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Ammonia-free method for synthesis of CdS nanocrystalline thin films through chemical bath deposition technique
Solid State Communications 149 (2009) 1765–1768
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Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Ammonia-free method for synthesis of CdS nanocrystalline thin films through chemical bath deposition technique M. Karimi a , M. Rabiee a , F. Moztarzadeh a , M. Bodaghi b , M. Tahriri a,∗ a
Biomaterial Group, Faculty of Biomedical Engineering, Amirkabir University of Technology, P. O. Box 15875-4413, Tehran, Iran
b
Ceramic Department, Materials and Energy Research Center, Tehran, Iran
article
info
Article history: Received 26 December 2008 Received in revised form 23 May 2009 Accepted 17 July 2009 by V. Pellegrini Available online 24 July 2009 PACS: 81.07._b 81.15._z Keywords: A. Nanostructures A. Semiconductors B. Chemical synthesis D. Optical properties
abstract The preparation of thin films of CdS by chemical bath deposition is mostly based on the utilisation of ammonia as a complexing agent for cadmium ions. Here we report on a technique based on sodium citrate dihydrate that eliminates the problems of ammonia volatility and toxicity. The crystallites with a size range of 10–20 nm in diameter with zinc blend (cubic) and wurtzite (hexagonal) crystal structures and strong photoluminescence were prepared from the mixture solutions of: cadmium chloride dihydrate as a cadmium source, thiourea as a sulfur source and sodium citrate dihydrate as a complexing agent for cadmium ions. The well-cleaned glass used as a substrate for thin film deposition. The obtained samples were characterized by the techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscope (AFM) and fluorescence spectroscopy. Also, the effect of two operating conditions, (i) pH, and (ii) the temperature of reaction on the synthesizing of CdS nanocrystals was examined. Finally, it was found that the CdS nanocrystals showed sharp excitation features and strong ’’band-edge’’ emission. Crown Copyright © 2009 Published by Elsevier Ltd. All rights reserved.
1. Introduction The synthesis of binary metal chalcogenides of group II–VI semiconductors in a nanocrystalline form has been a rapidly growing area of research due to their important nonlinear optical properties [1,2], luminescent properties [3,4], quantum size effect [5,6] and other important physical and chemical properties [7]. Cadmium sulfide (CdS) with a wide band gap of 2.4 eV (in bulk) is a very important technological semiconductor material which has been studied for decades. CdS in a nanocrystalline thin film form can be prepared by a variety of methods (both physical and chemical) like sol–gel [8], electrostatic deposition [9], electrochemical method [10,11], gas evaporation [12], micelles [13], CBD [14] etc. The CBD process is a simple and inexpensive technique to obtain homogeneous, hard, adherent, transparent and stoichiometric CdS nanocrystal thin films. Typically, chemically CdS nanocrystals are formed from the reaction between a cadmium salt and thiourea in an ammoniacal alkaline solution. The main role of ammonia in the CBD process is as a complexing agent for the cadmium ions in the reaction solution. It is clear that the preparation of CdS nanocrystals by
∗
Corresponding author. Tel.: +98 912 2388573; fax: +98 21 66468186. E-mail address: [email protected] (M. Tahriri).
CBD for large scale solar cell production represents a serious environmental problem because the employment of large amounts of ammonia, which is toxic and is highly volatile and harmful for the environment [15,16]. Some efforts have been dedicated to the investigation of CBD processes for the synthesis of good quality CdS nanocrystals, which reduce this environmental problem. One of the main approaches is the substitution of ammonia as the complexing agent of cadmium ions in the CBD process [15,17]. Here we report the properties of CdS nanocrystalline thin film synthesized by an ammonia-free CBD process employing sodium citrate dihydrate as the complexing agent substituting ammonia. We have found that good quality CdS nanocrystals can be obtained from a cadmium chloride, thiourea and sodium citrate dihydrate solution. The CdS nanocrystals obtained from this ammonia-free process showed properties comparable and even better than those of CdS nanocrystals obtained from an ammonia based CBD process.
