Nanoparticle Stacks on

Dec 26, 2014 - Abstract Image. Gradient refractive index layered antireflection (GRIL-AR) films were fabricated using a layer-by-layer (LbL) self-asse...
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Gradient Functional Characteristic of Polymer/Nanoparticle Stacks on a Polyethylene Naphthalate Film Kenta Fukada and Seimei Shiratori* School of Integrated Design Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa-ken 223-8522, Japan S Supporting Information *

ABSTRACT: Gradient refractive index layered antireflection (GRIL-AR) films were fabricated using a layer-by-layer (LbL) selfassembly method. As a substrate, we chose indium tin oxide−polyethylene naphthalate (ITO−PEN; the refractive index of PEN is 1.77). We deposited the GRIL-AR films on the PEN side of ITO−PEN, and the refractive index was gradually decreased from 1.59 to 1.35 by alternating deposition cycles of SiO2 as a low-refractive-index material and TiO2 as a high-refractive-index material. In the LbL process, each material was movable because it was coated in wet conditions; this may have led to fabrication of the gradient refractive index. When poly(ethylene imine) (PEI) was compared with poly(diallyldimethylammonium chloride), antireflection (AR) using PEI showed high abrasion durability because of the loopy structure of the polymer and a high polymer ratio in the film. With the drying process, the prepared film showed enhanced abrasion durability. GRIL-AR on ITO−PEN showed broad-band AR properties.

1. INTRODUCTION Currently, flexible substrates for displays and solar cells use indium tin oxide−polyethylene naphthalate (ITO−PEN), which is a transparent, heat-durable, and flexible conductive substrate also used for various other applications. However, the high refractive index of ITO−PEN (the refractive index of ITO is 1.88, and that of PEN films is 1.77) leads to reduced transparency. Therefore, the refractive index gap between air and the substrate needs to be reduced for enhanced transparency. To reduce the reflection loss, antireflection (AR) films have been extensively studied and fabricated by various methods.1−12 AR films have been used for applications such as solar cell surfaces and optical lenses. The fundamental structure of the AR film is a single-layered AR film. With a target wavelength λ, AR film thickness d, and refractive index n, the ideal structure is d = λ/4n, where n = (n1n0)1/2 (n1 is the refractive index of the substrate and n0 is the refractive index of air). Double-layered AR films composed of low and high refractive indices are able to enhance the transmittance of specific wavelengths; however, these AR coatings are not effective for oblique incident lights or the entire wavelength range. To enhance the AR properties for various incident angles and broad-band wavelengths, gradient refractive index layered antireflection (GRIL-AR) films are attracting increasing attention. There are both multilayered types1−3,6,8,10 and moth-eye types4,5,7,9 of GRIL-AR. The multilayered type of GRIL can be fabricated by stacks of materials with different refractive indices, such as SiO2, TiO2, and ZrO2. The fabrication methods include oblique-angle electron-beam (e-beam) deposition1 and the spin coating of various materials in turn.2,10 Oblique-angle e-beam deposition is useful for planar surfaces; however, it is difficult to coat curved surfaces uniformly. Multilayered AR coatings with wet processes such as spin coating do not require vacuum systems or etching processes;10 however, thermal expansion mismatch and interfacial instability are problems in the case of multilayered thin-film AR coating.3 AR coatings by chemical etching showed high performance; © 2014 American Chemical Society

