Transparent and superhydrophobic Ta<sub>2</sub>O<sub>5</sub> nanostructured thin films

Supone Manakasettharn; Tsung-Hsing Hsu; Graham Myhre; Stanley Pau; J. Ashley Taylor; Tom Krupenkin

doi:10.1364/OME.2.000214

1. Introduction

In nature there are many examples of highly textured or superhydrophobic surfaces that easily shed water. These surfaces typically consist of micron and submicron structures with low surface energy coatings. When water contacts the surface, a highly mobile rolling ball is formed with a contact angle greater than 150°. Plants such as the lotus leaf (Nelumbonucifera) have evolved these surfaces for self-cleaning, as the rolling water droplet collects particulates as it falls from the leaf [1]. By removing water in a similar manner, the surface can have anti-fogging properties. Recently it was shown that water droplets impinging on superhydrophobic surfaces at below freezing temperatures are able to recoil from the surface before freezing, thus preventing ice to accumulate [2]. Transparent superhydrophobic surfaces with self-cleaning and anti-fogging properties are especially useful for glass windows and optical coatings for lenses. Various methods for preparing transparent superhydrophobic surfaces have already been reported [3–23]. In addition to being transparent, superhydrophobic optical coatings should possess feature sizes of less than 200 nm in order to avoid scattering loss of visible light. It is also desirable that the film can be conformally coated on curved or complex topographical surfaces. The superhydrophobic Ta₂O₅ optical thin film, which is the subject of this work, possesses both of these properties.

Tantalum pentoxide (Ta₂O₅) is a transparent, low absorption, high refractive index material, which can be utilized as optical coatings from near ultraviolet (350 nm) to infrared (> 8 µm). The refractive index of Ta₂O₅ is in the range of 1.65-2.3 [24–27] and depends on preparation techniques. It has been extensively studied because of its promising optical, physical, chemical and electrical properties. The material is robust, has melting temperature of greater than 1785 °C [28,29], and is useful for applications requiring high temperature. It is insoluble in water, ethanol and most acids, excluding hydrofluoric acid [28]. Its dielectric constant of greater than 22 [30,31] is useful for electronic applications such as capacitors. Many different preparation techniques of Ta₂O₅ have been reported [32], none of which describe the fabrication of superhydrophobic surfaces.

2. Experimental section

The transparent Ta₂O₅ nanostructured thin films were fabricated using a multi-step anodization process of tantalum (Ta) thin films as shown in Fig. 1 . The Ta layer (150 nm thick) was sputter-deposited (DC power 1 kW, 5 mT Ar pressure) on a cleaned quartz wafer. An aluminum (Al) layer 600 nm thick then was sputter-deposited (DC power 1 kW, 8 mT Ar pressure) on top of the Ta layer without breaking vacuum. The anodization process to form the porous Al layer was similar to recipes which have previously been published [33–35]. The Al layer was anodized in 0.2 M oxalic acid with a constant current density of 10 mA/cm² while ramping the voltage up to 53 V. As soon as 53 V was reached, the anodization process was stopped preventing the thin Al film from being completely anodized. The thin top layer of Al₂O₃ was stripped in a chromium trioxide (CrO₃) / phosphoric acid (H₃PO₄) solution (2 g CrO₃ / 100 ml H₂O, 4.1 ml conc. H₃PO₄) at 70 °C for 15 min. The surface of the remaining Al layer is shown in Fig. 2(a) . The remaining Al layer was reanodized by sweeping the voltage from 0 V to 45 V (1 V/s) in 0.0005 M oxalic acid, and then the Al layer was reanodized in 0.2 M oxalic acid at 53 V with constant current density of 10 mA/cm² to completely oxidize the Al as shown in Fig. 2(b). When the applied voltage was stable at 53 V and the current density dropped to 0.5 mA/cm² or lower, the voltage was then increased to 200 V for 12 min to grow Ta₂O₅ posts in the Al₂O₃ pores as shown in Fig. 2(c). After that, the Al₂O₃ layer was stripped in the CrO₃/H₃PO₄ solution at 70 °C for 120 min. The remaining metallic Ta layer was reanodized in 0.2 M oxalic acid ramping the voltage to 200 V with a constant current density of 0.2 mA/cm². Complete oxidation of the film occurred at a voltage below 200 V, at which point the current would decrease. This process formed a transparent textured Ta₂O₅ thin film on a quartz substrate. To make the structure superhydrophobic a thin CF_x film was deposited using plasma excitation of C₄F₈.

