Structural and optical properties of nanocrystalline lead sulfide thin films prepared by microwave-assisted chemical bath deposition

Materials Science in Semiconductor Processing 16 (2013) 971–979

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Structural and optical properties of nanocrystalline lead sulfide thin films prepared by microwave-assisted chemical bath deposition A.S. Obaid a,b,n, M.A. Mahdi a,c, Y. Yusof a, M. Bououdina d,e, Z. Hassan a a

Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia Department of Physics, College of Sciences, University of Anbar, P.O. Box 55431, Baghdad, Iraq Physics Department, College of Science, Basrah University, Basrah, Iraq d Nanotechnology Centre, University of Bahrain, P.O. Box 32038, Kingdom of Bahrain e Department of Physics, College of Science, University of Bahrain, P.O. Box 32038, Kingdom of Bahrain b c

a r t i c l e i n f o

abstract

Available online 16 March 2013

Nanocrystalline PbS thin films have been successfully deposited on glass substrate from lead nitrate (Pb2 þ ions) and thiourea (S2  ions) precursors using MACB technique. The effects of molar concentration (0.02, 0.05, 0.075 and 0.1 M) on the structure and microstructure evolution were studied using X-ray diffraction (XRD), scanning electron microscopy, and atomic force microscopy. The optical properties were investigated using UV–vis spectrophotometer. Crystal size values obtained from XRD were compared with these calculated using atomic force microscopy (AFM). The values of optical band gaps were found to decrease as the ion source molar concentration increase. & 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Thin films B. Chemical synthesis C. Atomic force microscopy D. X-ray diffraction F. Optical properties

1. Introduction Lead sulfide (PbS) is an important binary IV–VI semiconductor material with direct narrow optical energy gap (0.41 eV at 300 K) and relatively large excitation Bohr radius (18 nm), which results in strong quantum confinement of both electrons and holes in nanosize structure. Thus, the value of the band gap can be simply controlled by modifying the particle size as well as the shape, according to the effective mass model [1]. Hence, there has been growing interest in developing new processing routes for the preparation of PbS at the nanoscale, in particular as thin films. Chemical-bath deposition (CBD) method has become an attractive method for the preparation of thin films due to several advantages compared to other techniques, including industrial-scaled manufacturing, low cost, suitability for large scale deposition areas, ability to deposit

n

Corresponding author. Tel.: þ60 174170405; fax: þ 60 46579150. E-mail address: [email protected] (A.S. Obaid).

1369-8001/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2013.02.005

thin films on different substrates, and flexibility of tuning thin film properties simply by controlling and adjusting the deposition experimental parameters [2]. Depending on the deposition conditions, the film growth can take place via ion-by-ion condensation or by adsorption of colloidal particles (cluster by cluster) from the solution onto substrates [2,3]. In general, the preparation of PbS films using CBD method requires low temperature and a long processing period of time [2,4]. Moreover, microwave irradiation is widely used to prepare organic and inorganic materials. When a material is exposed to electromagnetic wave, it will absorb the electromagnetic energy and transform it into thermal energy [5,6]. In this case, heat is generated from inside the material through the microwave irradiation, whereas in other methods, heat is usually transmitted from the outside of the material to the inside. Hence, the production of internal heat reduces the reaction time and energy cost, as well as allows the synthesis of new material [7]. Therefore the combination of both methods, called microwave assisted chemical bath deposition (MACBD) synthesis, is generally quite-fast, simple and

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energy-efficient. MACBD synthesis method has been developed and is widely used in the synthesis of thin films [8–10]. In this study, MACBD method was used to prepare nanocrystalline PbS thin films. The effect of molar concentration on structural and microstructural changes and hence their effects on optical properties, were investigated. 2. Experiment details 2.1. Preparation of PbS nanocrystalline thin films Microscope glass slides (75  25  2 mm) were used as substrates. First, substrates were washed with hot distilled

water, immersed in HCl 20% (diluted with 80% water solution) for 24 h, and washed with acetone. Then, substrates were cleaned ultrasonically with water for 20 min. The deposition was done in a reactive solution prepared in a 50 mL beaker which containing various molar concentrations (0.1, 0.075, 0.05, and 0.025 M) from both lead nitrate and thiourea (TU). Ammonium acetate [NH4CH3COO] was added as a buffer solution to control the rat reaction. The pH of all solutions was fixed at 12 by sodium hydroxide NaOH which represents the optimal value, and the total volume was 100 mL after water was added. Substrates were fixed vertically in the chemical bath and the beakers were placed in the microwave oven system (2.4 GHz) at 80 1C for a deposition time of 30 min by controlling the oven power. External power supply was used as a source of voltage to the microwave for controlling the deposition temperature. For the first 5 min of reaction time, the solution remained transparent, indicating the occurrence of decomposition reactions. Beyond 5 min, the solution turned dark gray, indicating the formation of PbS compound. After completion of the deposition time, the films were removed from the solution, rinsed ultrasonically in hot distilled water for 5 min, and then dried in air. Mirror-like gray thin film surfaces were obtained after removal of one side of the glass slide using cotton with HCL. 2.2. Characterizations

