Download Physical properties of silver oxide thin films by pulsed laser deposition

IOP PUBLISHING
JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 42 (2009) 135411 (6pp)
doi:10.1088/0022-3727/42/13/135411
Physical properties of silver oxide thin
films by pulsed laser deposition: effect of
oxygen pressure during growth
N Ravi Chandra Raju, K Jagadeesh Kumar and A Subrahmanyam1
Semiconductor Laboratory, Department of Physics, Indian Institute of Technology Madras,
Chennai 600036, India
E-mail: [email protected]
Received 20 February 2009, in final form 13 May 2009
Published 17 June 2009
Online at stacks.iop.org/JPhysD/42/135411
Abstract
Silver oxide thin films have potential applications in ultra-high density optical non-volatile
memories and in fluorescence imaging. In this paper, the physical properties of silver oxide
thin films prepared at room temperature by the pulsed laser deposition (PLD) technique with
varying oxygen pressure during growth are reported. The oxygen pressure in the growth
chamber is varied between 9 and 50 Pa. The x-ray diffraction (XRD) analysis showed that all
the films were polycrystalline. With increasing oxygen pressure in the growth chamber, it is
observed that (i) the hexagonal Ag2 O transforms to monoclinic AgO, (ii) the grain size in the
film increases from 59 to 200 nm, (iii) the surface roughness of the film increases from 9 to
42 nm, (iv) the resistivity of the films increases from 1 to 4 × 104 m, (v) the surface work
function of the films increases from 5.47 to 5.61 eV and (vi) the optical band gap of AgO thin
films decreases from 1.01 to 0.93 eV. Raman spectroscopy on AgO thin films shows low wave
number peaks corresponding to the stretching vibration of Ag–O bonds. This study shows that
single phase AgO thin films, a requirement for plasmonic devices, can be prepared at room
temperature by the PLD technique with an oxygen pressure of 20 Pa.
(Some figures in this article are in colour only in the electronic version)
It is known that the silver, because of its d-shell
electrons, exists in different oxidation states and forms several
oxides: AgO, Ag2 O, Ag3 O, Ag2 O3 . The formation of these
oxides depends upon the growth conditions/reaction kinetics:
availability of oxygen in the growth chamber and the energy
required for the oxidation. The surface morphology and
the nucleation kinetics of the silver oxide depend upon the
kinetic energy of the particles (silver and oxygen atoms or
silver oxide molecules) reaching the substrate. In the plasma
assisted growth process (such as in PLD and sputtering), the
oxidation of silver can take place in the plasma (plume) in the
gaseous phase because of the small mean free path (MFP) of
silver atoms; consequently, a large number of collisions with
oxygen atoms result in the silver oxide molecules reaching
the substrate. The MFP at 20 Pa is estimated to be 0.25 mm,
which would, at a plume length of 2 cm, amount to 80 collisions
within the plume. In the case where the availability of oxygen
is insufficient for the oxidation process (due to low oxygen
1. Introduction
Nano-crystalline silver oxide thin films, in view of their new
and novel applications, continue to attract the attention of
several investigators. Some of the emerging applications of
silver oxide thin films are the anti-bacterial activity [1], a
possible use in super-resolution near-field structure (RENS) for
optical read–write ultra-high density non-volatile memories,
in fluorescence imaging and the property of surface enhanced
Raman scattering (SERS) in plasmonic devices [2, 3]. The
decomposition of silver oxide (into oxygen and small metallic
silver particles at relatively low temperatures and high energy
optical beams) can create a strong light-scattering centre that
resolves small pits or marks beyond the diffraction limit. This
is a useful property in ultra-high-density optical data storage
applications [3].
1
Author to whom any correspondence should be addressed.
0022-3727/09/135411+06$30.00
1
© 2009 IOP Publishing Ltd
Printed in the UK
J. Phys. D: Appl. Phys. 42 (2009) 135411
N Ravi Chandra Raju et al
chamber pressures), metallic silver can also be deposited on
the substrate. This is an oversimplified picture of oxidation;
more details of the oxidation process of silver oxide are given
in [4]. It is known that the AgO phase is relatively stable at high
oxygen pressures and at low temperatures. The AgO exists
in different crystal systems such as cubic, monoclinic and
tetragonal [5–7]. Silver oxide thin films have been prepared
by several techniques: RF and dc sputtering [8, 9], thermal
evaporation [10], chemical synthesis [2] and by pulsed laser
deposition (PLD) [11]. Our earlier studies [8] on silver oxide
thin films prepared by the reactive dc magnetron sputtering
technique have shown that (i) the oxygen pressure in the
growth chamber (∼10−3 mbar) influences the formation of
different oxides of silver, (ii) at high oxygen flow rates in
the growth chamber (2.01 sccm, corresponds to a chamber
pressure of 7 × 10−3 mbar) and at a sputter power density
of 1.0 W cm−2 , stoichiometric Ag2 O is formed and (iii) the
mixed silver oxide films show p-type conductivity. The p-type
conductivity in these films has been explained on the basis
of Sanderson’s theory of partial ionic charge; that is, the
bond between the oxygen and silver contains both ionic and
covalent components. The mixed bonding is also supported
by the theoretical calculations of the electronic structure and
the bonding mechanism [12].
Continuing our pursuit towards silver oxide based
plasmonic devices [13], we aim to prepare single phase AgO
thin films at higher oxygen pressures (compared with that of
dc magnetron sputtering [5]) and at lower temperatures; one
of the suited techniques for these growth conditions is PLD.
In this paper, the effect of oxygen pressure (9 to 50 Pa) on
the growth, electrical and optical properties of silver oxide
thin films prepared at room temperature (300 K) by the PLD
technique is reported.
Dellasega et al recently reported the growth of silver
oxide thin films by the PLD technique, using the fourth
harmonic beam (λ = 266 nm) of the Nd : YAG laser with
a fluence of 1.6 J cm−2 and varying the oxygen pressure in
the chamber between 4 and 150 Pa [11]. These experimental
conditions have produced a varying visible plume length
and large droplets (∼0.2–2 µm) on the grown film. The
varying visible plume length has a significant influence on
the reaction and growth kinetics (in this investigation, a small
change in the plume length is observed but it could not be
recorded). However, Dellasega et al have not reported the
electrical and optical properties of the silver oxide thin films.
In this investigation, the energy of the laser is in the third
harmonic (1.06 J cm−2 ); thus, the deposition rates and the
oxidation kinetics are significantly different from the work of
Dellasega et al.
Figure 1. XRD patterns of silver oxide films prepared by PLD
technique at (a) 9 Pa, (b) 10 Pa, (c) 20 Pa, (d) 30 Pa, (e) 40 and
(f) 50 Pa oxygen chamber pressure.
Figure 2. GIXRD patterns of silver oxide films prepared at (a) 9 Pa
and (b) 10 Pa oxygen chamber pressure.
chamber is varied between 9 and 50 Pa using a mass flow
controller (MKS, Model 1179A; flow range 0–100 sccm).
The growth chamber is initially evacuated (with a Balzers
TURBOVAC 361 C) to a base vacuum of 2.0 × 10−4 Pa and
then pure oxygen gas is introduced by the mass flow controller.
The target assembly positioned in the growth chamber is fitted
with a microprocessor controlled stepper motor. During the
deposition, the target is rotated at 10 rpm. The target is a 3 cm
diameter pure silver (99.999%) disc. The laser beam is guided
2. Experimental
The silver oxide thin films are prepared at room temperature
(∼300 K) using a PLD unit equipped with a Nd : YAG
laser (Quanta-Ray INDI series, Spectra Physics). The
third harmonic beam of wavelength 355 nm and fluence of
1.06 J cm−2 (the pulse duration is 7 ns with 10 Hz repetition
rate) is used for the deposition. The oxygen pressure in the
2
J. Phys. D: Appl. Phys. 42 (2009) 135411
N Ravi Chandra Raju et al
Table 1. Summary of structural, morphological, compositional, electrical and optical data obtained for silver oxide films grown at various
oxygen pressures.
Pressure
(Pa)
Crystallite
size from
XRD
(nm)
Grain
size from
AFM
(nm)
RMS
roughness
(nm)
EDX measurements
At%
Work
function
Oxygen Silver (eV)
9
10
20
23
—
36
59 ± 5
93 ± 3
120 ± 15
8
12
29
38.6
43.8
56.0
61.4
56.2
44.0
30
41
150 ± 10
23
61.8
40
40
164 ± 11
33
50
31
200 ± 23
42
Conductivity at
300 K
(−1 m−1 )
Activation
energy
(eV)
Optical
band gap
(eV)
5.47
5.50
5.63
5.0 × 10−2
9.7 × 10−1
4.1 × 10−5
—
—
0.64 ± 0.04
38.2
5.56
2.4 × 10−5
0.66 ± 0.02
62.0
38.0
5.61
3.9 × 10−5
0.68 ± 0.05
63.3
38.4
5.61
2.6 × 10−5
0.75 ± 0.04
—
—
1.01
±0.02
0.97
±0.02
0.97
±0.02
0.93
±0.02
3. Results and discussion
Table 2. Lattice parameters obtained for silver oxide films at
various oxygen pressures.
In this experiment, the growth parameters: temperature
(300 K), fluence (1.06 J cm−2 ), substrate–target distance
(3.0 cm) and the deposition time (90 min) are kept constant;
the oxygen pressure in the growth chamber is varied in the
range from 9 to 50 Pa. The variation of oxygen pressure in
the growth chamber has two major effects: (i) the available
oxygen at the plume (the reaction zone) will be different
for different pressures resulting in the formation of different
valence states of the metal oxide and (ii) the MFP of the metal
oxide (formed during the deposition process) decreases with
increasing oxygen pressure in the chamber. The decrease
in the MFP affects the kinetic energy of the metal oxide
molecule reaching the substrate and consequently influences
the size, the orientation of the grains and the surface roughness
(morphology) of the films on the substrate via the growth
kinetics.
The thickness of the films is in the range 1.0–1.5 µm, thus,
the size effects in the electrical resistivity measurements are
eliminated [15]. As the oxygen pressure is significantly higher
compared with films prepared with the reactive dc magnetron
sputtering technique [8], the electrical resistivities of the silver
oxide thin films are much higher. Hall effect measurements
could not be performed.
All the silver oxide films grown are nano-crystalline;
the crystallite size increases from 23 to 41 nm with oxygen
pressure. Figure 1 shows the XRD patterns. As may be seen,
the films formed at a low chamber pressure of 9 Pa are Ag2 O
in the hexagonal crystal system with grains preferentially
oriented along (1 0 0); the average crystallite size determined
by Scherrer’s formula is 23 nm. The grazing incident XRD
indicates the presence of metallic silver for the films prepared
at chamber pressures of 9 and 10 Pa (figure 2). The oxygen
incorporation into the film increases with increasing pressure in
the chamber; the atomic percentages of oxygen and silver in the
films grown at different oxygen chamber pressures (measured
by EDX) are given in table 1. It may be mentioned that the
EDX data are only indicative.
At a chamber pressure of 10 Pa, the phase corresponding
to AgO (monoclinic crystal system oriented along the (1̄ 1 1)
plane) and Ag2 O are seen. At 20 Pa and above this pressure,
Lattice constants (Å)
Oxygen pressure
of chamber (Pa)
Crystallite phase
a
b
c
9
20
30
40
50
Bulk values [6]
H-Ag2 O
M-AgO
M-AgO
M-AgO
M-AgO
M-AgO
3.058
5.747
5.756
5.748
5.895
5.852
3.058
3.461
3.459
3.463
3.607
3.478
4.865
5.440
5.436
5.437
5.021
5.495
into the growth chamber through quartz lenses such that it is
incident on the target at 45◦ with the target normal. The time of
deposition is 90 min for all the films. The thickness of the films
is in the range 1.0–1.5 µm. Cleaned soda lime glass substrates
(10.0 mm×10.0 mm×1.3 mm) are placed parallel to the target
at a distance of 3.0 cm. The cleaning steps of the substrates
are Cedepol (a commercially available detergent), acetone, and
then the substrates are subjected to an ultrasonic bath followed
by chromic acid cleaning for 10 min and washed thoroughly
with de-ionized water. Finally the cleaned substrates were
dried with pure nitrogen gas.
