- Email: [email protected]
Optical properties of strongly luminescing mercaptoacetic-acid-capped ZnS nanoparticles
Journal of Luminescence 102–103 (2003) 768–773
Optical properties of strongly luminescing mercaptoacetic-acid-capped ZnS nanoparticles S. Wageha,b,*, Liu Shu-Mana, Fang Tian Youa, Xu Xu-Ronga a
Institute of Optoelectronic Technology, Northern Jiaotong University, Beijing, 100044, China b Faculty of Electronic Engineering, Minufiya University, Egypt
Abstract We synthesized zinc sulfide nanoparticles with mercaptoacetic acid as capping agent and obtained nanoparticles of different sizes. The optical properties of ZnS nanoclusters were studied by optical absorption and photoluminescence. High sample quality is reflected in sharp absorption features and strong band edge emission, which is tunable with particle size. The selectively excited luminescence properties and the coupling between electronic and vibrational excitation in ZnS nanoparticles are discussed. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.67.n; 78.67.Bf; 78.67.Hc Keywords: Nanoparticles; Optical absorption; Photoluminescence; X-ray
1. Introduction In the past decade a tremendous amount of research has been carried out on low-dimensional semiconductor systems. Colloidal semiconductor nanocrystallites have been studied intensively [1], and as there is a potential of application in nonlinear optical devices [2], focus has been on their optical properties. Such materials are promising for the production of optical sensitizers and photocatlysts [3–5], further, they can be used for light converting electrodes[6], electroluminescent application [7–9] and quantum devices [10–11].
*Corresponding author. Institute of Optoelectronic Technology, Northern Jiaotong University, Beijing, 100044, China. E-mail address: [email protected] (S. Wageh).
The optical properties can be tuned due to quantum-size effects, which effectively lead to a size-dependent variation of the band gap energy. Semiconductor nanocrystals can be prepared by several characteristic methods. Capped semiconductor nanocrystals made by colloidal method are of high quality and flexibility: they are highly monodisperse and can be treated as powders, or often have high quantum efficiency. Bulk ZnS is an important inorganic material for light emitting applications. It has a band gap of 3.7 eV at 300 K [12]. The optical interband transition corresponds to ultraviolet (UV) radiation. Previous studies of luminescence and absorption in colloidal ZnS have been performed for pure ZnS samples [13–15]. The reported luminescence spectra show a broad ultraviolet peak at around 420 nm. In this paper we report a synthesis of two sizes of ZnS quantum
0022-2313/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-2313(02)00639-7
S. Wageh et al. / Journal of Luminescence 102–103 (2003) 768–773
2. Experimental details ZnS nanoparticles having an average diameter ( hereafter called ZnS-I and ZnS-II, 35 and 44 A, respectively, were synthesized following the procedure reported earlier by Nakaoka et al. [16]. The synthesizing route followed a modification adopted by Ref. [17]. For ZnS-I: A solution of zinc acetate dehydrate and mercaptoacetic acid of dimethyl formamide (DMF) was adjusted to pH=8 with 2 M NaOH at N2 atmosphere. The solution was placed in three necked flask fitted with a septum and valves and was deareated with N2 bubbling for 10 min followed by dropwise addition of an aqueous solution of sodium sulfide (Na2S 9H2O). Then was refluxed at room temperature for 12 h. The particles were extracted by precipitation with addition of acetone solution. The resulting powder was separated by centrifuging and dried in vacuum at room temperature. For ZnS-II: The reaction mixture containing fixed amount of zinc acetate and thiourea in the presence of capping agent mercaptoacetic acid in an organic solvent dimethyl formamide (DMF) and refluxing the reaction mixture for 14 h under nitrogen atmosphere at room temperature. The particles were extracted as explained in synthesis ZnS-I. X-ray powder diffraction analysis was carried out using a Rigaku D/Max-2000 powder diffract-
ometer with CuKa radiation. Ultraviolet absorption spectra were measured on Shimadzu UV-3101 PC spectrometer. The photoluminescence (PL) and photoluminescence excitation (PLE) were taken by SPEX fluorology model FL3-21 spectrofluorometer, consisting of two double spectrometers. The first selects the desired excitation wavelength from the emission of xenon arc lamp. The second scans the emission spectrum with a photomultiplier tube. All optical measurements were conducted at room temperature.
