Alteration of crystal structure of bismuth confined in cylindrical nanopores

Article abstract of DOI:10.1016/j.cplett.2007.06.131

The effect of nanoscopic cylindrical confinement on the crystal structures and crystallization behaviors of Bi was investigated. While the crystallites of Bi nanorods were found to have a higher orientation proportional to the degree of confinement, an alteration of the crystal orientation between electrodeposited and recrystallized Bi nanorods was observed. Thermal analysis confirmed a structural difference between the as-deposited and recrystallized nanorods and that a crystallization mechanism where nucleation determines the overall rate of crystallization occurred during recrystallization. The crystal structure, including the preferred crystal orientation, is discussed in terms of the dominant crystallization mechanism under the cylindrical nanoconfinement.

Full text of DOI:10.1016/j.cplett.2007.06.131

Chemical Physics Letters 444 (2007) 130–134
www.elsevier.com/locate/cplett
Alteration of crystal structure of bismuth confined
in cylindrical nanopores
*
Kyong Wook Noh, Euntaek Woo, Kyusoon Shin
School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, Republic of Korea
Received 23 March 2007; in final form 16 June 2007
Available online 1 July 2007
Abstract
The effect of nanoscopic cylindrical confinement on the crystal structures and crystallization behaviors of Bi was investigated. While
the crystallites of Bi nanorods were found to have a higher orientation proportional to the degree of confinement, an alteration of the
crystal orientation between electrodeposited and recrystallized Bi nanorods was observed. Thermal analysis confirmed a structural dif-
ference between the as-deposited and recrystallized nanorods and that a crystallization mechanism where nucleation determines the over-
all rate of crystallization occurred during recrystallization. The crystal structure, including the preferred crystal orientation, is discussed
in terms of the dominant crystallization mechanism under the cylindrical nanoconfinement.
ꢀ 2007 Elsevier B.V. All rights reserved.
1. Introduction
materials is significant. As the crystal structure is influenced
by the nanoscopic confinement effects, it is possible to con-
trol the crystal structure by applying the appropriate nano-
structures, which may be used to fine-tune the properties of
the target material.
Nanoscopic materials, compared to their bulk counter-
parts, have peculiar properties which are important in
practical applications such as quantum-dot lasers and opti-
cal detectors. [1–3] Among the nanostructured materials,
interest in one-dimensional (1D) structures, e.g. wires,
rods, and tubes, has recently begun to increase significantly
[4] since unique studies on the anisotropic properties of
such nanomaterials can be performed. For device applica-
tions, 1D nanostructured materials are attractive in that
they can maintain high density without losing their proper-
ties. With the advent of nanometer-scale devices, the prop-
erties of nanoscale materials, especially those of metals and
semiconductors which are the key elements of such devices,
have become increasingly important. The thermal behav-
iors [5–8] and the magnetic and electrical properties
[9–11] are examples of fields that have been investigated
extensively. The properties of such crystalline materials
are greatly dependent on their crystal structure, and there-
fore investigation on the crystal structure of nanoscopic
Bismuth (Bi) is a semimetal with a rhombohedral struc-
ture and is characterized by many peculiar properties; it
exhibits small effective mass, low density of states, long
mean free path of the carriers, and a highly anisotropic
Fermi surface [12]. Due to these factors, Bi shows large
magnetoresistance (MR) and thermoelectric effects [12,13];
thus it is potentially a great material for application in mag-
netic sensors or thermoelectric devices. The possible appli-
cations of the special properties of Bi have stimulated
various investigations on MR and thermoelectric effects
[13–16]. In addition, the thermal properties and behaviors
of Bi nanostructures have been studied [6,8], and thin film
morphologies depending on the crystallization method were
also shown [17]. However, detailed understanding of the
crystal structure on nanoscopic scale is still demanded for
the sophisticated use of the special properties of Bi, espe-
cially for nanometer-scale structures of specific geometry.
While the crystal structure of nanoscopic materials in a
confined geometry has been known to be determined by the
geometry itself [18] or by the deposition methods [8],
*
Corresponding author. Fax: +82 2 8847355.
E-mail address: shin@snu.ac.kr (K. Shin).
0009-2614/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2007.06.131

