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Applied Physics A
Materials Science & Processing
ISSN 0947-8396
Appl. Phys. A
DOI 10.1007/s00339-014-8242-5
Controllable fabrication of highly orderedthin AAO template on Si substrate for
electrodeposition of nanostructures
Khaled M. Chahrour, Naser M. Ahmed,
M. R. Hashim, Nezar G. Elfadill &
M. A. Qaeed
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publication is available at link.springer.com.
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Controllable fabrication of highly ordered thin AAO templateon Si substrate for electrodeposition of nanostructures
Khaled M. Chahrour Naser M. Ahmed
M. R. Hashim Nezar G. Elfadill M. A. Qaeed
Received: 30 November 2013 / Accepted: 8 January 2014
Springer-Verlag Berlin Heidelberg 2014
Abstract In this work, simple fabrication of hexagonally
highly ordered porous anodic aluminum oxide (AAO) of Althin film (1 lm) on Si substrate is described using two-step
anodization method for electrochemical synthesis of
nanostructures. In this method, the templates were prepared
under the controllable conditions of the parameters, which
give rise to the possibility of highly ordered nanopore
arrays with a well aspect ratio. Pore widening was then
fulfilled in 5 wt% phosphoric acid solution at 25 C. The
pore diameter and spacing are proportional to the applied
voltage, which is due to the mechanical stress associated
with the volume expansion of the aluminum during the
anodization according to the mechanical stress model.
Pore-widening solution adjusted the pore diameter and
thinned the AAO barrier layer at room temperature under
the control of etching time. As an application, Cu nanorods
arrays embedded in anodic alumina (AAO) template were
fabricated by dc electrodeposition. The characterization of
the AAO templates and the Cu nanorods produced was
made by X-ray diffraction, field emission scanning
microscope, energy dispersive X-ray spectroscopy and
atomic force microscope (AFM). The images of AFM
show that porous AAO template under constant voltage is
40 V which presents the optimum ordering.
1 Introduction
Template technique is one of the most successful approa-
ches for obtaining size-controllable nanomaterials [1].
Recently, porous anodic aluminum oxide (AAO) templates
have received considerable attention in synthetic nano-
structure materials due to particular characters such as
controllable pore diameter and periodicity [2]. The porous
AAO template fabrication process and mechanisms of pore
formation have been studied [3, 4]. Using porous AAO
templates for nanostructure deposition needs no costly
nanolithography. There are two major kinds of porous AAO
templates, the first-type of porous AAO template is grown
on a bulk pure aluminum foil [5], while the second-type is
grown on a substrate such as silicon [3]. The second-type of
the template is preferable due to fact that this silicon sub-
strate usually functions as an electrode as well as a
mechanical support [6]. Most of the developed methods for
producing porous AAO templates generally yield highly
ordered arrays on bulk Al foil [7]. Until now, the fabrication
of highly ordered thin films of porous AAO on Si substrate
is difficult to be formed mainly due to the complicated
surface states (roughness and crystallite sizes) and non-
uniformity of the deposited Al film [8,9] and still remains a
major challenge from the scientific and technological point
of review [10]. In spite of the wide range of promising
applications of the AAO templates, the biggest problem
remains is the barrier layer formed at the bottom of the AAO
pores, which prevents direct physical and electrical contact
to the substrate [11]. To remove the barrier layer, various
techniques such as pore widening, cathodic polarization,
voltage drop, plasma assisted etching, etc. were employed
[1214]. However, there are some challenges posed by the
thin film AAO templates, including achieving highly-
ordered AAO templates with open-through pore structure.
K. M. Chahrour (&) N. M. Ahmed M. R. Hashim
N. G. Elfadill M. A. Qaeed
Nano-Optoelectronics Research and Technology Laboratory,
School of Physics, Universiti Sains Malaysia, 11800 Penang,
Malaysia
e-mail: [email protected]
1 3
Appl. Phys. A
DOI 10.1007/s00339-014-8242-5
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In this work, we show that highly ordered thin porous
AAO templates can be grown on Si substrate under con-
trollable condition sets of anodizing process with thin Al
films using two-step anodizing process [15], leading to
perfect hexagonal pore structure, since the ordering of the
pore arrangement of porous AAO templates control the
regularity of further nanostructures fabricated using AAO
templates as host material [16]. This work focused on theinfluence of voltage and pore-widening time on the struc-
ture, order of thin porous AAO template. The advantages
of our work over the past studies are that we used a simple
pore-widening solution for adjusting the pore diameter and
thinning the AAO barrier layer at room temperature under
control of etching time for only few minutes to prepare
nanostructures scaffolds with desirable pore diameters that
we need.
