A developed Ullmann reaction to III-V semiconductor nanocrystals in sealed vacuum tubes

Article abstract of DOI:10.1039/b809442j

Group III-V (13-15, III = Ga, In, and V = P, As) semiconductor nanocrystals were effectively obtained via a developed Ullmann reaction route through the reactions of preformed nanoscale metallic indium or commercial gallium with triphenylphosphine (PPh₃) and triphenylarsine (AsPh₃) in sealed vacuum quartz tubes under moderate conditions at 320-400 °C for 8-24 h. The developed synthetic strategy in sealed vacuum tubes extends the synthesis of III-V semiconductor materials, and the air-stable PPh₃ and AsPh₃ with low toxicity provide good alternative pnicogen precursors for the synthesis of III-V nanocrystals. The analysis of XRD, ED and HRTEM established the production of one-dimensional (1D) metastable wurtzite (W) InP, InAs and GaP nanostructures in the zinc blende (ZB) products. Further investigations showed that 1D W nanostructures resulted from kinetic effects under the moderate synthetic conditions employed and the steric effect of PPh₃ and AsPh₃, and that the tendency for the synthesis of III-V nanocrystals was in the orders of IIIP > IIIAs and GaV > InV on the basis of experiments and thermodynamic calculations. Meanwhile, the microstructures and growth mechanism of the III-V nanocrystals were investigated.

Full text of DOI:10.1039/b809442j

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PAPER
www.rsc.org/dalton | Dalton Transactions
A developed Ullmann reaction to III–V semiconductor nanocrystals
in sealed vacuum tubes†
Junli Wang and Qing Yang*
Received 3rd June 2008, Accepted 29th July 2008
First published as an Advance Article on the web 25th September 2008
DOI: 10.1039/b809442j
Group III–V (13–15, III = Ga, In, and V = P, As) semiconductor nanocrystals were effectively
obtained via a developed Ullmann reaction route through the reactions of preformed nanoscale
metallic indium or commercial gallium with triphenylphosphine (PPh3) and triphenylarsine (AsPh3) in
sealed vacuum quartz tubes under moderate conditions at 320–400 ◦C for 8–24 h. The developed
synthetic strategy in sealed vacuum tubes extends the synthesis of III–V semiconductor materials, and
the air-stable PPh3 and AsPh3 with low toxicity provide good alternative pnicogen precursors for the
synthesis of III–V nanocrystals. The analysis of XRD, ED and HRTEM established the production of
one-dimensional (1D) metastable wurtzite (W) InP, InAs and GaP nanostructures in the zinc blende
(ZB) products. Further investigations showed that 1D W nanostructures resulted from kinetic effects
under the moderate synthetic conditions employed and the steric effect of PPh3 and AsPh3, and that the
tendency for the synthesis of III–V nanocrystals was in the orders of IIIP > IIIAs and GaV > InV on
the basis of experiments and thermodynamic calculations. Meanwhile, the microstructures and growth
mechanism of the III–V nanocrystals were investigated.
as common organic compounds containing VA elements, and
Introduction
they can serve as the effective precursors of P and As sources
to synthesize III–V semiconductor nanocrystals. Meanwhile, the
use of low-toxicity and air-stable PPh3 and AsPh3 as alternative
pnicogen precursors can effectively avoid the employment of
EH3, (Na/K)3E, ECl3 (E = P, As), P4, As or organometallic
compounds of pnicogens such as tris(trimethylsilyl) phosphine
[P(TMS)3],6,8–10,12,29 tris(trimethylsilyl) arsine [As(TMS)3]11,27 and
trioctyl phosphine (TOP)32 with poor stability and high toxicity in
the synthesis of III–V nanocrystals. The specific configurations
of PPh3 and AsPh3 with high steric effects may induce some
exceptional structures and shapes for III–V nanocrystals. On the
basis of these ideas, we have successfully obtained InP and GaP
nanocrystals from commercial bulk metal indium and gallium
directly reacting with PPh3.33 Very recently, we have also synthe-
sized InP and InAs nanocrystals through the reduction of metal
chlorides with the above two triphenylpnictines in the presence of
KBH4 or Zn in benzene solution via a solvothermal process.34
To extend the work33,34 in depth, here we report a developed
Ullmann reaction route to III–V semiconductor nanocrystals in
sealed vacuum quartz tubes at 320–400 ◦C for 8–24 h through the
reactions of initial preformed metallic indium nanoparticles or
commercial gallium with PPh3 or AsPh3. The reactions involved
