Chemical Physics Letters 414 (2005) 107–110
www.elsevier.com/locate/cplett
Synthesis and photoluminescence property of nanostructured
sol–gel indium tin oxide film on glass
Susmita Kundu, Prasanta K. Biswas
*
Sol–Gel Division, Central Glass and Ceramic Research Institute, 196 Raja S.C. Mullick Road, Kolkata 700032, West Bengal, India
Received 3 August 2005
Available online 2 September 2005
Abstract
Sol–gel dip coated indium tin oxide film of In:Sn = 90:10 was prepared on glass at 350 °C from aqueous precursor. Nanostructured
feature and cubic phase of the film were characterized by scanning electron microscopy and X-ray diffraction studies, respectively. Average cluster of 4.7 nm diameter was measured by the transmission electron microscopy and it is near to Bohr radius of In2O3. The evaluated bandgap of the nanocluster (Eg = 4.0 eV) is consistent with the absorption band appearing for quantum confinement. The
confinement phenomenon resulted in photoluminescence (PL) when excited at different photoluminescence excitation wavelengths.
The PL bands have been identified as the emissions of bound and free excitons.
Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction
Quantum confinement effect of nanostructured semiconductors having size close to Bohr radius [1–5] presently
leads to the development of nonlinear optical (NLO) materials. These materials have large potentiality for photonic
devices [6,7]. Transparent conducting oxides (TCO) can
be promising NLO materials if their nanoclusters are suitably confined to generate excitons. Usually the excitons
formed by the combination of electron and hole on exposure to electromagnetic radiation have photoluminescence
property [7]. The added advantage of these materials is visible transmissivity due to their wide bandgap values [8]. Attempts have been made by various research groups [9–14] to
study nanosized TCO such as indium tin oxide (ITO) and its
base materials. They have concentrated on the quantum
confinement study [6,15] of the nanostructured SnO2,
In2O3 with little emphasis on indium tin oxide. Hence, we
have been interested in synthesizing nanostructured ITO
close to Bohr radius and characterizing photoluminescence
behavior to understand its quantum confinement effect.
*
Corresponding author.
E-mail address: [email protected] (P.K. Biswas).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.08.062
Regarding the synthesis of nanostructured ITO, thrust
has been given to its development in nanocluster, nanorod
and nanowire forms using co-precipitation [11], microemulsion [12], solvothermal [13] or carbon assisted processes
[14]. We have carried out a new synthesis route to prepare
ITO, i.e. Sn doped In2O3 (In:Sn = 90:10) nanoclusters in
thin film form using sol–gel precursor starting from an
aqueous solution of metal salts and an organic binder. We
also studied their photoluminescence exciting at different
wavelengths.
2. Experimental
The precursor sol for the indium tin oxide film was prepared from the aqueous solution of hydrated In(NO3)3 and
SnCl4 Æ 5H2O. The indium nitrate solution was synthesized
starting with indium ingots (SRL, India) and concentrated
nitric acid (A.R.). Then aqueous solution of SnCl4 Æ 5H2O
(A.R.) was added to the above, maintaining [16,17]
In:Sn = 90:10. Hydrolysis and condensation reaction was
accomplished by refluxing the mixed solution for 4 h. To
improve the wettability of the precursor, aqueous solution
of an organic binder, polyvinyl alcohol (PVA) (molecular
weight 22 000, BDH, UK) [–CH2–CH(OH)–]n was added.
S. Kundu, P.K. Biswas / Chemical Physics Letters 414 (2005) 107–110
Desired concentration of the sol (6.0 wt% equivalent
In2O3) was maintained by addition of deionized water
and the sol was aged for 48 h. Film of 2000 ± 5 Å thickness
(measured by Autogain L116B ellipsometer) was developed
on Heraus make (Germany) suprasil grade pure silica glass
by the dipping technique (Chemat 200, USA). The film was
initially dried up to 100 °C in an air oven and then cured at
350 °C with a soaking of 30 min when the nanocluster of
ITO was formed through the decomposition of PVA. The
surface morphology and the cluster size distribution of
nanostructured indium tin oxide film were studied by scanning electron microscopy (SEM) (Leo 400C) and transmission electron microscopy (TEM) (Jeol 2110), respectively.
