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Enhanced Photoluminescence and Characterization of Mn-Doped ZnS Nanocrystallites Synthesized in Microemulsion L. M. Gan,*,† B. Liu,† C. H. Chew,† S. J. Xu,‡ S. J. Chua,‡ G. L. Loy,§ and G. Q. Xu*,† Department of Chemistry, Center for Optoelectronics, Department of Electrical Engineering, and School of Biological Sciences, National University of Singapore, Republic of Singapore 119260 Received May 27, 1997. In Final Form: September 15, 1997X ZnS:Mn luminescent nanomaterials were first prepared in an inverse microemulsion at room temperature as well as under a hydrothermal condition. Mn-doped ZnS nanoparticles obtained are distributed from 3 to 18 nm in diameter as determined by transmission electron microscopy. The crystalline nature of the materials is clearly demonstrated by the X-ray diffraction results. Compared with Mn-doped ZnS materials synthesized through the conventional aqueous reaction, the nanoparticles prepared in a microemulsion show a significant enhancement in photoluminescence. In particular, the photoluminescence of particles prepared in microemulsion under hydrothermal treatment was found to be enhanced by a factor of 60 as compared to that of the material obtained through the direct aqueous reaction at room temperature. This dramatic increase in photoluminescence yield is attributed to the surface passivation of nanoparticles by the adsorption of surfactants in microemulsion, the formation of sphalerite with cubic zinc blende structure, and Mn migration into the interior lattice of ZnS host.
1. Introduction Semiconductor nanocrystals have attracted much attention in the past few years because of their unique physical properties such as size quantization,1,2 nonlinear optical behaviors,3,4 and unusual luminescence.5-7 Doped nanosize semiconductor, a new class of luminescent material, has been demonstrated to have a much higher quantum efficiency of photoluminescence as well as a luminescent decay several orders of magnitude faster than that for bulk crystals.7,8 To improve its applicability, great efforts have been made to search for materials with higher luminescence efficiency.9-11 Doping zinc sulfide with manganese is usually made by the thermal diffusion of Mn salt at high temperature.12,13 Such a thermal diffusion process is inferior to nanometersized particles because nanocrystals will coagulate and * To whom correspondence should be addressed. † Department of Chemistry. ‡ Department of Electrical Engineering. § School of Biological Sciences. X Abstract published in Advance ACS Abstracts, November 1, 1997. (1) Weller, H.; Schmidt, H. M.; Koch, U.; Fojtik, A.; Barel, S.; Henglein, A.; Kunath, W.; Weiss, K.; Diemann, E. Chem. Phys. Lett. 1986, 124, 557. (2) Rama, K. M. V.; Friesner, R. A. J. Chem. Phys. 1991, 95, 8309. (3) Huang, H. H.; Yan, F. Q.; Kek, Y. M.; Chew, C. H.; Xu, G. Q.; Ji, W.; Oh, P. S.; Tang, S. H. Langmuir 1997, 13, 172. (4) Hilinski, E. F.; Lucas, P. A.; Wang, Y. J. Chem. Phys. 1988, 89, 3435. (5) Spanhel, L.; Hnase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649. (6) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (7) Bhargava, B. N.; Gallagher,D.; Hong, X.; Nurmikko, A. Phys. Rev. Lett. 1994, 72, 416. (8) Bhargava, R. N.; Gallagher, D.; Welker, T. J. Lumin. 1994, 60 & 61, 275. (9) Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M. G.; Steigerwald, M. L.; Carroll, P. L.; Brus, L. E. J. Am. Chem. Soc. 1990, 112, 1327. (10) Saklal, K.; Cullum, B. S.; Angel, S.; Murphy, C. L. J. Phys. Chem. 1996, 100, 4551. (11) Henglein, A. Top. Curr. Chem. 1988, 143, 133. (12) Shanker, V.; Tanaka, S.; Shiki, M.; Deguchi, H.; Kobayashi, H.; Sasakura, H. Appl. Phys. Lett. 1984, 45, 960. (13) Chadha, S. S.; Vecht, A. In Electroluminescence. Springer proceedings in Physics; Springer: Berlin, 1988; Vol. 38, p 337.