in the synthesis can be simplified as follows:
Group III–V (13–15) compounds are important semiconduc-
tors with high electron mobilities and larger exciton diameters,
which make them widely-studied in electronic and optoelectronic
devices,1–3 high-speed electronics,4,5 and quantum size effects.6–12
Chemical synthesis of III–V semiconductor materials and recently
those within the nanometer scale can be carried out in solid,13,14
vapor15–20 or liquid phase, or in solution6–12,21–29 as well as their
hybrids.13–30 Many kinds of chemical reactions, such as ele-
mental reactions,13,23–25 metathesis reactions,14,21,22 organometallic
compound decomposition,6–12,18–20,26–30 and others15–17,31 are often
involved in their synthesis, and all of them can efficiently lead to
III–V semiconductor materials. However, the synthesis of III–
V semiconductors of high quality still meets many problems,
despite that there have been considerable efforts on the synthesis
of III–V semiconductors in recent years. Synthetic reactions in
the solid state must be conducted at high temperature, while
organometallic compounds are usually highly toxic and unstable
in air, which leads to complex synthesis procedures. Moreover, the
higher covalent nature and less-available precursors also arouse
many difficulties in the synthesis of III–V materials. Therefore, it is
of much importance to seek novel pnicogen (N, P, As, Sb) sources
and develop facile synthetic methods for III–V semiconductors
from the perspective of chemistry and materials.
Triphenylphosphine (PPh3) and triphenylarsine (AsPh3) are
widely used in organic synthesis and coordination chemistry
320-400 ∞C
in sealed, vacuum tubes
(1)
2III + 2VPh3 æææææææÆ2IIIV + 3Ph - Ph
where III = In, Ga, and V = P, As. Based on the above reac-
tions, namely developed Ullmann reactions, we have successfully
synthesized InP, InAs, GaP and GaAs nanocrystals. The designed
synthetic strategy is facile and versatile, and it has been proven to
be a good alternative route to III–V semiconductors and relevant
compounds.
Hefei National Laboratory for Physical Science at Microscale and Depart-
ment of Chemistry, University of Science and Technology of China (USTC),
Hefei, Anhui, 230026, P. R. China. E-mail: qyoung@ustc.edu.cn; Fax: +86
(0)551 3606266; Tel: +86 (0)551 3600243
† Electronic supplementary information (ESI) available: Energy-dispersive
X-ray spectroscopy (EDX) spectra of the GaP products and GaAs
products. See DOI: 10.1039/b809442j
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Interestingly, investigations indicated that metastable wurtzite
(W) InP, InAs and GaP were present in the as-synthesized one-
dimensional (1D) nanostructures (nanowires or nanorods) besides
the main zinc blende (ZB) phase. Further investigations suggested
that the moderately synthetic conditions (at 320–400 ◦C in sealed
vacuum quartz tubes) and the employment of PPh3 and AsPh3
were favorable for the growth of W structured III–V nanocrystals.
Moreover, the tendency for the synthesis of III–V nanocrystals
could be arranged in the following order: IIIP > IIIAs and GaV >
InV (III = Ga, In, and V = P, As) on the basis of experiments
and thermodynamic calculations. In addition, investigation also
suggested that the formation of the 1D III–V semiconductor
nanostructures was attributable to the solution–liquid–solid (SLS)
growth mechanism.6,26–29,35–38
images were recorded on an Oxford ISIS spectroscope attached to
the microscope for composition analysis. X-Ray photoelectron
spectroscopy (XPS) was taken on an ESCALab MKII X-ray
photoelectron spectrometer, using Al Ka X-ray as the excitation
source. The binding energies obtained in the XPS analysis were
calibrated against the C1s peak at 284.6 eV.
Results and discussion
In the developed Ullmann-reaction synthetic route, four typical
III–V compounds can be obtained. For the synthesis of InP and
InAs nanocrystals, the chemical reactions employed are as follows:
2In + 2PPh3 → 2InP + 3Ph-Ph
(2)
(3)
2In + 2AsPh3 → 2InAs + 3Ph-Ph
Experimental
The two reactions are feasible thermodynamically. For G◦
H◦ - T·S◦, in an isothermal process (T is constant), DrG◦
=
=
Synthesis of III–V semiconductor nanocrystals
DrH◦–T·DrS◦, and DrG◦ is determined by DrH◦, DrS◦ and T. If
the temperature (T) is low or not very high, the influence of T·DrS◦
on the above equation is rather small, and the value of the first
term on the right side of the equation dominates that of DrG◦.
