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Controlling Colour and Effective Refractive Index of MetalAnodic Aluminium Oxide-Al Nanostructures: Morphology of AAO Cristina Vicente Manzano, Daniel Ramos, Laszlo Pethö, Gerhard Bürki, Johann Michler, and Laetitia Vérnique Philippe J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11131 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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The Journal of Physical Chemistry

Controlling Colour and Effective Refractive Index of Metal-Anodic Aluminium Oxide-Al Nanostructures: Morphology of AAO

C.V. Manzano1,*, D. Ramos2, L. Pethö1, G Bürki1, J. Michler1, L. Philippe1. 1

Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for

Mechanics of Materials and Nanostructures, Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland 2

Bionanomechanics Lab, Instituto de Micro y Nanotecnología, IMN-CNM (CSIC), Isaac Newton 8 (PTM), Tres Cantos, Madrid, Spain

Abstract Metal-anodic aluminium oxide (AAO)-Al nanostructures have been deposited by using sputtering for the metal layer deposition and a two-step anodization process in different electrolytes to produce self-ordered anodic aluminium oxide films. The effect of the morphological parameters of AAO films (such as thickness, pore diameter, interpore distance and porosity) on the optical properties was studied. The UV-Vis reflectance properties as a function of the thickness for the different electrolytes of metal-AAO-Al films were analysed in order to obtain the colour diagrams and the effective refractive indices of the films. The effective refractive index was found to depend on the thickness and porosity of AAO films. The change in colour observed in metal-AAO-Al nanostructures is due to the thickness and porosity of the AAO films. In order to verify the experimental results, UV-Vis reflectance spectra of AAOAl films were simulated using commercially available finite element simulation software.

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1. Introduction In the last years, the development of brilliant colours has produced immense interest in generating the vivid colours found in nature. 1 There are two distinct approaches to fabricating artificially brilliant colours for industrial applications chemical and physical. For chemical coloration,2 pigments and/or dyes are used, and the colour is governed by the chemicals and their concentrations. The physical approach consists of fabricating anodic aluminium oxideAl films using new electrolytes such as etidronic acid,3 pulsed anodization4 or constructing metal-dielectric-metal nanostructures, which are more environmentally friendly than their chemical counterparts. In these novel materials, the observed colour is due to the interaction of light with periodic nanostructures5 of the anodic aluminium oxide. Similar behaviour is observed in nature; for example, the structural colours of butterflies have been attributed to a diverse range of physical mechanisms such as multilayer interference, diffraction, Bragg scattering, Tyndall scattering and Rayleigh scattering. 6 Such mechanisms also play an important role in the generation of brilliant colours for metal-AAO-Al nanostructures, and this is being extensively studied. These nanostructures can be used in different applications, such as surface plasmon resonance,7-9 optical interference, wavelength absorbers,2 RGB display devices10 and chemical11 optical sensors.12 Brilliant (structural) colours observed in metal-AAO-Al films depend on the optical interference (i.e. the metal layer

13-14

and the effective refractive index of the AAO-Al films)

which correlates with AAO morphological parameters. In a previous study by our group, the influence of the morphological parameters and compositional structure on the optical properties (reflectance) of AAO-Al films was studied. The reflectance was found to depend on the thickness of AAO films, and significantly, on the interpore distance.15 Moreover, a strong dependence of the reflectance on the anodization electrolyte was found in the UV region, leading to the observation that the chemical composition plays an important role. 15-16 For metalAAO-Al nanostructures, it is well established that the colours are tuned by the thickness of AAO films.7, 13-14, 17-18 Moreover, some studies have reported that the colour is tuned by the pore diameter19-20 or porosity;7, 10, 18 but some controversy still exists due to the fact that in these studies the pore diameter and the porosity are changing at the same time, since a chemical etching is performed to modify the pore diameter. To date, the complete influence of AAO films’ morphological parameters on colour has not been determined.

