Preparation and photoluminescence of single-crystal zinc selenide nanowires

Article abstract of DOI:10.1016/S0009-2614(03)01197-7

Single-crystal ZnSe nanowires have been synthesized via a thermochemical method in a N₂ atmosphere with CO and H₂ gases. The as-prepared ZnSe nanowires have a mean diameter of 40 nm and a length of 1 μm. The nanowires are single crystals with a hexagonal structure growing along the [001] direction. A self-catalyzed vapor-liquid-solid process is proposed for the formation of such nanowires. The as-prepared nanowires show two emission bands at ca. 447 and 617 nm.

Full text of DOI:10.1016/S0009-2614(03)01197-7

Chemical Physics Letters 377 (2003) 367–370
www.elsevier.com/locate/cplett
Preparation and photoluminescence of single-crystal
zinc selenide nanowires
*
Ying-Chun Zhu , Yoshio Bando
Advanced Materials Laboratory, National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
Received 4 June 2003; in final form 8 July 2003
Published online: 30 July 2003
Abstract
Single-crystal ZnSe nanowires have been synthesized via a thermochemical method in a N2 atmosphere with CO and
H2 gases. The as-prepared ZnSe nanowires have a mean diameter of 40 nm and a length of 1 lm. The nanowires are
single crystals with a hexagonal structure growing along the [0 0 1] direction. A self-catalyzed vapor–liquid–solid process
is proposed for the formation of such nanowires. The as-prepared nanowires show two emission bands at ca. 447 and
617 nm.
Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction
to the unique optical and electronic properties
demonstrated by these quantum-confined nano-
structures such as quantum dots (0D), quantum
wires (1D), and quantum sheets (2D) [7]. 1D
semiconductor nanostructures are of special in-
terest in the assembly of nanodevices, such as
logic circuits, nanosensors, nanolasers etc. [8–10].
1D nanoscale materials have been shaped
into various morphologies, such as nanotubes,
nanocables, nanowires, and nanobelts [11–13].
Many attempts have been made to prepare ZnSe
nanostructures and study their optical and elec-
tronic properties. ZnSe nanocrystallites have been
synthesized using surfactant-assisted chemistry
methods [14,15], a sonochemical method [16],
solvothermal routes [17], and a vapor-phase syn-
thesis etc. [18]. Most of these researches are fo-
cused on ZnSe nanoparticles, while there are few
reports on the preparation and characterization
Selenides as wide-band-gap II–VI semiconduc-
tor materials have been studied extensively due to
their wide applications in the fields of light-emit-
ting devices, solar cells, sensors, and optical re-
cording materials [1–4]. ZnSe is one of the
promising materials for fabrication of light-emit-
ting devices, such as blue–green laser diodes [1,2],
and turnable mid-IR laser sources for remote-
sensing applications [5]. ZnSe also has potential
application in optically controlled switching due to
its giant photoresistivity [6].
Nanoscale semiconductor materials represent
an area of ever-increasing research interest, due
* Corresponding author. Fax: +81-298-51-6280.
E-mail address: yingchunzhu@yahoo.com (Y.-C. Zhu).
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0009-2614(03)01197-7

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Y.-C. Zhu, Y. Bando / Chemical Physics Letters 377 (2003) 367–370
3. Results and discussion
of one-dimensional ZnSe nanostructures [6,19,20].
ZnSe nanowires or rods have been synthesized by
using a template method [6], a solvothermal
method [19] and vapor phase growth method [20].
In this Letter, we report the preparation, struc-
tural characterization, and the photoluminescence
(PL) property of zinc selenide nanowires by using
zinc selenide powders as precursors via a ther-
mochemical process. This process involves a self-
catalyzed vapor–liquid–solid (VLS) mechanism
without using additional catalysts such as gold
etc. [20], which means a low cost and high purity
of the products.
The TEM analysis shows that as-prepared
nanowires have a uniform morphology with a
mean diameter of 40 nm and a length of 1 lm, as
shown in Fig. 1. Fig. 2 shows the HRTEM image
and electron diffraction (ED) pattern of a ZnSe
nanowire. The ED pattern can be indexed as a
hexagonal phase with lattice constants of a ¼ 0:40
nm and c ¼ 0:65 nm, recorded from [1 )1 0] zone
axis. The measuring d-spacings of (1 1 0) and (0 0 2)
planes are 0.20 and 0.33 nm, respectively, which
are consistent with the ideal values of wurtzite
ZnSe (JCPDS Card: 80-0008). The axis direction
of the as-prepared ZnSe nanowires is generally
along [0 0 1], as confirmed by the ED pattern and
2. Experimental
ZnSe nanowires were prepared according to the
method described in our previous work [21]. An
induction furnace was used for the synthesis of
ZnSe nanowires. Graphite powders and fibers
were put into a graphite crucible, above which the
other crucible containing zinc selenide powders
was arranged. All the components were enclosed in
a graphite susceptor, and the susceptor was put in
the center of the furnace with a flowing N2 at-
mosphere (1.5 l/min). The temperature of the sus-
ceptor was controlled to ensure that graphite
powders were heated to 1600 °C, while the zinc
selenide powders were heated to 1250 °C. The N2/
H2O stream (obtained from bubbling N2 through
distilled water) was managed to pass through and
react with the graphite powders at 1600 °C for the
generation of CO and H2 gases. Next, the N2
stream, together with the CO and H2 gases, passed
through zinc selenide powders. After the process
was conducted for 2 h, the furnace was cooled
down. A yellow product was obtained after the
synthesis. After the product was dispersed into
ethanol, it was transferred onto a copper mesh.
The specimen was then characterized using a JEM-
3000F (JEOL) field emission high-resolution
transmission electron microscope (HRTEM) op-
erated at 300 kV, which was attached with an en-
ergy dispersive X-ray spectrometer (EDS). PL
measures were taken at room temperature using a
He–Cd laser line of 325 nm as the excitation
source.
Fig. 1. TEM morphology of ZnSe nanowires.
Fig. 2. HRTEM images of ZnSe nanowires. The inset is the
corresponding ED pattern with [1 )1 0] incidence. The d-spac-
ings of 0.33 and 0.20 nm correspond to the (0 0 2) and (1 1 0)
planes.

