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Synthesis and photoluminescent properties of doped ZnS nanocrystals capped by poly(vinylpyrrolidone)
Optical Materials 31 (2009) 1631–1635
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Synthesis and photoluminescent properties of doped ZnS nanocrystals capped by poly(vinylpyrrolidone) Michael W. Porambo, Anderson L. Marsh * Department of Chemistry, Lebanon Valley College, 101 N. College Ave., Annville, PA 17003, USA
a r t i c l e
i n f o
Article history: Received 3 July 2008 Received in revised form 17 February 2009 Accepted 18 March 2009 Available online 23 April 2009 PACS: 61.46.+w 61.72.Vv 78.55.m Keywords: Zinc sulfide Semiconductor nanocrystals Dopants Photoluminescence Poly(vinylpyrrolidone)
a b s t r a c t Zinc sulfide semiconductor nanocrystals doped with selected transition metal ions (Mn2+, Cu2+, and Ni2+) have been synthesized via a solution-based method utilizing low dopant concentrations (0–1%) and employing poly(vinylpyrrolidone) (PVP) as a capping agent. UV/Vis absorbance spectra for all of the synthesized nanocrystals show an exitonic peak at around 310 nm, indicating that the introduction of the dopant does not influence the particle size. Calculated particle sizes for undoped and doped nanocrystals are in the 4.3 nm size range. Photoluminescence spectra recorded for undoped ZnS nanocrystals, using an excitation wavelength of 310 nm, exhibit an emission peak centered at around 460 nm. When a dopant ion is included in the synthesis, peaks in the corresponding photoluminescence spectra are red-shifted. For Mn-doped nanocrystals, an intense peak centered at approximately 590 nm is found and is seen to increase in photoluminescence intensity with an increase in dopant concentration. In contrast, for Cudoped and Ni-doped nanocrystals, weaker peaks centered at around 520 and 500 nm, respectively, are observed and are noticed to decrease in photoluminescence intensity with an increase in dopant concentration. These results clearly show that careful control of synthetic conditions must be employed in the synthesis of doped semiconductor nanocrystals in order to obtain materials with optimized properties. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor nanocrystals continue to receive much attention due to unique optical and electronic properties that are dependent on a variety of material properties, such as size, shape, and composition [1–5]. These optical and electronic properties arise due to quantum confinement resulting from the nanometer size of the particle. A variety of synthetic methods exist for controlling the size, shape, and composition of these semiconductor nanocrystals, allowing for the tailoring of properties specific to the desired application [6,7]. For nanocrystals prepared by solution-based chemical methods, a capping agent, which adsorbs to the nanocrystal surface, is generally added both to control the size of the nanocrystal and to prevent agglomeration of the synthesized crystals. These adsorbates have been shown to alter the electronic structure of the nanocrystals [4,5]. Additionally, metal ion dopants added during synthesis may modify the luminescent properties of these semiconductor nanocrystals [3]. The resultant properties have allowed the use of these nanocrystals in applications ranging from biological sensors to optical displays. Thus, the synthesis and characterization of semiconductor nanocrystals of various sizes and * Corresponding author. Tel.: +1 717 867 6149; fax: +1 717 867 6075. E-mail address: [email protected] (A.L. Marsh). 0925-3467/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2009.03.013
compositions and with different capping agents remains an active area of current research. More specifically, the synthesis and characterization of colloidal zinc sulfide semiconductor nanocrystals with selected optoelectronic properties have been carried out [8]. A range of ZnS nanocrystals doped with different transition metal and rare-earth metal ions have been prepared using a variety of synthetic methods. For example, with the Mn2+ ion as a dopant, the photoluminescence of the nanocrystal is red-shifted from the blue region of the visible spectrum to the orange region [9–12]. Based on further experimental findings, the authors suggest that the position of the dopant ion, on the surface versus incorporated in the nanocrystal lattice, influences the resulting photophysical properties [12]. In addition to dopants, capping agents with organic functional groups have also been shown to modify luminescence properties [13,14]. Enhanced photoluminescence has been reported for doped ZnS nanocrystals capped with poly(vinylpyrrolidone) (PVP) [13]. The authors suggest that efficient energy transfer occurs between the polymer functional group adsorbed at the surface and the dopant centers in the nanocrystal. This result indicates that PVP may prove to be a suitable capping agent for semiconductor nanocrystals, particularly those targeted for applications such as photocatalysis in aqueous systems. Different synthetic methods for producing PVPcapped, undoped ZnS nanocrystals have recently been reported
