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SiO2 hybrid phosphor synthesized by in situ surface modification co-precipitation
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Journal of Luminescence 113 (2005) 69–78 www.elsevier.com/locate/jlumin
Luminescent properties of ZnS:Mn2+ nanocrystals/SiO2 hybrid phosphor synthesized by in situ surface modification co-precipitation Y. Hattoria, T. Isobea,, H. Takahashib, S. Itohb a
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3–14–1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan b Futaba Corporation, 1080 Yabutsuka, Chosei-mura, Chosei-gun, Chiba-ken 299-4395, Japan Received 31 March 2004; received in revised form 31 August 2004; accepted 31 August 2004 Available online 2 December 2004
Abstract Colloidal solution of ZnS:Mn2+ nanocrystals (2–5 nm in diameter) coated with silica was prepared by in situ surface modification co-precipitation in the presence of a surface modifier of 3-mercaptopropyl trimethoxysilane (MPS) and a dispersion stabilizer of sodium citrate, followed by addition of sodium silicate. The change in the optical absorption due to the interband transition of ZnS was measured during irradiation by a Xe lamp. No appreciable change in the optical absorption by irradiation was observed for the colloidal solution prepared in the presence of MPS and sodium citrate, while the absorbance decreased for the nanocrystals modified by either MPS or sodium citrate. This indicates that the photo-dissolution reaction of ZnS:Mn2+ is suppressed in the former by perfect surface modification. Formation of SiO2 layer around ZnS:Mn2+ nanocrystals induced quantum confinement effect and the passivation of surface defects to enhance the orange photoluminescence (PL) due to the d–d transition of Mn2+ ions. In addition, the broad cathodoluminescence (CL) spectrum was observed around 800 nm for ZnS:Mn2+ nanocrystals with SiO2 modification by electron bombardment at 10 kV. The difference in the PL and CL spectra might be attributed to the emission of isolated Mn2+ ions for PL and the emission of the locally concentrated Mn2+ ions for CL, although the CL might be the emission due to defects or impurities in the SiO2 layer. r 2004 Elsevier B.V. All rights reserved. PACS: 78.55.Et; 78.67.Bf; 81.07.Bc; 81.16.Be Keywords: Co-precipitation; Photoluminescence; Cathodoluminescence
Corresponding author. Tel.: +81 45 566 1554; fax: +81 45 566 1551.
E-mail address: [email protected] (T. Isobe). 0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.08.054
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1. Introduction
2. Experimental procedures
Photoluminescence (PL) due to the d–d transition of Mn2+ in ZnS:Mn2+ nanocrystals is enhanced by surface modification [1–9]. Significant roles of surface modification are quantum confinement effect and the passivation of surface defects. Most of the works concentrate on PL properties of ZnS:Mn2+ nanocrystals modified by organic materials [1–7]. When the cathodoluminescence (CL) is concerned, the electron-beam excitation degrades organic materials seriously. It is, therefore, necessary to use an inorganic material resistant to electron bombardment as a modifier. In fact, the effects of thin SiO2 coating on the degradation of the practical micrometer-size ZnS phosphor have been investigated for the low-voltage CL [10–12], but CL properties of ZnS:Mn2+ nanocrystals have not been investigated yet to our knowledge. Since the band gap of silica is wider than that of ZnS, the exciton formed by the interband excitation of ZnS could be completely confined inside the ZnS nanocrystal coated with SiO2. Silica coating of individual nanocrystals can prevent their aggregation, i.e., their direct contact to achieve stronger confinement. Therefore, if silica coating is applied on doped nanocrystals, we can expect luminescent enhancement as well as an improved resistance to electron bombardment. Recently, ZnS nanocrystals coated with SiO2 were prepared using tetraethyl orthosilicate (TEOS) [13,14] or SiO2 nanoparticles [15]. ZnS:Mn2+ nanocrystals embedded in SiO2 are also synthesized by sol–gel method, using TEOS [16,17]. Recently, Ethiraj et al. [18] reported that the ZnS:Mn2+ nanocrystal was stabilized by capping of thioglycerol, followed by addition of TEOS, to obtain ZnS:Mn2+ nanocrystals by silica coating, showing PL enhancement. Here we report in situ surface modification co-precipitation in the presence of a surface modifier and a dispersant, followed by addition of sodium silicate, to form ZnS:Mn2+ nanocrystals modified by SiO2, and discuss the effects of SiO2 coating on PL and CL properties of ZnS:Mn2+ nanocrystals.
