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Applied Surface Sciencejournal homepage: www.elsevier.com/locate/apsusc

Full Length Article

Large and constant absorption coeﬃcient in NbxTi1−xO2 thin ﬁlms
throughout the visible range

7

Adam E. Shimabukurob, Akihiro Ishiia, Itaru Oikawaa, Yusuke Yamazakic, Masaaki Imurac,⁎
Toshimasa Kanaic, Fumio S. Ohuchib, Hitoshi Takamuraa,aDepartment of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
Department of Materials Science and Engineering, College of Engineering, University of Washington, Seattle 98195, USAcThin Films Division, Nippon Electric Glass Co., Ltd., Nagahama 529-0292, Japanb

A R T I C LE I N FO

A B S T R A C T

Keywords:
Pulsed laser deposition
Non-dispersive media
NbO2
Ti4O7
Absorption coeﬃcient

We investigated the optical absorption properties of NbxTi1−xO2 thin ﬁlms, a solid solution combining the
reduced titanium and niobium oxide phases TiO2 and NbO2. The optical absorption properties of NbxTi1−xO2thin ﬁlms prepared by pulsed laser deposition at 600 °C were shown to be large in magnitude at an almost
constant value of ≈17 µm−1. Because this large absorption coeﬃcient is nearly independent of incident photon
energy in the visible range (400–700 nm), the NbxTi1−xO2 thin ﬁlms appear optically black. Flat and homogenous, optically black coatings like these are desirable for color isolation in ﬂat panel displays. The origin of ﬂatwavelength dispersion in NbxTi1−xO2 is the coexistence of semiconducting absorption and metallic light absorption mechanisms. Localized Nb-Nb dimers in the metallic NbxTi1−xO2 phase open an optical band gap which
gives rise to semiconducting behavior. We show here that reduction results in strong visible light sensitization innormally transparent Nb-Ti oxides.

1. Introduction
Optically absorbing coatings are important for applications both
future and present in the electronics display industry. Organic lightemitting displays (OLED) and liquid crystal displays (LCD), for example, require a black matrix coating to optically isolate the colors ofindividual pixels. Displays with good isolation have superior contrast
and can display dark colors. The black matrix also dictates the poweredoﬀ appearance of these displays. Ideal coatings for this application mustmeet two important criterion:(1) They must be homogeneous and ﬂat to avoid light scattering andfor compatibility with layered structures.
(2) They must have a large, consistent absorption coeﬃcient, α(E) toensure wavelengths in the visible light region are absorbed withconstant intensity.
Materials with such dispersion behavior appear exceptionally black.
Organic molecules with a variety of bond lengths and vibrational modes
are known to absorb many wavelengths of light in the visible spectrum[1,2]. Therefore, visible light absorbing coatings are typically madefrom organic resins and pigments. However, organic compounds have⁎poorer thermal stability and resistance to ultraviolet radiation thaninorganic metal oxides. Additionally, metal oxide coatings are compatible with many existing mass production facilities. A novel visiblelight-absorbing oxide ﬁlm with ﬂat wavelength dispersion should be thekey to improving the picture quality of LCD and OLED displays. It may
be possible to achieve such ﬂat wavelength dispersion in metal oxides
by combining an oxide with strong short wavelength absorption withanother oxide which has strong long wavelength absorption. Shortwavelenth absorption known as Tauc absorption [3–5] is typically applied to semiconductors while long wavelength absorption is known as
Drude absorption [3,6,7] is typical of metals.
It is known that oxygen vacancies increase optical absorption inmetal oxides by introducing free carriers and mid-gap defect levels[8–10]. A large amount of reduction has been show to lead to theperiodic ordering of oxygen vacancies in the form of crystallographic
shear planes [11]. For example, the Magnéli TinO2n-1 series crystallizes
into distorted rutile structures with shear planes along a pseudo-rutile caxis where edge-sharing octahedra become face-sharing in order toaccommodate reduced Ti3+ cations [12,13]. These phases are considered metallic as their Fermi energies lie just below their conduction
bands [14]. Accordingly, Magnéli titanium oxides exhibit free carrier
conductivity and optical absorption. This optical absorption is strongest

Corresponding author.
E-mail address: takamura@material.tohoku.ac.jp (H. Takamura).

https://doi.org/10.1016/j.apsusc.2018.09.036
Received 24 May 2018; Received in revised form 23 August 2018; Accepted 5 September 2018
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A.E. Shimabukuro et al.

