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Physica B 670 (2023) 415371
Available online 2 October 2023
0921-4526/© 2023 Elsevier B.V. All rights reserved.
Investigation of the structural, optical and thermoelectric performances of
ash-evaporated cobalt-doped copper sulde thin lms
L. Amiri
a
, A. Narjis
b
,
*
, M. Bousseta
a
, L. Nkhaili
a
,
**
, A. Tchenka
a
, S. Elmassi
a
, A. Alsaad
c
,
Hind Albalawi
d
, A. El kissani
a
, A. Outzourhit
a
a
Laboratory of Materials, Energy and Environment (LaMEE), Faculty of Science Semlalia. Cadi Ayyad University, P. O. Box 2390, Marrakech, 40000, Morocco
b
Laboratoire Mat´
eriaux et Energies Renouvelables (LMER), Universit´
e Ibn Zohr, B.P. 1136, Agadir, 80000, Morocco
c
Department of Physical Sciences, Jordan University of Science & Technology, P.O. Box 3030, Irbid, 22110, Jordan
d
Department of Physics, College of Sciences, Princess Nourah bint Abdulrahman University (PNU), P.O. Box 84428, Riyadh, 11671, Saudi Arabia
ARTICLE INFO
Keywords:
Co:CuS lms
Flash evaporation
Thermoelectric properties
Structural properties
Optical properties
ABSTRACT
In this study, we employed the ash evaporation method to deposit thin lms of pure copper sulde and cobalt-
doped copper sulde. We focused on evaluating the inuence of cobalt content, ranging from zero to 9%, on the
structural, optical, electrical, and thermoelectric characteristics of the synthesized thin lms. Our ndings
indicate that the passage from the copper-rich (covellite phase to the digenite or djurleite phases) can be carried
out by annealing or by doping. Raman’s investigation supports the discovered phase and shows that the A
1g
(LO)
mode intensity drops with doping. The gap energy was found to decrease by increasing the percentage of cobalt.
The annealing at 350 ◦C resulted in good thermoelectric properties, while the dominance of the djurleite phase,
resulted in the highest Seebeck coefcients.
1. Introduction
Currently, the high consumption of fossil fuel resources is reected in
the rise in the execution of renewable energies, such as solar and wind
energy [1–4]. Although these renewable energies have the property of
being environmentally friendly, they are expansive and require expan-
sive energy storage [5]. Wind energy suffers, in addition, from the
problem of moving parts, which causes noise pollution, contributing to
the degradation of the ecosystem.
Thermoelectricity proves to be a less expensive technology that ad-
dresses the issue of operation without moving parts and in conditions
not accessible by other types of energy, such as closed environments,
microscale devices, and space instruments. The durability of thermo-
electric (TE) materials [6,7] reinforces their usefulness and justies
intense research on this subject.
Whether it is a TE generator (Fig. 1(a)), converting the temperature
gradient into electricity, or a Peltier module that produces the inverse
conversion, the construction requires the implantation of the TE legs,
electrically in series and thermally in parallel, as shown in Fig. 1(a). The
current ows alternately through the N-type material, with Seebeck
coefcient
α
N
, electrical conduction
σ
N
, and thermal conduction λ
N
, and
the P-type material, having the analogous TE parameters
α
P
,
σ
P,
and λ
P
.
At the average temperature T, the efciency of a TE device is written:
η
=
η
C
ZT +1
√−1
ZT +1
√+TH
TC
(1)
where
η
C
is the Carnot efciency.
Taking into account Eq. (1), the efciency is plotted, in Fig. 1(b), as a
function of temperature and the gure of merit, ZT, of the TE couple. ZT
is dened as follows:
ZT =(
σ
N+
σ
P)(
α
N+
α
P)2
λN+λP
T(2)
Fig. 1(b) shows the importance of achieving high ZT. Therefore, a
good TE material should exhibit a high Seebeck coefcient and electrical
conductivity and a reduced thermal conductivity. The power factor
(
σα
2
) is approximately a parameter, allowing to judge on the TE ef-
ciency of a given material [8–11]. This means that the Seebeck coef-
cient and electrical conductivity are crucial parameters for assessing the
* Corresponding author.
