Photoluminescent properties of LiInO2 nanocrystals

Materials Letters 65 (2011) 2920–2922
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Materials Letters
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Photoluminescent properties of LiInO2 nanocrystals
Yu-Jen Hsiao a,⁎, Shang-Chou Chang b
a
b
National Nano Device Laboratories, Tainan City 74147, Taiwan
Department of Electrical Engineering, Kun Shan University, Da-Wan Road, Tainan City 71003, Taiwan
a r t i c l e
i n f o
Article history:
Received 30 November 2010
Accepted 20 February 2011
Available online 25 February 2011
Keywords:
LiInO2
Luminescence
Sol–gel process
a b s t r a c t
Synthesis and luminescence properties of LiInO2 nanocrystals by the sol–gel process were investigated. The
products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence spectroscopy and absorption spectra. The well-crystallized tetragonal LiInO2 can be obtained by heat
treatment above 600 °C from XRD. The excitation wavelengths at about 246 nm were associated with charge
transfer between In and O with In3+ ions in octahedral coordination. The PL spectra excited at 246 nm have a
broad and strong emission band maximum at 391 nm, corresponding to the self-activated luminescence. The
optical absorption spectra of the 600 °C sample exhibited the band gap energies of 3.7 eV.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Recently, the general formula of A+ 1B+ 3O2 compounds has
attracted interest in view of the correlation between their crystal
structure and physical properties [1–3]. LiInO2 is a candidate material
for a solid-state scintillator for solar neutrinos [4]. In order to improve
efﬁciency, knowledge of the basic physical properties of LiInO2, such
as optical properties, is required for the investigation of scintillators.
However, LiInO2 can also be a host for luminescent materials.
Varadaraju et al. [5] have synthesized and studied a series of Eu3+doped LiInO2 compounds and shown that LiIn0.95Eu0.05O2 is a
promising phosphor.
The main purpose of this work is to explore a sol–gel synthetic
route for the preparation of single phase LiInO2 oxides with a
tetragonal structure. The major advantage of sol–gel processing is that
it is a low-temperature process. In addition, chemically synthesized
ceramic powders often have better chemical homogeneity, ﬁner
particles and good control of particle morphology than those
produced by the mixed oxide route [6]. Therefore, the luminescence
and optical absorption properties of LiInO2 nanocrystals have been
investigated by a sol–gel method.
2. Experiments
The LiInO2 crystals were prepared by the sol–gel method using
lithium nitrate Li(NO3), indium nitrate In(NO3)3, ethylene glycol (EG)
and citric acid anhydrous (CA), purities of over 99.9%. A sufﬁcient
amount of citric acid was added to this as a chelating agent to form a
solution. Citric acid in the molar ratio of 3:2 to the total metal ions was
⁎ Corresponding author. Tel.: +886 6 5050650x6638.
E-mail address: [email protected] (Y.-J. Hsiao).
0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.matlet.2011.02.067
used for this purpose. EG was also added to the solution as a
stabilizing agent. The precursor containing Li and In was dried in an
oven at 120 °C for 10 h, and then the LiInO2 powders were obtained
after calcination at 500–700 °C for 3 h in air. The phase identiﬁcation
was performed by X-ray powder diffraction (Rigaku Dmax-33). The
morphology and microstructure were examined by transmission
electron microscopy (HR-TEM, HF-2000, Hitachi). The excitation and
emission spectra were recorded on a Hitachi-4500 ﬂuorescence
spectrophotometer equipped with a xenon lamp. The absorption
spectra were measured using a Hitachi U-3010 UV–Vis spectrophotometer. All of the above measurements were taken at room
temperature.
3. Results and discussion
XRD patterns of the precursor powders at heat treatment temperatures of 500–700 °C for 3 h are shown in Fig. 1. The amorphous metal–
organic gel was heat-treated to pyrolyze the organic components for
crystallization. Calcination at 500 °C produced an obvious interphase
In2O3, and a small amount of LiInO2 phase was also observed. The
content of LiInO2 phase had a rapid product at 550 °C. When the
precursor was sintered at temperatures above 600 °C, the samples
exhibited a single phase and all of the peaks were identiﬁed to be the
tetragonal LiInO2 phase (JCPDS ﬁle no. 76-0427). The variation of the
relative phase as a function of calcining temperature could be explained
by the homogenization of the composition with the enhancement of the
diffusion process.