2. Materials and methods 2.1. Synthesis of CdS nanocrystals All the chemicals were of reagent grade used without further purifications. CdS thin films have been deposited on the glass substrates using CBD at various temperatures from 60 ◦ C to 80 ◦ C. The substrates
0038-1098/$ – see front matter Crown Copyright © 2009 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.07.027
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Table 1 Samples synthesized under different pH and different temperature of reaction. Sample
pH
Temperature (◦ C)
A1 A2 A3 A4 A5
10 11 12 12 12
70 70 60 70 80
used for the deposition of CdS thin films were 50 × 50 × 1 mm glass slides. Before the deposition, the substrates were cleaned with ethanol and washed with de-ionized water and acetone, respectively, then etched with hydrofluoric acid (HF 5%), then again cleaned with ethanol and de-ionized water, respectively and finally dried in air. Stock solutions of 0.5 M CdCl2 ·2H2 O as a source of cadmium and 0.5 M C6 H5 Na3 O7 ·2H2 O (sodium citrate dihydrate) as a complexing agent were prepared. In the same way, stock solution of 0.5 M CH4 N2 S (thiourea) was prepared as a source of sulfur. Lastly, stock solution of 0.1 M NaOH (sodium hydroxide) was prepared as an agent for pH adjustment. At first, 20 ml of CdCl2 ·2H2 O was added into 20 ml C6 H5 Na3 O7 · 2H2 O and it magnetically stirred. Subsequently, 20 ml of CH4 N2 S was added into the mixture and the whole solutions magnetically stirred again. Two operating conditions, (i) pH, and (ii) the temperature of reaction were varied for different synthesis experiments as shown in Table 1. In the next step, the prepared substrates immerged in these solutions for 30 min. Eventually, these samples were dried in air.
a
20
25
30
35
40
45
50
55
60
65
70
75
80
55
60
65
70
75
80
2 Theta
b
2.2. Sample characterization The resulting samples were analyzed by X-ray diffraction (XRD) with Philips PW 3710. Transmission electron microscopy (TEM; CM200-FEG-Philips) was used for characterizing the particles. AFM imaging was performed using Multimode Scanning Probe Microscope with Nanoscope IIIa controller (Veeco Instruments, Santa Barbara, CA) with a vertical engagement (JV) 125 µm scanner. Images were collected using mainly tapping mode AFM because it is particularly well adapted to soft samples due to a nearly complete reduction of lateral forces. Silicon cantilevers with spring constant of 5.5 N/m and typical resonance frequency of 290 kHz were utilized. The fluorescence emission spectra were acquired at right angle on a Perkin–Elmer LS-55 fluorescence spectrophotometer. Excitation and emission fluorescence spectra of chemical bath deposited CdS thin films were measured at room temperature. The instrument parameters affecting photoluminescence intensity are as follows. Measurement type: wavelength scan, scan mode: excitation/emission, data mode: fluorescence, ex. slit: 5.0 nm, em. slit: 5.0 nm, PMT voltage: 950 V. The film thickness was measured using a profilometer (Dektak3, Veeco Inst.). 3. Results and discussions 3.1. XRD analysis Fig. 1 shows the XRD patterns of the powders prepared at the different pH, and temperature of reaction. CdS nanocrystalline films synthesized by CBD show a predominant single peak at 2θ = 27◦ which can be assigned to the (002) plane of hexagonal or the (111) plane of cubic CdS. With the increase of pH and temperature of reaction, there is an increase in intensity and sharpening of this peak, which is caused by improving crystallinity of the crystals and increasing crystallite size.
20
25
30
35
40
45
50 2 Theta
Fig. 1. X-ray patterns for the CdS nanocrystalline thin films in (a) different temperature (constant pH) and (b) different pH (constant temperature) of reaction. Peaks labeled with C and H indicate the cubic and hexagonal phases, respectively.
