Electronic and Optical Properties of Nanocristalline Wo3 Thin Films Studied by Optical Spectroscopy and Density Functional Calculations

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Electronic and optical properties of nanocrystalline WO3thin films studied by optical

spectroscopy and density functional calculations

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2013 J. Phys.: Condens. Matter 25 205502

(http://iopscience.iop.org/0953-8984/25/20/205502)

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IOP PUBLISHING JOURNAL OFPHYSICS:CONDENSEDMATTER

J. Phys.: Condens. Matter25 (2013) 205502 (11pp) doi:10.1088/0953-8984/25/20/205502

Electronic and optical properties of

nanocrystalline WO3thin films studied byoptical spectroscopy and densityfunctional calculations

Malin B Johansson1, Gustavo Baldissera2, Iryna Valyukh3,

Clas Persson2,4, Hans Arwin3, Gunnar A Niklasson1 and Lars Osterlund1

1 Division of Solid State Physics, Department of Engineering Sciences, The Angstrom Laboratory,Uppsala University, PO Box 534, SE-75121 Uppsala, Sweden2 Division of Applied Materials Physics, Department of Materials Science and Engineering, RoyalInstitute of TechnologyKTH, SE-100 44 Stockholm, Sweden3 Laboratory of Applied Optics, Department of Physics, Chemistry and Biology, Linkoping University,SE-58183 Linkoping, Sweden4 Department of Physics, University of Oslo, PO Box 1048 Blindern, NO-0316 Oslo, Norway

E-mail:[email protected]

Received 12 February 2013, in final form 22 March 2013

Published 25 April 2013

Online atstacks.iop.org/JPhysCM/25/205502

Abstract

The optical and electronic properties of nanocrystalline WO3

thin films prepared by reactive

dc magnetron sputtering at different total pressures (Ptot) were studied by optical spectroscopy

and density functional theory (DFT) calculations. Monoclinic films prepared at lowPtotshow

absorption in the near infrared due to polarons, which is attributed to a strained film structure.

Analysis of the optical data yields band-gap energiesEg 3.1 eV, which increase withincreasingPtotby 0.1 eV, and correlate with the structural modifications of the films. The

electronic structures of triclinic-WO3, and monoclinic- and-WO3were calculated using

the Green function with screened Coulomb interaction (GW approach), and the local density

approximation. The-WO3and -WO3phases are found to have very similar electronic

properties, with weak dispersion of the valence and conduction bands, consistent with a direct

band-gap. Analysis of the joint density of states shows that the optical absorption around the

band edge is composed of contributions from forbidden transitions (>3 eV) and allowed

transitions (>3.8 eV). The calculations show that Egin -WO3is higher than in the-WO3

and-WO3phases, which provides an explanation for thePtotdependence of the optical data.

(Some figures may appear in colour only in the online journal)

1. Introduction

Tungsten trioxide (WO3) is one of the most studied

electrochromic materials [1]. Recently, its applications

in renewable energy technology have been highlighted,

including solar hydrogen production, solar cells, and

photocatalysis [26]. However, WO3exists in several phases,

with similar crystal structures near room temperature,and is readily reduced to form sub-stoichiometric WO3x

structures. The WO3 structure is based on a common WO6octahedral structure arranged in at least five crystallographic

modifications. The thermodynamic stability of these (at 1 bar)

is reported to be monoclinic P21/c (-WO3) up to 230 K,

triclinic P1 (-WO3) between 230 and 300 K, monoclinic

P21/n (-WO3) between 300 and 623 K, orthorhombic

Pnma (-WO3) between 623 and 1020 K, tetragonal P4/ncc

(-WO3) between 1020 and 1171 K, and finally tetragonalP4/nmm(-WO3) up to the melting point at 1700 K [711].

10953-8984/13/205502+11$33.00 c 2013 IOP Publishing Ltd Printed in the UK & the USA
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J. Phys.: Condens. Matter25 (2013) 205502 M B Johanssonet al

Moreover, the structure of nano- and microcrystalline WO3depends sensitively on the preparation conditions, which

modifies the WO3 phase diagram [1215] and further

complicates the analysis of these materials.

The electronic structure of WO3 and sub-stoichiometric

WO3x is intimately linked to the structural properties,

and has been the subject of several studies [1, 4, 1622].The valence band (VB) consists mainly of O 2p orbitals,

and the conduction band (CB) consists mainly of W 5d

orbitals [1621]. Phase transitions result in changes of the

band-gap, Eg, due to modifications of the W 5d states.