Fig. 1. XRD of the four nanocrystalline PbS thin films deposited with different molar concentration: (a) 0.025, (b) 0.05, (c) 0.075, and (d) 0.1 M.

The film thickness was measured using an optical reflectometer (Filmetrics F20). The thickness of films was found to increase from 163 nm at a molar concentration of 0.025 M up to 195 nm at molar concentration of 0.1 M. The structural properties of the prepared thin films were examined using high-resolution X-ray diffraction (HR-XRD) using X’Pert Pro MRD diffractometer (PANalytical Company) system equipped with Cu-Ka radiation wavelength (l ¼0.15418 nm) operating at 40 kV and 20 mA. Morphology and microstructure of the films were investigated by scanning electron microscopy (SEM) using Jeol JSM-6460 LV microscope operating at 10 kV and attached to energy dispersive X-ray spectrometer (EDX) for elemental chemical composition determination. The surface morphology of thin films was investigated by atomic force microscopy (AFM) using NanoScope Analysis version1.20 (Veeco) scan mode Tapping (non-contact). Optical measurements were conducted at room temperature using Shimadzu UV–vis 1800 spectrophotometer at wavelengths ranging from 300 to 1100 nm.

Table 1 XRD data of nanocrystalline PbS thin films. Molar concentration (M)

Thickness (nm)

Average crystal size Cs (nm)

a(Ao) calculated

d( Ao )

d(Ao) Stander

hkl

TC%

0.025

163

10

5.936

0.05

177

10.5

5.931

0.075

186

12

0.1

195

13.6

3.431 2.971 3.424 2.970 3.435 2.970 3.429 2.969

3.424 2.969 3.424 2.969 3.424 2.969 3.424 2.969

111 200 111 200 111 200 111 200

1.26 2.61 6.24 5.64 6.71 4.89 7.46 4.35

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3. Results and discussion 3.1. Mechanism of PbS formation The microwave radiation interaction among the reactants during the synthesis involves rapid heating via energy transfer from microwaves to the material, either through resonance or relaxation [11]. Microwave irradiation induces interaction of the dipole moment of polar molecules or molecular ionic aggregates with alternating electronic and magnetic fields, causing molecular-level heating that leads to homogeneous and quick thermal reactions. Both temperature and concentration gradients can be avoided via microwave irradiation of reactants in polar solvents, thereby providing a uniform environment for nucleation [10]. During the process, microwave provides the energy for the decomposition of complexes, accelerates the nucleation of the PbS, and depresses

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the direct growth of the newborn PbS nuclei because of the intense friction and collisions of the molecules created by the microwave irradiation. The chemical reactions that occurring in the mixture of lead nitrate and sodium hydroxide create lead hydroxide [Pb(OH)2], which subsequently react with ammonia to produce lead tetramine complex [Pb(NH3)4]2 þ , as described by the reactions below [12]: Pb(NO3)2 þ2NaOH-Pb(OH)2 þ2NaNO3

(1)

Na4 PbðOHÞ6 is the complex formed will be dissociates to product Pb(HO)2 which is important product because it will be used in later reaction: Pb(OH)2 þ4NaOH-Na4Pb(OH)6

(2) 

Na4 PbðOHÞ6 -4Na þ þH2 O þHPbO2 þ 3OH

ð3Þ

Fig. 2. Scanning electron microscopy images of the four nanocrystalline PbS thin films deposited with different molar concentration: (a) 0.025, (b) 0.05, (c) 0.075, and (d) 0.1 M.

