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Study the effect of thiourea concentration on optical and structural properties of CdS-nanocrystalline thin films prepared by CBD technique
Accepted Manuscript Title: Study the Effect of Thiourea Concentration on Optical and Structural Properties of CdS - nanocrystalline thin films Prepared by CBD Technique Author: Rehana Zia Madeeha Riaz Quratulain Safia Anjum PII: DOI: Reference:
S0030-4026(16)30186-3 http://dx.doi.org/doi:10.1016/j.ijleo.2016.02.081 IJLEO 57418
To appear in: Received date: Accepted date:
23-1-2016 29-2-2016
Please cite this article as: R. Zia, M. Riaz,Quratulain, S. Anjum, Study the Effect of Thiourea Concentration on Optical and Structural Properties of CdS - nanocrystalline thin films Prepared by CBD Technique, Optik - International Journal for Light and Electron Optics (2016), http://dx.doi.org/10.1016/j.ijleo.2016.02.081 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Study the Effect of Thiourea Concentration on Optical and Structural Properties of CdS - nanocrystalline thin films Prepared by CBD Technique
ip t
Rehana Zia,Madeeha Riaz ,Quratulain, Safia Anjum
an
Abstract
us
Corresponding Author, Email: rzia1960 @gmail.com Corresponding Author: Dr. Rehana Zia Area Code: 54000 Phone no: 009242 99203801-9 Fax no: 00924299203087
cr
Department of Physics, Lahore College for Women University, Lahore, Pakistan
Nano crystalline CdS thin films have been deposited on glass substrate employing chemical bath
M
deposition technique by varying thiourea concentration in two different cadmium sources as cadmium chloride (S1) and cadmium acetate (S2). The aim of this research is to investigate the
d
effect of thiourea concentration on optical and structural properties of the deposited CdS thin
te
films. XRD analysis reveals that the CdS thin films fabricated by using S1 have zinc blende
Ac ce p
cubic structure and S2 have hexagonal as well as cubic structure. UV-VIS studies demonstrate that as the thiourea concentration increases transmission spectra shift towards the shorter wavelength and optical band gap of CdS thin films also increases from 2.45 - 2.71eV in S1 and 2.65-2.74eV in S2. In absorption spectra, a blue shift in the band gap was observed which indicates the formation of CdS nanoparticles measured as 31-9.91 nm in S1 and 30-15.02 nm in S2.
Key Words: Precipitation, Semiconductors, Optical properties, X-ray diffraction topography
1 Page 1 of 12
1. Introduction In solid state technology, nano crystalline semiconducting thin films play an important role in optoelectronic and magnetic devices due to low-priced production. Cadmium sulfide is among
ip t
the most promising direct band gap semiconductor due to its high coefficient of absorption [1].
cr
CdS is used in many applications such as window materials in CdTe/ CdS hetrojunction solar cells, semiconductor laser [2] display devices and in biological applications [3].
us
Many deposition techniques have been used to fabricate CdS thin film such as molecular beam epitaxy [4, 5], electrodeposition [6, 7], physical vapor deposition [8], chemical vapour deposition
an
[9], RF sputtering [10], spray pyrolysis [11], vacuum evaporation [12], screen printing [13].
M
Chemical bath-deposition (CBD) is one of the most promising methods for manufacturing large areas low-cost CdS thin films [14, 15]. In the present study CBD technique is used to produce
d
CdS nanocrystalline thin films. Over the years various cadmium ion sources have been used such
te
as cadmium iodide [16], cadmium sulfate [17], cadmium acetate, and cadmium chloride [18]. To study the effect of thiourea as a capping agent on the properties of CdS thin films were
Ac ce p
investigated by many workers [19, 20]. The purpose of this work is to provide a complete comparative study of the effect of thiourea concentration on structural and optical properties of CdS thin films prepared by CBD method where CdCl2 and Cd(CH3COO)2 act as cadmium ion source.
