Luminescent Antireflective Coatings with Disordered Surface

Feb 21, 2011 - tronic devices such as fluorescent lamps, cathode ray tubes, X-ray detectors, and flat panel displays. Recently, thin phosphor films...
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Luminescent Antireflective Coatings with Disordered Surface Nanostructures Fabricated by Liquid Processes Sota Tanaka and Shinobu Fujihara* Department of Applied Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan

bS Supporting Information ABSTRACT: Antireflective phosphor coatings having disordered surface nanostructures were fabricated by a sol-gel dip coating method and a subsequent hot water treatment. Thin films of a Bi3þ,Eu3þ-codoped YVO4 red phosphor were first prepared and effects of the addition of an aluminum source to precursor solutions on their microstructure and optical properties were examined. Optical transmittance of the YVO4:Bi3þ,Eu3þ film was lower than that of a bare quartz glass substrate due to a higher refractive index of YVO4. The addition of the aluminum source and the hot water treatment resulted in a considerable increase of transmittance and its smaller angular dependence, which could generate an antireflective effect by the phosphor thin films. Observation of the microstructure revealed that the hot water treatment brought a remarkable change in the surface as well as the cross-section structure in the aluminum-added YVO4:Bi3þ,Eu3þ film. The film density and hence the refractive index were gradually changed like a pseudo moth-eye structure, which explained the occurrence of the antireflective effect. The microstructural change was attributed to the dissolution of alumina present in the film and the reprecipitation of boehmite on the film surface during the hot water treatment. Photoluminescence of the YVO4:Bi3þ,Eu3þ film could also be enhanced by the antireflective effect due to the suppression of surface Fresnel reflection of incident light and total internal reflection of emitted light.

’ INTRODUCTION Inorganic luminescent materials, also designated as phosphors, can convert certain kinds of energy sources into electromagnetic radiation. Phosphors are commonly utilized as powders, which are widely applied in various sorts of optoelectronic devices such as fluorescent lamps, cathode ray tubes, X-ray detectors, and flat panel displays. Recently, thin phosphor films have received considerable attention for practical or potential use in field emission displays, X-ray imaging devices,1 solar spectral converters for photovoltaic cells,2 phosphor-converted lightemitting diodes (LEDs),3 and so on. Phosphor thin films have some outstanding advantages such as high contrast and resolution and superior thermal conductivity which cannot be attained in conventional powdery phosphors. One of the significant characteristics of flat and smooth phosphor thin films is their high transparency in broad wavelength ranges without the occurrence of light scattering. This property is indispensable for solar spectral converters to minimize optical losses and enhance efficiencies in photovoltaic cells.4 Against the expectations, however, the use of phosphor thin films has been impeded by the occurrence of high reflections, which is easily understood with the fundamental optical principles. Many phosphor materials are derived from densely packed crystals composed of heavy metal elements, which essentially have high polarizability and high refractive indices. A large difference in the refractive index between phosphor films and air then causes extreme Fresnel reflection of incident light from r 2011 American Chemical Society

air to films and total internal reflection of emitted light from films to air or substrates. Surface Fresnel reflection diminishes intensity of the incident light to be transmitted, leading to a great optical loss. It also prevents absorption of incoming excitation light for phosphors. Most of the emitted light from phosphors propagates inside films due to total internal reflection and therefore cannot be extracted efficiently. Surface Fresnel reflection is usually suppressed by employing antireflective coatings in optical devices. Many attempts have been made so far for fabricating effective antireflective coatings using materials with lower refractive indices,5 porous films,6 or multilayer structures.7 Suppression of total internal reflection has been studied intensively in LEDs. For example, gallium nitride (GaN) that is a basic material of LEDs has a high refractive index (n ≈ 2.5).8 Only 4% of the internal light can be extracted from the surface due to reflection.9 Accordingly, surface texturing10-12 and photonic crystal structures13,14 have been investigated in attempts to enhance light extraction efficiency. From these considerations, we paid attention to a moth-eye structure. In the 1960s it was discovered that moths can see objects even in the darkness of night due to the orderly bump structures of their cornea. Following this observation, it was shown that moth-eye mimicking structures also have an antireflective property. These structures form a gradual change in Received: October 18, 2010 Published: February 21, 2011 2929