however, the fabrication step requires toxic materials such as a HF etching solution.8 In the case of moth-eye AR coatings, GRILs were fabricated by mimicking a moth eye (pyramid structure). This structure was more effective for thermal and mechanical durability than other coatings because only one material was used; however, various structures were fabricated with vacuum methods such as e-beam lithography and dry etching,22−33 and some previous reports did not mention durability tests. As an application of AR coating, the solar cell surface was focused on; however, some solar cells have been fabricated without AR coatings because of the high production cost.8 Therefore, enhancement of the durability and low-cost fabrication methods is also strongly desired. In this study, we focused on enhancement of the durability of a GRIL-AR on a PEN film using an easy wet process. To achieve this purpose, we chose the layer-by-layer (LbL) fabrication method.13−21 A thin film was fabricated using a wet process by alternately depositing the substrate in cationic and anionic solutions. This method has been widely studied because it offers easy control of nanomaterial deposition on various substrates, including curved surfaces. To enhance the durability of GRIL-AR by the LbL method, we investigated ideal polymers as glues, the effect of the drying process, and the deposition of various materials to fabricate different gradient refractive index films. To obtain low-refractive-index materials, complex fabrication steps are conventionally needed, making them not cost-effective. Considering both the cost and ease of fabrication, we chose SiO2 and TiO2 as anionic materials because SiO2 has a refractive index near 1.4 and that of TiO2 is approximately 1.6. Finally, we varied the ratio of these materials in the AR film and obtained GRIL-AR Received: Revised: Accepted: Published: 979

October 24, 2014 December 17, 2014 December 26, 2014 December 26, 2014 DOI: 10.1021/ie504202m Ind. Eng. Chem. Res. 2015, 54, 979−986

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Industrial & Engineering Chemistry Research

Figure 1. Structure of fabricated films. In this study, we fabricated three types of AR films. The first is a 2layeres (PEI)-AR composed of (PEI/TiO2)6 + (PEI/SiO2)6. The refractive index was changed stepwise. The second is 2layers (PDDA)-AR, composed of (PDDA/TiO2)30 + (PDDA/SiO2)8, which also has a stepwise refractive index change. The difference between the two types of AR involved the use of either weak or strong polyelectrolyte polymers. The last type is GRIL-AR fabricated using PEI.

Figure 2. LbL method. We fabricated AR films using the LbL method. We chose ITO−PEN as the substrate. As a cationic solution, we used PEI(aq) or PDDA(aq). As an anionic solution, we used SiO2(aq) or TiO2(aq).

films using a wet process with easy fabrication and cost-effective materials.

Table 1. Fabrication Conditions

2. EXPERIMENTAL SECTION 2.1. Materials. We used materials without any purification. Poly(ethylene imine) (PEI; M w = 10000) and poly(diallyldimethylammonium chloride) (PDDA; Mw = 250000− 300000) were used as positively charged materials. PEI had a loopy structure because it is a weak polyelectrolyte and has a low charge density; conversely, PDDA had a train structure because it is a strong polyelectrolyte with a high charge density. Titanium(IV) bis(ammonium lactato)dihydroxide (TALH) and a SiO2 nanoparticle aqueous dispersion were prepared as negatively charged materials. Poly(4-styrenesulfonic acid) (PSS) and PDDA were used for the buffer layer of a quartz crystal microbalance (QCM). The concentrations of PEI, PDDA, TALH, and SiO2 were adjusted to 0.01 and 0.01 M and 1.0 and 0.2 wt %, respectively, with ultrapure water (>18 MΩ·cm). The pH values were 10.2, 5.5, 3.6, and 5.4, respectively. 2.2. Fabrication Method. Solutions were stirred for 24 h and used within several days. The ITO−PEN substrate was ultrasonically washed with ethanol for 5 min and with ultrapure water for 5 min two times. We covered the ITO side of ITO− PEN with masking tape. To prevent detachment, we removed the air between the ITO and masking tape by squeezing. The squeezing bar was rolled on the masking tape to remove air bubbles between the ITO and masking tape. By using this method, ITO was protected during the LbL process. The high-refractive-index layer was fabricated by dipping ITO−PEN in either PEI or PDDA and TALH solutions alternately with the LbL method. PEI or PDDA and SiO2 solutions were used for the low-refractive-index layer. To fabricate the GRIL, we changed the dipping conditions. Images of the fabricated samples and materials are shown in Figures 1 and 2. The deposition conditions are shown in Table 1. We refer to double-layered AR using PEI as “2layers (PEI)-AR”, doublelayered AR using PDDA as “2layers (PDDA)-AR”, and the