CBD-CdS crystals show three diffraction peaks at 27, 43 and 52.5◦ which are associated with the (002), (110) and (112) reflections of the hexagonal modification or (111), (220) and (331) reflections of the cubic CdS structure as shown in Fig. 1. With further investigations, it is noticed that the peak at about 2θ = 27◦ , which is contributed by the (002) plane in the hexagonal phase and the (111) plane in the cubic phase as mentioned before, is unusually narrow and strong, indicating a preferential growth along the orientation because the two planes describe the same direction [18]. It is worth mentioning that the preferred orientation factor f (002) of the (002) plane for the CdS nanoparticles can be calculated by evaluating the fraction of (002) plane intensity over the sum of intensities of all peaks within a given measuring 2θ range (20–80 ◦ ) [19]. It is mentionable that the identification and assignments of the observed diffraction patterns were made using the JCPDS data and by comparison with published results [20,21]. By examining the whole patterns it can be concluded that they are a mixture of two crystallographic phases: a hexagonal phase (wurtzite structure) and a cubic phase (zinc blend structure), which are closed to the values of JCPDS 41-1049 and 10-454. Also, it was found from XRD patterns that when the CdS cluster synthesized and named as A1, A2 and A3, they grow approximately in cubic phase. The
M. Karimi et al. / Solid State Communications 149 (2009) 1765–1768
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Table 2 Thickness of CdS thin film (sample A1) deposited with various deposition times. Deposition time (min)
Film thickness (nm)
15 30 60 90
175 252 346 502
samples synthesized at higher pH, named as A4 and A5, grow with mixed phases. Likewise, from XRD peaks and peak intensity observations, cubic phase is more dominating. This cubic phase indicates the predominance of a colloidal process in the deposition mechanism [22]. Finally, according to the above explanations, it was found that pH value and temperature of reaction are the key factors for phase modification [zinc blend (cubic) and wurtzite (hexagonal)]. In fact, due to change in preparative parameters, phase modification takes place.
1 µm
a
b
Fig. 2. (a) SEM micrograph of CdS nanocrystalline thin films and (b) TEM micrograph of CdS nanocrystalline thin film for sample A1.
300
3.2. Thickness of films The CdS film formation began from about 15 min after mixing the reactants, and was completed within about 90 min. The obtained CdS films were homogeneous, hard and had very good adhesion on to the glass substrate. In this section the results of the deposition time investigations is presented. Table 2 shows the evolution of the film thickness for sample A1 with the deposition time. As seen the film thickness increases with the deposition time. It is noticeable that in CBD technique, the film growth starts initially with an incubation period, where no clearly observable growth occurs. This period corresponds to the nucleation step followed by a subsequent linear growth. This growth process is reported by many authors [23]. 3.3. SEM and TEM observations The microstructures of these thin films prepared by the present process were examined by SEM and TEM. The SEM and TEM micrographs of synthesized thin films are shown in Fig. 2(a) and (b), respectively. The SEM micrographs show typical tightly adherent CdS films on the commercial glass substrates. In most cases the final films are homogeneous, without any crack, rather dense and exhibit almost complete coverage of the substrate. They are composed of largely irregular-shaped grains of diameter 100–1000 nm, with average thickness between 300 and 500 nm. Compact films of thickness greater than 1 µm are also possible. With the CBD process, much thinner films are obtained (50–100 nm), while a thicker film is usually associated with powdery deposits. The morphology of the thin film was examined by TEM imaging. The image of CdS thin films shows that all particles of thin films exhibited approximately spherical shape, and the crystal sizes are estimated to be 10 nm. 3.4. AFM analysis Surface morphology of the films deposited at different temperatures is shown in Fig. 3. AFM images are characterized by slight surface roughness with a uniform crack-free, densely packed microstructure. The surface roughness (RMS) of the films is calculated by using the equipment’s software routine. The surface roughness of film deposited (sample A1) is 4.91. Of course, at a microscopic scale, no surface appears perfectly smooth and all thin films exhibit surface irregularities and bumps that this case is not an exception. Eventually, it is worth mentioning that the roughness of the films can be explained as the z-value of the particles, which has a relation with the size of particles obtained from SEM.
200
2.5
2.5
100
2.0
2.0 1.5
1.5 1.0
1.0
0.5
0.5 0.0
0
0.0
Fig. 3. AFM micrograph of thin film (sample A1).
