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Abstract
A series of zinc oxide nanorods (ZnO NRs) were grown by chemical bath deposition and their optical properties as a retarder for display devices were investigated. The effect of the precursors, growth time, and calcination time on the morphology and the optical properties was examined. The vertically-grown ZnO NRs layer represented positive c-plate property. We simulated the antireflection property of a quasi-circular polarizer containing the ZnO NRs layer for an organic light emitting diode (OLED) display. The quasi-circular polarizer with the ZnO NRs layer showed better antireflection property than the polarizer containing no ZnO NRs layer.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Corrections
8 December 2020: A correction was made to add a link to the Supplemental document.
1. Introduction
The zinc oxide (ZnO) has been widely used as an attractive semiconductor material with high mobility and small reverse current [1–2]. This material plays an important role in many scientific fields due to its excellent electrical, optical, electrooptical, mechanical, and chemical properties. By changing the synthesis conditions, various morphologies of ZnO could be obtained such as microspheres [3], nanoparticles [4], nanorods [5], nanotubes [6], nanobelts [7], nanoneedles [8], nanopellets [9], nanoplates [10], nanoflower [11], snowflakes [12], etc. Among various kinds of ZnO allotropes, the ZnO nanorods (NRs) has attracted much attention because of its wide applications in light emitting diodes and lasers [13,14], light detectors [15], photocatalytic applications [16], solar cells [17], gas sensors [18], and power nanogenerators [19]. Because ZnO is optically transparent, it also has a great potential in various kinds of electrooptical applications [1, 2]. The wide band-gap (approximately 3.37 eV) property of ZnO NRs can be used for short wavelength electrooptical applications. The high exciton binding energy (approximately 60 meV) of ZnO is suitable for room temperature UV laser fabrication [2].
On the other hand, the application of the ZnO nanomaterials as display components has been rarely reported. In this paper, we synthesized ZnO NRs and examined their optical properties as an optical retarder. The vertically-grown ZnO NRs showed + c plate property due to its form birefringence [20,21]. We used the + c ZnO NRs layer as a retarder film for the quasi-circular polarizer (CP) which is used for eliminating the external reflection from the organic light-emitting diode (OLED) displays [22,23]. In these days, the display devices are rapidly integrated with various functions and sensors. The retarder films are desirable to be embedded inside of the display devices using similar processes for the construction of the active layer. Hence, the traditional retarder films which are fabricated by stretching of polymer films needs to be replaced to the scalable process which can be merged with the active layer process of display devices.
Because the mother glass length of the display devices is over 2 m, scalability of ZnO NRs are essential with a viewpoint of commercialization. Many approaches have been developed for growing ZnO NRs; pulsed-laser ablation [24], pulsed-laser deposition [25], vapor-liquid-solid deposition [26], chemical bath deposition [27], chemical vapor deposition [28], vapor-phase transport [29], galvanostatic deposition [30]. Certainly, the chemical bath deposition technique has better scalability and is suitable for the display application [31,32]. The chemical bath deposition technique includes two following steps: the formation of ZnO seed layer on substrates and the growth of ZnO NRs from the seed layer in aqueous solution of zinc precursors. The morphologies and physicochemical properties of ZnO NRs can be changed depending on the synthesis conditions [32–35]. Because the optical property such as the refractive index strongly depends on the morphology of the NRs, the relation between the synthesis condition and the optical properties needs to be investigated.
This paper is consisted of two parts. The first part is about the synthesis process of ZnO NRs on quartz substrate using the chemical bath deposition technique with zinc acetate and zinc nitrate precursors. The morphology, transparency, and retardation dispersion of ZnO NRs were investigated. The second part of the paper is about examination of the vertically-grown ZnO NRs as a retarder for the OLED display devices. The calculated results showed that the maximum and average reflectance of external light can reduced by 67 and 71%, respectively, by inserting the vertically-grown ZnO NRs layer.
2. Materials and methods
2.1. Materials
Zinc acetate dihydrate (Zn(CH3COO)2.2H2O, 5970-45-6), 2-methoxyethanol (CH3OCH2CH2OH, 109-86-4), zinc nitrate hexahydrate (Zn(NO3)2.6H2O, 10196-18-6), hexamethylenetetramine (C6H12N4, 100-97-0), ethanolamine (NH2CH2CH2OH, 141-43-5) were purchased from Sigma-Aldrich. Quartz wafer (2 inch, 500µmT, 29-01502-01) was obtained from iNexus, Inc (South Korea).