** Corresponding author.
E-mail addresses: a.narjis@uiz.ac.ma (A. Narjis), lahcen.nkhaili@uca.ac.ma (L. Nkhaili).
Contents lists available at ScienceDirect
Physica B: Condensed Matter
journal homepage: www.elsevier.com/locate/physb
https://doi.org/10.1016/j.physb.2023.415371
Received 3 June 2023; Received in revised form 5 September 2023; Accepted 30 September 2023
Physica B: Condensed Matter 670 (2023) 415371
2
TE performances. Copper sulde-based materials have, over the years,
shown good TE properties [8,12,13], which is reinforced by their
favorable properties such as low cost and low toxicity. Besides CuS
nanoparticles have been reported as promising materials in making
photocatalytic, electrochemical cells [14–17] and Li-ion batteries
[18–20].
The stability of copper sulde phases poses a signicant challenge,
casting doubt on their suitability for integration. The investigation into
deposition conditions for TE phases, such as digenite (Cu
1.8
S) and
djurleite (Cu
1.96
S), remains a highly relevant and actively researched
area. Doping strategy offers a possible solution to achieve high TE ef-
ciency, with high stability.
In our previous work [8], we noticed that the thermal ash evapo-
ration of CuS powder gave pure phases with promising properties of the
undoped CuS lms, in terms of exibility toward the optimization of
chemical and thermoelectric properties.
In this work, doping copper sulde lms by cobalt is observed to
alter the structural properties (lattice parameters, crystallite size), op-
tical properties (transmittance, energy band gap) and the TE perfor-
mances (Seebeck coefcient, electrical conductivity).
2. Experimental
2.1. Deposition procedure
Used powder are CuS, pure cobalt, and pure sulfur. Each one is with a
purity of 99% (Sigma Aldrich). The quantities of these powders were
determined based on the desired molar composition, calculated while
maintaining the ratios of atomic masses for Co, Cu, and S, as depicted in
the following reaction:
xCo +(1−x)CuS +xS→Cu1−xCoxS
Doping of the powder involves grinding the CuS, Co, and S powders
with a meticulously cleaned mortar for 15 min. The cobalt is utilized in
varying percentages of 0%, 5%, 7%, and 9% while maintaining a con-
stant total mass of 0.4 g.
The next step involves the cleaning of the substrates designated for
evaporating these powders through the ash evaporation technique. A
cleaned soda-lime glass substrate (initially sectioned into squares with a
surface of 1.2 ×1.2 cm
2
) where used for deposition. Subsequently, each
square undergoes sequential ultrasonic cleaning in distilled water,
acetone, ethanol, and distilled water, with a duration of 10 min dedi-
cated to each step. Following this, the squares were dried in an oven. The
taken vacuum pressure was 5 ×10−6 mbar and the distance between the
crucible and the substrate was xed at 12 cm.
2.2. Characterizations
The prepared thin lms were characterized by X-ray diffraction
(XRD), using a Smart lab SE diffractometer (Japan, Cu(K
α
), λ =1.5418
Å). The Raman spectroscopy was employed using a Sol instrument, with
λ =532 nm, to verify the X-ray data. Scanning Electron Microscopy
(SEM) was performed to study the surface morphology, using a
UV–Vis–NIR spectrometer, while the optical spectrum was measured
from 300 to 2000 nm in wavelength using a Shimadzu UV-PC
spectrophotometer.
The electrical properties (electrical conductivity, charge carrier
mobility, and carrier concentration) of the prepared lms were
measured using the Ecopia HMS-300 Hall measurement system at room
temperature, employing the Van der Pauw method [21].
The measurement of the Seebeck coefcient of each sample was
carried out using a lab-fabricated device whose functioning is detailed in
the Ref [22]. Indeed, the creation of the temperature difference by using
hot and cold copper blocks, results in creating a voltage between the
sides of the sample placed above. Then, the slope is used to determine
Fig. 1. (a) Representative scheme of the TE generator; (b) Efciency as a function of the mean temperature for the cold temperature 300 K.
