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Pb1−xFexS nanoparticle films grown from acidic chemical bath
Applied Surface Science 239 (2004) 1–4
Short communication
Pb1xFexS nanoparticle films grown from acidic chemical bath Rakesh K. Joshi*, G.V. Subbaraju, Renu Sharma, H.K. Sehgal Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Received in revised form 16 September 2003; accepted 26 March 2004 Available online 28 May 2004
Abstract Pb1xFexS (x ¼ 0:25, 0.50, 0.75) films were grown from an acidic chemical bath. Nanoparticle films were structurally characterized by XRD and TEM. Optical band gap of films is observed to vary from 1.65 to 1.42 eV with increase in their iron concentration from x ¼ 0:25 to 0.75 in the films. Increased optical band gap of the ternary films compared to the estimated bulk value is attributed to quantum confinement in the nanocrystals deposited on solid substrates. # 2004 Elsevier B.V. All rights reserved. PACS: 81.05.Hd; 81.05.Ys; 81.10.Dn Keywords: Ternary films; Solution growth
1. Introduction Growth of semiconductor nanocrystals on solid substrates is an emerging field of research to study the utility of a material in different technological applications. Several chemical methods have been reported for fabrication of nanostructured thin films. In chemical methods the growth of particles on substrates depends on dilution, pH and temperature of the chemical bath and it is also known to depend on the complexing agent used to control the growth of particles [1]. It is reported that the preparation of semiconductor thin films by chemical bath deposition in basic medium is based on utilization of ammonia as complexing agent for metal ions. Potassium or sodium hydroxides are also used to increase pH of the che-
*
Corresponding author. Tel.: þ91-11-26591349; fax: þ91-11-26581114. E-mail address: [email protected] (R.K. Joshi).
mical bath for the growth of semiconductors films [2]. We have also reported the growth of ternary Pb1xFexS nanopaticle films using a chemical bath with pH 9:5 [3]. Recently it is reported that the CdS nanocrystals can be grown from an ammonia-free alkaline chemical bath using nitrilotriacetic acid as a complexing agent for the Cd ions, which eliminates the problem of ammonia volatility and toxicity [2]. Therefore, in this context we made an attempt to grow nanoparticle thin films of ternary semiconductors without using an alkaline bath. In this paper we present, for the first time, synthesis of nanoparticle films of Pb1xFexS from an acidic chemical bath with ethylenediamine tetraacetic acid (EDTA) as complexing agent. Critical control of temperature and dilution of the bath are found necessary to achieve nanocrystallinity in the films. This method appears relatively easier than methods reported for the other semiconductor nanoparticle films. Structural and optical properties of the nanoparticle films grown from acidic chemical bath are discussed in this paper.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.240
2
R.K. Joshi et al. / Applied Surface Science 239 (2004) 1–4
2. Experimental The films were grown on glass and quartz substrates using M/25 lead acetate, M/20 thiourea and M/25 ferrous chloride aqueous solutions. These concentrations were observed to be appropriate for growth of homogeneous films. Lead acetate and thiourea were mixed in equal proportions and an appropriate quantity of FeCl2 was used to obtain x ¼ 0:25, 0.50, 0.75 in the films. EDTA, used as complexing agent for the metal ions, was mixed so as to achieve pH of 6.0 (0.1) of the bath solution. Cleaned glass, quartz and silicon substrates were suspended vertically in the solution and the temperature of the bath was maintained at 70 8C (0.5 8C). The solution was stirred continuously during growth. Growth of films proceeds due to thermally generated S2 ions in the solution bath. Presence of EDTA provides better adhesion and quality to the films. Deposition times were optimized to get film thickness of 70 nm for all values of x. Maximum thickness limit of the films obtained from a single growth step is observed to be less (90 nm) from the pH of 6.0 acidic bath as compared to those (250 nm) from pH of 9.5 from the alkaline bath. This could possibly be due to slow release of metal ions from the EDTA complex. Compositions of films were estimated from X-ray fluorescence spectroscopy data. Films were structurally characterized by glancing angle X-ray diffraction. Average grain size in each film was estimated by transmission electron microscope (TEM) in plane-view (PVTEM) mode, using Philips CM20 instrument operating at 200 keV. Optical transmittance (T) and reflectance (R) of the films were measured by UV-Vis-NIR spectrophotometer.