2layers (PEI)AR 2layers (PDDA)-AR GRIL-AR

sample name

(cation/anion) bilayers (PEI/TiO2)6 + (PEI/SiO2)6 (PDDA/TiO2)30 + (PDDA/SiO2)8 (PEI/TiO2)3 + (PEI/SiO2)1 + (PEI/TiO2)2 + (PEI/SiO2)2 + (PEI/TiO2)1 + (PEI/SiO2)3

The number of cycles was optimized by measuring the transparency, refractive index, and film thickness. To investigate the polymer effect, we fabricated two types of AR. Abrasion tests revealed that PEI has high durability, so we fabricated GRIL-AR using PEI.

gradient refractive index layer AR as “GRIL-AR”. Here, when we deposit material B over material A, we describe it as (A/B). If we repeat the sequence “n” times, we describe it as (A/B)n. 2.3. Equipment. The film thickness was determined by ellipsometry measurements (ULVAC ESM-1A). Optical characterization of multilayered films was carried out using an ultraviolet−visible (UV−vis) spectrometer (Shimadzu UV mini1240). Parallel and total transmittance was measured using a haze meter. Surface images were captured by field-emission scanning electron microscopy (FE-SEM; Hitachi S-4700). Depth profile analysis was carried out by glow discharge optical emission spectrometry (GDOES; Horiba). LbL deposition was carried out using an automatic dipping machine (nanofilm maker, TsubakiSNT).

3. RESULTS AND DISCUSSION 3.1. 2layers-AR. Adsorbed Amount. We first compared PEI and PDDA as cationic materials. The mass of each layer was measured by QCM, as shown in Figure 3. Figure 3a shows the results of PEI/TiO2 or PEI/SiO2 deposition, and Figure 3b shows the results of PDDA/TiO2 or PDDA/SiO2 deposition. From these results, it is clear that fewer TiO2 nanoparticles were deposited than SiO2 nanoparticles. The same trend was indicated with the film growth measurement results from Figure S3 in the 980

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Figure 3. QCM results for polymer and nanoparticle stacks: (a) (PEI/TiO2)6 + (PEI/SiO2)6; (b) (PDDA/TiO2)10 + (PDDA/SiO2)5. Adsorbed amounts were calculated from the frequency change of QCM. 1 Hz is nearly equal to 1 ng of adsorption.

Figure 6. Detachment of nanoparticles. After the deposition of polymers, decreased amounts were observed in Figure 5. We conclude that removal of the nanoparticles decreased the total amounts because of a difference in the solution pH.

Figure 4. Total amounts: 2−6 bilayers of (PEI/TiO2), 8−12 bilayers of (PEI/SiO2), 2−6 bilayers of (PDDA/TiO2), and 11−15 bilayers of (PDDA/SiO2), as shown in Figure 3, were used to calculate the total amount adsorbed.

Table 2. Polymer Ratio 5 bilayers

PEI/TiO2

PDDA/TiO2

PEI/SiO2

PDDA/SiO2

polymer ratio (%)

14.0

12.2

6.72

5.17

The polymer ratio was calculated from Figure 4. The polymer ratio using TiO2 was higher than that using SiO2. In the case of PDDA, the polymer ratio was lower than that using PEI because PDDA had a train structure and was not as efficiently adsorbed. Figure 7. Abrasion test with and without water. To investigate the effect of water on the film, we compared two types of AR with or without heat drying at 150 °C for 3 h. We used cotton fabrics applied at 10 g/cm2 for the durability test. The abrasion cycle was varied from 10 to 50 times.