Table 2 Variation of particle size and energy gap of PbS nanocrystalline thin films. Molar concentration (M)

Ps(nm) AFM

Rms(nm)

EDX Pb:S ratio

Eg (eV)

0.025 0.05 0.075 0.1

16 18 24 27

3.6 4.1 7 8.7

0.71 0.83 0.84 0.85

2.35 2.11 2.00 1.59

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Thiourea was dissolved in water to produce sulfide ions according to the following equations:

(4)

(5)

A complex solution composed of both Pb2 þ ions and TU leads to the formation of Pb-TU complex. The complex decomposes with the presence of heat because of the irradiation that forms PbS, where TU reacts with metal ions to form complexes with the capability to decay at high temperature and produce PbS [11]: 

HPbO2 þ SH -PbS þ 2HO

ð6Þ

3.2. Structural analysis: X-ray diffraction Fig. 1 shows the evolution of X-ray diffraction patterns of the prepared PbS films with different molar concentration. It is clear that all films are polycrystalline with good crystallinity, except for the molar concentration of 0.025 where the XRD pattern shows amorphous behavior (hallo) with low crystallinity (probably the film is a mixture of an amorphous and crystalline phases). The observed diffraction peaks were indexed within a cubic rock salt (NaCl) type structure, as confirmed using a standard card (ICCDPDF4 No.00-005-0592). The main features of the diffraction patterns are similar but the relative intensity of the diffraction peaks varies considerably with the variation of the molar concentration. Thus, changing the molar concentration has effect on the phase formation and its structural features, including crystallinity, without the appearance of new phases aside from cubic PbS. This observation means that no oxidation occurred during the preparation, thereby resulting in a good quality of the formed thin films. A preferred orientation of the deposited films along the (1 1 1) or (2 0 0) directions was observed depending on the molar concentration. The texture coefficient (TC) represents the texture of a particular plane, where the deviation from unity implies the direction of a preferred growth. Quantitative information on the preferential crystallites orientation was obtained from (hkl) and defined by the following equation [12]: IðhklÞ=Io ðhklÞ TCðhklÞ ¼ P  100% ð7Þ n IðhklÞ=Io ðhklÞ where I(hkl) is the measured relative intensity of a defined diffraction plane (hkl), Io(hkl) is its corresponding standard intensity, and n is the number of diffraction peaks. The value of TC(hkl)¼1 represents randomly oriented crystallites, while higher values indicate the abundance of grains oriented in a given (hkl) direction. The variation of TC for (1 1 1) and (2 0 0) diffraction peaks are reported in Table 1. The highest TC value was obtained

for (1 1 1) plane for all prepared nanocrystalline PbS thin films except for the molar concentration of 0.025 M. The lattice constant of the cubic rock salt structure is given by [13]: 2

2

2

a ¼ dðh þk þ l Þ

ð8Þ

where h, k and l are the Miller indices; and d is the interplanar spacing. It is found that the lattice constant increases with increasing molar concentration (Table 1), which clearly indicates that the crystallization and stress result in lattice compression [14]. The average crystallite size (Cs) of nanostructures PbS can be calculated from the well-known Scherrer formula [4]: CS ¼

kl bcos y

ð9Þ

where k is a constant (0.94), l is the XRD wavelength (0.15418 nm), and bis the full width at half maximum (FWHM) of a defined diffraction peak. The calculated crystallite size value increases from 14 up to 24 nm for the molar concentration ranging from 0.025 to 0.1 M (Table 1). This is in good agreement with the results reported in the literature, but the grain size was found to be very sensitive to the synthesis route and the experimental conditions and parameters. For example, Ding et al. [16] synthesized PbS nanocrystals of 10 nm in size in ethanol solvent using microwave heating for 20 min. Similarly, Zhao et al. [17] used microwave method for 30 min to obtain PbS thin films with grain size in the range of 30–50 nm. Kumara et al. [18] synthesized PbS nanoparticles with an average particle size of 40 nm by CBD.

3.3. Microstructural analysis: scanning electron microscopy The morphology of the nanocrystalline PbS thin films was investigated using SEM as shown in Fig. 2. Descriptions of the films in terms of size and morphology of nanocrystalline PbS for samples prepared using MACBD method. Fig. 2 illustrates that the structure variety resulting from changes in molar concentration. Fig. 2(c) and (d) show lager particle size distribution compared with the other sample, thus confirming the XRD results as shown in Table 2. In all cases the morphology tends towards small cubic like shaped particles, which indicates the nanocrystalline nature of the PbS thin films. It is observed that the films are very adherent to the glass substrate; with increasing molar concentration the films show more uniform surface morphology as shown in Fig. 2(d). SEM images revealed that thin films deposited at molar concentration (0.1 M) has good structure compared with the other thin films deposited at different molar concentration. Energy dispersive X-ray analysis spectra of PbS thin films are reported in Fig. 3 and the corresponding quantitative chemical analysis is reported in Table 2. The analysis confirms the presence of Pb and S elements and that the Pb:S ratio increases from 0.71 for the small molar concentration (0.025 M) to reach a steady value around 0.84 for higher molarity (0.1 M). It is very important to note that no additional peaks attributed to impurities or contamination are observed, thus confirming the purity of

