Tuning nanoporous anodic alumina distributed-Bragg reflectors with the number of anodization cycles and the anodization temperature
© Ferré-Borrull et al.; licensee Springer. 2014
Received: 16 May 2014
Accepted: 16 July 2014
Published: 21 August 2014
The influence of the anodization temperature and of the number of applied voltage cycles on the photonic properties of nanoporous anodic alumina-based distributed-Bragg reflectors obtained by cyclic voltage anodization is analyzed. Furthermore, the possibility of tuning the stop band central wavelength with a pore-widening treatment after anodization and its combined effect with temperature has been studied by means of scanning electron microscopy and spectroscopic transmittance measurements. The spectra for samples measured right after anodization show irregular stop bands, which become better defined with the pore widening process. The results show that with 50 applied voltage cycles, stop bands are obtained and that increasing the number of cycles contributes to enhancing the photonic stop bands (specially for the case of the as-produced samples) but at the expense of increased scattering losses. The anodization temperature is a crucial factor in the tuning of the photonic stop bands, with a linear rate of 42 nm/°C. The pore widening permits further tuning to reach stop bands with central wavelengths as low as 500 nm. Furthermore, the results also show that applying different anodization temperatures does not have a great influence in the pore-widening rate or in the photonic stop band width.
KeywordsNanoporous anodic alumina Distributed-Bragg reflectors Photonic properties tuning, Anodization temperature Pore widening Cyclic voltage anodization
Nanoporous anodic alumina (NAA) is a material of great interest in nanotechnology because of its cost-effective and easily up-scalable production techniques [1–3] and also because of its vast field of applications [4–8]. This material consists of an array of cylindrical pores in an aluminum oxide matrix obtained by electrochemical anodization of aluminum. In the appropriate fabrication conditions, the pores self-arrange in a triangular lattice with domains containing several hundreds of pores . This pore arrangement is usually obtained with three kinds of acid electrolytes (oxalic, phosphoric, or sulfuric) and in two different regimes, known as hard and mild anodization .
The photonic properties of NAA make this material specially interesting in optical applications such as biosensing [11–14] In previous works, the authors described the existence of photonic stop bands for light propagating inside the material  in a direction perpendicular to the pore axes, and also described a method to obtain distributed-Bragg reflectors (DBRs)  based on NAA. DBRs are dielectric multilayer structures [17–20] with a periodic variation of the refractive index in the direction perpendicular to the surface. This gives rise to photonic stop bands for light incident in a direction parallel to the pore axes. The central wavelength of such stop bands depends on the effective refractive index and on the optical thickness of each of the cycles, while the width of the bands is directly related with the contrast of the refractive index variations. Ideal photonic stop bands are achieved for infinite periodic structures [21, 22]. However, DBR structures are finite and consequently, the characteristics of the photonic stop band depend on the number of cycles they contain.
NAA-based DBR can be achieved by taking advantage of the fact that a wet etching applied after the anodization to enlarge the pore diameter (pore-widening step) has a different rate depending on the used anodization voltage . Thus, by combining a cyclic anodization voltage with a subsequent pore-widening step, tunable in-depth modulation of the pore diameter and effective refractive index variations are obtained. Other authors have reported on the fabrication of DBR structures by applying a cyclic anodization voltage [19, 20, 24] although they did not stress the importance of the pore-widening step in order to obtain the photonic stop bands.
Temperature is also a key factor in the fabrication of NAA structures [25, 26], as it is directly influencing the reaction speed. By lowering adequately the temperature, an increase in anodization voltage is possible so that hard-anodization NAA can be obtained without the need of an initial protective layer . The color of the NAA can also be influenced by temperature .
In this work, we study the influence of the number of cycles and of the anodization temperature on the optical properties of NAA-based DBR. We also study how the pore-widening step (necessary to obtain the well-defined photonic stop bands) can be combined with these parameters in order to adjust the stop band position of the fabricated structures.
