# Photo- and Electrochromic Properties of Activated Reactive EvaporatedMoO3 Thin Films Grown on Flexible Substrates

Authors: K. Hari Krishna; O. M. Hussain; C. Guillen
Journal: Research Letters in Nanotechnology
 (2008)

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## Abstract

The molybdenum trioxide (MoO3) thin films were grown onto ITO-coated flexible Kapton substrates using plasma assisted activated reactive evaporation technique. The film depositions were carried out at constant glow power and oxygen partial pressures of 8 W and
1×10−3 Torr, respectively. The influence of substrate temperature on the microstructural and optical properties was investigated. The
MoO3 thin films prepared at a substrate temperature of 523 K were found to be composed of uniformly distributed nanosized grains with an orthorhombic structure of
α-MoO3. These nanocrystalline
MoO3 thin films exhibited higher optical transmittance of about 80% in the visible region with an
evaluated optical band gap of 3.29 eV. With the insertion of 12.5 mC/cm2, the films exhibited an optical modulation of 40% in the visible region with coloration efficiency of
22 cm2/C at the wavelength of 550 nm. The
MoO3 films deposited at 523 K demonstrated better photochromic properties and showed highest
color center concentration for the irradiation time of 30 minutes at 100 mW/cm2.


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## Body

## 1. Introduction

In the  current  nanoscience and technology era, the
transition metal oxides (TMOs) constitute a fascinating and promising class of
inorganic solids that have received substantial attention of solid
state/materials chemists, due to their novel material characteristics. Among
the various transition metal oxides, molybdenum trioxide (MoO3) has
been recognized as a promising and persistent wide band gap material owing to
its quite motivating structural, chemical, chromogenic, catalytic, and optical properties. In particular, the orthorhombic layered α-MoO3 structure with high
electrochemical activity, high stability, and good coloration efficiencies of
MoO3  makes it useful for electro-, photo- and
gasochromic industrial applications. Recently, nanocrystalline molybdenum
trioxide in thin film configuration is considered as prospective chromogenic
material to adapt it as active layers in the field of advanced electrochromic
windows [1, 2]. Most of the researchers investigated extensively about the microstructural and electro-photochromic properties of
MoO3 thin
films grown onto various solid substrates using various physical/chemical vapor
thin films deposition techniques and ensure their efficiency and durability in
a device level [3, 4]. Nevertheless, at present the designing and fabrication
of thin film coatings on flexible substrates have grown worldwide into a major challenging and
novel research area for cutting-edge future-based technologies. These flexible
substrates are unique than solid glass substrates due to the following reasons:
(i) they are flexible, so they can bent and stick to any curved shape objects
without altering their basic properties, (ii) they are weight less, and (iii)
they are easy to carry and can be folded. Hence, deposition of thin film
coatings on flexible substrates goes hand-in-hand with the explosion of
scientific and technological breakthrough in microelectronics, electronic
information displays, optical memory devices, and nanotechnology. To the best
of our knowledge, reports are not available on the growth and chromic
application of
MoO3 thin films on flexible substrates. In addition,
despite various physical vapor deposition techniques, activated reactive
evaporation (ARE) is one of the plasma-assisted physical vapor thin-film
deposition techniques to grow nearly stoichiometric thin films with better
uniformity at relatively lower substrate temperatures with higher deposition
rates. In this deposition technique, the reaction occurs predominantly in
plasma; as a result, chemical   composition of the films can be controlled by
changing the ratio of reacting species. Hence, the present investigations aimed
at growing molybdenum trioxide thin films onto ITO-coated
flexible Kapton substrates using home-built activated reactive evaporation
technique and explicated their microstructural and optical properties as a
function of substrate temperature (Ts). The electrochromic and
photochromic properties of the nanocrystalline
MoO3 thinfilms
were investigated for their effective utilization in electrochromic windows.


## 2. Experimental Details

The plasma  assisted home built activated reactive evaporation technique
is adapted to prepare molybdenum trioxide thin films onto ITO-coated flexible
Kapton substrates. The ITO transparent conducting coating was deposited using
sputtering technique on polyimide flexible substrate which can be used up to 673 K as
substrates in the present investigation [5]. A constant and high density of plasma
was established between two electrodes at the glow power and oxygen partial
pressures of 8 W and 1×10−3 Torr, respectively. The film
depositions were carried out at various substrate temperatures ranging from
300 K to 600 K. The structural characteristics of the as-deposited
MoO3 thin
films were investigated using grazing incidence X-ray diffraction (GIXRD)
technique (Seifert
computerized X-ray diffractometer, model 3003 TT) with a grazing incidence of
angle 3°. Atomic force microscopy (AFM) (Digital Instrument:
Dimension 3100 series) was used to study the surface morphology  of the films in
a simple contact mode of operation. The optical measurements were carried out
by Hitachi U-3400 UV-Vis-NIR double beam spectrophotometer in the wave length
range of 300–1500 nm. The electrochromic studies were investigated
by a dry lithiation method, and the photochromic behavior is studied by
illuminating the specimens with a 100 W tungsten lamp. The intensity of
radiation during illumination at the surface of the film is kept at about 100 mW/cm2.


