The photoluminescence enhancement of Mn2+ ions and the crystal field in ZnS:Mn nanoparticles covered by polyvinyl alcohol

Abstract The polyvinyl alcohol (PVA)-capped ZnS:Mn nanoparticles with Mn content of 8 mol% and different PVA mass (denoted as ZnS:Mn/PVA) are synthesized by co-precipitation method, in which PVA solution is mixed from the beginning with the initial solutions used to synthesize ZnS:Mn nanoparticles. The microstructures, morphology and average crystalline size of ZnS:Mn/PVA nanoparticles were investigated by X-ray diffraction patterns, high resolution transmission electron microscopy, thermal gravimetric analysis (TGA) and differential thermal gravimetric analysis and Fourier transform infrared absorption spectra. The role of PVA to the photoluminescence (PL) of Mn2? ions in these nanoparticles at 300 K were studied by the PL and photoluminescence excitation (PLE) spectra. The investigated results show that the PVA covering for ZnS:Mn nanoparticles almost is not change the microstructure, morphology, the crystal field and the peak positions in their PL and PLE spectra, but the maximum intensity of peaks increased with PVA mass from 0.2 to 1.0 g. The clear peak positions in PLE spectra show that the energy levels of Mn2? ions were splitted into multiple levels in the ZnS:Mn crystal field, that its strength Dq was caculated. Furthermore, the effect of PVA on the PL enhancement of Mn2? ions in ZnS:Mn/PVA nanoparticles also was explained.

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1 23 Optical and Quantum Electronics ISSN 0306-8919 Volume 48 Number 7 Opt Quant Electron (2016) 48:1-15 DOI 10.1007/s11082-016-0622-y The photoluminescence enhancement of Mn2+ ions and the crystal field in ZnS:Mn nanoparticles covered by polyvinyl alcohol Dang Van Thai, Pham Van Ben, Tran Minh Thi, Nguyen Van Truong & Hoa Huu Thu 1 23 Your article is protected by copyright and all rights are held exclusively by Springer Science +Business Media New York. This e-offprint is for personal use only and shall not be self- archived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com”. The photoluminescence enhancement of Mn2+ ions and the crystal field in ZnS:Mn nanoparticles covered by polyvinyl alcohol Dang Van Thai1 • Pham Van Ben1 • Tran Minh Thi2 • Nguyen Van Truong1 • Hoa Huu Thu3 Received: 13 November 2015 / Accepted: 7 June 2016  Springer Science+Business Media New York 2016 Abstract The polyvinyl alcohol (PVA)-capped ZnS:Mn nanoparticles with Mn content of 8 mol% and different PVA mass (denoted as ZnS:Mn/PVA) are synthesized by co-pre- cipitation method, in which PVA solution is mixed from the beginning with the initial solutions used to synthesize ZnS:Mn nanoparticles. The microstructures, morphology and average crystalline size of ZnS:Mn/PVA nanoparticles were investigated by X-ray diffraction patterns, high resolution transmission electron microscopy, thermal gravimetric analysis (TGA) and differential thermal gravimetric analysis and Fourier transform infrared absorption spectra. The role of PVA to the photoluminescence (PL) of Mn2? ions in these nanoparticles at 300 K were studied by the PL and photoluminescence excitation (PLE) spectra. The investigated results show that the PVA covering for ZnS:Mn nanoparticles almost is not change the microstructure, morphology, the crystal field and the peak positions in their PL and PLE spectra, but the maximum intensity of peaks increased with PVA mass from 0.2 to 1.0 g. The clear peak positions in PLE spectra show that the energy levels of Mn2? ions were splitted into multiple levels in the ZnS:Mn crystal field, that its strength Dq was caculated. Furthermore, the effect of PVA on the PL enhancement of Mn2? ions in ZnS:Mn/PVA nanoparticles also was explained. Keywords Co-precipitation method  Nanoparticles  PL spectra  PLE spectra  Crystal field strength & Tran Minh Thi tranminhthi@hnue.edu.vn 1 Faculty of Physics, Hanoi University of Science, VNU, Hanoi, Vietnam 2 Faculty of Physics, Hanoi National University of Education, Hanoi, Vietnam 3 Faculty of Chemistry, Hanoi University of