Fig. 1. XRD traces for Pb1xFexS nanoparticle films: (a) x ¼ 0:25, (b) x ¼ 0:50, and (c) x ¼ 0:75.
˚ ). Observed increase in peak that of Pb2þ ions (1.20 A broadening in the XRD traces with increase in iron concentrations (x) in the films indicates decrease in particle size in the films. Average grain sizes estimated from peak broadening using Scherer equation [4] are observed to be 30, 24 and 21 nm in the x of 0.25, 0.50, 0.75 Pb1xFexS films respectively. Average grain sizes in the films estimated by TEM (33, 26 and 24 nm in the x of 0.25, 0.50 and 0.75 Pb1xFexS films respectively) agree well with those computed from XRD data. Typical bright-field image of the PVTEM taken
3. Results and discussion Analysis of the XRD data from the films indicates the presence of a single-phase ternary alloy of the type Pb1xFexS in the films. Fig. 1 shows X-ray diffraction (XRD) trace for the Pb1xFexS (x ¼ 0:25, 0.50, 0.75) films. All XRD peaks are observed to shift towards higher value of y resulting from the smaller lattice parameters for films with higher iron concentrations. Decrease in lattice parameter with x attributed to the ˚ ) in comparison to smaller ionic radii of Fe2þ (0.74 A
Fig. 2. TEM micrograph for Pb0.5Fe0.5S nanoparticle film.
R.K. Joshi et al. / Applied Surface Science 239 (2004) 1–4
3
(cm1) for the films. Fig. 3a shows the typical variation of reflectance and transmittance with wavelength for the Pb0.75Fe0.25S films. (ahn)n versus hn plots for n ¼ 2, 1/2, and 1/3 show linear behavior for n ¼ 2, only indicating presence of direct optical band gaps in films with all x. Fig. 3b shows hn versus (ahn)2 plots for the x of 0.25, 0.50, 0.75 Pb1xFexS films. Optical
with the electron beam direction close to the (1 1 0)zone axis in a strongly underfocused conditions for films with iron concentration x ¼ 0:50 is shown in Fig. 2. Optical transmittance (T) and reflectance (R) of the films measured by UV-Vis-NIR spectrophotometer were used to estimate the absorption coefficient a 50
Reflectance (R)
40
% R& T
Transmittance (T) 30
20
10
0 500
1000
1500
2000
2500
3000
Wavelength(nm)
(a) 13
1.0x10
12
x=0.25 x= 0.50 x=0.75
(αhν)2(eV/cm)2
8.0x10
12
6.0x10
12
4.0x10
12
2.0x10
0.0 0.0
(b)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
hν(eV)
Fig. 3. (a) Reflectance and transmittance vs. wavelength for Pb0.75Fe0.25S nanoparticle films. (b) hn vs. (ahn)2 plot for the Pb1xFexS nanoparticle films: (&) x ¼ 0:25; (*) x ¼ 0:50; (*) x ¼ 0:75.
