Y-branched Bi nanowires with metal-semiconductor junction behavior

Article abstract of DOI:10.1063/1.1779956

The synthesis of Y-branched Bi nanowires (NW) by electrochemical deposition in the Y-branched nanochannels of anodic aluminum oxide (AAO) templates was analyzed. The stem and branches of Y-branched Bi NW were found to be single crystalline. It was found f

Full text of DOI:10.1063/1.1779956

Y-branched Bi nanowires with metal–semiconductor junction behavior
Yongtao Tian, Guowen Meng, Sujit K. Biswas, Pulickel M. Ajayan, Shuhui Sun, and Lide Zhang
Citation: Applied Physics Letters 85, 967 (2004); doi: 10.1063/1.1779956
View online: http://dx.doi.org/10.1063/1.1779956
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APPLIED PHYSICS LETTERS
VOLUME 85, NUMBER 6
9 AUGUST 2004
Y-branched Bi nanowires with metal–semiconductor junction behavior
Yongtao Tian
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei,
Anhui 230031, People’s Republic of China
a)
Guowen Meng
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei,
Anhui 230031, People’s Republic of China, and Department of Materials Science and Engineering,
Rensselaer Polytechnic Institute, Troy, New York 12180
Sujit K. Biswas
Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute,
Troy, New York 12180
Pulickel M. Ajayan
Department of Materials Science and Engineering, Rensselaer Polytechnic Institute,
Troy, New York 12180
Shuhui Sun and Lide Zhang
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei,
Anhui 230031, People’s Republic of China
(
Received 8 March 2004; accepted 8 June 2004)
Y-branched Bi nanowires (NWs) embedded in anodic aluminum oxide templates were synthesized
by electrochemical deposition. Transmission electron microscope observations revealed that the
“
stem” and the “branches” of the Y-branched Bi NWs are about 80 and 50 nm in diameter,
respectively. Selected area electron diffraction studies showed that both the stem and the branches
are single crystalline. Current–voltage measurement revealed that the parallel Y-branched Bi NWs
have characteristics of conventional metal–semiconductor junctions. Our approach to produce
one-dimensional metal–semiconductor junctions using Y-branched NWs consisting of only one kind
of semimetal and without any external doping can be exploited to create metal–semiconductor
junctions of other semimetals, which may find various applications in nanodevices. © 2004
American Institute of Physics. [DOI: 10.1063/1.1779956]
Quasi-one-dimensional (1D) nanostructures have be-
come the focus of intensive research owing to their unique
applications in mesoscopic physics and fabrication of nan-
odevices. They provide a good system to study the depen-
dence of electronic and thermal transport or mechanical
properties on dimensional reduction, and play an important
role as both interconnects and functional units in fabricating
electronic, optoelectronic, electrochemical, and electrome-
two opposite sides of the templates. This will also provide a
convenient approach to study the transport properties of 1D
heterojunction nanostructures. Segmented 1D heterojunction
in straight nanochannels of an anodic aluminum oxide
1
3
(AAO) template, such as, multimetal NWs, metal–CdSe–
1
4
15
metal NWs, Ag–Si NWs and Ni–CNTs junctions,
1
6
17
Pt Si –Si NWs and p-n ZnO NW junctions, have been
6
5
fabricated. Furthermore, Y-junction CNT heterostructures
embedded in AAO template were also fabricated, and
showed intrinsic nonlinear transport and rectifying behavior
1
,2
chanical nanodevices. Longitudinal heterojunctions within
D nanostructures are important to electronic and optoelec-
1
1
8,19
tronic nanodevices. Until now, segmented heterojunctions in
at room temperature.
individual 1D nanostructures, such as carbon nanotubes
Bismuth (Bi) is a semimetal with unusual electronic
properties that result from its highly anisotropic Fermi sur-
face, low carrier concentration, small effective mass, and
3
(
CNTs) and Si nanowires (NWs), single-walled CNTs and
4
5
carbide NWs, n-p junction Si NWs, n-p junction InP
NWs, p-n junction GaN NWs, InAs–InP junction NWs,
ZnO–ZnMgO junction NWs, and superlattice NWs of
5
6
7–9
20
long mean-free path of the carriers. Transport properties of
1
0
21–23
Bi NWs have attracted a great deal of interest,
investigation results
and the
5
11,12
21–23
GaAs/GaP, and Si/SiGe,
have been synthesized. Al-
revealed that a semimetal-to-
though these randomly oriented intramolecular 1D hetero-
junctions can be used as building blocks for nanodevices and
device fabrication, however, the device fabrication involves
dispersing, manipulating, and assembling, and is still very
complicated and unreliable for practical application. If seg-
mented heterojunctions in 1D nanostructures are synthesized
within a template, then nanodevices consisting of 1D hetero-
structures can be realized simply by adding electrodes on the
semiconductor transition takes place as the diameter of Bi
NWs decreases. When the diameter of Bi NWs is reduced to
about 50 nm and smaller, the Bi NWs are found to be semi-
conducting, while Bi NWs with a diameter of 70 nm and
larger are metallic. Our previous studies on the transport
properties of straight Bi NWs revealed similar results. We
have now synthesized metal–semiconductor junctions in
Y-branched Bi NWs using only one kind of semimetal (Bi)
and without any external doping. This was made possible as
the Y-branched Bi NWs in AAO template have large-
diameter “stem” segments which are metallic, and small-
2
4
a)
Author to whom correspondence should be addressed; electronic mail:
gwmeng@issp.ac.cn; mengg2@rpi.edu
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003-6951/2004/85(6)/967/3/$20.00 967 © 2004 American Institute of Physics
58.109.139.195 On: Mon, 24 Nov 2014 13:49:10
0
1

