Composition-Tuned ConSi Nanowires:
Location-Selective Simultaneous Growth
along Temperature Gradient
Kwanyong Seo,†Sunghun Lee,†Hana Yoon,†Juneho In,†Kumar S. K. Varadwaj,†Younghun Jo,‡
Myung-Hwa Jung,§Jinhee Kim,?and Bongsoo Kim†,*
their constituent metal atoms can be par-
tially substituted by other metal atoms
while the same crystal structure is
maintained.1?3Because of this, transition
of physical properties. Fe5Si3, for example,
is a high-temperature ferromagnetic ma-
terial with Curie temperature (Tc) of 381 K,
and Fe3Si shows a half-metallic property
with Tcof 840 K.4?6On the other hand, ter-
nary silicides such as Fe1?xCoxSi exhibit un-
usual positive magnetoresistance (MR) and
a large anomalous Hall effect while display-
ing a helimagnetic ordering.7?9Such diver-
sities in the structures and physical proper-
ties make nanowires (NWs) of transition
terials in the fields of spintronics, thermo-
electrics, nanoelectronics, and field emis-
Most of metal silicide NWs reported so
far were synthesized in the gas phase.15?27
Because we have to maintain both the
metal to silicon ratio and degree of super-
saturation at appropriate values simulta-
neously, changing the composition of metal
silicide NWs in a wide range has been rather
difficult. Finding effective methods to syn-
thesize metal silicide NWs in various com-
positions is, therefore, indispensible for de-
velopment of practical nanodevices
utilizing versatile physical properties of
these NWs. We have not fully understood,
however, the crystal structures and physi-
cal properties of most of the metastable
phases of metal silicides even in a bulk form
because of their structural instability at
cally metastable modifications can be stabi-
ransition metal silicides can form
stable crystal structures over a wide
range of composition, and often
lized in nanostructures,28novel metal sili-
cide structures can be synthesized in the
form of NWs. Hence, synthesizing
composition-tuned metal silicide NWs
would provide opportunities to explore
various materials of novel and interesting
between the crystal structures and the
Here we report the simultaneous and
location-dependent selective synthesis of
ConSi NWs (n ? 1?3) and their correspond-
ing crystal structuresOsimple cubic (CoSi),
orthorhombic (Co2Si), and face-centered cu-
bic (fcc) (Co3Si)Ofollowing a composition
RESULTS AND DISCUSSION
We demonstrated previously that on
the Si substrate at a high temperature only
CoSi NWs are formed by the following reac-
2CoCl2(g) + 3Si(s) T 2CoSi(s) +
SiCl4(g) [on a Si substrate](1)
No other phases of cobalt silicide were ob-
tained on the Si substrate in the above reac-
tion because only the thermodynamically
*Address correspondence to
Received for review February 25, 2009
and accepted April 18, 2009.
Published online April 23, 2009.
10.1021/nn900191g CCC: $40.75
© 2009 American Chemical Society
KEYWORDS: nanowires · transition metal silicides · cobalt silicide · single-
crystalline · crystal engineering · metastable compounds
www.acsnano.orgVOL. 3 ▪ NO. 5 ▪ 1145–1150 ▪ 2009
most stable binary phase is produced in the solid-state
reaction of Si substrate and CoCl2. If we place a sapphire
substrate on top of a Si wafer (Figure 1), however, NWs
of a diverse cobalt?silicon ratio could be formed on
the sapphire substrate since Si atoms are supplied from
the SiCl4vapor instead of the Si substrate, and thus
the reaction follows a different mechanism (reactions 2
6CoCl2(g) + 3SiCl4(g) T 3Co2Si +
12Cl2(g) [on the sapphire 1] (2)
6CoCl2(g) + 2SiCl4(g) T 2Co3Si +
10Cl2(g) [on the sapphire 2] (3)
In this case, the sapphire substrate does not partici-
pate in the reaction but only plays a role as a support-
actions. Identical results were obtained when quartz
plates were employed in place of the sapphire
The main idea of this work is to control the compo-
sition of the cobalt silicide NWs by adjusting the SiCl4
concentration. When the CoCl2concentration is kept
constant in reactions 2 and 3, the cobalt?silicon com-
position ratio is determined by the concentration of
SiCl4, which is produced in reaction 1. The production
the reaction temperature. In our experimental setup,
the temperature inside the furnace increases toward
the center of the downstream zone (Figure 1b), thus the
concentration of SiCl4can be varied by adjusting the
position of the sapphire substrate. When the sapphire
substrate is placed in the lower temperature region
(sapphire 2 in Figure 1c), the Si wafer near the sap-
phire substrate is also at a lower temperature, leading
to less supply of SiCl4gas by reaction 1. In this case, co-
balt silicide NWs with higher cobalt?silicon ratio are
synthesized (reaction 3). On the other hand,
cobalt?silicon ratio of the NWs formed on the sap-
phire 1 substrate is lower than that on the sapphire 2
substrate because of its higher temperature. Hence, co-
balt silicide NWs of different elemental compositions
grow simultaneously on the sapphire substrates placed
along a temperature gradient (Figure 1c).