Carbon coated 300 mesh Cu grid were used for TEM
images. Crystalline phase was identified by XRD pattern
obtained from a X-ray diffractometer [Philips PW-1730
(Ni-filtered Cu Ka radiation)]. UV–Vis absorption and
photoluminescence (PL) spectra were recorded using Shimadzu UV–Vis–NIR (model 3101PC) spectrophotometer
and Perkin Elmer fluorimeter (LS55), respectively.
222
Intensity (a.u.)
108
400
440
25
30
35
40
45
50
622
55
60
65
2θ (degree)
Fig. 1. X-ray diffraction pattern of the ITO film cured at 350 °C.
3. Results and discussion
It is interesting to note that two specific functions of the
organic binder, PVA are in operation in the synthesis.
These are: (i) to increase wettability of the precursor due
to the increase of hydroxyl groups and (ii) to generate
nanoclusters through its decomposition. During thermal
curing of the sol–gel layer, decomposition of PVA starts
at 250 °C when the bond, C–O is possibly broken (Eq.
(1)) and the nanoclusters of the metal oxides are formed
in the PVA decomposed area.
Fig. 2. SEM image of the nanostructured ITO film.
> 250˚C
[-CH2-CH-]n
O-M
[-CH2-CH-]n
~
O-M
(M = In/Sn)
ð1Þ
It is also not unlikely to generate oxygen deficient entities
during the removal of PVA. All the diffraction lines in
XRD pattern (Fig. 1) are assigned to the bixbyite cubic
structure [16] of ITO with close resemblance to oxygen deficient In2Sn2O7x entities [18]. Microstructure of the film as
depicted by SEM image (Fig. 2) reveals the nanostructured
feature of ITO. The TEM image (Fig. 3a) has been utilized
to estimate the nanocluster size of ITO. The average size of
4.7 nm diameter was obtained from the size distribution of
the clusters depicted in the histogram (Fig. 3b). As we
understand, the Bohr radius of In2O3 is 2.14 nm [19] and
the dopant Sn4+ replaces the In3+ site in ITO having almost
similar size, it can be stated that the nanocluster size is
approaching to the Bohr radius of ITO. Hence, there is
large possibility of quantum confinement of the nanoclusters which results in formation of hydrogen like excitons
with the interaction of electromagnetic radiation. The
absorption spectrum in Fig. 4a gives a broad band at
around 310 nm (4.0 eV) which is due to the excitonic transition of the ITO nanoclusters from highest occupied
molecular orbital (HOMO) in the valence band to the low-
est unoccupied molecular orbital (LUMO) in the conduction band. The excitonic peak is in good agreement with
earlier reported results of In2O3 nanoclusters encapsulated
in mesoporous silica host [15]. We have also recorded
absorption spectra (not shown here) of pure silica glass
substrate and pure PVA film deposited on pure glass substrate in the same region. But we did not find any absorption behavior near 300 nm. This experiment confirmed that
the absorption band of ITO film at 310 nm is due to the
nanostructured feature. The excitonic transition displays
parabolic absorption and the relation (Eq. (2)) fits well to
the direct bandgap of typical semiconductors, where a is
the absorption co-efficient of the film, hm is the photon energy and E is the bandgap. We evaluated effective bandgap
of the nanoclusters from the plot, (ahm)2 vs. hm. From the
inflexions of the plot (Fig. 4b) it is clear that two entities
are present in the film.
ahm / ðhm EÞ
1=2
.
ð2Þ
We obtained two bandgap values at 3.60 and 4.0 eV as per
the selected regions, (3.50–3.90 eV) and (3.90–5.50 eV).
The 3.60 eV corresponds to the bandgap of the bulk ITO
(Eg (bulk), Eq. (3)) [8], whereas the blue shift 4.0 eV (Eg, Eq.
S. Kundu, P.K. Biswas / Chemical Physics Letters 414 (2005) 107–110
109
Fig. 3. (a) TEM image of ITO nanoclusters. Inset shows the corresponding high-resolution image of single crystal showing the lattice resolved planes,
(b) histogram showing the size distribution of nanoclusters.
b
a
2.0
120
1.5
2
(αhν) x 10
Intensity (a.u.)
10
80
1.0
40
0.5
0.0
200
0
300
400
500
600
700
800
3
4
5
6
7
hν(eV)
Wavelength (nm)
Fig. 4. (a) Absorption spectrum of ITO film, (b) plot of (ahm)2 vs. hm.