in the long wavelength near infrared region.
Another oxide with metallic behavior is NbO2. NbO2 is particularlyinteresting because it undergoes a transition from an insulating phase toa metallic phase at around 800 °C [15–17]. This transition is accompanied by a transformation from distorted rutile type to regular rutile
type. In the insulating phase, alternating short (2.7 Å) and long (3.3 Å)
Nb-Nb dimers exist along the c axis [17]. The alternating bond length ofthese dimers gives rise to a Peierls type instability along the c-axis bychanging domians of periodic potential in the lattice, causing an insulator to metallic transition [15,17,18]. The instability opens an appreciable band gap of indirect nature by splitting the Nb-4d bands[17,18]. While the exact value of this gap is unclear, calculations haveevaluated it as low as 0.1 eV and spectroscopy measurements as high as
1.3 eV have been reported [15,16,18,19]. The transformation from insulator to metal is based on the thermal dissolution of long Nb-Nb dimers [20]. Sakata found that over a large composition range, Nb and Ti
form a solid solution of NbxTi1−xO2. From 0 < x < 0.85, the solid
solution is metallic rutile type at temperatures as low as 500 °C [20,21].
Because the closed shell Ti structure is less likely to dimerize than open
shell Nb4+, the introduction of Ti lowers the transformation temperature from insulating to metallic. Sakata also reported a nonlinear increase in the c axis, with 0 < x < 0.85 indicating the persistence ofsome short Nb-Nb dimers. The existence gap forming short Nb-Nb dimers in a metallic phase makes the NbxTi1−xO2 system an excellent
candidate for a ﬁlm that displays both long and short (Drude and Tauc)wavelength absorption. However, the optical properties of this solid
solution have not been reported. Here we show that the optical absorption of NbxTi1−xO2 is characteristic of both short wavelength indirect gap absorption and long wavelength free carrier absorption. It isalso shown that redox interactions between Nb and Ti occur in the solid
solution.

Table 1
Naming conventions for thin ﬁlms and the compositions of their PLD targets.
SampleCationic mol% Ti intargets

Cationic mol% Nb intargets

Deposition Temperature
[°C]

T100
TN25
TN67
N100

100
75
33
0

0
25
67
100

600
600
600
600

peaks were deconvoluted using a rutile powder reference sample. An
Nb-3 s peak near 470 eV was included in the Ti-2p peak deconvolution
in order to obtain an accurate ﬁt for samples containing both Nb and Ti.
Spectroscopic ellipsometry was used to obtain the optical propertiesof the ﬁlm. Measurements of the phase and amplitude changes, Δ and ψ
were obtained at 50° and 70° from normal at a spectral range of
250–1000 nm. Our model included a 0.5 mm Cauchy ﬁlm as the substrate, the deposited ﬁlm and an air layer. The complex pseudo-dielectric function was modelled from this data and the refractive index(n) and extinction coeﬃcient (k) were derived from this function. The
absorption coeﬃcient is deﬁned by α = 2πk / λ . A multi oscillator
scheme was used to model the dielectric functions of the ﬁlms. TaucLorentz oscillators were used throughout except for the near infrared
region of the T100. A metallic type Drude oscillator was used for thisregion because of its good ﬁt to the data. In every case, the modelobtained a mean squared error below 10. The compositions of the deposited ﬁlms and the naming conventions are summarized in Table 1.
The ﬁlms are referred to by these names hereafter.
3. Results and discussion
The ﬁlms were deposited under a high vacuum in order to introduce
oxygen vacancies and reduce the oxide cations for visible light sensitization. While stoichiometric TiO2, Nb2O5 and Nb5+ doped TiO2 are
typically transparent the results of this study show that reduction ishighly eﬀective to sensitize these oxides to visible light [16,22–25].
Since the ﬁlms deposited at lower than 600 °C were largely amorphous,
only polycrystalline ﬁlms deposited at 600 °C were used in this study.
Grazing incidence XRD and Raman spectroscopy were used for structural characterization and phase identiﬁcation; these spectra are shownin Fig. 1.
The XRD spectrum of the N100 ﬁlm deposited under high vacuum at
600 °C was indexed to distorted rutile type (I41/a) NbO2. This is theinsulating phase of this ﬁlm, its XRD spectrum is shown in Fig. 1a. T100
was largely amorphous due to deposition on the amorphous glass
substrate, thus oxygen deﬁciency related stacking faults are randomly
distributed throughout the ﬁlm. However, the Raman spectrum of T100
shown in Fig. 1b, is consistent with the previously reported Raman
spectra of Magnéli type Ti4O7 [13] indicating localized Ti4O7 coordination. While N100 was the low temperature insulating phase of
NbO2, the TN25 and TN67 ﬁlms were indexed to the metallic regularrutile phase of NbxTi1−xO2 showin in Fig. 1a. The absence of the XRD
peaks attributed to (1 0 1) and (2 1 1) planes might indicate a preferredorientation near the surface where grazing incidence XRD analyzes.
This is likely because the planes perpendicular to the c-axis have lower
surface energy than the other planes in the rutile type crystal structure[26]. The peaks of the Raman spectra of TN25 and TN67 ﬁlms wereidentical at 304 and 722 cm−1, as shown in Fig. 1b, and agree reasonably well with the Raman data reported for metallic rutile typeNbO2 [27]. Broadening of these Raman peaks can be attributed to theatomic displacement of Nb due to local dimerization. The Raman
spectrum of our NbO2 ﬁlm matches previously reported data well [28].
The positions of the XRD peaks shown in Fig. 1a were used to calculate
the a axis lengths of TN67 and TN25, at 4.783 Å and 4.728 Å, respectively. These values are consistent with the measurements of the a axis
versus composition in NbxTi1−xO2 reported by Sakata [20]. Expansion