3. Results and discussion Small and wide-angle X-ray diffractomatry has been performed on the powders of ZnS nanoparticles, Fig. 1 shows wide-angle X-ray diffractions of ZnS-I and ZnS-II samples. We have also shown the pattern of bulk ZnS corresponding to zinc blend phase for comparison. The diffractograms confirmed the crystallinity of the ZnS nanoparticles. The two samples exhibited pure zinc-blende crystal structure and the three diffractions correspond to (1 1 1), (2 2 0) and (3 1 1) planes of ZnS, respectively. The broadness of the diffraction peak is due to finite size of the nanoparticles and this broadness increased for ZnS-I sample due to its smaller size as shown in Fig. 1. The mean nanocrystal size obtained from full-width at halfmaximum intensity of the (1 1 1) zinc blende reflection according to the Debeye-scherrer [18] equation L ¼ 0:9l=bCos y; where L is the
Intensity (a. U)
dots by using different source for chalcogenide (S2) and mercaptacetic acid as-capping agent. The optical properties of these nanocrystallite displayed sharp optical absorption edge and welldefined excitonic feature. Furthermore, photoluminescence spectra showed a very narrow emission peak at 330 nm with full-width at half-maximum 20 nm. We have studied photoluminescence spectra by selective excitation. The nanocrystallites were characterized by small-angle as well as wideangle X-ray diffraction. We also present a comparison of the sizes of nanocrystallites obtained from X-ray and different models that relate the optical band gap with the size of the crystallites.
769
Zn S-I
Zn S-II 111 200
220
311
Zinc Blende
2θ (degrees) Fig. 1. Wide-angle X-ray diffraction pattern of ZnS-I and ZnSII nanocrystallites. The bulk ZnS pattern to the zinc blende phase is also shown for comparison.
S. Wageh et al. / Journal of Luminescence 102–103 (2003) 768–773
coherence length, L ¼ 34D in the case of spherical crystallites, D is the crystallite size, b is the halfwidth of diffraction peak, l is the X-ray wave( and y is the angle of diffraction. length (1.54056 A) ( for The average crystallite sizes 33 and 44 A ZnS-I and II respectively. Fig. 2 shows the small angle X-ray differaction peaks corresponding to ZnS-I and ZnS-II, respectively. The data of small angle diffraction can be converted to the nearestneighbor distances of the clusters in the powdered sample by using Bragg equation. These distances we used as a measure of the mean particle size including legends. The diffraction pattern of ZnS-I samples consists of shoulder and two peaks clearly found at 2.52, 5.5, and 8.5, respectively, and is surprisingly, the second peak at the double angle of the first peak and the third peak at 2y increased three fold of the first one. According to the Bragg ( which is larger than law the size of ZnS-I is 35 A, that estimated from the wide-angle X-ray diffraction. This difference arises due to that the Scherrer method probes only the crystalline region and do not take into account the thickness of the capping layer. The X-ray differactogram of ZnS-II consists of shoulder and two peaks at 2.2, 6.85, and 9.1, respectively which are not related with each other. This disagreement between the positions of the peaks may be due to presence of defects such as staking faults and distortions they affect the diffraction line shape and position. The average particle size of ZnS-II, determined from the first ( The uv–vis absorption spectra of shoulder is 44 A. both ZnS-I and ZnS-II are shown in Fig. 3. These spectra of the nanocrystallites are distinctly
Intensity (a. U)
Zn S-I
Zn S-II
2
4
6 8 2θ (degrees)
10
Fig. 2. Small-angle X-ray diffraction pattern of ZnS-I and ZnSII nanocrystallites.
4.0 3.5 absorption (a. U.)
