Preparation of one dimensional Bi2O3-core/ZnO-shell structures by thermal evaporation and atomic layer deposition

Article abstract of DOI:10.1016/j.ssc.2008.11.037

One dimensional(1D) Bi₂O₃-core/ZnO-shell structures with uniform layer thicknesses were prepared by using a two step process consisting of thermal evaporation of Bi₂O₃ and ALD of ZnO. Highly straight 1D Bi₂

Full text of DOI:10.1016/j.ssc.2008.11.037

Solid State Communications 149 (2009) 315–318
Contents lists available at ScienceDirect
Solid State Communications
journal homepage: www.elsevier.com/locate/ssc
Preparation of one dimensional Bi O -core/ZnO-shell structures by thermal
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evaporation and atomic layer deposition
∗
Sunghoon Park, Jina Jun, Hoyun Woo Kim, Chongmu Lee
Department of Materials science and Engineering, Inha University, Incheon 402-751, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
2 3
One dimensional(1D) Bi O -core/ZnO-shell structures with uniform layer thicknesses were prepared by
Received 8 May 2008
using a two step process consisting of thermal evaporation of Bi2O3 and ALD of ZnO. Highly straight 1D
Bi2O3 structures were synthesized by thermal evaporation of Bi powders on c-plane Al2O3 substrates and
then ZnO layers were coated on them by ALD.
Accepted 24 November 2008 by R. Phillips
Available online 7 December 2008
SEM analysis shows that the oxygen partial pressure must be precisely controlled to obtain straight
Bi2O3 nanowires. According to the TEM analysis results the Bi2O3-core is a pure tetragonal β-Bi2O3
phase single crystal, while the ZnO-shell is amorphous. Photoluminescence (PL) measurement of the
ID core/shell nanostructures under excitation at 325 nm indicates that the critical number of ALD cycle
for which a transition from a violet emission characteristic of Bi2O3 to UV and green–yellow emissions
characteristic of ZnO occurs is less than 10, suggesting that the wavelength of the emission changes
abruptly with an increase in the number of ALD cycle.
PACS:
6
7
8
1.46.-w
8.55.-m
1.07.-b
Keywords:
A. Bi2O3
A. ZnO
C. One-dimensional structure
D. Thermal evaporation
©
2008 Elsevier Ltd. All rights reserved.
1
. Introduction
as demand for fabricating special nanowire structures increases,
development of methods not only for synthesizing a wide variety
of nanowires but also for modifying or improving the properties
of as-synthesized nanowires is becoming increasingly important.
For example, the wavelength of the light emitting from a radial
core/shell nanowire structure could be controlled by selecting
proper materials and thicknesses for the core and the shell.
Radial core-shell heterostructure in which the heterointerface
is parallel to the wire axis is one of the two basic nanowire
heterostructures together with axial heterostructures in which
the heterointerface is perpendicular to the wire axis. There are
many important reasons to make efforts to form controlled
radial heterostructures. One reason is protection of nanowires
from contamination or oxidation and passivation of interface
states abundant at the surface of nanowires which have a very
large surface-to-volume ratio [1]. Another is to give various
functions which can be effectively used for nanoscale electronic
or optoelectronic devices [2]. A dielectric shell could provide
electrical isolation. A shell dielectric could be used to form a
high-quality optical cavity around a small core nanowire acting
as the gain medium in nanowire FET applications. A dissimilar
semiconductor shell could be used to generate internal fields
perpendicular to the interface to provide confinement potentials
for carriers in either the core or the shell. By combining a radial
heterojunction with a layer of dopant atoms, a two-dimensional
cylindrical quantum well with high-mobility change carriers could
be made. A ferroelectric oxide shell offers the opportunity to
make nonvolatile RAM or tunable transceivers [2]. Furthermore,
Bismuth oxides are attractive materials owing to their spe-
cial physical properties such as significant energy band gap,
high refractive index, dielectric permittivity, and high oxygen-ion
conductivity, as well as marked photoconductivity and photolu-
minescence. These peculiar features make the bismuth oxides ap-
plicable to sensors, optical coatings, photovoltaic cells, microwave
integrated circuits, transparent ceramic glass manufacturing, cath-
ode ray tubes [3]. They are also used as the soft oxidation of hydro-
carbons and good electrolyte materials for the applications such
as solid oxide fuel cells and oxygen sensors [4]. The 1D nanos-
tructures of bismuth oxide have been prepared by using vari-
ous techniques such as hydrothermal synthesis, microemulsion,
metal-organic chemical vapor deposition, the chemical method,
the oxidative metal vapor transport deposition, the template-
based heat treatment, thermal oxidation [5]. However, to the best
of own knowledge, Bi2O3 nanowires have never been successfully
synthesized by using thermal evaporation which is known as the
simplest and commonest method for preparation of 1D nanostruc-
tures. Furthermore, no radial 1D structure with a Bi O core has
∗
Corresponding author. Tel.: +82 32 8607536; fax: +82 32 8737537.
E-mail address: cmlee@inha.ac.kr (C. Lee).
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been reported before. In this paper, we report on preparation of 1D
0
038-1098/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2008.11.037