3.5. Photophysical analysis Luminescence properties of samples are quite well investigated and described in numerous papers [19,24]. It is known that single crystals of CdS have luminescence in blue, green, red and infrared regions of spectra. In Figs. 4 and 5 normalized excitation and emission fluorescence spectra for the samples with different pH and temperature of reaction are presented. From Fig. 5, we can conclude that the photoluminescence emission peak shifts to longer wavelengths (red-shift) and that the photoluminescence intensity decreases as the pH and temperature of reaction increase. The photoluminescence emission process is induced by the surface defect recombination mechanism. It is notable that the CdS nanocrystals become bigger as the pH and temperature of reaction increase (according to the Scherrer equation D = kλ/β cos θ , where k is a constant (shape factor, about 0.9), λ is the Xray wavelength (1.5405 Å as mentioned before), β is the full width at half maximum (FWHM) of the diffraction line, and θ was the diffraction angle), so the photoluminescence intensity will decreases as the size of CdS nanocrystals increases. It is notable that the CdS nanocrystals become bigger as the pH and temperature of reaction increase, and the ratio of surface atoms to the total number of atoms in the CdS nanocrystals decreases, therefore, the photoluminescence intensity will decrease as the size of CdS nanocrystals increases [25]. The well-defined, technically attractive, blue and green emissions in the range of 420–540 nm are found to be of high intensity and broader and are attributed to the fundamental absorption in CdS. These emissions could be due to the band-edge related, and the surface defect state mediated recombination center in the conventional HOMOLUMO model. It is well known that surface defect states play important role in the luminescence properties of nanocrystals, which
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4. Conclusions
800 700
A1
In conclusion, the zinc blend and wurtzite structures of CdS nanocrystals with the size about 10–20 nm and strong blue and green photoluminescence were prepared directly via chemical bath deposition method. A wet-chemical synthetic route was taken to prepare CdS nanocrystalline thin films through the reaction of cadmium chloride dihydrate with thiourea. The resulting colloidal solution of CdS nanocrystals gave a bright photoluminescence emission at room temperature. Finally, it is mentionable that the CBD method may be extended to synthesis other inorganic materials such as ZnS, ZnO, CdO, TiO2 etc.
Intensity
600 A2
500
A3
400
A4 A5
300 200 100 200
250
300
350 Wavelength (nm)
400
450
450
References
Fig. 4. Excitation fluorescence spectra for samples: A1, A2, A3, A4 and A5.
900 A1 A2 A 3
800
A4
Intensity
700
A5
600 500 400 300 200 100 0
100
200
300
400 500 600 Wavelength (nm)
700
800
900
1000
Fig. 5. Emission fluorescence spectra for samples: A1, A2, A3, A4 and A5.
act as radiative or nonradiative centers [26]. The broadening of these peaks can be attributed to the inhomogeneities in the size and shape of the nanocrystallites (evident also from the TEM micrograph). Also, another peaks of relatively low intensity in the range of 610–750 nm bears the signature of the broad red emission (Lambe–Klick model) [27]. This could be due to the recombination of electrons trapped at sulfur (S) vacancies with valence-band-free holes. Nevertheless, the broad IR emission (related to e–h recombination trapped at the S and cadmium (Cd) vacancies, respectively) could not be verified in our samples because of the limit in the measurement of the PL apparatus.