770
Zn S-II
3.0 2.5
Zn S-I
2.0 1.5 1.0 0.5 0.0 300
350
400
Wavelength (nm) Fig. 3. UV–vis optical absorption spectra of ZnS-I and ZnS-II nanocrystallites.
different compared to the well-known featureless absorption edge of bulk ZnS that appears approximately at 340 nm. The spectra from nanocrystallites are structured with the absorption maximum at 291 and 306 nm for ZnS-I and ZnSII, respectively. Such sharp absorption spectra with clear excitonc feature indicate a narrow size distribution of the nanocrystallites. This structure in the absorption spectra corresponds to the 1Se– 1Sh exctionic transitions in the nanocrystallites. It is clear from the experimental spectra that there is a large shift of the absorption maximum of the nanocrystallites towards shorter wavelength. The relation between this shift and the crystallite size, according to [19,20] can be expressed as DE ¼
_2 p2 e2 1:786 2mR2 AR2 2 X 2n e sn þ ; R n R
where m is the reduced mass of electron and hole, (1=m ¼ 1=me þ 1=mh ), and A is the dielectric constant. The first term represents the confinement effect, the second one is the coulomb term and the third term is a result of a spatial correlation effect. This last size independent term is usually small but may be significant for semiconductors with small dielectric constant. (In ZnS, the effective mass m ¼ 0:176 me and e ¼ 8:3 [21].) Calculations based on the above equation yield the crystallite size 35 ( for ZnS-I and ZnS-II nanocrystallites, and 44 A respectively. Which are in a good agreement with
S. Wageh et al. / Journal of Luminescence 102–103 (2003) 768–773
Intensity (a. U.)
200000 180000 160000 140000 120000 100000 80000 60000 40000 20000 0
PLE PL
200
250
300 350 400 Wavelength (nm)
450
500
Fig. 4. Photoluminescence emission (PL) Spectra of ZnS-I sample with excitation wavelength (lexcitation ¼ 280 nm) and excitation (PLE) spectra of the same sample detected at 360 nm.
350000
Intensity (a. U.)
300000
pL PLE
250000 200000 150000 100000 50000 0
250 300 350 400 450 500 550 600 Wavelength (nm) Fig. 5. Photoluminescence emission (PL) Spectra of ZnS-II sample with excitation wavelength (lexcitation ¼ 305 nm) and excitation (PLE) spectra of the same sample detected at 360 nm.
PL Intensity (a. U.)
X-ray analysis. Otherwise we have calculated the crystallite size using tight binding approximation ( for ZnS-I and [22] they were about 30 and 37 A ZnS-II respectively. It is clear that these values are smaller than that estimated from Debye–Scherrer formula. There is a mismatch between the experiments and tight binding approximation this arises from the tight binding approximation does not take into account the Coulomb interactions. In Fig. 4 we compare the PL and PLE for ZnS-I sample. The stockes shift between the PL and PLE is about 0.1 eV .The Huang Rhys factors (or coupling constant) calculated from this stockes shift using Frank–Condon model is about 1.1. The PL spectrum consists of sharp peak around 330 nm showing electron hole recombination after relaxation (band edge emission) and a very small emission tail around 408 nm due to recombination via surface localized state (the trap state emission). The line width of the band edge emission (fullwidth at-half maximum) is about 20 nm. This small line width indicates that this sample has a very narrow size distribution. In Fig. 5 the PL and PLE spectra of ZnS-II, the photoluminescence has two peaks at different energies, the high energy at 345 nm with stockes shift from PLE about 0.30 eV. The Huang Rhys factors for this sample is 3.4, which is large compared with that one for ZnS-I sample. We attributed this large value of S for ZnS-II sample to extrinsic factors, such as the presence of charged point defects inside the dots
771
350000
Eexcitation 4.13 eV
300000 250000 200000 150000 100000 50000
4.96 eV
0 250
300
350 400 450 Wavelength (nm)
500
Fig. 6. Photoluminescence emission (PL) Spectra of ZnS-I sample with different excitation energy, starting from 4.95 to 4.13 eV with 10 nm steps.
that confirmed with the obtained results from X-ray analysis. More detailed information is obtained by measuring the PL at different excitation energy. This kind of measurement can reveal what species are responsible for the emission and it is also useful in checking the purity of the sample. Fig. 6 presenting a series of PL spectra was obtained at room temperature starting from 4.95 to 4.13 eV with 10 nm steps for ZnS-I sample. It is clear that the intensity of PL decreases with increasing the excitation energy. In contrast: the line width increases with tuning for higher energy of the absorption edge, that is due to all nanocrystals are excited simultaneously and the homogenous profile of luminescence is observed. On the other hand, the PL peak position is nearly independent
S. Wageh et al. / Journal of Luminescence 102–103 (2003) 768–773
772
PL Intensity (a. U.)