S0743-7463(97)00546-5 CCC: $14.00
melt even at a reduced temperature. Thus, it is necessary to dope ZnS chemically during precipitation at a relatively low temperature in order to obtain doped nanoparticles. Moreover, for quantum confinement in ZnS:Mn, a surfactant can be employed to act as a barrier, to prevent particle-particle contact or coagulation. Recently, several different ways like matrix-mediated growth technique,14 a chemical route,15 coating method,16 and sol-gel processing17 have been developed. The characteristics of these methods are that they require either strict conditions or higher temperature treatment which is not beneficial to the existence of nanoparticles. On the other hand, the solution-phase microemulsion technique18-20 provides an ideal medium for preparing Mn-doped ZnS nanoparticles. In addition, various oxides, complex oxides, and spherical sphalerite with narrow particle size distribution, single phase, and controlled particle morphology can be synthesized by the hydrothermal technique.21,22 In this work, we first report the results of Mn-doped ZnS nanomaterials prepared by combining the hydrothermal treatment with inverse microemulsion at 120 °C. The optically active luminescent centers were successfully doped into ZnS nanoparticles. The characterization of the particle size and crystalline structure was accomplished by the use of transmission electron microscopy (TEM) and X-ray diffraction (XRD), respectively. The studies on photoluminescence properties of Mn-doped ZnS were performed using a laser source plus SPEX monochromator. Our results demonstrate an enhanced pho(14) Gallagher, D.; Heady, W. E.; Racz, J. M.; Bhargava, R. N. J. Mater. Res. 1995, 10, 870. (15) Khosic, A. A.; Kunalu, M.; Gulwa, L.; Deshpande, S. K.; Bhagwat, U. A.; Sastry, M.; Kulkarni, S. K. Appl. Phys. Lett. 1995, 67, 2702. (16) Yu, I.; Isobe, T.; Senna, M.; Takahashi, S. Mater. Sci., Eng. 1996, B38, 177. (17) Tang, W.; Cameron, D. C. Thin Solid Films 1996, 280, 221. (18) Gan, L. M.; Li, T. D.; Chew, C. H.; Teo, W. K.; Gan, L. H. Langmuir 1995, 11, 3316. (19) Gan, L. M.; Chew, L. H. In Polymeric Materials Encyclopedia; Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; Vol. 6, p 4321. (20) Gan, L. M.; Zhang, L. H.; Chan, H. S. O.; Chew, C. H.; Loo, B. H. J. Mater. Sci. 1996, 31, 1071. (21) Dawson, W. J. Ceram. Bull. 1988, 67, 1673. (22) Qian, Y.; Su, Y.; Xie, Y.; Chen, Q.; Chen, Z. Mater. Res. Bull. 1995, 30, 601.
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toluminescence yield of the nanoparticles synthesized in microemulsion. 2. Experimental Section 2.1. Materials. Manganese(II) chloride (analytical reagent) and zinc sulfide (analytical reagent) were from Merck, sodium sulfide (analytical reagent) was from Hopkin & Williams LTD, petroleum ether (PE, boiling point 60-80 °C) was from BDH, nonionic surfactants of poly(oxyethylene)5 nonyl phenol ether (NP5) and poly(oxyethylene)9 nonyl phenol ether (NP9) were from Albright & Wilson Asia Pte Ltd, and acetone was from J. K. Baker. All reagents and solvents were used as received. Water was purified by a Milli Q water purification system (resistance 18.2 kΩ). 2.2. Microemulsion Phase Diagram. The single-phase microemulsion region was determined visually by titrating a specific amount of petroleum ether and the mixture (weight ratio 2:1) of NP5 and NP9 with aqueous solution containing 0.1 M ZnCl2/0.01 M MnCl2 or 0.1 M Na2S in a screw-capped tube at 30 °C. Each titration was thoroughly mixed using a Vortex mixer. The clear-turbid boundaries were established from the systematic titration. The transparent microemulsion region is represented by the shaded area as shown in Figure 1. 2.3. Synthesis of Mn-Doped ZnS Materials. Doped ZnS powders were prepared via four processing routes: (i) conventional reaction at room temperature, (ii) conventional reaction by hydrothermal treatment at 120 °C, (iii) inverse microemulsion at room temperature, and (iv) inverse microemulsion by hydrothermal treatment at 120 °C. For the conventional processing, zinc chloride (0.1 M) and manganese chloride (0.01 M) aqueous solutions were mixed with a mole ratio of 99% and 1%, respectively. Then a sodium sulfide (0.1 M) aqueous solution was added with constant stirring in a conical flask. For hydrothermal treatment, this mixture was put into a Teflonliner autoclave of 60 mL capacity and was heated at 120 °C for 15 h. After cooling, the sample was rinsed by distilled water and acetone, and dried in a vacuum oven at 30 °C. For the inverse microemulsion, petroleum ether (boiling point 60-80 °C) was used as the oil phase, and