Synthesis and magnetic properties of nickel nanoparticles deposited on the silicon nanowires

Article abstract of DOI:10.1016/j.jallcom.2011.09.047

Nickel (Ni) nanoparticles with sizes of ～35 nm were deposited on the surface of silicon nanowires (SiNWs) by electroless plating technique. The magnetic properties of Ni/SiNWs were investigated. The blocking temperature (T_B) of 370 K was obtained and confirmed by field-cooled (FC) and zero-field-cooled (ZFC) plots. The M-H hysteresis loops from 5 K to 400 K were measured. The saturation magnetization value was ～4.5 emu/g and the coercivity was ～375.3 Oe for the loop at 5 K, respectively. While for the loop at 400 K, these values were of ～2.6 emu/g and ～33.3 Oe, respectively. The temperature dependence of coercivity followed by the relation H_C(T) = H_C0[1 - (T/T_B)^1/2], indicating a superparamagnetic behavior. The magnetization of superparamagnetic grains in a magnetic field H was better described by Langevin function at 400 K. These novel magnetic properties of Ni/SiNWs were possibly attributed to the paramagnetic defects on the surface of SiNWs.

Full text of DOI:10.1016/j.jallcom.2011.09.047

Journal of Alloys and Compounds 511 (2012) 257–261
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis and magnetic properties of nickel nanoparticles deposited on the
silicon nanowires
Zhiliang Wanga,b, Junyu Zhu , Xuejiao Chen , Qiang Yan , Jian Zhang , Yun Chena,b
a
a
a
a,∗
a
Department of Electronic Engineering, Key Laboratory of Polar Materials and Devices (Ministry of Education of China), East China Normal University, 500 Dongchuan Road,
Shanghai 200241, China
b
School of Electronics and Information, Nantong University, 9 Seyuan Road, Nantong 226019, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 July 2011
Received in revised form
Nickel (Ni) nanoparticles with sizes of ∼35 nm were deposited on the surface of silicon nanowires (SiNWs)
by electroless plating technique. The magnetic properties of Ni/SiNWs were investigated. The blocking
temperature (TB) of 370 K was obtained and confirmed by field-cooled (FC) and zero-field-cooled (ZFC)
plots. The M–H hysteresis loops from 5 K to 400 K were measured. The saturation magnetization value
was ∼4.5 emu/g and the coercivity was ∼375.3 Oe for the loop at 5 K, respectively. While for the loop at
1
5 September 2011
Accepted 15 September 2011
Available online 21 September 2011
4
00 K, these values were of ∼2.6 emu/g and ∼33.3 Oe, respectively. The temperature dependence of coer-
1/2
civity followed by the relation HC(T) = HC0[1 − (T/TB) ], indicating a superparamagnetic behavior. The
magnetization of superparamagnetic grains in a magnetic field H was better described by Langevin func-
tion at 400 K. These novel magnetic properties of Ni/SiNWs were possibly attributed to the paramagnetic
defects on the surface of SiNWs.
Keywords:
SiNWs
Nickel nanoparticles
Paramagnetic defects
Superparamagnetic
Magnetic properties
© 2011 Elsevier B.V. All rights reserved.
1
. Introduction
actual application, it casts a shadow on the future applications
[
5,10,13].
SiNWs had huge surface-to-bulk ratio and were attracting more
Magnetic nanostructures had received increasing interest
because of both the richness of their physical properties and the
potential applications like sensors, medical diagnostics, and stor-
age systems [1–3]. Previous studies focused on the ferromagnetic
metals such as Fe, Co, and Ni had shown that the basic mag-
netic properties could vary significantly as a function of both
size and shape, especially, when those nanoparticles were dis-
tributed to the substrate of high specific surface area [4–10]. In this
condition, for nanoparticles, the thermal agitation would weaken
the interaction forces; prevent the existence of stable magnetiza-
tion and lead to the superparamagnetic (SPM) state [11–13]. The
transition temperature point between ferromagnetic and super-
paramagnetic state is the blocking temperature (TB). Nowadays,
lots of studies had been pursued to obtain the high TB values.
