Optical and Magnetic Properties of Mn-Doped ZnSNanoparticles Synthesized by a Hydrothermal Method Hong Van Bui1, Hoang Nam Nguyen1, Nam Nhat Hoang2, Thanh Trung Truong1, and Van Ben Pham
Trang 1Optical 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
1Vietnam National University-Hanoi University of Science, Hanoi, Vietnam
2Vietnam National University-University of Engineering and Technology, Hanoi, Vietnam
The Mn-doped ZnS nanoparticles with T d2− 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(CH 3 COO) 2 (0.1M), Mn(CH 3 COO) 2 (0.01M), and Na 2 S 2 O 3 (0.1 M)
as the precursors The appearance of characteristic photoemission bands of Mn 2 + (3d 5 ) 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 Mn 2 + (3d 10 ) ions substituted for Zn 2 + (3d 10 ) 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 INTRODUCTION
THE 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
promis-ing 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
paramag-netic or ferromagparamag-netic Peng et al [1] showed that ZnS:Mn
nanoparticles synthesized by a co-precipitation method were
paramagnetic at 2 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 5 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
meth-ods
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
substi-tuted into the sites of Zn2 + (3d10) ions in the ZnS crystal.
II EXPERIMENT The Mn-doped ZnS nanoparticles were synthesized as fol-lows First, we dissolved the high-purity precursor chemicals (>99.9%): Zn(CH3COO)2.2H2O, Mn(CH3COO)2·4H2O, and
Na2S2O3· 5H2O into the de-ionized water to obtain the solu-tions of Zn(CH3COO)20.1M (A), Mn(CH3COO)20.01M (B), and Na2S2O30.1M (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 min Slowly we dropped another 30-ml solution (C) into the solution (D) at continuous stirring for the next 60 min This final mixture was put into the Teflon-lined 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
4Na2S2O3 → Na2S+ 3Na2SO4+ 4S Zn(CH3COO)2+ Na2S→ ZnS ↓ +2CH3COONa Mn(CH3COO)2+ Na2S→ MnS ↓ +2CH3COONa 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.
Trang 2Fig 1 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 RESULTS ANDDISCUSSION
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 T d2−F43 m
symmetry and the calculated lattice constant a = 5.4130
Å When doping into ZnS with Mn content from 0.1 to 1
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
nanopar-ticles 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, andθ (rad) is the Bragg angle The
calcu-lated 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 1 mol%
Fig 2 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 3 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 2 TEM image of ZnS:Mn nanoparticles with Mn content of 0.5 mol%.
Fig 3 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 orange band at 585 nm appears in the PL spectra This yellow-orange band can be due to the radiation transition of electrons
in Mn2+(3d5) configuration [4T1(4G)→6A1(6S)] [9] As the
Mn content is increased, the doping of Mn2+ ions into ZnS matrix accumulates, therefore the intensity of the yellow-orange band develops but its position remains unchanged [Fig 3(b)–(e)]
Fig 4 shows the PLE spectra when monitoring the yellow-orange 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 elec-trons from 6A1(6S) ground state to 4E(4D); 4T2(4D); 4A1
(4G)-4E(4G);4T2(4G) 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 1 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)
Trang 3Fig 4 PLE spectra when monitoring the yellow-orange band of ZnS:Mn
nanoparticles with different Mn contents.
Fig 5 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 1 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
3d5electrons 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.
Fig 6 Magnetization curves of ZnS:Mn nanoparticles with different Mn contents.
Fig 6 shows the magnetization curves of ZnS:Mn nanopar-ticles with different Mn contents at 300 K At all contents
of Mn, the ZnS:Mn nanoparticles showed a weak ferro-magnetic response At a low Mn content of 0.1 mol%, the ZnS:Mn nanoparticles showed a saturated magnetization of
1.3×10−4emu/g at an applied field of 5×104Oe [Fig 6(a)] When Mn content increased to 0.2, 0.5, and 1 mol%, the saturated magnetization increased to 5.5 × 10−4, 8.8 × 10−4, and 14.1×10−4emu/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
so-called 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 CONCLUSION
By using the hydrothermal method from Zn (CH3COO)2 (0.1 M), Mn(CH3COO)2 (0.01 M), and Na2S2O3 (0.1 M) precursors, we have successfully prepared the Mn-doped ZnS nanoparticles that exhibited both ferromagnetism and
Trang 4enhanced 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.
This work was supported by the QG 12.03 Project
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