Single crystalline nanowires of lead: Large-scale synthesis, mechanistic studies, and transport measurements

Article abstract of DOI:10.1021/jp036758x

A solution-phase, precursor method has been demonstrated for the large-scale synthesis of single crystalline nanowires of lead with uniform diameters in the range 50-90 nm and lengths up to several millimeters. In a typical procedure, the nanowires were synthesized by refluxing a mixture of Pb(CH₃COO)₂?·3H₂O and poly-(vinyl pyrrolidone) (PVP) in ethylene glycol under N₂ for 90 min. Elemental lead was generated as a result of thermal decomposition of the precursor salt. Electron microscopic studies indicated that a solution-liquid-solid (SLS) growth mechanism was responsible for the nucleation and growth of lead atoms into single crystalline nanowires. When the hot reaction solution was injected into a cold ethanol bath through a syringe needle, bundles made of uniaxially aligned lead nanowires were formed. It was found that both composition and morphology of the product had a strong dependence on the reaction conditions that included the molar ratio between PVP and lead acetate, the reaction temperature, and the environment (nitrogen versus air). As the parameters were varied, nanostructures with a range of morphologies and compositions were observed as the products. Transport measurements on individual lead nanowires (as thin as a??50 nm in diameter) suggest a transition to the superconducting state around 7.13 K, a temperature similar to the value (7.20 K) reported for bulk lead.

Full text of DOI:10.1021/jp036758x

J. Phys. Chem. B 2004, 108, 8631-8640
8631
Single Crystalline Nanowires of Lead: Large-Scale Synthesis, Mechanistic Studies, and
Transport Measurements†
‡
‡
§
,‡
Yuliang Wang, Xuchuan Jiang, Thurston Herricks, and Younan Xia*
Department of Chemistry and Department of Materials Science and Engineering, UniVersity of Washington,
Seattle, Washington 98195-1700
ReceiVed: September 15, 2003; In Final Form: NoVember 13, 2003
A solution-phase, precursor method has been demonstrated for the large-scale synthesis of single crystalline
nanowires of lead with uniform diameters in the range 50-90 nm and lengths up to several millimeters. In
a typical procedure, the nanowires were synthesized by refluxing a mixture of Pb(CH
3
COO)
2
2
‚3H O and poly-
(
vinyl pyrrolidone) (PVP) in ethylene glycol under N for 90 min. Elemental lead was generated as a result
2
of thermal decomposition of the precursor salt. Electron microscopic studies indicated that a solution-liquid-
solid (SLS) growth mechanism was responsible for the nucleation and growth of lead atoms into single
crystalline nanowires. When the hot reaction solution was injected into a cold ethanol bath through a syringe
needle, bundles made of uniaxially aligned lead nanowires were formed. It was found that both composition
and morphology of the product had a strong dependence on the reaction conditions that included the molar
ratio between PVP and lead acetate, the reaction temperature, and the environment (nitrogen versus air). As
the parameters were varied, nanostructures with a range of morphologies and compositions were observed as
the products. Transport measurements on individual lead nanowires (as thin as ∼50 nm in diameter) suggest
a transition to the superconducting state around 7.13 K, a temperature similar to the value (7.20 K) reported
for bulk lead.
Introduction
conditions for electrochemical deposition. Fourth, any defects
on the surface of the channels would result in the formation of
One-dimensional (1D) nanostructures (in the form of wires,
rods, belts, and tubes) have received considerable attention in
recent years due to their novel properties and intriguing
applications in a number of different areas. Metal nanowires,
for example, have been the focus of many studies for their
important roles as active components and/or interconnects in
fabricating nanoscale electronic or optoelectronic devices.
Among various metals that have been studied as nanowires, lead
is a particularly attractive, important, and challenging target as
nanowires with poorly defined structures and shapes, i.e., kinks
or nonuniform thicknesses. The last but not least, additional
steps are often required in order to separate the nanowires from
the template and obtain individual nanowires for use in property
measurements or device fabrication.
1
2
We have recently demonstrated a solution-phase approach
that generated silver nanowires with uniform diameters and in
large quantities by reducing silver nitrate with ethylene glycol
5
3
in the presence of poly(vinyl pyrrolidone) (or PVP). In this
a result of its superconductivity and high reactivity. Uniform
so-called “polyol process”, which has been extensively exploited
by other groups for the synthesis of metal nanoparticles, ethylene
glycol serves as both reducing agent and solvent for the reaction.
The key component of this synthesis is PVP that acts as a
capping reagent to confine and direct the growth of seeds into
nanowires of lead are usually synthesized via electrochemical
deposition by templating against channels etched in various
porous membranes or atomic steps on various solid substrates.
4
Although the use of physical templates may have the advantages
of predefined 1D morphology and controllable wire diameters,
there are also a number of intrinsic shortcomings or limita-
tions: First of all, the total number of wires that can be produced
in each run of synthesis is restricted by the density of channels
in a membrane or the number of steps on a substrate. For a
typical polycarbonate or anodic alumina membrane, the density
1
D nanostructures. In the present study, the similar polyol
method (but with a different mechanism) was employed to
generate single crystalline nanowires of lead with long, uniform
diameters and in copious quantities. More specifically, the
production of lead nanoparticles, i.e., the initial step, was realized
via the thermal decomposition (instead of reduction as it was
for silver) of a lead precursor salt, Pb(CH3COO)2‚3H2O, in
boiling ethylene glycol under the protection of N2. Here ethylene
glycol merely served as a high-boiling-point solvent, which
could be refluxed at a temperature sufficiently high to initiate
and maintain the thermal decomposition of lead acetate. As
refluxing was continued, small nanoparticles of lead were
produced through homogeneous nucleation, and some of them
further grew into micrometer-sized crystals. These microcrystals
could serve as the roots in subsequent steps to initiate the growth
of lead nanowires. The small lead nanoparticles remained in
the solution would continuously provide a source of lead atoms
of pores is usually below 1012 /cm2. Second, it still remains a
grand challenge to form nanowires longer than 20 µm by
templating against porous membranes. Third, most of the
samples prepared by using the template-directed method were
polycrystalline, although it was possible (as demonstrated by
Schwarzacher et al.4 and Jalochowski et al.
c
4d,4e
) to generate
single crystalline lead nanowires by carefully controlling the
†
Part of the special issue "Alvin L. Kwiram Festschrift".
*
To whom correspondence should be addressed. E-mail: xia@
chem.washington.edu.