For example, for daily data storage, the blocking temperature of
the nanoparticles should be well above the room temperature in
order to have a stable data recorded capability [14,15]. So far,
for most materials, TB values obtained are not high enough for
and more attentions due to their nanoscale effect. The as-grown
SiNWs revealed several paramagnetic defects. A total spin density
18
−3
of 1.5 × 10 cm was found for the as-grown SiNWs, which were
paramagnetic defects of silicon nanowires [16]. SiNWs modified
with ferromagnetic metal particles could exhibit different behav-
iors and properties. For instance, Ingole’s group [17] reported that
Ni nanoparticles at the tips of silicon nanowires displayed a low
coercivity ferromagnetic behavior.
It was reported when the ferromagnetic particle interacted
with the superparamagnetic particle. The thermal stability of
the resulted composites could be affected [12]. Meanwhile, the
fluctuation amount of superparamagnetic particles could make
ferromagnetic particles magnetically soft [18]. So the interface
between Ni particles (ferromagnetism) and defects on the surface of
SiNWs (SPM) could provide extra interparticle interactions, which
can affect the anisotropy barrier and the spin behavior of magnetic
nanoparticles [19,20]. To our knowledge, so far the research in this
field is seldom found.
In this paper, we presented an easy-to-follow synthesis pro-
cedure to prepare the Ni/SiNWs. First the SiNWs were fabricated
by chemical etching. Then Ni was deposited onto the surface of
SiNWs by the electroless plating. We discussed the correlation
∗ Corresponding author. Tel.: +86 21 54345203; fax: +86 21 54345119.
E-mail addresses: jzhang@ee.ecnu.edu.cn, jzhang0002@gmail.com (J. Zhang).
0
925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.09.047

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Z. Wang et al. / Journal of Alloys and Compounds 511 (2012) 257–261
between microstructure, morphology and magnetic response of
Ni/SiNWs.
2. Experimental
2.1. Substrates and reagents
Double-sided polished p-type silicon-wafers (with ꢀ1 0 0ꢁ orientation and
∼
0.1–10 ꢀ cm resistivity) were used as the substrates for SiNWs growth. Chemicals
such as AgNO3, HF (>40%), HNO3 (65–68%), NiSO4·6H2O, (NH4)2SO4, sodium dodecyl
sulfate, NH4F, sodium succinate, Sodium citrate, and ammonia were all analytical
reagents and used as-received without further purification.
2.2. SiNWs growth and electroless plating nickel
Silicon-wafers were cleaned via standard RCA process. The chemical solution of
HF (20%):AgNO3 (35 mM) = 1:1 (volume ratio) was used as the etchant for SiNWs
preparation. The cleaned silicon wafers were etched for 60 min. After the formation
of SiNWs, nickel was deposited onto the SiNWs for 5 s. The detailed information
about SiNWs fabrication process and nickel electroless plating was reported in our
previous work [21,22].
2.3. Ni/SiNWs characterization
Fig. 1. X-ray diffraction pattern of Ni/SiNWs.
The crystalline phase of Ni/SiNWs was examined by X-ray diffraction (XRD)
using Cu K␣ radiation (D/MAX-2250 V, Rigaku Co., ꢁ = 0.15405 nm). The morpholo-
gies of prepared samples were observed by scanning electron microscopy (FE-SEM,
Philips XL30FEG). The magnetic properties of Ni/SiNWs were carried out under mag-
netic fields of up to 10 kOe in the temperature range from 5 to 400 K by a physical
property measurement system (Quantum Design PPMS-9). The ZFC and FC measure-
ments were done in the 100 Oe field. In ZFC measurements, the sample was cooled
to 4.2 K in zero applied fields and then 100 Oe was applied during the increase in
temperature. For FC measurements, the sample was cooled down to 4.2 K under
3
. Results and discussion
Fig. 1 shows XRD patterns of Ni/SiNWs. Except for the silicon
substrate peak Si (4 0 0), three Ni diffraction peaks correspond to
different crystallization directions of (1 1 1) at about 44.2 , (2 0 0) at
about 51.9 and (2 2 0) at about 76.4 were visible. All peaks were
consistent with the standard card, JCPDS 870712, indicating that
the as-deposited Ni had a face-centered-cubic (fcc) structure [23].
