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Organic molecules passivated Mn doped Zinc Selenide quantum dots and its properties
Applied Surface Science 257 (2011) 7699–7703
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Organic molecules passivated Mn doped Zinc Selenide quantum dots and its properties J. Archana a , M. Navaneethan a,b , S. Ponnusamy a , Y. Hayakawa b , C. Muthamizhchelvan a,∗ a b
Center for Material science and Nano devices, Department of Physics, SRM University, Kattankulathur, Chennai 603203, Tamil Nadu, India Research Institute of Electronics, Shizuoka University, 3-5-1, Johoku, Naka- ku Hamamatsu, Shizuoka 432-8011, Japan
a r t i c l e
i n f o
Article history: Received 4 September 2010 Received in revised form 24 January 2011 Accepted 2 April 2011 Available online 12 April 2011 Keywords: Mn doped Zinc Selenide Nanoparticles Wet chemical method Optical properties Structural properties
a b s t r a c t Quantum dots of Mn doped Zinc Selenide with N-Methylaniline as the capping agent was prepared by simple and inexpensive wet chemical method. Size of the particles observed by TEM was of the order of 2–4 nm which was well consistent with the size measured by UV analysis. The presence of paramagnetic substance Mn2+ in the ZnSe quantum dots was confirmed by EPR measurement. Mn doped ZnSe nanoparticles exhibited a strong blue emission that was strongly dependent upon the Mn dopant level and the surface passivation produced by N-Methylaniline. The stability of the product was studied by thermal analysis which shows that this product is highly suitable for opto-electronic applications. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, there has been considerable interest in semiconductors of nanometric dimensions due to their potential applications [1–3]. The nanometric semiconductor particles exhibit novel properties due to the large number of surface atoms and three dimensional confinement of electrons. Altering the size of the particles changes the degree of confinement of the electrons and affects the electronic structure of the solid, especially the band edges which are tunable with particle size [4,5]. Binary compound semiconductor nanomaterial possess many interesting potential applications in photo detectors, solar cells, light emitting diodes, mesocopic, electronic and optical devices [6–8]. Extensive studies have been reported, in particular, on the optical properties of transition metal doped semiconductor nanoparticles [9,10]. ZnSe is one of the II–VI compounds semiconductors which have the band gap of 2.7 eV and transmittance range is 0.5–22 m. It has high luminescence efficiency, low absorption coefficient and excellent transparency in the infrared region. Bhargava et al. reported for the first time that the photoluminescence of Mn2+ doped ZnS nanoparticles have enhanced quantum efficiency compared to the bulk crystal [11]. Mn2+ doped ZnSe nanoparticles have been realized a potential candidate for various applications such as highly emissive nanocrystals for bio imaging, wavelength
∗ Corresponding author. Tel.: +91 44 27456020; fax: +91 44 27452343. E-mail address: [email protected] (C. Muthamizhchelvan). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.04.012
tunable lasers, solar cells and diluted magnetic semiconductors for spintronics applications [12–15]. Therefore, it is very important to investigate how the doped semiconductor nanoparticles affect optical properties from the view points of basic physics and applications. It is aimed in the present work to study the effect of Mn2+ ion doped ZnSe quantum dots by simple wet chemical route with N-Methylaniline as the capping agent. The structure of the ZnSe:Mn2+ nanoparticles has been determined by X-ray diffractometer (XRD). The band gap was calculated from ultraviolet (UV) visible absorption spectrum. The luminescence property has been investigated using photoluminescence (PL) spectral studies. The presence of NMA and Mn2+ was confirmed by Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray analysis (EDAX) and electron paramagnetic resonance (EPR) measurements. The morphological characterization was carried out by transmission electron microscope (TEM) analysis. 2. Experimental method All the reagents were of analytical grade and used without further purification. 0.2 M solution of zinc acetate monohydrate (CH3 COO2 )Zn·H2 O) was prepared in 20 ml of distilled water and 0.02 M manganese sulphate (MnSO4 ) was added to the solution. After that 20 ml of solution of 0.2 M sodium selenite (Na2 SeO3 ·5H2 O) solution was added. Finally, 0.2 ml NMA was added drop wise to the same solution as a capping agent. It is stirred vigorously for 36 h using magnetic stirrer. The role of NMA is to sta-
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J. Archana et al. / Applied Surface Science 257 (2011) 7699–7703
(511) (531)
200
(400)
300
(440)
400
(220)
Intensity (cps)
500
(331)
(311)
600
100 0
20
30
40
50
60
2θ (degree) Fig. 1. XRD pattern of N-Methylaniline passivated ZnSe:Mn2+ Quantum dots.