‡
§
Department of Chemistry.
Department of Materials Science and Engineering.
1
0.1021/jp036758x CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/20/2004

8
632 J. Phys. Chem. B, Vol. 108, No. 25, 2004
Wang et al.
for prolonged wire growth through processes such as Ostwald
ripening. Experimental evidences also indicated that a solution-
liquid-solid (SLS) growth mechanism was involved in the
initiation and growth of single crystalline lead nanowires. In
general, each nanowire was composed of three components:
root (the microcrystal), stem (the nanowire), and tip (the liquid
droplet of lead). After the solution had been refluxed under N2
for 90 min, single crystalline nanowires of lead with diameters
in the range 50-90 nm and lengths up to several millimeters
could be easily separated from the hot reaction mixture by
centrifugation. Compared to template-directed synthesis, our
method allowed for the production of uniform lead nanowires
in bulk quantities (i.e., on a scale close to the Avogadro’s
number). Different reaction conditions (e.g., the molar ratio
between PVP and lead acetate, the temperature, and the reaction
environment) were all found to have a profound impact on the
morphologies and compositions of the final products. The
electron transport properties of these lead nanowires were also
studied in this work, and measurements on individual wires (with
diameters down to ∼50 nm) suggest a transition to the
superconducting state around 7.13 K.
quickly in a microcentrifuge (Model V, VMR Scientific) at a
speed of 10 000 rpm for 3 min. The collected sample was then
washed with ethanol and the dispersed on a silicon substrate or
TEM grid.
Formation of Bundles of Uniaxially Aligned Nanowires.
After the reaction had proceeded for 90 min, the hot reaction
mixture was drawn into a 10-mL glass syringe (MICRO-MATE
interchangeable, Popper & Sons) through a stainless steel needle.
Without delay, the solution was quickly injected into 500 mL
of cold ethanol hosted in a 1000-mL beaker, which had been
cooled in a freezer in advance. Precipitates with a threadlike
morphology could be observed instantly after the injection. The
supernatant was decanted carefully after all solids had settled
down to the bottom of the beaker. In the following step, fresh
ethanol was added slowly to the beaker to rinse the precipitates.
After filtration through a membrane, the nanowire bundles were
dried in a desiccator connected to an aspirator and stored in an
inert environment.
Transport Measurements. In a typical procedure, a freshly
prepared nanowire of lead was deposited from a very dilute
dispersion in ethanol (via slow solvent evaporation) onto four
gold electrodes (10 µm wide, 200 nm thick, and separated by
25 µm) that had been patterned on a glass substrate (micro slides
Experimental Section
2
947, Corning). Due to the low melting point of lead, a good
Chemicals and Materials. Anhydrous ethylene glycol (EG,
contact between the lead nanowire and the gold electrode could
be ensured via brief heating by focusing the electron beam of
a scanning electron microscope on the contacted region. For
the same reason, the currents used in all measurements were
kept below 1.5 µA to avoid possible melting of the nanowires.
Silver paste (Chemtronics, Kennesaw, GA) and copper wires
were used to connect the pads of gold electrodes to the
instrument. A Keithley 236 sourcemeter was used to supply the
current and voltages were measured using a Keithley 155
microvoltimeter. A continuous flow cryostat (SuperTran-VP,
Janis Research) was used to conduct the measurements of
resistance at temperatures down to 4.2 K.
Instrumentation. Scanning electron microscopy (SEM) im-
ages were taken using a field emission microscope (Sirion, FEI,
Portland, OR) operated at an acceleration voltage of 5 kV.
Transmission electron microscopy (TEM) images and selected
area electron diffraction (SAED) patterns were taken using a
JELO-EX II microscope operated at 80 kV. The samples were
prepared by placing one drop of the alcohol suspension on a
silicon substrate or TEM grid and dried in a desiccator connected
to an aspirator. High-resolution TEM (HRTEM) images were
taken using a TOPCON 002B microscope operated at 160 kV.
Powder X-ray diffraction (XRD) patterns were recorded from
a large quantity of lead nanowires supported on a glass slide
9
9.8%, Aldrich or Fisher), lead acetate trihydrate (Pb(CH3-
COO)2‚3H2O, 99+%, Aldrich), poly(vinyl pyrrolidone) (PVP,
Mw ≈ 55 000, Aldrich), and ethanol (200 proof, Aaper) were
all used as received without further purification.
Synthesis of Lead Nanowires. Uniform nanowires of lead
were generated by refluxing an ethylene glycol (Aldrich)
solution that contained both lead acetate (the precursor) and
PVP. In a typical synthesis, 0.1 g of Pb(CH3COO)2‚3H2O and
0.1 g of PVP were separately dissolved in 5 mL ethylene glycol
and then simultaneously added dropwise (using pipets) into 10
mL of boiling ethylene glycol hosted in a three-neck flask. The
reaction was protected with a continuous flow of N2 gas and
kept under constant magnetic stirring. A Glas-Col hemispherical
heating mantle (Series O, 380 W, ChemGlass) was used to
provide the heat required for this reaction. As the reaction
proceeded, the solution underwent gradual changes in color from
clear, colorless (0 min) to light yellowish opaque (∼25 min),
then brownish gray (∼45 min), and finally dark black turbid
(∼90 min). Darkening of the reaction mixture indicated the
appearance of lead due to the thermal decomposition of the
precursor salt and its continuous increase in quantities. After
the reaction had been refluxed for about 90 min, the final
product, i.e., lead nanowires, had to be collected by centrifuging
the hot solution at 4000 rpm. In more detail, the hot reaction
mixture was poured directly into a glass centrifugation tube
within a few seconds and immediately centrifuged for 3 min.
The supernatant was carefully decanted and ethanol was added
to wash off the PVP and residual EG from the collected black
precipitate (lead nanowires) in the tube. After another step of
centrifugation, the cleaned nanowires were collected and stored
in a desiccator under the protection of N2 for future use. If
necessary, the washing process could be repeated several more
times until all the residual PVP and EG had been removed. It
has been found that if the synthesis was carried out in EG that
was purchased from Fisher, additional 30-60 min may be
required in order to get complete reaction.
(micro slides 2947, Corning) using a Philips PW1710 diffrac-
tometer (Cu KR radiation, λ ) 1.540 56 Å) at a scanning rate
of 0.02° per second for 2θ in the range from 10° to 70°.
Results and Discussion
Microscopic Studies on the Formation of Lead Nanowires.