ZnS:Mn2+ nanocrystal/SiO2 hybrid particles were prepared by the modified method after LizMarza´n [19–21]. The deionized water (90 mL) in a reactor was bubbled by N2 gas (300 mL min1) with stirring for 90 min to remove the dissolved oxygen. Then, the following solutions were put into this water under N2 gas flow in the order: (1) 10 wt% sodium citrate aqueous solution (0.5 mL), (2) 0.4 M 3-mercaptopropyl trimethoxysilane (MPS) aqueous solution (1 mL), (3) 0.08 M Na2S aqueous solution (5 mL), and (4) mixed aqueous solution of 0.1 M zinc nitrate (3.6 mL) and 0.1 M manganese nitrate (0.4 mL). After 90 min, 5.4 wt% sodium silicate aqueous solution (4 mL) was put into the resulting ZnS:Mn2+ colloidal solution. After standing for more than 2 weeks, this colloidal solution was centrifuged at 12,000 rpm for 20 min and dried at 50 1C to obtain the powdered sample. The colloidal solution was dropped on a carbonprecoated Cu grid, blotted by filter paper and dried at 50 1C. This sample was observed by field emission transmission electron microscopy (FETEM) (Philips, TECNAI F20) at 200 kV. The particle size distribution and the zeta potential of ZnS:Mn2+/SiO2 colloidal solution were measured by the dynamic light scattering (DLS) method (Malvern, HPPS) and the laser doppler method (Malvern, Zetasizer 3000HS), respectively. The colloidal solution was preliminarily filtered before DLS measurement to eliminate exceptional large aggregates and unexpected dust. The crystalline structure was examined by X-ray diffractometry (XRD) (Rigaku, Rint 2200). Optical absorption spectra of solutions at room temperature were measured at a scanning speed of 40 nm min1, sampling intervals of 0.5 nm and a bandwidth of 1.0 nm by UV-visible absorption spectrometer (JASCO, V550). Emission and excitation spectra were measured at a scanning rate of 50 nm min1 by spectrophotofluorometer (JASCO, FP-6500). Excitation and emission bandwidths were 5 nm, and the sampling intervals, 0.5 nm. The angle between excitation and detection paths was 901, and the scattered excitation light was cut off by the optical filter in the detector side. For the samples
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excited by a nitrogen pulse laser (wavelength 337 nm), time-resolved emission spectra were captured by fluorescence lifetime measurement system C4780 (Hamamatsu Photonics) using 2D photon counting. The chemical states of sulfur were measured at 10–7 Pa by X-ray photoelectron spectroscopy (XPS) (JEOL, JPS-9000MX) using Mg Ka as an exciting radiation. The XPS data were collected at intervals of 0.05 eV and an energy-pass of 10 eV. The charge up was corrected using Au 4f1/2 peak at 83.8 eV. Electronic states of Mn2+ were investigated by electron spin resonance (ESR) spectroscopy (JEOL, JEPRE3X), where the microwave frequency was 9.5 GHz (Xband), the modulation width, 0.32 mT, and the modulation frequency, 100 kHz. The public ESR software [22] produced by the National Institute of Environmental Health Sciences was used for simulation of ESR signals. The CL spectrum was measured at 10 kV electron beam. The ZnS:Mn2+ nanocrystal/SiO2 hybrid phosphor was applied for vacuum fluorescent display [23].
3. Results and discussion 3.1. Particle properties The ZnS:Mn2+ colloidal solution prepared in the presence of both MPS and sodium citrate remains well-dispersed for more than 1 month, as shown in Fig. 1(d). This stability of dispersion is preserved after the addition of sodium silicate
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(Fig. 1(e)). In contrast, the sedimentation occurs immediately for the samples without any additives (Fig. 1(a)) and with the addition of only sodium citrate (Fig. 1(b)), and within two days for the sample prepared in the presence of only MPS (Fig. 1(c)). The zeta potential of ZnS:Mn2+/SiO2 colloidal solution is 39 mV, showing that the well-dispersion is attributed to the electrostatic repulsion between negatively charged particles. According to the FE-TEM micrographs (Figs. 2(a) and (b)), the particle size ranges from several nanometers to 100 nm. ZnS:Mn2+ nanocrystalline cores are embedded in silica matrix, as shown in Fig. 2(c). The individual ZnS:Mn2+ particles have one domain of single crystal, whose diameter ranges from 2 to 5 nm, being less than the exciton Bohr diameter (5 nm) of ZnS. This diameter’s range is close to the diameter, 2.7 nm, estimated from the optical absorption edge, using effective mass approximation [24]. The single core of ZnS:Mn2+ nanocrystal with a 1 nm thick silica shell is also observed by FE-TEM, as shown in Fig. 2(d). As shown in Fig. 3, DLS measurement also reveals the bimodal particle size distribution. We attribute the large peak to the multi-core/shell and the small peak to the single-core/shell. These results suggest that single-core/shell particles coalesce into a multi-core/shell particle. The interplanar spacing shown in the inset of Fig. 2(c) is likely to be close to d 102 ¼ 0:23 nm of wurzite or d 200 ¼ 0:27 nm of zinc blende. According to the XRD profiles (Fig. 4), the crystalline structure of ZnS:Mn2+ nanocrystal appears to be
Fig. 1. Photographs of the ZnS:Mn2+ colloidal solution prepared (a) without any additives, in the presence of (b) sodium citrate, (c) MPS, (d) sodium citrate and MPS, and (e) sodium citrate and MPS, followed by addition of sodium silicate.
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Fig. 2. TEM micrographs of ZnS:Mn2+ core/SiO2 shell hybrid particles.
Fig. 3. The particle size distribution measured by DLS for the ZnS:Mn2+/SiO2 colloidal solution.
Fig. 4. XRD profiles of ZnS:Mn2+ powders. (a) commercial bulk (zinc blende), (b) nanocrystals/SiO2. (b) is the profile obtained by subtracting the halo pattern due to silica from the original profile.
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identical with zinc blende (cubic structure) of the commercial bulk ZnS:Mn2+, although the XRD peaks of the former are much broader than those of the latter. 3.2. Roles of MPS and SiO2 layers Fig. 5 shows the change in the absorbance at 310 nm due to the interband transition of ZnS during irradiation by a Xe lamp. The ZnS:Mn2+ colloidal solution prepared in the presence of either sodium citrate (Fig. 5(a)) or MPS (Fig. 5(b)) exhibits the decrease in absorbance with increasing irradiation time. This is attributed to the photodissolution reaction of ZnS [20]. On the other hand, the decrease in absorbance is not observed for the ZnS:Mn2+ colloidal solutions prepared in the presence of both MPS and sodium citrate, irrespective of the addition of sodium silicate, as shown in Figs. 5(c) and (d). This verifies that the ZnS:Mn2+ nanocrystal is completely covered with MPS and SiO2 on an atomic scale. Fig. 6 shows XPS spectra of S 2p electrons for the bulk and the nanocrystal modified by SiO2. The XPS peak of the bulk at around 162 eV is assigned to S2. The XPS peak of the nanocrystal is broader than that of the bulk, and is decom-
Fig. 6. XPS spectra of S 2p electrons for ZnS:Mn2+ powders: (a) commercial bulk (zinc blende), (b) nanocrystals/SiO2. dots: experimental data; line: obtained by curve fitting. Two decomposed curves and their sum are shown in (b).
