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An inkjet printing method: Drop and Synthesis (DAS). Application to the synthesis of ZnS:Mn nano-phosphor with a pattern
Current Applied Physics 10 (2010) e109ee112
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
An inkjet printing method: Drop and Synthesis (DAS). Application to the synthesis of ZnS:Mn nano-phosphor with a pattern Hongki Cha 1, Sung Il Ahn 2, Kyung Cheol Choi* Dept. of Electrical Engineering, Korea Advanced Institute for Science & Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 December 2009 Received in revised form 3 May 2010 Accepted 10 June 2010 Available online 18 June 2010
An inkjet printing method was introduced to synthesize materials with a pattern, which have been very difficult to print by normal inkjet printers. Based on the proposed method, ZnS:Mn nano-phosphor was synthesized with a pattern by a chemical reaction with two different reactive inks. The nano-phosphor had mostly hexagonal-type structures with an average size of 20 nm. Photoluminescence and cathodoluminescence spectra showed an intensity resonance between peaks at 441 nm and 597 nm depending on a capping agent, which means that the Mn2þ ion was incorporated into the vacancy sites of ZnS lattice after the formation of vacancies in ZnS. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Inkjet Reactive ink ZnS:Mn Photoluminescence Cathodoluminescence
Inkjet printers are one of the most familiar household devices that can transfer visual data to various kinds of substrates. Since they were first developed in the 1970s, they have progressed from only printing on papers to being used in numerous technical fields. They are now known for their compatibility with various materials and devices including OLEDs, polymers, ceramics, metals, and other materials [1e5]. Inkjet printing technology is expected to replace some of the electrical device manufacturing processes such as photo-lithography, etching, and spin-coating. Compared to conventional manufacturing methods, inkjet printing can be carried out in low temperatures. Furthermore, chemical waste can be significantly reduced because inkjet printers can print only on the desired location without unnecessary post-processing that can damage the actual substrate and materials. Thus, we can print various chemicals on to the substrates that cannot bear high temperatures such as fabrics, papers, and polymers so that inkjet printers are expected to be used in the field of flexible electronics. Despite all the advantages of current inkjet printing technology, however, nozzle clogging has been the biggest problem of all. This is because the inks used for inkjet printing contain insoluble micro- or sometimes
* Corresponding author. Tel.: þ82 42 350 3482. E-mail addresses: [email protected] (H. Cha), [email protected] (S.I. Ahn), [email protected] (K.C. Choi). 1 Tel.: þ82 42 350 5482. 2 Tel.: þ82 42 350 8263. 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.06.010
nano-particles that can be precipitated during the printing process. Although nowadays many inks with nano-sized particles are being commercialized, these inks still have a high possibility to block micro-sized nozzles. In this paper, we introduce an inkjet printing method that potentially solves the nozzle clogging problem and applies to various research and industry areas. The patterned colloidal nanoparticles are formed in a single-step process via micro-chemical reactions between reactive ink-drops from different inkjet heads in a printer. With a kinetic energy of ink droplet and with diffusion energy due to concentration differences between two ink droplets, they can react with each other to form colloidal nano-particles after these inks are both dropped to the same spot. Thus, the proposed method here is termed Drop and Synthesis (DAS). Unlike conventional inkjet printing, colloidal nano-particles do not exist in any of the inks; consequently, the possibility of nozzle clogging is reduced significantly. Furthermore, designing a chemical reaction between the inks can give a patterned material that is hardly etched or printed. For these purposes, we studied on ZnS:Mn synthesized by DAS method, a thoroughly investigated wide-band-gap phosphor known for its high photoluminescence efficiency. Two kinds of inks were prepared corresponding to two different colors of EPSON Stylus Photo R290 inkjet printer, which has a piezoelectric inkjet head. As shown in Fig. 1, we inserted an ink containing zinc and manganese into the yellow cartridge, and an ink containing a sulfur source into the magenta cartridge. Then in the graphics editor, CMYK color patterns were adjusted to print