1600000 1400000
Eexcitation 3.44 eV
1200000 1000000 800000 600000 400000 200000
4.13 eV
0 300 350 400 450 500 550 600 650 Wavelength (nm) Fig. 7. Photoluminescence emission (PL) Spectra of ZnS-II sample with different excitation energy, starting from 3.44 to 4.13 eV with 10 nm steps.
Line Width (nm)
120
4. Conclusion
100
80
60 3.4
nearly equal the LO phonon in ZnS (43 meV). On the other hand by further increasing of the excitation energy the abrupt increase in the line width by 203 meV equals nearly the stretching vibration of C=O bond in mercaptoacetic acid this observation can be explained by the coupling between surface localized exciton and the local bonds of surface species (mercaptoacetic acid). Since Zn–S and C=O bonds are polar, the coupling between excitons and phonons occurs . through Frohlich intraction. These data suggest that the interaction between zinc and capping molecule is one of origin of the surface state and the broad PL band.
3.6 3.8 4.0 Excitation Energy (ev)
Fig. 8. Excitation energy dependence of the PL line width (fullwidth at half-maximum) for the second emission peak in ZnS-II sample.
of the excitation energy, suggesting that the origin of the emitting state is similar in all species. In Fig. 7a series of PL spectra were obtained at room temperature for ZnS-II sample starting from 4.13 eV going to lower energy with 5 nm steps. The higher and lower energy PL peaks slightly shifts to red upon lowering the excitation energy. These changes indicate that there is more than one emitting site in this sample, and the intensity ratio of the lower energy emission to the higher energy emission increases systematically upon increasing the excitation energy. In addition to the general increase in the PL intensity accompanied by red shift with decreasing excitation energy, as shown in Fig. 8, by increasing the excitation energy from 3.44 to 3.87 eV the line width of lower energy emission increased by nearly 46 meV which is
We have succeeded in synthesizing two different sizes of ZnS quantum dots with relatively narrow size distribution, using mercaptoacetic acid as capping agent from different source for chalcogenide (S2). X-ray diffraction provides definitive identification of the nanoparticles size and permits a determination of the average nanoparticle size. The average particle sizes calculated with different ( The optical techniques are about 33 and 44 A. absorption showed sharp absorption spectra with clear excitonc feature. The shift of the band gap with respect to the size of the nanocrystallites as observed in the optical absorption spectra of the nanocrystallites was compared with the results from model calculations that relate the band gap to the size of the crystallites. The PL spectrum for the sample synthesized by using Na2S 9H2O as a source for chalcogenide (S2) showed a sharp emission peak around 330 nm with line width 20 nm and small emission tail. While the sample synthesized using Thiourea as a source for chalcogenide (S2) showed two peaks, in which the lower energy peak has higher intensity than the higher energy peak.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant
S. Wageh et al. / Journal of Luminescence 102–103 (2003) 768–773
No. 2992530 and No. 19974002) and paper Foundation of Northern Jiaotong University.
References [1] H. Weller, Angew. Chem. Int. Ed. Engl. 32 (1993) 41. [2] L. Brus, Appl. Phys. A: Solids Surf. 53 (1991) 465. [3] A.J. Hoffman, G. Mills, H. Yee, M.R. Hoffmann, J. Chem. Phys. 96 (1992) 5546. [4] M. Kanemoto, T. Shiragami, C. Pac, S. Yanagida, J. Phys. Chem. 96 (1992) 3521. [5] S. Kuwabata, K. Nishida, R. Tsuda, H. Inoue, H. Yoneyama, J. Electrochem. Soc. 141 (1994) 1498. [6] M. Gaetzel, Platinum Met. Rev. 38 (1994) 151. [7] Y. Yang, S. Xue, S. Liu, J. Huang, J. Shen, Appl. Phys. Lett. 69 (1996) 377. [8] V. Colvin, M. Schlamp, A.P. Alivisatos, Nature (London) 370 (1994) 354. [9] B.O. Dabbousi, M.G. Bawendi, B.O. Onitsuka, M.F. Rubner, Appl. Phys. Lett. 66 (1995) 11.
