Solution synthesis of germanium nanowires using a Ge2+ alkoxide precursor

Article abstract of DOI:10.1021/ja058524s

A simple solution synthesis of germanium (Ge⁰) nanowires under mild conditions (<400 °C and 1 atm) was demonstrated using germanium 2,6-dibutylphenoxide, Ge(DBP)₂ (1), as the precursor where DBP = 2,6-OC₆H₃(C(CH₃)₃)₂. Compound 1, synthesized from Ge(NR₂)₂ where R = SiMe ₃ and 2 equiv of DBP-H, was characterized as a mononuclear species by single-crystal X-ray diffraction. Dissolution of 1 in oleylamine, followed by rapid injection into a 1-octadecene solution heated to 300 °C under an atmosphere of Ar, led to the formation of Ge⁰ nanowires. The Ge ⁰ nanowires were characterized by transmission electron microscopy (TEM), X-ray diffraction analysis, and Fourier transform infrared spectroscopy. These characterizations revealed that the nanowires are single crystalline in the cubic phase and coated with oleylamine surfactant. We also observed that the nanowire length (0.1-10 μm) increases with increasing temperature (285-315 °C) and time (5-60 min). Two growth mechanisms are proposed based on the TEM images intermittently taken during the growth process as a function of time: (1) self-seeding mechanism where one of two overlapping nanowires serves as a seed, while the other continues to grow as a wire; and (2) self-assembly mechanism where an aggregate of small rods (<50 nm in diameter) recrystallizes on the tip of a longer wire, extending its length.

Full text of DOI:10.1021/ja058524s

Published on Web 03/21/2006
Solution Synthesis of Germanium Nanowires Using a Ge2+
Alkoxide Precursor
Henry Gerung,*,† Timothy J. Boyle,*,‡ Louis J. Tribby,† Scott D. Bunge,‡,§
C. Jeffrey Brinker,†,‡ and Sang M. Han*,†
Contribution from the Department of Chemical and Nuclear Engineering, UniVersity of New
Mexico, 209 Farris Engineering Center, Albuquerque, New Mexico 87131, and AdVanced
Materials Laboratory, Sandia National Laboratories, 1001 UniVersity BouleVard SE,
Albuquerque, New Mexico 87106
Received December 15, 2005; E-mail: meister@unm.edu
Abstract: A simple solution synthesis of germanium (Ge0) nanowires under mild conditions (<400 °C and
1 atm) was demonstrated using germanium 2,6-dibutylphenoxide, Ge(DBP)2 (1), as the precursor where
DBP ) 2,6-OC6H3(C(CH3)3)2. Compound 1, synthesized from Ge(NR2)2 where R ) SiMe3 and 2 equiv of
DBP-H, was characterized as a mononuclear species by single-crystal X-ray diffraction. Dissolution of 1 in
oleylamine, followed by rapid injection into a 1-octadecene solution heated to 300 °C under an atmosphere
of Ar, led to the formation of Ge0 nanowires. The Ge0 nanowires were characterized by transmission electron
microscopy (TEM), X-ray diffraction analysis, and Fourier transform infrared spectroscopy. These
characterizations revealed that the nanowires are single crystalline in the cubic phase and coated with
oleylamine surfactant. We also observed that the nanowire length (0.1-10 µm) increases with increasing
temperature (285-315 °C) and time (5-60 min). Two growth mechanisms are proposed based on the
TEM images intermittently taken during the growth process as a function of time: (1) self-seeding mechanism
where one of two overlapping nanowires serves as a seed, while the other continues to grow as a wire;
and (2) self-assembly mechanism where an aggregate of small rods (<50 nm in diameter) recrystallizes
on the tip of a longer wire, extending its length.