To understand the reaction mechanism (such as the evolution
in morphology and composition), samples were collected at
different times and characterized by using both SEM and TEM.
Figure 1 shows SEM and TEM images of samples collected at
20, 30, 45, and 60 min after lead acetate and PVP had been
introduced, indicating all major distinctive morphologies as-
sociated with various stages of the reaction. Figure 1A shows
an SEM image of the solid sampled at 20 min, where platelike
whiskers with lengths around tens of micrometers and widths
of several micrometers were the major product. Further XRD
For electron microscopic studies, the samples were prepared
as follows: a small portion of the reaction mixture (∼1 mL)
was taken out of the flask during reaction using a pipet and
then transferred into a microcentrifugation tube and centrifuged

Single Crystalline Nanowires of Lead
J. Phys. Chem. B, Vol. 108, No. 25, 2004 8633
Figure 1. SEM images of four samples collected at different times after lead acetate had been introduced into the boiling ethylene glycol: (A) 20,
B) 30, (C) 45, and (D) 60 min. Each sample is characterized by distinctive morphologies and compositions: (A) platelike whiskers of basic lead
(
acetate; (B) whiskers of basic lead acetate and small (5-10 nm in diameter) nanoparticles of lead formed as a result of thermal decomposition (the
insets show a TEM image of the lead nanoparticles and their corresponding electron diffraction pattern); (C) a mixture of growing lead nanowires,
microcrystals, and nanoparticles; and (D) long nanowires of lead mixed with a small amount of lead microcrystals.
examination indicated that the composition of these whiskers
was basic lead acetate, an intermediate often involved in the
early stages of thermal decomposition of Pb(CH3COO)2‚3H2O.
As a matter of fact, a similar morphology was also observed
by Mohamed et al. in their studies on the thermal decomposition
of lead acetate in the solid phase.6 Figure 1B shows an SEM
image of the sample collected at 30 min, where the appearance
of many newly formed small nanoparticles can be easily
identified. A small amount of whiskers (similar to those shown
in Figure 1A, but with much smaller dimensions) could still be
found in this sample. Both TEM and diffraction studies (see
the insets of Figure 1B) indicated that the sizes of these small
particles were only a few nanometers across and they were
purely made of lead. As a result, the light yellowish color
observed for the reaction mixture can be attributed to the Mie
scattering of such nanoparticles of lead.
performed in a liquid phase such as in ethylene glycol (bp )198
°C) and under the protection of N2 gas. It is believed that the
minimum temperature required to decompose lead acetate into
metallic lead could be greatly reduced in the polyol process, as
it was now dissolved in a solution in the form of molecular
species. In addition, as discussed by Buhro et al. in their SLS
a
7
a
growth of semiconducting nanowires, the use of a heating
mantle (whose temperature could reach as high as 450 °C) might
generate local hot spots on the wall of reaction container with
their temperatures much higher than the boiling point of ethylene
glycol. The thermal decomposition of lead acetate could thus
be greatly facilitated due to the presence of such hot spots.
A control experiment was also carried out to verify that there
was, in fact, no reduction function associated with ethylene
glycol in the present synthesis. In this experiment, lead nitrite
that has a much higher decomposition temperature (∼470 °C)
was added (together with PVP) to ethylene glycol and refluxed
in the same way as we did for the thermal decomposition of
lead acetate. No production of lead was observed, even after
prolonged periods of heating to 5 h. This result suggested that
the production of lead was not due to the reduction of Pb2 by
ethylene glycol but rather was due to thermal decomposition.
In comparison with lead nitrate, lead acetate happened to
decompose at a sufficiently low temperature to be used with
ethylene glycol. For the sample collected at 45 min, the solution
exhibited a brownish gray color, and an SEM image of this
In the past, thermal decomposition of lead acetate in the solid
6
state has been studied by a number of groups. Depending on
the temperature (up to 450 °C) and environment (under N2 or
in air), both Pb and PbO had been identified as the final
products, together with several types of basic lead acetate as
the intermediates. According to these studies, the minimum
temperature required for the production of metallic lead was
around 325 °C. Here we found that lead could be formed as
the predominate product (together with the formation of acetic
acid as a byproduct) when the decomposition reaction was
+

8634 J. Phys. Chem. B, Vol. 108, No. 25, 2004
Wang et al.
Figure 2. (A) Typical SEM image of the as-synthesized lead nanowires that were 85 ( 5 nm in diameter and up to several millimeters in length.
The inset shows the cross section of a fractured nanowire. (B) XRD pattern recorded from the same batch of lead nanowires. (C) A TEM image
of nanowires taken from the same sample as shown in (A). The inset gives a microdiffraction pattern recorded by focusing the electron beam on
an individual wire. (D) HRTEM image taken from the edge of a lead nanowire. The fringe spacing along the [111] axis is indicated.
sample is given in Figure 1C. Compared to Figure 1B, no
whiskers of basic lead acetate existed anymore, suggesting that
all of them (produced in earlier stages) had been fully
decomposed into elemental lead. In addition to small lead
nanoparticles (similar to those appearing in Figure 1B), two new
types of morphologies were found predominantly in this
image: wirelike nanostructures of lead that represents the
synthetic target of this work and micrometer-sized crystals that
were also made of pure lead. For samples collected after the
solution had been refluxed for 60 min, a typical SEM image is
given in Figure 1D. Here we see mostly lead nanowires with
lengths of up to tens of micrometers, together with a small
portion of micrometer-sized crystals that were also observed
for the sample collected at 45 min. However, only a small
amount of lead nanoparticles (as those shown in Figure 1B)
could be found in this sample. As the reaction time was further
increased, the percentage of lead nanowires in the final product
and the lengths of these nanowires were both greatly increased.
For samples collected after the precursor salt had been intro-
duced for 90 min, the yield of lead nanowires could approach
and TEM (Figure S1) further indicated that about 80% of them
had their diameters confined to the range of 75 ( 10 nm, albeit
wires as thin as 30 nm could also be found in the final product.
Figure 2A shows the SEM image of a typical sample (after PVP
and EG had been removed), illustrating the uniformity, straight-
ness along the long axis, and the copiousness in quantity that
could be routinely achieved using this new synthetic approach.