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Fig. 1. The composite color of yellow and magenta resulted inkjet-printed ZnS:Mn. Zn:Mn was inserted in the yellow cartridge and S was inserted in the magenta cartridge. Then they were printed through different printer nozzles simultaneously (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
a composite color of yellow and magenta in order for the two inks to be printed out simultaneously. Finally, the inks were printed on a silicon substrate and dried on a hot plate for 10 min at 80 C in an atmospheric condition. The inks were prepared as follows. For the first ink, due to Mn concentration quenching results of PL of ZnS:Mn, the optimum amount of the dopant has not been agreed upon among researchers so far. Hurd et al. and Warren et al. reported the optimum amount to be w1 mol% with respect to the amount of Zn and S [6e7], and Migita et al. reported an optimum of 0.5e1.2 mol% depending on the material preparation [8]. In this experiment, therefore, we chose the Mn concentration to be 1 mol%. This corresponding amount of Mn (Mn2(CH3COO) 4H2O, Aldrich; 99.99%) was added to the prepared 0.1 M % of zinc acetate (Zn(CH3COO)2 2H2O, Aldrich 99%) solution consisted of 10% Ethylene glycol (Aldrich; 99.8%), 20% 2-Methoxyethanol (Aldrich; 99.3%), and 70% de-ionized water. Another ink was fabricated by adding sodium sulphide (Na2S9H2O, Aldrich; 98%) with identical volume concentration of Zn into the same mixture of solvents. We finished making these inks by adding an appropriate amount of poly-vinyl-pyrrolidone (PVP, Aldrich; average Mw w200,000) as a capping and viscosity control agent under stirring at 80 C for 30 min. Previously, there have been a number of papers reporting that while synthesizing ZnS:Mn, adding PVP not only controls particle size, but it also helps surface caps to passivate the defect states and dangling bond density [9e11]. However, although all the merits hold for our experiment as well, the main difference is that all of these wet chemical methods add PVP after synthesizing ZnS:Mn. On the other hand, we had to add PVP into Zn:Mn and S solutions separately since these solutions were to be printed out from respective print heads. Therefore, three pairs of inks were prepared, each of which contained 0.4 g, 0.8 g, and 1.2 g of PVP, to study the effects of PVP when generally added to reactants before synthesized into the desirable state. The inkjet-printed ZnS:Mn nano-particles were analyzed by the following equipment. Photoluminescence (PL) measurements were carried out at room temperature using a PSI Ltd. Co. PS-PLU-X1420 spectrophotometer equipped with a 500 W Arc Xe lamp excitation light source. A Rigaku D/MAX-RB conventional X-ray diffractometer (XRD) was also used to study the phase structure and crystallinity of the synthesized phosphor. Finally, we could determine the shape and particle size distribution of ZnS:Mn with a Hitachi S-4800 field emission scanning electron microscope (FESEM). Fig. 2 shows the SEM images of inkjet-printed ZnS:Mn nanoparticles. The average particle size was 20 nm. Reaction residues
Fig. 2. (a) SEM image of inkjet-printed ZnS:Mn on silicon substrate. Nano-particles were evenly formed throughout the substrate and (b) SEM images in a wider perspective.
including potentially un-reacted chemicals were observed underneath as-synthesized particles since no post-processing was carried out after drying the phosphor for an in-situ study. However, we did not observe any particle size distribution difference according to the amount of added PVP. Fig. 3 shows the XRD pattern of as-synthesized ZnS:Mn nano-particles. We also added the XRD peaks of wurtzite-type ZnS powder for a reference. Observation of XRD indicated that there were no significantly high peaks compared to the overall right-skew of the pattern due to PVP and reaction residues. The XRD pattern has a peak at 30.78 , which corresponds to the (101) plane of the standard wurtzite-type ZnS. In spite of this highest peak, as well as other minor peaks at 28.4 and 56.32 , the task of defining the ZnS crystal structure is difficult for two reasons. Firstly, the XRD patterns for all nanostructures are rather broad and have low intensities; and, secondly, the XRD peak
Fig. 3. XRD patterns of inkjet-printed ZnS:Mn showing a broad peak near 30.78 . The straight lines indicate XRD pattern of wurtzite ZnS as a reference.
