The use of single-source precursors for the solution-liquid-solid growth of metal sulfide semiconductor nanowires

Article abstract of DOI:10.1002/anie.200705142

(Figure Presented) All wired up! High-quality colloidal PbS and CdS nanowires were grown from Bi nanoparticles by the solution-liquid-solid (SLS) mechanism. The single-source-precursor strategy could provide a general approach for the synthesis of colloidal semiconductor nanowires.

Full text of DOI:10.1002/anie.200705142

Angewandte
Chemie
DOI: 10.1002/anie.200705142
Nanowire Growth
The Use of Single-Source Precursors for the Solution–Liquid–Solid
Growth of Metal Sulfide Semiconductor Nanowires**
Jianwei Sun and William E. Buhro*
Herein we describe the syntheses of high-quality PbS and CdS
nanowires by the solution–liquid–solid (SLS) mechanism
from single-source precursors. The SLS synthesis of nano-
wires offers several advantages over other methods, including
purposeful control of nanowire mean diameters, narrow
diameter distributions, small diameters in the quantum-
confinement regime, control of surface passivation, nanowire
solubility, and ease of implementation.[1] However, although
SLS growth has been applied to a wide range of II–VI,[2] III–
V,[3] and IV–VI[4] nanowire compositions, the method is not
yet generally applicable, as each new composition requires
extensive trial-and-error experimentation to identify appro-
priate precursors and reaction conditions. Our prior work has
employed separate metallic-element and nonmetallic-ele-
ment precursors to synthesize high-quality, compound–semi-
conductor nanowires.[2a,d,3d,g] Finding dual precursors with
appropriately balanced reactivities has often been a primary
origin of the extensive empirical experimentation required to
develop a given, successful SLS nanowire synthesis. In several
cases, such as for metal–sulfide semiconductors, the dual-
precursor SLS approach has failed to produce highly crystal-
line, diameter-controlled nanowires (see Figure S1 in the
Supporting Information).
We now report that the use of single-source precursors, in
which the metallic and nonmetallic semiconductor constitu-
ents are combined in a single molecule, solves the reactivity-
balance problem, resulting in the successful SLS growth of
PbS and CdS nanowires. Therefore, the single-source
approach has the potential to greatly extend the generality
of the SLS method for semiconductor–nanowire synthesis. As
single-source precursors have been extensively developed for
use in the MOCVD growth of thin films,[5] suitable precursors
for the SLS growth of many new nanowire compositions are
likely already available.
exciton generation was recently discovered in nanocrystals of
PbS and PbSe.[7] Specifically, up to four excitons (quantum
yield up to 430%) were produced by absorption of a single
photon in PbS nanocrystals.[7a] Although the ultrafast non-
radiative Auger recombination of multiexcitons prevents the
carriers from being harvested so far, this obstacle may be
overcome by using elongated nanostructures, such as nano-
rods and nanowires.[8] If the rate of Auger recombination is
significantly slower in 1D nanostructures, the efficiency of
photovoltaic energy conversion might be greatly enhanced.
Additionally, PbS has a relatively large exciton Bohr radius of
20 nm,[9] which makes it one of the ideal candidates for
investigating quantum-confinement effects in relatively large
nanostructures. The high quality of the PbS nanowires
reported here enabled observation of discrete excitonic
nanowire absorptions in the near infrared (NIR), allowing
quantitative evaluation of the quantum confinement. The
similar growth of high-quality CdS nanowires supports the
generality of the single-source SLS approach.
Efforts have been made to grow PbS and CdS nanowires
using the hard-template approach,[10] CVD and related
approaches,[11] the solvothermal method,[12] the nanoparticle-
induced anisotropic growth,[13] and surfactant-directed 1D
growth.[14] However, for all of these synthetic strategies
control over mean diameters and diameter distributions was
limited.