2.2. Synthesis of ZnO NRs/quartz wafer
2.2.1. Preparation of ZnO seed layer/quartz wafer
Preparation of zinc precursor solution: The solution was obtained by dissolving 2.2 g zinc acetate dihydrate [Zn(CH3COO)2.2H2O] as a precursor and 0.61 g ethanolamine (MEA) as a stabilizer to 100 ml of 2-ethoxyethanol. The prepared solution was stirred at 60 °C for 2 hours until a homogenous and transparent solution was obtained. Then, the solution was aged for 24 h at room temperature before using.
Preparation of ZnO seed layer: Quartz wafers were cut into 4 pieces/wafer and washed with 100 ml of acetone and 100 ml of ethanol with assistance of sonicator for 10 min to remove all oil on the surface. Then, the wafers were rinsed with deionized water 3 times and dried with compressed air. Few drops of zinc precursor solution were spread onto the cleaned wafer and then spin-coated at 750 rpm for 10s and 1500 rpm for 3s. After the spin coating, the wafer was dried at 100 °C for 1 min on a hot plate. The coating process was repeated twice to obtain homogenous ZnO seed layer. The coated wafer was annealed at 300 °C in a furnace for 1 h under air environment to remove organic residuals.
2.2.2. Growth of ZnO NRs
ZnO NRs were grown in the zinc acetate or the zinc nitrate aqueous solutions with the presence of hexamethylenetetramine (HMT). The concentrations of zinc ions and HMT in solutions were maintained at 16 mM and 25 mM, respectively. The coated wafer was vertically oriented with a Teflon holder and then immersed into a Teflon autoclave containing 150 ml of the prepared solutions. During the growth process, the prepared solutions were heated to 90 °C in oven and kept for 3, 4, 5 and 6 h to evaluate the effect of growth time. Then, the samples were rinsed with deionized water 3 times to remove residual salts from the surface and dried with compressed air. Prepared ZnO NRs samples from zinc nitrate and zinc acetate were named ZnON-NT and ZnON-AC, respectively, hereinafter.
2.3. Material characterization
Scanning electron microscopy (SEM, Hitachi S-4300SE, Hitachi, Ltd) was employed to analyze the surface morphology of the ZnO NRs. The structural properties of the ZnO NRs were characterized by X-ray diffraction (XRD, Rigaku Ultima III, Rigaku) with a Cu Kα radiation (λ = 1.54 Å). The morphology on the surface and the thickness of ZnO NRs were measured by atomic force microscopy (AFM, XE7, Park system). The transparency of samples was measured by a UV-VIS spectrophotometer (UV-2600, Shimadzu Corporation).
The out-of-plane retardation (Rth) dispersion was obtained by measuring the retardation (Re) at different incident angles θ from -30 ° to 30 ° with a step of 1 ° [36]. Rth is given by,
The refractive indices nx, ny, and nz were measured by fitting the measurement data, and the Rth was obtained from the refractive indices.
3. Results and discussion
3.1. Thickness and transparency of the ZnO seed layer
Figure 1 shows the surface topography and the transmittance (TR) of the ZnO seed layer. The 19-nm-thick ZnO seed layer was observed using AFM with the RMS of less than 2 nm, as shown in Fig. 1(a). Spin coating process results in a thinner seed layer as compared to the dip-coating process at the same concentration of zinc acetate precursor (0.1 M) [37]. TR was measured with the UV-Vis spectroscopy and it was slightly decreased less than 1% as shown in Fig. 1(b).
Fig. 1. (a) Cross section height profile of ZnO seed-coated wafer measured by AFM, (b) Transmittance of quartz and ZnO seed-coated wafer in the visible wavelength range.
3.2. Effect of zinc precursors and growth time on the thickness and optical properties of ZnO NRs
The top-view SEM images of the ZnO NRs synthesized with different zinc precursors are shown in Fig. 2 and Fig. S1. The morphology of ZnON-NT is clearly different from that of ZnON-AC. ZnO NRs grew in the random order with zinc nitrate precursor, while they gathered to small groups with zinc acetated precursor. The diameter of ZnO NRs is about 50 nm in both precursors. Because the diameter of ZnO NRs strongly depends on the grain size of the ZnO seed layer [38], therefore, the same ZnO seed preparation process is the reason of the similar diameter in both precursors. The oblique-view SEM images at 60 ° [ Fig. 3 and Fig. S2] also show the different morphologies of ZnO NRs between using zinc acetate precursor and using zinc nitrate precursors. The increasing size of gathered nanorods groups of ZnON-AC samples can be observed in both top-view and oblique view with the increase of growth time. Otherwise, the size of nanorods of ZnON-NT samples is similar at different growth time.