the mixture (weight ratio 2:1) of poly(oxyethylene)5 nonyl phenol ether (NP-5) and poly(oxyethylene)9 nonyl phenol ether (NP-9) as the surfactant phase. Two types of inverse microemulsion, noted as MA and MB, were prepared separately. Both MA and MB contain two common components, i.e., a surfactant mixture of NP-5 and NP-9 in a weight ratio of 2:1 and petroleum ether. The only difference is that MA consists of an aqueous solution of zinc chloride (0.1 M)/MnCl2 (0.01 M), whereas MB contains an aqueous solution of sodium sulfide (0.1 M). Equal amounts of MA and MB were then mixed with continuous stirring at room temperature. The microemulsion system can also be treated under hydrothermal conditions as mentioned above. The fine doped zinc sulfide particles were recovered by centrifugation and washed several times with acetone and then dried in a vacuum oven at 30 °C. 2.4. Characterization. The particle sizes of doped ZnS particles formed were determined by TEM with a JEOL-100CXII electron microscope at an accelerating voltage of 100 kV. For the inverse microemulsion, a drop of the mixed MA and MB was deposited on a copper grid coated with a thin film of Formvar and dried. For the conventional processing, a drop of aqueous solution was added. Powder X-ray diffraction (XRD) experiments were done on a Philips, PW 1729 X-ray generator using Cu KR radiation to identify the structure. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was employed to determine the elemental content of the ZnS:Mn system with use of a Plasmascan 710 instrument (Lab Tan Co.). The IR spectra of doped ZnS in microemulsion were obtained using a Fourier transform FTIR spectrometer (Perkin-Elmer Model 1600 series). 2.5. Photoluminescence Measurements. Photoluminescence measurements of Mn-doped ZnS particles were performed with the same amount of samples mounted on a metal holder with a hole. The excitation source was an Ar+ laser pumped Ti:sapphire laser. A SPEX 750M monochromator and a HAMAMATSU 1767 photomultiplier were used for the spectral analysis of the PL emission.
Figure 1. Phase diagram for the system consisting of petroleum ether-(NP-5+NP-9)-aqueous solutions of 0.1 M ZnCl2/0.01 M MnCl2 (solid line) and 0.1 M Na2S (dash line) at room temperature.
3. Results and Discussion 3.1. Phase Behavior of Inverse Microemulsion. An inverse microemulsion is a thermodynamically stable and optically isotropic dispersion of aqueous microdroplets in a continuous oil phase.20 The system is stabilized by surfactant molecules at the water-oil interface. Figure 1 shows the phase diagram for the system consisting of petroleum ether-(NP-5/NP-9)-aqueous solution of 0.1 M ZnCl2 and Na2S at room temperature. The microemulsion region is marked by the shaded area. Any mixtures of petroleum ether, surfactant (NP-5/NP-9), and aqueous solutions within the shaded region are clear. The transparency is apparently due to the small dispersion sizes (5-20 nm) of the aqueous phase.23 For a fixed oil/ surfactant weight ratio at 70/30, the maximum aqueous contents were limited to 47 wt % of 0.1 M ZnCl2/0.01 M MnCl2 and 48 wt % of 0.1 M Na2S for the formation of a microemulsion. In order to achieve a high yield of ZnS powder for a given amount of oil and surfactant, the microemulsion composition at point A, which consisted of 37.8% petroleum ether, 16.2% NP-5+NP-9, and 46 wt % aqueous section, was selected for the processing. 3.2. Morphology of Mn-Doped ZnS Particles. Typical transmission electron micrographs (TEM) for the samples of doped ZnS are displayed in Figure 2. It shows that the microemulsion systems at room temperature and with hydrothermal treatment produced ultrafine and agglomerate-free particles in the range of 3 to 18 nm in diameter, together with a narrow particle size distribution as indicated in parts c′ and d′ of Figure 2, respectively. This result is consistent with the microstructures of the system consisting of petroleum ether-NP5/NP9-aqueous phase.19 The particle size within the microemulsion region is limited by the nanometer aqueous droplets. In contrast, the particles prepared via conventional aqueous solutions were often coagulated in forming large aggregated clusters as shown in parts a and b of Figure 2. This system produced gelatinous precipitates immediately after mixing the aqueous ZnCl2 solution with the aqueous solution of Na2S. 3.3. Photoluminescence Properties. The results of photoluminescence (PL) studies on the samples synthesized with different processing routes are shown in Figure 3. All the emission spectra peak at around 590 (23) Langevin, D. Acc. Chem. Res. 1988, 21, 257.