Fig. 1 Schematics of the multi-step anodization process of an Al-Ta bilayer.

Download Full Size | PDF

Fig. 2 Top view SEM images of (a) the Al surface after the first Al₂O₃ layer was stripped, (b) the Al₂O₃ porous layer, (c) Ta₂O₅ nanoposts grown in the Al₂O₃ pores, and (d) the final nanostructured Ta₂O₅ thin film after the Al₂O₃ porous layer was stripped. (e) The cross-section view of the final nanostructured Ta₂O₅ thin film.

Download Full Size | PDF

Contact angles of water droplets were measured using an automated drop shape analyzer (FTA1000 C Frame, First Ten Angstroms, Portsmouth, Virginia, US). Contact angles were measured at five different points. Advancing and receding contact angles were measured by video recording the change in drop shape as the drop, attached to the injection needle, was dragged across the moving surface. The nanostructure was observed by SEM (Zeiss 1500XB). The transmittance spectrum was measured with UV/VIS/NIR spectrometer (Lambda 900, Perkin Elmer, Waltham, Massachusetts, US).

3. Results and discussion

The final structure, as shown in SEM micrographs in Figs. 2(d) and 2(e), consists of a layer of Ta₂O₅ nanoposts (often called nanograss [36]) on top of a continuous Ta₂O₅ layer supported by a quartz substrate. The Ta₂O₅ nanoposts are about 40 nm in diameter and 200 nm in height. The thickness of the continuous Ta₂O₅ layer is 200 nm. The height of the nanoposts is controlled by the anodization voltage. The thickness of the continuous Ta₂O₅ layer is controlled by the initial thickness of the deposited Ta layer. Post diameter and density are somewhat dependent on anodization conditions used to form the porous Al₂O₃ mask.

The Ta₂O₅ nanograss surface, which was cleaned using O₂ plasma, is highly hydrophilic. When a 10 μL water droplet was deposited on the surface, the diameter of the spreading droplet was about 10 mm. The water contact angle was estimated to be about 2.9° when assuming the spreading droplet possessed a hemispherical shape (see Fig. 3(a) ). Surfaces with contact angles less than 5° are often defined as superhydrophilic. However, the contact angle of a water droplet on a cleaned non-textured Ta₂O₅ surface was also less than 5°. The Ta₂O₅ nanostructured surface was easily made superhydrophobic by depositing a low surface energy CF_x coating using plasma deposition from C₄F₈. As shown in Fig. 3(b), the observed contact angle of water on the Ta₂O₅ nanostructured surface with the hydrophobic coating was 155 ± 2° with a hysteresis of 20 ± 2°, as compared with the water contact angle of 107 ± 2° for a non-textured surface with the same hydrophobic coating. The contact angle hysteresis [37] is the difference between the advancing angle and the receding angle which is observed when the front contact line of a droplet advances, and its back contact line recedes. If the hysteresis is high, a droplet will tend to be pinned on a surface. In our case, the water droplet on the textured surface was highly mobile typical of superhydrophobic surfaces. The observed contact angle _{_$θ$} of 155° is in good agreement with the contact angle predicted by the Cassie Baxter equation [38]:

\cos θ = f (\cos θ_{0} + 1) - 1

where _{_$f$} is the ratio between the surface area in contact with a droplet and the total projected area and _{$θ_{0}$} is the local contact angle or the contact angle formed on the posts. For our case, _{_$f$} is approximately equal to 0.126, and _{$θ_{0}$} is estimated to be ~107° or the contact angle observed on a similar flat surface. The observed contact angle can be increased by decreasing the diameter of the nanoposts or by increasing the average distance between them.

Fig. 3 (a) A clean Ta₂O₅ nanograss surface which is highly hydrophilic showing a contact angle of less than 3°. (b) The same surface rendered superhydrophobic by depositing a CF_x coating showing a contact angle 155 ± 2° with a hysteresis of 20°. (c) The transparent superhydrophobic nanograss film with two water droplets deposited on the surface.