Figure 2 shows morphologies and elemental com-
positions of the as-synthesized ConSi NWs examined
by field emission scanning electron microscopy (SEM),
transmission electron microscopy (TEM), and energy-
dispersive X-ray spectrometry (EDS). The SEM images
and low-resolution TEM images show NWs with lengths
of tens of micrometers. No secondary growth or extra
structural features were observed. Si concentration in
the NWs grown on the different substrates ranges from
about 50 to 15% by the TEM-EDS study (third column
in Figure 2). These elemental compositions are consis-
tent with the proposed reaction mechanisms. We note
that the density of the NWs decreases when the Co?Si
composition ratio in the NWs increases. This can be ex-
plained by the fact that the nucleation rate of NWs is
proportional to the amount of Si source when the pre-
cursor (CoCl2) concentration is fixed at a constant pre-
cursor temperature. NW growth process on a Si sub-
strate is primarily based on three steps: interdiffusion
of the reactant elements (Si and deposited Co) in the
solid state, nucleation, and growth of the crystalline
product. Only thermodynamically stable CoSi NWs can
grow in the diffusion-limited solid-state reactions at a
low degree of supersaturation.16In this case, the growth
rate along the axial direction would be much faster
than that on the sapphire substrate. This would sup-
press the radial growth, and only thin and long NWs can
grow on the Si substrate. In contrast, the NWs with
higher Co?Si compositions show larger diameters and
more straight morphology, which can be explained by
the gas-phase synthetic process of these NWs. The gas-
phase-based reactions facilitate atomic level mixing of
the precursors in the vapor phase on the sapphire sub-
strate. The precursor vapors would contribute to both
radial and axial growth, hence axial growth competes
with radial growth.29More detailed investigations are
required to explain these observed aspect ratio
TEM analysis of these NWs indicates that three kinds
of NWs, simple cubic CoSi, orthorhombic Co2Si, and fcc
Figure 1. Experimental setup. (a) Horizontal tube furnace with two indepen-
dently controlled heating zones. (b) Temperature profile indicates that the
°C. (c) Tilted view illustration of the substrate placement in panel a. A rectan-
gular Si wafer (50 mm ? 15 mm) kept at the downstream zone played a role of
Si source for NW synthesis. Co2Si NWs are grown on the sapphire 1 sub-
strate, CoSi NWs on the Si, and Co3Si NWs on the sapphire 2.
VOL. 3 ▪ NO. 5 ▪ SEO ET AL.www.acsnano.org
Co3Si, have been synthesized.
CoSi NWs are synthesized on
the Si wafer, as reported previ-
ously, and have a single-
crystalline B20 CoSi struc-
ture.25Only CoSi NWs, the
phase, can grow on a Si wafer
by the solid-phase reactions.
Figure 3 shows the crystal
structures of the NWs with Si
composition of 33%. The se-
lected area electron diffrac-
tion (SAED) study shows a
regular spot pattern (Figure
3a), revealing the single-
crystalline nature of the NWs.
The spots can be fully indexed
to the orthorhombic C37-
type Co2Si and demonstrate
that the NW growth is along
the  direction down the
[1¯30] zone axis. X-ray diffrac-
tion (XRD) data (Pbnm, JCPDS
04-0847)30and a high-
resolution TEM image of a
NW with clear lattice fringes
(Figure 3b) confirm again that
the NWs are composed of
single-crystalline Co2Si. The
lattice spacings of the planes
are measured to be 0.37 and
0.21 nm, agreeing well with
the spacings of the (001) and
(311) planes of an orthorhom-
bic Co2Si structure, respec-
tively. The two-dimensional
fast Fourier transform (FFT) of the lattice-resolved im-
ure 3b) can also be indexed to an orthorhombic
Figure 4 shows the high-resolution TEM image
and SAED patterns from various zone axes of the
NWs with Si composition of 15%. In spite of numer-
ous investigations on the crystallization and struc-
ture formation of Co?Si alloys, the Co-rich struc-
tures (especially over 70 at % Co case) have not
been fully understood. During the cooling process
in the bulk, metastable Co-rich structures are sepa-
rated into many stable phases such as Co and
Co2Si. While we initially considered standard Co
structures of hcp and fcc in order to analyze the
TEM results, we find no similarity between the ob-
served TEM results and the references. The simula-
tion by Malozemoff et al., however, indicated that
the Co3Si structure has almost the same lattice pa-
rameters as those of fcc Fe3Si.31Therefore, we tried
to identify the crystal structure of the Co-rich NWs indi-
rectly with reference to the fcc Fe3Si structure. We took
SAED patterns from the various zone axes to make
more accurate analysis of the crystal structure of the
Figure 2. Change of the morphology and elemental concentrations of Si in the NWs grown at different
growth environments. NWs grown on (a) Si wafer, (d) sapphire substrate 1, and (g) sapphire substrate 2
in Figure 1c. The low-resolution TEM images and TEM-EDS spectra corresponding to (a), (d), and (g) are
shown in the second and third column.
