On the optical properties of Ag^{+15} ion-beam irradiated TiO_{2} andSnO_{2} thin films

Abstract

The effects of 200-MeV Ag^{+15} ion irradiation on the optical properties of
TiO_{2} and SnO_{2} thin films prepared by using the RF magnetron sputtering
technique were investigated. These films were characterized by using UV-vis
spectroscopy, and with increasing irradiation fluence, the transmittance for
the TiO_{2} films was observed to increase systematically while that for
SnO_{2} was observed to decrease. Absorption spectra of the irradiated samples
showed minor changes in the indirect bandgap from 3.44 to 3.59 eV with
increasing irradiation fluence for TiO_{2} while significant changes in the
direct bandgap from 3.92 to 3.6 eV were observed for SnO_{2}. The observed
modifications in the optical properties of both the TiO_{2} and the SnO_{2}
systems with irradiation can be attributed to controlled structural
disorder/defects in the system.

Full text

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On the optical properties of Ag+15ion beam irradiated TiO2and SnO2thin films
Hardeep Thakur, K. K. Sharma, and Ravi Kumar
Center for Material Science and Engineering, National Institute of Technology, Hamirpur - 177 005, India
Pardeep Thakur
European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France∗
Abhinav Pratap Singh
Pohang Light Source, San31 Hyojadong, Namgu, Pohang -790 784, Republic of Korea
Yogesh Kumar
Material Science Division, Inter University Accelerator Centre, New Delhi 110067, India
Sanjeev Gautam and Keun Hwa Chae†
Nano Analysis Center, Korea Institute of Science and Technology, Seoul 136 − 791, Republic of Korea
(Dated: Received December 13, 2011)
The effects of 200 MeV Ag+15ion irradiation on the optical properties of TiO2 and SnO2 thin
films prepared by RF magnetron sputtering technique were investigated. These films were charac-
terized by the UV-vis spectroscopy and it was observed that with increase in irradiation fluence the
transmittance for the TiO2 films systematically increases while that for SnO2 decreases. Absorption
spectra of the irradiated samples showed a minor changes in indirect bandgap from 3.44 to 3.59 eV
for TiO2 while that for SnO2 significant modifications in the direct bandgap from 3.92 to 3.6 eV
were observed on increasing irradiation fluence. The observed modifications in the optical prop-
erties of both TiO2 and SnO2 systems with irradiation can be attributed to controlled structural
disorder/defects in the system.
PACS numbers: 78.66.Li; 68.55.-a; 78.20.Ci; 71.20.Nr
Keywords: SHI irradiation; optical properties; UV-vis; surface modifications
I.INTRODUCTION
Due to increasing interest in electronics and optoelec-
tronics, among the wide band gap semiconductors, TiO2
and SnO2 are being considered as the most promising
materials in view of their unique properties and vari-
ous future technological applications. These applications
boast their moderate price, high-volume, nontoxicity and
chemical stability. In addition, these materials offer the
possibility of integrating their magnetic and electronic
properties in spintronic devices, which make the use of
both spin and charge of the electrons [1, 2].
TiO2is a very interesting and versatile material with
a wide range of applications, including use in microelec-
tronics due to its high dielectric constant and in opti-
cal coatings because of its high refractive index [3–8].
It also has excellent optical transmittance in the visi-
ble and near-infrared region. TiO2exists in three crys-
talline polymorphs: rutile, anatase, and brookite with
their band gap values 3.03, 3.19, and 3.11 eV respec-
tively [9] . Among different TiO2 polymorphs, anatase
(tetragonal, D19
4h) is a metastable phase which contains
∗Diamond Light Source Ltd., Didcot, Oxfordshire, OX11 0DE, UK
†Electronic address: khchae@kist.re.kr(K.H. Chae); Phone/Fax:
++82-542791192/1599
four shared edges per octahedron (the highest condensa-
tion of TiO6 octahedra) and is known to be useful for
photocatalysis with response to ultraviolet photons. The
rutile (tetragonal, D14
4h) is thermodynamically most sta-
ble phase at all the temperatures and is formed by shar-
ing two edges per octahedron (the lowest condensation
of TiO6octahedra) with largest index of refraction. The
brookite (orthorhombic, D15
4h) is the most distorted phase
which shares three edges per octahedron. The proper-
ties of TiO2 are significantly dependent on the crystalline
phases, i.e.; anatase, rutile, or brookite and the morphol-
ogy of the material [10].
On the other hand, tin dioxide (SnO2) has been inves-
tigated in view of potential technological applications in
catalysis, gas sensor technology etc. [11–13], because of
high carrier density, optical transparency, wideband gap
(∼ 3.6 eV), and remarkable chemical and thermal stabil-
ities. SnO2 exists in the most important form of crys-
talline phase known as cassiterite with a rutile (tetrago-
nal, D14
4h) structure. Another form of SnO2with an or-
thorhombic structure is known to be stable only at high
pressures and temperatures.
Many deposition techniques (pulsed laser deposition,
sol-gel deposition etc.) have been employed to synthesize
TiO2and SnO2thin films, although magnetron sputter-
ing remains the preferred method due to better coating
uniformity, the process versatility, large area coatings and
more freedom in selecting of deposition conditions[14].
arXiv:1112.2320v1 [cond-mat.mtrl-sci] 11 Dec 2011