Science, VNU, Hanoi, Vietnam 123 Opt Quant Electron (2016) 48:362 DOI 10.1007/s11082-016-0622-y Author's personal copy 1 Introduction Recently, Mn doped ZnS nanoparticles (denoted by ZnS:Mn) is widely used in photonic, biological markers, photocatalytic applications and many other applications because they are wide band gap semiconductors with stable and strong PL intensity in the yellow-orange region and high luminescent efficiency (Bhargava et al. 1994; Bhargava 1996; Pouretedal et al. 2009; Chitkara et al. 2011; Sajan et al. 2015). To increase the application capacity from biological sensors to optical displays, the synthesis and characterization of semiconductor nanocrystals of various sizes and com- positions and with different capping agents remains an active area of current research. ZnS:Mn nanoparticles were often capped by surface active substances such as C2H4O2S thyoglycolic acid, (C6H9NO)n polyvinyl pyrrolidone (PVP) and [CH2CH(OH)]n polyvinyl alcohol (PVA) 3-mercaptopropionic acid (MPA) (Chitkara et al. 2011; Onwudiwe et al. 2014; Murugadoss 2010; Murugadoss et al. 2010; Thi et al. 2013; Hirankumar et al. 2005; Kareem et al. 2012; Zhou et al. 2015). Meanwhile, ZnS:Mn nanoparticles isolated with the environment, un-aggregation, thus the particle size reduced. In addition, the PL intensity increased due to excitation energy transfer from the surfactant to ZnS:Mn nanoparticles (Onwudiwe et al. 2014; Murugadoss et al. 2010; Zhou et al. 2015). Typical of polymers, PVA acts as a ligand and forms a bond with the metal ions by donor, acceptor interactions leading to the formation of a coordination sphere. In the polymer chain, the N and O atoms have lone pairs of electrons which could be used in the formation of the bond (Onwudiwe et al. 2014). In our earlier studies, we have reported the synthesis and the energy transition process in ZnS:Mn/PVP nanoparticles (Thi et al. 2013). PVP controls the growth of the particles by forming passivation layers around the particle core via coordination bond formation, in which PVP part acts as the head group, while the polyvinyl alcohol (PVA) part acts as the tail group (Onwudiwe et al. 2014). Furthermore, PVA with the energy band gap of 5.4 eV (Hirankumar et al. 2005) have the semi-crystalline nature of organic material (Wang et al. 2014), that is composed mainly of 1,3-diol linkage [–CH2–CH(OH)–CH2–CH(OH)–] but a few percent of 1,2-diol [–CH2–CH(OH)–CH(OH)–CH2–] occurred, thus the covered formation and their properties created by PVA around the ZnS:Mn particles are different than the PVP coating. However, PVA has received a significant amount of interest in both academic and industrial research for a long time due to its biocompatibility and degradability by certain bacteria. PVA has been widely used in the form of hydrogels in the biotechnology area, the topics of intense research due to their size-related electronic, magnetic and optical properties (quantum size effect) and their wide applications from optoelectronics to biology (Murugadoss et al. 2010; Hammad et al. 2015). Furthermore, there are not any papers that completely investigated the optical properties and calculated of the crystal field strength of ZnS:Mn/PVA nanoparticles. The capping of ZnS:Mn nanoparticles can be done by one of two methods: (1) dis- persing of ZnS:Mn nanoparticles into surfactant solution (Kareem et al. 2012) or (2) from the beginning, the surfactant solution and the initial solutions of nanoparticles simulta- neous are mixed each other (Chitkara et al. 2011). By second method, the ZnS:Mn nanoparticles formed in surfactant solution matrix, thus ZnS:Mn nanoparticles are un- aggregation to each other and its particle size decreased, the quantum confinement effect increased. The paper report the synthesis of ZnS:Mn/PVA nanoparticles by second method, in which PVA solution is mixed from the beginning with the initial solutions used to 362 Page 2 of 15 D. V. Thai et al. 123 Author's personal copy synthesize ZnS:Mn nanoparticles. The microstructures, morphology of ZnS:Mn/PVA nanoparticles, the PL enhancement of Mn2? ions by different PVA mass, the absorption and radiation transitions in the ZnS:Mn crystals and the crystal field strength Dq are investigated and explained. 