K.W. Noh et al. / Chemical Physics Letters 444 (2007) 130–134
131
detailed mechanism of the crystal structure formation on
nanoscopic scale is still unclear. In this study, we investi-
gate the crystal structures and thermal behaviors of Bi
within nanoscopic cylindrical confinements, which show
that the crystal structure of nanometer-scale materials in
a robust template can also be influenced by the crystalliza-
tion mechanism. Since the crystallization kinetics of mate-
rials under nanoscopic confinement is known to be
determined by the rate of nucleation as compared to the
growth-determining kinetics of the bulk [19], we expected
that the crystallization on nanoscopic scale could induce
orientation preferences of the crystal structures. From the
study on the crystallization behaviors of Bi in cylindrical
nanopores, insight on the crystal structures of metals and
other crystalline materials confined to nanoscopic cylindri-
cal geometries could be obtained, which can be utilized in
many applications concerning the control of crystal struc-
tures and even the optimization of their physical
properties.
Fig. 1. Schematic diagram of the XRD experiment.
and cooling scans were both conducted at a scanning rate
of 10 ꢁC/min.
2. Experimental
3. Results and discussion
Bismuth nanorods were fabricated by electrodeposition
using nanoporous anodic alumina as a template. The alu-
mina was prepared by the two-step anodization technique
[20,21]. After anodization, a protective coating of polysty-
rene was applied on the alumina and the leftover aluminum
was removed with a CuCl2/HCl solution. The obtained alu-
mina membranes were treated with a 0.1 M H3PO4 solu-
tion to remove the barrier layers. Afterwards, the
protective polystyrene was removed and Au was sputtered
onto one side of the membrane to serve as an electrode.
Electrodeposition of Bi was carried out in a two-electrode
cell with a Pt plate as the counter electrode. The electrolyte
was an aqueous solution containing 10 g/L BiCl3, 50 g/L
tartaric acid, and 95 g/L glycerol with a small amount of
35 wt% HCl added until the BiCl3 was completely dis-
solved. A direct current of 1.3 V was applied for 3 h at
room temperature.
Fig. 2a shows an FE-SEM image of the top view of a
nanoporous anodic alumina membrane. The nanopores
of the alumina are uniform in diameter (d ꢀ 77 2.4 nm)
and hexagonally closely packed with several micrometer-
scale grains. Fig. 2b is an FE-SEM image of the Bi nano-
rods. The nanorods have a high aspect ratio and the
diameter is uniform throughout the length of the nano-
rod. In addition, the diameter of the nanorods
(d ꢀ 79 2.1 nm) and the pores are the same, indicating
that the nanorods completely filled the pores, and, like
the pores, the diameter of the nanorods is also uniform.
A dramatic shift of the major direction of crystal orien-
tation after melting and recrystallization of the nanorods
was found by XRD experiments, which is shown in
Fig. 3. The distinctive difference in the XRD patterns
between the as-deposited and recrystallized nanorods is in
the diffraction of the (012) plane. While the (012) plane
normal of the as-deposited nanorods have a strong orienta-
tion preference in the direction of 40–45ꢁ from the cylindri-
cal axis, it has an orientation parallel to the cylindrical axis
for recrystallized nanorods. From the diffraction patterns,
it can be deduced that the three axes of the rhombohedral
unit cell are arranged so that the (110) plane is normal to
the cylindrical axis for the as-deposited nanorods. In the
mean time, for the recrystallized nanorods, the crystallites
are tilted to have the (012) plane normal to the cylindrical
axis. The crystal axes of both nanorods are depicted in the
upper-right corners of Fig. 3a and e.
Field emission scanning electron microscopy (FE-SEM,
JEOL JSM-6700F) was used to investigate the morphology
of the nanorods. To obtain free nanorods, the alumina
membrane was dissolved in a 3 wt% NaOH solution, and
the remaining nanorods were washed in deionized water
several times. For XRD measurements, the alumina mem-
branes containing the nanorods were cut into rectangular
shapes (5 mm · 2 mm) and any leftover metal on the sur-
face of the membrane was scraped away with a razor blade.
The XRD experiments were conducted at the 4C2 beamline
at Pohang Light Source. A schematic diagram of the XRD
experiments in this study is shown in Fig. 1. The incident
˚
beam (k = 1.38 A) hit the long side of the sample, perpen-
As shown in Fig. 3, the crystals of both as-deposited and
recrystallized nanorods appear to be more oriented as the
diameter of the nanorods becomes smaller, though there
is little difference between each other. In order to analyze
the degree of orientation more quantitatively, the intensity
was plotted along the circumference of the (012) diffraction
dicular to the pore axes, at an incident angle of ai = 0.3ꢁ
and the scattered X-rays were detected with a two-dimen-
sional area detector. Thermal analysis of the nanorods
was conducted by calorimetric measurements using the
Perkin–Elmer DSC-7 with a refrigerating cooler. Heating