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[10] E. Corcoran, Sci. Am. (1990) 74. [11] H. Sasakura, et al., J. Appl. Phys. 52 (1981) 6901. [12] G.F.J. Garlick, A.F. Gibson, J. Opt. Soc. Am. 39 (1945) 935. [13] W.G. Becker, A.J. Bard, J. Phys. Chem. 78 (1983) 4888. [14] A. Henglein, M. Gutierrez, Ber. Bunsenges. Phys. Chem. 87 (1983) 852. [15] H. weller, U. Koch, M. Gutierrez, A. Henglein, Ber. Bunsenges. Phys. Chem. 88 (1984) 649. [16] Y. Nakaoka, Y. Nosaka, Langmuir 13 (1997) 708. [17] T. vossmeyer, L. Katsikas, M. Giersig, I.G. Popovic, K. Diesner, A. Chemseddine, A. Eychmuller, H. Weller, J. Phys. Chem. 98 (1994) 7665. [18] A. Guinier, X-ray Diffraction, W.H. Freeman, San Francisco, CA, 1963. [19] R. Rossetti, R. Hull, J.M. Gibson, L.E. Brus, J. Chem. Phys. 82 (1985) 525. [20] J.P. Zheng, H.S. Kwok, J. Opt. Soc. Am. B 9 (11) (1992) 2047. [21] K. Sooklal, B.S. Cullum, S.M. Angel, et al., J. Phys. Chem. 100 (1996) 4551. [22] P.E. Lippens, M. Lanno, Phys. Rev. B. 39 (1989) 10935.
Optical properties of strongly luminescing mercaptoacetic-acid-capped ZnS nanoparticles S. Wageha,b,*, Liu Shu-Mana, Fang Tian Youa, Xu Xu-Ronga a
Institute of Optoelectronic Technology, Northern Jiaotong University, Beijing, 100044, China b Faculty of Electronic Engineering, Minufiya University, Egypt
Abstract We synthesized zinc sulfide nanoparticles with mercaptoacetic acid as capping agent and obtained nanoparticles of different sizes. The optical properties of ZnS nanoclusters were studied by optical absorption and photoluminescence. High sample quality is reflected in sharp absorption features and strong band edge emission, which is tunable with particle size. The selectively excited luminescence properties and the coupling between electronic and vibrational excitation in ZnS nanoparticles are discussed. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.67.n; 78.67.Bf; 78.67.Hc Keywords: Nanoparticles; Optical absorption; Photoluminescence; X-ray
1. Introduction In the past decade a tremendous amount of research has been carried out on low-dimensional semiconductor systems. Colloidal semiconductor nanocrystallites have been studied intensively [1], and as there is a potential of application in nonlinear optical devices [2], focus has been on their optical properties. Such materials are promising for the production of optical sensitizers and photocatlysts [3–5], further, they can be used for light converting electrodes[6], electroluminescent application [7–9] and quantum devices [10–11].
*Corresponding author. Institute of Optoelectronic Technology, Northern Jiaotong University, Beijing, 100044, China. E-mail address: [email protected] (S. Wageh).
The optical properties can be tuned due to quantum-size effects, which effectively lead to a size-dependent variation of the band gap energy. Semiconductor nanocrystals can be prepared by several characteristic methods. Capped semiconductor nanocrystals made by colloidal method are of high quality and flexibility: they are highly monodisperse and can be treated as powders, or often have high quantum efficiency. Bulk ZnS is an important inorganic material for light emitting applications. It has a band gap of 3.7 eV at 300 K [12]. The optical interband transition corresponds to ultraviolet (UV) radiation. Previous studies of luminescence and absorption in colloidal ZnS have been performed for pure ZnS samples [13–15]. The reported luminescence spectra show a broad ultraviolet peak at around 420 nm. In this paper we report a synthesis of two sizes of ZnS quantum
0022-2313/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-2313(02)00639-7
S. Wageh et al. / Journal of Luminescence 102–103 (2003) 768–773
2. Experimental details ZnS nanoparticles having an average diameter ( hereafter called ZnS-I and ZnS-II, 35 and 44 A, respectively, were synthesized following the procedure reported earlier by Nakaoka et al. [16]. The synthesizing route followed a modification adopted by Ref. [17]. For ZnS-I: A solution of zinc acetate dehydrate and mercaptoacetic acid of dimethyl formamide (DMF) was adjusted to pH=8 with 2 M NaOH at N2 atmosphere. The solution was placed in three necked flask fitted with a septum and valves and was deareated with N2 bubbling for 10 min followed by dropwise addition of an aqueous solution of sodium sulfide (Na2S 9H2O). Then was refluxed at room temperature for 12 h. The particles were extracted by precipitation with addition of acetone solution. The resulting powder was separated by centrifuging and dried in vacuum at room temperature. For ZnS-II: The reaction mixture containing fixed amount of zinc acetate and thiourea in the presence of capping agent mercaptoacetic acid in an organic solvent dimethyl formamide (DMF) and refluxing the reaction mixture for 14 h under nitrogen atmosphere at room temperature. The particles were extracted as explained in synthesis ZnS-I. X-ray powder diffraction analysis was carried out using a Rigaku D/Max-2000 powder diffract-