2. Experimental Details
CdS thin films were fabricated by using chemical bath deposition technique at three different concentrations of thiourea. Commercially available soda lime glass slides (35 x 25 x 1) mm were used as substrates for CdS thin films deposition. The glass substrates were cleaned with nitric acid for 48 hours, then rinsed with deionized water and finally desiccated in air prior to 2 Page 2 of 12
deposition. Thiourea (0.1M) was used as sulfur precursor while cadmium acetate and cadmium chloride (0.1 M each) as Cd precursors. Ammonia was added into the solution as complexing agent. First of all we prepared aqueous solutions of 0.3M ammonium chloride (NH4Cl) and 0.1M
ip t
solution of cadmium chloride (CdCl2) in 100 ml distilled water. After that 0.6 M ammonia solution was prepared and added into the solution as a complexing agent to set the value of pH to
cr
10. Finally 0.1M solution of thiourea ((NH2)2CS) was added and then stirred it with a magnetic
us
stirrer. The magnetic stirrer was turned off when a clear uniform bath is obtained and then the substrates of glass were placed vertically in the solution which kept at the temperature of 90oC.
an
The samples were removed from the solution after 6 hours. A thin, strongly adherent, dense, shiny, CdS layer was deposited on the glass substrate. Nano crystalline CdS thin films were
M
fabricated on three different thiourea (NH2)2CS concentrations in cadmium chloride (CdCl2)
d
samples termed as S1 and the samples in cadmium acetate (CdAc2) used as cadmium ion source
2.1 Chemical Reaction
te
as S2 were shown in table 1 and 2 respectively.
Ac ce p
CdS thin films were synthesized by the thiourea decomposition in an alkaline chemical bath solution containing a salt of cadmium. 1. A complex was formed by the reaction of ammonia with cadmium salts: CdCl2 + 4NH3
Cd (NH3)4 Cl2
(1)
2. On the catalytic surface of the glass substrate thiourea and Cd(NH3)42+ , Cl-, OH complex ions were diffused. [Cd (NH3)4]2+
Cd2+ + 4NH3
(2)
NH3 + H2O
NH4 +2 + OH-
(3)
Cd2+ + 2OH-
Cd (OH2)
(4)
3 Page 3 of 12
3. In the alkaline medium the thiourea dissociates on the catalytic surface of CdS: (NH2)2 CS + 2OH-
[HS+] + H2O + CH2N2
(5)
4. The ions of bivalent sulfide were produced: S-2 + H2O
(6)
ip t
[HS]+ + OH-
5. The particles of CdS were formed: CdS + NH3
(7)
cr
[Cd (NH3)4]+2 + S-2
us
The fabricated thin films of CdS were highly adhesive, homogeneous, reflecting, smooth and light yellow in color.
Optical Properties
M
3.1.
an
3. Results and Discussion
To study the optical properties of CdS thin films the transmission and absorption spectra were
d
obtained in the range of 300-900nm. It was found that in the wavelength range greater than 500
te
nm all nano crystalline thin films of CdS had very high optical transmittance. Figure 1 and 2 shows the Transmission spectra of CdS thin films of samples S1 and S2 that lies
Ac ce p
between 70-90% and 75-90% in visible range respectively. CdS thin films have high transmission in samples S2 as compared to sample S1. The rapid increase in transmission is the result of broad band gap of CdS by increasing thiourea concentration which results to increase the transmission in the visible region. Absorption spectrum of CdS thin films was achieved by measuring the electromagnetic radiations absorbance of uniform, nano crystalline films as showed in Figure 3and 4. It was observed that all the films have peaks at 500nm to 450nm which indicated that the absorption edge shifted toward blue. It was also seen that maximum absorption for all the CdS thin films with different concentration of thiourea was nearly 4%. This absorption edge shift towards blue
4 Page 4 of 12
can be endorsed to the exitonic absorbance at low dimensions due to the quantum confinement as compared to its bulk counterparts. The spectrum of absorption assured that absorption edges shifted toward the shorter wavelength as the concentration of thiourea increases. The same trend
ip t
has been reported by Jian, et. al [19] Abdullah, et.al [20] and Cortes, et.al [21].
The energy band gap values of the CdS nano crystalline thin films were also found from the
cr
transmission vs. wavelength as shown in figure 5and 6 for samples S1 and S2 respectively. The
us
energy band-gap values of deposited thin films were found by examining the optical data of photon energy hν and optical absorbance α using Taucs formula relation [22].
an
αhν = C ( hν – Eg )1/2
(8)
M
Eg is band gap energy and C is energy-independent constant. The graph linearity indicated that all the films had direct band transition. Optical band gap of CdS nanoparticals was found by
d
extrapolating the straight line segment of the (αhν)2 vs. (hν) to (hν) axis at α = 0.