◦
◦
◦
1
00 Oe and then magnetization versus temperature was measured in 100 Oe. The
dc magnetic relaxation experiments were done at 40 Oe.
Fig. 2. SEM cross sectional images of silicon nanowires with different magnification. SiNWs were aligned perpendicularly and no particles were deposited on them (a).
By the electroless plating, Ni particles were distributed on the surface of SiNWs (b). Partial enlargement of the red frame in (b) and Ni particles were noninteracting and
monodisperse (c). Partial enlargement of the red frame in (c) and the diameter range of Ni particles was ∼35 nm (d). (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of the article.)

Z. Wang et al. / Journal of Alloys and Compounds 511 (2012) 257–261
259
Fig. 5. Temperature dependence of coercivity for the Ni/SiNWs structure. The red
line shows the fit curve according to Eq. (1). (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of the article.)
−
1
Fig. 3. FC/ZFC plots at 100 Oe of Ni/SiNWs showing TB was 370 K (a). ꢂ was roughly
linear with temperature in agreement with the Curie–Weiss law (b).
Fig. 6. Normalized magnetization as a function of H/T at temperature 400 K.
Fig. 4. The M–H hysteresis loops of Ni/SiNWs taken at 5–400 K.
The average crystallite size could be estimated from XRD pattern
using the Scherrer relation [24]. The diameters determined from
the (1 1 1), (2 0 0) and (2 2 0) peaks were 35.4, 27.4 and 32.4 nm
respectively.
◦
◦
The broad SiO2 peaks observed at about 12.1 and 62.6 were
the amorphous native oxide at the surface of SiNWs [25], in which
a large number of defects located [16,25].
Several very weak peaks of nickel oxide were also detected from
XRD, indicating nickel oxide formed on the surface. Normally, the
influence of nickel oxide was seldom and negligible. Meanwhile,
the magnetic measurements stated below show that the exchange-
bias effect [24] was not found, indicating that NiO had not affected
the magnetic properties of Ni/SiNWs [27,28]. The data implies that
the face-centered-cubic, less oxidized Ni can be obtained under the
current synthesis conditions.
Fig. 2 shows the SEM cross sectional images of silicon nanowires
with different magnification. From Fig. 2(a), it could be seen
that SiNWs were aligned perpendicularly and no particles were
deposited on them. Fig. 2(b) displays that by the electroless plat-
ing, Ni particles were distributed on the surface of SiNWs. Fig. 2(c)
exhibits the partial enlargement of Fig. 2(b), which was the region
of the red frame. It was shown that Ni particles were noninteract-
ing and monodisperse in Fig. 2(c). Fig. 2(d) is the enlargement of
the red frame in Fig. 2(c). It could be observed that the Ni parti-
cles exhibited homogeneous spherical structure and the diameter
+
−
Fig. 7. Expanded view of hysteresis loops taken at 5 and 400 K. HC and HC were
defined in the text.
of Ni particles was ∼35 nm. The diameter of diameter values was
in agreement with XRD results stated previously.