As indicated by an arrow, there was also a small amount of
platelike structures in this sample, and the lead nanowires
seemed to grow out of their tapered corners. These plates, in
fact, had the same pure composition (Pb) as the particulate
products shown in Figure 1C,D. The only difference was that
each platelike crystal had at least one nanowire growing out of
its tapered corners. As discussed in more detail later, these
micrometer-sized plates actually served as the roots for the
growth of long, uniform lead nanowires during the reaction.
The inset of this image provides a closer look at the end of a
fractured nanowire, and it clearly indicates that each lead
nanowire synthesized using the present method had a rectangular
cross section. Figure 2B shows the XRD pattern taken from
the same batch of sample (in a bulk quantity), implying that
the nanowires obtained using this solution-phase method
crystallized purely in the face-centered cubic phase of lead.
Figure 2C shows a TEM image of several lead nanowires
that were prepared from the same batch of sample shown in
Figure 2A, and the uniformity in diameter along each individual
wire could also be seen from this image. The inset displays a
typical microdiffraction pattern that was obtained from an
individual nanowire when the beam was oriented parallel to
the [ 1h 11] axis. All the diffraction spots could be indexed to the
100%, with their lateral dimensions being uniformly distributed
in the range 50-90 nm and lengths up to several millimeters.
Structural Characterization of the Lead Nanowires. After
lead acetate had been refluxed with PVP in ethylene glycol for
9
0 min, the lead nanowires had to be harvested immediately
through centrifugation (at 4000 rpm) while the reaction mixture
was still hot. The collected solid was also washed with ethanol
to remove excess PVP and EG coated on the nanowires. The
as-obtained nanowires were often thinner than 90 nm in diameter
and with lengths up to several millimeters. Studies by SEM

Single Crystalline Nanowires of Lead
J. Phys. Chem. B, Vol. 108, No. 25, 2004 8635
face-centered cubic phase of lead. The diffraction pattern was
essentially unchanged as the electron beam was scanned across
each individual wire, suggesting that these lead nanowires were
single crystalline in structure. Electron diffraction studies also
indicated a growth direction along the [110] axis. The HRTEM
image shown in Figure 2D was taken from the edge of a
nanowire. With the use of fast Fourier-transfer technique, the
lattice spacing of this single crystalline nanowire could also be
resolved and indexed to that of face-centered cubic lead. Because
the melting point of lead is relatively low (328 °C), the nanowire
was highly susceptible to beam damage (especially for the
relatively thinner region like the edge of a wire) when it was
exposed to a flux of high-energy electrons. As a result, it is not
unusual to observe that the right edge of this wire appears to
have stacking defects, an artifact that might be caused by
electron-beam-induced damage.
are also shown in Figure 3F,G, where two additional TEM
images are given. Figure 3G, in particular, shows the TEM
image of a wire that was still in its early stage of growth and
both root and tip could be clearly observed in the same image.
The presence of a root, and especially a particulate tip, on the
opposite ends of a growing wire suggests that the growth of
lead nanowires could share a mechanism similar to the solution-
liquid-solid (SLS) process that has been explored by Buhro et
al. and Korgel et al. for the synthesis of highly crystalline
7
nanowires of various semiconductors. A typical and important
component of their synthesis was the use of a low melting point
metal that could exist in the form of liquid droplets during the
reaction. These liquid droplets could act as the solvent to
dissolve (or suck in) the desired material in the form of atomic
(or ionic) species. In the subsequent step, recrystallization could
lead to the formation of nanowires or filaments with diameters
of 10-150 nm and lengths up to several micrometers. In the
present synthesis, most of the small lead nanoparticles produced
via the thermal decomposition should be in their molten states
because of the relative low melting point of lead. For particles
that were able to grow into larger crystals through a homoge-
neous nucleation process, they should exist in the solid phase
due to their enlarged dimensions (on the scale of micrometers).
These large crystals could function as roots to initiate the growth
of nanowires. As shown in Figure 3A-G, the wire growth
normally started from the sharpest corner or thinnest end of a
micrometer-sized root. For these regions with small areas and
large curvatures, the surface atoms tended to be highly mobile
and reactive at the boiling point of EG. As a result, lead atoms
from the melted nanoparticles could be attracted to these reactive
sites and eventually formed liquid droplets. Once the sizes of
these liquid droplets had reached a critical value, the lead atoms
would start to crystallize into a solid at the interface between
the droplet and the root. As more lead atoms were driven into
the droplets via Ostwald ripening, the growth could continue
until all of the small lead nanoparticles had been completely
consumed. Figure 3H shows a schematic illustration of this
growth mechanism, where the root, stem, and tip of a nanowire
are all specified.
Mechanism for the Growth of Lead Nanowires. The
growth of lead nanowires included at least four distinctive
steps: (i) formation of elemental lead via the thermal decom-
position of lead acetate, (ii) growth of the lead atoms into
nanoscale particles through a homogeneous nucleation process,
(
(
iii) further growth of some lead nanoparticles into larger crystals
or roots) with their sizes on the scale of several micrometers,
and (iv) nucleation and growth of nanowires from the corners
of microcrystals at the expense of small nanoparticles as driven
8
by Ostwald ripening. This mechanism is consistent with all
the images shown in Figure 1. With the sample collected at 45
min as an example, lead particles of two different sizes
(micrometer versus nanometer) could be clearly seen in Figure
1
C. This mechanism also resembles the one involved in the
synthesis of silver nanowires, where silver particles of two
distinctive sizes were found to coexist in the early stage of the
reaction. It was further demonstrated that the silver particles of
larger sizes could serve as seeds to initiate the growth of
nanowires, as silver particles of smaller sizes were continuously
dissolved into the solution phase and regrew onto the surfaces
of larger ones as a result of Ostwald ripening. The growth
should, in principle, continue until all the small particles had
been completely consumed. However, there were some major
differences between these two systems: (i) the roots of lead
were single crystals with dimensions on the scale of micrometers
while the seeds of silver were multiple-twined particles (MTPs)
of only 20 to 30 nm in size; (ii) each lead nanowire was a single
crystal with a rectangular-shaped cross section while the silver
nanowire was multiply twined with a pentagonal cross section.
On the basis of these observations, the growth mechanisms
involved in these two systems should be different.