Therefore, the value of DrG◦ approaches that of DrH◦. Ignoring
the influence of T·DrS◦ on the reaction, we have calculated the
Gibbs energy change (DrG◦) in these reactions at 25 ◦C (and even
up to 400 ◦C).40 For the synthesis of InP, the Gibbs energy change
All materials were used as received without further purification.
In a typical synthesis of InP nanocrystals, the mixture of 0.114 g
newly-prepared indium nanocrystals39 and 0.320 g PPh3, of which
the molar ratio of indium to PPh3 was 1 : 1.2, was added
into a quartz ampullaceous tube (f8 mm ¥ 12 cm), which was
then evacuated and sealed. The tube was horizontally kept in a
resistance furnace at 400 ◦C for 8 h. The temperature increased
◦
from room temperature to 400 C in 80 min. After the tube was
of reaction 2 is calculated as follows: DrG◦ =DrH◦ - T·DrS◦
ª
ꢀ
ꢀ
naturally cooled to room temperature, the black products were
collected, washed with benzene, diluted hydrochloric acid and
absolute alcohol, and finally dried for further investigation.
As for the synthesis of InAs, GaP and GaAs nanocrystals,
similar synthesis procedures were employed by changing cor-
responding reaction precursors with various molar ratios. The
details of the conditions for the synthesis of III–V semiconductor
nanocrystals were summarized in Table 1.
DrH◦
=
DfH◦
–
DfH◦
= -343.6 kJ mol-1, while
(products)
(reactants)
the change in the Gibbs energy in reaction 3 for the synthesis of
InAs is about -439.0 kJ mol-1. The Gibbs energy changes in these
synthetic reactions are rather negative, which implies a vigorous
tendency for reactions (2) and (3) to progress towards the right
side. Similarly, the Gibbs energy changes of the reactions to GaP
and GaAs are also negative, and the corresponding reactions of
(4) and (5) (as shown below)
2Ga + 2PPh3 → 2GaP + 3Ph - Ph
(4)
(5)
Characterization
The phase purity and crystallizability of the as-prepared prod-
ucts were determined by powder X-ray diffraction (XRD) us-
ing a Philips X’Pert Pro Super diffractometer with graphite-
2Ga + 2AsPh3 → 2GaAs + 3Ph - Ph
can proceed toward the right side thermodynamically. It is
concluded that a general approach could be proposed for the
synthesis of group III–V semiconductor materials through such
designed chemical reactions in sealed vacuum tubes under mod-
erate conditions on the basis of the thermodynamic principle.
The XRD patterns of the as-synthesized products are shown
in Fig. 1, which indicates that all the III–V products are of good
˚
monochromatized Cu Ka radiation (l = 1.54178 A). Mor-
phologies and microstructures of the samples were examined by
transmission electron microscopy (TEM, Hitachi model H-800),
selected area electron diffraction (SAED) patterns, and high-
resolution TEM (HRTEM, JEOL-2010, used with an accelerating
voltage of 200 kV). Energy-dispersive X-ray spectroscopy (EDX)
Table 1 Synthetic conditions of III–V nanocrystals with various morphologies and crystallographic phasesa
Precursor molar ratio of III to V
Conditions
Products
Morphologies of the products
Phases of the products
1 : 1.2 (In:P)
1 : 1 (In:As)
2 : 1 (In:As)
1 : 1.2 (Ga:P)
1 : 1 (Ga:As)
2 : 1 (Ga:As)
3 : 1 (Ga:As)
400 ◦C, 8 h
370 ◦C, 24 h
350 ◦C, 24 h
380 ◦C, 12 h
380 ◦C, 24 h
360 ◦C, 24 h
360 ◦C, 24 h
InP
InAs + As
InAs
NWs
ZB and W NWs
ZB NPs and W segments
ZB NPs and W segments
ZB NPs and W NWs
ZB NPs
ZB NPs
ZB NPs
NPs and NRs
NPs and NRs
NPs and NWs
NPs
NPs
NPs
GaP
GaAs + As
GaAs + Astrace
GaAs
a NWs, NPs and NRs denote nanowires, nanoparticles and nanorods, respectively.
