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ZnS nanoparticles
159 Studies in Surface Science and Catalysis, volume 159 Hyun-Ku Rhee, In-Sik Nam Nam and Jong Moon Park (Editors) B.V. All All rights reserved © 2006 Elsevier B.V.
757
Synthesis and characterization of Mn, Pr doped ZnS and CdS/ ZnS nanoparticles Kwan Hwi Park", Hyun Uk Kanga, Jun Woo Leeb, Sang Sig Kim , Sung Hyun Kim** "Department of Chemical & Biological Engineering, Korea University, 1,5-Ka, Anam-dong, Sungbuk-ku, Seoul 136-701, Republic of Korea Department of Electrical Engineering, Korea University, 1,5-Ka, Anam-dong, Sungbuk-ku, Seoul 136-701, Republic of Korea Nanoparticles of Mn and Pr-doped ZnS and CdS-ZnS were synthesized by wet chemical method and inverse micelle method. Physical and fluorescent properties were characterized by X-ray diffraction (XRD) and photolumineseence (PL). ZnS nanoparticles annealed optically in air shows higher PL intensity than in vacuum, PL intensity of Mn and Pr-doped ZnS nanoparticles was enhanced by the photo-oxidation and the diffusion of luminescent ion. The prepared CdS nanoparticles show cubic or hexagonal phase, depending on synthesis conditions. Core-shell nanoparticles enhanced PL intensity by passivation. The interfacial state between CdS core and shell material was unchanged by different surface treatment. 1. INTRODUCTION For nanoparticles doped with luminescent ions, an efficient energy transfer occur from host to luminescent ions [1], ZnS doped with luminescent ions generates light with wavelengths in the visible range. Tm and Li-codoped, Mn-doped, and Sm-doped ZnS semiconductors emit blue, orange and red light respectively [2-3]. ZnS nanoparticles codoped with Mn and Pr may show stronger white emission than bulk ZnS powders codoped with these ions. Nevertheless, there is severe non-radiative recombination through the surface states of the nanoparticles. Optical annealing is one of methods to reduce the severe non-radiative recombination due to polymerization and photo-oxidation [4] and enhancement of the crystal quality of the nanoparticles [5]. Core/shell-type nanoparticles overcoated with higher band gap inorganic materials exhibit high PL quantum yield compared with uncoated dots due to elimination of surface non-radiative recombination defects. Such core/shell structures as CdSe/CdS [6] and CdSe/ZnS [7] have been prepared from organometallic precursors. In this study, we investigated the rare-earth material doping, optical annealing, and coreshell structure for enhancing the luminescent intensity. The synthesis of nanoparticles was first described, and the luminescent properties of undoped, doped ZnS nanoparticles optically annealed in vacuum or air were analyzed. Then, the crystalline and PL properties of CdS nanoparticles were discussed.
758
2. EXPERIMENTAL ZnS nanoparticles were synthesized by stirring the mixed solution which consisted and ZnfNCya'HaO. To synthesize Mn and Pr-doped ZnS nanoparticles, Pr(NO3)3-6H2O and MnfNCya-HaO were added to ZnCNOjJa'HzO aqueous solution and thereafter NajS solution was injected into the mixed solutions. The UV light of 325nm from a He-Cd laser was used as the light source for optical annealing, CdS-ZnS core-shell nanoparticles were synthesized by water/AOT/ether inverse micro-emulsion system. Cd(NO3)r4H2O, NaaS^HfeO, and Zn(NO3)z-4H2O as precursors of CdS-ZnS core-shell nanoparticles and dodecanethiol as a capping material were used. 3. RESULTS AND DISCUSSION 3.1. Characterization of Mn and Pr-doped ZnS nanoparticles PL spectra of the undoped ZnS nanoparticles optically annealed in air (a) and vacuum (b) are plotted in Fig. 1. PL intensity increases with optical annealing time. In the PL spectrum of the undoped ZnS nanoparticles, the emission band peaks appear at around 420nm, A comparison of Fig.la and lb reveals the more significant increase in PL intensity of the nanoparticles annealed in air than vacuum. Optical annealing in vacuum helps in enhancing the crystallinity. hi addition to the enhancement of the crystallinity, the optical annealing for ZnS nanoparticles in air induces the photo-oxidation of nanoparticles. PL intensity is increased more by photo-oxidation than the enhancement of crystal quality of the nanoparticles by the annealing. PL spectra of Mn-doped ZnS nanoparticles optically annealed in air (a) and in vacuum (b) are shown in Fig. 2. For Mn-doped ZnS nanoparticles, the PL band is seen at around 585nm. When Mn-doped ZnS nanoparticles were annealed in air, PL intensity is increased more significantly with UV irradiation time compared with ones annealed in vacuum. PL spectra of Pr-doped ZnS nanoparticles are shown in Fig. 3. The broad emission at 430 nm corresponds to the emission of the undoped ZnS nanoparticles. The other peak is related to the Pr-related complexes. The effect of the optical annealing in air is more notable than in vacuum on the enhancement of luminescent intensity. The increase of PL intensity for Prdoped ZnS nanoparticles in air is more rapid than undoped or Mn-doped ZnS nanoparticles.