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[15–19]. However, studies on the effect of dopant concentration for PVP-capped ZnS nanocrystals have been limited [20,21]. In order to better understand synthetic conditions needed to prepare these PVP-capped nanocrystals with selected optoelectronic properties, experiments systematically varying the dopant concentration present during synthesis must be performed. In this work, we report on the synthesis and optoelectronic characterization (UV/Vis absorption and photoluminescence spectroscopy) for PVP-capped zinc sulfide semiconductor nanocrystals prepared using 0–1% concentration of selected transition metal ions dopants (Mn2+, Cu2+, and Ni2+). Analysis of particle sizes using peak positions in UV/Vis absorption spectra indicate that all synthesized nanocrystals are approximately 4.3 nm in size. Undoped ZnS nanocrystals exhibit an emission peak centered at around 460 nm in the photoluminescence spectrum, while doped ZnS peaks are red-shifted to higher wavelengths. The photoluminescence peak centered at around 460 nm decreases in intensity with increasing concentration of the dopant. Only Mn-doped nanocrystals display a strong peak corresponding to the dopant ion. These results signify that PVP-capped zinc sulfide semiconductor nanocrystals may be synthesized with various dopant compositions that modify their optoelectronic properties without changing particle sizes of the synthesized nanocrystals. 2. Experimental Zinc acetate dihydrate, copper (II) acetate monohydrate, manganese (II) acetate tetrahydrate, nickel (II) acetate tetrahydrate, sodium sulfide nonahydrate, and 55,000 MW poly(vinylpyrrolidone) (PVP) were purchased from Aldrich Chemical and used without further purification. Solutions of 1.0 M Zn(C2H3O2)2, 0.85 M Na2S, and 0.010 M Cu(C2H3O2)2, Mn(C2H3O2)2, and Ni(C2H3O2)2 were prepared in deionized water. The synthetic methods used in this work have been based on previously reported procedures [11– 13,20–27]. Undoped ZnS nanocrystals were synthesized following a simple double replacement reaction in aqueous solution:
ZnðC2 H3 O2 Þ2 þ Na2 S þ PVP ! PVP—ZnS þ NaC2 H3 O2 To 5.0 mL of a 1.0 M Zn(C2H3O2)2 solution, 0.55 g PVP was added and dissolved while stirring continuously. Next, 5.0 mL of 0.85 M Na2S was added, and a white precipitate immediately formed. Samples of the precipitate were centrifuged at 3500 RPM for 10 min, followed by decanting of the liquid and washing of the remaining solid. This procedure was repeated two more times, with no washing after the final centrifugation. The nanocrystals were then analyzed by UV–Vis absorption and photoluminescence emission spectroscopies. Doped nanocrystals were synthesized in a similar manner. The procedure as detailed in the above paragraph was followed with a modification for the addition of the dopant ion. After 0.55 g PVP was dissolved in 5.0 mL of the Zn(C2H3O2)2 solution, a known volume of one of the doping reagents (Cu(C2H3O2)2, Mn(C2H3O2)2, or Ni(C2H3O2)2) was added to produce a known concentration (0– 1%) of dopant. Dopant percentages reported in this work are based on Zn:dopant cation ratios present during synthesis, and not on analysis of the final precipitated products. All subsequent steps of the synthesis and purification were carried out as described. UV/Vis absorbance spectra were recorded using an HP 8452A diode array spectrophotometer. Photoluminescence emission spectra were obtained over the 330–800 nm wavelength range with a Shimadzu RF-5301 PC spectrofluorometer operating with an excitation wavelength set at 310 nm. Care was taken to ensure that all samples were of the same concentration when recording photoluminescence spectra.
3. Results and discussion The UV/Vis absorbance spectrum for freshly-prepared undoped ZnS nanocrystals capped by PVP is shown in Fig. 1. As observed in the figure, the sample exhibits an exitonic peak at around 310 nm. This absorption spectrum has been used to calculate an approximate size for these nanocrystals using the Brus equation [28]. Based on the peak position in the absorbance spectrum, the particle size of these nanocrystals has been calculated to be 4.3 nm. This value is close to that reported previously for PVP-capped ZnS nanocrystals [13], and slightly higher than that reported for ZnS nanocrystals synthesized with the same Zn:S concentration ratio but capped with sodium polyphosphate [22]. No reason has been found in the literature as to the slightly larger particle size resulting from the use of PVP as a capping agent, but we speculate that it is most likely due to the differences in coordination between the metal ions and PVP versus sodium polyphosphate. The larger polymer chain may coordinate a higher concentration of metal ions, which leads to the formation of slightly larger nanocrystals. Despite this larger particle size, using PVP as a capping agent for ZnS nanocrystals does lead to the formation of stable nanocrystals. Aging experiments (data not shown) conducted over a period of days clearly indicate no change in the exitonic peak position, suggesting that the PVP prevents the nanocrystals from aggregating in solution. The photoluminescent properties of these undoped ZnS semiconductor nanocrystals capped with PVP have also been studied. The photoluminescence spectrum of undoped nanocrystals is displayed in Fig. 2. With the sample excited by 310 nm light, the spectrum contained a broad photoluminescence emission peak centered at around 460 nm. This wavelength is within the range of wavelengths reported in the literature for ZnS nanocrystals [9,12,14,15,17,19,20,24,26,27] Sulfur vacancies in the lattice have been suggested to be responsible for this photoluminescence emission [9,12]. The small peak at around 620 nm is believed to be the second order Rayleigh scattering peak from the excitation light, while the origin of the peak at around 375 nm is currently unknown. Both of these peaks appear in the photoluminescence emission spectra for doped nanocrystals as well. No emission bands resulting from the PVP capping agent are believed to be present, because of the selection of 310 nm as the excitation wave-
Fig. 1. UV/Vis absorbance spectrum for undoped PVP-capped ZnS nanocrystals.