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Fig. 2 XRD patterns of (a) the pure InAs sample produced using In and
AsPh3 taken in a ratio of 2 : 1; and (b) the pure GaAs sample produced
using Ga and AsPh3 with a ratio of 3 : 1.
displayed a 1D wire-like morphology, and the studies of ED and
HRTEM (Fig. 3c) established that the presence of W diffraction
peaks in the XRD pattern (Fig. 1a) was due to the production
of W InP nanowires, and that these W InP nanowires were of a
single-crystal nature in the diameter range of 30–150 nm. It is well-
known that InP is stable in ZB structure, especially in the bulk,41–45
and the sequence of stabilization for ZB structured MP (M= Ga,
Al, In) is on the order of GaP < AlP < InP.44 The structural
preference denotes the strong trend in the growth of InP with
Fig. 1 XRD patterns of (a) the InP sample synthesized at 400 ◦C for 8 h;
(b) the InAs sample synthesized at 370 ◦C for 24 h; (c) the GaP sample
synthesized at 380 ◦C for 12 h; and (d) the GaAs sample synthesized at
380 ◦C for 24 h. The diffraction peaks marked with asterisks are from the
W structure.
crystallizability. Fig. 1a and b are the XRD patterns for the I◦nP
◦
and InAs products synthesized at 400 C for 8 h and at 370 C
for 24 h, respectively. It is interesting to note that there are weak
W structured diffraction peaks besides the ones of the main ZB
phase presented in the patterns, which indicates that metastable W
InP and InAs were produced under current moderate conditions.
However, there is not any W GaP (Fig. 1c) and GaAs (Fig. 1d)
observed for the GaP and GaAs samples synthesized at 380 ◦C
for 12 h and at 380 ◦C for 24 h, respectively, under the limit of
detection of the diffractometer and there is only ZB GaP and
GaAs detected (Fig. 1c and d). Meanwhile, there is some As
impurity existing in the InAs and GaAs products because of the
observation of extra diffraction peaks of As in the patterns (Fig. 1c
and d). Elemental As impurity in the arsenides results from the
pyrogenation of AsPh3 at 350–400 ◦C. In the synthesis, increasing
the molar ratios of III to V can eliminate As impurity in the
products. As shown in Fig. 2, pure InAs and GaAs products were
obtained by changing the molar ratios of reaction precursors with
the excess IIIA metals, and detailed experimental conditions can
be seen in Table 1. It is also noted that there is a broad hump
for both GaP and GaAs samples near the 36◦ 2q angle in their
corresponding XRD patterns (Fig. 1c and 2b). The origin of this
hump is probably due to the generation of oxides (formation of
Ga2O3) and amorphous materials, and the similar phenomena
were reported previously in ref. 21.
The III–V semiconductor nanocrystals synthesized via the
present approach displayed a series of diverse morphologies
(shown in Table 1) although we intended to obtain 1D nanostruc-
tures. The detailed investigations were demonstrated as follows.
TEM images (Fig. 3a and b) showed that the InP products
Fig. 3 (a) TEM image of the InP nanowires synthesized at 400 ◦C for 8 h;
(b) TEM images of typical single-crystal W InP nanowires with the various
diameter of 31 nm, 80 nm and 110 nm, respectively; (c) HRTEM image
and ED pattern (the inset) recorded on W InP nanowires; (d) HRTEM
image and ED pattern (the inset) recorded on ZB InP nanowires.
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ZB structure, and it is difficult to obtain W structured InP single
crystals because of the high thermodynamic stability of ZB InP.41–45
In fact, many methods reported the production of InP nanowires
with the ZB phase. Recently, it is noted that InP nanowires with
W structure have been reported growing on InP(111)A substrates
by a selective area metalorganic vapor phase epitaxy (MOVPE)
process.46 However, the W structure of InP should be further
confirmed since the given ED pattern of the InP nanowires in ref.
46 is likely to be indexed as the diffraction spots of ZB structure
along the [112] projection rather than the [010] projection of W
structure. More recently, ultrafine InP nanowires fabricated by
MOVPE47,48 take ZB structure with many rotational twin blocks
consisting of W segments along the <111> growth directions.