2. Experimental
Films were synthesized by pulsed laser deposition (PLD). PLD targets were prepared using Nb(V) oxide powder (Kanto Chemical Ltd.:
purity 99.95%) and rutile Ti(IV) oxide powder (High Purity Chemical
Ltd.: purity 99.9%). Four targets, TiO2, Nb2O5, and TiNb2O7 and TiO2TiNb2O7 composite (25 cationic mol% Nb), were prepared by solid state
reaction at 1250 °C. All ﬁlms were deposited on amorphous alkali-freeglass(OA-10G,
NipponElectric
Glass
Co.,
Ltd.:
15 mm × 15 mm × 0.5 mm) using a KrF excimer laser (COMPexPro205, COHERENT, Inc,: λ = 248 nm) repeated at 5 Hz for 30 min
with a laser ﬂuence of 5.8 Jcm−2. The deposition chamber (PLAD-242,
AOV Co., Ltd.) was evacuated to an atmosphere of < 10−4 Pa. Thisvacuum condition was required in order to obtain NbO2 rather than
Nb2O5 and substoichiometric TiO2−x rather than TiO2. The glass substrates were heated to 600 °C by means of an infrared heating lamp.
This temperature was chosen because the phase study reported by
Sakata indicates both insulating NbO2 and metallic NbxTi1−xO2 arestable at this temperature [20,21]. Films deposited at 600 °C under ahigh vacuum were between 35 and 50 nm in thickness.
The structure of the as deposited ﬁlms were characterized withgrazing incidence X-ray diﬀraction (D8 Advance, Bruker AXS) with agrazing angle of 2° with a Cu-Kα radiation source (λ = 1.5418 Å) and
Raman spectroscopy (HR-800, Horiba Jobin Yvon S.A.S.) using a He-Ne
laser (λ = 632.8 nm). Surface morphology and average roughness were
measured by atomic force microscopy (AFM) (AFM, JSPM-5200. JEOL
Ltd.). The chemical states were measured using an XPS (Theta Probe,
Thermo Electron Corporation) with an Al-Kα source (λ = 1486.6 eV)from a ﬂood type electron gun and spectroscopic ellipsometry (M-2000,
J.A. Woollam Co., Ltd.). Due to the insulating nature of the sample
surfaces, XPS peak positions were corrected using a carbon 1 s reference
set at 285.00 eV. Gauss-Lorentz deconvolution was used for Nb-3d
peaks. This ﬁtting employed several ﬁxed parameters; between the 5/2
and 3/2 peaks the area ratio and energy separation were ﬁxed. Ti-2p
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A.E. Shimabukuro et al.

345

466

TN25

N100
T100

T100 (Ti4O7)
20

30

40

50

459

557

TN25

179

141

(200)

156

TN25

TN67

TN67
304

Intensity (a.u.)

N100 (NbO2)
(211)
(220)

(200)

(110)
(110)

579

408
N100

TN67

Intensity (a.u.)

635

813

(622)
(800)

(222)
(440)

(400)

RutileRutiletype NbxTi1-x
O22
1-xO

164

b)

Distorted
NbO
Distorted
rutileRutile
type NbO
2 2

722

a)
a)

50
60

1000

2Θ (degrees)

800

600

400

Raman shift (cm-1)

200

Fig. 1. Structural identiﬁcation data. (a) Thin ﬁlm XRD diﬀraction data with amorphous substrate background data subtracted, (b) Raman spectra of thin ﬁlms. The
diﬀuse peaks of TN67 and TN25 are caused by localized Nb-Nb dimers which inhibit long range order.

