Effect of nickel doping concentration on structural and magnetic properties of ultrafine diluted magnetic semiconductor ZnO nanoparticles

Article abstract of DOI:10.1016/j.jmmm.2009.06.055

The ZnO:Ni²⁺ nanoparticles of mean size 2-12 nm were synthesized at room temperature by the simple co-precipitation method. The crystallite structure, morphology and size were determined by X-ray diffraction (XRD) and high-resolution transmissi

Full text of DOI:10.1016/j.jmmm.2009.06.055

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Journal of Magnetism and Magnetic Materials 321 (2009) 3457–3461
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Effect of nickel doping concentration on structural and magnetic properties of
ultrafine diluted magnetic semiconductor ZnO nanoparticles
Ã
Prashant K. Sharma , Ranu K. Dutta, Avinash C. Pandey
Nanophosphor Application Centre, University of Allahabad, Allahabad 211002, India
a r t i c l e i n f o
a b s t r a c t
2+
Article history:
The ZnO:Ni nanoparticles of mean size 2–12 nm were synthesized at room temperature by the simple
co-precipitation method. The crystallite structure, morphology and size were determined by X-ray
diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The wurtzite
structure of ZnO gradually degrades with the increasing Ni doping concentration and an additional
Received 30 May 2009
Available online 23 June 2009
Keywords:
2
+
DMS
NiO-associated diffraction peak was observed above 15% of Ni doping. The change in magnetic
2
+
Ferromagnetism
VSM
behavior of the nanoparticles of ZnO with varying Ni doping concentration was investigated using a
vibrating sample magnetometer (VSM). Initially, these nanoparticles showed strong ferromagnetic
Raman Spectroscopy
2+
behavior, however, at higher doping percentage of Ni , the ferromagnetic behavior was suppressed and
paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring
2
+
Ni–Ni ions suppressed the ferromagnetism at higher doping concentrations of Ni
.
&
2009 Elsevier B.V. All rights reserved.
1
. Introduction
magnetic Mn, Fe, Co and Ni exhibit a number of unique magnetic,
magneto-optical and magnetotransport properties, applicable for
magneto-electronic and spintronic devices. Most of these studies
focused on Mn- and Fe-doped ZnO films, while fewer studies on
Ni-doped ZnO films have been reported [12]. The origin of RT FM
in TM-doped ZnO are sensitive to the synthesis conditions, and are
still debatable, thus, research work in this field is still in its
infancy and stalk of RT FM semiconductors, which are the
foundation for spintronics, are still exigency. Moreover, it also
seems that most studies focus on film materials, and ferromag-
netism could usually only be found in film materials. In the
present paper, we reveal a simple technique to synthesize
In recent years, the scientific community has paid much
attention to the synthesis and characterization of II–VI semi-
conductor materials at nanometer scale, due to their great
potential to test fundamental concepts of quantum mechanics
[
1,2] and because of their key role in various applications such as
solid-state lighting devices (LEDs), photonics [3], nanoelectronics
4], optoelectronics and data storage. ZnO is an important II–VI
semiconductor having a wide and direct bandgap (as wide as
.37 eV), equivalent to that of GaN [5]. Besides this, ZnO is
[
3
piezoelectric and optically transparent with a large exciton
binding energy of 60 MeV.
2
+
ZnO:Ni nanoparticles with excellent structural and magnetic
properties.
In recent past, room-temperature (RT) ferromagnetism (FM) in
transition metal (TM)-doped p-type ZnO has been repeatedly
predicted by several groups using various calculations/simulation
studies [6,7]. This makes ZnO one of the most promising materials
for potential applications in spintronics and as diluted magnetic
semiconductor (DMS) material. At the same time, besides
theoretical predictions, RT FM in TM-doped ZnO has been
reported by various experimentalists. Progress in producing
high-quality ZnO to obtain ferromagnetism at or above room
temperature by doping with 3d transition metals has been
highlighted in several papers [8–11]. It has been found that the
diluted magnetic semiconductors (DMSs) formed by replacing the
cations of III–V or II–VI nonmagnetic semiconductors by ferro-
2. Experimental details
2
+
For synthesis of ZnO:Ni
nanoparticles, the zinc acetate
ꢀ 2H O, nickel acetate 1-hydrate
2
O, potassium hydroxide KOH, citric acid,
dihydrate (99.2%) Zn(CH
99.4%) (CH COO)
Ni ꢀ H
3
COO)
2
2
(
3
2
methanol and ethanol were procured from E. Merck Limited,
India. All chemicals were of analytical reagent grade and were
directly used without any special treatment.
