Materials Chemistry and Physics 97 (2006) 71–80
Structural and optical properties of PbS thin films deposited
by chemical bath deposition
S. Seghaier a,∗ , N. Kamoun a , R. Brini b , A.B. Amara c
b
a Laboratoire de Physique de la Matière Condensée, Faculté des Sciences de Tunis, 2092 El Manar, Tunisia
Laboratoire de Photovoltaı̈que et des Matériaux Semi-conducteurs, Ecole Nationale, d’Ingénieurs de Tunis, El Manar, Tunisia
c Laboratoire de Physique des Matériaux, Faculté des Sciences de Bizerte, 7003 Zarzouna, Tunisia
Received 10 May 2005; received in revised form 22 July 2005; accepted 23 July 2005
Abstract
In the current paper lead sulphide (PbS) thin films have been deposited on glass slide substrates via a simple and easily controlled method,
proceeding large area films, using the chemical bath deposition technique. The reactive substances used to obtained the PbS layers were
(Pb(NO3 )2 ), (NaOH), (SC(NH2 )2 ) and H2 O for different concentration and dipping times, at constant room temperature. The structure was
determined by X-ray diffraction studies. The films are very adherent to the substrate and well crystallized according to the centered cubic
structure with the preferential orientation (2 0 0). The surface morphology and crystallite sizes were determined by scanning electron microscope measurements. The optical properties were carried out from spectroscopy measurements of transmission–reflection and ellipsometry.
The thickness of these films was controlled by changing the dipping times and the concentrations of the reaction solution. Experiments showed
that the growth parameters and thermal treatment influenced the structure, the morphology and the optical properties of PbS films.
© 2005 Elsevier B.V. All rights reserved.
Keywords: PbS thin films; Chemical bath deposition; Structure; Optical properties
1. Introduction
Lead sulphide (PbS) is an important direct narrow gap
semiconductor material with an approximate energy band
gap of 0.4 eV at 300 K and a relatively large excitation Bohr
radius of 18 nm [1]. These properties make PbS very suitable
for infrared detection application [2]. This material has also
been used in many fields such as photography [3], Pb2+ ionselective sensors [4] and solar absorption [5]. In addition, PbS
has been utilized as photoresistance, diode lasers, humidity
and temperature sensors, decorative and solar control
coatings [6,7]. These properties have been correlated with
the growth conditions and the nature of substrates. For these
reasons, many research groups have shown a great interest in
the development and study of this material by various deposition processes such as electrodeposition [8], spray pyrolysis
∗
Corresponding author. Tel.: +216 22536062; fax: +216 71 885073.
E-mail address: samia [email protected] (S. Seghaier).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2005.07.061
[9], photoaccelerated chemical deposition [6], microwaveheating [10,11] and chemical bath deposition (CBD) [12–16].
CBD method is presently attracting considerable attention,
as it does not require sophisticated instrumentation. It is relatively inexpensive, easy to handle, convenient for large area
deposition and capable of yielding good quality thin films.
The characteristics of chemically deposited PbS thin films
by CBD strongly depend on the growth conditions. In this
paper, we report the structural and optical properties of PbS
thin films obtained by CBD method at various concentrations
of the precursors and different deposition time.
2. Experimental
2.1. Synthesis of lead sulphide
The PbS thin films were grown on ordinary glass slide
(2.5 cm × 2.5 cm × 2 mm) substrates. The deposition was
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S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71–80
done in a reactive solution prepared in a 50 ml beaker containing lead nitrate, which concentration range [Pb(NO3 )2 ]
was set between 0.165 and 0.185 M. This reactive was mixed
in alkaline aqueous solution with thiourea at concentration
[SC(NH2 )2 ] varying between 0.09 and 0.20 M. The alkalinity is set using sodium hydroxide [NaOH] in the range
0.55–0.60 M. The bidistilled water was added to the solution
to achieve a total volume of 50 ml. Cleaned substrates was
vertically immersed into the solution. The beaker with the
reactive solution was immersed in water heating bath circulator placed on heating magnetic agitator and was maintained
at room temperature.
