InP nanowires from surfactant-free thermolysis of single molecule precursors

Article abstract of DOI:10.1039/c2dt30283g

Indium phosphide nanofibres were grown from a single-molecule precursor, [(PhCH₂)₂InP(SiMe₃)₂]₂, using hot injection techniques by a solution-liquid-solid (SLS) process, under surfactant-free conditions and without the use of protic additives. The fibres are 85-95 nm in diameter and grow from In metal droplets of 100 nm diameter. The length of the nanofibres is a function of the precursor injection temperature (rather than the growth temperature) and can be varied from 6000 nm at 210 °C to 1000 nm at 310 °C. The indium metal tip can be readily removed under mild, non-etching conditions by treatment with thiophenol-P(SiMe₃)₃ mixtures.

Full text of DOI:10.1039/c2dt30283g

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PAPER
InP nanowires from surfactant-free thermolysis of single molecule
precursors†‡
Chiranjib Banerjee,a David L. Hughes,a Manfred Bochmann*a and Thomas Nann*b
Received 8th February 2012, Accepted 5th April 2012
DOI: 10.1039/c2dt30283g
Indium phosphide nanofibres were grown from a single-molecule precursor, [(PhCH2)2InP(SiMe3)2]2,
using hot injection techniques by a solution–liquid–solid (SLS) process, under “surfactant-free”
conditions and without the use of protic additives. The fibres are 85–95 nm in diameter and grow from In
metal droplets of 100 nm diameter. The length of the nanofibres is a function of the precursor injection
temperature (rather than the growth temperature) and can be varied from 6000 nm at 210 °C to 1000 nm
at 310 °C. The indium metal tip can be readily removed under mild, non-etching conditions by treatment
with thiophenol–P(SiMe3)3 mixtures.
necessary to reduce the size of the In metal droplets, while
Introduction
IntBu3 precursors at 330 °C gave tadpole-like InP fibres attached
to large indium metal droplets.18 According to Fang et al. a
similar temperature regime is required to generate In/InP nano-
wires from InMe3 and P(SiMe3)3 in octadecene (ODE) in the
presence of myristic acid. Clean production of nanowires was
observed >280 °C, while at lower temperatures a mixture of
nanoparticles and nanowires was produced and no wire growth
was seen at 260 °C.19 By contrast, Weller and co-workers
recently reported the growth of In/InP nanorods from InMe3 and
trioctylphosphine in the presence of TOPO at the much lower
optimum growth temperature of 200 °C, while higher tempera-
tures increased the size of the In droplets an led to multiple
nanorod growth per droplet.20
In this paper we describe a fast and simple route to clean-sur-
faced InP nanofibres using a hot injection technique, without the
addition of strongly binding surfactant stabilisers and in the
absence of protic reagents and carboxylic acids. The lengths of
these wires could be easily controlled by adjusting the injection
temperature.
Group III–V semiconductor nanowires (NWs) are of interest in
applications such as optical devices, solar cells, interconnects,
plastic electronics and others.1–7 Several methodologies for their
synthesis have been developed, including vapour–liquid–solid
(VLS),8,9 solution–liquid–solid (SLS),10 laser catalytic growth
(LCG),11,12 oxide assisted growth13,14 and templating strat-
egies.15,16 The low toxicity of compounds such as InP compared
to CdSe nanostructures is a particular attraction. A number of
wet chemical methods for the bottom-up synthesis of InP NWs
have been reported. For example, Buhro et al. used mixtures of
IntBu3 and PH3 in the presence of protic reagents, as well as the
methanolysis of [tBu2InP(SiMe3)2]2, to obtain InP fibres of
10–100 nm diameter. They demonstrated that the protic additive,
methanol, cleaves the In–P and P–SiMe3 bonds, generating
[tBu2In(OMe)]2 and [tBu2In(PH2)]3 as intermediates. Nanofibre
growth occurred from In metal nanodroplets formed by reduction
of some of the precursor, and tert-alkyl substituents in particular
were thought to be linked to this reductive decomposition
pathway.10,17 Ahrenkiel et al. used a mixture of two molecular
precursors, viz. [tBu2InP(SiMe3)2]2 and [Cl2InP(SiMe3)2]2
together with donor surfactants such as dodecylamine and trioc-
tylamine in the presence of trioctylphosphine (TOP), trioctylphos-
phine oxide (TOPO) and MeOH at temperatures of up to 250 °C.