been relatively scarce despite their unique characteristics.6-10
Nanoscale Ge0 semiconductor systems are superior to Si0 for
Introduction
One-dimensional (1-D) semiconductor nanowires have been
actively investigated in an effort to utilize them for advanced
applications, such as liquid crystal displays,1 field effect
transistors,2 optical waveguides, laser diodes, and biological
sensors.3 The physical properties of nanowires at their charac-
teristic length scale, in contrast to their bulk behavior, are
strongly governed by the localized electrons confined within
the nanowires.4,5 This quantum confinement effect enables the
use of nanowires in diverse areas of electronics, optoelectronics,
and biological sensing. Among group 14 elements, both carbon
(C) and silicon (Si0) have been predominantly used for the
development of advanced nanoelectronic devices. For instance,
Si0 nanowires can serve simply as interconnects or as building
blocks for advanced logic devices.2,3 Preliminary results have
also been reported for the use of Si0 nanowires as sensors to
detect biological and chemical reagents.2,3 In comparison to C
and Si0, the synthesis and utilization of Ge0 nanowires have
(i) higher electron and hole mobility,11 (ii) a larger Bohr exciton
radius that yields a more pronounced quantum confinement
effect,12,13-15 and (iii) a predicted nanostructure with direct band
gap behavior, although bulk Ge0 is an indirect band gap
material.16
One reason for the scarce exploration of Ge0 nanowires as
an advanced material is due to the lack of a simple and benign
synthetic pathway to this morphology.17 The previously reported
routes cover a myriad of processes, including chemical vapor
deposition (CVD),6,18 laser ablation,19 vapor-liquid-solid
(6) Wu, Y. Y.; Yang, P. D. Chem. Mater. 2000, 12, 605-607.
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† University of New Mexico.
‡ Sandia National Laboratories.
§ Current address: Kent State University, Department of Chemistry, 305
William Hall, Kent, OH 44242.
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Germanium Nanowires
A R T I C L E S
(VLS) growth,20 templated supercritical fluid,21-24 and combina-
tion of these techniques. The VLS technique is most widely
used among these methods. Modified VLS techniques, such as
solution-liquid-solid (SLS) and supercritical-fluid-liquid-
solution (SFLS)6,25-27 growth techniques, have also been
developed. While these routes can yield Ge0 nanowires, they
require the use of metal catalysts (e.g., Au0 or Fe0) or
nonmetallic, high-boiling-point catalysts (e.g., trioctylamine) as
seed materials to initiate the growth and to define the growth
direction of Ge0 nanowires.28,29 Also, the use of high temperature
and pressure coupled with the volatility of the precursor could
be potentially hazardous.21 For the synthetic routes described
above, the typical Ge precursors are in the +4 oxidation state,
such as GeH4, GeI4,7 Ge(CH3)4,6 or GexFe1-x alloy.19 More
recently, a Ge precursor in the +2 oxidation state has found
utility in a CVD and SLS systems, using Fe0 or Bi0 as the
catalyst material.18,30 Despite the success of producing Ge0
nanowires by these techniques, the presence of foreign seed
particles adds complexity to the production of pure Ge0
nanowires and influences their electrical and optical properties.
In comparison, select solution routes can be simple and
effective for the production of high-quality Ge0 nanoparticles.
To date, very limited information has been available for the
production of Ge0 nanowires from these solution routes.31,32 One
reported solution route to Ge0 nanowires involves the reduction
of GeCl4 and phenyl-GeCl3 by Na0 in an alkane solvent at
elevated temperature and pressure.8 In this solution route, the
complexity of the reaction as well as the potential contamination
by metal reducing agents and salt byproducts limits the direct
use of the as-produced Ge0 nanowires for novel devices. An
ideal case for the synthesis of Ge0 nanowires would be a simple
synthesis conducted under relatively benign conditions associ-
ated with low pressure, low temperature, and no salt byproduct.
In this paper, we report a solution synthesis of Ge0 nanowires
from a newly characterized Ge2+ precursor, Ge(DBP)2 (1),
where DBP represents 2,6-OC6H3(C(CH3)3)2. Our approach
hinges on molecularly designed precursors to control their
reactivity and resulting nanostructure morphology. This con-
ceptual approach has been introduced in the synthesis of several
other systems, such as CdSe,33 PbSe, and Au.34,35 Previously,
we described the utility of Ge(N(SiMe3)2)2 to produce Ge0
nanocrystals.31 Herein, we tailor the precursor reactivity by the
use of 1 to form Ge0 nanowires. The synthesis and characteriza-
tion of both 1 and the subsequent Ge0 nanowires are described
below.