2 Experimental In order to investigate the role and influence of PVA to the properties of ZnS:Mn nanoparticles, the ZnS:Mn powder mass of 0.5 g is kept constant for all ZnS:Mn/PVA nanopowder samples, while the PVA mass changed from 0 to 1.5 g. By second method, ZnS:Mn/PVA nanopowder samples have been synthesized from the initial solutions Zn(CH3COO)2 0.1 M (A); Mn(CH3COO)2 0.1 M (B); Na2S 0.1 M (C), which were calcu- lated to create the nominal ZnS:Mn powder mass of 0.5 g with Mn content of 8 mol%, while PVA mass changed with values: 0; 0.2; 0.4; 0.6; 0.8; 1.0; 1.2 and 1.5 g. The process of this method was performed as following: the solutions A and Bwere mixed each other and stirred for 30 min to receive the solution D. The different PVA mass of 0.2; 0.4; 0.6; 0.8; 1.0; 1.2; 1.5 g in turn were stirred in distilled water of 50 mL for 3 h at 80 C, to obtained the solutions E, respectively. The solutions D and E were mixed and stirred each other for 1 h to receive the solutions F. The solution C was putted in the solutions F drop by drop and stirred for 1 h to create precipitation. The reaction equation occurred as follow: Zn CH3COOð Þ2þMn CH3COOð Þ2þ 2Na2S þ PVA ! ZnSMnSð Þ  PVA½  # þ 4CH3COONa [(ZnSMnS)-PVA] precipitations (8 samples of ZnS:Mn/PVA nanoparticles with nominal mZnS:Mn of 0.5 g and different PVA mass: mPVA = 0; 0.2; 0.4; 0.6; 0.8; 1.0; 1.2 and 1.5 g) were separated by centrifugation with speed of 2500 rpm and filtered, washed several times by distilled water. The ZnS:Mn/PVA nanoparticles obtained by drying at 80 C for 10 h and finely grind. The crystal structure and average crystalline size of these nanoparticles were investigated by X-ray diffraction patterns (XRD) recorded on XD8 Advance Bukerding using CuKa radiation (k = 1.5406 A˚, 2h = 10–70). The morphology of ZnS:Mn/PVA nanoparticles with different PVA mass were also demonstrated by HRTEM image on the high resolution transmission electron microscopy JEM-2100. The PL and PLE spectra of nanoparticles at 300 Kwere recorded onMS-257 Oriel, FL3-22 spectrometers, respectively, using 325 nm excitation radiations of He–Cd laser and XFOR-450 xenon lamp. Thermal gravimetric analysis (TGA) and differential thermal gravimetric analysis (DTG) were per- formed using Setaram instrumentation. The samples were placed in the specimen holder, in which the heating rate was set at 10 C/min and the measurements were carried out in argon gas ambient from 30 to 700 C. Fourier Transform Infrared (FT-IR) absorption spectra of the nanoparticles also at 300 K were recorded on spectrometer Nicolet 6700 FT-IR. 3 Results and discussions 3.1 Crystal structure and morphology of ZnS:Mn/PVA nanoparticles XRD pattern was used to characterize the original PVA powders (Fig. 1a). The XRD pattern of the original PVA powders shows a strong diffraction peak centered at 20 and a The photoluminescence enhancement of Mn2? ions and the Page 3 of 15 362 123 Author's personal copy weak diffusion diffraction peak centered at 41, indicating its semi-crystalline nature. But these diffraction peaks of PVA in XRD pattern of ZnS:Mn/PVA nanoparticles decrease strongly and become smooth range (Fig. 1b), while the peaks of (111), (220) and (311) belong to ZnS:Mn crystal phase. This phenomenon is suggesting a decrease in the degree of crystallinity of PVA derivatives in ZnS:Mn/PVA nanocomposite. The breakage of PVA crystal region should be attributed to the decrease of the intermolecular hydrogen bonding between PVA chains (Murugadoss et al. 2010; Wang et al. 2014) due to the embedment of ZnS:Mn nanoparticles in PVA matrix. Figure 2 present XRD patterns of ZnS:Mn/PVA (CMn = 8 mol%) nanoparticles with different PVA mass (0.0, 0.4, 0.8, 1.0, 1.5 g). These patterns include (111), (220) and (311) diffraction peak, in which intensity of (111) peak is greatest. This XRD patterns also showed that these nanoparticles are single phase with T2d  F43m symmetry cubic struc- ture. The strong and sharp diffraction peaks