968
Appl. Phys. Lett., Vol. 85, No. 6, 9 August 2004
Tian et al.
FIG. 1. A SEM image of the AAO template with Y-branched nanochannels,
the inset shows the close-up of the Y-branched nanochannels. The dashed
line shows where the Y branches start to grow.
diameter “branched” segments which are semiconducting. In
this letter, we report the fabrication of Y-branched Bi NWs,
and the characteristics of their electronic transport properties.
AAO templates with Y-branched nanochannels were fab-
1
8
ricated by using a similar method reported previously:
High-purity Al foils were anodized in 0.3 M oxalic acid so-
lution at 50 VDC for 4 h. After chemically removing the
original film in a mixed solution of phosphoric acid
͑
6 wt %͒ and chromic acid ͑1.8 wt %͒ at 60 °C for 6 h, a
second anodization was performed under the same condi-
tions for 4 h. The anodization voltage was then reduced to
ͱ
3
5 VDC (by a factor of 1/ 2), and another anodization was
performed for 4 h. This will lead to nearly all large diameter
FIG. 2. (a) XRD pattern taken from Y-branched Bi NWs embedded in AAO
template. (b) TEM image of a single Y-branched Bi NW. The four SAED
patterns were taken from the two branches, the stem, and the branch near the
branching point, respectively.
channels (formed at 50 V) branched into two smaller-
diameter channels (formed at 35 V). After the remaining Al
was removed in a saturated SnCl solution, the templates
1
8
4
were immersed in a 5 wt % phosphoric acid solution at
3
0 °C for 90 min to remove the barrier layer, and to widen
[Fig. 2(b)] reveals that the stem of the NW is about 80 nm in
dameter and the branches are about 50 nm in diameter, in
agreement with those of the Y-branched nanochannels in the
template. Selected area electron diffraction (SAED) patterns
taken from both the branches and the stem [Fig. 2(b)] reveal
that the rhombohedral lattice structure is preserved in the Bi
NWs, in agreement with XRD results. Along the axes of both
the stem and the branches, the diffraction patterns are iden-
tical, revealing that both the branches and the stem are es-
sentially single crystalline, and the Bi NWs preferentially
grow along the (101) stacking face. However, the diffraction
pattern taken from the branch near the branching point [Fig.
2(b)] is not regular, indicating that this domain is polycrys-
talline, which may be caused by variations in the growth
orientation of the branches.
the nanochannels homogeneously over the entire pore
1
2
length. Figure 1 is a scanning electron microscope (SEM)
image of the cross section of the AAO template with
Y-branched nanochannels. The inset of Fig. 1 shows the
close-up of the region between stems and branches. The re-
sulting AAO template consists of Y-branched nanochannels
with stems and branches about 80 and 50 nm in diameter,
respectively.
The electrochemical deposition of Bi into the
Y-branched nanochannels of AAO template was carried out
using a similar method to the one previously reported.
gold layer was sputtered onto the bottom side (with branched
channels) of the AAO template serving as the working elec-
trode in a three-electrode electrochemical cell. The electro-
lyte contained 75 g/l Bi͑NO ͒ ·5H O, 65 g/l KOH, 50 g/l
tartaric acid, and 125 g/l glycerol. The electrolyte was buff-
ered to pH=0.9 with HNO solution. The electrodeposition
2
4,25
A
The current–voltage ͑I–V͒ characteristic of the parallel
Y-branched Bi NWs embedded in AAO template was mea-
sured at room temperature, as illustrated in Fig. 3(a) (inset).
The result reveals that the I–V curve [Fig. 3(a)] is nonlinear
and asymmetrical about zero bias. This characteristic is
analogous to Schottky junction between a metal and a degen-
3
3
2
3
+
was performed under −30 mV relative to the Ag /AgCl ref-
erence electrode. X-ray diffraction (XRD) pattern [Fig. 2(a)]
taken from the Y-branched Bi NWs embedded in AAO tem-
plate shows that all peaks are located very close to the peak
positions of bulk Bi, revealing that the rhombohedral lattice
structure of bulk Bi is reserved in the Y-branched NWs. It
should also be noted that the strongest peak of the
Y-branched Bi NWs is (202), while other peaks are weak,
indicating that most of the Bi NWs are oriented perpendicu-
lar to the (202) lattice plane. After dissolving the AAO tem-
plate in a 5 wt % NaOH olution, transmission electron mi-
2
6
erate n-type semiconductor. The possible energy diagram
for the device is shown in Fig. 3(b) (inset). The observed
characteristics can be explained by assuming that the semi-
conductor is degenerate at room temperature with conduction
electrons available from the bottom of the conduction band
E to the Fermi energy E . The I–V curve shows a kink close
c
F
to zero bias on the positive side. This is highlighted on the
conductance curve, which was derived from the I–V curve
by numerical differentiation [Fig. 3(b)]. The conductance
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croscope (TEM) observation of a single Y-branched Bi NW
158.109.139.195 On: Mon, 24 Nov 2014 13:49:10

