Temperature dependent dielectric properties and AC conductivity studies on titanium niobium oxide (TiNb2O7) thin films

Journal Pre-proof Temperature dependent dielectric properties and AC conductivity studies on titanium niobium oxide (TiNb2 O7 ) thin films V. Daramalla, Soumyadeep Dutta, Krupanidhi S.B.

PII:

S0955-2219(19)30777-0

DOI:

https://doi.org/10.1016/j.jeurceramsoc.2019.11.032

Reference:

JECS 12857

To appear in:

Journal of the European Ceramic Society

Received Date:

5 August 2019

Revised Date:

8 November 2019

Accepted Date:

9 November 2019

Please cite this article as: { doi: https://doi.org/ This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Temperature dependent dielectric properties and AC conductivity studies on titanium niobium

ro of

oxide (TiNb2O7) thin films V. Daramalla1,3, * , Soumyadeep Dutta1,2, † , and Krupanidhi S.B.1 1

Quantum Structures and Device Lab, Materials Research Centre,

2

-p

Indian Institute of Science, Bangalore-560012 (India)

Centre for Nano Science and Engineering, Indian Institute of

3

re

Science, Bangalore-560012 (India)

Laboratory for Multiscale eXperiments, Thin Films and Interfaces

*

lP

group, Paul Scherrer Institute, Villigen-5232 (Switzerland) corresponding authors: [email protected], and [email protected]

na

†

Jo

ur

equal contribution

1

Abstract In the present work, the dielectric properties of TiNb2 O7 (TNO) thin films are studied for two different thicknesses in a Metal-Insulator-Metal configuration (Au/ TiNb2 O7 /Pt). The temperature dependence of the dielectric dispersion and the AC conductivity behavior is strongly dependent on two different thicknesses we considered in this study. Low thickness ( ∼ 236 nm) films shows

ro of

considerably less variation with temperatures than their thicker counterparts ( ∼ 480nm). These behaviors are explained on different physical basis – nearly constant loss (NCL) and Jonscher’s Universal Dielectric Response (UDR) for

the former and the latter, respectively. The dielectric constant of the films are typically large (∼ 59–73) and the dielectric loss are moderate (∼ 0.07–0.11).

-p

The AC conductivity values of the thin films lie in the range of about (10−8 to 10−9 ) at 10 kHz. The thicker films show show DC conductivity values which

re

are thermally activated with an activation energy of about 0.44 eV. This is slightly higher (about 0.1 eV) than ωp in Jonscher’s Universal response and this

lP

discrepancy is explained on the basis of mixed ionic electronic conduction in these complex oxide TNO films. The possible role of growth morphology in the dielectric response is also briefly mentioned.

na

Keywords— TiNb2 O7 thin films, high dielectric constant, dielectric relax-

Jo

ur

ation, pulsed laser deposition

2

1

Introduction

Titanium Niobium Oxide based materials are emerging as novel multi-functional metal oxides for potential applications such as energy storage, photo-catalysis and as high K-dielectrics.1–3 Owing to their layered crystal structures also known as WadsleyRuth shear structures, these materials offer mixed redox potentials, stable chemical states, high charge density, reversible kinetics and safety for their potential usage in energy storage and conversion devices.4, 5 Titanium niobium oxide based complex ox-

ro of

ides form three major systems of formula Me3 O7 (TiNb2 O7 ), Me12 O29 (Ti2 Nb10 O29 ), and MeO2 (TiNb24 O64 ) (Me = Ti, Nb) in TiO2 -Nb2 O5 equilibrium thermodynamic phase diagrams. These complex crystal structures of Titanium Niobium Oxides con-

sist of fragments of ReO3 -type structure, in the form of blocks via corner and edge shared metal–oxygen octahedra (MO6 ) along b-axis, in which different stoichiometric

-p

structure forms depending on the size of ReO3 blocks and the way they are joined to-

gether.6, 7 However, the fundamental understanding of structure-property correlation

re

is not well understood due to their complex crystal structures. Detailed investigations are henceforth needed in order to use them in various device technologies.