Fig. 2. The top-view SEM images of the ZnO NRs synthesized with different zinc precursors. (a), (b): the top-view SEM images of ZnON-NT samples at growth time of 3 h, and 6 h, respectively; (c), (d): the top-view SEM images of ZnON-AC samples at growth time of 3 h, 6 h, respectively. Scale bars correspond to 500 nm.
Fig. 3. The side-view (60 °) SEM images of the ZnO NRs synthesized with different zinc precursors. (a), (b): the oblique-view SEM images of ZnON-NT samples at growth time of 3 h, 6 h, respectively; (c), (d): the oblique-view SEM images of ZnON-AC samples at growth time of 3 h, 6 h, respectively. Scale bars correspond to 500 nm.
For investigating the size increase of ZnO NRs, the morphology on the surfaces of samples was characterized by AFM [ Fig. 4]. The period frequency of ZnO NRs was estimated via Gaussian function. The period frequencies of ZnON-NT samples were dispersed with the broad bands in all reaction time, while the period frequencies of ZnON-AC samples were shifted to lower values with the increase of reaction time. These results means that the size of ZnO NRs in the ZnON-NT sample is more homogenous and greater than the ones in ZnON-AC samples. These results are also consistent with SEM images.
Fig. 4. The period frequency of (a) ZnON-NT and (b) ZnO-AC samples counted from the AFM topography data.
The crystal structures of the ZnO nanoseeds and the ZnO NRs samples were investigated through XRD in the range of 2θ from 10 to 80 ° [ Fig. 5]. Only one broad peak of amorphous SiO2 wafer at 2θ=21.7 ° was observed in the ZnO nanoseeds [39]. All peaks of the ZnO NRs samples are consistent with the standard peaks of ZnO and presented a significantly strong peak between 34.5 ° to 34.6 ° at [002] plane. Other peaks had very low intensity to compare to the ones in [002] plane. These results mean that all ZnO NRs samples possessed hexagonal wurtzite structures with very coherent c-axis orientations. The element components of ZnO NRs samples at growth time of 6 h were characterized by EDS [Fig. S3]. The samples composed O and Zn from ZnO NRs, Si and O from quarzt wafer, C from carbon tape, and Pt from sputtered layer to enhance the conductivity of samples before measuring. The results confirms that pure ZnO NRs were synthesized using both zinc acetate and zinc nitrate precursors.
The thickness of the ZnO NRs layer was measured by AFM [ Fig. 6]. In both precursors, the thickness linearly increased with the growth time. The growth velocity of ZnO NRs using zinc acetate as precursor is faster than that using zinc nitrate as precursor. The different growth velocity using zinc nitrate and zinc acetate precursors can be explained by role of anion in the growth solution. In the solution, Zn2+ reacts with OH-, released from the hydrolysis of HMT, to form dissociated Zn(OH)42-. Zn(OH)42- moves to ZnO NRs, assembles and decomposes to ZnO for the growth of NRs [40,41]. Zinc nitrate released nitrate anion (NO3-), while zinc acetate released acetate anion (CH3COO-). Acetate ion, a kind of surfactant, decreased the tension and the surface energy of water [42] that enhance the dissociated Zn(OH)42- to ZnO NRs. In contrast, nitrate ion increased the tension and the surface energy of water [42] that prevent the dissociated Zn(OH)42- from ZnO NRs [40].
Fig. 6. (a) Surface topography of ZnON-NT samples, (b) ZnON-AC samples measured by AFM, (c) Thickness of ZnO NRs layer versus the growth time. Error in Fig. 6(c) was less than 5%.
Figure 7 corresponds to TR of the ZnO NRs samples. TRof the ZnO NRs layers significantly decreased with the increase of growth time in both precursors. TR of the ZnON-AC samples strongly decreased with growth duration to compare to that of ZnON-NT because of the faster increase of ZnO NRs layer thickness using zinc acetate as precursor. The visible light absorption of ZnO is strong at short wavelength and weak at long wavelength [43]. In ZnON-NT samples, TR of the samples with growth time of 4, 5 and 6 h are similar at long wavelength and decrease at short wavelength. It seems that the thickness of ZnON-NT samples increased with the growth duration, but their scattering kept similar. In contrast, the transparency of ZnON-AC samples significantly decreases in whole wavelength due to the bigger gathered groups playing as scattering centers (SEM images).