2 Experimental method
P-type (100) Si substrate with area of (1.5 cm 9 1.5 cm)
was cleaned with (RCA) method prior to deposit 20 nm of
Ti film using RF sputtering, subsequently an Al (purity
99.99 %) was evaporated by e-beam on Ti film with
thickness of 1 lm. All samples were annealed at 500 C
for 2 h in a conventional furnace under nitrogen ambient.
Our experimental results show that the use of a Ti adhesion
layer prevented the pealing-off the Al film from the Si
wafer during anodization. The anodizing process was per-
formed in a special design electrochemical cell using a
platinum rod as a cathode. The electric contact was made at
the backside of the Si substrate. The samples were anod-
ized in acidic aqueous solution of 0.3 M oxalic acid under
different voltages for a certain time. After the finishing ofthe first anodizing, an aluminum oxide layer was removed
by wet etching in aqueous solution of 6 wt% phosphoric
acid and 1.8 wt% chromic acid at 60 C for 30 min. The
second anodizing was carried out with the same control-
lable conditions of the first anodizing until all of the
residual aluminum was oxidized. Pore widening is con-
trolled by immersing the AAO template into 5 wt%
phosphoric acid solution at 25 C for different times. The
electrochemical cell was cooled with circulating water bath
system to ensure a constant temperature below 20 C.
During anodizing process, the electrolyte was vigorously
stirred. Some as-synthesized AAO templates are furtherannealed at 800 C for 5 h in atmosphere.
As an application, copper nanorods were deposited po-
tentiostatically, with AAO template serving as working
electrode, platinum rod and Ag/AgCl(sat)as the counter and
reference electrodes, respectively, using a EDAQ Model
potentiostat. The electrolyte was 0.45 M of CuSO2
Fig. 1 aFESEM images show top view and a cross-sectional view of
the AAO templates. AAO templates were prepared under parameter
conditions; time of first anodization is 10 min and anodizing voltage
40, 45, 50 V, respectively. Pore-widening time is 20 min, b curve
correlating mean pore diameter with voltage, andc AFM images (i )
AAO prepared under 50 V and (ii) AAO prepared under 40 V
K. M. Chahrour et al.
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dissolved in 3 M lactic acid. The solutions PH was
adjusted to 9 using sodium hydroxide. The electrodepos-
ition was performed at an applied potential of -1 V for
5 min. The temperature was maintained at 60 C, with
constant stirring throughout electrodeposition. Upon com-
pletion, the AAO template was rinsed by deionized water
and dried. The surface morphology and structure of the
fabricated AAO templates and the deposited Cu nanorodswere studied using FESEM, EDX, AFM and XRD.
3 Results and discussion
The morphology of the fabricated AAO templates was
characterized by FESEM and AFM images. Figure1a
shows typical top view and a cross-sectional view of the
AAO templates under 40, 45 and 50 V, respectively. The
results show distinctly that an ordered honeycomb structure
with uniformity in pore diameter and spacing can be fab-
ricated with two-step anodization under controllable con-ditions; also Fig.1a illustrates that the pore diameter and
interpore distance increase with increasing voltage. The
mean of pore diameter is 65 nm and mean of interpore
distance is 110 nm for AAO template prepared under 40 V.
AAO template prepared under 45 V has a mean pore size of
70 nm and a mean interpore distance of 115 nm, while
AAO template prepared under 50 V has a mean pore size of
85 nm and a mean interpore distance of 128 nm. A curve
correlating mean pore diameter with anodizing voltage
described in Fig.1b illustrates that the mean pore diameter
is increased with increasing the voltage. In addition, AAOsample prepared under lower voltage is more regular as
shown in AFM images (Fig. 1c). The mechanism of self-
organization is not fully understood, even though it can be
explained by the mechanical stress model proposed by
Jessensky et al. [4]. Pores are first formed at certain micro-
rough region where the current density is concentrated on
after the formation of steady oxide layer and then the pores
grow vertically to surface with equilibrium of field-
enhanced oxide dissolution at the oxide/electrolyte interface
and oxide growth at the metal/oxide interface, and the
horizontal growth is performed simultaneously. In this
process, the compressive stress between the pores which areassociated with the volume expansion of the aluminum
during anodization impulses the structural adjustment to
form honeycomb like pore arrays. With the enhancement of
Fig. 2 FESEM images show top view of the AAO templates. AAO
templates were prepared under parameter conditions; time of first
anodization is 10 min and anodizing voltage 40 V. Time of pore
widening t= 10, t= 15 and t= 20 min, respectively. Curve corre-
lating mean pore diameter with pore-widening time
Controllable fabrication of highly ordered thin AAO template on Si substrate
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anodizing voltage, the horizontal growth of the pores
increases leading to the enlargement of the pore diameter
and spacing. At the same time, the arrangement among the
pores cannot keep up with the growth velocity of AAO and
the ordering of this structure might decrease.
