The role of precursor-decomposition kinetics in silicon-nanowire synthesis in organic solvents

Article abstract of DOI:10.1002/anie.200463001

(Figure Presented) Under pressure! The synthesis of crystalline silicon nanowires can be carried out in organic solvents at reaction temperatures of up to 450-500°C under high-pressure conditions. Gold particles are used as seeds, and organosilanes are em

Full text of DOI:10.1002/anie.200463001

Angewandte
Chemie
Nanostructures
chemistry and move away from slow and expensive gas-phase
chemical vapor deposition (CVD).
The Role of Precursor-Decomposition Kinetics in
The colloidal synthesis of Si nanomaterials is extremely
Silicon-Nanowire Synthesis in Organic Solvents** challenging and represents to some extent a “holy grail” in
solution-based nanomaterials chemistry. In 2000, we showed
that crystalline Si nanowires could be synthesized in solution
Doh C. Lee, Tobias Hanrath, and Brian A. Korgel*
by using Au nanocrystals as seeds to lower the crystallization
[
14]
barrier and promote crystalline nanowire growth.
By
The “bottom-up” chemical synthesis of semiconductor nano-
wires has been developed as an alternative strategy to
conventional lithographic patterning for the preparation of
functional nanostructures suitable for applications such as
pressurizing the organic solvent, reaction temperatures
exceeding the Au/Si eutectic temperature (3638C) could be
[
15]
reached and “VLS-like” nanowire growth could be pro-
moted. We refer to this nanowire-growth mechanism as
[
1]
[2]
[3]
[16]
logic gates, memory devices, light-emitting devices,
supercritical fluid–liquid–solid (SFLS) synthesis.
[
4]
[5,6]
sensors, and photonic circuits.
Nanowire growth by the
In hindsight, we were very lucky to stumble across
diphenylsilane as a suitable Si precursor for the synthesis.
Very little is known about the chemistry of the relevant
silanes (that is, aryl- and alkyl-substituted silanes and
trisilane) in pressurized solvents at high temperature, and as
we show herein, other potential precursors that would appear
to be obvious choices fail completely. In fact, we have found
that the SFLS process is very sensitive to the precursor-
vapor–liquid–solid (VLS) mechanism has been very success-
ful for a variety of different materials, including Group IV,
[
7–12]
Group II–VI, and Group III–V semiconductors,
metal oxides.
and
[
13]
These nanowires are in many ways like
macromolecules, as they can be suspended in solvents and
deposited on substrates or mixed with polymers as compo-
sites. Ideally, the aim is to synthesize nanowires by solution
Figure 1. HRSEM images of the reaction products obtained from a) octylsilane, b) diethylsilane, c) tetraethylsilane, d) trisilane, e) phenylsilane,
and f) diphenylsilane injected into hexane at 4508C, approximately 7.2 MPa, and a concentration of 350 mm. The reactions were carried out for
5
min with a Au/Si molar ratio of 1:1000.
decomposition kinetics, and careful tuning of this process is
required to optimize the nanowire quality and prevent
unwanted homogeneous Si-particle nucleation. Herein, we
report on the relationship between the silane decomposition
chemistry and the quality of the Si nanowires produced by
SFLS by using Au-nanocrystal seeds.
Figure 1 shows scanning electron microscopy (SEM)
images of the solid product obtained from six different
Si precursors injected into anhydrous hexane at 4508C and
[
*] D. C. Lee, T. Hanrath, Prof. B. A. Korgel
Department of Chemical Engineering
Texas Materials Institute
Center for Nano- and Molecular Science and Technology
The University of Texas at Austin
Austin, TX 78712-1062 (USA)
Fax: (+1)512-471-7060
E-mail: korgel@mail.che.utexas.edu
[
**] This work was supported financially by the National Science
Foundation, the Welch Foundation, and the Advanced Materials
Research Center in collaboration with International SEMATECH. We
acknowledge fruitful discussions with A. E. Saunders and thank J. P.
Zhou for assistance with the high-resolution transmission electron
microscopy.
7
.2 MPa with dodecanethiol-coated Au nanocrystals with an
average diameter of approximately 4 nm. The silane concen-
tration in each case was 350 mm with a Au/Si molar ratio of
1:1000, and the reactions were carried out for 5 min. The
Angew. Chem. Int. Ed. 2005, 44, 3573 –3577
DOI: 10.1002/anie.200463001
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images in Figure 1 show the products obtained from the alkyl
silanes octylsilane, diethylsilane, and tetraethylsilane (Fig-
ure 1a–c, respectively). In all cases, the nanowire was formed
in extremely low yield or not at all. Only the monosubstituted
alkyl silane, octylsilane, produced a measurable amount of
crystalline Si nanowires, but in miniscule yield and with large
amounts of oligomeric silicon- and carbon-containing impur-
ities. The multisubstituted alkyl silanes, diethylsilane and
tetraethylsilane, did not produce any crystalline nanowires,
only curly amorphous wires in the case of diethylsilane and
amorphous particulates in the case of tetraethylsilane. It
appears that the SiꢀH bond is relatively reactive and labile,
but homolytic cleavage of the alkyl SiꢀC bond is too slow to
provide sufficient Si atoms for the Au-seed particles to sustain
crystalline nanowire growth.