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Any mechanism that affects the lattice structure of the
TiO2 and SnO2 systems also influences their electronic
structure and optical properties. Swift heavy ion (SHI)
irradiation is one of the mechanisms which have been
used to tailor the material properties by modifying its
electronic structure [14–16]. It is well documented that,
their radiation induced defects produced in the mate-
rial entirely depends upon the energy loss processes,
namely; nuclear energy loss (elastic process), and elec-
tronic energy loss (inelastic process) involved during the
passage of ion in the target material. In the high energy
regime, due to dense electronic excitations, SHI induces
point/cluster/columnar defects and structural disorder
depending upon the extent of electronic energy loss mech-
anism in the system. The previous studies on TiO2and
SnO2systems were only focused on the SHI induced mod-
ifications in the electronic structure, orbital anisotropy
and magnetic properties [15, 16]. Thus, other properties,
such as, optical properties, need further study.
This experimental study was conducted to observe
the changes in the optical transmittance, absorption and
band gaps of TiO2and SnO2thin films, irradiated with
200 MeV Ag+15ion beam at various irradiation fluences
ranging from 1 × 1011to 5 × 1012ions/cm2that deviate
from what is largely reported.
II.EXPERIMENT AND DISCUSSION
The pure Titanium (II) oxide (TiO) and SnO2 com-
pound (purity 99.9%) were used as the starting mate-
rials for the deposition of thin films. TiO2 and SnO2
films, approximately ∼ 100 nm thick, were deposited on
cleaned sapphire single crystal substrates by using RF
magnetron sputtering technique. The pure oxide materi-
als were ground into fine powder in an agate mortar and
then mixtures were pressed in the form of circular targets
of 50 mm diameter by applying a pressure of 5−6 tons in
a hydraulic press. The targets were sintered at 1000◦C
for ∼ 12 hours. Prior to filling up sputtering gas the
chamber was evacuated to a base pressure of ∼ 1.1×10−5
Torr by the turbo molecular pump. The deposition was
carried out in a partial pressure of 10 mTorr of oxygen
and Ar gasses mixture (1:1) keeping the substrate tem-
perature at 550◦C and RF power 100 W. After deposition
the films were annealed in-situ at 550◦C in oxygen for 1
h. The deposited thin films were irradiated with 200
MeV Ag+15ions to fluences of 1 × 1011, 1 × 1012, and
5 × 1012ions/cm2at room temperature (RT) using the
15UD tandem accelerator at Inter University Accelerator
Centre, New Delhi, India.
The structural analysis of the pristine and irradiated
TiO2and SnO2films has been carried out using the high
resolution x-ray diffraction (HRXRD) with λ = 1.5425˚ A,
at the bending magnet 10B XRS KIST-PAL beamline
of the Pohang Accelerator Laboratory (PAL), S. Korea.
The HRXRD profiles are shown in Fig. 1(a) for the pris-
tine and SHI irradiated TiO2 thin films. As shown in
Fig.
structure (JCPDS, Card No.84-1286) while on irradia-
tion at the highest SHI fluence of 5 × 1012ions/cm2,
the inset in the Fig.1a clearly shows mixed peaks of
the brookite (JCPDS, Card No.76-1937) and the rutile
phases of TiO2.The appearance of broader brookite peaks
clearly indicates that SHI has induced structural disorder
and/or strain in the films [16]. Figure 1b shows HRXRD
pattern of the pristine and SHI irradiated SnO2 thin
films collected at RT. For the pristine sample depicted
in Fig. 1b, it is evident that the characteristic peaks at
2θ = 24.20◦,41.40◦,41.90◦correspond to the reflections
from (110), (211), and (116) planes of the orthorhombic
phase (JCPDS, Card No.78-1063) of SnO2, respectively.
The inset in the Fig. 1b shows the extended view of
diffraction peaks of the pristine sample. At highest SHI
fluence of 5×1012ions/cm2, the irradiation causes partial
amorphization and/or strain in the SnO2 system. The
detailed structural verification of the SnO2orthorhombic
structure and the effect of SHI irradiation on its structure
is discussed elsewhere [17].
The optical transmittance of the pristine and irradi-
ated thin films of both oxides were measured by collect-
ing the transmittance spectra as a function of wavelength
in the range of 200 − 800 nm at RT with a resolution
of λ = 0.5 nm, using Hitachi-U3300 spectrophotome-
ter. Figure 2 presents optical transmittance spectra for
the pristine and SHI irradiated TiO2(upper panel) and
SnO2(bottom panel) thin films collected at RT. It is clear
from Fig.2 that the pristine TiO2film exhibits a trans-
mittance value of about 25% in the visible region at ∼ 470
nm. With increasing irradiation (SHI:1×1011−5×1012
ions/cm2) fluence the transmittance increases consider-
ably, which at highest fluence (SHI:5 × 1012ions/cm2)
acquires a value of ∼ 70%. The increase in the transmit-
tance with SHI fluence signifies that the transparency
of SHI irradiated TiO2 films is superior to that of the
pristine film. On the other hand, the pristine SnO2film
exhibits a systematic decrease in the transmittance with
increasing irradiation (SHI:1×1011−5×1012ions/cm2)
fluence. In the visible region, the transmittance for the
pristine SnO2film was of ∼ 50%, which is decreased to
∼35% for the highest SHI fluence. Moreover, at ∼ 350
nm the transmittance decreases quickly for all the sam-
ples for both the oxides materials and approaches to zero
at ∼ 300 nm. This fast decrease in the transmittance
is due to strong absorption of light in this region caused
by the excitation and migration of electrons from the va-
lence band to the conduction band. Typical oscillations
in the transmittance spectra particularly for SnO2 can
be due to interference of light transmitted through the
thin film and the substrate [18].
The previous studies[16, 17, 19] reveal that the irra-
diated TiO2 thin films for the highest SHI fluence ex-
hibit a mixed (dominating brookite + rutile) phase of
TiO2while the SnO2pristine film is composed of a pure
orthorhombic phase of SnO2and SHI has induced con-
trolled structural disorder (distortion in the SnO6octa-
1a, the pristine sample as a tetragonal anatase