te
The observed band gap values of deposited CdS nanoparticles of samples S1 and S2 were given in table 3. It shows that the band gap values of samples S1 lies in the range of 2.45 to 2.71eV and
Ac ce p
in the samples S2 were 2.65 to 2.74eV, which are closed to the values quoted in the literature [23, 24]. It was clearly seen that as the concentration of thiourea increases the value of the band gap of CdS nanoparticles increases as in figure 7. This observed shift indicates quantum size effect on the nano crystalline CdS thin films because as the thiourea concentration increases the size of the nanoparticles decreases and the band gap increases. The variation in the particle size with respect to the band gap energy was explained as the nanoparticles have the crystalline structure of their bulk counter parts and hence were characterized by the fully occupied valence band and an empty conduction band separated by the energy gap (Eg). However, the charge carriers in these bands e.g., the electrons in the conduction band and holes in the valence band experience
5 Page 5 of 12
an overall confining potential due to the finite size of these particles. As a result there will be size-dependent discrete states in the conduction and valence bands leading in the effective enhancement of the band gap; the so-called quantum size effect [25].
t
=
ip t
The thickness of the CdS thin films from two minima and maxima was calculated as, M λ1 λ2
(9)
cr
2 [n (λ1) λ2 – n (λ2) λ1]
us
In this equation, M was number of total oscillations occurring at λ1 and λ2 between two extremes with t and n determined accurately in the region where the transmission of film was measured.
an
The measured thickness of samples S1 and S2 was given in table 3. Figure 8 shows that as the concentration of thiourea increases the thickness of CdS thin films decreases. Generally the
M
thickness of CdS thin films is mainly depends upon grain size of the nanoparticles, similar trend
d
was observed by Lee, et.al [26]. It was reported in table 3a that in case of S1 thin films have high
te
values of thicknesses as compared to S2 films. The main factor was that the grain size of thin films prepared as S1 was greater than S2 and transmission from thicker films was always lesser
Ac ce p
then thinner films [27]. It was observed in figure 2 the transmission from samples S2
were
greater as compared to samples S1. 3.2. Structural Properties
XRD patterns of the CdS thin films fabricated as sample S1 and S2 are shown in figure 9 and 10. The XRD pattern for sample S1 shows that the prominent peak at 2Ѳ values of 26.7o scattering from the diffraction plane (111) which shift to 26.8o as the thiourea concentration increases and two other low diffraction intensity peaks were also observed at 44o and 52o, as scattering from the crystal planes (220), and (311) of zinc blende cubic phase of CdS according to the JCPDS card File No. 10-0454, suggesting that the nanoparticles were in cubic structure [28]. This CdS
6 Page 6 of 12
cubic phase is metastable, appearing in low-dimension structures such as nano crystalline thin films or systems [29, 30]. The X-ray diffraction pattern of sample S2 exhibited three main peaks at diffraction angles of
ip t
26.8o, 29.2o, and 52o that corresponds to the miller indices (002), (101) and (112) and matched with the JCPDS data (JCPDS No. 6-314) of hexagonal phase CdS. In the sample S2 based film, a
cr
fourth peak was also observed, which was found to be (200) plane, that was a characteristic peak
us
of cubic CdS. Hexagonal phase CdS thin films had been reported previously by some workers [31, 32] and cubic phases by others [33, 34].
an
The information about crystallite size was obtained from the broadening of the diffraction peaks. The size of the particle decreases as the width increases, and vice versa [35]. The value of d-
(10)
d
2dSinѲ = nλ
M
spacing was calculated by using Bragg’s law,
te
where n is the diffraction order, λ is the incident X-rays wavelength, d the distance between parallel plans to the incident beam axis and Ѳ is angle of diffraction. The values of d-spacing of
Ac ce p
sample S1 and S2 are given in table 4(a) 4(b) respectively. The lattice constant “a” for cubic planes of sample S1 were calculated from x-ray diffraction data by using the following relation [36]
1/d2 = (h2 + l2 + k2) / a2
(for cubic)
(11)
Where (h, k, l) is the miller indices and ‘a’ is lattice constant. The average crystallites size (D) of CdS nanoparticles from XRD patterns has been evaluated by Scherer’s equation [37]. D = kλ / β cosѲ
(12)
7 Page 7 of 12
Edited by Foxit Reader Copyright(C) by Foxit Software Company,2005-2007 For Evaluation Only.
Where λ is wavelength of X-rays (0.15418nm), K is a shape factor typically =0.94, Ѳ is the Bragg’s angle, β is the full-width at half maximum (FWHM) of the maximum intensity peak. The unit of β is converted into radian.