The substrates were subsequently taken out of the chemical bath after different dipping time tD (10 ≤ tD ≤ 70 min),
rinsed with bidistilled water, dried and placed into a desiccator. The reaction process for forming lead sulphide films is
considered as follows [17]:
Pb(NO3 )2 + 2NaOH → Pb(OH)2 + 2NaNO3
Pb(OH)2 + 4NaOH → Na4 Pb(OH)6
Na4 Pb(OH)6 → 4Na+ + HPbO2 − + 3OH− + H2 O
SC(NH2 )2 + OH− → CH2 N2 + H2 O + SH−
HPbO2 − + SH− → PbS + 2OH−
The resulting films were homogeneous, well adhered to
the substrate with darker surface like mirror. The thickness of each grown film was calculated from the density of
7.596 g cm−3 (of bulk PbS) and the mass of the deposit surface area that is determined by the double weight method.
2.2. Instruments
These films were structurally characterized by, X-ray
diffraction (XRD) using a bruker D8 Advance X-ray diffractometer with Cu K␣ irradiation (λ = 1.54060 Å) and by scanning electron microscopy (SEM) images carried out with XL
30/EDAX BSE microscope.
This study was complemented with ellipsometry and spectroscopy measurements. The ellipsometry analysis were performed at λ = 0.6328 Å using a L 117 Gaertner Manual Ellipsometer. The optical transmittance and reflectance spectrum
were examined on UV-3100 Schimadzu UV–vis spectrophotometer.
3. Results and discussion
Pb2+ ions sources, 0.10 M thiourea [SC(NH2 )2 ], as sulphurliberating solution and 0.57 M sodium hydroxide [NaOH]
that is used as complexing agent for Pb2+ ions. The deposition is carried out at a constant temperature TD equal to 25 ◦ C.
The substrates were subsequently taken out of the beaker after
dipping time from 10 to 70 min. Fig. 1 shows the XRD patterns of the PbS thin films deposited at several growth time,
in the above condition. The crystallography of the film is
good and characterized by three principal peaks corresponding to (1 1 1), (2 0 0) and (2 2 0) orientations. The width at
half maximum of the (2 0 0) line is in the range 0.130–0.259◦
for all time deposition, the values are indicative of the good
crystallinity.
By comparison with the data from JCDF cards No. 1-880,
all diffraction peaks can be indexed within experimental error
as a face-centered cubic structure of PbS. The narrow peaks
show that the materiel has good crystallinity preferentially
oriented along the (2 0 0) direction which is perpendicular to
the substrate and which intensity depends on tD value.
Table 1 displays the ratio values of the relative diffraction peaks intensities (I1 1 1 /I2 0 0 ) and (I2 2 0 /I2 0 0 ) for PbS
films produced for various dipping times. From the following table we can note that an increase in the deposition
time from 10 to 40 min is accompanied by an increase in
the I1 1 1 /I2 0 0 ratio. For tD values varying between 40 and
60 min, we notice that the peak (1 1 1) intensity is practically
constant, whereas the intensity of (2 0 0) increases reaching
its maximum for a growth time equal to 60 min. Both the
intensity ratio I1 1 1 /I2 0 0 and I2 2 0 /I2 0 0 for thin films grown
during 60 min, are equal to 0.280 and 0.094, these values are
lower than the theoretical one equal to 0.8 and 0.6, respectively, showing that the PbS samples grown by CBD have
(2 0 0) preferred orientation with strong intensity and full
width at half maximum. That can be related to the larger number of both Pb2+ and S2− ions in the reaction solution at tD
equal to 60 min. Similar results were obtained by ValenzuelaJauregui et al. [16] and Larramendi et al. [13]. However, for
tD equal to 70 min, the intensity of (2 0 0) decreases. In this
case, we notice also that the thin layers obtained are very
rough. We tried to make thin layers of PbS for tD more than
70 min and we noticed that the layer is detached from its substrate. Recently, rectangular and rod-like PbS crystals have
been successfully prepared in systems containing organic
polyamines with N-chelation properties such as triethylenetetramine [18] and ethylenedimine [19], respectively.