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FIG. 1:
(SHI:5×1012ions/cm2) TiO2 thin films and (b) pristine and
irradiated SnO2 thin films. The inset in Fig. 1(a): shows the
extended view of the mixed brookite and rutile phases of the
irradiated film and in (b): shows the extended view of the
orthorhombic phase of SnO2.
HRXRD pattern of: (a) pristine and irradiated
hedra) and/or strain in the films. The dominating struc-
ture of brookite phase in the SHI irradiated TiO2films
and orthorhombic distortions of the SnO2 lattice could
have an important implication on its electronic structure
and possible optical properties. Therefore, the observed
changes in the transmittance of the pristine TiO2 and
FIG. 2: Transmittance spectra for the pristine and irradiated
(SHI:1 × 1011− 5 × 1012ions/cm2) TiO2 (upper panel) and
the SnO2 (bottom panel) thin films collected at RT.
SnO2films with SHI fluence can be attributed to TiO6
and SnO6octahedral distortions respectively.
The absorption spectra for the pristine and irradiated
TiO2 and SnO2 films were measured by collecting the
absorbance as a function of wavelength at RT. The ab-
sorbance is given according to Eq.1.
?I
where I0is the intensity of incident radiation and I is the
transmitted intensity.
Figure 3 depicts the absorption spectra for the pris-
tine and SHI irradiated TiO2 (upper panel) and SnO2
(bottom panel) thin films. The optical absorption spec-
tra of TiO2showed a clear absorption edge at ∼352nm
for the pristine TiO2sample.With increasing irradiation
(SHI:1×1011−5×1012ions/cm2) fluence the absorption
edge is slightly shifted to the smaller wavelength side and
at the highest irradiation (SHI:5×1012ions/cm2) fluence
it acquires a value ∼ 345 nm. The observed values of the
absorption edge deviate from the reported values in the
literature [9]. For the SnO2system the optical absorp-
tion edge of the pristine film was found to be at ∼ 335
nm, which is shifted to ∼ 325 nm at the highest SHI
fluence. In this study, an anomalous trend was observed
for the optical absorption edges and the suppression in
the maximum absorption with increasing SHI fluence for
both the oxide materials. The absorption spectra of both
A = log
I0
?
(1)