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Langmuir average refractive indices between air and the inside eye, and minimize surface Fresnel reflection over a broad range of wavelengths and incidence angles.15-18 The gradual change in refractive indices is not only effective for suppressing surface Fresnel reflection but for total internal reflection. Some researchers have attempted to fabricate moth-eye structured LEDs and succeeded in increasing light extraction efficiency.19-22 Our approach is then fabricating phosphor thin films with surface nanostructures like the moth-eye structure. However, moth-eye structures are usually prepared by photolithography techniques, and it is therefore difficult to produce them over large areas and on curved surfaces. Recently, alumina (Al2O3) films with pseudo moth-eye structures have been reported by using a sol-gel process and a subsequent hot water treatment.23-25 The hot water treatment is very simple and a low cost process with immersing films in hot water. The resultant alumina films had a flower-like structure which worked as antireflective coatings since the structure exhibited a density gradient from the surface to the substrate. Here we propose the fabrication of a new type of phosphor thin films working also as antireflective coatings. YVO4:Bi3þ, Eu3þ was chosen as a phosphor material. Eu3þ-doped YVO4 is an efficient red-emitting phosphor due to excitation through energy transfer from strongly light-absorbing vanadate groups (VO43-) to Eu3þ activators. The Bi3þ incorporation in the YVO4 lattice makes the absorption band-edge of YVO4 shift to a longer wavelength, which is ascribed to a charge-transfer state involving Bi3þ and neighboring oxygen in (Y,Bi)VO4. Bi3þ can also act as a sensitizer for the Eu3þ red emission in the YVO4:Bi3þ,Eu3þ phosphor.26 In this work, YVO4:Bi3þ,Eu3þ films were first prepared by a sol-gel method with a dip-coating technique. Although a subsequent hot water treatment was carried out, no significant change was observed in the film structure. We then conceived a composite structure where water-soluble components were embedded in the YVO4:Bi3þ,Eu3þ films. It was then found that an addition of aluminum sources in precursor solutions for preparing the YVO4:Bi3þ,Eu3þ films was effective to produce disordered surface nanostructures after the hot water treatment. A mechanism of dissolution-reprecipitation in water was proposed according to structural and compositional analyses. Results of optical characterization demonstrated that our films worked as highly luminescent antireflective coatings.

’ EXPERIMENTAL SECTION Preparation of Films. A coating solution for YVO4:Bi3þ,Eu3þ phosphor films was prepared from pertinent metal acetates, metavanadate, a polymer additive, and solvents. A 0.864 mmol sample of yttrium acetate tetrahydrate (Y(CH3COO)3 3 4H2O, 99.9%; Wako Pure Chemical Industries, Japan), 0.096 mmol of europium acetate trihydrate (Eu(CH3COO)3 3 3H2O, 99.9%; Soekawa Chemical, Japan), 0.240 mmol of bismuth(III) acetate (Bi(CH3COO)3, 99.99þ%; Aldrich Chemical, USA), 1.20 mmol of ammonium metavanadate (NH4(VO3), 99.0%, Wako), and 1.20 g of poly(ethylene glycol) (average molecular weight: 9000, Merck, Germany) were dissolved in a mixed solvent composed of 8 mL of isopropyl alcohol (98%, Taisei Kagaku, Japan), 2.0 mL of acetylacetone (99.0%, Wako), and 2.0 mL of nitric acid (Taisei Kagaku). The resultant solution was aged at 60 °C for 3 h, and then a dark-green colored transparent coating solution was obtained. The coating solution was dip-coated on quartz glass substrates 1 mm in thickness using a Micro Speed Dip Coater (MS215, Asumi Giken Ltd., Japan) at a withdrawal speed of 0.800 mm/s. The coated