by the AFM images (Fig. 2). The variance in TN25 and TN67 absorption
coeﬃcient throughout the visible spectrum is in the range of 3 μm−1.
The absorption spectra of our thin ﬁlms was compared to Si, titanium nitride and Fe3O4 dispersion data from the literature [30–32]. Our
experimental absorption spectra along with literature absorption
spectra are plotted in the visible region in Fig. 4. From this image, it isclear that the absorption coeﬃcients of both TN25 and TN67 are verylarge and almost constant. While the wavelength disperson of Fe3O4,
which is used commercially as a black pigment, is also very ﬂat, themagnitude of its absorption is very small [33].
Since absorption coeﬃcient uniformity and magnitude are important characteristics of absorbing ﬁlms, a numerical assessment ofthese quantities is required. We describe the magnitude of absorption
spectra using the ﬁlm absorption coeﬃcient value at 550 nm, the wavelength at which the human retina is most sensitive [34]. The uniformity of a spectrum can be described quantitatively by the average
deviation from the absorption coeﬃcient at 550 nm. The average deviation is the absolute sum of deviations from the value at 550 nm
normalized by the number of data points in the set. For this analysis, weused only data in the visible range (400–700 nm). These values aremarked by colored squares in Fig. 4 and are tabulated numericallyalong with average deviations in Table 2. A material with ‘blackness’of the a axis with increasing Nb content is related to the incorporationof Nb in the lattice.
Since optical absorption is also dependent on surface roughness, weanalyzed surface morphology with AFM. The air-oxide interface of very
rough surfaces causes non specular absorption and reﬂection which
does not represent the optical response of a ﬁlm’s bulk [29]. Atomic
force microscopy analysis on our ﬁlms yielded acceptable surface
roughness values below 1 nm. TN25 had an average surface roughnessof 0.51 nm while TN67 had an average surface roughness of 0.68 nm.
Micrographs of the ﬁlm surfaces are shown in Fig. 2. Such small surface
deviations are not signiﬁcant sources of error for our absorption measurements, and suggest these thin ﬁlms are compatible with the layered
structures.
Film absorption spectra obtained from spectroscopic ellipsometry
and actual ﬁlm appearances are shown in Fig. 3. N100 appears slightly
red because of its strong absorption in the blue region but weak absorption in the red region. T100 appears slightly blue due to its strong
red region absorption but weak blue region absorption. TN25 and TN67
have strong absorption in both blue and red regions and accordingly,
the ﬁlms appear black with no coloration. For these NbxTi1−xO2 thinﬁlms, no light scattering is visible and no surface roughness layers are
required for the analysis with spectroscopic ellipsometry as indicatedFig. 2. AFM images of (a) TN67 and (b) TN25. In both cases, the average surface roughness in less than 1 nm.
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A.E. Shimabukuro et al.

35

a)

T100
TN25
TN67
N100

Magnéli-type
Ti4O7(T100)

Rutile-type
NbxTi1-xO2(TNxx)

Distorted rutile
NbO2(N100)-1absorption coefficient, α (µm )

30

b)

25

20

15

10

5

Blue-Black
400

600

Optically Black

Red-Black800

λ (nm)

Fig. 3. (a) Absorption spectra derived from spectroscopic ellipsometry with the visible spectrum represented graphically. (b) Film appearance and color transmittance. TN67 and TN25 rutile type NbxTi1−xO2 ﬁlms appear optically black because their absorption coeﬃcients are almost constant in the visible spectrum.

suitable for the electronic display industry should have a large absorption coeﬃcient at 550 nm with a small average deviation from thatvalue. The large absorption coeﬃcients, at 550 nm, of both TN25 andTN67 are comparable to that of metallic titanium nitride, and theirsmall average deviations are comparable to Fe3O4. This analysis shows
the rutile type NbxTi1−xO2 is an excellent candidate for optically black
absorbing layers because of its large, almost constant absorption coefﬁcient throughout the visible range.
The two primary mechanisms responsible for optical absorption
near the band edge are the primary interband transition and transitions
due to localized states. These localized transitions create Urbach tails
and result in a smaller amorphous band gap than the crystalline bandgap [35–38]. For the ﬁlms deposited in this study, the absorption edge
is outside the visible range, and the primary character we observe is theabsorption tail. This is clear from the spectral shapes in Fig. 3a. The
shapes of these tails is related to the electronic structure and the type oftransition which results in the tails.
The metallic absorption tail can be clearly diﬀerentiated from thesemiconducting tail. Tauc suggested the energy dependence of the absorption tail in amorphous semiconductors is a (ω)Tauc ∝ ω2 [35]. Although it is only applicable to allowed type transitions in indirect gap
semiconductors, this relationship has been used extensively to approximate the optical band gap with reasonable accuracy [3,39,40]. A
similar relationship was later derived for indirect gap semiconductors
1with forbidden type transitions by Nakahara et al. where a (ω) ∝ ω3 near2
the absorption edge and a (ω) ∝ ω3 at lower energies [41]. It then follows that the absorption tail of indirect gap semiconductors increases
with frequency, according to a (ω) ∝ ωn where n > 1 or decreases withthe wavelength, according to a (λ ) ∝ λn where n < 1.
Because of their large free carrier concentration, the absorption tailenergy dependence can also be derived for semiconductors which behave like metals [3]. Typically, the plasma frequency, ωp , of metals iswell into the ultraviolet region and reﬂectance is dominant in thevisible range. However, for metal-like semiconductors, we can apply
Drude conductivity, σDrude = (ne 2τ )/(m∗ (1−iωτ )) to the complex di∼∼ (ω) + ik(ω)]2 = εc + 4πiσ /ω where ω is inelectric function, ε (ω) = [ncident angular frequency, τ is the scattering time, m* is the eﬀective∼ (ω) and ∼k (ω) are the complex remass, εc is the core polarizability, nfractive index and extinction coeﬃcient, respectively. Given the re∼lationship, α = (2ωk (ω))/ c and assuming ωτ is much larger than 1 in
the visible range for a semiconductor with large free carrier

