Thermal properties of Bi nanowire arrays with different orientations and diameters

Article abstract of DOI:10.1021/jp0652493

The thermal properties of single-crystalline Bi nanowire arrays with different orientations and diameters were studied by differential scanning calorimeter and in situ high-temperature X-ray diffraction. Bi nanowires were fabricated by a pulsed electrodeposition technique within the porous anodic alumina membrane. The relationships between the orientation and diameter of Bi nanowires and the corresponding thermal properties are deduced solely from experimental results. It is shown that the melting point decreases with decreasing nanowire diameter, and there is an anisotropic thermal expansion property of Bi nanowires with different orientations and diameters. The transition of the thermal expansion coefficient from positive at low temperature to negative at high temperature for Bi nanowire arrays was analyzed and discussed.

Full text of DOI:10.1021/jp0652493

J. Phys. Chem. B 2006, 110, 26189-26193
26189
Thermal Properties of Bi Nanowire Arrays with Different Orientations and Diameters
Yonggang Zhu, Xincun Dou, Xiaohu Huang, Liang Li, and Guanghai Li*
Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanostructure Institute of
Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P. R. China
ReceiVed: August 14, 2006; In Final Form: September 29, 2006
The thermal properties of single-crystalline Bi nanowire arrays with different orientations and diameters were
studied by differential scanning calorimeter and in situ high-temperature X-ray diffraction. Bi nanowires
were fabricated by a pulsed electrodeposition technique within the porous anodic alumina membrane. The
relationships between the orientation and diameter of Bi nanowires and the corresponding thermal properties
are deduced solely from experimental results. It is shown that the melting point decreases with decreasing
nanowire diameter, and there is an anisotropic thermal expansion property of Bi nanowires with different
orientations and diameters. The transition of the thermal expansion coefficient from positive at low temperature
to negative at high temperature for Bi nanowire arrays was analyzed and discussed.
TABLE 1: Deposition Parameters of Bi Nanowire Arrays
with Different Orientations
Introduction
Nanowires are very interesting building blocks for nanoscale
devices.1 Devices and components such as field effect transistors,
decoders, inverters, UV sensors, LEDs, lasers, chemical sensors,
and biosensors2-10 based on nanowires have been demonstrated.
The nanowires could be used as components to create electrical
circuits that are capable of being formed into extremely small
circuits.11 The exact knowledge of thermal properties of
nanowires will be very important for application in nanowire
devices.
The semimetal Bi is often selected for the study of funda-
mental quantum transport phenomena due to the very small
electron density, highly anisotropic Fermi surface, small effec-
tive carrier masses, and long carrier mean-free path.12,13 The
electronic, magnetic, and thermoelectric properties of Bi nanow-
ires have been extensively studied,14-17 but the thermal proper-
ties of Bi nanowires are still widely unexplored. The physical
properties of Bi nanowires strongly depend on not only the
diameter but also the orientation of nanowires as well. Previous
experimental studies focused mainly on the effect of diameter,
and no report was found on the orientation of Bi nanowires. In
our previous report the lattice parameter and thermal expansion
coefficient of [110]-oriented Bi nanowires with a diameter
ranging from 10 to 250 nm have been reported.18 In this paper,
as an extension of our previous study, we report the thermal
properties of Bi nanowire arrays with different orientations and
diameters.
voltage (V)
pulsed time (ms)
duty (%)
orientation
-1.5
-1.9
-1.1
80
50
40
75
75
25
[104]
[110]
[202]
serve as the cathode in a two-electrode plating cell, and a
graphite plate was used as the counter electrode.
The electrolyte for the deposition of Bi nanowires contains a
mixture of BiCl3 (40 g/L), tartaric acid (50 g/L), glycerol (100
g/L), NaCl (70 g/L), and HCl (1 mol/L). The pH of the
electrolyte was adjusted to about 0.9 by adding appropriate
amounts of aqueous ammonia (5 mol/L) in order to avoid
corrosive attack on the AAM. The pulsed electrodeposition was
carried out under modulated voltage control, and detailed
preparation conditions of Bi nanowires with different orienta-
tions can be found in Table 1.
The growth orientation of Bi nanowires was characterized
using a X-ray diffractometer (Philips X’Pert) with Cu KR1
radiation. The morphologies of the Bi nanowire arrays were
observed with field-emission scanning electron microscopy (FE-
SEM, JEOL JSM-6700F). For X-ray diffraction (XRD) mea-
surements the overfilled nanowires on the surface of the AAM
template were mechanically polished away. The melting point
of Bi nanowire arrays was measured by differential scanning
calorimeter (Pyris Diamond DSC). In situ high-temperature
XRD measurement was performed in the temperature range from
300 to 500 K under a high-vacuum atmosphere for the Bi
nanowires inside the AAM. Temperatures were kept constant
at each point for 20 min before measurement, and scans were
carried out for 20° < 2θ < 80°. For SEM observations, the
AAM was partly dissolved with 5 wt % NaOH solution and
then carefully rinsed with deionized water several times.
Experimental Section
The anodic alumina membrane (AAM) was prepared using
a two-step anodization process as described previously.19-23
After the second anodization, the AAM was etched by saturated
SnCl4 solution to remove the remaining aluminum. The alumina
barrier layer was then dissolved in a 5 wt % H3PO4 solution at
36 °C for 40 min. The AAMs with pore sizes of about 12, 36,
and 60 nm were prepared. Finally, a layer of Au film (about
200 nm in thickness) was sputtered on one side of the AAM to
Results and Discussion
The XRD patterns of Bi nanowires with different orientations
and diameters are shown in Figure 1. It can be seen that all
diffraction peaks can be indexed to the rhombohedral Bi (JCPDS
No. 85-1331). The sharp and narrow XRD peaks indicate that
* To whom correspondence should be addressed. Fax: +86-551-
5591434. E-mail: ghli@issp.ac.cn.
10.1021/jp0652493 CCC: $33.50 © 2006 American Chemical Society
Published on Web 11/30/2006