The crystal structure and surface morphology of the
films are evaluated by x-ray diffraction (XRD) (X’ Pert PRO
Diffractometer, PANalytical Products) with Cu Kα radiation
(1.5406 Å) and atomic force microscopy (Digital instruments,
Nano scope 3100) in contact mode. The thickness of the films
is measured by a cross sectional scanning electron microscope
(Philips FEI QUANTA 200). The optical transmittance (T %)
of the films in the wavelength range 600–2500 nm is measured
using a double beam spectrophotometer (JASCO V-570). The
Raman scattering spectroscopy studies have been carried out
by using an Ar ion laser with a 488 nm excitation source of a
power 25 mW (Jobin Yvon Model HR800UV). The electrical
conductivity measurements have been carried out on the films
using an electrometer (Keithley-614) in the two point probe
technique in the temperature range 300 to 600 ± 5 K. The
surface work function has been measured (with an accuracy
of 0.01 eV) by a reed-type Kelvin probe [14].
3
J. Phys. D: Appl. Phys. 42 (2009) 135411
N Ravi Chandra Raju et al
Figure 3. AFM images (1 µm×1 µm) of silver oxide films prepared from 9 to 50 Pa oxygen chamber pressure.
only AgO (monoclinic with preferential orientation of (1 1 1))
is formed; the Ag2 O phase is completely absent. Dellasega
et al have observed that 4 Pa is the threshold pressure to
obtain pure AgO (at a higher fluence: 1.6 J cm−2 ). The
formation of Ag–O dimers has been detected by optical
spectroscopy in the expanding plume of Ag and an Ag doped
YBCO target ablated in oxygen atmosphere [16]. The lattice
constants (table 2) for monoclinic AgO (primitive cell formula:
Ag4 O4 ) are comparable to those of the bulk silver oxide [17].
With increasing oxygen pressure till 50 Pa the crystallite size
calculated by the Scherrer formula increases from 36 to 41 nm
(table 1).
Figure 3 shows AFM images scanned over a 1 µm × 1 µm
area. The oxygen pressure in the chamber influences the
surface morphology of the silver oxide thin films. For the
film deposited at 9 Pa, the presence of small particles on the
surface has been observed (figure 3); the oxygen content on
these particles is found to be significantly lower (as evaluated
by SEM and EDX, not shown here). In conjunction with the
GIXRD (figure 2) for these samples, it may be possible to
attribute these small particles to metallic silver. The metallic
silver cannot be due to the droplet production in the PLD
technique (as no metallic silver is seen for higher pressures of
oxygen), but it may be due to the impossibility to transform all
Ag to an oxide at the given low oxygen pressure. At a chamber
pressure of 20 Pa, clear grain growth formation corresponding
to AgO may be seen. The roughness of the films increases
with the oxygen pressure (table 1). The fairly smooth and
non-columnar growth may be attributed to the lower fluence
of the ablated laser beam.
Figure 4 shows the optical transmission spectra of silver
oxide films in the wavelength range 600–2500 nm. A low
transmission for the films grown at chamber pressures 9 and
10 Pa may be attributed to the presence of metallic silver (the
metallic silver also is seen in GIXRD: figure 2). The optical
band gap of the transmitting thin films calculated from the
optical absorption spectra (using a Tauc plot) decreases slightly
(from 1.01 to 0.93 eV for AgO monoclinic structure: table 1)
with increasing oxygen pressure.
Since the oxygen pressure in the growth chamber is rather
high, the silver oxide formed will be rich in oxygen and
consequently the films show high electrical resistance; the
conductivity values of the films at room temperature (300 K)
are given in table 1. In order to understand the nature of
the conductivity (metallic or semiconducting) of monoclinic
silver oxide films, dc conductivity measurements have been
performed in the temperature range 300–600 K; the films show
a non-linear and semiconducting nature (figure 5). In the
Arrhenius plot of the samples grown with an oxygen chamber
pressure 20–50 Pa a linear dependence of the natural logarithm
of the conductivity on 1/T in a very narrow region from 310
to 360 K can be observed. This indicates the presence of
an activation energy, in this case mainly due to the hopping
of electrons either from the defect sites or from the surface
states into the conduction band. It is known that the activation
energy (Ea ) cannot be calculated in a very narrow range of
temperatures. However, in order to estimate the order of
activation energy in the silver oxide, an attempt has been made
to calculate the activation energy using the Arrhenius equation
σ = σ0 exp(−Ea /kB T ),
4
(1)
J. Phys. D: Appl. Phys. 42 (2009) 135411
N Ravi Chandra Raju et al
Figure 6. Raman spectra of silver oxide films prepared at 9 Pa and
30 Pa oxygen chamber pressure.
Figure 4. Optical transmittance spectra of silver oxide films
prepared from 9 to 50 Pa oxygen chamber pressures and the inset of
figure shows the Tauc plot for the films deposited from 20 to 40 Pa
oxygen chamber pressures.
The surface work function (ϕF ) of these silver oxide thin
films has been evaluated by measuring the contact potential
difference (CPD) with a stainless steel reference electrode
using the Kelvin probe set up [14]. The work function of
the film (ϕF ) is calculated using the equation
qVCPD = ϕR − ϕF ,
(2)
where ϕR is the work function of the reference electrode. The
surface work function, as is well known, depends upon the
surface roughness, inhomogeneity and non-stoichiometry of
the surfaces; the values reported in this paper are the average
taken over an area of 2 mm × 2 mm. The work function
values (±0.01 eV) change with oxygen pressure in the range:
5.4–5.6 eV. (for a ready reference, the work function of pure
Ag is 4.3 eV [20] and for reactive dc magnetron sputtered
cubic Ag2 O films: 4.8–5.2 eV [8]). Interestingly, there is a
correlation between the average surface roughness (measured
by AFM) and the surface work function (table 1).
Figure 6 shows the Raman spectra for the films deposited
at 9 and 30 Pa oxygen chamber pressure. At a low oxygen
pressure of 9 Pa, the presence of metallic silver and the mixed
phase of Ag2 O and AgO may be the reasons for not obtaining
clear Raman peaks. At 30 Pa, the Raman peaks observed
correspond to the AgO phase: 216, 300, 379, 429, 467 and
487 cm−1 and these Raman peaks match well with the peaks
reported in the literature [19].
Figure 5. Arrhenius plot of the conductivity ln(σ ) versus 1000/T
for pure monoclinic AgO films.
where σ is the conductivity at temperature T , σ0 is a constant,
kB is the Boltzmann constant and T is the absolute temperature.
The activation energy ranges between 0.64 and 0.75 eV. The
possible defects in these silver oxide thin films are (i) oxygen
vacancies, (ii) a mixed valence state (ionic and covalent) of
the silver to oxygen [8, 12] and (iii) the grain boundaries.
The activation energies observed in this work (though very
approximate values) are comparable to the reported values
[18]. Another interesting observation from the temperature
dependent conductivity is that at 454 ± 3 K, the conductivity
changes by almost two orders of magnitude (figure 5). This
change may be attributed to the decomposition of AgO into
silver and oxygen [19].
4. Conclusions
Silver oxide thin films have been prepared at room temperature
by the PLD technique. The oxygen gas pressure in the
growth chamber is varied in the range 9–50 Pa. At low
oxygen pressures (9 and 10 Pa), both Ag2 O and AgO are
formed. At higher oxygen pressures, starting at 20 Pa, pure
monoclinic AgO with semiconducting nature is formed. The
thickness (1.0–1.5 µm), optical band gap (∼1.0 eV) and the
5
J. Phys. D: Appl. Phys. 42 (2009) 135411
N Ravi Chandra Raju et al
work function (5.4–5.6 eV) are found to vary with oxygen
pressure.
[11] Dellasega D, Facibeni A, Fonzo F D, Russo V, Conti C,
Ducati C, Bassi A L and Bottani C E 2009 Appl. Surf. Sci.
255 5248
[12] Deb A and Chatterjee A K 1998 J. Phys.: Condens. Matter
10 11719
[13] Tominaga J 2003 J. Phys.: Condens. Matter 15 R1101
[14] Kumar C S, Subrahmanyam A and Majhi J 1996 Rev. Sci.
Instrum. 67 805
[15] Rossnagel S M and Kuan T S 2004 J. Vac. Sci. Technol. B
22 240
[16] Kumar D, Oktyabrsky S, Kalyanaraman R, Narayan J,
Apte P R, Pinto R, Manoharan S S, Hegde M S, Ogale S B
and Adhi K P 1997 Mater. Sci. Eng. B 45 55
[17] JCPDS card 89-3722
[18] Garner W E and Reeves L W 1954 Trans. Faraday Soc.
50 254
[19] Waterhouse G I N, Bowmaker G A and Metson J B 2001
Phys. Chem. Chem. Phys. 3 3838
[20] Tjeng L H, Meinders M B J, Elp J V, Ghijsen J and
Sawatzky G A 1990 Phys. Rev. B 41 3190
References
[1] Antelman M S 1993 US Patent 5.211.855
[2] Peyser L A, Vinson A E, Bartko A P and Dickson R M 2001
Science 291 103
[3] Fujimaki M, Awazu K and Tominaga J 2006 J. Appl. Phys.
100 074303
[4] Rehren C, Muhler M, Bao X, Schlögl R and Ertl G 1991
Z. Phys. Chem. 174 11
[5] JCPDS card 76-1489
[6] JCPDS card 75-0969
[7] JCPDS card 84-1108
[8] Barik U K, Srinivasan S, Nagendra C L and Subrahmanyam A
2003 Thin Solid Films 429 129
[9] Her Y C, Lan Y C, Hsu W C and Tsai S Y 2004 Japan. J.
Appl. Phys. 43 267
[10] Al-kuhaili M F 2007 J. Phys. D: Appl. Phys. 40 2847
6
Silver oxide (AgO) thin films for Surface
Enhanced Raman Scattering (SERS) studies
N. Ravi Chandra Raju*, K. Jagadeesh Kumar and A. Subrahmanyam
Semiconductor Lab, Department of Physics, Indian Institute of Technology, Madras 600 036, India;
Email: [email protected]
ABSTRACT
Present paper reports the use of photo-activated silver oxide thin films for the study of
Surface Enhanced Raman Scattering (SERS). The silver oxide thin films grown by
pulsed laser deposition (PLD) are excited with 488 nm wavelength of power density
47 watts/cm2 (for 5 minutes) to produce nano silver clusters. Rhodamine 6G (10-7 M)
is employed for detecting the enhanced Raman signal.
Key words: SERS, silver oxide thin films, Pulsed Laser Deposition, Rhodamine 6 G
Surface Enhanced Raman Scattering (SERS) is based on the plasmonic behavior of
the metals. SERS is a well known and highly sensitive probing technique being used
for bio-medical applications [1, 2]. Pure Silver is a preferred ‘SERS active substrate’
[3]; however, preserving the virgin surface of silver and reproducing a near identical
silver surface is a challenge. Since silver is photo-active, it is possible to dissociate
silver oxide into pure nano metallic silver. Thus, it is proposed that one can employ
silver oxide and use suitable wavelength to produce nano clusters of pure silver which
can be used to enhance the Raman signal.