We have also noticed that a large number of the lead roots
displayed a planar or platelike morphology, and the wires
growing out of the thin ends of these plates seemed to be
coplanar with their roots. This is supported by the SEM image
at the bottom of Figure 3I, which gives the side view of a
nanowire that was still at its early stage of growth. The SAED
patterns acquired from the root and stem regions of another
nanowire are also shown as the insets of Figure 3I. Both of
them exhibited the same type of diffraction spots with a
hexagonal symmetry that are normally obtained when the beam
direction is parallel to the [111] axes. Since the root of this
nanowire had a platelike morphology, the placement of this plate
should be parallel to the surface of the TEM grid. It is thus
reasonable to conclude that both top and bottom surfaces of
the thin plate were terminated in the {111} planes and that the
wire growing out of the end of this plate was enclosed by the
same set of planes. If we take a closer look at the pattern
obtained from the root, it also exhibited some features that were
not presented in the pattern taken from the stem. One feature is
the appearance of diffraction rings that are usually associated
with a polycrystalline material. Analysis on the d values of these
rings indicated that they all originated from the face-centered
cubic lead, similar to the index shown in Figure 1B. The
appearance of these rings could be due to the deposition of some
small lead nanoparticles onto the relatively large surfaces of
As we have noted in Figure 2A, the lead nanowire was often
initiated from one of the corners of a micrometer-sized, platelike
crystal. To better understand the growth mechanism, we
carefully examined the sample collected at 60 min (the same
as shown in Figure 1D) by electron microscopy and diffraction.
Figure 3A-C shows the SEM images of several typical
examples, illustrating how the nanowires were initiated and grew
from the corners of roots. Despite their differences in sizes and
shapes for the micrometer-sized crystals, all of them could serve
as roots for the initiation of nanowire growth. Figure 3D shows
the SEM image of a nanowire that had been fractured (by
sonication) at a position close to the root. It is clear that this
nanowire also exhibits a rectangular cross section similar to the
one illustrated in the inset of Figure 2A. For the nanowire
indicated by an arrow in Figure 3E, a spherical particle was
also observed at the end opposite to the root. The inset of this
image gives a blow-up view of the tip. Similar tip structures

8636 J. Phys. Chem. B, Vol. 108, No. 25, 2004
Wang et al.
Figure 3. (A-D) SEM images of several lead nanowires, with focus on their roots. (E) The SEM image of a growing lead nanowire, showing both
root and tip. The inset gives a magnified view of the tip. (F) A TEM image of two nanowires whose tips are terminated in droplets. (G) TEM image
of another growing nanowire that was still at its early stage of growth, with its root, stem (wire), and tip all clearly recognizable. (H) Schematic
illustration of the proposed mechanism responsible for nanowire growth. (I) TEM image of a growing nanowire and the SAED patterns (insets)
recorded from its root and stem (wire), respectively. The side view SEM image of another wire at its early stage of growth, indicating that both root
and stem of this wire were confined to the same plane.
the platelike root during sample preparation. Another feature is
that there was also another set of diffraction spots with
hexagonal symmetry located in the inner area of the diffraction
pattern (as marked by a circle). These spots could be attributed
to the (1/3){422} reflection that is forbidden for a face-centered
cubic lattice. Because of the close match of their d values (2.90
Å versus 3.03 Å), these six spots overlapped with the {111}
diffraction ring that originated from the small lead nanoparticles.
As has been discussed in previous work on silver or gold
As a result, the side surfaces of a silver nanowire, enclosed by
{100} planes, could be preferentially stabilized; while the ends,
terminated in {111} planes, could be kept active for continuous
growth. However, the PVP seems to play a different role in the
synthesis of lead nanowires. As discussed above, the formation
of highly anisotropic nanostructures was mainly directed by the
liquid droplet at the tip of each growing nanowire. In this case,
the kinetic effect (as mediated by the adsorption of PVP) seems
to only play a minor role in inducing and maintaining 1D
growth. We believe that the major function of PVP was to
prevent the lead nanoparticles (which might also exist as liquid
droplets in the boiling EG) from aggregating into large entities
in the nucleation stage. As a result, only a small portion of the
lead nanoparticles could grow into micrometer-sized crystals
(or roots) and most of them were left in the solution to serve as
a source for lead atoms in the growth step.
The percentage of lead nanowires in the final product, as well
as the morphology of other lead structures produced during the
reaction, was found to strongly depend on the molar ratio
between PVP and lead acetate. When the ratio was around 3.5,
the content of lead nanowires contained in the as-synthesized
sample could approach 100%, as demonstrated by Figure 2A.
However, if the ratio was decreased or increased to other values,
lead structures of different morphologies were also found in
the final product, with typical examples including belts and
hexagonal (or triangular) thin plates. Depending on the exact
value of this ratio, these new types of morphologies could even
9
nanostructures, the apperance of these forbidden spots was often
associated with thin structures that were enclosed by atomically
flat top and bottom facets. It thus provides another piece of
evidence to support our claim that the root is characterized by
a platelike morphology, with a pair of flat {111} surfaces
oriented parallel to the TEM grid.
Influence of the Molar Ratio between PVP and Lead
Acetate. The exact role of PVP in controlling the formation of
lead nanowires is yet to be completely understood. As estab-
lished in a recent study related to the growth of silver
5
d
nanowire, the PVP molecules were able to modulate the
growth kinetics of a face-centered cubic metal by interacting
with various crystallographic facets at different strengths. For
example, the PVP molecules were found to interact more
strongly with the {100} facets than with the {111} ones. This
selectivity can be attributed to the difference in configuration
for the atoms on these surfaces, which may enhance or hinder
their coordination to the repeating units of PVP macromolecules.

Single Crystalline Nanowires of Lead
J. Phys. Chem. B, Vol. 108, No. 25, 2004 8637
their sizes were normally on the micrometer-sized scales. This
result suggests that, without the protection of PVP, most of the
nanoparticles produced in the early stage preferred to grow into
micrometer-sized roots, and only a small portion of the lead
nanoparticles, which were still dispersed in the solution, could
provide a very limited source of lead necessary for the
continuous growth of leas nanowires. When this ratio was
increased from 3.5 to 9 and 18, the final products were found
to exist as sheetlike structures (Figure 4B) and hexagonal (or
triangular) flakes (Figure 4C), respectively. Electron diffraction
patterns taken from these structures indicate that they were all
made of pure lead, with the flat surfaces being {111} planes.
The appearance of these sheetlike structures might reflect the
difference in interaction between PVP and various crystal-
lographic planes, as we discussed for the preparation of silver
nanostructures. A systematic study on this matter is under way.
