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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 27 — Dec. 19, 2011
  • pp: 25935–25943
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Urchin-aggregation inspired closely-packed hierarchical ZnO nanostructures for efficient light scattering

Yeong Hwan Ko and Jae Su Yu  »View Author Affiliations


Optics Express, Vol. 19, Issue 27, pp. 25935-25943 (2011)
http://dx.doi.org/10.1364/OE.19.025935


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Abstract

We reported the enhancement of light scattering in the urchin-aggregation shaped closely-packed hierarchical ZnO nanostructures, fabricated by a simple and scalable process based on the hydrothermal method utilizing the silica microspheres monolayer as a two-dimensional periodic template. From theoretical predictions, the diffuse light scattering is closely related to the size of silica microspheres as light diffusion centers. Moreover, the ZnO nanorod arrays on silica microspheres monolayer provide the further enhancement of light scattering. The experimentally fabricated urchin-aggregation shaped ZnO nanostructures using silica microspheres of 970 nm indicated a high density of ZnO nanorods with a wide bending angle, which led to the largely increased photoluminescence intensity and a high transmittance haze ratio of > 70% in the wavelength range of 400-900 nm in keeping with a high total transmittance. The contact angles of a water droplet on the surface of the samples were also explored.

© 2011 OSA

1. Introduction

Meanwhile, sunlight (solar radiation) is becoming more important energy resource, so that many research efforts are focusing on the study of photon managements, i.e. collecting, trapping, and concentrating the sunlight in photovoltaic devices [18

18. B. V. Andersson, D. M. Huang, A. J. Moulé, and O. Inganäs, “An optical spacer is no panacea for light collection in organic solar cells,” Appl. Phys. Lett. 94(4), 043302 (2009). [CrossRef]

21

21. T. Minemoto, C. Okamoto, S. Omae, M. Murozono, H. Takakura, and Y. Hamakawa, “Fabrication of spherical silicon solar cells with semi-light-concentration system,” Jpn. J. Appl. Phys. 44(7A), 4820–4824 (2005). [CrossRef]

]. Recently, for collecting the sunlight, the absorption was maximized by the antireflection coating layer with tuning the shape and geometry at the surface of device [22

22. S. E. Han and G. Chen, “Optical absorption enhancement in silicon nanohole arrays for solar photovoltaics,” Nano Lett. 10(3), 1012–1015 (2010). [CrossRef] [PubMed]

25

25. Y. M. Song, S. J. Jang, J. S. Yu, and Y. T. Lee, “Bioinspired parabola subwavelength structures for improved broadband antireflection,” Small 6(9), 984–987 (2010). [CrossRef] [PubMed]

] and the light trapping or concentrating could be efficiently achieved by coating or embedding the metal nanoparticles which induce the localized surface plasmon resonance modifying the light path of photons or enhancing the light intensity near the metal nanoparticles [26

26. H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9(3), 205–213 (2010). [CrossRef] [PubMed]

29

29. J. Y. Lee and P. Peumans, “The origin of enhanced optical absorption in solar cells with metal nanoparticles embedded in the active layer,” Opt. Express 18(10), 10078–10087 (2010). [CrossRef] [PubMed]

]. In utilizing the ZnO nanostructures, there have been also intensive efforts to manage the photons of sunlight by reducing the surface reflection and enhancing the light scattering [30

30. Y. J. Lee, D. S. Ruby, D. W. Peters, B. B. McKenzie, and J. W. P. Hsu, “ZnO nanostructures as efficient antireflection layers in solar cells,” Nano Lett. 8(5), 1501–1505 (2008). [CrossRef] [PubMed]

33

33. Z. Jehl, J. Rousset, F. Donsanti, G. Renou, N. Naghavi, and D. Lincot, “Electrodeposition of ZnO nanorod arrays on ZnO substrate with tunable orientation and optical properties,” Nanotechnology 21(39), 395603 (2010). [CrossRef] [PubMed]