ometer with CuKa radiation. Ultraviolet absorption spectra were measured on Shimadzu UV-3101 PC spectrometer. The photoluminescence (PL) and photoluminescence excitation (PLE) were taken by SPEX fluorology model FL3-21 spectrofluorometer, consisting of two double spectrometers. The first selects the desired excitation wavelength from the emission of xenon arc lamp. The second scans the emission spectrum with a photomultiplier tube. All optical measurements were conducted at room temperature.
3. Results and discussion Small and wide-angle X-ray diffractomatry has been performed on the powders of ZnS nanoparticles, Fig. 1 shows wide-angle X-ray diffractions of ZnS-I and ZnS-II samples. We have also shown the pattern of bulk ZnS corresponding to zinc blend phase for comparison. The diffractograms confirmed the crystallinity of the ZnS nanoparticles. The two samples exhibited pure zinc-blende crystal structure and the three diffractions correspond to (1 1 1), (2 2 0) and (3 1 1) planes of ZnS, respectively. The broadness of the diffraction peak is due to finite size of the nanoparticles and this broadness increased for ZnS-I sample due to its smaller size as shown in Fig. 1. The mean nanocrystal size obtained from full-width at halfmaximum intensity of the (1 1 1) zinc blende reflection according to the Debeye-scherrer [18] equation L ¼ 0:9l=bCos y; where L is the
Intensity (a. U)
dots by using different source for chalcogenide (S2) and mercaptacetic acid as-capping agent. The optical properties of these nanocrystallite displayed sharp optical absorption edge and welldefined excitonic feature. Furthermore, photoluminescence spectra showed a very narrow emission peak at 330 nm with full-width at half-maximum 20 nm. We have studied photoluminescence spectra by selective excitation. The nanocrystallites were characterized by small-angle as well as wideangle X-ray diffraction. We also present a comparison of the sizes of nanocrystallites obtained from X-ray and different models that relate the optical band gap with the size of the crystallites.
769
Zn S-I
Zn S-II 111 200
220
311
Zinc Blende
2θ (degrees) Fig. 1. Wide-angle X-ray diffraction pattern of ZnS-I and ZnSII nanocrystallites. The bulk ZnS pattern to the zinc blende phase is also shown for comparison.
S. Wageh et al. / Journal of Luminescence 102–103 (2003) 768–773
coherence length, L ¼ 34D in the case of spherical crystallites, D is the crystallite size, b is the halfwidth of diffraction peak, l is the X-ray wave( and y is the angle of diffraction. length (1.54056 A) ( for The average crystallite sizes 33 and 44 A ZnS-I and II respectively. Fig. 2 shows the small angle X-ray differaction peaks corresponding to ZnS-I and ZnS-II, respectively. The data of small angle diffraction can be converted to the nearestneighbor distances of the clusters in the powdered sample by using Bragg equation. These distances we used as a measure of the mean particle size including legends. The diffraction pattern of ZnS-I samples consists of shoulder and two peaks clearly found at 2.52, 5.5, and 8.5, respectively, and is surprisingly, the second peak at the double angle of the first peak and the third peak at 2y increased three fold of the first one. According to the Bragg ( which is larger than law the size of ZnS-I is 35 A, that estimated from the wide-angle X-ray diffraction. This difference arises due to that the Scherrer method probes only the crystalline region and do not take into account the thickness of the capping layer. The X-ray differactogram of ZnS-II consists of shoulder and two peaks at 2.2, 6.85, and 9.1, respectively which are not related with each other. This disagreement between the positions of the peaks may be due to presence of defects such as staking faults and distortions they affect the diffraction line shape and position. The average particle size of ZnS-II, determined from the first ( The uv–vis absorption spectra of shoulder is 44 A. both ZnS-I and ZnS-II are shown in Fig. 3. These spectra of the nanocrystallites are distinctly
Intensity (a. U)
Zn S-I
Zn S-II
2
4
6 8 2θ (degrees)
10
Fig. 2. Small-angle X-ray diffraction pattern of ZnS-I and ZnSII nanocrystallites.
