Rapid oxidation of InP nanoparticles in air

Article abstract of DOI:10.1016/j.ssc.2006.12.033

InP nanoparticles (NPs) in the size range of 1.5-3?nm were synthesized using colloidal chemistry methods. Exposure of these NPs to air resulted in rapid oxidation, as shown by transmission electron microscopy. Diffraction and spectroscopic measurements confirmed the formation of In₂O₃. Similar behavior was observed for commercial InP NPs, even when capped with a ZnS shell. Photoluminescence studies suggest that the oxidation occurs even while InP NPs are still dispersed in hexane, albeit at a much slower rate.

Full text of DOI:10.1016/j.ssc.2006.12.033

Solid State Communications 141 (2007) 624–627
www.elsevier.com/locate/ssc
Rapid oxidation of InP nanoparticles in air
a,∗
a
b
b
b
b
b
J. Jasinski , V.J. Leppert , Si-Ty Lam , G.A. Gibson , K. Nauka , C.C. Yang , Zhang-Lin Zhou
a
School of Engineering, University of California, Merced, 5200 N. Lake Road, Merced, CA 95343, USA
b
Hewlett-Packard Laboratories, 1501 Page Mill Road, Palo Alto, CA 94304, USA
Received 30 November 2006; accepted 21 December 2006 by R. Phillips
Available online 7 January 2007
Abstract
InP nanoparticles (NPs) in the size range of 1.5–3 nm were synthesized using colloidal chemistry methods. Exposure of these NPs to air
resulted in rapid oxidation, as shown by transmission electron microscopy. Diffraction and spectroscopic measurements confirmed the formation
of In O . Similar behavior was observed for commercial InP NPs, even when capped with a ZnS shell. Photoluminescence studies suggest that
2
3
the oxidation occurs even while InP NPs are still dispersed in hexane, albeit at a much slower rate.
ꢀ
c 2007 Elsevier Ltd. All rights reserved.
PACS: 61.16.Bg; 61.46.+w; 61.66.Fn; 78.55.-m
Keywords: A. Nanostructures; A. Semiconductors; C. Transmission electron microscopy
1
. Introduction
2. Experimental
Semiconductor nanoparticles (NPs) have attracted a great
deal of interest in recent years due to quantum confinement
effects that arise when the size of a nanoparticle becomes
comparable to or smaller than the effective Bohr radius [1]. Due
to quantum confinement, semiconductor nanoparticles have
interesting, size-dependent optical properties [2,3].
InP NPs with sizes ranging between 1.5 and 3 nm were
synthesized by a microwave enhanced chemical reaction
process. The chemical system used here was similar to that
reported by Battaglia et al. [12]. In a typical process, a 1:3 molar
ratio of indium acetate, [In(CH3COO)3], and hexadecanoic
acid (also known as palmitic acid), [CH3(CH2)14COOH], was
mixed in octadecene, [CH3(CH2)16CH3]. The mixture was
NPs of II–VI semiconductors, especially CdSe, are the most
studied and utilized so far [2,4]. However, there is growing
interest in the fabrication and application of NPs of other
semiconductors, such as III–V’s. InP is one of the most
attractive materials among them, due to its electronic properties,
good lattice match with low-band-gap compounds, and non-
toxicity. InP is also widely used in optical fiber communication,
solar cells, millimeter-wave amplifiers and a number of other
optoelectronic devices [5–7]. Recently, there have been several
reports on the synthesis and characterization of nanocrystalline
InP [8–23]. However, there are still a number of problems with
the synthesis of InP nanocrystals that require more work to be
done. Here, we will focus on their easy oxidation, which may
be partly responsible for the low emission efficiency and aging
of these NPs.
◦
heated to ∼130 C for 2 h under N2 flow to obtain an optically
clear solution. Subsequently, a 4 ml of the clear solution was
transferred to a 10 ml glass tube and was then placed into a
microwave reactor. A microwave power of up to 300 W was
◦
applied to the reactor to heat the solution to 275 C, and this
temperature was maintained at all times during the process.
◦
Once the temperature reached 275 C, a 1 ml mixture of
tris(trimethylsilyl)phosphine [P[(CH3)3Si]3] in octadecene was
injected into the solution in the glass tube. The molar ratio of
tris(trimethylsilyl)phosphine to indium acetate was 1:1. Under
the applied microwave energy, chemical reaction occurred
immediately and formed InP. As the reaction continued, NPs
formed and changed in size and size distribution. The NP size
and distribution depend on process parameters such as reaction
temperature, time, and various chemical concentrations. For a
∗
Corresponding author. Tel.: +1 209 228 4411; fax: +1 209 228 4047.
E-mail address: jjasinski@ucmerced.edu (J. Jasinski).
0
038-1098/$ - see front matter ꢀc 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2006.12.033

