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

DSpace at VNU: The photoluminescence enhancement of Mn2+ ions and the crystal field in ZnS:Mn nanoparticles covered by p...

Opt Quant Electron (2016)48:362 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 Thai1 • Pham Van Ben1 • Tran Minh Thi2 Nguyen Van Truong1 • Hoa Huu Thu3 • Received: 13 November 2015 / Accepted: June 2016 Ó Springer Science+Business Media New York 2016 Abstract The polyvinyl alcohol (PVA)-capped ZnS:Mn nanoparticles with Mn content of 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 Keywords Co-precipitation method Á Nanoparticles Á PL spectra Á PLE spectra Á Crystal field strength & Tran Minh Thi tranminhthi@hnue.edu.vn Faculty of Physics, Hanoi University of Science, VNU, Hanoi, Vietnam Faculty of Physics, Hanoi National University of Education, Hanoi, Vietnam Faculty of Chemistry, Hanoi University of Science, VNU, Hanoi, Vietnam 123 362 Page of 15 D V Thai et al 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 compositions 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) dispersing 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 simultaneous 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 unaggregation 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 123 The photoluminescence enhancement of Mn2? ions and the… Page of 15 362 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 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 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 calculated to create the nominal ZnS:Mn powder mass of 0.5 g with Mn content of 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 B were mixed each other and stirred for 30 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 h at 80 °C, to obtained the solutions E, respectively The solutions D and E were mixed and stirred each other for h to receive the solutions F The solution C was putted in the solutions F drop by drop and stirred for h to create precipitation The reaction equation occurred as follow: ZnCH3 COOị2 ỵ MnCH3 COOị2 ỵ 2Na2 S ỵ PVA ! ẵZnSMnSị PVA # ỵ 4CH3 COONa [(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 ˚ , 2h = 10°–70°) The morphology of ZnS:Mn/PVA using CuKa radiation (k = 1.5406 A 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 K were recorded on MS-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 performed 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 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 123 362 Page of 15 D V Thai et al 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 present XRD patterns of ZnS:Mn/PVA (CMn = 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  symmetry cubic strucshowed that these nanoparticles are single phase with T2d À F43m 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: Dẳ 0:9k b cos h 1ị where, k(nm) is the wavelength of CuKa radiation; b(rad) is the full width at haft maximum 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 XRD patterns of PVA (a) and ZnS:Mn/PVA nanoparticles (mPVA = 1.0 g) (b) 123 The photoluminescence enhancement of Mn2? ions and the… Fig XRD patterns of ZnS:Mn/PVA nanoparticles with different PVA mass: a mPVA = g; c mPVA = 0.4 g; e mPVA = 0.8 g; f mPVA = 1.0 g; h mPVA = 1.5 g Page of 15 362 (111) h 1.5g f 1.0g e 0.8g c 0.4g a 0.0g (220) Intensity (a.u) (311) h f e c a 20 30 40 50 60 70 2θ (degree) Fig HRTEM images of ZnS:Mn nanoparticles (a–c) and ZnS:Mn/PVA nanoparticles with mPVA = g (d–f) 123 362 Page of 15 D V Thai et al However, the lattice constant of ZnS:Mn/PVA nanoparticles with different PVA mass is ˚ , this value is approximately to standard lattice conalmost unchanged with a = 5.373 A ´˚ 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 shows HRTEM images of the ZnS:Mn and ZnS:Mn/PVA nanoparticles with mPVA = 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 calculated 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, 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% 20 Fig TGA and DTG spectra of PVA 0.2 TGA and DTG of PVA in Argon -6% -44.7% 140oC -0.2 -40 -0.4 -60 -49.3% -80 -0.6 TGA -100 -0.8 