30enitridium
Titan

absorption coefficient,( m-1)

25

TN67

20

TN25
15

NbO2 (N100)
10

Ti 4O 7

0
(T10

)

5

Si
0
400

450

Fe3O4
500

550

600

650

700

(nm)
Fig. 4. Absorption spectra in the visible range from experimental (solid) andliterature (dashed) data. Colored squares mark the absorption coeﬃcient at
550 nm. (For interpretation of the references to colour in this ﬁgure legend, thereader is referred to the web version of this article.)
Table 2
Tabulated values for absorption coeﬃcient at 550 nm and the average deviation
within the visible range.
Material

Absorption coeﬃcient at 550 nm,
α(550 nm) [µm−1]

Average deviation[µm−1]

T100 (Ti4O7)
TN25
TN67
N100 (NbO2)
Fe3O4 [24]Titanium nitride[23]
Si [22]

5.94
17.62
19.10
12.77
1.19
22.51

2.59
0.74
0.83
0.96
0.28
4.64

0.42

1.21

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A.E. Shimabukuro et al.

concentration, it can be concluded that very simplistic free carrier absorption in semiconductors is described qualitatively by αDrude (ω) ∝ ω−2or αDrude (λ ) ∝ λ2 [3].
From the spectra in Figs. 3a and 4, it is clear that Ti4O7 (T100) has ametal like absorption tail where α increases with wavelength. Similarly,
metallic titanium nitride has an absorption tail which increases withwavelength. As expected, Si has a semiconducting band tail, and NbO2
(N100), which is known to be an indirect gap semiconductor, also has asemiconducting band tail [15]. Since the Peierls gap in NbO2 opens in
the Nb-4d orbital, the transition should be forbidden [15,17,18]. Accordingly, the absorption tail of N100 appears to have two regimes ofa (λ ) ∝ λn .
The absorption tails of the TN25, TN67 and Fe3O4 do not obey the
aforementioned power laws. Optical absorption in magnetite Fe3O4
originates from distinct O-2p to Fe-4s and Fe-3d to Fe-4s transitions[42]. Furthermore, both Fe2+ and Fe3+ exist simultaneously [42]. The
result is an absorption tail that is neither speciﬁcally semiconducting or
metallic. We believe a similar duality is responsible for the opticalabsorption characteristics of rutile type NbxTi1−xO2 (TN25 and TN67).
It is likely that a Peierls gap in the Nb-4d orbital in the metallic
NbxTi1−xO2 phase gives the solid solution both semiconducting andmetallic absorption. By tuning the Nb-Ti ratio and thus the amount ofband opening Nb-Nb dimers, the uniformity of the visible range absorption spectra can be controlled.
Absorption in TN25 and TN67 ﬁlms is more complicated than asimple mixture of N100 and T100. Using the measured absorption
spectra of T100 and N100, the mole fraction was synthesized based on
the addition absorption curves for TN25 and TN67. Curve synthesis forTN25 was carried out by scaling the absorption values of T100 by 75%
and adding them to the values of N100 scaled to 25%. A similar procedure was used for TN67, using 33% and 67%, respectively. These
synthesized curves are shown in Fig. 5. The deviation between measured absorption spectra and synthesized absorption spectra for TN25
and TN67 is likely due to slight oxygen non-stoichiometry and interactions between Nb and Ti cations.
XPS measurements gave insight into the chemical interactions
taking place in the NbxTi1−xO2 solid solution. The integration of deconvolved metal peaks and O-1s peaks yielded average values forcomposition and cationic valencies. The calculated average chemical
states of are as follows: T100 → Ti3.35+, N100 → Nb4.25+, TN67 →Ti3.76+&Nb4.33+. These values are in good agreement with the stoichiometry of the ﬁlm phases: Ti4O7, NbO2, Nb.67Ti.33O2. Since the XPS

4. Conclusion
We studied the optical absorption properties of the reduced titanium
and niobium oxide phases Ti4O7 and NbO2 as well as the solid solutionof the two. The visible spectrum absorption coeﬃcient power law,α (λ ) ∝ λn where n = −2 for semiconductors and n = 2 for metals applies reasonably well to insulating NbO2 and metallic Ti4O7. The absorption spectra of solid solution NbxTi1−xO2 appears to be a combination of both types. Due to its large, nearly constant absorption
coeﬃcient throughout the visible range, this rutile type NbxTi1−xO2 isan excellent candidate for optically black absorbing layers. Localized
Nb-Nb dimers in the metallic NbxTi1−xO2 phase open an optical band
gap resulting in the coexistence of semiconducting and metallic absorption mechanisms in a single phase. The extent of this semiconducting and metallic character can be manipulated by controlling
the Nb-Ti ratio. The Nb-Ti solid solution was found to be rather complex
from an electronic perspective because of cationic reduction and
40

a)