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the prepared thin films as confirmed previously by XRD analysis. 3.4. Microstructural analysis: atomic force microscopy The surface images of the prepared PbS thin films were recorded in an area of 5  5 mm, see Fig. 4. AFM images confirmed that the films have a smooth surface, good adherence to the substrate, and narrow particles size distribution. The images showed also that with increased molar concentration, the surface roughness increased due to increased particles size. The average particles size was measured using AFM images, and found to be higher than that determined by XRD. This finding indicated that a number of nanocrystallites (oriented in the same plane) coalesced to form PbS nanocrystals. Abbas et al. [14] have also reported that the average grain size of their PbS thin films observed by AFM was greater than that calculated using the Debye–Scherrer relation. Fig. 5 shows the histogram distribution. It can be observed that the average particle size increased with increasing molar concentration, which may be due to the increased film thickness (Table 2). SEM and AFM images revealed that the thin film deposited has narrow particles size distribution. 3.5. Optical properties The transmission spectra of PbS nanocrystalline thin films deposited with different molar concentration, recorded at room temperature in the wavelength range of 300–1100 nm, showed a decrease in transmission wavelength with increasing molar concentration, due to

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the increase in the thickness, which leads to a decrease in light scattering losses [19] as shown in Fig. 6. The results can be explained through the film morphology, as the film thickness increases the grain size increases, as mentioned previously; the surface roughness increases with increasing grain size thus leading to a decrease in the transmission. Also, it is noticed that all the films have low transmittance in the ultraviolet UV region below 400 nm, which is due to the strong absorbance in this region. Thereafter, the transmission increases with increasing wavelength towards the NIR regions. It should be emphasized that as the molar concentration increases, the transparency window decreases due to the increase of thickness with molar concentration. The absorbance spectra of nanocrystalline PbS thin films deposited at different molar concentration were displayed in Fig. 7. It is observed that the shapes of the curves were similar although differences in absorbency were observed. The absorbance for all films increased with the molar concentration due to the increase of the film’s thickness, where the thicker films have more atoms present hence more states are available for the photon energy to be absorbed [20]. It can be concluded that PbS thin films have the high absorbing nature. However, all films show low absorption over the longer wavelength region. The effect of change of molar concentration can be observed in the long wavelength portion of the spectra, where interference effects took place. The energy gaps of PbS thin films were determined by Tauc formula [5]: ðahnÞ ¼ AnðhnEg Þm

ð10Þ

Fig. 3. EDX spectra of the four nanocrystalline PbS thin films deposited with different molar concentration: (a) 0.025, (b) 0.05, (c) 0.075, and (d) 0.1 M.

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Fig. 4. AFM images (2D and Ps distribution) for nanocrystalline PbS thin films with different molar concentration: (a) 0.025, (b) 0.05, (c) 0.075 , and (d) 0.1 M.

where An is a constant, a is the absorption coefficient, hnis the incident photon energy, and the parameter m depends on the transmission type (equals to 12for allowed direct transmission). Fig. 8 shows a typical plot of ðanhnÞ2 vshnwhich allow estimating experimentally the value of the band gap. The energy gap was determined by taking the extrapolation of the linear portion. The electronic transition occurs between the valence band and conduction bands at the absorption edge corresponding to the minimum energy difference between the lowest energy of the conduction band and the highest energy of the valence band in the crystalline materials [21]. In the case of the prepared PbS thin films, the maximum of the valence band and the minima of the conduction band lie at the same wave vector k value of E–k band diagram and hence the transitions are direct type [21]. It is found that the value of Egdecreases from 2.35 to 1.59 eV with increasing molar concentration, due to the crystallite size-dependant properties of the energy band gap [20]. The variation inEg with molar concentration is summarized in Table 2, which are higher than that for bulk, i.e. 0.41 eV, this shift is due to quantum size effect [22]. Similar results were reported in the literature [15,23]. As the particle size changes below a certain limiting size associated with its exciton Bohr radius, the spacing between levels in the band also changes, thus Eg changes

[5]. The particle size can be calculated from the relationship between the particle radius and the band gap of nanostructured material given by the Brus Model [23]: 2