In the present paper, the silver oxide thin films (~ 200 nm thickness) are prepared
by pulsed laser deposition technique using Nd: YAG laser of 355 nm wavelength on
polished crystalline Si-substrates. The film growth conditions and basic
characterization are given in our earlier paper [4]. Rhodamine 6G of 10-7 M
concentration is dispersed on the film and subsequently dried. The Raman
measurements are performed with 488 nm Ar-ion laser excitation source (Jobin Yvon
Model HR800UV). The Raman band of silicon substrate at 521 cm-1 is used to
calibrate the spectrometer. The laser is focused using 100 X magnification objective
lens , the signal is collected (10 seconds) in a backscattering geometry and guided to
Peltier-cooled charge-coupled device (CCD) detector. The scan range is: 100 - 1700
cm-1. Scanning electron microscopy (SEM) images were captured using a Philips FEI
QUANTA 200.
Figure 1 shows the Raman spectra of pure silver oxide thin film prior to the
disbursement of Rhodamine 6G on to the film surface. The bands at 217, 302, 379,
429, and 487 cm-1 corresponds to vibrational modes of Ag and oxygen in AgO phase
of silver oxide [4]. The intense band at 521 cm-1 corresponds to Si-Si vibrational mode
of Si substrate. Raman studies have been performed on the Rhodamine 6G dispersed
silver oxide films, in two stages: first, an instant exposure (0 minutes) and second,
2
TO BE INSERTED
ON THE FIRST
OF EACH PAPER
) to 5 minutes (Fig.2). As
exposing the film (toCREDIT
488LINE
nm(BELOW)
of power
density
47PAGE
watts/cm
CP1267, XXII International Conference on Raman Spectroscopy
edited by P. M. Champion and L. D. Ziegler
© 2010 American Institute of Physics 978-0-7354-0818-0/10/$30.00
1005
Downloaded 18 Aug 2010 to 203.237.47.242. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions
anticipated, the instant exposure does not show the Stokes frequencies corresponding
to Rhodamine 6G; the 5 minutes exposure clearly enhances the Raman signal. An
optical image of the 5 minute exposure sample reveals the silver metal nano clusters
(Inset in Fig. 2 (b)). This observation is confirmed by SEM (Fig.2 (c)) and optical
absorption (figure not given). The enhancement in the 5 minute exposure silver oxide
is due to the photo-dissociation of silver oxide into silver nano-clusters. It is observed
that photo-dissociation did not take place till 5 minutes of exposure to 488 nm. These
observations have been reproduced several times and with the same results (within
experimental error). This formation of silver nano structures under focused laser
irradiation (beyond a threshold of 5 minutes) is responsible for the plasmonic nature
and consequently for the enhancement of Raman intensity of Rhodamine 6G stokes
frequencies [5, 6].
FIGURE 1. Raman spectra of silver
oxide thin film coated on Silicon
FIGURE 2. Raman spectra of Rhodamine 6G
on silver oxide thin film, Inset of figure shows
the optical microscope images (a) before, (b)
after irradiation of 5 min and (c) SEM image
after 5 min of laser irradiation.
REFERENCES
1. M. Fleischmann, P. J. Hendra, A. J. Mcquillan, Chem. Phys. Lett. 26, 163-166 (1974).
2. K. Kneipp, H. Kneipp, I. Itzkan, R. R. Dasari, M. S. Feld, J. Phys: Condens. Matter. 14, R597-R624
(2002).
3. X. Bao, M. Muhler, Th. Schedel-Niedrig, R. Schlogl, Phys. Rev. B, 54, 2249-2262 (1996).
4. N. R. Raju, K. J. Kumar, A. Subrahmanyam, J. Phys. D: Appl. Phys., 42, 135411 (6pp)
(2009).
5. D. Buchel, C. Mihalcea, T. Fukaya, N. Atoda, J. Tominiaga, Appl. Phys. Lett.,79, 620-622 (2001).
6. Y. Iwanabe, T. Horiuchi, J. Tominaga, D. Buchel, C. Mihalcea, Jpn. J. Appl. Phys., 42,
L1208-L1209 (2003).
1006
Downloaded 18 Aug 2010 to 203.237.47.242. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions
Research Article
Received: 27 September 2010
Accepted: 21 December 2010
Published online in Wiley Online Library: 23 February 2011
(wileyonlinelibrary.com) DOI 10.1002/jrs.2895
Photodissociation effects on pulsed laser
deposited silver oxide thin films: surfaceenhanced resonance Raman scattering
N. Ravi Chandra Raju∗ and K. Jagadeesh Kumar
Temporal Raman scattering measurements with 488, 532 and 632 nm excitation wavelengths and normal Raman studies by
varying the power (from 30 W/cm2 to 2 MW/cm2 ) at 488 nm were performed on silver oxide thin films prepared by pulsed-laser
deposition. Initially, silver oxide Raman spectra were observed with all three excitation wavelengths. With further increase
in time and power, silver oxide photodissociated into silver nanostructures. High-intensity spectral lines were observed at
1336 ± 25 and 1596 ± 10 cm−1 with 488 nm excitation. No spectral features were observed with 633 nm excitation. Surfaceenhanced resonance Raman scattering theory is used to explain the complex behavior in the intensity of the 1336/1596 cm−1
c 2011 John Wiley & Sons, Ltd.
lines with varying power of 488 nm excitation. Copyright Keywords: photodissociation; surface-enhanced resonance Raman scattering; pulsed-laser deposition; silver oxide thin film; silver
nanostructures
Introduction
J. Raman Spectrosc. 2011, 42, 1505–1509
Experimental
The details of AgO thin film preparation using Nd : YAG laser pulses
have been presented in our earlier paper.[10] All the silver oxide
thin films of thickness ∼300 nm were deposited on borosilicate
glass (BSG) slides.
The Raman spectra of silver oxide thin films in the wavenumber
region 200–3000 cm−1 were measured at room temperature
(300 K) with a Jobin Yvon, Model HR 800UV, Raman spectrometer
as a function of time (0–30 min) by three different excitation
wavelengths: 488 nm (Ar-ion laser), 633 nm (He–Ne laser) and
532 nm (Nd : YAG laser). Spectral resolutions (with a holographic
grating of 1800 lines/mm) of 0.6 and 1 cm−1 have been achieved
with slit openings of 100 and 200 µm, respectively. A dry Olympus
100× objective (numerical aperture (NA) = 0.9) and a dry
Nikon 20× objective (NA = 0.4) were used. The Raman band
of monocrystalline silicon at 520 cm−1 was used as the calibration
∗
Correspondence to: N. Ravi Chandra Raju, Department of Physics, Indian
Institute of Technology Madras, Chennai 600036, India.
E-mail: [email protected]
Department of Physics, Indian Institute of Technology Madras, Chennai 600036,
India
c 2011 John Wiley & Sons, Ltd.
Copyright 1505
Surface-enhanced Raman scattering (SERS) is a phenomenon
due to the interaction of light with the plasmons in metallic
nanostructures. Silver (Ag) and gold (Au) nanostructures are
widely used as SERS substrates.[1,2] Ag supports stronger plasmonic
fields and hence the corresponding SERS enhancement is higher
than that of Au. It is because Au has a higher dielectric loss
due to interband damping of plasmon resonance.[1] Since Ag is
highly reactive, it is rather difficult to avoid its interaction with
ambient gases. As a result of this, the surface of silver always
contains an uncontrolled thin oxide layer (a few monolayers)
along with other physisorbed organic molecules.[3,4] Again, Ag
being multivalent, it forms several oxides.[5,6] These factors limit
the use of silver nanostructures as SERS substrates. Realization of
a good and reproducible SERS substrate can be achieved by using
silver oxide rather than Ag,[7] which can be prepared by several
techniques.[8 – 10]
The interpretation of SERS spectra collected on silver oxide
substrate demands careful attention of understanding the background problems.[11,12] It is known that the bandgap of silver oxide
is ∼550 nm. So the photodissociated silver nanostructures from
silver oxide under laser irradiation[13 – 19] lead to fluorescence of
silver nanoclusters,[20 – 22] which are responsible for enhancing the
Raman signals of the adsorbents.[7,23] One such example of wrong
interpretation was the highly intense photoactivated fluorescence
of silver oxide at 522 and 529 nm under irradiation with 488 nm
laser reported by Chuang et al.[24] The Raman signals of carbonate
at 1336 cm−1 (523 nm) and 1596 cm−1 (529 nm) with 488 nm
excitation on the silver nanostructures were reported.[25 – 32] It is
a well-known fact that the Raman shift (υ = υ ± υ ) is constant
irrespective of the excitation wavelength used. Based on the
Raman shift observed at different excitation wavelengths, we can
easily distinguish the Raman and the fluorescence signals.
With these points in mind, in this article we report the
experimental results of temporal Raman studies on silver oxide
thin films prepared by pulsed-laser deposition (PLD) with three
different (488, 532 and 632 nm) excitation wavelengths. Also, we
have studied the Raman spectra of silver oxide films by varying
the power of the 488 nm excitation. The plasmonic nature of silver
nanostructures under laser illumination was confirmed by optical
absorption spectroscopy and X-ray diffraction (XRD) studies.
Surface-enhanced resonance Raman scattering (SERRS) theory
is used to explain the complex behavior in the intensity of the
1336/1596 cm−1 lines when the power of the 488 nm excitation
line is varied.
N. R. C. Raju and K. Jagadeesh Kumar
reference. The signal was collected (10 s) in a backscattering
geometry and guided to an air-cooled (1024 × 256 pixels of
26 µm) charge-coupled device (CCD) detector. The laser power on
the sample was measured by a PM 100D console with a C-series
photodiode sensor (Thorlabs GmbH).
XRD studies on silver oxide thin films were carried out by
a PANalytical X Pert PRO diffractometer with Cu Kα radiation
(1.5406 Å). The optical absorption studies were performed with a
double beam spectrophotometer (JASCO V-570). Scanning electron microscopy (SEM) images were captured using a Philips FEI
QUANTA 200 instrument. X-ray photoelectron spectroscopy (XPS)
studies were carried out on an OMICRON EA-125 photoelectron
spectrometer at a base pressure ∼1 × 10−10 Torr. Al Kα X-rays,
with the source operated at an emission current of 10 mA and
an anode voltage of 10 kV, constituted the probing beam. No
sputter-etching was performed[6] on the films. The Au 4f7/2 line at
84.0 eV was kept as the external reference. Graphitic C 1s line at
284.7 eV was chosen as the internal reference.
Results and Discussion
The reported characteristic Raman lines for silver oxide are 216,
300, 379, 429, 467 and 487 cm−1 with the excitation wavelength
of 488 nm.[10]
Excitation with 488 nm
Figure 1(a) presents the Raman signal of silver oxide thin films with
488 nm excitation (at a power density of 82 kW/cm2 ). At t = 0 min
(immediately after the laser irradiation), the characteristic intense
lines of AgO at 216 and 429 cm−1 were observed. At t = 2 min,
the 216 and 429 cm−1 peaks of AgO completely disappeared and
new peaks at 992 cm−1 (513 nm), 1336 cm−1 (523 nm), 1596 cm−1
(529 nm) and 1970 cm−1 (540 nm) started showing up. At higher
times, up to 30 min, only two peaks were observed at 1336 cm−1
(523 nm) and 1596 cm−1 (529 nm) with enhanced intensity.
Interestingly, the broad bands at 1340 and 1590 cm−1 on Ag
surfaces have also been reported to be the SERS lines due to the
presence of carbon.[25 – 32] The presence of carbon in the present
study has been confirmed by XPS studies (Fig. 2). The intensity
of the C 1s peak is found to be 25% of the Ag 3d peak. The other
peaks correspond to Ag 3d5/2 , Ag 3d3/2 , Ag 3p3/2 and Ag 3p1/2 .