high static electric force leads to high adsorption. The first layer of PEI/TiO2 and the seventh layer of PEI/TiO2 had slightly less deposition of nanoparticles compared with the other layers. This was because of the difference in the deposition materials. QCM measurement determined that the first layer was deposited on the PDDA/PSS buffer layer and the seventh layer was deposited on the PEI/TiO2 layers. We conclude that the difference in the surface charge or surface structure affected the deposition of materials. This phenomenon was also observed in the first layer of PDDA/TiO2 and the 11th layer of PDDA/SiO2. Concentration of the Polymer. We calculated the amount of polymer and nanoparticles in these films from Figures 3 and 4. We used 2−6 bilayers of (PEI/TiO2), 8−12 bilayers of (PEI/ SiO2), 2−6 bilayers of (PDDA/TiO2), and 11−15 bilayers of (PDDA/SiO2) and calculated the total amounts. The total amounts were 1680, 4719, 1747, and 3689 ng, respectively. The polymer ratio was calculated and is shown in Table 2. The polymer ratios of PEI/TiO2 and PEI/SiO2 were higher than those of PDDA/TiO2 and PDDA/SiO2. The polymer ratio decreased with the use of SiO2.

Figure 5. QCM results with the drying process. The deposition results for PDDA/SiO2 are shown. The polymer was adsorbed in the red area, and nanoparticles were adsorbed in the blue area. The substrate was dried at room temperature in the white regions. For example, 0−60 s, polymer deposition; 61−150 s, water rinse; 151−3750 s, drying at room temperature; 3751−3810 s, nanoparticle deposition; 3811−3900 s, water rinse; 3901−7500 s, drying at room temperature. These steps were performed a total of six times.

Supporting Information (SI). The film thickness of (PEI/TiO2)6 was 102 nm, and the film thickness of (PEI/SiO2)6 was 134 nm. The ζ potentials of TiO2 and SiO2 were −29.0 and −48.4 mV, respectively, as shown in Figure S1 in the SI, and this difference caused variation in film growth because, in the LbL process, a 981

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Figure 8. Abrasion test using PEI and PDDA. (a) Every sample was dried at 150 °C for 3 h. We compared 2layers (PEI)-AR and 2layers (PDDA)-AR. We used cotton fabrics applied at 100 g/cm2. The abrasion cycle was varied from 10 to 50 times. (b) The film fabricated using PEI was well mixed with polymer and nanoparticles. This was because of the structure of PEI, i.e., a loopy structure. Compared with PDDA, which had a train structure, PEI showed better abrasion test results.

Figure 9. Before and after annealing at 150 °C. (a) Transmittance of ITO−PEN, ITO−PEN with two layers (PEI)-AR, and ITO−PEN with GRIL-AR. (b) Film thickness and refractive index of 6 bilayers of SiO2/PEI, 8 bilayers of SiO2/PDDA, 6 bilayers of TiO2/PEI, and 30 bilayers of TiO2/PDDA. After annealing at 150 °C for 3 h, there were no clear differences.

structure, the film thickness was enhanced; however, the train structure did not appear to enhance the film thickness. The polymer ratio was higher for PEI than it was for PDDA; however, PEI was suitable for fabricating the low-refractive-index layer because the film growth rate was high and the AR properties were almost identical with those obtained using PDDA. From Figure S3 in the SI, it can be determined that the refractive indices of (PEI/SiO2)6 and (PDDA/SiO2)8 are 1.35 and 1.37, respectively. The transmittances at 550 nm were 84.4% and 84.07%, respectively, as seen in Figure S2 in the SI. Effect of the Water Content. We also investigated the effect of the water content in the film because in the LbL process we fabricated the AR film in wet conditions. We attempted to

Layered Structures. Taking into consideration film thicknesses and deposition amounts, we suggest a film deposition mechanism. The film thicknesses of (PEI/TiO2)6, (PEI/SiO2)6, (PDDA/TiO2)30, and (PDDA/SiO2)8 were 102, 134, 107, and 111 nm, respectively. In the case of PEI, the polymer ratio was high and the film growth rate was also high. In the case of PDDA, the polymer ratio was low and the film growth rate was also low. We estimated that the difference in the deposition mechanism was a result of the different polymers. PEI is a weak polyelectrolyte with a loopy structure, whereas PDDA is a strong polyelectrolyte with a train structure. The difference in structure contributed to the film growth. In the case of the loopy 982

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Figure 10. SEM images. A cross-sectional SEM image of GRIL-AR is shown. The bottom area was rich in TiO2 nanoparticles, and the top area was rich in SiO2 nanoparticles. The fabrication condition of GRIL-AR was as follows. PEI/TiO2 3 bilayers + PEI/SiO2 1 bilayer + PEI/TiO2 2 bilayers + PEI/SiO2 2 bilayers + PEI/TiO2 1 bilayer + PEI/SiO2 3 bilayers.

