Spectroscopic ellipsometry determination of the optical constants of titanium-dopedWO3 films made by co-sputter depositionM. Vargas, E. J. Rubio, A. Gutierrez, and C. V. Ramana Citation: Journal of Applied Physics 115, 133511 (2014); doi: 10.1063/1.4869665 View online: http://dx.doi.org/10.1063/1.4869665 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/13?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Optical properties of nanocrystalline WO3 and WO3-x thin films prepared by DC magnetron sputtering J. Appl. Phys. 115, 213510 (2014); 10.1063/1.4880162 Optical properties of WO3 thin films using surface plasmon resonance technique J. Appl. Phys. 115, 043104 (2014); 10.1063/1.4862962 Dynamic in situ spectroscopic ellipsometric study in inhomogeneous TiO 2 thin-film growth J. Appl. Phys. 108, 013522 (2010); 10.1063/1.3457839 Electrical and optical properties of Sn doped CuInO 2 thin films: Conducting atomic force microscopy andspectroscopic ellipsometry studies J. Appl. Phys. 106, 053709 (2009); 10.1063/1.3211941 Real-time spectroscopic ellipsometry study of Ta–Si–N ultrathin diffusion barriers J. Vac. Sci. Technol. A 23, 1359 (2005); 10.1116/1.1996612

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Spectroscopic ellipsometry determination of the optical constantsof titanium-doped WO3 films made by co-sputter deposition

M. Vargas, E. J. Rubio, A. Gutierrez, and C. V. Ramanaa)

Department of Mechanical Engineering, University of Texas at El Paso, El Paso, Texas 79968, USA

(Received 5 January 2014; accepted 15 March 2014; published online 7 April 2014)

Titanium (Ti) doped tungsten oxide (WO3) thin films were grown by co-sputter deposition of W

and Ti metal targets. The sputtering powers to the W and Ti were kept constant at 100 W and 50 W,

respectively, while varying the growth temperature (Ts) in the range of 25–400 �C. The structural

quality of Ti-doped WO3 films is dependent on Ts. Ti-doped WO3 films grown at Ts< 400 �C were

amorphous. A temperature of 400 �C is critical to promote the structural order and formation of

monoclinic, nanocrystalline films. The optical constants and their dispersion profiles determined

from spectroscopic ellipsometry indicate that there is no significant inter-diffusion at the

film-substrate interface for W-Ti oxide film growth of �40 nm. The index refraction (n) at

k¼ 550 nm varies in the range of 2.15–2.40 with a gradual increase in Ts. Lorentz-Lorenz analysis

(n(k)¼ 550 nm) of the data indicates the gradual improvement in the packing density coupled

with structural transformation accounts for the observed optical quality of the Ti-doped WO3 films

as a function of Ts. A correlation between the growth conditions and optical constants is discussed.VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4869665]

I. INTRODUCTION

Tungsten oxide (WO3) is an important “chromogenic”

material because of the coloration effects associated with vari-

ous processes.1–6 Low-dimensional structures, such as nano-

rods, nanowires, nano-particles, and ultra-thin films, of WO3

are recently gaining considerable interest for applications in

optoelectronics, microelectronics, photovoltaic devices, catal-

ysis, and integrated sensors.2–40 Optical and electrical proper-

ties of WO3 thin films are quite interesting for utilization in

smart window technology for energy-efficient architecture of

buildings and automobiles, flat-panel displays, optical mem-

ory and writing–reading–erasing devices, and electronic infor-

mation displays.1–6 Most recently, WO3 based materials are

considered and demonstrated to be attractive for application in

photoelectrochemical cells (PECs) for hydrogen production

by water-splitting.19–21 In addition, recently, application of

WO3 films in emerging dye-sensitized solar cells (DSSC)

technology has been demonstrated.22,23

The objective of the present work is to derive a compre-

hensive understanding of the structure and optical properties

of Ti-doped WO3 films deposited by co-sputtering of Ti and

W metals. The relevance and current interest in Ti-doped

W-oxide films are due to the following considerations.