26190 J. Phys. Chem. B, Vol. 110, No. 51, 2006
Zhu et al.
Figure 3. DSC traces of Bi nanowire arrays with different diameters
together with those that form empty AAM. (inset) Those with the same
diameter but different orientations. The curves were shifted in the Y
axis direction for clarity.
Figure 1. XRD patterns of Bi nanowire arrays with different
orientations and diameters: (1) 12, (2) 36, and (3-5) 60 nm.
Figure 4. High-temperature XRD patterns of Bi nanowire arrays with
a diameter of 12 nm at different temperatures.
Figure 3 shows heating measurement DSC curves of Bi
nanowire arrays with different diameters. There is one endo-
thermic peak located about the melting point of Bi that appears
in each curve, and the peak is related to the melting fusion of
Bi nanowires. The melting points of Bi nanowires with 12, 36,
and 60 nm in diameter are found to be 536, 539, and 542 K,
respectively, which all are lower than that of bulk Bi (544 K).
The fact that the melting point of Bi nanowires with the same
diameter but different orientation nearly has the same value,
see the inset in Figure 3, indicates that the melting point of Bi
nanowires is size dependent and independent of the orientation
and shifts to low temperature with decreasing diameter. The
difference in the heat of fusion with different diameters is
considered the main reason for the size-dependent melting
point.24 The size-dependent melting point also has been observed
in Zn nanowire arrays.25 Being embedded inside the AAM, the
thermal stress caused by the different thermal expansion
coefficient between the nanowires and AAM will affect the
melting point of the nanowires; however, these factors do not
affect the comparison of the relative melting points of the
nanowires with different diameters.
Figure 2. FESEM photographs of Bi nanowire arrays with different
diameters: (a) 36 and (b) 60 nm.
the nanowires have good crystalline order, and the peaks at about
2 θ ) 38.1°, 39.7°, and 48.9° are very strong as compared with
other peaks, indicating a highly preferential orientation of the
nanowires, respectively, along the [104], [110], and [202]
direction. No position shift of the XRD diffraction peak was
found for Bi nanowires with the same orientation but different
diameters. Clearly, the five Bi nanowire arrays can be divided
into two groups, one with the same orientation but different
diameters (curves 1-3 in Figure 1) and another with the same
diameter but different orientations (curves 3-5 in Figure 1).
The FE-SEM images of Bi nanowires are shown in Figure
2. One can see that high filling, ordered, and uniform Bi
nanowires were produced in the AAMs. Complete filling of Bi
nanowires into the pores of AAM can be clearly observed in
Figure 2b after AAM was partly dissolved.
Figure 4 shows XRD patterns of the Bi nanowire array with
a diameter of 12 nm at different temperatures, which shows
that though there are delicate changes in the peak shape, the Bi
nanowires preserve a rhombohedral lattice structure as compared
with the standard diffraction of bulk bismuth, and no structure