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substrates were dried at 90 °C for 10 min and then immediately annealed at 750 °C for 15 min in air followed by quenching to obtain crystalline phosphor thin films. The thickness of the films was controlled by repetitive coating, drying, and annealing. Another coating solution was prepared for composite YVO4:Bi3þ, Eu3þ films containing aluminum compounds. 1.20 mmol of basic aluminum acetate n-hydrate (Al2O(CH3COO)4 3 nH2O, Wako) was added further to the above-mentioned coating solution used for the YVO4:Bi3þ,Eu3þ films. The amount of Al (vanadium:aluminum = 1:1 in moles) had been optimized in our preliminary experiments. The deviation from this molar ratio resulted in inferior optical properties of the films. The same coating, drying, and annealing procedure was carried out as that described above to obtain the composite phosphor films (tentatively designated as Al-added YVO4:Bi3þ,Eu3þ films). A hot water treatment (HWT) for the films obtained was carried out by immersing them in 20 mL of ion-exchanged water at 80 °C for 30 min. In our preliminary experiments, we had examined variations of temperatures between 60 and 90 °C and duration between 0.5 and 60 min for the hot water treatment. The best results were obtained with the temperature of 80 °C and the duration of 30 min, which is described in this report. The films were finally dried at room temperature. Preparation of Powders. Powder samples were also prepared from the precursor solutions to examine changes occurring during the hot water treatment in detail. The coating solutions for the YVO4:Bi3þ, Eu3þ films and the Al-added YVO4:Bi3þ,Eu3þ films were concentrated in a rotary evaporator at 70 °C. Dried gels thus obtained were annealed at 750 °C for 30 min in air followed by quenching. The hot water treatment for the powder samples was carried out by immersing 0.3 g of them in ion-exchanged water at 80 °C for 30 min. The powders were collected by centrifugation and then dried at room temperature. Characterization. Crystalline phases of the films and powders were identified by X-ray diffraction (XRD) using an X-ray diffractometer (D8, Bruker AXS, Japan) with Cu KR radiation. The morphology and thickness of the films were observed with a field-emission scanning electron microscope (FE-SEM; S-4700, Hitachi, Japan). Energy dispersive X-ray analysis was performed with an equipment embedded in another field-emission scanning electron microscope (FE-SEM, Sirion, FEI, USA). Optical transmittance spectra were recorded with a UV-vis spectrophotometer (U-3300, Hitachi, Japan). Incidence angles of a light beam were varied to evaluate optical properties of the films precisely. P-polarized light and s-polarized light were separated by polarizing filters. Photoluminescence (PL) spectra were measured at room temperature with a spectrofluorophotometer (RF-5300PC, Shimadzu, Japan) using a xenon lamp (150 W) as a light source. To evaluate surface emissions exactly, the PL intensity was normalized by a unit area by using a black masking (see Figure S1, Supporting Information). Careful attention was paid to avoid any misinterpretation of the spectra possibly coming from the apparatus and measurement conditions. Emission scans were performed with a 1.5 nm band-pass emission slit. A filter was used to remove a second-order peak of the excitation light. The mechanical strength of the films was evaluated with scratching resistance by using a pencil hardness tester at a load of 750 g (see Supporting Information). The environmental durability of the films was tested under exposure to the weather outside the building. The photoluminescence intensity and the optical transmittance of the films were measured every 10 days for a period of 140 days.

’ RESULTS AND DISCUSSION The preliminary experiments revealed that the annealing temperature of 750 °C was enough high to crystallize the films and powders in a tetragonal YVO4 phase. Effects of the hot water treatment on the crystalline phase in the samples were then 2930

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Figure 3. Incidence-angle dependence of optical transmittance spectra of the Al-added YVO4:Bi3þ,Eu3þ film (a, c) before and (b, d) after the hot water treatment for (a, b) p-polarized and (c, d) s-polarized light.

Figure 1. XRD patterns of YVO4:Bi3þ,Eu3þ (without Al) and Al-added YVO4:Bi3þ,Eu3þ (with Al) recorded for (a) the films and (b) the powders before and after the hot water treatment (HWT).

