- Email: [email protected]
Thinning anodic aluminum oxide films and investigating their optical properties
Materials Letters 65 (2011) 1648–1650
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Thinning anodic aluminum oxide films and investigating their optical properties Sorina Garabagiu ⁎, Gheorghe Mihailescu National Institute for R&D of Isotopic and Molecular Technologies, Donath Str, No. 65-103, P.O. Box 700, 400293 Cluj-Napoca, Romania
a r t i c l e
i n f o
Article history: Received 8 October 2010 Accepted 20 February 2011 Available online 25 February 2011 Keywords: Porous material Anodic aluminum oxide Thin film Transmission
a b s t r a c t In this study, we propose the determination of the dissolution rate of anodic aluminum oxide barrier layer, using a new, simple electrochemical setup and the transmission spectrum of alumina, recorded before and after several successive partial removals of barrier layer of the thin film. By dissolving the barrier layer and then thinning the alumina membrane, some changes appear in the optical transmission spectrum, in both experimental and calculated examples, which provide us information on the dissolution rate of alumina barrier layer. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Materials and methods
In the recent years, nanoscale materials, especially 1D nanostructures, have attracted extensive attention, due to their electronic and optical properties, which differ essentially from the bulk material, and also due to their potential applications in nanodevices. The unique properties of the nanostructure or even of an integral functional unit consisting of multiple nanostructures are the result of collective behavior and interaction of a group of nanoelements acting together and producing responses of the system as a whole. Alumina membrane is a close-packed array of hexagonally arranged cells containing pores in each cell-center, which is obtained by anodizing aluminum. The depth of fine parallel channels can exceed 100 μm, a characteristic that makes alumina one of the most desired nanostructures with a high aspect ratio and a high pore density (109–1012 pores/cm2). A template synthesis of 1D nanostructures has proven to be an elegant, inexpensive and technologically simple approach for the fabrication of various nanoscale sophisticated materials. Anodic aluminum oxide (AAO) membranes are convenient template for synthesizing highly ordered, vertically standing nanowires in a ceramic matrix. The wires are electrically isolated from each other by surrounding alumina [1–3]. In this study, we propose the determination of AAO barrier layer dissolution rate, using the transmission spectrum of AAO (both experimental and simulated), recorded before and after controllable removal of barrier layer of the thin film. Optical properties of anodic aluminum oxide thin films are important for various applications, including photonic crystals [4] and metamaterials [5].
All materials were purchased from Sigma-Aldrich, except for aluminum (99.9% purity) that was purchased from AlRo Slatina (Romania). The aluminum foil was first annealed for an hour, at 400 °C. X-ray diffraction pattern proves that after this process the crystalline structure of aluminum is broken and replaced with an amorphous one. After that, aluminum foil was electrochemically polished (at 20 V continuous current) using a mixture of perchloric acid and ethanol (1:4 vol. ratio) at 2 °C [1], to obtain a mirror-finish. For the self-ordering of pores, a two step anodization process was performed in 0.3 M oxalic acid. The process developed at 4 °C. The applied potential was 40 V. After 4 h of anodization, the first anodized layer was removed in a phosphoric and chromic acid mixture. The second anodization process was performed in the same conditions for an hour and a half. Residual aluminum was removed in a mixture of hydrogen chloride and copper chloride. The thickness of the obtained alumina films was 3 ± 0.1 μm. In order to control the dissolution of alumina we used a double electrolytic cell (see Fig. 1). One of the compartments contains phosphoric acid (5%) that dissolves the alumina, and the other contains potassium chloride (1 M), with no effect on the alumina membrane. The AAO membrane was placed between the two compartments, with the barrier layer faced to the acid. The acid dissolves slowly the membrane, first the barrier layer and then the porous layer. An AC potential was applied between the two platinum electrodes (0.5 or 1 V). The current that passes through the double cell was recorded using a PC connected multimeter HC-3500 T. The optical transmission spectra were measured at normal incidence over the wavelength range 700–2400 nm, using a UV–vis–NIR spectrophotometer Jasco V-5600.
⁎ Corresponding author. Tel.: +40 264584037; fax: +40 264420042. E-mail address: [email protected] (S. Garabagiu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.02.068
S. Garabagiu, G. Mihailescu / Materials Letters 65 (2011) 1648–1650
1649
Fig. 1. Experimental setup for dissolving barrier layer of AAO. Fig. 2. Current intensity–time plot of AAO membrane.