The SEM image of LiInO2 as-synthesized nanocrystals produced at
600 °C is shown in Fig. 2. The circular particles seem to be distributed
homogeneously, with diameters of about 60 nm. The big particles
were condensed by assembled nanograins, and it is conjectured that
the assemble effect arising from the nanocrystals is responsible for the
decrease in surface energy.
Y.-J. Hsiao, S.-C. Chang / Materials Letters 65 (2011) 2920–2922
Fig. 1. X-ray diffraction patterns of LiInO2 precursor powders annealed at (a) 500 °C,
(b) 550 °C, (c) 600 °C, and (d) 700 °C for 3 h.
Fig. 3 presents the excitation and emission spectra of the LiInO2
samples at 600 °C. The photoluminescence results reveal that the
sample prepared at 600 °C exhibits the absorption intensity at 246 nm
by monitoring ﬂuorescence at a wavelength of 391 nm. The peak is
associated with the direct excitation of the LiInO2 host itself via the
charge transfer (CT) transition between In and O. The crystal structure
of LiInO2 is the NaCl layer type structure with In3+ ions in octahedral
coordination ﬁelds [7]. Therefore, the main peaks of excitation, at
about 246 nm, are associated with the charge transfer bands of
octahedral indium group in the LiInO2 system. The PL spectra show a
broad and blue emission peak at about 391 nm. Kuriyama [8]
indicated that the LiInO2 thin ﬁlm had a luminescent center, which
may be oxygen vacancies, with a broad emission band at about
550 nm at temperature 20 K. Here, the edge-shared InO6 groups are
efﬁcient luminescent centers for the emission, which may be ascribed
to self-trapped exciton recombination [9]. However, LiInO2 phosphors
exhibited a peak at a wavelength of 391 nm, which is associated with
charge transfer bands of In atoms in octahedral sites.
We measured the UV–Vis absorption spectra of the as-prepared
LiInO2 nanocrystals and estimated the band gap from the absorption
onset in Fig. 4. The optical absorption band has a maximum at 335 nm,
Fig. 2. SEM image of as-synthesized nanocrystals at 600 °C.
2921
Fig. 3. (a) The room temperature excitation (λem = 383 nm) and (b) emission
(λex = 327 nm) spectra of pure LiInO2 phosphors heat-treated at 600 °C.
corresponding well with the excitation spectrum. For a direct band
gap semiconductor, the absorbance in the vicinity of the onset due to
the electronic transition is given by the following equation:
α=
1 = 2
C hν−Eg
hν
ð4Þ
where α is the absorption coefﬁcient, C is the constant, hν is the
photon energy and Eg is the band gap. The inset of Fig. 4 shows the
relationship of (αhν)2 and hν. Extrapolation of the linear region gives
a band gap of 3.7 eV, and this is close to the results reported recently
by a solid state reaction method [7].
Fig. 4. Absorption spectra of the LiInO2 nanopowders annealed at 600 °C for 3 h at room
temperature. The inset is the behavior of optical absorption as the function of photon
energy for LiInO2 nanocrystals.
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Y.-J. Hsiao, S.-C. Chang / Materials Letters 65 (2011) 2920–2922
4. Conclusions
LiInO2 nanocrystal was prepared by a sol–gel synthesis using Li
(NO3) and In(NO3)3. The well-crystallized tetragonal LiInO2 can be
obtained by heat treatment above 600 °C from XRD. The excitation
wavelengths at about 246 nm are associated with charge transfer
between In and O with In3+ ions in octahedral coordination. The PL
spectra under 246 nm excitation showed a broad emission peak at
about 391 nm which originated from the octahedral indium group.
The visible light absorption edge of the 600 °C sample was at 335 nm,
which corresponds to the band gap energy of 3.7 eV.
Acknowledgment
The authors would like to thank the National Science Council of the
Republic of China, Taiwan, for ﬁnancially supporting this research
under contract no. NSC 99-2221-E-492-023.
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