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KKU Journal of Basic and Applied Sciences
Journal homepage: jbas.kku.edu.sa Vol 2, 2 (2016) 18-22
P a g e | 18
Investigating Effects of Manganese Dopant on the Structure, Morphology and Optical Properties of the Nano Lead Sulfide Film
S. AlFaify*
Department of Physics, College of Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia
Keywords: CBD method, nano PbS film, Mn doping, Structure, Morphology, Optical properties
1. INTRODUCTION
Lead sulfide (PbS) film has been shown to be of great interest for many years for researchers and developer alike aiming to utilize such material in many promising and modern applications. Lead sulfide is a binary (IV–VI) semiconducting material that has a narrow optical band energy gap of 0.41eV at room temperature and under normal conditions [1,2]. PbS materials have simple crystalline structures, usually of cubic crystalline phases. Moreover, PbS maintained relatively a large Bohr radius of about 18 nm which promotes strong quantum confinement effects desirably needed for varieties of cutting edge technologies [3] – [4]. The nano thin film of PbS, in particular, has attracted considerable attentions in recent years due to its low-toxicity and cost-effective materials and due to its advanced applications in solar cell fabrication, gas sensing and infra-red and near infra-red detections [5] – [9]. For harnessing PbS potentials on targeted applications, it is important to have effective means for tuning and tailoring the material structural, morphological, optical, and electronical properties. Fortunately, it can be done by utilizing the right methods of deposition and/or by doping with suitable impurities. Hence, many modern methods for PbS thin film depositions were introduced by many scientists
and researchers from around the globe including electrodeposition [10], atomic layer deposition (ALD) [11], spray pyrolysis [12], ionic layer adsorption and reaction (SILAR) [13], microwave heating [14], and chemical bath deposition (CBD) [15] – [19] techniques. Distinguishably, the CBD method is standing out as an attractive technique for the preparation of thin nano and micro-crystalline films in general. Since, it is relatively cost effective, simple to handle and control, suitable for industrial scale deposition on large areas, and capable of yielding high quality nano/micro structure materials [20]. In addition to the favorably designated method of deposition, researchers have successfully utilized CBD method to control qualities and properties of PbS by doping as well. In fact, doping is a key requirement for many modern and futuristic applications. PbS as other similar materials can be doped with many elements to enhance and control its physical properties. The nature and concentrations of the carefully chosen dopants are of great importance when considering particular properties and potential applications of nano PbS films.
Therefore, in this work, good qualities PbS and Mn-doped PbS nano films were successfully prepared on glass substrates by chemical bath deposition (CBD) techniques. Manganese has been utilized as a dopant of choice due to its outstanding characteristics such as small ionic radius of 0.066 nm [21], Received: April 30, 2016 / Revised: July 20, 2016 / Accepted: August 03, 2016 / Published: December 01, 2016
Abstract: The Manganese doped nano lead sulfide (Mn:PbS) films were synthesized on glass substrates by chemical bath deposition (CBD) method at ambient temperature and pressure conditions. Effects of the Mn dopant with varying content into structure, morphology and optical properties of the nano PbS films were investigated. The refined Rietveld X-ray diffraction (XRD) and atomic force microscopy (AFM) analyses showed all deposited films exhibiting polycrystalline nature with cubic rock-salt structure. The optical characterizations of the prepared film provide insight on the allowed electronic transitions taking places during interaction with the incoming electromagnetic waves. The direct band gap values are found to be in the range of 0.75 to 0.86 eV depending on the dopant Mn content. Correlation between refractive index and energy bandgap values were investigated as well. Based on atomic theoretical models, refractive Indies (n) and high frequency dielectric constant (ε∞) values are calculated using the obtained data of energy bandgaps as functions of the Mn doping content. In general, Mn doping has structure-properties correlating effects on the nano PbS film and can be used to tune the film’s texture between nano and micro crystalline structure.
P a g e | 19 and high activation energy [22]. The structural and
morphological parameters of the as deposited Mn doped PbS nano films such as crystalline phase, lattice constants, grain size, and surface roughness are investigated and correlated to the dopant Mn content. Moreover, the effects of the Mn dopant on the refractive indices (n) and high frequency dielectric constants (ε∞), calculated theoretically based on well-known atomic models, of the nano PbS film are studied.