bilize the particles against aggregation. The resultant product was dried at 180 ◦ C for 10 h. The structure of N-Methylaniline passivated ZnSe: Mn2+ quantum dots were characterized by X-ray diffraction (XRD) using X’per PRO (PANalytical) advanced X-ray Diffractometer with CuK␣ ˚ with 2 ranging between 20–80◦ at the radiation ( = 1.5406 A), ◦ scanning rate of 0.05 per 2 s. Optical properties were measured using Perkin Elmer lambda 5 UV–visible spectrophotometer. The PL spectrum was obtained by Flurolog-3 spectrophotometer (Jobin Yvon) equipped with a He–Cd lamp as the excitation source in the range 200–900 nm. Morphology studies were made using a JEOL 2010 electron microscope with an accelerating voltage of 200 kV. Histogram of the particles size distribution was measured by iTEM software which is attached with TEM. The EDAX spectrum was obtained by Hitachi S-3400. The FTIR spectrum were recorded using Perkin Elmer Spectrophotometer in the range of 400–4000 cm−1 . The electron paramagnetic resonance (EPR) spectrum was recorded on a Burker EPR spectrometer (ER083CS) operating at an X-band (v = 9.77 GHz) with a 100 kHz magnetic field modulation. Thermal analysis were carried out on SDT Q600 (TA) apparatus with a heating rate 10 ◦ C/min in nitrogen gas atmosphere. Alumina was used as a reference. 3. Results and discussion
Fig. 2. UV–visible absorption spectrum (inset: Band gap plot) of N-Methylaniline passivated ZnSe:Mn2+ Quantum dots.
indicates the quantum confinement of the particles. The radius of the nanocrystals R can be calculated using the following equation Eq. (2). 2
E = Eg +
2h ¯ 2R2
1 me
+
1 mh
−
1.8e2 + smaller terms εR
(2)
where E is the energy of the first excited state, Eg = 2.7 eV is the band gap energy of bulk ZnSe, me * and mh *are the effective masses of the electron and hole in ZnSe (me = 0.15 mo , mh = 0.66 mo , where mo = 9.11 × 10−28 gm, the free electron mass)[17], respectively the value of --C = 9.2, is the semiconductor dielectric constant, h ¯ = 6.58 × 10−16 eV is the reduced Plank constant, and e = 1.6 × 10−19 C is the electron charge. The size of the particle is calculated as 2.7 nm which is almost equal to the value observed in the TEM. Photoluminescence spectrum of N-Methylaniline passivated ZnSe: Mn2+ quantum dots is shown in Fig. 3. Here the excitation wavelength was at 250 nm and the emission peaks were found at different wavelengths such as 376 nm, 400 nm, 422 nm, 505 nm, 609 nm. The sharp band edge emission is centered at 376 nm, the peak centered at 400 nm and 420 nm are due to the trap state emission. The other peak centered at 505 nm is usually defect related and may originate from the self activated centers and the broad-
3.1. Structural and optical properties
5
8x10
376
5
7x10
400
5
PL Intensity (a.u)
XRD pattern of N-Methylaniline passivated ZnSe:Mn2+ quantum dots is shown in Fig. 1. All the reflection peaks can be indexed ˚ to cubic system with the lattice constant of (a = b = c = 5.65 A), which is in good agreement with the reported data (JCPDS card no: 010690). No other peaks related to impurities were detected. UV–visible absorption spectrum of N-Methylaniline passivated ZnSe: Mn2+ quantum dots is shown in Fig. 2. It exhibits absorption edge at 300 nm which is blue shifted from that of the bulk ZnSe whose absorption edge is located at 460 nm [16]. The band gap of Mn2+ doped ZnSe quantum dots are derived based on the well established equation Eq. (1).