335oC -120 100 200 300 420oC 400 500 Temperature (oC) 123 600 700 -1.0 DTG TGA (%) -20 0.0 DTG The photoluminescence enhancement of Mn2? ions and the… Page of 15 362 10 Fig TGA and DTG of ZnS:Mn/PVA nanoparticles 0.1 TGA and DTG of ZnS:Mn/PVA in Argon DTG 0.0 -7% 248oC -10 -0.1 -5.4% -0.2 DTG TGA (%) 379oC -11.6% -20 TGA -0.3 -0.4 102oC -30 100 200 300 400 500 600 700 Temperature (oC) 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 comparison 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 characteristic 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 123 362 Page of 15 D V Thai et al Fig FT-IR spectra of PVA (a) and ZnS:Mn/PVA nanoparticles (mPVA = g) (b) Fig The schema of PVAcapped ZnS:Mn nanoparticles particle size has reduced due to un-aggregation of ZnS:Mn nanoparticles The schema of PVA-capped ZnS:Mn nanoparticles is presented in Fig 3.3 The photoluminescence enhancement of Mn21 ions in ZnS:Mn/PVA nanoparticles Figure 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 123 2.0x10 PL intensity (a.u) Fig PL spectra of ZnS:Mn/ PVA nanoparticles with different PVA mass a mPVA = g; b mPVA = 0.2 g; c mPVA = 0.4 g; d mPVA = 0.6 g; e mPVA = 0.8 g; f mPVA = 1.0 g; g mPVA = 1.2 g and h mPVA = 1.5 g 1.6x10 1.2x10 8.0x10 PL intensity (a.u) The photoluminescence enhancement of Mn2? ions and the… Page of 15 362 2.0x104 603 f 1.8x10 1.6x104 1.4x104 0.0 0.4 0.8 1.2 1.6 Mass of PVA(g) e g d c h b a a 0.0 g b 0.2 g c 0.4 g d 0.6 g e 0.8 g f 1.0 g g 1.2 g h 1.5 g 440 4.0x10 0.0 350 400 450 500 550 600 650 700 750 Wavelength (nm) 9.8 1.6x10 LnIPL 9.4 PL intensity (a.u) 603 a 0.05 W/cm i h b 0.08 W/cm g e 0.19 W/cm 9.6 1.2x10 9.2 9.0 8.8 8.4 -0.8 -0.4 0.0 8.0x10 0.0 350 0.4 0.8 1.2 LnIep 400 450 f e d g 0.26 W/cm h 0.31 W/cm i 0.33 W/cm c b a 440 d 0.15 W/cm f 0.24 W/cm 8.6 4.0x10 c 0.12 W/cm 500 550 600 650 700 750 Wavelength (nm) Fig PL spectra of ZnS:Mn/PVA (mPVA = g) with different excitation powder densities nanoparticles, the intensity of blue band changes little, but the intensity of yellow-orange band increased with the increasing of PVA mass from 0.2 g and reached the maximum at PVA mass of g (Fig 8b–f), then it decrease with the increasing of PVA mass to 1.5 g (Fig 8g, h) The dependence of yellow-orange band intensity on PVA mass is present in inset of Fig However, the peak positions of blue and yellow-orange bands almost unchanged with the change of PVA mass (Fig 8) The PL spectra of ZnS:Mn/PVA nanoparticles with PVA mass of g by different excitation power densities of 325 nm radiation were presented in Fig It is clear that the intensity of yellow-orange band is increased with the increase of excitation power densities from 0.05 to 0.33 W/cm2, but its peak position still unchanged The dependence of PL intensity on excitation power density is given by the equation IPL ¼ A Á Inep , where IPL is PL 123 362 Page 10 of 15 D V Thai et al intensity, Iep is excitation power density, A is a constant and n is a coefficient, that depended on radiation nature By the linearization of this equation (inset of Fig 9), the calculated result for ZnS:Mn/PVA nanoparticles received the value n & 0.9, which is appropriate to the result of other authors (Chen et al 2004) 3.4 The photoluminescence excitation spectra and crystal field strength In order to absorption transitions of Mn2? ions in ZnS:Mn/PVA nanoparticles and calculation of crystal field strength, PLE spectra investigated using excitation radiation of xenon lamp Figure 10 show PLE spectra monitored at the yellow-orange PL band of ZnS:Mn/PVA nanoparticles with different PVA mass For ZnS:Mn nanoparticles, this spectrum appeared the peak about of 341 nm with great intensity and the other peaks at about of 395, 430, 468 and 492 nm with smaller intensity, in which the peaks of 468 and 492 nm appeared very clearly (Fig 10a) The peak of 341 nm (3.633 eV) is attributed to the near edge absorption transition of ZnS crystals because the photon energy corresponding to this transition is very near the width of its band gap (Cadis et al 2010) The lines of 395, 430, 468 and 492 nm are assigned to the absorption