Bullet [20] and Stashans and Lunell [18]studied the influence

of alkali intercalation on cubic, room-temperature monoclinic

and Perovskite structures by semi-empirical calculations.

They showed that distortion of the ideal cubic structure to a

monoclinic structure increased Eg from 1.5 to 2.4 eV with

an up-shift of the W 5d states. Sub-stoichiometric WO3xgives rise to broad absorption in the near infrared (NIR)

region due to the electron transfer from W5+ to neighboringW6+ sites [23]. Excess electrons can be localized (smallpolarons), or delocalized over several neighboring ions (large

polarons) [1,17].

Reported experimental values of Eg in WO3 vary

considerably, in the range from 2.6 to about 3.2 eV [1, 4,

15,16,22,24]. This is partly due to variations of the WO3structure, but also due to the model which has been used

to extract Eg. In particular, values of Eg < 3 eV are often

obtained assuming an indirect band-gap [4, 15, 24]. The

importance of surface area and interphase boundaries on the

W charge state is well documented (see [1] and references

therein). Oxygen deficient WO3x and associated extended

defects has an impact on color and electrical properties,and give e.g. rise to observed color changes from slight

yellow to greenish for WO3 and WO3x, respectively. Innanostructured WO3 films prepared by reactive sputtering,

Eg has been reported to correlate with the O2 sputtering

pressure and the O vacancy concentration [15]. Changes

in Eg, d-band occupancy, and the positions of the VB

maximum and the CB minimum have been observed for

different phases of WO3[17,25]. Calculations of cubic WO3using density functional theory (DFT) calculations have been

reported to underestimate Eg with about 0.6 eV compared

with experimental values of Eg 2.6 eV, inferred for an

indirect band-gap [19]. The band dispersion near the band-gapregion for the -WO3, -WO3, -WO3 and -WO3 phases

are found to be small using ab initiocalculations[26], which

is a consequence of the small differences in bond angles and

lattice constants among the different phases.

In this paper we report on the electronic and optical

properties of well characterized, nanocrystalline WO3 films

prepared by reactive dc magnetron sputtering. We examine

the experimentally determined absorption coefficient and

dielectric constants obtained from optical spectrophotometry

and ellipsometry, and relate this to specific WO3 phase

compositions, without making any assumptions on the

character of the optical band-gap transition. The results are

compared with calculations performed with the self-consistentGW approximation. Using this quasi-particle approach, which

goes beyond DFT,Egis much better described compared with

regular DFT band structures.

2. Materials and methods

2.1. Sample preparation

The tungsten oxide (WO3) thin films were prepared on

13 mm diameter CaF2 substrates (Crystran Ltd) by reactive

dc magnetron sputtering using a versatile deposition system

based on a Balzers UTT 400 unit. The sputter target was

a 5 cm diameter plate of tungsten with 99.95% purity

(Plasmaterials). Sputtering was conducted in an Ar and

O2 plasma. The purity of the gases was 99.998% and the

sputtering power was 200 W for all the films. The O2/Ar ratio

was kept at 0.43, and the samples were sputtered at a substrate

temperature of Ts= 553 K and subsequently post-annealedat Ta= 673 K for 1 h ex situ. The CaF2 substrates werecleaned with de-ionized water and ethanol before sputtering.

Samples were sputtered at working pressures, Ptot= 10,15, 20, 25 and 30 mTorr, respectively, and the deposition

rate varied as a function of the working pressure from

36 nm min1 at Ptot= 10 mTorr to 11 nm min1 atPtot= 30 mTorr. The sputtering conditions employed for thedifferent WO3 films are summarized in table 1. WO3 thin

films sputtered on CaF2 substrates were highly transparent.

Films prepared at low Ptot exhibited a slightly bluish color,

indicating sub-stoichiometry [23,27]. The samples sputtered

atPtot = 20, 25 and 30 mTorr were slightly yellowish and red,characteristic of stoichiometric WO3.