Fig. 1. PL spectra of undoped ZnS nanoparticles optically annealed in air (a) and in vacuum (b)
wavelanBin(rim)
Wavolsngth (nm)
Fig. 2. PL spectra of Mn-doped ZnS nanopartioles optically annealed in air (a) and vacuum (b)
759
HO
AM 4H MO 1 »
«O HO TH
Wavelonglti (TIITI}
Wavelength (rim)
Fig. 3. PL spectra of Pr-doped ZnS nanoparticles optically annealed in air (a) and vacuum (b)
Wavelength (ntnj
Wavelength (Dm)
Fig. 4. PL spectra of ZnS: (Mn, Pr) nanoparticles optically annealed in air (a) and vacuum (b) Fig. 4 shows PL spectra of Mn and Pr-codoped ZnS nanoparticles optically annealed in air and vacuum. Mn and Pr-codoped ZnS nanoparticles emit light of white color. The PL intensity of the Pr-related peaks increased more rapidly than that of Mn-related peak, for the codoped ZnS nanoparticles annealed in air. The different rates may be associated with the luminescent ions. Pr-related complexes are increased with the increasing UV irradiation time, but Mn ions are constant, hi case of the annealing in vacuum, Pr-related peaks are initially weaker in intensity than Mn-related peaks due to small Pr-related complexes. 3.2. Characterization of the CdS-ZnS core-shell nanoparticles Fig. 5 shows XRD patterns of CdS nanoparticles. For the synthesized nanopartices, a mean crystallite size (D) calculated by the Scherrer formula is between 3 and 5nm [8]. XRD pattern of hexagonal CdS in pattern of (a), (b) and (c) of Fig. 5 consists of two broad peak at 25-30° and 44-52°. The peak at 25-30° is due to the convolution of (002) and (101) peak of hexagonal phase. The peak at 44—52° may be due to the convolution of (110), (103), and (112) peak of hexagonal phase. XRD pattern of Fig. 5(d) shows convolution of cubic and hexagonal phase. The peak of Fig. 5(d) at 25-30° maybe due to both (101) peak of hexagonal structure and (111) peak of cubic. The peak of Fig. 5(d) at 44-52° may be convolution of (103) peak of hexagonal structure and (220) and (311) peak of cubic structure. The structure of CdS changes to cubic phase with increasing concentration of cadmium nitride. The cubic structure of (111), (220), and (311) peak are shown in XRD pattern of CdS synthesized in ultrasonic condition (c) 0.02M Precursor
(101) (002)
(d) [Cd]:[S]=2:1 Condition
(b) 0.1M AOT
(111)
(a) Basic Condition
(100)
(112) (200)
20
(e) Ultrasonic Condition (200)
(110) (103) (102)
30
40 2θ
50
(220) (311)
(201) (004) (202)
60 20
(222) 30
40
50
60
2θ
Fig. 5. X-ray diffraction pattern of the Cadmium Sulfide nanoparticles woe synthesized in 0.05M AOT and 0.04M precursors condition (a), 0.1M AOT condition (b), 0.02M precursors condition (c), (CdCNO3)3-4H2O): (Na2S-9Hj0)=2:1 condition (d), and ultrasonic condition(e)
760 140000
140000
(a)
120000
100000 PL Intensity
PL Intensity
(b)
120000
100000 80000 60000 CdS/ZnS QDs
40000
80000 After Reaction
60000 40000
After Rising
20000 20000
400
500 600 Wavelength (nm)
After Sintering
0
CdS QDs
0
700
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 6. PL spectra of CdS nanopartieies and CdS-ZnS core-shell nanoparticles (a) and CdS-ZnS coreshell nanoparticles by surface treatment (b) Fig, 6 shows PL spectra of CdS nanoparticles and CdS-ZnS core-shell nanoparticles. In PL spectrum of CdS nanoparticles, the emission band is seen at around 400nm, The emission band of CdS-ZnS core-shell nanoparticles is higher than that of CdS ones at around 400nm. The PL enhancement of CdS-ZnS core-shell nanoparticles is due to passivation which means that surface atoms are bonded to the shell material of similar lattice constant and much larger band gap [9]. Although the surface treatment conditions are different, the emission band of CdS-ZnS core-shell nanoparticles is same in PL spectra of Fig. 6(b). This indicates that interfacial state between CdS core and shell material was unchanged by different surface treatment. 