For the diameter (<50 nm) of Ni nanoparticles, superparam-
agnetism had been reported. Peng’s group [29] reported that
the fcc nickel nanoparticles synthesized in pure oleylamine

2
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Z. Wang et al. / Journal of Alloys and Compounds 511 (2012) 257–261
−
1
Fig. 8. FC/ZFC plots at 100 Oe of Ni/Si showing TB was 230 K (a) ꢂ was roughly linear with temperature in agreement with the Curie–Weiss law (b). The M–H hysteresis
loops of Ni/Si taken at 5–400 K (c).
exhibited a rather broad size distribution in the range of 10–50 nm
and the splitting ZFC–FC magnetization curves reached a crossing
point around 300 K, indicating a blocking temperature above room
temperature. Jardim’s group [5] also reported that the fcc nickel
the anisotropy barrier [31]. It was well known that at nanoscale
confinement, where the size was of the order of 5–50 nm, the ther-
mal activation energy overcome the cohesive energy of fluctuating
magnetic domains. Above a certain blocking temperature, the ade-
quate energy for the alignment of particle moments in an applied
magnetic field was supplied [29]. As a result, at T > TB, the hysteretic
behavior was suppressed and the system behaves as a strong para-
magnet or superparamagnetism. The temperature dependence of
coercivity was given by the relation [30]:
nanoparticles embedded in a SiO amorphous matrix had been pre-
2
pared which exhibit superparamagnetism above TB < 40 K. So it was
meaningful to study the magnetic properties of ∼35 nm Ni particles
deposited on the surface of SiNWs.
Each silicon nanowire could be seen as core/shell structure
ꢀ
ꢃ
[
25,30]. The loose and rough shell (mean roughness height 1–5 nm)
ꢁ
ꢂ
k
T
contained a large number of interface states or defects. This rough-
ness might be attributed to randomness of the lateral oxidation and
etching in the corrosive aqueous solution or slow HF etching and
faceting of the lattice during synthesis. These defects located in the
oxide and at the interface of the crystalline core to the surround-
ing oxide were proved to possess paramagnetic properties [16]. By
the electroless plating, it was thought that Ni particles were pref-
erentially deposited on the surface defect regions of SiNWs. The
paramagnetic defects and magnetic particles would interact and
affect the Ni/SiNWs magnetic properties.
The magnetic properties of fcc-Ni nanoparticles decorated
SiNWs were investigated. The temperature dependent of the
magnetization (M–T) and hysteresis loops (M–H) curves were
obtained by the zero-field-cooled (ZFC) and the field-cooled (FC)
measurement. It was known that the main features of the super-
paramagnetic systems as follows: (1) the ZFC curves are rounded
at the blocking temperature TB, defined as the temperature of the
maximum, indicating a blocking process of the small particles,
and (2) above TB, a paramagnetic like behavior can be found, i.e.,
Curie–Weiss law is satisfied [5]. From ZFC and FC curves as shown
in Fig. 3, Ni nanoparticles decorated SiNWs composite turns from
ferromagnetic to superparamagnetic and the blocking temperature
HC = HC0 1 −
(1)
TB
where HC0 was zero temperature coercivity, the exponent value k
was 0.5 for an assembly of aligned particles and 0.77 for randomly
oriented particles [31]. Referring to Fig. 6, the red line showed
the fitting result according to Eq. (1) using the blocking temper-
ature as the fitting parameter. In Ni/SiNWs studied, when k was
0
.5, the experimental data fitted very well to the above theoretical
relation. From the fitting curve, the coercivity at T = 0 K (HC0) for
nanoparticles was ∼394.2 Oe while the blocking temperature was
3
83.1 K. The blocking temperature obtained is in agreement with
FC/ZFC measurement (370 K in Fig. 3). So it is also concluded that
Ni nanoparticles in our study were well aligned.
For T ≥ TB, the magnetization of superparamagnetic grains in a
magnetic field H was better described as Langevin function [5]:
ꢁ
ꢂ
ꢁ
ꢂ
ꢁ
ꢂ
M
ꢃH
ꢃH
1
= L
= coth
−
(2)
MS
B
k T
B
k T
ꢃH/kBT
where k is Boltzmann constant and ꢃ is the effective moment.
