Antimony nanowires self-assembled from Sb nanoparticles

Article abstract of DOI:10.1021/jp047375h

For the first time, we introduced a simple method of synthesizing segregated thin antimony nanowires based on the principle that nanoparticles can spontaneously self-assemble into crystalline nanowires (a??20 nm) in the absence of any rigid templates at room temperature. By collecting electron energy loss spectra from individual Sb nanowires with different diameters, we investigated the effect of nanowire diameter on plasmon excitations in Sb nanowires. As the diameter of Sb nanowire decreases, we find that the peak energy of surface plasmon shifts toward the lower energy.

Full text of DOI:10.1021/jp047375h

J. Phys. Chem. B 2004, 108, 16723-16726
16723
Antimony Nanowires Self-Assembled from Sb Nanoparticles
Ye Wu Wang,† Byung Hee Hong,† Ju Young Lee,† Jeong-Sun Kim,‡ Geun Hong Kim,‡ and
Kwang S. Kim*,†
National CreatiVe Research InitiatiVe Center for Superfunctional Materials, Department of Chemistry, DiVision
of Molecular and Life Sciences, Pohang UniVersity of Science and Technology, San 31, Hyojadong,
Namgu, Pohang 790-784, Korea, and Agency for Defense DeVelopment, P.O. Box 35-5,
Yuseong, Daejeon 305-600, Korea
ReceiVed: June 17, 2004; In Final Form: August 20, 2004
For the first time, we introduced a simple method of synthesizing segregated thin antimony nanowires based
on the principle that nanoparticles can spontaneously self-assemble into crystalline nanowires (∼20 nm) in
the absence of any rigid templates at room temperature. By collecting electron energy loss spectra from
individual Sb nanowires with different diameters, we investigated the effect of nanowire diameter on plasmon
excitations in Sb nanowires. As the diameter of Sb nanowire decreases, we find that the peak energy of
surface plasmon shifts toward the lower energy.
Introduction
Of special interest for studies of electronic properties is the
electron energy loss spectroscopy (EELS) technique, which is
One-dimensional (1D) nanostructured materials including
nanowires, nanorods, nanobelts, and nanotubes have received
much attention due to their peculiar physical properties and
potential applications as interconnect and functional units in
fabricating electronic, optoelectronic, electrochemical, thermo-
electric, and electro-mechanical nanodevices.1,2
useful for probing collective excitations of electrons (plasmons).
In dimensionally restricted materials, the position and width of
the plasmon peak provide an evidence of the influence of
quantum confinement and surface effect on charge carriers.9 In
this paper, to observe the effect of nanowire diameter on
plasmon excitations in Sb nanowires, we collected EELS spectra
from individual Sb nanowires with different diameters. It has
been found that as the diameter of Sb nanowire decreases, the
peak energy of surface plasmon shifts toward the lower energy.
In particular, semimetallic nanowires including bismuth,
antimony, and bismuth-antimony alloys are of interest because
of their small effective mass and large mean-free path, which
make these nanowires become an interesting system for studying
quantum confinement effects.3 In addition, these nanomaterials
have been suggested to be responsible for the enhancement of
thermoelectric properties at room temperature.4 Recently, a few
studies have focused on these semimetallic nanowire arrays
prepared in anodic alumina membrane by using the vapor phase
deposition and pulsed electrodeposition techniques,5 and the
solvothermal process has also been employed to prepare bismuth
nanowires and nanotubes in the absence of rigid template.6 These
fabrication methods lead to Sb nanowires with large diameters,
which are often in the form of aggregated bundles. Defects on
the nanochannel surfaces of rigid template may lead to the
formation of poorly defined nanowires that often exhibit kinks
and uneven thickness. To measure the absolute resistivity of
an individual nanowire with uniform diameter, it is desirable
to remove the templates to recover the individual nanowires.7
To avoid the template removal step, several chemical approaches
have recently been explored to directly obtain individual
nanowires for measuring their absolute resistivity.8 Thus, it is
of importance to find mild, simple, and low-cost routes to
directly synthesize bare Sb nanowires. Here, for the first time,
we introduce a simple method to synthesize segregated thin bare
antimony nanowires. The antimony nanoparticles can spontane-
ously self-assemble into thin crystalline nanowire (∼20 nm) in
the absence of any rigid templates at room temperature.
Experimental Section
All the chemical reagents used in this experiment were
analytical grade. The detailed synthetic procedure of Sb nano-
wires with diameters about 20 nm is as follows: 0.5 mmol of
PVP (Mw ) 55 000) and 0.05 mmol of SbCl3 were dissolved
in 50 mL of N,N-dimethylformamide (DMF), which was stirred
for 15 min. Then, 0.25 mL of 1.8 M NaBH4 aqueous solution
was added all at once into the mixed solution of PVP and SbCl3
with constant stirring. Within a few minutes, the colorless
solution darkened to a brown color. After stirring for 10 min,
the solution was allowed to age in darkness at room temperature
for 8 days. When the molar ratio between the PVP and SbCl3
increased to 100, the Sb nanowires with large diameters (300
nm) were synthesized by using the same procedure.
The synthesized Sb nanowires were characterized using a
high-resolution transition electron microscope (HRTEM, JEOL-
2010F), EELS attached to HRTEM, and field-emission scanning
electron microscope (FE-SEM, JEOL 6400). For SEM observa-
tion, the solution aged for 8 days was centrifuged. Then, the
obtained precipitate containing Sb nanowires was redispersed
in a small amount of DMF and dropped onto silicon substrate
for SEM analysis.
Results and Discussion
† Pohang University of Science and Technology, Korea.
‡ Agency for Defense Development, Korea.
* Corresponding author: Tel +82-54-279-2110; Fax +82-54-279-8137;
e-mail kim@postech.ac.kr.
The synthesis of Sb nanowires began with the preparation of
Sb nanoparticles, with subsequent aging treatment of the
synthesized Sb nanoparticle solution at room temperature for
10.1021/jp047375h CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/30/2004