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R.K. Joshi et al. / Applied Surface Science 239 (2004) 1–4
band gap EgN of Pb1xFexS nanoparticle films was observed to decrease with an increase in iron concentration x in the films. Values of EgN for the x of 0.25, 0.50 and 0.75 films are observed to be 1.65, 1.51 and 1.42 eV respectively. Decrease in optical band gap of the ternary alloys with increase in iron concentration suggests alloying between FeS nanoparticles (bulk Eg ¼ 0:04 eV [5]) with PbS nanoparticles (bulk Eg ¼ 0:41 eV; 2 nm average grain size Eg ¼ 5:4 eV [6]) to form the ternary Pb1xFexS nanoparticles. The larger optical band gap in the Pb1xFexS nanoparticle films, like that reported for PbS nanoparticles [7–9] and CdSe, CdS, ZnSe, and ZnS nanoparticles [10], is attributed to the quantum confinement effect [10,11]. The ratio EgN/Eg, which suggests the extent of quantum size effect, is observed to increase with increase in iron concentration (x) in the nanoparticle films. The particle size effect on optical band gap is observed to be more pronounced in the ternary Pb1xFexS nanoparticle films in comparison to that reported for the PbS nanoparticle films with same crystallite size. A band gap of EgN 1.4 eV reported from electrodeposited PbS nanoparticles (24 nm) [8] is 3.4 times the band gap of bulk PbS (0.41 eV), whereas 24 nm average grain size (estimated by TEM) Pb0.25Fe0.75S films have an EgN of 1.42 eV which is 10.31 times the estimated bulk value (using Vegard’s law) of 0.137 eV. The experimental data from PbS nanoparticle films [8] interpreted on the basis of hyperbolic band model [7,11] using the expression Eg2 ðrÞ ¼ Eg2 þ
2 h2Eg ðp=rÞ2 m e
(where Eg is the band gap of for the bulk semiconductor, r the particle radius and m e the effective mass of electron) gives m e /me value of 5:8 104 (using Eg ðrÞ ¼ 1:4 eVand r ¼ 24 nm). The same model used for Pb0.25Fe0.75S nanoparticle films gives the value of m e /me of 1:7 104 . This extremely low values of m e /me could possibly be due to nonparabolicity of the conduction band, finite value of the barrier potential and difference between electron masses inside and outside the nanocrystals [12–14]. (ahn)2 versus hn curves in Fig. 3b do not show a normal Urbach tail on the lower energy side of the absorption threshold. The absorption band whose intensity increases with increase in iron concentration
in the films just next to the absorption threshold on the lower energy side is possibly due to excitons in the nanocrystals. Excitons have also been reported from CdSe [15], CdS, ZnSe and ZnS nanocrystallites [9].
4. Conclusions In summary, Pb1xFexS (x ¼ 0:25, 0.50, 0.75) films have been successfully grown from an acidic chemical bath. Optical band gap EgN of films can be varied from 1.65 to 1.42 eV with increase in iron concentration from x ¼ 0:25–0.75 in the films. Increase in optical band gap of the ternary films is attributed to quantum confinement in the nanocrystals.
Acknowledgements Authors are grateful to Dr. Aloke Kanjilal of Department of Physics and Astronomy, University of Aarhus, Denmark, for the TEM measurements. References [1] P. Neˇ mec, I. Neˇ mec, P. Naha´ lkova´ , K. Knı´eˇ ek, P. Maly´, J. Cryst. Growth 240 (2002) 484. [2] M.B. Ortun˜ o Lo´ pez, J.J. Valenzuela-Ja´ uregui, M. SoteloLerma, A. Mendoza-Galva´ n, R. Ramı´rez-Bon, Thin solid Film 403/404 (2002) 9. [3] R.K. Joshi, H.K. Sehgal, J. Cryst. Growth 247 (2003) 425. [4] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, 1967. [5] J.R. Gosselin, M.G. Townsend, R.J. Trembly, Solid State Commun. 19 (1976) 799. [6] R. Thielsch, T. Bo¨ hme, R. Reiche, D. Schla¨ fer, H.D. Bauer, H. Bo¨ ttcher. Nanostruct. Mater. 10 (1998) 131. [7] K.K. Nanda, F.E. Kruis, H. Fissan, M. Acet, J. Appl. Phys. 91 (2002) 2315. [8] K.K. Nanda, S.N. Sahu, Appl. Phys. Lett. 79 (2001) 2743. [9] R.K. Joshi, A. Kanjilal, H.K. Sehgal, Appl. Surface Sci. 221 (2004) 43. [10] N. Chestnoy, R. Hull, L.E. Brus, J. Chem. Phys. 85 (1986) 2237. [11] Y. Wang, A. Suna, W. Mahler, R. Kawoski, J. Chem. Phys. 87 (1987) 7315. [12] Y. Kayanumma, Momiji, Phys. Rev. B 41 (1990) 10261. [13] D.B. Tran Thoai, Y.Z. Hu, S.W. Koch, Phys. Rev. B 42 (1990) 11261. [14] P. Lavallard, J. Cryst. Growth 184 (1998) 352. [15] P.E. Lippens, M. Lannoo, Phys. Rev. B 41 (1990) 6079.
