Sn-doped polyhedral In2O3 particles: Synthesis, characterization, and origins of luminous emission in wide visible range

Article abstract of DOI:10.1016/j.jssc.2011.11.019

Sn-doped octahedronal and tetrakaidecahedronal In₂O₃ particles were successfully synthesized by simple thermal evaporation of indium grains using SnO as dopant. Structural characterization results demonstrated that the Sn-doped tetrakaidecahedronal In₂O₃ particle had additional six {001} crystal surfaces compared with the octahedronal one. The luminous properties of both samples were characterized by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. A broad visible luminous emission around 570 nm was observed. Studies revealed that the emission consisted of three peaks of 511 nm, 564 nm, and 622 nm, which were attributed to radioactive recombination centers such as single ionized oxygen vacancy, indium interstitial, and antisite oxygen, respectively. We believe that the Sn donor level plays an important role in the visible luminous emission.

Full text of DOI:10.1016/j.jssc.2011.11.019

Journal of Solid State Chemistry 186 (2012) 182–188
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Sn-doped polyhedral In2O3 particles: Synthesis, characterization, and origins
of luminous emission in wide visible range
Yunqing Zhu, Yiqing Chen n
School of Materials Science and Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei, Anhui 230009, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Sn-doped octahedronal and tetrakaidecahedronal In2O3 particles were successfully synthesized by
simple thermal evaporation of indium grains using SnO as dopant. Structural characterization results
demonstrated that the Sn-doped tetrakaidecahedronal In2O3 particle had additional six {001} crystal
surfaces compared with the octahedronal one. The luminous properties of both samples were
characterized by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. A broad visible
luminous emission around 570 nm was observed. Studies revealed that the emission consisted of three
peaks of 511 nm, 564 nm, and 622 nm, which were attributed to radioactive recombination centers
such as single ionized oxygen vacancy, indium interstitial, and antisite oxygen, respectively. We believe
that the Sn donor level plays an important role in the visible luminous emission.
Received 17 June 2011
Received in revised form
7 November 2011
Accepted 11 November 2011
Available online 4 December 2011
Keywords:
Thermal evaporation
ITO particles
Luminous emission
Cathodoluminescence spectroscopy
& 2011 Elsevier Inc. All rights reserved.
1. Introduction
2. Experimental section
As an important and well-known semiconducting oxide (TCO),
indium oxide (IO) has been broadly applied to displays [1,2],
organic light-emitting diodes (OLEDs) [3], solar cells [4,5], func-
tional glass [6,7], and energy efficient windows [8,9]. Researchers
have synthesized pure octahedronal IO particles and IO pyramids
by liquid-phase method or by physical evaporation of indium
with or without Au as catalyst [10–16]. IO octahedrons precipi-
tated on Sn-doped indium oxide substrate were also reported
[17]. The Sn-doped indium oxide known as indium tin oxides
(ITO) is another kind of promising n-type transparent semicon-
ductor material in screen display techniques [18,19]. ITO powder
of high purity and fine grain size is largely in demand for the
preparation of ITO targets [20,21]. However, few articles were
reported on the polyhedral ITO particles.
In this article, we developed a simple thermal evaporation of
indium grains to prepare Sn-doped octahedronal and tetrakaide-
cahedronal In2O3 particles using SnO as dopant without the
presence of Au catalyst. Apart from photoluminescence (PL)
spectroscopy, cathodoluminescence (CL) spectroscopy was also
employed to characterize the luminous emission features of
samples. Both samples exhibit broad visible luminous emission
peak around 570 nm, which has potential application in screen
display techniques.
2.1. Synthesis
The synthesis of samples was carried out in a horizontal
alumina tube furnace. High purity indium grains (5 N) and a
mixture of SnO powder and graphite powder (both are analytic
purity) were separately placed in a ceramic boat, which was
located in the heating center of alumina tube. A Si substrate was
put 25 cm away from the ceramic boat to collect the products.
Prior to heating, the alumina tube was sealed and pumped down
to a pressure of 10ꢀ3 Torr. Argon (Ar) gas was then introduced
with a flow rate of 40 standard cubic centimeters per minute
(sccm). The temperature was increased to 1000 1C at a rate of
6 1C/min and the argon gas was switched off. A flow of argon and
oxygen (O2) gas was introduced at a flow rate of 50 sccm. The O2
partial pressure was 5% in the Ar/O2 carrier gas with a constant
total pressure of 200 Torr. After 50 min of typical growth process,
the Si substrate was cooled and removed from the furnace.
A yellow layer (sample A) was found on the surface of the Si
substrate. The procedure for synthesis of sample B is the same as
above except that the Si substrate was 2.5 cm farther from the
heating center than the case of sample A.
2.2. Characterization
The samples were characterized by field emission scanning
electron microscopy (FESEM, JEOL-JSM-6700F) equipped with energy
dispersive spectrometer (EDS, Link ISIS, Oxford), x-ray diffraction
(XRD, D/MAX2500VL/PC), and high resolution transmission electron
n Corresponding author. Fax: þ86 551 290 1362.
E-mail address: chenyq63@126.com (Y. Chen).
0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.11.019