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S. Park et al. / Solid State Communications 149 (2009) 315–318
nanostructures of Bi2O3-core/ZnO-shell by using a two-step pro-
cess of thermal evaporation of Bi2O3 and atomic layer deposition
(
ALD) of ZnO.
2. Experimental
Preparation of Bi2O3-core/ZnO-shell basically consists of two
steps: synthesis of Bi2O3 nanowires by using thermal evaporation
and coating of the Bi2O3 nanowires with ZnO thin films by using
ALD. First, the gold(Au)-coated Al2O3(0001) substrate was put on
top of an alumina boat loaded with pure Bi powders positioned
at the center of a quartz tube furnace. The furnace was ramped
◦
up to 900 C and maintained the temperature for 2h with an
ambient gas (Ar + O2) held at a constant total pressure of 2 Torr.
Next, the Si substrate with the as-synthesited Bi2O3 nanowires was
transferred to an ALD chamber. Diethylzinc(DEZn) and H2O were
alternately fed into the chamber through separate inlet lines and
nozzles. Typical pulse times were 0.2, 0.2 and 2 s for DEZn feed,
H2O feed, and the reactants purge, respectively. The details of the
sample preparation procedures and the experimental set-ups are
described elsewhere.
The morphology and size of the final products were examined
using scanning electron microscopy (SEM, Hitachi S-4200). Glanc-
◦
ing angle (0.5 ) X–ray diffraction (XRD) analysis was performed
to investigate the phases of the obtained products. High resolution
transmission electron microscopy (HRTEM) and selected area elec-
tron diffraction (SAED) were carried out on Philips CM-200 elec-
tron microscope at an acceleration voltage of 200 kV. The samples
used for characterization were dispersed in absolute ethanol and
ultrasonicated before SEM and TEM observations. The photolumi-
nescence (PL) measurement was conducted at room temperature
by using a He–Cd laser (325 nm) as an excitation source.
◦
Fig. 1. SEM images of Bi2O3 nanowires grown by thermal evaporation at 550
C
with the oxygen partial pressure fixed at 1.5% (a) on (0001) Al2O3 and (b) on (100) Si.
heteronucleation [6], the energy barrier for nucleation decreases
and thus the nucleation rate increases as the wettability between
a condensate and a substrate increases. There must be a factor
in the Bi2O3–Al2O3 binary system other than lattice mismatch
that enhances wettability between Bi2O3 nuclei and the Al2O3
substrate.
3
. Results and discussion
Figs. 1(a) and (b) display SEM images of the as–synthesized
Bi2O3 nanowires on Si(100) and Al2O3(0001) substrates, respec-
tively. We can see a distinct difference between the two SEM im-
ages. The nanowires grown on the Si(100) substrate are much
shorter than those on the Al2O3(0001) substrate. Also many Bi2O3
particles are seen on the Al2O3 substrate surface, which suggests
following two things :
A quantitative analysis of chemical compositions was per-
formed by EDS point scanning on Fig. 2 shows that element Au as
well as Bi and O exist, indicating that the spherical nanoparticles at
the tip of nanowires are composed of Au, Bi, and O components. The
existence of Au in the nanoparticles is an evidence that the growth
of the Bi2O3 nanowire is via VLS mechanism. The growth mecha-
nism of one-dimensional nanostructures could be different for dif-
ferent process parameters. Particularly it depends on the growth
temperature. The VLS mechanism in which ID structures grow with
an assistance of catalysts is dominant at lower temperatures be-
cause thermal energy is not enough for the nucleation and growth
of the structure. In contrast, the VS mechanism is dominant at a
higher temperatures where sufficient thermal energy is provided.
As the process parameters for the Bi2O3 nanowire growth in this
work is basically the same as those in our work for other nanowire
growth, there is no reason why the growth mechanism differs.
The structure and phase purity of the products were examined
by XRD. Fig. 3 shows a typical XRD pattern of the as-synthesized
(
1) Bi2O3 nanowires nucleate and grow as particles at the
substrate surface first and then grow in a length direction.
2) The growth rate of Bi2O3 nanowires is much lower on the
Si(100) substrate than on the Al2O3(0001) substrate.
(
As is well known, thermal evaporation is the most widely used
technique in synthesizing nanowires of various materials. A variety
of oxide nanowires have been reported to be synthesized by using
the thermal evaporation technique, yet there has been almost no
report on the growth of Bi2O3 nanowires by thermal evaporation.
β-Bi2O3 has a tetragonal structure with lattice constants a =
5
.63 Å and c = 7.74 Å whereas Al2O3 has a hexagonal close-packed
structure with lattice constants a = 4.76 Å and c = 12.99 Å.
The mismatch between the c of β-Bi2O3 (5.63 Å) and the a of Si
(
5.43 Å) is only 3.6%, while that between the c of β-Bi2O3 (5.63
◦
Å) and the a of Al2O3 (4.76 Å) is 15.5%. Hence, Al2O3 has much
larger lattice mismatch with β-Bi2O3 than Si. Therefore, Al2O3 has
not been considered as a candidate for a substrate material for
Bi2O3 nanowire growth based on thermal evaporation of bismuth
or bismuth oxide powders, which may be the reason why synthesis
of Bi2O3 nanowires by using a thermal evaporation method has
not been successful yet. It is not understood at the moment why
the growth rate of Bi2O3 nanowires is far lower on the Si substrate
than on the Al2O3 substrate. According to the capillarity theory for
samples at 550 C for 2 h. All the reflection peaks except two
Au-related peaks due to the Au catalyst can be readily indexed
to the tetragonal β-Bi2O3 with lattice constants a = 7.742 Å
and c
=
5.631 Å (JCPDS 78-1793). No other phase except
Au used as a catalyst was detected in Fig. 4 indicating the
high purity of the final products. As is well known, α-Bi2O3 is
the only low temperature phase, while β-, γ -, and δ-Bi2O3 [7]
and ω-Bi2O3 [8] are high temperature structures. It is known
that tetragonal β-Bi2O3 with a distorted defect-fluorite structure

