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CuO nanostructures grown by the SILAR method: Influence of Pb-doping on the morphological, structural and optical properties
Journal of Alloys and Compounds 619 (2015) 378–382
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
CuO nanostructures grown by the SILAR method: Influence of Pb-doping on the morphological, structural and optical properties F. Bayansal a,b,⇑, Y. Gülen c, B. Sß ahin b, S. Kahraman a,b, H.A. Çetinkara b a
Department of Metallurgical and Materials Engineering, Faculty of Technology, Mustafa Kemal University, Hatay, Turkey Department of Physics, Faculty of Arts and Sciences, Mustafa Kemal University, Hatay, Turkey c _ Turkey Department of Physics, Faculty of Arts and Sciences, Marmara University, Istanbul, b
a r t i c l e
i n f o
Article history: Received 2 July 2014 Received in revised form 8 September 2014 Accepted 8 September 2014 Available online 16 September 2014 Keywords: CuO Nanostructure Doping Optical properties Electron microscopy
a b s t r a c t CuO nanostructures with and without Pb were synthesized by the Successive Ionic Layer Adsorption and Reaction method. The films were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and ultraviolet–visible spectrophotometry. Scanning electron microscopy results showed that the morphology of the film surface was changed from plate-like to coral-like nanostructures with increasing Pb concentration. The X-ray diffraction patterns showed the monoclinic crystal 1 1Þ and (1 1 1). Furthermore, ultraviolet–visible spectra showed structure with preferential planes of ð1 that the band gap of the films was tailored by Pb doping. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In the recent years many efforts have been devoted to grow, characterize and describe the physical and chemical properties of transition-metal oxides because of their potential applications in a variety of functional devices. Among these transition-metal oxides copper oxide (CuO), one of the important p-type semiconductors with a narrow band gap of 1.2–1.9 eV [1–3], has attracted much attention. It has potential applications in many fields such as catalysts [4], field emission devices [5], gas sensors [6], biosensors [7], high temperature superconductors [8], photovoltaic devices [9], energy storage [10] and spintronics [11]. Most of these applications require copper oxide on nanoscale size. Therefore many recent efforts have been concentrated on the fabrication techniques that could change the nanostructure and thus enhance the performance of the devices. CuO films have been synthesized by many different methods which are thermal oxidation [12], electrodeposition [13], dip coating [14], chemical vapor deposition [15], magnetron sputtering [16], chemical bath deposition (CBD) [17] and Successive Ionic Layer Adsorption and Reaction (SILAR) [18]. In these methods one of Cu, CuO, Cu2O and Cu(OH)2 phases were obtained ⇑ Corresponding author at: Department of Metallurgical and Materials Engineering, Faculty of Technology, Mustafa Kemal University, Hatay, Turkey. Tel.: +90 326 2455845; fax: +90 326 2455867. E-mail address: [email protected] (F. Bayansal). http://dx.doi.org/10.1016/j.jallcom.2014.09.085 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
[12,16,19]. But generally in most of the low temperature methods these four phases were obtained together. Therefore, in order to convert the other phases into CuO further annealing processes in oxygen or air atmosphere were applied to the samples after the growth. On the other hand, among these techniques SILAR is a promising technique because it is a simple, safe, environmental friendly, suitable to mass production, low temperature and cost effective solution method. The microstructure and growth steps reasonably affect the physical and chemical properties of the metal oxide films. Moreover, doping of semiconductors with some certain elements offers an effective approach to adjust their properties which is crucial for their practical applications [20]. Although a number of researchers performed doping, it is still remains a challenge to achieve high quality of crystalline films with excellent physical and chemical properties of doped CuO nanostructures. Pb is one of the doping elements that were used in many metal oxide growth processes [21,22]. But, to the best of our knowledge there are no reports on the synthesis and properties of Pb-doped CuO thin films. In the present paper we report for the first time synthesis of CuO nanostructures with different doping concentrations of Pb by the SILAR method. The morphological, compositional, structural and optical properties of the films have been investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and UV–vis. spectrophotometry.