J. Jasinski et al. / Solid State Communications 141 (2007) 624–627
625
Fig. 1. (a) PL spectrum from a typical InP NP sample. (b) Low-magnification
TEM micrograph and (c) corresponding SAED pattern of InP NPs. Due to the
very small size of the NPs, only the (111) ring of InP is well resolved. Two other
pronounced rings visible in this pattern originate from graphite nanosheets,
which are often present on the holey carbon support films used for the study.
Fig. 2. (a) Low-magnification, bright-field micrograph of an In O NP cluster.
2
3
(
b) HRTEM micrograph of one of these NPs. The inset shows a Fourier
◦
typical synthesis process, the reaction temperature was 275 C
and the time was 5 min.
Transform of the image. Indices corresponding to strong spots (i.e. best
resolved lattice planes) are marked. (c) SAED pattern taken from a freshly
prepared InP NP sample. (d) SAED pattern for the same sample after several
days in air. (e) and (f) SAED patterns of oxidized commercial InP and
InP(core)–ZnS(shell) NPs, respectively. (g) Intensity distribution measured
from the pattern shown in (d) (gray line), plotted along with X-ray diffraction
Photoluminescence (PL) was measured at room temperature
using a CW 454 nm Ar laser and a spectrometer equipped
with a photomultiplier detector. JEOL JEM2010 and JEM3010
transmission electron microscopes (TEMs) with LaB6 guns
and operating at 200 and 300 kV, respectively, were used for
structural characterization of the NPs. An EDX study was
performed in a CM200 Field Emission Gun (FEG) Philips
microscope operating at 200 kV with a nanoprobe diameter
of 1.4 nm. Electron Energy-Loss Spectroscopy (EELS) and
Energy-Filtered TEM (EFTEM) were carried out in the
JEM2010 microscope, which was equipped with a model 2000
Gatan Image Filter (GIF). TEM specimens were prepared by
dispersing NPs in either hexane or toluene, followed by drop
casting onto a commercially available amorphous holey carbon-
coated 300 mesh copper or gold grid. The grids were quickly
transported to the microscope column and studied as soon as the
solvent evaporated. Only samples selected for the aging study
were stored in air before investigating them in the microscope.
intensities for In O (ICDC PDF card No. 01-071-2194) (dark bars).
2
3
was probably due to the phase modification caused by the
air-induced oxidation. In fact, only a few samples exhibited
the InP ring pattern. An example is shown in Fig. 1(c). The
smallest, most intense ring in this pattern, representing a d-
spacing of about 3.29 A˚ , originates from the (111) InP planes.
The ring is very broad, which is in agreement with a very
small NP size. Because of the small NP size and consequent
broadening of the SAED pattern, the other allowed rings,
i.e. (200), (220), (311), etc., are nearly indistinguishable from
the background. The two other strong rings visible in the pattern
˚
(Fig. 1(c)), corresponding to d-spacings of 2.13 and 1.23 A,
are not associated with InP NPs. They instead originate from
clusters of graphite nanosheets [24], that were often found on
the holey carbon support films. These rings were used for the
internal camera constant calibration of the SAED pattern.
In addition to InP, a second type of NP was observed in
almost all freshly prepared samples. These were bigger and
tended to form clusters. Moreover, larger clusters of these NPs
were observed on the grids after storing them in air for a few
days. A bright-field image of such a cluster in a four day old
sample is shown in Fig. 2(a). A high-resolution transmission
electron microscope (HRTEM) image of one NP from this
cluster is displayed in Fig. 2(b). SAED patterns taken from
a freshly prepared grid from the area where the second type
of NP was present show several spotty rings at d-spacing
values different from those of InP (Fig. 2(c)). After storing
the sample in air for several days, the SAED patterns became
much more intense, with additional rings appearing (Fig. 2(d)).
3
. Results and discussion
A typical PL spectrum is shown in Fig. 1(a). The energy of
this peak corresponds to an average particle diameter of about
.7–1.8 nm [17]. A low-energy tail observed in the PL spectra
1
of as-synthesized particles was most likely due to defect states.
TEM studies of these samples confirmed very small sized
NPs, of about 2 nm. A low-magnification TEM micrograph of
such NPs in a typical sample is shown in Fig. 1(b). Structural
identification of the NPs was initially difficult due to their
very small diameters and because of the fast oxidation that
will be discussed later in the paper. For the most part, even
freshly prepared TEM samples did not reveal InP rings in
their selected area electron diffraction (SAED) patterns. This