Figure 2. Optical transmittance spectra of the YVO4:Bi3þ,Eu3þ (without Al) and the Al-added YVO4:Bi3þ,Eu3þ (with Al) films before and after the hot water treatment (HWT). A spectrum of the bare quartz glass substrate is also shown for comparison.

examined. Figure 1 compares XRD patterns of the YVO4:Bi3þ, Eu3þ and the Al-added YVO4:Bi3þ,Eu3þ films and powders before and after the hot water treatment. The number of coating times was once for these films. All the patterns can be indexed with the tetragonal YVO4 phase (JCPDS No. 17-341). In spite of the addition of basic aluminum acetate n-hydrate in the solutions, there appears no peak related to aluminum compounds in the Al-added YVO4:Bi3þ,Eu3þ films and powders. However, the presence of aluminum was confirmed by the energy dispersive X-ray analysis and its relatively low crystallinity was verified in a sol-gel-derived alumina gel powder, as discussed later. As to the YVO4 phase, no significant change is observed before and after the hot water treatment for all the

samples. This indicates that YVO4 is not reactive with water and the crystal structure of the samples is maintained, which is of prime importance for the occurrence of red emissions from the doped Eu3þ ions. Transparency of the films was evaluated first by measuring optical transmittance at normal incidence. Figure 2 shows optical transmittance spectra of the YVO4:Bi3þ,Eu3þ and the Al-added YVO4:Bi3þ,Eu3þ films before and after the hot water treatment. A spectrum of the quartz glass substrate is also recorded for comparison. Because of surface reflection, the transmittance of the SiO2 glass (n = 1.51) is approximately 92-93% in the measured wavelength range. When the substrate is coated with the YVO4:Bi3þ,Eu3þ film, the transmittance is decreased due to the increased reflection with the higher refractive index of YVO4 (theoretically, n = 2.01). After the hot water treatment, the spectrum coincides with that before the treatment, indicating that the film structure was not changed against the optical properties. In contrast, the Al-added YVO4:Bi3þ,Eu3þ film exhibits a remarkable change by the hot water treatment. As shown in Figure 2, the transmittance of the hot-water treated, Aladded YVO4:Bi3þ,Eu3þ film exceeds that of the substrate, which is attributed to appearance of an antireflective effect. Two plausible reasons for this antireflective effect found in the highrefractive index material are the homogeneously increased porosity or the gradually changed density of the films. The former brings the lower refractive index according to the extended Lorentz-Lorenz formula, while the latter leads to the gradient refractive index. To clarify this obscurity, we investigated an angular dependence of optical transmittance of the Al-added YVO4:Bi3þ,Eu3þ films before and after the hot water treatment. Results are shown in Figure 3 for p-polarized and s-polarized light with five incidence angles between 0 and 60°. Note that the angle of 0° corresponds to the normal incidence. Transmittance of the film for p-polarized light (Figure 3a) before the hot water treatment increases with increasing the incidence angle up to 45° in a whole 2931

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Figure 4. Optical transmittance spectra of the thicker YVO4:Bi3þ,Eu3þ (without Al) and the thicker Al-added YVO4:Bi3þ,Eu3þ (with Al) films, which were obtained by repeating the coating procedure five times, before and after the hot water treatment (HWT). The spectrum of the bare quartz glass substrate is also shown for comparison.