Experimental Details
The synthesis was conducted at 285-315 °C under 1 atm of Ar in
a vacuum/Schlenk line setup, typically used for solution synthesis of
nanoparticles and nanowires.36 Note: no metal catalysts were used, and
no salt byproducts were formed. All anhydrous solvents were purchased
in sure-seal bottles. The following chemicals were purchased from
Aldrich and used without further purification: GeCl2‚dioxane, LiN-
(SiMe3)2, and oleylamine. Ge(N(SiMe3)2)2 was synthesized following
literature reports.37
Fourier transform infrared (FTIR) transmission spectra were obtained
on a Bruker Vector 22 spectrometer using KBr pellets pressed under
an Ar atmosphere and handled under an atmosphere of flowing N2.
Elemental analyses were performed on a Perkin-Elmer 2400 CHN-
S/O elemental analyzer. Thermogravimetry analysis and differential
thermal analysis (TGA/DTA) were performed on a TA Instrument STD
2960 at 15 °C/min ramp rate under Ar atmosphere. Nuclear magnetic
resonance (NMR) data were collected on flame-sealed samples dis-
solved in toluene-d8 using a Bruker 250 MHz NMR.
Ge(OC6H3(C(CH3)3)-2,6)2 (1). A solution of DBP-H (0.525 g, 2.54
mmol) dissolved in toluene was slowly added by pipet to a stirring
solution of Ge(N(SiMe3)2)2 (0.500 g, 1.27 mmol) in toluene. The
mixture was heated to 50 °C for 15 min to drive the reaction into
completion and then allowed to cool to room temperature. X-ray quality
pale yellow crystals of 1 were isolated by slow evaporation of the
1
volatile component of the mixture. Yield: 0.335 g (67.0%). H NMR
(400.1 MHz, tol-d8): δ 7.37-7.29 (br m, 3-H, OC6H3(C(CH3)3)2) and
1.54 (s, 18 H, OC6H3(C(CH3)3)2)). 13C{1H} NMR (100.0 MHz, tol-
d8); δ 140.6, 125.9 (partial, OC6H3(C(CH3)3)2), 35.44 (OC6H3-
(C(CH3)3)2), and 32.9 (OC6H3(C(CH3)3)2). FTIR (KBr, cm-1): 3088(s),
2965(sh, s), 2380(s), 2044(s), 1914(s), 1859(s), 1794(s), 1696(s), 1641-
(s), 1578(s), 1469(sh, s), 1414(m), 1359(sh, s), 1316(s), 1218(m), 1109-
(m), 1019(s), 844(m), 746(m), 659(s), and 583(s). Elemental analysis
calculated for C20H26GeO2: 69.59% C and 8.76% H. Found 69.36% C
and 8.78% H.
General X-ray Crystal Structure Information. Using a Bruker
AXS diffractometer, a single crystal of 1 was mounted onto a thin glass
fiber from a pool of Fluorolube and immediately placed under a liquid
N2 stream. The radiation source was graphite-monochromatized Mo
KR (λ ) 0.7107 Å). The lattice parameters were optimized from a
least-squares calculation on carefully centered reflections. Lattice
determination and data collection were carried out using SMART
Version 5.054 software. Data reduction was performed using SAINT
Version 6.01 software. The structure refinement was performed using
XSHELL 3.0 software. The data were corrected for absorption, using
the SADABS program within the SAINT software package. General
collection parameters for 1 are shown in Table 1. Additional information
concerning the structure of these compounds can be found in the
Supporting Information or by accessing the final CIF files through the
Cambridge Crystallographic Data Base.
(19) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208-211.
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Soc. 2003, 125, 6284-6288.
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Morris, M. A.; Erts, D.; Holmes, J. D. J. Mater. Chem. 2004, 14, 585-
589.
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W. J.; Holmes, J. D. J. Mater. Chem. 2005, 15, 4809-4815.
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J. J.; Wu, B.; Holmes, J. D. J. Mater. Chem. 2005, 15, 2408-2413.
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2004, 108, 9574-9587.
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ed.; VCH: New York, 1994.
