Photochromic Mechanism in a-WO3 Thin Films Based on Raman ...

Journal of the Korean Physical Society, Vol. 48, No. 6, June 2006, pp. 1657∼1660
<strong>Photochromic</strong> <strong>Mechanism</strong> <strong>in</strong> a-WO 3 Th<strong>in</strong> <strong>Films</strong> <strong>Based</strong>
on Raman Spectroscopic Studies
Kyungmoon Kim, Chungwon Seo and Hyeonsik Cheong ∗
Department of Physics, Sogang University, Seoul 121-742
Se-Hee Lee
National Renewable Energy Laboratory, Golden, CO 80401, U.S.A.
(Received 21 November 2005)
The photochromic mechanism <strong>in</strong> a-WO 3 th<strong>in</strong> films prepared by thermal evaporation has been
<strong>in</strong>vestigated us<strong>in</strong>g Raman scatter<strong>in</strong>g measurements. The Raman spectra of uv-irradiated th<strong>in</strong>
films show a weak peak at 450 cm −1 . This peak is known to be related to the gasochromic
and electochromic coloration and is attributed to the formation of W 5+ = O bonds. The I-V
characteristic of uv-irradiated a-WO 3 th<strong>in</strong> films was measured to study the change <strong>in</strong> the electrical
properties due to photochromic coloration. The uv-irradiated films show enhanced conductivity
due to excited carriers.
PACS numbers: 82.47.Tp, 78.66.Li, 78.30.Ly
Keywords: Photochromism, Tungsten oxide, Raman spectroscopy
I. INTRODUCTION
<strong>Photochromic</strong> materials, which change color upon exposure
to light, have been widely <strong>in</strong>vestigated because
of their potential technological applications <strong>in</strong> displays
and large-area optical devices [1–3]. Amorphous tungsten
oxide (a-WO 3 ) is a well known material exhibit<strong>in</strong>g
electrochromic and gasochromic properties and show<strong>in</strong>g
photochromic properties. It is the most widely studied
material for solid-state electrochromic device applications,
and its gasochromic properties, especially its sensitivity
to hydrogen gas, has made it the prime material
for hydrogen-sensor applications [4]. Because the change
<strong>in</strong> the optical transmittance can be measured remotely
by us<strong>in</strong>g optical fibers, the need for electric currents <strong>in</strong>
the sens<strong>in</strong>g area can be elim<strong>in</strong>ated [5]. Whereas electrochromic
devices, such as smart w<strong>in</strong>dows, need an external
applied voltage to operate [6], photochromic devices
do not require external electrical power to change
color; to achieve a color change, a light source with appropriate
wavelength is all that is needed. a-WO 3 th<strong>in</strong>
films are among the most promis<strong>in</strong>g candidates for photochromic
device applications [4,7–9].
The electrochromic and the gasochromic coloration
mechanisms of a-WO 3 th<strong>in</strong> films have been <strong>in</strong>vestigated
us<strong>in</strong>g <strong>in</strong>-situ Raman spectroscopy [10,11], and the two
phenomena are based on the same mechanism. The color
∗ Correspond<strong>in</strong>g Author: hcheong@sogang.ac.kr
change <strong>in</strong> an electrochromic film is directly related to the
double <strong>in</strong>jection/extraction of electrons and ions <strong>in</strong> the
films, which can be written <strong>in</strong> a simplified form as [12]
xM + + xe − + WO 3 ⇄ M x WO 3 ,
where M = H, Li, etc. The <strong>in</strong>jected electrons reduce W 6+
ions to W 5+ , and small polaron transitions between W 6+
and W 5+ ions are responsible for the color change [10,11].
In the photochromic process <strong>in</strong>duced by uv absorption,
electron-hole pairs are created, and these electron-hole
pairs may play a role similar to that of the electron-ion
pairs <strong>in</strong> the electrochromic or the gasochromic processes.
In this study, we used Raman scatter<strong>in</strong>g measurements
and I-V measurements on a-WO 3 films to <strong>in</strong>vestigate the
photochromic mechanism and the changes <strong>in</strong> the electrical
properties due to uv irradiation.
II. EXPERIMENT
The a-WO 3 films were prepared, by us<strong>in</strong>g vacuum
thermal evaporation, on polished sta<strong>in</strong>less steel for Raman
scatter<strong>in</strong>g measurements and on glass substrates
for I-V measurements. The films were deposited by us<strong>in</strong>g
thermal evaporation of WO 3 powder with a purity
of 99.995 %. The thickness of the samples was about
300 nm. Alum<strong>in</strong>ium electrodes were deposited on the
WO 3 layers for I-V measurements. The thickness of the
alum<strong>in</strong>ium layer was approximately 40 nm.