2.2. Materials characterization

The structure of the films were determined by grazing

incidence x-ray diffraction (GIXRD), using a Siemens D5000

Th2Th instrument, employing parallel-plate sollers which

had a resolution of 0.14 and 0.3 (2), respectively. Thegrain size determined from the XRD data was in the

range 1555 nm for all films, and suggests that their

physical properties should be similar to the corresponding

bulk materials. Composition and density were determined

using Rutherford backscattering spectroscopy (RBS) on films

deposited on carbon substrates. The RBS measurements were

carried out at the Uppsala Tandem Accelerator Laboratoryusing 4He ions with an energy of 2 MeV. An azimuth of

7 was applied to the sample holder to avoid the risk ofchanneling into the crystals. The ions were backscattered at

an angle of= 172. Data analysis was performed with theprogram SIMNRA [28]. The physical properties of the WO3films are compiled in table 1. A thorough characterization

of the WO3 films has been presented elsewhere, where the

dependence of the physical properties of the films on the

deposition conditions is explained in more detail [27].

2.3. UV/vis/NIR spectrophotometry

Optical measurements in the 3002500 nm wavelengthregion were performed using a Perkin-Elmer Lambda 900

2


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Table 1. Sputter deposition parameters and physical properties of WO3 films prepared by reactive dc magnetron sputtering in this study.

Sample Thickness Refractive index Grain size

Crystal phase

Density

Ptot(mTorr) dSa (nm) dC

b (nm) na Dc (nm) d (g cm3)

10 1016 1016 2.14 15 6.5815 996 1050 2.07 28 20 877 772 2.03 35 , 25 789 774 2.10 49 , 30 857 819 2.01 55 , () 6.38

a From Swanepoel analysis of UVvis transmittance data [29]in the range 500600 nm.b From the Cauchy model applied to ellipsometry data [30].c From the WilliamsonHall method [31].d From RBS measurements [28].

double-beam UV/vis/NIR spectrophotometer, which was

equipped with an integrating sphere attachment, using a

Spectralon reflectance standard. The refractive index, n, and

thickness, d, of WO3 films on CaF2 were calculated from

the interference fringe method of Swanepoel using data fromtransmittance measurements[29]. The absorption coefficient,

, can be determined from the spectral transmittance, T(),

and reflectance,R(), namely[32].

() = 1d

ln

1R()

T()

. (1)

The band-gap energy,Eg, can be obtained from the equation:

E= A(EEg)p (2)whereEis the photon energy,A is a constant, and the exponent

pdepends on the type of optical transition[33]. Depending on

the type of electronic transition in bulk semiconductors, theexponent isp = 2, 1/2, 3 and 3/2 for indirect allowed, directallowed, indirect forbidden and direct forbidden transitions,

respectively, using the parabolic band approximation around

the absorption threshold. Equation (2) can be recast in the

form

d(ln(E))

d(E)= p

(EEg). (3)

A plot of d ln(E)/d(E)as a function ofEyields a divergence

at Eg, which can thus be determined without assumptions

about the nature of the optical transition, as previously

demonstrated for nanostructured CdSZnO thin films [34]. Itshould be noted that equations (2) and(3) are strictly valid

only for E> Eg. However, the absorption tail commonly

present in optical spectra atE< Egwill result in a low positive

value of the derivative in equation(3). Therefore the band-gap

is determined by the maximum in the plot of d(ln(E))/d(E)

versus E. With Eg determined in this manner for all WO3films, corresponding p values may be determined from the

slope of ln(E)versus ln(EEg).

2.4. Ellipsometry

To extend the optical characterization further out into the

ultraviolet region we have used spectroscopic ellipsometry(SE). The ellipsometry parameters and have been

measured between 0.62 and 5.5 eV with a variable angle

rotating analyzer ellipsometer (VASE from J.A. Woollam Co.,

Inc.). First, transmittance at normal incidence was measured

for the CaF2 substrate and for WO3 films deposited on CaF2.

Then, the back side of the samples was covered with Scotchtape in order to avoid incoherent backside reflections. SE

measurements in reflection mode were performed at several

different angles of incidence between 50 and 75 andapproximately at the same point of the sample where

transmittance measurements were made. All measurements

were performed at room temperature.

Ellipsometry is an indirect method, and an appropriate

model, based on the physical properties of the samples, must

be constructed to interpret the data. Optical constants (=1+ i2 and N=

= n+ ik) were obtained through

the complex reflection coefficients, Rp and Rs, from the

fundamental ellipsometry equation [35]:

Rp

Rs= tanexp(i), (4)

where the subscripts p and s correspond to polarization of

the electric field of the light parallel and perpendicular to the

plane of incidence, respectively. The experimental data were

then fitted to the models to reproduce measured and

using the LevenbergMarquardt regression algorithm[36].