These authors found the indium chloride precursor to be
Results and discussion
The indium dibenzyl single-molecule precursor [(PhCH2)2InP-
(SiMe3)2]2 (1) was chosen as a halide-free starting material,
initially in the expectation that reductive elimination of the
benzyl substituents might be more facile than the decomposition
of tBu derivatives and might therefore result in a reduction of the
initiation temperature. The compound is readily prepared from
InCl3, benzyl Grignard and P(SiMe3)3; the crystal structure is
shown in Fig. 1.
Thermolysis of 1 on injection into hot methyl myristate leads
to two decomposition processes, the reduction of In(III) to In(0)
which gives nanosized metal droplets, and the formation of
indium phosphide, which grows as fibres from the molten metal
surface. Thermogravimetric analysis (Fig. 2) shows that 1 incurs
aWolfson Materials and Catalysis Centre, School of Chemistry,
University of East Anglia, Norwich NR4 7TJ, UK. E-mail:
m.bochmann@uea.ac.uk; Fax: +44 (0)1603 592044
bIan Wark Research Institute, University of South Australia, Mawson
Lakes Blvd, Mawson Lakes, SA 5095, Australia. E-mail:
Thomas.Nann@unisa.edu.au
†Dedicated to Professor David Cole-Hamilton on the occasion of his
retirement and in recognition of his outstanding contributions to inor-
ganic and materials chemistry.
‡CCDC 858944. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2dt30283g
7244 | Dalton Trans., 2012, 41, 7244–7248
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Fig. 1 Molecular structure of [(PhCH2)2InP(SiMe3)2]2 (1), showing
the atomic numbering scheme. Selected bond lengths (Å) and angles (°):
In–P 2.6085(5); In–P′ 2.6160(4); In–C(3) 2.2279(15); In–C(4) 2.2013
(14); P–In–P′ 86.652(14); In–P–In′ 93.348(14); C(3)–In–C(4) 110.00(6).
Fig. 3 Dependence of InP NW average lengths with growth
temperature.
in our case, which provide for a different reaction path to InP
formation.
The formation of NWs depends on the minimum injection
temperature of 210 °C, but not on the growth temperatures. No
NWs resulted if the injection temperature was below 210 °C, but
they can be grown even at 160 °C once sufficient nucleation has
taken place.
Effect of temperature
The morphology of nanowires can depend on many factors, such
as the concentration of the precursors, growth duration, tempera-
ture, and concentration of surfactants. Although empirical results
have been published in the past showing a dependence of the
diameter of nanowires on temperature,19 to the best of our
knowledge the effect of the growth temperature on the lengths of
nanowires has not been systematically investigated. The results
are shown in Fig. 3.
Fig. 2 TGA diagram of [(PhCH2)2InP(SiMe3)2]2; heating rate 10 °C
min−1 (N2 flow rate 60 cm3 min−1).
The length of the InP nanofibres decreases with rising injec-
tion temperature. As the temperature increases, so does the
number of indium nanodroplets, at the expense of InP growth,
resulting in shorter fibres being formed at higher temperatures.
Fig. 4 shows the TEM images of nanofibres grown at three
different injection temperatures, viz. low (210 °C), medium
(240 °C) and high (310 °C).
a 66% weight loss on heating to 209 °C, probably due to the
loss of volatiles such as PhCH2SiMe3, PhCH2CH2Ph and
PhCH2P(SiMe3)2 (which approximate to 72 wt%), and shows
that formation of InP and In(0) occurs at this temperature. While
it was not our objective to determine the decomposition mechan-
ism, under the high temperature conditions two competing
decomposition pathways seem likely, leading to In metal and
InP, respectively:
Morphology and microstructure
½ðPhCH2Þ2InPðSiMe3Þ2ꢀ2 ! 2PhCH2PðSiMe3Þ2
þ PhCH2CH2Ph þ 2In0
The nanowires synthesized by this method were several μm long
but with fairly constant diameters of 85–95 nm at all tempera-
tures. In their as-made state they are attached to indium metal
particles of ca. 100 nm diameter. Electron diffraction studies
show that the lattice parameters of these fibres match those of
cubic InP. Fig. 5 shows the X-ray diffraction (XRD) pattern of a
typical sample of InP nanofibres and confirms the presence of
both InP and In metal.