The structure was solved using direct methods, which yielded the
heavy atoms, along with a number of the C and O atoms. Subsequent
Fourier synthesis yielded the remaining atom positions. The H atoms
were fixed in positions of ideal geometry and refined within the
XSHELL software. These idealized H atoms had their isotropic
temperature factors fixed at 1.2 or 1.5 times the equivalent isotropic U
of the C atoms to which they were bonded. The final refinement of
each compound included anisotropic thermal parameters on all non-
(36) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993,
115, 8706-8715.
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A R T I C L E S
Gerung et al.
Table 1. Data Collection Parameters for 1
formula
formula weight
C28H42GeO2
483.21
crystal system
space group
monoclinic
P2(1)/n
a, Å
b, Å
c, Å
13.620(4)
13.810(4)
14.056(4)
95.534(5)
2631.3(12)
4
â, deg
V, Å3
Z
T (K)
203(2)
calculated density, mg/m3
absorption coefficient, mm-1
number of reflections
number of observed reflections
R1a (%) (all data)
wR2b (%) (all data)
1.220
1.185
18605
4774
6.16 (9.54)
13.92 (15.86)
Figure 1. Structure plot of 1. Thermal ellipsoids of heavy atoms plot is
drawn at 30% level. Carbons are shown as ball-and-stick for clarity.
nucleation sites were formed upon injection.31 This led to rapid
consummation of the precursor and caused uncontrolled growth
of nanoparticles.38 To obtain a nanowire morphology, it is
critical to control the rate of nucleation site generation, while
maintaining sufficient precursor feedstock to continue the 1-D
growth.38 Two main factors that affect the reactivity of the
precursor are the Ge-ligand bond strength and the steric
hindrance of the ligand. Therefore, we chose alkoxides as the
precursor, whose Ge-O dissociation energy is approximately
157 kcal/mol.39 This dissociation energy is much greater than
the 55 kcal/mol39 of the Ge-N bond in Ge(N(SiMe3)2)2. Pendant
chain manipulation is also much easier with alkoxides than with
amides due to the larger number of alcohols readily available.
Precursor Synthesis. Numerous ligands, such as ORdOC-
(C(CH3)3)2,40 2,6-OC6H2(C(CH3)3)2-4-(CH3)-4,41 2,4,6-OC6H3-
(CH3)3,42 2,6-OC6H3(CH(CH3)2)2,42 2,6-OC6H3(C6H5)2,42 and
2,3,5,6-OC6H3(C6H5)4,42 were found to be useful for isolation
of solvent-free Ge(OR)2. However, the absence of a systematic
variation in the ligands made it difficult to correlate ligand
effects on the final material physical properties. Therefore, we
undertook the systematic synthesis of Ge(OR)2 to tailor the
reactivity, using a variety of alkoxide ligands produced from
an amide alcohol metathesis route shown in eq 1. The majority
of this family of Ge(OR)2 compounds will be reported shortly;
however, the dibutylphenoxide (DBP) derivative proved to have
the characteristics that were favored for nanowire growth and
will be discussed exclusively at this time.
2
2
a R1 ) ∑||F0| - |Fc||/∑|F0| × 100. b wR2 ) |∑ w(F0 - Fc )2/
2
∑(w|F0| )2]1/2 × 100.
hydrogen atoms. Final crystal solutions that contain alkoxides are well-
known to possess ligand positional disorder issues. Additional infor-
mation concerning the data collection and final structural solutions of
1 can be found in the Supporting Information.
Ge0 Nanowire Synthesis. In a vial, 1 (0.393 g, 1.00 mmol) was
dissolved in oleylamine (2.69 g, 10.0 mmol) and stirred until all the
material was completely dissolved forming a pale yellow solution. In
general, this precursor solution was rapidly injected into a three-neck
flask containing noncoordinating solvent 1-octadecene (11.5 mL, 17.0
mmol) heated to 300 °C. The temperature was measured by a K-type
thermocouple and regulated with a variable voltage regulator (Variac)
heater by an Omega positive-feedback temperature controller 9100A
with 3 °C accuracy. Upon injection, the temperature decreased to 280
°C and recovered to 300 °C within 1 min, wherein the color of the
solution changed immediately to dark purple. At selected intervals, 1
mL aliquots were taken and immediately quenched in ∼5 mL of CHCl3.