Appl. Phys. Lett., Vol. 85, No. 6, 9 August 2004
Tian et al.
969
respectively. I–V measurement revealed that the parallel
Y-branched Bi NWs have characteristics of conventional
metal–semiconductor junctions. Our result showed that 1D
metal–semiconductor junctions could be realized by
Y-branched NWs consisting of only one kind of semimetal
(Bi) and without any external doping, accordingly other
semimetals should be considered for creating metal–
semiconductor junctions with Y-branched NWs. These semi-
metal Y-branched NWs with metal–semiconductor junction
characteristics may find various applications in nanodevices.
The authors would like to thank Professor Wenhai Song
and Yuping Sun for assistance. This work was supported by
the Natural Science Foundation of China (Grant No.
1
0374092), and the National Major Project of Fundamental
Research: Nanomaterials and Nanostructures (Grant No.
9994506). Two of the authors (G.W.M. and P.M.A) are
1
grateful to RPI and Philip Morris USA for financial support.
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Our Y-branched Bi NWs with diameters of about 80 and
0 nm for the stem and the branches respectively, show
22
5
2
2
3
4
metal–semiconductor junction properties without any exter-
nal doping.
In conclusion, we have synthesized Y-branched Bi NWs
by electrochemical deposition in the Y-branched nanochan-
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