4. CONCLUSION Undoped, Mo, and Pr-doped ZnS nanoparticles synthesized by wet chemical method were optically annealed in air or vacuum. PL emission increased with annealing time. This increase is attributed to the photo-oxidation, enhancement in the crystal quality, and diffusion of the luminescent ions. PL intensity of nanoparticles annealed in air increased more significantly due to the photo-oxidation compared with the nanoparticles annealed in vacuum, Mn and Prcodoped ZnS nanoparticles emitted white light due to the effects of dopants. The optical annealing enhanced the emission intensity. CdS and CdS-ZnS core-shell nanoparticles were synthesized by inverse micelle method, Crystallinity of CdS nanoparticles was hexagonal structure under the same molar ratio of Cd and S precursor. However it was changed easily to cubic structure under the condition of sonieation or higher concentration of Cd than S precursor. The interfacial state between CdS core and shell material was unchanged by different surface treatment. REFERENCES [1] R.N. Bhargava and D. Gallagher, Phys. Rev. Lett., 72 (1994) 416. [2] L.V. Zavyalova, A.K, Savin and GS. Svechnikov, Displays, 18 (1997) 73. [3] A.B. Stambouli, S. Hamzaoui and M, Bouderbala, Thin Solid Films, 283 (1996) 204. [4] T. Tang, M. Yang, and K. Chen, Ceramics Int., 26 (2000) 153. [5] A.L. Stepanov, and V.N. Popok, Surf. Coat. Tech., 185 (2004) 30. [6] C. Choo, T. Sakamoto, M. Tohara, K. Tanaka, R. Nakata and N. Okuyama, Surf. Sci., 445 (2000) 480. [7] X.G. Peng, M.C. Schlamp, A.V. Kadavanieh, and A.P. Alivisatos, J. Am. Chem. Sac., 119 (1997) 7019. [8] M. L. Cum, G. Leo, M. Alvisi, A. Agostiano, M. Delia Monica and L. Vasanelliz, J. Coll. Inter. Sci., 243 (2001) 165. [V] k.K. Sony and S.I I. Lee. Curr. Appl. Phys.. 1 (2000) 107.
757
Synthesis and characterization of Mn, Pr doped ZnS and CdS/ ZnS nanoparticles Kwan Hwi Park", Hyun Uk Kanga, Jun Woo Leeb, Sang Sig Kim , Sung Hyun Kim** "Department of Chemical & Biological Engineering, Korea University, 1,5-Ka, Anam-dong, Sungbuk-ku, Seoul 136-701, Republic of Korea Department of Electrical Engineering, Korea University, 1,5-Ka, Anam-dong, Sungbuk-ku, Seoul 136-701, Republic of Korea Nanoparticles of Mn and Pr-doped ZnS and CdS-ZnS were synthesized by wet chemical method and inverse micelle method. Physical and fluorescent properties were characterized by X-ray diffraction (XRD) and photolumineseence (PL). ZnS nanoparticles annealed optically in air shows higher PL intensity than in vacuum, PL intensity of Mn and Pr-doped ZnS nanoparticles was enhanced by the photo-oxidation and the diffusion of luminescent ion. The prepared CdS nanoparticles show cubic or hexagonal phase, depending on synthesis conditions. Core-shell nanoparticles enhanced PL intensity by passivation. The interfacial state between CdS core and shell material was unchanged by different surface treatment. 1. INTRODUCTION For nanoparticles doped with luminescent ions, an efficient energy transfer occur from host to luminescent ions [1], ZnS doped with luminescent ions generates light with wavelengths in the visible range. Tm and Li-codoped, Mn-doped, and Sm-doped ZnS semiconductors emit blue, orange and red light respectively [2-3]. ZnS nanoparticles codoped with Mn and Pr may show stronger white emission than bulk ZnS powders codoped with these ions. Nevertheless, there is severe non-radiative recombination through the surface states of the nanoparticles. Optical annealing is one of methods to reduce the severe non-radiative recombination due to polymerization and photo-oxidation [4] and enhancement of the crystal quality of the nanoparticles [5]. Core/shell-type nanoparticles overcoated with higher band gap inorganic materials exhibit high PL quantum yield compared with uncoated dots due to elimination of surface non-radiative recombination defects. Such core/shell structures as CdSe/CdS [6] and CdSe/ZnS [7] have been prepared from organometallic precursors. In this study, we investigated the rare-earth material doping, optical annealing, and coreshell structure for enhancing the luminescent intensity. The synthesis of nanoparticles was first described, and the luminescent properties of undoped, doped ZnS nanoparticles optically annealed in vacuum or air were analyzed. Then, the crystalline and PL properties of CdS nanoparticles were discussed.
