Optical and structural properties of lead sulphide (PbS) thin films synthesized by chemical method

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Vol. 10(13), pp. 385-390, 16 July, 2015
DOI: 10.5897/IJPS2015.4354
Article Number: 6773C7D54070
ISSN 1992 - 1950
Copyright ©2015
Author(s) retain the copyright of this article
http://www.academicjournals.org/IJPS

### _Full Length Research Paper_

## International Journal of Physical Sciences

# Optical and structural properties of lead sulphide (PbS) thin films synthesized by chemical method

B. A. Ezekoye 1 *, T. M. Emeakaroha 1 , V. A. Ezekoye 1 , K. O. Ighodalo 1 and P. O. Offor 2

1Crystal Growth and Characterization Laboratory, Department of Physics and Astronomy, University of Nigeria, Nsukka,
Enugu State, Nigeria.
2Department of Metallurgical and Materials Engineering, University of Nigeria.

Received 21 April, 2015; Accepted 15 June, 2015

The influence of dip times on the lead sulphide (PbS) thin films deposited on glass slide substrates via
chemical bath deposition (CBD) technique using chemical precursors, nitrate , and thiourea,

) in alkaline medium at 300 K was investigated. The optical, structural and morphological
studies were performed by UV-vis spectophotometry, X-ray diffraction (XRD) and scanning electron
microscopy (SEM) respectively. The XRD showed films of cubic (galena), crystalline in nature with the
preferential (111) orientation. The optical studies showed films of direct band gaps in the range of 1.59-
1.65 eV.

Key words: Lead sulphide, thin films, lead sulphide (PbS), X-Ray diffraction, scanning electron microscope
(SEM), bandgap.

INTRODUCTION

Lead sulphide (PbS) is an important IV-VI group 2007), and successive ionic layer adsorption and reaction
chalcogenides semiconductor that has attracted (Puiso et al., 2003; Gulen, 2014; Pawar et al., 2013).
considerable attention in the recent times due to its Chemical bath method is a very simple, relatively cost
numerous optical and opto-electronic properties and effective, convenient for large area scaling and is used in
useful applications in solar cells, optoelectronic devices, the deposition of good quality thin films with physical and
photoconductors, sensors and infrared detector devices chemical properties comparable to other methods. In the
(Chattarki et al., 2012; Koao et al., 2014; Preetha et al., last decade, there has been a renewed interest in this
2015). PbS thin films has direct optical bandgap that can method, mainly associated with its remarkable success in
be changed from 0.39 up to 5.20eV (Koao et al., 2014). depositing semiconductor layers in thin film photovoltaic
PbS thin films have been deposited through various cells. By chemical bath deposition (CBD), the crystallites
deposition processes such as electrodeposition (Osherov can be varied by controlling deposition parameters
et al., 2007), spray pyrolysis (Rajashree et al., 2014; (Abbas et al., 2011). Researchers observed that thermal
Thangaraju and Kaliannan, 2000), chemical bath treating process has effect on the rate of absorptivity of
deposition (Koao et al., 2014; Preetha et al., 2015; PbS thin films and consequently influence the optical
Garcia-Valenzuela et al., 2013; Fernandez-Lima et al., characterization of chemically deposited (Thangaraju and

2007), and successive ionic layer adsorption and reaction
(Puiso et al., 2003; Gulen, 2014; Pawar et al., 2013).

considerable attention in the recent times due to its Chemical bath method is a very simple, relatively cost
numerous optical and opto-electronic properties and effective, convenient for large area scaling and is used in
useful applications in solar cells, optoelectronic devices, the deposition of good quality thin films with physical and
photoconductors, sensors and infrared detector devices chemical properties comparable to other methods. In the
(Chattarki et al., 2012; Koao et al., 2014; Preetha et al., last decade, there has been a renewed interest in this
2015). PbS thin films has direct optical bandgap that can method, mainly associated with its remarkable success in
be changed from 0.39 up to 5.20eV (Koao et al., 2014). depositing semiconductor layers in thin film photovoltaic
PbS thin films have been deposited through various cells. By chemical bath deposition (CBD), the crystallites
deposition processes such as electrodeposition (Osherov can be varied by controlling deposition parameters
et al., 2007), spray pyrolysis (Rajashree et al., 2014; (Abbas et al., 2011). Researchers observed that thermal
Thangaraju and Kaliannan, 2000), chemical bath treating process has effect on the rate of absorptivity of
deposition (Koao et al., 2014; Preetha et al., 2015; PbS thin films and consequently influence the optical
Garcia-Valenzuela et al., 2013; Fernandez-Lima et al., characterization of chemically deposited (Thangaraju and

*Corresponding author. E-mail: benjamin.ezekoye@unn.edu.ng
[Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution](http://creativecommons.org/licenses/by/4.0/deed.en_US)
[License 4.0 International License](http://creativecommons.org/licenses/by/4.0/deed.en_US)