16724 J. Phys. Chem. B, Vol. 108, No. 43, 2004
Wang et al.
Figure 1. TEM images of Sb nanoparticles. (a) Low-magnification
TEM image of Sb nanoparticles in starting solution. The inset shows
the selected area electron diffraction pattern of these nanoparticles. (b)
High-resolution TEM image of a 4 nm nanoparticle.
Figure 3. (a) Electron energy loss spectrum taken from Sb nanowire
with diameter about 300 nm and its Gaussian-fit band with five peaks
(1, 2, 3, 4, 5). (b) Series of electron energy loss spectra taken from Sb
nanowires with nominal diameters of 300 and 20 nm.
in Figure 2. The FE-SEM observation (Figure 2a) reveals that
the synthesized products consisted of a large quantity of
nanowires with typical length of several micrometers. The
isolated single Sb nanowires with a few micrometers in length
were easily found in TEM images shown in Figure 2b,c. The
inset in Figure 2c shows high magnification of the single Sb
nanowire with diameter about 20 nm. A HRTEM image of this
single nanowire (Figure 2c) is shown in Figure 2d, which
provides further insight into the structure of Sb nanowires.
Dislocations and stacking faults were rarely observed in the
HRTEM image. The spacing of the lattice fringes was found to
be about 0.37 nm, which indexes to the (003) spacing of
rhombohedral Sb.10 The longitudinal axis of the nanowire is
perpendicular to the 003 direction of the Sb crystal lattice.
These results indicate that individual Sb nanowires are practi-
cally defect-free single crystals.
A representative EELS spectrum from the Sb nanowire with
diameter of about 300 nm is shown in Figure 3a. Since the peaks
in this spectrum were overlapped and some of the peaks were
somewhat obscured by the tail of nearby higher peaks, we fit
the spectrum to the sum of peaks. After decomposing spectrum
into five peaks by the best fitting with Gassian function, except
the zero-loss peak (1), we can easily identify an Sb surface
plasmon at about 10 eV (peak 2), the volume plasmon at about
16 eV (peak 3),11 and a carbon film bulk plasmon at about 22
eV (peak 4).12 One weak higher-energy loss peak is at about
32 eV (peak 5), and it is identified as the ionization edge.
Figure 2. SEM and TEM images of Sb nanowires after aging the
starting solution for 8 days: (a) FE-TEM image of the synthesized Sb
nanowires; (b, c) typical TEM images of isolated single Sb nanowires
with inset (c) showing the high-magnification image of the nanowire;
(d) HRTEM image of the Sb nanowire (c).
about 8 days. To investigate the growth mechanism of Sb
nanowires, we aged the solution containing Sb nanoparticles
with different durations. The TEM image of Sb nanoparticles
in the starting solution is shown in Figure 1a. The average size
of nanoparticles is about 4 nm. The inset in Figure 1a shows
the selected area electron diffraction pattern of these nanopar-
ticles, indicating that the Sb nanoparticles possess high crystal-
linity and single rhombohedral phase. The diffraction rings of
this pattern are (003), (012), and (110) (from inner to exterior).
The HRTEM image (Figure 1b) of a Sb nanoparticle exhibits
fringes (d ) 0.18 nm) that index to the (006) spacing of
rhombohedral Sb, according to JCPDS card no. 85-1324.10
After aging the starting solution for 8 days, single crystalline
Sb nanowires were formed, and their morphologies are shown