Either Ti metal doping or capping is proved to alter the elec-

tronic structure in order to benefit the properties and device

performance.41–51 Enhancements in the electrical conductiv-

ity, luminescence, sensing ability, and photocatalytic per-

formance are the benefits expected and/or reported in the

literature under controlled Ti incorporation or TiO2-coupling

with other oxides.41–51 When Ti is sputtered onto ZnO at

400 �C, the near band edge luminescence enhances five times

in Ti/ZnO nanorod heterostructures.42 For the specific

case of Ti- and W-oxides, it has been reported that coupling

TiO2 photocatalyst with WO3 can extend the optical absorp-

tion to the visible region to enhance the photo-catalytic

efficiency.43–48 Recently, studies focused on TiO2-WO3 sys-

tem proved the enhanced photocatalytic performance of such

materials.43–49 The key to obtain such enhanced performance

is the specific composition and associated effect on the elec-

tronic structure and band gap.43–49 Also, it has been demon-

strated that W-Ti mixed oxides exhibit enhanced selectivity

and sensitivity to certain chemicals for their utilization in

integrated sensors.50,51 In our most recent work, using the

approach of W-Ti alloying in targets utilized for

sputter-deposition, enhanced electrical conductivity of 5%

Ti-doped WO3 compared with that of pure WO3 films has

been demonstrated.52,53 Similarly, the high end of Ti(20%)

into WO3 results in fully disordering, which results in

the formation of amorphous W-Ti-O films with optical prop-

erties tuned in between in TiO2 and WO3 by selectively

choosing the deposition temperature.41 Tuning the structure

and properties of such W-Ti-O films for applications in

inorganics and organics based photovoltaics has been

proposed.41 These considerations clearly demonstrate that

investigating the fundamental aspects of microstructure evolu-

tion and structure-property relationships in W-Ti-O films may

provide opportunities to tailor the microstructure and optical

properties of the materials for utilization in contemporary and

emerging energy technologies. Specifically, if tuned for com-

position similar to Al-doped ZnO, a set of materials based on

Ti-doped WO3 or selective composition in the W-Ti mixed

oxides can serve as transparent conducting electrodes for

application in emerging field of organics-based photovoltaics

and optoelectronics technologies. In this paper, we report on

the optical properties of Ti-doped WO3 co-sputtered films

determined using spectroscopic ellipsometry (SE). Obviously,

a)Author to whom correspondence should be addressed. Electronic mail:

rvchintalapalle@utep.edu

0021-8979/2014/115(13)/133511/6/$30.00 VC 2014 AIP Publishing LLC115, 133511-1

JOURNAL OF APPLIED PHYSICS 115, 133511 (2014)

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the fundamental knowledge of the structure and optical con-

stants of these Ti-doped WO3 films will allow us to under-

stand their electronic behaviour and design the materials for a

given technological application.

II. EXPERIMENTAL

A. Film growth

Titanium (Ti) doped WO3 thin films were deposited

onto silicon (Si) wafers by radio-frequency (RF)

(13.56 MHz) magnetron sputtering. The Si(100) wafers and

quartz substrates were cleaned by RCA and chemical clean-

ing, respectively, as reported elsewhere.52–55 After cleaning,

all the substrates were dried with nitrogen before introducing

them into the vacuum chamber, which was initially evac-

uated to a base pressure of �10�6 Torr. The co-sputtering

was performed using the W and Ti metal targets

(Plasmaterials, Inc.) of 200 diameter and 99.95% purity. The

targets were placed on a 2-in. sputter gun, which was corre-

spondingly placed at a distance of 7 cm from the substrate.

The sputtering powers applied to W and Ti targets are 100 W

and 50 W, respectively. A sputtering power of 40 W was ini-

tially applied to the targets while introducing high purity ar-

gon (Ar) into the chamber causing plasma ignition. Once

ignited, the power was increased to the desired set value for

each target and oxygen (O2) was released into the chamber

for reactive deposition. The flow of the Ar and O2 and their

ratio were controlled using MKS mass flow meters. Before

each deposition, both the targets were pre-sputtered for

10 min using Ar alone with the shutter above the gun closed.

The samples were deposited at different temperatures (Ts)

varying from room temperature (RT¼ 27 �C) to 400 �C. The

deposition was made to obtain a film thickness of �40 nm.