3. Theory The measurement of the transmission T of light through a parallelfaced dielectric film in the region of transparency is sufficient to determine the real and imaginary parts of the complex refractive index, as well as the thickness d of the film. The alumina membrane is considered to be a thin film, standing freely in air. The film has the thickness d and the complex refractive index n = n − ik, where n is the refractive index and k is the extinction coefficient which can be expressed in terms of the absorption coefficient (α). The refraction index of the surrounding medium is n0 = 1 [6]. There is a variation in the thickness of the film, due to anodization parameters (both formation and dissolving of alumina) and to the porous structure of alumina membrane. Inhomogeneities in the film have a large influence on the optical transmission spectrum. Alumina membranes are regarded as non-uniform thin films with respect to their thickness. Taking in consideration the effects of thickness variation, Swanepoel analyzed the transmission spectrum of amorphous silicon thin films [6,7]. The envelopes around the maxima and the minima of the spectrum are considered to be continuous functions of λ and n(λ). In order to determine the thickness and optical constants from the transmission spectrum of AAO films, a modified Swanepoel method was used [7,8]. Using Swanepoel modified method, Zhao and coworkers calculated the optical constants (refractive index n and extinction coefficient k as functions of wavelength λ) of anodic aluminum oxide prepared in a mixture of electrolytes (oxalic and sulfuric acids) [8]. Xu and coworkers studied the optical properties of anodic aluminum oxide prepared in oxalic acid, with different thicknesses and porosities. Thus, considering the contribution of barrier layer to the transmission spectrum and the porosity of alumina membrane, transmission becomes [9]:
When the amplitude of oscillations is weak, the accuracy of the calculated values is strongly affected. When there is a great number of extrema in the spectrum, this leads to reduced errors while determining thickness and the order numbers of corresponding extrema [10]. 4. Results and discussion The experimental setup was developed to control the removal of barrier layer of alumina, and then thinning the porous layer. The three domains of the plot in Fig. 2 correspond as following: the first one is the time that the barrier layer completely removed, then the increasing slope corresponds to opening of pores, and when all pores are opened, the current intensity becomes constant (the third region of the plot) and the membrane is thinning. The dissolving process is not influenced by the potential difference between the two platinum electrodes in the cell, result proved by applying short potential pulses between the electrodes (1-min pulse at every 30 min) (see Fig. 3). Neither the applied potential nor temperature influences the shape of the plot, but increasing the applied potential increases the current intensity and increasing temperature accelerates the process [9]. Fig. 4 shows the good fitting results between the experimental and calculated transmission spectrum, from which the wavelengthdependent refractive index and the thickness of alumina thin film are obtained. Using modified Swanepoel method, from the transmission spectrum is obtained: film thickness = 3050 nm (corresponds to the
T = Tpores p + Tbarrier ð1−pÞ where p = porosity. The refractive index function dependence of the wavelength is: 2
nðλÞ = a = λ + b The thickness d of the layer can be calculated from two maxima or two minima using equation: d = mλ1 λ2 = ½2ðnðλ1 Þλ2 −nðλ2 Þλ1 Þ where m is the number of oscillations between the two extrema.
Fig. 3. Current intensity–time plot of AAO membrane with potential pulses.
1650
S. Garabagiu, G. Mihailescu / Materials Letters 65 (2011) 1648–1650
Fig. 4. Experimental and calculated transmission spectrum of AAO and the variation of the refractive index (inset).
