IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 2400504 Optical and Magnetic Properties of Mn-Doped ZnS Nanoparticles Synthesized by a Hydrothermal Method Hong Van Bui1 , Hoang Nam Nguyen1, Nam Nhat Hoang2, Thanh Trung Truong1 , and Van Ben Pham1 Vietnam Vietnam National University-Hanoi University of Science, Hanoi, Vietnam National University-University of Engineering and Technology, Hanoi, Vietnam The Mn-doped ZnS nanoparticles with Td2 − F43m cubic structure and an average crystalline size of about 16 nm were synthesized using the hydrothermal method at 220 °C for 15 h from Zn(CH3 COO)2 (0.1M), Mn(CH3 COO)2 (0.01M), and Na2 S2 O3 (0.1 M) as the precursors The appearance of characteristic photoemission bands of Mn2+ (3d5 ) ions at 390, 430, 467, and 493 nm in the photoluminescence excitation spectra while monitoring the yellow-orange band at 585 nm showed that the Mn2+ (3d10 ) ions substituted for Zn2+ (3d10 ) ions in ZnS matrix and caused the ferromagnetism of Mn-doped ZnS nanoparticles The dependence of photoluminescence, photoluminescence excitation spectra, and magnetization curves on Mn content and the wavelength of excitation radiation were reported Index Terms— Nanoparticle, photoluminescence, photoluminescence excitation I I NTRODUCTION T HE MN-DOPED ZnS nanomaterial (denoted ZnS:Mn) is an interesting diluted magnetic semiconductor with both optical and magnetic properties that can be observed when Mn2+ (3d5) magnetic ions partially substitute for Zn2+ (3d10) ions in the ZnS mother matrix [1]–[5] Because the local magnetic moment of the Mn2+ (3d5) ions is nonzero, the s-d exchange interaction between 3d electrons of Mn2+ ions and the conduction electrons or d-d exchange interaction between the Mn2+ ions themselves arises [6], [7] Thus, interesting optical and magnetic properties appear such as strong luminescence in the yellow-orange region, a long emission lifetime, reduction of photoluminescence intensity in the applied magnetic field, and ferromagnetism at room temperature [1]–[8] Therefore, this material is very promising for applications in optoelectronics such as luminescence diode, LED, color display, and magneto-optical control devices [9]–[12] Depending on doping, ZnS:Mn material may be paramagnetic or ferromagnetic Peng et al [1] showed that ZnS:Mn nanoparticles synthesized by a co-precipitation method were paramagnetic at K By using a co-precipitation method, Vinotha et al [2] and Ragan et al [3] also synthesized the ZnS:Mn nanoparticles that showed ferromagnetism even at 300 K [2], [3] Using a vapor phase chemical method, Kang et al [4] prepared the ZnS:Mn nanoparticles that showed the ferromagnetism at and 300 K Notably, Sarkar et al [5] discovered that the luminescence intensity reduction of the yellow-orange band assigned to the ferromagnetic phase’s Mn2+ ions in ZnS lattice increased as the applied magnetic field increased The nanowires, nanorods, and thin films of ZnS:Mn possessing the ferromagnetism at room temperature have recently been prepared [13]–[16] by the chemical methods Manuscript received November 9, 2013; accepted January 6, 2014 Date of current version June 6, 2014 Corresponding author: H Van Bui (e-mail: buihongvan2011@gmail.com) Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org Digital Object Identifier 10.1109/TMAG.2014.2300187 In this paper, we present our results on the optical and magnetic properties of the ZnS:Mn nanoparticles that were synthesized by the hydrothermal method The obtained results