6x10
5
5x10
422
505
5
4x10
5
3x10
5
609
2x10
5
A(h − Eg ) ˛= h
1x10
1/2
(1)
where ˛, Eg and A are the absorption coefficient, band gap and constant, respectively. By extrapolating the linear region in the plots of (˛h)2 versus h (inset of Fig. 2.), the band gap value is estimated as 4.2 eV. But the band gap of the bulk ZnSe is 2.7 eV. This blue shift
0 250
300
350
400
450
500
550
600
650
700
Wavelength (nm) Fig. 3. Photoluminescence spectrum of N-Methylaniline passivated ZnSe:Mn2+ Quantum dots.
J. Archana et al. / Applied Surface Science 257 (2011) 7699–7703
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Fig. 4. EDAX spectrum of N-Methylaniline passivated ZnSe:Mn2+ Quantum dots.
shown in Fig. 5. This FTIR analysis is undertaken in order to establish the chemisorbed N-Methylaniline on the surface of nanoparticles. This analysis could provide the evidence that N-Methylaniline acts as a ligand to control the size of the nanoparticles. In this spectrum the peaks, observed at 3403 cm−1 , 1579 cm−1 and 1025 cm−1 are attributed due to N-H and C-N stretching of N-Methylaniline. This clearly confirms that N-Methylaniline covers the surface of ZnSe: Mn2+ nanoparticles.
100
1025
60 40
3.3. Formation mechanism
20 0 4000
3500
1579
3403
Transmittance (%)
80
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 5. FTIR spectrum of N-Methylaniline passivated ZnSe:Mn2+ Quantum dots.
ened peak observed at 609 nm is due to the deep level formed by the incorporation of Mn2+ in the ZnSe quantum dots [17,18].
Fig. 6 indicates the formation mechanism of NMA passivated ZnSe: Mn2+ quantum dot. ZnSe is formed using zinc acetate and sodium selenium as precursors. When manganese sulphate is introduced to the above solution which leads to the formation of ZnSe: Mn2+ . In addition to that, when NMA is added; it attaches to the surface of ZnSe: Mn2+ nanoparticles due to its chemisorptions property and also due to the affinity of the nitrogen in the NMA molecule. This forms a co-ordination bond between the surfaces of the ZnSe: Mn2+ nanoparticles. 3.4. Morphological study
3.2. Spectroscopic studies Fig. 4 describes the EDAX spectrum of Mn2+ doped ZnSe nanoparticles. It indicates that the nanoparticles contain Zn, Se and Mn. The atomic percentage of Zn: Se: Mn is 55.19: 43.12: 1.69. It is noted that no impurity in the nanoparticles was observed with in the detection limit of EDAX. FTIR spectrum of Mn2+ doped ZnSe is
Fig. 7(a) shows the TEM micrograph of N-Methylaniline passivated ZnSe: Mn2+ quantum dots. The dispersed quantum dots can be clearly seen and size of the quantum dots is in the range of 2–5 nm. The size distribution is fairly narrow as illustrated by particle size histograms shown in Fig. 7(b) and most of the quantum dots are in the size of 2 nm. High resolution transmission electron
Fig. 6. The schematic diagram for the formation mechanism of N-Methylaniline passivated ZnSe:Mn2+ Quantum dots.
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J. Archana et al. / Applied Surface Science 257 (2011) 7699–7703
Fig. 7. (a) TEM image, (b) Histogram of particle size distribution, (c) HRTEM images and (d) SAED image of N- Methylaniline passivated ZnSe:Mn2+ Quantum dots.
microscopy images are shown in Fig. 7(c). Higher magnification images showed that the size of the particles is about 2–5 nm. Also it indicates that the quantum dots are crystalline. Fig. 7(d) is a typical selected–area electron diffraction (SAED) pattern of the ZnSe: Mn2+ quantum dots. These pattern spots can be readily indexed as the (3 1 1), (4 0 0), (3 3 1), (4 4 0), (5 1 1), (5 3 1) planes for the cubic structure of ZnSe.