F. Bayansal et al. / Journal of Alloys and Compounds 619 (2015) 378–382
379
Fig. 1. Scanning electron microscopy images and energy dispersive spectroscopy results of pure and Pb-doped nanostructures.
2. Experimental details Synthesis of pure and Pb-doped CuO nanostructured thin films were carried out in a solution containing Cu2+ (for pure samples) or Cu2+:Pb2+ (for doped samples) ions. The raw materials used in the experiments were analytical grade reagents, purchased from Sigma–Aldrich Company and Merck KGaA and used without further purification. In order to deposit pure films first 0.1 M copper chloride solution was prepared with copper(II) chloride dehydrate (CuCl22H2O) and 100 ml double distilled water (18.2 MX cm). Then it was stirred in a magnetic stirrer at room temperature for a few min. in order to get a transparent and well-dissolved solution. After stirring, pH value of the solution was adjusted to 10.0 by adding aqueous ammonia and then it was heated up to 90 °C. During the experiments the temperature was kept as constant. Microscope glass slides which were cleaned with a certain recipe given in [17] are used as substrates. In order to get 1–2 lm thick films 10 cycles of SILAR were applied. A SILAR cycle can be described as follows: the substrates were dipped into the solution for 30 s. Then they were taken out from the bath and immediately dipped into hot water (90 °C) for another 30 s. Finally the films were dried at room temperature for a day. After drying, the samples were cleaned ultrasonically for 5 min. in order to detach bigger and tightly bonded particles. For the doped samples the same procedure was applied, but this time different concentrations of Pb(NO3)2 were added to 0.1 M copper chloride solutions. In order to get 1, 2, 4, 8 and 16 at.% of Pb in the solutions the molarities were adjusted
to 0.001, 0.002, 0.004, 0.008 and 0.016 respectively. The rest of the experiments were remained as the same. A PhilipsXL30S FEG scanning electron microscope (SEM) was operated at an acceleration voltage of 15 kV for morphological imaging. The compositions of Pb ions were investigated through energy dispersive X-ray spectroscopy. The crystal structures of the samples were examined by Rigaku SmartLab X-ray diffractometer (Cu Ka radiation, k = 1.540056 Å). A scan rate of 0.01°/s was applied to record the patterns in the 2h range of 30–80°. Optical studies were conducted at room temperature by using a Thermo Scientific Genesys 10S UV–vis. spectrophotometer.
3. Results and discussion 3.1. Morphological studies Scanning electron microscopy was employed to analyze the morphological properties of pure and Pb-doped samples. Fig. 1 shows the morphologies of pure and Pb-doped (1, 2, 4, 8 and 16 at.%) films. The insets in the figures show the elemental distribution (EDS results) of the films. From these graphs it is obvious
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F. Bayansal et al. / Journal of Alloys and Compounds 619 (2015) 378–382
are much stronger than that of the other peaks which indicate that they are preferential crystal planes of the nanostructures. As seen from the figure Pb doping (even in small percentages) caused a 1 1Þ and (1 1 1) peak intensities. But further great decrease in the ð1 1 1Þ increase in doping concentration caused an increase in the ð1 and (1 1 1) peak intensities. These mean that the crystalline quality of the films first was decreased suddenly with a small Pb doping and then increased slowly with increasing doping percentages. None of the patterns showed any of Pb phases which mean the Pb phases were very small and disappeared in the noise signal. The average grain sizes (D) of the structures was calculated from the peak full width at the half maximum (FWHM) of a peak (b), using the Debye–Scherrer’s equation [24]:
_ (111) (111) _ (311) _ (113) (220)
(110) _ (202) (020)
16% doped
8% doped
Intensity (a.u.)