## 3. Results and Discussion

## 3.1. Structural Studies

Figure1 shows the X-ray diffraction pattern of
MoO3 thin films grown at
various substrate temperatures by maintaining constant glow power and oxygen partial
pressures of 8 W and 1×10−3 Torr, respectively. The films deposited
at room temperature were found to be amorphous, and the onset of
crystallization in MoO3 films is observed at around 473 K. This may
be due to increased kinetic energy of the ionized species in presence of plasma
(which is higher than the kinetic energy of evaporated species in the thermal
evaporation), which enhances ad-atom mobility on the substrate surface. The
enhancement of crystallinity in the films is observed with the augmentation of
substrate temperature to the higher value of 523 K. As a result, the respective
GIXRD pattern of the films exhibited (020), (110), (040), and (060) Bragg
reflections, and the evaluated lattice parameters which are a=0.395nm, b=1.384nm, and c=0.368nm are in good agreement with powder diffraction data and
attributed to the orthorhombic structure of α-MoO3 (JCPDS card no. 05-0508). The
crystallite size was estimated using Scherer’s formula and found to be about
65 nm for the films deposited at 523 K.Figure 1
The X-ray diffraction patterns
ofMoO3 thin films.


## 3.2. Surface Studies

The surface topographical investigations of tungsten trioxide thin films were
carried out as a function of substrate temperature using atomic force
microscopy (AFM). The smooth and featureless AFM surface morphological image of
the films grown atTs<423K supports the amorphous nature of the
films. The films deposited at substrate temperature of 523 K (see Figure 2) are
observed to be composed of needle-like morphology and nanosized grains of about
60 nm which are
fused compactly together, and the root-mean-square surface roughness of the
films is 3 nm. The temperature dependence of surface morphological features of
the films can be explained as follows. The evaporated species interact with the
established plasma and reach the surface of the substrates maintained at higher
substrate temperature (Ts=523K) and acquire larger thermal energy
and mobility. This process leads to the enhancement of the diffusion distance, initiates
the nucleation, and increases the island formation in order to grow continuous
film.Figure 2
The
AFM picture of theMoO3 thin film grown at the substrate temperature of 523 K.


## 3.3. Optical and Electrochromic Properties

The
optical transmittance characteristics of activated reactive evaporated molybdenum
trioxide thin films are investigated as a function of substrate temperature.
The fundamental absorption corresponding to the sharp decrease in transmittance
is noticed in the wavelength range 300–400 nm. TheMoO3 films grown at lower
substrate temperatures were observed to be light bluish in color indicating the presence of
oxygen deficiency  in the films. The increase in optical transmittance of the
films is noticed with the increase of substrate temperature to the higher
values, and the observed fundamental absorption edge shifted towards lower
wavelength side, by indicating the increase in optical band gap values. The
nanocrystalline molybdenum trioxide  thin films grown at Ts=523K exhibited
higher optical transmittance of 80% in the visible region as shown in Figure 3. The estimated optical band gap value increased from 3.18 eV to 3.29 eV with the
increase of substrate temperature to the higher values from 300 K to 523 K,
respectively. The increase of optical band gap values as a function of
substrate temperature can be explained as follows. The
MoO3 films
prepared at lower substrate temperatures (<423K) may contain reduced
oxidation states of “Mo” such as Mo5+ and Mo4+ states
which are closely related to oxygen vacancies formed in the films during
deposition. These  oxygen ion vacancies present in the films are able to capture one or two electrons and as a
result excited states of electrons trapped at Mo5+ sites commence to
overlap with the empty “4d” states on the neighboring Mo6+ sites. Resultantly, the oxygen vacancies
occupied by the electrons act as donor centers and form a narrow donor band in
the forbidden gap at about 0.3 eV below the conduction band. This donor band
deeply extends into the main band gap and causes lower optical band gap value of the amorphous
MoO3 films grown at lower substrate temperatures. With the increase
of substrate temperature to the higher values, the oxygen deficiency in the films decreases and was
responsible for the degradation of Mo5+ states in the films [6].Figure 3
The
optical transmittance spectra ofMoO3 thin films.The
electrochromic studies were carried out for the nanocrystallineMoO3 thin films deposited at Ts=523K on ITO-coated flexible substrates
using dry lithiation method [7]. In this method, LiNbO3 powder was
heat treated in vacuum to expel lithium atoms, which inserts into MoO3 thin films kept at temperature of 373 K, to give coloration for the films. The
respective optical modulation in optical transmittance of the films in colored
state is shown in Figure 3. When lithium atoms reach the exposed MoO3 thin films surface, they diffuse into the films and become responsible for the
following reaction:(1)Li⟷Li++e−,MoO3+x(Li++e−)⟷LixMoO3. No change in thickness of the films after Li-ion
intercalation into the film confirms the diffusion of Li+ ions into
the films. However, the color of the film appeared blue with the insertion of
Li+ ions which just similar to the MoO3 films lithiated
using wet method. The quartz monitor is used for the measurement of the film
thickness and lithiation process control. The thickness of the films and the quantity
of the lithium intercalated into the film are measured by monitoring the
frequency change of the quartz crystal as the mass of MoO3, and
Li layers are deposited, respectively. The degree of such lithiation was
measured by noting the change in the quartz crystal thickness (the effective
mass) and calibrated against the electrochemical insertion. The maximum
coloration studied here is for 20 nm effective mass thickness of lithium which
corresponds to approximately 12.5 mC/cm2 as varied from the
electrochemical method. These films showed an average optical modulation (40%) in
the visible region, and the coloration efficiency (CE) of MoO3 films
was evaluated by using relation CE=OD/Qin, where OD is the optical
density and Qin (mC cm−2) is the charge injected during
the coloration cycle. The MoO3 films grown at Ts=523K demonstrated
maximum CE value of 22 cm2/C at the  wavelength of (λ) 550 nm than the conventional MoO3 films. Hsu et al. [8] reported 45% of optical modulation with coloration
efficiency of 23.7 cm2/C at the wavelength of 550 nm for the sol-gel
spin-coated MoO3 thin films annealed at 573 K and  Sivakumar et al.
[9] reported maximum coloration efficiency of 30 cm2/C at the
wavelength of 633 nm for e-beam evaporated MoO3 thin films grown onto
FTO-coated glass substrates at room temperature. During intercalation process,
the injected electrons into nanocrystalline MoO3 films are localized
at Mo atom thus creating the Mo5+ sites and polarize their
surrounding lattice to form small polarons. As a result, small overlap occurs
between the wave functions corresponding to adjacent sites which are conductive
for polaron formation. The incident photons are absorbed by these small
polarons and can hop from an Mo5+ site to a neighboring Mo6+ (intervalence charge transformation) by the absorption of incident photon
energy. Therefore, electrochromism of nanocrystalline MoO3 thin
films is attributed to the small polaron absorption.