Fig. 7. ESR spectra of ZnS:Mn2+ powders: (a) commercial bulk, (b) nanocrystals/SiO2.
Fig. 5. Change in the absorbance at 310 nm due to the interband transition of ZnS during irradiation by a Xe lamp: (a) sodium citrate, (b) MPS, (c) sodium citrate and MPS, and (d) sodium citrate and MPS, followed by addition of sodium silicate.
posed to at least two components by curve fitting, as shown in Fig. 6(b). These are possibly assigned to two kinds of sulfurs originating from zinc sulfide and MPS ((CH3O)3Si(CH2)3SH). No XPS peak due to S6+ is observed around 168 eV for both samples. This indicates that the removal of dissolved oxygen by N2 gas bubbling prevents the oxidation of ZnS:Mn2+ nanocrystal and that oxygen in silica does not directly interact with sulfur in ZnS:Mn2+. This differs from the XPS
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Fig. 8. ESR spectra of ZnS:Mn2+: (a) measured spectrum of nanocrystals/SiO2, (b)–(d) simulated spectra. (b) the summation of sextet signal and singlet broad signal, (c) sextet signal, (d) singlet broad signal.
Scheme 1. Formation process of ZnS:Mn2+ nanocrystal/SiO2 hybrid phosphor.
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results of ZnS:Mn2+ nanocrystal modified by poly(acrylic acid) (PAA), where a part of S2 is oxidized to S6+ by interaction with the oxygen of carboxyl group in PAA [5]. Fig. 7 shows ESR spectra of ZnS:Mn2+ of commercial bulk and nanocrystal modified by SiO2. The sextet signal I indicated in Fig. 7 is observed for both samples. This signal corresponds to the interior site of Mn2+ substituted for Zn2+ with four-fold S2 coordination [6]. The hyperfine coupling constant, |A|, of both samples is 6.9 mT, indicating that the crystal structure of both samples is zinc blende. This is consistent with XRD results. The ESR spectrum (Fig. 8(a)) measured in the wide region is close to the simulated spectrum (Fig. 8(b)) obtained by summation of the sextet signal (Fig. 8(c)) and the singlet broad signal (Fig. 8(d)). Since this simulation does not consider the second perturbation, the simulated spectrum does not perfectly fit the actual result. However, it is obvious that the singlet broad signal is overlaid with signal I for the nanocrystal modified by SiO2. This broad signal is attributed to locally concentrated Mn2+ ions with exchange and/or dipole interaction between Mn2+ ions, whereas signal I is attributed to isolated Mn2+ ions [25]. In our previous work, signal II corresponding to the surface Mn2+ sites in the vicinity of oxygen is observed for ZnS:Mn2+ nanocrystal modified by PAA [6]. In contrast, no signal II is observed for ZnS:Mn2+ nanocrystals modified by SiO2. These results verify that oxygen in silica is not directly bonded to Mn2+ and hence that SH groups of MPS are bonded to metallic ions of ZnS:Mn2+. This is consistent with the XPS results. Based on the results mentioned in Sections 3.1 and 3.2, the formation process of ZnS:Mn2+ nanocrystals modified by SiO2 is summarized in Scheme 1. 3.3. Photoluminescence and cathodoluminescence properties Fig. 9 shows the PL spectra due to the d–d transition of Mn2+ by the excitation of interband of ZnS. The PL intensity for the colloidal solution of ZnS:Mn2+/SiO2 (Fig. 9(a) line 2) is 2.4 times higher than that for the sample prepared without
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Fig. 9. PL spectra (a) and PLE spectra (b) for the ZnS:Mn2+ colloidal solution prepared (1) without any additives, (2) in the presence of sodium citrate and MPS, followed by the addition of sodium silicate. The optimum wavelength of PLE spectrum was chosen as the excitation wavelength of PL spectrum. Peak wavelength: (1) lem ¼ 579 nm; lex ¼ 328 nm; (2) lem ¼ 576 nm; lex ¼ 311 nm:
any additives (Fig. 9(a) line 1). The optimum excitation wavelength, 311 nm, for ZnS:Mn2+/ SiO2 (Fig. 9(b) line 2) is 33 nm shorter than that of ZnS:Mn2+ commercial bulk, 344 nm. But, the optimum excitation wavelength (328 nm) of the sample prepared without any additives (Fig. 9(b) line 1) is 17 nm longer than that of ZnS:Mn2+/ SiO2. This indicates that the direct contact of ZnS:Mn2+ nanocrystal by aggregation causes weak confinement for the former. Accordingly, the PL enhancement and the blue shift of optimum excitation wavelength by silica coating are attributed to stronger quantum confinement and surface
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passivation. In addition, in situ surface modification, i.e., the preparation method in the presence of additives, may prevent the growth of nanocrystals to cause the reduction of particle size, leading to the larger blue shift for ZnS:Mn coated with SiO2. The powdered sample prepared by drying the colloidal solution of ZnS:Mn2+/SiO2 exhibits the PL spectrum peaking at 577 nm. The excitation peak for this emission is located at 321 nm, being 23 nm shorter than that of bulk. Thus, as a result of formation of SiO2 layer around ZnS:Mn2+ nanocrystal, the blue shift can be preserved after drying. The difference in blue shift between the powder and the colloidal solution would be attributed to the change in—Si–O–Si—networks in silica during drying and hence the change in
interaction at the interface between ZnS:Mn2+ and SiO2. The time-resolved PL spectra due to the d–d transition of Mn2+ in the nanosecond and microsecond ranges are shown in Fig. 10A for the colloidal solution of ZnS:Mn2+/SiO2. Based on the curve fitting using the equation of I ¼ I 0 þ I 1 expðt=t1 Þ þ I 2 expðt=t2 Þ; the PL decay curve (Fig. 10C) is composed of two decay time constants of t1 ¼ 0:3 ms and t2 ¼ 1:9 ms: These decay time constants are close to t1 ¼ 0:2 ms and t2 ¼ 1:4 ms for our bulk sample. Based on the analysis of decay times on single Mn2+ ions and Mn2+–Mn2+pairs of ZnS:Mn2+ bulk [26,27], fast and slow decay components are assigned to the locally concentrated Mn2+ ions and the isolated Mn2+ ions, respectively. As a result, nanosizing
Fig. 10. Time-resolved PL spectra (A, B) of ZnS:Mn2+/SiO2 colloidal solution and their decay curves (C, D). (A) accumulated spectra for 0–0.3 ms (a), 0.4–0.7 ms (b), 3.2–3.5 ms (c); (B) accumulated spectra for 0–2 ns (a), 3–5 ns (b), 11–13 ns (c). (C) integral PL intensity from 550 to 630 nm; (D) integral PL intensity from 405 to 600 nm.
