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This work is inspired by biological nanostructured surfaces possessing structural color generation and wettability control. Nanostructured films consisting of a tantalum pentoxide (Ta2O5) nanopost array (nanograss) formed on top of a continuous Ta2O5 layer on a reflective Ta thin-film have been fabricated and investigated. A non-iridescent coloration typical for short-range order was produced by the nanograss while iridescent coloration was produced by the underlying continuous Ta2O5 layer. When the nanograss surface is wetted with organic liquids, a different non-iridescent blue color appears. Moreover, this blue color can be dynamically and reversibly switched on and off by applying electrical current to an indium-tin-oxide electrode.
© 2014 Optical Society of America
1. Introduction
The bright coloration, found in a broad range of different biological species from frogs, to birds, to insects, to cephalopods, is often created by the interplay between coherent (phase dependent) and incoherent (phase independent) light scattering from nano-scale structural elements. For iridescent structures the color of the light changes when viewed at different angles, while for non-iridescent structures the color does not change with the viewing angle. Structures, such as quasi-ordered nanostructures of avian feather barbs that exhibit only short-range order tend to produce non-iridescent color [1–3]. To the contrary, well ordered structures, such as laminar reflective or crystal-like arrays in avian feathers and butterfly wings generate strongly iridescent coloration [1]. Not surprisingly, many brightly-colored biological tissues exhibit nanostructures with elaborate combinations of both short and long-range order. Such a combined approach provides great flexibility in selecting the most evolutionary advantageous coloration strategy.
Besides color production, micro and nanostructured surfaces are commonly used by plants and animals for a totally different purpose that is to control wettability. In particular, butterfly and cicada wings [4], Namib Desert beetle elytra [5], and water strider legs [6,7] are reported to have superhydrophobic properties. Inspired by this multifunctional approach, encountered in nature, we have investigated the interplay between the short-range and the long-range order in the structural color generation exhibited by the artificial nanostructured surfaces. Moreover, the interplay between optical properties of the nanostructured surfaces, and their wetting behavior, has been investigated as well.
2. Experimental Section
In this work we developed a novel nanostructured film consisting of tantalum pentoxide (Ta2O5) nanograss formed on top of a continuous Ta2O5 layer [8]. The Ta2O5 nanograss structure was fabricated by using a multi-step anodization process of a Ta thin film deposited on Si through a porous Al2O3 mask. After anodization the Al2O3 was removed leaving the Ta2O5 nanograss structure. The complete fabrication process is described in Ref. 8. The height and spacing of the posts were controlled by the anodization conditions. The dimensions of the nanostructure were obtained from SEM (Zeiss 1500XB) micrographs. Typical diameters of the posts ranged from 40 to 100 nm with heights from 100 to 200 nm. The spacing between the posts tended to be roughly 3 times their diameter. SEM micrographs of one structure are shown in Figs. 1(b) and (d).
Fig. 1 (a) Schematic illustrating the structure of the nanostructured thin film and the interaction of light at its surface: thin film interference from the continuous Ta2O5 layer and scattering by the Ta2O5 nanograss. (b) SEM cross-section image of the nanograss structure. (c) Top view optical image of yellow nanograss structure. (d)Top view SEM image of the Ta2O5 nanograss. (e) Optical image of methanol spot on the Ta2O5 nanograss substrate in ambient light. (f) Top view optical image of methanol spot taken with direct illumination by the fiber optic light. (g) Optical image of methanol spot taken at 30° with respect to normal to the surface, under direct illumination by the fiber optic light..
After the nanograss film was fabricated, it was cleaned with O2 plasma causing the surface to become superhydrophilic [8]. In order to investigate the interaction of liquids with the nanostructured surface, droplets of various organic liquids, such as methanol, ethanol, isopropyl alcohol (IPA), acetone, toluene, chloroform, and water were deposited on the nanograss surface. As the liquid drop evaporated, a thin uniform liquid film was formed within the nanostructure, creating a blue colored spot. When the color of the liquid film was uniform, a quartz wafer was positioned on top of the liquid film and the nanograss surface to reduce the evaporation rate of the thin liquid film. The transmittance spectra of dry nanograss and of the blue spot formed on the wet nanograss were measured with UV/VIS/NIR spectrometer (Lambda 900, Perkin Elmer, Waltham, Massachusetts, US).