Short communication
Pb1xFexS nanoparticle films grown from acidic chemical bath Rakesh K. Joshi*, G.V. Subbaraju, Renu Sharma, H.K. Sehgal Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Received in revised form 16 September 2003; accepted 26 March 2004 Available online 28 May 2004
Abstract Pb1xFexS (x ¼ 0:25, 0.50, 0.75) films were grown from an acidic chemical bath. Nanoparticle films were structurally characterized by XRD and TEM. Optical band gap of films is observed to vary from 1.65 to 1.42 eV with increase in their iron concentration from x ¼ 0:25 to 0.75 in the films. Increased optical band gap of the ternary films compared to the estimated bulk value is attributed to quantum confinement in the nanocrystals deposited on solid substrates. # 2004 Elsevier B.V. All rights reserved. PACS: 81.05.Hd; 81.05.Ys; 81.10.Dn Keywords: Ternary films; Solution growth
1. Introduction Growth of semiconductor nanocrystals on solid substrates is an emerging field of research to study the utility of a material in different technological applications. Several chemical methods have been reported for fabrication of nanostructured thin films. In chemical methods the growth of particles on substrates depends on dilution, pH and temperature of the chemical bath and it is also known to depend on the complexing agent used to control the growth of particles [1]. It is reported that the preparation of semiconductor thin films by chemical bath deposition in basic medium is based on utilization of ammonia as complexing agent for metal ions. Potassium or sodium hydroxides are also used to increase pH of the che-
*
Corresponding author. Tel.: þ91-11-26591349; fax: þ91-11-26581114. E-mail address: [email protected] (R.K. Joshi).
mical bath for the growth of semiconductors films [2]. We have also reported the growth of ternary Pb1xFexS nanopaticle films using a chemical bath with pH 9:5 [3]. Recently it is reported that the CdS nanocrystals can be grown from an ammonia-free alkaline chemical bath using nitrilotriacetic acid as a complexing agent for the Cd ions, which eliminates the problem of ammonia volatility and toxicity [2]. Therefore, in this context we made an attempt to grow nanoparticle thin films of ternary semiconductors without using an alkaline bath. In this paper we present, for the first time, synthesis of nanoparticle films of Pb1xFexS from an acidic chemical bath with ethylenediamine tetraacetic acid (EDTA) as complexing agent. Critical control of temperature and dilution of the bath are found necessary to achieve nanocrystallinity in the films. This method appears relatively easier than methods reported for the other semiconductor nanoparticle films. Structural and optical properties of the nanoparticle films grown from acidic chemical bath are discussed in this paper.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.240
2
R.K. Joshi et al. / Applied Surface Science 239 (2004) 1–4
2. Experimental The films were grown on glass and quartz substrates using M/25 lead acetate, M/20 thiourea and M/25 ferrous chloride aqueous solutions. These concentrations were observed to be appropriate for growth of homogeneous films. Lead acetate and thiourea were mixed in equal proportions and an appropriate quantity of FeCl2 was used to obtain x ¼ 0:25, 0.50, 0.75 in the films. EDTA, used as complexing agent for the metal ions, was mixed so as to achieve pH of 6.0 (0.1) of the bath solution. Cleaned glass, quartz and silicon substrates were suspended vertically in the solution and the temperature of the bath was maintained at 70 8C (0.5 8C). The solution was stirred continuously during growth. Growth of films proceeds due to thermally generated S2 ions in the solution bath. Presence of EDTA provides better adhesion and quality to the films. Deposition times were optimized to get film thickness of 70 nm for all values of x. Maximum thickness limit of the films obtained from a single growth step is observed to be less (90 nm) from the pH of 6.0 acidic bath as compared to those (250 nm) from pH of 9.5 from the alkaline bath. This could possibly be due to slow release of metal ions from the EDTA complex. Compositions of films were estimated from X-ray fluorescence spectroscopy data. Films were structurally characterized by glancing angle X-ray diffraction. Average grain size in each film was estimated by transmission electron microscope (TEM) in plane-view (PVTEM) mode, using Philips CM20 instrument operating at 200 keV. Optical transmittance (T) and reflectance (R) of the films were measured by UV-Vis-NIR spectrophotometer.
