- Email: [email protected]
Microwave assisted synthesis of ZnS quantum dots using ionic liquids
Materials Letters 89 (2012) 316–319
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Microwave assisted synthesis of ZnS quantum dots using ionic liquids Robina Shahid a,n, Mikhail Gorlov b,nn, Ramy El-Sayed a, Muhammet S. Toprak a, Abhilash Sugunan a, Lars Kloo b, Mamoun Muhammed a a b
Division of Functional Materials, Department of Materials and Nanophysics, KTH Royal Institute of Technology, Stockholm, Sweden Applied Physical Chemistry, Department of Chemistry, KTH Royal Institute of Technology, Stockholm, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 August 2012 Accepted 31 August 2012 Available online 7 September 2012
In this work we report results from microwave (MW) assisted synthesis of highly crystalline ZnS quantum dots (QDs) using ionic liquid (ILs) as MW absorbing medium. Two types of ionic liquids, imidazolium and phosphonium based, were used. The QDs are less than 5 nm in size and of wurtzite ZnS type, as characterized by high-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) pattern. The optical properties were investigated by UV–vis absorption and show a blue shift in absorption as compared to bulk wurtzite ZnS due to quantum confinement effects. The photoluminescence (PL) spectra of the QDs show different trap state emissions. & 2012 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline materials Semiconductors Chemical synthesis Electron microscopy Crystal structure Optical properties
1. Introduction Microwave irradiation (MWI) is an emerging efficient heating method, where a dielectric heating mechanism involving dipolar polarization and ionic conduction allows faster reactions with higher yields and higher purities without high vacuum requirements. Furthermore, MWI methods are unique in providing upscaled processes without suffering unwanted thermal gradient effects, thus leading to a potentially industrially important advancement in the large-scale production of nanomaterials [1]. Ionic liquids (ILs) are recognized as ‘green’ alternative media with respect to volatile and often toxic organic solvents. ILs unusual properties, such as high polarity, negligible vapor pressure, high ionic conductivity, good solvent properties for both organic compounds and electrolytes and high thermal stability. These properties make them attractive as environmentally benign solvents for organic chemical reactions, polymer synthesis, chemical separations, and electrochemical applications [2]. ILs typically consists of an organic and rather bulky cation and a small inorganic anion. ILs are normally classified according to the organic cation. Two major classes of ionic liquids are based on the phosphonium or imidazolium cations together with a variety of anions ranging from simple anions, such as halides, to more complex ones, such as bis(trifluoromethanesulfonyl)amide [3].
n
Corresponding author. Tel.: þ468 790 8157; fax: þ 468 790 9072. Corresponding author. Tel.: þ 468 790 8125; fax: þ 468 21 2626. E-mail addresses: [email protected] (R. Shahid), [email protected] (M. Gorlov).
nn
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.143
From the perspective of MW chemistry, one of the important advantages of ILs is their high polarizability making them good media for absorbing MWs [4]. Therefore the MW assisted IL method enable ‘‘green’’ or more sustainable synthetic routes for the preparation of nanoparticles [5]. The microwave assisted ionic liquid (MAIL) method has been used for the synthesis of metal, [6–8] metal oxide [9–11], metal fluoride [12], metal sulfide [13] and semiconductor nanoparticles [14]. However, there are few reports on the synthesis of QDs [15] by the (MAIL) method. Therefore, in this work we report on the synthesis of ZnS QDs using MW irradiation as heating source in controlled reactions with ILs as MW absorbing medium.