Y. Zhu, Y. Chen / Journal of Solid State Chemistry 186 (2012) 182–188
183
Fig. 1. FESEM images of (a) sample A and (b) sample B with high magnification images in the insets. EDS spectra (c) from sample A and (d) from sample B.
microscopy (HRTEM, JEOL JSM-2010) equipped with EDS. PL spectro-
scopy was performed on a steady-state spectrofluorometer (FLUOR-
OLOG-3-TAU) at room temperature using a He–Cd laser as the
excitation source. CL spectroscopy was performed at room tempera-
ture by means of a Gatan MonoCL system attached to a scanning
electron microscope. The electron paramagnetic resonance (EPR)
spectra were recorded on an Electron Spin Resonance Spectrometer
(JES-FA200) at a modulation frequency of 100 kHz with modulation
amplitude of 0.1 mT and a sweep width of 30 mT. Raman spectra
were obtained under air condition from a Laser Confocal Raman
Microscope (LABRAM-RH) with an excitation wavelength of
514.5 nm.
3. Results and discussion
The morphologies of sample A and sample B are shown in
Fig. 1(a) and (b), respectively. Fig. 1(a) reveals the octahedronal
particles attached by tiny particles. In Fig. 1(b), the tetrakaideca-
hedronal particles exhibit six more surfaces than the octahedro-
nal particles in Fig. 1(a). The diameters of two kinds of particles
are averagely estimated about 450 nm. The EDS spectra of both
samples were obtained under FESEM. It was found that sample A
and sample B consist of elemental In, Sn, and O shown in
Fig. 1(c) and (d), respectively.
XRD patterns in Fig. 2 prove that both samples are body
centered cubic indium oxides phase with lattice parameter of
a¼1.011 nm (JCPDS card no. 89-4595). In the present work, SnO
is utilized as dopant since the SnO can decompose to Sn and SnO2
above 300 1C as follows: [22,23]
2SnO(g)¼Sn(l)þSnO2(s)
(1)
(2)
SnO2(s)"SnO(g)þ1/2O2(g)
Considering the synthesis temperature of 1000 1C, it is inevi-
Fig. 2. XRD patterns of (a) sample A and (b) sample B.
table that peaks from SnO2 and Sn appear in the XRD patterns.