The film thickness was verified with various analytical meth-

ods to ensure that films are reasonably with a constant thick-

ness for comparison of the data and analysis presented, and

to understand the effect of growth temperature on the struc-

ture and properties.

B. Characterization

The grown Ti-doped WO3 films were characterized by

performing structural and optical measurements. X-ray dif-

fraction (XRD) measurements on the samples grown on

Si(100) were performed by using a Bruker D8 Advance x-ray

diffractometer. In order to avoid interference by the substrate

and obtain diffraction pattern of the coatings, grazing inci-

dence X-ray diffraction (GIXRD) was performed on the films.

All the measurements were made ex-situ as a function of Ts.

GIXRD patterns were recorded using Cu Ka radiation

(k¼ 1.54056 �̊A) at RT. A high voltage of 40 kV was used to

generate the X-rays. The GIXRD patterns were recorded

employing the X-ray beam fixed at a grazing incidence of 1�.The scanning was performed in a 2h range of 15–70� using

the “detector scan” mode, where the detector was independ-

ently moved in the plane of incidence to collect the diffraction

pattern. The optical properties and surface/interface character-

istics were probed by SE. The measurements were made using

a J. A. Woollam VVASE spectroscopic ellipsometer operating

in the wavelength range of 250–1350 nm with a step size of

2 nm and at angles of incidence of 65�, 70�, and 75�, near the

Brewster’s angle of silicon. Number of revolutions per mea-

sure was set at 20. The ellipsometry data analysis was per-

formed using commercially available WVASE32 software.56

III. RESULTS AND DISCUSSION

The GIXRD patterns of WO3 films are shown in Fig. 1

as a function of Ts. The XRD patterns of Ti-doped WO3

films grown at Ts< 400 �C did not show any peaks indicat-

ing their amorphous (a-WO3) nature. The XRD peak corre-

sponding to monoclinic WO3 (m-WO3) phase appears for

Ti-doped WO3 films when Ts¼ 400 �C. The intense peak at

23.1� corresponds to diffraction from (002) planes. For pure

WO3 films, this peak corresponding to m-WO3 phase appears

when the Ts¼ 100 �C.54,55 However, initially it was broad

and then becomes narrower and intense with increasing Ts.

An increase in the average crystallite-size and preferred ori-

entation of the film along (002) with increasing Ts especially

for WO3 films grown at Ts� 200 �C was evident in our ear-

lier studies.54,55 In the present case of Ti-doped WO3

co-sputtered films, absence of diffraction peaks in GIXRD

patterns, it can be concluded that the films are completely

amorphous for Ts< 400 �C. Higher Ts (400 �C) required to

obtain nanocrystalline Ti-doped WO3 films compared with

pure WO3 films is attributed due to the Ti-doping.

Co-sputtering of Ti and W to obtain doping of Ti into WO3

prevents the crystallization and thus induces the amorphous

nature to the resulting Ti-doped WO3 films.

Optical constants of the Ti-doped WO3 co-sputtered

films were primarily probed by SE, which measures the rela-

tive changes in the amplitude and phase of the linearly polar-

ized monochromatic incident light upon oblique reflection

from the sample surface. The experimental parameters

obtained by SE are the angles W (azimuth) and D (phase

change), which are related to the microstructure and optical

properties, defined by57–60

q ¼ Rp=Rs ¼ tan W expðiDÞ; (1)

where Rp and Rs are the complex reflection coefficients of

the light polarized parallel and perpendicular to the plane of

FIG. 1. The GIXRD patterns of Ti-doped WO3 films co-sputtered at various

temperatures. The patterns confirm the absence of any diffraction peaks indicat-

ing the characteristic amorphous nature of the films grown at Ts¼RT-300 �C.

Samples deposited at Ts¼ 400 �C are nanocrystalline.

