Superconducting properties of lead nanowires fabricated by electrochemical deposition

Article abstract of DOI:10.1016/j.physc.2005.02.040

Pb nanowires were fabricated by electrochemical deposition inside the nanopores of polycarbonate membranes. The diameter of fabricated nanowires ranged from 80 to 100 nm, and length is 5 μm on average. Temperature dependence of resistance was measured in magnetic fields by two-point contact method. It is found that the nanowires show a broad transition with higher critical fields compared with typical values of bulk Pb, and that the nanowires belong to Type II superconductors.

Full text of DOI:10.1016/j.physc.2005.02.040

Physica C 426–431 (2005) 268–272
www.elsevier.com/locate/physc
Superconducting properties of lead nanowires fabricated
by electrochemical deposition
a,b,
b
b
a
b
S. Ishii
*, E.S. Sadki , S. Ooi , Y. Ochiai , K. Hirata
a
Graduate School of Science and Technology, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan
b
Superconducting Materials Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan
Received 23 November 2004; accepted 14 February 2005
Available online 1 July 2005
Abstract
Pb nanowires were fabricated by electrochemical deposition inside the nanopores of polycarbonate membranes. The
diameter of fabricated nanowires ranged from 80 to 100 nm, and length is 5 lm on average. Temperature dependence of
resistance was measured in magnetic fields by two-point contact method. It is found that the nanowires show a broad
transition with higher critical fields compared with typical values of bulk Pb, and that the nanowires belong to Type II
superconductors.
ꢀ 2005 Elsevier B.V. All rights reserved.
PACS: 74.25.Fy; 74.40.+k
Keywords: Lead; Superconducting nanowire; Electrochemical Deposition
1. Introduction
development in fine processing technologies, many
researchers have become more interested with
nanometer size and fine-shaped superconducting
materials, as well as high-Tc materials. New differ-
ent phenomena are expected in such small size and
low dimensional superconductors, as well as in met-
als and semiconductors. Understanding the physics
of low dimensional superconductors, especially,
nanowires and thin films, is very important to deve-
lop a new-generation of superconducting nanode-
vices. There have been some interesting theoretical
predictions for the properties of superconducting
Since the discovery of high-Tc superconductors
[1], new superconducting materials have been ex-
plored extensively. Recently, as the size of elec-
tronic devices has been miniaturized due to the
*
Corresponding author. Address: Superconducting Materials
Center, National Institute for Materials Science, 1-2-1 Sengen,
Tsukuba 305-0047, Japan. Tel.: +81 29 859 2361; fax: +81 29
859 2301.
E-mail address: ishii.satoshi@nims.go.jp (S. Ishii).
0921-4534/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2005.02.040