These results confirm the presence of a surface carbonate layer
on the silver oxide film. This carbonate layer is picked by the
active surface of silver oxide from the ambient. Thus, the peaks
observed at 1336 cm−1 (523 nm) and 1596 cm−1 (529 nm) by the
Raman excitation wavelength of 488 nm may be attributed to
the enhanced Raman lines of carbon or carbon-related molecules
adsorbed on the silver oxide film. The reported intense ‘photoactivated fluorescence’ of silver oxide at 522 nm (1336 cm−1 ) and
529 nm (1596 cm−1 ) under the irradiation of a 488 nm laser by
Chuang et al.[24] might be an inadvertent mistaken identification.
Excitation with 532 nm
1506
Figure 1(b) shows the Raman spectra of silver oxide thin films
excited with 532 nm irradiation (1.3 MW/cm2 ). Even at t = 0 min,
the peaks at 971 cm−1 (560 nm), 1362 cm−1 (572 nm), 1603 cm−1
(580 nm) and 1782 cm−1 (587 nm) started to appear, and were
seen up to 30 min of irradiation. Since the power of the 532 nm
wileyonlinelibrary.com/journal/jrs
Figure 1. Raman spectra of silver oxide thin films with (a) 488 nm
(82 kW/cm2 ) (b) 532 nm (1.3 MW/cm2 ) and (c) 633 nm (40 kW/cm2 ) laser
excitation sources at different exposure times.
line is high compared to that of the 488 nm line, silver oxide gets
photodissociated into silver and hence the spectral lines of silver
oxide are not observed at t = 0 min. Taking all the accuracies in
the measurement, one may safely say that the peaks at ∼1336 and
∼1596 cm−1 are still present with 532 nm irradiation.
Excitation with 633 nm
Figure 1(c) presents the Raman signal of silver oxide with 633 nm
excitation (40 kW/cm2 ). At t = 0 min, the characteristic intense
c 2011 John Wiley & Sons, Ltd.
Copyright J. Raman Spectrosc. 2011, 42, 1505–1509
Photo-dissociation effects on pulsed laser deposited silver oxide thin films
appear and were present up to 2 MW/cm2 . The peak intensities of
1336/1596 cm−1 showed a nonlinear variation.
The observation of equal Raman shifts (υ = υ ± υ ) of 1336
and 1596 cm−1 with 488 nm and 532 nm excitation confirms that
the lines are due to Raman scattering and not to fluorescence.
Thus the observed Raman lines at 1336 and 1596 cm−1 can be
attributed to the SERS associated with the disorder band of carbon
or carbon related-molecules attached (adsorbed/physisorbed) to
the silver oxide surface (as confirmed by the XPS studies). The
observation that the peaks at 523 nm (1336 cm−1 ) and 529 nm
(1596 cm−1 ) occur only with the 488 and 532 nm excitation but
not with 633 nm suggests that a resonance phenomenon is taking
place. The resonance depends upon the cluster size, the excitation
wavelength and the corresponding coupling coefficient of the
resonance Raman scattering (RRS).
The enhanced resonance Raman scattering cross-section
σ ERRS (λL , λ) is given as[33]
Figure 2. X-ray photoelectron spectra of silver oxide thin films.
Figure 3. Raman spectra of silver oxide thin films with 488 nm laser
excitation for different power densities. Spectra were collected at the
instant of incident irradiation.
lines of AgO at 216 and 429 cm−1 were observed, which
disappeared after 6 min of laser irradiation. Moreover, no spectral
features were identified in the range 1000–2000 cm−1 till the
exposure time of 30 min.
Variation of Incident Power for 488 nm Irradiation
J. Raman Spectrosc. 2011, 42, 1505–1509
(1)
where, M1 (λL ) is the enhancement factor induced by the coupling
of the plasmon resonance with the incident or excitation light,
M2 (λ) is the enhancement factor induced by coupling of the
plasmon resonance with the Raman scattered light and σ RRS (λL , λ)
is the RRS cross-section spectrum of a molecule free from surface
enhancement.
From Eqn (1), it is clear that the enhancement due to the
presence of the monolayer on the silver oxide surface is dependent
on (1) the excitation wavelength, (2) surface plasmon resonance
absorption (size dependence) of silver nanostructures induced
during the laser irradiation and (3) the Raman shift (υ = υ − υ )
of the adsorbent (monolayer of carbonate) present on the surface
of the silver oxide film.
Metallic silver nanostructure, a result of photodissociation of
silver oxide, is the building block for plasmon resonance. XRD and
optical absorption studies confirm the presence of metallic silver
on laser-irradiated silver oxide thin films. Figure 4(a) and(b) shows
the morphology change (SEM images) observed before and after
30 min of irradiation of the silver oxide thin films. The XRD study
further confirms the formation of silver nanostructures in the silver
oxide film by the irradiation of laser, as shown in Fig. 5. Thus there
is enough evidence to state that silver oxide photodissociates
into silver nanostructures to give rise to plasmonic bands. The
plasmon resonance can be identified by the optical absorption
spectra of the irradiated silver oxide thin films as a function of
time. The optical absorption spectrum of silver oxide thin films
irradiated with 532 nm at 1.3 MW/cm2 up to 30 min is given in
Fig. 6. The characteristic Ag plasmon resonance band is seen in
the range 402–454 nm. The variation of the Ag nanocluster size
under laser irradiation and photo (and thermal) dissociation may
be responsible for the complex shifts in the plasmon resonance.[26]
The observed surface plasmon band (402–454 nm) is very near
the Raman excitation wavelength of 488 nm. Hence, there is
always a chance of a resonance taking place. So from Eqn (1), the
possibility for getting more enhancement cross-section should
happen under the 488 nm excitation, which is confirmed by the
observed higher intensity of 1336/1596 cm−1 peaks with 488 nm
compared to 532 nm. If the size of the Ag nanoclusters changes,
one would anticipate a corresponding change in the SERRS
signals, provided the excitation wavelength is near the resonance
frequencies. Additional contribution of the SERRS signals may also
be from the molecular dissociation of hydrocarbon. Thus SERRS
c 2011 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/jrs
1507
Figure 3 presents the Raman signal as a function of incident
power (from 30 W/cm2 to 2 MW/cm2 ) of the 488 nm irradiation.
Every reading was recorded on a fresh surface of the silver oxide
sample. From 30 W/cm2 to 3 kW/cm2 , no spectral features were
observed in the 1000–2000 cm−1 region. At 88 kW/cm2 , two
peaks at 1336 cm−1 (523 nm) and 1596 cm−1 (529 nm) started to
σ ERRS (λL , λ) = M1 (λL )M2 (λ)σ RRS (λL , λ)
N. R. C. Raju and K. Jagadeesh Kumar
Figure 5. XRD of silver oxide film (a) before and (b) after 30 min irradiation
with 532 nm laser excitation.
Figure 4. SEM image of silver oxide film (a) before and (b) after 30 min
irradiation with 532 nm laser excitation.
may be responsible for the complex behavior of the intensity of
1336/1596 cm−1 line with power of 488 nm excitation.
Figure 6. Optical absorption spectra of silver oxide thin films at different
irradiation times (1–30 min) with 532 nm laser line. The inset shows the
optical absorption spectra of a silver oxide thin film before irradiation.
Conclusions
Temporal Raman scattering measurements with three different
excitation wavelengths (488, 532, and 633 nm) have been
performed on silver oxide thin films prepared by PLD. Observation
of the constant Raman shift of the 1336/1596 cm−1 lines
with 488 nm and 532 nm excitation wavelengths confirmed as
Raman lines. The plasmonic nature of photodissociated silver
nanostructures from silver oxide was confirmed. Intensity variation
of the 1336/1596 cm−1 lines with power of 488 nm and the
nonobservation of these spectral lines with 633 nm are explained
using SERRS theory.
Acknowledgements
1508
We thank Prof. Dr. A. Subrahmanyam, Indian Institute of Technology Madras (IITM), Chennai, for technical support and discussions.
wileyonlinelibrary.com/journal/jrs
We also thank the International Advanced Research Centre for
Powder Metallurgy and New Materials (ARCI), Hyderabad, India,
for the XPS measurements.
References
[1] P. K. Jain, X. H. Hung, I. H. El-Sayed, M. A. El-Sayed, Plasmonics 2007,
2, 107.
[2] S. Link, M. A. El-Sayeed, Annu. Rev. Phys. Chem. 2003, 54, 331.
[3] X. Bao, M. Muhler, T. Schedel-Niedrig, R. Schlogl, Phys. Rev. B 1996,
54, 2249.
[4] M. Erol, Y. Han, S. K. Stanley, C. M. Stafford, H. Du, S. Sukhishvili,
J. Am. Chem. Soc. 2009, 131, 7480.
[5] J. F. Pierson, C. Rousselot, Surf. Coat. Technol. 2005, 200, 276.
[6] M. Bielmann, P. Schwaller, P. Ruffieux, O. Groning, L. Schlapbach,
P. Groning, Phys. Rev. B 2002, 65, 235431.
c 2011 John Wiley & Sons, Ltd.
Copyright J. Raman Spectrosc. 2011, 42, 1505–1509
Photo-dissociation effects on pulsed laser deposited silver oxide thin films
[7] D. Buchel, C. Mihalcia, T. Fukaya, N. Atoda, J. Tominaga, T. Kikukawa,
H. Fuji, Appl. Phys. Lett. 2001, 79, 620.
[8] J. Tominaga, S. Haratani, K. Uchiyama, S. Takayama, Jpn.J.Appl.Phys.
1992, 31, 2757.
[9] S. Haratani, J. Tomonaga, H. Dohi, S. Takayama, J. Appl. Phys. 1994,
76, 1297.
[10] N. R. Raju, K. J. Kumar, A. Subrahmanyam, J. Phys. D: Appl. Phys. 2009,
42, 135411.
[11] R. Aroca, Surface-enhanced Vibrational Spectroscopy, John Wiley &
Sons, Ltd.: Chichester, 2006.
[12] A. Otto, J. Raman Spectrosc. 2002, 33, 593.
[13] O. L. A. Monti, J. T. Fourkas, D. J. Nesbitt, J. Phys. Chem. B 2004, 108,
1604.
[14] C. D. Geddes, A. Parfenov, J. R. Lakowicz, J. Fluorescence 2003, 13,
297.
[15] X. Wu, E. K. L. Yeow, Nanotechnology 2008, 19, 035706.
[16] Y. Iwanabe, T. Horiuchi, J. Tominaga, D. Buchel, C. Mihalcea, Jpn. J.
Appl. Phys. 2003, 42, L1208.
[17] R. S. Eachus, A. P. Marchetti, A. A. Muenter, Annu. Rev. Phys. Chem.
1999, 50, 117.
[18] M. L. Jacobson, K. L. Rowlen, Chem. Phys. Lett. 2005, 401, 52.
[19] M. L. Jacobson, K. L. Rowlen, J. Phys. Chem. B 2006, 110, 19491.
[20] L. A. Peyser, T. H. Hee, R. M. Dickson, J. Phys. Chem. B 2002, 106,
7725.
[21] A. Mooradian, Phys. Rev. Lett. 1969, 22, 185.
[22] A. Subrahmanyam, P. Suman Kumar, IETE J. Res. 2006, 52, 365.
[23] E. C. Leru, P. G. Etchegoin, Principles of Surface-enhanced Raman
Spectroscopy and Related Plasmonic Effect, Elsevier: The Netherlands,
2009.
[24] C.-M. Chuang, M.-C. Wu, W.-F. Su, K.-C. Cheng, Y.-F. Chen, Appl. Phys.
Lett. 2006, 89, 061912.
[25] J. C. Tsang, J. E. Demuth, P. N. Sanda, J. R. Kirtley, Chem. Phys. Lett.