4.0 3.5 absorption (a. U.)
770
Zn S-II
3.0 2.5
Zn S-I
2.0 1.5 1.0 0.5 0.0 300
350
400
Wavelength (nm) Fig. 3. UV–vis optical absorption spectra of ZnS-I and ZnS-II nanocrystallites.
different compared to the well-known featureless absorption edge of bulk ZnS that appears approximately at 340 nm. The spectra from nanocrystallites are structured with the absorption maximum at 291 and 306 nm for ZnS-I and ZnSII, respectively. Such sharp absorption spectra with clear excitonc feature indicate a narrow size distribution of the nanocrystallites. This structure in the absorption spectra corresponds to the 1Se– 1Sh exctionic transitions in the nanocrystallites. It is clear from the experimental spectra that there is a large shift of the absorption maximum of the nanocrystallites towards shorter wavelength. The relation between this shift and the crystallite size, according to [19,20] can be expressed as DE ¼
_2 p2 e2 1:786 2mR2 AR2 2 X 2n e sn þ ; R n R
where m is the reduced mass of electron and hole, (1=m ¼ 1=me þ 1=mh ), and A is the dielectric constant. The first term represents the confinement effect, the second one is the coulomb term and the third term is a result of a spatial correlation effect. This last size independent term is usually small but may be significant for semiconductors with small dielectric constant. (In ZnS, the effective mass m ¼ 0:176 me and e ¼ 8:3 [21].) Calculations based on the above equation yield the crystallite size 35 ( for ZnS-I and ZnS-II nanocrystallites, and 44 A respectively. Which are in a good agreement with
S. Wageh et al. / Journal of Luminescence 102–103 (2003) 768–773
Intensity (a. U.)
200000 180000 160000 140000 120000 100000 80000 60000 40000 20000 0
PLE PL
200
250
300 350 400 Wavelength (nm)
450
500
Fig. 4. Photoluminescence emission (PL) Spectra of ZnS-I sample with excitation wavelength (lexcitation ¼ 280 nm) and excitation (PLE) spectra of the same sample detected at 360 nm.
350000
Intensity (a. U.)
300000
pL PLE
250000 200000 150000 100000 50000 0
250 300 350 400 450 500 550 600 Wavelength (nm) Fig. 5. Photoluminescence emission (PL) Spectra of ZnS-II sample with excitation wavelength (lexcitation ¼ 305 nm) and excitation (PLE) spectra of the same sample detected at 360 nm.
PL Intensity (a. U.)
X-ray analysis. Otherwise we have calculated the crystallite size using tight binding approximation ( for ZnS-I and [22] they were about 30 and 37 A ZnS-II respectively. It is clear that these values are smaller than that estimated from Debye–Scherrer formula. There is a mismatch between the experiments and tight binding approximation this arises from the tight binding approximation does not take into account the Coulomb interactions. In Fig. 4 we compare the PL and PLE for ZnS-I sample. The stockes shift between the PL and PLE is about 0.1 eV .The Huang Rhys factors (or coupling constant) calculated from this stockes shift using Frank–Condon model is about 1.1. The PL spectrum consists of sharp peak around 330 nm showing electron hole recombination after relaxation (band edge emission) and a very small emission tail around 408 nm due to recombination via surface localized state (the trap state emission). The line width of the band edge emission (fullwidth at-half maximum) is about 20 nm. This small line width indicates that this sample has a very narrow size distribution. In Fig. 5 the PL and PLE spectra of ZnS-II, the photoluminescence has two peaks at different energies, the high energy at 345 nm with stockes shift from PLE about 0.30 eV. The Huang Rhys factors for this sample is 3.4, which is large compared with that one for ZnS-I sample. We attributed this large value of S for ZnS-II sample to extrinsic factors, such as the presence of charged point defects inside the dots
771
350000
Eexcitation 4.13 eV
300000 250000 200000 150000 100000 50000
4.96 eV
0 250
300
350 400 450 Wavelength (nm)
500
Fig. 6. Photoluminescence emission (PL) Spectra of ZnS-I sample with different excitation energy, starting from 4.95 to 4.13 eV with 10 nm steps.