(3)) is resembling to the absorption band due to excitonic
transition. It is expected that some amount of ITO are of larger cluster size which is responsible for the appearance of
inflexion in the 3.50–3.90 eV region of the plot, (ahm)2 vs.
hm. However, this value is useful to calculate the nanocluster
size. According to Brus formula (Eq. (3)) on exciton transition [1], the nanocluster size can be evaluated if the effective
bandgap of nanocluster and that of bulk are known.
Eg ¼ Eg
ðbulkÞ
þ h2 ð1=me þ 1=mh Þ=8R2 1.8e2 =eR þ ;
ð3Þ
where Eg is effective bandgap of nanocluster (quantum dot),
Eg (bulk) is the direct bandgap energy of bulk system, R is the
nanocluster radius, e is the dielectric constant of the system,
me and mh are the effective masses of electron and hole of
exciton. Using the evaluated bandgap values in Eq. (3) and
considering e, me and mh of In2O3 [15] as 9, 0.3 m0 (m0 = mass
of electron) and 0.6 m0, respectively, we calculated nanocluster size (diameter) because ITO retains its crystalline structure even with dopant incorporation in In2O3. The
evaluated cluster size is 3.7 nm which is quite close to the
average value, 4.7 nm measured from the TEM image.
To understand the confinement behavior we have recorded the photoluminescence (PL) spectra (Fig. 5a) of
the film excited at the bandgap energy (Eg = 4.0 eV, i.e.
310 nm, kex) of the excitons. Three PL bands appeared at
423, 486, 529 nm with a shoulder at 439 nm. If the PL be
fixed at 423 nm, we obtained four photoluminescence excitation (PLE) bands at 270, 278, 290 and 310 nm. It is already noted by other workers [20,21] that in the
nanoclusters of semiconductors associated with defect
110
S. Kundu, P.K. Biswas / Chemical Physics Letters 414 (2005) 107–110
a
b
(B)
290
310
423
362
(A)
λ1
λ
d 2
c λ3
b
a
Intensity (a.u)
Intensity (a.u)
270
278
439
486
λ1=387 nm
λ2=394 nm
λ3=403 nm
280
320
(B)
420
439
(A)
486
529
529
a
b
c,d
360 400 440 480
Wavelength (nm)
520
300
350
400 450 500
Wavelength (nm)
550
600
Fig. 5. (a) Photoluminescence (PL) spectra (A) of the ITO film excited at: a, 310 nm; b, 290 nm; c, 278 nm; d, 270 nm, Photoluminescence excitation (PLE)
spectra (B) of the ITO film fixing the PL band at 423 nm. (b) PL spectrum (A) of the ITO film excited at 362 nm; PLE spectrum (B) of the same film fixing
the PL band at 439 nm.
centers, emissions for both bound and free excitons would
appear when kex is equivalent to Eg and emissions for only
free excitons in the case kex is >Eg or <Eg. Hence, we may
predict that the PL bands at 423, 486 and 529 nm are for
bound excitons trapped in the defect centers of ITO. We
also excited at other PLE wavelength, kex = 270, 278,
290 nm which are >Eg and obtained one additional PL
band at 400 nm which shifts to lower energy with
decreasing excitation energy. This band may be ascribed
to the free excitons and the shift with variation of excitation energy is due to the change in nanocluster size [22]
as revealed from the histogram of the TEM picture. In
the case of excitation at kex = 362 nm, <Eg two prominent
PL bands appeared (Fig. 5b) at 420 and 439 nm with two
shoulders at 486, 529 nm. The 439 nm band is possibly
due to the defect of In–O vacancy [23]. The PL band at
400 nm for free exciton could not be traced out in this
case due to its overlapping with the excitation band.
4. Conclusion
Incorporation of PVA in the precursor solution of hydrated indium nitrate and tin chloride, led to the wettable
sol–gel film which transformed to nanostructured ITO on
thermal curing at 350 °C. The TEM image revealed the formation of nanoclusters of ITO in thin film form. The observed size being close to Bohr radius responded to the
quantum confinement effect of the material. The photoluminescence behavior of the film excited at kex equivalent
to Eg or >Eg or <Eg, suggested the presence of excitons
in bound or in free state.
Acknowledgements
Authors are thankful to Dr. H.S. Maiti, Director,
CGCRI, Kolkata for his constant encouragement to carry
out this work under CTSM Programme [#CMM 0022 (1)].
One of the authors (S.K.) thanks to CSIR, India for offering her JRF fellowship.
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