½ðPhCH2Þ2InPðSiMe3Þ2ꢀ2 ! 4PhCH2SiMe3 þ 2InP
Growth will then take place from InP dissolved on the surface
of liquid In droplets.
The observation of facile nanofibre growth in non-protic
media differs from the report by Fang et al. who found myristic
acid to be necessary for nanowire growth, while in its absence
mainly InP nanoparticles were obtained.19 We believe these
differences may be due to the use of single-molecule precursors
Fig. 6 shows a high-resolution TEM micrograph of a typical
nanofibre. The <001> lattice fringes of the InP (zinc blende
structure) are well aligned and the nanowire grew apparently
along its unit cell axes. The distance between 10 lattice fringes is
2.9 nm, which corresponds well with the expected value of
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Fig. 4 TEM micrographs of InP nanofibres synthesized at injection temperatures of 210, 240 and 310 °C.
2.844 nm (InP has a lattice constant of 0.5688 nm). There was
no evidence for an oxide or amorphous surface layer.
The nanowires formed were very stable, even in absence of
any surfactants. The solvent has no influence over the growth
process, as the same morphology was obtained from reactions
conducted in methyl myristate and in a non-coordinating
solvent, 1-octadecene (ODE), although in the latter case the uni-
formity of the wires was slightly poorer.
Removal of the In metal tip
The indium metal attached to the nanowires can act as charge
acceptor and affect the physical properties of the wires. Thus the
presence or absence of the metal tips becomes potentially impor-
tant. Buhro et al. used metallic bismuth for NW growth,21
whereas, Ahrenkiel et al. used indium metal particles as seeds to
grow InP nanorods.18 In both cases etching techniques have
been used for metal removal, which not only dissolved the metal
Fig. 5 X-ray diffraction pattern of InP nanofibres. Red: InP reflections
(card no. 70-2902); blue: In metal reflections (card no. 05-0642).
7246 | Dalton Trans., 2012, 41, 7244–7248
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Fig. 7 TEM image showing InP nanofibres (grown at 250 °C) after
Fig. 6 HRTEM micrograph of a single InP nanofibre, showing the
removal of In metal particles.
<001> lattice planes and distance corresponding to cubic InP.
temperature, from about 1000 nm at 320 °C to 6000 nm at
210 °C. No growth was observed at injection temperatures below
210 °C. The indium metal tip can be removed under mild con-
ditions by conversion to InP nanoparticles, which are readily
separated by centrifugation. We are currently exploring potential
applications of these intriguing materials.
but also corroded the surface of the nanorods. Moreover, Ahren-
kiel’s procedure also involves the use of HF and mercury.
We found that the metal tip can be conveniently removed in a
one-pot reaction by utilizing the reactivity differences between
InP and In metal towards P(SiMe3)3. When P(SiMe3)3 was
added to the solution of pre-formed nanowires in the presence of
reagent capable of protolysing the P–Si bonds, in this case thio-
phenol, at a temperature well above the melting point of indium
metal (m.p. 156.6 °C), the metal droplets were converted to InP
nanoparticles. The reducing nature of this process means that
any oxide layer that may be present is also removed. Heating the
mixture above the melting point of indium is necessary for
efficient conversion into InP nanoparticles under these con-
ditions. Fig. 7 shows the TEM micrograph of the metal-free InP
nanowires. The inset image confirms that there is no indium left
at the base of an individual wire. There was no evidence that
removal of the In tip affected the length of the nanowires. Some
twisting occurred after several washing and purification pro-
cedures. XRD confirmed that the wires are composed of InP,
with no evidence for the presence of In2S3.