experimental determination: 3 ± 0.1 μm). The variation in the thickness of the film is Δd = 110 ± 10 nm. Other result obtained from the simulation is the barrier layer thickness: 70 ± 25 nm (in agreement with literature [1], for the parameters we used). Correlation coefficient between experimental and calculated spectrum is r2 = 0.97988, which is related to a very good correlation. Cauchy interpolation of complex refractive index calculated from our simulations has the coefficients: a = 26140.336, b = 1.556 (see the inset in Fig. 4). Refractive index values are in good agreement with other studies made on anodic aluminum oxide [11]. Experimental measurements involve modifications of both film thickness and barrier layer thickness, but modification of film thickness has a larger influence on the transmission spectrum, due to a larger contribution of the porous layer to the transmission. Using the experimental setup for dissolving the barrier layer, measuring transmission spectra after several dissolving periods for the same membrane, and calculating the thickness of the films obtained, can be concluded that the dissolution rate of alumina is 0.5 ± 0.1 nm/min, at room temperature (23 °C). This value is in good agreement with bibliographic source [9]. 5. Conclusions We developed a new method to control the dissolution of barrier layer of AAO, and to thin the film, using a simple, electrochemical setup. By thinning the barrier layer and alumina membrane, some changes appear in the optical transmission spectrum, in both experimental and calculated examples. It is possible to correlate the experimental setup
for dissolution of alumina with the results obtained from the transmission spectrum, to determine the dissolution rate of alumina. From the simulations, we conclude the following: – increasing the thickness of the film will increase the number of interference fringes in the transmission spectrum, which is in good agreement with both experimental and analytical results; and – decreasing the barrier layer thickness will increase the optical transmission through the membrane, due to decreasing the overall thickness. Acknowledgement This work was supported by the National Authority for Scientific Research of Romania. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Ali Eftekhari. Nanostructured materials in electrochemistry. WILEY-VCH; 2008. Jinxia Xu, Xinmin Huang. Mater Lett 2008;62:1491–4. Yang Rui, Sui Chunhong, Gong Jian, Qu Lunyu. Mater Lett 2007;61:900–3. Mizeikis V, Mikulskas I, Tomadiunas R, Juodkazis S, Matsuo S, Misawa H. Jpn J Appl Phys 2004;43(6A):3643–7. Kanungo J, Schilling J. Appl Phys Lett 2010;97:021903. Swanepoel R. J Phys E Sci Instrum 1983;16:1214–22. Swanepoel R. J Phys E Sci Instrum 1984;17:896–903. Zhao Li-Rong, Wang Jian, Li Yan, Wang Chen-Wei, Zhou Feng, Liu Wei-Min. Phys B 2010;405:456–60. Xu WL, Chen H, Zheng MJ, Ding GQ, Shen WZ. Opt Mater 2006;28:1160–5. Manifacier JC, Gasiot J, Fillard JP. J Phys E Sci Instrum 1976;9:1002–4. Ji Nan, Ruan Weidong, Wang Chunxu, Lu Z, Zhao B. Langmuir 2009;25(19): 11869–73.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Thinning anodic aluminum oxide films and investigating their optical properties Sorina Garabagiu ⁎, Gheorghe Mihailescu National Institute for R&D of Isotopic and Molecular Technologies, Donath Str, No. 65-103, P.O. Box 700, 400293 Cluj-Napoca, Romania
a r t i c l e
i n f o
Article history: Received 8 October 2010 Accepted 20 February 2011 Available online 25 February 2011 Keywords: Porous material Anodic aluminum oxide Thin film Transmission
a b s t r a c t In this study, we propose the determination of the dissolution rate of anodic aluminum oxide barrier layer, using a new, simple electrochemical setup and the transmission spectrum of alumina, recorded before and after several successive partial removals of barrier layer of the thin film. By dissolving the barrier layer and then thinning the alumina membrane, some changes appear in the optical transmission spectrum, in both experimental and calculated examples, which provide us information on the dissolution rate of alumina barrier layer. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Materials and methods
In the recent years, nanoscale materials, especially 1D nanostructures, have attracted extensive attention, due to their electronic and optical properties, which differ essentially from the bulk material, and also due to their potential applications in nanodevices. The unique properties of the nanostructure or even of an integral functional unit consisting of multiple nanostructures are the result of collective behavior and interaction of a group of nanoelements acting together and producing responses of the system as a whole. Alumina membrane is a close-packed array of hexagonally arranged cells containing pores in each cell-center, which is obtained by anodizing aluminum. The depth of fine parallel channels can exceed 100 μm, a characteristic that makes alumina one of the most desired nanostructures with a high aspect ratio and a high pore density (109–1012 pores/cm2). A template synthesis of 1D nanostructures has proven to be an elegant, inexpensive and technologically simple approach for the fabrication of various nanoscale sophisticated materials. Anodic aluminum oxide (AAO) membranes are convenient template for synthesizing highly ordered, vertically standing nanowires in a ceramic matrix. The wires are electrically isolated from each other by surrounding alumina [1–3]. In this study, we propose the determination of AAO barrier layer dissolution rate, using the transmission spectrum of AAO (both experimental and simulated), recorded before and after controllable removal of barrier layer of the thin film. Optical properties of anodic aluminum oxide thin films are important for various applications, including photonic crystals [4] and metamaterials [5].