]. However, it is difficult to generate sufficient light scattering using only a vertically-aligned 1D ZnO nanostructure because of its high verticality and limited surface area [34

34. R. Tena-Zaera, J. Elias, and C. Lévy-Clément, “ZnO nanowire arrays: optical scattering and sensitization to solar light,” Appl. Phys. Lett. 93(23), 233119 (2008). [CrossRef]

]. In this paper, we demonstrated considerable improvements of the light scattering in urchin-aggregation shaped closely-packed ZnO nanostructures with a periodic alignment as 3D building blocks on indium tin oxide (ITO) coated glass by a simple procedure based on the dip-coated monolayer of silica microspheres and a subsequent wet-chemical growth of ZnO nanorods with a thin sputtered aluminum-doped ZnO (AZO) seed layer by hydrothermal method, together with theoretical analysis. For a deep understanding of urchin-aggregation shaped ZnO nanostructures on their optical characteristics, the finite difference time domain (FDTD) simulation was also performed [35

35. K. Yee, “Numerical solution of initial boundary value problems involving Maxwell’s equations in isotropic media,” IEEE Trans. Antenn. Propag. 14(3), 302–307 (1966). [CrossRef]

].

2. Simulation of urchin-aggregation shaped ZnO nanostructures

The structure as integrated building blocks should be properly optimized to modify the behavior of light because it is closely correlated to the light diffusion. Figure 1
Fig. 1 Calculated electric fields of light passing through the (a) ITO/glass, (b) silica microspheres (300 nm)/ITO/glass, (c) silica microspheres (1 μm)/ITO/glass, and (d) ZnO nanorods (0.8 μm height, 50 nm size)/silica microspheres (1 μm)/ITO/glass. The thickness of ITO film is fixed at 200 nm.
shows the calculated electric fields of light passing through the (a) ITO/glass, (b) silica microspheres (300 nm)/ITO/glass, (c) silica microspheres (1 μm)/ITO/glass, and (d) ZnO nanorods (0.8 μm height, 50 nm size)/silica microspheres (1 μm)/ITO/glass. The thickness of ITO films is fixed at 200 nm, which has been widely used as a TCO coating layer in photovoltaic and optoelectronic devices. In FDTD simulations, the amplitude of y-polarized electric field was calculated for the incident plane wave with a normalized Gaussian beam profile propagating in the z direction at λ ~600 nm. The refractive indices of glass, ITO, silica microspheres, and ZnO nanorods were 1.46, 1.88, 1.54, and 1.99, respectively. For the bare ITO coated glass in Fig. 1(a), the incident light propagated smoothly without any change of its propagation direction. This behavior confirms that the light interference rarely occurs. When the monolayer of silica microspheres was employed on ITO coated glass, however, the light interference patterns could be observed as shown in Fig. 1(b) and Fig. 1(c). For 300 nm silica microspheres on ITO/glass, the light interfered within nearly 1 μm distance from the surface, but it returned to be z-propagating plane wave at the longer distance, as can be seen in Fig. 1(b). It is found that the subwavelength (i.e. smaller than the wavelength of light) microspheres monolayer could not efficiently diffuse the light. This reason is that the subwavelength structure suppresses all but zeroth-order diffraction. As shown in Fig. 1(c), in contrast, the light passing through the 1 μm silica microspheres monolayer on ITO/glass evidently caused strong interference patterns at both near and far distances from the surface, which allows for efficient diffuse light scattering. This means that the structure with a longer period than the light wavelength is more beneficial to the light diffusion. The use of the ZnO nanorod arrays (NRAs) on the 1 μm silica microspheres monolayer (i.e. urchin-aggregation shaped ZnO nanostructures) further enhances the light interference and diffusing properties as shown in Fig. 1(d). Thus, the urchin-aggregation shaped ZnO nanostructures may act as light diffusion centers to highly improve the light scattering.