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Figure 2. TEM of Mn-doped ZnS particles prepared through (a) conventional reaction at room temperature, (b) conventional reaction with hydrothermal treatment, (c) microemulsion at room temperature, and (d) microemulsion with hydrothermal treatment. (c′) and (d′) are the corresponding particle size distribution of the samples in (c) and (d), respectively.
nm are attributable to the transition from 4T1 to 6A1 state in Mn Td symmetry,24 which is characteristic of Mn2+ within a crystalline ZnS host matrix. The concentration of reagents used in our preparations of ZnS:Mn in microemulsion would provide 1 mol % manganese if all the manganese chloride was incorporated in the ZnS. The (24) Brus, L. IEEE J. Quantum Electron. 1986, 22, 1909.
actual doping concentration is approximately 0.81 mol % Mn measured by ICP-AES, which nearly closes with the measured values for bulk ZnS:Mn (0.87%).25 Two trends are noted from this figure. One is that the PL intensities of spectra c and d are much higher than (25) Gallagher, D.; Heady, W. E.; Racz, J. M.; Bhargava, R. N. J. Mater. Res. 1995, 10, 870.
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Figure 3. Photoluminescence (PL) of ZnS:Mn2+ prepared by the respective route described in Figure 2 with excitation intensity in 4 W/cm2.
Figure 5. XRD patterns for the dried powders prepared by the respective route described in Figure 2.
Figure 4. PL spectra of ZnS:Mn2+ at different excitation intensities of argon laser. The sample was prepared in microemulsion by hydrothermal treatment. Inset: the peak intensity versus the excitation density.
those of spectra a and b, showing the significant enhancement of photoluminescence yield for samples produced in microemulsion. The other is that the hydrothermal treatment can further increase the PL intensity of the samples as evidenced from the comparison of spectra a and b as well as c and d. We observed a dramatic increase (about 60-fold) in the luminescent intensity of Mn2+ emission for ZnS:Mn2+ prepared in microemulsion with hydrothermal treatment as compared with that obtained from the conventional aqueous reaction at room temperature. To demonstrate its single photon process, the photoluminescence spectrum of ZnS:Mn2+ at various excitation intensities has been done and is shown in Figure 4. It is noted that the PL intensity increases linearly with the excitation intensity as indicated in the inset of Figure 4, suggesting the involvement of a single photon process in the initial excitation. PL intensity depends on the
population of the states involved.26 With increasing the excitation density, the average occupation number on the Mn2+ excited state, 4T1, is higher, resulting in the improvement of PL intensity. The possible correlation between the photoluminescence and the structures of the Mn-doped ZnS synthesized through various routes were investigated by XRD. By comparing the XRD patterns between (d) and (c) as shown in Figure 5, it indicates that the sample (d) obtained by hydrothermal microemulsion route has a better crystalline structure compared with the sample (c) obtained from microemulsion at room temperature. A similar trend is also observed for the sample (a) produced from conventional aqueous reactions at room temperature and the sample (b) with hydrothermal treatment. This result indicates that concentrated Mn species are activated to migrate into the interior of ZnS16 under hydrothermal condition. In addition to the identical ionic states of the migrating Mn2+ and host lattice Zn2+, their ionic radii are also similar, i.e., 0.83 Å (Zn2+) and 0.80 Å (Mn2+). It is therefore anticipated that Mn2+ migration takes place by the simultaneous replacement of Zn2+, i.e., partial substitution of ZnS with MnS. Under the effect of crystal field in host ZnS, Mn2+ ions can luminesce effectively. The structural transition from amorphous to nanocrystalline sphalerite with fine cubic zinc blende structure is of benefit to luminescence.27 Our work as presented in Figures 3 and 5 confirms this structural effect on photoluminescence property. This means that the nanopar(26) Laiho, R.; Pavlov, A.; Hovi, O.; Tsuboi, T. Appl. Phys. Lett. 1993, 63, 275. (27) Chadha, S. S. In Solid State Luminescence: Theory, materials and Devices; Kitai, A. H., Ed.: Hhapman & Hall: London, 1993; p 166.