Eg ðnanoÞ ¼ Eg ðblukÞ þ

h p2 1 1 1:78e2 ð þ n Þ 2 mn m zR 2R e b

ð11Þ

where R is the radius of the particle, mne and mnb are the effective masses of the electron in the conduction band and of the hole in the valence band respectively; and z is the dielectric constant of PbS (its value is 17.3). The second term represents quantum localization (i.e. the kinetic energy), which shifts Eg to higher energies. The third term arises from the screened Coulomb interaction between the electron and the hole, which shifts Eg to lower energies. It is noticed that the values of crystallite size obtained by Brus model are higher, almost double, than that determined by XRD analysis, but the general variation with molarity M is exactly identical; with increasing the molar concentration, the particles size increases, from 26 up to 36 nm (Table 2). 4. Conclusion Nanocrystalline PbS thin films of good quality were successfully prepared using the microwave-assisted chemical bath deposition method with different molar

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Fig. 5. Crystal sizes distribution of the four nanocrystalline PbS thin films deposited with different molar concentration: (a) 0.025, (b) 0.05, (c) 0.075, and (d) 0.1 M.

Fig. 6. Transmission versus wavelength (l) of the four nanocrystalline PbS thin films deposited with different molar concentration: (a) 0.025, (b) 0.05, (c) 0.075, and (d) 0.1 M.

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Fig. 7. Absorbance versus wavelength (l) of the four nanocrystalline PbS thin films deposited with different molar concentration: (a) 0.025, (b) 0.05, (c) 0.075, and (d) 0.1.

Fig. 8. Optical band gap energy of the four nanocrystalline PbS thin films deposited with different molar concentration: (a) 0.025, (b) 0.05, (c) 0.075, and (d) 0.1 M.

concentration. XRD analysis revealed that the thin films crystallize within NaCl-type structure with a preferred orientation depending on the molar concentration. The value of crystallite size determined by XRD and Brus

model was found to increase with increasing molar concentration. SEM analysis revealed the formation of cubiclike shaped particles with uniform size distribution whereas the size increases also with increasing molarity,

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in agreement with XRD analysis. AFM analysis confirms the formation of smooth surfaces with good adherence to the substrate, a narrow particle’s size distribution and that roughness slightly increases with increasing molarity. The value of the energy gap is greatly influenced by the change of the molar concentration; it decreases by almost half when increasing molar concentration from 0.025 to 0.1 M, i.e. 2.35 and 1.59 eV, respectively.

Acknowledgments The authors gratefully acknowledge support from ERGS grant and Universiti Sains Malaysia. References [1] A.S. Obaid, M.A. Mahdi, Z. Hassan, Nanocoral PbS thin film growth by solid-vapor deposition, Optoelectronics and Advanced Materials– Rapid 6 (2012) 422–426. [2] E. Pentia, L. Pintilie, I. Matei, T. Botila, E. Ozbay, Chemically prepared Nanocrystalline PbS thin films, Optoelectronics & Advanced Materials 3 (2001) 525–530. [ 3] C.D. Lokhande, Chemical deposition of metal chalcogenide thin films, Materials Chemistry and Physics 27 (1991) 1–43. [4] M. Sharon, K.S. Ramaiah, M. Kumar, M. Neumann-Spallartm, C. Levy, Electrodeposition of lead sulphide in acidic mediumlctrod, Electroanalytical Chemistry 436 (1997) 49–52. [5] K.S. Suslick, R.E. Cline, and D.A. Hammerton, Determination of Local Temperatures caused by acoustic cavitation, Ultrason. Symp. Proc, 21 (1985) 1116–1121. [6] X.-H. Liao, N.-Y. Chen, S. Xu, S.-B. Yang, J.-J. Zhu, A microwave assisted heating method for the preparation of copper sulfide nanorods, Journal of Crystal Growth 252 (2003) 593–598. [7] D. Chen, J.-R. Zhang, J.-J. Zhu, Microwave-assisted synthesis of metal sulfides in ethylene glycol, Materials Chemistry and Physics 82 (2003) 206–209. [8] V.P. Singh, R.S. Singh, G.W. Thompson, V. Jayaraman, S. Sanagapalli, V.K. Rangari, Characteristics of nanocrystalline CdS films fabricated by sonochemical, microwave and solution growth methods for solar cell applications, Solar Energy Materials and Solar Cells 81 (2004) 293–303. [9] X. Liao, J. Zhu, W. Zhong, H.-Y. Chen, Synthesis of amorphous Fe O nanoparticles by microwave irradiation, Materials Letters 50Z (2001) 341–346.

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