Trisilane is very reactive and decomposes rapidly at
temperatures above 3508C to produce silicon in nearly 100%
[
17]
yield. However, trisilane does not form Si nanowires in the
presence of the Au nanocrystals. The reaction produces
micrometer-sized amorphous Si colloids (Figure 1d); surpris-
ingly, the same product was obtained in the absence of
[
18]
Au nanocrystals. Unlike the alkyl or aryl silane precursors,
trisilane can undergo thermolysis through heterogeneous
insertion at hydrogen-terminated sites on Si surfaces, which is
a process that can lead to rapid particle formation once
[
19]
Figure 2. High-resolution TEM (HRTEM) images of Si nanowires pro-
duced by SFLS from Au nanocrystals and diphenylsilane at 4508C.
Under these conditions, SFLS yields predominantly h111i-oriented
nanowires, as shown in (a) and (b); however, limited numbers of
h110i- and h211i-oriented nanowires are also found with diameters
smaller than 10 nm (c, d). The fast Fourier transforms (FFTs) of the
image in (a) are shown in the inset.
amorphous Si colloids have nucleated.
Furthermore, the
SiꢀSi bonds in trisilane are very stable and do not dissociate at
the typical SFLS reaction temperatures of approximately
4
50–5008C. Therefore, dehydrogenation of trisilane at these
reaction temperatures leaves a “bare” Si trimer, which
appears not to dissolve in the Au-nanocrystal seeds. Trisilane
can also be deposited on the sidewalls of
any nanowires that have not fully formed
to give rise to a thick amorphous coating.
The alkyl silanes are not sufficiently reac-
tive to produce high-quality crystalline
nanowires by Au-seeded SFLS and trisi-
lane is too reactive, which leads to the
homogeneous particle formation of amor-
phous Si structures.
Aryl silanes exhibit suitable reactivity
for the formation of high-quality nano-
wires. Figure 1e,f shows the Si products
obtained from phenylsilane and diphenyl-
silane: Both precursors yield large quanti-
ties of Si nanowires as the primary reaction
product. As shown in Figure 2, the nano-
wires formed by using diphenylsilane as
the precursor are crystalline with a dia-
mond cubic structure and few defects. The
alkyl- and aryl-substituted organosilanes
exhibit qualitatively different reactivities at approximately
Scheme 1. Bimolecular disproportionation of a) phenylsilane and b) diphenylsilane. Silane
decomposes at temperatures above approximately 3508C to produce Si atoms.
[
24]
have not been well-studied, the mechanism appears to be
associative and involves the exchange of a hydrogen atom and
a phenyl group. The resonance effect of the benzene ring
lowers the activation energy for the disproportionation
reaction, whereas the alkyl group does not give rise to a
4
508C. The SiꢀC bonds in the alkyl-substituted silanes have
lower dissociation enthalpies than those in the aryl-substi-
[
20,21]
tuted silanes;
however, the aryl-substituted silanes can
undergo disproportionation reactions of the type shown in
Scheme 1, and therefore the aryl group is a more reactive
[
23]
resonance effect.
The alkyl-substituted silanes can only
[
22,23]
substituent under the nanowire growth conditions.
decompose by homolytic dissociation, which is very slow at
these temperatures; however, the aryl silanes undergo a series
Although the exact details of the disproportionation reactions
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of disproportionation reactions that yield silane and tetra-
phenylsilane as the final reaction products. Tetraphenylsilane
Figure 3 shows HRSEM images of the reaction products
synthesized from phenylsilane (Figure 3a–c) and diphenylsi-
lane (Figure 3d–e) at different reaction temperatures
between 400 and 5008C. When the synthesis was performed
at 3508C—just below the bulk-phase Si/Au eutectic temper-
ature (3638C)—from either phenylsilane or diphenylsilane,
no significant quantities of solid product were yielded (the
results are not shown). In the reactions carried out at just
above the eutectic temperature (4008C), nanowires did not
form and only particulate materials that were poorly defined
in structure, and so could not be clearly observed by HRSEM,
were formed. This result is in stark contrast to the Au-
nanocrystal-promoted SFLS synthesis of Ge nanowires,
[
24,25]
is very stable and does not decompose below 5008C,
and
[
26]
silane decomposes to silicon at approximately 3508C. On
the basis of this proposed mechanism, silane generated in situ
during the reaction promotes nanowire growth.
As expected from the proposed disproportionation mech-
anism for the decomposition of the aryl silanes, reactants with
higher phenyl substitution give the product in lower yields. As
shown in Scheme 1, phenylsilane requires only one dispro-
portionation step to form silane, in contrast to diphenylsilane,
which requires two consecutive reactions. Phenylsilane was
found to lead to higher product yields than diphenylsilane.