Rth of the ZnO NRs samples is shown in Fig. 8(a) and 8(b). Re was zero when the light was normally incident, which means no inplane birefringence (nx=ny). Re was increasing with θ, which means nz>nx,y and Rth<0. Therfore, the ZnO NRs layer corresponds to optically + c-plate (nz>nx=ny). The absolute magnitude of Re decrease with the increase of wavelength, i.e., positive dispersion was observed [21]. Re increased with the growth time or thickness in both precursors. The relation between Rth and the layer thickness at wavelength of 550 nm is shown in Fig. 8(c). In ZnON-NT samples, the length of NRs increase holding similar growth time. Thus, the absolute magnitude of Re linearly increases with growth time.
Fig. 8. (a) The retardation dispersion of ZnON-NT samples, (b) The retardation dispersion of ZnON-AC samples NRs, (c) The retardation values at wavelength of 550 nm versus ZnO NRs layer thickness using different precursors.
The origin of the retardation in ZnO NRs layers seems relate to the form birefrigence. The compostion of isotropic medium with uniformly distributed aligend particles can be shown the birefringence [20,21]. The ZnO NRs layer can be assume as composition of parallel aligned cylinderical ZnO NRs in air. The composite medium that consist with parallel cylinders could be an uniaxial retarder and optical axis is along the c-axis of the cylinder. Thefore, the ZnO NRs can work as c-plate beacuse ZnO NRs are aligned perpendicular to the quartz surface (z-axis).
3.3. Effect of calcination on the optical properties
The morphology, transparency and retardation dispersion of ZnO NRs after calcination at 300 °C for 30 min were investigated [41]. The SEM images of the ZnON-NT and the ZnON-AC samples at 3 h before and after calcination are shown in Fig. 9. No difference point can be observed in both samples.
Fig. 9. The top-view SEM images of samples grew for 3 h. (a) ZnON-NT before calcination, (b) ZnON-NT after calcination, (c) ZnON-AC before calcination, (d) ZnON-AC after calcination. Scale bars correspond to 500 nm.
The crystal structure of the ZnO NRs samples after calcination were characterized through XRD [ Fig. 10]. The sharp peak in the (002) plane is still observed at 2θ=34.55 °. This means that the c-axis orientation was not changed by calcination. In addition, the intensities of other peaks at 31.9 °, 36.4 °, 47.6 °, 62.9 °, and 72.6 °, corresponding to (100), (101), (102), (110) and (103), respectively, increased in all samples. This result means that the particle sizes of ZNO NRs increase after calcination due to coalescence of small crystallites [41]. However, TR of ZnO NRs did not change after calcination [ Fig. 11]. The reason is the forming of perfect structure of ZnO NRs crystals (XRD pattern) during the growing time in hydrothermal conditions. Therefore, there is few defect in the structure and the calcination has less affect on reparing the structure, which helps increase TR.
Fig. 11. The transparency of ZnO NRs samples before and after calcination. (a) ZnON-NT, (b) ZnON-AC; Dot line (
Figure 12 shows Re of the ZnO NRs samples before and after calcination. The absolute value of Re was slightly increased at whole measured wavelengths after calcination [Fig. 12]. The calcination process presumably enhances the anisotropy of the ZnO NRs, resulting in increase of refractive index along the surface normal direction.
Fig. 12. The retardation dispersion of ZnO NRs NRs samples before and after calcination. (a) ZnON-NT, (b) ZnON-AC; Dot line (
3.4. Application to the display retarder film
Let us move on the application of ZnO NRs as the display retarder film. Recently, the OLED displays have been widely used in various area. Because a metal cathode layer is located inside of the OLED panel, the external light is reflected, degrading the image quality. In order to elliminate the reflection of the external light, an antireflection (AR) film which is optically a quasi-circular polarizer is attached on the OLED display panel. The quasi-circular polarizer is composed of a linear polarizer (LP) and a quarter wave plate (QWP) [21]. The external light is circular polarized after passing through the LP and the QWP. The handedness of the circular polarized light is reversed after reflection on the metal cathode. Then, the reversely-circular polarized light is finally absorbed in the LP. This is certainly valid for the normally incident light. However, Re of an obliquely incident light is dependent on the incident angle θ as shown in Eq. (2) and the external light cannot be completely eliminated.