However, we have determined the average crystallite sizes
L by measuring full width at half of the (2 0 0) peaks and by
Table 1
Variation of PbS peak orientation degree as a function of the deposition time
(tD )
3.1. Structure and surface morphology
3.1.1. Effect of the deposition time
To optimize the deposition time, we produced PbS thin
layers on glass from 0.175 M lead nitrate [Pb(NO3 )2 ] as
tD (min)
I1 1 1 /I2 0 0
I2 2 0 /I2 0 0
10
20
30
40
50
60
70
0.421
0.185
0.423
0.184
0.808
0.333
0.853
0.322
0.557
0.297
0.280
0.094
1.391
0.383
S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71–80
73
Fig. 1. X-ray diffraction spectra of PbS layer, CBD-prepared with [Pb(NO3 )2 ] = 0.175 M, [NaOH] = 0.57 M, [SC(NH2 )2 ] = 0.1 M, TD = 25 ◦ C and for various
deposition time (tD ).
using the Scherrer formula [20]:
L=
0.9λ
cos θ0 ∆(2θ)
For different deposition time, comparable L values are measured from SEM micrograph as shown in Table 2.
Table 2
Effect of the deposition time on PbS grain size deduced from MEB and XRD
analysis
tD (min)
Grain size (Å) SEM
L (Å) of (2 0 0) XRD
40
50
60
70
204.2
208.4
250.0
250.2
320.0
312.9
291.7
312.9
On the other hand, SEM analysis gives the morphology of
the PbS nanocrystals. The prepared thin films have a pyramidal shape with a rather compact surface, which does not show
any fissures, faults and disturbances. Fig. 2 shows the thickness variation of PbS thin films as a function of deposition
time. The curve shows an initial slow growth involving the
creation of nucleation centers; it corresponds to the induction
phase. The growth phase (20–60 min) at which the thickness
is obtained in a linear dependence. They increase steadily
with dipping time up to 60 min to reach the max value of
771.560 nm, so while the film thickness increases the grain
size goes up (see Table 2), and the terminal phase in which
the growth slows down. We can conclude the uniformity, the
good adhesion and the best crystallinity of PbS on glass sub-
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S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71–80
strate is obtained for an immersion time equal to 60 min from
the bath described above.
Fig. 2. Thickness of PbS films as a function of deposition time.
3.1.2. Effect of the lead nitrate concentration
In this study we set deposition time tD at its optimal value
equal to 60 min and we tried to change the [Pb(NO3 )2 ] from
0.165 to 0.185 M per step of 0.005 M. The XRD patterns of
as-prepared PbS are shown in Fig. 3 all diffraction peaks,
can be indexed to the pure cubic phase of PbS. No peak of
any other phase was detected. The strong intensity and narrow
peaks show that the material has good crystallinity and (2 0 0)
preferred orientation. The crystallography of the film is good
and characterized by three peaks corresponding to (1 1 1),
(2 0 0) and (2 2 0) orientations. The (2 0 0) reflection peak
has very strong intensity and the full width at half maximum
of line is the order of 0.130◦ for a concentration equal to
0.170 M. This indicates a good preferential orientation of the
grains with c-axis perpendicular to the plane of the substrate.
The values of the intensity ratios I1 1 1 /I2 0 0 and I2 2 0 /I2 0 0 for
lead nitrate concentrations equal to 0.170 M are in the order of
0.661 and 0.371, respectively, as it indicated in Table 3. The
Fig. 3. XRD pattern of PbS thin layers, CBD-prepared on glass substrate with [NaOH] 0.57 M, [SC(NH2 )2 ] = 0.1 M, tD = 60 min, TD = 25 ◦ C and for various
Pb(NO3 )2 concentration.
S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71–80
Table 3
Variation of PbS peak orientation degree as a function of the lead nitrate
concentration
[Pb(NO3 )2 ] (M)
I1 1 1 /I2 0 0
I2 2 0 /I2 0 0
0.165
0.170
0.175
0.180
0.185
0.866
0.338
0.661
0.371
0.280
0.094
0.367
0.190
0.886
0.381
75
(2 0 0) preferential direction is also observed with a middle
intensity [16,17].