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the systems reveal that the films grown under the same
parametric conditions have low absorbance in the visi-
ble/near infrared region while absorbance is high in the
ultraviolet region. Since SHI induces a controlled struc-
tural disorder (TiO6and SnO6octahedral distortions) in
both the systems, modification in the absorption spectra
can be correlated to changes in the electronic structure
as a result of lowering in the orbital symmetry (i.e. its s-,
p-, and d-like character) via strong hybridization effects
after irradiation.
FIG. 3: Optical absorption spectra for the pristine and irra-
diated (SHI: 1×1011−5×1012ions/cm2) TiO2 (upper panel)
and the SnO2 (bottom panel) thin films collected at RT.
The variation of absorption coefficient as a function
of photon energy for allowed indirect transitions [19] is
given by Eq. 2,
α = Bi(hν − Eg± Ep)2
and for allowed direct transitions by Eq. 3.
(2)
α = Bd(hν − Eg)1/2
where α is the absorption coefficient, Biand Bdare con-
stants for indirect and direct transitions, h is Planck’s
constant, ν is the frequency, Epis the photon energy in-
volved in the indirect transition, and Egis the band gap
energy. The absorption coefficient α is obtained from
Beer’s law,
(3)
I = I0exp(−αt)
In the Eq. 4, t is the thickness of the measured sample.
(4)
The relationship between the absorbance A, absorption
coefficient α, and thickness of the film t is given according
to Eq. 5.
?A
A plot of α1/2versus energy was used to obtain the value
of indirect band gap and α2versus energy was used for
the direct band gap by extrapolating the linear portion
of the curves to zero absorption.
α = 2.303
t
?
(5)
FIG. 4: Plots of: (a) (αhν)1/2versus hν for the pristine and
SHI irradiated TiO2 thin films and (b) (αhν)2versus hν for
the pristine and SHI irradiated SnO2 thin films collected at
RT.
TABLE I: The band gap values of 100 nm thick pristine TiO2
and SnO2 films and irradiated by 200 MeV Ag+15ion beam
Irradiation fluence Band gap values
(ions/cm2)
Pristine
1 × 1011
1 × 1012
5 × 1012
TiO2
3.44
3.57
3.50
3.59
SnO2
3.92
3.78
3.86
3.60
The variations in the band gap for the pristine and SHI
irradiated TiO2thin films are depicted in the Fig.4a. The