Thermal Properties of Bi Nanowire Arrays
J. Phys. Chem. B, Vol. 110, No. 51, 2006 26191
Figure 6. Temperature dependences of lattice parameter (a) and
thermal expansion coefficient (b) of Bi nanowires with the same
orientation but different diameters.
Figure 5. Temperature dependences of lattice parameter (a) and
thermal expansion coefficient (b) of Bi nanowires with the same
diameter but different orientations.
change has happened in the entire measuring temperature range.
It is worth noting that the sample temperature is generally lower
than the measured temperature due to the sample setup in the
XRD measurement. Nevertheless, this factor does not affect the
change trend of the measured lattice parameter with temperature.
The existence of the XRD trace at 538 K might indicate this
temperature difference, though the temperature is higher than
the melting point obtained by DSC measurement.
The lattice parameter, d, can be calculated from the XRD
pattern and Bragg equation, 2d sin θ ) λ. The exact position
of the XRD peak of Bi nanowires at each temperature has been
calibrated against the XRD peak taken on Pt. After fitting the
experimental data with the fourth-order polynomial, d )
4
n)0
∑
anTn, the obtained coefficients, an, can be used to
calculate the thermal expansion coefficient. The thermal expan-
Figure 7. Temperature dependences of lattice parameter of Bi
nanowires with a diameter of 60 nm measured at the first (a) and second
time (b).
sion coefficient is defined by26
1 ∂d
1
R )
)
(a1 + 2a2T + 3a3T2 + 4a4T3)
(1)
TABLE 2: Lattice Parameter of Bi Nanowires at Room
Temperature Before and After High-Temperature XRD
Measurements
d0 ∂T d0
Figure 5 shows temperature dependences of lattice parameter
and thermal expansion coefficient of Bi nanowire arrays with
the same diameter (60 nm) but different orientations. One can
see that the lattice parameter nonlinearly depends on temperature
and increases with temperature at low temperature after reaching
a maximum value and then decreases at high temperature. The
thermal expansion coefficient also nonlinearly depends on
temperature, and there is a transition from a positive thermal
expansion coefficient at low temperature to a negative one at
high temperature. The transition temperature is about 345, 459,
and 491 K for Bi nanowires with an orientation of [104], [202],
and [110], respectively. An anisotropic thermal expansion
behavior of Bi nanowires with different orientations was
demonstrated for the first time in the present study.
The temperature dependences of lattice parameter and thermal
expansion coefficient of Bi nanowires with the same orientation
([202]) but different diameters are shown in Figure 6. For [202]-
orientated Bi nanowires, the transition temperature of the thermal
expansion coefficient from positive to negative is about 409,
425, and 459 K with a diameter of 12, 36, and 60 nm,
respectively. The result shows that the transition temperature
shifts to high temperature with increasing diameter of Bi
nanowires, which has the same feature as reported in our
previous study.18
12 nm
(202)
36 nm
(202)
60 nm
(202)
60 nm
(110)
60 nm
(104)
sample
bulk
before
after
1.8624
1.8626
1.8606
2.2665
2.2742
2.2736
2.3579
2.3670
2.3667
1.8639
1.8624
1.8646
1.8645
nanowires with [202] orientation the second time after the first
time measurement, and the result is shown in Figure 7. It is
worth noting that the maximum measuring temperature (538
K) is lower than the melting point of the nanowires (542 K),
and thus, no melting happened after the first time measurement.
From Figure 7 one can clearly see three features: (1) the lattice
parameter of the Bi nanowires at the second time is lower than
that at the first time in the whole temperature range studied,
(2) the temperature dependence of the lattice parameter mea-
sured at the second time becomes weak as compared with that
at the first time, (3) the lattice parameters in the second time
measurement at temperature higher than 500 K are even lower
than that at room temperature. The lattice parameters of all Bi
nanowires at room temperature before and after high-temperature
measurements are summarized in Table 2. From this table one
can see that the room-temperature lattice parameter measured
the second time is always smaller than at the first time for all
nanowires studied. This result indicates that the defects, such
as vacancies, impurities, and surface defects, might affect the
thermal properties of the nanowires.
To study the influence of defects and stresses on the thermal
properties, we measured the lattice parameter of 60 nm Bi