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To understand the effect of the morphological parameters on the colour observed in metal-AAO-Al films, it is very important to know the effect of the morphological parameters on the effective refractive index of the AAO, due to the fact that the observed colour depends on the effective refractive index according to the Bragg reflection: ∑ 2𝑛 · 𝑑 · cos 𝜃 = 𝑚 · 𝜆

(1)

where 𝑛 is the effective refractive index, 𝑑 is the thickness, 𝜃 is the reflection angle, m is the order number and 𝜆 is the reflective peak wavelength, which gives the colour observed. In most of the studies found in literature, the effective refractive index is obtained from the Maxwell-Garnett equation21 using the refractive index of bulk alumina (1.77);22-23 but in this approximation the thickness of AAO is not taken into account and, furthermore, the refractive index of bulk alumina is given for crystalline alumina, while an amorphous structure is formed in the case of anodized alumina. J. De Laet et al. reported the modelling of the optical properties of AAO films using spectroscopic ellipsometry.24-26 Moreover, the anisotropy of the effective refractive index of AAO/Al films was studied from the same group. 27 Other technique to determine the effective refractive index of thin films is using the reflectance spectra; this technique provides direct and accurate values of the effective refractive index and at the same time the RGB values of the samples and the effective refractive index can be obtained. By this method M. Shaban et. al observed that the effective refractive index of the AAO-Al films decreases (from 1.68 to 1.38) when the thickness increases (from 400 to 1200 nm),8 but the dependence of the effective refractive index on different individual AAO films morphological parameters has not been thoroughly studied. In this study, metal-anodic aluminium oxide (AAO)-Al nanostructures were obtained by two steps: 1) chromium thin films were deposited using sputtering deposition and 2) selfordered AAO films were fabricated using a two-step anodization process in different electrolytes in order to control the pore diameter, interpore distance and porosity. The UV-Vis reflectance was analysed for varying thicknesses and different electrolytes. The effective refractive index of AAO-Al films as a function of thickness, electrolyte, pore diameter, interpore distance and porosity was calculated from the UV-Vis reflectance. The aims of this work are to study the effects of morphological parameters (pore diameters, interpore distance, porosity, nanostructure order) on the colours and on the effective refractive index of AAO films. This study provides an important perspective of how the colour can be tuned by the morphological parameters of AAO films through the effective refractive index, which can be used in different areas such as surface plasmon resonance, optical interference, wavelength absorbers, RGB display devices and optical sensors. 3



2. Experimental methods 2.1. Fabrication of AAO films and Cr deposition Self-ordered anodic aluminium oxide (AAO) films were fabricated using a two-step anodization process28 following the same conditions used in a previous manuscript of our group.15 Different pore diameters and interpore distances were obtained using oxalic acids (0.3 M H2C2O4, 40 V, 3 °C ), sulfuric acids (0.3 M H2SO4, 25 V, 0 °C) and ethylene glycol containing sulfuric acid (50 wt.% of ethylene glycol and 10 wt.% of H2SO4, 19 V, 0 °C).29-31 The thickness of the AAO films was controlled by the second anodization time. A thin chromium film of 8 nm was deposited using an Alliance-Concept DP 650 DC magnetron sputtering equipment on top of AAO in order to obtain different colours for the films. The sputtering pressure and the power were 5·10-3 mbar and 350 W, respectively. Chromium was chosen due to its significant interference improvement and colour tuning capabilities.17 Metal-anodic aluminium oxide (AAO)-Al nanostructures are formed by 8 nm of Cr, AAO, and mirror Al (which was obtained using an electropolishing process on Al foil).

2.2. Characterization of AAO films Morphological characterization was conducted using field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) with a 1.5 kV accelerating voltage. In order to obtain an accurate measure of the thickness, cross-sectional images were taken using a focused ion beam (FIB) Lyra instrument (TESCAN, Brno, Czech Republic) with a gallium source at 30 kV and 180-400 pA. 2 µm of platinum was deposited to protect the surface prior to the FIB cutting. Reflectance spectroscopy measurements on AAO films were performed using a PerkinElmer Lambda 900 UV-Vis spectrophotometer ranging from 300 to 1000 nm. The colours are normally evaluated using RGB (red, green and blue) values, which is the primary colour model to describe all the colours of the visible light spectrum. Additionally, Yxy values represent another colour model that describes luminance (Y) and chromaticity (xy). From the UVVis reflectance values of the nanostructures, the Yxy values using the CIE 1931 colour space were calculated. The effective refractive index of the films was obtained from the reflectance measurements; it is given by the following equation: 4