2. Experimental Two MW assisted ionlic liquid (MAIL) synthesis systems were developed; first method denoted as MAIL-I involving inorganic precursors and the IL-1, secondly, a method denoted MAIL-O, in which organic precursors and three ILs, IL-1, IL-2 and IL-3 were used. The Molecular structure and the constituting ions of three ILs are shown in Table 1. For the first system, MAIL-I, the inorganic salt ZnCl2 and sulfur powder were used as Zn and S sources, respectively. Equimolar quantities of ZnCl2 and sulfur powder were added to 5 mL of IL-1. The solution was heated to 250 1C under MW irradiation under stirring for different reaction times ranging from 1 to 10 min. Subsequently, ethanol was added to precipitate the QDs from the
R. Shahid et al. / Materials Letters 89 (2012) 316–319
317
Table 1 Molecular structure and constituting ions of ILs used in the MWA synthesis of QDs. IL-1
IL-2
IL-3
Fig. 1. (a) TEM image of sample after 1 min reaction time made by method MAIL-1. Inset is image of one of the rod. (b) FTIR spectra of IL and nanoparticles synthesized at 60 min.
cooled solution. The product was then centrifuged and re-dispersed in ethanol. In the second system, MAIL-O, the organic compounds Zn-oleate and TOP-S (see below) were used as Zn and S sources, respectively. Zn-oleate and TOP-S stock solutions were prepared as follows: 0.407 g ZnO was added to 10 mL of oleic acid, in a MW heating vial of 10–20 mL volume and heated at 250 1C for 2 h, in order to obtain zinc oleate (Zn-oleate). Trioctylphosphine sulfide (TOP-S) was obtained by adding 0.16 g sulfur powder to 10 mL of TOP and the solution was then stirred at room temperature for 24 h. The solutions were stored under nitrogen. In a typical synthesis, equal volumes of Zn-oleate and TOP-S stock solution were added to 10 mL of IL in a 20 mL MW vial. The solution was MW irradiated at 250 1C for different reaction times ranging from 5 min to 1 h under continuous stirring. Subsequently, ethanol was added to precipitate the QDs from the cooled solution. The product was centrifuged and re-dispersed in different solvents.
3. Results and discussion Fig. 1(a) shows the TEM images of the sample made by method MAIL-I using IL-1 after a reaction time of 1 min. At that point, the reaction is not complete; mainly due to the low solubility of sulfur in the solvent used. An EDX analysis of the rod like structures shows that they mainly consist of F and Zn. Considering the co-existence of Zn and F, which originates from the [BF4] anion, the rod-like nanocrystals are likely to consist of Zn(BF4)2. FTIR spectra of the particles and ILs heated at 250 1C for same reaction times showed the presence of IL components on the surface of the particles. These results confirm that there is no observable effect of MW heating on the structure of the ILs. The QDs synthesized by MAIL-I using a reaction time of 10 min and by MAIL-O using a reaction time of 60 min are shown in the
TEM images in Fig. 2(a) and (b), respectively. For both systems the synthesized ZnS QDs have a size smaller than 5 nm and exhibit a wurtzite crystal structure of ZnS. The insets in Fig. 2 are the SAED patterns of the ZnS QDs with diffraction rings indexed to the (101) (103) and (201) crystal planes of the wurtzite structure of ZnS (JCPDS no. 001-0677). Fast Fourier transforms (FFT) performed on randomly selected individual particles from HRTEM image data also revealed d-spacing of planes matching the wurtzite structure of the ZnS QDs. The QDs synthesized by the two methods show an absorption shoulder at 310 nm and 300 nm, respectively, as shown in Fig. 2(c) and (d). The absorption is slightly blue-shifted as compared to the absorption of bulk wurtzite ZnS at 326 nm, due to quantum confinement effect. The blue-shift in the absorption edge (bandgap widening) can also be attributed to the large free carrier concentrations due to defects on the surface of the particles and the existence of potential barriers. The electric fields arising from these factors in the disordered state may result in an increase in the optical band gap [16]. Surface defects are extremely sensitive to the synthesis conditions as demonstrated for nanograined nitrides and oxides as they can dilute much more impurities than the bulk phase [17–19]. Our methods create ZnS QDs with surface defects, which are evidently sensitive to synthesis conditions. Such methods can be very useful to synthesize doped QDs (d-dots) and investigate the effect of synthesis conditions on optical properties. Fig. 2(c) shows the PL emission spectrum of QDs synthesized by method MAIL-I with an excitation wavelength of 280 nm, and it displays a narrow emission at 310 nm corresponding to band edge emission together with a broad emission with a prominent peak at 420 nm corresponding to Zn vacancy acceptor level [20]. The small peak at 490 nm may be attributed to metal ion impurities in the sample [21]. The PL spectra of QDs synthesized by the method MAIL-O show two emission peaks centered at 340 nm and 460 nm, as shown in Fig. 2(d), with excitation wavelength of 310 nm. The
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R. Shahid et al. / Materials Letters 89 (2012) 316–319
Fig. 2. HRTEM images of ZnS QDs synthesized by method MAIL-I (a) and MAIL-O (b). Insets in (a) and (b) are the SAED patterns of ZnS QDs. UV–vis absorption and PL spectra of ZnS QDs synthesized by the methods MAIL-O (c) and MAIL-I (d).