The characterizations of the as deposited nano films were done using XRD, AFM and optical measurements. Results and discussions are going to provide valuable information on the correlation nature of structure and properties of the nano PbS film as functions of the carefully utilized method of deposition and choosing dopant.
2. FILMSPREPARATIONS
Both pure and Mn doped PbS layers were synthesized on normal glass substrates by the well-known CBD method. The reactive solution was made by the sequential adding of an alkaline aqueous solution of about 0.17 molar (M) lead nitrate [Pb(NO3)2], 0.57 M sodium hydroxide [NaOH], and 0.1 M thiourea [SC(NH2)2] into a 50 ml beaker. To prepare the Mn2+
doped films, MnCl2 was also added into the main solution. pH value was set at 12 by adding NaOH to the reaction bath as well. The cleansed glass substrates were introduced vertically into the bath solutions. The whole deposition set up including beakers, bath solutions, and the slowly dipped glass substrates were immersed into hot water bath circulator placed on a heating magnetic agitator to maintained temperature and pressure at the ambient conditions. The reaction and growth process of the nano lead sulfide film can be described as follow [23]:
Pb(NO3)2 + 2NaOH → Pb(OH)2 + 2NaNO3
Pb(OH)2 + 4NaOH → Na4Pb(OH)6
Na4Pb(OH)6 → 4Na+ + HPbO2− + 3OH− + H2O SC(NH2)2 + OH− → CH2N2 + H2O + SH− HPbO2− + SH− → PbS + 2OH−
After deposition, the grown films were cleaned and rinsed with deionized water to get rid of residuals due to bath solution and loosely adhered PbS particles on the film’s surface. Grayish/black colored films were obtained in one hour time. The variation of the Mn dopant into the PbS films was carried out by changing molar ratio of the Pb(NO3)2 and MnCl2 to obtained respectively 1 at. %, 2 at. %, 3 at. % , 4 at.
% and 5 at. % Mn doped PbS films.
3. RESULTSANDDISCUSSIONS
3.1. Structural Properties
3.1.1. X-Ray Diffraction (XRD) Analysis
The XRD patterns of the Mn−doped PbS films are shown in Fig. 1 as a function of Mn content. The XRD patterns present the same peaks corresponding to the cubic rock-salt PbS structure imposed upon a broad halo related to the substrate (glass). The films exhibit a good crystallinity and a (200) preferred plane orientation along the c-axis normal to the substrate. For 1% Mn film, however, the (111) peak intensity increases and reaches a value comparable to that of (220) as confirmed by the intensity ratio I(111)/I(200) which
is about 1.005 (Table I or Fig. 2).
Fig. 1. XRD patterns of Mn doped PbS as a function of Mn content.
TABLE I: VARYING PbS PEAK’S ORIENTATION DEGREE AS A FUNCTION OF Mn CONTENT
Fig. 2. Varying PbS peak’s orientation degree as function of Mn content.
The Rietveld refinements of the X ray diffraction patterns were performed with the introduction of the PbS phase (space group and lattice parameter a0=5.936Å). The Rietveld refinement of the 1 at. % Mn doped PbS thin film is shown in Fig. 3. Depending on the Mn content, the films exhibit either a nanocrystalline (pure PbS, 2 at.% Mn and 3 at.
% Mn) or microcrystalline structure (1, 4, and 5 at.% Mn) as revealed by the variation of the crystallite size (Fig. 4).The most/highest quality of crystallinity is obtained in the 5 at. % Mn doped PbS film.
KKU Journal of Basic and Applied Sciences
Fig. 3. The Rietveld refinement of the XRD pattern of 1at. % Mn doped PbS.
Fig. 4. Evolution of the crystallite size as a function of Mn content.
