Quenching of the surface-state-related photoluminescence in Ni-coated ZnO nanowires

Article abstract of DOI:10.1016/j.physb.2010.08.036

Nickel-coated ZnO nanowires (NWs) were fabricated by electrodepositing Ni particles on ZnO NW arrays. The morphological, magnetic, and photoluminescent properties of the Ni-coated ZnO NWs were investigated. The Ni particles were deposited on the ZnO NWs' surface along its length to form a Ni/ZnO shell-core structure. The Ni-coated ZnO NWs exhibited more isotropic characteristic than the electrodeposited Ni films owing to the isotropic sphere structure of the Ni particles. A strong ultraviolet emission can be obtained from the Ni-coated ZnO NWs, while the green emission related to surface states was quenched by the passivated layer.

Full text of DOI:10.1016/j.physb.2010.08.036

Physica B 405 (2010) 4551–4555
Contents lists available at ScienceDirect
Physica B
journal homepage: www.elsevier.com/locate/physb
Quenching of the surface-state-related photoluminescence
in Ni-coated ZnO nanowires
Yang Tang a,b, Dongxu Zhao a,n, Jiying Zhang a, Dezhen Shen a
a Key Laboratory of Excited State Processes, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 16 East Nan-Hu Road,
Open Economic Zone, Changchun 130033, People’s Republic of China
b Graduate School of the Chinese Academy of Sciences, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Nickel-coated ZnO nanowires (NWs) were fabricated by electrodepositing Ni particles on ZnO NW
arrays. The morphological, magnetic, and photoluminescent properties of the Ni-coated ZnO NWs were
investigated. The Ni particles were deposited on the ZnO NWs’ surface along its length to form a Ni/ZnO
shell-core structure. The Ni-coated ZnO NWs exhibited more isotropic characteristic than the
electrodeposited Ni films owing to the isotropic sphere structure of the Ni particles. A strong
ultraviolet emission can be obtained from the Ni-coated ZnO NWs, while the green emission related to
surface states was quenched by the passivated layer.
Received 12 October 2009
Received in revised form
2 August 2010
Accepted 12 August 2010
Keywords:
ZnO nanowire
Ni-coated
& 2010 Elsevier B.V. All rights reserved.
Electrodeposition
Surface states
1. Introduction
nickel particles were electrodeposited on ZnO NW arrays to form
a Ni/ZnO shell-core NW structure. It is expected that the ability to
grow magnetic-metal/semiconductor NW heterostructures can
improve the luminescence properties by surface passivation and
increase versatility and power of these building blocks for
applications in nanoscale spintronics and optical nanodevices
[15]. In addition, magnetic fields have been used to control
the direction and the position of magnetic nanowires and Ni
end-capped nonmagnetic nanowires recently [17–19]. Therefore,
it is expected that the Ni-coated ZnO NWs can be applied to make
a matrix of devices as well as a single device using magnetic
alignment.
As a wide band-gap semiconductor, one dimensional ZnO
nanostructures have aroused intense theoretical and experimental
research interests worldwide in recent years because of their
potential applications as ultraviolet lasers [1], field-effect transistors
[2], gas sensors [3], field-emission displays [4], and nanogenerators
[5,6]. Owing to the large aspect ratio, the luminescence properties of
ZnO nanowires (NWs) may degrade as result of the unpassivated
surface states serving as the nonradiative recombination centers
[7,8]. The surface passivation is adopted to generate desired NW
surface characteristics and improve luminescence properties [9]. The
optical quenching with increasing temperature have been reported
in Ni-nanodot-coated ZnO NWs [10]. As an important transitional
metal, Ni has attracted great attention in terms of its expected
applications in high-density magnetic recording, magnetic sensors,
catalysis, and Ni-based batteries [11–14]. The fabrication of
magnetic materials/semiconductor heterostructures is of particular
interest in nanoscale spintronics [15]. The Ni-passivated ZnO NWs
not only exhibit the improvement of the photoluminescence
property by the surface passivation, but also are regarded as a
formation of junction, which may serve as a basic unit for the
optoelectronic nanodevices.
2. Experimental methods
Our study is based on the aligned ZnO NWs arrays grown on
p-Si (1 0 0) substrate by vapor transport process. The silicon
substrate was pre-coated with a ZnO thin layer. The details of the
NW growth were concluded in Refs. [20,21]. After the fabrication of
ZnO nanowires arrays, the wafers were then transferred to the
electrodeposition cell with aqueous electrolytes containing 0.01 M
Ni(CH3COO)2 as the source of metal ions and 0.05 M CH3COONH4 as
the supporting electrolyte. A Pt counter electrode and an Ag/AgCl
reference electrode were adapted. The electrodeposition was carried
out at a constant potential of ꢀ1950 mV at room temperature. The
growth time was held for 10 min. The surface morphology of the
nanostructures was observed by field emission scanning electron
Electrochemical techniques are particularly attractive for the
electrodeposition of metallic nanostructures [16]. In this paper
n Corresponding author. Tel.: +86 431 86176322; fax: +86 431 84627031.
E-mail address: dxzhao2000@yahoo.com.cn (D. Zhao).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2010.08.036