revealed without doubt that the Mn2+ (3d5 ) ions were substituted into the sites of Zn2+ (3d10) ions in the ZnS crystal II E XPERIMENT The Mn-doped ZnS nanoparticles were synthesized as follows First, we dissolved the high-purity precursor chemicals (>99.9%): Zn(CH3COO)2 2H2 O, Mn(CH3 COO)2 · 4H2 O, and Na2 S2 O3 · 5H2 O into the de-ionized water to obtain the solutions of Zn(CH3 COO)2 0.1M (A), Mn(CH3 COO)2 0.01M (B), and Na2 S2 O3 0.1M (C) solutions Next, by mixing B with A in the specified molar ratios we obtained a 30-ml solution (D), which was stirred for 60 Slowly we dropped another 30-ml solution (C) into the solution (D) at continuous stirring for the next 60 This final mixture was put into the Teflonlined chamber steel vessel with an enclosed lid, after which the mixture was annealed at 220 °C for 15 h In the hydrothermal process, the ZnS:Mn nanoparticles are formed according to 4Na2 S2 O3 → Na2 S + 3Na2 SO4 + 4S Zn(CH3 COO)2 + Na2 S → ZnS ↓ +2CH3 COONa Mn(CH3 COO)2 + Na2 S → MnS ↓ +2CH3 COONa After reaction, the chamber was left to cool down to room temperature and the obtained product was precipitated, then filtered and washed several times by distilled water and CS2 The resulting powder was then dried at 60 °C for 10 h in ambient condition The crystalline structure of the product was studied by using the X-ray diffraction method (XRD) on the XD8-Advance Buker system with Cu-Kα radiation (λ = 1.54056 Å) The surface morphology was examined with the transmission electron microscope (TEM) JEM-1010 The photoluminescence (PL) and photoluminescence excitation (PLE) spectra at 300 K were recorded using 325-nm excitation radiation from a He–Cd laser and using radiation from a XFOR-450 xenon lamp on the Oriel-Spec MS-257, FL3-22 spectrometers, respectively The magnetization curves were 0018-9464 © 2014 IEEE Personal use is permitted, but republication/redistribution requires IEEE permission See http://www.ieee.org/publications_standards/publications/rights/index.html for more information 2400504 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 Fig TEM image of ZnS:Mn nanoparticles with Mn content of 0.5 mol% Fig XRD patterns of ZnS and ZnS:Mn nanoparticles with different Mn contents recorded by VSM mode in Physical Properties Measurement System, PPMS Evercool II, Quantum Design III R ESULTS AND D ISCUSSION 1) Structure and Morphology of Nanoparticles: Fig 1(a) shows XRD patterns of ZnS nanoparticles It consists of diffraction peaks corresponding to (111), (220), and (311) reflection planes, where (111) peak has the strongest intensity XRD patterns showed that the ZnS nanoparticles crystallized into a form of polycrystals in the cubic phase with Td2 − F43 m symmetry and the calculated lattice constant a = 5.4130 Å When doping into ZnS with Mn content from 0.1 to mol%, the diffraction peak positions and lattice constant are almost unchanged [Fig 1(b)–(e)] because of the small doping content and the nearly equal ionic radius of Mn2+ (0.89 Å) and Zn2+ (0.88 Å) These values are in good agreement with the ones from the JCPDS card No 05-0566, where a = 5.4060 Å The average crystalline size of ZnS and ZnS:Mn nanoparticles were obtained from fitting the XRD peak profiles and from the Debye-Scherrer formula: D = 0.9λ/β cos θ , where D (Å) is the crystalline size, λ(Å) is the X-ray wavelength of CuKα , β (rad) is the full-width at half-maximum (FWHM) of the diffraction line, and θ (rad) is the Bragg angle The calculated values showed that the ZnS and ZnS:Mn