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𝑛

=

( (

×

)

+ 𝑠𝑖𝑛 𝜃

)

(2)

where N is the number of fringes, 𝜆 is the maximum wavelength, 𝜆 is the minimum wavelength, 𝜃 is the angle of incidence and t is the thickness of the films. The experimental errors of the effective refractive index were calculated using the propagation of the uncertainties, taking into account the random error obtained by performing multiple measurements of the thickness and reflectance spectra measurements. To validate the experimental results, theoretical simulations of reflectance were performed using the commercially available Radio Frequency Module of Comsol Multiphysics Finite Element Method (FEM) software. For the purpose of mimicking the actual experimental sample, the entire system was modelled as a nanostructured AAO layer with a semiinfinite aluminium substrate underneath and a semi-infinite air layer above. We are using the FEM to directly solve the Fresnel equations for our actual experimental setup. The dispersion relation of the materials has been taken into account, i.e. the optical properties of the materials have been included in the simulation by introducing a wavelength dependency of the refractive index and absorption for each layer, n(λ) and k(λ). 32-33 The FEM calculation requires one simulation of the entire structure for each wavelength,34 therefore, in order to minimize the computational cost, we have taken advantage of the geometrical symmetries of the periodic structure, reducing the dimension of the problem just to a single unit cell containing only one pore with a lateral size equal to the interpore distance. By using these symmetries, the light was injected into the system by means of a periodic port, which allows for easily calculating the diffraction orders and the reflectance was calculated through the absolute value of the quadratic value of element 11 of the scattering matrix.”

3. Results and discussion 3.1 AAO film surface morphology Brilliant colours found in metal-AAO-Al nanostructures depend on the metal layer composition, the metal layer thickness and the effective refractive index of the AAO-Al films. In order to study the morphological parameters, which affect the effective refractive index of the AAO-Al films, the metal layer was fixed as a thin film (8 nm) of chromium. The morphological parameters are the thickness (t), ions trapped in the AAO structure, pore diameter (Dp), interpore distance (Dint) and porosity (P) (see Figure 1). 5



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Figure 1. Scheme of the morphological parameters of the AAO films which affect the effective refractive index of Cr-AAO-Al nanostructures.

AAO films with different thicknesses (from ~ 430 to ~ 1400 nm) were first anodized to study the effect of the thickness on the effective refractive index. The influence of the ions trapped in the AAO structure was analysed by performing anodization in different electrolytes. The porosity of the AAO films was adjusted to be about 10 % in order to study its effect on the effective refractive index. The correlation of the pore diameter and pore distance was studied by anodizing AAO films under different conditions. Figure 2 shows field emission scanning electron microscopy (FE-SEM) images of AAO films anodized in different anodization conditions. Figure 2(a)-(c) shows the top-view of the AAO films, while Figure 2(d) shows the cross-sectional images of one of the AAO films. In all cases, pores in the AAO films are organized hexagonally within ordered domains. Morphological characterization, the pore diameter, interpore distance, porosity and thickness were analysed by FE-SEM images. Accurate measurements of the pore diameter and porosity were obtained from an image treatment analysis. The pore diameter is calculated from the pore distribution obtained from the digitally analysed image.15 The analysis of the porosity was carried out using FE-SEM images, where the magnification was dependent on the pore diameter (50 kX, 70 kX, and 100 kX) (see Figure 2). The porosity, P, of the AAO templates is given by the following equation: 𝑃=

/

(3)

√

where Dp is the pore diameter and Dint is the interpore distance.35 For all cases, the porosity is approximately 10%, which is equivalent to the porosity value under self-ordered anodization conditions.

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Figure 2. FE-SEM micrographs of AAO films anodized in different electrolytes: (a) oxalic acid, (b) sulfuric acid, (c) ethylene glycol containing sulfuric acid, and (d) cross-section of an AAO film after FIB cutting.