lP

Among the three complex TNO oxide systems, TiNb2 O7 is of a special interest due to its low stoichiometric formula (TiO2 :Nb2 O5 ); it can be studied as a prototypical system. TiNb2 O7 (TNO) is a monoclinic-layered crystal structure with 3 X 3

na

ReO3 shear structure with TiO6 , NbO6 octahedra sharing via corner and edges in disordered manner.8 In recent years, the energy storage performance of monoclinic TNO material have been extensively studied in various forms such as bulk particles, vari-

ur

ous form of micro- and nano-strcutures.9–14 A significant progress has been achieved for using TNO based materials in rechargeable Li-ion batteries.15–21 Interestingly, the wide-bandgap semiconducting nature ( ∼ 2.9 eV), and highly optical transparent prop-

Jo

erties of TNO oxide materials can be well-employed for next-generation transparent oxide-semiconductors, and electrochromic devices.22 Despite this, studies on electrical or dielectric properties of TNO complex oxide materials are not well understood or rarely available in the literature. The earliest dielectric studies on TNO oxides are to be found by Cava et.al.23 The TiO2 -Nb2 O5 crystallographic shear structures shows enhanced dielectric properties (dielectric constant κ ∼ 100) in bulk ceramic oxide form

3

then that of well-known Nb2 O5 oxide systems that resembles similar crystal structure. The dielectric constant and low dielectric loss (dissipation factor) of bulk TNO shear structures can be improved by aliovalent substition of Nb5+ by Ta4+ which reduces the oxygen vacancy concentrations. Lashtabeg et.al., (2003) studied the DC electrical conductivity of doped-niobium titanates in a reduced environment for using them as current collectors in solid oxide fuel cells.24 Xing et. al., (2013) investigated the possible defect chemistry in mixed-ionic TiNb2 O7 ceramic pellets via measuring the

ro of

electronic conductivity as a function of temperature in various gaseous environments.25 Very recently, Chang et.al., studied the dielectric properties of sol-gel derived TNO thin films as potential high-K gate dielectric oxide for usage in low-power

consumption transparent oxide electronics.26 These amorphous TNO thin films are

studied in Au/TNO/ITO (indium tinoxide) device configuration which shows high

-p

dielectric constant (> 30 at 1 MHz frequency) with low leakage current density (<1

X 10–6 A/cm2 ) with more than 80% transmittance in the visible region. Neverthe-

re

less, electrical transport properties cannot be overlooked in any of such applications; dielectric relaxation and AC conductivity are some of the major areas which need further elaborations. In our earlier work, we have successfully fabricated monoclinic

lP

TNO crystalline thin films and demonstrated them as novel anode materials for Li-ion micro-batteries.27, 28 On the other hand, our present investigations are mainly focused on studying the temperature dependent dielectric properties of polycrystalline TNO

na

thin films. These properties of TNO thin films are studied in metal-insulator-metal (Au/TNO/Pt) device configuration. Polycrystalline TNO thin films showed some interesting dielectric properties, which are very sensitive to their morphological features.

ur

Our systematic experimental investigation confirms that the TNO films exhibited very stable (temperature independent) and enhanced dielectric constant (∼ 59 - 73) up to

160◦ C. In addition, the origin of frequency dependent dielectric dispersion, and AC

Jo

conductivity behavior in TNO thin films are also discussed.