3. Results and discussion

3.1. Fabrication of urchin-aggregation shaped ZnO nanostructures

Figure 2
Fig. 2 Schematic diagram and SEM images of the fabrication process of urchin-aggregation shaped ZnO nanostructures on ITO/glass by the hydrothermal method using a thin sputtered AZO seed layer. The SEM images show (i) the monolayer of silica microspheres of 970 nm on ITO/glass, (ii) the deposited AZO on upper middle part of silica microspheres of 970 nm, and (iii) the ZnO NRAs on AZO/silica microspheres of 970 nm/ITO/glass.
shows the schematic diagram of the fabrication processes of urchin-aggregation shaped ZnO nanostructures on ITO/glass by the hydrothermal method using a thin sputtered AZO seed layer. The corresponding scanning electron microscope (SEM) images are also shown. The SEM images show (i) the monolayer of silica microspheres of 970 nm on ITO/glass, (ii) the deposited AZO layer on upper middle part of silica microspheres of 970 nm, and (iii) the ZnO NRAs on AZO/silica microspheres of 970 nm/ITO/glass. After cleaning the ITO/glass, the monolayer of silica microspheres was deposited by a dip coating technique [36

36. S. M. Yang, S. G. Jang, D. G. Choi, S. R. Kim, and H. K. Yu, “Nanomachining by colloidal lithography,” Small 2(4), 458–475 (2006). [CrossRef] [PubMed]

]. For an efficient deposition, the hydrophilic surface of ITO/glass was prepared. To form the monolayer of silica microspheres, the samples were vertically dipped into the silica microsphere colloidal solutions and then pulled up very slowly. The silica microspheres with different diameters of 320 nm, 540 nm, and 970 nm were used. As shown in Fig. 2, the silica microspheres monolayers were formed in a periodic two-dimensional hexagonal closely-packed arrangement. For growing the ZnO NRAs on silica microspheres by the hydrothermal growth, a thin AZO seed layer was deposited on the monolayer of silica microspheres by radio-frequency (RF) magnetron sputtering method. The middle upper part of silica microspheres could be finely covered by the AZO films due to the excellent step coverage ability in sputtering process as shown in the magnified SEM image of Fig. 2(i). Then, the AZO coated samples were dipped into the equimolar growth solution (10 mM zinc nitrate hydrate, 10 mM hexamethylenetetramine, 1 liter deionized water) at 85-88 °C for 10 hour. After drying the samples on a hot plate, the urchin-aggregation shaped ZnO nanostructures on the monolayer of silica microspheres on ITO/glass were fabricated.

Figure 3(a)
Fig. 3 (a) Top-view and cross-sectional SEM images of the ZnO NRAs on (i) AZO/ITO/glass and AZO/silica microspheres of (ii) 320 nm, (iii) 540 nm, and (iv) 970 nm/ITO/glass, and (b) PL spectra of the corresponding samples. The 2θ scan XRD patterns are also shown in the inset of (b).
shows the top-view and cross-sectional SEM images of the ZnO NRAs on (i) AZO/ITO/glass and AZO/silica microspheres of (ii) 320 nm, (iii) 540 nm, and (iv) 970 nm/ITO/glass. The thickness of AZO seed layer was approximately 20 nm, which was estimated from the calibrated thickness monitored by a quartz crystal oscillator. The photoluminescence (PL) spectra of the corresponding samples are shown in Fig. 3(b). The 2θ scan X-ray diffraction (XRD) patterns are also shown in the inset of Fig. 3(b). When the ZnO NRAs were grown on the flat AZO seed layer, they were vertically aligned with a dominant growing direction of c-axis in the wurzite structure as can be seen in the inset of Fig. 3(b) [32

32. J. Y. Chen and K. W. Sun, “Growth of vertically aligned ZnO nanorod arrays as antireflection layer on silicon solar cells,” Sol. Energy Mater. Sol. Cells 94(5), 930–934 (2010). [CrossRef]