S. Ishii et al. / Physica C 426–431 (2005) 268–272
269
nanowires, markedly the so-called ‘‘phase slip’’ in
the superconducting order-parameter, which can
be attributed to thermal or quantum phase slips
[2]. Therefore further understanding will provide a
possibility for the realization of superconducting
nanodevices. In this study, we report on a fabrica-
tion method for Pb nanowires with a small diame-
ter, and on their temperature dependence of
resistance, which is measured in magnetic fields, as
an example to study transport properties in small
size superconductors.
2. Experimental
Fig. 1. SEM picture of polycarbonate surface after deposition
of gold. The diameter of nanopores in the membranes was
There are several methods to fabricate nano-
wires. Lithographic method followed by lift-off
process can produce nanowires through mask
and along patterns. Physical and chemical milling
method can also be used with etching to reduce
the dimension of the deposited materials. But such
methods require a long processing time, even for
the fabrication of only one nanowire. Electro-
chemical deposition is one of the most reasonable
methods. It can produce large quantity of nano-
wires with uniform diameter and length simulta-
neously. Furthermore, diameter and length can
be controlled by selecting templates with a desired
size. Therefore, we selected for this work an elec-
trochemical deposition technique for the fabrica-
tion of nanowires.
Lead nanowires are fabricated by electrochemi-
cal deposition inside nanopores of polycarbonate
membranes. Commercially available membranes
with thickness of 20 lm were used. The diameter
of nanopores in the membranes was 50 nm, and
their density was 3.13 · 105 mmꢀ2 (Fig. 1). The
membrane was used as a working electrode in a
three-electrode configuration system for the depo-
sition. Carbon plate was used as a counter elec-
trode, and Ag/AgCl electrode was used as a
reference electrode in this system. Voltage was
biased between the working electrode and the
counter electrode keeping the potential of the
working electrode at ꢀ0.5 V versus the reference
electrode by a potentiostat. Electrolyte bath, in
which the working and the counter electrodes were
dipped, was filled with an aqueous solution of
50 nm, and their density was 3.13 · 105 mmꢀ2
.
40.4 g/l Pb(BF4)2, 33.6 g/l HBF4, and 15 g/l
H3BO3. The other bath is filled with 3 M KCl solu-
tion, in which Ag/AgCl electrode was dipped, and
was connected to the electrolyte bath via a salt
bridge.
Before the electrochemical deposition, gold was
deposited on both sides of the membrane by ion
plating, and used as a working electrode, since
polycarbonate was an insulator. Another purpose
of depositing gold was to avoid etching of the
membrane surface, as it was easy to melt in acids,
such as aqueous solution in the electrolyte bath.
After filling Pb into the nanopores, Pb surged
over the nanopores and covered the top surface
of the membrane. It made the area of electrode-
position broader, and may have produced an in-
crease in the deposition current. Even though
there have been some reports that at the comple-
tion of nanopores filling-up it was possible to de-
tect a sudden increase of current [3], such an
increase could not be detected in our experiment.
The current increased gradually, and saturated
ultimately when the nanoporous were filled-up
with Pb. The electrochemical deposition was
stopped when the saturation of current was de-
tected. Average current density was 2.05 · 10ꢀ2
mA/mm2 at the beginning of the electrochemical
deposition, and 3.69 · 10ꢀ2 mA/mm2 at the end.
Electrochemical deposition was carried out for

270
S. Ishii et al. / Physica C 426–431 (2005) 268–272
30 min on average. The electrochemical deposi-
tions were done several times by using the same
aqueous solution in the bath. The concentration
of the aqueous solution was changed as the num-
ber of depositions increased. The change of con-
centration influenced only the deposition time
and current density.
In order to confirm the size and length of Pb
nanowires inside the pores of membrane, the
nanowires were retrieved from the membrane.
Firstly the membrane was dissolved in chloroform
at 60 ꢁC and stirred at 200 rpm for 30 min. Then,
the nanowires in this solution were dispersed onto
Si substrates by spin coating, and observed by
scanning electron microscope (SEM). X-ray dif-
fraction (XRD) was also performed to character-
ize the crystal structure of the nanowires.
Electrical transport measurements were carried
out to compare the property of nanowires with
that of bulk samples. Temperature dependence of
resistance in Pb nanowires was measured by using
a liquid helium cryostat system at temperatures
from 1.8 K to 10 K, applying magnetic fields up
to 16 kOe. Two-point method with the electrodes
on the top and bottom of the membrane was used
in this work to measure Pb nanowires inside mem-
branes. The contacts were put on both sides of the
membrane by silver paste. All obtained data in-
clude the contact resistance between silver paste
and lead. Thus a number of Pb nanowires is indeed
connected to the silver paste. It means that a num-
ber of Pb nanowires inside the membrane were
measured in parallel at the same time.
Fig. 2. SEM picture of Pb nanowires and clusters dispersed
onto Si substrate after dissolution of polycarbonate membrane.
The diameter of nanowires distributes from 80 nm to 100 nm,
and the length is longer than 5 lm. The inset shows the
nanowires connected with each other by spilled Pb from
nanopores. They are connected with each other at the top or
bottom by Pb layer made of spilled Pb from nanopores.
500
Pb
Pb
400
Pb
Pb
300
Pb
200
Pb
100
20
30
40
50
60
[deg]
2
θ θ
/
70
80
90
Fig. 3. XRD pattern of Pb nanowires and clusters. There are
peaks originating from single crystal and background from
amorphous. It is not certain that these peaks are attributed to
Pb nanowires.
3. Results and discussion
The nanowires obtained by the electrochemical
deposition are shown in Fig. 2. It includes a lot of
short nanowires and residual Pb. XRD was mea-
sured to verify the composition of the nanowires
and residual Pb as shown in Fig. 3. Some peaks
are observed, and they can be attributed to Pb
crystals. The nanowires and residual Pb do not
contain oxidized Pb or other compounds. There
is also a background in the diffraction pattern
which originates from amorphous material. Fur-
ther studies are needed to clarify the crystallinity
of the nanowires, such as by transmission electron
microscope. The diameter of Pb nanowires ranged
from 80 to 100 nm, and most of Pb nanowires are
about 90 nm. Even though the diameter of nanop-
ores of the membrane was 50 nm, the diameter of
Pb nanowires obtained was larger than that. It
seems that the aqueous solution soaked into the
nanopores and etched the inside of nanopores to