[1] L.E. Brus, Appl. Phys. A 53 (1991) 465–474. [2] Y. Wang, N. Herron, J. Phys. Chem. 95 (1991) 525–532. [3] R.B. Kale, S.D. Sartale, B.K. Chougule, C.D. Lokhande, Semicond. Sci. Technol. 19 (2004) 980–986. [4] A. Vadivel Muruga, R.S. Sonawane, B.B. Kale, S.K. Apte, A.V. Kulkarni, Mater. Chem. Phys. 71 (2001) 98–102. [5] C.B. Murry, C.R. Kagan, M.G. Bawendi, Science 270 (1995) 1335–1338. [6] R. Rossetti, R. Hill, J.M. Gibson, L.E. Brus, J. Chem. Phys. 82 (1995) 552–559. [7] S. Mann, Nature 322 (1988) 119–124. [8] A.V. Rao, G.M. Pajonk, N.N. Parvathy, Mater. Chem. Phys. 48 (1997) 234–239. [9] O.V. Salata, P.J. Dobson, P.J. Hull, J.L. Hutchinson, Thin Solid Films 251 (1994) 1–3. [10] M. Innocenti, S. Cattarin, M. Cavallini, F. Loglio, M.L. Foresti, J. Electroanal. Chem. 532 (2002) 219–225. [11] M. Cavallini, M. Facchini, C. Albonetti, F. Biscarini, M. Innocenti, F. Loglio, E. Salvietti, G. Pezzatini, M.L. Foresti, J. Phys. Chem. C 111 (2007) 1061–1064. [12] T. Arai, T. Yoshida, T. Ogawa, J. Appl. Phys. 26 (1987) 396–404. [13] P. Lianos, J.K. Thomas, Chem. Phys. Lett. 125 (1986) 299–302. [14] M.G. Sandoval-Paza, M. Sotelo-Lermab, A. Mendoza-Galvana, R. Ramírez-Bon, Thin Solid Films 515 (2007) 3356–3362. [15] D.S. Boyle, A. Bayer, M.R. Heinrich, O. Robbe, P.O. O’Brien, Thin Solid Films 361–362 (2000) 150–154. [16] D. Hariskos, M. Powalla, N. Chevaldonnet, D. Lincot, A. Schindler, B. Dimmler, Thin Solid Films 387 (2000) 179–181. [17] H. Zhang, X. Ma, D. Yang, Mater. Lett. 58 (2003) 5–9. [18] X. Moa, C. Wang, L. Hao, M. You, Y. Zhu, Z. Chen, Y. Hu, Mater. Res. Bull. 36 (2001) 1925–1930. [19] P.K. Vidhyadharan-Pillai, K.P. Vijayakumar, J. Mater. Sci. Lett. 13 (1994) 1725–1726. [20] S.A. Al Kuhaimi, Vacuum 51 (1998) 349–355. [21] P.N. Gibson, M.E.O. Zsan, D. Lincot, P. Cowache, D. Summa, Thin Solid Films 361 (2000) 34–40. [22] A. Cortes, H. Gomez, R.E. Marroti, G. Riveros, E.A. Dalchiele, Sol. Energy Mater. Sol. Cells 82 (2004) 21–34. [23] G. Hodes, Chemical solution deposition of semiconductor film, Marcel Dekker, New York, 2002. [24] H. Metin, R. Esen, J. Cryst. Growth 258 (2003) 141–148. [25] Y. Chen, X. Ji, S. Jiang, Q. Sun, B. Jiang, Colloid Polym. Sci. 281 (2003) 386–389. [26] M.G. Bawendi, P.J. Carroll, W.L. Wilson, L.E. Brus, J. Chem. Phys. 96 (1992) 946–954. [27] K.V. Shalimova, V.N. Khirin, Russian Phys. J. 12 (1969) 487–491.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Ammonia-free method for synthesis of CdS nanocrystalline thin films through chemical bath deposition technique M. Karimi a , M. Rabiee a , F. Moztarzadeh a , M. Bodaghi b , M. Tahriri a,∗ a
Biomaterial Group, Faculty of Biomedical Engineering, Amirkabir University of Technology, P. O. Box 15875-4413, Tehran, Iran
b
Ceramic Department, Materials and Energy Research Center, Tehran, Iran
article
info
Article history: Received 26 December 2008 Received in revised form 23 May 2009 Accepted 17 July 2009 by V. Pellegrini Available online 24 July 2009 PACS: 81.07._b 81.15._z Keywords: A. Nanostructures A. Semiconductors B. Chemical synthesis D. Optical properties
abstract The preparation of thin films of CdS by chemical bath deposition is mostly based on the utilisation of ammonia as a complexing agent for cadmium ions. Here we report on a technique based on sodium citrate dihydrate that eliminates the problems of ammonia volatility and toxicity. The crystallites with a size range of 10–20 nm in diameter with zinc blend (cubic) and wurtzite (hexagonal) crystal structures and strong photoluminescence were prepared from the mixture solutions of: cadmium chloride dihydrate as a cadmium source, thiourea as a sulfur source and sodium citrate dihydrate as a complexing agent for cadmium ions. The well-cleaned glass used as a substrate for thin film deposition. The obtained samples were characterized by the techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscope (AFM) and fluorescence spectroscopy. Also, the effect of two operating conditions, (i) pH, and (ii) the temperature of reaction on the synthesizing of CdS nanocrystals was examined. Finally, it was found that the CdS nanocrystals showed sharp excitation features and strong ’’band-edge’’ emission. Crown Copyright © 2009 Published by Elsevier Ltd. All rights reserved.