4

2

Experimental Details

TNO thin films were grown using pulsed laser deposition method. A pulsed laser of 248 nm wavelength, 20 ns pulse width was employed to ablate the TNO target. Whereas TNO target was synthesized via conventional solid-state reaction. The details of experimental procedure and PLD growth conditions were described elsewhere.28 In brief, platinized silicon Pt (200)/TiO2 /SiO2 /Si(100) substrates were cleaned and boiled in acetone, isopropyl alcohol (IPA) for 5 to 10 minutes for removing organic

ro of

contamination and then dried using dry N2 gas. As-cleaned substrates and TNO target were loaded minutes in PLD chamber and then the chamber was evacuated to

base pressure of 5X10−6 mbar, prior to TNO film growth. All TNO films were grown in oxygen partial pressure (PO2 ) of 1.3X10–1 mbar, growth temperature 750◦ C, laser

fluence of 4.6 J/cm2 , and 7 Hz laser frequency. The substrate to TNO target distance

-p

was kept at ∼ 5 cm during the PLD growth. Two TNO samples were grown with

different thickness, labelled as TNO20 (20 mins, ∼ 236 nm), and TNO40 (40 mins,

re

∼ 480 nm), respectively. In the discussion that follows, we will refer to these films interchangeably as TNO20 or thinner films and TNO40 or thicker films, respectively.

lP

The structure, morphology of as-grown TNO films were characterized using X-ray diffraction (Cu Kα1 1.54059 ˚ A), field emission scanning electron microscopy (FEI Inspect F50) and atomic force microscopy (Veeco dII system), respectively. The

na

dielectric properties of TNO films were measured in standard Metal-Insulator-Metal (MIM) device configuration. Agilent 4294A impedance analyzer was employed for dielectric measurements of TNO films. For temperature dependent dielectric mea-

ur

surements, a Karl Suss probe-station PM5 (with thermal chuck) controlled with an ATT temperature controller was used. The frequency dispersion data was acquired with a 4294A precision impedance analyzer from Agilent. The main discussion in this

Jo

manuscript pertains to the frequency range of 1 kHz – 1 MHz with additional sweeps into the lower frequency range of 40 Hz reported separately in the supplementary information. Signals were averaged for multiple sweep cycles for noise reduction and also several devices on a single source were probed for reproducibility. For the purposes of dielectric relaxation studies it is necessary to study the

5

complex dielectric permittivity (˜  ) defined by: ˜ = 0 − i00

(1)

The impedance analyzer has the provision to present this basic information in the form of different equivalent circuit models. We found it convenient and simple to choose the Cp -G model which reports the data in the form of a parallel combination

3

Results and Discussion

Structure and morphology of TNO thin films

Jo

ur

na

lP

re

-p

3.1

ro of

of capacitance (reactive component) and conductance (resistive) component.

Figure 1: X-ray diffraction patterns of polycrystalline TNO20 and TNO40 thin films grown on Pt(200)/TiO2 /SiO2 /Si (100) substrates The structural information of as-grown PLD TNO thin films with different thicknesses are obtained by X-ray diffraction analysis, given in Figure 1. As-grown

6

TNO films shows two major X-ray reflections at 2θ of 23.6◦ , 47.6◦ which are corresponding to (¯ 111) and (020) planes of monoclinic TNO crystal structure with C 2/m or A 2/m symmetry. The remaining X–ray reflections which appeared at 2θ of 33◦ , 39.9◦ , 41.8◦ , and 46.5◦ belong to platinized silicon substrates. Other low intensity X-ray reflections are observed (in case of TNO20) films at 24.3◦ (011), 35.5◦ (602), and 38.6◦ (¯ 611) also confirms the polycrystalline nature of as-grown TNO films.27, 28 The surface morphologies and cross-sectional analysis of TNO films are studied using FESEM analysis which are shown in Figure (2) (a)-(d). From this figure we observe

ro of

that thin TNO (236 nm) (part (a)) film shows a uniform surface morphology with homogeneous grain size distribution whereas the thicker counterpart TNO (480 nm)

(part (c)) films displays more spread in the grain size distribution. The AFM mor-

phology of PLD grown TNO films grown with different deposition times are shown in