M.W. Porambo, A.L. Marsh / Optical Materials 31 (2009) 1631–1635
Fig. 2. Photoluminescence emission spectrum of undoped PVP-capped ZnS nanocrystals.
length [13]. With an understanding of the optoelectronic properties of the undoped ZnS nanocrystals now established, next the effect of the addition of dopant ions during the synthesis will be examined. Absorbance spectra have been recorded for all of the doped ZnS nanocrystals (data not shown). All doped samples exhibited an absorbance peak at approximately the same wavelength observed for the undoped samples, indicating that at these concentrations doping does not affect the size of the nanocrystals. This result is in accord with those observed previously for ZnS nanocrystals synthesized with Mn2+, Cu2+, and Ni2+ dopants at similar concentrations [11,12,17,19,22,24,26,27]. Because of the relative similarity in size of the dopant ions to that of Zn2+, as well as the small concentrations used in doping, there is little to no change in the lattice size within the nanocrystal. As a result, the size of the nanocrystal stays relatively constant over the range of dopants and dopant concentrations used in this work. The addition of dopant ions in the ZnS crystal structure, however, changes the photoluminescent emission properties of the nanocrystals. Photoluminescence spectra for Mn-doped ZnS nanocrystals capped by PVP may be seen in Fig. 3. As observed in the figure, while the Mn-doped nanocrystals retain the photoluminescence peak of an undoped sample at 460 nm, the Mn dopant adds a very intense peak centered at around 590 nm. This finding is consistent with those reported previously for Mndoped ZnS nanocrystals [9–12,21,23], with the emission centered at around 590 nm due to the 4T1–6A1 transition in Mn2+ [12]. The spectra show that as the concentration of Mn2+ present during synthesis is increased, the intensity of the 590 nm peak is increased while the intensity of the 460 nm peak is decreased, further confirming the peak assignments. For PVP-capped ZnS nanocrystals doped with Mn in much larger concentrations (10–40%), no blue emission was observed in the spectrum [21]. In the current work, however, the blue emission (peak centered at around 460 nm) is still present due to the low concentration of dopant used. Newly-synthesized Cu-doped nanocrystals continue this observed shift in photoluminescence, but at a somewhat weaker intensity. Photoluminescence spectra for Cu-doped ZnS nanocrystals capped by PVP may be viewed in Fig. 4. As found in the figure, a weak peak centered at around 520 nm does emerge due to the Cu2+ dopant, as reported previously in the literature [24–26]. This
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Fig. 3. Photoluminescence emission spectra for PVP-capped ZnS nanocrystals prepared using selected manganese dopant concentrations.
Fig. 4. Photoluminescence emission spectra for PVP-capped ZnS nanocrystals prepared using selected copper dopant concentrations.
transition has been suggested to occur between a sulfur vacancy and the t2 level of Cu2+ [26]. Although the sample still exhibits a peak centered at around 460 nm that is characteristic of undoped ZnS nanocrystals, this peak is almost non-existent at the higher dopant concentration. In addition, the overall peak intensity decreases as the concentration of the Cu2+ ion present during synthesis is increased. This observation is in contrast to one described in previously published work for similar dopant concentrations [26]. Still, the authors did note that higher dopant concentrations (>1%) lead to a decrease in photoluminescence intensity. Zinc sulfide nanocrystals doped with Ni2+ ions also show behavior similar to that for the Cu-doped nanocrystals described in the above paragraph. Photoluminescence spectra for Ni-doped ZnS nanocrystals capped by PVP are located in Fig. 5. As observed in the figure, the Ni-doped nanocrystals also exhibit a red-shifted peak centered at around 500 nm, but as with the Cu-doped nano-
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and 2 1025), decreasing the likelihood of the formation of MnS as a precipitate over the formation of Mn-doped ZnS nanocrystals. In the case of the Cu(II) ion, however, the Ksp value for CuS (8 1037) is much lower than any of the values for ZnS, increasing the probability for formation of CuS as a precipitate. The same may be said for the Ni(II) ion, where Ksp values (4 1020, 1.3 1025, and 3 1027) are similar in magnitude to those for ZnS. These differences in solubility product constants likely lead to the formation of other sulfide precipitates, and not ZnS nanocrystals. 4. Conclusions
Fig. 5. Photoluminescence emission spectra for PVP-capped ZnS nanocrystals prepared using selected nickel dopant concentrations.