The optical properties of the CaF2 substrate were

determined by using the Cauchy model for the refractive index

nand assumingk= 0, namely [30]

n = A + B2+ C

4 (5)

where is the wavelength and A,Band Care fit parameters.

The optical constants for the CaF2 substrate were kept

constant in the further analysis.

The electronic structure of WO3 exhibits a complicated

behavior in the region close to the band-gap, as discussed

below in section 3. As a consequence, we could not obtain

sufficiently good fits of the dielectric function to standard

models over the whole energy range. Instead we analyzed

the ellipsometric data in two steps. First, the thickness and

refractive index of WO3, as well as parameters representing

the roughness at the film/air interface were determined fromexperimental data in the region between 2.2 and 2.8 eV, where

3


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Figure 2. Close up on the XRD spectra for differentPtotin the characteristic 2regions for WO3: (a) 21.525.5 and (b) 3136.

to previous studies, which show that the monoclinic - and

-WO3structures are stabilized in microcrystalline WO3[12].

While peak 4, for the film prepared at 10 mTorr, appears

to coincide with the 33.34 reflection corresponding to the

(112) plane in -WO3, the large broadening of peaks 1, 2and3, rather suggests that the structure of this film is mainly

composed of a slightly sub-stoichiometric (WO3x, x< 0.1),strained monoclinic phase, which is supported by a strain

analysis [27]. As we shall see in section 3.3 below, this

interpretation of the phase composition at Ptot= 10 mTorr,which is slightly different from our previous analysis [27],

is qualitatively supported by the ab initio calculations. In

addition, the position of peak 3 is shifted towards lower

diffraction angle, compared with the database value of the

monoclinic -WO3(200) planes at 24.37(figures1and2), at

higherPtot [9]. This can be attributed to an increased fraction

of the inter-mixed monoclinic -WO3 phase with increasing

Ptot, since -WO3 is the only phase which exhibits such a

small shift in diffraction angle due to the (110) planes located

at 24.11 [50]. We note, however, that the position of peak3for the film prepared at Ptot= 30 mTorr deviates somewhatfrom the films prepared at Ptot= 20 and 25 mTorr. This isalso reflected in the optical properties of this particular film

(see below), which suggests that the -WO3 composition in

the Ptot= 30 mTorr film is slightly lower. In contrast, thepeak 3 position for the film sputtered at Ptot= 15 mTorrshows very good agreement with a monoclinic -WO3phase,

and indicates that the transition from pure monoclinic -WO3to an inter-mixed monoclinic -WO3 phase occurs at Ptot

=20 mTorr. Detailed analysis of XRD data reveal that the thinfilms have a preferred growth direction that changes with

increasing Ptot from[202] to [200] [27]. In summary, theXRD results show that phase composition of the nano-WO3films changes as a function of sputtering conditions, and

result in several co-existing phases with the main components

being a strained monoclinic phase at Ptot 15 mTorr, anda (unstrained) monoclinic -WO3 phase inter-mixed with a

minor monoclinic-WO3phase atPtot 20 mTorr (table1).

3.2. Optical properties

Transmittance and reflectance of the WO3 films weremeasured in the wavelength range 3002500 nm. Figure 3

Figure 3. Transmittance and reflectance of WO3 films on CaF2substrates deposited at working pressures Ptotof 10 and 30 mTorr.

shows T and R for WO3 thin films prepared at Ptot 10 and

30 mTorr. The diffuse transmittance and reflectance were

found to be


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1 2 3 40

20

40

60

80

100

Energy (eV)

Absorptance(%)

10

30

25

20

15

a)

mTorr

1 2 3 4 50

1

2

3

4

5

6

x 107

Energy (eV)

(m1)

30

25

20

15

10

b)

mTorr

Figure 4. (a) Absorptance, and (b) absorption coefficient , calculated from spectrophotometry (0.53.65 eV) and ellipsometrymeasurements (3.55.5 eV) of WO3 thin films prepared at different working pressure Ptot.

1 2 3 42

1

0

1

2

3

4

5

6

Energy (eV)

d(ln(E))/d(E)

10 mTorr, Eg= 3.10 eV15 mTorr, E

g= 3.12 eV

20 mTorr, Eg= 3.13 eV



Figure 5. Derivative of optical absorption coefficient times energyd(ln(E))/d(E)as a function ofE(3)for WO3 thin films preparedat differentPtotbetween 10 and 30 mTorr.

has been associated with the order of 1 at.% O vacancies in

the films[27].The band-gap Eg for the different nano-WO3 thin films

prepared at different Ptot were estimated from the UVvis

data using equation (3) [34]. Figure 5 shows a plot ofd(ln(E))/d(E) versus E, where the peak position gives Eg.