S. Ishii et al. / Physica C 426–431 (2005) 268–272
271
make the diameter larger. The length is larger than
clean
300
200
100
0
5 lm, and the longest one was about 10 lm. When
the membrane was melted, Pb nanowires in the
chloroform were stirred to make the dissolution
more effective. Most of Pb nanowires seem to
break up in this process. Some clusters of Pb nano-
wires were also shown in the inset of Fig. 2. They
were connected with each other at the top or bot-
tom by a thin Pb layer made of spilled Pb from the
nanopores.
ξ
ξ
(T)
GL
(T)
GL
dirty
GL
ξ
(T)
Fig. 4 shows the temperature dependence of
resistance in magnetic fields from 0 to 16 kOe. A
decrease of resistance could be observed. This de-
crease is attributed to the superconducting transi-
tion of Pb, since this drop of resistance begin
from near Tc of 7.2 K of bulk Pb. But the transi-
tion was broader to lower temperatures. Pb nano-
wires show superconductivity even at higher
magnetic field up to 14 kOe at 1.8 K, while Hc of
bulk value is 0.8 kOe at 0 K [4].
0
2
4
6
8
Temperature [K]
Fig. 5. Temperature dependence of coherence length. Dotted
clean
line shows n ðTÞ, dashed line show nGL(T) and solid line
GL
dirty
GL
dirty
shows n ðTÞ respectively. nGL(T) shows good fitting for
GL
clean
n
ðTÞ with a parameter l of 1.15 nm. Even though we tried to
GL
fit n ðTÞ to nGL(T), the parameter n0 of coherence length
obtained from fitting was 1152 nm. It is appropriate that
nanowires seem to be in the dirty limit.
To confirm the superconductivity of Pb nano-
wires, we discuss the upper critical fields value be-
low. A criterion was defined on the onset of
superconducting transition to determine the upper
critical field Hc2(T). The Ginzburg–Landau coher-
ence length nGL(T) was estimated from critical
fields Hc2(T). nGL(T) is related to Hc2(T) by the
formula Hc2(T) = U0/2pn2(T) [5], where U0 is the
magnetic flux quantum. The obtained nGL(T) is
plotted in Fig. 5. Furthermore, nGL(T) is related
to the BCS coherence length n0. In the clean limit
(l ꢁ n0), coherence length is expressed as
n
ðTÞ ¼ 0.74n0=ð1 ꢀ T=TcÞ1=2, and for dirty lim-
1=2
clean
GL
1=2
it (l ꢂ n0), expressed as ndirtyðTÞ ¼ 0.86ðn0lÞ
=
GL
ð1 ꢀ T =TcÞ near Tc where l is the electron mean
free path [5]. The n0 of Pb was used as 87 nm [4].
The difference between nGL(T) and ncleanðT Þ is
clean
GL
found to be large near Tc. Then, we tried to fit
the formula n ðTÞ to nGL(T) obtained above
clean
GL
with the parameter n0. The n0 obtained from the
fitting was 1152 nm. Thus n ðTÞ formula is inad-
GL
equate. It is appropriate that the nanowires used in
this work are in the dirty limit. Then, nGL(T) ob-
tained from the experiments is fitted by the for-
1.0
16
dirty
GL
0.9
0.8
0.7
0.6
12
10
9
8
6
5
4
3
2
mula of n ðTÞ for dirty superconductor near Tc
dirty
GL
with parameter l. Obtained n ðTÞ is shown as a
solid curve in Fig. 5. As the result of this fitting,
l was 1.15 nm. l is 10.43 nm for bulk Pb at 77 K,
when l is estimated by using a Drude relaxation
time of 0.57 · 10ꢀ14 s [6]. The l of nanowires
obtained from fitting is much smaller than bulk
value. This decrease of l seems to be attributed
to disorder in the nanowires, such as granularity
and defects.
1
0 [KOe]
0
2
4
6
8
10
Temperature [K]
For dirty superconductors, magnetic penetra-
tion depth k(T) is expressed as k(T) = kL(T)(1 +
0.75n0/l)1/2 near Tc from the BCS theory [5], where
Fig. 4. Temperature dependence of resistance in magnetic
fields. Large broadening can be seen below Tc. Increase of
Hc2(T) can also be seen.