Influence of the Reaction Temperature. Temperature was
found to be another key factor that could affect the composition
and morphology of the resultant product. Ethylene glycol had
to be heated to its boiling point (i.e., ∼198 °C) in order to initiate
and maintain the decomposition of lead acetate into elemental
lead. At temperatures lower than this value (e.g., at 190 °C),
no elemental lead could be produced in the EG solution. Instead,
several other types of lead salts (e.g., basic lead acetate and
lead glycolate) were obtained as the final product, depending
on the reaction time and temperature (in the range of 160 to
1
90 °C).
It was found that variation in temperature might also change
the direction of reaction in the present synthesis. For example,
after the reaction had proceeded for about 90 min and nanowires
of lead had been produced in the form of a dark black, turbid
solution, the reaction mixture could quickly fade into a clear
light-yellow color within less than 10 min if heating was turned
off. If this light yellow solution was heated to boil again, its
dark color would be restored, and lead nanowires (of the quality
and quantity similar to the sample shown in Figure 2A) could
be obtained as the final product. We believed that the cause of
this phenomenon was a result of the balance between two
possible directions of a reversible reaction. When the temper-
ature was sufficiently high, it could provide the heat required
for the thermal decomposition of lead acetate, and elemental
lead could be obtained as the predominant product, together
with acetic acid as a major byproduct. If the temperature was
lowered, the reaction direction would be reversed, and the lead
nanowires would react with the acetic acid to produce lead
acetate as the product. As shown by Sankarapapavinasam et al.
in their studies on the corrosion rate of Pb in acetic acid,10 at a
relatively high temperature (e.g., immediately after heating was
turned off), this reverse reaction was much faster when
compared to the case at room temperature. As a result, the color
of reaction solution quickly faded into light yellow. Since lead
acetate can be easily dissolved in ethylene glycol, the resultant
reaction mixture appeared as a clear solution. Figure 5 shows
TEM images of a sample prepared from the light yellow
solution, indicating the existence of very small nanoparticles
with sizes around several nanometers. Electron diffraction
pattern (the inset) taken from an assembly of these particles
indicated that they were purely made of face-centered cubic lead.
The light yellow color exhibited by the reaction solution could
be attributed to the Mie scattering of the small lead nanoparticles
stabilized by PVP macromolecules (same as for the sample
obtained at the early stage of this synthesis). Since some of the
acetate acid produced via thermal decomposition of lead acetate
had been evaporated from the reaction mixture during the
Figure 4. SEM images of three as-obtained samples illustrating the
variation in morphology when nonoptimized molar ratios between PVP
and Pb(CH COO) were used: (A) 0; (B) 9; (C) 18. These structures
3 2
were all made of pure lead, as confirmed by electron diffraction studies.
be obtained as the predominant ones in the final product. In
these cases, lead nanowires only existed in relatively low
quantities. Figure 4 shows the SEM images of three other
samples that were prepared under a condition similar to the one
used for Figure 2A, except that the molar ratio between PVP
and lead acetate was varied to values other than 3.5. Figure 4A
shows a typical image of the product where no PVP was added
to the reaction mixture. This image implies that the same type
of nanostructures as those shown in Figure 2A was formed,
and some of them even showed the same root and stem
structures when examined closely. The percentage of such
wirelike structures were, however, much lower. The majority
of structures in this sample exhibited irregular morphology and

8638 J. Phys. Chem. B, Vol. 108, No. 25, 2004
Wang et al.
Figure 5. Influence of temperature on the morphology of resultant
Pb nanostructures. The TEM image was taken from a sample prepared
after the reaction solution had been cooled to room temperature. The
insets give an enlarged view of the small nanoparticles and their
corresponding SAED pattern.
refluxing process, elemental lead should be in excess when
compared with the amount of acetic acid. As a result, a certain
amount of the lead nanoparticles remained in the solution even
after the reverse reaction had been completed. As limited by
the reverse reaction, the separation of lead nanowires from the
reaction mixture had to be completed within a couple of minutes
while the solution was still hot.
Stability of the Lead Nanowires under Ambient Condi-
tions. The lead nanowires had to be synthesized under the
protection of an inert gas such as nitrogen or argon. Otherwise,
elemental lead resulting from the thermal decomposition would
be quickly oxidized at elevated temperatures (in particular, in
the presence of CO2 and O2). After the nanowires had been
separated from the reaction solution, it was also necessary to
store them under an inert gas environment to prevent their
surfaces from oxidation. As shown by Schwarzacher et al. in
their TEM studies,4d a thin layer of PbO2 would be readily
formed on the surface of a lead nanowire after it had been
exposed to air for a certain period of time. Here we also found
that the lead nanowires tended to slowly change their morphol-
ogy by reacting with gaseous species in the environment. Basic
lead carbonate was identified as the major product by XRD
studies, no matter if the lead nanowires were stored as a dry
sample or as a suspension in ethanol. Figure 6A shows the SEM
image of a dry sample after the lead nanowires had been exposed
to the ambient environment of a chemical laboratory for 5 days.
This image clearly indicates that the majority of lead nanowires
had been converted to micrometer-sized flakes, whose appear-
ance could be well-resolved in the inset. Figure 6B gives a
typical XRD pattern recorded from this sample, indicating the
coexistence of both elemental lead and basic lead salt, Pb3-
Figure 6. (A) SEM image of the same lead nanowires shown in Figure
2A, after the sample had been left in air for several days. The inset
gives a closer look at the newly formed flakes of basic lead carbonate.
(B) XRD pattern obtained from this sample, indicating the coexistence
of both lead nanowires (dots) and basic lead carbonate particles (stars).
reaction). An alternative way to gather lead nanowires from the
hot EG solution was to quickly quench the reaction solution by
pouring the boiling mixture into a large quantity of cold ethanol.
Here the ethanol played a role to abruptly reduce the temperature
of the reaction system and to dilute the acetic acid to such an
extent that the reverse reaction rate can be minimized. The solid
that precipitated out in this process could then be collected using
conventional centrifugation. We occasionally observed that the
precipitate quenched out in the ethanol existed in a threadlike
morphology, which could be as long as ∼1 cm and even visible
to the naked eye. A closer examination by SEM indicated that
these threads were, in fact, thick bundles of many nanowires
that were uniaxially aligned along other longitudinal axes. If
we used a syringe to siphon the hot solution and immediately
injected it into cold ethanol through a needle, most of the
precipitates quenched out would exist in the threadlike morphol-
ogy. They all contained bundles of uniaxially aligned nanowires
when examined under an electron microscope. Figure 7 shows
the low-magnification (Figure 7A) and high-magnification
(Figure 7B) SEM images of a typical sample. From these two
images, we can also readily appreciate the copiousness in
quantity, uniformity in size, and relatively long length associated
with the nanowires contained in the bundles, as well as the
uniaxial alignment for nanowires within each bundle.