The band-gap obtained in this manner is in the range of

Eg 3.103.18 eV and increases with increasingPtot.The Eg values obtained from the analysis in figure 5

can be used to find the exponent p in equation (3) by a

plot of ln(E) versus ln(E Eg) as shown in figure 6. Itis evident from figure 6that this linearization procedure, as

proposed by Panda et al [34] is ambiguous and is not valid

in an extended region away from Eg. Different p values in

different energy ranges are obtained, which prevents definitive

determination ofp nearEg from the simple analysis given by

equation(3). Thus the simple parabolic band model is shown

to be inadequate to describe the optical properties of the WO3films nearEgand this will be discussed further below.

15.5

16

16.5

17

ln(EEg) [eV]

ln(

E)

10 mTorr

15 mTorr

20 mTorr

25 mTorr

30 mTorr

2.5 2 1.5 1

Figure 6. A plot of ln(E)versus ln(EEg)for the WO3 thinfilms withPtot= 10, 15, 20, 25, and 30 mTorr.

Table 2. The maxima of the two Gaussian functions G1and G2 thatwere fitted to ellipsometry data for WO3films deposited at differentworking pressures,Ptot.

Ptot(mTorr) 10 15 20 25 30

G1 (eV) 4.57 4.60 4.67 4.67

4.5

G2 (eV) 6.07 6.03 6.48 6.15 5.85

Figure 7 shows 1 and 2, respectively, for the WO3films on CaF2 substrates between 3.5 and 5.5 eV, as obtained

from ellipsometry data, using the point-by-point method.

A KramersKronig consistent model dielectric function

based on two Gaussian absorption bands were fitted to the

experimental data with a good agreement as seen in figure8.

In table2the Gaussian peak positions obtained in this manner

are displayed for all films.

It is evident from figure 8 that 2 in the WO3 thin

films consists of two peaks in the UV region, one in therange 4.54.7 eV and the other at 6.06.5 eV (table 2). With

6


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3.5 4 4.5 5 5.52

3

4

5

6

7

8

1

Photon energy (eV)

25

30

20

15

10

a)

mTorr

3.5 4 4.5 5 5.50

1

2

3

4

5

6

2

Photon energy (eV)b)

mTorr10

15

30

20

25

Figure 7. The real (a) and imaginary part (b) of the dielectric function obtained from ellipsometry for WO3films deposited at differentworking pressuresPtot.

Figure 8. The optical constants of a WO3 film sputtered at aworking pressure ofPtot= 20 mTorr fitted to a model dielectricfunction with two KramersKronig consistent Gaussian functions.A decomposition into the Gaussians is also shown.

increasing Ptot the absorption in the UV region decreases,

and the positions of the absorption peaks in the UV region

are shifted to higher energies. The 2 peak positions for the

film prepared at Ptot= 30 mTorr was obtained by employingan additional Gaussian peak in the 12 eV region to be

able to accurately model the 2 data. Despite this it is seen

that Gaussian peaks in the UV region are red-shifted forthis film, which appears to falter the Ptot trend in table 2.

This is, however, in qualitative agreement with the XRD data

presented above, which suggested that the film prepared at

Ptot= 30 mTorr has a structure similar to those prepared atlower Ptot and thus contains less of the -phase. In figure 7

it is also seen that 2 decreases with increasing Ptot, in good

agreement with the data in figure4(b). This may be explained

by increasing film porosity with increasingPtot. Although this

is qualitatively also corroborated by the RBS data (table 1),

the small density change cannot fully explain the large

decrease of the optical constants. Further investigations of the

film porosity should be performed to explore the morphologydependence. The blue-shift of2 as a function ofPtot close

to the band edge relates to a change of electronic structure,

which is elaborated in section3.3.