1980, 76, 54.
[26] W. A. Weimer, M. J. Dyer, Appl. Phys. Lett. 2001, 79, 3164.
[27] C. E. Taylor, S. D. Garvey, J. E. Pemberton, Anal. Chem. 1996, 68,
2401.
[28] I. Mrozek, C. Petten Kofer, A. Otto, Surf. Sci. 1990, 238, 192.
[29] A. Otto, Surf. Sci. 1978, 75, L392.
[30] D. Lin-Vien, N. Colthup, W. Fately, J. Grasselli, The Handbook of
Infrared and Raman Characteristic Frequencies of Organic Molecules,
Academic Press: New York, 1991.
[31] Y. W. Alsmeyer, R. L. McCreery, Anal. Chem. 1991, 63, 1289.
[32] X. Q. Cui, C. M. Li, H. Bao, X. Zheng, J. Zang, C. P. Ooi, J. Guo, J. Phys.
Chem. C 2008, 112, 10730.
[33] K.-I. Yoshida, T. Itoh, V. Biju, M. Ishikawa, Y. Ozaki, Phys. Rev. B 2009,
79, 085419.
1509
J. Raman Spectrosc. 2011, 42, 1505–1509
c 2011 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/jrs
Applied Surface Science 257 (2011) 3075–3080
Contents lists available at ScienceDirect
Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
Thickness dependent physical and photocatalytic properties of ITO thin films
prepared by reactive DC magnetron sputtering
K. Jagadeesh Kumar, N. Ravi Chandra Raju, A. Subrahmanyam ∗
Semiconductor Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e
i n f o
Article history:
Received 25 August 2010
Received in revised form 22 October 2010
Accepted 22 October 2010
Available online 30 October 2010
Keywords:
ITO thin films
Magnetron sputtering
Photocatalysis
Thickness
Work function
a b s t r a c t
Transparent and conducting indium tin oxide (ITO) thin films were deposited on glass substrates by
reactive DC magnetron sputtering at room temperature. The effect of thickness (165–1175 nm) on the
physical (structural, optical, electrical) and photo catalytic properties of ITO thin films were investigated
systematically. It is observed that with increasing thickness (i) the films turn from amorphous to polycrystalline with a preferential orientation along (4 4 0) direction, (ii) the average grain size and RMS
roughness increases from 35 nm to 100 nm and 2.3 nm to 8.6 nm respectively, (iii) the optical band gap
decreases from 3.65 eV to 3.45 eV and (iv) the relative density (calculated from the refractive index data)
decreases. Four probe and Hall effect measurements show a low resistivity (4.5 × 10−4 cm), mobility
(26 cm2 /V s) and high carrier concentration (5.3 × 1020 cm−3 ) values for film with a thickness 545 nm. The
work function of ITO films measured by Kelvin probe method varies with thickness. The photocatalytic
activity (PCA) of ITO thin films was studied by the degradation of Rhodamine B in water; highest PCA is
shown for the films of 545 nm thickness. Present work shows that the ITO is a promising photocatalytic
material for the degradation of organic compounds.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Indium tin oxide (ITO) is a degenerate, direct and wide band
gap semiconductor; extensively used in flat panel displays, electro chromic devices, solar cells, anti reflection coatings and gas
sensor applications [1–5] because of its unique transparent and
conducting properties when compared to other transparent conducting oxide films such as SnO2 and ZnO [6]. ITO is formed by
substitutional doping of tin (Sn) in the indium oxide (In2 O3 ) lattice,
which replaces the In3+ atoms from the cubic bixbyite structure of
In2 O3 . The high conductivity of ITO is due to the contribution of
substitutional Sn and oxygen vacancies [7].
ITO thin films are grown by several deposition techniques such
as electron beam evaporation, DC/RF magnetron sputtering, pulsed
laser deposition, ion beam deposition, reactive thermal evaporation, spray-pyrolysis and chemical vapour deposition [8–15].
Among these techniques, magnetron sputtering is industrially
viable and widely used to produce large area and good quality
films. The properties of ITO films are strongly dependent on the
process parameters like oxygen partial pressure, substrate temperature, deposition rate, target-substrate distance and the nature of
the substrates (glass, poly-ethylene, etc.). The dependence of these
∗ Corresponding author. Tel.: +91 44 22575876; fax: +91 44 22574852.
E-mail address: [email protected] (A. Subrahmanyam).
0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2010.10.119
parameters on the film growth has been well established [14–19].
Besides the process parameters, the thickness also influences the
properties of ITO films [20–26].
In order to explore the oxidative photocatalytic property of
these ITO thin films, one needs large surface area and sufficient
number of photo-generated holes onto the surface. The photogeneration of carriers depends upon the absorption coefficient and
the depth of penetration of the light in the semiconductor. Thus, one
parameter (from among the several other parameters) that influences the efficacy of photocatalytic activity is the film thickness
[27,28].
In the present work, we report the effect of thickness on the
structural, electrical, optical and photocatalytic properties of ITO
thin films prepared at room temperature by reactive DC magnetron
sputtering on glass substrates. An attempt has been made to understand the relation between physical and photocatalytic properties
of ITO thin films.
2. Experimental details
2.1. Thin film preparation
The ITO thin films have been prepared at room temperature (300 K) by reactive DC magnetron sputtering from a metallic
(In:Sn – 90:10 wt%, 99.99% purity) rectangular (15 in. × 5 in.) target in a commercial sputtering system (ANELVA model SPC-530H).
The sputter chamber is initially evacuated to a base pressure of
3076
K.J. Kumar et al. / Applied Surface Science 257 (2011) 3075–3080
Fig. 1. Chemical structure of the Rhodamine B-dye molecule.
3.0 × 10−6 Torr. The flow rates of argon (sputter gas) and oxygen
(reactive gas) are controlled through independent mass flow controllers (MKS model-1179A); the partial pressure of oxygen (ppo)
is maintained at 3.2 × 10−4 Torr. The chamber pressure during the
sputter process is kept at 2.2 × 10−3 Torr.
The target was powered through an arc free magnetron power
supply (Advanced Energy, model MDX-10K) with a power density
of 0.20 W/cm2 ; the low power density is intentional (our experience is that such low power densities give very smooth surface and
preferential orientation of the grains). The growth rate of the films
is ∼0.3 nm/s. The target to substrate distance was kept constant
at 6.0 cm. Prior to the deposition, the soda lime glass substrates
(2.5 cm × 2.5 cm) were cleaned with a dilute detergent solution,
chromic acid, rinsed in de-ionized water, finally dried with nitrogen
(N2 ) gas respectively and then introduced into the growth chamber.
In order to vary the thickness of the ITO films, the deposition time
is varied between 10 min and 60 min in steps of 10 min (±15 s);
all the other growth conditions are kept nearly identical. The samples are labeled as S1–S6, S1 corresponding to a deposition time
of 10 min (thickness ts1 = 165 nm) and S6 (ts6 = 1175 nm), 60 min.
Before each deposition, the target was pre-sputtered with argon gas
for 2 min in order to remove the surface oxide layer and to ensure
the near virgin state of the target. All the data reported have shown
reproducibility within the experimental error.
2.2. Thin film characterization
The amorphous/crystalline nature of the ITO thin films was
investigated at 300 K by X-ray diffraction (Philips X’pert Pro) with
Cu K␣ radiation ( = 1.5406 Å). The surface morphology of the
films was observed by atomic force microscopy (Digital instruments Nanoscope IV) in contact mode. The optical transmittance
measurements were carried-out with a UV–vis double beam
spectrophotometer (Jasco V-570) in the wavelength region of
300–2500 nm at normal incidence, taking cleaned soda lime glass as
reference. The thickness and refractive index (over the wavelength
range 400–800 nm) of the ITO films were measured by reflectometry technique (Filmetrics F-20) with an accuracy of ±5 nm and
±5% respectively. The four probe electrical resistivity and Hall
Fig. 2. The X-ray diffraction pattern of the ITO films for different thicknesses.
Effect measurements were carried out at 300 K (Lakeshore, Model
7604). The surface work function of the ITO films was measured
with an accuracy of 1 meV using Kelvin probe technique [29]. An
attempt has been made on ITO films to measure the photoconductivity (Keithley SMU 238), photo-Hall effect (Lakeshore -7604) and
photo-Kelvin probe with 254 nm UV excitation.
2.3. Characterization of photocatalytic property
The photocatalytic activity (PCA) of ITO thin films was studied by the degradation of Rhodamine-B (RhB) dye. RhB is a very
stable, non-volatile dye (Fig. 1), widely used in the textile industry. The experiments were conducted with 12.0 ml of RhB solution
(2 × 10−5 M) in a Petri dish. The ITO samples (2.5 cm × 2.5 cm) were
immersed in RhB solution and irradiated with the 6 W monochromatic UV source of 254 nm wavelength (Sankyo Denki-G6T5). The
distance between the samples and the UV lamp is 8.5 cm. The
photon flux incident on the catalyst was measured using Power
meter (Thor labs – PM100A, detector: S120VC) and it is found to be
1.78 × 1016 photons/cm2 /s. The concentration change of RhB was
monitored in the visible absorbance intensity at 554 nm as a function of irradiation time using a spectrophotometer (Jasco V-570)
and the number of RhB molecules degraded/oxidized by photocatalytic action is evaluated. For comparison, the photocatalytic
activity of a commercial ITO sample (of thickness 400 nm) has also
been measured.
3. Results and discussion
3.1. Structural properties
Fig. 2 shows the XRD pattern of the ITO thin films for different
thicknesses. For sample S1 (ts1 = 165 nm); the absence of crystalline
Table 1
Summary of structural and optical properties of ITO thin films prepared by reactive DC magnetron sputtering.
Sample
S1
S2
S3
S4
S5
S6
Commercial ITO
Thickness (nm)
165
380
545
732
950
1175
400
XRD crystallite
size (nm)
–
45
48
47
37
38
–
AFM
Grain size (nm)
RMS roughness (nm)
–
35
50
71
83
100
116
–
2.3
3.3
5.0
8.6
8.3
1.8
Optical band gap (eV)
Relative density
(%) @ 550 nm
3.65
3.60
3.53
3.50
3.47
3.45
3.50
0.94
0.93
0.93
0.91
0.90
0.88
0.90
K.J. Kumar et al. / Applied Surface Science 257 (2011) 3075–3080
3077
Fig. 3. AFM-images (2 ␮m × 2 ␮m) of the ITO thin films for different thicknesses.
behavior is possibly due to X-ray amorphous nature of the sample. As thickness increases, the S2 (ts2 = 380 nm) sample shows
diffraction peak along the (2 2 2) and (4 4 0) directions, the (2 2 2)
peak has the preferred orientation (maximum intensity). Further increase in the thickness (S3–S6, ts3 = 545 nm; ts6 = 1175 nm)
show additional diffraction peaks indicating the formation of
crystallites in (2 1 1), (3 2 1), (4 0 0), (4 1 1), (4 3 1), (5 4 1), (6 2 2)
and (6 3 1) orientations with a preferred orientation along (4 4 0)
direction. These changes in the ITO crystalline growth on glass
substrates prepared at room temperature (300 K) and their orientation are in good agreement with the reported values [26].
The average crystallite size ‘D’ was calculated using Scherrer
formula [30],
D=
0.9
ˇ cos (1)
3078
K.J. Kumar et al. / Applied Surface Science 257 (2011) 3075–3080
F-20). The obtained refractive index values of the films were in
agreement with the reported values [23]. The relative density: the
ratio between the densities of the film and bulk were calculated
using Lorentz–Lorentz relationship [31], given in Table 1.
P=
Fig. 4. The optical transmission spectra of ITO thin films for different thicknesses.
where = 0.154 nm and ˇ is the full width at half maximum
(FWHM) of the (4 4 0) peak at the diffraction angle of 2. The calculated values of the crystallite sizes are listed in Table 1.
