing of the sol−gel coating. During this curing step, organic material was removed through pyrolysis, which made an additional process step for removal of the polymer template obsolete.49 The resulting ARC consists of inorganic materials, and is therefore assumed to be stable against UV irradiation. After curing, glass coated with a sol−gel ARC comprising hollow silica spheres was obtained.49 The resulting coated float glass slides displayed a low level of reflection of about 1% combined with a high level of scratch resistance: the coatings were not visually damaged in a steel wool test using a load of 250 g.47 They reported similar results using cationic polymer particles as core material (Figure 8).47,49 In 2009, Buskens et al. reported two ARCs based on polymer−silica core−shell particles that were commercialized by DSM: Claryl and KhepriCoat.49,50 Claryl is an ARC on glass for indoor applicationpicture and art glazingwhile KhepriCoat is an ARC on glass for outdoor application−cover glass for photovoltaic modules. In addition to a low reflection and high level of scratch resistance, the group of Buskens managed to optimize the coating with respect to ease of cleaning and outdoor durability.49,50 They reported that the KhepriCoat ARC passed all accelerated aging tests required for photovoltaic modules (as specified in IEC 61215), indicating a long-term outdoor stability.49 When switching from glass to polymer substrates, coating formulations comprising polymer−silica core−shell particles cannot be applied anymore, since removal of the core material through pyrolysis is not an option due to the temperature sensitivity of the substrate. Other potential removal steps, such as solvent extraction or etchingdepending on the type of core materialinvolve rather high processing costs and are therefore undesired. An acceptable solution would be the use of coating formulations comprising hollow or solvent filled spheres. In 2010, the group of Rubner reported an ARC for PMMA for the UV−vis range comprising hollow silica spheres.51 This ARC reduced the minimum reflection of PMMA from 7% to 0.5%. The wavelength for which the reflection was minimized could be varied from the near-UV into the VIS. Also, hollow polymer spheres were applied in ARCs. ARCs comprising polymer materials may be sensitive for UV irradiation, which can affect their durability. In 2011, Sun et al. reported the preparation of hollow polymer spheres using interfacially confined reversible addition−fragmentation chain transfer (RAFT) miniemulsion polymerization.52 They prepared particles with an outer diameter in the range of 68 to 180 nm, and with a void fraction up to 0.58, and applied them for the preparation of quarter-wave ARCs. To date, however, large scale processes for the synthesis of hollow or solvent filled spheres in dispersion are rather limited, since most synthesis routes developed to date are technically challenging and (too) expensive. Most synthesis routes reported to date are multistep, multipot syntheses, involve tailored monomers or polymers, comprise process steps that are difficult to scale up, and require large amounts of surfactants.53 The latter strongly limit the formulation window of these particles, and need to be removed in an additional and costly process step. Hence, in spite of the large number of reports published on the synthesis of hollow spheres to date,54 there is still a need for one-pot synthesis routes that can be G


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Figure 9. (a) Silicated CNCs, (b) highly porous ultralow refractive index coatings comprising CNC−silica core−shell particles, and (c) hollow silica rods obtained through pyrolysis of cellulose, developed by Buskens et al. Reproduced with permission from ref 63. Copyright Elsevier, 2015.

Figure 10. Transmission of uncoated glass, glass coated with CNC−silica core−shell particles using a coating speed of 100 mm·min−1 and 500 mm· min−1 (uncured), and corresponding samples heated at 450 °C (cured). Reproduced with permission from ref 63. Copyright Elsevier, 2015.

2.3.4. Non-Spherical Particles. To achieve a robust quarterwave ARC on float glass with an ideal refractive index of 1.23, a nanoporous layer comprising only colloidal particles is insufficient. A robust ARC should consist of a layer of colloidal particles infused with binder. The binder acts as a glue, and glues the particles together and to the glass substrate, which is required to realize a scratch resistant and durable coating. Infusion of a porous particle layer with binder inevitably results in a loss of porosity and, consequently, an increase in refractive index of the coating. Since an ideal quarter-wave ARC on float

silica matrix. It is reasonable to expect that this will make the coatings even more mechanically fragile (cf. nanotextured graded-index coatings). This texture may also have an effect on pollution and ease of cleaning of the ARCs (cf. nanotextured graded index coatings). In conclusion, the use of sacrificial particle templates in inorganic matrices could be an alternative to the use of hollow silica spheres in ARCs on glass, for applications that allow a refractive index of 1.29 or higher. The technology is of no use for temperature-sensitive polymer substrates (cf. polymer−silica core−shell particles). H


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Figure 11. Relationship between reflection (R) of a float glass substrate coated with a quarter-wave coating and the refractive index of the coating (nC). Typical refractive index of magnesium fluoride, and the four types of porous coatings discussed in this article are indicated.