Fig. 2. XRD patterns of the as-deposited Co:CuS thin lms.
Table 1
Structural parameters of pure and Co doped CuS lms.
Sample 2θ
1
(◦)
at (110)
2θ
2
(◦) at
(102)
a
(110)
c
(102)
d
(110)
d
(102)
D
(102)
(nm)
CuS-0%
Co
48.0 29.3 3. 9 16.7 1.9 3.04 39.8
CuS-5%
Co
48.2 29.5 3.8 16.0 1.9 3.03 45.7
CuS-7%
Co
48.2 29.6 3.8 15. 8 1.9 3.02 28.7
CuS-9%
Co
48.3 29.6 3.8 15.8 1.9 3.01 29.4
L. Amiri et al.
Physica B: Condensed Matter 670 (2023) 415371
3
the Seebeck coefcient using the equations detailed in the Ref [22].
3. Results and discussion
3.1. Effect of cobalt doping
Fig. 2 displays the X-ray diffraction (XRD) patterns of the as-
deposited thin lms, each containing varying percentages of cobalt
doping. The observed peaks consistently exhibit a hexagonal covellite
phase, as conrmed by comparison with the standard JCPDS card No.
01–0752. Upon increasing the cobalt doping level to 9%, the presence of
the digenite phase becomes evident. Importantly, as the doping per-
centage increases, the peak positions gradually shift towards higher 2θ
values, indicating a reduction of the inter-planar spacing. This phe-
nomenon can be attributed to the contrasting ionic radii of Co
2+
(0.058
nm) and Cu
2+
(0.065 nm) ions [23].
The parameters a and c of the hexagonal structure were estimated
from the inter-planar spacing of the peaks (110) and (102) using the
Braggs law of diffraction (λ =2dsinθ) and the following formula [24]:
1
d2=4
3[h2+hk +k2
a2]+l2
c2(3)
The calculated results are listed in Table 1. As the ionic radius of
Co
2+
is lower than that of Cu
2+
, a and c decrease as the cobalt per-
centage increases.
The crystallite size was calculated using the Debay-Scherrer formula
from the total width at half-max (FWHM) of the intense peak (102):
D102 =0.9λ
β102 cos θ(4)
The average crystallite size of the undoped lm is 39.8 nm. The
doping at 5% enhances this size, then, further increasing the cobalt
percentage (at 7%) causes a considerable decrease in crystal size. Finally
doping at 9% causes a not signicant rise.
These XRD results are supported by the Raman spectra, shown for
various cobalt percentages in Fig. 3(a–c).
Two peaks are present in the undoped samples. The rst peak,
appearing at 258 cm
−1
, is attributed to the transverse optical mode A
1
g
(TO), associated with the Cu–S stretching vibrational mode. The second
peak, located at 467.44 cm
−1
, corresponds to the longitudinal optical
mode A
1 g
(LO) related to the S–S stretching vibrational mode. These
results ensure the purity of the synthesized covellite phase. With an
increase in the cobalt percentage, the intensity of the peaks decreases
(Fig. 3(a)) until the rst peak disappears, which may explained by the
incorporation of into the CuS lattice [23].
Doping also inuences the peak position, as depicted in Fig. 3(c).
These positions are determined by tting the most intense peak, as
Fig. 3. (a) Raman spectra of the prepared samples; (b) Fitted main peak; (c) FWHM and peak position variations as a function of Co percentage.
L. Amiri et al.
Physica B: Condensed Matter 670 (2023) 415371
4
illustrated in Fig. 3(b). It is notable that the doping up to 7%, almost
restores the peak’s position. The FWHM is also calculated for these
peaks, and it is discovered that this evolves inversely to the position of
this peak.
The SEM images of the prepared samples are shown in Fig. 4. It is
seen that the cobalt doping strongly inuences the morphology of the
CuS lms.