Figure S1 in supplementary information. The as-grown TNO films show well-defined

-p

crystalline grain morphologies with different size distribution. The root mean square (RMS) roughness of TNO thin films increased significantly from ∼ 14 nm (TNO20)

re

to ∼ 30 nm (TNO40) with increased TNO film thickness. From AFM and FESEM analysis, we conclude that thin TNO samples grow with different grain sizes and displayed distorted spherical shape grains in the range of few tens of nanometer. On the

lP

other hand, thick TNO films developed larger spherical grain size of ∼ 50 – 100 nm range with non-homogeneous grain distribution forming large porous island-like grains on film’s surface. Moreover, FESEM analysis on thick TNO films (480 nm) reveal two

na

distinctive morphologies: i) a typical columnar grain morphology, due to layer-plusisland (2D/3D) or Stranski-Krastanov type growth, followed by, ii) a well separated spherical grain morphology due to island (2D) or Volmer-Weber type growth above

ur

thickness (h) i.e. ≥ 236 nm (TNO20) and ≤ 480 nm (TNO40) as calculated using

cross-sectional SEM analysis. This suggests that there could be a threshold film thick-

Jo

ness above which the film microstructure, grain size, and surface roughness drastically changes due to the change in growth modes or mixed growth process phenomenon.29, 30

Nonetheless, systematic studies should be carried out with few more film thicknesses for understanding this interesting growth behavior, which is beyond the scope of this work. Here our main focus is to investigate the temperature dependent dielectric properties and AC conductivity of as-grown TNO films.

7

ro of -p re lP na

Jo

ur

Figure 2: Surface morphological features of a) TNO 20 film, and c) TNO40 film; whereas b) TNO20 ( 236 nm) and d) TNO40 ( 480nm) displays corresponding SEM cross-sectional analysis

8

3.2

Dielectric Properties of TNO Films

3.2.1

Dielectric Constant and Loss of TNO Thin Films

To initiate the study of dielectric properties, we first present a representative view of the dielectric constant and dielectric loss of the TNO films deposited to thickness of 236

re

-p

ro of

nm and 480 nm which are shown in Figure 3(a) and Figure 3(b) below, respectively.

lP

Figure 3: The comparison of the dielectric constant (a) and loss (tan δ) (b) for the thin (TNO20) and thick (TNO40) films at room temperature .

We can immediately observe that they typically exhibit dielectric constant at the higher side (κ ∼ 59 − 73) and dielectric losses are also moderately high (tan δ ∼

na

0.07 − 0.11). There are two noteworthy points from this Figure. Firstly, there is a very weak peak (shown by arrows in Figure (3) (b) that occur at about 10 kHz. We comment on the origin of the peak when we discuss the AC conductivity studies in

ur

section 3.2.2 because its origin belongs to that part proper. Besides this peak, there is a sharp peak at about 300 kHz which is accompanied by a correspondingly sharp

Jo

discontinuity in the dielectric constant (see Figure 3(a)). The location of this peak in the frequency spectrum remains invariant with respect to the thickness of the films, or their variety. Thus the origin of this peak could be attributed only to extrinsic factors, possibly due to inductive effects from the measurement fixture. This makes the high frequency dielectric response a convolution of the intrinsic material response and the frequency response of the external fixture. This somewhat obscures the interpretation of dielectric properties at higher frequencies. To alleviate such an ambiguity, we have

9

extended the measurement of dielectric properties unto lower frequencies (40 Hz - 110 kHz) so as to extend the dynamic range of the dielectric response of the material. These results are to be found in the supplementary information (Figures S2 - S3). Taking into account such measurements, we can comment on the general behavior of the dielectric constant and dissipation factor. In the intermediate frequency range (from about 300 kHz – 1 kHz) the dielectric response is mostly flat in the frequency range. Below this frequency range, the dielectric constant shows some enhancement, and the higher the temperature, the stronger is this enhancement. The Kramers-

ro of

Kronig relation implies a dispersion in the real part of the dielectric constant will be accompanied by a peak in its imaginary component (or the dissipation factor) which

is indeed what we observe as the weak peak in the dissipation factor. Once again, we defer the discussion to later a section wherein the AC conductivity behavior is dealt

-p

with.