These samples were used for TEM analyses. After the reaction was
completed in 60 min, the solution was extracted twice with a 50:50
vol % mixture of CHCl3/CH3OH, and the final Ge0 nanowires in CHCl3
were then precipitated by the addition of excess acetone. After
centrifugation, the dark Ge0 nanowires in a powder form can be
rediluted in a variety of solvents, including CHCl3, toluene, and hexanes.
Characterization. A JEOL 2010 high-resolution transmission
electron microscope (HRTEM) with acceleration voltage of 200 keV
was used to image Ge0 nanowires and to obtain their selected area
electron diffraction (SAED) pattern. TEM sample preparation consisted
of dissolving Ge0 nanowires in CHCl3, toluene, or hexane. The
dissolution was followed by dispersing a drop of the solution onto a
holey-carbon copper grid and allowing it to dry completely. In addition,
the powder was characterized by Cu KR radiation from a Siemens 500
diffractometer with 0.2° step size and 10 s dwell time. The Ge0 nanowire
sample was placed on a zero-background X-ray diffraction (XRD)
holder, which minimized the background noise against XRD signal.
Ge(NR2)2 + 2H - DBP f Ge(DBP)2 + 2HNR2 (1)
Compound 1 was synthesized according to eq 1, and the
structure plot of 1 is shown in Figure 1. Upon addition of the
alcohol to the pale yellow solution of Ge(NR2)2 in toluene, the
reaction mixture turned darker yellow. After stirring for several
minutes, the reaction was heated to ensure that complete
exchange had occurred, while no additional changes in color
were noted. The reaction was then allowed to cool to room
temperature and set aside until X-ray quality crystals were
formed.
Results and Discussion
We have previously demonstrated the formation of Ge0
nanoparticles through the reduction of Ge2+ amide precursor
Ge(N(SiMe3)2)2.31 Our approach was to design Ge2+ precursors
at the molecular level, thus controlling their reactivity and
resulting morphologies. Following this approach, we sought
alternative Ge2+ precursors that could be tailored to generate
Ge0 nanowires instead of particles. A number of important
characteristics, such as general structure, pendant ligand chains,
decomposition temperature, and solubility, were considered in
designing the precursors. For Ge0 nanoparticles, we reasoned
that, due to the high reactivity of Ge(N(SiMe3)2)2, numerous
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A R T I C L E S
The FTIR transmission spectrum of the bulk powder indicated
that the amide was fully substituted by the alkoxides. The
structure of 1 was solved by single-crystal X-ray analysis as a
mononuclear species with a bent geometry and is similar to the
literature derivatives.41 The bond distances and angles were
found to be in agreement with the literature structures.40-42
Elemental analysis confirmed that the bulk sample was consis-
tent with single-crystal structure. Since the thermogravimetry
analysis (TGA) data of 1 (Supporting Information) revealed a
decomposition temperature around 200 °C, it was necessary to
use solvents with a boiling point higher than this temperature
for nanomaterial production. The noncoordinating 1-octadecene
was selected as the solvent.31
Ge0 Nanowires. The TEM image of typical Ge0 nanowires
isolated after synthesis is shown in Figure 2a. The SAED pattern
shown in the inset of Figure 2a displays typical diffraction spots
of {111} and {220} planes, which indicate that the Ge0
nanowires are in cubic phase. The nanowires’ anisotropic growth
can reach up to 10 µm in length, while the diameter ranges
from 15 to 25 nm. Observations around the nanowires also
reveal the presence of smaller nanorods, which can be attributed
to the arrested growth of the nanorods that formed during the
initial nucleation stage. The HRTEM images in Figure 2b and
2c indicate that the nanowires are crystalline. The HRTEM
image in Figure 2b shows lattice fringes with d spacing of 3.26
Å, which corresponds to {111} planes, growing in the [110]
direction. A corresponding diffraction pattern obtained by fast
Fourier transformation (FFT) is shown in the inset. Similarly,
another HRTEM image in Figure 2c also shows visible lattice
fringes of the {111} plane but with [111] growth direction.