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Y. Tang et al. / Physica B 405 (2010) 4551–4555
microscopy (FESEM, Hitachi S-4800) and energy-dispersive X-ray
(EDX) attached to the SEM. Magnetic characteristics were studied
the capillary force. According to Fig. 2(d), the electrodeposited Ni
particle has a typical diameter of 260 nm. The ZnO NWs in the
electrodeposition process could be viewed as nano-electrode
arrays. The electrodeposition process is a complex multi-step
procedure. The changes in the electrolyte bulk phase and the
electrode interface are continuous during the deposition process.
In our previous report NH4þ ions in the electrolyte play a role to
assist the Ni deposition [22]. The Ni2+ and its ammonium group
complex in the electrolytes transfer to the vicinity of the electrode
interface. The ions discharge at the interface, then generate
the absorbed atoms. The Ni electrodeposition process in the
electrolyte is as follows [24]:
using
a vibrating sample magnetometer (VSM) (Lake Shore
Company) at room temperature. Micro-PL spectra were measured
using the 325 nm line of a He–Cd laser as an excitation source.
3. Results and discussion
When Ni was electrodeposited directly on silicon, uniform Ni
thin films can be obtained. The substrate surface is entirely
covered by the compact Ni films. The details of the Ni films
electrodeposited on silicon were concluded in Ref. [22]. The ZnO
NWs grown on the silicon substrate form a matrix, indicated in
Fig. 1(a). The ZnO NW has a typical diameter of 70 nm and the
[Ni(NH3)2]2+ ¼Ni2+ +2NH3
Ni2+ +2e-Ni
(1)
(2)
length of 4 mm. After the Ni electrodeposition for 10 min, there
were great changes in morphology. Ni particles can be observed
on the top and lateral surface of ZnO NWs as shown in Fig. 1(b).
The previous smooth ZnO NW surfaces became rough owing to
the electrodeposited Ni particles. The Ni-coated ZnO NW arrays
turned out to be bundled together under the capillary force. The
bundling behavior of ZnO NWs by capillary force was included in
our previous report [23]. There were two kinds of Ni particles on
ZnO NW arrays. At first, the Ni particles were deposited on the
ZnO NW surfaces along its length to form a Ni/ZnO shell-core
structure. When the ZnO NWs were bundled together under the
capillary force, the Ni started to deposit on the top area of the ZnO
NW bundles. As a result, the Ni micro-particle blocks formed as
shown in Fig. 1(c). The inset image of Fig. 1(c) illustrates that the
block is consisted of compact Ni grains. There were no Ni micro
particles on the bundles that included less ZnO NWs in Fig. 1(d).
The ZnO NW bundles composed of a certain quantity of NWs
acted as the nucleation sites for the following growth of Ni micro-
particle blocks. For the shell-core structure in Fig. 2(a), the ZnO
NW was entirely covered by the string bead-like Ni nanoparticles.
The boundary between the adjacent Ni particles was obvious in
Fig. 2(b). At the top area of ZnO NW, the density of the Ni particles
was larger, which is depicted in Fig. 2(c). The circle area in
Fig. 2(c) indicates that some ZnO NWs could break as a result of
A negative potential (ꢀ1.95 V) makes the reactions go along
the path (1)-(2), which results in the growth of the Ni particles
over the whole surface of the ZnO NWs.
Fig. 3(a) shows the EDS result of the as-synthesized ZnO NWs.
In addition to the Si signal from the substrate, there were only Zn
and O signals. After the Ni electrodeposition, the Ni signals were
obvious in Fig. 3(b). No other signals except for Zn, O, and Ni were
observed.
Fig.
4 shows the room temperature hysteresis loops of
Ni-coated ZnO NWs while the applied magnetic field is parallel
and perpendicular to the sample surface. The coercive force and
remanence ratio were 93 Oe and 15% while the magnetic field was
parallel to the sample surface and 74 Oe and 9.2% while the
applied field perpendicular to the surface. Both coercive forces
were smaller than that of Ni bulk materials (100 Oe) but the
remanences were larger than that of the bulk Ni (4.9%) [25].
The magnetic properties of nanomaterials are dependent on the
shape, crystallinity, magnetization directions, and so on [26].
Because Ni exhibits a small magnetocrystalline anisotropy, the
magnetic response is expected to be dominated by the shape
anisotropy. It was reported that electrodeposited Ni films
had strong shape anisotropy exhibiting characteristic of easy
Fig. 1. (a) SEM images of the as-synthesized ZnO NW arrays; (b), (c), and (d) SEM images of the Ni/ZnO shell-core NWs. Inset image of (c): an enlarged view of the Ni
micro-particle block.