crystals, the intensity is weak compared to that for the Mn-doped nanocrystals. At present in the literature, this 500-nm transition has been assigned to d–d optical transitions of Ni2+ centers in the nanocrystals [27]. As with Cu-doped nanocrystals, the overall photoluminescence intensity decreases with an increase in dopant concentrations, but to a constant level for the range of concentrations used in this work. Previous work for Ni-doped ZnS nanocrystals with low dopant concentrations has shown an increase in photoluminescence intensity for these nanocrystals compared to undoped nanocrystals, and then a decrease in photoluminescence intensity for higher dopant concentrations [27]. In light of the present results for undoped and doped ZnS nanocrystals capped with PVP, it is clear that synthetic conditions play a role in determining the resulting optoelectronic properties of the prepared nanocrystals. As discussed in the above paragraphs, Mn-doped ZnS nanocrystals capped with PVP display an increase in photoluminescence intensity with an increase in dopant ion concentration. In contrast, both Cu-doped and Ni-doped ZnS nanocrystals capped with PVP exhibit a decrease in photoluminescence intensity with an increase in dopant ion concentration. Quenching of the blue photoluminescence has been reported previously for mercaptoethanol-capped ZnS nanocrystals doped with Ni2+ and Fe2+ cations [29]. The authors suggest that Ni and Fe ions introduce a more positive charge in the lattice, relative to Zn ions, which contributes to the decrease in photoluminescence intensity. They also indicated that for Fe-doped nanocrystals, clustering of Fe ions in the lattice contributes to the quenching. In the case of Cu-doped nanocrystals, as mentioned previously, quenching was observed at higher dopant concentrations [26]. The authors proposed that the formation of CuS nanocrystals lead to the observed decrease in photoluminescence intensity. The precipitation of CuS nanocrystals could be due to the differences in solubility product constant (Ksp) values for the corresponding metal sulfides. In the work described here, we hypothesize that the observed decrease in photoluminescence intensity for Cu-doped and Nidoped ZnS nanocrystals is most likely the result of differences in solubility product constants [30], and not concentration quenching. In the case of the Mn(II) ion, Ksp values for MnS (3 1011 and 3 1014) are much higher than those for ZnS (3 1023
Zinc sulfide semiconductor nanocrystals doped with Mn2+, Cu2+, and Ni2+ have been synthesized in solution and capped with PVP. Results from UV/Vis absorption spectra show that the presence of the dopant does not alter the size of the nanocrystals. However, photoluminescence emission spectra clearly show that doped nanocrystals emit at higher wavelengths compared to the undoped ZnS nanocrystals. For Mn-doped nanocrystals, a broad peak at 590 nm increases in intensity with an increase in the concentration of the dopant ion. On the contrary, for both Cu-doped and Nidoped nanocrystals, weaker peaks centered at around 520 and 500 nm, respectively, decrease in intensity with an increase in the concentration of the dopant ion. Ongoing work is focusing on using these synthesized nanocrystals for photocatalytic removal of chlorinated phenols from aqueous solutions. Preliminary experiments clearly demonstrate much better performance for Mndoped ZnS nanocrystals as compared to Cu-doped or Ni-doped nanocrystals. Taken together, these results clearly suggest that the synthetic conditions for preparing PVP-capped ZnS nanocrystals with selected dopant compositions that alter the nanocrystal optoelectronic properties must be carefully controlled to optimize material performance. Acknowledgement The authors acknowledge support from the Neidig Endowed Research Fund of the Lebanon Valley College Department of Chemistry. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
M.L. Steigerwald, L.E. Brus, Acc. Chem. Res. 23 (1990) 183. A.P. Alivisatos, J. Phys. Chem. 100 (1996) 13226. A.L. Efros, M. Rosen, Annu. Rev. Mater. Sci. 30 (2000) 475. F. Seker, K. Meeker, T.F. Kuech, A.B. Ellis, Chem. Rev. 100 (2000) 2505. M.A. El-Sayed, Acc. Chem. Res. 37 (2004) 326. C.B. Murray, C.R. Kagan, M.G. Bawendi, Annu. Rev. Mater. Sci. 30 (2000) 545. Y. Jun, J.-H. Lee, J. Choi, J. Cheon, J. Phys. Chem. B 109 (2005) 14795. H. Hu, W. Zhang, Opt. Mater. 28 (2006) 536. W.G. Becker, A.J. Bard, J. Phys. Chem. 87 (1983) 4888. R.N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416. A.A. Khosravi, M. Kundu, B.A. Kuruvilla, G.S. Shekhawat, R.P. Gupta, A.K. Sharma, P.D. Vyas, S.K. Kulkami, Appl. Phys. Lett. 67 (1995) 2506. K. Sooklal, B.S. Cullum, S.M. Angel, C.J. Murphy, J. Phys. Chem. 100 (1996) 4551. K. Manzoor, S.R. Vadera, N. Kumar, T.R.N. Kutty, Solid State Commun. 129 (2004) 469. X. Zhang, H. Song, L. Yu, T. Wang, X. Ren, X. Kong, Y. Xie, X. Wang, J. Lumin. 118 (2006) 251. R. He, X. Qian, J. Yin, H. Xi, L. Bian, Z. Zhu, Coll. Surf. A: Phys. Eng. Aspects 220 (2003) 151. L. Wang, X.-T. Tao, J.-X. Yang, Y. Ren, Z. Liu, M.-H. Jiang, Opt. Mater. 28 (2006) 1080. G. Ghosh, M.K. Naskar, A. Patra, M. Chatterjee, Opt. Mater. 28 (2006) 1047. R. Maity, U.N. Maiti, M.K. Mitra, K.K. Chattopadhyay, Physica E 33 (2006) 104. S.K. Panda, A. Datta, S. Chaudhuri, Chem. Phys. Lett. 440 (2007) 235. K. Manzoor, S.R. Vadera, N. Kumar, T.R.N. Kutty, Mater. Chem. Phys. 82 (2003) 718. N. Karar, F. Singh, B.R. Mehta, J. Appl. Phys. 95 (2004) 656. J. Chow, N.N. Dingra, E. Baker, B.C. Helmly, D.A. Nivens, W.E. Lynch, J. Photochem. Photobiol. A 173 (2005) 156. J.F. Suyver, S.F. Wuister, J.J. Kelly, A. Meijerink, Nano Lett. 1 (2001) 429.