nanoparticles exhibited almost the same average crystalline size of about 16 nm This value did not change as Mn content increased from 0.1 to mol% Fig shows the TEM image of ZnS:Mn nanoparticles with Mn content of 0.5 mol% It revealed that the nanoparticles are quasi-spheres with the particle size ranging from 30 to 40 nm This value is larger in comparison to the one obtained from the calculation of peak profiles 2) Optical and Magnetic Properties: Fig shows the PL spectra of ZnS and ZnS:Mn nanoparticles with different Mn contents when excited by a 325-nm radiation from the He–Cd laser In the PL spectra of ZnS nanoparticles, there is a green band around 505 nm [Fig 3(a)] This band can be assigned to self-active centers, that is, vacancies of Zn, S, and to their interstitials and surface states in ZnS crystal [17] Fig PL spectra of ZnS and ZnS:Mn nanoparticles with different Mn contents While doping Mn into ZnS with a content of 0.1 mol%, the green band is almost extinguished, and a broad yelloworange band at 585 nm appears in the PL spectra This yelloworange band can be due to the radiation transition of electrons in Mn2+ (3d5) configuration [4 T1 (4 G) →6 A1 (6 S)] [9] As the Mn content is increased, the doping of Mn2+ ions into ZnS matrix accumulates, therefore the intensity of the yelloworange band develops but its position remains unchanged [Fig 3(b)–(e)] Fig shows the PLE spectra when monitoring the yelloworange band at 585 nm when excited by the radiation of xenon lamp At the Mn content of 0.1 mol%, besides a broad band with strong intensity at 335 nm (3.7015 eV) assigned to a near band edge absorption of ZnS crystal [18], bands appeared with weaker intensity at 390, 430, 467, and 493 nm [Fig 4(a)] These bands are related to the absorption transitions of electrons from A1 (6 S) ground state to E(4 D); T2 (4 D); A1 (4 G)-4 E(4 G); T2 (4 G) exited states of Mn2+ (3d5) ions in ZnS crystal, respectively [called absorption bands of Mn2+ (3d5)] [19], [20] When Mn content is increased from 0.1 to mol%, the intensity of these bands increases but their positions remain almost constant [Fig 4(b)–(d)] This result shows that the Mn2+ (3d5) ions are well substituted into the Zn2+ (3d10) BUI et al.: OPTICAL AND MAGNETIC PROPERTIES OF MN-DOPED ZNS NANOPARTICLES Fig PLE spectra when monitoring the yellow-orange band of ZnS:Mn nanoparticles with different Mn contents Fig PL spectra of ZnS:Mn nanoparticles at the Mn content of 0.5 mol% excited by different excitation radiations of a xenon lamp sites and their vacancies in the ZnS crystal However, at a Mn content of mol%, the near band edge absorption that shifts toward a longer wavelength at 340 nm, may be due to the s-d exchange interaction between conduction electrons and 3d5 electrons of Mn2+ ions [Fig 4(d)] [6] Using in turn the radiations of 325, 335, 390, 430, 467, and 493 nm of the xenon lamp, which correspond to the bands in the PLE spectra to excite the ZnS:Mn nanoparticles with Mn content of 0.5 mol%, we obtained only a yellow-orange band at 585 nm at an intensity according to the wavelength of the excitation radiation (Fig 5) The intensity appeared strong when excited by the radiations of 335 and 325 nm (the photon energy is approximately equal to the band gap of ZnS [Fig 5(a) and (b)] and decreased gradually when excited by the radiations of 390, 493, 467, and 430 nm (that is the photon energy is smaller than the band gap of ZnS [Fig 5(c)–(f)] This provided evidence to show that there are two different absorption