Fig. 5 shows the variation of the lattice parameter against the Mn content in the Mn doped PbS structure. The increase of the lattice parameter of 1 at. % Mn, 3 at. % Mn and 5at. % Mn doped PbS can be related to the expansion of the crystal lattice. The Mn atom (radius 0.14 nm) has a strong preference for occupying the PbS sites in the rock salt crystal structure by replacing Pb (radius 0.18 nm) or S (radius 0.10 nm) atoms. Hence, Mn induces changes in the composition of the nanocrystalline PbS film. Since the atomic radii are in the order of Pb>Mn>S, the expansion of the lattice parameter might be related to the replacement of sulfur atoms by manganese ones. The relative change of the lattice parameter compared to that of the pure crystal (а0) defined by Δа = (a-a0)/a0, reaching as much as ~ 0.11% and 0.12% for 3 at. % Mn and 5 at.% Mn doped PbS, respectively.
Fig. 5. Variation of the lattice parameter versus Mn content.
3.1.2. Atomic Force Microscopy(AFM)
The AFM analysis was carried out on the pure and Mn doped PbS films. The obtained micrographs have been analyzed using Gwyddion software utilized under the GUN general public license [24]. Fig. 6 shows the pure as well as doped PbS films micro/nanostructures at varying concentrations of the Mn dopant. Inset images simulate the average grain/crystal size and highest of each studied film.
Fig. 6. AFM micrographs of the pure and Mn doped PbS films. Inset image on each micrograph is a colored representation of the average
grain size and height from the analysis of the film’s structure.
Fig. 7 has revealed the estimated average grain size as well as surface roughness for each studied film with a clear dependency on the dopant Mn content. The obtained results correlate very well with the XRD analyses shown earlier. The effect of the Mn dopant content on the as deposited films resembles to a great extent the pattern seen in the XRD studied. The PbS films doped with 0 at. %, 2 at. %, and 3 at.%
of Mn appear to be of nanocrystalline structures. While the films deposited with 1 at.%, 4 at.%, and 5 at.% of Mn content turning relatively into microcrystalline materials, to check the film’s structural behaver due to different concentration of the Mn dopant compare between Fig. 6 and Fig. 7 (a). Moreover, AFM analysis showed the mean square roughness of the as deposited films doped with varying Mn content to be of the range of 15 to 19 nm as clear for the data of the Fig. 7 (b).
0 1 2 3 4 5
60 80 100 120 140
(a) (b)
equivlent disc raduis [
r eq] (nm)
Mn content (at.%)
0 1 2 3 4 5
15 16 17 18 19 20
surface roughness [
R ms
] (nm)
Mn content (at.%)
Fig. 7. AFM analysis data as functions of Mn content for the pure and Mn doped PbS films: (a) the average grain sizes represented by the
estimated radius and (b) the mean square roughness for the studied films.
3.2. Optical Properties
Transmission coefficient spectra measured for pure as well as Mn doped PbS nano films are represented in Fig. 8 the optical spectra were taken in the UV-VIS-NIR regions between 250 and 2500 nm. Result showed all films having low transmittance rate in the UV region below 400 nm, due to strong
250 500 750 1000 1250 1500 1750 2000 2250 2500 0
10 20 30 40 50
Pure PbS 1 at% Mn 2 at% Mn 3 at% Mn 4 at% Mn 5 at% Mn
Transmittance (%)
(nm)
P a g e | 21 (4)
(3) Fig. 8. Optical transmission spectra of PbS thin films; pure and doped
with varying content of Mn.
absorbance in that region. However, transmittance rate is generally enhanced gradually toward the near IR region. From Fig. 8, it is apparent as well that the transmission coefficient of the PbS film is behaving as a function of the Mn doping concentration with films
doped with 0 at.%, 2 at.%, 3 at.%, and 4 at.% Mn given the highest rates.
Although PbS is known to be a material of interest in the near IR region of the electromagnetic spectrum, recently many researchers are trying to tune its optical properties, in particular, energy band gap for the PbS to be useful material in the UV and visible regions as well. Therefore, It is important to understand the doping effect on the band gap energy value of the Mn:PbS film under investigations.
For the electronic transitions taking place within the studied film’s material when interact with incident light, the fundamental absorption coefficient (α) is obtained as a function on the photon energies using UV-Vis spectrometer.