1/d2 = [4/3 (h2 + hk + k2) + l2/c2 ] / a2
(for hexagonal)
(13)
(14)
us
c = 1.633a
cr
diffraction data by using the equation 13 and 14 [38].
ip t
The lattice constant “a” and “c” for hexagonal planes of sample S2 were determined from X-ray
films prepared by S1 and S2 was given in table 4.
an
The calculated value of d-spacing, lattice constant, β (FWHM), X-ray domain size for CdS thin
The figure 11 shows that the thiourea concentration increases the grain sizes decreases. The
M
nanoparticles have narrower size distribution and optimized stability. There are two reasons
d
behind this,
structure,
Cd
Ac ce p
NH2+
te
(1) The ‘S’ atom in the thiourea can effectively chelated with Cd and created the following
S
C
NH2
The competitive reactions between Cadmium, thiourea and sulfide lead to the narrower size distribution and smaller size of nanoparticles. (2) The surface of nanoparticles becomes positive when the nanoparticles of CdS have been capped by thiourea. Due to positive surface, the stronger repulsion of each particle prevents the nanoparticles from aggregation. Same trend has been reported by Zhang [39] and Jian, et.al [19].
8 Page 8 of 12
It has been argued that in these semiconductor nanoparticles the hyperbolic band model gave a better fit to the observed quantum size effect as compared to the effective mass approximation. So it was employed in this work to estimate the size of the nanocrystallites [40]. Egn = [Egb2 + 2h Egb (π / R) 2 / m*] 1/2
ip t
(15)
Where Egb is the band gap for bulk CdS which is 2.42eV, R is the particle's radius and m* was the
cr
effective electron mass. Taking m*= 0.2me for CdS, where me was the mass of a free electron.
us
The values of average grain size calculated by using equation 16 are in comparison with those of using Scherer relation were given in table 5.
an
4. Conclusions
Fabrication of CdS thin film nan-ocrystals network by using CBD technique is simple and
M
economical. CdS thin films of samples S1 and S2 shows high transmission. The transmission in
d
the sample S2 was better than that of sample S1 and the band gap increased by increasing
te
thiourea concentration. All thin films showed the absorption edge shifted toward blue which was endorsed to the exitonic absorbance at low dimensions due to the quantum confinement as
Ac ce p
compared to its bulk counterparts. It was also observed that as the concentration of thiourea increases, band gap increases while the grain size of nanoparticles decreases in both types of thin films. This increment in effective band gap yields CdS thin films a more efficient window material for photovoltaic applications.
References
[1] J Xiaoxia et al. Sol. Energy Mater. Sol. Cells 94 338 (2010) [2] X Dunan, Y Huang, R Agarwal, C Lieber Nature 421 241 (2003) [3] S B Santos et al. Appl. Surf. Sci. 255 796 (2008) [4] R Banerjee, R Jayakrisnan, P Ayyub J.Physic.Condens.Matter. 12 10647 (2000) 9 Page 9 of 12
[5] J W Choi, A Bhupathiraju, M A Hasan, M J Lannon J. Crystal Growth. 255 1 (2003) [6] L Brus Appl. Phys. 53 465 (1991) [7] S Chaure, B N Chaure, K R Pandey J. Nanosci. Nanotechnol. 7 945 (2007)
[9]
ip t
[8] K K Nanda, N S Sarangi, N S Sahu Nano Structured Material. 10 1401(1998)
D Barreca, A Gasparotto, C Maragno, E Tondello, C Sada Chem. Vapour Depos. 4
cr
229(2004)
us
[10] S B Moon, H J Lee, H Jung Thin Solid Films. 511 299 (2006)
[11] R D Acosta, R C Magana, I A Martinez, A Maldonado Sol. Energy Mater. Sol. Cells 82 1
an
(2004)
[12] B Z He, L G Zhao, J W Weng, Y P Du, G Shen, R G Han Vacuum 79 1 (2005)
M
[13] J Nanda, S K Narayan, A B Kuruvilla, L G Murthy, D D Sarma J App. Phys. 72 11(1998)
R H Moutinho, G R Dhere, K Ramanathan, P Sheldom, L L Kazmerski Proceedings of
te
[15]
d
[14] A Catalano Sol. Energy Mater. Sol. Cells 205 41 (1996)
25th IEEE Photovoltaic Specialists Conference 945 (1996) T Nakanishi and K Ito Sol. Energy Mater. Sol. Cells 35 171(1994)
[17]
H Metin and R Esen Semicond. Sci. Technol, 18 647(2003)
Ac ce p
[16]