Next, the temperature dependent dielectric properties of the thin films are

re

shown in Figure (4) for TNO20 and Figure (5) for TNO40, respectively. The legends for the various temperature traces are synchronized with each other for ease of comparison. We clearly notice the weak loss peak that we mentioned earlier in Figure (3) (b) persists

lP

for higher temperatures in both cases. and is identifiable at least until 140◦ C and 100◦ C for the case of TNO20 and TNO40, respectively. The behavior of the dielectric constant is quite similar for the two cases though the dielectric loss behaviors show

na

some differences. To facilitate this discussion, we need to examine the dielectric loss

Jo

ur

and the permittivity in conjunction with each other.

10

ro of

na

lP

re

-p

Figure 4: (a) Dielectric constant and (b) dissipation factor for the thinner films (TNO20) and their temperature dependent behaviors. The small arrow in part (B) marks the position of the weak loss peak.

Jo

ur

Figure 5: (a) Dielectric constant and (b) dissipation factor for the 480 nm film (TNO40) and their temperature-dependent behaviors. Again, the arrow marks the position of a weak dielectric loss peak.

11

An important quantity characterizing dielectric response is the complex susceptibility (χ) which can be obtained in terms of the complex dielectric function as follows: χ = ˜ − 1

(2)

The Universal Dielectric Response which was advocated by Jonscher31 as representative of a wide variety of dielectric materials can be expressed in terms of real and imaginary components of the susceptibility as follows:

(3)

ro of

χ00 (ω)/χ0 (ω) = cot(nπ/2)

where the exponent (n) takes on values depending on the material at hand and the frequency range in which the same is probed. In particular, we note that low values

AC conductivity analysis

re

3.2.2

-p

of n (n  1) is indicative of Low-Frequency Dispersive (LFD) behavior.

TNO, like many other high-k dielectric materials is expected to have some

lP

conductivity contribution due to ionic transport, in addition to the usual electronic contribution, especially at relatively higher temperatures. Ionic hopping, as a form of ionic conduction, is closely associated with the dielectric matrix in which ion hopping takes place.32 Thus, dielectric relaxation studies should play an important role in

na

elucidating the prevalence of mixed ionic/electronic conduction. Such studies are generally carried out in the form of AC conductivity analysis, which provide us with a wealth of information regarding both the relevant relaxation mechanisms as well

ur

as providing some key estimates of parameters very relevant for the ionic hopping processes such as the activation energy for ionic hopping rates and the approximate

Jo

value of DC or frequency independent conductivity values.33 A key feature of the dielectric response of ionic conductors is embodied in

the UDR response mentioned earlier. The qualifier “universal” is justified in view of the very broad range of dielectric materials that this law applies to, irrespective of their chemical nature, or of their physical state.31 For ionic conductors, this law can

12

be written in this following form:34, 35 χ00 (ω) ∝ (ω/ωp )n1 −1 + (ω/ωp )n2 −1

(4)

where ωp is the frequency corresponding to a dipolar loss peak. In the case of ionic solids, this can be identified with the ionic hopping rate. The conductivity of the films (σ) is related to the imaginary part (00 ) of the complex permittivity (˜ ) by the following equation: σ(ω) = ω × 00 (ω)

ro of

(5)

where (ω) is the radian frequency. From this we can can see that the dielectric response shows two distinct dispersive regions characterized by the exponents n1 and n2 (refer

to eqn. (4). It is significant that the exponent n1 is frequently not equal to zero. If

it were, the data would always show a frequency-independent or DC characteristic at

-p

sufficiently low frequency. On the other hand, we obtain in the case of TNO40 films,

low values of n (less compared to 1) indicative of incipient low-frequency dispersive

re

behavior.