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Y. Zhu, Y. Chen / Journal of Solid State Chemistry 186 (2012) 182–188
Fig. 3. HRTEM images taken from (a) octahedronal particle A and (b) tetrakaidecahedronal particle B displayed in the TEM images in the insets with corresponding SAED
patterns. EDS results from (c) octahedronal particle A, (d) tetrakaidecahedronal particle B, and (e) the tiny particles in sample A (inset: TEM image of a typical tiny particle).
Fig. 3(a) and (b) is the HRTEM images of an octahedronal
particle A and a tetrakaidecahedronal particle B, respectively.
Corresponding selected area electron diffraction (SAED) patterns
were indexed. Analysis of the HRTEM images and the SAED
patterns demonstrate that both particles are single crystal. In
particle A, the interplanar distance of 0.42 nm corresponds to the
separation of (121) crystal face. It is in good agreement with the
standard value of 0.4131 nm (JCPDS card no. 89-4595) within the
measuring error. The angle between the indexed crystal face
(121) and (ꢀ12ꢀ1) in SAED pattern was 70.531, which is in
accord with the apex corner angle of an octahedronal structure. In
particle B, the edge along [110] direction indicates that the
tetrakaidecahedronal particle is grown along o0014 with six
apex corners truncated to add six more {001} crystal surfaces
than the octahedronal one. Fig. 3(c) and (d) is the point scanning
EDS spectra recorded on particle A and particle B, respectively.
Fig. 3(e) shows the EDS spectrum from tiny particles with a
typical diameter of about 100 nm displayed in the inset. All the
EDS results indicate that both kinds of particles including the tiny
particles in sample A are made of In, Sn, and O. Based on the
above observations, we conclude that the octahedronal particles
and the tetrakaidecahedronal particles are ITO single crystal.

Y. Zhu, Y. Chen / Journal of Solid State Chemistry 186 (2012) 182–188
185
appears in CL image as Fig. 5(f) shows. The point 5 in dark area of
Fig. 5(h) has poor CL performance shown in Fig. 5(k).
The origins of luminous emission of samples were studied by
analysis of various measurement results including EDS, Raman
spectra, and EPR. According to the analysis of the EDS spectra
recorded under HRTEM, the octahedronal and tetrakaidecahedro-
nal particles have the atomic ratios of indium to oxygen (In:O)
about 77:18 and 67:27, respectively. The nonstoichiometry is
further confirmed by the Raman spectra recorded on both
samples. As Fig. 6 shows, there is a strong Raman peak at
306.3 cmꢀ1, which was reported to be very sensitive to the
presence of oxygen vacancies [24]. Gauss fit results exhibit two
weak shoulders at 325 cmꢀ1 and 329 cmꢀ1 in sample A and
sample B, respectively. Kumar et al. reported a shoulder peak at
321.2 cmꢀ1 in octahedronal IO particles [14]. The shoulder peak
can be attributed to the nonstoichiometry caused by oxygen
vacancy or the quantum confinement effect [25,26]. Considering
the size of the particles compared with typical Bohr radius of
In2O3 (2.14 nm), quantum confinement effect can be ruled out in
our samples. The peaks at 325 cmꢀ1 and 329 cmꢀ1 arise from the
nonstoichiometry, which agrees with the EDS observation. Singly
ionized oxygen vacancies (VdO), which are paramagnetic F-centers,
are expected to yield a single EPR signal centered near the free
electron g-value (2.0023) [27]. The g factors obtained from EPR
spectra in Fig. 7 indicate the existence of VdO in both samples. In
pure IO, it has been demonstrated, by means of EPR measure-
ments and proper thermal treatments [28,29], that a band at
430 nm (2.88 eV) originates from the oxygen vacancy, which acts
as the recombination center and may introduce an acceptor level
in the band gap of IO. Similar shallow acceptor level was also