[18] M Rami, E Benamar, M Fahoume, F Chraibi, A Ennaoui Solid State Sciences 179 (1999) [19] Y Jian-Xi, Z Gao-Ling, H Gao-Rong Microelectronic Engineering 66 115 (2003) [20] A M Abdullah, A Hussam, S J J Abdual Journal of the association of Arab universities for basic and applied science 11 27 (2012) [21] A Cortes, H G!omez, E R Marotti, G Riveros, A E Dalchiele, Sol. Energy Mater. Sol. Cells 82 21 (2004)
10 Page 10 of 12
[22] T Tauc, F Abeies Optical Properties of solids (Amsterdam North Holland IOP Publishing Ltd) (1970) C R Weast CRC Handbook of Chemistry and Physics (Cleveland: 57th Ed. CRC Press,
Cleveland) 102(1976) [24] M Savelli, J Bougnot Solar energy converssion 31 213(1979)
cr
[25] J S Ikhmayies and N R Ahmad-Bitar Physica B. 405 3141 (2010)
ip t
[23]
us
[26] J Lee and T Tsakalakos Nanostructured materials 8 381 (1997)
[27] N Gaewdang and T Gaewdang Technical digest of the international 14 123 (2004)
an
[28] P Rodrgues, M O Aguirre, E San-Martin, G Gonzalez Appl. Surf. Sci. 255 740 (2008) [29] D Lincot, M Froment, H Cachet Advances in Electrochemical Science and Engineering 6
d
[31]
R J Wen, G W Lin, J Y Jiang, H X Sorensen Appl. Phys. Lett 67 831 (1995) A Bandaranayake, P Kapmann, B Cowach, R Mokili, D Ortega, J Vedel 12th Europ.
te
[30]
M
165 (1999)
Photovoltaic solar energy conference, Amesterdam 664 (1994)
[33]
D Lincot, R O Bortges, F M Phil B Msg 68 185 (1993)
Ac ce p
[32]
L O D Melo, O Hernandez, Z Angel, R L Morales, M Beccerril, E Vasco Appl. Phys.
Lett. 65 1278 (1994) [34]
C P Riek and B S Bentjen Chem. Mater. 5 43 (1993)
[35]
B D Cullity Elements of X-ray Diffraction (USA: Addison Wesley Publishing Company
Inc) (1956) [36]
C Kittel 1996. Introduction to Solid State Physics (USA: Addison Wesley Publishing
Company Inc) (1996)
11 Page 11 of 12
[37] B D Cullity Elements of X-ray Diffraction (USA: Addison Wesley Publishing Company Inc) (1972) [38]
W Li, J Yang, Z Sun, L Feng, J Zhang, W LiLi International journal of photoenergy 5
ip t
(2011) [39] Y Zhang, J Chin J. inorg. Chem. 15 595 (1999)
te
d
M
an
us
cr
K K Nanda, N S Sarangi, S Mohanty, N S Sahu Thin Solid Films 322 21(1998)
Ac ce p
[40]
12 Page 12 of 12
S0030-4026(16)30186-3 http://dx.doi.org/doi:10.1016/j.ijleo.2016.02.081 IJLEO 57418
To appear in: Received date: Accepted date:
23-1-2016 29-2-2016
Please cite this article as: R. Zia, M. Riaz,Quratulain, S. Anjum, Study the Effect of Thiourea Concentration on Optical and Structural Properties of CdS - nanocrystalline thin films Prepared by CBD Technique, Optik - International Journal for Light and Electron Optics (2016), http://dx.doi.org/10.1016/j.ijleo.2016.02.081 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Study the Effect of Thiourea Concentration on Optical and Structural Properties of CdS - nanocrystalline thin films Prepared by CBD Technique
ip t
Rehana Zia,Madeeha Riaz ,Quratulain, Safia Anjum
an
Abstract
us
Corresponding Author, Email: rzia1960 @gmail.com Corresponding Author: Dr. Rehana Zia Area Code: 54000 Phone no: 009242 99203801-9 Fax no: 00924299203087
cr
Department of Physics, Lahore College for Women University, Lahore, Pakistan
Nano crystalline CdS thin films have been deposited on glass substrate employing chemical bath
M
deposition technique by varying thiourea concentration in two different cadmium sources as cadmium chloride (S1) and cadmium acetate (S2). The aim of this research is to investigate the
d
effect of thiourea concentration on optical and structural properties of the deposited CdS thin
te
films. XRD analysis reveals that the CdS thin films fabricated by using S1 have zinc blende
Ac ce p
cubic structure and S2 have hexagonal as well as cubic structure. UV-VIS studies demonstrate that as the thiourea concentration increases transmission spectra shift towards the shorter wavelength and optical band gap of CdS thin films also increases from 2.45 - 2.71eV in S1 and 2.65-2.74eV in S2. In absorption spectra, a blue shift in the band gap was observed which indicates the formation of CdS nanoparticles measured as 31-9.91 nm in S1 and 30-15.02 nm in S2.