Download Full Size | PDF

The transmittance (_$T$) of the transparent multilayered Ta₂O₅ film (as shown in Fig. 3(c)) can be estimated by calculating the reflectance (_{_$R$}) and subtracting it from one, assuming the absorption is small. For light at normal incidence, the observed reflectance is given by [39]

R = r e^{i z} \times {(r e^{i z})}^{*}

where _{_$r$} is the amplitude and _{$e^{i z}$} is the phase of the light wave and _{_{${(r e^{i z})}^{*}$}} is the complex conjugate. In this case of three interfaces (i.e. by a double layer), the reflectance is given by [39]

r e^{i z} = \frac{r_{1} + r_{2} e^{- i Δ_{1}} + r_{3} e^{- i (Δ_{1} + Δ_{2})} + r_{1} r_{2} r_{3} e^{- i Δ_{2}}}{1 + r_{1} r_{2} e^{- i Δ_{1}} + r_{1} r_{3} e^{- i (Δ_{1} + Δ_{2})} + r_{2} r_{3} e^{- i Δ_{2}}}

where _{_{$r_{k} = (n_{k - 1} - n_{k}) / (n_{k - 1} + n_{k})$}} is the amplitude of the reflectance at the interface _$k$, _{_{$Δ_{k} = 4 π n_{k} d_{k} / λ$}} is the phase change of the light wave at the interface _$k$, _{_$k$} = 0, 1, 2, 3, _{$n_{k}$} is the refractive index in medium _{_$k$}, and _{$d_{k}$} is film thickness of layer _$k$. In our case, we assumed the first, second, third, and fourth layers are air, Ta₂O₅ nanograss, Ta₂O₅ continuous, and air layers, respectively. The 500 μm thick quartz substrate was not included in the calculation since its thickness is much larger than the coherence length of the visible light source (here a tungsten halogen lamp), which is just a few micrometers and within which thin-film interference is still strong. The refractive indices of air and Ta₂O₅ continuous layer are well known. The effective refractive index of Ta₂O₅ nanograss layer was calculated from [40]

n_{e f f} - j \frac{α_{e f f}}{2 k_{0}} = \sqrt{\frac{ε_{e f f}}{ε_{0}}}

where _{$k_{0} = 2 π / λ$}, _{_$λ$} is the wavelength of light, _{$ε_{0}$} is the permittivity of free space, _{_{$α_{e f f}$}} is the effective absorption coefficient of the medium. The effective dielectric function of the medium _{$ε_{e f f}$} can be approximated by [40]

f_{0} \frac{ε_{0} - ε_{e f f}}{ε_{0} + 2 ε_{e f f}} + f_{2} \frac{ε_{2} - ε_{e f f}}{ε_{2} + 2 ε_{e f f}} = 0

where _{$f_{0, 2}$} and _{_{$ε_{0, 2}$}} are volume fraction and dielectric functions of air medium and Ta₂O₅ medium.

The calculated transmittance of the Ta₂O₅ bi-layer film is shown in Fig. 4(a) . Two cases are shown for films with 90 nm thick nanograss layer on top of a continuous Ta₂O₅ layer with thickness of 300 nm and 410 nm. The measured transmittance of quartz and the two Ta₂O₅ films are shown in Fig. 4(b). Good qualitative agreement between the calculated and measured transmittance curves was obtained. Similar calculation shows that the thickness of the continuous Ta₂O₅ layer has the major affect on the reflectance as compared to the nanograss thickness. About 10% of the incident light is reflected at the quartz Ta₂O₅ interface which is not taken into account in the calculated curves. For much of the visible range the index of refraction for Ta₂O₅ remains relatively constant. For example, if the index of refraction of Ta₂O₅ at 550 nm is taken as 1.807 and the volume fraction of the nanograss as 0.15, the calculated index of refraction of the nanograss layer is 1.103.

Fig. 4 (a) The predicted transmittance spectra of two films with different layer thickness: the red curve is for a film consisting of a continuous Ta₂O₅ layer 300 nm thick and a Ta₂O₅ nanograss layer of 90 nm thick and the blue curve is for a film with a Ta₂O₅ continuous layer of 410 nm and a nanograss layer of 90 nm. (b) Measured transmittance spectra of quartz (green) and transparent nanograss films (blue, green) with the same dimensions.