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K.W. Noh et al. / Chemical Physics Letters 444 (2007) 130–134
Fig. 2. SEM image of the top view of a nanoporous anodic alumina membrane (a), and the fabricated Bi nanorods (b).
pattern, and the full width at half maximum (FWHM) of
the peaks was obtained. The FWHM to the size of the
nanorods is plotted in Fig. 4. Clearly, the FWHM
decreases as the diameter decreases, which indicates that
the degree of orientation is higher for nanorods of smaller
diameters. This suggests that the crystallization of the
nanorods is influenced by the confined geometry. The little
difference of FWHM between the two differently crystal-
lized nanorods implies that such confinement effects take
place in a similar degree regardless of the crystallization
method.
The difference in crystal structure gives rise to the alter-
ation of thermal behaviors, as depicted in Fig. 5a. The
lower melting point and smaller heat of fusion of nano-
scopic materials, which has already been reported [5,7,8],
could easily be observed. As the pore diameter decreases,
the melting point and heat of fusion of the as-deposited
nanorods are depressed. Melting occurs in the range of
264–271 ꢁC for 78 nm nanorods and steadily decreases with
diameter to have 261–264 ꢁC for 26 nm nanorods. In addi-
tion, the melting endotherms of the recrystallized nanorods
were observed at slightly lower (0.5–2 ꢁC) temperature
ranges than that of the as-deposited nanorods. The depres-
sion of the melting point can be explained by the fact that
the influence of the interfacial free energy becomes more
important when the size of the crystallites becomes smaller
[5]. The interfacial area increases as the size of the crystal-
Fig. 3. XRD patterns of the Bi nanorods of various sizes, as-deposited
(a)–(d), and recrystallized from melt (e)–(h). In the upper-right margin of
(a) and (e), the schematic diagrams of the unit cell axes of both the as-
deposited and recrystallized crystallites are shown, respectively.
Fig. 4. FWHM of the (012) diffraction peak verses diameter of the Bi
nanorods, as-deposited and recrystallized from melt.

K.W. Noh et al. / Chemical Physics Letters 444 (2007) 130–134
133
Fig. 5. DSC thermograms of bulk Bi and Bi nanorods of various sizes. (a) Is the heating scan and (b) is the cooling scan. In (a), the solid lines are from as-
deposited nanorods and the dotted lines are from recrystallized nanorods.
lite decreases; the relation between the melting temperature
and the crystallite size can be described by the Gibbs–
Thomson equation [22]. Assuming that the crystallites are
cylindrical in shape with a radius of r and a length of l,
the equation can be written as follows:
molecules in cylindrical nanopores are typical examples
that exhibit such crystallization where nucleation deter-
mines the overall crystallization rate [18,19,23]. Two
crystallization peaks can be observed in the cooling ther-
mograms, one around 90 ꢁC and another around 150 ꢁC,
as indicated in Fig. 5b; there is a possibility that some por-
tion of the crystallites in the nanorods could have nucleated
in a different way, but the exact cause of this phenomenon
is yet to be accounted for. Direct current electrodeposition
into nanopores, on the other hand, can be a growth-dom-
inant process, and even single crystalline nanorods can be
formed [24,25]. Once initial nucleation occurs and the crys-
tallites cover the Au electrode at the bottom of the pores,
there would be few independent nucleation sites, and there-
fore growth along the walls of the pores is thought to be
favored.
ꢀ
ꢁ
r1
r2
Tm ¼ Tom 1 ꢁ
ꢁ
ð1Þ
lqDHof rqDHof
Here, Tm is the melting temperature of the nanorods while
Tom is the melting point of an infinitely large crystal, r1 and
r2 are the interfacial energies of the top and bottom planes
and the curved wall of the cylindrical crystallites, respec-
tively, q is the density, and DHof is the heat of fusion. It
can be seen from the equation that the melting temperature
decreases as the size of the crystallite decreases. The slightly
lower melting temperatures of the recrystallized nanorods
can be ascribed to a difference in the crystallite size and/
or interfacial energies. The different crystallization condi-
tions could have induced a difference in the crystallite size.
Also, different crystal orientations change the interface be-
tween adjacent crystallites, and between the crystallites and
pore walls, which would alter the values of r1 and r2, the
interfacial energies. While these are thought to be the cause
of the different melting temperatures, it is unclear presently
which is the main factor. A more intensive investigation on
the properties of the crystallites needs to be carried out in
order to confirm the exact nature of this matter.
4. Conclusion
Conclusively, electrodeposited Bi nanorods acquire dif-
ferent crystal orientations and melting temperatures when
they are recrystallized from the melt. This is due to the fact
that the crystallization mechanism of Bi during electrode-
position into nanoscale cylindrical pores and recrystalliza-
tion from the melt is different. For recrystallized nanorods,
crystallization is governed by nucleation due to confine-
ment, while the crytstal structure of as-deposited nanorods
is thought to be driven by the growth process.
The difference in crystal structure can be ascribed to a
difference in the crystallization mechanism, which can be
observed by the cooling experiments, shown in Fig. 5b. It
can be seen that a high degree of supercooling (over
150 ꢁC) occurs during the crystallization of the nanorods.
Such high degree of supercooling indicates that the crystal-
lization is governed by the nucleation process [18,19,23].
Besides showing a high degree of supercooling, the peaks
are also quite broad compared to the bulk counterparts.
The very narrow crystallization peak of the bulk indicates
that the growth of the crystals is an extremely fast proce-
dure; thus some other factor, i.e. the nucleation of crystals,
makes the crystallization peaks of the nanorods broader,
meaning that nucleation is influential in the overall crystal-
lization [19]. Isolated metallic microdroplets and chain
Acknowledgements
This work was supported by the Korean Government
through KRF (MOEHRD, KRF-2006-331-D00160) and
KOSEF (R01-2006-000-10749-0). The experimental sup-
port from the 4C2 and 8C1 beamlines at Pohang Light
Source is gratefully acknowledged.
References
[1] V.I. Klimov et al., Science 290 (2000) 314.
[2] J. Phillips, J. Appl. Phys. 91 (2002) 4590.
[3] K. Shin, H.Q. Xiang, S.I. Moon, T. Kim, T.J. McCarthy, T.P.
Russell, Science 306 (2004) 76.