A recent theoretical calculation suggests that the stability trend
of ZB and W structured nanowires depends on the diameter
of nanowires, and W nanowires are stabilized in small diameter
(<32 nm for InP).45 It is very significant to obtain metastable W
InP single-crystal nanowires with large diameter from the reaction
of indium nanoparticles with PPh3, which is clearly confirmed by
XRD, ED and HRTEM in the present work. The synthesis of
single-crystal W InP nanowires with large diameters up to 150 nm
will promote the studies on the polytypism and the relative stability
between ZB and W structures of InP crystals. The investigations of
XRD and HRTEM also demonstrated that the ZB InP nanowires
are of high crystallizability and single-crystal nature (Fig. 3d).
Fig. 4 demonstrates typical TEM images of the InAs products,
showing that the products are in the shapes of both nanoparticles
and nanorods. The analysis of ED and HRTEM indicated that
InAs nanoparticles took the ZB phase, and that W structured
segments, stacking faults and rotational twins were often present
in the InAs nanorods along the growth direction, which resulted
in the appearance of W diffraction peaks in the XRD pattern
of the InAs sample. Fig. 4c–f shows ED patterns and HRTEM
images taken from InAs nanorods. The ED pattern (Fig. 4c) clearly
presents one set of basic diffraction spots and two sets of additional
spots. The basic set is indexed as the ZB InAs diffraction pattern
viewed along the [110] direction. One set of additional spots
corresponds to the ZB InAs (111) twinning diffraction pattern
viewed along the [110] direction, and the other set corresponds to
the W InAs diffraction pattern viewed along the [2110] direction
(Fig. 4c). The corresponding HRTEM image is shown in Fig. 4d.
Fig. 4e is the TEM image and ED pattern of another InAs
nanorod. The ED pattern can be well indexed as the ZB (111)
twinning diffraction pattern projected along the [110] direction,
which indicates that the InAs nanorod consists of many rotational
twins. The corresponding HRTEM image is shown in Fig. 4f, and
it clarifies that the InAs nanorods comprise many W structured
segments, stacking faults and rotational twins along the growth
direction (the [111] direction). The special microstructures in the
InAs nanorods can be attributed to the small energy difference
between the W and ZB structures (DEW–ZB). According to the
recent theoretical calculation,45 the DEW–ZB of InAs nanowires will
become smaller than that of the bulk InAs, which might cause
the W structure incorporating ZB structured nanowires. At the
same time, the current synthetic conditions, for example, the low
temperature of 320–380 ◦C and low pressure in sealed vacuum
tubes, facilitate the growth of metastable W structured segments
in the InAs nanorods. The observation of such microstructures in
the present work is close to the literature reported previously,45,49
Fig. 4 (a) TEM image of the as-synthesized InAs nanoparticles; (b) TEM
image of InAs nanorods; (c) ED pattern and (d) HRTEM image of a typical
InAs nanorod; (e) TEM image and ED pattern (the inset) of another
individual InAs nanorod; (f) HRTEM image recorded on the InAs rod
shown in (e), from which we can see that rotational twins dominate in the
InAs nanorod along the growth direction (the [111] direction).
The similarly-relevant reasons can also induce the growth of single-
crystal W InP nanowires in the synthesis of InP.
The purity and surface composition of the InP and InAs samples
were analyzed with XPS. Fig. 5 shows the high-resolution XPS
spectra of the P2p region from the InP products and the As 3d
region from the InAs ones. In the P2p close-up spectrum, there are
two obvious peaks. The one at 128.53 eV corresponds to P from
InP, and the other one at 133.37 eV corresponds to P from oxidized
P species. The P peak from InP is much stronger than that from
oxidized species (Fig. 5a), demonstrating the high purity of the
as-synthesized InP products. As shown in Fig. 5b, three kinds of
As peaks appear in the As region of the InAs products. The peaks
at 40.67 eV, 41.36 eV and 44.41 eV can well be indexed as the
peaks of As from InAs,50 elemental As and As2O3,51 respectively.
The presence of As peaks from InAs and elemental As in the XPS
spectrum agrees well with the XRD results (Fig. 1b).