(CO3)2(OH)2. It was clear that the newly formed flakes shown
in Figure 6A corresponded to the basic lead salt, which is a
documented product for the reaction between Pb and H2O under
ambient conditions (i.e., in the presence of both O2 and CO2
gases).
Formation of Bundles of Uniaxially Aligned Lead Nanow-
ires. The collection of lead nanowires from the hot reaction
mixture was normally performed by centrifugation and then
washed with ethanol to remove physically adsorbed PVP. As
mentioned earlier, the collection process had to be performed
quickly to avoid any possible reverse reaction between lead and
acetic acid (another product of the thermal decomposition
Electron Transport Properties of Individual Lead Nanow-
ires. We note that several other groups have already observed
the transition from metal to superconductor for lead nanowires
prepared by templating against channels in porous membranes.4
All of their measurements were, however, performed on arrays
a-c

Single Crystalline Nanowires of Lead
J. Phys. Chem. B, Vol. 108, No. 25, 2004 8639
Figure 7. (A) Low-magnification and (B) high-magnification SEM
images of a bundle of lead nanowires that was formed by injecting the
wire dispersion into cold ethanol through a syringe needle. Note that
the nanowires were uniaxially aligned within the bundle.
Figure 8. (A) Typical I-V curve for a lead nanowire ∼50 nm in
diameter at room temperature. (B) A plot showing the dependence of
resistance on temperature for the same lead nanowire tested in (A).
The resistance was normalized against the value recorded at room
temperature. The inset in (B) shows an SEM image of the typical setup
used for the four-probe transport measurement, where a single lead
nanowire was deposited on four gold electrodes equally separated by
of multiple nanowires by directly attaching contacts to the top
and bottom surfaces of a membrane template. The measured
resistance was, therefore, an averaged response from a large
number of nanowires that were inevitably different in diameter,
structure, composition, and crystallinity. Most recently, Michotte
et al. reported their study on the electron transport properties
of individual lead nanowires, where the wires were also
embedded in the porous polycarbonate membrane used for
electrochemical deposition.11 By introducing a clever design for
the contact, these authors were able to observe the transport
properties of individual nanowires. Since the lead nanowires
synthesized by the present method were single crystalline in
structure and characterized by uniform diameters, it is clearly a
big advantage to directly use them for electron transport studies.
We studied the electron transport properties of as-synthesized
lead nanowires using the four-probe technique. Because of the
relatively high reactivity of these nanowires, freshly prepared
samples (within a few hours after synthesis) were used for all
measurements. We have also concentrated on individual wires,
rather than bundles or aggregates of nanowires. The contacts
in such a measurement were realized by first depositing a single
lead nanowire through solvent evaporation on the four gold
electrodes that had been patterned in advance on a glass slide,
and then thermally annealing the regions in physical contact
via brief exposure to the electron beam of a scanning electron
microscope. Due to the relatively low melting point of lead,
this process assured a good contact between the lead nanowire
and the gold electrode. Note that exposure to the electron beam
for a prolonged period should be avoided. Otherwise, the leads
could melt as a result of resistive heating. Wang et al. also
2
5 µm. The electrodes denoted with I+ and I- were used as sources
for current feeding; V+ and V- were used for measuring the potential
drop. The resistances recorded here exhibited a clear transition to the
superconducting state at a critical temperature similar to that of bulk
lead.
described a similar observation in their TEM study.12 As
restricted by the low melting point of lead, the testing current
used in all measurements had to be kept below 1.5 µA to avoid
any possible melting of the nanowires.
Depending on the lateral dimensions of the lead nanowire,
its resistance measured at room temperature could vary in the
range from hundreds of ohms to several kiloohms. The current-
voltage (I-V) plots from these measurements exclusively
exhibited a linear dependence for every single nanowire we have
tested. Figure 8A shows the typical plot for a nanowire whose
lateral dimension was around 50 nm. This I-V plot recorded
at room temperature gives a resistance around 2.5 kΩ, and a
4
conductivity of 4.0 × 10 S/cm could be derived from the liner
plot, which is very close to the value reported for bulk lead
4
(4.8 × 10 S/cm). We further studied the electron transport
properties associated with individual lead nanowires of various
diameters at lower temperatures. It was found that the resistance
of an individual nanowire responded accordingly to the tem-
perature: the lower the temperature was, the smaller the

8640 J. Phys. Chem. B, Vol. 108, No. 25, 2004
Wang et al.
resistance would be. When the temperature dropped to ∼8 K,
its resistance was only about 10% of the value recorded at room
temperature (Figure 8B). A sharp change in resistance (5-6
times when compared to the resistance at 8 K) was observed
when the temperature was reduced to 7 K, indicating that the
nanowire had entered a superconducting state. As the temper-
ature was further decreased, the ratio between RT and R8K
continued to decrease, but at a slower rate. In the work by
Michotte et al., a similar result was also obtained for individual
lead nanowires of ∼40 nm in diameter. However, the resistance
never dropped to zero in all our measurements. This observation
might be related to the fact that the resistive heating could slowly
transform the wire surface to a thin layer of lead oxide while
the temperature was still above the transition temperature. It
has been shown in previous studies that the transition temper-
ature sometime decreased as the diameter of nanowires (prepared
on SEM measurements. This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. This work has been supported in part
by an AFOSR-DURINT subcontract from SUNY Buffalo, a
Career Award from the National Science Foundation (DMR-
9983893), and a Fellowship from the David and Lucile Packard
Foundation. Y.X. is an Alfred P. Sloan Research Fellow and a
Camille Dreyfus Teacher Scholar. We thank Professor David
Cobden for suggesting the synthetic target described in this
article. Y.W. thanks Drs. Yugang Sun and Brian Mayers for
their help with TEM and HRTEM.
References and Notes
(1) See, for example: (a) a special issue on one-dimensional nano-
structures: AdV. Mater. 2003, 15, 341. (b) A special issue on nanowires:
MRS Bull. 1999, 24, 20. (c) Hu, J.; Odom, T.; Lieber, C. Acc. Chem. Res.
1999, 32, 435.
4
c,13
via electrodeposition against templates) was reduced.