30

( m-1)

TN67 SYNTHESIZEDTN67 MEASURED

absorption coefficient,absorption coefficient,( m-1)

40

measurent is a surface sensitive method, we expect the bulk to be more
reduced than the measurements indicate. For example, the XPS spectrum of N100 shown in Fig. 6c exhibits a high concentration of Nb5+
which should not be present in NbO2. This large concentration of Nb5+
must be due to ﬁlm storage in open atmosphere.
XPS measurements were important to understand cationic interactions in NbxTi1−xO2. The Nb2+ state in TN67 not seen in N100 can beseen in Fig. 6. Additionally, more of the Ti cations were oxidized in
TN67 than in T100. We see that Ti tends to reduce Nb in solution. Thisis likely attributed the formation of a fully coordinated Ti octahedron
which is more energetically favorable than a fully coordinated Nb octahedron according to the reported free energy of formation values[43]. From the valency information alone, inferences can be made regarding the band structure of NbxTi1−xO2: namely, the conduction
band consists of TiO2-like Ti-3d orbitals, NbO2-like Nb-4d orbitals andprobably some hybridization of the two. Due to the contribution of NbNb dimers, there is also a small gap in the conduction band. Combined,
these conduction band features give rise to the non-typical absorption
spectra of the TN25 and TN67 ﬁlms. It is possible that other as-yet
unknown chemical mechanisms are at play in the NbxTi1−xO2 solid
solution. Some research has suggested Nb-Ti hybridization forms a nonbonding band near the valence band [44]. We can speculate that exotic
orbital hybridizations or charge transfer complexes exist, but only in
depth ab initio calculations can shed light on these processes.

20

10

b)
TN25 SYNTHESIZEDTN25 MEASURED

30

20

10

300

400

500

600

700

800

300

(nm)

400

500

600

(nm)

Fig. 5. Measured and synthesized absorption spectra for (a) TN67 and (b) TN25.
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A.E. Shimabukuro et al.

a)

b)
Nb5+

Intensity (a.u.)

Ti3+,2+
Nb

3+

Ti4+

Nb 3s

Nb4+
Nb2+

212

210

208

206

204

202 475470

Eb (eV)

465

460

455

460

455

Eb (eV)

c)

d)
Nb5+

Intensity (a.u.)

Ti3+,2+
Nb3+
Ti4+
Nb4+

212

210

208

206

204

202 475470

Eb (eV)465

Eb (eV)

Fig. 6. XPS spectra of thin ﬁlms (a) Nb-3d of TN67 (b) Ti-2p of TN67 (c) Nb-3d of N100 (d) Ti-2p of T100. In TN67, Nb is more reduced than in N100 and Ti is lessreduced than in T100.

Referencesoxidation since Ti tends to reduce Nb. Due to this eﬀect, the absorption
spectra of the NbxTi1−xO2 is diﬀerent from that of numerical combination of NbO2 and Ti4O7. Further investigation is necessary to elucidate how this cation interaction impacts the band structure, and, inturn, the optical absorption properties of the solid solution.

[1] S. Bum Yuk, W. Lee, J. Woong Namgoong, J. Choi, J. Bok Chang, S. Hun Kim, J. Yun
Kim, J. Pil Kim, Synthesis and characterization of bay-substituted perylene dyes for
LCD black matrix of low dielectric constant, J. Incl. Phenom. Macrocycl. Chem. 82(2015) 203–212.
[2] J. Choi, W. Lee, J.W. Namgoong, T.-M. Kim, J.P. Kim, Synthesis and characterization of novel triazatetrabenzcorrole dyes for LCD color ﬁlter and black matrix, Dye.
Pigment. 99 (2013) 357–365.
[3] M. Dresselhaus, G. Dresselhaus, S. Cronin, A. Gomes Souza Filho, in: Solid State
Properties, 2018.
[4] B. von Blanckenhagen, D. Tonova, J. Ullmann, Application of the Tauc-Lorentz
formulation to the interband absorption of optical coating materials, Appl. Opt. 41(2002) 3137–3141.
[5] L.V. Rodríguez-de Marcos, J.I. Larruquert, Analytic optical-constant model derived
from Tauc-Lorentz and Urbach tail, Opt. Express 24 (2016) 28561–28572.
[6] K. Tamura, K. Hirakawa, Y. Shimada, Drude absorption and electron localization in
GaAs/AlGaAs superlattices, Phys. B Condens. Matter. 272 (1999) 183–186.
[7] D. Svintsov, V. Ryzhii, T. Otsuji, Negative dynamic Drude conductivity in pumped
graphene, Appl. Phys. Express 7 (2014) 115101.
[8] C. Singh, S. Nozaki, S. Rath, Spectroscopic ellipsometry study of the free-carrier andband-edge absorption in ZnO thin ﬁlms: eﬀect of non-stoichiometry, J. Appl. Phys.
118 (2015) 195305.
[9] X. Chen, L. Liu, P.Y. Yu, S.S. Mao, Increasing solar absorption for photocatalysis
with black hydrogenated titanium dioxide nanocrystals, Science 331 (2011)