3.5. Electron paramagnetic resonance (EPR) EPR is a unique tool to monitor the real environment of paramagnetic species in the local environment (crystal lattice). Fig. 8 shows a typical EPR spectrum of N-Methylaniline ZnSe: Mn2+ quantum dots. The figure illustrates a single broadened Lorentizian band which indicates the domination of Mn2+ –Mn2+ exchange dipolar
J. Archana et al. / Applied Surface Science 257 (2011) 7699–7703
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135 ◦ C–200 ◦ C region is 4% and it can be attributed the decomposition of N-Methylaniline molecules by increasing the temperature because the boiling point of NMA is 195 ◦ C; further 21% of weight loss occurred in the region of 200 ◦ C–395 ◦ C, it is due to the evaporation of the NMA molecules from the surface of the ZnSe:Mn2+ quantum dots. Normally, thermal stability of the nanomaterials is quite lower than that of bulk. TGA results suggests that the synthesized NMA passivated ZnSe:Mn2+ quantum dots is highly suitable for opto-electronic applications in the room temperature range. 4. Conclusion
Fig. 8. EPR spectrum of N-Methylaniline passivated ZnSe:Mn2+ Quantum dots.
100
Weight ( %)
90
80
Mn2+ doped ZnSe quantum dots have been successfully synthesized by simple wet chemical route. NMA acts as template in controlling the growth of ZnSe: Mn2+ quantum dots. The present study illustrates the incorporation of Mn2+ ions in ZnSe lattice confines the particle size without appreciable decrease in PL intensity. XRD reveals that the synthesized ZnSe: Mn2+ quantum dots were well formed without impurity. The TEM micrograph indicates the distribution of particle size of the order of 2–5 nm. The FTIR and EDAX measurement confirmed the presence of NMA and Mn2+ in the synthesized quantum dots. The substitutional incorporation of Mn2+ in to the ZnSe quantum dot is confirmed by EPR measurement. The UV and PL spectrum shows the blue shift which further confirms the quantum dots is comparatively smaller. Since there is high luminescence property it can be used in many applications especially in solar cells. References [1] [2] [3] [4] [5]
70
60 100
200
300
400
500
[6] [7]
Temperature (°C) [8] Fig. 9. TGA Traces of N-Methylaniline passivated ZnSe:Mn2+ Quantum dots.
interactions. Mn has the tendency to form pairs and clusters with an increase in the number of Mn atoms and reduces the hyperfine interactions and also represents the partial segregation of Mn2+ at the surface of ZnSe nanocrystals [19,20]. The presence of Mn2+ in ZnSe is in good agreement with the stoichiometric ratio from the EDAX spectrum
[9] [10] [11] [12] [13] [14] [15] [16]
3.6. Thermal study
[17]
The thermal stability of the synthesized NMA capped ZnSe:Mn2+
[18]
quantum dots is shown in Fig. 9. Three weight loss events at 135 ◦ C–200 ◦ C, 200 ◦ C–395 ◦ C and 395 ◦ C–500 ◦ C were observed in the TGA trace. The percentage of the weight loss in the
[19] [20]
Y. Nakaoka, Y. Nosaka, Langmuir 13 (1997) 708–713. A.P. Alivisatos, Science 271 (1996) 933–937. X. Duan, Y. Huang, R. Agarwal, C.M. Lieber, Nature 421 (2003) 241–245. M. Lazell, P. O’Brien, J. Mater. Chem. 231 (1999) 1381–1382. Z.F. Ren, Z.P. Huang, J.W. Ku, J.H. Wang, P. Bush, M.P. Siegal, P.N. Provencio, Science 282 (1998) 1105–1107. A.G. Stanelyin, R. Wolfe (Eds.), App. Solid State Sci. 15 (1975). R. Leon, P.M. Petroff, D. Leonard, J.L. Merz, P.M. Petroff, Science 267 (1995) 1966–1968. F.T. Quinlan, J. Kuther, W. Tremel, W. Knoll, S. Risbud, P. Stroeve, Langmuir 16 (2000) 4049–4051. A.B. Panda, S. Acharya, S. Efriman, Y. Golan, Laangmuir 23 (2007) 765–770. H. Luo, J.K. Furdyna, Semicond. Sci. Technol. 