These directions are consistent with the reported growth
direction of Ge0 nanowires via other methods. Two XRD
measurements, conducted immediately after the synthesis
(Figure 3), show the presence of {111}, {220}, {311}, {400},
and {331} planes of the cubic phase Ge0 nanowires. The
presence of these XRD peaks after the 60 day exposure to the
ambient air suggests that the Ge0 nanowires are well passivated
by the amine surfactant, which minimizes surface oxidation of
Ge0 nanowires. The adsorption of amines on the surface of Ge
has been previously demonstrated.31,43 In comparison to the
FTIR transmission spectrum of oleylamine (Figure 4a), the FTIR
transmission spectrum of the Ge0 nanowires, taken after washing
with CHCl3 and CH3OH, reveals the presence of characteristic
peaks of amine surfactant,31,43 such as N-H stretching at 3460
cm-1 and N-H scissor modes at 1640 cm-1 (Figure 4b). This
result further verifies that the Ge0 nanowires are encapsulated
by oleylamine on the surface.
The dimensions of Ge0 nanowires were found to depend
strongly on both the time and temperature of the reaction. Figure
5 captures the temporal evolution of Ge0 nanowires over the
temperature range from 285 to 315 °C. When the synthesis was
conducted at a low temperature (285 °C), no definitive shape
formation was observed (Figure 5a) after 5 min; however, after
60 min, Ge0 rods e50 nm in length with 6 nm in diameter were
observed (Figure 5b). At a higher temperature (300 °C), Ge0
rods, whose diameter and length are 10-17 and 20-50 nm,
respectively, formed within 5 min (Figure 5c). After 60 min,
the length of Ge0 nanowires was an order of magnitude longer
Figure 2. (a) Ge0 nanowires (inset, typical SAED pattern); (b) HRTEM
image of Ge0 nanowires in the [110] direction (inset, FFT image); (c)
HRTEM image of Ge0 nanowires in the [111] direction.
(200-400 nm), while maintaining the 20 nm diameter (Figure
5d). Increasing the temperature even higher to 315 °C resulted
in the formation of substantially longer Ge0 nanowires with the
20 nm diameter. The length of these nanowires increased from
400 nm (5 min, Figure 5e) to 10 µm (60 min, Figure 5f),
depending on the reaction time. As the temperature is increased,
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2002, 124, 4027-4038.
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A R T I C L E S
Gerung et al.
Figure 6. Two different growth mechanisms are observed: (a) self-seeding
mechanism with the diffraction patterns of the head (inset (i)), the interface
(inset (ii)), and the tail (inset (iii)) shows {111} planes, and (b) self-assembly
growth showing aggregates of nanorods at the tip of the nanowire.
molecular capping agent. The lone electron pair in the amines
provides the necessary electron charge for the reduction process,
which can be observed by the change from yellowish to dark
color during the reaction. We speculate that, upon mixing of
oleylamine with 1, an intermediate Ge compound is formed,
containing both Ge-N and Ge-O bonds. The color of the
compound after mixing remains constant, which suggests that
there is no change in the oxidation state of the Ge precursor.
Upon injection into the boiling solvent, several reactions most
likely occur simultaneously, including the reduction of 1 by the
amine, dissociation of Ge-N and Ge-O bonds, and nucleation
of Ge0 nanostructure. The higher bond strength of Ge-O bonds
and the steric hindrance from the ligands increase the activation
energy barrier for the reduction process, which translates to
higher temperature needed to reduce the precursor and to start
the nucleation process. The precursor reactivity governs the
production rate of nucleation sites. To support this mechanism,
a control experiment was conducted by replacing the oleylamine
surfactant with 1-dodecanethiol surfactant. The result showed
that no reduction process occurred because of the high bond
strength of Ge-S and suggested that amine surfactant is a more
suitable surfactant.45,46 The complete reduction of 1 into Ge0
was confirmed by the repeatable observation of lattice fringes
in the TEM images (Figure 2) and XRD diffraction peaks
(Figure 3) that are consistent with Ge0 and not with GeOx. The
excess amine surfactant present in the system served as a
capping agent around the nanowires. Mui et al. have shown
that methylamine chemisorbs on the Ge(100) surface in the gas
phase without undergoing N-H dissociation.43 Instead of
dissociative adsorption, the lone pair electrons of the amines
bond to the Ge0 surface.