]. While, the ZnO NRAs grown on silica microspheres were formed as the urchin-aggregation shaped architectures along the hexagonal closely-packed arranged monolayers. As the diameter of silica microspheres was increased, the ZnO nanorods were more densely integrated on the AZO surface covering the silica microspheres because of their larger surface area for growing ZnO NRAs. As can be seen in Fig. 3(b), the PL peak intensity was enhanced with increasing the diameter of silica microspheres due to the increased density of ZnO NRAs. From XRD patterns, the (100), (101), and (102) XRD peaks of ZnO were increased with increasing the diameter of silica microspheres. It is noticeable that the ZnO NRAs were well aligned with a wide range of bending angles because the ZnO nanorods were more radically extended over a large surface area during the growth. This may provide a superior morphology for efficient light scattering due to the wider bending angle of the aligned ZnO NRAs [33

33. Z. Jehl, J. Rousset, F. Donsanti, G. Renou, N. Naghavi, and D. Lincot, “Electrodeposition of ZnO nanorod arrays on ZnO substrate with tunable orientation and optical properties,” Nanotechnology 21(39), 395603 (2010). [CrossRef] [PubMed]

].

3.2. Characterization of optical property for urchin-aggregation shaped ZnO nanostructures

To provide a better insight into the improvement of light scattering in urchin-aggregation shaped ZnO nanostructures, the calculated transmittance haze ratios (HT = TD/TT) for the ITO/glass and ZnO NRAs on ITO/glass without and with silica microspheres were compared in Fig. 5
Fig. 5 Measured HT as a function of wavelength for the ITO/glass, ZnO NRAs on AZO/ITO/glass, and ZnO NRAs on AZO/silica microspheres of 320 nm, 540 nm, and 970 nm/ITO/glass, The insets show oblique-view SEM images and photographs of the water droplets for ZnO NRAs on ITO/glass with and without silica microspheres of 970 nm.
. The insets show the oblique-view SEM images of ZnO NRAs on ITO/glass without and with silica microspheres of 970 nm. The photographs of the water droplets on the samples are also shown in the inset. For ITO coated glass, as expected, the HT was very low. The ZnO NRAs on AZO/ITO/glass also exhibited low HT values below 27%. Instead, the incorporation of silica microspheres into the structures leads to a significant increase in HT. The HT was increased with increasing the diameter of microspheres due to the enhanced TD. The urchin-aggregation shaped ZnO nanostructure with 970 nm silica microspheres yielded a high HT value of > 70% in the wavelength range of 400-900 nm, indicating the fairly high improvement in light trapping and scattering abilities. Moreover, the urchin-aggregation shaped ZnO nanostructures give rise to a high density of ZnO NRAs, keeping a high total transmittance over a wide wavelength region. On the other hand, the surface wettability is an important property of solid surfaces. The nanostructures can be also used to modify the surface property. In order to explore the surface property of urchin shaped ZnO nanostructures compared to the conventional ZnO NRAs, the contact angle of water droplets on the sample surface was determined by using a contact angle measurement system. The urchin-aggregation shaped ZnO nanostructures provided an improved surface hydrophobicity, which may be useful for photovoltaic applications [37

37. J. Zhu, C. M. Hsu, Z. Yu, S. Fan, and Y. Cui, “Nanodome solar cells with efficient light management and self-cleaning,” Nano Lett. 10(6), 1979–1984 (2010). [CrossRef] [PubMed]

]. In view of the surface macroscopic properties on wettability, for ZnO NRAs on AZO/ITO/glass, the water droplet spread rapidly out, corresponding to a contact angle of 36.65° which is close to the hydrophilic, whereas forthe urchin-aggregation shaped ZnO nanostructure the contact angle was increased to 90.51°, as shown in the inset of Fig. 5. For silica microspheres of 320 nm and 540 nm, the urchin-aggregation shaped ZnO nanostructures also exhibited the increased contact angles of 88.37° and 90.45°, respectively (not shown here). The contact angle was almost not changed when the size of silica microspheres in urchin-aggregation shaped ZnO nanostructures was varied from 320 nm to 970 nm. This indicates that the modified surface of integrated ZnO NRAs on silica microspheres improves the hydrophobicity by preventing the penetration of water into the surface, which is desirable for certain applications.