10 (1995) 1041–1048. R.N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416–419. N. Pradhan, X. Peng, J. Am. Chem. Soc. 129 (2007) 3339–3347. S.B. Mirov, V.V. Fedorov, I.S. Moskalev, D.V. Martyshkin, IEEE. J. Sel. Top. Quantum Electron 13 (2007) 810–822. H. Yang, S. Santra, P.H. Holloway, J. Nanosci. Nanotechnol. 5 (2005) 1364–1375. D.A. Bussian, S.A. Crooker, M. Yin, M. Brynda, A.L. Efros, V.I. Klimov, Nat. Mater. 8 (2009) 35–40. Aparna C. Deshpande, Shashii B. Singh, Majid Kazemian Abyaneh, Renu Pasricha, S.K. Kulkarni, Mater. Lett. 62 (2008) 3803–3805. Saim M. Emin, Norihito Sogoshi, Seilichrio Nakabayashi, Takashi Fujihara, Ceco D. Dushkin, J. Phys. Chem. C 113 (2009) 3998–4007. H.C. Liang, H. Xiang, M.Z. Rong, M.Q. Zhang, H.M. Zeng, S.F. Wang, Q.H. Gong, Chin. Phys. Lett. 19 (2002) 871–874. I. Zutic, J. Fabian, S.D. Sarma, Rev. Mod. Phys. 76 (2004) 323–410. D. Sayan Bhattacharyya, A. Zitoun, J. Gedanken, Phys. Chem. C 112 (2008) 7624–7630.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Organic molecules passivated Mn doped Zinc Selenide quantum dots and its properties J. Archana a , M. Navaneethan a,b , S. Ponnusamy a , Y. Hayakawa b , C. Muthamizhchelvan a,∗ a b
Center for Material science and Nano devices, Department of Physics, SRM University, Kattankulathur, Chennai 603203, Tamil Nadu, India Research Institute of Electronics, Shizuoka University, 3-5-1, Johoku, Naka- ku Hamamatsu, Shizuoka 432-8011, Japan
a r t i c l e
i n f o
Article history: Received 4 September 2010 Received in revised form 24 January 2011 Accepted 2 April 2011 Available online 12 April 2011 Keywords: Mn doped Zinc Selenide Nanoparticles Wet chemical method Optical properties Structural properties
a b s t r a c t Quantum dots of Mn doped Zinc Selenide with N-Methylaniline as the capping agent was prepared by simple and inexpensive wet chemical method. Size of the particles observed by TEM was of the order of 2–4 nm which was well consistent with the size measured by UV analysis. The presence of paramagnetic substance Mn2+ in the ZnSe quantum dots was confirmed by EPR measurement. Mn doped ZnSe nanoparticles exhibited a strong blue emission that was strongly dependent upon the Mn dopant level and the surface passivation produced by N-Methylaniline. The stability of the product was studied by thermal analysis which shows that this product is highly suitable for opto-electronic applications. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, there has been considerable interest in semiconductors of nanometric dimensions due to their potential applications [1–3]. The nanometric semiconductor particles exhibit novel properties due to the large number of surface atoms and three dimensional confinement of electrons. Altering the size of the particles changes the degree of confinement of the electrons and affects the electronic structure of the solid, especially the band edges which are tunable with particle size [4,5]. Binary compound semiconductor nanomaterial possess many interesting potential applications in photo detectors, solar cells, light emitting diodes, mesocopic, electronic and optical devices [6–8]. Extensive studies have been reported, in particular, on the optical properties of transition metal doped semiconductor nanoparticles [9,10]. ZnSe is one of the II–VI compounds semiconductors which have the band gap of 2.7 eV and transmittance range is 0.5–22 m. It has high luminescence efficiency, low absorption coefficient and excellent transparency in the infrared region. Bhargava et al. reported for the first time that the photoluminescence of Mn2+ doped ZnS nanoparticles have enhanced quantum efficiency compared to the bulk crystal [11]. Mn2+ doped ZnSe nanoparticles have been realized a potential candidate for various applications such as highly emissive nanocrystals for bio imaging, wavelength