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optical transitions for TiO2have been shown to be pre-
dominantly indirect [20, 21] while that for SnO2 direct
[22]. The overall values obtained for all the irradiation
fluences are higher than the reported values for TiO2,
usually reported around 3.2 eV. The highest band gap
value was obtained for the sample irradiated at fluence
5 × 1012ions/cm2at ∼ 3.6 eV. The band gap values in-
creased until a SHI fluence of 1 × 1012ions/cm2, where
it is slightly decreased from the irradiated sample at SHI
fluence of 1 × 1011ions/cm2, 3.6 − 3.5 eV. For the SnO2
irradiated samples (see Fig. 4b the similar trend is fol-
lowed as that of TiO2 irradiated samples. The highest
value for the band gap of SnO2sample is for the pristine
sample with a value of 3.92 eV. The values decreased un-
til a SHI fluence of 1 × 1012ions/cm2, where the value
slightly increases to 3.86 eV. For the irradiated sample
with a fluence of 5 × 1012ions/cm2, the value again de-
creases to 3.6 eV. Table I shows the variations in the band
gap energy with irradiation fluence of both the TiO2and
SnO2systems. HRXRD data indicates that SHI creates
controlled structural disorder in lattice of oxide materials.
This can be responsible for the generation of defect lev-
els near the conduction band, i.e., shallow energy levels,
which can give rise to a transition from valence band to
these levels instead of a band-to-band transition. Due to
shallow levels, the band gap is effectively changed. This
decrease of the band gap gives an indication of the stoi-
chometric deviation of the irradiated SnO2and increase
in the oxygen vacancies in the SnO2lattice. A similar be-
haviour was also reported in a previous work [23], where
the effect of In doping concentration on the optical band
gap of nano-SnO2was investigated as a function of cal-
cination temperature. Another possible explanation for
the changes in the band gap value can be due to the
fact that the density of surface states in the SnO2lattice
induced are modified on irradiation.
It is observed that the values of optical absorption
edges obtained in our experiments deviate to the known
values reported in the literature [9, 24]. The obvious band
gap energy changes of the both oxide systems indicate the
possibility of band gap engineering in the as-deposited
thin films by means of SHI irradiation. To understand
the observed modifications in the optical properties it
is necessary to analyze the possible implications of ion
transport through the film. When 200 MeV Ag15+ions
pass through the oxide films, it loses its energy by colli-
sions with nuclei (nuclear stopping power Sn) and inelas-
tic collisions with electrons (electronic stopping power
Se). In the present case, for a TiO2 film the value of
Se∼ 12.96 keV/nm and Sn∼ 32.30 eV/nm, while for a
SnO2film the value of Se∼ 12.72 keV/nm and Sn∼ 36
eV/nm as calculated by stopping and range of ions in
matter software [25]. The vertical line marks 200 MeV
of the incident ion energy, the energy used in the present
work (see Fig. 5). From these values, it can be seen that
at higher energies, the electronic energy loss dominates
over nuclear energy loss and at energy of few hundreds of
keV, the opposite is true. Thus, the inelastic electronic
FIG. 5: The electronic and nuclear energy losses of 200 MeV
Ag+15ions as a function of ion energy inside TiO2 and SnO2
targets.
collision process is the dominant energy loss mechanism,
which induces point and columnar defects and can lead to
increase in defect density and modification in the lattice
structure [26].
III.CONCLUSIONS
In this study, the transmittance of the TiO2thin films
increased with increasing SHI fluence, while the transmit-
tance of the SnO2thin films have been found to decrease.
Both oxide systems showed improvements in the trans-
mittance as the irradiation fluence were increased. An
anomalous trend was observed for the optical absorption
edges with increasing SHI fluence for both the oxide ma-
terials. For irradiated TiO2thin films, it has been shown
that the optical band gap values for the indirect transi-
tions are much higher than the expected values reported.
The highest value of the band gap was achieved in the
SHI irradiated sample at fluence 5×1012ions/cm2, while
the lowest value of the band gap was observed in the pris-
tine sample. Similar trend was followed in the irradiated
SnO2samples. However, for irradiated SnO2thin films
the band gap values on average decreased.
From the results, it can be concluded that SHI do not
have critical effects on the pristine TiO2samples, while
for SnO2, significant modifications in the optical proper-
ties has been observed. These observed changes in the

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optical properties of the both pristine TiO2 and SnO2
films with SHI fluence can be attributed to controlled
structural disorder/defects in the system. Our results
show a direct linkage between SHI induced structural dis-
order/defects and the modifications in the optical prop-
erties of the oxide materials.
Acknowledgments
The authors would like to thank the Inter University
Accelerator Centre, New Delhi, India and Korea Insti-
tute of Science and Technology (KIST-2V02083), Seoul
136-791, Republic of Korea for experimental supports.
Department of Science and Technology (DST), Govern-
ment of India, is acknowledged for supporting this work
under Project No. S2/SR/CMP-0051/2007.
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