H. Cha et al. / Current Applied Physics 10 (2010) e109ee112
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Fig. 4. (a) PL spectra of inkjet-printed ZnS:Mn with excitation wavelength of 325 nm. The magnitudes of orange and violet peaks were inversely proportional and proportional to the amount of added PVP, respectively. Inset is a PLE spectra of inkjet-printed ZnS:Mn with the emission wavelength of 597 nm, and (b) Normalized CL spectra at 597 nm (dashed lines) and 441 nm (solid lines), respectively. (c) PL spectra of ZnS:Mn with PVP added after phosphor synthesis (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
centers for the cubic and hexagonal types of ZnS are very close. We conducted an additional experiment to verify the crystalline structure. Nano-phosphors were synthesized by a conventional method using the same composition as in this work. We then examined the crystalline structure. The structure appears to have a broad peak and a cubic ZnS structure. However, there may be some differences between the ZnS samples synthesized by the DAS method and those synthesized by the conventional method [12]. Fig. 4(a) is PL spectra of inkjet-printed ZnS:Mn with three different amounts of PVP; 0.4 g, 0.8 g, and 1.2 g, respectively. It showed an orange peak at 597 nm and a weak blue peak at 441 nm upon photo-excitation. At first, the orange peak tended to decrease as the amount of added PVP increased. The orange emission was attributed to the 4T1 to 6A1 transition of Mn2þ ions [13]. Since PVP acts as a capping agent, it appeared that PVP interrupts more Mn2þ from being diffused as the amount of added polymer increased.
While the orange peak decreased, on the contrary, the weak violet peak tended to increase. There have been a number of studies claiming that the blue emission results from the “self-activated” colloidal ZnS due to lattice vacancies [14e15]. Integrated intensities of the curves of Fig. 4(a) were approximately the same for all three cases which stated a PL intensity resonance between peaks at 441 nm and 597 nm depending on PVP content. The CL spectra showed a similar intensity resonance to that of PL as shown in Fig. 4 (b). These results indicated an interesting spectroscopic clue how the Mn2þ ions incorporated into the ZnS lattice. We could consider three different cases of Mn2þ incorporation mechanism during the formation of ZnS:Mn phosphor: replacement of Zn2þ after ZnS lattice formation, competition with Zn2þ during ZnS lattice formation, and direct incorporation into the zinc deficiency site after ZnS lattice formation. The PL results in Fig. 4 show a broad peak at around 450 nm. This peak apparently indicates a sulfur
Fig. 5. (a) Converting IDMP Lab logo colors to composite color of yellow and magenta. (b) The logo printed on a silicon substrate. (c) The logo illuminated with 324 nm UV hand lamp in darkness (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
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H. Cha et al. / Current Applied Physics 10 (2010) e109ee112
vacancy and dangled sulfur bonds on the surface of the ZnS. The decrease of PL in the blue area is related to a reduction of the two possible defects. Thus, the resonance of PL and CL is simultaneous result of the second and third cases. In addition, in order to support our assertion, we carried out a supplementary experiment. We again fabricated both inks without inserting PVP and they were mixed and thoroughly stirred for 30 min at 80 C. Finally, we added three different amounts of PVP 0.4 g, 0.8 g, and 1.2 g into the precipitated phosphors. Fig. 4(c) is the PL spectra of these phosphors, and there were no differences among these three samples. Therefore, we could conclude that PVP actually determined the degree of Mn2þ incorporation with ZnS lattices and consequently led to PL peak differences of inkjet-printed ZnS:Mn phosphors, although PVP didn’t have any role in controlling the PL intensity of ZnS:Mn formed by the conventional synthesis method, except for extremely high concentrations. In the meantime, we also prepared a sample of a patterned logo of our laboratory. Fig. 5(a) shows how we changed the whole color of our laboratory logo into the composite color of yellow and magenta. After the pattern was dried on a hot plate, we illuminated the logo by exciting the inkjet-printed phosphor with 324 nm UV lamp, and we could observe the patterned phosphor emitting light from Fig. 5(b) and Fig. 5(c). In summary, an inkjet printing method called Drop and Synthesis (DAS) was proposed for the purpose of reducing the probability of nozzle clogging, making insoluble materials possible to print, and studying growth mechanisms of some useful materials using conventional inkjet printers. By using two inkjet nozzles, ZnS:Mn nano-phosphor was studied on its growth mechanism depending on a capping agent of PVP. A SEM image indicates that the synthesized phosphor had an average of 20 nm size. PVP, which was added to each ink, acted as a capping agent, but at the same time it prevented Mn2þ from being properly doped inside ZnS colloids. This led to lower PL and CL orange peaks at 597 nm, but led to higher blue peak. The PL and CL results confirm that Mn2þ ions were competitively incorporated into the ZnS lattice with Zn2þ during the formation of the ZnS lattice and directly incorporated into the zinc deficiency site after the formation of the ZnS lattice. Unlike conventional inkjet printing, which is used to print alreadysynthesized materials, the proposed method concentrates on the fact that the desired materials could be synthesized on the substrate surface after they are printed out from different nozzles.