133511-2 Vargas et al. J. Appl. Phys. 115, 133511 (2014)

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incidence, respectively. The spectral dependencies of the

ellipsometric parameters, W and D, determined for Ti-doped

WO3 films grown at various temperatures are shown in

Fig. 2. The spectral dependencies of ellipsometric parame-

ters W (azimuth) and D (phase change) can be fitted with

appropriate models to extract film thickness and the optical

constants, i.e., the refractive index (n) and extinction coeffi-

cient (k), based on the best fit between experimental and

simulated spectra.57,58

The curves obtained for co-sputtered Ti-doped WO3

films indicate (Fig. 2) a reasonable agreement between the

experimental and simulation data. The Levenberg-Marquardt

regression algorithm was used for minimizing the

mean-squared error (MSE)57

MSE ¼ 1

2N �M

Xn

i¼1

Wexp: � Wcalc:ð Þrexp

Wi

( )224

þDexp: � Dcalc:

� �rexp

Di

( )2 #; (2)

where Wexp, Wcalc and Dexp, Dcalc are the measured (experi-

mental) and calculated ellipsometry functions, N is the num-

ber of measured W, D pairs, M is the number of fitted

parameters in the optical model, and r are standard devia-

tions of the experimental data points. Extracting meaningful

physical and optical parameters from SE requires the con-

struction of an optical model of the sample which generally

accounts a number of distinct layers with individual optical

dispersions. Interfaces between these layers are optical

boundaries at which light is refracted and reflected according

to the Fresnel relations. The optical stack model used to

simulate the spectra purpose of determining the optical con-

stants of Ti-doped WO3 co-sputtered films is schematically

shown in Fig. 3. The model contains, from top, Ti-doped

WO3 film, SiO2 interface, and Si substrate. The surface and

interface roughness were also considered in order to accu-

rately fit the experimental data. Tauc–Lorentz (TL) model

was used and empirical parameterization is based on the

Tauc expression for the imaginary part (e2) of the dielectric

function.42 For a single transition, the complex dielectric

function e2 is defined as57

e2 Eð Þ ¼ AL E0 C ðE � EgÞ2

ðE2�Eo2 Þ2 þ C2 E2

� 1E

" #; (3)

where E0 is the resonance energy, Eg represents band

gap energy, E photon energy, and AL, C are the amplitude

and broadening coefficient of e2 peak, respectively,

FIG. 2. The spectral dependence of W and D for Ti-doped WO3 films grown at various temperatures. The experimental data obtained and modeling curves are

shown.

FIG. 3. The optical stack model of the Ti-doped WO3 thin film sample con-

structed for ellipsometry data analysis.

133511-3 Vargas et al. J. Appl. Phys. 115, 133511 (2014)

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The aforementioned model allowed for the determination

of the optical constants, n and k, as well as thickness verifi-

cation. Note that the real and imaginary parts of the dielec-

tric function are related to n and k as e1¼ n2� k2; e2¼ 2nk.

The set of Ti-doped WO3 films was modeled with TL

oscillators while attempting to locate the best fit for the data

by minimizing the MSE. The modeling parameters are listed

in Table I. The governing oscillator of the genosc layer, the

TL oscillator has four unique parameters associated with it

and they are the amplitude (AL) of the e2 peak, the half width

(C) of the e2 peak, fixed center energy (E0) of the TL peak,

and the Tauc gap (Eg). This particular model employed for

the set of Ti doped WO3 films has been proven effective, in

the literature, to model reasonably transparent conducting

oxides.41,58,61–63 The microstructure information, specifi-

cally, film thickness and interfacial oxide thickness of

Ti-doped WO3 films were determined from SE analysis. The

variation of film thickness as a function of growth tempera-

ture is shown in Fig. 4. It is evident that the film thickness is

nearly constant with increasing Ts. Most important is that the

interfacial oxide (SiO2) is limited to 2–3 nm at the interface.

This result was consistent with our earlier reports for either

pure or T-doped WO3 films, where there was no significant

interfacial oxide growth.41

The most important optical parameters determined from

SE data are the optical constants, namely, the index of

refraction (n) and extinction coefficient (k). The dispersive

optical constants of Ti-doped WO3 co-sputtered films are

shown in Figs. 5 and 6. Spectral dependence of the extinction

coefficient (k) determined from SE data for Ti-doped WO3

films is shown in Fig. 5. It is evident that “k” values are low

and very close to zero in most part of the spectrum (Fig. 5)

which indicates very low optical losses due to absorption.