mechanisms: a near band edge absorption and an absorption caused by the Mn2+ ions, where the near band edge absorption dominates 2400504 Fig Magnetization curves of ZnS:Mn nanoparticles with different Mn contents Fig shows the magnetization curves of ZnS:Mn nanoparticles with different Mn contents at 300 K At all contents of Mn, the ZnS:Mn nanoparticles showed a weak ferromagnetic response At a low Mn content of 0.1 mol%, the ZnS:Mn nanoparticles showed a saturated magnetization of 1.3 × 10−4 emu/g at an applied field of × 104 Oe [Fig 6(a)] When Mn content increased to 0.2, 0.5, and mol%, the saturated magnetization increased to 5.5 × 10−4 , 8.8 × 10−4 , and 14.1×10−4 emu/g, respectively [Fig 6(b)–(d)] The weak ferromagnetism appeared with well-defined hysteresis loops and is caused by the existence of exchange pairs between the Mn ions in the lattice of ZnS When Mn2+ ions exist in ZnS matrix at low content, there are two possible ferromagnetic interactions that can occur One is due to the ferromagnetic exchange between Mn2+ ions themselves, that is mediated by the neighbor S2− ions (Mn2+ −S2− −Mn2+ ) and the other is the interaction mediated by their near neighbor native defects such as S vacancies (Mn2+ −[S]−Mn2+) The socalled Anderson’s super exchange takes place where the strong hybridization occurs between the d shell of Mn2+ ions and the p shell of their near neighbor S2− ions [15] However, as seen in Fig 3, the number of defects decreased when Mn doped into ZnS matrix The peak at 505 nm almost disappeared when Mn2+ was doped Thus, the interaction mediated by defects in the crystal may not be the major interaction The increase of the saturated magnetization at the increasing Mn content indicates that the Mn2+ (3d5) ions may successfully replace the Zn2+ (3d10) ions in ZnS matrix These results support the above discussion about the optical properties, when the intensity of both the orange-yellow band in the PL spectra and the photoemission bands in the PLE spectra increase together as the Mn content increases IV C ONCLUSION By using the hydrothermal method from Zn (CH3 COO)2 (0.1 M), Mn(CH3 COO)2 (0.01 M), and Na2 S2 O3 (0.1 M) precursors, we have successfully prepared the Mn-doped ZnS nanoparticles that exhibited both ferromagnetism and 2400504 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 enhanced emissions in the visible range The substitution of Mn2+ (3d5 ) ions created characteristic bands at 585 nm in PL spectra and at 390, 430, 467, and 493 nm in the PLE spectra, simultaneously with a weak ferromagnetism that saturated at about 14.1 × 10−4 emu/g ACKNOWLEDGMENT This work was supported by the QG 12.03 Project R EFERENCES [1] W Q Peng, S C Qu, G W Cong, X Q Zhang, and Z G Wang, “Optical and magnetic properties of ZnS nanoparticles doped with Mn2+ ,” J Cryst Growth, vol 282, no 1, pp 179–185, 2005 [2] P Vinotha, B Lakshmi, K S Raj, and K Ramachandran, “Synthesis and characterization of nano ZnS doped with Mn,” Cryst Res Technol., vol 44, no 2, pp 153–158, 2009 [3] M Ragan, G Kalaiselvan, S Arumugam, N Sankar, and K Ramachandran, “Room temperature ferromagnetism in Mnx Zn1−x S (x=0.00-0.07) nanoparticles,” J Alloys Compounds, vol 541, no 1, pp 222–226, 2012 [4] T Kang, J Sung, W Shim, H Moon, J Cho, Y Jo, et al., “Synthesis and magnetic properties of single-crystalline Mn/Fe-doped and co-doped ZnS nanowires and nanobelts,” J Phys Chem C, vol 113, no 14, pp 5352–5357, 2009 [5] I Sarkar, M K Sanyal, S Takeyama, S Kar, H Hirayama, H Mino, et