Tuning optical properties of photonic crystal of anodic alumina and the influence of electrodeposition

Article abstract of DOI:10.1149/1.3223992

In this paper, the tuning optical properties in a photonic crystal of an anodic alumina were investigated together with the influence of the metal electrodeposition on the photonic crystal with an Al substrate. The transmission spectra of the photonic cry

Full text of DOI:10.1149/1.3223992

Tuning Optical Properties of Photonic Crystal of Anodic Alumina
and the Influence of Electrodeposition
X. Hu, Z. Y. Ling, T. L. Sun and X. H. He
J. Electrochem. Soc. 2009, Volume 156, Issue 11, Pages D521-D524.
doi: 10.1149/1.3223992
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Journal of The Electrochemical Society, 156 ͑11͒ D521-D524 ͑2009͒
D521
0013-4651/2009/156͑11͒/D521/4/$25.00 © The Electrochemical Society
Tuning Optical Properties of Photonic Crystal of Anodic
Alumina and the Influence of Electrodeposition
X. Hu,z Z. Y. Ling, T. L. Sun, and X. H. He
Department of Electronic Materials Science and Engineering, South China University of Technology,
Guangzhou 510640, China
In this paper, the tuning optical properties in a photonic crystal of an anodic alumina were investigated together with the influence
of the metal electrodeposition on the photonic crystal with an Al substrate. The transmission spectra of the photonic crystal show
the redshift of the transmission dip with increasing anodic voltage and the blueshift of the color with observable angles obeying
the Bragg law. The electrodeposited metal layer strongly absorbs the reflecting light from the Al substrate, and it makes the
photonic crystals with an Al substrate show vivid colors.
© 2009 The Electrochemical Society. ͓DOI: 10.1149/1.3223992͔ All rights reserved.
Manuscript submitted July 6, 2009; revised manuscript received August 17, 2009. Published October 1, 2009.
Photonic crystals have been the focus of many researchers over
the past two decades because of their applications ranging from
optical communications to chemical and biological sensors.1,2 The
fabrication of photonic crystal structures operating at visible and
near-infrared wavelengths is a challenging task for nanotechnology
research. The numerous nanofabrication strategies being pursued to
fabricate photonic crystals include mainly two categories: microli-
thography and self-assembly.3 However, sophisticated equipment is
required for microlithography, and the self-assembly method has
limitations such as difficulty in engineering precise defects.4
The photonic bandgap in anodic aluminum oxide was first ob-
served by Masuda et al.5 in 1999. Wang et al.6 fabricated a photonic
crystal in anodic alumina by adjusting the anodizing cell voltage
periodically in the process of electrochemical oxidation and follow-
ing chemical etching. Lee et al.7 reported an approach termed “pulse
anodization” comprising the mild and hard anodization processes to
get a periodical structure. More recently, Losic and Losic8 fabricated
three-dimensional periodical porous structures by combining the cy-
clic anodization approach and chemical etching.
In the industrial application, anodic oxide on aluminum can be
colored by various methods such as electrolytic coloring.9 In elec-
trolytic coloring, small particles of metal are deposited at the bottom
of the pores of the oxide film. Chen et al.10 reported that the color
tuning in thin films of nickel nanowires grown inside an anodic
alumina oxide template obeys the rule of interference. The photonic
crystal of porous alumina has great potential for the coloring of
anodized aluminum11 for practical application. But, the color cannot
be obtained from the photonic crystal with an Al substrate because
of the strong light reflection from the interface between the oxide
film and the Al.
H2SO4, 4–6 g/L citric acid, and 15–25 g/L thiourea. The residual
aluminum on the back side was removed by immersing it into a
saturated CuCl2 solution. The morphology of the membranes was
examined with an LEO1530VP field-emission scanning electron mi-
croscope. The optical characteristics were measured with a spectro-
photometer ͑TU-1901͒, and the reflection spectra were performed
under a 5° incident angle. The optical images were characterized by
an optical digital camera.
Results and Discussion
Anodization was conducted under a periodic cell voltage, as
shown in Fig. 1a. The cell voltage decreases linearly from U1͑U0
+ 10 V͒ to U2͑U0 − 10 V͒ in 40 s followed by a steady state at U2
for 30 s, and then it increases sinusoidally from U2 to U1 in 10 s.