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S. Ishii et al. / Physica C 426–431 (2005) 268–272
to be 26.7 kOe. This value is close to the experi-
mental value of Hc2(0) as shown in Fig. 6.
40
30
20
10
4. Conclusion
Pb nanowires were fabricated by electrochemi-
cal deposition. The diameter of obtained nano-
wires distributes from 80 to 100 nm, and the
length was more than 5 lm. Unusual behavior
was observed in magnetic transport measurement
below Tc. Transition was broadened widely below
Tc and upper critical field Hc2(T) was much larger
than the value of bulk Pb.
The temperature dependence of coherence
length was estimated from experiment, which
seemed to deviate from that of clean limit. So
nanowires were assumed to be in the dirty limit.
The mean free path l obtained was 1.15 nm, which
is much smaller than n0 of bulk Pb. This small va-
lue of l seemed to enhance the value of j to make
Pb nanowires behave as Type II superconductor.
There are still unclear questions, for example,
how dimensionality influences the superconduc-
ting property of Pb nanowire. Further experiments
are needed, such as directional dependence of
resistance on magnetic field.
0
0
2
4
6
8
Temperature [K]
Fig. 6. Magnetic phase diagram of Pb nanowires obtained from
magnetic transport measurements. Dots shows the upper
critical field Hc2(T) from transport measurements. Dashed line
shows the gradient of Hc2(T) obtained from fitting near Tc.
kL(T) is the London penetration depth, and is ex-
pressed as kL(T) ꢃ kL(0)[1 ꢀ( T/Tc)4]ꢀ1/2. kL(0) is
39 nm for bulk Pb [4]. The Ginzburg–Landau
parameter is defined as j = k(T)/n(T), and j has
pffiffi
a critical value of 2=2 [5]. Above this value, a
superconductor belongs to Type II and below this
value to Type I. The j, estimated by using nGL(T)
and l obtained above, shows much larger value
pffiffiffi
than 2=2. Thus, nanowires in this work behave
as Type II superconductors, even though the bulk
is a Type I superconductor.
References
The DC current was biased to deposit Pb onto
nanopores in this work, while there have been
some report that single crystal nanowires were fab-
ricated by applying AC current and polycrystal for
DC current [7]. There is a high possibility that
nanowires fabricated in this work were made of
polycrystal and made electron mean free path l
shorter. Therefore j was enhanced and it is suggest
that nanowires behave like Type II super-
conductor.
The upper critical field Hc2(0) is given by the
formula, Hc2(0) = ꢀ0.6913(dHc2/dT) near Tc for
Type II superconductors in the dirty limit [8,9].
The value of dHc2/dT is obtained from Fig. 6. As
a result, Hc2(0) of the Pb nanowires is determined
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