M.W. Porambo, A.L. Marsh / Optical Materials 31 (2009) 1631–1635 [24] A.A. Khosravi, M. Kundu, L. Jatwa, S.K. Deshpande, U.A. Bhagwat, M. Sastry, S.K. Kulkarni, Appl. Phys. Lett. 67 (1995) 2702. [25] A.A. Bol, J. Ferwerda, J.A. Bergwerff, A. Meijerink, J. Lumin. 99 (2002) 325. [26] W.Q. Peng, G.W. Cong, S.C. Qu, Z.G. Wang, Opt. Mater. 29 (2006) 313. [27] P. Yang, M. Lü, D. Xü, D. Yuan, J. Chang, G. Zhou, M. Pan, Appl. Phys. A 74 (2002) 257.
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[28] L. Brus, J. Phys. Chem. 90 (1986) 2555. [29] P.H. Borse, N. Deshmukh, R.F. Shinde, S.K. Date, S.K. Kulkarni, J. Mater. Sci. 34 (1999) 6087. [30] D.C. Harris, Quantitative Chemical Analysis, seventh ed., W.H. Freeman, New York, 2007. pp. AP9–AP10.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis and photoluminescent properties of doped ZnS nanocrystals capped by poly(vinylpyrrolidone) Michael W. Porambo, Anderson L. Marsh * Department of Chemistry, Lebanon Valley College, 101 N. College Ave., Annville, PA 17003, USA
a r t i c l e
i n f o
Article history: Received 3 July 2008 Received in revised form 17 February 2009 Accepted 18 March 2009 Available online 23 April 2009 PACS: 61.46.+w 61.72.Vv 78.55.m Keywords: Zinc sulfide Semiconductor nanocrystals Dopants Photoluminescence Poly(vinylpyrrolidone)
a b s t r a c t Zinc sulfide semiconductor nanocrystals doped with selected transition metal ions (Mn2+, Cu2+, and Ni2+) have been synthesized via a solution-based method utilizing low dopant concentrations (0–1%) and employing poly(vinylpyrrolidone) (PVP) as a capping agent. UV/Vis absorbance spectra for all of the synthesized nanocrystals show an exitonic peak at around 310 nm, indicating that the introduction of the dopant does not influence the particle size. Calculated particle sizes for undoped and doped nanocrystals are in the 4.3 nm size range. Photoluminescence spectra recorded for undoped ZnS nanocrystals, using an excitation wavelength of 310 nm, exhibit an emission peak centered at around 460 nm. When a dopant ion is included in the synthesis, peaks in the corresponding photoluminescence spectra are red-shifted. For Mn-doped nanocrystals, an intense peak centered at approximately 590 nm is found and is seen to increase in photoluminescence intensity with an increase in dopant concentration. In contrast, for Cudoped and Ni-doped nanocrystals, weaker peaks centered at around 520 and 500 nm, respectively, are observed and are noticed to decrease in photoluminescence intensity with an increase in dopant concentration. These results clearly show that careful control of synthetic conditions must be employed in the synthesis of doped semiconductor nanocrystals in order to obtain materials with optimized properties. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor nanocrystals continue to receive much attention due to unique optical and electronic properties that are dependent on a variety of material properties, such as size, shape, and composition [1–5]. These optical and electronic properties arise due to quantum confinement resulting from the nanometer size of the particle. A variety of synthetic methods exist for controlling the size, shape, and composition of these semiconductor nanocrystals, allowing for the tailoring of properties specific to the desired application [6,7]. For nanocrystals prepared by solution-based chemical methods, a capping agent, which adsorbs to the nanocrystal surface, is generally added both to control the size of the nanocrystal and to prevent agglomeration of the synthesized crystals. These adsorbates have been shown to alter the electronic structure of the nanocrystals [4,5]. Additionally, metal ion dopants added during synthesis may modify the luminescent properties of these semiconductor nanocrystals [3]. The resultant properties have allowed the use of these nanocrystals in applications ranging from biological sensors to optical displays. Thus, the synthesis and characterization of semiconductor nanocrystals of various sizes and * Corresponding author. Tel.: +1 717 867 6149; fax: +1 717 867 6075. E-mail address: [email protected] (A.L. Marsh). 0925-3467/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2009.03.013