Key Words: Precipitation, Semiconductors, Optical properties, X-ray diffraction topography
1 Page 1 of 12
1. Introduction In solid state technology, nano crystalline semiconducting thin films play an important role in optoelectronic and magnetic devices due to low-priced production. Cadmium sulfide is among
ip t
the most promising direct band gap semiconductor due to its high coefficient of absorption [1].
cr
CdS is used in many applications such as window materials in CdTe/ CdS hetrojunction solar cells, semiconductor laser [2] display devices and in biological applications [3].
us
Many deposition techniques have been used to fabricate CdS thin film such as molecular beam epitaxy [4, 5], electrodeposition [6, 7], physical vapor deposition [8], chemical vapour deposition
an
[9], RF sputtering [10], spray pyrolysis [11], vacuum evaporation [12], screen printing [13].
M
Chemical bath-deposition (CBD) is one of the most promising methods for manufacturing large areas low-cost CdS thin films [14, 15]. In the present study CBD technique is used to produce
d
CdS nanocrystalline thin films. Over the years various cadmium ion sources have been used such
te
as cadmium iodide [16], cadmium sulfate [17], cadmium acetate, and cadmium chloride [18]. To study the effect of thiourea as a capping agent on the properties of CdS thin films were
Ac ce p
investigated by many workers [19, 20]. The purpose of this work is to provide a complete comparative study of the effect of thiourea concentration on structural and optical properties of CdS thin films prepared by CBD method where CdCl2 and Cd(CH3COO)2 act as cadmium ion source.
2. Experimental Details
CdS thin films were fabricated by using chemical bath deposition technique at three different concentrations of thiourea. Commercially available soda lime glass slides (35 x 25 x 1) mm were used as substrates for CdS thin films deposition. The glass substrates were cleaned with nitric acid for 48 hours, then rinsed with deionized water and finally desiccated in air prior to 2 Page 2 of 12
deposition. Thiourea (0.1M) was used as sulfur precursor while cadmium acetate and cadmium chloride (0.1 M each) as Cd precursors. Ammonia was added into the solution as complexing agent. First of all we prepared aqueous solutions of 0.3M ammonium chloride (NH4Cl) and 0.1M
ip t
solution of cadmium chloride (CdCl2) in 100 ml distilled water. After that 0.6 M ammonia solution was prepared and added into the solution as a complexing agent to set the value of pH to
cr
10. Finally 0.1M solution of thiourea ((NH2)2CS) was added and then stirred it with a magnetic
us
stirrer. The magnetic stirrer was turned off when a clear uniform bath is obtained and then the substrates of glass were placed vertically in the solution which kept at the temperature of 90oC.
an
The samples were removed from the solution after 6 hours. A thin, strongly adherent, dense, shiny, CdS layer was deposited on the glass substrate. Nano crystalline CdS thin films were
M
fabricated on three different thiourea (NH2)2CS concentrations in cadmium chloride (CdCl2)
d
samples termed as S1 and the samples in cadmium acetate (CdAc2) used as cadmium ion source
2.1 Chemical Reaction
te
as S2 were shown in table 1 and 2 respectively.
Ac ce p
CdS thin films were synthesized by the thiourea decomposition in an alkaline chemical bath solution containing a salt of cadmium. 1. A complex was formed by the reaction of ammonia with cadmium salts: CdCl2 + 4NH3
Cd (NH3)4 Cl2
(1)
2. On the catalytic surface of the glass substrate thiourea and Cd(NH3)42+ , Cl-, OH complex ions were diffused. [Cd (NH3)4]2+
Cd2+ + 4NH3
(2)
NH3 + H2O
NH4 +2 + OH-
(3)
Cd2+ + 2OH-
Cd (OH2)
(4)
3 Page 3 of 12
3. In the alkaline medium the thiourea dissociates on the catalytic surface of CdS: (NH2)2 CS + 2OH-
[HS+] + H2O + CH2N2
(5)
4. The ions of bivalent sulfide were produced: S-2 + H2O
(6)
ip t
[HS]+ + OH-
5. The particles of CdS were formed: CdS + NH3
(7)
cr
[Cd (NH3)4]+2 + S-2
us
The fabricated thin films of CdS were highly adhesive, homogeneous, reflecting, smooth and light yellow in color.