2
+
Synthesis of ZnO:Ni nanoparticles were carried out using the
same technique followed by us [11,13] for the synthesis of ZnO:Fe
2+
and ZnO:Cu nanoparticles. For synthesizing ZnO:Ni , 0.5 M of
zinc acetate and 0.01 M of nickel acetate were taken in 100 ml of
methanol and dissolved with continuous stirring for 2 h at room
Ã
Corresponding author. Tel./fax: +91532 2460675.
E-mail address: prashantnac@gmail.com (P.K. Sharma).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2009.06.055

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temperature (solution A). Simultaneously, 140 mmol KOH solution
was prepared in 100 ml of methanol with refluxing through water
condenser with constant stirring for 2 h at 501 C (solution B). Now,
mix the solution A and B with constant stirring for 2 h. This
mixing was done while refluxing through water condenser at
the positions of 31.631, 34.501, 36.251, 47.501, 56.601, 62.801,
66.361, 67.921 and 68.911, which are in good agreement with the
˚
standard JCPDS file for ZnO (JCPDS 36-1451, a ¼ b ¼ 3.249 A,
˚
c ¼ 5.206 A) and can be indexed as the hexagonal wurtzite
structure of ZnO having space group P63mc. Furthermore, it can
2
+
5
01 C. Final solution was allowed to cool at room temperature and
be seen that at higher doping percentages of Ni (15% and 20% in
present case), a new phase emerges at 43.231 in the XRD spectra.
This new phase in the XRD spectra corresponds to NiO (2 0 0)
(matched with JCPDS 78-0643), which may be due to the
aged overnight. This solution was centrifuged and washed several
times with absolute ethanol and water in order to remove
unnecessary impurities. The obtained white product was placed
in a vacuum oven for 24 h at 501 C to get white powders of
2
+
formation of NiO from remaining un-reacted Ni ions present
in the solution. All the available reflections of the present XRD
phases have been fitted with Gaussian distribution. The
broadening of XRD peaks (i.e. Scherrer’s broadening) attributes
2
+
ZnO:Ni
. Similar procedure was followed for synthesis of
2+
2+
ZnO:Ni with varying Ni doping percentage, i.e. 1%, 2%, 3%,
5
%, 8%, 10%, 15% and 20% named as RNi 1, RNi 2, RNi 3, RNi 4, RNi 5,
2
+
2+
RNi 6, RNi 7 and RNi 8, respectively.
nanosized formation of ZnO:Ni . The particle size, d, of ZnO:Ni
nanoparticles were estimated by Debye–Scherrer’s equation
2.1. Characterization used
0 l
B cos y
:9
d ¼
2+
The prepared ZnO:Ni nanoparticles were characterized by X-
ray diffraction (XRD), transmission electron microscopy (TEM)
and energy-dispersive X-rays (EDX), in order to elaborate
structural properties in a precise manner for various doping
percentage of Ni. XRD was performed on Rigaku D/max-2200 PC
where, d is the particle size,
l
the wavelength of radiation used,
y
the Bragg angle and B is the full-width at half-maxima (FWHM)
on 2 scale. The crystallite size was estimated for the strongest
y
X-ray diffraction, corresponding to (0 0 2) peak at 36.251, and was
found to vary from 12 to 2 nm for samples RNi1–RNi8.
diffractometer operated at 40 kV/20 mA, using CuK
˚
a1
radiation
with wavelength of 1.54 A in the wide angle region from 251 to 701
XRD pattern not only showed decreasing crystallite size for
on 2y scale. The size and morphology of prepared nanoparticles
2
+
increasing doping percentage of Ni , but a significant degradation
in crystallinity with enhanced peak broadening was also observed
were found using a transmission electron microscope (model
2
Technai 30 G S-Twin electron microscope) operated at 300 kV
2
+
with increasing Ni doping. Moreover, it can also be seen that as
the Ni doping percentage increases, the wurtzite structure of ZnO
starts gradual degradation. The degradation in crystallinity was
observed from sample RNi 1–RNi 8 and enhancement in the peak
broadening was also observed indicating more nano-nature of the
higher Ni content samples. This was due to distortion in the host
ZnO lattice, because of the introduction of a foreign impurity i.e.