The surface morphology of ITO thin films with different thicknesses was observed by atomic force microscopy (AFM). All the
films have smooth surface morphology (as anticipated due to the
low power density) and Fig. 3 shows the AFM images (scan area:
2 ␮m × 2 ␮m) of the ITO thin films (S2–S6). The average grain size
and RMS roughness increases with thickness (Table 1). The increase
in roughness is due to the aggregation of grains or the increased
crystallinity with increasing thickness.
3.2. Optical properties
Fig. 4 shows the optical transmission spectra of the ITO thin
films for S1–S6. It is seen that the transmission edge shifts towards
longer wavelengths (red shift) with increasing thickness. All the
films show a good transparency (∼80%) in the visible region and
the transmission decreases, slowly in the visible region and very
fast in the near infrared (NIR) region with thickness. The continuous decrease of transmission in the NIR region is attributed to free
carrier absorption. The optical band gap (Eg ) was evaluated from
Tauc plot for direct allowed transitions [(˛h)2 vs. (h)] and the
values are given in Table 1.
Fig. 5 shows the refractive index (n) of ITO films with different
thicknesses measured using reflectometry technique (Filmetrics
f
b
=
n2f − 1
n2f + 2
n2b + 2
n2b − 1
(2)
where f , b are the film and bulk density for ITO thin films. nf
is the film refractive index, nb = 2.19 is the bulk refractive index
at 550 nm [32]. Though the relationship (2) is derived from the
microscopic theory of Clausius–Mossosti, it may be applied to the
present case for the thin films as an approximation (and for a physical understanding). The decrease in relative density of the films
with increasing thickness may be attributed to the decrease in packing fraction (equivalently, enhancement in the porous nature). The
pore size (the surface area) plays an important role in determining
the photocatalytic action of a catalyst.
3.3. Electrical properties
The electrical properties of ITO thin film depend upon the oxygen ion vacancies and the concentration density of substitutional
tin in the film. There is a trade off between carrier concentration
and mobility for achieving the low resistivity [33]. The resistivity, carrier concentration and mobility values of the ITO thin films
were evaluated by four probe conductivity measurements and Hall
effect, listed in Table 2. The low resistivity of 4.5 × 10−4 cm,
high carrier concentration of 5.3 × 1020 cm−3 and a low mobility
of 26 cm2 /V s is observed for a thickness of 545 nm.
The electrical transport, in general, is analyzed through the different scattering mechanisms operative in these ITO thin films.
Following the earlier analysis of scattering mechanisms [34,35], it
is possible to infer that the grain boundary scattering in the present
investigation has negligible contribution (because of the mean free
path of the electrons ( ∼ 4 nm) in the ITO films is much less than
the grain size (Table 1)). The variation in the Hall mobility with the
thickness in these ITO films being very small, it may be inferred
that the ionized impurity density along the thickness is essentially
constant; consequently, the ionized impurity scattering seems to
be fairly constant [34]. The surface scattering, dependant on the
surface roughness, may also contribute to the electrical transport
in these ITO thin films (Table 1).
The surface work function of the ITO thin films was measured
from the contact potential difference (CPD) measurements using
Kelvin probe method [29]. All the measurements were performed
in the ambient atmosphere and at room temperature. The stainless
steel electrode (work function ϕR = 4.83 eV) was used as reference
and the work function of the ITO thin films (ϕS ) was calculated using
the following equation,
qVCPD = ϕR − ϕS
(3)
The work function values are presented in Table 2.
3.4. Photocatalytic degradation
Fig. 5. Variation of refractive index as a function of wavelength for different thicknesses of ITO thin films.
The photocatalytic activity (PCA) of ITO thin films deposited
at oxygen pressure of 2.2 × 10−3 Torr (and power density of
0.2 W/cm2 ) increases with thickness up to 545 nm and further
increase in the thickness resulting in a decrease in the activity.
This observation is in good agreement with the results reported
by Nam et al. [36] and Chen et al. [28]. This enhancement in PCA
may be attributed to (i) the enhancement in the surface area (both
by the surface roughness and by the increase in the pore size
with thickness) and (ii) due to the complete absorption of incident
K.J. Kumar et al. / Applied Surface Science 257 (2011) 3075–3080
3079
Table 2
Summary of electrical and photocatalytic properties of ITO thin films prepared by reactive DC magnetron sputtering.
Sample
Work function (eV)
Resistivity
(10−4 cm)
Mobility (cm2 /V s)
Carrier
concentration
(1020 cm−3 )
No. of
Rhodamine B
molecules
oxidized
(1013 cm−2 s−1 )
Rate constant
(k)
(10−4 min−1 )
S1
S2
S3
S4
S5
S6
Commercial ITO
4.84
4.86
4.76
4.84
4.89
4.90
4.84
7.4
5.5
4.5
5.0
5.3
5.2
8.5
30
29
26
31
30
28
26
2.8
3.9
5.3
4.0
3.9
4.3
2.6
0.52
1.42
2.15
1.86
1.58
1.72
0.81
0.66
3.78
6.00
5.10
3.26
3.11
1.92
Fig. 6. Optical absorption co-efficient ˛, at 350 nm for ITO thin films as a function
of thickness.
light (254 nm) across the thickness to produce the photo generated electron–hole pairs. The incident light produces maximum
number of electron–hole pairs for an optimum thickness (Fig. 6);
with further increase in the thickness, the generation rate, though,
Fig. 7. Photocatalytic degradation of Rhodamine B for different thicknesses of ITO
thin films. The inset shows the absorption spectra of S3 sample for different time
intervals.
Fig. 8. Comparison of photocatalytic activities of ITO (S3), ITO (S2), commercial ITO
and Rhodamine B.
remains constant, the migration length of the carriers to the surface
of the catalyst (more precisely, the semiconductor–liquid interface)
increases where in the carriers experience higher recombination
rates, resulting in a decrease in the photocatalytic activity.
Fig. 7 shows the photocatalytic degradation curves of ITO thin
films for different thicknesses. The inset shows the degradation of
Rhodamine B for S3 sample at different time intervals. The rate
constant for the photocatalytic action was calculated using pseudo
Fig. 9. Energy band diagram of Rhodamine B and ITO sample.
3080
K.J. Kumar et al. / Applied Surface Science 257 (2011) 3075–3080
first order reaction [37],
ln
C 0
C
= kt
4. Conclusions
(4)
where C0 and C are concentration of RhB at t = 0 min and after
t = t min of catalytic action and k is the rate constant respectively.
The rate constants for different thicknesses are given in Table 2.
Fig. 8 shows the comparison of photocatalytic degradation of RhB
solution for the ITO prepared in the present study (sample S3, S2),
commercial ITO thin film and RhB without catalyst (ITO thin film). It
is seen that the commercial ITO film has shown an activity less than
that of ITO-S2 sample (of comparable thickness), may be because
the commercial ITO is prepared at a different substrate temperature.
The efficacy of the PCA depends upon the effective number of
holes (having velocity more than the threshold) participating in the
oxidation process. Typically, the photo-excited valence-band holes
at the semiconductor–liquid interface initiate the oxidation process
with the OH– group. The degradation process steps are:
h
ITO−→ITO(h+ + e− )
(5)
O2 + e− → O−
2
(6)
+
OH− + h → OH∗
(7)
OH∗ + RH → R∗ + H2 O → CO2 + H2 O + mineral acids
(8)
where RH represents the organic compound (Rhodamine B) to be
degraded ITO in contact with RhB forms a semiconductor–liquid
interface. The charge transfer process across the interface is mainly
governed by the barrier potential in the semiconductor and the
redox potentials of the liquid. The charge transfer process across the
semiconductor–liquid interface is quite complex [38,39]. Hinckley
and Haneman [40] have proposed a general theory for the charge
transfer parameters at the semiconductor–liquid interface. These
theories are of limited applicability as ITO being a degenerate wide
band gap semiconductor, the photo-generation of electron–hole
pairs influence the valence band significantly. From the energy of
the lowest unoccupied orbital (LUMO) and energy of the highest
occupied orbital (HOMO) of RhB and ITO [41,42], a band diagram of the semiconductor–liquid junction: ITO/RhB is constructed
(Fig. 9).
An attempt has been made to quantify the PC activity in ITO
thin films. We have calculated the number of RhB molecules
degraded in the photocatalytic process. The photon flux of
1.78 × 1016 photons/cm2 /s (of 254 nm wavelength) is incident
through the liquid (RhB) to the catalyst: ITO. Assuming a quantum
efficiency of 60% (since ITO is direct gap semiconductor), the e–h
pair generation rate is about 1 × 1015 photons/cm2 /s. The electron
concentration, being of the order of 1020 (from Hall measurements),
the quasi static Fermi level (during illumination of 254 nm light) is
mostly controlled by the photo generated holes, consequently, only
valence band bending is affected by the incident light.
Thus, 1015 holes/cm2 /s generated in the bulk of the thin film
participated in the PCA and have oxidized about 1013 molecules
of RhB/cm2 /s (Table 2); the number of RhB molecules degraded in
PCA was calculated using the optical density (absorbance) and the
molar concentration. This efficacy of ITO, considering the heavy
non-stoichiometry and surface defects, is quite significant. For a
realistic quantification, one needs the charge density at the surface of the semiconductor. We have made an attempt to quantify
the PCA results by measuring the (i) photoconductivity, (ii) photoexcited (254 nm wavelength) Hall effect and photo-excited Kelvin
probe measurements on these ITO samples to estimate the photogenerated hole (charge) density; however, our instrumentation
could not detect the changes.
The present work has shown that with increase in the thickness,
the reactive DC magnetron sputtered ITO films exhibit significant
changes in the structural (crystallization, RMS roughness), optical
(absorption, refractive index) and electrical (carrier concentration,
work function) properties. The increase in the grain size and RMS
roughness, along with the relative density (calculated from refractive index data) has shown (by the photocatalytic activity of ITO
thin films) that the surface properties and the pore size in the
films are influenced by the thickness. The high photocatalytic activity is observed for an optimum film thickness of 545 nm. One of
the important conclusions of the study is that ITO can be used as
promising photocatalytic material.
References
[1] H. Kim, A. Pique, J.S. Horwitz, H. Mattoussi, H. Murata, Z.H. Kafafi, D.B. Chrisey,
Appl. Phys. Lett. 74 (1999) 3444–3446.
[2] V. Teixeira, H.N. Cui, L.J. Meng, E. Fortunato, R. Martins, Thin Solid Films 420
(2002) 70–75.
[3] J.B. Chu, S.M. Huang, H.B. Zhu, X.B. Xu, Z. Sun, Y.W. Chen, F.Q. Huang, J. NonCryst. Solids 354 (2008) 5480–5484.
[4] H.P. Lobl, M. Huppertz, D. Mergel, Surf. Coat. Technol. 82 (1996) 90–98.
[5] T. Sako, A. Ohmi, H. Yumoto, K. Nishiyama, Surf. Coat. Technol. 142/144 (2001)
781–785.
[6] J.W. Bae, H.J. Kim, J.S. Kim, N.E. Lee, G.Y. Yeom, Vacuum 56 (2000) 77–81.
[7] E. Terzini, P. Thilakan, C. Minarini, Mater. Sci. Eng. B 77 (2000) 110–114.
[8] X.D. Liu, E.Y. Jiang, D.X. Zhang, J. Appl. Phys. 104 (2008), 073711 (5pp).
[9] H.C. Lee, J.Y. Seo, Y.W. Choi, D.W. Lee, Vacuum 72 (2004) 269–276.
[10] J.O. Park, J.H. Lee, J.J. Kim, S.H. Cho, Y.K. Cho, Thin Solid Films 474 (2005)
127–132.