Furthermore, dynamic color control of the blue spot was investigated by using the following procedure. When the color of the organic liquid film, such as methanol, was uniform, an indium-tin-oxide (ITO) electrode was positioned on top of the methanol film and the nanograss surface. When electrical current about 0.1 A was applied to the 1 mm wide ITO electrode (220 Ω resistance), the blue spot would disappear. When the electrical current was turned off, the blue spot would reappear, thus allowing dynamic control of the coloration. To create the optimal lighting conditions for the blue spot observation, the fiber optic light (Schott KL 2500 LCD, Germany with Osram 64653 250W/24V GX5.3 lamp) was used.
3. Results and Discussion
The nanostructured film used in this work is formed by the integration of two different structural elements – an array of 50-100 nm diameter Ta2O5 posts and an underlying laminar thin-film stack. A typical structure is shown in Figs. 1(b) and 1(d). The Ta2O5 posts exhibit short range order while the underlying continuous film stack is characterized by thin-film interference. The incident light interacts with both types of structures simultaneously generating a combination of iridescent and omnidirectional coloration as schematically shown in Fig. 1(a). The iridescent color of the dry nanograss sample is yellow, when viewed under 30° with respect to the normal to the surface, as shown in Fig. 1(c). This yellow appearance is predominantly determined by interference of light with the underlying film stack, which consisted of 120 nm thick Ta2O5 layer deposited on a reflective Ta thin-film. The top layer of dry nanograss contributed little to the observed color. This can be demonstrated by comparing the reflectance spectrum of the nanostructured film possessing a yellow appearance with the reflectance spectrum of a smooth planar 120 nm thick Ta2O5 film (refractive index: n = 1.78-1.87 in visible light) anodically grown on top of a reflective Ta layer. As shown in Fig. 2 the two spectra are quite similar in the visible range.
Fig. 2 Measured reflectance spectra of a planar Ta2O5 film, dry Ta2O5 nanograss, and the blue spots created by wetting the nanograss with various organic liquids (methanol, ethanol, IPA, acetone, toluene, and chloroform). The reflectance spectra were taken at 30° with respect to the normal to the surface.
One of the most interesting properties of this film is the change of its optical properties between the dry and the wet states. In particular, when wetted by an organic liquid, such as methanol (n = 1.31-1.37) or even water, the nanograss surface exhibits light blue coloration in ambient light and intense blue coloration when directly illuminated with a fiber optic light, as shown in Figs. 1(e),1(f), and 1(g). The nanograss surface was wetted with a number of other organic liquids besides methanol: ethanol (n = 1.36-1.37), IPA (n = 1.37), acetone (n = 1.35-1.36), toluene (n = 1.5), and chloroform (n = 1.45-1.5), all of which exhibited the same blue coloration similar to methanol, as demonstrated by comparing their reflectance spectra shown in Fig. 2.