The onset or sharp increase in “k” at short wavelengths is

due to the fundamental absorption across the band gap. An

understanding of the structural quality of Ti-doped WO3

films can also be derived from the dispersion profiles of k(k).

Specifically, the curves (Fig. 5) indicate that the k value of

the Ti-doped WO3 co-sputtered films is almost zero in the

visible and near infrared spectral regions, while for photon

energies towards ultraviolet region, “k” increases sharply.

The k(k) behavior is obviously related to the optical quality

of the films. Strong absorption with no weak shoulders or

tailing behavior for the Ti-doped WO3 films can be attributed

to the high quality of the grown layers with a very high trans-

parency. The dispersion profiles of index of refraction (n)

determined from SE data for Ti-doped WO3 films are shown

in Fig. 6. The results indicate a similar behavior as noted in

k(k). The “n” dispersion curves also indicate a sharp increase

at shorter wavelengths corresponding to fundamental absorp-

tion of energy across the band gap. However, the effect of Ts

is evident in the dispersion curves (Fig. 6), where there is an

increase in “n” values with increasing Ts.

In order to understand the physics of Ti-doped WO3

films and the effect of Ts on their optical constants, the “n”

variation of the films at k¼ 550 nm with Ts is shown in

Fig. 7. At a k¼ 550 nm, the “n” values increase from 2.15 to

2.40 with increasing Ts from 25 to 400 �C. For comparison,

at 550 nm, the “n” values of bulk, crystalline WO3 (Refs. 1

and 64) and TiO2 (Ref. 65) are 2.2 and 2.5, respectively. It

can be seen (Fig. 7) that the data exhibit a linear dependence

on Ts indicating the functional Ts-n linear relationship of

Ti-doped WO3 co-sputtered films. A simple model can be

formulated to explain the observed functional Ts-n

FIG. 4. Thickness of Ti-doped WO3 co-sputtered films grown at various Ts.

TABLE I. Ellipsometry modelling parameters of W-Ti-O films.

Temperature (�C)! RT 200 300 400

Oscillator 1 Tauc-Lorentz Tauc-Lorentz Tauc-Lorentz Tauc-Lorentz

AL 97.736 6 2.42 108.01 6 1.1 104.35 6 1.26 98.145 6 2.24

E0 4.3604 6 0.00958 4.4931 6 0.0037 4.3571 6 0.0046 4.5279 6 0.00735

C 1.7857 6 0.034 1.9407 6 0.0151 1.6371 6 0.014 1.4972 6 0.0186

Eg 3.113 6 0.00959 3.093 6 0.00398 2.9853 6 0.00551 2.8673 6 0.0129

FIG. 5. The k(k) curves of Ti-doped WO3 films grown at various Ts. The

extremely low values of k at k> 400 nm can be attributed to the transparent

nature.

133511-4 Vargas et al. J. Appl. Phys. 115, 133511 (2014)

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relationship in co-sputtered Ti-doped WO3 films. Evident

from the results, the optical constants and their dispersion

behavior depend on Ts. GIXRD measurements demonstrated

that the Ti-doped WO3 co-sputtered films grown at

Ts< 400 �C are completely amorphous while Ts¼ 400 �Ccrystallizes the film in a monoclinic phase. The observed

increase in “n” values when Ti-doped WO3 films are grown

at higher Ts can be attributed to the improved packing den-

sity of the films. However, we believe that the packing den-

sity slightly improves but not significantly. Perhaps, the

temperature is not enough to promote the structural order but

is sufficient to increase the mobility of ad-atoms to join to-

gether to increase the packing density of the materials in the

films leading to observed increase in “n” values. In order to

confirm the validity of this hypothesis, the relative density

(which is the ratio of film density to that bulk of the material)

of Ti-doped WO3 co-sputtered films has been determined

from Lorentz-Lorentz relation66

P ¼qf

qb

¼ ðnf �1Þ2

ðnf þ 1Þ2� ðnb þ 1Þ2

ðnb � 1Þ2

" #; (4)

where q is the density, nf is the refractive index of the deposited

Ti-doped WO3 film, and nb is the refractive index of the bulk

material. The measured “n” values from SE are used for

Ti-doped WO3 films. The functional dependence of the relative

density of Ti-doped WO3 films on the Ts is shown in Fig. 8. It

can be seen that there is a direct correlation between the index

of refraction, relative density, and growth temperature suggest-

ing that the improved packing density accounts for the observed

optical constants and their behavior in Ti-doped WO3 films.