We believe that this method is one of the pathways to overcome and solve problems of conventional inkjet printing technology. Acknowledgement This research was supported by Grant Nos. R11-2007-04502001-0 and N01090243 from the Basic Research Program of the Korea Science and Engineering Foundation, and Brain Korea 21 Project, the School of Information Technology, KAIST in 2008. References [1] P. Calvert, Inkjet printing for materials and devices, Chem. Mater. 13 (2001) 3299e3305. [2] H.P. Le, Progress and trends in ink-jet printing technology, J. Imaging Sci. Technol. 42 (1998) 49e62. [3] S.C. Chang, J. Liu, J. Bharathan, Y. Yang, J. Onohara, J. Kido, Multicolor organic light-emitting diodes processed by hybrid inkjet printing, Adv. Mater. 11 (1999) 734e737. [4] B.-J. de Gans, P.C. Duineveld, Inkjet printing of polymers: state of the art and future developments, Adv. Mater. 16 (2004) 203e213. [5] H. Sirringhaus, T. Kawase, R.H. Friend, T. Shimoda, M. Inbasekaran, W. Wu, E.P. Woo, High-resolution inkjet printing of all-polymer transistor circuits, Science 290 (2000) 2123e2126. [6] J.M. Hurd, C.N. King, Physical and electrical characterization of co-deposited ZnS: Mn electroluminescent thin film structures, J. Electron. Mater. 8 (1979) 879e891. [7] A.J. Warren, C.B. Thomas, H.S. Reehal, P.R.C. Stevens, A study of the luminescent and electrical characteristics of films of ZnS doped with Mn, J. Lumin. 28 (1983) 147e162. [8] M. Migita, O. Kanehisa, M. Shiiki, H. Yamamoto, The preparation of ZnS: Mn electroluminescent layers by MOCVD using new manganese sources,, J. Cryst. Growth 93 (1988) 686e691. [9] N. Karar, S. Raj, R. Singh, Properties of nanocrystalline ZnS: Mn, J. Cryst. Growth 268 (2004) 585e589. [10] N. Kara, F. Singh, B.R. Mehta, Structure and photoluminescence studies on ZnS: Mn nanoparticles, J. Appl. Phys. 95 (2004) 656e660. [11] K. Manzoor, S.R. Vadera, N. Kumar, T.R.N. Kutty, Energy transfer from organic surface adsorbate-polyvinylpyrrolidone molecules to luminescent centers in ZnS nanocrystals, Solid State Commun. 129 (2004) 469e473. [12] J. Baars, G. Brandt, Structural phase transitions in ZnS, J. Phys. Chem. Solids 34 (1973) 905e909. [13] R.N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Optical properties of manganese-doped nanocrystals of ZnS, Phys. Rev. Lett. 72 (1994) 416e419. [14] W.G. Becker, A.J. Bard, Photoluminescence and photoinduced oxygen adsorption of colloidal zinc sulfide dispersions, J. Phys. Chem. 87 (1983) 4888e4893. [15] J. Lee, S. Lee, S. Cho, S. Kim, I.Y. Park, Y.D. Choi, Role of growth parameters on structural and optical properties of ZnS nanocluster thin films grown by solution growth technique, Mater. Chem. Phys. 77 (2002) 254e260.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
An inkjet printing method: Drop and Synthesis (DAS). Application to the synthesis of ZnS:Mn nano-phosphor with a pattern Hongki Cha 1, Sung Il Ahn 2, Kyung Cheol Choi* Dept. of Electrical Engineering, Korea Advanced Institute for Science & Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 December 2009 Received in revised form 3 May 2010 Accepted 10 June 2010 Available online 18 June 2010
An inkjet printing method was introduced to synthesize materials with a pattern, which have been very difficult to print by normal inkjet printers. Based on the proposed method, ZnS:Mn nano-phosphor was synthesized with a pattern by a chemical reaction with two different reactive inks. The nano-phosphor had mostly hexagonal-type structures with an average size of 20 nm. Photoluminescence and cathodoluminescence spectra showed an intensity resonance between peaks at 441 nm and 597 nm depending on a capping agent, which means that the Mn2þ ion was incorporated into the vacancy sites of ZnS lattice after the formation of vacancies in ZnS. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Inkjet Reactive ink ZnS:Mn Photoluminescence Cathodoluminescence