Figure 5. FE-SEM images of the YVO4:Bi3þ,Eu3þ films (a, b) before and (c, d) after the hot water treatment, and the Al-added YVO4:Bi3þ, Eu3þ films (e, f) before and (g, h) after the hot water treatment. These films were obtained by repeating the coating procedure five times.

wavelength range. This phenomenon is due to a decrease in reflectance of p-polarized light up to the Brewster’s angle. On the contrary, the film after the hot water treatment shows almost the

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same transmittance spectra regardless of the incidence angle for p-polarized light (Figure 3b). For s-polarized light, the film before the hot water treatment shows a large decrease of the transmittance with increasing the incidence angle (Figure 3c), which is reasonably explained by the increased reflection.25 The film after the hot water treatment keeps higher transmittance against the varied incidence angle (Figure 3d). This smaller angular dependence of transmittance is a typical feature of films having graded refractive index.27 Next we fabricated thicker films by repeating the coating procedure five times to observe the morphology and thickness of the films. This experiment had also the aim to enhance the photoluminescence intensity as described later. Figure 4 shows optical transmittance spectra of the resultant thick YVO4:Bi3þ, Eu3þ and Al-added YVO4:Bi3þ,Eu3þ films before and after the hot water treatment. Generally, optical transmittance spectra of thicker films exhibit interference when they are uniform throughout the depth direction and have a constant refractive index.1 Actually the YVO4:Bi3þ,Eu3þ film shows interference fringe, and the spectral structure does not change before and after the hot water treatment. The Al-added YVO4:Bi3þ,Eu3þ film also shows clear interference fringe before the hot water treatment, indicating the optical uniformity of the film. The hot water treatment is effective to eliminate this interference, as evidenced by the increased transmittance of the Al-added YVO4:Bi3þ,Eu3þ film with the antireflective effect. These results demonstrate that we have succeeded in fabricating the highly transparent thick phosphor film having the gradient refractive index. This is of fundamental significance for managing to enhance photoluminescence properties and keep the high transparency at the same time. The increased film thickness is also effective to increase the absorption of excitation light in the transparent phosphor films. Effects of the hot water treatment on the film microstructure were examined by FE-SEM observation. Panels a and b in Figure 5 are surface and cross-section FE-SEM images, respectively, of the YVO4:Bi3þ,Eu3þ film before the hot water treatment. The surface appears granular with accumulated particles approximately 50 nm in size. The thickness is observed to be 239 nm after five repetitions of the coating procedure. As expected from the optical characterization, the microstructure of the film does not show any significant change after the hot water treatment as shown in panels c and d in Figure 5. The change in the thickness (235 nm after the hot water treatment) is also negligible, implying that the YVO4:Bi3þ,Eu3þ film does not react with water at 80 °C. A dramatic change is observed in the Al-added YVO4:Bi3þ,Eu3þ films by the hot water treatment. Before the hot water treatment, the Al-added YVO4:Bi3þ,Eu3þ film exhibits a relatively flat and smooth surface as shown in panel e in Figure 5. The difference in the surface morphology between the films without and with the Al addition may come from the precipitation and segregation of amorphous or nanocrystalline aluminum compounds at grain boundaries of the YVO4:Bi3þ, Eu3þ particles. It is then suggested that the Al-added YVO4:Bi3þ, Eu3þ film contains both YVO4:Bi3þ,Eu3þ particles and aluminum compounds. Panel f in Figure 5 reveals that the thickness of the Al-added YVO4:Bi3þ,Eu3þ film (300 nm) is slightly larger than that of the YVO4:Bi3þ,Eu3þ film. This explains the different interference fringes found in Figure 4. Panel g in Figure 5 shows a dramatically changed surface morphology with disordered nanostructures composed of thinner flakes. As mentioned above, the film with this microstructure has the antireflective effect without the occurrence of light scattering. The film thickness is considerably 2932

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Figure 7. XRD patterns of the alumina gel powders, which were obtained by annealing at 750 °C, before and after the hot water treatment.