4% doped
2% doped
1% doped
D¼
40
50
60
70
ð1Þ
where k is the wavelength of X-ray radiation, h is the Bragg’s angle of the peaks and b is the angular width of peaks at FWHM. Each Xray diffraction peak obtained in a diffractometer is broadened due to instrumental and physical factors (grain size and lattice strains). The microstrain (e) and dislocation density (q) for the CuO films were calculated using the following equations [24]:
Pure
30
0:94k b cos h
80
e¼
2θ (degree)
b cos h 4
ð2Þ
15e aD
ð3Þ
and Fig. 2. X-ray diffraction patterns of pure and Pb-doped CuO films.
q¼ that the Pb concentrations of 1, 2, 4, 8 and 16 at.% in the growth solutions provided dopings of 0.27, 0.37, 0.36, 0.46 and 0.75 at.% of Pb in the films, respectively. If we further increase the concentration of Pb in the growth solution we can obtain highly Pb-doped CuO films just by looking at this linear behavior. As seen from the SEM figures the obtained films are crack free with nano-sized particles. As the Pb doping increases the film morphology changes from plate-like nanostructures as in [23] to coral-like nanostructures. From the SEM figures it is clear that a small amount of Pb (0.27 at.%) changed the surface morphology. Further dopings had very little effects on the morphology but they caused a decrease in the thickness of the nanostructures that is they became thinner with increasing Pb doping. As seen in the following sections this change affected the band gap of the films strongly.
where a is the lattice constant. The calculated average grain sizes (D) of the CuO nanostructures are given in Table 1. As seen from the table, grain sizes of the films were first increased (0.27 at.% Pb) suddenly then decreased slowly with increasing Pb doping. This result can also be supported by the SEM measurements. As expected, both the behavior of microstrain and dislocation density values of the films are in reverse manner with the grain size values. For all the calculations (grain size, microstrain and dislocation den 1 1Þ, (1 1 1), sity) the first dominant five peaks that are (1 1 0), ð1 0 2Þ and (0 2 0) are used. From a detailed investigation of XRD patð2 terns, a shift in peak positions was determined towards lower 2h values with increasing Pb doping. This shift indicates the presence of a decrease in lattice strains in the structures [25].
3.2. Structural studies
Studying the optical absorption provide us simple methods for explaining properties related to the band structure and band gap energy of the materials. In order to measure the band gap energies of the films a Thermo Scientific Genesys 10S UV–vis. spectrophotometer was used in the wavelength range of 190–1100 nm. The optical absorption in the UV–vis. region is dominated by the optical band gap (Eg) of a semiconductor that is related to the optical absorption coefficient (a) and the incident photon energy (ht) by the following relation
The crystal structure and phase purity of the films has been characterized by a Rigaku SmartLab X-ray diffractometer (Cu Ka radiation, k = 1.540056 Å). Fig. 2 shows the XRD patterns of pure and Pb-doped (1, 2, 4, 8 and 16 at.%) films obtained at an operating voltage and current of 40 keV and 30 mA respectively. The 2h range of 30–80° was recorded with the scan rate of 0.01°/s. All the major diffraction peaks in the patterns can be indexed to monoclinic CuO crystal structure (JCPDS Card Number: 01-080 1 1Þ and (1 1 1) peaks 0076). It is found that the intensities of ð1
3.3. Optical studies
ðahtÞ ¼ C ht Eg
m
ð4Þ
Table 1 Pb concentration, band gap and structural parameters of the films. Pb concentration (at.%)
Eg (eV)
Average grain size (nm)
Microstrain (103)
Dislocation density (1012/cm2)
0.00 0.27 0.37 0.36 0.46 0.75
1.43 1.80 1.76 1.72 1.68 1.65
9.94 17.22 16.21 15.79 13.07 8.98
3.896 2.150 2.254 2.297 4.083 5.119
143.20 41.85 45.43 46.79 253.20 281.30
F. Bayansal et al. / Journal of Alloys and Compounds 619 (2015) 378–382
Pure,
381
the figure it is seen that the band gap was increased suddenly with a small Pb doping (0.27 at.%) and then decreased slowly with increasing doping. These results are also found to be parallel to the results obtained by SEM and XRD analysis.