## 3.4. Photochromic Properties

The photochromic
properties of ARE deposited nanocrystallineMoO3 thin films were
performed as a function of irradiation time interval from 10 minutes to 150 minutes.
Figure 4 indicates the optical absorption spectra of as-deposited and
irradiated ITO-coated Kapton/MoO3 samples. The broad faint
absorption band near 850 nm is observed for as-deposited films. The observed
intensity of broadband absorption increased with the increase of irradiation
time. The irradiation of these films tends to increase anion vacancies (Vo+)
owing to the capture of released electrons by the oxygen vacancies. The
absorption band with a peak at 850 nm (G-band at 1.46 eV) may be attributed to
the Vo+ type
whose level lies close to the bottom of the conduction band. The color of the
films after irradiation arises from the transfer of electrons from valence 2p
oxygen orbital to empty Mo6+ (4d) level which gives rise to an
incorporation of lower valence Mo5+ within the lattice. The color of
the film changes from light blue to deep blue color with the increase of
irradiation time. Hence, the illumination produces free electrons which are
trapped by ion vacancies thereby forming color centers. The estimation of color
center concentration was estimated by using Smakula’s equation. The values of
the refractive index and the oscillator strength for MoO3 are taken
as 1.9 and 0.18, respectively. The integral is evaluated in the spectral range
400–1200 nm, where
color centers are generated by using the Gaussian approximation as shown in Figure
4. The absorption G-band whose maximum is located in the near infrared region (≈1.46eV) is considered to originate from the
intervalence transfer of electrons localized on transition metal ions. The
shape of this absorption band is expected to be Gaussian from the intervalence transfer
model. The asymmetric shape in the spectra is interpreted as the presence of
random potential fluctuations in the films. Figure 5 shows the variation of
color center concentration with irradiation time in MoO3 film. The concentration of color
centers is shown to vary nonlinearly with irradiation time. For flexible
Kapton/MoO3, samples irradiated for 30 minutes exhibited highest
color center concentration of 1.3×1017cm−3. From Figure 5, it is evident
that there is a threshold time for which the concentration reaches its maximum
value. When films irradiated beyond the threshold time, the color center
concentration notably decreases due to the destruction of color centers.Figure 4
The optical
absorption spectra at various irradiation times.Figure 5
The variation of color center
concentration as a function of  irradiation time.


## 4. Conclusions

The
nanocrystallineMoO3 thin films were deposited by using home built
activated reactive evaporation technique. The films grown at substrate
temperature of 523 K demonstrated orthorhombic structure of α-MoO3, and the grain growth was
observed to be needle-like morphology with 3 nm rms roughness, which might have
slightly decreased the electrochromic properties. The as-deposited MoO3 films at substrate temperature of 523 K were showed nearly 80% of optical
transmittance in the visible region and demonstrated 40% of optical modulation
during electrochromic studies with coloration efficiency of 22 cm2/C
at the wavelength of 550 nm. The nanocrystalline MoO3 films prepared
onto flexible Kapton substrates demonstrated better photochromic properties
with maximum color center concentration of 1.3×1017cm−3 when exposed for 30 minutes at an intensity of radiation 100 mW/cm2.



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*Source: 217510-2008-12-04.xml*