compositions and with different capping agents remains an active area of current research. More specifically, the synthesis and characterization of colloidal zinc sulfide semiconductor nanocrystals with selected optoelectronic properties have been carried out [8]. A range of ZnS nanocrystals doped with different transition metal and rare-earth metal ions have been prepared using a variety of synthetic methods. For example, with the Mn2+ ion as a dopant, the photoluminescence of the nanocrystal is red-shifted from the blue region of the visible spectrum to the orange region [9–12]. Based on further experimental findings, the authors suggest that the position of the dopant ion, on the surface versus incorporated in the nanocrystal lattice, influences the resulting photophysical properties [12]. In addition to dopants, capping agents with organic functional groups have also been shown to modify luminescence properties [13,14]. Enhanced photoluminescence has been reported for doped ZnS nanocrystals capped with poly(vinylpyrrolidone) (PVP) [13]. The authors suggest that efficient energy transfer occurs between the polymer functional group adsorbed at the surface and the dopant centers in the nanocrystal. This result indicates that PVP may prove to be a suitable capping agent for semiconductor nanocrystals, particularly those targeted for applications such as photocatalysis in aqueous systems. Different synthetic methods for producing PVPcapped, undoped ZnS nanocrystals have recently been reported
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[15–19]. However, studies on the effect of dopant concentration for PVP-capped ZnS nanocrystals have been limited [20,21]. In order to better understand synthetic conditions needed to prepare these PVP-capped nanocrystals with selected optoelectronic properties, experiments systematically varying the dopant concentration present during synthesis must be performed. In this work, we report on the synthesis and optoelectronic characterization (UV/Vis absorption and photoluminescence spectroscopy) for PVP-capped zinc sulfide semiconductor nanocrystals prepared using 0–1% concentration of selected transition metal ions dopants (Mn2+, Cu2+, and Ni2+). Analysis of particle sizes using peak positions in UV/Vis absorption spectra indicate that all synthesized nanocrystals are approximately 4.3 nm in size. Undoped ZnS nanocrystals exhibit an emission peak centered at around 460 nm in the photoluminescence spectrum, while doped ZnS peaks are red-shifted to higher wavelengths. The photoluminescence peak centered at around 460 nm decreases in intensity with increasing concentration of the dopant. Only Mn-doped nanocrystals display a strong peak corresponding to the dopant ion. These results signify that PVP-capped zinc sulfide semiconductor nanocrystals may be synthesized with various dopant compositions that modify their optoelectronic properties without changing particle sizes of the synthesized nanocrystals. 2. Experimental Zinc acetate dihydrate, copper (II) acetate monohydrate, manganese (II) acetate tetrahydrate, nickel (II) acetate tetrahydrate, sodium sulfide nonahydrate, and 55,000 MW poly(vinylpyrrolidone) (PVP) were purchased from Aldrich Chemical and used without further purification. Solutions of 1.0 M Zn(C2H3O2)2, 0.85 M Na2S, and 0.010 M Cu(C2H3O2)2, Mn(C2H3O2)2, and Ni(C2H3O2)2 were prepared in deionized water. The synthetic methods used in this work have been based on previously reported procedures [11– 13,20–27]. Undoped ZnS nanocrystals were synthesized following a simple double replacement reaction in aqueous solution:
ZnðC2 H3 O2 Þ2 þ Na2 S þ PVP ! PVP—ZnS þ NaC2 H3 O2 To 5.0 mL of a 1.0 M Zn(C2H3O2)2 solution, 0.55 g PVP was added and dissolved while stirring continuously. Next, 5.0 mL of 0.85 M Na2S was added, and a white precipitate immediately formed. Samples of the precipitate were centrifuged at 3500 RPM for 10 min, followed by decanting of the liquid and washing of the remaining solid. This procedure was repeated two more times, with no washing after the final centrifugation. The nanocrystals were then analyzed by UV–Vis absorption and photoluminescence emission spectroscopies. Doped nanocrystals were synthesized in a similar manner. The procedure as detailed in the above paragraph was followed with a modification for the addition of the dopant ion. After 0.55 g PVP was dissolved in 5.0 mL of the Zn(C2H3O2)2 solution, a known volume of one of the doping reagents (Cu(C2H3O2)2, Mn(C2H3O2)2, or Ni(C2H3O2)2) was added to produce a known concentration (0– 1%) of dopant. Dopant percentages reported in this work are based on Zn:dopant cation ratios present during synthesis, and not on analysis of the final precipitated products. All subsequent steps of the synthesis and purification were carried out as described. UV/Vis absorbance spectra were recorded using an HP 8452A diode array spectrophotometer. Photoluminescence emission spectra were obtained over the 330–800 nm wavelength range with a Shimadzu RF-5301 PC spectrofluorometer operating with an excitation wavelength set at 310 nm. Care was taken to ensure that all samples were of the same concentration when recording photoluminescence spectra.