Our synthetic strategy is outlined in Scheme 1. The PbS
and CdS nanowires were grown by the conventional SLS
mechanism,[1] except that the elements of the semiconductor
phases were derived from the thermal decomposition of the
single-source diethyldithiocarbamate precursors [Pb-
(S2CNEt2)2] and [Cd(S2CNEt2)2], respectively. These precur-
sors have been previously used for the MOCVD growth of
PbS and CdS thin films,[15] the vapor–liquid–solid (VLS)
growth of CdS nanowires,[16] and the solution-based growth of
PbS[17] and CdS[18] nanostructures. Bismuth nanoparticles
were used to catalyze SLS growth, as we have found them
Semiconductor nanowires have drawn increasing interest
for their potential applications in electronic and photonic
devices[6] such as field-effect transistors, light-emitting diodes,
logic gates, lasers, waveguides, and solar cells. Multiple-
[*] J. Sun, Prof. W. E. Buhro
Department of Chemistry and Center for Materials Innovation
Washington University
1 Brookings Drive, St. Louis, MO 63130 (USA)
Fax: (+1)314-935-4481
E-mail: buhro@wustl.edu
[**] We thank the National Science Foundation (grant no. CHE-
0518427) for support of this work, and Richard A. Loomis and
John G. Glennon for assistance with NIR spectroscopy.
Scheme 1. Solution–liquid–solid (SLS) growthof metal sulfide nano-
wires catalyzed by Bi nanoparticles by using [M(S2CNEt2)2] (M=Pb,
Cd) single-source precursors.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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to be the most generally useful catalysts.[1] The minimum,
The lengths of the nanowires prepared in TOPO solvent were
typically several micrometers (see Figure S3 in the
controlled nanowire diameters that can typically be achieved
from Bi catalysts is approximately 5 nm, because the small Bi
nanoparticles (d < 5 nm) required to produce smaller nano-
wire diameters are very unstable with respect to agglomer-
ation.
In a typical synthesis of PbS nanowires, [Pb(S2CNEt2)2]
was first dissolved in phenyl ether with warming, and
combined with a tri-n-octylphosphine (TOP) solution of Bi
catalyst nanoparticles. The mixture was then rapidly injected
into a pre-heated tri-n-octylphosphine oxide (TOPO) solvent
or TOPO/TOP mixed solvent at a desired temperature (in the
range of 210–3008C), held at this temperature for 5 min, and
allowed to cool.
Supporting Information). However, use of the TOPO/TOP
mixed solvent enabled controlled growth of shorter nano-
wires (see Figure S4 in the Supporting Information), with
mean lengths dependent on the TOPO/TOP ratio. Lower
TOPO/TOP ratios produced shorter nanowires. Long nano-
wires tended to form bundles, likely as a result of strong Van
der Waals attractions between the nanowires.[19] Catalyst
nanoparticles were observed at one end of the wires (see
Figure S5 in the Supporting Information), confirming the SLS
growth mechanism.[1] Control experiments conducted without
Bi-catalyst nanoparticles produced no nanowire growth (see
Figure S6 in the Supporting Information).
Representative TEM images of the PbS nanowires shown
in Figure 1 indicate that the diameters were varied over the
range of 9–31 nm. The nanowires exhibited uniform diame-
ters along their lengths, and the standard deviation in the
diameter distributions was less than 30% of the mean
diameter (see Figure S2 in the Supporting Information).
The mean nanowire diameters were controlled by varying
the reaction temperature, and by using an additional surfac-
tant, n-tetradecylphosphonic acid (TDPA) (see Table S1 in
the Supporting Information). Larger nanowire diameters
were favored at higher reaction temperatures using Bi
nanoparticles of a given diameter. Thus, 10.3 nm Bi nano-
particles produced 11.0 nm and 13.3 nm nanowire diameters
at 232 and 2778C, respectively. Reactions carried out at about
2108C were unsuccessful. The use of a small amount of TDPA
in the synthesis resulted in 8.7 nm PbS nanowires. However,
the amount of TDPAwas critical; TDPA/Cd ratios larger than
1 produced short wires, cubes, and irregularly shaped
particles. Another commonly used naowire surfactant, n-
hexadecylamine (HDA), was found to be synthetically
detrimental in this system (see Figure S7 in the Supporting
Information). The naowire diameters were typically larger
than the initial diameters of the Bi nanoparticles, especially
for smaller-diameter Bi nanoparticles. The largest nanowires
(31 nm diameter) were obtained from the smallest (4.3 nm) Bi
nanoparticles. A minority population of 8.9 nm nanowires
was also produced under these conditions. The results
indicated that the nanowires grew from larger Bi nano-
particles formed by nanoparticle agglomeration. Because the
smallest Bi nanoparticles exhibit the strongest agglomeration
tendencies, the best nanowire diameter control was achieved
with the larger-diameter catalyst nanoparticles.