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does not shorten the lifetime of Mn2+ emission, as suggested by other researchers [28–32]. In the nanosecond range, the broad PL peak was observed at 500 nm, as shown in Fig. 10B. This might be attributed to the defect-related emission of ZnS. Its decay time constants are 0.8 ns and 5.4 ns, which are obtained by curve fitting of the decay curve shown in Fig. 10D. The broad peak at 500 nm observed in the time-resolved spectra (Fig. 10B) appears to be too weak to observe in the time-averaged spectrum (Fig. 9(a)). Therefore, the researchers [1,33] who reported that the decay time due to the Mn2+ emission was shortened from ms to ns by nanosizing, probably observed the tail of the broad defect-related emission, as already suggested by other workers [28,30]. The ZnS:Mn2+ bulk shows the CL peak at 570 nm when excited by 10 kV electron-beam, as shown by the dotted line in Fig. 11(a). In contrast, ZnS:Mn2+ nanocrystal modified by SiO2 shows the broad asymmetric CL peak at 800 nm by the same excitation, as shown by the solid line in Fig. 11(a). This CL peak wavelength of ZnS:Mn2+ nanocrystal/SiO2 hybrid phosphor is higher than the PL peak wavelength, 577 nm. The CL might be attributed to defects or impurities in the SiO2 layer around ZnS:Mn2+ nanocrystals. Toyama et al. have reported that ZnS:Mn2+(2.5 nm)/ Si3N4(0.6 nm) multilayer exhibits two peaks at 610 and 700 nm for PL and at 626 and 700 nm for electroluminescence (EL) [34,35]. They conclude that the peak at 700 nm would be assigned to Mn2+–Mn2+ pairs by comparison with the EL spectrum of ZnS:Mn2+ thin film. Accordingly, the difference between CL and PL spectra in this work also might be explained by the main emission from isolated Mn2+ ions for PL and the main emission from the locally concentrated Mn2+ ions for CL. The ZnS:Mn2+ nanocrystal/SiO2 hybrid phosphor was applied for vacuum fluorescent display (VFD), and its spectrum is shown by the solid line in Fig. 11(b). The emission peaks for VFD device are observed at 440 and 640 nm under the condition of filament voltage X10 V. The latter is attributed to the d–d transition of Mn2+ ions, and the former is attributed to the emission due to self-activated centers related with S2 defects. In contrast, the emission is observed at 585 nm for
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Fig. 11. (a) CL spectra of the samples excited by electron beam at 10 kV. (b) CL spectra for VFD device. (solid line) ZnS:Mn2+ nanocrystal/SiO2 hybrid phosphor; (dotted line) commercial bulk ZnS:Mn2+.
bulk ZnS:Mn2+ in VFD device, as shown by the dotted line in Fig. 11(b). Heat treatment in vacuum at 500 1C during VFD fabrication process possibly promotes the diffusion of Mn2+ into ZnS to decrease the degree of concentrated Mn2+ ions, so that the peak wavelength of the emission in VFD is shorter than that of CL peak shown by the solid line in Fig. 11(a). During heat treatment at 500 1C in the fabrication process of VFD, ZnS:Mn2+ nanocrystal would be partly oxidized by a trace of oxygen gas and/or by oxygen in silica at the interface to form S2 defects, resulting in the emission at 440 nm. Therefore, ZnS:Mn2+ nanocrystal/SiO2 hybrid phosphor appears to be appropriate for practical application without heat treatment.
4. Conclusion ZnS:Mn2+ multi-core/SiO2 shell hybrid particles are prepared by in situ surface modification
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co-precipitation in the presence of MPS and sodium citrate, followed by addition of sodium silicate. Surface modification of ZnS:Mn2+ nanocrystals by silica induces PL enhancement by quantum confinement effect and surface passivation.