IV. CONCLUSIONS

Ti-doped WO3 films were fabricated using

sputter-deposition by varying deposition temperature in the

range of 25–400 �C. The structure and optical properties of

the grown Ti-doped WO3 films were evaluated as a function

of growth temperature. Crystal structure analyses indicate

and confirm that Ti-doped WO3 co-sputtered films were

amorphous at up to Ts¼ 400 �C, at which point formation

of nanocrystalline, monoclinic films occurs. The effect

Ti-incorporation and associated amorphization is dominant

except at Ts¼ 400 �C. The dispersive optical constants were

evaluated and a correlation between Ts and optical property

of the Ti-doped WO3 films was established based on the spec-

troscopic ellipsometry results. Ti-doped WO3 films grown at

Ts¼ 25–400 �C are highly transparent and exhibit low optical

losses in the visible and near infrared regions. The SE data

indicate that the index of refraction of the samples increases

from 2.15 to 2.40 with increasing Ts from 25 to 400 �C.

Relative density determined from Lorentz-Lorentz relation

using the measured “n” also correlates with proposed mecha-

nism of improved packing density in the films with increasing

Ts leading to the linear trend observed in index of refraction

values of Ti-doped WO3 co-sputtered films.

ACKNOWLEDGMENTS

This work is supported by the National Science

Foundation (NSF) with NSF-PREM Grant No. DMR-1205302.

1C. G. Granqvist, Handbook of Inorganic Electrochromic Materials(Elsevier, New York 1995).

FIG. 6. The dispersion profiles of index refraction of Ti-doped WO3 films

grown at various Ts. The sharp increase in index of refraction at k� 400 nm

is due to fundamental absorption across the band gap of the films.

FIG. 7. The functional dependence of index of refraction measured at

k¼ 550 nm on the Ts for Ti-doped WO3 co-sputtered films. A gradual

increase in index of refraction suggests improved packing density of the

films with Ts.

FIG. 8. The functional dependence of relative density of the Ti-doped WO3

co-sputtered films on Ts. The relative density values were calculated from

Lorentz-Lorentz model.

133511-5 Vargas et al. J. Appl. Phys. 115, 133511 (2014)

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2Y. Zhang, S. H. Lee, A. Mascarenha, and S. K. Deb, Appl. Phys. Lett. 93,

203508 (2008).3C. V. Ramana, S. Utsunomiya, R. C. Ewing, C. M. Julien, and U. Becker,

Phys. Chem. B 110, 10430 (2006).4L. Berggren, J. C. Jonsson, and G. A. Niklassoon, J. Appl. Phys. 102,

083538 (2007).5Y. S. Lin, H. T. Chen, and J. Y. Lai, Thin Solid Films 518, 1377 (2009).6D. Lu, J. Chen, H. J. Chen, J. Gong, S. Z. Deng, N. S. Xu, and Y. L. Liu,

Appl. Phys. Lett. 90, 041919 (2007).7D. S. Lee, S. D. Han, J. S. Huh, and D. D. Lee, Sens. Actuators, B 60, 57

(1999).8S. H. Baeck, T. Jaramillo, G. D. Stucky, and E. W. McFarland, Nano Lett.

2, 831 (2002).9D. S. Lee, K. H. Nam, and D. D. Lee, Thin Solid Films 375, 142 (2000).

10L. Wang, S. E. Teleki, S. E. Pratsinis, and P. I. Gouma, Chem. Mater. 20,

4794 (2008).11H. Kawasaki, J. Namba, K. Iwatsuji, Y. Suda, K. Wada, K. Ebihara, and T.

Ohshima, Appl. Surf. Sci. 197–198, 547 (2002).12R. Ionescu, E. Llobet, J. Brezmes, X. Vilanova, and X. Correig, Sens.