Optical Properties
M
3.1.
an
3. Results and Discussion
To study the optical properties of CdS thin films the transmission and absorption spectra were
d
obtained in the range of 300-900nm. It was found that in the wavelength range greater than 500
te
nm all nano crystalline thin films of CdS had very high optical transmittance. Figure 1 and 2 shows the Transmission spectra of CdS thin films of samples S1 and S2 that lies
Ac ce p
between 70-90% and 75-90% in visible range respectively. CdS thin films have high transmission in samples S2 as compared to sample S1. The rapid increase in transmission is the result of broad band gap of CdS by increasing thiourea concentration which results to increase the transmission in the visible region. Absorption spectrum of CdS thin films was achieved by measuring the electromagnetic radiations absorbance of uniform, nano crystalline films as showed in Figure 3and 4. It was observed that all the films have peaks at 500nm to 450nm which indicated that the absorption edge shifted toward blue. It was also seen that maximum absorption for all the CdS thin films with different concentration of thiourea was nearly 4%. This absorption edge shift towards blue
4 Page 4 of 12
can be endorsed to the exitonic absorbance at low dimensions due to the quantum confinement as compared to its bulk counterparts. The spectrum of absorption assured that absorption edges shifted toward the shorter wavelength as the concentration of thiourea increases. The same trend
ip t
has been reported by Jian, et. al [19] Abdullah, et.al [20] and Cortes, et.al [21].
The energy band gap values of the CdS nano crystalline thin films were also found from the
cr
transmission vs. wavelength as shown in figure 5and 6 for samples S1 and S2 respectively. The
us
energy band-gap values of deposited thin films were found by examining the optical data of photon energy hν and optical absorbance α using Taucs formula relation [22].
an
αhν = C ( hν – Eg )1/2
(8)
M
Eg is band gap energy and C is energy-independent constant. The graph linearity indicated that all the films had direct band transition. Optical band gap of CdS nanoparticals was found by
d
extrapolating the straight line segment of the (αhν)2 vs. (hν) to (hν) axis at α = 0.
te
The observed band gap values of deposited CdS nanoparticles of samples S1 and S2 were given in table 3. It shows that the band gap values of samples S1 lies in the range of 2.45 to 2.71eV and
Ac ce p
in the samples S2 were 2.65 to 2.74eV, which are closed to the values quoted in the literature [23, 24]. It was clearly seen that as the concentration of thiourea increases the value of the band gap of CdS nanoparticles increases as in figure 7. This observed shift indicates quantum size effect on the nano crystalline CdS thin films because as the thiourea concentration increases the size of the nanoparticles decreases and the band gap increases. The variation in the particle size with respect to the band gap energy was explained as the nanoparticles have the crystalline structure of their bulk counter parts and hence were characterized by the fully occupied valence band and an empty conduction band separated by the energy gap (Eg). However, the charge carriers in these bands e.g., the electrons in the conduction band and holes in the valence band experience
5 Page 5 of 12
an overall confining potential due to the finite size of these particles. As a result there will be size-dependent discrete states in the conduction and valence bands leading in the effective enhancement of the band gap; the so-called quantum size effect [25].
t
=
ip t
The thickness of the CdS thin films from two minima and maxima was calculated as, M λ1 λ2
(9)
cr
2 [n (λ1) λ2 – n (λ2) λ1]
us
In this equation, M was number of total oscillations occurring at λ1 and λ2 between two extremes with t and n determined accurately in the region where the transmission of film was measured.
an
The measured thickness of samples S1 and S2 was given in table 3. Figure 8 shows that as the concentration of thiourea increases the thickness of CdS thin films decreases. Generally the
M
thickness of CdS thin films is mainly depends upon grain size of the nanoparticles, similar trend
d
was observed by Lee, et.al [26]. It was reported in table 3a that in case of S1 thin films have high
te
values of thicknesses as compared to S2 films. The main factor was that the grain size of thin films prepared as S1 was greater than S2 and transmission from thicker films was always lesser
Ac ce p
then thinner films [27]. It was observed in figure 2 the transmission from samples S2
were
greater as compared to samples S1. 3.2. Structural Properties
XRD patterns of the CdS thin films fabricated as sample S1 and S2 are shown in figure 9 and 10. The XRD pattern for sample S1 shows that the prominent peak at 2Ѳ values of 26.7o scattering from the diffraction plane (111) which shift to 26.8o as the thiourea concentration increases and two other low diffraction intensity peaks were also observed at 44o and 52o, as scattering from the crystal planes (220), and (311) of zinc blende cubic phase of CdS according to the JCPDS card File No. 10-0454, suggesting that the nanoparticles were in cubic structure [28]. This CdS
6 Page 6 of 12
cubic phase is metastable, appearing in low-dimension structures such as nano crystalline thin films or systems [29, 30]. The X-ray diffraction pattern of sample S2 exhibited three main peaks at diffraction angles of
ip t
26.8o, 29.2o, and 52o that corresponds to the miller indices (002), (101) and (112) and matched with the JCPDS data (JCPDS No. 6-314) of hexagonal phase CdS. In the sample S2 based film, a
cr
fourth peak was also observed, which was found to be (200) plane, that was a characteristic peak
us
of cubic CdS. Hexagonal phase CdS thin films had been reported previously by some workers [31, 32] and cubic phases by others [33, 34].