Antimony Nanowires
J. Phys. Chem. B, Vol. 108, No. 43, 2004 16725
driving force of nanoparticle self-organization. In the present
case, one possible function of PVP is to kinetically control the
growth rates of the different crystalline planes by interacting
with these planes through adsorption and desorption, which is
related to the “poisoning” mechanism,19-21 leading to a high
anisotropic growth. The presence of short nanorod and linear
nanostructures in Figure 4 indicates that the nanowires formed
not through point-initiated vectorial growth but rather by the
recrystallization of multiple nanoparticles in linear aggregate
that fused gradually into one crystal. Therefore, with the help
of the appropriate concentration of PVP and the strong metallic
bonding between the Sb nanoparticles, these nanoparticles
coalesced and aggregated orientationally to form linear nano-
structures in aging process22 and then recrystallized into
anisotropic crystal, i.e., nanowires in the following aging stage.
Conclusions
Figure 4. TEM image of intermediate state (by aging the starting
solution for 4 days) of Sb nanoparticle-nanowire transition. Nanorod
and nanorod-like structures of aggregated Sb nanoparticles are marked
by A and B arrows, respectively.
In conclusion, we reported in this paper a simple method to
prepare Sb nanowires. It has been found that the peak energy
of surface plasmon significantly shifts toward the lower energy
with the decrease in diameter of Sb nanowire. We believe that
the present method would be extended to synthesize Sb alloys
or other semimetal nanowires by choosing appropriate concen-
tration of PVP and aging duration.
EELS spectra collected from Sb nanowires with two different
diameters, nominally 300 and 20 nm, are shown in Figure 3b.
In this figure, the energy of the surface plasmon peak shifts
(from 10 to 7.5 eV) toward the lower energy, as the diameter
of Sb nanowires decreases. This red shift is similar to the result
of the theoretical calculation for the surface plasmon in metal
clusters due to the quantum size effect and the large ratio of
surface to volume.13 In metal nanowires, it is well-known that
the surface-to-volume ratio also becomes significant as the
nanowire diameters become very small. Therefore, when the
diameter of Sb nanowire changes from 300 to 20 nm, the peak
energy of surface plasmon shifts toward the lower energy.
To investigate the intermediate steps for transition from
nanoparticles to nanowires, the aliquots of the solution in the
early stages of the synthesis process (first 4 days) were examined
by TEM (Figure 4). It can be seen that a Sb nanorod (denoted
as A) has formed, and some Sb nanoparticles orientationally
aggregate along the short nanorod to form linear nanostructure.
In other parts of this figure, some nanoparticles also aggregate
orientationally and form linear nanostructures (denoted as B).
In the continued aging process, these linear nanostructures
recrystallize into nanowire crystals. Thus, the oriented aggrega-
tion of Sb nanoparticles seems to be the prerequisite leading to
crystalline fusion in our experimental conditions.
Acknowledgment. This work was supported by KOSEF/
CRI and BK21.
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