3.3. Electronic structure

Figure 9 shows the calculated band structure of triclinic

phase and both monoclinic and phases in WO3thin films

obtained with the GW approach (circles in figure 9). For

comparison, we present also the LDA results (solid lines),

where the absolute energies of the CBs have been shifted by a

constant in order to force the LDA band-gap energy to agree

with that of GW. Overall, LDA and GW show very similar

band curvatures. The triclinic and the monoclinic -phase

crystalline structures have both flat energy bands along the

(001) direction, and the CB at the Z point has a comparableenergy level to the CB at the point; this is in agreement with

earlier studies [17,26,51]. The calculated band-gap energies

are very comparable for these two structures: Eg= 3.04 eV(-point) for the triclinic-WO3and Eg= 2.93 eV (-point)for the monoclinic -WO3 obtained from the single-particle

transitions across the point (table 3). Monoclinic -WO3has a very flat band in the (110) direction and the phase

shows an indirect gap ofEg = 3.27 eV with the VB maximumat the A point (1/2 1/2 0). The energy difference between

the indirect gap at the A point and the direct -point gap

is however only 0.06 eV, and the direct transitions at the

point will completely dominate the optical absorption.These calculated GW band-gap energies are larger than those

found in the literature [17,26], since the GW quasi-particle

correction increases the band-gap. Our values are comparable

with results obtained from hybrid functional calculations

that also generate rather accurate band-gap energies [51].

Moreover, variations in having direct/indirect transitions for

the different tungsten phases are also found in the literature,

but these deviations are normally small, and most likely this

is a consequence of the flat curvatures of the bands [25,26,

51].

It is evident from the band structure of the three

phases (figure9) that even for relatively low photon energies

aroundEg 3.5 eV both direct and indirect transitions maycontribute to the optical absorption. However, it is expected

7


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0 1 2 3 4 5 6

0

2

4

6

8

10

12

Dielectric

function

Energy (eV)

phase 2(E)

phase 2(E)

phase 2(E)

phase 1(E)

phase 1(E)

phase 1(E)

Figure 11. Dielectric function of the triclinic (thin black),monoclinic(red thicker) and monoclinic (green thicker) phasesobtained from GW calculations. The dash-dotted curves representthe real part,1(E), of the dielectric function,

(E) = 1(E)+ i2(E), whereas the solid curves are the imaginarypart,2(E).

from deeper lying VB states evidenced in figures 10 and 11,

whereas the shoulder in the 45 eV region correspond to

transitions just below the VB maximum, in good agreement

with the ellipsometry results (figure7and table2).

The trends in the dielectric functions are further

emphasized in the absorption coefficients (E), shown in

figure12for the three WO3structures. Here, the joint density

of states (JDOS) is also presented. JDOS can represent

direct VB to CB transitions when the transition rules are

neglected, and thus by comparing the JDOS with (E) onecan understand the strength of the optical transitions. The

JDOS spectra demonstrate that all three WO3structures have a

threshold at a lower energy than the true threshold for optical

absorption. This indicates that the structures exhibit a band

edge region, where the absorption is dominated by forbidden

optical transitions. It is noticeable that the threshold energy

estimated from the JDOS extrapolation (figure 12) as well

as from the band structure (figure 9) indicates that Eg3 eV for the -WO3 phase (table3), which is the dominant

phase for the films prepared at low Ptot. The monoclinic

-phase has a 0.30.4 eV larger band-gap compared with

the other two phases, but overall all three structures havesimilar absorption coefficients. This agrees well with the

analysis of the spectrophotometry data presented above, and

give further support that the -phase, which develops at

highPtot, yields films with different electronic properties and

higher band-gap than those prepared at low Ptot. Finally, we

comment on the influence of O vacancies. Defects, such as O

vacancies may also affect the electronic structure and modify

the band edge positions [22]. In our case, this may affect

the WO3 films prepared at low Ptot= 10 mTorr, where aslight sub-stoichiometry (of the order of 1 at.%) was inferred,

but not as much for the films prepared at high Ptot. Thus

the Eg values determined experimentally at low Ptot may be

expected to be more uncertain than those at high Ptot, andthis may also explain the smaller shifts of Eg versus Ptot

3 3.5 4 4.5 50

10

20

30

40

50

Absorptioncoef

ficient(10

JDOS(ar

b.units)

4/cm)

Energy (eV)

phase

phase

phaseJDOS

Eg

Eg

Eg

Figure 12. Absorption coefficient(E)(solid curves) near theband-gap energy for the triclinic (black), monoclinic (red) andmonoclinic (green) structures. In order to analyze transitionprobabilities, the corresponding joint density of states, JDOS

(dash-dotted curves) is shown in the same graph in arbitrary units.

observed experimentally compared to the predicted calculated

Eg for the associated WO3 phases (table3). Regardless, this

will not change the conclusions about the different band-gaps

(in the range 0.30.4 eV) in the monoclinic epsilon phase

compared to the triclinic and monoclinic gamma phases, nor

the comparisons of the dielectric function between DFT and

the optical measurements.