Making use of equation (5) we calculate the conductivity as a function of

lP

frequency for both TNO20 and TNO40 samples and is shown in Figure (6). Firstly, there is a general tendency for the conductivity to increase with increasing temperatures. The low frequency region (marked as region ‘I’) show the maximum variation

na

with temperature. Secondly, there is a very weak discrete Debye-like peak observed at low frequencies at ≈ 10 kHz.36 This weak peak was also noted earlier with reference to the loss data shown in Figure (3). This peak also could be related to discrete defect

ur

relaxations that are also observed in other ionic solids at similar frequencies.23, 36 This peak is more evident for thicker films (TNO40) and particularly, at lower temperatures

Jo

(see supplementary Figure S2-S3). In spite of the above similarities, there are some important differences be-

tween these samples with regard to their low-frequency behavior (region ”I”). For one, the dispersion for the thinner films Figure 6(a) is much less with temperature as compared to part Figure 6(b). As a consequence of this, the low frequency conductivity increases much faster for the (thicker) samples. Another important difference in the low-frequency dispersive behavior is noted if we compare the dielectric loss behaviors

13

ro of

Figure 6: Conductivity comparison for the (a) thinner (236nm) films and (b) thicker (480nm) films. The demarcation of the three regions is carried out based on the nature of the dispersion with frequency and temperature

for these films (compare Figure (4) and Figure (5). At least below temperatures above

-p

which direct current conduction becomes predominant, TNO20 films show a decrease

in dielectric loss with decreasing frequency whereas for TNO40 it follows the opposite trend. The latter behavior is more in keeping with the UDR behavior whereas, for the

re

former case, electrode polarization seems important.

To clarify the origin of this behavior in a more unambiguous manner, we

lP

plot the complex impedance spectra for these samples as a function of temperature (see Figure (S5) in supplementary information). From this diagram, the following points are noted. First, at the lowest temperature, the spectra is parallel to the Z’=Z”

na

line, but is displaced from it. This parallelism decreases with increasing frequency. Secondly, the curves becomes displaced towards the Z’=Z” line, as the temperature increases. Thirdly, at the highest temperatures under study, the impedance curve

ur

shows a curved region, at the lowest frequencies. This may be due to additional defect

Jo

relaxations at higher temperatures. Parallelism with the Z’=Z” line is indicative of UDR behavior and the

impedance spectrum provides a further vindication of this hypothesis, especially for the low and intermediate frequencies range, and if the temperatures are not too high. The stage is now set to examine the conductivity data in the light of equation (4). We take the conductivity data below 140◦ C and try to estimate important parameters that are thermally activated. We follow the method outlined in35 for this analysis. In brief,

14

the method consists of fitting two straight lines at the two dispersive regions (following a different frequency exponent “n”) and finding their point of intersection (the abscissae of which correspond to the ionic hopping frequency ωp and the ordinate represents twice the assumed DC conductivity). In applying this method, care is required to be exercised in defining and identifying the low frequency dispersive regions. Thus for instance, the lowest temperature data at 27◦ C shows almost no evidence of LFD. As the temperature increases, there is an incipient flattening of the characteristics at the lowest frequencies. As the temperature further increases, this region becomes more

ro of

prominent, corresponding to an “inner” movement of ωp . We show in the following

Jo

ur

na

lP

re

-p

figure the manner in which the aforementioned procedure is executed.

15

ro of -p

lP

re

Figure 7: Straight line fitting in accord with Jonscher’s Law (4) of the conductivity data pertaining to the thicker films (see figure 6). (inset, bottom right) The variation with temperature exponents “n1” and “n2” corresponding to the low and high frequency regions, respectively. (inset, top left) The ωp that are derived from the fitting are shown in an Arrhenius format. Apart from an outlier which is marked in red, the other show an activated behavior We have only chosen these temperatures because outside of this temperature

na

range, the identification with either the high or the low frequency dispersive region becomes ambiguous. The inset to this plot shows the variation of the exponents n1 and n2 . As expected,37 the high frequency exponent (n2 ) shows a steady decrease with

ur

temperature though the trend with respect to n1 is less well-defined. The reasons for

this can be the presence of the discrete low-frequency dielectric loss peak mentioned