The starting and nal cobalt concentrations (obtained by the EDAX
spectra) in the CuS lms are shown in Table 2. It is seen that the nal
cobalt content is slightly lower than the starting one. This means that the
cobalt atoms are not all included in the reaction which gave rise to the
obtained crystalline lattice. The difference was expected since the
melting temperature of cobalt (1495 ◦C) is higher than that of copper
(1085 ◦C).
For the various doping percentages, Fig. 5 displays the transmission
spectra of Co:CuS thin lms in the 300–2000 nm range. It is shown that
Fig. 4. SEM images of the Co:CuS lms for various cobalt percentages: (a) 0%, (b) 5%, (c) 7% and 9%.
Table 2
Starting and nal elemental compositions of the prepared Co:CuS lms.
Starting composition (at %) Obtained composition (at %)
Cu Co Cu Co
100 0 100 0
95 5 96.58 3.42
93 7 94.55 5.45
91 9 94.16 5.84
Fig. 5. Co:CuS thin lms’ optical transmission spectrum. Inset: Band gap as a
function of Co content.
L. Amiri et al.
Physica B: Condensed Matter 670 (2023) 415371
5
the transmission decreases as the doping percentage increases. With
increasing the cobalt concentration, the absorption edge moves toward
longer wavelengths.
Based on this spectrum, the gap energy is estimated using the Tauc
plots. It is found that by cobalt doping, the gap energy decreases (inset of
Fig. 5).
At room temperature, the Hall effect method was used to measure the
electrical characteristics of the deposited lms. Table 3 displays the
electrical properties of these lms (conductivity, mobility and
concentration of holes). The positive Hall coefcient proves that all lms
exhibit p-type conduction, which is well expected. It is worth noticing
that the conductivity generally increases with doping. The same evolu-
tion is observed for mobility, while the concentration is almost
invariant.
Fig. 6 shows the evolution of the Seebeck coefcient as a function of
temperature. For the undoped lm and the doped lms,
α
varies be-
tween 5.5 and 10
μ
V/K, except for the CuS-9% Co lm, which presents
Table 3
Electrical proprieties of Co:CuS thin lms.
Sample Conductivity (Ω
−1
.
cm
−1
)
Mobility
(cm
2
V
−1
s
−1
)
p ( ×10
17
cm
−3
)
CuS-0% Co 9.5 ×10
2
1.7 4.9
CuS-5% Co 7.1 ×10
2
1.5 4.7
CuS-7% Co 7.4 ×10
2
1.3 5.7
CuS-9% Co 1.1 ×10
3
2.3 4.5
Fig. 6. Seebeck coefcient as a function of temperature.
Fig. 7. XRD patterns of the Co:CuS lms annealed at 350
◦C.
Fig. 8. Effect of annealing on optical transmission and energy band gap.
Table 4
Effect of annealing on electrical properties.
Samples Conductivity (Ω
−1
.
cm
−1
)
Mobility
(cm
2
V
−1
s
−1
)
p ( ×10
17
cm
−3
)
CuS-0% Co 2.8 ×10
2
0.6 9.2
CuS-5% Co 67.1 0.2 2.9
CuS-7% Co 86 0.3 2.1
CuS-9% Co 3 ×10
2
0.3 8.7
Fig. 9. Seebeck coefcient as a function of temperature for the lms annealed
at 350
◦C.
L. Amiri et al.
Physica B: Condensed Matter 670 (2023) 415371
6
higher Seebeck coefcients than the others, perhaps because of the
beginning of the formation of the digenite phase (Cu
1.8
S) which is
known to present a large Seebeck coefcient compared to that of the
covellite phase (CuS) [9]. The positive values of
α
conrm the p-type
conduction in the synthesized Co:CuS lms.
3.2. Effect of annealing
Based on the ndings of prior investigations, showing that the best
TE properties are obtained by moving from the covellite to digenite or
djurleite phases [8], a temperature of 350 ◦C, under vacuum, is selected
to study the inuence of the measurement temperature on the thermo-
electric properties.
