– x
The effect of the cobalt ferrites nanoparticles (CoFe2O4)
on the porous silicon deposited by spin coating
Sana. Ben Khalifa a,b,*, Malek Gassoumi c,d, Anis Ben Dhahbi a,e, Faisal Alresheedi c,
Amera Zain elAbdeen Mahmoud a, Lotfi Beji b
a
Department of Physics, College of Sciences & Arts at Ar-Rass, Qassim University, Ar Rass 51921, Saudi Arabia
of Energy and Materials (LabEM), ESSTHS, University of Sousse, 4011 H. Sousse, Tunisia
b Laboratory
c
Department of Physics, College of Sciences, Qassim University, P.O. 6644, Buryadh 51452, Saudi Arabia
Unit Advanced Materials and Nanotechnologies, University of Kairouan, BP 471, Kasserine 1200, Tunisia
d Research
e
Unit for Research in Nuclear and High Energy, Physics Faculty of Sciences of Tunis, University of Tunis El Manar, 2092
Tunis, Tunisia
Received 27 November 2019; revised 16 December 2019; accepted 17 December 2019
KEYWORDS
Ferrites
nanoparticles;
Porous
silicon;
SEM;
XRD;
Reflectance;
Ellipsometry
Abstract In this paper, we investigate a new composite that consists
of ferrites nanoparticles on porous silicon. Ferrites of cobalt
nanoparticles (CoFe2O4) were embedded using spin coating method
into porous Silicon (PSi) which is prepared by electrochemical
etching of p-type Si (1 0 0) wafers.
Scanning electron microscopy (SEM) and X-Ray diffraction (Cu-Ka
radiation) results has shown the presence of nanoparticles with
Please cite this article in press as: S.B. Khalifa et al., The effect of the cobalt ferrites nanoparticles (CoFe 2O4) on the porous silicon deposited by spin coating, Alexandria Eng.
J. (2019), https://doi.org/10.1016/j.aej.2019.12.031
2
S.B. Khalifa et al.
crystallite sizes about 7–9 nm. The SEM Image CoFe2O4 nanoparticles spin coated on ptype porous silicon shows that most of the particles are aggregated and exhibit a compact
arrangement of the nanoparticles with roughly spherical shape. Nonetheless, X-Ray
diffraction results have shown that the film have a good crystalline quality.
For the optical properties of the composite, it will be studied using various techniques
such as ellipsometry and Reflectance by a UV–VIS spectrophotometer. The reflectance
study illustrates the ability of the nanoparticles dispersion to produce optical antireflection
properties on the porous silicon. We also deduce that the reflectance of the nanoparticles
dispersed on porous Si is higher than the reflectance of the nanoparticles dispersed on Si
wafer; it is in fact due to the enormous number of nucleation sites in the case of porous
Si. The optical parameters such the refractive index and the extinction coefficient were
determined using the ellipsometry spectroscopy.
2019 Faculty of Engineering, Alexandria University. Production and hosting by
Elsevier B.V. This is an
open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
* Corresponding author at: Department of Physics, College of Sciences & Arts at Ar-Rass, Qassim University, Ar Rass 51921, Saudi Arabia. E-mail
address: sanaa.benkhalifa@gmail.com (S.B. Khalifa).
Peer review under responsibility of Faculty of Engineering, Alexandria University.
https://doi.org/10.1016/j.aej.2019.12.031
1110-0168 2019 Faculty of Engineering, Alexandria University. Production and hosting by Elsevier B.V.
This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Please cite this article in press as: S.B. Khalifa et al., The effect of the cobalt ferrites nanoparticles (CoFe 2O4) on the porous silicon deposited by spin coating, Alexandria Eng.
J. (2019), https://doi.org/10.1016/j.aej.2019.12.031
1.