758
2. EXPERIMENTAL ZnS nanoparticles were synthesized by stirring the mixed solution which consisted and ZnfNCya'HaO. To synthesize Mn and Pr-doped ZnS nanoparticles, Pr(NO3)3-6H2O and MnfNCya-HaO were added to ZnCNOjJa'HzO aqueous solution and thereafter NajS solution was injected into the mixed solutions. The UV light of 325nm from a He-Cd laser was used as the light source for optical annealing, CdS-ZnS core-shell nanoparticles were synthesized by water/AOT/ether inverse micro-emulsion system. Cd(NO3)r4H2O, NaaS^HfeO, and Zn(NO3)z-4H2O as precursors of CdS-ZnS core-shell nanoparticles and dodecanethiol as a capping material were used. 3. RESULTS AND DISCUSSION 3.1. Characterization of Mn and Pr-doped ZnS nanoparticles PL spectra of the undoped ZnS nanoparticles optically annealed in air (a) and vacuum (b) are plotted in Fig. 1. PL intensity increases with optical annealing time. In the PL spectrum of the undoped ZnS nanoparticles, the emission band peaks appear at around 420nm, A comparison of Fig.la and lb reveals the more significant increase in PL intensity of the nanoparticles annealed in air than vacuum. Optical annealing in vacuum helps in enhancing the crystallinity. hi addition to the enhancement of the crystallinity, the optical annealing for ZnS nanoparticles in air induces the photo-oxidation of nanoparticles. PL intensity is increased more by photo-oxidation than the enhancement of crystal quality of the nanoparticles by the annealing. PL spectra of Mn-doped ZnS nanoparticles optically annealed in air (a) and in vacuum (b) are shown in Fig. 2. For Mn-doped ZnS nanoparticles, the PL band is seen at around 585nm. When Mn-doped ZnS nanoparticles were annealed in air, PL intensity is increased more significantly with UV irradiation time compared with ones annealed in vacuum. PL spectra of Pr-doped ZnS nanoparticles are shown in Fig. 3. The broad emission at 430 nm corresponds to the emission of the undoped ZnS nanoparticles. The other peak is related to the Pr-related complexes. The effect of the optical annealing in air is more notable than in vacuum on the enhancement of luminescent intensity. The increase of PL intensity for Prdoped ZnS nanoparticles in air is more rapid than undoped or Mn-doped ZnS nanoparticles.
Fig. 1. PL spectra of undoped ZnS nanoparticles optically annealed in air (a) and in vacuum (b)
wavelanBin(rim)
Wavolsngth (nm)
Fig. 2. PL spectra of Mn-doped ZnS nanopartioles optically annealed in air (a) and vacuum (b)
759
HO
AM 4H MO 1 »
«O HO TH
Wavelonglti (TIITI}
Wavelength (rim)
Fig. 3. PL spectra of Pr-doped ZnS nanoparticles optically annealed in air (a) and vacuum (b)
Wavelength (ntnj
Wavelength (Dm)
Fig. 4. PL spectra of ZnS: (Mn, Pr) nanoparticles optically annealed in air (a) and vacuum (b) Fig. 4 shows PL spectra of Mn and Pr-codoped ZnS nanoparticles optically annealed in air and vacuum. Mn and Pr-codoped ZnS nanoparticles emit light of white color. The PL intensity of the Pr-related peaks increased more rapidly than that of Mn-related peak, for the codoped ZnS nanoparticles annealed in air. The different rates may be associated with the luminescent ions. Pr-related complexes are increased with the increasing UV irradiation time, but Mn ions are constant, hi case of the annealing in vacuum, Pr-related peaks are initially weaker in intensity than Mn-related peaks due to small Pr-related complexes. 3.2. Characterization of the CdS-ZnS core-shell nanoparticles Fig. 5 shows XRD patterns of CdS nanoparticles. For the synthesized nanopartices, a mean crystallite size (D) calculated by the Scherrer formula is between 3 and 5nm [8]. XRD pattern of hexagonal CdS in pattern of (a), (b) and (c) of Fig. 5 consists of two broad peak at 25-30° and 44-52°. The peak at 25-30° is due to the convolution of (002) and (101) peak of hexagonal phase. The peak at 44—52° may be due to the convolution of (110), (103), and (112) peak of hexagonal phase. XRD pattern of Fig. 5(d) shows convolution of cubic and hexagonal phase. The peak of Fig. 5(d) at 25-30° maybe due to both (101) peak of hexagonal structure and (111) peak of cubic. The peak of Fig. 5(d) at 44-52° may be convolution of (103) peak of hexagonal structure and (220) and (311) peak of cubic structure. The structure of CdS changes to cubic phase with increasing concentration of cadmium nitride. The cubic structure of (111), (220), and (311) peak are shown in XRD pattern of CdS synthesized in ultrasonic condition (c) 0.02M Precursor