S. Park et al. / Solid State Communications 149 (2009) 315–318
317
Fig. 2. An EDS spectrum at the tip of a Bi2O3 nanowire.
Fig. 4. (a) TEM image, (b) HRTEM image and (c) SAED pattern of a Bi2O3-core/ZnO-
shell nanowire structure.
Fig. 3. The XRD pattern of the Bi2O3 nanowires grown by thermal evaporation on
◦
(
0001) Al2O3 at 550 C at an oxygen partial pressure of 1.5%.
Fig. 4(a) is TEM image of a Bi2O3-core/ZnO-shell nanowire
transforms into stable monoclinic α-Bi2O3 at about 870 K [9].
Formation of tetragonal β-Bi2O3 at 923 K on cooling cubic δ- Bi2O3
has been reported to be observed before [10]. In the case of our
products β-Bi2O3 seems to exist as a metastable phase at room
temperature. The dominant crystal structure of Bi2O3 nanowires
at ambient conditions reported as yet is α-Bi2O3. The Bi2O3
nanowires synthesized by using hydrothermal [11], chemical [12],
metal organic chemical vapor deposition (MOCVD) [13] and
atmospheric pressure CVD techniques [14] was reported to have
an α-Bi2O3 phase, while Bi2O3 nanotubes synthesized by oxidizing
the bismuth nanotubes or nanowires in air [15] and Bi2O3
nanowires synthesized by using an oxidative metal vapor transport
deposition technique [16] to have β-Bi2O3 phase. On the other
hand, Kumari et al. [17] reported that one-dimensional nanowires
and nanoflowers of Bi2O3 synthesized by using an oxidative metal
vapor transport deposition technique had both α- and β-Bi2O3
phases. Judging from these reports and our results in this work,
we may conclude that Bi2O3 nanowires have a stable α-Bi2O3 or
metastable β-Bi2O3 phase depending on the synthesis method or
the synthesis process conditions.
structure prepared by a two-step process consisting of thermal
◦
evaporation of Bi powders in an oxygen atmosphere at 900
C
◦
and atomic layer deposition of ZnO at 170 C using DEZn and
H2O as a precursor of Zn and an oxidant, respectively. This
TEM image clearly displays that a Bi2O3-core with a diameter
of about 130 nm is surrounded by a ZnO-shell with a thickness
of 15–20 nm. The thicknesses of the Bi2O3-core and the ZnO-
shell are uniform all the way along the nanowire length direction.
High-resolution TEM(HRTEM) images (Fig. 4(b)) and selected area
electron diffraction (SAED) patterns (Fig. 4(c)) recorded from the
core layer reveal that an individual Bi2O3 nanowire core is a
single crystal of the tetragonal structure with lattice constants
a = 7.742 Å and c = 5.631 Å and Bi2O3 nanowires grow along
the (101) direction. From the HRTEM image, clear lattice fringes
perpendicular to the nanowire length direction are observed,
indicating that the nanowire is of a single crystalline nature. The
interplanar spacing is 0.32 nm, which is corresponding to the
(201) crystallographic plane of the tetragonal β-Bi2O3 lattice. The
associated SAED pattern (Fig. 4(c)) recorded, with the incident