∗ Corresponding author. Tel.: +91 44 27456020; fax: +91 44 27452343. E-mail address: [email protected] (C. Muthamizhchelvan). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.04.012
tunable lasers, solar cells and diluted magnetic semiconductors for spintronics applications [12–15]. Therefore, it is very important to investigate how the doped semiconductor nanoparticles affect optical properties from the view points of basic physics and applications. It is aimed in the present work to study the effect of Mn2+ ion doped ZnSe quantum dots by simple wet chemical route with N-Methylaniline as the capping agent. The structure of the ZnSe:Mn2+ nanoparticles has been determined by X-ray diffractometer (XRD). The band gap was calculated from ultraviolet (UV) visible absorption spectrum. The luminescence property has been investigated using photoluminescence (PL) spectral studies. The presence of NMA and Mn2+ was confirmed by Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray analysis (EDAX) and electron paramagnetic resonance (EPR) measurements. The morphological characterization was carried out by transmission electron microscope (TEM) analysis. 2. Experimental method All the reagents were of analytical grade and used without further purification. 0.2 M solution of zinc acetate monohydrate (CH3 COO2 )Zn·H2 O) was prepared in 20 ml of distilled water and 0.02 M manganese sulphate (MnSO4 ) was added to the solution. After that 20 ml of solution of 0.2 M sodium selenite (Na2 SeO3 ·5H2 O) solution was added. Finally, 0.2 ml NMA was added drop wise to the same solution as a capping agent. It is stirred vigorously for 36 h using magnetic stirrer. The role of NMA is to sta-
7700
J. Archana et al. / Applied Surface Science 257 (2011) 7699–7703
(511) (531)
200
(400)
300
(440)
400
(220)
Intensity (cps)
500
(331)
(311)
600
100 0
20
30
40
50
60
2θ (degree) Fig. 1. XRD pattern of N-Methylaniline passivated ZnSe:Mn2+ Quantum dots.
bilize the particles against aggregation. The resultant product was dried at 180 ◦ C for 10 h. The structure of N-Methylaniline passivated ZnSe: Mn2+ quantum dots were characterized by X-ray diffraction (XRD) using X’per PRO (PANalytical) advanced X-ray Diffractometer with CuK␣ ˚ with 2 ranging between 20–80◦ at the radiation ( = 1.5406 A), ◦ scanning rate of 0.05 per 2 s. Optical properties were measured using Perkin Elmer lambda 5 UV–visible spectrophotometer. The PL spectrum was obtained by Flurolog-3 spectrophotometer (Jobin Yvon) equipped with a He–Cd lamp as the excitation source in the range 200–900 nm. Morphology studies were made using a JEOL 2010 electron microscope with an accelerating voltage of 200 kV. Histogram of the particles size distribution was measured by iTEM software which is attached with TEM. The EDAX spectrum was obtained by Hitachi S-3400. The FTIR spectrum were recorded using Perkin Elmer Spectrophotometer in the range of 400–4000 cm−1 . The electron paramagnetic resonance (EPR) spectrum was recorded on a Burker EPR spectrometer (ER083CS) operating at an X-band (v = 9.77 GHz) with a 100 kHz magnetic field modulation. Thermal analysis were carried out on SDT Q600 (TA) apparatus with a heating rate 10 ◦ C/min in nitrogen gas atmosphere. Alumina was used as a reference. 3. Results and discussion
Fig. 2. UV–visible absorption spectrum (inset: Band gap plot) of N-Methylaniline passivated ZnSe:Mn2+ Quantum dots.