1. Introduction The synthesis of binary metal chalcogenides of group II–VI semiconductors in a nanocrystalline form has been a rapidly growing area of research due to their important nonlinear optical properties [1,2], luminescent properties [3,4], quantum size effect [5,6] and other important physical and chemical properties [7]. Cadmium sulfide (CdS) with a wide band gap of 2.4 eV (in bulk) is a very important technological semiconductor material which has been studied for decades. CdS in a nanocrystalline thin film form can be prepared by a variety of methods (both physical and chemical) like sol–gel [8], electrostatic deposition [9], electrochemical method [10,11], gas evaporation [12], micelles [13], CBD [14] etc. The CBD process is a simple and inexpensive technique to obtain homogeneous, hard, adherent, transparent and stoichiometric CdS nanocrystal thin films. Typically, chemically CdS nanocrystals are formed from the reaction between a cadmium salt and thiourea in an ammoniacal alkaline solution. The main role of ammonia in the CBD process is as a complexing agent for the cadmium ions in the reaction solution. It is clear that the preparation of CdS nanocrystals by
∗
Corresponding author. Tel.: +98 912 2388573; fax: +98 21 66468186. E-mail address: [email protected] (M. Tahriri).
CBD for large scale solar cell production represents a serious environmental problem because the employment of large amounts of ammonia, which is toxic and is highly volatile and harmful for the environment [15,16]. Some efforts have been dedicated to the investigation of CBD processes for the synthesis of good quality CdS nanocrystals, which reduce this environmental problem. One of the main approaches is the substitution of ammonia as the complexing agent of cadmium ions in the CBD process [15,17]. Here we report the properties of CdS nanocrystalline thin film synthesized by an ammonia-free CBD process employing sodium citrate dihydrate as the complexing agent substituting ammonia. We have found that good quality CdS nanocrystals can be obtained from a cadmium chloride, thiourea and sodium citrate dihydrate solution. The CdS nanocrystals obtained from this ammonia-free process showed properties comparable and even better than those of CdS nanocrystals obtained from an ammonia based CBD process.
2. Materials and methods 2.1. Synthesis of CdS nanocrystals All the chemicals were of reagent grade used without further purifications. CdS thin films have been deposited on the glass substrates using CBD at various temperatures from 60 ◦ C to 80 ◦ C. The substrates
0038-1098/$ – see front matter Crown Copyright © 2009 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.07.027
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M. Karimi et al. / Solid State Communications 149 (2009) 1765–1768
Table 1 Samples synthesized under different pH and different temperature of reaction. Sample
pH
Temperature (◦ C)
A1 A2 A3 A4 A5
10 11 12 12 12
70 70 60 70 80
used for the deposition of CdS thin films were 50 × 50 × 1 mm glass slides. Before the deposition, the substrates were cleaned with ethanol and washed with de-ionized water and acetone, respectively, then etched with hydrofluoric acid (HF 5%), then again cleaned with ethanol and de-ionized water, respectively and finally dried in air. Stock solutions of 0.5 M CdCl2 ·2H2 O as a source of cadmium and 0.5 M C6 H5 Na3 O7 ·2H2 O (sodium citrate dihydrate) as a complexing agent were prepared. In the same way, stock solution of 0.5 M CH4 N2 S (thiourea) was prepared as a source of sulfur. Lastly, stock solution of 0.1 M NaOH (sodium hydroxide) was prepared as an agent for pH adjustment. At first, 20 ml of CdCl2 ·2H2 O was added into 20 ml C6 H5 Na3 O7 · 2H2 O and it magnetically stirred. Subsequently, 20 ml of CH4 N2 S was added into the mixture and the whole solutions magnetically stirred again. Two operating conditions, (i) pH, and (ii) the temperature of reaction were varied for different synthesis experiments as shown in Table 1. In the next step, the prepared substrates immerged in these solutions for 30 min. Eventually, these samples were dried in air.