To obtain accurate measurements of the thickness, FIB milling was performed (see Figure S1 of Supporting information), and FE-SEM images were taken. The pore diameter, interpore distance, porosity and thickness, with corresponding errors, are collected in Table 1 for the various AAO films. Table 1. Morphological parameters of the AAO films anodized in different anodization conditions. Dp

Dint

P

Thickness

32 ± 3 nm

101 ± 10 nm

13.6%

437 ± 1.5 nm

32 ± 3 nm

101 ± 10 nm

13.6%

614 ± 3.0 nm

32 ± 3 nm

101 ± 10 nm

13.6%

935 ± 1.8 nm

32 ± 3 nm

101 ± 10 nm

13.6%

1222 ± 6.6 nm

23 ± 2 nm

60 ± 6 nm

11.5%

671 ± 2.2 nm

23 ± 2 nm

60 ± 6 nm

11.5%

785 ± 1.2 nm

23 ± 2 nm

60 ± 6 nm

11.5%

983 ± 4.8 nm

23 ± 2 nm

60 ± 6 nm

11.5%

1333 ± 5.8 nm

17 ± 2 nm

48 ± 5 nm

13.2%

435 ± 3.3 nm

H2SO4 + Ethylene

17 ± 2 nm

48 ± 5 nm

13.2%

518 ± 3.8 nm

glycol

17 ± 2 nm

48 ± 5 nm

13.2%

763 ± 5.8 nm

17 ± 2 nm

48 ± 5 nm

13.2%

872 ± 4.2 nm

Oxalic acid

H2SO4

7



3.2 Reflectance, colour diagram and effective refractive index of AAO films The UV-Vis reflection spectra of AAO films anodized under different anodization conditions were measured. Figure 3 shows the reflectance from 300 nm to 1000 nm as a function of thickness for each electrolyte (approximately same porosity, 10 %). As shown in Figure 3, UV-Vis reflectance spectra present Fabry-Perot interference due to the thickness of the AAO films. It is well known that this Fabry-Perot interference is produced by the interference of light between the metal (Al)/AAO film and air/AAO film interfaces and it is normally observed when the thickness of the AAO films is less than 2 µm.15 The maximum of the reflectance is similar in all cases, and the number of fringes is higher when the AAO films are thicker. In all cases, after the deposition of Cr onto the AAO-Al films, a blue-shift and a decrease in the reflectance is observed due to the plasmonic effects. Similar reflectance spectra were obtained in previous studies.7, 36 Oxalic acid

100

40

20

60

40

20

0 300

671 nm 785 nm 983 nm 1333 nm Cr/671 nm Cr/785 nm Cr/983 nm Cr/1333 nm

80

Reflectance (%)

60

Sulfuric acid

100

437 nm 614 nm 935 nm 1222 nm Cr/437 nm Cr/614 nm Cr/935 nm Cr/1222 nm

80

Reflectance (%)

0 400

500

600

700

800

900

1000

300

400

500

 (nm)

600

700

800

900

1000

 (nm) 100

Ethylene glycol containing sulfuric acid 413 nm 435 nm 763 nm 880 nm Cr/413 nm Cr/435nm Cr/763 nm Cr/880 nm

80

Reflectance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

40

20

0 300

400

500

600

700

800

900

1000

 (nm)

Figure 3. UV-Vis reflectance spectra for AAO-Al films anodized in different electrolytes and with different thicknesses before (continuous lines) and after (dotted lines) chromium deposition.

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From the reflectance spectra of the metal (Cr)-AAO-Al nanostructures (see dotted lines in Figure 3), Yxy values were obtained using the CIE 1931 colour space. The first conclusion referring to the morphological parameters of AAO films is that the change of the colour depends on its thickness, as it was observed previously in numerous studies. 7, 14, 17 The colour coordinates are presented in Figure 4 in the CIE 1931 colour diagram. Different colours, xy values, were obtained in Cr-AAO-Al nanostructures by tuning the thickness and anodization conditions of the AAO films.

Figure 4. Representation of Cr-AAO-Al nanostructures in the CIE 1931colour diagram.

Brilliant or structural colours observed in metal-AAO-Al nanostructures depend on the effective refractive index of the AAO-Al films according to the Bragg reflection (equation 1). Figure 5 shows the effective refractive index as a function of the thickness of the AAO films anodized in different anodization conditions (similar porosity).