TEM images of the as-synthesized GaP and GaAs nanocrystals
with curved shapes, which are fused from nanoparticles, displayed
in Fig. 6a and b. The composition of these nanocrystals was
close to 1 : 1 by checking with EDX (see the ESI†). We note
that there are a small amount of GaP nanowires in the GaP
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products. The analysis of SAED patterns and HRTEM
images shows that GaP nanowires take W structure and grow in
the [0001] direction (shown in Fig. 6c and d), while the curved
GaP nanocrystals take the ZB structure. Because of the low
yield of W GaP nanowires there are no obvious diffraction
peaks of W structure observed in the XRD pattern (Fig. 1c)
for the as-synthesized GaP sample. Moreover, no typical GaAs
nanowires or nanorods were detected by TEM investigations. The
low yields of 1D GaP and GaAs nanostructures are probably
relevant to the large size and the poorly catalytic activity of
commercial metallic gallium, while nanoscale metallic indium as
a catalyst performs better for the anisotropic growth of 1D InP
crystallites28,29 and other nanostructures.31
In the synthetic procedures, new-prepared metallic indium
nanoparticles were used to synthesize InP and InAs since the
indium nanoparticles with high reactivity can induce the synthesis
to proceed sufficiently. However, it is difficult to prepare metallic
gallium within the nanometer scale except a recent vapor route31
because of the too low melting point of gallium. In the route,
commercial gallium was used for the synthesis of GaP and GaAs,
and excess gallium was employed for the synthesis of GaAs
nanocrystals to make the reaction progress effectively (see Table 1).
It is also found that longer reaction time was required for the
synthesis of GaP and GaAs. The reaction temperature of 320–
400 ◦C was chosen for the synthesis of III–V nanocrystals, and
◦
a high temperature above 420 C will lead to lots of amorphous
carbon in the products, while at the low temperature below 320 ◦C
the synthetic reactions can not be performed sufficiently, and
will lead to the formation of amorphous indium and gallium
pnicogenides mixed with the final products.
The investigations also showed that the degree of the difficulty
in the synthesis of III–V compounds via the present synthetic route
is in the following order: InP > InAs and GaP > InP. This trend
could be approved with some designed experiments, determined
by XRD. Fig. 7a is the XRD pattern of the products synthesized
from the new-prepared indium nanoparticles reacting with the
mixture of PPh3 and AsPh3, where the molar ratio of PPh3 to
AsPh3 is 4 : 1. The XRD pattern indicates that InAs dominates
Fig. 5 Higher resolution XPS spectra of (a) the P 2p region from the InP
products; and (b) the As 3d region from the InAs products obtained using
In : AsPh3 = 1 : 1.
Fig. 7 (a) XRD pattern of the products synthesized from the reaction of
new-prepared indium nanoparticles with the mixture of PPh3 and AsPh3
with the molar ratio of 4 : 1. The inset is the enlarged part of the XRD
pattern from 20◦ to 30◦ (2q), indicating that InAs dominates the products;
(b) XRD pattern of the products obtained from the reaction of PPh3 with
the mixture of metallic In and Ga with the molar ratio of 1 : 1.
Fig. 6 TEM images of (a) the GaP sample, and (b) the GaAs sample;
(c) TEM image of an individual W GaP nanowire; (d) HRTEM image and
ED pattern (the inset) recorded on the GaP nanowire shown in (c).
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the products, which means that the synthesis of InAs is much
easier than that of InP. Basis on the thermodynamic calculations,
the trend in the Gibbs energy changes of the reactions for the
system.59–61 The as-synthesized III–V nanocrystals have broad size
distributions, which can be ascribed to heterogeneous nucleation
and catalysts of metal droplets with a diverse size. It is no doubt
that the current work provide an alternatively creative route to
group III–V semiconductors and relevant compounds although
the improvements on the controllable synthesis along with their
properties are needed in the coming investigations.
synthesis of InP and InAs is DrG◦ (-439.0 kJ mol-1 at 25 ◦C)InAs
<
DrG◦ (-343.6 kJ mol-1 at 25 ◦C)InP. The experimental results well
match with the thermodynamic calculations. We also find that it
is more difficult to obtain GaP than InP, which can be confirmed
by the XRD pattern of the products obtained from the reaction of
PPh3 with the mixture of metallic In and Ga with the equal molar
ratio (shown in Fig. 7b). The degree of the difficulty in the synthesis
of III–V compounds can be summarized as follows: GaV > InV
and IIIP > IIIAs (III = Ga, In, and V = P, As). Interestingly,
this order is also consistent with the trend in bond energies (Eb)
of group III–V compounds:52 Ga–V > In–V and III–P > III–As
(III = Ga, In, and V = P, As).