Figure 3. Detailed structural analysis of the NWs with Si concentration of 33 at %,
shown in Figure 2d. (a) SAED pattern of the NW from the [1¯30] zone axis. (b) High-
resolution TEM image. The labeled distances of 0.37 and 0.21 nm correspond to
the (001) and (311) planes, respectively, and the arrow shows  direction. In-
set in (b) shows the FFT.
www.acsnano.orgVOL. 3 ▪ NO. 5 ▪ 1145–1150 ▪ 2009
NWs (Figure 4b?d) and found that all these clear SAED
patterns match well to the fcc Fe3Si structure. Accord-
ing to this analysis, the NWs grow along the  direc-
tion. Interestingly, Si compositions of these Co-rich
NWs are in a broad range from 10 to 20% as revealed
by repeated TEM-EDS measurements, while the NWs
keep the same crystal structure. This result is consis-
tent with the fact that Si concentration in metastable
Co3Si ranges from 10 to 20% in the phase diagram.32On
the other hand, Si compositions for the CoSi and Co2Si
NWs show generally 50 and 33%, respectively, with
much less deviations. The synthesis of Co3Si NW shows
for the first time that single-crystalline Co3Si fcc struc-
ture can exist at room temperature.
We measured the electrical and magnetic proper-
ties of the Co2Si NWs to find out how the physical prop-
erties of NWs vary with the change of composition
and crystal structure of the NWs. Figure 5a shows the
temperature dependence of magnetization measured
at 100 Oe for as-grown Co2Si NW ensemble on a sap-
phire substrate. The high-temperature data are well fit-
ted by the Curie?Weiss law, M ? Cp/(T ? ?p), where Cp
is the Curie constant and ?pis the paramagnetic Curie
temperature. We obtained the ?pvalue of ?366.7 K. We
note that the inverse magnetization starts to deviate
from linearity for T ? Tc? 115 K,
which is attributable to the ferromag-
netic ordering. This ferromagnetic sig-
nature is verified from the magnetic
hysteresis loop (Figure 5b). The mag-
netic moment of the Co2Si NW en-
semble was obtained by subtracting
the diamagnetic contribution from
the sapphire substrate. The magneti-
zation measured below Tcexhibits a
rapid and nonlinear increase at lower
fields and a linear increase at higher
fields. This indicates the ferromag-
netic property of the Co2Si NWs, al-
though the magnetization curve does
not show a typical shape of ferromag-
netic materials. On the other hand,
the magnetization curves measured
above Tcshow almost linear response
to the magnetic field, which is a typi-
cal paramagnetic behavior.
Figure 5c displays electrical trans-
port data from the single NW device
fabricated by standard e-beam lithog-
raphy. The linear current versus volt-
age behavior at room temperature in-
dicates an ohmic contact between
the NW and electrodes. The resistivi-
ties of the two- and four-probe con-
figurations are 546 and 200 ??·cm,
respectively, which match well with
that reported for bulk single-crystal-
Figure 4. Detailed structural analysis of the NWs with Si concentra-
tion of 15 at %, shown in Figure 2g. (a) High-resolution TEM image.
The arrow shows the  direction. Inset in (a) shows the FFT. SAED
patterns of Co3Si NW from the various zone axes: (b) , (c) [11¯2¯],
and (d) [11¯1¯].
Figure 5. Electrical and magnetic properties of Co2Si NWs. (a) Inverse magnetization vs T for
the NW ensemble. Inset shows a plot of M vs T obtained from the NW ensemble at an applied
field of 100 Oe. Arrow indicates the Curie temperature of 115 K. (b) Plot of M as a function of H
set, upper left), with four- (line 1) and two- (line 2) probe measurements, respectively. R vs T
curve shows typical metallic behavior (inset, lower right). (d) MR (%) curves of a single Co2Si NW
at various temperatures.