[11] J.H. Kim, K.A. Jeon, G.H. Kim, S.Y. Lee, Appl. Surf. Sci. 252 (2006) 4834–4837.
[12] D. Kim, Y. Han, J.S. Cho, S.K. Koh, Thin Solid Films 377/378 (2000) 81–86.
[13] A. Amaral, P. Brogueira, C.N. de Carvalho, Surf. Coat. Technol. 125 (2000)
151–156.
[14] A. El Hichou, A. Kachouane, J.L. Bubendorff, M. Addou, J. Ebothe, M. Troyon, A.
Bougrine, Thin Solid Films 458 (2004) 263–268.
[15] Y.C. Park, Y.S. Kim, H.K. Seo, S.G. Ansari, H.S. Shin, Surf. Coat. Technol. 161 (2000)
62–69.
[16] C. Guillen, J. Herrero, J. Appl. Phys. 101 (2007), 073514 (7pp).
[17] C.G. Choi, K. No, W.J. Lee, H.G. Kim, S.O. Jung, W.J. Lee, W.S. Kim, S.J. Kim, C.
Yoon, Thin Solid Films 258 (1995) 274–278.
[18] L. Kerkache, A. Layadi, A. Mosser, J. Alloys Compd. 485 (2009) 46–50.
[19] C.H. Yi, Y. Shigesato, I. Yasui, S. Takaki, Jpn. J. Appl. Phys. 34 (1995) L244–
L247.
[20] Z. Qiao, R. Latz, D. Mergel, Thin solid films 466 (2004) 250–258.
[21] M.Z. Gao, R. Job, D.S. Xue, W.R. Fahrner, Chin. Phys. Lett. 25 (2008) 1380–1383.
[22] L. Hao, X. Diao, H. Xu, B. Gu, T. Wang, Appl. Surf. Sci. 254 (2008) 3504–3508.
[23] C. Guillen, J. Herrero, Semicond. Sci. Technol. 23 (2008), 075002 (5pp).
[24] D.H. Kim, M.R. Park, H.J. Lee, G.H. Lee, Appl. Surf. Sci. 253 (2006) 409–411.
[25] H. Kim, J.S. Horwitz, G. Kushto, A. Pique, Z.H. Kafafi, C.M. Gilmore, D.B. Chrisey,
J. Appl. Phys. 88 (2000) 6021–6025.
[26] C. Liu, T. Matsutani, T. Asanuma, K. Murai, M. Kiuchi, E. Alves, M. Reis, J. Appl.
Phys. 93 (2003) 2262–2266.
[27] J. Sheng, J. Karasawa, T. Fukami, J. Mater. Sci. Lett. 16 (1997) 1709–1711.
[28] D. Chen, F. Li, A.K. Ray, AIChE J. 46 (2000) 1034–1045.
[29] A. Subrahmanyam, C. Suresh Kumar, Kelvin Probe for Surface Engineering, CRC
Press, Florida, 2009.
[30] B.D. Cullity, Elements of X-ray Diffraction, 2nd ed., Additson-Wesley, Reading,
MA, 1978.
[31] W. Heitmann, Thin Solid Films 5 (1970) 61–67.
[32] J.K. Kim, C. Sameer, M.F. Schubert, E.F. Schubert, A.J. Fischer, M.H. Crawford, J.
Cho, H. Kim, C. Sone, Adv. Mater. 20 (2008) 801–804.
[33] H. Kim, J.S. Horwitz, Appl. Phys. Lett. 78 (2001) 1050–1052.
[34] Y. Shigesato, S. Takaki, T. Haranou, Appl. Surf. Sci. 48/49 (1991) 269–275.
[35] P. Manivannan, A. Subrahmanyam, J. Phys. D: Appl. Phys. 26 (1993) 1510–1515.
[36] H.J. Nam, T. Amemiya, M. Murabayashi, K. Itho, J. Phys. Chem. B 108 (2004)
8254–8259.
[37] R. Pan, S. Pan, J. Zhou, Y. Wu, Appl. Surf. Sci. 255 (2009) 3642–3647.
[38] A. Wieckowski (Ed.), Interfacial Electrochemistry: Theory, Experiment and
Applications, Marcel Dekker Inc, New York, 1999.
[39] N.S. Lewis, Annu. Rev. Phys. Chem. 42 (1991) 543–580.
[40] S. Hinckley, D. Haneman, Appl. Surf. Sci. 22/23 (1985) 1075–1082.
[41] P. Ruankham, C.S. Kung, N. Mangkorntong, P. Mangkorntong, S. Choopun, CMU.
J. Nat. Sci. 7 (1) (2008) 177–183.
[42] W. Lee, R.S. Mane, S.H. Lee, S.H. Han, Electrochem. Commun. 9 (2007)
1502–1507.
This article appeared in a journal published by Elsevier. The attached
copy is furnished to the author for internal non-commercial research
and education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling or
licensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of the
article (e.g. in Word or Tex form) to their personal website or
institutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies are
encouraged to visit:
http://www.elsevier.com/copyright
Author's personal copy
Surface & Coatings Technology 205 (2011) S261–S264
Contents lists available at ScienceDirect
Surface & Coatings Technology
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 / s u r f c o a t
Properties of pulsed reactive DC magnetron sputtered tantalum oxide (Ta2O5) thin
films for photocatalysis
K. Jagadeesh Kumar ⁎, N. Ravi Chandra Raju, A. Subrahmanyam
Department of Physics, Indian Institute of Technology Madras, Chennai-600036, India
a r t i c l e
i n f o
Article history:
Received 15 September 2010
Accepted in revised form 16 March 2011
Available online 23 March 2011
Keywords:
Photocatalysis
Ta2O5
Work function
Rhodamine B
Pulsed sputtering
a b s t r a c t
Tantalum oxide (Ta2O5) is a wide band gap semiconductor, known for a wide range of applications in many
areas. The present investigation reports the effect of pulsing on the physical and photocatalytic properties of
Ta2O5 thin films. The samples are prepared at room temperature on quartz and ITO substrates by pulsed
reactive direct current (DC) magnetron sputtering. The pulsing frequency is varied between 5 kHz and
100 kHz. The photocatalytic activity is measured by the Rhodamine B dye. The thicknesses of all films were
kept constant of ~ 500 nm. The microstructure obtained by X-ray diffraction is amorphous for all the samples.
A lowest surface roughness of 4.62 nm for 50 kHz pulsing frequency is seen in atomic force microscopy
measurements. The calculated relative density, optical band gap and the surface work function varies with the
pulsing frequency. The sample prepared at 50 kHz pulsing frequency shows high photocatalytic activity:
2.59 × 1013 number of Rhodamine B molecules were oxidized per an incident photon flux of 1.78 × 1016 photons/s at 254 nm. The pulsed-Ta2O5 thin films were compared with continuous DC-Ta2O5 films and showed
that pulsing (target power) gives the enhanced film properties, leading to better photocatalytic properties.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Tantalum oxide (Ta2O5) is a wide band gap semiconductor having
very interesting properties like, high dielectric constant, high
refractive index, low optical absorption coefficient and high chemical
stability; these properties are being used in many applications,
memory devices, coatings on photographic lenses, electrochromic
devices, photocatalysis etc., just to name a few [1–4]. Ta2O5 is also a
bio-compatible material. Among the several oxide based photocatalysts, tantalum based compounds have attracted much attention and
are widely studied for the water treatment and hydrogen generation
from water. It is due to the relatively high conduction band (ECB) level
made up of Ta5d orbital and low valance band (EVB) level made up of
O2P orbital with respect to the standard hydrogen electrode (SHE)
potential [5].
Ta2O5 thin films were grown by various physical and chemical
vapour deposition techniques, such as electron beam evaporation,
pulsed laser deposition, reactive magnetron sputtering, ion beam
deposition, sol–gel method, and anodization [1–6]. Depending upon
the deposition techniques and deposition parameters, the films show
a variation in its physical and photocatalytic properties. The variation
in the photocatalytic activities of these thin films are related to its
physical properties, namely, the surface area, the surface roughness,
porosity, the crystalline nature etc. Among the above mentioned
⁎ Corresponding author. Tel.: + 91 44 22574865; fax: + 91 44 22574852.
E-mail address: [email protected] (K.J. Kumar).
0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2011.03.052
deposition techniques, reactive DC magnetron sputtering is widely
used to deposit large area thin films. Recently, pulsed reactive DC
magnetron sputtering is shown to yield high quality oxide films [7]. In
this pulsing technique, the polarity of the target power oscillates
between positive and negative potential, thus, reduces the formation
of insulating layer on the target during deposition. The combination of
pulsing parameters (reverse time, reverse voltage and the pulse
frequency) can influence significantly the microstructure and surface
properties of the thin films, consequently, the physical properties and
the photocatalytic properties [8–11].
In the present work, we report the effect of pulsing frequency (5–
100 kHz) on the physical and photocatalytic properties of reactive DC
magnetron sputtered Ta2O5 thin films deposited at room temperature
on quartz and ITO substrates; compared the properties with
continuous DC – Ta2O5 films.
2. Experimental details
The tantalum oxide thin films were prepared on quartz and ITO
coated glass substrates at room temperature from a metallic Ta
(99.999%) target in a commercial vacuum system (Hind Hivac, Model
19F9). The base pressure of sputtering chamber was 1 × 10−6 Torr.
The flow rates of argon (sputter gas) and oxygen (reactive gas) are
controlled through independent mass flow controllers (UNIT model –
1100); the working pressure of the chamber is kept at 0.1 mTorr. The
target to substrate distance was kept at 8 cm. When DC power was
applied to target (200 W) the reverse voltage and reverse time was
fixed at 15% and 2 μs, the pulsing frequency was varied from 5 to
Author's personal copy
S262
K.J. Kumar et al. / Surface & Coatings Technology 205 (2011) S261–S264
100 kHz. Deposition time was controlled to obtain a film thickness of
~ 500 nm (Table 1). All the growth conditions are kept nearly identical
while the pulsing frequency is varied.
During the sputtering process, the optical emission spectrum of
plasma is recorded using a fiber optic spectrophotometer (Ocean
Optics – HR 2000) interfaced to a computer. The amorphous/
crystalline nature of the Ta2O5 thin films were investigated by X-ray
diffraction (Philips X'pert Pro) with Cu Kα radiation (λ = 1.5406 Å).
The surface morphology of the films was observed by atomic force
microscopy (Digital instruments Nanoscope IV) in contact mode. The
optical transmittance measurements were carried-out with a UV–VISNIR double beam spectrophotometer (Jasco V-570) in the wavelength
region of 200–800 nm at normal incidence, taking cleaned quartz
plate as the reference. The thickness and refractive index (over the
wavelength range 400–800 nm) of the Ta2O5 films were measured by
reflectometry technique (Filmetrics F-20) with an accuracy of ±5 nm
and ±5% respectively. The surface work functions of Ta2O5 thin films
are measured from the contact potential difference (CPD) measurement using the Kelvin probe method with an accuracy of 1 meV. The
stainless steel electrode (work function φR = 4.83 eV) is used as
reference and the work function of the Ta2O5 thin films (φS) is
calculated using the relation: qVCPD = φR − φS [12].
The photocatalytic activity (PCA) of Ta2O5 thin films was studied
by the degradation of Rhodamine-B (RhB) dye. The experiments were
conducted with the Ta2O5 samples (3.9 cm × 0.9 cm) immersed in a
cuvette having 3.5 ml of RhB solution (2 × 10−5 M); irradiated with
254 nm wavelength UV radiation (Sankyo Denki-G6T5). The distance
between the samples and UV lamp is 8.0 cm. The photon flux incident
on the catalyst was measured using Power meter (Thor labs –
PM100A, detector: S120VC) and it is found to be 1.78 × 1016 photons/
cm2/s. The concentration change of RhB was monitored in the visible
absorbance intensity at 554 nm as a function of irradiation time (in
intervals of 30 min and for a total time of 2 h) using a double beam
spectrophotometer (Jasco V-570) and the number of RhB molecules
degraded in the photocatalytic action is evaluated. For comparison,
the PCA of as-prepared continuous DC magnetron sputtered Ta2O5 is
also measured.