Introduction
Porous silicon (PSi) has been stimulating a great interest to silicon-based technologies. It
exhibits, in fact, exceptional characteristics which makes it suitable for potential and
various applications such as microelectronics, optoelectronics, solar cells, biomedicine,
etc. [1,2]. Besides, its high surface area makes it also appropriate to put up one or more
materials leading to a radical evolution of its properties [2]. Moreover, adding the
appropriate material deposited on porous materials and choosing the major factors that
affect the deposition method which may alter the final properties of the deposited thin
films brings by the scientific community’s interest [3–5]. In fact, the combination of
nanoparticles-porous semiconductors allowed the enhancement of their functionality [6].
Particularly, the combination of nanoparticles and porous Silicon, it gives the possibility
to obtain multifunctional nano-composite with various properties [7]. These
nanocomposites are a favorable candidate to several prospect applications in different
scientific and technological fields, as antireflection coatings and high reflectivity mirrors,
optoelectronics, catalysis, gas sensors, electronic and biomedicine [8–12].
Actually, the chosen nanoparticle is Ferrites nanoparticles. Owing to its various
characteristics, it has been widely used in many applications, such as humidity sensors,
photo detectors, biomedical, electronic and optoelectronics devices [13–17]. Indeed,
Ferrites nanoparticles corresponding to the AFe2O4 formula are widely known for their
exceptional characteristics as the electrical, optical, and magnetic ones [18–20]. Among
Please cite this article in press as: S.B. Khalifa et al., The effect of the cobalt ferrites nanoparticles (CoFe 2O4) on the porous silicon deposited by spin coating, Alexandria Eng.
J. (2019), https://doi.org/10.1016/j.aej.2019.12.031
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S.B. Khalifa et al.
the various ferrites nanoparticles, CoFe2O4 has been approved as the best candidate for
various potential applications in comparison with others magnetite nanoparticles [21,22].
Few studies however, have been done on the improving the porous substrate properties
by ferrites nanoparticles [23]. Briefly, the properties of cobalt ferrite nanoparticles
dispersed in a porous silicon, which keeps them apart, have not been studied yet. That is
why the significance of this research is the deposition of cobalt ferrites nanoparticles on
porous Silicon and then the study of its various properties; structural, and optical also
aiming to study the enhancement of the new nanocomposites (CoFe2O4/PSi).
2.
Experimental detail
2.1. Formation of porous silicon by electrochemical anodization
The formation of porous Silicon (PSi) by electrochemical etching of p-type Si (1 0 0)
wafer has been performed. The etching solution was prepared from a mixture of
hydrofluoric acid and ethanol (HF/C2H5OH/H2O). The etching process was assured by
applying a current anodization density of 90 mA/cm2 during 5 min. Then the prepared
porous silicon was rinsed using deionized distilled water and dried with nitrogen.
2.2. Preparation of CoFe2O4 nanoparticles
The CoFe2O4 nanoparticles were formed by a one-pot solvothermal method, by dissolving
acetylacetonates of iron and cobalt in benzyl alcohol [24].
Please cite this article in press as: S.B. Khalifa et al., The effect of the cobalt ferrites nanoparticles (CoFe 2O4) on the porous silicon deposited by spin coating, Alexandria Eng.
J. (2019), https://doi.org/10.1016/j.aej.2019.12.031
2.3. Dispersion of CoFe2O4 nanoparticles by spin coating
Firstly, the nanoparticles in powder form need to be dissolved in solvent mixture before
spin- coating. The mixture of nanoparticles was then poured using a spin coater with a
speed of 2000 rpm and with spin-up time of 20 s. After depositing, using an oven to
remove the residual solvent, the obtained sample was dried at 60 C for 3 min and then
annealed at a temperature of 350 C for 15 min.
2.4. Characterizations
The obtained composites were assessed by X-ray diffraction (XRD) with Cu-Ka line
radiation (k = 1.54056 A˚ ). Then the morphology of the samples was studied by scanning
electron microscopy (SEM). And the optical study was performed by Reflectance
Ultraviolet–Visible spectroscopy (UV–Vis) and Spectroscopic Ellipsometry (SE).
3.