Fig. 1. XRD traces for Pb1xFexS nanoparticle films: (a) x ¼ 0:25, (b) x ¼ 0:50, and (c) x ¼ 0:75.
˚ ). Observed increase in peak that of Pb2þ ions (1.20 A broadening in the XRD traces with increase in iron concentrations (x) in the films indicates decrease in particle size in the films. Average grain sizes estimated from peak broadening using Scherer equation [4] are observed to be 30, 24 and 21 nm in the x of 0.25, 0.50, 0.75 Pb1xFexS films respectively. Average grain sizes in the films estimated by TEM (33, 26 and 24 nm in the x of 0.25, 0.50 and 0.75 Pb1xFexS films respectively) agree well with those computed from XRD data. Typical bright-field image of the PVTEM taken
3. Results and discussion Analysis of the XRD data from the films indicates the presence of a single-phase ternary alloy of the type Pb1xFexS in the films. Fig. 1 shows X-ray diffraction (XRD) trace for the Pb1xFexS (x ¼ 0:25, 0.50, 0.75) films. All XRD peaks are observed to shift towards higher value of y resulting from the smaller lattice parameters for films with higher iron concentrations. Decrease in lattice parameter with x attributed to the ˚ ) in comparison to smaller ionic radii of Fe2þ (0.74 A
Fig. 2. TEM micrograph for Pb0.5Fe0.5S nanoparticle film.
R.K. Joshi et al. / Applied Surface Science 239 (2004) 1–4
3
(cm1) for the films. Fig. 3a shows the typical variation of reflectance and transmittance with wavelength for the Pb0.75Fe0.25S films. (ahn)n versus hn plots for n ¼ 2, 1/2, and 1/3 show linear behavior for n ¼ 2, only indicating presence of direct optical band gaps in films with all x. Fig. 3b shows hn versus (ahn)2 plots for the x of 0.25, 0.50, 0.75 Pb1xFexS films. Optical
with the electron beam direction close to the (1 1 0)zone axis in a strongly underfocused conditions for films with iron concentration x ¼ 0:50 is shown in Fig. 2. Optical transmittance (T) and reflectance (R) of the films measured by UV-Vis-NIR spectrophotometer were used to estimate the absorption coefficient a 50
Reflectance (R)
40
% R& T
Transmittance (T) 30
20
10
0 500
1000
1500
2000
2500
3000
Wavelength(nm)
(a) 13
1.0x10
12
x=0.25 x= 0.50 x=0.75
(αhν)2(eV/cm)2
8.0x10
12
6.0x10
12
4.0x10
12
2.0x10
0.0 0.0
(b)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
hν(eV)
Fig. 3. (a) Reflectance and transmittance vs. wavelength for Pb0.75Fe0.25S nanoparticle films. (b) hn vs. (ahn)2 plot for the Pb1xFexS nanoparticle films: (&) x ¼ 0:25; (*) x ¼ 0:50; (*) x ¼ 0:75.