Normalized magnetization as a function of H/T at temperature
400 K is shown in Fig. 6. The experimental results fitted to
Eq. (2) very well. The best-fit parameter of magnetic moment,
B
−
16
4
(
TB) was estimated to be 370 K. Above TB, the reciprocal of suscepti-
ꢃ = 4.9 × 10
erg/Oe = 5.3 × 10 ꢃ , was obtained, where ꢃ was
B
B
−1
bility, ꢂ , was roughly linear with temperature, in agreement with
the Curie–Weiss law, which provided an evidence for the suggested
superparamagnetic behavior.
Bohr Magneton. Such high value of magnetic moment had been
reported for similar nanosystems that show superparamagnetic
behavior [32]. It was concluded that a large number of paramag-
netic defects on the surface of silicon nanowires could give rise
to such high magnetic moment which in turn could explain the
superparamagnetism in Ni/SiNWs.
The M–H hysteresis loops at 5–400 K were measure. And the
5
–400 K loops are shown in Fig. 4. For the loop at 5 K, the values
of saturation magnetization and coercivity were ∼4.5 emu/g and
375.3 Oe, respectively, whereas for the loop at 400 K, the values
∼
From the X-ray diffraction pattern showed previously in Fig. 1,
NiO was also detected. It is reported that NiO/Ni composites exhib-
ited an exchange bias effect due to interfacial interaction between
ferromagnetic Ni and antiferromagnetic NiO [5]. Fig. 7 is the partial
enlarged drawing of the hysteresis loops. Defining HC and HC as
the coercive fields with decreasing and increasing fields, respec-
tively, a measure of the symmetry of the M–H curves was given
were ∼2.6 emu/g and ∼33.3 Oe, respectively. With the temperature
increasing, the values of coercivity and saturation magnetization
decreased due to the thermal activation effect, as shown in Fig. 4.
Fig. 5 shows the relationship between the coercivity values and
temperature. A monotonic decrease in coercivity with tempera-
ture was observed. The reason could be explained by considering
the effects of thermal fluctuation of the blocked moment across
+
−
+
−
by ꢄHC = (HC + H )/2. The hysteresis loops displayed in Fig. 5
C

Z. Wang et al. / Journal of Alloys and Compounds 511 (2012) 257–261
261
clearly showed that these loops were symmetric about zero field
(Grant No. 09ZZ46) and the Education Committee of Jiangsu
Province (Grant No. 11KJB510023) and Natural Science Foundation
of Nantong University (Grant No. 10Z023 and 10Z025).
(
ꢄH ∼ 1 Oe), indicating no exchange bias effect was found and NiO
C
had not effect the magnetic properties of magnetic.
In order to probe the extra interparticle interactions provided by
Ni/SiNWs, Ni nanoparticles were also deposited on the bare silicon
wafer with the same electroless plating process and the magnetic
properties of the resulted Ni/Si structures were also examined. The
results are summarized in Fig. 8. The result indicated that TB of Ni/Si
was ∼230 K, which is much lower than TB of Ni/SiNWs, ∼370 K. In
addition, at 5 K, the saturation magnetization value was ∼3.7 emu/g
and the coercivity was ∼216.4 Oe, respectively. While at 400 K, the
saturation magnetization value was ∼1.4 emu/g and the coerciv-
ity was near zero. All these values are smaller than those obtained
from Ni/SiNWs structures. Therefore, the novel magnetic proper-
ties of Ni/SiNWs structure were attributed to the introduction of
SiNWs. According to literatures [12,16], the magnetic properties
are reported to be related to the surface defects on SiNWs.
References
[
[
[
1] H. Wang, Y. Yu, Y. Sun, Nano 6 (2011) 1–17.
2] N. Negulyaev, V. Stepanyuk, W. Hergert, Phys. Rev. Lett. 106 (2011) 037202.
3] J. Cai, Phys. Rev. Lett. 106 (2011) 100501.
[4] C.M. Liu, Y.C. Tseng, C. Chen, Nanotechnology 20 (2009) 415703.