Meanwhile, one can recognize that Re can be independent of θ provided that the ($n_\textrm{z}^2 - {n_\textrm{x}}{n_\textrm{y}}$) term becomes zero in Eq. (2). This condition is approximately equivalent to zero Rth condition [21]. Thus, the external light can be completely removed regardless of the incident angle provided that Rth of the QWP is zero. Generally, the QWP layer is optically + a plate whose slow axis is located in the surface plane. Rth of the + a plate is also positive and its magnitude is half of inplane Re. On the other hand, the ZnO NRs is optically + c plate whose refractive index along the surface normal direction nz is greater than the others. Thus, Rth of the + c plate is negative. Hence, the net Rth of the QWP can be reduced by stacking the ZnO NRs on the QWP layer. Practically, Rth of the ZnO-stacked QWP can be zero by adjusting the thinkness of the ZnO layer.
We calculated the reflectance from the OLED panels where the conventional CP film and the CP film with the ZnO NR layer were attached [ Fig. 13]. The reflectance was simulated by a commercial program TechWiz LCD 1D (Sanayi System). We used Jones matrix method for the calculation [44]. The details of optical backgrounds used in the simulation was described in the supplement materials. A metal reflector, QWP, LP were consecutively stacked from bottom to top. The transmission axis of polarizer was at 0 ° and the optic axis of QWP was at 45 °. The refractive indices ne, no and thickness of QWP were 1.5, 1.4 and 1375 nm, respectively. The structure of the CP film with ZnO NR layer is same with the conventional CP film, while the ZnO NR layer is inserted between QWP and LP. The refractive indices nz, nx=ny, and thickness of the ZnO NR were 2.15, 1.85 and 188.33 nm, respectively. Rth of ZnO NR layer was 56.50 nm. We used the refractive indices and the thickness of the fabricated ZnO NR layer whose Rth was closest to the half of Re.
Fig. 13. Polar contour of reflectance in OLED AR films consisted of (a) conventional LP + QWP, and (b) LP + ZnO NR + QWP layers.
As shown in Fig. 13, the reflection from the OLED panel was much decreased after inserting the ZnO NR layer. The maxiumum light leakage and the average reflectance of the conventional CP film were 7.3 and 1.4%, respectively, while those of the proposed CP film with ZnO NRs were 2.0 and 0.4%, respectively [Fig. 13]. Thus, the maximum and average reflectance reduced by 67 and 71%, respectively. The viewing angle dependence of the reflectance was also much improved by introduction the ZnO NR layer.
In order to clarify the optical principle for the reduction of the viewing angle dependence of the reflectance, we analyzed the polarization state of light using Poincare sphere [ Fig. 14]. The external light is incident at 70 ° in Fig. 14. The blue and red line and dot represent polarization state before and after reflection, respectively. After passing through the polarizer the polarization state is at S1 axis. The polarization state of light is rotated by the phase retardation of QWP in counter clockwise (CCW) direction around the optic axis. When the light is obliquely incident, the optic axis of QWP is tilted and the retardation is changed. As a result, the polarization state deviates from the S1 axis resulting in light leakage [Fig. 14(a)]. On the other hand, the ZnO NR layer rotates the polarization state after QWP in CCW direction along the meridian of the Poincare sphere. Consequently, final polarization state after passing through QWP became closer to S1 axis hence the light leakage is decreased [Fig. 14(b)].
Fig. 14. Change of the polarization state of external light on Poincare sphere. (a) QWP and (b) QWP with the ZnO NR layer attached on the OLED panel.
4. Conclusions
In this paper, we synthesized ZnO NRs and investigated their optical properties as a display retarder. We investigated the effect of the growth conditions on the morphology and the optical property of the ZnO NRs layer. The ZnO NRs which were vertically grown on the quartz wafer showed + c plate property with a negative sign of Rth. The birefringence of the ZnO NRs was about 0.3 which was large enough for the display application with smaller thickness about 50 nm. The antireflection property of the quasi-circular polarizer where the ZnO + c plate was inserted showed a smaller reflectance.
Funding
Ministry of Trade, Industry and Energy (20011031); National Research Foundation of Korea (2019H1A2A1074764, 2019R1A2B5B01069580, 2019R1A6A1A09031717).
Acknowledgements
This study was supported by the National Research Foundation (NRF, 2019R1A2B5B01069580, 2019R1A6A1A09031717, 2019H1A2A1074764), and Ministry of Trade, Industry, and Energy (20011031).
Disclosures
The authors declare no conflicts of interest.
See Supplement 1 for supporting content.
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): before calcination, solid line (
): after calcination.