Acknowledgements We thank Dr. H. Watanabe and Dr. K. Suzuki at Hamamatsu Photonics K.K. for the timeresolved PL measurement. We also thank Sysmex Corp. for helping the measurement of zeta potential. References [1] R.N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416. [2] M. Tanaka, Y. Masumoto, Chem. Phys. Lett. 324 (2000) 249. [3] S.W. Lu, B.I. Lee, Z.L. Wang, W. Tong, B.K. Wagner, W. Park, C.J. Summers, J. Lumin. 92 (2001) 73. [4] A.A. Bol, A. Meijerink, J. Phys. Chem. B 105 (2001) 10197. [5] M. Konishi, T. Isobe, M. Senna, J. Lumin. 93 (2001) 1. [6] T. Igarashi, M. Ihara, T. Kusunoki, K. Ohno, T. Isobe, M. Senna, J. Nanoparticle Res. 3 (2001) 51. [7] T. Kubo, T. Isobe, M. Senna, J. Lumin. 99 (2002) 39. [8] W. Chen, R. Sammynaiken, Y. Huang, J. Appl. Phys. 88 (2000) 5188. [9] L. Cao, J. Zhang, S. Ren, S. Huang, Appl. Phys. Lett. 80 (2002) 4300. [10] W. Park, B.K. Wagner, G. Russell, K. Yasuda, C.J. Summers, Y.R. Do, H.G. Yang, J. Mater. Res. 15 (2000) 2288. [11] Y.R. Do, D.H. Park, H.G. Yang, W. Park, B.K. Wagner, K. Yasuda, C.J. Summers, J. Electrochem. Soc. 148 (2001) G548. [12] A.D. Dinsmore, D.S. Hsu, H.F. Gray, S.B. Qadri, Y. Tian, B.R. Ratna, Appl. Phys. Lett. 75 (1999) 802. [13] M.H. El-Khair, L. Xu, K.J. Chen, Y. Ma, Y. Zhang, M.H. Li, X.F. Huang, Chin. Phys. Lett. 19 (2002) 967.
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Journal of Luminescence 113 (2005) 69–78 www.elsevier.com/locate/jlumin
Luminescent properties of ZnS:Mn2+ nanocrystals/SiO2 hybrid phosphor synthesized by in situ surface modification co-precipitation Y. Hattoria, T. Isobea,, H. Takahashib, S. Itohb a
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3–14–1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan b Futaba Corporation, 1080 Yabutsuka, Chosei-mura, Chosei-gun, Chiba-ken 299-4395, Japan Received 31 March 2004; received in revised form 31 August 2004; accepted 31 August 2004 Available online 2 December 2004
Abstract Colloidal solution of ZnS:Mn2+ nanocrystals (2–5 nm in diameter) coated with silica was prepared by in situ surface modification co-precipitation in the presence of a surface modifier of 3-mercaptopropyl trimethoxysilane (MPS) and a dispersion stabilizer of sodium citrate, followed by addition of sodium silicate. The change in the optical absorption due to the interband transition of ZnS was measured during irradiation by a Xe lamp. No appreciable change in the optical absorption by irradiation was observed for the colloidal solution prepared in the presence of MPS and sodium citrate, while the absorbance decreased for the nanocrystals modified by either MPS or sodium citrate. This indicates that the photo-dissolution reaction of ZnS:Mn2+ is suppressed in the former by perfect surface modification. Formation of SiO2 layer around ZnS:Mn2+ nanocrystals induced quantum confinement effect and the passivation of surface defects to enhance the orange photoluminescence (PL) due to the d–d transition of Mn2+ ions. In addition, the broad cathodoluminescence (CL) spectrum was observed around 800 nm for ZnS:Mn2+ nanocrystals with SiO2 modification by electron bombardment at 10 kV. The difference in the PL and CL spectra might be attributed to the emission of isolated Mn2+ ions for PL and the emission of the locally concentrated Mn2+ ions for CL, although the CL might be the emission due to defects or impurities in the SiO2 layer. r 2004 Elsevier B.V. All rights reserved. PACS: 78.55.Et; 78.67.Bf; 81.07.Bc; 81.16.Be Keywords: Co-precipitation; Photoluminescence; Cathodoluminescence
Corresponding author. Tel.: +81 45 566 1554; fax: +81 45 566 1551.
E-mail address: [email protected] (T. Isobe). 0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.08.054
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1. Introduction
2. Experimental procedures
Photoluminescence (PL) due to the d–d transition of Mn2+ in ZnS:Mn2+ nanocrystals is enhanced by surface modification [1–9]. Significant roles of surface modification are quantum confinement effect and the passivation of surface defects. Most of the works concentrate on PL properties of ZnS:Mn2+ nanocrystals modified by organic materials [1–7]. When the cathodoluminescence (CL) is concerned, the electron-beam excitation degrades organic materials seriously. It is, therefore, necessary to use an inorganic material resistant to electron bombardment as a modifier. In fact, the effects of thin SiO2 coating on the degradation of the practical micrometer-size ZnS phosphor have been investigated for the low-voltage CL [10–12], but CL properties of ZnS:Mn2+ nanocrystals have not been investigated yet to our knowledge. Since the band gap of silica is wider than that of ZnS, the exciton formed by the interband excitation of ZnS could be completely confined inside the ZnS nanocrystal coated with SiO2. Silica coating of individual nanocrystals can prevent their aggregation, i.e., their direct contact to achieve stronger confinement. Therefore, if silica coating is applied on doped nanocrystals, we can expect luminescent enhancement as well as an improved resistance to electron bombardment. Recently, ZnS nanocrystals coated with SiO2 were prepared using tetraethyl orthosilicate (TEOS) [13,14] or SiO2 nanoparticles [15]. ZnS:Mn2+ nanocrystals embedded in SiO2 are also synthesized by sol–gel method, using TEOS [16,17]. Recently, Ethiraj et al. [18] reported that the ZnS:Mn2+ nanocrystal was stabilized by capping of thioglycerol, followed by addition of TEOS, to obtain ZnS:Mn2+ nanocrystals by silica coating, showing PL enhancement. Here we report in situ surface modification co-precipitation in the presence of a surface modifier and a dispersant, followed by addition of sodium silicate, to form ZnS:Mn2+ nanocrystals modified by SiO2, and discuss the effects of SiO2 coating on PL and CL properties of ZnS:Mn2+ nanocrystals.