Figure 12. Refractive index by GDOES. To investigate the ratio of Si and Ti in 2layers (PEI)-AR and GRIL-AR, we used GDOES. From eq 1, we calculated the refractive index. In the case of GRIL-AR, the refractive index changed gradually; however, the refractive index changed in a stepwise fashion in the case of 2layers (PEI)-AR.

estimate the film condition in the wet process by QCM, as shown in Figure 5. The fabrication method was PDDA(aq) 1 min/water rinse 30 s (×3)/drying 1 h/SiO2(aq) 1 min/water rinse 30 s (×3)/drying 1 h, and these cycles were performed six times. We dried the films at room temperature. During the drying process, the QCM was not measured precisely because it can be measured only in the solution or in the atmosphere. When we focused on the deposition of PDDA, the deposition amount was decreased. The deposition of PDDA was measured from the difference after the deposition of nanoparticles and after the deposition of PDDA. We suggest that this was because of the detachment of nanoparticles during the deposition of PDDA, as shown in Figure 6. This detachment was considered to be the reason for the nonuniform films. To investigate the effect of the water content, we performed an abrasion test against (PDDA/TiO2)30 + (PDDA/SiO2)8 before and after heat drying. In this experiment, we used ITO−PEN, which has high annealing durability, so we dried the prepared film at 150 °C for 3 h. We abraised these films with cotton fabric. The abrasion pressure was 10 g/cm2 (≑1 kPa). The results are shown in Figure 7. After the heat-drying process, the film had a high abrasion stability. From these results, we concluded that the contained water was not suitable for solid films. This wet film was weak against abrasion because the wetness assisted the mobility of the polymers and the film was easily damaged. From these observations, we considered that the contained water was not suitable for the abrasion test and it was better to dry the films. Abrasion Durability. We wanted to focus on the durability of GRIL-AR. To achieve this purpose, we compared PEI and PDDA to select a suitable polymer for GRIL-AR. The results of the

abrasion test are shown in Figure 8. We selected these polymers because PEI is a weak polyelectrolyte and PDDA is a strong polyelectrolyte. Normally, weak polyelectrolytes change the structure from loopy to train with changes in the pH. The reason for this transformation is the ionization degree. Conversely, strong polyelectrolytes have a train structure because the ionization degree of the polymer does not change with the pH. The loopy structure has a small charge density, which contributes to the formation of a thick film. The train structure has a large charge density, which leads to the formation of a thin film. Twolayered AR using PEI treated at 150 °C for 3 h had higher abrasion durability compared with that made using PDDA. The film with PEI formed a complex with polymer and nanoparticles because of its loopy structure. This well-mixed structure contributed to the adhesion of each layer. The film made using PDDA had weak abrasion durability because each layer was separated by a strong polyelectrolyte. The strong polyelectrolyte was not combined in each layer and had no effect on the adhesion between polymer and nanoparticles. Low adhesion force led to weak abrasion resistance. High-Temperature Durability. From investigation of the annealing treatment of each AR film, we determined that the annealing process was important for enhancing durability. In this experiment, we chose ITO−PEN as a substrate because of its high temperature durability, which made it suitable for the annealing process. We decided to use PEI and the annealing

Figure 11. GDOES of 2layers (PEI)-AR and GRIL-AR. (a) (PEI/TiO2)6 + (PEI/SiO2)6 and (b) GRIL are shown. The intensity and etching time were normalized. 983