Eg=1.43
0.27% Pb, Eg=1.80 0.37% Pb, Eg=1.76 0.36% Pb, Eg=1.72 0.46% Pb, Eg=1.68 0.75% Pb, Eg=1.65
4. Conclusions
( α hv)2(a.u.)
As a conclusion the authors have presented the first report on the morphological, structural and optical properties of Pb-doped CuO nanostructured thin films. The films were synthesized by the SILAR method on the glass substrates successfully. By changing the doping percentages CuO nanostructures of different morphologies were grown. Also the XRD results showed that the crystal quality of the films was modified by Pb doping. From the UV–vis. spectra it can be concluded that the band gap of the films can be changed by tailoring the morphology and Pb doping concentrations of the films. Acknowledgement This work is partially supported by the Scientific Research Commission of Mustafa Kemal University (Project No. 279). References
1
1.2
1.4
1.6
1.8
2
2.2
Eg (eV) 2
Fig. 3. Comparison of (aht) vs. ht plots of pure and Pb-doped CuO films.
1.9
Eg (eV)
1.8
1.7
1.6
1.5
1.4 0
4
8
12
16
Pb concentration in the solution (%) Fig. 4. Band gap values of the CuO films as a function of Pb concentration.
where C is an energy independent constant and m is an index which depends on the kind of optical transition that prevails. Specifically, n is 1/2, 3/2, 2 and 3 when the transition is direct-allowed, directforbidden, indirect-allowed, and indirect-forbidden, respectively. CuO is known to be a direct-allowed semiconductor [26], and hence a graph was plotted (Fig. 3) with (aht)2 (where m = 1/2) vs. photon energy (ht) as a function of Pb doping. By using this graph the band gap values can be determined by extrapolating the straight line portion. The Eg of pure CuO film was found to be 1.43 eV which is in accordance with [17]. On the other hand the Eg values were found to be 1.80, 1.76, 1.72, 1.68 and 1.65 eV for the films having 0.27, 0.37, 0.36, 0.46 and 0.75 at.% of Pb, respectively. The band gap values vs. Pb concentration in the growth solution are plotted in Fig. 4. From
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
CuO nanostructures grown by the SILAR method: Influence of Pb-doping on the morphological, structural and optical properties F. Bayansal a,b,⇑, Y. Gülen c, B. Sß ahin b, S. Kahraman a,b, H.A. Çetinkara b a
Department of Metallurgical and Materials Engineering, Faculty of Technology, Mustafa Kemal University, Hatay, Turkey Department of Physics, Faculty of Arts and Sciences, Mustafa Kemal University, Hatay, Turkey c _ Turkey Department of Physics, Faculty of Arts and Sciences, Marmara University, Istanbul, b
a r t i c l e
i n f o
Article history: Received 2 July 2014 Received in revised form 8 September 2014 Accepted 8 September 2014 Available online 16 September 2014 Keywords: CuO Nanostructure Doping Optical properties Electron microscopy
a b s t r a c t CuO nanostructures with and without Pb were synthesized by the Successive Ionic Layer Adsorption and Reaction method. The films were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and ultraviolet–visible spectrophotometry. Scanning electron microscopy results showed that the morphology of the film surface was changed from plate-like to coral-like nanostructures with increasing Pb concentration. The X-ray diffraction patterns showed the monoclinic crystal 1 1Þ and (1 1 1). Furthermore, ultraviolet–visible spectra showed structure with preferential planes of ð1 that the band gap of the films was tailored by Pb doping. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In the recent years many efforts have been devoted to grow, characterize and describe the physical and chemical properties of transition-metal oxides because of their potential applications in a variety of functional devices. Among these transition-metal oxides copper oxide (CuO), one of the important p-type semiconductors with a narrow band gap of 1.2–1.9 eV [1–3], has attracted much attention. It has potential applications in many fields such as catalysts [4], field emission devices [5], gas sensors [6], biosensors [7], high temperature superconductors [8], photovoltaic devices [9], energy storage [10] and spintronics [11]. Most of these applications require copper oxide on nanoscale size. Therefore many recent efforts have been concentrated on the fabrication techniques that could change the nanostructure and thus enhance the performance of the devices. CuO films have been synthesized by many different methods which are thermal oxidation [12], electrodeposition [13], dip coating [14], chemical vapor deposition [15], magnetron sputtering [16], chemical bath deposition (CBD) [17] and Successive Ionic Layer Adsorption and Reaction (SILAR) [18]. In these methods one of Cu, CuO, Cu2O and Cu(OH)2 phases were obtained ⇑ Corresponding author at: Department of Metallurgical and Materials Engineering, Faculty of Technology, Mustafa Kemal University, Hatay, Turkey. Tel.: +90 326 2455845; fax: +90 326 2455867. E-mail address: [email protected] (F. Bayansal). http://dx.doi.org/10.1016/j.jallcom.2014.09.085 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