Actuators, B 95, 177 (2003).13S. C. Moulzolf, S. Ding, and R. J. Lad, Sen. Actuators, B 77, 375 (2001).14M. Satnkova, X. Vilanova, E. Llobet, J. Calderer, C. Bittencourt, J. J.

Pireaux, and X. Correig, Sens. Actuators, B 105, 271 (2005).15G. Xie, J. Yu, X. Chen, and Y. Jiang, Sens. Actuators, B 123, 909 (2007).16S. Santucci, C. Cantalini, M. Crivellari, L. Lozzi, L. Ottaviano, and M. J.

Passacantaso, J. Vac. Sci. Technol. A 18, 1077 (2000).17P. I. Gouma and K. Kalyanasundaram, Appl. Phys. Lett. 93, 244102

(2008).18T. P. Huelser, A. Lorke, P. Ifeacho, H. Wiggers, and C. J. Schulz, J. Appl.

Phys. 102, 124305 (2007).19B. Marsen, E. L. Miller, D. Paluselli, and R. E. Rocheleau, Int. J.

Hydrogen Energy 32, 3110 (2007).20D. Paluselli, B. Marsen, E. L. Miller, and R. E. Rocheleau, Electrochem.

Solid State Lett. 8, G301 (2005).21Y. Sun, C. J. Murphy, K. R. Reyes-Gil, E. A. Reyes-Garcia, J. M.

Thornton, N. A. Morris, and D. Raftery, Int. J. Hydrogen Energy 34, 8476

(2009).22H. Zheng, Y. Tachibana, and K. Kalantar-Zadeh, Langmuir 26, 19148

(2010).23H. Zheng, J. Z. Ou, M. S. Strano, R. B. Kaner, A. Mitchell, and K.

Kalantar-Zadeh, Adv. Funct. Mater. 21, 2175 (2011).24J. Z. Ou, S. Balendhram, M. R. Field, D. G. McCulloch, A. S. Zoolfakar,

R. A. Rani, S. Zhuiykov, A. P. O’Mullane, and K. Kalantar-Zadeh,

Nanoscale 4, 5980 (2012).25M. R. Waller, K. T. Townsend, J. Zhao, E. M. Sabio, R. L. Chamousis, N.

D. Browning, and F. E. Osterloh, Chem. Mater. 24, 698 (2012).26S. J. Wang, W. J. Lu, G. Cheng, K. Cheng, and X. H. Jiang, Appl. Phys.

Lett. 94, 263106 (2009).27J. H. Ha, P. Muralidharan, and D. K. Kim, J. Alloys Compd. 475, 446

(2009).28J. Zhang, J. Tu, X. Xia, X. Wang, and C. Gu, J. Mater. Chem. 21, 5492

(2011).29Z. Jiao, X. W. Sun, J. Wang, L. Ke, and H. V. Demir, J. Phys. D: Appl.

Phys. 43, 285501 (2010).30J. Wang, E. Khoo, P. S. Lee, and J. Ma, J. Phys. Chem. 112, 14306 (2008).31D. Ma, G. Shi, H. Wang, Q. Zhang, and Y. Li, J. Mater. Chem. 1, 684

(2013).32A. Phuruangrat, D. J. Ham, S. J. Hong, S. Thongtem, and J. S. Lee,

J. Mater. Chem. 10, 1983 (2010).

33J. Su, X. Feng, J. D. Sloppy, L. Guo, and C. A. Grimes, Nano Lett. 11, 203

(2011).34L. Li, Y. Zhang, X. Fang, T. Zhai, M. Liao, X. Sun, Y. Koide, Y. Bando,

and D. Golberg, J. Mater. Chem. 21, 6525 (2011).35C. W. Lai and S. Sreekantan, Int. J. Hydrogen Energy 38, 2156 (2013).36W. L. Kwong, N. Savvides, and C. C. Sorrell, Electrochim. Acta 75, 371

(2012).37V. S. Vidyarthi, M. Hofmann, A. Savan, K. Sliozberg, D. K€onig, R.

Beranek, W. Schuhmann, and A. Ludwig, Int. J. Hydrogen Energy 36,

4724 (2011).38C. Santato, M. Odziemkowski, M. Ulmann, and J. Augustynski, J. Am.