Download Full Size | PDF

The calculated index of refraction of the nanograss layer from Eq. (4) is almost the same as the index of refraction of air since most of the volume of the nanograss layer consists of air. Consequently, the nanograss layer does not have a major influence on the optical transmittance spectrum based on the calculation from Eq. (2) and Eq. (3). One may vary anodization conditions to increase the volume fraction of the solid nanograss resulting in the increase of the index of refraction of the nanograss layer. If its index of refraction is high enough, the nanograss layer may affect the optical properties of Ta₂O₅ nanostructured thin films. However, an increase of the volume fraction of the nanograss also would affect the observed contact angle of water on the Ta₂O₅ nanostructured surface as indicated by Eq. (1). To maintain superhydrophobic surfaces (_$θ$ ≥ 150° at _{_{$θ_{0}$}} = 107°), _$f$ should be less than 0.189 which limits the maximum index of refraction of the nanograss layer.

4. Conclusion

In conclusion, we have demonstrated the fabrication of transparent Ta₂O₅ nanostructured thin films by using a multi-step anodization process of sputter-deposited Ta thin films. The transparent films can easily be made highly water repellent or superhydrophobic by depositing a low surface energy coating. The films have been characterized by measuring water contact angles and by obtaining optical transmittance spectra and SEM micrographs. The measured contact angles and transmittance curves are in good agreement with calculations. Our simple low-cost process can potentially be used on an industrial scale to fabricate superhydrophobic transparent Ta₂O₅ nanostructured thin films to create durable optical coatings with self-cleaning and anti-fogging properties.

Acknowledgments

This work has been funded by the United States Air Force Office of Scientific Research (AFOSR) Multi-University Research Initiative (MURI) Program under Award FA9550-09-1-0669-DOD35CAP. G. Myhre acknowledges funding from the Arizona Technology Research Infrastructure Fund (TRIF). The authors thank the Wisconsin Center for Applied Microelectronics (WCAM) and Materials Science Center (MSC) at the University of Wisconsin-Madison for processing assistance.

References and links

1. W. Barthlott and C. Neinhuis, “Purity of the sacred lotus, or escape from contamination in biological surfaces,” Planta 202(1), 1–8 (1997). [CrossRef]

2. L. Mishchenko, B. Hatton, V. Bahadur, J. A. Taylor, T. Krupenkin, and J. Aizenberg, “Design of ice-free nanostructured surfaces based on repulsion of impacting water droplets,” ACS Nano 4(12), 7699–7707 (2010). [CrossRef] [PubMed]

3. K. Tadanaga, N. Katata, and T. Minami, “Formation process of super-water-repellent Al₂O₃ coating films with high transparency by the sol-gel method,” J. Am. Ceram. Soc. 80(12), 3213–3216 (1997). [CrossRef]

4. A. Nakajima, A. Fujishima, K. Hashimoto, and T. Watanabe, “Preparation of transparent superhydrophobic boehmite and silica films by sublimation of aluminum acetylacetonate,” Adv. Mater. (Deerfield Beach Fla.) 11(16), 1365–1368 (1999). [CrossRef]

5. M. Miwa, A. Nakajima, A. Fujishima, K. Hashimoto, and T. Watanabe, “Effects of the surface roughness on sliding angles of water droplets on superhydrophobic surfaces,” Langmuir 16(13), 5754–5760 (2000). [CrossRef]

6. A. Nakajima, K. Hashimoto, T. Watanabe, K. Takai, G. Yamauchi, and A. Fujishima, “Transparent superhydrophobic thin films with self-cleaning properties,” Langmuir 16(17), 7044–7047 (2000). [CrossRef]

7. K. Tadanaga, J. Morinaga, A. Matsuda, and T. Minami, “Superhydrophobic-superhydrophilic micropatterning on flowerlike alumina coating film by the sol-gel method,” Chem. Mater. 12(3), 590–592 (2000). [CrossRef]

8. H. M. Shang, Y. Wang, S. J. Limmer, T. P. Chou, K. Takahashi, and G. Z. Cao, “Optically transparent superhydrophobic silica-based films,” Thin Solid Films 472(1-2), 37–43 (2005). [CrossRef]