[12,16,19]. But generally in most of the low temperature methods these four phases were obtained together. Therefore, in order to convert the other phases into CuO further annealing processes in oxygen or air atmosphere were applied to the samples after the growth. On the other hand, among these techniques SILAR is a promising technique because it is a simple, safe, environmental friendly, suitable to mass production, low temperature and cost effective solution method. The microstructure and growth steps reasonably affect the physical and chemical properties of the metal oxide films. Moreover, doping of semiconductors with some certain elements offers an effective approach to adjust their properties which is crucial for their practical applications [20]. Although a number of researchers performed doping, it is still remains a challenge to achieve high quality of crystalline films with excellent physical and chemical properties of doped CuO nanostructures. Pb is one of the doping elements that were used in many metal oxide growth processes [21,22]. But, to the best of our knowledge there are no reports on the synthesis and properties of Pb-doped CuO thin films. In the present paper we report for the first time synthesis of CuO nanostructures with different doping concentrations of Pb by the SILAR method. The morphological, compositional, structural and optical properties of the films have been investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and UV–vis. spectrophotometry.
F. Bayansal et al. / Journal of Alloys and Compounds 619 (2015) 378–382
379
Fig. 1. Scanning electron microscopy images and energy dispersive spectroscopy results of pure and Pb-doped nanostructures.
2. Experimental details Synthesis of pure and Pb-doped CuO nanostructured thin films were carried out in a solution containing Cu2+ (for pure samples) or Cu2+:Pb2+ (for doped samples) ions. The raw materials used in the experiments were analytical grade reagents, purchased from Sigma–Aldrich Company and Merck KGaA and used without further purification. In order to deposit pure films first 0.1 M copper chloride solution was prepared with copper(II) chloride dehydrate (CuCl22H2O) and 100 ml double distilled water (18.2 MX cm). Then it was stirred in a magnetic stirrer at room temperature for a few min. in order to get a transparent and well-dissolved solution. After stirring, pH value of the solution was adjusted to 10.0 by adding aqueous ammonia and then it was heated up to 90 °C. During the experiments the temperature was kept as constant. Microscope glass slides which were cleaned with a certain recipe given in [17] are used as substrates. In order to get 1–2 lm thick films 10 cycles of SILAR were applied. A SILAR cycle can be described as follows: the substrates were dipped into the solution for 30 s. Then they were taken out from the bath and immediately dipped into hot water (90 °C) for another 30 s. Finally the films were dried at room temperature for a day. After drying, the samples were cleaned ultrasonically for 5 min. in order to detach bigger and tightly bonded particles. For the doped samples the same procedure was applied, but this time different concentrations of Pb(NO3)2 were added to 0.1 M copper chloride solutions. In order to get 1, 2, 4, 8 and 16 at.% of Pb in the solutions the molarities were adjusted
to 0.001, 0.002, 0.004, 0.008 and 0.016 respectively. The rest of the experiments were remained as the same. A PhilipsXL30S FEG scanning electron microscope (SEM) was operated at an acceleration voltage of 15 kV for morphological imaging. The compositions of Pb ions were investigated through energy dispersive X-ray spectroscopy. The crystal structures of the samples were examined by Rigaku SmartLab X-ray diffractometer (Cu Ka radiation, k = 1.540056 Å). A scan rate of 0.01°/s was applied to record the patterns in the 2h range of 30–80°. Optical studies were conducted at room temperature by using a Thermo Scientific Genesys 10S UV–vis. spectrophotometer.