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R.K. Joshi et al. / Applied Surface Science 239 (2004) 1–4
band gap EgN of Pb1xFexS nanoparticle films was observed to decrease with an increase in iron concentration x in the films. Values of EgN for the x of 0.25, 0.50 and 0.75 films are observed to be 1.65, 1.51 and 1.42 eV respectively. Decrease in optical band gap of the ternary alloys with increase in iron concentration suggests alloying between FeS nanoparticles (bulk Eg ¼ 0:04 eV [5]) with PbS nanoparticles (bulk Eg ¼ 0:41 eV; 2 nm average grain size Eg ¼ 5:4 eV [6]) to form the ternary Pb1xFexS nanoparticles. The larger optical band gap in the Pb1xFexS nanoparticle films, like that reported for PbS nanoparticles [7–9] and CdSe, CdS, ZnSe, and ZnS nanoparticles [10], is attributed to the quantum confinement effect [10,11]. The ratio EgN/Eg, which suggests the extent of quantum size effect, is observed to increase with increase in iron concentration (x) in the nanoparticle films. The particle size effect on optical band gap is observed to be more pronounced in the ternary Pb1xFexS nanoparticle films in comparison to that reported for the PbS nanoparticle films with same crystallite size. A band gap of EgN 1.4 eV reported from electrodeposited PbS nanoparticles (24 nm) [8] is 3.4 times the band gap of bulk PbS (0.41 eV), whereas 24 nm average grain size (estimated by TEM) Pb0.25Fe0.75S films have an EgN of 1.42 eV which is 10.31 times the estimated bulk value (using Vegard’s law) of 0.137 eV. The experimental data from PbS nanoparticle films [8] interpreted on the basis of hyperbolic band model [7,11] using the expression Eg2 ðrÞ ¼ Eg2 þ
2 h2Eg ðp=rÞ2 m e
(where Eg is the band gap of for the bulk semiconductor, r the particle radius and m e the effective mass of electron) gives m e /me value of 5:8 104 (using Eg ðrÞ ¼ 1:4 eVand r ¼ 24 nm). The same model used for Pb0.25Fe0.75S nanoparticle films gives the value of m e /me of 1:7 104 . This extremely low values of m e /me could possibly be due to nonparabolicity of the conduction band, finite value of the barrier potential and difference between electron masses inside and outside the nanocrystals [12–14]. (ahn)2 versus hn curves in Fig. 3b do not show a normal Urbach tail on the lower energy side of the absorption threshold. The absorption band whose intensity increases with increase in iron concentration
in the films just next to the absorption threshold on the lower energy side is possibly due to excitons in the nanocrystals. Excitons have also been reported from CdSe [15], CdS, ZnSe and ZnS nanocrystallites [9].
4. Conclusions In summary, Pb1xFexS (x ¼ 0:25, 0.50, 0.75) films have been successfully grown from an acidic chemical bath. Optical band gap EgN of films can be varied from 1.65 to 1.42 eV with increase in iron concentration from x ¼ 0:25–0.75 in the films. Increase in optical band gap of the ternary films is attributed to quantum confinement in the nanocrystals.
Acknowledgements Authors are grateful to Dr. Aloke Kanjilal of Department of Physics and Astronomy, University of Aarhus, Denmark, for the TEM measurements. References [1] P. Neˇ mec, I. Neˇ mec, P. Naha´ lkova´ , K. Knı´eˇ ek, P. Maly´, J. Cryst. Growth 240 (2002) 484. [2] M.B. Ortun˜ o Lo´ pez, J.J. Valenzuela-Ja´ uregui, M. SoteloLerma, A. Mendoza-Galva´ n, R. Ramı´rez-Bon, Thin solid Film 403/404 (2002) 9. [3] R.K. Joshi, H.K. Sehgal, J. Cryst. Growth 247 (2003) 425. [4] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, 1967. [5] J.R. Gosselin, M.G. Townsend, R.J. Trembly, Solid State Commun. 19 (1976) 799. [6] R. Thielsch, T. Bo¨ hme, R. Reiche, D. Schla¨ fer, H.D. Bauer, H. Bo¨ ttcher. Nanostruct. Mater. 10 (1998) 131. [7] K.K. Nanda, F.E. Kruis, H. Fissan, M. Acet, J. Appl. Phys. 91 (2002) 2315. [8] K.K. Nanda, S.N. Sahu, Appl. Phys. Lett. 79 (2001) 2743. [9] R.K. Joshi, A. Kanjilal, H.K. Sehgal, Appl. Surface Sci. 221 (2004) 43. [10] N. Chestnoy, R. Hull, L.E. Brus, J. Chem. Phys. 85 (1986) 2237. [11] Y. Wang, A. Suna, W. Mahler, R. Kawoski, J. Chem. Phys. 87 (1987) 7315. [12] Y. Kayanumma, Momiji, Phys. Rev. B 41 (1990) 10261. [13] D.B. Tran Thoai, Y.Z. Hu, S.W. Koch, Phys. Rev. B 42 (1990) 11261. [14] P. Lavallard, J. Cryst. Growth 184 (1998) 352. [15] P.E. Lippens, M. Lannoo, Phys. Rev. B 41 (1990) 6079.