al., “Suppression of Mn photoluminescence in ferromagnetic state of Mn-doped ZnS nanocrystals,” Phys Rev., vol 79, no 5, pp 054410-1–054410-6, 2009 [6] A Twardowski, T Dietl, and M Demianiuk, “The study of the s-d type exchange interaction in Zn1−x Mnx Se mixed crystals,” Solid State Commun., vol 48, no 10, pp 845–848, 1983 [7] S Sapra, J Nanda, A Anand, S V Bhat, and D D Sarma, “Optical and magnetic properties of manganese-doped zinc sulfide nanoclusters,” J Nanosci Nanotech., vol 3, no 5, pp 392–400, 2003 [8] A A Bol and A Meijerink, “Long-lived Mn2+ emission in nanocrystalline ZnS:Mn2+ ,” Phys Rev B, vol 58, no 24, pp 15997–16000, 1998 [9] R N Bhargava, D Gallagher, X Hong, and A Nurmikko, “Optical properties of manganese-dped nanocrystals of ZnS,” Phys Rev Lett., vol 72, no 3, pp 416–419, 1994 [10] H Yang, S Santra, and P H Holloway, “Syntheses and application of Mn-doped II-VI semiconductor nanocrystals,” Nanosci Nanotechnol., vol 5, no 9, pp 1364–1375, 2005 [11] T Toyama, D Adachi, and H Okamoto, “Electroluminescent devides with nanostructured ZnS:Mn emission layer operated at 20 V0− p ,” in Proc Mat Res Soc Symp., vol 621 2000, pp Q 4.4.1–Q4.4.6 [12] J.-S Hu, L.-L Ren, Y.-G Guo, H.-P Liang, A.-M Cao, L.-J Wan, et al., “Mass production and high photocatalytic activity of ZnS nanoporous nanoparticles,” Angew Chem Int Ed., vol 44, no 8, pp 1269–1273, 2005 [13] S Senthilkumaar, R T Selvi, N G Subramaniam, and T W Kang, “Facile synthesis and magnetic properties of maganese doped ZnS nanorods,” Superlattices Microstruct., vol 51, no 1, pp 73–79, 2012 [14] J Cao, D Han, B Wang, L Fan, H Fu, M Wei, et al., “Low temperature synthesis, photoluminescence, magnetic properties of the transition metal doped wurtzite ZnS nanowires,” J Solid State Chem., vol 200, pp 317–322, Apr 2013 [15] M Wei, J Yang, Y Yan, L Yang, J Cao, H Fu, et al., “Influence of Mn ions concentration on optical and magnetic properties of Mn-doped ZnS nanowires,” Phys E, vol 52, pp 144–149, Aug 2013 [16] M El-Hagary and S Soltan, “Absence of room temperature ferromagnetism in Mn-doped ZnS nanocrystalline thin film,” Solid State Commun., vol 155, no 1, pp 29–33, 2013 [17] W Chen, Z Wang, Z Lin, Y Xu, and L Lin, “Photoluminescence of ZnS clusters in zeolite-Y,” J Mater Sci Technol., vol 13, no 5, pp 397–404, 1997 [18] A I Cadis, E J Popovici, E Bica, I Perhaita, L Barbu-Tudoran, and E Indrea, “On the preparation of manganese-doped zinc sulphide nnocrystalline powders using the wet-chemical synthesis route,” Chalcogenide Lett., vol 7, no 11, pp 631–640, 2010 [19] T Kushida, Y Tanaka, and Y Oka, “Excited-state absorption spectra of ZnS:Mn,” Solid State Commun., vol 14 no 7, pp 617–620, 1974 [20] W Chen, R Sammynaiken, Y Huang, J.-O Malm, R Wallenberg, J O Bovin, et al., “Crystal field, phonon coupling and emission shift of Mn2+ in ZnS:Mn nanoparticles,” J Appl Phys., vol 89, no 2, pp 1120–1129, 2001 ... 3 (a) ] This band can be assigned to self-active centers, that is, vacancies of Zn, S, and to their interstitials and surface states in ZnS crystal [17] Fig PL spectra of ZnS and ZnS: Mn nanoparticles. .. crystals,” Solid State Commun., vol 48, no 10, pp 845–848, 1983 [7] S Sapra, J Nanda, A Anand, S V Bhat, and D D Sarma, Optical and magnetic properties of manganese-doped zinc sulfide nanoclusters,”... excitation radiation (Fig 5) The intensity appeared strong when excited by the radiations of 335 and 325 nm (the photon energy is approximately equal to the band gap of ZnS [Fig 5 (a) and (b)] and