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S. Park et al. / Solid State Communications 149 (2009) 315–318
peak for an ALD cycle of 20 may be attributed to the spontaneous
emission from the free exciton. With continued increases in ALD
cycle from 20 to 30, 40 and 50 the NBE peak rapidly increases. This
rapid increase in the NBE peak height can be explained by exciton-
exciton scattering process, in which one of the two excitons obtains
energy from the other and scatters into a higher exciton energy
state. The changing trend of PL emission spectrum tells us that it
is not easy to control the wavelength of the emitted light from the
Bi2O3-core/ZnO-shell 1D structure to fall in the range from 450 nm
to 550 nm by changing the number of ALD cycle (or the thickness
of the ZnO-shell). Since the wavelength of the main emission peak
changes very abruptly with an increase in the number of ALD cycle.
4. Conclusions
Highly straight single crystal Bi2O3 nanowires with sizes of
0–300 nm in diameter and 50–100 in length were synthesized
5
on Au-coated c–plane sapphire (Al2O3) substrates by thermal
evaporation using Bi powders. One dimensional(1D) Bi2O3−
core/ZnO-shell structures with uniform layer thicknesses were
subsequently prepared by coating the Bi2O3 nanowires with
ZnO using atomic layer deposition (ALD). The crystalline nature
of the 1D core/shell structure was revealed by high resolution
transmission electron diffraction(SAED). X–ray diffraction(XRD)
results indicate that the Bi2O3-core is a pure tetragonal β-
Bi2O3 phase single crystal, while the ZnO-shell is amorphous.
EDS analysis confirms that the Bi2O3 nanowires grow via
vapor–liquid–solid(VLS) mechanism in thermal evaporation of Bi
powders. The photoluminescence(PL) spectra of the 1D core/shell
structures show that a violet emission at ∼450 nm characteristic
of Bi2O3 changes to UV and green–yellow emissions at 370 and
590 nm, respectively, characteristic of ZnO very abruptly within
10 cycles as the number of ALD cycles increases.
Fig. 5. A PL spectrum of the nanowires grown by thermal evaporation on (0001)
◦
Al2O3 at 550 C at an oxygen partial pressure of 1.5%.
electron beam parallel to the (010) direction and perpendicular to
¯
¯
the nanowire length direction was indexed for the [1 2 2] zone axis
of the crystalline β-Bi2O3. The well defined SAED pattern confirms
that the Bi2O3 nanowire is monocrystalline. Differently from the
Bi2O3-core, the ZnO-shell seems to be amorphous as no fringe
pattern is observed in the ZnO shell area of the TEM image.
Fig. 5 represents the room temperature PL spectra of 1D Bi2O3-
core/ZnO-sell structures. For the 0 cycle of ZnO ALD corresponding
to the Bi2O3 nanowire not coated with ZnO at all, there exist a
broad emission band with a main peak centered around 450 nm
and a shoulder at a bout 425 nm, in the blue region. A 324nm
He–Cd laser was used as an excitation source for the PL. Similar
blue emissions were observed for Bi2O3 nanonods synthesized
by authors using an MOCVD technique and Bi2O3 nanoparticles
synthesized using a microemulsion technique [18]. On the other
hand, there is also a recent report that broad PL peaks centered at
around 588.6 and 598.7 nm obtained for Bi2O3 structures seems
to be related to the electrovalency of Bi ions [19]. It is known that
the emission of Bi ion is assigned to its electrovalency, which arises
from different S-P transitions [20]. However, the luminescence of
Bi ions usually appears in the visible wavelength region such as
Acknowledgment
This work was supported by Korean Science and Engineering
Foundation (KOSEF) through ‘2007 National Research Laboratory
Program’.
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