High yield solution-liquid-solid synthesis of germanium nanowires

Article abstract of DOI:10.1021/ja055850z

High yields of crystalline Ge nanowires were synthesized for the first time in a conventional solvent of trioctylphosphine by disproportionating GeI₂ in the presence of Bi nanoparticle growth seeds at 350 °C and atmospheric pressure. Copyright

Full text of DOI:10.1021/ja055850z

Published on Web 10/20/2005
High Yield Solution-Liquid-Solid Synthesis of Germanium Nanowires
Xianmao Lu, Dayne D. Fanfair, Keith P. Johnston,* and Brian A. Korgel*
Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and
Technology, The UniVersity of Texas at Austin, Austin, Texas 78712-1062
Received August 25, 2005; E-mail: kpj@che.utexas.edu; korgel@mail.che.utexas.edu
Scheme 1. Binary Phase Diagram of Ge-Bi and Ge Nanowire
Growth Mechanism
Here we report the high yield solution-liquid-solid synthesis
of crystalline Ge nanowires. Bi nanocrystals are used as seeds to
promote nanowire growth in trioctylphosphine (TOP) from decom-
posing GeI2 at ∼350 °C. This is the first example of a group IV
nanowire synthesis in a conventional solvent under atmospheric
pressure.
Research in semiconductor nanowires has been fueled by their
unique optical, electronic, and mechanical properties and their
potential use in optoelectronic devices, chemical and biological
sensors, computing devices, and photovoltaics.1 Semiconductor
nanowires have been synthesized under a wide range of conditions,
from high temperature (<1100 °C) gas-phase reactions1,2 to
relatively low temperature (>250 °C) solution-phase conditions.1,3-5
Solution-phase routes to semiconductor nanowires are particularly
interesting due to their potential for good size and shape control,
chemical surface passivation, colloidal dispersibility, and high
throughput continuous processes.
Nanocrystals and nanowires of group IV materials, such as C,
Si, and Ge, have been extremely challenging to synthesize in
solution due to their high crystallization energy barrier and the
propensity of those elements to form stable oligomeric species with
hydrocarbons. In the past, our approach to crystalline Si6 and Ge
nanowires7 and multiwall carbon nanotubes8 has been to use
supercritical solvents that are heated well above their boiling points
to temperatures between 350 and 650 °C by pressurization. These
temperatures exceed the decomposition temperatures of organosilane
and organogermane precursors and the Au:Si and Au:Ge eutectic
temperatures (∼360 °C), making it possible to promote Au
nanocrystal-seeded nanowire growth.9 Although very high quality
nanowires are obtained, it would be more desirable to synthesize
crystalline nanowires under milder conditions to alleviate solvent
decomposition and safety concerns.
injecting the precursor/nanocrystal mixture swiftly into preheated
TOP at 365 °C, the reaction vessel was stirred for 10 min at 350
°C. The resulting black precipitate of nanowires was washed with
toluene, chloroform, and ethanol. The best Ge nanowire product
was obtained using a GeI2:Bi mole ratio of 80:1. Scanning electron
microscopy (SEM) of the reaction product synthesized from GeI2
and Bi nanoparticles in TOP at 350 °C showed a large amount of
nanowires (Figure 1). The wires were long and generally straight,
ranging in diameter from 20 to 150 nm with an average of ∼50
nm, with aspect ratios exceeding 100. The average length of the
nanowires is greater than 5 µm, and many are longer than 10 µm.
The relatively broad diameter distribution is primarily the result of
the Bi nanocrystals used to seed the reactionsalthough the Bi
nanocrystals were relatively size-monodisperse to begin with, Bi
nanocrystals are notoriously difficult to stabilize and aggregate
quickly during the nanowire synthesis, leading to a broad nanowire
diameter distribution. The product was nearly free of contaminants,
The low melting metal, Bi, forms a eutectic with Ge at 271.4
°C (Scheme 1),10 which is well within the temperature window for
conventional solvents. Buhro4 first showed that Bi nanocrystals
could seed SLS growth of CdSe nanowires, which Fanfair and
Korgel5 later extended to group III-V nanowire synthesis. On the
basis of the Bi:Ge phase diagram, Bi is also a good candidate for
Ge nanowire synthesis. Ge nanowire synthesis in a conventional
solvent also requires a precursor that is sufficiently reactive at ∼350
°C. We recently identified GeI2 as an effective precursor for Ge