The expected rock-salt structure of the PbS nanowires was
confirmed by powder X-ray diffraction (XRD, see Figure S8
in the Supporting Information) and high-resolution trans-
mission electron microscopy (HRTEM, Figure 1e). The
single-crystalline nature of the nanowires was clearly evi-
denced in the HRTEM image (Figure 1e), in which the {200}
lattice fringes were well resolved and exhibited the correct
spacing of 0.292 nm.[20] The fast-Fourier-transform (FFT)
pattern of the PbS nanowire lattice was indexed to the rock-
salt structure (inset, Figure 1e), and indicated a [200] growth
direction for the nanowires.
Strong quantum confinement in the PbS nanowires was
confirmed by NIR absorption spectroscopy. A typical spec-
trum (Figure 2) of an 11.2 nm-diameter specimen clearly
shows two well-resolved excitonic features. The position of
the first excitonic feature was determined to be 2343 nm by
background fitting and subtraction (see Figure S9 in the
Supporting Information).[2a] This feature is strongly blue-
shifted in comparison to the PbS bulk absorption edge at
Figure 1. Representative TEM images of PbS nanowires of various
diameters (d). a) d=(8.7Æ2.4) nm (Æ28%), b) d=(11.2Æ2.3) nm
(Æ21%), c) d=(16.1Æ2.9) nm (Æ18%), and d) d=(31.2Æ3.9) nm
(Æ13%). e) A lattice-resolved HRTEM image of a 15 nm diameter PbS
nanowire. The inset shows the indexed fast-Fourier-transform (FFT)
pattern of the image, indicating that the nanowire grows along the
[200] direction.
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7–11 nm than did the VLS method using the same precur-
sor.[16] The standard deviations in the diameter distributions
of the SLS-grown nanowires were below 20% of the mean
diameters (see Figure S10 in the Supporting Information),
which were narrower than the distributions achieved by the
VLS method. The wurtzite crystal structure of the nanowires
was confirmed by XRD (see Figure S11 in the Supporting
Information) and by the FFTpattern from an HRTEM image
(Figure 3d inset). The well-resolved lattice fringes perpen-
dicular to the growth direction in the HRTEM image
corresponded to a spacing of 0.330 nm, consistent with the
(002) planes in hexagonal CdS.[21] The indexed FFT pattern
confirmed the [002] nanowire growth direction.
In conclusion, we have demonstrated that colloidal PbS
and CdS semiconductor nanowires may be grown by the SLS
mechanism using single-source precursors. Excellent diame-
ter control was achieved. The high quality of the PbS
nanowires enabled us to observe well-resolved NIR excitonic
absorption features, confirming strong quantum confinement.
These PbS and CdS nanowires are candidates for further
fundamental studies, and for potential applications in solar
cells[7a] and photodetectors.[22] Moreover, the results described
here strongly suggest that the single-source SLS strategy may
extend to other colloidal semiconductor nanowire systems
that are not easily prepared by a dual-source precursor
approach.
Figure 2. NIR absorption spectrum of colloidal PbS nanowires witha
mean diameter of 11.2 nm, dispersed in carbon tetrachloride. The first
excitonic feature, centered at 2343 nm, is identified by an arrow. The
inset shows a TEM image of this sample.
3024 nm, corresponding to a quantum confinement of DEg =
119 meV. As expected, because the bulk exciton Bohr radius
in PbS is quite large (20 nm), 11.2 nm diameter PbS nanowires
are strongly confined. The photoluminescence of the PbS
nanowires was not investigated.