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J. Jasinski et al. / Solid State Communications 141 (2007) 624–627
shown. This pattern contains the features expected of an In2O3
NP.
The rapid oxidation of InP NPs observed in this study
occurred after solvent evaporation when a solution of NPs was
drop-casted onto a substrate and exposed to air. However, it
seems that oxidation, at a much lower rate, may also take place
while InP NPs are suspended in hexane. This is suggested by
our PL study. The gray-line spectrum shown in Fig. 1 originates
from the same sample as the dark-line spectrum shown in the
same plot, but was collected three weeks later. It can be seen
that the peak intensity decreased by about 15%–20%, the peak
became broader, and the emission feature between 600 and
6
50 nm became more pronounced. All these changes suggest
that there is a degradation of the emission coming from the NPs
caused by partial oxidation.
Fig. 3. (a) EDX spectrum collected in nanoprobe mode from a cluster of large
particles found on a freshly prepared TEM grid. (b) EELS spectrum of the
There is some controversy in the literature regarding the
role of the oxide layer at the surface of InP NPs. Guzelian
et al., for example, suggested that formation of the oxidation
layer at the surface of such NPs substantially increases
the photoluminescence efficiency, which typically is rather
poor (less than 1%) for as-prepared NPs [17]. On the other
hand, several studies have shown that the photoluminescence
efficiency of InP nanocrystals can be drastically improved,
to about 20%–30%, by etching or photoetching with fluorine
compounds [10,27]. Micic et al. believe that upon such etching
fluoride ions fill phosphorus vacancies on the surface of InP
NPs, and that they also replace oxygen in the oxide layer.
There are also a number of studies showing that the optical
properties of InP NPs can be improved and controlled by
fabricating advanced core–shell nanocrystal structures, where
a shell of a higher bandgap semiconductor is grown on the
core NP. PL efficiencies of 5%–10% and ∼20% were achieved
when InP NPs were coated with ZnCdSe2 [28] or ZnS [29]
shells, respectively. We have found, however, that the core–shell
structure does not completely prevent particles from oxidizing.
This was ascertained through a study of commercial InP–ZnS
core–shell NPs. As shown in Fig. 2(f), an SAED pattern from
this sample, after drop-casting onto a TEM grid and exposure
to air, consists of ring patterns of In2O3 and wurzite ZnO. This
suggests that both the shell and the core of these NPs were
oxidized.
In O NP cluster (the carbon edge originates from the supporting holey carbon
2
3
film). (c) Bright-field image and (d) chemical map of the In O NP cluster.
2
3
The radial distribution of the intensity measured from such a
pattern (after background removal) is plotted in Fig. 2(g) (gray
line), along with the X-ray diffraction intensities for In2O3
(
ICDC PDF card No. 01–071–2194) (dark bars). There is very
good agreement in the peak location and intensity distribution
between these two diffraction data sets, indicating that In2O3
NPs are formed on our TEM grids. This identification is
consistent with the HRTEM image shown in Fig. 2(b), where
the measured d-spacing of the (400) planes agrees with that
for In2O3. EDX and EELS results are also consistent with this
identification, as described in the following.
A typical EDX spectrum, collected in the nanoprobe mode
from a characteristic large NP, is shown in Fig. 3(a). A strong
In peak is accompanied by a strong O peak, as expected for
indium oxide. The C and Si peaks come from the supporting
holey carbon film whereas the Cu peak is an artifact originating
from the copper grid.
An EELS spectrum collected from a cluster of In2O3 NPs
is shown in Fig. 3(b). There are three ionization edges visible
in this pattern: carbon K-edge, oxygen K-edge and indium M-
edge. The carbon K-edge originates from the holey carbon
support film, whereas the two other edges derive from the
cluster of NPs. The fine structure of the oxygen K-edge and
indium M-edge corresponds to that of In2O3 [25].
The three-window EFTEM method [26] was used to form a
chemical map from a cluster of In2O3 NPs. Due to the overlap
of the oxygen K-edge and the indium M-edge, individual
maps of oxygen and indium could not be created. Instead, a
chemical map showing the location of either of the elements
was constructed. This map and a corresponding bright-field
image are shown in Fig. 3(d) and (c), respectively. It can be
observed that all the particles are composed of oxygen and/or
indium, in agreement with our other results.
Fast oxidation of InP NPs and their transformation into
In2O3 when exposed to air is not a unique property of our
InP NPs. A study of commercial (Evident Technologies) InP
NPs yielded similar results. The evidence is shown in Fig. 2(e),
where the SAED pattern from the commercial NPs sample is
Fast oxidation of InP NPs is undesirable from a
technological point of view. However, it can provide a new way
of fabricating NPs of In2O3, which is an n-type, wide band-gap
(
∼3.6 eV) semiconductor widely used in solar cells, flat panel
displays [30], and transparent conductors [31]. Nanocrystalline
In2O3 is also used as a gas-sensing material because it exhibits
excellent sensitivity for gases such as NO2 [32,33], NH3 [34],
O3 [35], and H2 [36]. Despite the importance of this material,
there are only a few reports of the fabrication of In2O3 NPs
by either pulsed laser ablation [37] or colloidal chemistry [38,
3
9]. Therefore, the oxidation of InP NPs observed in this work
suggests new methods for producing this material.
4. Summary
Rapid oxidation of small InP nanoparticles (NPs) produced
by colloidal chemistry was observed after exposure to air. The

J. Jasinski et al. / Solid State Communications 141 (2007) 624–627
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[
[
[
[
#
DE-AC02-05CH11231 and the Imaging and Microscopy
Facility at the University of California, Merced are gratefully
acknowledged for the use of their instruments.
[
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