Jo

earlier. In any event, this point needs further understanding before the trend could be interpreted in more physical terms. The ωp or the “onset frequency” as it is sometimes also called35 is also shown in an Arrhenius format as an inset to the same figure. From

this we calculate activation energy for the onset frequency to be about 0.37 eV. To correlate this result with the DC activation energies, we show in the following Figure (8) the values of conductivity as a function of temperature for different frequencies. When plotted in an Arrhenius format, a clear trend of activated transport behaviour

16

emerges at the lowest frequencies. It is reasonable to assume the activation energy at this low frequency corresponds, in some measure, to the “actual” DC activation energy. This activation energy is calculated to be about 0.44 eV, somewhat higher than that of ωp . Materials in which conduction occurs predominantly by ionic hopping, there is believed to be a close relation38 between ωp and another parameter which is known as the carrier jump frequency (Γ). The former controls the dielectric relaxation of the ionic lattice environment whereas the latter controls the DC transport behavior (ref). In such situations the DC activation energy and the activation energy for ωp are found

ro of

to be very near to each other.36 The fact that this is not true in our present case is an

lP

re

-p

indication that in addition to ionic hopping, electronic transport may also be involved.

Jo

ur

na

Figure 8: The Arrhenius representation of the temperature dependence of the sample conductivity for various frequencies. Note the indication of the conductivity to be thermally activated for the lowest frequencies

17

The same type of analysis is not very pertinent for the thinner films (TNO20) because as we had noted earlier, they show less conformity to the UDR behavior, which forms the basis for the current analysis. Instead, we note from Figure 6(a) that the AC conductivity curves display a striking invariance with respect to temperature (27 – 160 ◦ C) (region ”II”) which warrants a more detailed analysis of the same. We extract the exponent “n” of the frequency dependent conductivity exponent for region “II” and it comes very near to 1. Now, we note from eq. (5) that an exponent of 1 corresponds to a frequency independence of the dielectric loss, so that this kind of

ro of

behavior is aptly termed as Nearly Constant-Loss (NCL) behavior.39, 40 NCL behaviors have been the subject of extensive research and different phenomenological models have

been proposed to explain their behaviors.41 A striking feature of NCL behavior is their near temperature independence, much in accord with what we find here. They are

believed to be the result of complex many-body ionic interactions, commonly observed

-p

in samples of high dopant concentrations, conditions in which their mutual interactions

become highly important. Another remarkable feature of NCL behavior is that they

re

persist until cryogenic temperatures, unlike the case of hopping conduction. This point to their origin being more dielectric in nature, in contrast to the conventional ionic hopping regime observed for the thicker films. It would be interesting to further

lP

analyze the effects of grain microstructure as a function of film thickness in an attempt

4

na

to uncover the reason for the transition from one conductivity regime to another.

Conclusions

ur

In conclusion, we report here the dielectric behaviors of Titanium Niobium Oxide films grown by laser ablation method. Detailed studies were carried out with regard to their

Jo

dielectric properties which revealed several important aspects of their transport behavior. The regimes of dielectric behavior were very different in the thick (TNO40) and the thin (TNO20) counterparts. The thick films of thickness 480nm show very evident signatures of ionic hopping motion embodied in the Jonscher’s Universal Dielectric Response (UDR) as revealed by both temperature as well as frequency dependencies of the dielectric properties. These films show significant low-frequency dispersion behavior and at higher temperatures, grain boundary effects dominate the low-frequency

18

response. DC transport as well as ionic hopping both seem thermally activated with a slightly higher activation energy of the former. This could be due to mixed ionic and electronic transport in the films. Thinner films (236nm) on the other hand, show surprising invariance of the dielectric properties with temperature, especially in the mid-frequency range of the measured frequency spectrum. This invariance, coupled with the constancy of the loss in this frequency range points to the Nearly Constant Loss behavior for these thinner films. It would be an interesting study in itself to understand and model this crossover between these two regimes as the thickness of the

ro of

films is varied and to further analyze what other factors are critical in the development of the same.