To demonstrate the potential generality of the single-
source SLS synthetic strategy, CdS nanowires were also
similarly grown from [Cd(S2CNEt2)2]. Representative TEM
images (Figure 3a–c) show that this SLS strategy afforded
high-quality nanowires with smaller diameters in the range of
Experimental Section
Materials: Lead(II) nitrate (99.999%, Aldrich), cadmium chloride
(99.995%, Strem), sodium diethyldithiocarbamate trihydrate (ACS
grade, Alfa Aesar), phenyl ether (99%, Acros), tri-n-octylphosphine
(TOP, 90%, Aldrich), tri-n-octylphosphine oxide (TOPO, 99%,
Aldrich), and n-tetradecylphosphonic acid (TDPA, PolyCarbon
Industries) were purchased and used as received. Technical-grade
TOPO (90%, Aldrich) was vacuum distilled before use. The single-
source precursors [M(S2CNEt2)2] (M = Cd,[23] Pb[17a]) were prepared
by literature methods. The Bi-nanoparticle stock solutions were
prepared as previously described.[2a] All procedures for the synthesis
of nanowires were conducted under dry, O2-free N2(g).
Synthesis of PbS nanowires: In a typical synthesis, TOPO (90%-
purity grade, 4.0 g) was loaded into a Schlenk reaction tube with a
small magnetic stir bar. Separately, [Pb(S2CNEt2)2] (20 mg,
0.04 mmol) and phenyl ether (0.8 g) were combined in a small vial.
In a separate small vial, a Bi-nanoparticle stock solution (15 mg) was
combined with TOP (0.5 g). The reaction tube was then inserted in a
pre-heated salt bath (NaNO3/KNO3, 46:54 by weight) at 2508C for at
least 5 min to allow the temperature to equilibrate. The Bi-nano-
particle mixture was loaded into a 2 mL syringe. The [Pb(S2CNEt2)2]
mixture was gently heated with a heat gun until a homogeneous
solution was obtained (< 1 min). This hot solution was loaded into the
syringe containing the Bi nanoparticles. The contents of the syringe
were rapidly injected into the pre-heated, stirred TOPO. The reaction
mixture turned black within 10 s. After the reaction mixture was
stirred for 5 min, the reaction tube was withdrawn from the salt bath
and allowed to cool. While cooling, toluene ( ꢀ 5 mL) was added to
prevent solidification. The conditions and results for various PbS
nanowire syntheses are summarized in Table S1 in the Supporting
Information.
Figure 3. Representative TEM images of CdS nanowires of various
diameters (d). a) d=(6.9Æ1.3) nm (Æ19%), b) d=(7.8Æ1.2) nm
(Æ15%), and c) d=(11.0Æ1.8) nm (Æ16%). d) A lattice-resolved
HRTEM image of a 9 nm diameter CdS nanowire. The inset shows the
indexed fast-Fourier-transform (FFT) pattern of the image, indicating
that the nanowire grows along the [002] direction.
Synthesis of CdS nanowires: In a typical synthesis, TDPA (7 mg,
0.025 mmol) and TOPO (99%-purity grade, 4.0 g) were loaded into a
Schlenk reaction tube with a small magnetic stir bar. Separately,
[Cd(S2CNEt2)2] (40 mg, 0.1 mmol) and TOP (0.8 g) were combined in
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Communications
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homogeneous solution. The reaction tube was then inserted in a pre-
heated salt bath (NaNO3/KNO3, 46:54 by weight) at 2508C for at least
5 min to allow the temperature to equilibrate. The contents of the vial
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preheated, stirred reaction tube. The reaction mixture turned a
yellowish color within 10 s. After the reaction mixture was stirred for
5 min, it was withdrawn from the salt bath and allowed to cool. While
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Isolation and characterization: The nanowires were separated
from the cooled reaction mixtures by addition of methanol
( ꢀ 10 mL). The precipitated wires were collected by centrifugation
and the supernatant was decanted. The wires were then redispersed in
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Keywords: bismuth· colloids · crystal growth· nanowires ·
semiconductors
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