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Fig. 1. Raman spectroscopy setup for the photochoromism
study.
Figure 1 shows the Raman spectroscopy setup used <strong>in</strong>
this study. The Raman spectra were taken <strong>in</strong> the quasibackscatter<strong>in</strong>g
geometry us<strong>in</strong>g the 514.5-nm l<strong>in</strong>e of an Ar
ion laser with 50 mw focused to a l<strong>in</strong>e of 5 mm × 100 µm
as the excitation source. The scattered light was filtered
with a holographic edge filter, dispersed by a Spex 0.55-m
spectrometer, and detected with a liquid-nitrogen-cooled
back-illum<strong>in</strong>ated charge-coupled-device (CCD) detector
array. Both the spectral resolution and the accuracy <strong>in</strong>
the Raman shift are estimated to be 2 cm −1 . As the
uv source, the 325-nm l<strong>in</strong>e of a He-Cd laser with about
4 mW was used. The two laser beams are aligned to
be col<strong>in</strong>ear as they enter the focus<strong>in</strong>g cyl<strong>in</strong>drical lens.
This ensures that the Raman spectrum is measured from
the same spot as that subjected to the uv irradiation.
The Raman spectra were measured before and after uv
irradiation.
I-V measurements were carried out us<strong>in</strong>g a Stanford
Research Systems model PS325 high-voltage power supply
and an Agilent 34410A digital multimeter. The samples
for the I-V measurements had alum<strong>in</strong>ium electrodes,
and the gap between them was about 1 mm. The I-V
curves were measured before, after, and dur<strong>in</strong>g uv irradiation.
III. RESULTS AND DISCUSSION
Figure 2 shows the Raman spectra for evaporated a-
WO 3 films irradiated with uv light for different time durations:
(a) 0 sec (as-deposited), (b) 30 sec, (c) 1 m<strong>in</strong>,
(d) 2 m<strong>in</strong>, (e) 5 m<strong>in</strong>, (f) 10 m<strong>in</strong>, (g) 20 m<strong>in</strong>, (h) 30 m<strong>in</strong>,
and (i) 60 m<strong>in</strong>. The spectrum of the as-deposited film
(a) shows a broad peak at 770 cm −1 due to the W 6+ −O
s<strong>in</strong>gle bonds. There is also a relatively sharp peak at 950
cm −1 , which has been assigned to the stretch<strong>in</strong>g mode
of the W 6+ = O double bonds <strong>in</strong>volv<strong>in</strong>g term<strong>in</strong>al oxygen
Fig. 2. Raman spectra of a-WO 3 films measured after uv
irradiation. The durations of the uv irradiation are (a) 0 sec
(as-deposited), (b) 30 sec, (c) 1 m<strong>in</strong>, (d) 2 m<strong>in</strong>, (e) 5 m<strong>in</strong>, (f)
10 m<strong>in</strong>, (g) 20 m<strong>in</strong>, (h) 30 m<strong>in</strong>, and (i) 60 m<strong>in</strong>.
Fig. 3. Detailed comparison of Raman spectra of a-WO 3
films before and after uv irradiation. The arrow <strong>in</strong>dicates the
450 cm −1 peak related to W 5+ = O bonds.
atoms on the surfaces of clusters and microvoid structures
<strong>in</strong> the film [10,13–16].
Figure 3 compares the Raman spectra taken before
and after an uv irradiation of 60 m<strong>in</strong>. The Raman spectrum
of the uv-irradiated film shows a peak at 450 cm −1 .
This peak also appears <strong>in</strong> electrochromic or gasochromic
coloration of a-WO 3 th<strong>in</strong> films and has been attributed
to W 5+ = O double bonds [10,15,16] formed by reduction
of W 6+ = O as electrons are <strong>in</strong>serted [16,17].
With uv irradiation, the overall <strong>in</strong>tensities of the two
ma<strong>in</strong> Raman peaks decrease due to the reduction of W 6+
ions to W 5+ ions. The Raman <strong>in</strong>tensities of W 6+ −O and
W 6+ = O bonds, and the ratio of the <strong>in</strong>tegrated <strong>in</strong>tensity
of the W 6+ = O bond peak at 950 cm −1 to that of
the O-W 6+ −O peak at 770 cm −1 are plotted <strong>in</strong> Figure
4. The data show that with uv irradiation, the W 6+ =
O/W 6+ −O ratio <strong>in</strong>creases. The <strong>in</strong>crease <strong>in</strong> the W 6+ =

<strong>Photochromic</strong> <strong>Mechanism</strong> <strong>in</strong> a-WO 3 Th<strong>in</strong> <strong>Films</strong>· · · – Kyungmoon Kim et al. -1659-
Fig. 4. Raman <strong>in</strong>tensities of the W 6+ −O and the W 6+ =
O bonds and the ratio W 6+ = O/W 6+ −O as functions of the
uv irradiation time.