Here, U0 is defined to be a benchmark anodic voltage. The color is
expected to be brighter for the broader reflection peak, which is
obtained by adjusting the anodization voltage. Therefore, 1 V is
reduced for U0 after 200 periods, as shown in Fig. 1b. Samples
1#–7# correspond to the initial benchmark anodic voltages ͑U0͒ of
39, 38, 37, 36, 35, 34, and 33 V. The characteristics of the current
density time can be explained by the change in the voltage and the
thickness of the barrier, which is determined by the thinning of the
field-assisted dissolution; the film growth and the details for the
characteristics are reported in Ref. 12. The sketch map of the micro-
structure of the photonic crystal is also shown in Fig. 1c. The mi-
crostructures shown in Fig. 2 are observed in the middle of the
fracture surface. To get statistically significant data on the distance
between the branched channels, 10 periodicities were calculated. As
shown in Fig. 2, the average distances between the branched chan-
nels of 2#, 3#, 5#, and 7# are 226, 215, 184, and 157 nm. The
distance between the branched channels is controlled by adjusting
the initial benchmark anodic voltage ͑U0͒.
In the present work, the tuning of optical characterization was
obtained by adjusting the function of anodization voltage, and the
aluminum was colored for photonic crystal of anodized alumina by
metal electrodeposition.
The transmission spectra for the film are shown in Fig. 3a. In the
present work, photonic crystals of anodized alumina with a series of
wavelengths of the transmission dip were obtained by adjusting the
anodization voltage, and all the intensities of the transmission dip
were kept at a low value. A redshift of the transmission dip is found
for the increasing anodization voltage. The diffraction is similar to
X-ray diffraction and optical reflection spectra in thin film. The fol-
lowing equation should be considered
Experimental
For the synthesis of a porous alumina membrane, high purity
aluminum sheets ͑99.99%͒ were degreased in acetone and alcohol
separately, and then the sample was electropolished in a mixture of
C2H5OH and HClO4 ͑ratio by volume was 4:1͒ to smooth the sur-
face. Anodization was conducted in 0.3 M H2C2O4 solution at 30°C
under a periodic cell for 267 min. Finally, the voltage was lowered
stepwise down to 3 V to thin the bottom barrier layer. The general
porous alumina was conducted under 40 V for 2 h at 30°C. Elec-
trodeposition was carried out with a constant current density
͑0.5 mA/cm2͒ ͑1 min for photonic crystal and 3 min for general
porous alumina͒. The electrolyte used to deposit had the following
composition: 30–50 g/L NiSO4, 10–15 g/L SnSO4, 20–25 g/L
2d͑n22 − n21 sin2 ␪͒ = m␭
͓1͔
1/2
where n2 and n1 are the average refractive indexes of the alumina
membrane and air, respectively, ␪ is the incidence angle, and d is the
distance between the branched channels, as shown in Fig. 1c and 2.
␭ is the wavelength of the diffracted light in air, usually character-
ized by the minimum in transmission or maximum in reflectance
spectra. m is an order of diffraction, and the transmission dips in
samples 1#–7# represent the first order of the diffraction. It is sup-
posed that the thickness of the film is proportional to the integral of
current density to time
z E-mail: huxing@scut.edu.cn
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D522
Journal of The Electrochemical Society, 156 ͑11͒ D521-D524 ͑2009͒
50
50
40
30
20
40
30
20
10
0
4000
8000
12000
16000
Time(s)
(a)
Figure 2. The cross-sectional scanning electron microscopy images of the
alumina membrane ͑the middle of the fracture surface͒.
50
50
40
30
20
U1
40
30
20
10
100
80
U2
60
40
80
120
160
200
240
Time(s)
20
6# 5#
500
7#
2#
1#
4#
(b)
3#
600
0
300
400
700
800
Wavelength(nm)
(a)
0.2
0.4
0.6
0.8
1.
7500.0
1#
700
650
600
550
500
450
2#
3#
(c)
4#
Figure 1. Curves of ͑a͒ the periodic cell voltage time and current density
time and ͑b͒ the enlarged part; ͑c͒ illustrations of the main channel and the
branched channel in a photonic crystal of anodic alumina.
5#
6#
7#
Q =
idt
͓2͔
͵
33
34
35
36
37
38
39
where Q is the charge passed during time ͑⌬t͒ and i is the current
density at time ͑t͒. The distance between the branched channels is
proportional to the current density and time. The wavelength of the
dips increases with the increasing benchmark anodic voltage shown
in Fig. 3b because of the increasing distance between the branched
channels ͑Fig. 2͒, which could be reflected by the current density
͑not shown͒.12
Voltage(V)
(b)
Figure 3. ͑a͒ Tuning optical transmission spectra of the anodic alumina; ͑b͒
the wavelength of transmission dip as the function of the initial benchmark
anodic voltage.
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Journal of The Electrochemical Society, 156 ͑11͒ D521-D524 ͑2009͒
D523
100
6#
3#
5#
80
60
40
20
0
2#
4#
7#
(a)
(b)
300
400
500
600
700
800
2 #
Wavelength(nm)
(a)
Figure 4. ͑Color online͒ Photographs of photonic crystal ͑a͒ with black
background at different observable angle ͑reflection angle͒ and ͑b͒ with Al
substrate.