a
20
25
30
35
40
45
50
55
60
65
70
75
80
55
60
65
70
75
80
2 Theta
b
2.2. Sample characterization The resulting samples were analyzed by X-ray diffraction (XRD) with Philips PW 3710. Transmission electron microscopy (TEM; CM200-FEG-Philips) was used for characterizing the particles. AFM imaging was performed using Multimode Scanning Probe Microscope with Nanoscope IIIa controller (Veeco Instruments, Santa Barbara, CA) with a vertical engagement (JV) 125 µm scanner. Images were collected using mainly tapping mode AFM because it is particularly well adapted to soft samples due to a nearly complete reduction of lateral forces. Silicon cantilevers with spring constant of 5.5 N/m and typical resonance frequency of 290 kHz were utilized. The fluorescence emission spectra were acquired at right angle on a Perkin–Elmer LS-55 fluorescence spectrophotometer. Excitation and emission fluorescence spectra of chemical bath deposited CdS thin films were measured at room temperature. The instrument parameters affecting photoluminescence intensity are as follows. Measurement type: wavelength scan, scan mode: excitation/emission, data mode: fluorescence, ex. slit: 5.0 nm, em. slit: 5.0 nm, PMT voltage: 950 V. The film thickness was measured using a profilometer (Dektak3, Veeco Inst.). 3. Results and discussions 3.1. XRD analysis Fig. 1 shows the XRD patterns of the powders prepared at the different pH, and temperature of reaction. CdS nanocrystalline films synthesized by CBD show a predominant single peak at 2θ = 27◦ which can be assigned to the (002) plane of hexagonal or the (111) plane of cubic CdS. With the increase of pH and temperature of reaction, there is an increase in intensity and sharpening of this peak, which is caused by improving crystallinity of the crystals and increasing crystallite size.
20
25
30
35
40
45
50 2 Theta
Fig. 1. X-ray patterns for the CdS nanocrystalline thin films in (a) different temperature (constant pH) and (b) different pH (constant temperature) of reaction. Peaks labeled with C and H indicate the cubic and hexagonal phases, respectively.
CBD-CdS crystals show three diffraction peaks at 27, 43 and 52.5◦ which are associated with the (002), (110) and (112) reflections of the hexagonal modification or (111), (220) and (331) reflections of the cubic CdS structure as shown in Fig. 1. With further investigations, it is noticed that the peak at about 2θ = 27◦ , which is contributed by the (002) plane in the hexagonal phase and the (111) plane in the cubic phase as mentioned before, is unusually narrow and strong, indicating a preferential growth along the orientation because the two planes describe the same direction [18]. It is worth mentioning that the preferred orientation factor f (002) of the (002) plane for the CdS nanoparticles can be calculated by evaluating the fraction of (002) plane intensity over the sum of intensities of all peaks within a given measuring 2θ range (20–80 ◦ ) [19]. It is mentionable that the identification and assignments of the observed diffraction patterns were made using the JCPDS data and by comparison with published results [20,21]. By examining the whole patterns it can be concluded that they are a mixture of two crystallographic phases: a hexagonal phase (wurtzite structure) and a cubic phase (zinc blend structure), which are closed to the values of JCPDS 41-1049 and 10-454. Also, it was found from XRD patterns that when the CdS cluster synthesized and named as A1, A2 and A3, they grow approximately in cubic phase. The
M. Karimi et al. / Solid State Communications 149 (2009) 1765–1768
1767
Table 2 Thickness of CdS thin film (sample A1) deposited with various deposition times. Deposition time (min)
Film thickness (nm)
15 30 60 90
175 252 346 502
samples synthesized at higher pH, named as A4 and A5, grow with mixed phases. Likewise, from XRD peaks and peak intensity observations, cubic phase is more dominating. This cubic phase indicates the predominance of a colloidal process in the deposition mechanism [22]. Finally, according to the above explanations, it was found that pH value and temperature of reaction are the key factors for phase modification [zinc blend (cubic) and wurtzite (hexagonal)]. In fact, due to change in preparative parameters, phase modification takes place.
1 µm
a
b
Fig. 2. (a) SEM micrograph of CdS nanocrystalline thin films and (b) TEM micrograph of CdS nanocrystalline thin film for sample A1.