transitions of the 3d5 electrons from 6A1(6S) ground state to 4 E( D), 4T2(4D), 4A1(4G)–4E(4G) and 4T2(4G) excited states of Mn2? ions in ZnS crystals (called Mn2? absorption band), respectively (Chen et al 2001) These results prove that Mn2? ions were replaced on some positions of the Zn2? ions in ZnS crystal lattice This ˚ ) very close to that of replacement ability is rather high because radii of Mn2? ion (0.89 A ˚ ) Zn2? ion (0.88 A For ZnS:Mn/PVA nanoparticles with PVA mass from 0.2 to 1.5 g, the intensity of near edge gap absorption band and the intensity of Mn2? absorption band increased and achieved maximum at PVA mass of g (Fig 10b–e), then that decrease with the increasing of PVA mass to 1.5 g (Fig 10f, g) This result also is similar as PL spectra of ZnS:Mn/PVA nanoparticles (Fig 8) However, the peak position of the near gap 345 PLE intensity (a.u) 1.2x107 1.0x107 8.0x10 e 468 d c f 492 e g b d 430 b a 6.0x106 341 a 4.0x106 c f g a 0.0g b 0.2g c 0.4g d 0.6g e 1.0g f 1.2g g 1.5g 400 450 500 550 Excitation wavelength (nm) 2.0x106 468 395 430 492 0.0 350 400 450 500 550 Excitation wavelength (nm) Fig 10 The PLE spectra monitored at the yellow-orange PL band of ZnS:Mn/PVA nanoparticles with different PVA mass: a mPVA = g; b mPVA = 0.2 g; c mPVA = 0.4 g; d mPVA = 0.6 g; e mPVA = 0.8 g; f mPVA = 1.0 g; g mPVA = 1.2 g and h mPVA = 1.5 g 123 The photoluminescence enhancement of Mn2? ions and the… Page 11 of 15 362 absorption band was shifted towards the longer wavelengths about of 341 nm (3.633 eV) for ZnS:Mn and 345 nm (3.591 eV) for ZnS:Mn/PVA, this may be due to the s–d exchange interaction between the conduction electrons and 3d electrons of Mn2? ions (Twardowski et al 1983), meanwhile, the peak positions of Mn2? absorption bands are nearly unchanged This proves that PVA unmodified the peak positions of Mn2? absorption bands In other words, the crystal field of ZnS:Mn nanoparticles unchanged with the increase of PVA mass Due to Mn2? ions substituted into some of Zn2? ion positions in ZnS crystals and the interaction of Mn2? ions with other ions in ZnS crystal, the energy levels of free Mn2? ion were splitted into multiple levels in the crystal field (Fazzio et al 1984) The crystal field of ZnS:Mn is characterized by the Racah parameters B, C and the crystal field strength Dq, in which B, C attributed to the interaction between 3d electrons of the Mn2? ions together, while Dq characterized the interaction of Mn2? ions with surrounding other ions in ZnS crystal Based on the clear appearance of Mn2? absorption peaks in PLE spectra, the parameters B, C and Dq were determined by the following equations (Chen et al 2000, 2001) 2ị 17B ỵ 5C ẳ E3 100D2q 14B ỵ 5C E2 ị 22B ỵ 7C E2 ị ẳ 10B ỵ 5C ẳ E1 12B2 E2 22B 7Cị 13B ỵ 5C E2 ð3Þ ð4Þ where E1, E2, E3 are photon energies that attributed to absorption transitions of electrons from 6A1(6S) ground state to 4A1(4G)–4E(4G), 4T2(4G) and 4E(4D) excited states of Mn2? ions, respectively, in ZnS crystals From the photon energy values corresponding to wavelengths of 395, 468 and 492 nm in the PLE spectra of ZnS:Mn/PVA nanoparticles and the Eqs (2)–(4), the parameters B, C and Dq were calculated and received the values: B = 546.3 cm-1, C = 3181.5 cm-1 (c = C/B = 5.8), Dq = 515.6 cm-1 These results are agreement with the reference (Chen et al 2001) However, in comparison with the crystal field strength of bulk sample (Dq = 667 cm-1) (Chen et al 2000, 2001), the value Dq of ZnS:Mn/PVA nanoparticles is smaller This result has relation to the reduction of particle size caused by the PVA capping on ZnS:Mn nanoparticles The weakening of crystal field strength is due to size effect of nanoparticles, in which there is no interaction of distant neighbors or this interaction is much weaker in comparison with bulk samples (Chen et al 2001; Borse et al 1999) On the other hand, the particle size more decreased, the number of Mn2? ions in crystal lattice at close surface of ZnS:Mn nanoparticles more increased, thus the PL intensity increase To determine the influence of excitation wavelengths, the PL spectra of ZnS:Mn/PVA nanoparticles