Thin films of nanocrystallines lead sulfide PbS have been

deposited on glass substrates at different deposition

temperatures of 30, 40, 50 and 60 °C. The films were

synthesized by chemical bath deposition (CBD) an aqueous

solution contained lead acetate, thiourea, Tri-sodium citrate

and sodium hydroxide. Tri-sodium citrate has been used as a

reducing and stabilizing agent. X-Ray diffraction, UV-vis-

NIR-Infared spectrophotometry and scanning electron

microscopy were used to investigate the effect of deposition

temperature on the characteristic properties of the PbS thin

films. The one hand, X-Ray diffraction measurements have

revealed that the increasing in deposition temperature caused

the increasing in grains sizes, and these sizes were found to be

between 39.46 to 45.35 nm. In the other hand, the optical

band gap Eg decreased from 0.97 eV to 0.87 eV when the

deposition temperature increased. This value of Eg can make

the material useful in various optoelectronic applications.

In the last years, the global needs for high-quality

semiconductors, which will have to be cheaper at the same

time, pose major challenges to experts and scientists around

the world. Among these thin-film semiconductor materials,

which have inevitably emerged in the last ten years decades

due to its low manufacturing cost and energy efficiency in

various technological applications such as: Infrared radiation

detectors [1,2], Optical switches [3], controllers of the

concentration of heavy metals [4] as well as solar absorbers

[5]; we find lead chalcogenides materials (PbS, PbSe and

PbTe).

In particular, lead sulphide PbS which has a direct band gap

and variant in massive value from 0.41eV to 2.3eV at 300 °K

[6,7]. This material belongs to group IV-VI of the elements of

the periodic table. The elaboration of PbS thin films is done

by several physical and chemical methods of deposition

which counts as a manufacturing advantage of these layers.

Among these methods of deposition we find: Hot-wall epitaxy

[8], and pulsed laser deposition [9], molecular-beam epitaxy

[8], Vacuum evaporation [10], electro deposition [11], spray

pyrolysis [12], successive ionic layer adsorption and reaction

(SILAR) [13], hydrothermal method [14] and chemical bath

deposition (CBD) [15–17].

In our Work, we used the chemical bath deposition method

(CBD) to deposit the PbS thin films. Our choice, returns to

the characteristic advantages of this method; by dint of the

simple processes of this method we can control the

parameters of deposition such as the deposition time, the

temperature of the solution, the PH, as well as the stirring

speed, in order to have more or less similar results to those

deposited materials by physical methods.

The PbS thin films were developed on normal glass slide

substrates. The deposition was done in a volume of 90 ml of a

reactive solution for an immersion time of 40 minutes,

containing 0.5M the molarity of lead acetate

((PbCH3COOH).3H2O), 1.2M the molarity of thiourea

(NH2CSNH2) and 0.5M the molarity of Tri-sodium citrate

(C6H5Na3O7) in a temperature range of 30°C to 60°C in

10°C increments with a pH fixed in 11.9. The pH adjustment

of the solution was done using the sodium hydroxide (NaOH),

the tri-sodium citrate (TSC) has been used as a complexing

agent. The substrates are cleaned with HCl for 10 min, then

with ethanol and acetone in 10 min successively and finally

with distilled water, after that, the substrate is immersed in the

solution vertically. After depositing of PbS on the glass

substrate, the film is rinsed by the distilled water several times

to remove the weakly particles of PbS bound to the surface of

the layer and allowed to dry.

PbS formation in the CBD method take place only when the

ionic product [Pb+2]*[S-2] exceeds the solubility of PbS

(Ksp≈10-28 at 25 ° C) [18]; therefore, the ionic concentration

of lead and sulfur must be controlled very carefully during

growth. The growth of the film takes place by ion-by-ion or

by a cluster mechanism.

Croissance ion par ion du PbS:

Croissance cluster par cluster du PbS:

Table.1 The crystallite size, the microstrain, the dislocation

density and band gap energy Eg values of the PbS thin films

deposited at different temperatures.

Little variations in grain sizes of nanoparticles can give rise to

significant modifications in the optical properties of PbS thin

films [19]. The dependence of grain size with temperature can

be explained by thermodynamic effects, in fact that the larger

crystallites are more stable than the smaller ones which are

more kinetically favored at lower temperatures [20] and also,

the microstrain values decrease from 0.878*10-3 to 0.770*10-

3 when the deposition temperature increases in the one hand.

In the other, the dislocation density decrease when the

deposition temperature increases.

Fig. 1. The variation of the refractive index n as a function of

the substrate temperature T (a) and as a function of the energy

of the band gap Eg (b) of the nanocrystalline thin films PbS.

The variation of the refractive index n as a function of the gap

energy Eg by the three models of the PbS thin layers is

presented in Fig. 1. It is clear in this figure that the values of

the refractive index are increased from 3.14 to 3.42 with

increasing substrate temperature. However, the rate of

increase of n seems to depend on the models used. Since n is

strongly connected with the energy band gap Eg, it can be

deduced that the material of a smaller band gap has a greater

value of n in all the doped PbS thin films. This result is in

good agreement with that found by Gassoumi et al [21].

The structural, optical and morphological properties of the

PbS thin films produced by the chemical bath method (CBD)

in different deposition temperatures of 30 °C to 60 °C

increment of 10 °C are presented.

The results obtained by the X-ray diffraction analysis shows

that the material crystallizes in a NaCl structure with a lattice

parameter a = 5.9240 Å. The exploitation of these results

also, shows that the preferred crystallographic orientation of

the films are (002), the crystallite sizes increases from 39.46

nm to 45.35 nm when the temperature increase from 30 °C to

60 °C, given in table 2. In addition, little variations in grain

sizes of nanoparticles can give rise to significant

modifications in the optical properties of PbS thin films [19].

The band gap energy decreases from 0.97eV to 0.87eV

when the deposition temperature increase

ACKNOWLEDGMENTThis work was supported by CNRST PPR/37/2015 project

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Sample TD (°C) Crystallite

Size (D) (nm)

Microstrain

ε (×10-3)

Dislocation

(×1013 lines m-2)

Band gap

(eV)

(a) 30 39.46 0.878 0.641 0.97

(b) 40 42.73 0.813 0.552 0.92

(c) 50 44.18 0.791 0.526 0.91

(d) 60 45.35 0.770 0.501 0.87

30 35 40 45 50 55 60

3,10

3,15

3,20

3,25

3,30

3,35

3,40

3,45

Re

fra

cti

ve

in

de

x (

n)

Temperature (°C)

Moss model

Ravinda et al model

Herve and Vandamme model

0,85 0,90 0,95 1,00

3,10

3,15

3,20

3,25

3,30

3,35

3,40

3,45

Re

fra

ctiv

e i

nd

ex

(n

)

The band gap Eg (eV)

Moss model

Ravinda et al model

Herve and Vandamme model