found by authors in the investigation into the surface of tin oxide
annealed at as high as 1000 1C for 24 min. The increased band
edge emission is located at 451 nm (2.75 eV) [30]. Thus, the
430 nm emission is from IO particles rather than tin oxide.
However, due to Sn doping in our samples A and B, the Sn donor
level is introduced into the band gap of IO particles [31]. When
electrons from the donor level recombine with photon-excited
holes in the valence band, the 432 nm emission can be observed
in our samples [32]. Sun et al. reported a 423 nm band edge
emission in ITO dots [33]. The weak blue emission at 432 nm in
Fig. 4(b) is then related to Sn donor level but is not a band edge
emission.
Fig. 4. (a) Excitation spectra monitored at 570 nm, (b) PL spectra excited by laser
of different wavelengths. The spectrum from sample excited by 350 nm
A
excitation laser is selected for Gauss fit processing. All curves are normalized.
In the present work, we consider that the Sn doping plays an
important role in CL performance of samples A and B. It is
well-known that a pure IO crystal belongs to a cubic bixbyite-
type structure (Ia3 space group) with two nonequivalents ‘‘In’’
sites. Three quarters of the tetrahedral sites are filled with oxygen
ions. One kind of ‘‘In1’’ site is coordinated 6-fold in a regular
oxygen octahedron, whereas the other ‘‘In2’’ site is surrounded by
six oxygen atoms at three different distances [34]. The diameter
of bivalent cations 112 pm is longer than that of the tetravalent
ones 69 pm, which is close to the trivalent indium cations (80 pm)
[35]. It should be easy for tetravalent tin cations from the
decomposition of SnO to substitute the ‘‘In2’’ site, which results
in a large repulsive force arise from the additional positive charge.
The electrons will be released into the Sn donor level discussed
latter to balance this extra charge [31]. These additional electrons
may provide brightness in CL image of samples A and B.
The CL emission in IO samples were attributed to the oxygen
vacancies or indium–oxygen vacancy centers [29,36]. In the
typical CL measurement, when the cathode electron ray shoots
on the samples, large amount of electrons are captured by the VdO,
which forms the donor center (VXO), which may introduce a donor
level below the bottom of conductor band in IO and the electrons
on acceptor levels introduced by defects will combine the holes
on the top of valance band, which lead to the formation of
The luminous properties of samples were investigated by PL
and CL spectroscopy. Fig. 4(a) is the excitation spectra monitored
at 570 nm. Fig. 4(b) is the PL spectra showing a broad visible
emission around 570 nm. All spectra are normalized. It can be
seen in Fig. 4(a) that there are two strongest peaks at 315 nm and
325 nm for samples A and B, respectively, and two sub-strong
peaks at 350 nm for both samples. The exciting wavelengths
315 nm and 325 nm are switched to 350 nm for samples A and
B. Both samples exhibit two additional PL peaks at 432 nm in PL
spectra shown in Fig. 4(b).
To study the local luminous emission features, CL spectroscopy
was utilized. The influence of the tiny particles in sample A was
specially investigated. Three test points are purposely selected
from area of few tiny particles (point 2) to that of rich tiny
particles (point 3) in the secondary electron images shown in
Fig. 5(a) and (c). All the CL spectra also exhibit a broad visible
emission band centered at around 570 nm in Fig. 5(i). No evident
discrimination is detected. The results prove that the tiny ITO
particles have little effect on the CL property. It was discovered
that sample A had a stronger PL peak at 432 nm (2.87 eV) than
that of sample B displayed in Fig. 4(b) as well as a uniform bright
CL image in Fig. 5(b) and (d). In sample B, however, darkness