(101) (002)
(d) [Cd]:[S]=2:1 Condition
(b) 0.1M AOT
(111)
(a) Basic Condition
(100)
(112) (200)
20
(e) Ultrasonic Condition (200)
(110) (103) (102)
30
40 2θ
50
(220) (311)
(201) (004) (202)
60 20
(222) 30
40
50
60
2θ
Fig. 5. X-ray diffraction pattern of the Cadmium Sulfide nanoparticles woe synthesized in 0.05M AOT and 0.04M precursors condition (a), 0.1M AOT condition (b), 0.02M precursors condition (c), (CdCNO3)3-4H2O): (Na2S-9Hj0)=2:1 condition (d), and ultrasonic condition(e)
760 140000
140000
(a)
120000
100000 PL Intensity
PL Intensity
(b)
120000
100000 80000 60000 CdS/ZnS QDs
40000
80000 After Reaction
60000 40000
After Rising
20000 20000
400
500 600 Wavelength (nm)
After Sintering
0
CdS QDs
0
700
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 6. PL spectra of CdS nanopartieies and CdS-ZnS core-shell nanoparticles (a) and CdS-ZnS coreshell nanoparticles by surface treatment (b) Fig, 6 shows PL spectra of CdS nanoparticles and CdS-ZnS core-shell nanoparticles. In PL spectrum of CdS nanoparticles, the emission band is seen at around 400nm, The emission band of CdS-ZnS core-shell nanoparticles is higher than that of CdS ones at around 400nm. The PL enhancement of CdS-ZnS core-shell nanoparticles is due to passivation which means that surface atoms are bonded to the shell material of similar lattice constant and much larger band gap [9]. Although the surface treatment conditions are different, the emission band of CdS-ZnS core-shell nanoparticles is same in PL spectra of Fig. 6(b). This indicates that interfacial state between CdS core and shell material was unchanged by different surface treatment. 4. CONCLUSION Undoped, Mo, and Pr-doped ZnS nanoparticles synthesized by wet chemical method were optically annealed in air or vacuum. PL emission increased with annealing time. This increase is attributed to the photo-oxidation, enhancement in the crystal quality, and diffusion of the luminescent ions. PL intensity of nanoparticles annealed in air increased more significantly due to the photo-oxidation compared with the nanoparticles annealed in vacuum, Mn and Prcodoped ZnS nanoparticles emitted white light due to the effects of dopants. The optical annealing enhanced the emission intensity. CdS and CdS-ZnS core-shell nanoparticles were synthesized by inverse micelle method, Crystallinity of CdS nanoparticles was hexagonal structure under the same molar ratio of Cd and S precursor. However it was changed easily to cubic structure under the condition of sonieation or higher concentration of Cd than S precursor. The interfacial state between CdS core and shell material was unchanged by different surface treatment. REFERENCES [1] R.N. Bhargava and D. Gallagher, Phys. Rev. Lett., 72 (1994) 416. [2] L.V. Zavyalova, A.K, Savin and GS. Svechnikov, Displays, 18 (1997) 73. [3] A.B. Stambouli, S. Hamzaoui and M, Bouderbala, Thin Solid Films, 283 (1996) 204. [4] T. Tang, M. Yang, and K. Chen, Ceramics Int., 26 (2000) 153. [5] A.L. Stepanov, and V.N. Popok, Surf. Coat. Tech., 185 (2004) 30. [6] C. Choo, T. Sakamoto, M. Tohara, K. Tanaka, R. Nakata and N. Okuyama, Surf. Sci., 445 (2000) 480. [7] X.G. Peng, M.C. Schlamp, A.V. Kadavanieh, and A.P. Alivisatos, J. Am. Chem. Sac., 119 (1997) 7019. [8] M. L. Cum, G. Leo, M. Alvisi, A. Agostiano, M. Delia Monica and L. Vasanelliz, J. Coll. Inter. Sci., 243 (2001) 165. [V] k.K. Sony and S.I I. Lee. Curr. Appl. Phys.. 1 (2000) 107.