samples, the transmission is increased compared to the transmission of uncoated float glass. In all cases, the refractive index is below 1.10, as confirmed by ellipsometry. Upon pyrolysis of cellulose at 450 °C, the refractive index and coating thickness decreased, resulting in a shift of λmax to shorter wavelengths and lower transmission. The resulting coatings consisted of hollow silica nanorods (Figure 9c). Since the CNC-based particle layer fulfills the requirements of a particle layer suited for infusion with binder to realize a robust quarter-wave ARC with refractive index 1.23, this system could become an interesting alternative to quarter-wave ARCs comprising hollow nanospheres (Figure 11). Currently, we are studying the porosity of these particle layers in more detail, focusing on the relationship between porosity and coating thickness, the potential presence of a refractive index gradient induced by the surface roughness, and their impact on the optics of the layers. To fully comprehend the optics of these and other types of highly porous particle layers, optical simulations using, e.g., rigorous coupled wave analysis (RCWA) may be required. Furthermore, the impact of binder infusion on the refractive index of the coating should be studied to validate whether it is possible to optimize this system to achieve robust ARCs with a low level of rest reflection. In addition, the balance between optical properties and mechanical robustness should be investigated, and compared to the other three technologies discussed in this article. Moreover, there is a high need for alternative systems comprising nonspherical building blocks other than CNCs. 2.3.5. Multifunctional Quarter-Wave ARCs. Recently, various research groups active in the field of quarter-wave ARCs focused on the possibility to realize multiple functionalities in one single coating. Gao et al. described the fabrication of superhydrophobic ARCs.64 After preparation of the ARC using a combination of three types of silica nanoparticles, leading to a coating with a distinct surface texture, they deposited 1H,1H,2H,2H-perfluorooctyltriethoxysilane via chemical vapor deposition. In that way, they realized a coating on float glass with a maximum transmission of 95.3% and a contact angle of 153° (sliding angle ≤5°), thus combining

glass has a refractive index of 1.23, the porous layer comprising only particles should have a refractive index of ≤1.13 to accommodate sufficient binder to yield a mechanically robust ARC with a refractive index of 1.23. The position of the transmission maximum, λmax, is determined by the thickness of the particle layer and the resulting ARC, which should be adjustable in the range of 80 to 150 nm to shift λmax to its desirable position in the visible. An interesting approach to lower the refractive index of a base coating comprising only particles to ≤1.13 could be the use of nonspherical particles, which are expected to generate more porous packings than spheres. Until recently, however, even through use of anisometric particles with a high aspect ratio such as needleshaped cellulose nanocrystals (CNCs), it has not been possible to produce base coatings with sufficient porosity. Two groups reported the use of CNCs in ARCs. Podsiadlo et al. reported the manufacturing of quarter-wave coatings through layer-bylayer deposition of poly(ethylene imine) and CNCs.61 To realize a coating thickness in the quarter-wave regime for visible light, they deposited 12 or more bilayers. The lowest refractive index achieved in their study was 1.28. Qi et al. reported a similar layer-by-layer approach, resulting in systems with 11 or more bilayers.62 The lowest refractive index achieved in their study was 1.41. In 2015, for the first time, our group developed a highly porous, ultralow refractive index particle layer with an adjustable thickness between 101 and 239 nm and a refractive index between 1.03 and 1.10 in one single dip-coating step using needle-shaped CNC−silica core−shell particles with an aspect ratio of about 25 (Figure 9).63 This particle layer fulfills the requirements of a particle layer suited for infusion with binder to realize a robust quarter-wave ARC with refractive index 1.23. The transmission curves of the particle layers on float glass, obtained through dip-coating from a dispersion of silicated CNCs in ethanol, displayed an inverted V-shape, which is typical for quarter-wave coatings (Figure 10). Based on the increase of coating thickness with increasing dip speed, the position of the wavelength of maximum transmission (λmax) shifts from the visible to the near IR. For all four coated glass I


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colloidal particles, in particular hollow spheres and nonspherical particles, seem to be most promising. ARCs on glass, based on polymer−silica core−shell particles that are treated at elevated temperature to cure the sol−gel coating and simultaneously remove the polymer core through pyrolysis, have been commercialized in the past decade. The same concept has been demonstrated on polymer substrates using solvent-filled or hollow spheres. However, large-scale processes for the synthesis of hollow or solvent-filled spheres in dispersion are rather limited, since most synthesis routes developed to date are technically challenging and (too) expensive. Hence, there is a need for one-pot synthesis routes that can be carried out in standard industrial reactors, using only low-cost and commercially available starting materials, and avoiding the use of large amounts of surfactants. An alternative approach to produce highly porous, lowrefractive index coatings is packing of nonspherical nanoparticles. Very recently, quarter-wave coatings based on needleshaped CNCs with a refractive index ≤1.10 have been reported. In principle, these coatings allow the addition of other coating ingredients, such as a binder, while simultaneously lowering the reflection up to a refractive index of 1.23. However, tuning of the refractive index still remains to be demonstrated. Furthermore, the porosity of such layer should be studied in more detail, and the balance between reflection and mechanical performance should be studied in comparison to ARCs comprising hollow silica nanospheres, to determine which system provides the best balance between these two key properties. Additionally, there is a high need for coatings comprising nonspherical nanoparticles other that CNCs. In addition to the specific needs for further research discussed above, it is a generic trend in the field of functional coatings to combine multiple functionalities in one single coating layer. Although several examples of such ARCs have been reported and addressed, it is worthwhile to search for additional combinations. A second generic trend in functional coatings that may affect the field of ARCs involves the development of adaptive systems, in which the functionality adapts to specific changes in the environment. This requires the integration of the ability to sense the change, and consequently respond to it, in one coating system. An example of such a system could be a quarter-wave ARC that can sense the wavelength of the incident light, and in response change its optical thickness to maximize the transmission for that specific wavelength. In conclusion, in spite of the maturity of ARCs as research area, a variety of important research questions related to graded index and quarter-wave coatings remain unanswered. In addition, the drive toward multifunctional and adaptive coating systems provides interesting directions for future research.