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Industrial & Engineering Chemistry Research Table 3. Refractive Index Determined by Ellipsometry 2layers (PEI)-AR layer 1 was fixed layer 2 layer 1 substrate

layer 2 was fixed

2layers (PEI)-AR

GRIL-AR

n

d

n

d

n

d

n

d

X = 1.37 1.59 1.77

Y = 106 102

1.35 X = 1.63 1.77

134 Y = 113

X = 1.53 X = 1.53 1.77

Y = 119 Y = 119

X = 1.58 X = 1.58 1.77

Y = 117 Y = 117

In the case of 2layers (PEI)-AR where “layer 1 was fixed”, we inserted the value of layer 1 for (PEI/SiO2)6 from Figure S3 in the SI and measured layer 2. In the case of 2layers (PEI)-AR where “layer 2 was fixed”, we inserted the value of layer 2 for (PEI/TiO2)6 from Figure S3 in the SI and measured layer 1. We also tried to measure the average values for 2layers (PEI)-AR and GRIL-AR.

results of an annealing test for the PET film in Figure S7 in the SI. After heating at 150 °C for 3 h, the transmittance of PET was drastically decreased. We investigated single-layered AR, shown in Figure 9b, by measuring the refractive index and film thickness. There were no clear differences after annealing at 150 °C for 3 h. Therefore, we conclude that we have achieved enhancement of durability without changing the refractive index or transparency. 3.2. GRIL-AR. Refractive Index. An image of GRIL-AR is shown in Figure 10. From this cross-sectional SEM image, it is possible to observe TiO2 and SiO2. However, it was difficult to determine the ratio of each material. To measure the refractive index of each AR film, we used GDOES analysis. The measured values are shown in Figure 11. In this figure, the interface of the PEN substrate and the AR film showed a maximum at the carbon peak. The refractive index was calculated using eq 1.

Figure 13. Abrasion test (GRILs). The durability of GRIL-AR was investigated and parallel transmittance, total transmittance, and haze were measured using a haze meter. We used cotton fabric applied at 10 g/cm2. The abrasion cycle was varied from 10 to 50 times. Even after abrasion, the total transmittance was over 80%.

refractive idex = [1.35(normalized intensity of Si) + 1.59(normalized intensity of Si)] /normalized intensities of Si and Ti

(1)

The refractive index with only the low-refractive-index layer was 1.35, and the refractive index with only the high-refractive-index layer was 1.59, as shown in Figure S3 in the SI. The refractive indices of 2layers (PEI)-AR and GRIL-AR were calculated, and the results are shown in Figure 12. Compared with 2layers (PEI)AR, the refractive index of GRIL-AR exhibited a gradient because, in the LbL process, each material was mobile because of the wet conditions, which led to the fabrication of a mixed area. We also attempted to measure the refractive index and film thickness by ellipsometry. When we considered 2layers (PEI)AR as two separate layers, we succeeded in measuring each value, as shown in Figure 12 and Table 3. However, when we considered 2layers (PEI)-AR as one layer, it was difficult to measure the precise refractive index and film thickness because of the large refractive index gap. This was the same situation encountered for GRIL-AR; i.e., measuring the averaged refractive index was difficult. Using GDOES analysis, we succeeded in measuring the stepwise refractive index of 2layer (PEI)-AR and a gradient refractive index for GRIL-AR. Durability. To investigate the abrasion durability of GRIL-AR, we performed an abrasion test. The film was dried at 150 °C for 3 h. We made abrasion tests for these films with cotton fabric. The abrasion pressure was 10 g/cm2 (≑1 kPa). The results are shown in Figure 13. The abrasion durability was not as high as it was with 2layers (PEI)-AR or 2layers (PDDA)-AR. The reason for this may be the difference in the size of the nanoparticles. The haze value increased; however, the transmittance remained over 80%. This was because of scattering light, which contributed to

Figure 14. Vertical light transmittance of GRIL-AR. Using GRIL-AR, AR regions were enlarged, especially that of the visible wavelength.