9



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Figure 5. Effective refractive index as a function of the thickness for the different anodization conditions, and then different electrolytes: oxalic, sulfuric, ethylene glycol containing sulfuric acid; and optical images of Cr-AAO-Al films as a function of the thickness.

The effective refractive index depends on the thickness and porosity, as shown in Figure 5. The effective refractive index is smaller when the AAO films are thicker. This behaviour is explained by equation 2, where effective refractive index varies inversely with the thickness of AAO films. Depending on the anodization conditions (electrolyte, voltage, temperature) the equations of the effective refractive index as a function of the thickness (see equation 2) are different for self-ordered conditions (~ 10% of porosity): = (−1.03 ∙ 10

𝑛

= (−3.02 ± 9.73 ∙ 10

)𝑥 𝑡 + (1.65 ±0.09)

for H2SO4

𝑛

= (−9.58 ± 2.35 ∙ 10

)𝑥 𝑡 + (2.30 ±0.18)

for H2SO4 and ethylene glycol

± 1.40 ∙ 10

)𝑥 𝑡 + (2.38 ±0.13)

for H2C2O4

𝑛

(4)

where t is the thickness of the films in nanometres. This method to obtain the refractive index is one of the most simple and accurate techniques. The thicknesses and effective refractive indices of the AAO films are collected in table 2. The values of the refractive index are in the same range with the values obtained in 10


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previous work on reflectance spectra.8 As shown in table 2, the error of the effective refractive index calculated from the reflectance spectra measurements is very low for all cases. Table 2. Thickness and effective refractive index with the corresponding error of the AAO films. Thickness

neff

437 ± 1.5 nm

2.01 ± 7.31 x 10-3

614 ± 3.0 nm

1.65 ± 8.33 x 10-3

935 ± 1.8 nm

1.42 ± 3.01 x 10-3

1222 ± 6.6 nm

1.17 ± 6.45 x 10-3

671 ± 2.2 nm

1.49 ± 5.14 x 10-3

785 ± 1.2 nm

1.37 ± 4.58 x 10-3

983 ± 4.8 nm

1.34 ± 6.99 x 10-3

1333 ± 5.8 nm

1.26 ± 5.69 x 10-3

435 ± 3.3 nm

1.99 ± 1.56 x 10-2

H2SO4 + Ethylene

518 ± 3.8 nm

1.70 ± 1.15 x 10-2

glycol

763 ± 5.8 nm

1.55 ± 1.21 x 10-2

872 ± 4.2 nm

1.48 ± 7.70 x 10-3

H2C2O4

H2SO4

Additionally, in Figure 5 it is shown that when the AAO effective refractive index is around 2, a gold-orange colour is obtained; for values between 1.8-1.6, a blue colour is observed; when the effective refractive index is around 1.5, the films display pink colours and for values between 1.4-1.2, a green colour is perceived. This relation between the effective refractive index and colours is valid when the AAO porosity is approximately 10% and 8 nm of Cr above AAO-Al films is deposited. In conclusion, the effective refractive index depends on the thickness and porosity, but not on the pore diameter, interpore distance or electrolyte. The thickness and porosity of the AAO films are the two morphological parameters which affect the colour of metal-AAO-Al nanostructures. There are different combination of thicknesses and porosities which give the same refractive index and consequently the same colour. In order to confirm that the colour and the effective refractive index depends on the thickness and porosity, anodization conditions were adjusted to obtain AAO films with the same thickness and porosity, but different pore diameter and interpore distance (different electrolytes). 11



Figure 6. UV-Vis reflectance spectra for AAO-Al films anodized in different electrolytes with similar thicknesses and porosities. In the inset, optical images of the films are shown.