suggest that the obtained products are well crystallized with ZnS:Mn crystal phase, without any strange phase is observed. It has to be noticed that for XRD patterns of ZnS:Mn/PVA nanoparticles, the width at haft maximum of diffraction peaks are larger than those observed for ZnS:Mn crystallites (Fig. 2a) and suggesting that the particle size becomes slightly smaller. Further, there is no significant change in the position of the peaks. It shows that the PVA capping agent unchanged the phase of ZnS:Mn nanoparticles. The lattice constant and average crystalline sizes were determined from XRD patterns and using Debye-Sherrer formula: D ¼ 0:9k b cos h ð1Þ where, k(nm) is the wavelength of CuKa radiation; b(rad) is the full width at haft maxi- mum and h (rad) is the diffraction angle. The results showed that the average crystalline size of uncapped ZnS:Mn nanoparticles is about of 3.6 nm, but this size decreased to 2.6–2.7 nm when PVA mass increased from 0.2 to 1.5 g. These size decrease are explained due to PVA solution mixed from the outset with the initial solutions used to synthesize ZnS:Mn/PVA nanoparticles, thus the formation of ZnS:Mn nanoparticles happened in PVA matrix simultaneously with the breakage of the crystal region of PVA. Meanwhile, PVC capping prevented the aggregation and growth of ZnS:Mn nanoparticles. Fig. 1 XRD patterns of PVA (a) and ZnS:Mn/PVA nanoparticles (mPVA = 1.0 g) (b) 362 Page 4 of 15 D. V. Thai et al. 123 Author's personal copy 20 30 40 50 60 70 In te ns ity (a .u ) 2θ (degree) (111) (220) (311) h. 1.5g f. 1.0g e. 0.8g c. 0.4g a. 0.0g a c e f h Fig. 2 XRD patterns of ZnS:Mn/PVA nanoparticles with different PVA mass: a mPVA = 0 g; c mPVA = 0.4 g; e mPVA = 0.8 g; f mPVA = 1.0 g; h mPVA = 1.5 g Fig. 3 HRTEM images of ZnS:Mn nanoparticles (a–c) and ZnS:Mn/PVA nanoparticles with mPVA = 1 g (d–f) The photoluminescence enhancement of Mn2? ions and the Page 5 of 15 362 123 Author's personal copy However, the lattice constant of ZnS:Mn/PVA nanoparticles with different PVA mass is almost unchanged with a = 5.373 A˚, this value is approximately to standard lattice con- stant of ZnS (JCPDS Card. No. 05-0566, a = b = 5.406 A˚ ´ ). From the lattice constant a, the lattice spacing of ZnS:Mn/PVA nanoparticles with cubic structure has been determined about of 0.31 nm. Figure 3 shows HRTEM images of the ZnS:Mn and ZnS:Mn/PVA nanoparticles with mPVA = 1 g. HRTEM image of ZnS:Mn nanoparticles includes a some of crystal planes (Fig. 3a) distributed in different directions, that proves the polycrystalline structure of ZnS:Mn nanoparticles. The HRTEM image in Fig. 3b and the corresponding fast Fourier transform image (FFT) in Fig. 3c (Wang et al. 2006) show that the lattice spacing was estimated to be around 0.31 nm. This value agrees with the (111) lattice spacing of ZnS, ZnS:Mn crystals (about of 0.30–0.31 nm) (Han et al. 2014; Son et al. 2007), which cal- culated from XRD patterns. The above results are also correct for ZnS:Mn/PVA nanoparticles (Fig. 3d–f). Thus, the covering of ZnS:Mn nanoparticles by PVA almost did not affect their microstructure and morphology. 3.2 Analyses of TGA, DTG and FT-IR spectra The PVA capping of ZnS:Mn nanoparticles was proved by TGA, DTG and FT-IR spectra. Figures 4, 5 show TGA and DTG spectra of PVA and ZnS:Mn/PVA nanoparticles at a heating rate of 10 C/min and in the range from room temperature to 750 C. A derivative weight loss curve can be used to tell the point at which weight loss is most apparent. TGA and DTG curves of PVA and ZnS:Mn/PVA revealed three main weight loss regions. In the TGA curve of PVA (Fig. 4), the initial weight loss for pure PVA about of -6 wt% occurredat the temperature region from 30 to 190 C, in which there is the endothermic peak at 140 C in the DTG curve, due to the evaporation of the trapped water from PVA. It was also observed that the major weight losses about of -44 wt% have occurred in the second range from 250 to 370 C. This is due to the degradation of O–H chains of PVA in this range with sharp endothermic peak of 335 C in the DTG curve (Ahad et al. 2012; Alkan and