Figure 6. Energy dispersive X-ray analysis line scans performed for Al and V across the cross-section in the direction from the substrate to the surface for the thicker Al-added YVO4:Bi3þ,Eu3þ films (a) before and (b) after the hot water treatment.

increased up to 562 nm as found in panel h in Figure 5. Moreover, the film is porous near the surface and dense at the interface with the substrate, resulting in a continuous decrease in the film density like the pseudo moth-eye structure. This observation agrees with the interpretation of the optical transmittance spectra that indicate the gradual change in the refractive index. A compositional analysis was carried out for the Al-added YVO4:Bi3þ,Eu3þ films before and after the hot water treatment. Figure 6 depicts results of energy dispersive X-ray analysis line scans across the cross-section in the direction from the substrate to the film surface. Vanadium was chosen as a marker for the YVO4:Bi3þ,Eu3þ phase. Before the hot water treatment, aluminum and vanadium distribute homogeneously throughout the film. Notably, the elemental distribution is changed after the hot water treatment with the amount of aluminum increasing from the substrate to the film surface. Tadanaga28 reported the fabrication of alumina gel films with a flower-like structure by immersing sol-gel-derived alumina gel films in boiling water. The flower-like structure consisted of boehmite (AlOOH) nanocrystals, which were supposed to be formed with the dissolution-reprecipitation process through the reaction between the sol-gel Al2O3 films and H2O. We consider that a similar reaction takes place in our Al-added YVO4:Bi3þ,Eu3þ films with the possible Al2O3/YVO4:Bi3þ,Eu3þ composite structure. Since YVO4 is not reactive with water, nearly amorphous Al2O3 is once dissolved in water and reprecipitated on the film surface during the hot water treatment. The inhomogeneous distribution of aluminum in the film after the hot water treatment is a result of the reprecipitation of the boehmite crystals. To confirm the validity of this mechanism, we prepared alumina powders by annealing alumina gels at 750 °C for testing the effect of the hot water treatment. The alumina gels had been obtained by concentrating a solution consisting of basic aluminum acetate

n-hydrate, poly(ethylene glycol), isopropyl alcohol, acetylacetone, and nitric acid at 70 °C. Figure 7 shows XRD patterns of the alumina powders before and after the hot water treatment. The pattern of the alumina powder before the treatment indicates the considerably low crystallinity with only two distinguishable peaks corresponding to a γ-Al2O3 phase (JCPDS No. 50-741). This also explains the absence of diffraction peaks due to aluminum compounds in Figure 1. The pattern of the powder after the hot water treatment shows clearly diffraction peaks corresponding to an AlOOH phase (Boehmite, JCPDS No. 21-1307). The peaks due to the γ-Al2O3 phase are also observed. It is then suggested that the flake-like particles precipitated on the surface of the Aladded YVO4: Bi3þ,Eu3þ film after the hot water treatment are the mixture of boehmite and γ-Al2O3. In the common photoluminescence measurement, emission intensities are determined by the number of those photons that are reaching the detector from every part of the films. In the present case, the intensity per unit surface area (see the experimental technique in the Supporting Information) is the measure that tells how much light is extracted from the film surface without propagating to the substrate edge. Figure 8 compares photoluminescence emission spectra (λex = 290 nm) and excitation spectra (λem = 619 nm) of the YVO4:Bi3þ,Eu3þ and the Aladded YVO4:Bi3þ,Eu3þ films before and after the hot water treatment. The photoluminescence emission spectra of all the films exhibit similar sharp peaks, which originate from electronic transitions of Eu3þ, at 593 nm for the 5D0 f 7F1 transition and 615 and 619 nm for the 5D0 f 7F2 transition.29 The photoluminescence excitation spectra of the films show a relatively broad excitation band ranging from 220 to 370 nm. This band is assigned to the overlap of the VO43- absorption and the charge transfer transition from O2- to Eu3þ. That is, the excitation band due to the charge transfer from oxygen ligands to the central vanadium atom inside the VO43- ion is located at 272 nm, and the O2- f Eu3þ charge transfer band is located at 290 nm.30,31 Furthermore, the band is broadened up to 370 nm by the chargetransfer state involving Bi3þ and neighboring oxygen in (Y, Bi)VO4. For the YVO4:Bi3þ,Eu3þ films, the photoluminescence intensity does not change by the hot water treatment, as expected from the unchanged optical transmittance and microstructure. The Al-added YVO4:Bi3þ,Eu3þ film before the treatment shows higher photoluminescence intensity than the YVO4:Bi3þ,Eu3þ film because of the increased film thickness from 239 to 300 nm. It is known that the film thickness can be controlled by the viscosity and concentration of coating solutions and the 2933