indicates the quantum confinement of the particles. The radius of the nanocrystals R can be calculated using the following equation Eq. (2). 2
E = Eg +
2h ¯ 2R2
1 me
+
1 mh
−
1.8e2 + smaller terms εR
(2)
where E is the energy of the first excited state, Eg = 2.7 eV is the band gap energy of bulk ZnSe, me * and mh *are the effective masses of the electron and hole in ZnSe (me = 0.15 mo , mh = 0.66 mo , where mo = 9.11 × 10−28 gm, the free electron mass)[17], respectively the value of --C = 9.2, is the semiconductor dielectric constant, h ¯ = 6.58 × 10−16 eV is the reduced Plank constant, and e = 1.6 × 10−19 C is the electron charge. The size of the particle is calculated as 2.7 nm which is almost equal to the value observed in the TEM. Photoluminescence spectrum of N-Methylaniline passivated ZnSe: Mn2+ quantum dots is shown in Fig. 3. Here the excitation wavelength was at 250 nm and the emission peaks were found at different wavelengths such as 376 nm, 400 nm, 422 nm, 505 nm, 609 nm. The sharp band edge emission is centered at 376 nm, the peak centered at 400 nm and 420 nm are due to the trap state emission. The other peak centered at 505 nm is usually defect related and may originate from the self activated centers and the broad-
3.1. Structural and optical properties
5
8x10
376
5
7x10
400
5
PL Intensity (a.u)
XRD pattern of N-Methylaniline passivated ZnSe:Mn2+ quantum dots is shown in Fig. 1. All the reflection peaks can be indexed ˚ to cubic system with the lattice constant of (a = b = c = 5.65 A), which is in good agreement with the reported data (JCPDS card no: 010690). No other peaks related to impurities were detected. UV–visible absorption spectrum of N-Methylaniline passivated ZnSe: Mn2+ quantum dots is shown in Fig. 2. It exhibits absorption edge at 300 nm which is blue shifted from that of the bulk ZnSe whose absorption edge is located at 460 nm [16]. The band gap of Mn2+ doped ZnSe quantum dots are derived based on the well established equation Eq. (1).
6x10
5
5x10
422
505
5
4x10
5
3x10
5
609
2x10
5
A(h − Eg ) ˛= h
1x10
1/2
(1)
where ˛, Eg and A are the absorption coefficient, band gap and constant, respectively. By extrapolating the linear region in the plots of (˛h)2 versus h (inset of Fig. 2.), the band gap value is estimated as 4.2 eV. But the band gap of the bulk ZnSe is 2.7 eV. This blue shift
0 250
300
350
400
450
500
550
600
650
700
Wavelength (nm) Fig. 3. Photoluminescence spectrum of N-Methylaniline passivated ZnSe:Mn2+ Quantum dots.
J. Archana et al. / Applied Surface Science 257 (2011) 7699–7703
7701
Fig. 4. EDAX spectrum of N-Methylaniline passivated ZnSe:Mn2+ Quantum dots.
shown in Fig. 5. This FTIR analysis is undertaken in order to establish the chemisorbed N-Methylaniline on the surface of nanoparticles. This analysis could provide the evidence that N-Methylaniline acts as a ligand to control the size of the nanoparticles. In this spectrum the peaks, observed at 3403 cm−1 , 1579 cm−1 and 1025 cm−1 are attributed due to N-H and C-N stretching of N-Methylaniline. This clearly confirms that N-Methylaniline covers the surface of ZnSe: Mn2+ nanoparticles.
100
1025
60 40
3.3. Formation mechanism
20 0 4000
3500
1579
3403
Transmittance (%)
80
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 5. FTIR spectrum of N-Methylaniline passivated ZnSe:Mn2+ Quantum dots.
ened peak observed at 609 nm is due to the deep level formed by the incorporation of Mn2+ in the ZnSe quantum dots [17,18].
Fig. 6 indicates the formation mechanism of NMA passivated ZnSe: Mn2+ quantum dot. ZnSe is formed using zinc acetate and sodium selenium as precursors. When manganese sulphate is introduced to the above solution which leads to the formation of ZnSe: Mn2+ . In addition to that, when NMA is added; it attaches to the surface of ZnSe: Mn2+ nanoparticles due to its chemisorptions property and also due to the affinity of the nitrogen in the NMA molecule. This forms a co-ordination bond between the surfaces of the ZnSe: Mn2+ nanoparticles. 3.4. Morphological study
3.2. Spectroscopic studies Fig. 4 describes the EDAX spectrum of Mn2+ doped ZnSe nanoparticles. It indicates that the nanoparticles contain Zn, Se and Mn. The atomic percentage of Zn: Se: Mn is 55.19: 43.12: 1.69. It is noted that no impurity in the nanoparticles was observed with in the detection limit of EDAX. FTIR spectrum of Mn2+ doped ZnSe is
Fig. 7(a) shows the TEM micrograph of N-Methylaniline passivated ZnSe: Mn2+ quantum dots. The dispersed quantum dots can be clearly seen and size of the quantum dots is in the range of 2–5 nm. The size distribution is fairly narrow as illustrated by particle size histograms shown in Fig. 7(b) and most of the quantum dots are in the size of 2 nm. High resolution transmission electron
Fig. 6. The schematic diagram for the formation mechanism of N-Methylaniline passivated ZnSe:Mn2+ Quantum dots.