For the synthesis of NAA-based DBR, we have used high-purity Al substrates (99.99%) of 500-μm thickness from Sigma-Aldrich (St. Louis, MO, USA). A pretreatment is required to meliorate the physical properties of the commercial Al substrate: first, the Al substrates were rinsed in deionized water, then cleaned with ethanol and rinsed in deionized water again, then dried with N2 and stored in a dry environment. Then, the surface roughness was reduced by an electropolishing process performed at room temperature and at 20 V for 4 min in a 1:4 v/v mixture of perchloric acid and ethanol. The sense of the stirrer was switched every 1 min. After electropolishing, the samples were cleaned in water. A first anodization was performed on the electropolished Al surface using 0.3 M oxalic acid (H2C2O4) solution at a temperature of 7°C. The anodization process was carried out in a PVC cell cooled by a circulating system (Thermo Scientific, Waltham, MA, USA) with continuous stirring, which ensured a stabilized temperature within an accuracy of less than 0.5°C. The working surface area of the samples was 1.4 cm2. A Pt grid was used as a cathode, and the distance between the two electrodes was about 2 cm. The electrochemical process was controlled by a lab-view program that saved the data of current and voltage and the amount of charge flown through the system every 200 ms. The process was carried out at a constant voltage (V) of 40 V for 20 h. The resulting nanostructure after this first anodization step is a thin film of alumina with disordered pores at the top but self-ordered pores at the bottom. This alumina film was dissolved by wet chemical etching at 70°C in a solution of chromic and phosphoric acids (0.4 M H3PO4 and 0.2 M H3CrO4), stirred at 300 rpm for 4 h. A number of samples were prepared in order to examine the effect of the applied number of cycles (NC) and of the anodization temperature (Tanod).
In order to examine the effect of the number of cycles, two types of samples having different NC were fabricated. A detail of the applied anodization voltage to one of the samples is shown in Additional file 1: Figure S1 where Figure S1(a) in Additional file 1 represents the voltage profile of entire anodization process with 50 cycles, while Figure S1(b) in Additional file 1 represents the voltage profile of one cycle. The anodization process started at 20 V and it lasted until a charge of 2 C flowed through the system. In this way, a self-ordered layer of vertical pores was obtained. To obtain the DBR structure, after this anodization at 20 V, the cyclic anodization process started immediately. Each cycle consisted of three phases: (I) a linear increasing ramp from 20 to 50 V, at a rate of 0.5 V/s, (II) an interval at 50 V for certain time duration to flow a given charge Q0 through the system, and (III) a subsequent linear decreasing ramp from 50 to 20 V at 0.1 V/s. The increasing and decreasing ramps were chosen as the fastest possible ramps in order to maintain the continuity of the anodization process. After the cyclic anodization steps finished, a final anodization voltage of 20 V was applied until 2 C of charge flowed through the system. After the anodization, a wet etching to increase pore radius (pore-widening step) was performed with 5 wt.% phosphoric acid (H3PO4) at 35°C. This pore widening was applied for different times, tPW. Samples with NC = 50 and NC = 150 cycles were obtained, with a Q0 = 0.5 C. On the other hand, samples with Q0 = 0 C were produced at four different anodization temperatures: Tanod = 8, 9, 10, and 11°C.
Results and discussion
Average stop band width and corresponding standard deviation as a function of the pore-widening time
Pore-widening time (min)
Average stop band width (nm)
Stop band width standard deviation (nm)
In this work, we analyzed the influence of the anodization temperature and of the number of applied voltage cycles on the photonic properties of NAA-based DBRs obtained by cyclic voltage anodization. In previous works, it was shown that DBR structures with stop bands can be obtained by the application of an anodization based in the repetition of voltage cycles between 20 and 50 V in 0.3 M oxalic acid. It was also shown that the application of a pore-widening step after anodization is crucial in order to obtain well-defined stop bands with low transmittance and high reflectance. In this work, these nanoporous structures have been obtained in the range of temperatures between 8°C and 11°C, for 50 and 150 applied voltage cycles and pore-widening times up to 27 min. The effect of these parameters on the morphologic and photonic properties of the nanostructures has been studied by means of SEM and spectroscopic transmittance measurements.
The results show that 50 applied voltage cycles are enough to produce stop bands and that increasing the number of cycles has two opposite effects: on one hand, an enhancement of the photonic stop bands is observed, in particular specially for the case of the as-produced samples, which is much better defined for samples with higher number of cycles. On the other hand, scattering losses are observed in the spectra caused by the irregular interfaces between cycles observed in the SEM images. Such losses increase with increasing number cycles and the corresponding interfaces.
Increasing the anodization temperature produces a remarkable shift of the photonic stop band central wavelength, with a linear rate of 42.5 nm/°C. On the other hand, a change in anodization temperature does not influence noticeably the obtained stop band widths or the rate of the subsequent pore widening. These three facts suggest that anodization temperature has a strong effect on the pore growth rate during anodization, but a small influence on the pore diameter or morphology. With this, it is also put into evidence that a precise control and stabilization of the temperature along the whole fabrication process is crucial to ensure accuracy in the tuning of the photonic stop bands.
nanoporous anodic alumina
- N C :
number of anodization voltage cycles
- T anod :
- t PW :
scanning electron microscopy
This research was supported by the Spanish Ministerio de Economía y Competitividad through the grant number TEC2012-34397 and the Generalitat de Catalunya through the grant number 2014-SGR-1344.
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