antireflective with self-cleaning functionality. The group of Vollmer reported a superhydrophobic coating consisting of porous silica capsules with a raspberry like surface structure, yielding a self-cleaning coated glass with increased transmission in the wavelength regime between 330 and 410 nm.65 Additional transparent superhydrophobic coatings were reported by Barshilia and co-workers.66−68 Zhang et al. also realized a self-cleaning ARC.69 This was based on a coating system combining silica and titania nanoparticles. The authors managed to include sufficient titania to ensure self-cleaning functionality based on its photocatalytic activity, while keeping the refractive index sufficiently low to establish good antireflective properties. The group of Rubner reported the preparation of an antifogging ARC, prepared through combined use of silica nanoparticles and a polycation.70 They achieved coatings that combine a high level of transmission (above 99%) and a water droplet contact angle of less than 5°. The latter causes the antifogging properties. This combination of antifogging and antireflective functionalities combined in a quarter-wave coating seems somewhat contradictory, however, since the water uptake by the coating required to realize the antifogging effect simultaneously results in an increase in refractive index, which leads to a deterioration of the optical properties of the coating.

3. CONCLUSIONS AND FUTURE WORK Three main coating concepts exist to lower the reflection at the interface of a transparent substrate and air: multilayer interference coatings, graded index coatings, and quarter-wave coatings. Multilayer interference coatings are mechanically robust and durable, and their optics can be fine-tuned for the application of choice. They have been on the market for several decades, albeit solely for applications in which the ARC represents a high added value. Markets in which the added value of the ARC is low to medium require a low-cost coating, which is easily applicable to high volumes (multimillion m2 p.a.) of large substrates (>1 m2). An example of an ARC that is potentially low-cost when compared to a multilayer interference stack, is a graded index coating, merely because it consists of one single coating layer. However, such nanotextured moth-eye ARCs, typically prepared through NIL, are not yet ready for use in large-scale applications. The main reasons for this are the lack of suitable liquid coating materials, which are qualified for this application, and the lack of a commercial scale NIL process enabling texturing of coatings on high volumes of large substrates, and at low cost. Additionally, it is still unclear whether the mechanical robustness required for successful application of surfacetextured ARCs can be realized, due to the intrinsic fragility of the surface nanotexture. This texture may also have an effect on pollution and ease of cleaning of the ARCs. Depending on the chemical composition of the coating, the geometry of the surface texture, and the characteristics of the pollutant, the texture may positively or negatively contribute to pollution and ease of cleaning. An example of an ARC which is potentially low-cost when compared to a multilayer interference stack, and does not require surface texturing, is a quarter-wave coating. Quarterwave coatings on float glass and transparent polymers with a refractive index of about 1.5 require a refractive index of 1.23. To realize such low refractive index, it is essential to introduce pores in coatings. Although there are multiple ways of introducing pores in quarter-wave coatings, introduction of

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. de. Phone: +31888662990. Notes

The authors declare no competing financial interest. J


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Biographies

developed metal oxide catalysts based at the company Bio Nano Consulting and the London Centre for Nanotechnology. Since 2012, he has been working at TNO on applied colloid chemistry and nanomaterials synthesis.

Pascal Buskens received his Ph.D. in chemistry from RWTH Aachen University. In 2006, he started working at DSM, where he headed a research group on functional coatings. Since 2011, he has worked at The Netherlands Organisation for Applied Scientific Research (TNO), where he currently holds the position of principal scientist for the research area colloids and interfaces. In addition, he holds a position as a research group leader at DWI − Leibniz Institute for Interactive Materials e.V. His main areas of interest are nanoparticle synthesis, nanostructured coatings, and optical materials.

Zeger Vroon received his Ph.D. from the University of Twente. In 1995, he started working at TNO. Since 2008, his research focuses on light management in solar cells, reliability of photovoltaic cells and modules, and integration of photovoltaic modules in the built environment. Since 2009, he has held a part-time position as a lecturer at Zuyd University of Applied Sciences.

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REFERENCES

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