One of the important aspects of this study was to better understand the physical nature of the observed blue spot. We observe that the blue color of the spot does not change with viewing angle, as shown in Fig. 1(f), (g), and that the dry samples show only a yellow interference color. This indicates that the blue spot may be caused by the back scattered light from nanograss filled with the organic liquid. As a first step towards testing this hypothesis, we theoretically analyzed coherent light scattering by the Ta2O5 nanograss array along the lines of the approach, developed by Prum and Torres [1]. Our calculations were carried out using the computational software program Mathematica [9]. First, the top view SEM image of the Ta2O5 nanograss or nanopost array (Fig. 1(d)) was analyzed with the two-dimensional discrete Fourier transform (2D DFT) to convert the original spatial domain of the nanopost array to the frequency domain. The results of the 2D DFT analysis of the SEM image are presented as the 2D DFT power spectrum which can be seen in Fig. 3(a).The power spectrum is the squared magnitudes of the complex coefficients in the 2D DFT function. The ring-shaped distribution in the 2D DFT power spectrum demonstrates the existence of the short-range order and confirms the absence of the long-range order. The distribution of power density among different spatial frequencies observed in Fig. 3(a) can be plotted as the radial average of the power spectrum shown in Fig. 3(b). To express the results in terms of wavelength, the spatial frequency is inverted and then multiplied by twice the weighted average refractive index. A weighted average refractive index for the dry nanograss is calculated by assigning the refractive index of Ta2O5 to the nanoposts and the refractive index of air to the surroundings. Analysis of the 2D DFT power spectrum (green dot and dash line) shown in Fig. 3(c) indicates that for the dry nanograss film the maximum scattering occurs in the UV region (100 nm – 200 nm wavelength). Thus, the observed iridescent color of the dry samples is predominantly determined by interference of light with the underlying laminar Ta2O5 thin-film stack. The situation is very different for the cases when the nanopost array is wetted by an organic liquid, such as methanol. The presence of the liquid changes the weighted average refractive index of the nanograss film (since there is a large difference in refractive index between methanol and air) and shifts the scattering peak of the 2D DFT power spectrum (blue dot and dash line) shown in Fig. 3(c) towards the visible wavelength region. This causes non-iridescent blue coloration that largely obscures the interference color.
Fig. 3 (a) 2D DFT of the Ta2O5 nanograss (shown in Fig. 1(d)). (b) Radial average of the power spectrum. (c) Scattered spectra (dots) predicted from the Fourier analysis of the SEM image (Fig. 1(d)); dashed lines used for guiding. (d) Scattering spectra obtained from the reflectance measurements.
Thus we were able to predict the nanograss scattering spectrum using the 2D DFT power spectrum of the nanograss structure. We can also predict the scattering spectrum of the dry nanopost array and the blue spot by deconvoluting the measured reflectance spectra of the nanostructured film and the planar Ta2O5 thin-film stack. As shown in Fig. 1(a) we assume that there is no light absorption in the nanostructured film, and we assume that the scattering loss occurs twice, when the light enters and exits the nanostructured film. We then model the reflectance of light from the nanostructured film by taking into account that the light is lost by scattering both while entering and while exiting the film,
where is the ratio of the interference light intensity () to incident light intensity () and is the ratio of the scattered light intensity () to the incident light intensity (). For , we used the measured reflectance spectrum (black line) of the 120 nm smooth planar Ta2O5 layer on a reflective Ta thin-film as shown in Figs. 4(a) and 4(c). The function is not a priori known. In order to determine , we first represented it a fitting function that is capable of reproducing the general shape of the scattered spectra in Fig. 3(c) obtained from the Fourier analysis of the SEM image. The following fitting function was found to be particularly successful in reproducing the scattered spectra in Fig. 3(c)where is wavelength and are constants. We then optimized to achieve the best possible agreement between the predicted reflectance spectra, , (green line in Fig. 4(b) and intense blue line in Fig. 4(d)) and the measured reflectance spectra (orange line in Fig. 4(b) and light blue line in Fig. 4(d)). The resulting constants for are as follows: for dry nanograss scattering and for wet nanograss scattering . The results for predicted scattered spectra, , are shown in Figs. 4(a) and 4(b) and Fig. 3(d). They are very similar to the scattering spectra in Fig. 3(c) predicted from the 2D DFT analysis. However, some differences do exist. In particular, the shoulders of the predicted scattered spectra, , are different, but the shoulders of the predicted scattered 2D DFT power spectra are the same. The detailed analysis of these differences goes beyond the scope of this paper and will be published elsewhere.Fig. 4 (a) The predicted scattered spectrum (green) of the dry nanograss film and the measured reflectance spectrum (black) of the planar Ta2O5 thin-film stack. (b) The measured (orange) and predicted (green) reflectance spectra of the Ta2O5 nanograss. (c) The predicted scattered spectrum (intense blue) of the blue spot and the measured reflectance spectrum (black) of the planar Ta2O5 thin-film stack. (d) The measured (light blue) and predicted (dark blue) reflectance spectra of the wet blue spot of methanol on Ta2O5 nanograss.