an
The information about crystallite size was obtained from the broadening of the diffraction peaks. The size of the particle decreases as the width increases, and vice versa [35]. The value of d-
(10)
d
2dSinѲ = nλ
M
spacing was calculated by using Bragg’s law,
te
where n is the diffraction order, λ is the incident X-rays wavelength, d the distance between parallel plans to the incident beam axis and Ѳ is angle of diffraction. The values of d-spacing of
Ac ce p
sample S1 and S2 are given in table 4(a) 4(b) respectively. The lattice constant “a” for cubic planes of sample S1 were calculated from x-ray diffraction data by using the following relation [36]
1/d2 = (h2 + l2 + k2) / a2
(for cubic)
(11)
Where (h, k, l) is the miller indices and ‘a’ is lattice constant. The average crystallites size (D) of CdS nanoparticles from XRD patterns has been evaluated by Scherer’s equation [37]. D = kλ / β cosѲ
(12)
7 Page 7 of 12
Edited by Foxit Reader Copyright(C) by Foxit Software Company,2005-2007 For Evaluation Only.
Where λ is wavelength of X-rays (0.15418nm), K is a shape factor typically =0.94, Ѳ is the Bragg’s angle, β is the full-width at half maximum (FWHM) of the maximum intensity peak. The unit of β is converted into radian.
1/d2 = [4/3 (h2 + hk + k2) + l2/c2 ] / a2
(for hexagonal)
(13)
(14)
us
c = 1.633a
cr
diffraction data by using the equation 13 and 14 [38].
ip t
The lattice constant “a” and “c” for hexagonal planes of sample S2 were determined from X-ray
films prepared by S1 and S2 was given in table 4.
an
The calculated value of d-spacing, lattice constant, β (FWHM), X-ray domain size for CdS thin
The figure 11 shows that the thiourea concentration increases the grain sizes decreases. The
M
nanoparticles have narrower size distribution and optimized stability. There are two reasons
d
behind this,
structure,
Cd
Ac ce p
NH2+
te
(1) The ‘S’ atom in the thiourea can effectively chelated with Cd and created the following
S
C
NH2
The competitive reactions between Cadmium, thiourea and sulfide lead to the narrower size distribution and smaller size of nanoparticles. (2) The surface of nanoparticles becomes positive when the nanoparticles of CdS have been capped by thiourea. Due to positive surface, the stronger repulsion of each particle prevents the nanoparticles from aggregation. Same trend has been reported by Zhang [39] and Jian, et.al [19].
8 Page 8 of 12
It has been argued that in these semiconductor nanoparticles the hyperbolic band model gave a better fit to the observed quantum size effect as compared to the effective mass approximation. So it was employed in this work to estimate the size of the nanocrystallites [40]. Egn = [Egb2 + 2h Egb (π / R) 2 / m*] 1/2
ip t
(15)
Where Egb is the band gap for bulk CdS which is 2.42eV, R is the particle's radius and m* was the
cr
effective electron mass. Taking m*= 0.2me for CdS, where me was the mass of a free electron.
us
The values of average grain size calculated by using equation 16 are in comparison with those of using Scherer relation were given in table 5.
an
4. Conclusions
Fabrication of CdS thin film nan-ocrystals network by using CBD technique is simple and
M
economical. CdS thin films of samples S1 and S2 shows high transmission. The transmission in
d
the sample S2 was better than that of sample S1 and the band gap increased by increasing
te
thiourea concentration. All thin films showed the absorption edge shifted toward blue which was endorsed to the exitonic absorbance at low dimensions due to the quantum confinement as
Ac ce p
compared to its bulk counterparts. It was also observed that as the concentration of thiourea increases, band gap increases while the grain size of nanoparticles decreases in both types of thin films. This increment in effective band gap yields CdS thin films a more efficient window material for photovoltaic applications.
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