Y. Tang et al. / Physica B 405 (2010) 4551–4555
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Fig. 2. SEM images of the Ni-coated ZnO NWs.
Fig. 3. EDS spectrum of (a) the as-synthesized ZnO NW arrays and (b) the Ni-coated ZnO NWs.
Fig. 4. VSM hysteresis loop of the Ni-coated ZnO NWs. The magnetic field was applied parallel (a) and perpendicular (b) to the substrate surface.
magnetized axis (Mr/Ms¼0.67) while the applied magnetic field
was parallel to the film plane and the characteristic of hard
magnetized axis (smaller than 0.02) while the applied field was
perpendicular to the film plane [27]. However, the Ni/ZnO shell-
core NWs in this paper exhibited more isotropic characteristic
(much smaller contrast of the remanences under parallel and
perpendicular magnetic fields) than the electrodeposited Ni films.
It is attributed to the isotropic sphere structure of the Ni particles
electrodeposited on ZnO NWs. The more isotropic behavior in the
magnetic response was observed in Ni hollow sphere arrays [28].

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Y. Tang et al. / Physica B 405 (2010) 4551–4555
Fig. 5. Room temperature photoluminescence spectrum of (a) the as-synthesized ZnO NW arrays (b) the Ni-coated ZnO NWs.
Jung et al. [15] evaporated Ni on the tips of ZnO nanorods and
investigated the structural and magnetic properties of the
heterostructure. However, it was not possible for the Ni particles
to cover the whole surface of the ZnO nanorods using the
evaporation method. An alternative way is to treat the NiO-coated
ZnO NWs by thermal annealing with the H2 reduction process
[10]. With the thermal-reduction process, NiO changed to Ni
nanodot. However, there were some space between the Ni
nanodots and the atomic layer deposition (ALD) process was
essential for coating NiO on ZnO NWs. Our results by electro-
chemical deposition take advantages of the low cost, the low-
temperature process, the possibility in preparing large scale
materials, and the complete coating on the NWs’ surface. As a
low-temperature process, the electrodeposition can avoid inter-
diffusion and compound formation at the interfaces so that an
abrupt and clean interface could be created, which is necessary for
the injection of spin-polarized currents from the ferromagnet to
the semiconductor [29].
Photoluminescence (PL) measurements of the Ni-coated ZnO
NWs were conducted under excitation of a 325 nm He–Cd laser at
room temperature. As shown in Fig. 5(a), the PL spectrum of the
as-synthesized ZnO NWs shows a strong ultraviolet emission line
from ZnO-free exciton and a negligibly weak green emission band
around 525 nm. The intensity ratio of the UV emission and the
visible emission of the as-synthesized ZnO NWs is 1.35. After the
electrodeposition of Ni on ZnO NW arrays, a great decrease of
deep level emission in the PL spectrum of the Ni-coated ZnO NWs
is illustrated in Fig. 5(b). The intensity ratio of the UV emission
and the visible emission of the Ni/ZnO shell-core NWs is 32,
which is almost 24 times larger than that of the as-synthesized
ZnO NWs. All spectra were obtained in one session under an
identical excitation and collection conditions. Because the
as-synthesized ZnO NWs has a large aspect ratio, the surface
approaches the bulk and the luminescence properties of ZnO NWs