[5] F.C. Fonseca, G.F. Goya, R.F. Jardim, Phys. Rev. B 66 (2002) 104406.
[6] N. Grobert, W.K. Hsu, Y.Q. Zhu, Appl. Phys. Lett. 75 (1999) 3363–3365.
[7] X.Y. Zhang, G.H. Wen, Y.F. Chan, Appl. Phys. Lett. 83 (2003) 3341–3343.
[8] H. Zeng, M. Zheng, R. Skomski, J. Appl. Phys. 87 (2000) 4718–4720.
[9] S. Balakrishnan, Y.K. Gun’ko, T.S. Perova, Small 2 (2006) 864–869.
10] P. Granitzer, K. Rumpf, A.G. Roca, J. Magn. Magn. Mater. 322 (2010)
[
[
1343–1346.
11] W. Chen, S. Zhang, H.N. Bertram, J. Appl. Phys. 71 (1992) 5579–5584.
[12] V.L. Safonov, H.N. Bertram, J. Appl. Phys. 93 (2003) 8474–8476.
[
13] X. Xiang, G. Fan, J. Fan, J. Alloys Compd. 499 (2010) 30–34.
[
14] S. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000)
1989–1992.
4
. Conclusion
[15] M. Pan, H. Liu, J. Wang, J. Jia, Q. Xue, J. Li, Nano Lett. 5 (2005) 87–90.
[
16] A. Baumer, M. Stutzmann, M.S. Brandt, Appl. Phys. Lett. 85 (2004)
43–945.
17] S. Ingole, P. Manandhar, J.A. Wright, Appl. Phys. Lett. 94 (2009) 223118.
45 (1992)
9
In summary, we had prepared the electroless plating Ni particles
[
on the surface of SiNWs which exhibited superparamagnetic above
TB. The interface between Ni particles (ferromagnetic) and defects
on the surface of SiNWs (SPM) could provide extra interparticle
interactions, which could affect the anisotropy barrier and the spin
behavior of magnetic nanoparticles. The magnetic characterization
indicated that Ni/SiNWs could be very useful for high temperature
magnetic applications.
[18] S. Gangopadhyay, G.C. Hadjipanayis, B. Dale, Phys. Rev.
778–9787.
B
9
[
[
19] V. Skumryev, S. Stoyanov, Y. Zhang, Nature 423 (2003) 850–853.
20] E. De Biasi, C.A. Ramos, R.D. Zysler, Phys. Rev. B 65 (2002) 144416.
[21] L. Wan, W. Gong, K. Jiang, Appl. Surf. Sci. 254 (2008) 4899–4907.
[
[
[
22] B. Tao, J. Zhang, F. Miao, H. Li, Sens. Actuators B 136 (2009) 144–150.
23] J. Gong, L. Wang, Y. Liu, J. Alloys Compd. 457 (2008) 6–9.
24] D.X. Chen, O. Pascu, A. Roig, J. Magn. Magn. Mater. 322 (2010) 3834–3840.
[25] A.I. Hochbaum, R. Chen, R.D. Delgado, Nature 451 (2008) 163–167.
[
[
[
[
[
27] Y.Z. Zhou, J.S. Chen, B.K. Tay, Appl. Phys. Lett. 90 (2007) 043111.
28] S. Kar, V. Singh, J. Alloys Compd. 509 (2011) 3582–3586.
29] Y.Z. Chen, D.L. Peng, D. Lin, Nanotechnology 18 (2007) 505703.
30] S.N. Mohammad, J. Appl. Phys. 108 (2010) 034311.
31] K. Maaz, A. Mumtaz, S.K. Hasanain, J. Magn. Magn. Mater. 322 (2010)
Acknowledgments
This work is supported by the National Natural Science
Foundation of China (Grant Nos. 60672002 and 61076070), Inno-
vation Program of Shanghai Municipal Education Commission
2199–2202.
[
32] M.A. Khadar, V. Biju, A. Inoue, Mater. Res. Bull. 38 (2003) 1341–1349.