Ni doping. This is mainly because of decrease in nucleation and
subsequent growth rate due to increasing Ni doping percentage
accelerating voltage by dissolving the as-synthesized powder
sample in ethanol and then placing a drop of this dilute ethanolic
solution on the surface of copper grid. Room-temperature
magnetization measurement was carried out using a vibrating
sample magnetometer (VSM, ADE Magnetics, USA) with pressed
pellets of prepared powdered samples. Raman spectra were taken
+
with a Reinshaw micro-Raman spectroscope using 514 nm Ar
laser as excitation source.
2+
due to the size difference of Zn and Ni ions. Ionic radius of Ni is
2
+
˚
˚
0
.68 A and is larger than that of Zn i.e. 0.60 A and lowers the
3
. Results and discussion
Fig. 1 shows the XRD pattern of the ZnO:Ni nanoparticles
reaction rate.
2+
The variations in XRD results were well supported by TEM
measurements. Fig. 2 shows the representative TEM image of the
prepared samples. The morphology of all the samples was found
to be spherical in nature having diameters ranging from 15 to
synthesized in the current work. XRD spectra show broad peaks at
3
nm for different samples. Fig. 2 clearly shows that the diameters
of these spherical nanoparticles were in agreement with those
obtained using XRD results. Fig. 2(f) shows the representative
selected area electron diffraction (SAED) pattern for sample RNi 1,
i.e. 1% Ni-doped ZnO samples. This SAED pattern showed that the
prepared ZnO nanoparticles were polycrystalline in nature.
Figs. 2(g) and 2(h) show high-resolution transmission electron
microscopic (HRTEM) images of RNi 1 and RNi 8 samples,
RNi8
RNi7
RNi6
RNi5
˚
respectively. The imaged lattice spacing 2.3 A (Fig. 2(g)) for RNi
(1% Ni) sample, corresponds to the (0 0 2) planes of hexagonal
wurtzite structure of ZnO while a remarkable deviation in the
1
RNi4
RNi3
˚
imaged lattice spacing 2.16 A (Fig. 2(h)) was observed for RNi 8
20% Ni) sample. HRTEM measurements showed a remarkable
shrink in imaged d-spacing for higher doping percentage of Ni
samples RNi 7 and RNi 8), whereas for the other samples no
(
RNi2
RNi1
(
significant change in imaged d-spacing were observed as
compared to standard ZnO wurtzite structure i.e. the effect of Ni
doping is dominant and appeared only at higher doping
percentages which is in good agreement with XRD, Raman and
VSM results (to be discussed later on). This result further
supported our claim that this distortion in the host ZnO lattice
2
5
30
35
40
45
50
55
60
65
70
2
θ (degree)
Fig. 1. X-ray diffraction spectra of ZnO for different doping percentage of Ni. The
XRD spectra showed crystalline nature having hexagonal wurtzite structure of ZnO
having space group P63mc. An additional peak, corresponding to NiO (2 0 0) phase
was observed at 43.231.
2+
arises due to the introduction of a foreign impurity i.e. Ni doping
at higher doping percentage. Again the origin of this distortion in

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3459
Fig. 2. Representative TEM images of ZnO:Ni nanoparticles, for sample RNi 1(a), RNi 2(b), RNi 4(c), RNi6(d), RNi 8(e), respectively, (f) represents the SAED pattern for
sample RNi 1. Representative HRTEM images of ZnO:Ni nanoparticles, for sample (g) RNi 1 and (h) RNi 8, respectively.