Fig. 3. HRTEM images of ZnS QDs synthesized by method MAIL-O using (a) IL-2 (b) IL-3. Insets in (a) and (b) are the SAED patterns of ZnS QDs. UV–vis absorption and PL spectra of ZnS QDs synthesized by MAIL-O using (c) IL-2 and (d) IL-3.
emission peak at 460 nm is related to the recombination of electrons at sulfur vacancy donor levels (V(S)) with holes trapped at zinc vacancy acceptor levels (V(Zn)), and the peak at 340 nm is attributed to band edge emission peak [22]. In the synthesis of ZnS QDs using phosphonium based ILs; IL-2 and IL-3, the formed ZnS QDs could not be separated from the solution. This is actually a rather general drawback of IL-based synthesis, where products may be difficult to isolate; due to their very low vapor pressure that prevents evaporation of the reaction solvent, the IL. The formation of QDs was confirmed by the HRTEM images and SAED patterns obtained from samples with very low concentration, Fig. 3(a) and (b). The SAED patterns were indexed to the wurtzite crystal form of ZnS. The estimated size of
the ZnS QDs obtained from various TEM images is 2.770.4 nm and 2.370.1 nm obtained from the MAIL-O using IL-2 and IL-3, respectively. The UV absorption spectra show a blue-shift in absorption as compared to bulk wurtzite ZnS due to quantum confinement effect. PL spectra show broad emission with peak centered at 380 nm for ZnS QDs synthesized in IL-2 and 400 nm for ZnS QDs synthesized in IL-3, as shown in Fig. 3(c) and (d). The low surface tension of ILs, values between alkanes and water [23], leads to high nucleation rates and, in consequence, to small particles. Furthermore, high viscosity of ILs cause slow growth rates of particles. Therefore ILs may act multiple roles as high MW absorber, stabilizer and designer solvents.
R. Shahid et al. / Materials Letters 89 (2012) 316–319
4. Conclusion Wurtzite ZnS QDs with sizes smaller than 5 nm were synthesized by MW assisted synthesis using different ILs. ILs act as high MW absorber, stabilizer and designer solvents. The synthesized QDs show quantum confinement effect due to their small size. The new methods of synthesis represent a green alternative to conventional high temperature thermal decomposition methods using volatile organic solvents. The presented methods of synthesis can also be used for synthesis of other semiconductor nanocrystals.
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Acknowledgments
[16] [17]
The author thanks Dr. Wubeshet Sahle for his help in TEM analysis; Higher Education Commission of Pakistan and Swedish Research Council for providing funding for this work.
[18]
References
[20] [21]
[1] [2] [3] [4]
Panda AB, Glaspell G, El-Shall MS. J Am Chem Soc 2006;128:2790–1. Li Z, Jia Z, Luan Y, Mu T. Curr Opin Solid State Mater Sci 2008;12:1–8. Gardas RL, Coutinho JAP. Fluid Phase Equilib 2008;263:26–32. Yin S, Luo Z, Xia J, Li HJ. Phys Chem Solids 2010;71:1785–8.