4. Conclusion

We have shown that the optical and electronic properties

of nanocrystalline WO3 thin films prepared by reactive

dc magnetron sputtering depend sensitively on the total

pressure, Ptot, in the preparation process. While the

morphology and phase composition of the films depend

strongly on the sputtering conditions, the electronic structure

resembles their bulk phase counterparts, with close to

stoichiometric composition. We have shown that a low-

temperature monoclinic phase (-phase) co-exists with

the monoclinic (-phase) WO3 at high Ptot. At low

Ptot the main constituent is a strained phase, which

exhibits pronounced near infrared absorption. The electronic

properties of monoclinic and triclinic WO3 were calculated

within the GW approximation. By comparison with optical

spectrophotometry and ellipsometry data it is shown that the

optical band-gap increases for films prepared at high Ptotdue

to the presence of -WO3, which inter-mixes with -WO3.

Analysis of the calculated density of states shows that the

optical absorption aroundthe band edge, which is apparent

between 3 and 4 eV in optical spectroscopy, is composed of

contributions from forbidden transitions (with a threshold at

around 3 eV), and allowed, direct optical transitions (with a

threshold at3.8 eV). These results may reconcile previousdiscrepancies of reported band-gap values for micro- and

nanocrystalline WO3, and facilitate rational fabrication ofWO3films with controlled electronic properties.

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[37] Bruggeman D A G 1935 Berechnung verschiedenerphysikalischer Konstanten von heterogenen Substanzen I.Dielektrizitatskonstanten und Leitfahigkeiten derMischkorper aus isotropen SubstanzenAnn. Phys.Lpz.5Folge 2463679

[38] Aspnes D E and Theeten J B 1979 Investigation of effective-medium models of microscopic surface roughness by

spectroscopic ellipsometryPhys. Rev.B 203292302[39] Fujiwara H, Koh J, Rovira P I and Collins R W 2000Assessment of effective-medium theories in the analysis ofnucleation and microscopic surface roughness evolution forsemiconductor thin filmsPhys. Rev.B 611083244

[40] Gupta S, Dudipala A, Williams O A, Haenen K andBohannan E 2008Ex situvariable angle spectroscopicellipsometry studies on chemical vapor depositedboron-doped diamond films: layered structure and modelingaspectsJ. Appl. Phys.104112

[41] Kresse G and Furthmuller J 1996 Efficient iterative schemesforab initiototal-energy calculations using a plane-wavebasis setPhys. Rev.B 541116986

[42] Kresse G and Joubert D 1999 From ultrasoft pseudopotentialsto the projector augmented-wave methodPhys. Rev.B

59175875[43] Blochl P E 1994 Projector augmented-wave method Phys. Rev.

B501795379[44] Gajdos M, Hummer K, Kresse G, Furthmuller J and

Brechstedt F 2006 Linear optical properties in theprojector-augmented wave methodology Phys. Rev.B7345112

[45] Shishkin M and Kresse G 2006 Implementation andperformance of the frequency-dependent GW methodwithin the PAW frameworkPhys. Rev.B74035101

[46] Hedin L 1965 New methods for calculating the one-particleGreens function with application to the electron gasproblemPhys. Rev.139A796823

[47] Cazzanelli E, Vinegoni C, Mariotto G, Kuzmin A and

Purans J 1999 Low-temperature polymorphism in tungstentrioxide powders and its dependence on mechanicaltreatmentsJ. Solid State Chem. 1432432

[48] Filho A G S, Filho J M, Freire V N, Ayala A P, Saski J M,Freire P T C, Melo F E A, Julio J F and Gomes U U 2001Phase transition in WO3 microcrystals obtained by sinteringprocessJ. Raman Spectrosc.326959

[49] Salje E K H, Rehmann S, Pobell F, Morris D, Knight K S,Herrmannsdoerfer T and Dove M T 1997 Crystal structureand paramagnetic behaviour of-(WO3x)J. Phys.:Condens. Matter9 656377

[50] Salje E K H 1994 Polarons and pipolarons in tungsten oxide,WO3x Eur. J. Solid State Inorg. Chem. 3180521