It is common for such material to show a linear dependency in the near IR region, in which the linear portion is an indicator for the allowed transition [25]. Graphing relation between (αhν)m and photon energy (hν), with h beings Planck constant and m= 2 for direct transition and = 1/2 for the indirect electronic transition, one can obtained band gap values [26].
Extrapolating the linear portion of the graph to the hν axis, we can obtain the direct band gaps of the Mn:PbS film, which is turned out to be in the range of 0. 75 to 0.86 eV depending on the doping concentration, as shown in Fig. 9. This variation in the band gap values can be explained in term of the enhancement in grain sizes at different Mn doping content. It is well known that the energy band gap value of a semiconductor is generally affected by varying parameters including temperature, grain size, residual strain, defects, impurities, disorder at the grain boundaries, etc.
Fig. 9. Variation of optical band gap of PbS thin films, pure and doped with varying content of Mn.
The optical constants are crucial parameters when trying
to characterization films and explore their potential applications in modern optoelectronics, as they describe the optical behavior and dielectric functions of the film’s materials. In particular, refractive index (n) is important when designing for example hetero-structure lasers, advanced optoelectronic devices, and solar cells. Many attempts have been made to established accurate relationship between energy bandgap (Eg) and optical refractive index (n) for the semiconductor materials.
Hence, we try to calculate refractive index of the Mn:PbS
films from the earlier obtained values of the band gab energy using Moss relation [27] that is given by the following atomic model:
n4 Eg = K (1)
where the constant K is originally given as 95 eV.
Ravindra et al [28] [29] have proposed a linear relation relating the energy gap Eg and refractive index n of a material as shown below:
n = α + βEg (2) with α = 4.16 and β= -0.85 eV-1
Another theoretical model between refractive index and band gap energy for a material was also proposed by Herve and Vandamme [30] based on the classical oscillator theory:
2
2 1
g
n A
E B
Where constant A found to be of the same value as the hydrogen ionization energy 13.6 eV and constant B = 3.4 eV.
Therefore, using the above mentioned relations one can calculate the refractive index of the Mn: PbS film at different Mn doing concentration, see Table II. The n values found to be decreasing from 3.52 to 3.24 with increasing Mn content.
However, rate of change appears to be dependent on the utilized model of calculations. In general, refractive index seems to be strongly correlating to the bandgap energy value.
In addition, the smaller bandgap energy is corresponding to the largest values of n in the studied Mn doped PbS films.
The dielectric function of a solid material is also important when trying to understand and design electronic device and properties. Hence, the high frequency dielectric constant (ε∞) for the Mn doped PbS thin films were estimated from the relation [31]:
n
2
Where n is being the obtained refractive index.
For different Mn content, the calculated Eg, n, and ε∞ values of the studied films were presented in Table II.
TABLE II: LIST OF THE (Eg), (n), and (
) VALUES OF THE Mn DOPED PbS THIN FILMS; PURE AND DOPED WITH VARYING CONTENT OF Mn4. CONCLUSION
The CBD method as an efficient and cost effective technique was successfully applied to synthesized Mn doped PbS thin layers on glass substrates at ambient temperature and
0.6 0.8 1.0 1.2
0.0 2.0x1013 4.0x1013 6.0x1013
(h)2 (eV2cm-1)
h(eV) Pure PbS
1at% Mn 2at% Mn 3at% Mn 4at% Mn 5at% Mn
Mn Mn Mn Mn Mn
KKU Journal of Basic and Applied Sciences
atmosphere pressure. The influences/effects of the varying Mn doping content on the structure, morphology and optical properties of the PbS nano film were thoroughly investigated.
The detailed X- ray diffraction and atomic force microscopy analyses showed Mn doped PbS thin films having a polycrystalline structure. Depending on the Mn doping concentration, the film’s structure can be microcrystalline or nanocrystalline material. The obtained bandgap values of the Mn doped PbS films are in the range of 0.75 to 0.86 eV while the refractive index (n) of the films found to be decreasing from 3.52 to 3.24 with increasing Mn content. Therefore, it can be concluded that the Mn doping content in the nano PbS film is playing a vital role on the characteristic parameters of the film and that the CBD method is a suitable technique for synthesizing Mn-doped PbS films to be utilized into many modern and cutting edge applications.
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