The presence of Cu line traces in EDX spectra could be attributed
due to the signal arising from the copper grid used for the
measurement. Fig. 4 shows the dependence of magnetization at
room temperature with applied magnetic field (M–H loop) for
Zn
sample (a) RNi 2 i.e. 2% Ni doping, (b) RNi 4 i.e. 5% Ni doping, (c)
RNi 6 i.e. 10% Ni doping and (d) RNi 8 i.e. 20% Ni doping,
respectively.
A
clear hysteresis loop, with noticeable coercivity, was
observed. Initially, ferromagnetic nature was observed for 1–5%
Ni (RNi 1–RNi 4)-doped samples and again for further high doping
of Ni (RNi 4–RNi 8) paramagnetic nature dominates. Initially,
these nanoparticles showed strong ferromagnetic behavior, how-
ever, at higher doping percentage of nickel the ferromagnetic
behavior was suppressed and paramagnetic nature was observed.
The enhanced antiferromagnetic interaction between neighboring
Ni–Ni ions suppressed the ferromagnetism at higher doping
concentrations of Ni.
O
O
Zn
Zn
O
O
Cu
Ni
Ni
Si
Cu Zn
Ni
Ni
Ni
1
.0k 2.0k 3.0k 4.0k 5.0k 6.0k 7.0k 8.0k 9.0k 10.0k
Energy (eV)
Clear hysteresis loops with coercivities 12.0, 10.2, 1.2 and
0.8 mT were observed for RNi 2, RNi 4, RNi 6 and RNi 8 samples,
Fig. 3. Representative EDX spectra of RNi 2 sample, i.e. 2% Ni-doped samples. EDX
respectively. Their corresponding magnetizations of remanence
measurements on single nanoparticle found that zinc and nickel are homo-
ꢂ3
ꢂ3
ꢂ3
ꢂ3
were 10.1 ꢁ10 , 9.8 ꢁ 10 , 7.28 ꢁ 10
and 5.92 ꢁ10 emu/g,
2
+
geneously distributed throughout the ZnO:Ni nanoparticles.
respectively. The noticeable coercivity of M–H loop could be
attributed to strong ferromagnetism at room temperature. The
narrow hysteresis implies a small amount of dissipated energy in
repeatedly reversing the magnetization that is important for quick
magnetization and demagnetization of the samples synthesized.
The ferromagnetic behavior can be attributed to the presence of
small magnetic dipoles located at the surface of nanocrystals,
which interacts with their nearest neighbors inside the crystal.
Consequently, the interchange energy in these magnetic dipoles
making other neighboring dipoles oriented in the same direction.
In nanocrystals, surface-to-volume ratio increases, so the popula-
tion of magnetic dipoles oriented in the same direction will
increase at the surface. Thus, the sum of the total amount of
dipoles oriented along the same direction will increase subse-
quently. In short, the crystal surface will be usually more
magnetically oriented.
host ZnO lattice could be attributed to the size difference of ionic
2+
2+
radius of both Zn and Ni ions as discussed earlier.
2+
In order to confirm the presence of Ni in the synthesized ZnO
nanoparticles, EDX measurements were performed. Fig. 3 shows
the representative EDX spectra of RNi 2 sample, i.e. 2% Ni-doped
samples. From the similarity of the Zn and Ni peak intensity line
traces, it is clear that after the synthesis process, zinc and nickel
were homogenously distributed inside the nanoparticle. From the
2+
EDX line traces it can also be concluded that Ni was successfully
substituted into the crystal structure of ZnO nanoparticles. The
2+
estimated amount of Ni ions were nearly 2%. EDX measurements
2+
on single ZnO:Ni nanoparticles found that zinc and nickel were
homogeneously distributed throughout the whole nanoparticle.