9. H. Yabu and M. Shimomura, “Single-step fabrication of transparent superhydrophobic porous polymer films,” Chem. Mater. 17(21), 5231–5234 (2005). [CrossRef]

10. J. Fresnais, J. P. Chapel, and F. Poncin-Epaillard, “Synthesis of transparent superhydrophobic polyethylene surfaces,” Surf. Coat. Tech. 200(18-19), 5296–5305 (2006). [CrossRef]

11. M. Kemell, E. Färm, M. Leskelä, and M. Ritala, “Transparent superhydrophobic surfaces by self‐assembly of hydrophobic monolayers on nanostructured surfaces,” Phys. Status Solidi A 203(6), 1453–1458 (2006). [CrossRef]

12. C. Su, J. Li, H. Geng, Q. Wang, and Q. Chen, “Fabrication of an optically transparent super-hydrophobic surface via embedding nano-silica,” Appl. Surf. Sci. 253(5), 2633–2636 (2006). [CrossRef]

13. J. Bravo, L. Zhai, Z. Wu, R. E. Cohen, and M. F. Rubner, “Transparent superhydrophobic films based on silica nanoparticles,” Langmuir 23(13), 7293–7298 (2007). [CrossRef] [PubMed]

14. N. Vourdas, A. Tserepi, and E. Gogolides, “Nanotextured super-hydrophobic transparent poly (methyl methacrylate) surfaces using high-density plasma processing,” Nanotechnology 18(12), 125304 (2007). [CrossRef]

15. X. Zhang, H. Kono, Z. Liu, S. Nishimoto, D. A. Tryk, T. Murakami, H. Sakai, M. Abe, and A. Fujishima, “A transparent and photo-patternable superhydrophobic film,” Chem. Commun. (Camb.) (46): 4949–4951 (2007). [CrossRef] [PubMed]

16. K. C. Chang, Y. K. Chen, and H. Chen, “Fabrication of highly transparent and superhydrophobic silica-based surface by TEOS/PPG hybrid with adjustment of the pH value,” Surf. Coat. Tech. 202(16), 3822–3831 (2008). [CrossRef]

17. J. T. Han, S. Y. Kim, J. S. Woo, and G. W. Lee, “Transparent, conductive, and superhydrophobic films from stabilized carbon nanotube/silane sol mixture solution,” Adv. Mater. (Deerfield Beach Fla.) 20(19), 3724–3727 (2008). [CrossRef]

18. Q. F. Xu, J. N. Wang, I. H. Smith, and K. D. Sanderson, “Superhydrophobic and transparent coatings based on removable polymeric spheres,” J. Mater. Chem. 19(5), 655–660 (2009). [CrossRef]

19. Y. Li, F. Liu, and J. Sun, “A facile layer-by-layer deposition process for the fabrication of highly transparent superhydrophobic coatings,” Chem. Commun. (Camb.) (19): 2730–2732 (2009). [CrossRef] [PubMed]

20. X. Y. Ling, I. Y. Phang, G. J. Vancso, J. Huskens, and D. N. Reinhoudt, “Stable and transparent superhydrophobic nanoparticle films,” Langmuir 25(5), 3260–3263 (2009). [CrossRef] [PubMed]

21. A. Venkateswara Rao, S. S. Latthe, D. Y. Nadargi, H. Hirashima, and V. Ganesan, “Preparation of MTMS based transparent superhydrophobic silica films by sol-gel method,” J. Colloid Interface Sci. 332(2), 484–490 (2009). [CrossRef] [PubMed]

22. J. Yang, Z. Zhang, X. Men, and X. Xu, “Fabrication of stable, transparent and superhydrophobic nanocomposite films with polystyrene functionalized carbon nanotubes,” Appl. Surf. Sci. 255(22), 9244–9247 (2009). [CrossRef]

23. M. Im, H. Im, J. H. Lee, J. B. Yoon, and Y. K. Choi, “A robust superhydrophobic and superoleophobic surface with inverse-trapezoidal microstructures on a large transparent flexible substrate,” Soft Matter 6(7), 1401–1404 (2010). [CrossRef]