[19]
[22] [23]
319
Palou RM. J Mex Chem Soc 2007;51:252–64. Kim KS, Demberelnyamba D, Lee H. Langmuir 2004;20:556–60. Dash P, Scott RWJ. Mater Lett 2011;65:7–9. Marquardt D, Xie Z, Taubert A, Thomann R, Janiak C. Dalton Trans 2011;40: 8290–3. Cao SW, Zhu YJ. Acta Mater 2009;57:2154–65. Goharshadi EK, Abareshi M, Mehrkhah R, Samiee S, Moosavi M, Youssefi A, et al. Mater Sci Semicond Process 2011;14:69–72. Ding K, Miao Z, Liu Z, Zhang Z, Han B, An G, et al. Am Chem Soc 2007;129: 6362–3. Jacob DS, Bitton L, Grinblat J, Felner I, Koltypin Y, Gedanken A. Chem Mater 2006;18:3162–8. Jiang Y, Zhu YJ. Chem Lett 2004;33:1390–1. Gerbec JA, Magana D, Washington A, Strouse GF. J Am Chem Soc 2005;127: 15791–800. Xiao L, Shen H, Hagen RV, Pan J, Belkoura L, Mathur S. Chem Commun. 2010;46:6509–11. Ramana CV, Smith RJ, Hussain OM. Phys Stat Sol (a) 2003;199:R4–6. Straumal B, Mazilkin A, Protasova S, Myatiev A, Straumal P, Goering E, et al. Phys Status Solidi B 2011;248:1581–6. Straumala BB, Myatievb AA, Straumalb PB, Mazilkina AA, Protasovaa SG, Goeringd E, et al. JETP Lett 2010;92:396–400. Straumal BB, Protasova SG, Mazilkin AA, Baretzky B, Myatiev AA, Straumal PB, et al. Mater Lett 2012;71:21–4. Yao WT, Yu SH, Wu QS. Adv Funct Mater 2007;17:623–31. Murase N, Jagannathan R, Kanematsu Y, Watanabe M, Kurita A, Hirata K, et al. J Phys Chem B 1999;103:754–60. Manzoor K, Vadera SR, Kumar N, Kutty TRN. Mater Chem Phys. 2003;82: 718–25. Sedev. R. Curr Opin Colloid Interface Sci 2011;16:310–6.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Microwave assisted synthesis of ZnS quantum dots using ionic liquids Robina Shahid a,n, Mikhail Gorlov b,nn, Ramy El-Sayed a, Muhammet S. Toprak a, Abhilash Sugunan a, Lars Kloo b, Mamoun Muhammed a a b
Division of Functional Materials, Department of Materials and Nanophysics, KTH Royal Institute of Technology, Stockholm, Sweden Applied Physical Chemistry, Department of Chemistry, KTH Royal Institute of Technology, Stockholm, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 August 2012 Accepted 31 August 2012 Available online 7 September 2012
In this work we report results from microwave (MW) assisted synthesis of highly crystalline ZnS quantum dots (QDs) using ionic liquid (ILs) as MW absorbing medium. Two types of ionic liquids, imidazolium and phosphonium based, were used. The QDs are less than 5 nm in size and of wurtzite ZnS type, as characterized by high-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) pattern. The optical properties were investigated by UV–vis absorption and show a blue shift in absorption as compared to bulk wurtzite ZnS due to quantum confinement effects. The photoluminescence (PL) spectra of the QDs show different trap state emissions. & 2012 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline materials Semiconductors Chemical synthesis Electron microscopy Crystal structure Optical properties
1. Introduction Microwave irradiation (MWI) is an emerging efficient heating method, where a dielectric heating mechanism involving dipolar polarization and ionic conduction allows faster reactions with higher yields and higher purities without high vacuum requirements. Furthermore, MWI methods are unique in providing upscaled processes without suffering unwanted thermal gradient effects, thus leading to a potentially industrially important advancement in the large-scale production of nanomaterials [1]. Ionic liquids (ILs) are recognized as ‘green’ alternative media with respect to volatile and often toxic organic solvents. ILs unusual properties, such as high polarity, negligible vapor pressure, high ionic conductivity, good solvent properties for both organic compounds and electrolytes and high thermal stability. These properties make them attractive as environmentally benign solvents for organic chemical reactions, polymer synthesis, chemical separations, and electrochemical applications [2]. ILs typically consists of an organic and rather bulky cation and a small inorganic anion. ILs are normally classified according to the organic cation. Two major classes of ionic liquids are based on the phosphonium or imidazolium cations together with a variety of anions ranging from simple anions, such as halides, to more complex ones, such as bis(trifluoromethanesulfonyl)amide [3].