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0
0
0
0
0
0
0
0
0
0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
.10
.09
.08
.07
.06
.05
.04
.03
.02
.01
.00
0
.10
.08
0
0.05
0.03
0.00
-0.03
-
-
-
-
-
-
-
-
-
-
-
-
-
0.05
0.08
0.10
-
400
-200
0
200
400
-400
-200
0
200
400
Applied Field (mT)
Applied Field (mT)
0
.10
.08
.06
0
0
0
0
0
.10
.08
.05
.03
.00
0
0
0.04
0.02
0.00
-0.02
-
0.03
0.05
0.08
0.10
-
-
-
-
0.04
0.06
0.08
0.10
-
-
-
-
400
-200
0
200
400
-
400
-200
0
200
400
Applied Field (mT)
Applied Field (mT)
Fig. 4. Room-temperature M–H loop for as-synthesized ZnO:Ni nanoparticles. (a) For RNi 2, i.e. 2%, (b) for RNi 4, i.e. 5%, (c) for RNi 6, i.e. 10% and (d) for RNi 8, i.e. 20% Ni-
doped samples, Observed clear hysteresis indicates ferromagnetic nature of the prepared nanoparticles at room temperature.
Raman spectroscopic studies were employed to understand the
effect of nickel doping on microscopic structure and vibrational
properties of prepared ZnO:Ni nanoparticles. Several literature
show that in hexagonal wurtzite ZnO following fundamental
second-order vibration modes of ZnO. These results further
2
+
confirm that the ZnO:Ni
nanoparticles are composed of
2+
hexagonal wurtzite structure. Some other peaks at positions
ꢂ1
1347 and 1426 cm , were also observed for higher doping
optical bands should exist, as according to the group theory: E
2
percentage of nickel.
For RNi 1 sample, very strong and sharp Raman peaks were
observed, indicating good wurtzite structure. The sharpest and
ꢂ1
ꢂ1
ꢂ1
(
(
low) at 101 cm , E
2
(high) at 437 cm , A
1
(TO) at 380 cm , A
1
ꢂ1
ꢂ1
ꢂ1
1 1
LO) at 574 cm , E (TO) at 407 cm and E (LO) at 583 cm .
ꢂ1
The low-frequency E
heavy Zn sub-lattice and the high-frequency E
2
mode is associated with the vibration of
mode involves
strongest peak at about 437 cm is the strongest Raman mode in
2
wurtzite crystal structure. However, as the Ni content increases to
5% and above, the Raman line of E2 (high) mode becomes broad
and weak, which means that the wurtzite crystalline structure of
ZnO might have been weakened by higher Ni doping concentra-
tion. It is well known that lattice defects and disorder could
usually be introduced by exotic ions doping, defect-induced
Raman modes would usually appear in defective crystals because
the Raman selection rules are relaxed [18]. For high-concentra-
tion-doped crystal, the translational invariance of the crystal
lattice is weakened and scattering events from the whole Brillouin
zone are possible.
only the oxygen atoms. Raman Spectra (Fig. 5) showed a strong
ꢂ1
Raman shift signal at ꢃ437 cm , which is due to E
2
(high)
mode of ZnO nanoparticles. These results were consistent with
already reported works [14–17]. The second-order vibrations
were at 208, 334 and 1050–1200 cm . Presence of small peak
ꢂ1
ꢂ1
at ꢃ583 and 378 cm confirmed that oxygen deficiency is quite
1 1
low in the nickel-doped ZnO and are due to E (LO) and A (TO)
modes of ZnO, respectively, this peak diminishes gradually
with increasing concentration of Ni, the dopant. The other
peaks appearing in the low-frequency region corresponds to the

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3461
tween neighboring Ni–Ni ions suppressed the ferromagnetism at
higher doping concentrations of nickel.
RNi8
Acknowledgements
RNi6
RNi4
Authors are thankful to DST and CSIR, India for the financial
assistance during the current work. Prof. S.C. Kashyap at IIT Delhi
is thankfully acknowledged for the VSM measurements of all the
samples.
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RNi1
[
[
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+
Fig. 5. Representative Raman spectra of ZnO:Ni nanoparticles with various
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We have successfully synthesized the ultrafine diluted mag-
2+
netic semiconductors nanoparticles of ZnO:Ni of mean size
12 nm at room temperature. XRD analysis and Raman study
2
2+
show wurtzite structures for all the ZnO:Ni nanoparticles. With
increasing Ni concentration, the wurtzite structures degrade
gradually. Initially, these nanoparticles showed strong ferromag-
netic behavior, however, at higher doping percentage of Ni the
ferromagnetic behavior was suppressed and paramagnetic nature
was observed. The enhanced antiferromagnetic interaction be-
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