Results and discussion
3.1. Scanning electron microscopy (SEM)
CoFe2O4 nanoparticles are shown in Fig. 1. The SEM Image shows spherical particles
having a regular form. Indeed, the Fig. 1 shows highly concentrated particles because of
its continuous magnetic moment. As a result, each particle is continuously magnetized
and coalesced with other particles.
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J. (2019), https://doi.org/10.1016/j.aej.2019.12.031
6
S.B. Khalifa et al.
Fig. 2 shows the typical image of CoFe2O4 nanoparticles deposited on p-type porous
silicon. We also noticed large aggregates. The Image of the sample reveals that most of
the particles are aggregated and display a condensate arrangement of nanoparticles with
roughly spherical shape and narrow size distribution (see Fig. 3.)
3.2. X-ray diffraction analysis
The CoFe2O4 nanoparticles deposited on Silicon substrate (CoFe2O4 /Si) and the CoFe2O4
nanoparticles deposited on porous Silicon (CoFe2O4/PSi) crystalline structure were
observed using X-Ray Diffraction (XRD). All peaks confirm the presence of CoFe2O4
phase which was according to ICDD cards.
The average diameter, D, of as-deposited CoFe2O4 nanoparticles was calculated from
XRD line broadening by the classical Debye-Scherer formula [25,26]: kk
D¼
ð1Þ bcosh
where D is the crystallite size, k the Scherer’s factor (0. 9), h the angle of the diffraction,
b the full width at half-maximum (FWHM) of the peak (3 1 1) calculated using Gaussian
fit, and k the wavelength of X-ray (1.54056 A˚ ).
The Values of D, for CoFe2O4 nanoparticles deposited on different substrates are shown
in Table 1. As shown (Table 1), both samples show nano-range crystallinity. The high
intensity peak is observed at 2h = 35o, corresponds to the ferrite phase of CoFe2O4. The
Please cite this article in press as: S.B. Khalifa et al., The effect of the cobalt ferrites nanoparticles (CoFe 2O4) on the porous silicon deposited by spin coating, Alexandria Eng.
J. (2019), https://doi.org/10.1016/j.aej.2019.12.031
variation of D may be attributed to the difference in the driving force for grain boundaries
motion and retarding force by pores [27].
The effect of the cobalt ferrites nanoparticles (CoFe2O4)
3
Fig. 1Top view SEM images of the CoFe2O4 nanoparticles.
CoFe2O4/Si poreux ((incidence rasante))
CoFe2O4/p-Si (substrate)
15
20
25
30
35
40
45
50
55
60
65
2θ (degree)
Fig. 2 Top view SEM images of the
CoFe2O4 0film spin coated on porous ptype silicon formed in the HF-Et-OH
electrolyte.
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8
S.B. Khalifa et al.
Fig. 3
XRD
nanoparticles
patterns
for
CoFe2O4
deposited
on
different
substrates Si and Si porous.
Then the interplanar spacing (d) values were determined using Bragg’s law as follows:
nk = 2dsinh. (Table 1)
Also, the lattice constant, a, for (3 1 1) plane, of asdeposited CoFe2O4 ferrite nanoparticles
has been calculated using the following equation [28]:
a ¼ dph2 þ k2 þ l2ffi
ð2Þ
where h, k and l are Miller indices.
Then we can determine the ion jumps length in A-site LA (tetrahedral) and B-site LB
(octahedral) by the following equations [29].
LA ¼ 0:25app3ffi
ð3Þ
LB ¼ 0:25a
2ffi ð4Þ
The obtained results for LA and LB of the samples deposited on different substrates were
summarized in Table 1. It is then noted that the ion jumps lengths (the hopping length), and
the lattice constant were reduced for CoFe2O4/porous Si comparing to CoFe2O4/Si. This
reduction is attributed to the change of the physical properties of the ferrite composite.
Please cite this article in press as: S.B. Khalifa et al., The effect of the cobalt ferrites nanoparticles (CoFe 2O4) on the porous silicon deposited by spin coating, Alexandria Eng.