n
Corresponding author. Tel.: þ468 790 8157; fax: þ 468 790 9072. Corresponding author. Tel.: þ 468 790 8125; fax: þ 468 21 2626. E-mail addresses: [email protected] (R. Shahid), [email protected] (M. Gorlov).
nn
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.143
From the perspective of MW chemistry, one of the important advantages of ILs is their high polarizability making them good media for absorbing MWs [4]. Therefore the MW assisted IL method enable ‘‘green’’ or more sustainable synthetic routes for the preparation of nanoparticles [5]. The microwave assisted ionic liquid (MAIL) method has been used for the synthesis of metal, [6–8] metal oxide [9–11], metal fluoride [12], metal sulfide [13] and semiconductor nanoparticles [14]. However, there are few reports on the synthesis of QDs [15] by the (MAIL) method. Therefore, in this work we report on the synthesis of ZnS QDs using MW irradiation as heating source in controlled reactions with ILs as MW absorbing medium.
2. Experimental Two MW assisted ionlic liquid (MAIL) synthesis systems were developed; first method denoted as MAIL-I involving inorganic precursors and the IL-1, secondly, a method denoted MAIL-O, in which organic precursors and three ILs, IL-1, IL-2 and IL-3 were used. The Molecular structure and the constituting ions of three ILs are shown in Table 1. For the first system, MAIL-I, the inorganic salt ZnCl2 and sulfur powder were used as Zn and S sources, respectively. Equimolar quantities of ZnCl2 and sulfur powder were added to 5 mL of IL-1. The solution was heated to 250 1C under MW irradiation under stirring for different reaction times ranging from 1 to 10 min. Subsequently, ethanol was added to precipitate the QDs from the
R. Shahid et al. / Materials Letters 89 (2012) 316–319
317
Table 1 Molecular structure and constituting ions of ILs used in the MWA synthesis of QDs. IL-1
IL-2
IL-3
Fig. 1. (a) TEM image of sample after 1 min reaction time made by method MAIL-1. Inset is image of one of the rod. (b) FTIR spectra of IL and nanoparticles synthesized at 60 min.
cooled solution. The product was then centrifuged and re-dispersed in ethanol. In the second system, MAIL-O, the organic compounds Zn-oleate and TOP-S (see below) were used as Zn and S sources, respectively. Zn-oleate and TOP-S stock solutions were prepared as follows: 0.407 g ZnO was added to 10 mL of oleic acid, in a MW heating vial of 10–20 mL volume and heated at 250 1C for 2 h, in order to obtain zinc oleate (Zn-oleate). Trioctylphosphine sulfide (TOP-S) was obtained by adding 0.16 g sulfur powder to 10 mL of TOP and the solution was then stirred at room temperature for 24 h. The solutions were stored under nitrogen. In a typical synthesis, equal volumes of Zn-oleate and TOP-S stock solution were added to 10 mL of IL in a 20 mL MW vial. The solution was MW irradiated at 250 1C for different reaction times ranging from 5 min to 1 h under continuous stirring. Subsequently, ethanol was added to precipitate the QDs from the cooled solution. The product was centrifuged and re-dispersed in different solvents.