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from physical factors and is explained as follows. First, the absorption of the excitation light is increased by decreasing the surface Fresnel reflection of the phosphor film (excitation enhancement). Second, total internal reflection of the emission light is reduced by the gradual change in the refractive index (emission enhancement). Figure 9 shows photographs of the films under illumination with the fluorescent lamp and the UV lamp. The variation of brightness of the red luminescence agrees with the photoluminescence intensity shown in Figure 8. On the assumption of practical applications, we examined the mechanical strength and the environmental durability of the Aladded YVO4:Bi3þ,Eu3þ film after the hot water treatment. It was shown that the film had 4B pencil hardness against scratching. Although annealing the film at high temperatures could improve the hardness to some extent (see Figure S2, Supporting Information), we still need to find techniques for increasing the scratch resistance. On the other hand, the film exhibited the excellent environmental durability (see Figure S3, Supporting Information). The photoluminescence intensity maintained approximately 94% of its initial value after 140 days. The optical transmittance was hardly changed during this period. Since the present liquid processes based on the sol-gel coating and the hot water treatment are easily adoptable to large area substrates, we believe that the luminescent antireflective coatings will find industrial applications such as spectral converters for many kinds of light sources in the near future. Figure 8. (a) PL emission and (b) PL excitation spectra of the thicker YVO4:Bi3þ,Eu3þ (without Al) and the thicker Al-added YVO4:Bi3þ, Eu3þ (with Al) films, which were obtained by repeating the coating procedure five times, before and after the hot water treatment (HWT).

Figure 9. Photographs comparing the brightness of the red luminescence from the thicker YVO4:Bi3þ,Eu3þ (without Al) and the thicker Aladded YVO4:Bi3þ,Eu3þ (with Al) films, which were obtained by repeating the coating procedure five times, before and after the hot water treatment (HWT), under the illumination with the fluorescent lamp and the UV (λ = 300 nm) lamp.

withdrawal speed in the dip-coating procedure. The addition of the aluminum source to the coating solution for the YVO4:Bi3þ, Eu3þ films seems to increase the concentration of the coating solution. After the hot water treatment, the photoluminescence intensity of the Al-added YVO4:Bi3þ, Eu3þ film is much increased as seen in both the emission and the excitation spectra. The integrated photoluminescence intensity of the Al-added YVO4:Bi3þ,Eu3þ film after the hot water treatment is 2.34 times higher than that of the film before the treatment. Judging from the XRD analysis (Figures 1 and 7), YVO4:Bi3þ,Eu3þ reacts neither with the aluminum compound nor water, which excludes the possibility of chemical modification of the phosphor crystal. The increased photoluminescence intensity therefore comes

’ CONCLUSIONS The YVO4:Bi3þ,Eu3þ films with a pseudo-moth-eye structure consisting of flake-like particles were fabricated by the addition of the aluminum source to the coating solution and the hot water treatment of the annealed films. The gradient density and refractive index of the films were evidenced by the microstructural observation and the optical characterization. As a result, the films exhibited the antireflective effect in spite of the high refractive index of the YVO4 crystal. The suppression of surface Fresnel reflection and total internal reflection led to the enhanced photoluminescence properties in the highly transparent phosphor thin films. Our coatings can therefore work multifunctionally and will find applications, for example, in spectral irradiance converters. Further work is ongoing to apply the developed technique to other phosphor materials which have different excitation and emission properties. ’ ASSOCIATED CONTENT

bS

Supporting Information. The experimental technique for the photoluminescence measurement (Figure S1), the evaluation of the mechanical strength (Figure S2), and the environmental durability test of the film (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors thank Dr. K. Tadanaga (Osaka Prefecture University, Japan) for the fruitful discussions. 2934

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dx.doi.org/10.1021/la200149y |Langmuir 2011, 27, 2929–2935