Figure 6(a) shows the reflectance for AAO films anodized in oxalic acid and ethylene glycol containing sulfuric acid; the thickness (437 ± 1.5 nm – 435 ± 3.3 nm) and the porosities (13.6%-13.2%) are similar. The effective refractive indexes are 2.01 ± 7.31 x 10-3 and 1.99 ± 1.56 x 10-2 for oxalic acid and ethylene glycol containing sulfuric acid, respectively. These effective refractive indexes correspond to gold colour when 8 nm of chromium layer is deposited above these AAO films. Figure 6(b) shows the reflectance for AAO films anodized in oxalic and sulfuric acids. In this case, the thicknesses (935 ± 1.8 nm -983 ± 4.8 nm) and the porosities (13.6% and 11.5%) are also similar. The effective refractive indexes are 1.42 ± 3.01 x 10-3 and 1.34 ± 6.99 x 10-3 for oxalic and sulfuric acids, respectively. These effective refractive indexes correspond to green colour when 8 nm of chromium layer is deposited above these AAO films. Thus, it was verified that the effective refractive index depends only on the thickness and porosity. With the purpose of validating the experimental results, Comsol simulations of the reflectance as a function of wavelength were performed. As it was stated above, the effective refractive index or the reflectance only depends on the porosity and the thickness. In order to confirm this, two distinct simulations of the reflectance were performed: different pore diameter and interpore distance while keeping the porosity and thickness constant (see Figure 7(a)) and sweeping the thickness while keeping the porosity constant (see Figure 7(b)).

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a

b

1.0

0.8

1.0

0.8

Reflectance (%)

Reflectance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Dp=35nm,Dint=98nm,bl=46nm,h=650nm Dp=23nm,Dint=65nm,bl=29nm,h=650nm Dp=17nm,Dint=49nm,bl=22nm,h=650nm

0.6

0.4

0.2

450 nm 650 nm 850 nm 1050 nm

0.6

0.4

0.2

0.0 300

400

500

600

700

800

0.0 300

900

400

 (nm)

500

600

700

800

900

 (nm)

Figure 7. (a) UV-Vis reflectance spectra simulated by Comsol for AAO-Al films with different morphologies, but with the same thickness; and (b) UV-Vis reflectance spectra simulated by Comsol AAO-Al films, with different thicknesses.

In the first case, three different pore diameters (17 nm, 23 nm, and 35 nm) with the corresponding interpore distances (49 nm, 65 nm, 98 nm) were used to maintain the porosity, while fixing the thickness to 650 nm. It is verified that the reflectance spectrum is the same when the porosity and the thickness are the same. In the second case, the AAO films had 35 nm pore diameters and 98 nm interpore distances, while the thicknesses were variated from 450 nm to 1050 nm. The reflectance spectrum changes when the thickness changes.

4. Conclusions Metal(Cr)-AAO-Al nanostructures have been fabricated using sputtering deposition and a two-step anodization process with different electrolytes. The morphological parameters, thickness, electrolyte, pore diameter, interpore distance and porosity of the AAO films were studied. The UV-Vis reflectance as function of the thickness for the different electrolytes of metal(Cr)-AAO-Al films was analysed, thus obtaining the colour diagrams and the effective refractive indices of AAO-Al films. In conclusion, the effective refractive index and the colour obtained depend on the thickness and porosity of AAO films, but not on the pore diameter, interpore distance or electrolyte used. This study clarified the morphological parameters which are controlling the change of colour observed in metal-AAO-Al nanostructures, while using fabrication methods, aluminium anodization and sputtering deposition, which are scalable to industrial processes. The colour tuning observed in metal-AAO-Al nanostructures is due to the metal layer deposited on the AAO-Al films and the thickness and porosity of the 13



AAO films. The anodization conditions used don’t affect the colours observed; the important parameters are the porosity and thickness of the AAO films. This conclusion was validated experimentally and theoretically using Comsol simulations. This study elucidates for the first time the effects of morphological parameters on the effective refractive index of AAO films.

AUTHOR INFORMATION Corresponding Author: *Email: [email protected]. ORDIC: 0000-0001-5708-6544

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors would like to acknowledge financial support from Commission for Technology and Innovation (CTI-16977.2 PFNM-NM) of the Swiss Confederation. C.V.M. would like to acknowledge funding from the Marie Curie Actions cofound program of the EU's Seventh Research Framework Program (FP7).

Supporting information Figure S1 Field emission scanning electron microscopy (FE-SEM) image of AAO film after focus ion beam (FIB) cutting.

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Controlling the Color and Effective Refractive Index ... - ACS Publications

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