Acknowledgments
This work has been ﬁnancially supported in part by JSPS KAKENHI
grant number 18H03832. AS would like to thank the research placement done by Junior Year Programs in English (JYPE) at Tohoku
University. AI would like to express his appreciation for the ﬁnancialfrom the Grant-in-Aid for JSPS Research Fellows and theInterdepartmental Doctoral Degree Program for Multi-dimensional
Materials Science Leaders in Tohoku University. The authors would liketo acknowledge the vitally important encouragement and support madethrough the University of Washington-Tohoku University: Academic
Open Space (UW-TU:AOS). They also would like to thank Ms. Y. Ohiraand Mr. N. Akao for technical support in taking XPS measurements.
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A.E. Shimabukuro et al.

[27] B.X. Huang, K. Wang, J.S. Church, Y.-S. Li, Characterization of oxides on niobium
by raman and infrared spectroscopy, Electrochim. Acta 44 (1999) 2571–2577.
[28] Y. Zhao, Z. Zhang, Y. Lin, Optical and dielectric properties of a nanostructured
NbO2 thin ﬁlm prepared by thermal oxidation, J. Phys. D. 37 (2004) 3392.
[29] B.J. Stagg, T.T. Charalampopoulos, Surface-roughness eﬀects on the determinationof optical properties of materials by the reﬂection method, Appl. Opt. 30 (1991)
4113–4118.
[30] D.E. Aspnes, A.A. Studna, Dielectric functions and optical parameters of Si, Ge, GaP,
GaAs, GaSb, InP, InAs, and InSb from 1.5 to 6.0 eV, Phys. Rev. B. 27 (1983)
985–1009.
[31] J. Pﬂüger, J. Fink, W. Weber, K.P. Bohnen, G. Crecelius, Dielectric properties of
TiCx, TiNx, VCx, and VNx from 1.5 to 40 eV determined by electron-energy-loss
spectroscopy, Phys. Rev. B. 30 (1984) 1155–1163.
[32] M.R. Querry, Optical Constants, Kansas City, 1985.
[33] N. Mufti, T. Atma, A. Fuad, E. Sutadji, Synthesis and characterization of black, redand yellow nanoparticles pigments from the iron sand, AIP Conf. Proc. 1617 (2014)
165–169.
[34] X. Zhang, A. Bradley, L.N. Thibos, Achromatizing the human eye: the problem ofchromatic parallax, J. Opt. Soc. Am. A. 8 (1991) 686–691.
[35] J. Tauc, Optical properties and electronic structure of amorphous Ge and Si, Mater.
Res. Bull. 3 (1968) 37–46.
[36] E.A. Davis, N.F. Mott, Conduction in non-crystalline systems V. Conductivity, optical absorption and photoconductivity in amorphous semiconductors, Philos. Mag.
A J. Theor. Exp Appl. Phys. 22 (1970) 903–922.
[37] I. Bonalde, E. Medina, M. Rodríguez, S.M. Wasim, G. Marín, C. Rincón, A. Rincón,
C. Torres, Urbach tail, disorder, and localized modes in ternary semiconductors,
Phys. Rev. B. 69 (2004) 195201.
[38] S.J. Ikhmayies, R.N. Ahmad-Bitar, A study of the optical bandgap energy and
Urbach tail of spray-deposited CdS: In thin ﬁlms, J. Mater. Res. Technol. 2 (2013)
221–227.
[39] B.D. Viezbicke, S. Patel, B.E. Davis, D.P. Birnie III, Evaluation of the Tauc method
for optical absorption edge determination: ZnO thin ﬁlms as a model system, Phys.
Status Solidi. 252 (2015) 1700–1710.
[40] S. Tanemura, L. Miao, P. Jin, K. Kaneko, A. Terai, N. Nabatova-Gabain, Opticalproperties of polycrystalline and epitaxial anatase and rutile TiO2 thin ﬁlms by rf
magnetron sputtering, Appl. Surf. Sci. 212–213 (2003) 654–660.
[41] J. Nakahara, K. Kobayashi, A. Fujii, Indirect-Forbidden Transitions in TlCl and TlBr,
J. Phys. Soc. Jpn. 37 (1974) 1312–1318.
[42] A. Schlegel, S.F. Alvarado, P. Wachter, Optical properties of magnetite (Fe3O4), J.
Phys. C Solid State Phys. 12 (1979) 1157.
[43] Ellingham Diagram, Encycl. Inorg. Bioinorg. Chem. (2011), https://doi.org/10.
1002/9781119951438.eibd0278.
[44] S.A. Chambers, Y. Gao, Y.J. Kim, M.A. Henderson, S. Thevuthasan, S. Wen,
K.L. Merkle, Geometric and electronic structure of epitaxial NbxTi1-xO2 on
TiO2(110), Surf. Sci. 365 (1996) 625–637.