3. Results and discussion 3.1. Morphological studies Scanning electron microscopy was employed to analyze the morphological properties of pure and Pb-doped samples. Fig. 1 shows the morphologies of pure and Pb-doped (1, 2, 4, 8 and 16 at.%) films. The insets in the figures show the elemental distribution (EDS results) of the films. From these graphs it is obvious
380
F. Bayansal et al. / Journal of Alloys and Compounds 619 (2015) 378–382
are much stronger than that of the other peaks which indicate that they are preferential crystal planes of the nanostructures. As seen from the figure Pb doping (even in small percentages) caused a 1 1Þ and (1 1 1) peak intensities. But further great decrease in the ð1 1 1Þ increase in doping concentration caused an increase in the ð1 and (1 1 1) peak intensities. These mean that the crystalline quality of the films first was decreased suddenly with a small Pb doping and then increased slowly with increasing doping percentages. None of the patterns showed any of Pb phases which mean the Pb phases were very small and disappeared in the noise signal. The average grain sizes (D) of the structures was calculated from the peak full width at the half maximum (FWHM) of a peak (b), using the Debye–Scherrer’s equation [24]:
_ (111) (111) _ (311) _ (113) (220)
(110) _ (202) (020)
16% doped
8% doped
Intensity (a.u.)
4% doped
2% doped
1% doped
D¼
40
50
60
70
ð1Þ
where k is the wavelength of X-ray radiation, h is the Bragg’s angle of the peaks and b is the angular width of peaks at FWHM. Each Xray diffraction peak obtained in a diffractometer is broadened due to instrumental and physical factors (grain size and lattice strains). The microstrain (e) and dislocation density (q) for the CuO films were calculated using the following equations [24]:
Pure
30
0:94k b cos h
80
e¼
2θ (degree)
b cos h 4
ð2Þ
15e aD
ð3Þ
and Fig. 2. X-ray diffraction patterns of pure and Pb-doped CuO films.
q¼ that the Pb concentrations of 1, 2, 4, 8 and 16 at.% in the growth solutions provided dopings of 0.27, 0.37, 0.36, 0.46 and 0.75 at.% of Pb in the films, respectively. If we further increase the concentration of Pb in the growth solution we can obtain highly Pb-doped CuO films just by looking at this linear behavior. As seen from the SEM figures the obtained films are crack free with nano-sized particles. As the Pb doping increases the film morphology changes from plate-like nanostructures as in [23] to coral-like nanostructures. From the SEM figures it is clear that a small amount of Pb (0.27 at.%) changed the surface morphology. Further dopings had very little effects on the morphology but they caused a decrease in the thickness of the nanostructures that is they became thinner with increasing Pb doping. As seen in the following sections this change affected the band gap of the films strongly.
where a is the lattice constant. The calculated average grain sizes (D) of the CuO nanostructures are given in Table 1. As seen from the table, grain sizes of the films were first increased (0.27 at.% Pb) suddenly then decreased slowly with increasing Pb doping. This result can also be supported by the SEM measurements. As expected, both the behavior of microstrain and dislocation density values of the films are in reverse manner with the grain size values. For all the calculations (grain size, microstrain and dislocation den 1 1Þ, (1 1 1), sity) the first dominant five peaks that are (1 1 0), ð1 0 2Þ and (0 2 0) are used. From a detailed investigation of XRD patð2 terns, a shift in peak positions was determined towards lower 2h values with increasing Pb doping. This shift indicates the presence of a decrease in lattice strains in the structures [25].