Benlikaya 2009). In the third temperature range from 370 to 730 C with the sharp endothermic peak of 420 C in the DTG curve, the weight loss about of -49.3 wt% 0 100 200 300 400 500 600 700 -120 -100 -80 -60 -40 -20 0 20 Temperature (oC) TG A (% ) -44.7% -49.3% TGA and DTG of PVA in Argon TGA -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 -6% D TG DTG 335oC420oC 140oC Fig. 4 TGA and DTG spectra of PVA 362 Page 6 of 15 D. V. Thai et al. 123 Author's personal copy may correspond to the decomposition of CC, CO, CH backbones of PVA in the TGA curve. At the temperature upper 730 C PVA completely decomposed. For the TGA curve of ZnS:Mn/PVA (Fig. 5), the weight loss about of -7 % wt% due to the evaporation of trapped water occurred at the temperature region from 30 to 190 C, in which there is the sharp endothermic peak at 102 C in the DTG curve. In the second temperature region from 180 to 310 C, the weight loss of ZnS:Mn/PVA is about of -6.4 wt%, meanwhile this weight loss is about of -11.6 wt% in the third temperature region from 310 to 750 C. However, due to the decrease of the intermolecular hydrogen bonding between PVA chains by the embedment of ZnS:Mn nanoparticles in PVA matrix, the endothermic peaks (248 and 379 C) occurred at lower temperatures than that in com- parison with pure PVA sample (in DTG curves of Figs. 4, 5). The degradation of O–H chains, the backbones of CH, CO, and CC of PVA occurred from 180 up to 750 C, after that the mass of ZnS:Mn remained about of 77 wt%. In FT-IR spectrum of PVA (Fig. 6a), the typical peaks of this spectrum were assigned to the stretching vibrations of groups: OH at 3450 cm-1; CH/CH2 at 2954 cm -1; C–O at 1108 cm-1; and the bending vibration of water absorbed by PVA at 1638 cm-1, in which OH group has largest absorptance (Venyaminov et al. 1997; Ilcin et al. 2010; Wong et al. 2009). For ZnS:Mn/PVA nanoparticles, on basic, its FT-IR spectrum also shows charac- teristic peaks of PVA. Besides that, the additional appearance of some peaks in this spectrum, that were assigned to the stretching and bending vibrations of CH group at 1544 cm-1; 1419 cm-1; the stretching vibration of oxygen at 1006 cm-1; Zn–S at 620, 472 cm-1 (Fig. 6b) (Baishya and Sarkar 2011). However, the vibration of OH group is shifted towards the shorter wavenumber at 3410 cm-1. This result shows the bonds between OH group of PVA and ZnS:Mn nanoparticles. The appearances of peaks of CH group at 1544 and 1419 cm-1 in FT-IR spectra of ZnS:Mn/PVA nanoparticles are the result of coordinate bonding between PVA and Zn (Thottoli and Achuthanunni 2013). The analyzed results of the XRD patterns, TGA, DTG, FT-IR spectra of ZnS:Mn/PVA nanoparticles proved that the PVA capping agent unchanged the phase of ZnS:Mn nanoparticles. The embedment of ZnS:Mn nanoparticles into PVA matrix caused the decrease of the intermolecular hydrogen bonding between PVA chains, simultaneous their 0 100 200 300 400 500 600 700 -30 -20 -10 0 10 -7% Temperature (oC) TG A (% ) -5.4% -11.6% DTG TGA 102oC 248oC 379oC TGA and DTG of ZnS:Mn/PVA in Argon -0.4 -0.3 -0.2 -0.1 0.0 0.1 D TG Fig. 5 TGA and DTG of ZnS:Mn/PVA nanoparticles The photoluminescence enhancement of Mn2? ions and the Page 7 of 15 362 123 Author's personal copy particle size has reduced due to un-aggregation of ZnS:Mn nanoparticles. The schema of PVA-capped ZnS:Mn nanoparticles is presented in Fig. 7. 3.3 The photoluminescence enhancement of Mn21 ions in ZnS:Mn/PVA nanoparticles Figure 8 shows the PL spectra of ZnS:Mn/PVA nanoparticles with different PVA mass excited by 325 nm radiation of He–Cd laser. The PL spectrum of ZnS:Mn nanoparticles appeared the blue band at about of 440 nm with little intensity and the yellow-orange band at about of 603 nm with greater intensity (Fig. 8a). The blue band is attributed to Zn, S vacancies and their interstitial atoms (Denzler et al. 1998), while the yellow-orange band assigned to the radiation of Mn2? ions [4T1( 4G) ? 6A1( 6S)] in ZnS crystals (Bhargava et al. 1994). For the ZnS:Mn/PVA Fig. 6 FT-IR spectra of PVA (a) and ZnS:Mn/PVA nanoparticles (mPVA = 1 g) (b) Fig. 7 The s