nanocrystal synthesis at 300 °C in TOP, with high yields and
minimal organic contamination.11
Figure 1 shows Ge nanowires produced by injecting GeI2 along
with sterically stabilized 20 nm diameter Bi nanocrystals into TOP
at 365 °C on a Schlenk line (see Supporting Information for details).
The Bi nanocrystals were synthesized by room temperature reduc-
tion of bismuth(III) 2-ethylhexanoate (Bi[OOCCH-(C2H5)C4H9]3)
in dioctyl ether in the presence of TOP as described in ref 5. TOP
serves as a capping ligand that prevents significant nanocrystal
aggregation but does not interfere with nanowire growth. After
Figure 1. (A-C) SEM images of Ge nanowires made from GeI2 in TOP
at 350 °C with Bi nanocrystals as growth seeds (GeI2:Bi ) 80:1 n:n). SEM
reveals a high yield of high quality Ge nanowires with an average diameter
of 50 nm. (C) A representative Ge nanowire shows a Bi nanoparticle at its
tip. (D) Ge nanowires synthesized at a lower mole ratio of GeI2 to Bi of
50:1 (n:n) were of lower quality.
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C O M M U N I C A T I O N S
Scheme 1 outlines the nanowire growth mechanism. Two key
aspects of the chemistry enable the production of crystalline Ge
nanowires in TOP: (1) the low temperature Bi:Ge eutectic at ∼270
°C, and (2) the highly reactive organic-free Ge precursor, GeI2.
GeI2 disproportionates to Ge and GeI4 with relatively high chemical
yield at temperatures greater than ∼330 °Csfar below its boiling
point at 550 °C and the boiling point of many widely used solvents,
such as tri-n-octylphosphine (TOP). GeI2 also forms a very soluble
complex of (GeI2)‚R3P,13 when mixed with TOP, which dissociates
at 120 °C, making it an ideal precursor for SLS Ge nanowire
synthesis. Control experiments where GeI2 was injected without
Bi nanocrystals into TOP at 365 °C showed Ge nanoparticle
formation in just 5 min, indicating rapid homogeneous GeI2
decomposition. With Bi present in the concentrations used for
nanowire synthesis, Ge nanoparticles were not observed. Although
small Ge clusters may nucleate homogeneously, their interfacial
free energy is greatly lowered by incorporation into the much larger
20 nm Bi particles. Heterogeneous GeI2 decomposition on the Bi
seed particle surface could also occur.
Figure 2. High-resolution TEM images of the Ge nanowires. (A) A 15
nm nanowire with 111 growth direction. (Inset) FFT indicates the nanowire
was imaged down the [110] zone axis. The forbidden spot of [002h] is from
double diffraction of [1h11h] and [11h1h]. (B) Ge nanowire with a tip showing
presence of Bi by EDS analysis. (Inset) A lower magnification TEM image
of the wire with 111 growth direction.
It should be possible to extend the approach to Si, as Bi:Si also
has a eutectic temperature at ∼270 °C. The key to Si nanowire
synthesis will be the identification of a suitable reaction pathway
to produce a high yield of Si from molecular precursors at
temperatures below 350 °C. In addition to Bi, other low melting
metals, such as In (156 °C)3 and Sn (231 °C),14 form liquid alloys
with Ge/Si at mild temperatures, making them promising candidates
for group IV nanowire growth seeds, as well.
Figure 3. XRD of Ge nanowire shows diamond cubic crystal structure of
Ge with {111}, {220}, and {311} peaks. The peaks labeled with / are
from hexagonal GeO2.
Acknowledgment. This material is based upon work supported
in part by the DOE Office of Basic Energy Sciences (DE-FG02-
04ER15549), the Robert A. Welch Foundation, the NSF, and the
Advanced Materials Research Center in collaboration with Inter-
national SEMATECH.
and the Ge nanowire reaction yield is very high: 7.5 mg of pure
Ge nanowires was obtained in a typical reaction with 0.6 mg of Bi
nanoparticles and 75 mg of GeI2 in 5 mL of TOP, corresponding
to a yield of ∼40%. Bi particles could be observed at the ends of
some Ge nanowires, as shown in Figure 1C. Energy-dispersive
X-ray spectroscopy (EDS) mapping confirmed that the particles
are composed of Bi (SI-2). At a lower GeI2:Bi ratio (GeI2:Bi )
50:1), both straight and tortuous wires were observed (Figure 1D)s
the tortuous wires have a similar average diameter but shorter length
(<1 µm).
Supporting Information Available: Synthesis details, HRTEM
image showing 110 growth direction of a Ge nanowire, and EDS of
a Ge nanowire with Bi tip. This material is available free of charge via
the Internet at http://pubs.acs.org.
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