World Journal Of Engineering
Cr nanostructures fabricated based on porous anodic alumina membranes
Hany Hamdy 1*, Sang-Wan Ryu 2, Mohamed Shaban 1†
1
Department of Physics, Faculty of Science, Beni-Suef University, Beni-Suef, Egypt
2
Department of Physics, Chonnam National University, Gwangju, South Korea
* E-mail:
†
hshamdy@hotmail.com,
E-mail: mssfadel@yahoo.com; mshaban_bsu@bsu.edu.eg
In this paper, we propose an efficient and effective method to fabricate highly ordered Cr nanoarrays of
different morphologies. Nanoporous hexagonal array of Cr nanoparticles with sub-gaps less than 15 nm and particle
size less than 50 nm are fabricated on the top surface of porous anodic alumina membrane (PAA) fabricated with the
assistance of cathodic polarization and pore widening process. In addition, V-Shape Cr-nanoarrays are fabricated on
the top surface of PAA membrane prepared without the assistance of cathodic polarization and pore widening. A
field emission scanning electron microscope (FE-SEM) and atomic force microscope (AFM) were used to
characterize the morphologies and structures of the Cr nanoarrays and the PAA membranes.
1. INTRODUCTION
In recent years, fabrication of noble metal nanoarrays
has attracted much interest owing to their potential use in
many applications.1,2 However, there is a strong demand for
cheap and reliable material that can be patterned in two and
three dimensions. We believe that chromium is in many
ways an excellent material for fabrication of nanostructures.
The grain size is typically very small (of order 5 nm or less),
the surface mobility is very low, and the stability under
exposure to air is excellent. In addition, Chromium has
found various applications in many fields including
metallurgy,
catalysis,
magnetic
fields,
and
semiconductors.3-6 Moreover, it is widely accepted that 2D
and 3D nanostructures are important components for
nanoscale devices with many promising applications, due to
their large surface areas and other unique properties.7-9 In
other hand, porous anodic alumina (PAA) membranes have
been widely used as templates in the fabrication of various
nanostructures because of the cheap equipment, easy
technology, high-throughput, and high controllability of the
process.10,11 Consequently, it will be very interesting to
fabricate 2D Cr nanoarrays based on PAA membranes.
2. EXPERIMENTAL DETAILS
Two step anodization was utilized to fabricate
highly ordered PAA membranes. 10,11 The first anodization
process was performed at 40 V in 0.3 M oxalic acid at 9 oC
for 3 h. After the removal of the aluminum oxide, the
second anodization was performed at the same conditions
for 5 min followed by barrier-thinning process.11 In order to
study the effects of the combined cathodic polarization and
pore widening processes on the morphology of the
fabricated Cr nanostructures another set of samples was
fabricated with the assistance of 10 min cathodic
polarization and 70 min pore widening process under the
same experimental conditions.11
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Cr thin layers were sputtered on the PAA membranes by
means of r.f. magnetron sputtering. Magnetron sputtering
was carried out in pure argon with an r.f. power of 150 W
for varying amounts of time. The total gas pressure during
deposition was 5 mTorr. The substrate temperature during
the sputtering was set at room temperature.
A field emission scanning electron microscope (FESEM), and atomic force microscopy (AFM) were used to
characterize the morphologies and structures of the PAA
membranes and the sputtered thin films.
3. RESULTS AND DISCUSSION
3.1. Nanoporous hexagonal array of Cr nanoparticles
Figure 1(a) illustrates a typical SEM top view of PAA
membrane anodized for 5 min and pores widened for 70 min after
10 min cathodic polarization.
Hexagonal aligned nanopores were formed in the PAA
membrane. The pore diameter is about 79 nm, the interpore
distance is 100 nm, and the pore density is approximately
1.1 × 1010 cm-2. A scraggly surface with nanoprotuberances
can be seen clearly at the oblique view AFM of the PAA
membrane as shown in figure 1(b). Around each nanopore
there are six active nanodots with a hexagonal
arrangement.12
Fig. 1. (a) Top view SEM image and (b) oblique view AFM image
of blank PAA membrane anodized for 5 min and pores widened
for 70 min after 10 min cathodic polarization.
World Journal Of Engineering
Figs. 2(a) and (b) show oblique view AFM images of
PAA membranes anodized for 5 min and sputter-coated with
Cr for 40 and 120s, respectively. The insets of Figs. 2(a)
and ( b) show top view SEM images. It is seen that each of
the six active dots surrounding the pores have acted as seed
for growing a Cr nanoparticle on top of PAA membrane.
There are also very narrow gaps, sub-gaps ≤ 15 nm,
between the neighboring Cr nanoparticles as seen in the
oblique and cross-sectional SEM images in Fig.2(c and d).
The possibility of tuning the size of these nanoparticles as
well as the distance between them is depending on the
length of the sputtering time. Additionally, the pore center
surrounded, and to some degree covered, by the Cr
nanoparticles. The mean height of the Cr nanoparticles is
increased from 30 to 70 nm whereas the diameter is
increased from 35 to 49 nm with increasing the sputtering
time from 40 to 160sec. The mechanism of formation of
nanoparticles by self agglomeration on porous anodic
alumina membranes was discussed in our previous report.13
23 nm. This can be attributed to the conformable and tight
adherence of the Cr film to the pore surface. In addition, the
rate of growth of V-shape height is estimated to be ~
0.113nm/s.
Fig.3. SEM images of PAA template anodized for 5 min (a) top
view, and (b) cross-sectional view.
Fig.4. Cross-sectional SEM images of PAA membranes, anodized
for 5 min after sputtering with Cr for (a) 80, and (b) 160 sec.
4. CONCLUSION
Aligned 2D- Cr nanostructures have been fabricated on
surface of PAA membranes by r.f. magnetron sputtering.
The sputtering time and PAA structural parameters can have
great effects on the morphology of the resulting
nanostructures. The synthesis method used in this study has
several merits such as low cost, mass production, reduced
impurities, and tailored dimensions of the resulting Cr
nanostructures.
Fig.2. AFM images of PAA membranes after sputter-coating with
Cr for (a) 40, (b) 120 sec; (c) oblique angle and (d) cross- sectional
view SEM. The insets of (a and b) show top view SEM images.
3.2 V-Shape Cr nanoarrays
Figure 3 illustrates SEM images (a) top view and (b)
cross-sectional view of blank PAA membrane anodized for
5 min and prepared without the assistance of cathodic
polarization and pore widening processes. The pore
diameter is about 31 nm, whereas the interpore distance and
the pore density are the same as in Fig.1. In this case the six
active dots around each nanopore are grown with larger size
but not sharper as in Fig.1.
Figure 4 shows cross-sectional SEM images of PAA
membranes sputter-coated with Cr for 80, and 160s,
respectively. It is seen that each of the six active dots
surrounding the pores have acted as seed for growing a Cr
nanoparticle on top. In this case, the size of the active dots
is very large and distance between them is very small, so
the neighbored Cr nanoparticles are side-by-side
coalescence and tailored to the "V" shape as the sputtering
time increases.
From the microscopic analysis of the SEM images, a
reduction in the pore diameter is observed from about 31 to
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