Figure 3. XRD of Ge0 nanowires shows the cubic structure and the stability
in air after 60 days.
Figure 4. (a) FTIR transmission spectrum of oleylamine; (b) FTIR
transmission spectrum of Ge0 nanowires.
Nanowires’ Growth Mechanism. Understanding the growth
mechanism of these Ge0 nanowires is necessary in order to
garner greater control over the nanowire dimensions. From
temporal investigations, we observed two potential growth
mechanisms: (a) self-seeding (Figure 6a) mechanism and (b)
self-assembly mechanism (Figure 6b). Both mechanisms showed
approximately equal probability of occurrence.
For the self-seeding mechanism, we examined the diffraction
patterns generated by FFT of selected areas in the TEM images
of the wire [inset of Figure 6a (i-iii)] to determine whether
Figure 5. The effect of temperature on the growth of Ge0 nanowires. At
285 °C after (a) 5 min and (b) 60 min, at 300 °C after (c) 5 min and (d) 60
min, and at 315 °C after (e) 5 min and (f) 60 min.
the axial growth of the nanowires is favored over the radial
growth. The formation of the Ge0 nanowires in this system is
influenced not only by the reactivity of 1 but also by the
surfactant passivation. The role of surfactant in the solution
growth of nanowires has been investigated in other systems,44
wherein amine surfactant acts as a reducing agent and a
(44) Law, M.; Goldberger, J.; Yang, P. D. Annu. ReV. Mater. Res. 2004, 34,
83-122.
(45) Kosuri, M. R.; Cone, R.; Li, Q. M.; Han, S. M.; Bunker, B. C.; Mayer, T.
M. Langmuir 2004, 20, 835-840.
(46) Han, S. M.; Ashurst, W. R.; Carraro, C.; Maboudian, R. J. Am. Chem.
Soc. 2001, 123, 2422-2425.
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Germanium Nanowires
A R T I C L E S
Figure 7. (a) The snapshot shows an overlap between two Ge0 nanorods.
(b) One of the rods continues to grow in the [100] direction to form a
nanowire.
Figure 9. Self-assembly at the nanowire tip: (i) low-resolution TEM image;
(ii) SAED of the tip; and (iii) SAED of the nanowire.
and cause the nanowire to grow anisotropically. From the image,
we observe a clear outline of individual single-crystalline
nanorods that, over time, these nanorods align themselves in
the growth direction, start to recrystallize, and form domains
of single-crystalline nanowires. The HRTEM image of a section
along the wire is shown in Figure 8b. The lattice fringes in
Figure 8b as well as FFT images in the inset (i-iii) indicate
that the recrystallization occurs at the boundaries between
nanorods and an individual wire, forming a single-crystalline
{111} domain along the Ge0 nanowires. From this result, we
propose that the Ge monomers in solution insert themselves at
the interface of coalescing nanorods and/or at the interface
between a rod and a wire. Similar nanowire growth mechanisms
have been observed in the self-assembly of ellipsoid Ag0
nanocrystals to form Ag0 nanowires.48 Figure 9 offers further
support of the self-assembly mechanism. In inset (i), a Ge0
nanowire longer than 5 µm has been formed with nanorod
aggregates still present at the tip. The SAED image in inset (ii)
taken from the tip of the nanowire shows the presence of
multidomain, single-crystalline nanorods with {111}, {220}, and
{311} planes. The SAED image in inset (iii) taken from the
section along the nanowire represents a later stage in the wire
growth, where these nanorods recrystallize into a growing single-
crystalline Ge0 nanowire.
Figure 8. (a) Self-assembly process of an aggregate of Ge0 nanorods. (b)
HRTEM image indicating formation of {111} planes across the nanorods.
Insets (i), (ii), and (iii) show the diffraction patterns of {111} planes across
the nanorods.
the head (seed) and tail (wire) exist as a single crystal. The
FFT images revealed that the head (i), the interface (ii), and
the tail (iii) share a common crystallographic plane (111), which
indicate that they are epitaxially connected, single-crystalline
Ge0. The contrast between the head and the tail observed in the
TEM images is due to the difference in their thickness.