This
(
2) (a) Kovtyukhova, N.; Mallouk, T. Chem. Eur. J. 2002, 8, 4355.
dependence suggests the introduction of disorder into a nanowire
(
b) Murphy, C.; Jana, N. AdV. Mater. 2002, 14, 80. (c) El-Sayed, M. Acc.
1
3
and thus a weakening in the superconducting property. For
all lead nanowires with diameters down to ∼50 nm, we did not
observe any significant difference in their critical transition
temperatures. It was demonstrated that all of them exhibited a
sharp drop in resistance at a critical temperature similar to the
value reported for bulk lead (7.2 K). It is believed that the single
crystalline structure of our nanowires might be the major reason
for the elimination of the size-dependent effect.
Chem. Res. 2001, 34, 257. (d) Song, J.; Wu, Y.; Messer, B.; Kind, H.;
Yang, P. J. Am. Chem. Soc. 2001, 123, 10 398. (e) Martin, C.; Mitchell, D.
Electroanal. Chem. 1999, 21, 1.
(3) Kittel, C. Introduction to Solid State Physics; John Wiley & Sons:
New York, 1996.
(4) (a) Dubois, S.; Michel, A.; Eymery, J.; Duvail, J.; Piraux, L. J.
Mater. Res. 1999, 14, 665. (b) Michotte, S.; Piraux, L.; Dubois, S.; Pailloux,
F.; Stenuit, G.; Govaerts, J. Physica C 2002, 377, 267. (c) Yi, G.;
Schwarzacher, W. Appl. Phy. Lett. 1999, 74, 1746. (d) Jalochowski, M.;
Bauer, E. Surf. Sci. 2001, 480, 109. (e) Jalochowski, M.; Bauer, E. Prog.
Surf. Sci. 2001, 67, 79. (f) Pang, Y.; Meng, G.; Zhang, L.; Shan, W.; Gao,
X.; Zhao, A.; Mao, Y. J. Phys.: Condens. Matter 2002, 14, 11 729. (g)
Pang, Y.; Meng, G.; Zhang, L.; Qin, Y.; Gao, X.; Zhao, A.; Fang, Q. AdV.
Func. Mater. 2002, 12, 719.
Conclusions
We have demonstrated a simple and practical route to the
large-scale production of single crystalline nanowires of lead
as uniform sample and with diameters ranging from 50 to 90
nm. Some of these nanowires could be grown up to several
millimeters in length. The formation of such highly anisotropic
nanostructures in an isotropic medium was believed to involve
a combination of Ostwald ripening (responsible for the transport
of material) and the so-called solution-liquid-solid mechanism
(
5) (a) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2,
1
65. (b) Sun, Y.; Yin, Y.; Mayers, B.; Herricks, T.; Xia, Y. Chem. Mater.
2
002, 14, 4736. (c) Sun, Y.; Xia, Y. AdV. Mater. 2002, 14, 833. (d) Sun,
Y.; Mayers, B.; Herricks, T.; Xia, Y. Nano Lett. 2003, 3, 955.
(
6) (a) Mohamed, M.; Halawy, S.; Ebrahim, M. Thermochim. Acta
1
994, 236, 249. (b) Leibold, R.; Huber, F. J. Therm. Anal. 1980, 18, 493.
(
c) Mu, J.; Perlmutter, D. Thermochim. Acta 1981, 49, 207.
(7) (a) Trentler, T.; Hickman, K.; Goel, S.; Viano, A.; Gibbons, P.;
Buhro, W. Science 1995, 270, 1791. (b) Trentler, T.; Goel, S.; Hickman,
K.; Viano, A.; Chiang, M.; Beatty, A.; Gibbons, P.; Buhro, W. J. Am. Chem.
Soc. 1997, 119, 2172. (c) Markowitz, P.; Zach, M.; Gibbons, P.; Penner,
R.; Buhro, W. J. Am. Chem. Soc. 2001, 123, 4502. (d) Holmes, J.; Johnston,
K.; Doty, R.; Korgel, B. Science 2000, 287, 1471.
(to account for the unidirectional growth). The nanowires were
shown to exhibit a transition into the superconducting state at
a critical temperature of 7.13 K, and no significant difference
was observed for nanowires with diameters down to ∼50 nm.
This observation further confirms that the lead nanowires
synthesized using the present method were single crystals, and
there were essentially no defects in each individual wire. It is
believed that this synthetic route could be potentially extended
to several other metals (e.g., Bi and Sn) that are also character-
ized by relatively low melting points. The availability of such
uniform nanowires in high qualities and large quantities will
certainly help us achieve a better understanding on the electron
transport properties of these solid materials at reduced dimen-
sionalities and sizes.
(
8) Roosen, A.; Carter, W. Physica A 1998, 261, 232.
(9) (a) Jin, R.; Cao, Y.; Mirkin, C.; Kelly, K.; Schatz, G.; Zheng, J.
Science 2001, 294, 1901. (b) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (c)
Sun, Y.; Mayers, B.; Xia, Y. Nano Lett. 2003, 3, 675. (d) Maillard, M.;
Giorgio, S.; Pileni, M.-P. AdV. Mater. 2002, 14, 1084. (e) Kirkland, A.;
Jefferson, D.; Duff, D.; Edwards, P.; Gameson, I.; Johnson, B.; Smith, D.
Proc. R. Soc. London A 1993, 440, 589. (f) Allpress, J.; Sanders, J. Surf.
Sci. 1967, 7, 1.
(10) Sankarapapavinasam, S.; Ahmed, M. J. Electrochem. Soc. India
1
990, 39, 251.
11) (a) Michotte, S.; Matefi-Tempfli, S.; Piraux, L. Appl. Phys. Lett.
003, 82, 4119. (b) Michotte, S.; Matefi-Tempfli, S.; Piraux, L. Physica C
(
2
2003, 391, 369.
(
(
12) Pan, Z.; Dai, Z.; Wang, Z. Appl. Phys. Lett. 2002, 80, 309.
13) (a) Graybeal, J.; Mankiewich, P.; Dynes, R.; Beasley, M. Phys.
ReV. Lett. 1987, 59, 2697. (b) Sharifi, F.; Herzog, A.; Dynes, R. Phys. ReV.
Lett. 1993, 71, 428. (c) Herzog, A.; Xiong, P.; Sharifi, F.; Dynes, R. Phys.
ReV. Lett. 1996, 76, 668.
Supporting Information Available: Histogram of the
diameter distributions for more than 50 lead nanowires based