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Y. Zhu, Y. Chen / Journal of Solid State Chemistry 186 (2012) 182–188
Fig. 5. Secondary electron images of (a), (c) sample A and (e), (g) sample B with corresponding CL images (b), (d) and (f), (h). (i) CL spectra of the three selected points in
sample A. (j) Normalized curves point 2 and point 4 with different slops at both lower and higher energy sides. (k) Gauss fit CL spectra of point 2 in sample A and point 4 in
sample B. The CL spectra from point 5 in sample B exhibits relatively low intensity.

Y. Zhu, Y. Chen / Journal of Solid State Chemistry 186 (2012) 182–188
187
extinction state in bandgap. When the electrons transit from the
donor level to the defects levels, the emission gives out the visible
CL image. In our ITO samples, the Sn donor level is considered to
be significant in the role of CL performance since the additional
positive charge from tetravalent tin ions. A broader EPR band in
sample A was observed. It indicates a higher density of VdO.
According to the Heisenberg’s uncertainty principle, the electron
lifetime on a certain level and the energy difference between
levels E can be written as
E ꢁ h. If the band width in EPR
spectrum is described by the frequency width E=hÞ), the
¼ ð
relationship between frequency width and lifetime is written
as Þ. Compared with sample B, the broader band width of
ꢁ ð1=
t
d
td
d
n
(
dn
d
t
d
n
dn
t
sample A suggests a shorter lifetime of electrons on the acceptor
level of VdO. It means that the electrons can be more quickly
activated to Sn donor level in sample A when exposed to the
cathode ray and radiatively transit back to the acceptor level of VOd
or other defect levels. The emission gives out an uniform bright CL
image whereas the narrow EPR band width of sample B with
lower density of VdO leads to the darkness in CL image shown in
Fig. 5(f) and (h) as well as poor CL performance in Fig. 5(k).
It was reported that the visible emission band at around
590 nm IO nanowires shifts to the higher energies at ꢁ2.9 eV
since Sn doping [33]. In our ITO particles, the PL band at
432 nm(2.87 eV) appears and the PL emission at 590 nm in
octahedronal IO particles shifts to higher energy side around
570 nm in our polyhedral ITO particles. There is noise in CL spectra
in Fig. 5. Two curves (points 2 and 4) are normalized in Fig. 5(j).
The curves exhibit different slopes in the lower energy side of the
band as well as in the band in the higher energy side, which
indicates that there are different contributions to the emission
from both lower and higher energy sides. The Gaussian deconvolu-
tion on the band centered at around 570 nm shown in the
Fig. 5(k) as well as Fig. 4(b) exhibits three Gaussian peaks at
around 511 nm (2.43 eV) in the higher energy side, 564 nm
(2.2 eV) near the center, and 622 nm (2 eV) in the lower energy
side. Mazzera et al. pointed out that the emission band at 630 nm
is referred to some possible defects such as interstitial oxygen (Oi),
Indium vacancy (VIn) or antisite oxygen (OIn) rather than VO in IO
nanowires [29], and this band also shifts to higher energy side
about 622 nm in our ITO particles. Therefore, we propose that the
appearance of 432 nm band and blue shift of the emissions band at
590 nm and 630 nm is the consequence of Sn doping. In the
experiment, oxygen gas can be from the carrier gas or the released
oxygen gas shown in the chemical reaction (2). At high tempera-
ture of 1000 1C, oxygen gas will decomposed to oxygen atoms.
When the oxygen atoms substitute some indium cations and form
OIn, the indium cations become interstitials in crystal lattice. The
possibility of the formation of VIn is then lowered. In addition, the
oxygen vacancy can provide site for oxygen atom to fill in. The Oi
will not greatly exist. We propose that the peak at 622 nm is
assigned to OIn in polyhedral ITO particles. The Gaussian peak at
564 nm, which is the near the 570 nm is associated with the Ini3þ
[14]. The intensity of Gaussian peak at 511 nm to the intensity of
Gaussian peak at 622 nm is calculated based on the CL spectra in
Fig. 5(k). The value was found obviously lower in sample B (0.195)
than that in sample A (0.366). The low VdO density in sample B is
probably responsible for the result.
Fig. 6. Raman spectra recorded on sample A and sample B. The peaks in the 280–
380 cmꢀ1 range are displayed in the inset.
Fig. 7. EPR spectra with g factors 1.997 (sample A) and 2.000 (sample B).
Fig. 8. Schematic diagrams of the radioactive recombination processes in IO and ITO. The reported emissions are indicated with references.

188
Y. Zhu, Y. Chen / Journal of Solid State Chemistry 186 (2012) 182–188
Based on the analysis above, schematic diagrams of the radio-
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active recombination processes in IO and ITO were derived in
Fig. 8. It can be calculated that the Sn donor level in ITO is about
0.42–1.42 eV below the conduction band edge. In the tin oxide,
the VXO donor level was demonstrated up to 0.3 eV below the
bottom of conduction band by means of electron spin resonance
measurements [37]. In our ITO samples, the Sn donor level is
important for the luminous emission in visible range. The major
part of the broad visible luminous emission originates from
indium interstitial and antisite oxygen in our samples.
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Sn-doped polyhedral In2O3 particles were synthesized by
simple thermal evaporation of indium grain using SnO as dopant.
The Sn-doped tetrakaidecahedronal In2O3 particle had six more
{0 0 1} crystal faces than the octahedronal one. The Sn donor level
is important for the luminous emission in visible range. The major
part of the broad visible luminous emission originates from
indium interstitial and antisite oxygen in our samples. With more
singly ionized oxygen vacancies and tin doping, ITO particles can
exhibit a better CL performance in wide visible range, which
satisfies the requirement of screen display technique.
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Acknowledgment
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This work was financially supported by the National Natural
Science Foundation of China (NSFC, No. 21071039).
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