100
80
60
40
20
0
100
80
60
40
20
0
A
B
7#
5#
6#
The photographs of the photonic crystal with a black background
are shown in Fig. 4a. The anodic alumina with the color from blue
͑7#͒ to red ͑2#͒ is obtained. The color of photonic crystal is strongly
dependent on the observable angle, and the blueshift in color is
reasonably well described by Bragg’s law,13 as shown in Fig. 4a.
Though the vivid color is observed for the photonic crystal with a
black background, the color of the photonic crystal with an Al sub-
strate is not visible, as shown in Fig. 4b. In chroma theory,14 the
incorporation of white light decreases the purity of the color. As
shown in Fig. 5a, the reflection spectra of the porous alumina with
an Al substrate include the diffraction of the porous alumina and the
reflection from the alumina film/Al interface. The strong reflection
from the Al substrate results in invisible color of the photonic crys-
tal.
4#
3#
300 400 500 600 700 800
Wavelength(nm)
2#
300
400
500
600
700
800
Wavenumber(nm)
To color the photonic crystal with Al substrate, the reflection
from Al has to be suppressed and the diffraction from the photonic
crystal should not be affected. An effective way is to introduce an
absorption layer at the bottom of the anodic alumina. To absorb the
light, nickel is always used for the black coloring process, as in
thermal energy production from solar energy.15 In the present work,
tin/nickel is used for the deposition in the porous alumina.16 With
enough deposition time, the reflection from Al substrate can be sup-
pressed seriously. The black was obtained in the general porous
alumina template, as shown in the inset of Fig. 5b. The discussion
on the details of the deposition is beyond the scope of the present
paper.
As shown in Fig. 5b, after the deposition, the intensity of reflec-
tion from the Al substrate is reduced by more than 30% compared
with that before deposition. It indicates a strong absorption for the
deposited Sn/Ni layer. The difference in the intensity of the reflec-
tion peak between Fig. 5a and b may be from a discrepancy in the
samples such as a contamination or bend. The photographs of pho-
tonic crystal with electrodeposited metal having tuned optical prop-
erties are shown in Fig. 6a. After deposition, the color evolved from
blue to red in the photonic crystal with Al substrate. The difference
between Fig. 4a and 6a is possibly from the resident light reflection
from the Al substrate, as shown in Fig. 5b. It also indicates that the
color could be tuned by adjusting the reflection from the Al sub-
strate, which is controlled by the deposition time. As shown in Fig.
6a, with the increasing observable angle, the color of the photonic
crystal could change from blue to black or from black to red ͑not
shown͒ obeying the Bragg law.
(b)
Figure 5. Optical reflection spectra of ͑a͒ photonic crystal with Al substrate
and ͑b͒ electrodeposited photonic crystal with Al substrate ͓The inset shows
the reflection spectra of general porous alumina with Al substrate ͑A͒ with-
out electrodeposition and ͑B͒ with electrodeposition͔.
The sketch map of the optical processes is also illustrated in Fig.
6b. When the light beam is incident on the anodic alumina, the light
obeying the Bragg law is reflected; the remaining beam is attenuated
during propagation through the deposition layer; the beam passed
through the deposition layer is reflected on the interface and then
suppressed seriously when passing through the deposition layer
again; all the light attenuates when propagating through the alumina
for the absorption and scattering; the light reflects on the interface of
air/alumina and air/deposition, but it could be neglected. Finally, the
emission light from the photonic crystal includes the diffraction light
and the fading reflection light from Al substrate.
Conclusions
In the present paper, the tuning of optical properties in photonic
crystal of anodic alumina was obtained by adjusting the anodization
voltage, and the aluminum was colored for photonic crystal with
metal electrodeposition. The transmission spectra of photonic crystal
show the redshift of the reflection peak with the increasing anodic
voltage and the blueshift of the color with observable angles corre-
sponding to the Bragg law. The variation in the optical properties of
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Journal of The Electrochemical Society, 156 ͑11͒ D521-D524 ͑2009͒
electrodeposited metal layer strongly absorbs the reflecting light
from the Al substrate, and it makes the photonic crystals with an Al
substrate show vivid colors. It is expected that the present process
can open up an area for the industrial application of photonic crys-
tals.
Acknowledgment
This work was supported by the Scientific Research Foundation
for the Returned Overseas Chinese Scholars, State Education Min-
istry and the Scientific and Technological Planning Project of
Guangdong Province ͑no. 2007A010500012͒.
South China University of Technology assisted in meeting the publication
costs of this article.
(a)
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photonic crystal with the variation in periodic anodizing is due to
the variation in the distance between the branched channels. The
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