500

400

300

200

100

1000

900

(111)

(111) 40 min 40 MINS
1a1a

800

700

600

500

400

300

200

100

(200)

40 min 40 MINS 60 MINS60 min

800

1a1a (200) 1b1b

60 MINS60 min
1b1b

(200)

(220)

(311)

(222)

(220)

(311)

(222)

0 10 20 30 40 50 60 70 80 90

2 (DEGREES)

(200) (220)

500

400

300

200

100

(111)

0
0 20 40 60 80 100

2 (DEGREES)

80 MINS80 min
1c1c

~~(~~ 311)

(222)

0
0 20 40 60 80

2 (DEGREES)

Figure 1. XRD Pattern of CBD PbS Thin Films for (A) 40 min deep time, (B) 60 min and (C) 80 min.

Kaliannan, 2000)., 10 ml of 0.8 M sodium hydroxide ( and

In the present study, the authors investigated the
influence of dip times on the lead sulphide (PbS) thin
films deposited on glass slide substrates via CBD
technique using chemical precursors, nitrate,
and thiourea, ) in alkaline medium at 300K.

MATERIALS AND METHODS

Lead sulphide thin films were deposited on glass substrate by the
chemical bath method. The precursor chemicals were lead nitrate
(, thiourea and sodium hydroxide .

Thiourea is used as our sulphide ion source and leads nitrate as
our lead ion source. The deposition process is based on slow
release of and ions in the solution which condensed on

the substrate. Lead sulphide were constituted from a solution of 10
ml of 0.1 M lead nitrate  (Pb(NO3)2),  10 ml  of  0.8 M  thiourea

distilled water of 40 ml were added to the solution making it total of
70 ml. Cleaned substrates were vertically immersed into the
solution and was maintained at room temperature. The substrates
were subsequently taken out of the chemical bath after 40, 60 and
80 min dip time, rinsed with distilled water and dried. The resulting
films were uniform, homogeneous and well adhered to the
substrate with dark surface. The suggested reactions are as follows
(Tohidi et al., 2014; Osherov et al., 2007).

Pb(NO3)2 + 4NaOH → [Pb(OH)4] 2- + 4Na + +2NO 3-

[Pb(OH)4] 2- + SC(NH2)2 → PbS + 2OH - +CN2H2 + 2H2O

RESULTS AND DISCUSSION

Figure 1 shows the X-ray diffraction patterns for PbS thin

#### B A C

Figure 2. SEM images for PbS deposited thin films for (A) 40 min deep time, (B) 60 min and (C) 80 min.

films according to the standard X-ray diffraction data files θ=is Bragg’s diffraction angle in degrees and β=is Full
with reference No.03-065-0692. The diffraction peaks of width at half maximum of the peak in radians.
the cubic thin films were found at peaks (111), Figure 2 shows the SEM
(200), (220), (311), (222), which corresponds to 2θ method. It shows that the PbS thin films were
angles ranging from 25.98-70.95 for all the samples. The homogeneous in nature and sparsely packed crystallites
as-prepared films have (111) preferential orientation, which appear to be randomly oriented with irregular and
cubic and polycrystalline in nature and lattice constant spherical shape of similar sizes distribution for film
5.9360 Å, with CuKα irradiation (λ=1.5443Å). deposited by CBD. This agrees with other reports of
The crystalline size of the deposited films for all the Castillo et al. (2014) and Jana et al. (2008).
samples were calculated using Full Width Half Maximum Figure 3 shows the plots from optical studies for the
data (FWHM) and Debye-Scherer formula and their grain absorption measurements carried out in the wavelength
size are in within the ranges of (12-27) nm (Chaudhuri et range of 200 to 1100 nm. Figure 3a shows the variation
al., 2005; Abbas et al., 2011). of absorbance with wavelength for the deposited PbS thin

D =

?????0.9 ?

0.9 ?

(1)

Where D=Grain Size, λ=is the wavelength of CuKα used,

θ=is Bragg’s diffraction angle in degrees and β=is Full
width at half maximum of the peak in radians.

Figure 2 shows the SEM thin films using CBD
method. It shows that the PbS thin films were
homogeneous in nature and sparsely packed crystallites
which appear to be randomly oriented with irregular and
spherical shape of similar sizes distribution for film
deposited by CBD. This agrees with other reports of
Castillo et al. (2014) and Jana et al. (2008).

Figure 3 shows the plots from optical studies for the
absorption measurements carried out in the wavelength
range of 200 to 1100 nm. Figure 3a shows the variation
of absorbance with wavelength for the deposited PbS thin
films at different dip times from 450 to 100 nm. The
sample A (40 min) exhibited absorbance with peak of
about 0.98 (a.u) corresponding to wavelength of about
480 nm which is in the near infrared region. Sample B (60
min) shows maximum absorbance of about 750 nm at

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

CBD 80MINS C
CBD 60MINS A
CBD 40MINS B
3A

100

(SAMPLE C)
(SAMPLE A)
(SAMPLE B)
3B

0.0
500 600 700 800 900 1000 1100

WAVELENGTH(nm)

100

200 400 600 800 1000

WAVELENGTH(nm)

(SAMPLE C)
(SAMPLE A)
(SAMPLE B)
3C

~~0~~
200 400 600 800 1000

WAVELENGTH(nm)

Figure 3. Absorbance, reflectance and transmittance spectra (200-1100 nm) of the PbS CBD deposited thin films.