3.2. Structural studies
Studying the optical absorption provide us simple methods for explaining properties related to the band structure and band gap energy of the materials. In order to measure the band gap energies of the films a Thermo Scientific Genesys 10S UV–vis. spectrophotometer was used in the wavelength range of 190–1100 nm. The optical absorption in the UV–vis. region is dominated by the optical band gap (Eg) of a semiconductor that is related to the optical absorption coefficient (a) and the incident photon energy (ht) by the following relation
The crystal structure and phase purity of the films has been characterized by a Rigaku SmartLab X-ray diffractometer (Cu Ka radiation, k = 1.540056 Å). Fig. 2 shows the XRD patterns of pure and Pb-doped (1, 2, 4, 8 and 16 at.%) films obtained at an operating voltage and current of 40 keV and 30 mA respectively. The 2h range of 30–80° was recorded with the scan rate of 0.01°/s. All the major diffraction peaks in the patterns can be indexed to monoclinic CuO crystal structure (JCPDS Card Number: 01-080 1 1Þ and (1 1 1) peaks 0076). It is found that the intensities of ð1
3.3. Optical studies
ðahtÞ ¼ C ht Eg
m
ð4Þ
Table 1 Pb concentration, band gap and structural parameters of the films. Pb concentration (at.%)
Eg (eV)
Average grain size (nm)
Microstrain (103)
Dislocation density (1012/cm2)
0.00 0.27 0.37 0.36 0.46 0.75
1.43 1.80 1.76 1.72 1.68 1.65
9.94 17.22 16.21 15.79 13.07 8.98
3.896 2.150 2.254 2.297 4.083 5.119
143.20 41.85 45.43 46.79 253.20 281.30
F. Bayansal et al. / Journal of Alloys and Compounds 619 (2015) 378–382
Pure,
381
the figure it is seen that the band gap was increased suddenly with a small Pb doping (0.27 at.%) and then decreased slowly with increasing doping. These results are also found to be parallel to the results obtained by SEM and XRD analysis.
Eg=1.43
0.27% Pb, Eg=1.80 0.37% Pb, Eg=1.76 0.36% Pb, Eg=1.72 0.46% Pb, Eg=1.68 0.75% Pb, Eg=1.65
4. Conclusions
( α hv)2(a.u.)
As a conclusion the authors have presented the first report on the morphological, structural and optical properties of Pb-doped CuO nanostructured thin films. The films were synthesized by the SILAR method on the glass substrates successfully. By changing the doping percentages CuO nanostructures of different morphologies were grown. Also the XRD results showed that the crystal quality of the films was modified by Pb doping. From the UV–vis. spectra it can be concluded that the band gap of the films can be changed by tailoring the morphology and Pb doping concentrations of the films. Acknowledgement This work is partially supported by the Scientific Research Commission of Mustafa Kemal University (Project No. 279). References
1
1.2
1.4
1.6
1.8
2
2.2
Eg (eV) 2
Fig. 3. Comparison of (aht) vs. ht plots of pure and Pb-doped CuO films.
1.9
Eg (eV)
1.8
1.7
1.6
1.5
1.4 0
4
8
12
16
Pb concentration in the solution (%) Fig. 4. Band gap values of the CuO films as a function of Pb concentration.
where C is an energy independent constant and m is an index which depends on the kind of optical transition that prevails. Specifically, n is 1/2, 3/2, 2 and 3 when the transition is direct-allowed, directforbidden, indirect-allowed, and indirect-forbidden, respectively. CuO is known to be a direct-allowed semiconductor [26], and hence a graph was plotted (Fig. 3) with (aht)2 (where m = 1/2) vs. photon energy (ht) as a function of Pb doping. By using this graph the band gap values can be determined by extrapolating the straight line portion. The Eg of pure CuO film was found to be 1.43 eV which is in accordance with [17]. On the other hand the Eg values were found to be 1.80, 1.76, 1.72, 1.68 and 1.65 eV for the films having 0.27, 0.37, 0.36, 0.46 and 0.75 at.% of Pb, respectively. The band gap values vs. Pb concentration in the growth solution are plotted in Fig. 4. From
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