300
3.2. Thickness of films The CdS film formation began from about 15 min after mixing the reactants, and was completed within about 90 min. The obtained CdS films were homogeneous, hard and had very good adhesion on to the glass substrate. In this section the results of the deposition time investigations is presented. Table 2 shows the evolution of the film thickness for sample A1 with the deposition time. As seen the film thickness increases with the deposition time. It is noticeable that in CBD technique, the film growth starts initially with an incubation period, where no clearly observable growth occurs. This period corresponds to the nucleation step followed by a subsequent linear growth. This growth process is reported by many authors [23]. 3.3. SEM and TEM observations The microstructures of these thin films prepared by the present process were examined by SEM and TEM. The SEM and TEM micrographs of synthesized thin films are shown in Fig. 2(a) and (b), respectively. The SEM micrographs show typical tightly adherent CdS films on the commercial glass substrates. In most cases the final films are homogeneous, without any crack, rather dense and exhibit almost complete coverage of the substrate. They are composed of largely irregular-shaped grains of diameter 100–1000 nm, with average thickness between 300 and 500 nm. Compact films of thickness greater than 1 µm are also possible. With the CBD process, much thinner films are obtained (50–100 nm), while a thicker film is usually associated with powdery deposits. The morphology of the thin film was examined by TEM imaging. The image of CdS thin films shows that all particles of thin films exhibited approximately spherical shape, and the crystal sizes are estimated to be 10 nm. 3.4. AFM analysis Surface morphology of the films deposited at different temperatures is shown in Fig. 3. AFM images are characterized by slight surface roughness with a uniform crack-free, densely packed microstructure. The surface roughness (RMS) of the films is calculated by using the equipment’s software routine. The surface roughness of film deposited (sample A1) is 4.91. Of course, at a microscopic scale, no surface appears perfectly smooth and all thin films exhibit surface irregularities and bumps that this case is not an exception. Eventually, it is worth mentioning that the roughness of the films can be explained as the z-value of the particles, which has a relation with the size of particles obtained from SEM.
200
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Fig. 3. AFM micrograph of thin film (sample A1).
3.5. Photophysical analysis Luminescence properties of samples are quite well investigated and described in numerous papers [19,24]. It is known that single crystals of CdS have luminescence in blue, green, red and infrared regions of spectra. In Figs. 4 and 5 normalized excitation and emission fluorescence spectra for the samples with different pH and temperature of reaction are presented. From Fig. 5, we can conclude that the photoluminescence emission peak shifts to longer wavelengths (red-shift) and that the photoluminescence intensity decreases as the pH and temperature of reaction increase. The photoluminescence emission process is induced by the surface defect recombination mechanism. It is notable that the CdS nanocrystals become bigger as the pH and temperature of reaction increase (according to the Scherrer equation D = kλ/β cos θ , where k is a constant (shape factor, about 0.9), λ is the Xray wavelength (1.5405 Å as mentioned before), β is the full width at half maximum (FWHM) of the diffraction line, and θ was the diffraction angle), so the photoluminescence intensity will decreases as the size of CdS nanocrystals increases. It is notable that the CdS nanocrystals become bigger as the pH and temperature of reaction increase, and the ratio of surface atoms to the total number of atoms in the CdS nanocrystals decreases, therefore, the photoluminescence intensity will decrease as the size of CdS nanocrystals increases [25]. The well-defined, technically attractive, blue and green emissions in the range of 420–540 nm are found to be of high intensity and broader and are attributed to the fundamental absorption in CdS. These emissions could be due to the band-edge related, and the surface defect state mediated recombination center in the conventional HOMOLUMO model. It is well known that surface defect states play important role in the luminescence properties of nanocrystals, which
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4. Conclusions
800 700
A1
In conclusion, the zinc blend and wurtzite structures of CdS nanocrystals with the size about 10–20 nm and strong blue and green photoluminescence were prepared directly via chemical bath deposition method. A wet-chemical synthetic route was taken to prepare CdS nanocrystalline thin films through the reaction of cadmium chloride dihydrate with thiourea. The resulting colloidal solution of CdS nanocrystals gave a bright photoluminescence emission at room temperature. Finally, it is mentionable that the CBD method may be extended to synthesis other inorganic materials such as ZnS, ZnO, CdO, TiO2 etc.
Intensity
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References
Fig. 4. Excitation fluorescence spectra for samples: A1, A2, A3, A4 and A5.
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Fig. 5. Emission fluorescence spectra for samples: A1, A2, A3, A4 and A5.
act as radiative or nonradiative centers [26]. The broadening of these peaks can be attributed to the inhomogeneities in the size and shape of the nanocrystallites (evident also from the TEM micrograph). Also, another peaks of relatively low intensity in the range of 610–750 nm bears the signature of the broad red emission (Lambe–Klick model) [27]. This could be due to the recombination of electrons trapped at sulfur (S) vacancies with valence-band-free holes. Nevertheless, the broad IR emission (related to e–h recombination trapped at the S and cadmium (Cd) vacancies, respectively) could not be verified in our samples because of the limit in the measurement of the PL apparatus.
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