3. Results and discussion Fig. 1(a) shows the TEM images of the sample made by method MAIL-I using IL-1 after a reaction time of 1 min. At that point, the reaction is not complete; mainly due to the low solubility of sulfur in the solvent used. An EDX analysis of the rod like structures shows that they mainly consist of F and Zn. Considering the co-existence of Zn and F, which originates from the [BF4] anion, the rod-like nanocrystals are likely to consist of Zn(BF4)2. FTIR spectra of the particles and ILs heated at 250 1C for same reaction times showed the presence of IL components on the surface of the particles. These results confirm that there is no observable effect of MW heating on the structure of the ILs. The QDs synthesized by MAIL-I using a reaction time of 10 min and by MAIL-O using a reaction time of 60 min are shown in the
TEM images in Fig. 2(a) and (b), respectively. For both systems the synthesized ZnS QDs have a size smaller than 5 nm and exhibit a wurtzite crystal structure of ZnS. The insets in Fig. 2 are the SAED patterns of the ZnS QDs with diffraction rings indexed to the (101) (103) and (201) crystal planes of the wurtzite structure of ZnS (JCPDS no. 001-0677). Fast Fourier transforms (FFT) performed on randomly selected individual particles from HRTEM image data also revealed d-spacing of planes matching the wurtzite structure of the ZnS QDs. The QDs synthesized by the two methods show an absorption shoulder at 310 nm and 300 nm, respectively, as shown in Fig. 2(c) and (d). The absorption is slightly blue-shifted as compared to the absorption of bulk wurtzite ZnS at 326 nm, due to quantum confinement effect. The blue-shift in the absorption edge (bandgap widening) can also be attributed to the large free carrier concentrations due to defects on the surface of the particles and the existence of potential barriers. The electric fields arising from these factors in the disordered state may result in an increase in the optical band gap [16]. Surface defects are extremely sensitive to the synthesis conditions as demonstrated for nanograined nitrides and oxides as they can dilute much more impurities than the bulk phase [17–19]. Our methods create ZnS QDs with surface defects, which are evidently sensitive to synthesis conditions. Such methods can be very useful to synthesize doped QDs (d-dots) and investigate the effect of synthesis conditions on optical properties. Fig. 2(c) shows the PL emission spectrum of QDs synthesized by method MAIL-I with an excitation wavelength of 280 nm, and it displays a narrow emission at 310 nm corresponding to band edge emission together with a broad emission with a prominent peak at 420 nm corresponding to Zn vacancy acceptor level [20]. The small peak at 490 nm may be attributed to metal ion impurities in the sample [21]. The PL spectra of QDs synthesized by the method MAIL-O show two emission peaks centered at 340 nm and 460 nm, as shown in Fig. 2(d), with excitation wavelength of 310 nm. The
318
R. Shahid et al. / Materials Letters 89 (2012) 316–319
Fig. 2. HRTEM images of ZnS QDs synthesized by method MAIL-I (a) and MAIL-O (b). Insets in (a) and (b) are the SAED patterns of ZnS QDs. UV–vis absorption and PL spectra of ZnS QDs synthesized by the methods MAIL-O (c) and MAIL-I (d).
Fig. 3. HRTEM images of ZnS QDs synthesized by method MAIL-O using (a) IL-2 (b) IL-3. Insets in (a) and (b) are the SAED patterns of ZnS QDs. UV–vis absorption and PL spectra of ZnS QDs synthesized by MAIL-O using (c) IL-2 and (d) IL-3.
emission peak at 460 nm is related to the recombination of electrons at sulfur vacancy donor levels (V(S)) with holes trapped at zinc vacancy acceptor levels (V(Zn)), and the peak at 340 nm is attributed to band edge emission peak [22]. In the synthesis of ZnS QDs using phosphonium based ILs; IL-2 and IL-3, the formed ZnS QDs could not be separated from the solution. This is actually a rather general drawback of IL-based synthesis, where products may be difficult to isolate; due to their very low vapor pressure that prevents evaporation of the reaction solvent, the IL. The formation of QDs was confirmed by the HRTEM images and SAED patterns obtained from samples with very low concentration, Fig. 3(a) and (b). The SAED patterns were indexed to the wurtzite crystal form of ZnS. The estimated size of
the ZnS QDs obtained from various TEM images is 2.770.4 nm and 2.370.1 nm obtained from the MAIL-O using IL-2 and IL-3, respectively. The UV absorption spectra show a blue-shift in absorption as compared to bulk wurtzite ZnS due to quantum confinement effect. PL spectra show broad emission with peak centered at 380 nm for ZnS QDs synthesized in IL-2 and 400 nm for ZnS QDs synthesized in IL-3, as shown in Fig. 3(c) and (d). The low surface tension of ILs, values between alkanes and water [23], leads to high nucleation rates and, in consequence, to small particles. Furthermore, high viscosity of ILs cause slow growth rates of particles. Therefore ILs may act multiple roles as high MW absorber, stabilizer and designer solvents.