[51] Wang F, Di Valentin C and Pacchioni G 2011 Electronic and

structural properties of WO3: a systematic hybrid DFTstudyJ. Phys. Chem.C 16834553[52] Gonzalez-Borrero P P, Sato F, Medina A N, Baesso M L,

Bento A C, Baldissera G, Persson C, Niklasson G A,Granqvist C G and Ferreira da Silva A 2010 Opticalband-gap determination of nanostructured WO3film Appl.Phys. Lett.96061909

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http://dx.doi.org/10.1002/andp.19354160705http://dx.doi.org/10.1002/andp.19354160705http://dx.doi.org/10.1002/andp.19354160705http://dx.doi.org/10.1103/PhysRevB.20.3292http://dx.doi.org/10.1103/PhysRevB.20.3292http://dx.doi.org/10.1103/PhysRevB.61.10832http://dx.doi.org/10.1103/PhysRevB.61.10832http://dx.doi.org/10.1063/1.2990058http://dx.doi.org/10.1063/1.2990058http://dx.doi.org/10.1103/PhysRevB.54.11169http://dx.doi.org/10.1103/PhysRevB.54.11169http://dx.doi.org/10.1103/PhysRevB.59.1758http://dx.doi.org/10.1103/PhysRevB.59.1758http://dx.doi.org/10.1103/PhysRevB.50.17953http://dx.doi.org/10.1103/PhysRevB.50.17953http://dx.doi.org/10.1103/PhysRevB.73.045112http://dx.doi.org/10.1103/PhysRevB.73.045112http://dx.doi.org/10.1103/PhysRevB.74.035101http://dx.doi.org/10.1103/PhysRevB.74.035101http://dx.doi.org/10.1103/PhysRev.139.A796http://dx.doi.org/10.1103/PhysRev.139.A796http://dx.doi.org/10.1006/jssc.1998.8061http://dx.doi.org/10.1006/jssc.1998.8061http://dx.doi.org/10.1002/jrs.727http://dx.doi.org/10.1002/jrs.727http://dx.doi.org/10.1088/0953-8984/9/31/010http://dx.doi.org/10.1088/0953-8984/9/31/010http://dx.doi.org/10.1021/jp201057mhttp://dx.doi.org/10.1021/jp201057mhttp://dx.doi.org/10.1063/1.3313945http://dx.doi.org/10.1063/1.3313945http://dx.doi.org/10.1063/1.3313945http://dx.doi.org/10.1063/1.3313945http://dx.doi.org/10.1021/jp201057mhttp://dx.doi.org/10.1021/jp201057mhttp://dx.doi.org/10.1088/0953-8984/9/31/010http://dx.doi.org/10.1088/0953-8984/9/31/010http://dx.doi.org/10.1002/jrs.727http://dx.doi.org/10.1002/jrs.727http://dx.doi.org/10.1006/jssc.1998.8061http://dx.doi.org/10.1006/jssc.1998.8061http://dx.doi.org/10.1103/PhysRev.139.A796http://dx.doi.org/10.1103/PhysRev.139.A796http://dx.doi.org/10.1103/PhysRevB.74.035101http://dx.doi.org/10.1103/PhysRevB.74.035101http://dx.doi.org/10.1103/PhysRevB.73.045112http://dx.doi.org/10.1103/PhysRevB.73.045112http://dx.doi.org/10.1103/PhysRevB.50.17953http://dx.doi.org/10.1103/PhysRevB.50.17953http://dx.doi.org/10.1103/PhysRevB.59.1758http://dx.doi.org/10.1103/PhysRevB.59.1758http://dx.doi.org/10.1103/PhysRevB.54.11169http://dx.doi.org/10.1103/PhysRevB.54.11169http://dx.doi.org/10.1063/1.2990058http://dx.doi.org/10.1063/1.2990058http://dx.doi.org/10.1103/PhysRevB.61.10832http://dx.doi.org/10.1103/PhysRevB.61.10832http://dx.doi.org/10.1103/PhysRevB.20.3292http://dx.doi.org/10.1103/PhysRevB.20.3292http://dx.doi.org/10.1002/andp.19354160705http://dx.doi.org/10.1002/andp.19354160705http://dx.doi.org/10.1002/andp.19354160705

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Electronic and Optical Properties of Nanocristalline Wo3 Thin Films Studied by Optical Spectroscopy and Density Functional Calculations