24. S. J. Ingrey, W. D. Westwood, Y. C. Cheng, and J. Wei, “Variable refractive index and birefringent waveguides by sputtering tantalum in O₂-N₂ mixtures,” Appl. Opt. 14(9), 2194–2198 (1975). [CrossRef] [PubMed]

25. F. Z. Tepehan, F. E. Ghodsi, N. Ozer, and G. G. Tepehan, “Optical properties of sol–gel dip-coated Ta₂O₅ films for electrochromic applications,” Sol. Energy Mater. Sol. Cells 59(3), 265–275 (1999). [CrossRef]

26. W. Cheng, S. Chi, and A. Chu, “Effect of thermal stresses on temperature dependence of refractive index for Ta₂O₅ dielectric films,” Thin Solid Films 347(1-2), 233–237 (1999). [CrossRef]

27. K. Schmitt, K. Oehse, G. Sulz, and C. Hoffmann, “Evanescent field sensors based on tantalum pentoxide waveguides–a review,” Sensors (Basel Switzerland) 8(2), 711–738 (2008). [CrossRef]

28. P. Patnaik, Handbook of Inorganic Chemicals (McGraw-Hill, 2003).

29. W. Martienssen and H. Warlimont, Springer Handbook of Condensed Matter and Materials Data (Springer, 2005).

30. D. Husted, L. Gruss, and T. Mackus, “Electrical properties of anodic oxide films of Ta, Nb, Zr, Ti, W, and V formed by the ion‐cathode method,” J. Electrochem. Soc. 118(12), 1989–1992 (1971). [CrossRef]

31. L. Young, “The determination of the thickness, dielectric constant, and other properties of anodic oxide films on tantalum from the interference colours,” Proc. R. Soc. A 244(1236), 41–53 (1958). [CrossRef]

32. C. Chaneliere, J. L. Autran, R. A. B. Devine, and B. Balland, “Tantalum pentoxide (Ta₂O₅) thin films for advanced dielectric applications,” Mater. Sci. Eng. Rep. 22(6), 269–322 (1998). [CrossRef]

33. A. Mozalev, M. Sakairi, I. Saeki, and H. Takahashi, “Nucleation and growth of the nanostructured anodic oxides on tantalum and niobium under the porous alumina film,” Electrochim. Acta 48(20-22), 3155–3170 (2003). [CrossRef]

34. C. Wu, F. Ko, and H. Hwang, “Self-aligned tantalum oxide nanodot arrays through anodic alumina template,” Microelectron. Eng. 83(4-9), 1567–1570 (2006). [CrossRef]

35. A. Mozalev, A. J. Smith, S. Borodin, A. Plihauka, A. W. Hassel, M. Sakairi, and H. Takahashi, “Growth of multioxide planar film with the nanoscale inner structure via anodizing Al/Ta layers on Si,” Electrochim. Acta 54(3), 935–945 (2009). [CrossRef]

36. T. N. Krupenkin, J. A. Taylor, T. M. Schneider, and S. Yang, “From rolling ball to complete wetting: the dynamic tuning of liquids on nanostructured surfaces,” Langmuir 20(10), 3824–3827 (2004). [CrossRef] [PubMed]

37. G. McHale, N. J. Shirtcliffe, and M. I. Newton, “Contact-angle hysteresis on super-hydrophobic surfaces,” Langmuir 20(23), 10146–10149 (2004). [CrossRef] [PubMed]

38. A. B. D. Cassie and S. Baxter, “Wettability of porous surfaces,” Trans. Faraday Soc. 40, 546–551 (1944). [CrossRef]

39. H. Anders, ThinFilms in Optics (Focal Press, 1967).

40. D. A. G. Bruggeman, “Berechnung verschiedener physikalischer Konstanten von heterogenen Substanzen. I. Dielektrizitätskonstanten und Leitfähigkeiten der Mischkörper aus isotropen Substanzen,” Ann. Phys. 416(7), 636–664 (1935). [CrossRef]

Transparent and superhydrophobic Ta₂O₅ nanostructured thin films

Abstract

1. Introduction

2. Experimental section

3. Results and discussion

4. Conclusion

Acknowledgments

References and links

Cited By

Figures (4)

Equations (5)

Optical Materials Express