134
K.W. Noh et al. / Chemical Physics Letters 444 (2007) 130–134
[4] Y.N. Xia et al., Adv. Mater. 15 (2003) 353.
[5] S.L. Lai, J.Y. Guo, V. Petrova, G. Ramanath, L.H. Allen, Phys. Rev.
Lett. 77 (1996) 99.
[6] E.A. Olson, M.Y. Efremov, M. Zhang, Z. Zhang, L.H. Allen, J. Appl.
Phys. 97 (2005) 034304.
[7] X.W. Wang, G.T. Fei, K. Zheng, Z. Jin, L. De Zhang, Appl. Phys.
Lett. 88 (2006) 173114.
[8] Y.G. Zhu, X.C. Dou, X.H. Huang, L. Li, G.H. Li, J. Phys. Chem. B
110 (2006) 26189.
[9] F.J. Himpsel, J.E. Ortega, G.J. Mankey, R.F. Willis, Adv. Phys. 47
(1998) 511.
[10] C. Durkan, M.E. Welland, Phys. Rev. B 61 (2000) 14215.
[11] Z.B. Zhang, X.Z. Sun, M.S. Dresselhaus, J.Y. Ying, J. Heremans,
Phys. Rev. B 61 (2000) 4850.
[12] F.Y. Yang, K. Liu, K.M. Hong, D.H. Reich, P.C. Searson, C.L.
Chien, Science 284 (1999) 1335.
[14] J. Heremans, C.M. Thrush, Y.M. Lin, S. Cronin, Z. Zhang, M.S.
Dresselhaus, J.F. Mansfield, Phys. Rev. B 61 (2000) 2921.
[15] Y.T. Tian, G.W. Meng, S.K. Biswas, P.M. Ajayan, S.H. Sun, L.D.
Zhang, Appl. Phys. Lett. 85 (2004) 967.
[16] A. Boukai, K. Xu, J.R. Heath, Adv. Mater. 18 (2006) 864.
[17] M.K. Zayed, H.E. Elsayed-Ali, J. Appl. Phys. 99 (2006) 123516.
[18] M. Steinhart, P. Goring, H. Dernaika, M. Prabhukaran, U. Gosele,
E. Hempel, T. Thurn-Albrecht, Phys. Rev. Lett. 97 (2006) 027801.
[19] E. Woo, J. Huh, Y.G. Jeong, K. Shin, Phys. Rev. Lett. 98 (2007) 136103.
[20] H. Masuda, K. Fukuda, Science 268 (1995) 1466.
[21] H. Masuda, M. Satoh, Jpn. J. Appl. Phys., Part 2 35 (1996) L126.
[22] A.K. Jena, M.C. Chaturvedi, Phase Transformation in Materials,
Prentice Hall, Englewood Cliffs, N.J., 1992.
[23] D. Turnbull, R.E. Cech, J. Appl. Phys. 21 (1950) 804.
[24] M.L. Tian, J.U. Wang, J. Kurtz, T.E. Mallouk, M.H.W. Chan, Nano
Lett. 3 (2003) 919.
[13] J. Heremans, C.M. Thrush, Phys. Rev. B 59 (1999) 12579.
[25] H. Pan et al., J. Phys. Chem. B 109 (2005) 3094.