As shown in Fig. 4a fat uniform structure can be seen,
which in a compact way, carpets the substrate surface with
lead sulphide crystallites of various size. So Table 4 shows
that the mean size of PbS crystallites, depends on the
Pb(NO3 )2 concentration in the bath solution. Besides, dispersed small spherical structures can also be observed, which
seem to be formed by cluster of microparticles. These disturbances on surfaces are less marked for a [Pb(NO3 )2 ]
equal to 0.170 M. Nevertheless, the pyramidal crystallites
Fig. 4. SEM surface micrograph of PbS thin films deposited on glass substrate prepared with [NaOH] = 0.57 M, [SC(NH2 )2 ] = 0.1 M, TD = 25 ◦ C and for various
[Pb(NO3 )2 ]: (a) 0.165 M, (b) 0.170 M, (c) 0.175 M, (d) 0.180 M and (e) 0.185 M.
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S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71–80
Table 4
Mean PbS grain size as a function of lead nitrate concentration deduced from
SEM and XRD analysis
[Pb(NO3 )2 ] (M)
Grain size (Å) SEM
L (Å) of (2 0 0) XRD
0.165
0.170
0.175
0.180
0.185
250
256.9
280
316.3
320
312.9
170.750
158.5
208.333
208.4
are uniformly distributed on the surface make thin film with
strong compactness. The coalescence of crystallites on thin
film surface affirms their privileged orientation along (2 0 0)
direction. With lead nitrate concentration equal to 0.175 M
and dipping time of 40 min, Mahmoud and Hamid [17] show
the presence of randomly oriented inter-grown aggregate
crystals in a heterogeneous size distributed on the surface
film.
Elsewhere the study of the dependence of the PbS thickness values on the lead nitrate concentration in same preceding conditions is reported by Fig. 5. While the [Pb(NO3 )2 ]
exceeds 0.170 M, the thickness of PbS layer decreases. The
result seems to confirm that the nucleation rate is mainly regulated by the concentration of Pb2+ ions in solution, which
results in the development of PbS complex. We can note
that for a [Pb(NO3 )2 ] equal to 0.170 M, the quantity of
Fig. 5. PbS film thickness as a function of lead nitrate concentration.
Pb2+ is high, so it favourites the growth of lead sulphide
thin films with a maximum thickness above 826 nm. As far
as lead nitrate concentration increases, PbS is formed and
the film is removed from its substrate then, so the solution becomes depleted of ions, resulting in a lower rate of
deposition.
Fig. 6. XRD pattern of PbS thin films deposited on glass with [Pb(NO3 )2 ] = 0.170 M, [SC(NH2 )2 ] = 0.1 M, TD = 25 ◦ C, tD = 60 min and for various sodium
hydroxide concentration [NaOH]: (a) 0.56 M, (b) 0.57 M, (c) 0.58 M and (d) 0.59 M.
S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71–80
3.1.3. Effect of the sodium hydroxide concentration
The presence of the sodium hydroxide in this prepared
CBD bath is very vital for the deposition of PbS thin films,
so it assures the precipitation of Na(OH)2 and the hydrolysis of thiourea. In this study, we have set the deposition time
and the lead nitrate concentration at the previous optimal values according to the structural and morphological analyses.
These values are, respectively, 60 min and 0.170 M. We have
varied the [NaOH] in the interval [0.55–0.60 M] with a step
of 0.01. In addition, we note by variation of the basic nature
reagent, that the pH of the solution varies from 13.27 to 13.33.
Fig. 6 proves strongly the effect of the [NaOH] on PbS
thin films structure. The spectra of the samples deposited
with a [NaOH] less than 0.57 M, is practically amorphous. It
shows small and broad peaks indicative of a film poor quality.
Similarly, Fig. 7 shows that the film thickness is rather low so
we concluded that lower concentration of sodium hydroxide
gives lower growth rates.
For [NaOH] equal to 0.57 M, X-ray diffraction patterns
show the appearance of the PbS diffraction lines (Fig. 6b).
The resulting film showed good adhesion to the glass substrate with thickness equal to 826 nm (Fig. 7). While the
[NaOH] increases from 0.57 M, we note that the intensity
of the all PbS peaks gradually decreases.
From this analysis we can conclude that the (2 0 0) direction is the preferential orientation of PbS. However, the well
crystallinity obtained for a sodium hydroxide concentration
equal to 0.57 M, setting the solution pH to 13.