5. Conclusion

We fabricated the urchin-aggregation shaped closely-packed hierarchical ZnO nanostructures by simply growing the ZnO nanorods with a thin AZO seed layer onto the monolayer of dip-coated silica microspheres on ITO/glass using the hydrothermal method. The effect of silica microspheres and ZnO NRAs on the light scattering of ITO/glass samples was investigated, with theoretical simulations. The urchin-aggregation shaped ZnO nanostructure would result in a significant enhancement in TD over a wide range of wavelengths. This interpretation was supported by the numerical simulations. It was found that the diffuse light scattering was affected by the size of silica microspheres. The TD was increased with increasing the diameter of silica microspheres, indicating a similar tendency with the theoretical results. For 970 nm silica microspheres, a HT value as high as > 70% at wavelengths of 400-900 nm was achieved, keeping a high total transmittance. Additionally, the hydrophobicity of the sample was improved compared to the ZnO NRAs on AZO/ITO/glass. These results suggest that the urchin-aggregation shaped hierarchical ZnO nanostructures with large light scattering efficiency are a very promising candidate for various photovoltaic and optoelectronic device applications.

Acknowledgment

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2011-0026393).

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J. H. Kim and K. J. Yong, “Mechanism study of ZnO nanorod-bundle sensors for H2S gas sensing,” J. Phys. Chem. C 115(15), 7218–7224 (2011). [CrossRef]

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J. Y. Park, D. E. Song, and S. S. Kim, “An approach to fabricating chemical sensors based on ZnO nanorod arrays,” Nanotechnology 19(10), 105503 (2008). [CrossRef] [PubMed]

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J. Elias, C. Lévy-Clément, M. Bechelany, J. Michler, G. Y. Wang, Z. Wang, and L. Philippe, “Hollow urchin-like ZnO thin films by electrochemical deposition,” Adv. Mater. 22(14), 1607–1612 (2010). [CrossRef] [PubMed]

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B. V. Andersson, D. M. Huang, A. J. Moulé, and O. Inganäs, “An optical spacer is no panacea for light collection in organic solar cells,” Appl. Phys. Lett. 94(4), 043302 (2009). [CrossRef]

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T. Minemoto, C. Okamoto, S. Omae, M. Murozono, H. Takakura, and Y. Hamakawa, “Fabrication of spherical silicon solar cells with semi-light-concentration system,” Jpn. J. Appl. Phys. 44(7A), 4820–4824 (2005). [CrossRef]

22.

S. E. Han and G. Chen, “Optical absorption enhancement in silicon nanohole arrays for solar photovoltaics,” Nano Lett. 10(3), 1012–1015 (2010). [CrossRef] [PubMed]

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26.

H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9(3), 205–213 (2010). [CrossRef] [PubMed]

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J. Y. Wang, F. J. Tsai, J. J. Huang, C. Y. Chen, N. Li, Y. W. Kiang, and C. C. Yang, “Enhancing InGaN-based solar cell efficiency through localized surface plasmon interaction by embedding Ag nanoparticles in the absorbing layer,” Opt. Express 18(3), 2682–2694 (2010). [CrossRef] [PubMed]

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30.

Y. J. Lee, D. S. Ruby, D. W. Peters, B. B. McKenzie, and J. W. P. Hsu, “ZnO nanostructures as efficient antireflection layers in solar cells,” Nano Lett. 8(5), 1501–1505 (2008). [CrossRef] [PubMed]

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J. Y. Chen and K. W. Sun, “Growth of vertically aligned ZnO nanorod arrays as antireflection layer on silicon solar cells,” Sol. Energy Mater. Sol. Cells 94(5), 930–934 (2010). [CrossRef]

33.