J. (2019), https://doi.org/10.1016/j.aej.2019.12.031
In addition, we can evaluate the Strain, e, and the dislocation density, d, using the
following Equations [30,31]:
k
1
e ¼b
ð5Þ
Dcosh
tanh
1
d¼
2
ð6Þ
D
e and
d values are given in Table 1.
From the table, it is recognizing that the lattice strain as well as the dislocation density
decrease for CoFe2O4/porous Si comparing to CoFe2O4/Si which can be due to strain
release in the structure. Then the CoFe2O4/porous Si is more relaxed. And we can say that
the CoFe2O4/porous Si structure have best crystallinity. Indeed the decrease of the lattice
strain and the dislocation density is attributed to the decrease of the crystal defects and
the stress [32,33].
3.3. Reflectance study
The Fig. 4 show the UV– visible reflectance spectra. The interference pattern is evident
over the entire wavelength range scanned for all the reflectance spectrum; several
interference fringes have been shown, the fringe spacing increasing with wavelength as
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S.B. Khalifa et al.
1
0
expected; we note the increase of the envelope of the reflection spectrum with the
wavelength; which is explained by the decrease of the absorption and scattering.
For the porous Si sample, the fringes of the interference are due to the light coming
from the upper surface of the porous silicon and from the interface between the porous
layer and the silicon substrate, which can be explained as Fabry-Perot interference [34].
The Fig. 4 illustrates the ability of the nanoparticles dispersion to obtain optical
antireflection properties on the porous silicon. The spectrum exhibits a reflectance of less
than 20% in the wave range of 200–1000 nm showing that an excellent antireflection
effect is acquired through dispersion of nanoparticles.
The reflectance spectrum of CoFe2O4/PSi and CoFe2O4 /Si shows the presence of
interference fringes systems for the porous composites. In the case of the Si wafer after
depot of the nanoparticles, we noticed the appearance of Oscillation with a Large Period.
This reveals the presence of a thick layer on the surface of Si.
We also deduce that the reflectance of the CoFe2O4 nanoparticle dispersed on porous
silicon is higher than the reflectance for the nanoparticles dispersed on Si wafer; this is
due to the high number of nucleation sites in the case of porous
Si.
Table 1 The crystallite size, D, strain, e, dislocation density, d, d spacing, lattice
parameter a, and hopping length LA and LB for CoFe2O4 films deposited on different
substrates Si and Si porous.
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Sample CoFe2O4 Bragg’s Crystallite Strain dislocation
d
a
LA
LB
deposited on
angle
size(D)
substrate of
(2H)
(nm)
Si porous
35.36
6.59
0.023 2.30
0.252 0.837 0.362 0.296
4.28
0.055 5.45
0.253 0.840 0.364 0.297
Si
(e)
density(d)
(nm) (nm) (nm)
1016 [m2]
The low value of reflectance in the case of the Si wafer after depot of the nanoparticles
is due to roughness and absorption in this area for the cobalt ferrites. Indeed, when the
absorption increases, the amplitude of the fringes decreases and when the absorption
increases sufficiently, the fringes disappear. 3.4. The technique of spectroscopic
ellipsometry
To identify the properties of the film that depend on the polarization change in the light
that interact with the sample structure the spectroscopic Ellipsometry (SE) was performed.
The SE measurements allow us to determine the film thickness as well as the optical
constants and other physical properties [35,36]. These properties are found by using the
typical measurement expressed as two values: Psi (W) and Delta (D) which described the
change in polarization that occurs when the light beam interact with a sample surface. In
fact, the incident light beam contains two components: parallel (p) and perpendicular (s)
components which changing the polarization. The properties of the sample is given by the
ratio, through the following equation (Eq. (7)):
R ¼ rp=rs ¼ tanexpð Þi
ð7Þ
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S.B. Khalifa et al.
1
2
rp and rs are the Fresnel reflection coefficients for the p- and s-
30
20
Porous Si
CoFe2O4/p-Si
CoFe2O4/Porous Si
10
0
500
600
700
800
900
1000
Wavelength(nm)
Fig. 4 UV– visible reflectance spectra of the porous p-type silicon formed in the HF-EtOH electrolyte (red line), CoFe2O4 thin film spin coated on p-type silicon (green line) and
CoFe2O4 thin film spin coated on p-type porous silicon (blue line).
polarized light respectively. Where, W is the angle calculated from the amplitude ratio
between p- and s-polarizations, and the D is the phase difference between the two
components.
