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Abstract
The present work is aimed to fabricate and characterize two dimension
photonic crystals composed of metal- dielectric multilayers namely, gold
nanorods/porous anodic alumina (Au/PAA) composites, as well as Au
decorated PAA membranes. Industrially pure aluminum foil in a 0.3M oxalic
acid has been used effectively for forming such self-ordered PAA
membranes. Tuning of the height and pore diameter for PAA with a highly
uniform, ordered, nanoporous, arrayed structure has been successfully dealt
with. The obtained results showed that:
1- The second anodization process and pore widening times were found to be
the key factors affecting the PAA height and pore diameter.
2-The PAA height increases linearly with the second anodization time and
the etching rate was found to be 64 nm /min.
3-The pore diameter of PAA is regularly and monotonously widened in pore
widening solution from 27 to 61 nm at rate of 0.435 nm /min.
The subsequent target of the present work is to remove entirely the
barrier layer that exists between the oxide film and Al substrate. The
cathodic polarization and the pore widening processes were combined and
applied successfully to remove entirely such barrier layer. Processes
optimization lead to the following:
1-The complete removal of the barrier layer occurred with cathodic
polarization of 7.5 min and pore widening of 70 min.
2- It was possible to control the pore diameter from 70 to 90 nm without any
damage to the PAA template.
3- The main advantages of these templates are;
(i) sufficient substrate adhesion,
(ii) uniform removal of the oxidized barrier layer,
(iii) no corrosive pits on the Al substrate at the optimization
conditions,
(iv) better size control,
(v) more uniformity.
The above mentioned advantages make these PAA templates adequate
for using them efficiently to make highly ordered nanoarrays (i.e., with high
fill factor and high uniformity). This was achieved by using DC
electrodeposition technique, the experimental work yielded;
1-Highly ordered gold nanorods with uniform rod diameter and height
are successfully deposited inside these PAA templates.
2-Detailed spectral reflection studies of dense and uniform Au/PAA
composites and freestanding Au nanorods arrays revealed the presence
of three resonance minima.
3- The position and reflection intensity of such minima are clearly
dependent on aspect ratio, polarization direction, angle of incidence
and dielectric environment.
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4- This results reveal the existence of a stronger fundamental
transverse SPR than was previously reported in addition to higher
order transverse SPR.
In addition, and by using r.f. magnetron sputtering, successful
decoration of the interior walls of these PAA membrane with metal
nanoparticles simultaneously with the formation of a uniform ultrathin porous
film of gold nanoparticles with sub-gap < 25 nm on the surface of the
membrane, have been achieved. The coated PAA membrane exhibited a
brilliant and tunable interference color with much higher saturation. Detailed
spectral reflection studies of these structures revealed the role of LSP and
PSP on the enhancement of optical multilayer interference. Moreover, the
presently obtained results include data on how the reflection spectra depend
on structures of porous Au nanoparticles arrays and PAA membrane.
The experimental results have been quantitatively analyzed based on a
suggested four-layer model. The analysis indicated that;
1- For porous multilayer metallic dielectric arrays, the strong
electromagnetic coupling between LSP and PSP with interference
bands leads to saturation of interference color.
2- A reflection phase difference close to 180o was observed for the
samples coated for longer than 60 s relative to those coated for less
than 60 s. This is interpreted as a result of the transition from
dipole-dipole interaction to conductively coupled regime.
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3- The sample color is mainly affected by many parameters such as
Au thickness (sputtering time), pore diameter of PAA membrane
(pore widening time), thickness of PAA membrane (anodizing
time), and angle of incidence.
from the results and conclusions mentioned above it is obvious that;
The fabrication approaches used in this study are simple, rapid, economical,
and reproducible. Therefore, the proposed structures can serve as building
blocks for nanophotonic and nanoelectronic devices.