Fig. 7 displays the registered XRD patterns, the degenite phase pre-
dominates in the undoped lm. However, as the doping rate rises, this
phase gradually diminishes until it is almost completely absent at 9%.
Since only the covellite phase exhibits peaks in all phases of copper
suldes, all of these lms have an amorphous character in Raman
spectra.
The transmission spectra and gap evolution as a function of the
doping percentage are shown in Fig. 8, respectively. The gap is reduced
by cobalt doping, and the transmission spectra exhibit the same evolu-
tion observed for the as-deposited lms, with the exception that the
intensity is higher, which is due to the increase of the sulfur escaping
during annealing [17].
To access the impact of annealing on the electrical properties of the
produced layers, Table 4 shows the evolution of the hole concentration,
conductivity, and mobility as a function of the doping. It is observed that
by cobalt doping, the electrical conductivity increases and was previ-
ously achieved for the layers without annealing, the conductivity is
greatly increased by doping.
Fig. 9 displays the Seebeck coefcient of the lms annealed at 350 ◦C
as a function of the cobalt percentage and temperature. When compared
to the undoped and 5% cobalt-doped, the lms doped at 7 and 9%
exhibit higher Seebeck coefcients. This is because they are dominated
by the djurleite phase, which is known to have a higher Seebeck coef-
cient than Cu
1.8
S.
We further investigate the effect of annealing. Table 5 lists the
Electrical, optical and thermoelectric properties of the synthesized Co:
CuS lms for various cobalt percentages. The effect of annealing on
electrical properties is complicated. Indeed, while the crystallinity im-
proves, the oxidation of the surface appears which reduces the electrical
conductivity. On the other hand, the electrons given up by the cobalt
take part in the reactions, which causes the phase change. This phase
change also inuences the mobility of the carriers. In other words,
several combined effects alter the electronic properties during annealing
(phase change, mobility, crystallinity, oxidation …). This complicates
the physical explanation. Nevertheless, we draw from this study that the
annealing at 350 ◦C of the compound with CuS-9% Co as the percentage
of cobalt results in the high room temperature thermoelectric factor
power 21.1
μ
W m
−1
K
−2
. On the other hand, the energy band gap de-
creases with annealing. This is due to interferences of the atomic or-
bitals, as explained in Ref. [25].
4. Conclusion
Through the combination of copper sulde and cobalt powders, we
successfully synthesized Co:CuS lms via ash evaporation. Structural,
optical, and thermoelectric properties are shown to depend on the
annealing and the percentage of cobalt. In particular, it is shown that the
passage from the copper-rich (covellite phase to the digenite or djurleite
phases) can be carried out by annealing or by doping. Annealing at
350 ◦C results in good thermoelectric properties, while doping at 7–9%
showed the highest Seebeck coefcients, due to the dominance of the
djurleite phase.
Credit author statement
Lahoucine Amiri: Conceptualization, Roles/Writing - original draft,
Writing-review & editing. Abdelfattah Narjis: Resources, Formal anal-
ysis, review & editing. Mohamed Bousseta: Data curation. Lahcen
Nkhaili: Resources, Investigation, Methodolgy, formal analysis. Abde-
laziz Tchenka: Software. Said Elmassi: Formal analysis. Ahmed Alsaad:
Investigation, Visualisation. Hind Albalawi: Resources, Formal analysis.
Abdelkader El Kissani: Resources, Methodolgy. Abdelkader Outzourhit:
Supervision.
Declaration of competing interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Data availability
No data was used for the research described in the article.
Acknowledgements
This research was funded by the Princess Nourah bint Abdulrahman
University Researchers Supporting Project number (PNURSP2023R7),
Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia
Prof. Ahmad Alsaad would like also to acknowledge Jordan Uni-
versity of Science and Technology (JUST) in Jordan, the Deanship of
Scientic Research, particularly for the support [grant number 325/
2021].
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Electrical, optical and thermoelectric properties of the as-deposited and annealed Co:CuS lms for various cobalt percentages.
Cobalt percentage phases P ( ×10
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