with PVA mass of g were investigated Figure 11 shows the PL spectra of these nanoparticles by using in turn the excitation wavelengths of 325, 345 and 468 nm of xenon lamp, corresponding to the typical absorption peaks in PLE spectra The results showed that PL peak position of yellow-orange band remains unchanged but its intensity depends on the excitation wavelength This peak has the greatest intensity when the nanoparticles excited by 345 nm wavelength, then by 325 nm wavelength (Fig 11a, b) and the smallest intensity by 468 nm wavelength (Fig 11c) The unchanged peak position of the yellow-orange band is also evidence that Mn2? ions were doped into ZnS crystal lattice 123 362 Page 12 of 15 D V Thai et al The results of PLE and PL spectra for ZnS:Mn/PVA by different excitation wavelengths may be showed two excitation mechanisms for 3d electrons of Mn2? ions (Chen et al 2001): (1) the indirect excitation and (2) the direct excitation The excitation wavelengths of 325 nm (3.812 eV), 345 nm (3.591 eV) with photon energy in the order of the band gap of ZnS host semiconductor belong to the indirect excitation While the excitation wavelength of 468 nm (2.647 eV) belongs to the direct excitation, because its photon energy is smaller than the band gap of ZnS The intensity of the yellow-orange band by excitation wavelength of 325 nm or 345 nm is larger than that by wavelength of 468 nm This issue shows that the probability of indirect excitation is greater than the direct excitation The pure PVA is the semi-crystalline organic material and high transparency in the whole visible range (Hirankumar et al 2005; Wang et al 2014; Rudko et al 2015) The embedment of ZnS:Mn nanoparticles into PVA matrix caused on the surface of ZnS:Mn nanoparticles due to (C2H3OH)n molecules with OH hydroxyl groups polarized strongly, in which its electrons bonded with Zn2? ions to form OH-Zn2? links surrounded the nanoparticles (Murugadoss et al 2010; Alkan and Benlikaya 2009; Schmidt et al 2012) This phenomenon caused the decrease of the intermolecular hydrogen bondings between PVA chains and the crystalline degree of PVA, thus the surrounding environment of ZnS:Mn nanoparticles become to no transparency in the whole visible range Simultaneous ZnS:Mn nanoparticles are un-aggregation to each other and its particle size decreased, meanwhile the excitation energy transfer from ZnS host semiconductor to Mn2? ions are also more efficient and the intensity of PL and PLE spectra of Mn2? bands increased at PVA mass of g Due to the decrease of ZnS:Mn nanoparticles size from 3.6 to 2.6 nm with the increase of PVA mass from to g in synthesized process, the intensity increase of Mn2? bands in PL, PLE spectra are the evidence of the quantum confinement effect of nanoparticle With larger mass of PVA, the processes of non-radiation transfer are increased, so the PL and PLE intensities of Mn2? ions in ZnS:Mn/PVA nanoparticles reduced (Murugadoss et al 2010) The diagram of absorption and radiation transitions in the ZnS:Mn crystals shown in Fig 12 Fig 11 PL spectra of ZnS:Mn/ PVA nanoparticles (mPVA = g) using excitation radiations: 345 (a), 325 (b) and 468 nm (c) 123 The photoluminescence enhancement of Mn2? ions and the… Page 13 of 15 362 Fig 12 The diagram of absorption, radiation transitions of Mn2? ions in ZnS:Mn crystals Conclusion By co-precipitation method, ZnS:Mn/PVA nanoparticles were synthesized successfully with their microstructure and morphology almost were not affected by PVA capping PVA polymer chain contributed 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(2015) doi:10.1016/j jlumin.2014.09.053 123 ... 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 Experimental In order to investigate... This enhancement caused by the co-ordinate bonding between OH groups of PVA and Zn2? ions of ZnS:Mn nanoparticles created into the surrounding OH-Zn2? links that have increased the excitation... with the increase of PVA mass Due to Mn2? ions substituted into some of Zn2? ion positions in ZnS crystals and the interaction of Mn2? ions with other ions in ZnS crystal, the energy levels of