3. Results and discussion
The growth parameter in the present investigation is the pulsing
frequency in DC magnetron sputtering. An increase in the pulsing
frequency from 5 kHz to 100 kHz results in a significant decrease in
the deposition rate (Fig. 1). It is mainly attributed to the rise in the
power consumption required for the pulse generation as the pulse
frequency increases resulting a decrease in the deposition rate. The
inset of Fig. 1 shows typical optical emission spectra (OES) of the
plasma at 50 kHz pulsing; the intense lines are indexed using
SPECLINE database. OES is used mainly to ascertain the repeatability
of the growth parameters. It is observed that the intensity of the peaks
for each of the identified lines increase with increasing pulsing
frequency.
Since the films are grown at room temperature, the X-ray
diffraction of Ta2O5 thin films as a function of pulsing frequency on
quartz and ITO coated glass substrates show amorphous nature; no
significant influence of pulsing frequency (figure not presented). All
the films have shown smooth surface morphologies, Fig. 2 shows the
AFM image (scan area: 2 μm × 2 μm) for 50 kHz sample; the RMS
roughness decreases with increasing pulsing frequency: from 5.39 nm
at 5 kHz to 4.80 nm at 100 kHz. However, lowest RMS roughness of
4.62 nm is seen at 50 kHz.
Fig. 3 shows the transmission spectra of Ta2O5 samples prepared
on different substrates at 50 kHz, 100 kHz respectively; the samples
have shown good transparency (~80%) in the visible region; the sharp
decrease in the transparency in UV region is due to the fundamental
absorption edge of semiconductor. The optical band gap (Eg) was
evaluated from Tauc plot for direct allowed transitions [(αhν)2 vs. hν]
and the values are given in Table 1.
The measured refractive index (Filmetrics – F20) increases
from 1.95 to 2.10 in-case of quartz substrate and from 2.01 to 2.12
for ITO substrate with increasing pulsing frequency up to 50 kHz,
further increase in pulse frequency results in a decrement in the
refractive index. The refractive index data is used to calculate the
relative film density (Table
1) using
Lorentz-Lorenz relationship [13],
!
!
n2f 1
n2b þ 2
ρf
i.e.,P ¼
¼ 2
, where ρf, ρb are the film and bulk
ρb
nf þ 2
n2b 1
density for Ta2O5 thin films. nf is the film refractive index, nb = 2.16 is
the bulk refractive index at 550 nm [14]. Though the above
relationship is derived from the microscopic theory of Clausius–
Mossosti, it may be applied to the present case for the thin films as an
approximation (and for a physical understanding). The relative film
density is a qualitative measure of the porosity (a measure of the
surface area) of the film which is a very important parameter for
photocatalytic experiments.
The surface work function (and the red-ox potential of the liquid)
is the one, which determines the (photo-generated) charge transfer
process across the semiconductor – liquid interface in photocatalytic
action. The measured surface work function by Kelvin probe method
is presented in Table 1. The work function for Ta2O5 films grown on
quartz substrates show higher values compared to those grown on
ITO substrates. Also the pulsing frequency introduces measurable
changes in the surface work function of Ta2O5 thin films.
Fig. 4 shows the photocatalytic degradation curves of Ta2O5 thin
films deposited on quartz substrates for different pulsing frequencies.
For comparison, the photocatalytic degradation of RhB solution for the
continuous DC magnetron sputtered Ta2O5 and RhB without catalyst
(Ta2O5 thin film) is also shown in the Fig. 4. The inset of Fig. 4 shows
the degradation of Rhodamine B (RhB) for 50 kHz–Ta2O5/quartz thin
Table 1
Summary of optical, electrical and photocatalytic properties of Ta2O5 thin films prepared by pulsed reactive DC magnetron sputtering.
Sl. no Pulsing frequency Sample
(kHz)
1
DC
2
5
3
25
4
50
5
75
6
100
Ta2O5 – Q
Ta2O5 – ITO
Ta2O5 – Q
Ta2O5 – ITO
Ta2O5 – Q
Ta2O5 – ITO
Ta2O5 – Q
Ta2O5 – ITO
Ta2O5 – Q
Ta2O5 – ITO
Ta2O5 – Q
Ta2O5 – ITO
Thickness Relative density @
(nm)
550 nm
Optical band gap Work function
(eV)
(eV)
504.0
501.7
513.1
507.5
513.4
503.8
536.9
525.6
537.8
516.4
525.9
530.0
4.30
3.52
4.35
3.33
4.34
3.35
4.30
3.36
4.33
3.36
4.32
3.36
0.93
0.99
0.87
0.91
0.92
0.94
0.96
0.98
0.95
0.95
0.94
0.95
–
5.44
4.97
5.48
4.94
5.35
4.90
5.04
5.01
5.06
5.01
5.22
No. of molecules degraded
Photocatalytic rate
constant (10−3 min−1) (1013 cm−2 s−1)
3.80
3.70
3.68
2.30
4.29
3.19
5.43
3.22
4.57
2.80
2.88
2.31
2.00
1.93
1.97
1.32
2.22
2.02
2.59
1.73
2.27
1.58
1.58
1.30
Author's personal copy
K.J. Kumar et al. / Surface & Coatings Technology 205 (2011) S261–S264
S263
Fig. 1. The variation of deposition rate as a function of pulsing frequencies for Ta2O5 thin
films. The inset shows the optical emission spectra (OES) of the pulsing plasma for
50 kHz.
Fig. 3. The transmission spectra of Ta2O5 films deposited on (a) quartz and (b) ITO
substrates for different pulsing frequencies.
film at different time intervals. The rate constant for the photocatalytic action, a measure of the photocatalytic activity of the
semiconductor, is calculated using pseudo first order reaction [15]:
1nð CC0 Þ ¼ kt, are listed in Table 1. where Co and C are concentration of
RhB at t = 0 min and after t = t minutes of catalytic action and k is the
rate constant respectively. It is seen that the sample at 50 kHz shows
high activity for both the substrates.
The efficacy of the photocatalytic activity (PCA) depends upon the
effective number of photo-generated electrons or holes (having
velocity more than the threshold) participating in the redox process
across the semiconductor – liquid interface. Typically, the photoexcited conduction band electrons, valence-band holes at the
semiconductor – liquid interface initiate the reduction – oxidation
process. The typical degradation process of RhB is as follows:
O2 þ e →O2
hυ
þ
Ta2 O5 → Ta2 O5 h þ e
ð1Þ
Fig. 2. AFM – images (2 μm × 2 μm) of the Ta2O5 films deposited on quartz substrate for
50 kHz pulsing frequency.
−
−
−
þ
ð2Þ
⁎
OH þ h →OH
⁎
⁎
OH þ RhB→R þ H2 O→CO2 þ H2 O þ mineral acids
ð3Þ
ð4Þ
We made an attempt to quantify the PCA in Ta2O5 thin films. Ta2O5
is an oxidative photocatalyst: only photo-generated holes participate
in the catalytic action. The number of RhB molecules degraded in the
photocatalytic process is calculated using the optical density (absorbance) and the molar concentration. The photon flux of
1.78 × 1016 photons/cm2 /s (of 254 nm wavelength) is incident
through the liquid (RhB) to the catalyst (Ta2O5). Assuming a quantum
efficiency of 60% (which is absolutely arbitrary but it is realistic
estimate since Ta2O5 is direct gap semiconductor), the e–h pair
generation rate is about 1 × 1015 photons/cm2/s. These photo-generated
holes after traversing through the bulk of the film (undergoing
recombination and scattering processes) reach the catalytic surface to
oxidize ~1013 molecules of RhB/cm2/s (Table 1). It is interesting to note
that pulsing did influence the PCA as shown in Table 1.
Fig. 4. The photocatalytic degradation curves of Ta2O5 thin films deposited on quartz
substrates for different pulsing frequencies. The inset shows the absorption spectra of
Rhodamine B for 50 kHz sample at different time intervals.
Author's personal copy
S264
K.J. Kumar et al. / Surface & Coatings Technology 205 (2011) S261–S264
Thus, the pulsing frequency (among the many other process
parameters) is anticipated to influence the microstructure (roughness), optical (relative density, band gap) and electrical (surface work
function) properties of samples grown on quartz and ITO substrates,
mainly because the pulsing is highly effective in reducing the arcing
during reactive sputtering [8–11]. One important observation is that
the substrate also influences the properties of the grown films, in
specific, the optical band gap of the Ta2O5 films at any given pulsing
frequency. The results show that an optimized film properties for the
high photocatalytic activity is observed at a pulsing frequency of
50 kHz. The high photocatalytic activity of Ta2O5 on quartz substrates
compared with the ITO substrates might be due to the differences in
the work function of both substrates, a charge-transfer across the
semiconductor – liquid interface. Also it is showed that pulsed
magnetron sputtering gives better photocatalytic properties in
comparison with the continuous DC – Ta2O5 films.
4. Conclusions
In conclusion, the room temperature deposited pulsed magnetron
sputtered Ta2O5 is a good photocatalytic material for the degradation
of organic compounds from water. These samples showed better
photocatalytic properties on quartz substrate and the high photocatalytic activity is obtained for the pulsing frequency of 50 kHz, with
optimized film properties.
References
[1] C. Bartic, H. Jansen, A. Campitelli, S. Borghs, Org. Electron. 3 (2002) 65.
[2] S. Boughaba, M.U. Islam, G.I. Sproule, M.J. Graham, Surf. Coat. Technol. 120–121
(1999) 757.
[3] C. Corbella, M. Vives, A. Pinyol, I. Porqueras, C. Person, E. Bertran, Solid State Ionics
165 (2003) 15.
[4] T. Sreethawong, S. Ngamsinlapasathian, Y. Suzuki, S. Yoshikawa, J. Mol. Catal. A
Chem. 235 (2005) 1.
[5] W.-J. Chun, A. Ishikawa, H. Fujisawa, T. Takata, J.N. Kondo, M. Hara, M. Kawai, Y.
Matsumoto, K. Domen, J. Phys. Chem. B 107 (2003) 1798.
[6] W. Wei, J.M. Macak, P. Schmuki, Electrochem. Commun. 10 (2008) 428.
[7] P.J. Kelly, R.D. Arnell, J. Vac. Sci. Technol. A 17 (3) (1999) 945.
[8] R.D. Arnell, P.J. Kelly, J.W. Bradley, Surf. Coat. Technol. 188–189 (2004) 158.
[9] J.W. Bradley, T. Welzel, J. Phys. D: Appl. Phys. 42 (2009) 0930018 (23 pp).
[10] I. Safi, Surf. Coat. Technol. 127 (2000) 203.
[11] J. Musil, J. Lestina, J. Vlcek, T. Tolg, J. Vac. Sci. Technol. A 19 (2) (2001) 420.
[12] A. Subrahmanyam, C. Suresh Kumar, Kelvin Probe for Surface Engineering, CRC
Press, Florida, 2009.
[13] W. Heitmann, Thin Solid Films 5 (1970) 61.
[14] E. Franke, C.L. Trimble, M.J. Devries, J.A. Woollam, M. Schubert, F. Frost, J. Appl.
Phys. 88 (2000) 5166.
[15] R. Pan, S. Pan, J. Zhou, Y. Wu, Appl. Surf. Sci. 255 (2009) 3642.