The (SE) spectral dependencies of W and D for as-deposited CoFe2O4 nanoparticles are
represented in Fig. 5. Then, they were fitted to extract the refractive index (n) and extinction
coefficient (k). The fit is validated by checking the root Mean Square Error function (MSE):
1
MSE ¼ 2N M
2
2
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Xi j1 @ WimodrWexp;iWiexp! þ DimodrDexp;j Diexp!A1
ð8Þ ¼
To quantify the difference between a theoretical model and an experimental data, it is
crucial to calculate the error [37–38]. In this study, The Mean Squared Error (MSE) is
estimated by (Eq. (8)) [40,40].
As shown in Fig. 5, it’s clear, that the fitted model and the measured data agree very well.
Indeed, the curves shapes are correctly reproduced.
The fitted optical indices, such as the extinction coefficients k and the refractive index,
of the as-deposited CoFe2O4 nanoparticles, have been deduced from W and D and presented
in Fig. 6. As observed, the peak of refractive index appears at 400 nm (about 3.1 eV). Then
the refractive index decreases with increasing wavelength. In addition, the refractive index
The effect of the cobalt ferrites nanoparticles (CoFe2O4)
5
Fig. 5 w and D spectra obtained from spectroscopic ellipsometry for as- deposited CoFe2O4
nanoparticles sample at an angle of incidence of 60. The continuous lines represent the
model fit.
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S.B. Khalifa et al.
1
4
Fig. 6 Refractive index n and extinction coefficient k as a function of the wavelength of asdeposited CoFe2O4 nanoparticles sample.
Table 2 Optical constants of CoFe2O4
nanoparticles
by
Ellipsometry
measurements.
S4
Thickness (A˚ )
109.86
MSE
0.700
Roughness (A˚ )
161.49
n @ 632.8 nm
1.373
k @ 632.8 nm
0.093
depends slightly on the photon energy for the long wavelengths. It can be noted that the
extinction coefficient gradually decreases with increasing the wavelength.
The disperse curve of the extinction coefficient k decreases sharply for the short
wavelengths then it is almost flat above 900 nm. The thickness and the MSE expressing the
accuracy of the fit are summarized in Table 2.
4.
Conclusions
In conclusion, ferrites nanoparticles were deposited on the Porous Si using spin coating
method. The SEM Image CoFe2O4 spin coated on p-type porous silicon reveals that most
Please cite this article in press as: S.B. Khalifa et al., The effect of the cobalt ferrites nanoparticles (CoFe 2O4) on the porous silicon deposited by spin coating, Alexandria Eng.
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of the particles are aggregated and exhibit a compact arrangement of the nanoparticles with
roughly spherical shape and narrow size distribution. XRD study revealed high crystallinity
of CoFe2O4 on porous silicon (CoFe2O4/PSi). The lattice constant were reduced for
CoFe2O4/porous Si comparing to CoFe2O4/Si.
Optical properties of the composite are studied using Reflectance and Ellipsometry. The
Reflectance study illustrates the ability of the nanoparticles dispersion to produce optical
antireflection properties on the porous silicon. We deduce that the reflectance of the
nanoparticles dispersed on porous Si is higher than the reflectance of the nanoparticles
dispersed on Si wafer, which is due to the high nucleation sites in the case of porous Si.
From ellipsometry measurements, extinction coefficient and refractive index have been
deduced according to Cauchy model with an excellent MSE value (0.7) and the assumed
thickness of the film close to 10.9 nm; this confirms the observation made by XRD.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge Qassim University, represented by the deanship of
Scientific Research, on the material support for this research under the number (5015alrasscac2018-1-14-S) during the academic year 1439 AH/2018 AD.
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S.B. Khalifa et al.
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