Chem. Soc. 123, 10639 (2001).39R. Kirchgeorg, S. Berger, and P. Schmuki, Chem. Commun. 47, 1000

(2011).40H. J. Chen, N. S. Xu, S. Z. Deng, D. Y. Lu, Z. L. Li, J. Zhou, and J. Chen,

Nanotechnology 18, 205701 (2007).41C. V. Ramana, G. Baghmar, E. J. Rubio, and M. J. Hernandez, ACS Appl.

Mater. Interfaces 5, 4659 (2013).42M. Mahanthi, T. Ghosh, and D. Basak, Nanoscale 3, 4427 (2011).43M. W. Xiao, L. S. Wang, X. J. Huang, Y. D. Wu, and Z. Dang, J. Alloys.

Compd. 470, 486 (2009).44J. Hong and W. I. Lee, Chem. Mater. 18, 847 (2006).45V. Keller, P. Bernhardt, and F. Garin, J. Catal. 215, 129 (2003).46M. Kobayashi and K. Miyoshi, Appl. Catal. B 72, 253 (2007).47S. Higashimoto, M. Sakiyama, and M. Azuma, Thin Solid Films 503, 201

(2006).48W. Smith and Y. Zhao, J. Phys. Chem. C 112, 19635 (2008).49W. Smith, A. Wolcott, R. C. Fitzmorriz, J. Z. Zhang, and Y. Zhao,

J. Mater. Chem. 21, 10792 (2011).50M. Ferroni, D. Boscarino, E. Comini, D. Gnani, V. Guidi, G. Martinelli, P.

Nelli, V. Rigato, and G. Sberveglieri, Sens. Actuators, B 58, 289 (1999).51M. Gerlich, S. Kornely, M. Fleishcher, H. Mixner, and R. Kassing, Sens.

Actuators, B 93, 503 (2003).52N. R. Kalidindi, F. S. Manciu, and C. V. Ramana, ACS Appl. Mater.

Interfaces 3, 863 (2011).53N. R. Kalidindi, K. Kamala Bharathi, and C. V. Ramana, Appl. Phys. Lett.

97, 142107 (2010).54S. K. Gullapalli, R. S. Vemuri, and C. V. Ramana, Appl. Phys. Lett. 96,

171903 (2010).55R. S. Vemuri, K. Kamala Bharathi, S. K. Gullapalli, and C. V. Ramana,

ACS Appl. Mater. Interfaces 2, 2623 (2010).56J. A. Woollam Co., Inc., Guide to Using WVASE32 Spectroscopic

Ellipsometry Data Acquisition and Analysis Software (J. A. Woollam

Company, Incorporated, 2008).57G. E. Jellison, Jr., Thin Solid Films 313–314, 33 (1998).58H. Fujiwara, Spectroscopic Ellipsometry: Principles and Applications

(John Wiley & Sons, Inc., 2007).59C. V. Ramana, S. Utsunomiya, R. C. Ewing, U. Becker, V. V. Atuchin, V.

Aliev, and V. N. Kruchinin, Appl. Phys. Lett. 92, 011917 (2008).60V. H. Mudavakkat, V. V. Atuchin, V. N. Kruchinin, A. Kayani, and C. V.

Ramana, Opt. Mater. 34, 893 (2012).61J. Tauc, R. Grigorovici, and A. Vancu, Phys. Status Solidi B 15, 627 (1966).62P. I. Rovira and R. W. Collins, J. Appl. Phys. 85, 2015 (1999).63H. Fujiwara and M. Kondo, Phys. Rev. B 71, 075109 (2005).64K. Hong, K. Kim, S. Kim, I. Lee, H. Cho, S. Yoo, H. W. Choi, N. Y. Lee,

Y. H. Tak, and J. L. Lee, J. Phys. Chem. C 115, 3453 (2011).65U. Diebold, Surf. Sci. Rep. 48, 53 (2003).66M. Born and E. Wolf, Principles of Optics (Cambride Unversity Press,

Cambridge, 1999).

133511-6 Vargas et al. J. Appl. Phys. 115, 133511 (2014)

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