that confirmed with the obtained results from X-ray analysis. More detailed information is obtained by measuring the PL at different excitation energy. This kind of measurement can reveal what species are responsible for the emission and it is also useful in checking the purity of the sample. Fig. 6 presenting a series of PL spectra was obtained at room temperature starting from 4.95 to 4.13 eV with 10 nm steps for ZnS-I sample. It is clear that the intensity of PL decreases with increasing the excitation energy. In contrast: the line width increases with tuning for higher energy of the absorption edge, that is due to all nanocrystals are excited simultaneously and the homogenous profile of luminescence is observed. On the other hand, the PL peak position is nearly independent
S. Wageh et al. / Journal of Luminescence 102–103 (2003) 768–773
772
PL Intensity (a. U.)
1600000 1400000
Eexcitation 3.44 eV
1200000 1000000 800000 600000 400000 200000
4.13 eV
0 300 350 400 450 500 550 600 650 Wavelength (nm) Fig. 7. Photoluminescence emission (PL) Spectra of ZnS-II sample with different excitation energy, starting from 3.44 to 4.13 eV with 10 nm steps.
Line Width (nm)
120
4. Conclusion
100
80
60 3.4
nearly equal the LO phonon in ZnS (43 meV). On the other hand by further increasing of the excitation energy the abrupt increase in the line width by 203 meV equals nearly the stretching vibration of C=O bond in mercaptoacetic acid this observation can be explained by the coupling between surface localized exciton and the local bonds of surface species (mercaptoacetic acid). Since Zn–S and C=O bonds are polar, the coupling between excitons and phonons occurs . through Frohlich intraction. These data suggest that the interaction between zinc and capping molecule is one of origin of the surface state and the broad PL band.
3.6 3.8 4.0 Excitation Energy (ev)
Fig. 8. Excitation energy dependence of the PL line width (fullwidth at half-maximum) for the second emission peak in ZnS-II sample.
of the excitation energy, suggesting that the origin of the emitting state is similar in all species. In Fig. 7a series of PL spectra were obtained at room temperature for ZnS-II sample starting from 4.13 eV going to lower energy with 5 nm steps. The higher and lower energy PL peaks slightly shifts to red upon lowering the excitation energy. These changes indicate that there is more than one emitting site in this sample, and the intensity ratio of the lower energy emission to the higher energy emission increases systematically upon increasing the excitation energy. In addition to the general increase in the PL intensity accompanied by red shift with decreasing excitation energy, as shown in Fig. 8, by increasing the excitation energy from 3.44 to 3.87 eV the line width of lower energy emission increased by nearly 46 meV which is
We have succeeded in synthesizing two different sizes of ZnS quantum dots with relatively narrow size distribution, using mercaptoacetic acid as capping agent from different source for chalcogenide (S2). X-ray diffraction provides definitive identification of the nanoparticles size and permits a determination of the average nanoparticle size. The average particle sizes calculated with different ( The optical techniques are about 33 and 44 A. absorption showed sharp absorption spectra with clear excitonc feature. The shift of the band gap with respect to the size of the nanocrystallites as observed in the optical absorption spectra of the nanocrystallites was compared with the results from model calculations that relate the band gap to the size of the crystallites. The PL spectrum for the sample synthesized by using Na2S 9H2O as a source for chalcogenide (S2) showed a sharp emission peak around 330 nm with line width 20 nm and small emission tail. While the sample synthesized using Thiourea as a source for chalcogenide (S2) showed two peaks, in which the lower energy peak has higher intensity than the higher energy peak.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant
S. Wageh et al. / Journal of Luminescence 102–103 (2003) 768–773
No. 2992530 and No. 19974002) and paper Foundation of Northern Jiaotong University.
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