3. Results and discussion The UV/Vis absorbance spectrum for freshly-prepared undoped ZnS nanocrystals capped by PVP is shown in Fig. 1. As observed in the figure, the sample exhibits an exitonic peak at around 310 nm. This absorption spectrum has been used to calculate an approximate size for these nanocrystals using the Brus equation [28]. Based on the peak position in the absorbance spectrum, the particle size of these nanocrystals has been calculated to be 4.3 nm. This value is close to that reported previously for PVP-capped ZnS nanocrystals [13], and slightly higher than that reported for ZnS nanocrystals synthesized with the same Zn:S concentration ratio but capped with sodium polyphosphate [22]. No reason has been found in the literature as to the slightly larger particle size resulting from the use of PVP as a capping agent, but we speculate that it is most likely due to the differences in coordination between the metal ions and PVP versus sodium polyphosphate. The larger polymer chain may coordinate a higher concentration of metal ions, which leads to the formation of slightly larger nanocrystals. Despite this larger particle size, using PVP as a capping agent for ZnS nanocrystals does lead to the formation of stable nanocrystals. Aging experiments (data not shown) conducted over a period of days clearly indicate no change in the exitonic peak position, suggesting that the PVP prevents the nanocrystals from aggregating in solution. The photoluminescent properties of these undoped ZnS semiconductor nanocrystals capped with PVP have also been studied. The photoluminescence spectrum of undoped nanocrystals is displayed in Fig. 2. With the sample excited by 310 nm light, the spectrum contained a broad photoluminescence emission peak centered at around 460 nm. This wavelength is within the range of wavelengths reported in the literature for ZnS nanocrystals [9,12,14,15,17,19,20,24,26,27] Sulfur vacancies in the lattice have been suggested to be responsible for this photoluminescence emission [9,12]. The small peak at around 620 nm is believed to be the second order Rayleigh scattering peak from the excitation light, while the origin of the peak at around 375 nm is currently unknown. Both of these peaks appear in the photoluminescence emission spectra for doped nanocrystals as well. No emission bands resulting from the PVP capping agent are believed to be present, because of the selection of 310 nm as the excitation wave-
Fig. 1. UV/Vis absorbance spectrum for undoped PVP-capped ZnS nanocrystals.
M.W. Porambo, A.L. Marsh / Optical Materials 31 (2009) 1631–1635
Fig. 2. Photoluminescence emission spectrum of undoped PVP-capped ZnS nanocrystals.
length [13]. With an understanding of the optoelectronic properties of the undoped ZnS nanocrystals now established, next the effect of the addition of dopant ions during the synthesis will be examined. Absorbance spectra have been recorded for all of the doped ZnS nanocrystals (data not shown). All doped samples exhibited an absorbance peak at approximately the same wavelength observed for the undoped samples, indicating that at these concentrations doping does not affect the size of the nanocrystals. This result is in accord with those observed previously for ZnS nanocrystals synthesized with Mn2+, Cu2+, and Ni2+ dopants at similar concentrations [11,12,17,19,22,24,26,27]. Because of the relative similarity in size of the dopant ions to that of Zn2+, as well as the small concentrations used in doping, there is little to no change in the lattice size within the nanocrystal. As a result, the size of the nanocrystal stays relatively constant over the range of dopants and dopant concentrations used in this work. The addition of dopant ions in the ZnS crystal structure, however, changes the photoluminescent emission properties of the nanocrystals. Photoluminescence spectra for Mn-doped ZnS nanocrystals capped by PVP may be seen in Fig. 3. As observed in the figure, while the Mn-doped nanocrystals retain the photoluminescence peak of an undoped sample at 460 nm, the Mn dopant adds a very intense peak centered at around 590 nm. This finding is consistent with those reported previously for Mndoped ZnS nanocrystals [9–12,21,23], with the emission centered at around 590 nm due to the 4T1–6A1 transition in Mn2+ [12]. The spectra show that as the concentration of Mn2+ present during synthesis is increased, the intensity of the 590 nm peak is increased while the intensity of the 460 nm peak is decreased, further confirming the peak assignments. For PVP-capped ZnS nanocrystals doped with Mn in much larger concentrations (10–40%), no blue emission was observed in the spectrum [21]. In the current work, however, the blue emission (peak centered at around 460 nm) is still present due to the low concentration of dopant used. Newly-synthesized Cu-doped nanocrystals continue this observed shift in photoluminescence, but at a somewhat weaker intensity. Photoluminescence spectra for Cu-doped ZnS nanocrystals capped by PVP may be viewed in Fig. 4. As found in the figure, a weak peak centered at around 520 nm does emerge due to the Cu2+ dopant, as reported previously in the literature [24–26]. This
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Fig. 3. Photoluminescence emission spectra for PVP-capped ZnS nanocrystals prepared using selected manganese dopant concentrations.