The image in Figure 7a is a snapshot of the self-seeding
crystal formation taken after 5 min, where two crystalline Ge0
nanorods are overlapping. One rod functions as a seed with
arrested growth, while the other continues to grow as a wire.
When we extend the reaction time to 60 min, the head and tail
configuration clearly emerges with the wire growing in the [100]
direction (Figure 7b). These observations support that Ge0
nanowires grow in low Miller index directions, such as [100],
[110], and [111].47 The diameter of the seed as well as the wire
is maintained, indicating that the surface of the seed is well
passivated by the surfactant. The HRTEM image of the seed
taken at a tilt angle (inset of Figure 7b) reveals that the seed is
single-crystalline Ge0. From these images, the self-seeding
growth mechanism appears to be similar to the VLS growth
mechanism, except that the Ge0 serves as its own catalyst (self-
seeded).
Conclusions
The simple solution synthesis of Ge0 nanowires via the use
of Ge2+ precursor has been described. The main reason for using
molecularly designed Ge2+ precursors is that their reduction
potential is significantly lower than that of Ge4+ 49
We found
.
In contrast, a self-assembly growth mechanism (Figure 6b)
was also noted, where an aggregate of smaller rods can be seen
near the tip, but not along the side of the nanowires. The
commonly observed presence of these aggregates suggests that
they recrystallize on the tips to extend the length of Ge0
nanowires. The TEM image in Figure 8a is another snapshot
taken during the reaction, where several Ge0 nanorods aggregate
that, with less energy required to reduce the precursor to Ge0,
the synthesis can be conducted at lower temperatures (300-
315 °C) and pressure (1 atm) under Ar without the use of any
potentially contaminating seed materials.
Precursor 1 is found to play a crucial role in the formation
of Ge0 nanowires in this system. The stronger Ge-ligand bond
(48) Korgel, B. A.; Fitzmaurice, D. AdV. Mater. 1998, 10, 661-665.
(49) Bard, A. J.; Parsons, R.; Jordan, J. Standard Potentials in Aqueous Solutions,
1st ed.; IUPAC (Marcel Dekker): New York, 1985.
(47) Mullin, J. W. Crystallization, 3rd ed.; Butterworth-Heinemann Ltd: Great
Britain, 1993.
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A R T I C L E S
Gerung et al.
and increased steric hindrance assist in decreasing the precursor
reactivity, which limits the degree of nucleation and allows for
controlled nanowire growth. We surmise that the free lone-pair
electrons on the nitrogen facilitates both the reduction of Ge2+
as well as the surface passivation of nanowires by the amine.
Higher temperatures accelerate the growth and result in longer
nanowires.
Two growth mechanisms are observed in this system: (i) self-
seeding mechanism and (ii) self-assembly mechanism. Self-
seeding mechanism relies on the overlap between two Ge0
moieties at the initial stage of growth, providing a growth
platform for subsequent nanowire growth. The lower potential
of the overlapping area serves as a sink for the monomer to be
incorporated into the growing nanowire. In the self-assembly
mechanism, the nanowire growth occurs when the nanorods
aggregate at the tip of the nanowire and undergo a recrystalli-
zation process to form a single-crystalline material, while being
incorporated into the growing plane of the Ge0 nanowires.
Acknowledgment. The authors would like to acknowledge
Ying Bing Jiang for his help with the TEM images, and NSF
CAREER (DMR-0094145) for the partial support of this project.
This work was partially funded by the National Institutes of
Health (NIH) through the NIH Roadmap for Medical Research,
Grant No. 1 R21 EB005365-01. Information on this RFA
(Innovation in Molecular Imaging Probes) can be found at http://
grants.nih.gov/grants/guide/rfa-files/RFA-RM-04-021.html. San-
dia is a multiprogram laboratory operated by Sandia Corporation,
a Lockheed Martin Company, for the United States Department
of Energy under Contract No. DE-AC04-94AL85000.
Supporting Information Available: X-ray crystallographic
files in CIF format, and the TGA data for compound 1. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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