O/W 6+ −O ratio <strong>in</strong> the Raman spectrum of uv-irradiated
a-WO 3 is similar to what is observed <strong>in</strong> electrochemical
<strong>in</strong>tercalation of Li <strong>in</strong>to a-WO 3 . For Li concentration x
up to x = 0.05 <strong>in</strong> a-Li x WO 3 , this ratio <strong>in</strong>creases monotonically
[11,18]. This <strong>in</strong>dicates that electrons have converted
more W 6+ <strong>in</strong>to W 5+ <strong>in</strong> W 6+ −O sites than <strong>in</strong> W 6+
= O sites [11]. Unlike Li <strong>in</strong>tercalation, however, the Raman
spectra of uv-irradiated a-WO 3 do not show the
peak due to W 5+ −O bonds [10]. This is probably due
to the fact that the degree of coloration <strong>in</strong> the current
photochromic experiment is lower than that <strong>in</strong> Li <strong>in</strong>tercalation
experiments. In fact, the aforementioned <strong>in</strong>crease
<strong>in</strong> the W 6+ = O/W 6+ −O ratio corresponds to the low
Li-concentration region <strong>in</strong> electrochemical <strong>in</strong>tercalation
[11]. For low Li concentrations, the Raman peak due to
W 5+ −O bonds is difficult to resolve because of its large
l<strong>in</strong>ewidth. The band-gap energy of a-WO 3 is 3.25 eV
[1]. With uv irradiation, optically exited electron-hole
pairs are created <strong>in</strong> a-WO 3 . The electrons reduce W 6+
ions to W 5+ ions, result<strong>in</strong>g <strong>in</strong> coloration of the film. The
role of photo-excited holes is not clear. A model suggests
that holes decompose the water <strong>in</strong> WO 3 <strong>in</strong>to protons and
oxygen radicals; thus, the created protons are said to be
<strong>in</strong>volved <strong>in</strong> the double <strong>in</strong>jection process [7,12]. However,
this models needs to be tested more rigorously.
Figure 5 shows the I-V characteristics of a-WO 3 films
before/dur<strong>in</strong>g/after uv irradiation. After uv irradiation,
the conductivity <strong>in</strong>creases dramatically. If uv irradiation
is left on dur<strong>in</strong>g the I-V measurement, the conductivity
is even higher. The <strong>in</strong>crease <strong>in</strong> the conductivity can be
understood <strong>in</strong> terms of photo-excited electrons and holes
<strong>in</strong> a-WO 3 . In the current double <strong>in</strong>jection model of coloration,
the electrons are trapped by tungsten ions, but
holes (or protons) are not bound. Therefore, even after
uv irradiation, the conductivity rema<strong>in</strong>s at the <strong>in</strong>creased
level. Dur<strong>in</strong>g the uv irradiation, both electrons and holes
can participate <strong>in</strong> the electrical conduction, so the conductivity
is even higher.
Fig. 5. I-V curve of a-WO 3 films before/dur<strong>in</strong>g/after uv
irradiation.
IV. SUMMARY
The photochromic effect and the electrical properties
of amorphous tungsten-oxide films have been studied
us<strong>in</strong>g Raman spectroscopy and I-V measurements.
Amorphous tungsten-oxide films show photochromic coloration
caused by uv irradiation. With uv irradiation,
the Raman peaks related to W 6+ −O (770 cm −1 ) and
W 6+ = O (950cm −1 ) decrease and a peak at 450 cm −1
due to W 5+ = O bond appears. This effect is expla<strong>in</strong>ed
<strong>in</strong> terms of the reduction of tungsten ions as <strong>in</strong> the case of
electrochromic or gasochromic colorations. The <strong>in</strong>crease
<strong>in</strong> the electrical conductivity for the uv-irradiated films
is consistent with this explanation.
ACKNOWLEDGMENTS
This work was performed for the Hydrogen Energy
R&D Center, one of the 21st Century Frontier R&D
Programs, funded by the M<strong>in</strong>istry of Science and Technology
of Korea, and was supported <strong>in</strong> part by a Korea
Research Foundation grant (KRF-2004-005-C00002) and
by Sogang University.
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