Conclusions
In summary, a new general strategy was put forward for the syn-
thesis of III–V semiconductor nanocrystals through a developed
Ullmann reaction from metallic indium or gallium with PPh3 or
AsPh3 in evacuated sealed quartz tubes at 320–400 ◦C for 8–
24 h. These synthesis reactions are thermodynamically feasible
and easy to be realized kinetically, by which InP, InAs, GaP
and GaAs nanocrystals have successfully been synthesized. The
investigations of XRD, ED and HRTEM established the presence
of metastable structures of W InP, InAs and GaP, which was
induced by the high steric effects of PPh3 and AsPh3 as well as
the low temperatures and pressure employed. It was found that
the tendency in difficulty for the synthesis III–V semiconductor
nanocrystals was as follows: IIIP > IIIAs (III = In, Ga) and
GaV > InV (V = P, As). This work offered a good candidate for the
studies on the polytypism and the relative stability between ZB and
W phases of III–V semiconductors, and the employment of low
temperature, sealed vacuum tubes and stable pnicogen precursors
(PPh3 and AsPh3) with high safety largely simplified the synthetic
procedures. The present synthetic strategy can be easily extended
to synthesize other phosphides and arsenides, and other III–V
semiconductors such as GaN, InN and InSb from precursors of
NPh3, SbPh3, or even other triarylpnictines, and it would promote
the development of methodologies for the synthesis of compound
semiconducting nanomaterials.
In the present process, the formation of 1D InP, InAs and
GaP nanostructures is attributed to the solution–liquid–solid
(SLS) growth mechanism6,26–29,35–38, due to the metallic droplets
of metals existing in the process, and the nanocrystals with W
structure or segments are resulted from the kinetic growth under
moderate conditions along with the use of PPh3 and AsPh3. At
temperatures of 320–400 ◦C, metallic indium and gallium become
melting droplets, and they act as not only reactants but catalysts
in the growth of 1D nanostructures. As reactants, the metals
serve as the precursors for the synthesis of III–V nanocrystals as
reaction performed,26–29 and as catalysts, they confine the growth
of InP, InAs or GaP nanocrystals elongated along a specific
direction to form 1D nanostructures based on the SLS growth
model.6,26–29,35–38 Meanwhile, PPh3 and AsPh3 with the high steric
effects53 can affect the shape and crystallographic phase of InP,
InAs and GaP nanocrystals with favorable anisotropic shapes.
Previously, trioctylphosphine (P(C8H17)3, TOP) and trioctylamine
(N(C8H17)3, TOA) were often employed as additives to direct
the growth of semiconductor nanocrystals with controllable size,
shape and phase in the synthesis,8–12,28,29,32,54 and group II–VI and
III–V nanostructures could be well tuned through the use of
trialkylphosphines based on the control of these additives with spe-
cific steric effects.6,8–12,27–29,54–57 For another example, GaP nanorods
were synthesized with W structure along with nanospheres with
ZB structure.54 The phase transformation from the ZB GaP to the
W structure was kinetically affected by using different stabilizers
such as TOA and hexadecylamine (HDA) with diverse steric
effects.54 Furthermore, trialkylphosphines and their similarities
prove to be useful pnicogen sources for the synthesis of group
III–V nanocrystals.6,8–12,27–29,32–34,58 The configurations of PPh3 and
AsPh3 used in the present route are close to their similarities such
as TOP, TOA, and etc., and they will further confine the growth of
1D nanostructures by suppressing atomic processes to metastable
structure due to their highly steric effects besides the directing
effects of the catalyst of metallic droplets in the SLS growth regime.
It is concluded that the steric effects of the VA precursors (PPh3 and
AsPh3) along with the low synthesis temperatures and pressures in
sealed vacuum tubes cooperatively arouse nanocrystals growing
in the metastable W phase from the aspect of kinetics. In
addition, the viscous liquid consisting of unreacted PPh3 or AsPh3
and byproducts including biphenyl, terphenyl, and so on33 also
played important roles in the growth of 1D nanostructures with
metastable W structures, rotational twins and stacking faults, since
mass transfer in crystal growth is quietly limited in a high viscous
Acknowledgements
Financial support from the National Nature Science Foundation
of China (Grants No. 20571068, 20621061), the Chinese Ministry
of Education (NCET2006–0552), and Anhui Provincial Natural
Science Foundation (070414194) is gratefully acknowledged.
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