High-resolution SEM (HRSEM) analysis of the Si nanowires
produced from diphenylsilane at 5008C also showed that a
significantly greater amount of carbonaceous by-products
formed relative to those produced from phenylsilane. Perhaps
because of its additional phenyl moiety, diphenylsilane
exhibits an increased likelihood to form carbonaceous by-
products as well as nanowires. For both diphenylsilane and
phenylsilane, there appears to be a “threshold” concentration
[
27]
which are routinely grown at 3858C in very high quality.
Since the Au/Ge system exhibits a similar eutectic temper-
[
27]
ature to that of the Au/Si system (3618C), similar results for
the Si nanowires would be expected. The significantly lower
growth temperature of the Ge nanowires appears to be
directly related to the higher reactivity of the aryl germanes
relative to the aryl silanes; therefore, the slow precursor-
degradation kinetics appear to limit the Si-nanowire growth
at temperatures just above the Au/Si eutectic temperature.
The reaction temperature must reach approximately 4508C
(
approximately 120 mm for phenylsilane) below which little
or no nanowire product is formed.
Figure 3. HRSEM images of the Si product obtained from phenylsilane (a–c) or diphenylsilane (d–f) in hexane at 4008C (a, d), at 4508C (b, e),
and at 5008C (c, f). For both precursors, reaction temperatures of at least 4508C are required to form nanowires.
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for high-quality crystalline Si nanowires to be produced.
cross-sectional interface (Figure 4b) but possess “curved”
interfaces instead that appear to undergo reconstruction to
form flat Si (111)/Au interfaces at the tip, as observed by Wu
However, further increases in reaction temperature do not
improve the nanowire growth: Phenylsilane produces nano-
wires at 5008C but with a relatively high proportion of
carbon-containing amorphous Si by-products, and reactions
at temperatures higher than 5008C result in significant
pyrolysis of hexane.
[
29]
et al.
for Si nanowires grown by Au-seeded VLS with
h110i orientation. The influence of the liquid–crystal interface
and the fact that the Si (111)/Au interface exhibits the lowest
free energy relative to other possible interfaces has been well-
[
28]
As shown in Figures 2 and 4, h111i is the predominant
established from early work on Si whiskers. The stability of
the Si (111)/Au interface is further confirmed by our obser-
vations of a migrating Si/Au interface when it is exposed to
the electron beam in TEM. Long exposure time of the Si/
Au interface to the electron beam results in the generation of
sufficient thermal energy for the Au interface to migrate
approximately 14 nm into the nanowire (Figure 5). The
growth direction for Si nanowires synthesized at 4508C from
Figure 5. Au tip at the end of a Si nanowire exposed to a converged
electron beam at 200 kV after a) 0, b) 1.5, c) 3, and d) 4.5 min. The
Au interface migrates into the Si nanowire until it penetrates by
approximately 14 nm. The nanowire maintains the sharp {111} inter-
face at the Au/Si tip. Scale bar: 5 nm.
interface remains sharp and atomically smooth despite this
progression. Compare this interfacial structure to the curved
Au/SiO interface that forms at the tip of a Si nanowire after
2
two months of exposure to air (Figure 4d). The difference in
Figure 4. HRTEM images of Au-seed particles at Si-nanowire tips.
a) HRTEM image of several nanowires with Au tips. b) The “curved”
Au/Si interface of a h211i-oriented nanowire. c) Au tip at the end of a
h111i-oriented Si nanowire. d) A nanowire that was exposed for two
months to air that has oxidized at the Au/Si interface and the nanowire
surface. e) Au tip at the end of a h111i-oriented Si nanowire. f) The
interface at the Au tip in (e) at a greater magnification of 1000000ꢀ.
the interfacial energy of the Au/SiO interface relative to the
2
Au/Si interface is reflected in the qualitatively different
structures formed.
In summary, aryl silanes are effective precursors for the
growth of crystalline Si nanowires by Au-nanocrystal-seeded
SFLS, whereas alkyl silanes and trisilane are not. The quality
of the Si product is related directly to the decomposition
chemistry of the precursor: The precursor must be sufficiently
reactive to saturate the Au-nanocrystal seeds with Si atoms
and promote nanowire growth, but not so reactive that
homogeneous particle nucleation and sidewall deposition
overcome the metal-particle-directed crystallization. To put
these findings into perspective, low-melting-point metals,
such as In and Bi, have recently been used successfully as
seeds to lower the metal/semiconductor eutectic temperature
to below 3008C and enable conventional solvents to be used
Au nanocrystals and either phenylsilane or diphenylsilane. A
few nanowires could be found with h110i or h112i growth
directions. The preference for the h111i growth direction in
the Si nanowires is consistent with Si whiskers grown in the
gas phase by Au-seeded VLS at similar reaction temper-
[
28]
atures. Transmission electron microscopy (TEM) imaging
of the Au/Si tip of the nanowires grown by SFLS reveals a flat,
atomically abrupt interface with a Si (111) surface. Nanowires
with h112i or h110i growth directions do not exhibit this flat
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Received: December 20, 2004
Published online: May 4, 2005
Keywords: nanowires · precursors · silicon · supercritical fluids ·
.
synthetic methods
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