Z. Jehl, J. Rousset, F. Donsanti, G. Renou, N. Naghavi, and D. Lincot, “Electrodeposition of ZnO nanorod arrays on ZnO substrate with tunable orientation and optical properties,” Nanotechnology 21(39), 395603 (2010). [CrossRef] [PubMed]

34.

R. Tena-Zaera, J. Elias, and C. Lévy-Clément, “ZnO nanowire arrays: optical scattering and sensitization to solar light,” Appl. Phys. Lett. 93(23), 233119 (2008). [CrossRef]

35.

K. Yee, “Numerical solution of initial boundary value problems involving Maxwell’s equations in isotropic media,” IEEE Trans. Antenn. Propag. 14(3), 302–307 (1966). [CrossRef]

36.

S. M. Yang, S. G. Jang, D. G. Choi, S. R. Kim, and H. K. Yu, “Nanomachining by colloidal lithography,” Small 2(4), 458–475 (2006). [CrossRef] [PubMed]

37.

J. Zhu, C. M. Hsu, Z. Yu, S. Fan, and Y. Cui, “Nanodome solar cells with efficient light management and self-cleaning,” Nano Lett. 10(6), 1979–1984 (2010). [CrossRef] [PubMed]

OCIS Codes
(050.1950) Diffraction and gratings : Diffraction gratings
(220.2740) Optical design and fabrication : Geometric optical design
(310.1210) Thin films : Antireflection coatings

ToC Category:
Diffraction and Gratings

History
Original Manuscript: July 25, 2011
Revised Manuscript: November 18, 2011
Manuscript Accepted: November 21, 2011
Published: December 6, 2011

Citation
Yeong Hwan Ko and Jae Su Yu, "Urchin-aggregation inspired closely-packed hierarchical ZnO nanostructures for efficient light scattering," Opt. Express 19, 25935-25943 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-27-25935


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References

  1. A. B. Djurišić and Y. H. Leung, “Optical properties of ZnO nanostructures,” Small2(8-9), 944–961 (2006). [CrossRef] [PubMed]
  2. Q. Zhang, C. S. Dandeneau, X. Zhou, and G. Cao, “ZnO nanostructures for dye-sensitized solar cells,” Adv. Mater.21(41), 4087–4108 (2009). [CrossRef]
  3. S. H. Ko, D. H. Lee, H. W. Kang, K. H. Nam, J. Y. Yeo, S. J. Hong, C. P. Grigoropoulos, and H. J. Sung, “Nanoforest of hydrothermally grown hierarchical ZnO nanowires for a high efficiency dye-sensitized solar cell,” Nano Lett.11(2), 666–671 (2011). [CrossRef] [PubMed]
  4. X. W. Sun, J. Z. Huang, J. X. Wang, and Z. Xu, “A ZnO nanorod inorganic/organic heterostructure light-emitting diode emitting at 342 nm,” Nano Lett.8(4), 1219–1223 (2008). [CrossRef] [PubMed]
  5. A. M. C. Ng, Y. Y. Xi, Y. F. Hsu, A. B. Djurišić, W. K. Chan, S. Gwo, H. L. Tam, K. W. Cheah, P. W. K. Fong, H. F. Lui, and C. Surya, “GaN/ZnO nanorod light emitting diodes with different emission spectra,” Nanotechnology20(44), 445201 (2009). [CrossRef] [PubMed]
  6. Y. Li, F. D. Valle, M. Simonnet, I. Yamada, and J. J. Delaunay, “High-performance UV detector made of ultra-long ZnO bridging nanowires,” Nanotechnology20(4), 045501 (2009). [CrossRef] [PubMed]
  7. Y. Y. Lin, C. W. Chen, W. C. Yen, W. F. Su, C. H. Ku, and J. J. Wu, “Near-ultraviolet photodetector based on hybrid polymer/zinc oxide nanorods by low-temperature solution processes,” Appl. Phys. Lett.92(23), 233301 (2008). [CrossRef]
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