Figure 15. Oblique light transmittance of GRIL-AR. (a) System for measuring transmittance against oblique incident lights. The light source was a semiconductor laser (λ = 632.8 nm). (b) Black dottted line: ITO−PEN. Blue line: ITO−PEN with 2layers (PEI)-AR. Red line: ITO−PEN with GRIL-AR.

process to fabricate durable GRIL-AR. We investigated the differences before and after the annealing process, as shown in Figure 9a. From this figure, it can be seen that there were no differences with the use of ITO−PEN. We also showed the 984

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GDOES analysis, QCM of GRIL-AR and surface image of each layer, and SEM images of the LbL film. This material is available free of charge via the Internet at http://pubs.acs.org.

transmittance even after abrasion. This makes GRIL-AR suitable for applications such as solar cells. Transparency. Gradually changing the refractive index layers was useful for GRIL-AR. Using GRIL-AR, the AR region, especially in the visible-light wavelengths, was enlarged, as shown in Figure 14. We conclude that the gradient refractive structure of GRIL-AR led to a smaller refractive index gap in the AR film. Conversely, 2layers (PEI)-AR had a stepwise refractive index structure, which resulted in a refractive index gap in the AR film. An increasing refractive index gap leads to increased reflection, which is a detriment to AR because incident light is reflected at different wavelengths by materials with different refractive indices. Using GRIL-AR, the AR region was enlarged, which led to a decrease in the refractive index gap. We also tried to measure the transmittance against oblique lights; however, we were not able to get s large difference compared with the case of vertical light, as shown in Figure 15. By using AR coatings, the transmittance against oblique incident light was enhanced. On the other hand, however, it was difficult to get the difference of 2layers (PEI)-AR and GRIL-AR. We considered that this was due to two reasons, the interface and the inside of the AR film. As the first reason, in this study we got a refractive index region from 1.35 to 1.59 in the case of GRIL-AR; however, the upside refractive index should be 1.0 for air, and the bottom side refractive index should be 1.77 for the PEN film. The refractive index gaps at the interfaces of air and the surface of GRIL-AR and the bottom of GRIL-AR and the PEN film were still high. As the second reason, GRIL-AR was fabricated by six layers and had five small stepwise refractive indices oriented from the interface of PEI/TiO2 and PEI/SiO2. From Figure 12, the stepwise indices were found at 60, 115, 140, 160, 180 nm in the case of GRIL-AR. We considered that a gradient refractive index structure was needed to obtain higher transparency compared with 2layers (PEI)-AR, and these stepwise indices were not suitable for oblique incident light. That is why we had reflection loss even in the case of GRILAR, and it was difficult to get the significant difference of 2layers (PEI)-AR and GRIL-AR by a photodiode detector; however, according to the UV−vis measurement, transmittance of GRILAR was higher than that of 2layers (PEI)-AR. Especially we need to reduce the refractive index of SiO2 for future research. The fabrication of hollow SiO2 nanoparticles,34 which has lower refractive index than SiO2 nanoparticles, may be a candidate for making GRIL-AR.

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*Tel.: +81-45-566-1602. Fax: +81-45-566-1602. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSIONS In this study, we fabricated AR films with polymer and nanoparticle stacks on a PEN film using the LbL method. We compared AR films fabricated using PEI or PDDA. PEI had better durability because the loopy structure contributed to the mixing of nanoparticles and polymers. We also investigated the effect of the drying process and determined that the durability was enhanced by drying. Using these results, gradually changing refractive index layers using various nanoparticles were fabricated using PEI and a drying process. The refractive index was changed by deposition conditions and measured by GDOES. In the case of visible-wavelength regions, transmittance was enhanced because of the broad-band AR properties of GRIL-AR.

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Measured ζ potentials of materials, transmittance, film thickness, and refractive index of fabricated films, single layer measured by 985

DOI: 10.1021/ie504202m Ind. Eng. Chem. Res. 2015, 54, 979−986

Article

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DOI: 10.1021/ie504202m Ind. Eng. Chem. Res. 2015, 54, 979−986