746–750 (80).
[10] H. Fujiwara, M. Kondo, Eﬀects of carrier concentration on the dielectric function of
ZnO: Ga and In2O3: Sn studied by spectroscopic ellipsometry: analysis of free-carrier
and band-edge absorption, Phys. Rev. B. 71 (2005) 75109.
[11] A.M. Stoneham, P.J. Durham, The ordering of crystallographic shear planes: theoryof regular arrays, J. Phys. Chem. Solids. 34 (1973) 2127–2135.
[12] L. Liborio, G. Mallia, N. Harrison, Electronic structure of the Ti4O7 Magnéli phase,
Phys. Rev. B. 79 (2009) 245133.
[13] M. Watanabe, Raman spectroscopy of charge-ordered states in Magnéli titaniumoxides, Phys. Status Solidi. 6 (2009) 260–263.
[14] H.K. Ardakani, Electrical and optical properties of in situ “hydrogen-reduced” titanium dioxide thin ﬁlms deposited by pulsed excimer laser ablation, Thin Solid
Films 248 (1994) 234–239.
[15] A. O’Hara, T.N. Nunley, A.B. Posadas, S. Zollner, A.A. Demkov, Electronic andoptical properties of NbO2, J. Appl. Phys. 116 (2014) 213705.
[16] Z. Weibin, W. Weidong, W. Xueming, C. Xinlu, Y. Dawei, S. Changle, P. Liping,
W. Yuying, B. Li, The investigation of NbO2 and Nb2O5 electronic structure by XPS,UPS and ﬁrst principles methods, Surf. Interface Anal. 45 (2013) 1206–1210.
[17] V. Eyert, The metal-insulator transition of NbO2: An embedded Peierls instability,
EPL Europhys. Lett. 58 (2002) 851.
[18] F.J. Wong, N. Hong, S. Ramanathan, Orbital splitting and optical conductivity of theinsulating state of NbO2, Phys. Rev. B. 90 (2014) 115135.
[19] J. Zhang, K.J. Norris, G. Gibson, D. Zhao, K. Samuels, M.M. Zhang, J.J. Yang,
J. Park, R. Sinclair, Y. Jeon, Z. Li, R.S. Williams, Thermally induced crystallizationin NbO2 thin ﬁlms, Nature 6 (2016) 34294.
[20] K. Sakata, Study of the phase transition in NbxTi1-xO2, J. Phys. Soc. Jpn. 26 (1969)1067.
[21] K. Sakata, Structural features of NbxTi1-xO2, Acta Crystallogr. Sect. B. 35 (1979)
2836–2842.
[22] J. Arbiol, J. Cerdà, G. Dezanneau, A. Cirera, F. Peiró, A. Cornet, J.R. Morante,Eﬀects of Nb doping on the TiO2 anatase-to-rutile phase transition, J. Appl. Phys. 92(2002) 853–861.
[23] T. Hitosugi, H. Kamisaka, K. Yamashita, H. Nogawa, Y. Furubayashi, S. Nakao,
N. Yamada, A. Chikamatsu, H. Kumigashira, M. Oshima, Y. Hirose, T. Shimada,T. Hasegawa, Electronic band structure of transparent conductor: Nb-doped anatase
TiO2, Appl. Phys. Express. 1 (2008) 111203.
[24] Y. Furubayashi, T. Hitosugi, Y. Yamamoto, K. Inaba, G. Kinoda, Y. Hirose,
T. Shimada, T. Hasegawa, A transparent metal: Nb-doped anatase TiO2, Appl. Phys.
Lett. 86 (2005) 252101.
[25] A. Ishii, K. Kobayashi, I. Oikawa, A. Kamegawa, M. Imura, T. Kanai, H. Takamura,
Low-temperature preparation of rutile-type TiO2 thin ﬁlms for optical coatings byaluminum doping, Appl. Surf. Sci. 412 (2017) 223–229.
[26] H. Perron, C. Domain, J. Roques, R. Drot, E. Simoni, H. Catalette, Optimisation of
accurate rutile TiO2 (110), (100), (101) and (001) surface models from periodic
DFT calculations, Theor. Chem. Acc. 117 (2007) 565–574.

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