All materials were purchased from Sigma-Aldrich, except for aluminum (99.9% purity) that was purchased from AlRo Slatina (Romania). The aluminum foil was first annealed for an hour, at 400 °C. X-ray diffraction pattern proves that after this process the crystalline structure of aluminum is broken and replaced with an amorphous one. After that, aluminum foil was electrochemically polished (at 20 V continuous current) using a mixture of perchloric acid and ethanol (1:4 vol. ratio) at 2 °C [1], to obtain a mirror-finish. For the self-ordering of pores, a two step anodization process was performed in 0.3 M oxalic acid. The process developed at 4 °C. The applied potential was 40 V. After 4 h of anodization, the first anodized layer was removed in a phosphoric and chromic acid mixture. The second anodization process was performed in the same conditions for an hour and a half. Residual aluminum was removed in a mixture of hydrogen chloride and copper chloride. The thickness of the obtained alumina films was 3 ± 0.1 μm. In order to control the dissolution of alumina we used a double electrolytic cell (see Fig. 1). One of the compartments contains phosphoric acid (5%) that dissolves the alumina, and the other contains potassium chloride (1 M), with no effect on the alumina membrane. The AAO membrane was placed between the two compartments, with the barrier layer faced to the acid. The acid dissolves slowly the membrane, first the barrier layer and then the porous layer. An AC potential was applied between the two platinum electrodes (0.5 or 1 V). The current that passes through the double cell was recorded using a PC connected multimeter HC-3500 T. The optical transmission spectra were measured at normal incidence over the wavelength range 700–2400 nm, using a UV–vis–NIR spectrophotometer Jasco V-5600.
⁎ Corresponding author. Tel.: +40 264584037; fax: +40 264420042. E-mail address: [email protected] (S. Garabagiu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.02.068
S. Garabagiu, G. Mihailescu / Materials Letters 65 (2011) 1648–1650
1649
Fig. 1. Experimental setup for dissolving barrier layer of AAO. Fig. 2. Current intensity–time plot of AAO membrane.
3. Theory The measurement of the transmission T of light through a parallelfaced dielectric film in the region of transparency is sufficient to determine the real and imaginary parts of the complex refractive index, as well as the thickness d of the film. The alumina membrane is considered to be a thin film, standing freely in air. The film has the thickness d and the complex refractive index n = n − ik, where n is the refractive index and k is the extinction coefficient which can be expressed in terms of the absorption coefficient (α). The refraction index of the surrounding medium is n0 = 1 [6]. There is a variation in the thickness of the film, due to anodization parameters (both formation and dissolving of alumina) and to the porous structure of alumina membrane. Inhomogeneities in the film have a large influence on the optical transmission spectrum. Alumina membranes are regarded as non-uniform thin films with respect to their thickness. Taking in consideration the effects of thickness variation, Swanepoel analyzed the transmission spectrum of amorphous silicon thin films [6,7]. The envelopes around the maxima and the minima of the spectrum are considered to be continuous functions of λ and n(λ). In order to determine the thickness and optical constants from the transmission spectrum of AAO films, a modified Swanepoel method was used [7,8]. Using Swanepoel modified method, Zhao and coworkers calculated the optical constants (refractive index n and extinction coefficient k as functions of wavelength λ) of anodic aluminum oxide prepared in a mixture of electrolytes (oxalic and sulfuric acids) [8]. Xu and coworkers studied the optical properties of anodic aluminum oxide prepared in oxalic acid, with different thicknesses and porosities. Thus, considering the contribution of barrier layer to the transmission spectrum and the porosity of alumina membrane, transmission becomes [9]:
When the amplitude of oscillations is weak, the accuracy of the calculated values is strongly affected. When there is a great number of extrema in the spectrum, this leads to reduced errors while determining thickness and the order numbers of corresponding extrema [10]. 4. Results and discussion The experimental setup was developed to control the removal of barrier layer of alumina, and then thinning the porous layer. The three domains of the plot in Fig. 2 correspond as following: the first one is the time that the barrier layer completely removed, then the increasing slope corresponds to opening of pores, and when all pores are opened, the current intensity becomes constant (the third region of the plot) and the membrane is thinning. The dissolving process is not influenced by the potential difference between the two platinum electrodes in the cell, result proved by applying short potential pulses between the electrodes (1-min pulse at every 30 min) (see Fig. 3). Neither the applied potential nor temperature influences the shape of the plot, but increasing the applied potential increases the current intensity and increasing temperature accelerates the process [9]. Fig. 4 shows the good fitting results between the experimental and calculated transmission spectrum, from which the wavelengthdependent refractive index and the thickness of alumina thin film are obtained. Using modified Swanepoel method, from the transmission spectrum is obtained: film thickness = 3050 nm (corresponds to the
T = Tpores p + Tbarrier ð1−pÞ where p = porosity. The refractive index function dependence of the wavelength is: 2
nðλÞ = a = λ + b The thickness d of the layer can be calculated from two maxima or two minima using equation: d = mλ1 λ2 = ½2ðnðλ1 Þλ2 −nðλ2 Þλ1 Þ where m is the number of oscillations between the two extrema.