R. Shahid et al. / Materials Letters 89 (2012) 316–319
4. Conclusion Wurtzite ZnS QDs with sizes smaller than 5 nm were synthesized by MW assisted synthesis using different ILs. ILs act as high MW absorber, stabilizer and designer solvents. The synthesized QDs show quantum confinement effect due to their small size. The new methods of synthesis represent a green alternative to conventional high temperature thermal decomposition methods using volatile organic solvents. The presented methods of synthesis can also be used for synthesis of other semiconductor nanocrystals.
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Acknowledgments
[16] [17]
The author thanks Dr. Wubeshet Sahle for his help in TEM analysis; Higher Education Commission of Pakistan and Swedish Research Council for providing funding for this work.
[18]
References
[20] [21]
[1] [2] [3] [4]
Panda AB, Glaspell G, El-Shall MS. J Am Chem Soc 2006;128:2790–1. Li Z, Jia Z, Luan Y, Mu T. Curr Opin Solid State Mater Sci 2008;12:1–8. Gardas RL, Coutinho JAP. Fluid Phase Equilib 2008;263:26–32. Yin S, Luo Z, Xia J, Li HJ. Phys Chem Solids 2010;71:1785–8.
[19]
[22] [23]
319
Palou RM. J Mex Chem Soc 2007;51:252–64. Kim KS, Demberelnyamba D, Lee H. Langmuir 2004;20:556–60. Dash P, Scott RWJ. Mater Lett 2011;65:7–9. Marquardt D, Xie Z, Taubert A, Thomann R, Janiak C. Dalton Trans 2011;40: 8290–3. Cao SW, Zhu YJ. Acta Mater 2009;57:2154–65. Goharshadi EK, Abareshi M, Mehrkhah R, Samiee S, Moosavi M, Youssefi A, et al. Mater Sci Semicond Process 2011;14:69–72. Ding K, Miao Z, Liu Z, Zhang Z, Han B, An G, et al. Am Chem Soc 2007;129: 6362–3. Jacob DS, Bitton L, Grinblat J, Felner I, Koltypin Y, Gedanken A. Chem Mater 2006;18:3162–8. Jiang Y, Zhu YJ. Chem Lett 2004;33:1390–1. Gerbec JA, Magana D, Washington A, Strouse GF. J Am Chem Soc 2005;127: 15791–800. Xiao L, Shen H, Hagen RV, Pan J, Belkoura L, Mathur S. Chem Commun. 2010;46:6509–11. Ramana CV, Smith RJ, Hussain OM. Phys Stat Sol (a) 2003;199:R4–6. Straumal B, Mazilkin A, Protasova S, Myatiev A, Straumal P, Goering E, et al. Phys Status Solidi B 2011;248:1581–6. Straumala BB, Myatievb AA, Straumalb PB, Mazilkina AA, Protasovaa SG, Goeringd E, et al. JETP Lett 2010;92:396–400. Straumal BB, Protasova SG, Mazilkin AA, Baretzky B, Myatiev AA, Straumal PB, et al. Mater Lett 2012;71:21–4. Yao WT, Yu SH, Wu QS. Adv Funct Mater 2007;17:623–31. Murase N, Jagannathan R, Kanematsu Y, Watanabe M, Kurita A, Hirata K, et al. J Phys Chem B 1999;103:754–60. Manzoor K, Vadera SR, Kumar N, Kutty TRN. Mater Chem Phys. 2003;82: 718–25. Sedev. R. Curr Opin Colloid Interface Sci 2011;16:310–6.