ZnS:Mn2+ nanocrystal/SiO2 hybrid particles were prepared by the modified method after LizMarza´n [19–21]. The deionized water (90 mL) in a reactor was bubbled by N2 gas (300 mL min1) with stirring for 90 min to remove the dissolved oxygen. Then, the following solutions were put into this water under N2 gas flow in the order: (1) 10 wt% sodium citrate aqueous solution (0.5 mL), (2) 0.4 M 3-mercaptopropyl trimethoxysilane (MPS) aqueous solution (1 mL), (3) 0.08 M Na2S aqueous solution (5 mL), and (4) mixed aqueous solution of 0.1 M zinc nitrate (3.6 mL) and 0.1 M manganese nitrate (0.4 mL). After 90 min, 5.4 wt% sodium silicate aqueous solution (4 mL) was put into the resulting ZnS:Mn2+ colloidal solution. After standing for more than 2 weeks, this colloidal solution was centrifuged at 12,000 rpm for 20 min and dried at 50 1C to obtain the powdered sample. The colloidal solution was dropped on a carbonprecoated Cu grid, blotted by filter paper and dried at 50 1C. This sample was observed by field emission transmission electron microscopy (FETEM) (Philips, TECNAI F20) at 200 kV. The particle size distribution and the zeta potential of ZnS:Mn2+/SiO2 colloidal solution were measured by the dynamic light scattering (DLS) method (Malvern, HPPS) and the laser doppler method (Malvern, Zetasizer 3000HS), respectively. The colloidal solution was preliminarily filtered before DLS measurement to eliminate exceptional large aggregates and unexpected dust. The crystalline structure was examined by X-ray diffractometry (XRD) (Rigaku, Rint 2200). Optical absorption spectra of solutions at room temperature were measured at a scanning speed of 40 nm min1, sampling intervals of 0.5 nm and a bandwidth of 1.0 nm by UV-visible absorption spectrometer (JASCO, V550). Emission and excitation spectra were measured at a scanning rate of 50 nm min1 by spectrophotofluorometer (JASCO, FP-6500). Excitation and emission bandwidths were 5 nm, and the sampling intervals, 0.5 nm. The angle between excitation and detection paths was 901, and the scattered excitation light was cut off by the optical filter in the detector side. For the samples
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excited by a nitrogen pulse laser (wavelength 337 nm), time-resolved emission spectra were captured by fluorescence lifetime measurement system C4780 (Hamamatsu Photonics) using 2D photon counting. The chemical states of sulfur were measured at 10–7 Pa by X-ray photoelectron spectroscopy (XPS) (JEOL, JPS-9000MX) using Mg Ka as an exciting radiation. The XPS data were collected at intervals of 0.05 eV and an energy-pass of 10 eV. The charge up was corrected using Au 4f1/2 peak at 83.8 eV. Electronic states of Mn2+ were investigated by electron spin resonance (ESR) spectroscopy (JEOL, JEPRE3X), where the microwave frequency was 9.5 GHz (Xband), the modulation width, 0.32 mT, and the modulation frequency, 100 kHz. The public ESR software [22] produced by the National Institute of Environmental Health Sciences was used for simulation of ESR signals. The CL spectrum was measured at 10 kV electron beam. The ZnS:Mn2+ nanocrystal/SiO2 hybrid phosphor was applied for vacuum fluorescent display [23].
3. Results and discussion 3.1. Particle properties The ZnS:Mn2+ colloidal solution prepared in the presence of both MPS and sodium citrate remains well-dispersed for more than 1 month, as shown in Fig. 1(d). This stability of dispersion is preserved after the addition of sodium silicate
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(Fig. 1(e)). In contrast, the sedimentation occurs immediately for the samples without any additives (Fig. 1(a)) and with the addition of only sodium citrate (Fig. 1(b)), and within two days for the sample prepared in the presence of only MPS (Fig. 1(c)). The zeta potential of ZnS:Mn2+/SiO2 colloidal solution is 39 mV, showing that the well-dispersion is attributed to the electrostatic repulsion between negatively charged particles. According to the FE-TEM micrographs (Figs. 2(a) and (b)), the particle size ranges from several nanometers to 100 nm. ZnS:Mn2+ nanocrystalline cores are embedded in silica matrix, as shown in Fig. 2(c). The individual ZnS:Mn2+ particles have one domain of single crystal, whose diameter ranges from 2 to 5 nm, being less than the exciton Bohr diameter (5 nm) of ZnS. This diameter’s range is close to the diameter, 2.7 nm, estimated from the optical absorption edge, using effective mass approximation [24]. The single core of ZnS:Mn2+ nanocrystal with a 1 nm thick silica shell is also observed by FE-TEM, as shown in Fig. 2(d). As shown in Fig. 3, DLS measurement also reveals the bimodal particle size distribution. We attribute the large peak to the multi-core/shell and the small peak to the single-core/shell. These results suggest that single-core/shell particles coalesce into a multi-core/shell particle. The interplanar spacing shown in the inset of Fig. 2(c) is likely to be close to d 102 ¼ 0:23 nm of wurzite or d 200 ¼ 0:27 nm of zinc blende. According to the XRD profiles (Fig. 4), the crystalline structure of ZnS:Mn2+ nanocrystal appears to be
Fig. 1. Photographs of the ZnS:Mn2+ colloidal solution prepared (a) without any additives, in the presence of (b) sodium citrate, (c) MPS, (d) sodium citrate and MPS, and (e) sodium citrate and MPS, followed by addition of sodium silicate.
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Fig. 2. TEM micrographs of ZnS:Mn2+ core/SiO2 shell hybrid particles.
Fig. 3. The particle size distribution measured by DLS for the ZnS:Mn2+/SiO2 colloidal solution.
Fig. 4. XRD profiles of ZnS:Mn2+ powders. (a) commercial bulk (zinc blende), (b) nanocrystals/SiO2. (b) is the profile obtained by subtracting the halo pattern due to silica from the original profile.