are dominated by properties of surfaces. Within an effective
distance from the surface the excited carriers would diffuse to
the surface and recombine at the surface defects [30]. Therefore,
the visible emission in the as-synthesized ZnO NWs mostly
originates from surface states owing to their relatively large
aspect ratio. In addition, van Dijken et al. [31] also proposed a
kind of mechanism for the visible green emission. That is, the
photogenerated hole was trapped by surface states or defects
(probably O2ꢀ/Oꢀ). The surface trapped hole tunneled back into
the bulk volume, where it recombined with an electron to form
the Vonn center. Finally, the green emission was assigned to the
recombination of a shallowly trapped electron with a deeply
trapped hole in a Vonn center. After the electrodeposition of Ni, an
interface between the Ni shell and the ZnO NW core formed,
which greatly passivated the surface states of the ZnO NWs.
The excited carriers that diffuse to the surface were quenched by
passivated layer. Otherwise, it is impossible for the trapped hole
by the surface states tunnels back to bulk volume to form Vonn
center. According to the mechanism of visible emission and the
model of a core-shell structure for the Ni/ZnO shell-core NWs, a
strong ultraviolet emission from the Ni/ZnO shell-core NWs was
expected, while the green emission would be quenched, which
agrees with the PL result.
4. Conclusions
In conclusion,
a structure of Ni-coated ZnO NWs was
fabricated by electrodepositing Ni particles on ZnO NW arrays.
The Ni particles were deposited on the ZnO NW surfaces along its
length to form a Ni/ZnO shell-core structure. The Ni/ZnO shell-
core NWs exhibited more isotropic characteristic than the
electrodeposited Ni films owing to the isotropic sphere structure
of the Ni particles. The Ni-coated ZnO NWs showed a strong
ultraviolet emission, while the green emission related to surface
states was quenched by the passivated layer. The improvement of
the Ni-coated ZnO NWs in optical quality is expected to combine
the merits of both high quality ZnO NWs and magnetic Ni
particles. They could exhibit some new physical properties and
stimulate further investigations. For example, the Ni/ZnO shell-
core structure, which is a radial structure, namely nanocables, can
provide a more efficient injection current with respect to the
crossed and the axial structures [32,33]. The magnetic-semicon-
ductor nanowire structures might be used as components for
nanoscale spin-valve transistors, spin light-emitting diodes,
nonvolatile storage, and logic devices [15]. In addition, the
magnetic Ni/ZnO structure could be controlled to build a matrix
of nanodevices using magnetic alignment. Therefore it is expected
that the ability to grow magnetic Ni/ZnO NW structure can
improve the luminescence properties by surface passivation and
increase versatility and power of these building blocks for
applications in nanoscale spintronics or optical nanodevices.
Acknowledgements
This work was supported by the Key Project of National Natural
Science Foundation of China under Grant nos. 60336020 and
50532050, the ‘‘973’’ Program under Grant no. 2006CB604906, the
CAS Innovation Program, the National Natural Science Foundation
of China under Grant nos. 60429403, 60506014, 50402016, and
10674133.

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