Inkjet printers are one of the most familiar household devices that can transfer visual data to various kinds of substrates. Since they were first developed in the 1970s, they have progressed from only printing on papers to being used in numerous technical fields. They are now known for their compatibility with various materials and devices including OLEDs, polymers, ceramics, metals, and other materials [1e5]. Inkjet printing technology is expected to replace some of the electrical device manufacturing processes such as photo-lithography, etching, and spin-coating. Compared to conventional manufacturing methods, inkjet printing can be carried out in low temperatures. Furthermore, chemical waste can be significantly reduced because inkjet printers can print only on the desired location without unnecessary post-processing that can damage the actual substrate and materials. Thus, we can print various chemicals on to the substrates that cannot bear high temperatures such as fabrics, papers, and polymers so that inkjet printers are expected to be used in the field of flexible electronics. Despite all the advantages of current inkjet printing technology, however, nozzle clogging has been the biggest problem of all. This is because the inks used for inkjet printing contain insoluble micro- or sometimes
* Corresponding author. Tel.: þ82 42 350 3482. E-mail addresses: [email protected] (H. Cha), [email protected] (S.I. Ahn), [email protected] (K.C. Choi). 1 Tel.: þ82 42 350 5482. 2 Tel.: þ82 42 350 8263. 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.06.010
nano-particles that can be precipitated during the printing process. Although nowadays many inks with nano-sized particles are being commercialized, these inks still have a high possibility to block micro-sized nozzles. In this paper, we introduce an inkjet printing method that potentially solves the nozzle clogging problem and applies to various research and industry areas. The patterned colloidal nanoparticles are formed in a single-step process via micro-chemical reactions between reactive ink-drops from different inkjet heads in a printer. With a kinetic energy of ink droplet and with diffusion energy due to concentration differences between two ink droplets, they can react with each other to form colloidal nano-particles after these inks are both dropped to the same spot. Thus, the proposed method here is termed Drop and Synthesis (DAS). Unlike conventional inkjet printing, colloidal nano-particles do not exist in any of the inks; consequently, the possibility of nozzle clogging is reduced significantly. Furthermore, designing a chemical reaction between the inks can give a patterned material that is hardly etched or printed. For these purposes, we studied on ZnS:Mn synthesized by DAS method, a thoroughly investigated wide-band-gap phosphor known for its high photoluminescence efficiency. Two kinds of inks were prepared corresponding to two different colors of EPSON Stylus Photo R290 inkjet printer, which has a piezoelectric inkjet head. As shown in Fig. 1, we inserted an ink containing zinc and manganese into the yellow cartridge, and an ink containing a sulfur source into the magenta cartridge. Then in the graphics editor, CMYK color patterns were adjusted to print
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H. Cha et al. / Current Applied Physics 10 (2010) e109ee112
Fig. 1. The composite color of yellow and magenta resulted inkjet-printed ZnS:Mn. Zn:Mn was inserted in the yellow cartridge and S was inserted in the magenta cartridge. Then they were printed through different printer nozzles simultaneously (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
a composite color of yellow and magenta in order for the two inks to be printed out simultaneously. Finally, the inks were printed on a silicon substrate and dried on a hot plate for 10 min at 80 C in an atmospheric condition. The inks were prepared as follows. For the first ink, due to Mn concentration quenching results of PL of ZnS:Mn, the optimum amount of the dopant has not been agreed upon among researchers so far. Hurd et al. and Warren et al. reported the optimum amount to be w1 mol% with respect to the amount of Zn and S [6e7], and Migita et al. reported an optimum of 0.5e1.2 mol% depending on the material preparation [8]. In this experiment, therefore, we chose the Mn concentration to be 1 mol%. This corresponding amount of Mn (Mn2(CH3COO) 4H2O, Aldrich; 99.99%) was added to the prepared 0.1 M % of zinc acetate (Zn(CH3COO)2 2H2O, Aldrich 99%) solution consisted of 10% Ethylene glycol (Aldrich; 99.8%), 20% 2-Methoxyethanol (Aldrich; 99.3%), and 70% de-ionized water. Another ink was fabricated by adding sodium sulphide (Na2S9H2O, Aldrich; 98%) with identical volume concentration of Zn into the same mixture of solvents. We finished making these inks by adding an appropriate amount of poly-vinyl-pyrrolidone (PVP, Aldrich; average Mw w200,000) as a capping and