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J. Archana et al. / Applied Surface Science 257 (2011) 7699–7703
Fig. 7. (a) TEM image, (b) Histogram of particle size distribution, (c) HRTEM images and (d) SAED image of N- Methylaniline passivated ZnSe:Mn2+ Quantum dots.
microscopy images are shown in Fig. 7(c). Higher magnification images showed that the size of the particles is about 2–5 nm. Also it indicates that the quantum dots are crystalline. Fig. 7(d) is a typical selected–area electron diffraction (SAED) pattern of the ZnSe: Mn2+ quantum dots. These pattern spots can be readily indexed as the (3 1 1), (4 0 0), (3 3 1), (4 4 0), (5 1 1), (5 3 1) planes for the cubic structure of ZnSe.
3.5. Electron paramagnetic resonance (EPR) EPR is a unique tool to monitor the real environment of paramagnetic species in the local environment (crystal lattice). Fig. 8 shows a typical EPR spectrum of N-Methylaniline ZnSe: Mn2+ quantum dots. The figure illustrates a single broadened Lorentizian band which indicates the domination of Mn2+ –Mn2+ exchange dipolar
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135 ◦ C–200 ◦ C region is 4% and it can be attributed the decomposition of N-Methylaniline molecules by increasing the temperature because the boiling point of NMA is 195 ◦ C; further 21% of weight loss occurred in the region of 200 ◦ C–395 ◦ C, it is due to the evaporation of the NMA molecules from the surface of the ZnSe:Mn2+ quantum dots. Normally, thermal stability of the nanomaterials is quite lower than that of bulk. TGA results suggests that the synthesized NMA passivated ZnSe:Mn2+ quantum dots is highly suitable for opto-electronic applications in the room temperature range. 4. Conclusion
Fig. 8. EPR spectrum of N-Methylaniline passivated ZnSe:Mn2+ Quantum dots.
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Mn2+ doped ZnSe quantum dots have been successfully synthesized by simple wet chemical route. NMA acts as template in controlling the growth of ZnSe: Mn2+ quantum dots. The present study illustrates the incorporation of Mn2+ ions in ZnSe lattice confines the particle size without appreciable decrease in PL intensity. XRD reveals that the synthesized ZnSe: Mn2+ quantum dots were well formed without impurity. The TEM micrograph indicates the distribution of particle size of the order of 2–5 nm. The FTIR and EDAX measurement confirmed the presence of NMA and Mn2+ in the synthesized quantum dots. The substitutional incorporation of Mn2+ in to the ZnSe quantum dot is confirmed by EPR measurement. The UV and PL spectrum shows the blue shift which further confirms the quantum dots is comparatively smaller. Since there is high luminescence property it can be used in many applications especially in solar cells. References [1] [2] [3] [4] [5]
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[6] [7]
Temperature (°C) [8] Fig. 9. TGA Traces of N-Methylaniline passivated ZnSe:Mn2+ Quantum dots.
interactions. Mn has the tendency to form pairs and clusters with an increase in the number of Mn atoms and reduces the hyperfine interactions and also represents the partial segregation of Mn2+ at the surface of ZnSe nanocrystals [19,20]. The presence of Mn2+ in ZnSe is in good agreement with the stoichiometric ratio from the EDAX spectrum
[9] [10] [11] [12] [13] [14] [15] [16]
3.6. Thermal study
[17]
The thermal stability of the synthesized NMA capped ZnSe:Mn2+
[18]
quantum dots is shown in Fig. 9. Three weight loss events at 135 ◦ C–200 ◦ C, 200 ◦ C–395 ◦ C and 395 ◦ C–500 ◦ C were observed in the TGA trace. The percentage of the weight loss in the
[19] [20]
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