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identical with zinc blende (cubic structure) of the commercial bulk ZnS:Mn2+, although the XRD peaks of the former are much broader than those of the latter. 3.2. Roles of MPS and SiO2 layers Fig. 5 shows the change in the absorbance at 310 nm due to the interband transition of ZnS during irradiation by a Xe lamp. The ZnS:Mn2+ colloidal solution prepared in the presence of either sodium citrate (Fig. 5(a)) or MPS (Fig. 5(b)) exhibits the decrease in absorbance with increasing irradiation time. This is attributed to the photodissolution reaction of ZnS [20]. On the other hand, the decrease in absorbance is not observed for the ZnS:Mn2+ colloidal solutions prepared in the presence of both MPS and sodium citrate, irrespective of the addition of sodium silicate, as shown in Figs. 5(c) and (d). This verifies that the ZnS:Mn2+ nanocrystal is completely covered with MPS and SiO2 on an atomic scale. Fig. 6 shows XPS spectra of S 2p electrons for the bulk and the nanocrystal modified by SiO2. The XPS peak of the bulk at around 162 eV is assigned to S2. The XPS peak of the nanocrystal is broader than that of the bulk, and is decom-
Fig. 6. XPS spectra of S 2p electrons for ZnS:Mn2+ powders: (a) commercial bulk (zinc blende), (b) nanocrystals/SiO2. dots: experimental data; line: obtained by curve fitting. Two decomposed curves and their sum are shown in (b).
Fig. 7. ESR spectra of ZnS:Mn2+ powders: (a) commercial bulk, (b) nanocrystals/SiO2.
Fig. 5. Change in the absorbance at 310 nm due to the interband transition of ZnS during irradiation by a Xe lamp: (a) sodium citrate, (b) MPS, (c) sodium citrate and MPS, and (d) sodium citrate and MPS, followed by addition of sodium silicate.
posed to at least two components by curve fitting, as shown in Fig. 6(b). These are possibly assigned to two kinds of sulfurs originating from zinc sulfide and MPS ((CH3O)3Si(CH2)3SH). No XPS peak due to S6+ is observed around 168 eV for both samples. This indicates that the removal of dissolved oxygen by N2 gas bubbling prevents the oxidation of ZnS:Mn2+ nanocrystal and that oxygen in silica does not directly interact with sulfur in ZnS:Mn2+. This differs from the XPS
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Fig. 8. ESR spectra of ZnS:Mn2+: (a) measured spectrum of nanocrystals/SiO2, (b)–(d) simulated spectra. (b) the summation of sextet signal and singlet broad signal, (c) sextet signal, (d) singlet broad signal.
Scheme 1. Formation process of ZnS:Mn2+ nanocrystal/SiO2 hybrid phosphor.
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results of ZnS:Mn2+ nanocrystal modified by poly(acrylic acid) (PAA), where a part of S2 is oxidized to S6+ by interaction with the oxygen of carboxyl group in PAA [5]. Fig. 7 shows ESR spectra of ZnS:Mn2+ of commercial bulk and nanocrystal modified by SiO2. The sextet signal I indicated in Fig. 7 is observed for both samples. This signal corresponds to the interior site of Mn2+ substituted for Zn2+ with four-fold S2 coordination [6]. The hyperfine coupling constant, |A|, of both samples is 6.9 mT, indicating that the crystal structure of both samples is zinc blende. This is consistent with XRD results. The ESR spectrum (Fig. 8(a)) measured in the wide region is close to the simulated spectrum (Fig. 8(b)) obtained by summation of the sextet signal (Fig. 8(c)) and the singlet broad signal (Fig. 8(d)). Since this simulation does not consider the second perturbation, the simulated spectrum does not perfectly fit the actual result. However, it is obvious that the singlet broad signal is overlaid with signal I for the nanocrystal modified by SiO2. This broad signal is attributed to locally concentrated Mn2+ ions with exchange and/or dipole interaction between Mn2+ ions, whereas signal I is attributed to isolated Mn2+ ions [25]. In our previous work, signal II corresponding to the surface Mn2+ sites in the vicinity of oxygen is observed for ZnS:Mn2+ nanocrystal modified by PAA [6]. In contrast, no signal II is observed for ZnS:Mn2+ nanocrystals modified by SiO2. These results verify that oxygen in silica is not directly bonded to Mn2+ and hence that SH groups of MPS are bonded to metallic ions of ZnS:Mn2+. This is consistent with the XPS results. Based on the results mentioned in Sections 3.1 and 3.2, the formation process of ZnS:Mn2+ nanocrystals modified by SiO2 is summarized in Scheme 1. 3.3. Photoluminescence and cathodoluminescence properties Fig. 9 shows the PL spectra due to the d–d transition of Mn2+ by the excitation of interband of ZnS. The PL intensity for the colloidal solution of ZnS:Mn2+/SiO2 (Fig. 9(a) line 2) is 2.4 times higher than that for the sample prepared without
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Fig. 9. PL spectra (a) and PLE spectra (b) for the ZnS:Mn2+ colloidal solution prepared (1) without any additives, (2) in the presence of sodium citrate and MPS, followed by the addition of sodium silicate. The optimum wavelength of PLE spectrum was chosen as the excitation wavelength of PL spectrum. Peak wavelength: (1) lem ¼ 579 nm; lex ¼ 328 nm; (2) lem ¼ 576 nm; lex ¼ 311 nm:
any additives (Fig. 9(a) line 1). The optimum excitation wavelength, 311 nm, for ZnS:Mn2+/ SiO2 (Fig. 9(b) line 2) is 33 nm shorter than that of ZnS:Mn2+ commercial bulk, 344 nm. But, the optimum excitation wavelength (328 nm) of the sample prepared without any additives (Fig. 9(b) line 1) is 17 nm longer than that of ZnS:Mn2+/ SiO2. This indicates that the direct contact of ZnS:Mn2+ nanocrystal by aggregation causes weak confinement for the former. Accordingly, the PL enhancement and the blue shift of optimum excitation wavelength by silica coating are attributed to stronger quantum confinement and surface