VOL. 3 ▪ NO. 5 ▪ SEO ET AL. www.acsnano.org
line Co2Si (190 ??·cm).33The higher value of two-
probe resistivity is due to the contact resistance be-
tween the NW and electrodes. The resistance decreases
monotonically with decreasing temperature from room
temperature to 2 K. This is a typical metallic behavior.
Figure 5d shows the standard longitudinal magnetore-
sistance (MR) data measured at various temperatures.
The MR data are taken from a single Co2Si NW device
with four-probe. The MR measured below Tcshows a
small peak structure around 2 T, which corresponds to
the saturation field of magnetization. The MR ratio at 9
T is estimated to be 0.09% at 10 K and 0.04% at 50 K.
The monotonic decrease of MR with increasing field is
attributed to the reduction of magnetic scattering by
magnetic ordering. No MR effect is observed above 50
K. The MR ratio is within our instrumental accuracy
It has been very difficult to synthesize Co2Si even in
bulk due to its thermodynamic instability. While few re-
ports are available referring to the physical properties
of Co2Si, conclusions are rather vague.33?35It is conjec-
tured that the low Tcand weak ferromagnetic proper-
ties of Co2Si measured in this work are attributable to
the orthorhombic crystal structure.36,37Simple cubic
CoSi is normally diamagnetic. For CoSi NWs with diam-
eters less than 30 nm, the high percentage of surface
Co atoms induces strong ferromagnetic properties of
CoSi NWs,25which could not be confirmed in the case
of Co2Si NWs that are about 80 nm thick. Measurements
of the physical properties of Co3Si NWs are underway.
We have reported simultaneous synthesis of high-
density freestanding single-crystalline ConSi NWs (n ?
1?3) on different substrates. Composition-tuned selec-
tive growth of cobalt silicide NWs is achieved by plac-
ing sapphire substrates along a temperature gradient.
TEM-EDS studies of the NWs indicated that Si concen-
tration in the NWs grown on the series of substrates
ranges from about 50 to 15%. Detailed crystal struc-
tures of the NWs are analyzed by high-resolution TEM,
revealing that three kinds of NWs, simple cubic CoSi,
orthorhombic Co2Si, and fcc Co3Si, have been synthe-
sized. We have further reported that the crystal struc-
ture of the NWs affects the physical properties of the
NWs indicated by the electrical and magnetic proper-
ties measurements on the Co2Si NWs. These results in-
dicate the possibility that metal silicide NWs of diverse
electrical and magnetic properties can be synthesized
by tuning the composition.
Growth of ConSi Nanowires. Single-crystalline CoSi, Co2Si, and
Co3Si NWs were synthesized in a horizontal hot-wall two-zone
furnace with a 1 in. diameter inner quartz tube, as shown in Fig-
ure 1a. The setup is equipped with pressure and mass flow con-
trollers. The upstream (US) zone and downstream (DS) zone were
used for vaporization of precursor and NW growth, respectively.
the source of Si. The Co2Si and Co3Si NWs were grown on c-plane
sapphire substrates placed on the Si wafer. This scheme pro-
vides convenient control of the Si composition in the NW. Anhy-
drous CoCl2powder (99.999%, Sigma-Aldrich) in an alumina
boat was placed at the center of the US zone. The carrier argon
gas was supplied through a mass-flow controller at the rate of
150?200 sccm. The substrates were placed at ?10 cm from the
precursor position in the DS zone. Temperatures of the US zone
and DS zone were maintained at 610 and 900 °C, respectively, for
20 min of reaction time, while the pressure was maintained at
500 Torr during the reaction. No catalyst was used for the NW
Characterization. X-ray diffraction (XRD) patterns of the
specimen were recorded on a Rigaku D/max-rc (12 kW) dif-
fractometer operated at 40 kV and 80 mA with filtered
0.15405 nm Cu K? radiation. Field emission scanning elec-
tron microscope (FESEM) images of ConSi NWs were taken on
a Phillips XL30S. Transmission electron microscope (TEM)
and high-resolution TEM (HRTEM) images and selected area
electron diffraction (SAED) patterns were taken on a JEOL
JEM-2100F transmission electron microscope operated at 200
kV. After nanostructures were dispersed in ethanol, a drop
of the solution was put on the holey carbon coated copper
grid for the preparation of TEM analysis.
Acknowledgment. This research was supported by KOSEF
through NRL (ROA-2007-000-20127-0), SRC (R11-2005-008-
00000-0), and CNMT (08K1501-02210) from MEST, Korea. M.-H.J.
is supported by the Sogang University Research Grant
Supporting Information Available: XRD spectrum of the Co2Si
NWs on c-plane sapphire substrate. This material is available
free of charge via the Internet at http://pubs.acs.org.
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