Y.-C. Zhu, Y. Bando / Chemical Physics Letters 377 (2003) 367–370
369
HRTEM image. The perfect diffraction pattern
and lattice fringes indicate that the nanowire is a
single crystal.
such as gold metal etc., as reported in many cases
[20,21]. The overall process of the formation of
ZnSe nanowires may involve the following reac-
tions
A VLS mechanism is proposed for the forma-
tion of ZnSe nanowires, as confirmed by the evi-
dence shown in Fig. 3. Fig. 3a shows the HRETM
image of ZnSe nanowires in the intermediate
growth stage. The columns of the nanowires are
crystal ZnSe, which are composed of Zn and Se
with a ratio of 1:1, as confirmed by the EDS in
Fig. 3b. The tops of the nanowires have hemi-
sphere morphology. The hemisphere is an amor-
phous phase and is mainly composed of Zn, as
confirmed by the EDS in the inset of Fig. 3b. The
EDS is performed with a nanobeam of 0.6 nm. The
formation process of the ZnSe nanowires is a self-
catalyzed process, without additional catalysts
ZnSe $ ZnSe
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ðsÞ
ðgÞ
ZnSe $ Zng þ Se
ðgÞ
ðgÞ
ZnSeðg;sÞ þ H2 $ ZnðgÞ þ H2Se
Zn ! Zn
ðgÞ
ðsÞ
The ZnSe powders evaporated into vapors,
which may be further decomposed into Zn, Se, and
other clusters during the heating process. Zn va-
pors were solidified to form liquid Zn-rich nano-
droplets in the low temperature regions due to the
low melting point of Zn (ca. 410 °C) as shown in
Fig. 3a. The Zn-rich droplets act as a catalyst for
the formation of ZnSe nanowires. The Zn, Se and
other gaseous clusters were dissolved into the
nano-droplets forming eutectic composites. When
ZnSe was supersaturated in the droplet, a ZnSe
nanowire grew out of the droplet. The growth axis
of ZnSe nanowires is along the [0 0 1] direction,
which is consistent with its crystallographic char-
acteristics. The hexagonal ZnSe crystal can be re-
garded as a stacking of {ZnSe4} tetrahedra by
sharing their common corners. In the [0 0 1] di-
rection of the hexagonal ZnSe crystal, each tetra-
hedron has a corner that favors a rapid growth due
to its stronger bond force comparing with the at-
oms at other positions [22,23]. The atmosphere is
an important factor for the formation of sulfides
and selenides nanostructures [24–26], because
nanoscaled sulfides and selenides are easily oxi-
dized. In this work, the atmosphere was controlled
by mixing N2 with reductive agents CO and H2
produced in situ.
The as-prepared ZnSe nanowires show strong
emission at room temperature, as shown in Fig. 4.
The PL spectrum of ZnSe nanowires presents two
emission bands around 447 and 617 nm when ex-
cited at 325 nm. The emission at 447 nm is very
weak comparing with that at 617 nm. The weak
emission band around 447 nm is usually attributed
to the edge emission of ZnSe [27]. The strong
orange emission at about 617 nm is associated with
Fig. 3. (a) HRTEM images and (b) EDS of ZnSe nanowires in
the intermediate growth stage. The column of the nanowires is
single-crystal ZnSe, the top is Zn-rich amorphous phase which
is in liquid state during the growth stage of ZnSe nanowires.

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Y.-C. Zhu, Y. Bando / Chemical Physics Letters 377 (2003) 367–370
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Acknowledgements
This work was supported by the Japan Science
Promotion Society (JSPS) fellowship at the Na-
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