Although AAO template prepared after two-step
anodization possesses controllable pore diameter and
periodicity, further pore adjustment can be actualized byexposure to phosphoric acid, which is called pore wid-
ening. The FESEM images of the AAO templates were
prepared under 40 V after pore widening for 10, 15 and
20 min at 25 C as shown in Fig. 2a. The pore diameter
increases, while the pore spacing almost keeps unchanged.Fig. 3 XRD spectra of AAO templatea with annealing andbwithout
annealing
Fig. 4 a, b FESEM image shows the deposition of Cu inside of AAO template and EDX spectra. c, d FESEM image shows a cross-sectional
view of Cu nanorods perpendicular to the Si substrate after removal of AAO template and EDX spectra, respectively
K. M. Chahrour et al.
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A curve correlating mean pore diameter with widening
time described in Fig.2b illustrates that the mean pore
diameter is monotonously increased with increasing the
pore-widening time. The increase of pore diameter after
the thinning of the outer and inner surface of AAO tem-
plate might be due to the wet etching effect of phosphoricacid. This provides a convenient route to prepare AAO
template of any desired pore diameter that we need. Then,
any desired nanostructure diameter can be grown in its
pore. Figure3 shows XRD spectra of AAO template after
annealing, the reflection peak of (222) corresponding to
the Al2O3 phase appear [17], which implies that the AAO
transforms from amorphous to crystalline after annealing
at 800 C.
With the aid of AAO template and electrodeposition
process, highly ordered and vertical arrays of Cu nanorods
can be prepared. Figure4a, b shows the plane view for
AAO template on a Si substrate after deposition of Cu,almost all the nanopores were filled by Cu and confirmed
with the EDX spectra. The lengths of Cu nanorods grown
inside the AAO template were longer than the depth of
AAO template whereby all the nanorods showed sign of
overgrowth. Figure4c shows the Cu nanorods remained
vertically to the Si substrate and could be observed clearly.
It is worth noting that the heights of the nanorods were
uniform and remained separated from each other after the
removal of the AAO template. The length of the nanorods
was 1.25 lm and the approximate diameter was 50 nm.
Note that some of the Cu nanorods near the front edge were
broken during the splitting process for the preparation ofcross-sectional FESEM samples. Figure4d shows EDX
spectra that confirm the compositions of the nanorods on Si
substrate after the removal of the AAO template. Figure5
shows the X-ray diffraction patterns of copper nanorods
embedded in AAO template. The nanorods were poly-
crystalline with cubic structure indicated by the presence of
two prominent peaks close to 2h angles of 43.41 and
50.60, corresponding to Cu (111) and Cu (200) diffrac-
tions, respectively.
4 Conclusions
Highly ordered pore arrays in AAO templates with uniform
pore size and vertically aligned nanotubes with well aspect
ratio were successfully fabricated by anodization of thin Al
film on Si wafer. Anodizing voltage and time of pore
widening are explored in our experimental conditions.
FESEM analysis show that the pore diameter depends onboth anodizing voltage and time of pore widening, also the
AAO template prepared under 40 V present the optimum
ordering as shown in the AFM images. The relation
between the ordering of the pore arrays and anodizing
voltage is explained by a growthdissolution model. A
simple wet etching process is used for thinning the bottom
barrier layer of AAO template and widening of any desired
pore diameter that we need under a control of etching time.
FESEM analysis show that highly orderly self-aligned Cu
nanorods have been prepared on AAO template/Si sub-
strate using electrochemical process. Finally, stirring and
maintaining the electrolyte below 20 C are critical steps inobtaining ordered pore arrays.
Acknowledgments We gratefully acknowledge the support of the
School of Physics, University Saince Malaysia under short term Grant
No. 304/PFIZIK/6312076.
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Fig. 5 XRD spectra of copper nanorods embedded in AAO template
Controllable fabrication of highly ordered thin AAO template on Si substrate
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