viscosity control agent under stirring at 80 C for 30 min. Previously, there have been a number of papers reporting that while synthesizing ZnS:Mn, adding PVP not only controls particle size, but it also helps surface caps to passivate the defect states and dangling bond density [9e11]. However, although all the merits hold for our experiment as well, the main difference is that all of these wet chemical methods add PVP after synthesizing ZnS:Mn. On the other hand, we had to add PVP into Zn:Mn and S solutions separately since these solutions were to be printed out from respective print heads. Therefore, three pairs of inks were prepared, each of which contained 0.4 g, 0.8 g, and 1.2 g of PVP, to study the effects of PVP when generally added to reactants before synthesized into the desirable state. The inkjet-printed ZnS:Mn nano-particles were analyzed by the following equipment. Photoluminescence (PL) measurements were carried out at room temperature using a PSI Ltd. Co. PS-PLU-X1420 spectrophotometer equipped with a 500 W Arc Xe lamp excitation light source. A Rigaku D/MAX-RB conventional X-ray diffractometer (XRD) was also used to study the phase structure and crystallinity of the synthesized phosphor. Finally, we could determine the shape and particle size distribution of ZnS:Mn with a Hitachi S-4800 field emission scanning electron microscope (FESEM). Fig. 2 shows the SEM images of inkjet-printed ZnS:Mn nanoparticles. The average particle size was 20 nm. Reaction residues
Fig. 2. (a) SEM image of inkjet-printed ZnS:Mn on silicon substrate. Nano-particles were evenly formed throughout the substrate and (b) SEM images in a wider perspective.
including potentially un-reacted chemicals were observed underneath as-synthesized particles since no post-processing was carried out after drying the phosphor for an in-situ study. However, we did not observe any particle size distribution difference according to the amount of added PVP. Fig. 3 shows the XRD pattern of as-synthesized ZnS:Mn nano-particles. We also added the XRD peaks of wurtzite-type ZnS powder for a reference. Observation of XRD indicated that there were no significantly high peaks compared to the overall right-skew of the pattern due to PVP and reaction residues. The XRD pattern has a peak at 30.78 , which corresponds to the (101) plane of the standard wurtzite-type ZnS. In spite of this highest peak, as well as other minor peaks at 28.4 and 56.32 , the task of defining the ZnS crystal structure is difficult for two reasons. Firstly, the XRD patterns for all nanostructures are rather broad and have low intensities; and, secondly, the XRD peak
Fig. 3. XRD patterns of inkjet-printed ZnS:Mn showing a broad peak near 30.78 . The straight lines indicate XRD pattern of wurtzite ZnS as a reference.
H. Cha et al. / Current Applied Physics 10 (2010) e109ee112
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Fig. 4. (a) PL spectra of inkjet-printed ZnS:Mn with excitation wavelength of 325 nm. The magnitudes of orange and violet peaks were inversely proportional and proportional to the amount of added PVP, respectively. Inset is a PLE spectra of inkjet-printed ZnS:Mn with the emission wavelength of 597 nm, and (b) Normalized CL spectra at 597 nm (dashed lines) and 441 nm (solid lines), respectively. (c) PL spectra of ZnS:Mn with PVP added after phosphor synthesis (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
centers for the cubic and hexagonal types of ZnS are very close. We conducted an additional experiment to verify the crystalline structure. Nano-phosphors were synthesized by a conventional method using the same composition as in this work. We then examined the crystalline structure. The structure appears to have a broad peak and a cubic ZnS structure. However, there may be some differences between the ZnS samples synthesized by the DAS method and those synthesized by the conventional method [12]. Fig. 4(a) is PL spectra of inkjet-printed ZnS:Mn with three different amounts of PVP; 0.4 g, 0.8 g, and 1.2 g, respectively. It showed an orange peak at 597 nm and a weak blue peak at 441 nm upon photo-excitation. At first, the orange peak tended to decrease as the amount of added PVP increased. The orange emission was attributed to the 4T1 to 6A1 transition of Mn2þ ions [13]. Since PVP acts as a capping agent, it appeared that PVP interrupts more Mn2þ from being diffused as the amount of added polymer increased.