Fig. 4. Photoluminescence emission spectra for PVP-capped ZnS nanocrystals prepared using selected copper dopant concentrations.
transition has been suggested to occur between a sulfur vacancy and the t2 level of Cu2+ [26]. Although the sample still exhibits a peak centered at around 460 nm that is characteristic of undoped ZnS nanocrystals, this peak is almost non-existent at the higher dopant concentration. In addition, the overall peak intensity decreases as the concentration of the Cu2+ ion present during synthesis is increased. This observation is in contrast to one described in previously published work for similar dopant concentrations [26]. Still, the authors did note that higher dopant concentrations (>1%) lead to a decrease in photoluminescence intensity. Zinc sulfide nanocrystals doped with Ni2+ ions also show behavior similar to that for the Cu-doped nanocrystals described in the above paragraph. Photoluminescence spectra for Ni-doped ZnS nanocrystals capped by PVP are located in Fig. 5. As observed in the figure, the Ni-doped nanocrystals also exhibit a red-shifted peak centered at around 500 nm, but as with the Cu-doped nano-
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and 2 1025), decreasing the likelihood of the formation of MnS as a precipitate over the formation of Mn-doped ZnS nanocrystals. In the case of the Cu(II) ion, however, the Ksp value for CuS (8 1037) is much lower than any of the values for ZnS, increasing the probability for formation of CuS as a precipitate. The same may be said for the Ni(II) ion, where Ksp values (4 1020, 1.3 1025, and 3 1027) are similar in magnitude to those for ZnS. These differences in solubility product constants likely lead to the formation of other sulfide precipitates, and not ZnS nanocrystals. 4. Conclusions
Fig. 5. Photoluminescence emission spectra for PVP-capped ZnS nanocrystals prepared using selected nickel dopant concentrations.
crystals, the intensity is weak compared to that for the Mn-doped nanocrystals. At present in the literature, this 500-nm transition has been assigned to d–d optical transitions of Ni2+ centers in the nanocrystals [27]. As with Cu-doped nanocrystals, the overall photoluminescence intensity decreases with an increase in dopant concentrations, but to a constant level for the range of concentrations used in this work. Previous work for Ni-doped ZnS nanocrystals with low dopant concentrations has shown an increase in photoluminescence intensity for these nanocrystals compared to undoped nanocrystals, and then a decrease in photoluminescence intensity for higher dopant concentrations [27]. In light of the present results for undoped and doped ZnS nanocrystals capped with PVP, it is clear that synthetic conditions play a role in determining the resulting optoelectronic properties of the prepared nanocrystals. As discussed in the above paragraphs, Mn-doped ZnS nanocrystals capped with PVP display an increase in photoluminescence intensity with an increase in dopant ion concentration. In contrast, both Cu-doped and Ni-doped ZnS nanocrystals capped with PVP exhibit a decrease in photoluminescence intensity with an increase in dopant ion concentration. Quenching of the blue photoluminescence has been reported previously for mercaptoethanol-capped ZnS nanocrystals doped with Ni2+ and Fe2+ cations [29]. The authors suggest that Ni and Fe ions introduce a more positive charge in the lattice, relative to Zn ions, which contributes to the decrease in photoluminescence intensity. They also indicated that for Fe-doped nanocrystals, clustering of Fe ions in the lattice contributes to the quenching. In the case of Cu-doped nanocrystals, as mentioned previously, quenching was observed at higher dopant concentrations [26]. The authors proposed that the formation of CuS nanocrystals lead to the observed decrease in photoluminescence intensity. The precipitation of CuS nanocrystals could be due to the differences in solubility product constant (Ksp) values for the corresponding metal sulfides. In the work described here, we hypothesize that the observed decrease in photoluminescence intensity for Cu-doped and Nidoped ZnS nanocrystals is most likely the result of differences in solubility product constants [30], and not concentration quenching. In the case of the Mn(II) ion, Ksp values for MnS (3 1011 and 3 1014) are much higher than those for ZnS (3 1023
Zinc sulfide semiconductor nanocrystals doped with Mn2+, Cu2+, and Ni2+ have been synthesized in solution and capped with PVP. Results from UV/Vis absorption spectra show that the presence of the dopant does not alter the size of the nanocrystals. However, photoluminescence emission spectra clearly show that doped nanocrystals emit at higher wavelengths compared to the undoped ZnS nanocrystals. For Mn-doped nanocrystals, a broad peak at 590 nm increases in intensity with an increase in the concentration of the dopant ion. On the contrary, for both Cu-doped and Nidoped nanocrystals, weaker peaks centered at around 520 and 500 nm, respectively, decrease in intensity with an increase in the concentration of the dopant ion. Ongoing work is focusing on using these synthesized nanocrystals for photocatalytic removal of chlorinated phenols from aqueous solutions. Preliminary experiments clearly demonstrate much better performance for Mndoped ZnS nanocrystals as compared to Cu-doped or Ni-doped nanocrystals. Taken together, these results clearly suggest that the synthetic conditions for preparing PVP-capped ZnS nanocrystals with selected dopant compositions that alter the nanocrystal optoelectronic properties must be carefully controlled to optimize material performance. Acknowledgement The authors acknowledge support from the Neidig Endowed Research Fund of the Lebanon Valley College Department of Chemistry. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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