3.1.4. Effect of the thiourea concentration
In this study [SC(NH2 )2 ] varies from 0.09 to 0.2 M. The
XRD pattern of the films is shown in Fig. 8. From this study,
we note that for a concentration value different from 0.1 M
the films are amorphous with weak thicknesses as shown in
Fig. 9. However, for thiourea concentration equal to 0.1 M
the layers essentially consists of PbS compound (Fig. 8),
as shown by the three principal orientations (1 1 1), (2 0 0)
Fig. 7. Thickness of PbS films as a function of sodium hydroxide concentration.
77
Fig. 8. XRD pattern of PbS thin layers, CBD-prepared on glass substrate
with [NaOH] = 0.57 M, [Pb(NO3 )2 ] = 0.170 M, tD = 60 min, TD = 25 ◦ C and
for various SC(NH2 )2 concentration.
and (2 2 0) characterizing a cubic structure. These diffraction peaks are rather narrow, indicating a good crystallinity
with a (2 0 0) preferential orientation. By examination of the
effect of [SC(NH2 )2 ] on the film thickness we can deduce that
if [SC(NH2 )2 ] increases from 0.09 to 0.1 M, the film thickness increases from 305 to 826 nm. For concentration value
exceeding 0.1 M, we note a reduction in the layer thickness
and the film has bad crystallization. Under these optimal conditions determined in this work the PbS film studied by EDAX
analysis (energy dispersive spectrometry) confirmed that the
film is only composed by PbS compound with a concentration ratio practically equal to the theoretical value [Pb]/[S] = 1
(see Fig. 10).
3.1.5. Annealing effect
We study the effect of the heat
nitrogen during 1 h at 200 ◦ C on the
sulphide carried out from deposition
mal conditions: [Pb(NO3 )2 ] = 0.170 M,
treatment under
structure of lead
bath with opti[NaOH] = 0.57 M,
Fig. 9. PbS film thickness as a function of thiourea concentration.
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S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71–80
Fig. 10. EDS analysis of as-deposited layer on glass with [Pb(NO3 )2 ] =
0.170 M, [SC(NH2 )2 ] = 0.1 M, [NaOH] = 0.57 M, tD = 60 min and TD =
25 ◦ C.
[SC(NH2 )2 ] = 0.1 M, tD = 60 min at room temperature. Thin
layer is well crystallized in the cubic structure before heat
treatment (Fig. 11a), but after heating (Fig. 11b) all peaks
corresponding to PbS compound have disappeared without
trace. This result can be explained by an increase of the disorder in the layer so the film becomes amorphous and/or by
a decrease of thin layer thickness after annealing.
Fig. 12. Transmittance spectra of PbS thin layers, CBD-prepared on
glass substrate with [NaOH] = 0.57 M, [SC(NH2 )2 ] = 0.1 M, tD = 60 min,
TD = 25 ◦ C and for various Pb(NO3 )2 concentration.
In this work, we study the influence of the deposition
parameters on the optical properties of chemically deposited
PbS films. The first part will be devoted to the study of the
effect of lead nitrate concentration and the effect of heat treatment on the transmittance and reflectance spectra. The second
part will proceed in determining the refractive index n1 and
the extinction coefficient k1 in various points of coordinates
(x, y) on PbS thin layer surface using an extinction ellipsometer.
3.2.1. Transmittance and reflectance measurements
Optical measurements were undertaken in order to compare the optical performances of PbS films prepared with
different values of lead nitrate concentration.
Transmittance and reflectance measurements at nearnormal incidence were performed over a spectral ranging
between 300 and 1800 nm on PbS thin films deposited on
glass substrate with optimal deposition parameters, determined according to X-ray diffraction analysis but with different values of lead nitrate concentration varying from 0.165
to 0.185 M.