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passivation. In addition, in situ surface modification, i.e., the preparation method in the presence of additives, may prevent the growth of nanocrystals to cause the reduction of particle size, leading to the larger blue shift for ZnS:Mn coated with SiO2. The powdered sample prepared by drying the colloidal solution of ZnS:Mn2+/SiO2 exhibits the PL spectrum peaking at 577 nm. The excitation peak for this emission is located at 321 nm, being 23 nm shorter than that of bulk. Thus, as a result of formation of SiO2 layer around ZnS:Mn2+ nanocrystal, the blue shift can be preserved after drying. The difference in blue shift between the powder and the colloidal solution would be attributed to the change in—Si–O–Si—networks in silica during drying and hence the change in
interaction at the interface between ZnS:Mn2+ and SiO2. The time-resolved PL spectra due to the d–d transition of Mn2+ in the nanosecond and microsecond ranges are shown in Fig. 10A for the colloidal solution of ZnS:Mn2+/SiO2. Based on the curve fitting using the equation of I ¼ I 0 þ I 1 expðt=t1 Þ þ I 2 expðt=t2 Þ; the PL decay curve (Fig. 10C) is composed of two decay time constants of t1 ¼ 0:3 ms and t2 ¼ 1:9 ms: These decay time constants are close to t1 ¼ 0:2 ms and t2 ¼ 1:4 ms for our bulk sample. Based on the analysis of decay times on single Mn2+ ions and Mn2+–Mn2+pairs of ZnS:Mn2+ bulk [26,27], fast and slow decay components are assigned to the locally concentrated Mn2+ ions and the isolated Mn2+ ions, respectively. As a result, nanosizing
Fig. 10. Time-resolved PL spectra (A, B) of ZnS:Mn2+/SiO2 colloidal solution and their decay curves (C, D). (A) accumulated spectra for 0–0.3 ms (a), 0.4–0.7 ms (b), 3.2–3.5 ms (c); (B) accumulated spectra for 0–2 ns (a), 3–5 ns (b), 11–13 ns (c). (C) integral PL intensity from 550 to 630 nm; (D) integral PL intensity from 405 to 600 nm.
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does not shorten the lifetime of Mn2+ emission, as suggested by other researchers [28–32]. In the nanosecond range, the broad PL peak was observed at 500 nm, as shown in Fig. 10B. This might be attributed to the defect-related emission of ZnS. Its decay time constants are 0.8 ns and 5.4 ns, which are obtained by curve fitting of the decay curve shown in Fig. 10D. The broad peak at 500 nm observed in the time-resolved spectra (Fig. 10B) appears to be too weak to observe in the time-averaged spectrum (Fig. 9(a)). Therefore, the researchers [1,33] who reported that the decay time due to the Mn2+ emission was shortened from ms to ns by nanosizing, probably observed the tail of the broad defect-related emission, as already suggested by other workers [28,30]. The ZnS:Mn2+ bulk shows the CL peak at 570 nm when excited by 10 kV electron-beam, as shown by the dotted line in Fig. 11(a). In contrast, ZnS:Mn2+ nanocrystal modified by SiO2 shows the broad asymmetric CL peak at 800 nm by the same excitation, as shown by the solid line in Fig. 11(a). This CL peak wavelength of ZnS:Mn2+ nanocrystal/SiO2 hybrid phosphor is higher than the PL peak wavelength, 577 nm. The CL might be attributed to defects or impurities in the SiO2 layer around ZnS:Mn2+ nanocrystals. Toyama et al. have reported that ZnS:Mn2+(2.5 nm)/ Si3N4(0.6 nm) multilayer exhibits two peaks at 610 and 700 nm for PL and at 626 and 700 nm for electroluminescence (EL) [34,35]. They conclude that the peak at 700 nm would be assigned to Mn2+–Mn2+ pairs by comparison with the EL spectrum of ZnS:Mn2+ thin film. Accordingly, the difference between CL and PL spectra in this work also might be explained by the main emission from isolated Mn2+ ions for PL and the main emission from the locally concentrated Mn2+ ions for CL. The ZnS:Mn2+ nanocrystal/SiO2 hybrid phosphor was applied for vacuum fluorescent display (VFD), and its spectrum is shown by the solid line in Fig. 11(b). The emission peaks for VFD device are observed at 440 and 640 nm under the condition of filament voltage X10 V. The latter is attributed to the d–d transition of Mn2+ ions, and the former is attributed to the emission due to self-activated centers related with S2 defects. In contrast, the emission is observed at 585 nm for
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Fig. 11. (a) CL spectra of the samples excited by electron beam at 10 kV. (b) CL spectra for VFD device. (solid line) ZnS:Mn2+ nanocrystal/SiO2 hybrid phosphor; (dotted line) commercial bulk ZnS:Mn2+.
bulk ZnS:Mn2+ in VFD device, as shown by the dotted line in Fig. 11(b). Heat treatment in vacuum at 500 1C during VFD fabrication process possibly promotes the diffusion of Mn2+ into ZnS to decrease the degree of concentrated Mn2+ ions, so that the peak wavelength of the emission in VFD is shorter than that of CL peak shown by the solid line in Fig. 11(a). During heat treatment at 500 1C in the fabrication process of VFD, ZnS:Mn2+ nanocrystal would be partly oxidized by a trace of oxygen gas and/or by oxygen in silica at the interface to form S2 defects, resulting in the emission at 440 nm. Therefore, ZnS:Mn2+ nanocrystal/SiO2 hybrid phosphor appears to be appropriate for practical application without heat treatment.
4. Conclusion ZnS:Mn2+ multi-core/SiO2 shell hybrid particles are prepared by in situ surface modification
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co-precipitation in the presence of MPS and sodium citrate, followed by addition of sodium silicate. Surface modification of ZnS:Mn2+ nanocrystals by silica induces PL enhancement by quantum confinement effect and surface passivation.
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