While the orange peak decreased, on the contrary, the weak violet peak tended to increase. There have been a number of studies claiming that the blue emission results from the “self-activated” colloidal ZnS due to lattice vacancies [14e15]. Integrated intensities of the curves of Fig. 4(a) were approximately the same for all three cases which stated a PL intensity resonance between peaks at 441 nm and 597 nm depending on PVP content. The CL spectra showed a similar intensity resonance to that of PL as shown in Fig. 4 (b). These results indicated an interesting spectroscopic clue how the Mn2þ ions incorporated into the ZnS lattice. We could consider three different cases of Mn2þ incorporation mechanism during the formation of ZnS:Mn phosphor: replacement of Zn2þ after ZnS lattice formation, competition with Zn2þ during ZnS lattice formation, and direct incorporation into the zinc deficiency site after ZnS lattice formation. The PL results in Fig. 4 show a broad peak at around 450 nm. This peak apparently indicates a sulfur
Fig. 5. (a) Converting IDMP Lab logo colors to composite color of yellow and magenta. (b) The logo printed on a silicon substrate. (c) The logo illuminated with 324 nm UV hand lamp in darkness (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
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H. Cha et al. / Current Applied Physics 10 (2010) e109ee112
vacancy and dangled sulfur bonds on the surface of the ZnS. The decrease of PL in the blue area is related to a reduction of the two possible defects. Thus, the resonance of PL and CL is simultaneous result of the second and third cases. In addition, in order to support our assertion, we carried out a supplementary experiment. We again fabricated both inks without inserting PVP and they were mixed and thoroughly stirred for 30 min at 80 C. Finally, we added three different amounts of PVP 0.4 g, 0.8 g, and 1.2 g into the precipitated phosphors. Fig. 4(c) is the PL spectra of these phosphors, and there were no differences among these three samples. Therefore, we could conclude that PVP actually determined the degree of Mn2þ incorporation with ZnS lattices and consequently led to PL peak differences of inkjet-printed ZnS:Mn phosphors, although PVP didn’t have any role in controlling the PL intensity of ZnS:Mn formed by the conventional synthesis method, except for extremely high concentrations. In the meantime, we also prepared a sample of a patterned logo of our laboratory. Fig. 5(a) shows how we changed the whole color of our laboratory logo into the composite color of yellow and magenta. After the pattern was dried on a hot plate, we illuminated the logo by exciting the inkjet-printed phosphor with 324 nm UV lamp, and we could observe the patterned phosphor emitting light from Fig. 5(b) and Fig. 5(c). In summary, an inkjet printing method called Drop and Synthesis (DAS) was proposed for the purpose of reducing the probability of nozzle clogging, making insoluble materials possible to print, and studying growth mechanisms of some useful materials using conventional inkjet printers. By using two inkjet nozzles, ZnS:Mn nano-phosphor was studied on its growth mechanism depending on a capping agent of PVP. A SEM image indicates that the synthesized phosphor had an average of 20 nm size. PVP, which was added to each ink, acted as a capping agent, but at the same time it prevented Mn2þ from being properly doped inside ZnS colloids. This led to lower PL and CL orange peaks at 597 nm, but led to higher blue peak. The PL and CL results confirm that Mn2þ ions were competitively incorporated into the ZnS lattice with Zn2þ during the formation of the ZnS lattice and directly incorporated into the zinc deficiency site after the formation of the ZnS lattice. Unlike conventional inkjet printing, which is used to print alreadysynthesized materials, the proposed method concentrates on the fact that the desired materials could be synthesized on the substrate surface after they are printed out from different nozzles.
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