Fig. 12 shows the optical transmission spectra of representative samples of lead sulphide coatings on glass from
baths prepared at various lead nitrate concentration. We can
explain the decrease of transmission for [Pb(NO3 )2 ] equal to
0.165, 0.180 and 0.185 M by the presence of very thin layer
Fig. 11. XRD pattern of PbS thin layers, CBD-prepared on glass substrate with [NaOH] = 0.57 M, [Pb(NO3 )2 ] = 0.170 M, [SC(NH2 )2 ] = 0.10 M,
TD = 25 ◦ C and tD = 60 min, before thermal treatment (a) and after heat treatment under nitrogen for 1 h at 200 ◦ C (b).
Fig. 13. transmission and reflection spectra of PbS thin layers
deposited on glass with [NaOH] = 0.57 M, [Pb(NO3 )2 ] = 0.170 M,
[SC(NH2 )2 ] = 0.100 M, TD = 25 ◦ C and tD = 60 min: (a) before and (b) after
annealing under nitrogen at 200 ◦ C during 1 h.
3.2. Optical properties
S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71–80
of Pb(OH2 ) on the film surface. Such films produce considerably lower specular reflectance and transmittance due to
diffuse reflection from the surface. It is known that Pb(OH2 )
formation can be reduced or avoided by minimizing the quantity of Pb2+ ions in the bath as well as by optimizing the OH−
ion concentration in the bath so that the solubility product
for the Pb(OH2 ) is not exceeded [21]. This result can be also
explained by the reduction of the thickness for [Pb(NO3 )2 ]
equal to 0.165, 0.180 and 0.185 M as can bee seen in
Fig. 5.
Fig. 13 shows the modifications of the transmittance and
reflectance spectra of the PbS with [Pb(NO3 )2 ] = 0.170 M,
after heat treatment at 200 ◦ C under nitrogen during 1 h, we
observe a decrease of T and an increase of R values over the
whole spectral range. The result can be interpreted by the
PbS layers being amorphous after heat treatment as shown in
Fig. 11b.
79
We notice the presence of rather weak interference fringes
on both the T and R spectra, which confirm that the PbS films
have uniform thickness and good surface homogeneity.
3.2.2. Ellipsometry measurements
In this section we determined the complex index
N1 = n1 − ik1 by measuring the ellipsometric parameter Ψ
and ∆ at a wavelength of 633 nm and for different point surface, of PbS thin films realized in optimal conditions.
Fig. 14 displays the surface topography of the refraction
index n1 (x, y) and the extinction coefficient k1 (x, y) as a function of point surface coordinates (x, y) of the PbS thin films
before and after annealing. As seen in this figure, the topography of n1 and k1 along the surface is quite uniform but it
get more homogeneous after thermal treatment (Fig. 14b).
The ellipsometric analysis shows that the reflective index n1
and k1 vary, respectively, between the following ranges n1
Fig. 14. Surface topography of the refraction index n1 and the extinction coefficient k1 of PbS layers CBD-deposited on glass for [NaOH] = 0.57 M,
[Pb(NO3 )2 ] = 0.170 M, [SC(NH2 )2 ] = 0.100 M, TD = 25 ◦ C and tD = 60 min: (a) before and (b) after heat treatment under nitrogen at 200 ◦ C during 1 h.
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S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71–80
between them is due to the uncertainties of measurement.
There is a good agreement between SEM and ellipsometric
analysis. The stoichiometric compound is also confirmed by
the EDAX measurements.
From these studies, we were able to determine this quality
of the surface and, there for a good epitaxy.
References
Fig. 15. SEM micrograph of the PbS fractured film CBD-deposited on
glass for [NaOH] = 0.57 M, [Pb(NO3 )2 ] = 0.170 M, [SC(NH2 )2 ] = 0.100 M,
TD = 25 ◦ C and tD = 60 min.
from 2.5 to 3 and k1 from 1.5 to 2. Elsewhere at a wavelength
of 550 nm bulk PbS is a highly absorbing material with a
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4. Conclusions
In this study, we have presented a chemical bath deposition
method to elaborate the PbS crystallites. From structural analysis we can conclude that the best crystallinity and the great
adhesion of PbS on glass substrate is obtained by employing 0.170 M of lead nitrate, 0.57 M of sodium hydroxide
and 0.1 M of thiourea as reactants, at room temperature for
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have determined by doubles weighting is practically equal
to 826 nm and by SEM micrograph we have found 833 nm
(Fig. 15). These two values are comparable and the difference
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