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Figure 6. FTIR spectrum of ZnS:Mn2+ powder produced in microemulsion by hydrothermal treatment.
ticles with a better crystalline structure prepared by a hydrothermal process give a higher photoluminescence intensity as compared with those respective samples produced at room temperature. However, we do not observe any obvious structural phase difference in the samples by the hydrothermal route either in a microemulsion (Figure 5d) or through conventional reaction (Figure 5b). Similarly, no structural phase difference was detected from the samples prepared at room temperature by two processes (Figure 5c and Figure 5a). This observation implies that the structure of the sample strongly depends on the processing temperature, rather than the reaction system, i.e., microemulsion or the conventional reaction. On the other hand, by comparing the intensity of PL between spectra d vs b and c vs a in Figure 3, a significant enhancement of PL for samples produced in an inverse microemulsion can be noticed, indicating the existence of the other factor affecting the photoluminescence in addition to the structural effect. The surface passivation of the ZnS:Mn may also be crucial for improving the intensity of luminescence. This can be derived from surfactant molecules in microemulsion that provide a chemical surface passivation, resulting in enhancing luminescence intensity. It is wellknown that the emission spectrum and efficiency are very sensitive to the nature of the nanoparticle surface,14 due to the presence of gap surface states arising from surface nonstoichiometry, unsaturated bonds, etc. Thus, the control of the surface property in particular is the key to produce highly luminescent ultrafine crystals. In this work, the use of surfactant in microemulsion can reduce the number of dangling bonds on the particle surfaces which are believed to provide surface trap states for nonradiative recombination and enhance the luminescent efficiency. To further decrease the contribution of the surface-related nonradiative recombination, an impurity of surfactant in a quantum dot was introduced, in
conducting the dominant recombination route that transferred from the surface states to the impurity states. Thus, the radiative efficiency of the impurity-induced emission increases dramatically.28 This mechanism, perhaps, is especially important for the nanoparticles on which a relatively great number of surface sites are present. In order to confirm the possible surface passivation for the particles prepared in the microemulsion, the Fourier transform infrared spectrum of a sample is shown in Figure 6. The absorption at 1190.3 and 1108.5 cm-1 corresponding to the asymmetric stretching modes of aromatic ether are clearly observed together with the stretching vibration at 1616.7 cm-1 associated with the benzenoid group. The values of 3.0 wt % C and 1.2 wt % H analyzed by ICP-AES in the sample further indicate the existence of surfactants. The adsorption of the surfactants of NP-5 and NP-9 on ZnS:Mn nanocrystalline particles is thus confirmed, and this leads to the surface passivation. Conclusions Manganese-doped nanocrystals of ZnS have been prepared in an inverse microemulsion by the hydrothermal technique. The PL intensity of nanocrystalline ZnS:Mn2+ synthesized by this new method is enhanced by a factor of 60 as compared to the sample prepared in a conventional aqueous reaction at room temperature. This dramatic increase in PL yield is attributable to the surface passivation of the ZnS:Mn nanocrystals and the formation of cubic zinc blende crystallinity in addition to the migration of Mn species into the host lattice of ZnS. A single photon process is involved in the initial excitation of the photoluminescence process. LA9705468 (28) Bhargava, R. N. J. Lumin. 1996, 70, 85.