The interplay between non-iridescent coloration produced by the nanograss layer and iridescent coloration produced by the underlying laminar Ta2O5 thin-film stack potentially opens a possibility of a dynamic color control. Indeed, it was observed that the blue color of the wet nanostructured film can be dynamically and reversibly switched on and off by applying electrical current to an ITO electrode positioned on top of the nanograss surface as shown in Fig. 5.. The color switching occurred in the area not covered by the electrode, as shown in Fig. 5(a).
Fig. 5 (a) Electrically-induced switching of the methanol-induced coloration. (b) Schematics illustrating how the methanol distribution inside the nanograss layer changes when the electric current applied to an ITO electrode is switched off and on.
As shown in Fig. 5(a), when the electrical current is not applied to the ITO electrode placed on top of the nanograss surface, the methanol inside the nanograss layer is uniformly distributed as indicated by a uniform blue coloration, which is only observed for thin methanol layers within the nanograss. When the electrical current is applied to the ITO electrode, it appears that the methanol flows underneath the ITO electrode. This type of behavior might be associated with the resistive heating of the ITO electrode by the electrical current. The heated ITO electrode creates a thermal gradient which causes a variation in the surface tension of the methanol, and this may induce the methanol to flow into the hot nanograss directly underneath the ITO electrode, possibly due to increased wicking associated with the reduced surface tension of the methanol located under the hot electrode, as shown in Fig. 5(b). This thermal-gradient-induced flow may be related to the phenomenon of thermocapillarity [10–17]. To further test whether the origin of the electrically induced color switching mechanism is consistent with the hypothesis of the thermocapillary flow, a beam of fiber optic light was used to heat the nanograss and the methanol film, which also resulted in the reversible switching of the blue color, further suggesting that the induced color switching mechanism is due to thermocapillary flow.
The details of the fluid distribution inside the nanograss layer require some additional investigation. In this initial study we assumed that within the blue spot region the fluid fills most of the available space between the nanograss columns, as it is often the case in the wicking phenomenon, This assumption is consistent with the observation that the evaporation of the blue spot proceeds by uniform shrinking of the spot diameter, without any noticeable change in its optical properties. Further research in this area will be focused on obtaining a better quantitative understanding of the physics of the electrically-controlled film wetting and elucidating its potential connection to the thermocapillary phenomenon.
4. Conclusion
We have studied the influence of nanostructured topography on the generation of iridescent and non-iridescent color. Samples combining both the short-range order nanograss layer and the underlying long-range order thin-film stack have been fabricated. We have experimentally observed the spectra of the samples and have determined that non-iridescent coloration is produced by the nanograss layer and the iridescent coloration is produced by the underlying thin-film stack. The scattering peak of non-iridescent color is in the near UV region for the dry nanograss film, while this scattering peak is shifted towards the visible wavelength region for the nanograss film wetted by an organic liquid providing blue coloration. We have developed a theoretical model capable of describing the generation of structural color, which is in good agreement with the observed spectra. We have also demonstrated the electrically-induced switching of the sample color from non-iridescent blue coloration to the iridescent color, produced by thin-film interference, by applying electrical current to an ITO electrode positioned on top of the nanograss surface. We hope this work will provide a pathway to novel optofluidic devices.
Acknowledgments
This work has been supported by the United States Air Force Office of Scientific Research (AFOSR) Multi-University Research Initiative (MURI) Program Award # FA9550-09-1-0669-DOD35CAP. The authors thank G. Myhre and S. Pau for discussions and technical assistance. The authors also acknowledge the Wisconsin Center for Applied Microelectronics (WCAM) and Materials Science Center (MSC) at UW-Madison for processing assistance.
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