Experimental section
All manipulations were performed under dry nitrogen, using
standard Schlenk techniques. Solvents were pre-dried over
sodium wire and distilled under argon over sodium (toluene),
sodium–potassium alloy (light petroleum, bp 40–60 °C), or
sodium–benzophenone (diethyl ether, diphenyl ether). Dichloro-
methane was dried over calcium hydride. Deuterated solvents
were stored over activated 4 Å molecular sieves and degassed by
several freeze–thaw cycles. InCl3 (Aldrich 98%) was used as
received, P(SiMe3)3 was prepared following a literature pro-
cedure.26 Methyl myristate was dried over molecular sieves and
degassed by several freeze–thaw cycles. 1H and 31P NMR
spectra were recorded using a Bruker DPX300 spectrometer.
Transmission electron microscopy (TEM) images were recorded
using a JEM-2000 EX Electron Microscope (Jeol Ltd) at
200 kV; TEM samples were prepared by placing InP nanowires
dispersed in dichloromethane on 300 mesh copper grids with
Holey carbon films. Thermogravimetric analysis was conducted
using a Hi-Res TGA 2950 instrument (TA Instruments).
Conclusions
The thermal decomposition of dialkylindium phosphide single-
source precursors in hot weakly or non-coordinating solvents
provides a simple, surfactant-free method for the growth of InP
nanowires. This reduces the scope of any lattice doping by het-
eroatoms. To the best of our knowledge, such surfactant-less
growth of nanowires synthesized through the SLS mechanism
has not previously been reported.21–25 The synthesis is fast, at
the time scale of minutes, and gives high-quality crystalline
wires. The method avoids the need for preparing metal seed
particles to induce NW growth. The lengths of the nanofibres
can be conveniently controlled by adjusting the injection
Synthesis of [(PhCH2)2InP(SiMe3)2]2 (1)
To a suspension of InCl3 (1.52 g, 6.86 mmol) in 50 mL dry
diethyl ether at −78 °C was added freshly prepared PhCH2MgCl
in Et2O (1.14 M, 12.04 mL, 13.7 mmol). The stirred mixture
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 7244–7248 | 7247

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was allowed to warm slowly to room temperature. A white pre-
cipitate formed. The solvent was removed under vacuum, the
precipitate was dried, and 50 mL of dry toluene and 2 mL of
P(SiMe3)3 (6.87 mmol) were added. The mixture was heated to
reflux for 8 h, during which time the colour turned pale-yellow.
Removal of volatiles afforded a pale-yellow residue which was
extracted with dry toluene (4 × 20 mL). The combined extracts
were reduced to ∼15 mL. Cooling to −25 °C gave large colour-
less crystals of 1 (1.42 g, 1.5 mmol, 43.7%), m.p. 209 °C
(decomp.). 1H NMR (300 MHz, CD2Cl2): δ 7.07 (m, 4H, m-Ph),
6.87 (m, 6H, o- and p-Ph), 2.33 (s, 4H, CH2), 0.32 (t, 18H,
SiMe3, JHP = 5.6 Hz). 13C{1H} NMR (CD2Cl2, 75.5 MHz): δ
146.27 (Ph, ipso-C), 129.46, 128, 68 (Ph, o-C, m-C), 122.72
(Ph, p-C), 25.75 (t, JCP = 17.4 Hz, CH2), 4.58 (t, JCP = 9.3 Hz,
SiMe3). 31P NMR (121.5 Hz, CD2Cl2): δ −220.3. Anal. Found:
C, 51.05; H, 7.06. Calcd for C20H32InPSi2: C, 50.63; H, 6.80%.
in idealised positions and their Uiso values were set to ride on the
U
eq values of the parent carbon atoms. Computer programs used
in this analysis have been noted above, and were run through
WinGX.29 Crystal data: C40H64In2P2Si4, M = 948.85, triclinic,
ˉ
space group P1 (no. 2), Z = 1, a = 10.6674(9), b = 10.7597(2),
c = 12.1674(12) Å, α = 102.721(9), β = 113.424(5), γ = 106.954
(1)°, U = 1132.32(15) Å3, T = 140 K, 23 201 reflections
measured, 6588 unique (Rint = 0.034) which were used in all cal-
culations. The final wR2 was 0.053 (all data).
Acknowledgements
This work was in part supported by the Royal Society and the
University of East Anglia. CB thanks UEA for a studentship.
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7248 | Dalton Trans., 2012, 41, 7244–7248
This journal is © The Royal Society of Chemistry 2012