26192 J. Phys. Chem. B, Vol. 110, No. 51, 2006
Zhu et al.
There are always certain defects and lattice stresses in Bi
nanowires, and with decreasing diameter the surface defects and
tension will increase substantially. Defect and stress might be
significant in determining the thermal behavior of nanowires.
They are ‘frozen’ around room temperature. Timmesfeld and
co-workers27 studied the influence of vacancies on the thermal
expansion of the solid state and found that the change of
crystalline volume caused by vacancies is
The thermal expansion or thermal contraction upon heating
depends on the balance between phonon modes with positive
and negative Grueneisen parameters. Transverse acoustic (TA)
modes might exhibit negative Grueneisen parameters related
to the increase of the restoring forces with increased tension.34
Because TA modes represent the lowest frequency modes,
thermal contraction is more often seen only well below the
Debye temperature. In the present study the transition of the
thermal expansion coefficient from positive to negative is well
above the Debye temperature of bulk Bi (119 K). Nevertheless,
in nanometer scale, for Bi nanowires some unusual low-energy
phonons might be activated at temperatures well above the
Debye temperature and contribute to the thermal contraction,35
and these unusual low-energy phonons might be diameter
dependent. The smaller the diameter, the lower the temperature
of the appearance of the unusual low-energy phonons. Study
of the vibration modes of Bi nanowires at different temperatures
is needed to understand the thermal contraction of Bi nanowires
with increasing temperature.
∆V ) BV0 exp[-Q/kT]
(2)
where Q, B, V0, k, and T are, respectively, the formed energy
of vacancies, constant, crystalline volume at 0 K, Boltzmann
constant, and temperature, which shows that the lattice parameter
will shrink with increasing temperature or upon annealing.
Actually our first time XRD measurement is equivalent to an
anneal process, which will partly eliminate the vacancies in Bi
nanowires, and thus the lattice parameter will decrease at the
second time measurement.
Being embedded inside the pores of the AAM, the nanowires
would experience biaxial compressive stresses in the inward
radial direction and induced tensile uniaxial stresses in the
growth direction due to both the restriction of the AAM wall
and the thermal stress by the different thermal expansion
coefficient between the nanowires and AAM, which will affect
the thermal expansion behavior of Bi nanowires and lead to an
increase in the thermal expansion coefficient. Nevertheless, the
thermal stress at the direction normal to the nanowire axis is
much higher than that along the nanowire, and restriction of
the AAM wall will mainly affect transverse expansion of the
nanowires, while in our study we measure the longitudinal
expansion and thus the influence of the restriction of the AAM
wall, and the thermal stress on the thermal expansion of the Bi
nanowires is much less.
As is well known, most materials usually show a positive
thermal expansion property with increasing temperature. This
behavior can be understood by considering the effects of the
anharmonic potential on the equilibrium lattice separations and
is usually characterized by the Grueneisen parameter (for bulk
Bi the Grueneisen parameter is always positive, so a positive
thermal expansion coefficient is obtained).28 The negative
thermal expansion, which represents lattice contraction with
temperature, was also observed among anisotropic systems,29,30
where contraction along one crystallographic direction was
usually accompanied by expansion along the others.31
Anisotropic properties of the thermal expansion of Bi
nanowires can be understood by properties of binding forces
between atoms. Different crystal directions have different
packing densities, which is a significant factor in affecting the
thermal expansion coefficient. The smaller the packing density,
the larger the thermal expansion coefficient.32 With increasing
temperature, the thermal activation motion of atoms in Bi
nanowires will increase and the electronic cloud of the crystal
lattice will expand.33 If the packing density is large, i.e., the
space that is not filled by atoms in the crystal cell is small, the
probability of inflated electronic clouds overlapping with each
other will be large, the repulsion forces between atoms will be
large, and thus the thermal expansion coefficient will be large.
In contrast, if the packing density is small, the probability of
inflated electronic clouds overlapping with each other will be
small and thus the thermal expansion coefficient will be small.
The packing density of the [202] direction of Bi nanowires is
the highest and thus has the lowest thermal expansion coef-
ficient, while [104] Bi nanowires have the highest thermal
expansion coefficient in the room-temperature region.
In our previous study18 the transition of the thermal expansion
coefficient from positive at low temperature to negative at high
temperature for Bi nanowire arrays was attributed to the effects
of electronic excitations on the equilibrium lattice separation.
In nanowires, the energy level spacing of the spatially confined
valence electrons depends on the temperature. Lattice shrinking
results in an increase in the level separation when the temper-
ature is increased, which, on one hand, reduces the number of
electrons occupying the excited states as dictated by the Fermi-
Dirac factor and, on the other hand, raises the thermal energy
of individual electrons in the excited states. These two factors
compete delicately to achieve a lower electronic potential energy
that results in the crossover from thermal expansion to thermal
contraction.
Conclusions
We successfully prepared Bi nanowire arrays with different
orientations and diameters by pulsed electrodeposition in the
pores of AAM. It was found that the melting point of Bi
nanowires decreases with decreasing nanowire diameter. Our
results clearly show an anisotropic thermal expansion property
of 60 nm Bi nanowires with different orientations, and there is
a transition from a positive thermal expansion coefficient at low
temperature to a negative one at high temperature for Bi
nanowire arrays with different orientations and diameters. Our
results proved for the first time from experimental data that the
physical properties of Bi nanowires depend strongly on the
orientation of the nanowires.
Acknowledgment. This work was supported by the National
Natural Science Foundation of China (no. 10474098) and the
National Major Project of Fundamental Research for Nanoma-
terials and Nanostructures (no.: 2005CB623603).
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