0.99 (au) in the visible region. Sample C (80 min) shows in the infrared region. This shows that the reflectance
maximum absorbance with peak of about 0.98 (au) spectra is correct because it is opposite of transmittance.
corresponding to wavelength of 760 nm which is also in This is done using Equation (2) (Castillo et al., 2014;
the near infrared region. The absorbance of the entire Abbas et al., 2011).
sample is about the same (au); this is because of the
complexing agent employed during the deposition 1−? 2,exp ⁡( −4?? )?
(Castillo et al., 2014; Abbas et al., 2011; Jana et al., T =

)

2008). 1−? 2 exp ⁡( −8?? ? (2)
Figure 3b shows the reflectance spectra of PbS thin
films at different dip times. All the samples show high Where t = thickness and ? = wavelength.
reflectance in the visible region which rapidly decreased The absorption coefficient α associated with the strong
in the near infrared region. This high reflectance and absorption region of the film was calculated from
absorbance in the visible region make the thin film good absorbance (A) and the thin film thickness (t) which was
material for anti-reflection coating and also for solar calculated using the relation below (Manouchehri et al.,
thermal applications, the films can be employed as a 2014):
solar control coating, also applied in infrared (IR)
detectors (Castillo et al., 2014; Abbas et al., 2011; Jana (3)
et al., 2008; Larramendi et al., 2001).

in the infrared region. This shows that the reflectance
spectra is correct because it is opposite of transmittance.
This is done using Equation (2) (Castillo et al., 2014;
Abbas et al., 2011).

1−? 2,exp ⁡( −4?? )?

T =

1−? 2,exp ⁡( −4?? )?

)
1−? 2 exp ⁡( −8?? ?

1−? 2 exp ⁡( −8?? ? (2)

Where t = thickness and ? = wavelength.

The absorption coefficient α associated with the strong
absorption region of the film was calculated from
absorbance (A) and the thin film thickness (t) which was
calculated using the relation below (Manouchehri et al.,
2014):

Figure 3c shows the transmittance spectra of PbS thin
films at different dip times. All the samples show low
transmittance in the visible region which rapidly increased

(3)

The absorption coefficient α was analyzed using the
following expression for optical absorption of
semiconductors (Manouchehri et al., 2014):

0.00005

0.00004

0.00003

0.00002

0.00001

0.00000

1.60E-010

1.40E-010

1.20E-010

1.00E-010

8.00E-011

6.00E-011

4.00E-011

2.00E-011

0.00E+000

1.0 1.2 1.4 1.6 1.8 2.0

hv(eV)

1.0 1.2 1.4 1.6 1.8 2.0

hv(eV)

0.00008

0.00007

0.00006

0.00005

0.00004

0.00003

0.00002

0.00001

0.00000
1.0 1.2 1.4 1.6 1.8

hv(eV)

Figure 4. Plots of _(αhv)_ _2_ versus _h_  for PbS thin films for (A) 40 min deep time, (B) 60 min and (C) 80 min.

(4)

where = constant, = energy band gap and  = 1

for allowed direct band gap semiconductor.

Figure 4 shows, Plots of (αhv) 2 versus _h_ for PbS thin
films at different dip times.

The values obtained for the direct band gap of thin
films deposited by chemical bath lies within the range of
1.59-1.65eV. A close observation of the band gap range
shows that increase in deposition time increases the
band gap.

Figure 5 shows the plot of thickness against the
deposition time. The thickness increased with the
deposition; time increases (Manouchehri et al., 2014;
Abbas et al., 2011), until after 1100 nm and remained

almost constant.

Conclusion

Chemical bath deposition (CBD) method has been
successfully used to deposit thin films. The optical
adsorption, morphological and structural studies of the
thin films were carried out. The results obtained from the
XRD for the thin films was found to be polycrystalline
in nature and grown in cubic crystal structure (galena),
with grain sizes of (12 to 27 nm). The properties of high
absorbance and high reflectance in the visible region, low
absorbance and high reflectance in the near infrared
region make the film a good material for anti-reflection
coating and for solar thermal applications and infrared
(IR) sensors.

120

0
400 600 800 1000 1200

THICKNESS (nm)

Figure 5. Plot of Deposition time Vs Thickness of PbS thin films.

Conflict of Interest

doi:10.1016/j.physe.2008.04.015.

The authors declare no conflict of interests.

Electrochem. Sci. 9:1747-1757.

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