Fig. 3. Current intensity–time plot of AAO membrane with potential pulses.
1650
S. Garabagiu, G. Mihailescu / Materials Letters 65 (2011) 1648–1650
Fig. 4. Experimental and calculated transmission spectrum of AAO and the variation of the refractive index (inset).
experimental determination: 3 ± 0.1 μm). The variation in the thickness of the film is Δd = 110 ± 10 nm. Other result obtained from the simulation is the barrier layer thickness: 70 ± 25 nm (in agreement with literature [1], for the parameters we used). Correlation coefficient between experimental and calculated spectrum is r2 = 0.97988, which is related to a very good correlation. Cauchy interpolation of complex refractive index calculated from our simulations has the coefficients: a = 26140.336, b = 1.556 (see the inset in Fig. 4). Refractive index values are in good agreement with other studies made on anodic aluminum oxide [11]. Experimental measurements involve modifications of both film thickness and barrier layer thickness, but modification of film thickness has a larger influence on the transmission spectrum, due to a larger contribution of the porous layer to the transmission. Using the experimental setup for dissolving the barrier layer, measuring transmission spectra after several dissolving periods for the same membrane, and calculating the thickness of the films obtained, can be concluded that the dissolution rate of alumina is 0.5 ± 0.1 nm/min, at room temperature (23 °C). This value is in good agreement with bibliographic source [9]. 5. Conclusions We developed a new method to control the dissolution of barrier layer of AAO, and to thin the film, using a simple, electrochemical setup. By thinning the barrier layer and alumina membrane, some changes appear in the optical transmission spectrum, in both experimental and calculated examples. It is possible to correlate the experimental setup
for dissolution of alumina with the results obtained from the transmission spectrum, to determine the dissolution rate of alumina. From the simulations, we conclude the following: – increasing the thickness of the film will increase the number of interference fringes in the transmission spectrum, which is in good agreement with both experimental and analytical results; and – decreasing the barrier layer thickness will increase the optical transmission through the membrane, due to decreasing the overall thickness. Acknowledgement This work was supported by the National Authority for Scientific Research of Romania. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Ali Eftekhari. Nanostructured materials in electrochemistry. WILEY-VCH; 2008. Jinxia Xu, Xinmin Huang. Mater Lett 2008;62:1491–4. Yang Rui, Sui Chunhong, Gong Jian, Qu Lunyu. Mater Lett 2007;61:900–3. Mizeikis V, Mikulskas I, Tomadiunas R, Juodkazis S, Matsuo S, Misawa H. Jpn J Appl Phys 2004;43(6A):3643–7. Kanungo J, Schilling J. Appl Phys Lett 2010;97:021903. Swanepoel R. J Phys E Sci Instrum 1983;16:1214–22. Swanepoel R. J Phys E Sci Instrum 1984;17:896–903. Zhao Li-Rong, Wang Jian, Li Yan, Wang Chen-Wei, Zhou Feng, Liu Wei-Min. Phys B 2010;405:456–60. Xu WL, Chen H, Zheng MJ, Ding GQ, Shen WZ. Opt Mater 2006;28:1160–5. Manifacier JC, Gasiot J, Fillard JP. J Phys E Sci Instrum 1976;9:1002–4. Ji Nan, Ruan Weidong, Wang Chunxu, Lu Z, Zhao B. Langmuir 2009;25(19): 11869–73.