Author Contributions

V.D. conceived the idea, designed the project, and performed all the experiments.

-p

Whereas S.D. has given the analysis on the dielectrics results, and S.B.K supervised the project, providing all the necessary facilities for executing the research project. All

Authors declare no conflict of interest.

lP

Acknowledgements

re

the authors discussed, and contributed equally for the final version of the manuscript.

Authors would like to acknowledge the Micro and Nano Characterization facility (MNCF), IISc-Bengalore for providing the materials characterization facilities. V.D.

na

thanks the Swiss Government Excellence Scholarship (2018-19) and S.B.K acknowledge the support of the J.C. Bose fellowship (India). V.D. expresses a special thanks to Prof. Dr. Lippert Thomas (PSI-Switzerland) for providing financial support for

ur

presenting this work at the Joint International Conference of Electroceramics (ICE) -

Jo

2019, Lausanne meeting, Switzerland.

References [1] Reddy MV, Subba Rao GV, Chowdari BVR. Metal Oxides and Oxysalts as Anode Materials for Li Ion Batteries. Chemical Reviews. 2013 07;113(7):5364– 5457. Available from: https://doi.org/10.1021/cr3001884.

19

[2] Rani RA, Zoolfakar AS, O’Mullane AP, Austin MW, Kalantar-Zadeh K. Thin films and nanostructures of niobium pentoxide: fundamental properties, synthesis methods and applications. J Mater Chem A. 2014;2:15683–15703. Available from: http://dx.doi.org/10.1039/C4TA02561J. [3] Wang B, Huang W, Chi L, Al-Hashimi M, Marks TJ, Facchetti A. High-k Gate Dielectrics for Emerging Flexible and Stretchable Electronics.

Chemical Re-

views. 2018 06;118(11):5690–5754. Available from: https://doi.org/10.1021/

ro of

acs.chemrev.8b00045. [4] Aravindan V, Lee YS, Madhavi S. Research Progress on Negative Electrodes

for Practical Li-Ion Batteries: Beyond Carbonaceous Anodes. Advanced Energy

Materials. 2015 2019/06/12;5(13):1402225. Available from: https://doi.org/ 10.1002/aenm.201402225.

-p

[5] Ishihara A, Tamura Y, Chisaka M, Ohgi Y, Kohno Y, Matsuzawa K, et al.

Titanium-Niobium Oxides as Non-Noble Metal Cathodes for Polymer ElecCatalysts. 2015;5(3):1289–1303.

re

trolyte Fuel Cells.

Available from: https:

//www.mdpi.com/2073-4344/5/3/1289. [6] Wadsley AD.

Mixed oxides of titanium and niobium. II. The crystal

lP

structures of the dimorphic forms Ti2Nb10O29. 1961 2019/06/12;14(6):664–670.

Acta Crystallographica.

Available from: https://doi.org/10.1107/

S0365110X6100200X.

na

[7] Iijima S. High-Resolution Electron Microscopy of Crystal Lattice of TitaniumNiobium Oxide. Journal of Applied Physics. 1971;42(13):5891–5893. Available from: https://doi.org/10.1063/1.1660042.

ur

[8] Gasperin M. Refinement of the structure of TiNb2O7 and the distribution of cations. Journal of Solid State Chemistry. 1984;53(1):144 – 147. Available from:

Jo

http://www.sciencedirect.com/science/article/pii/002245968490238X.

[9] Cava RJ, Murphy DW, Zahurak SM. Phases Based on Niobium Oxide.

Lithium Insertion in Wadsley-Roth

Journal of The Electrochemical Society.

1983;130(12):2345–2351. Available from: http://jes.ecsdl.org/content/130/ 12/2345.abstract.

20

[10] Han JT, Huang YH, Goodenough JB. New Anode Framework for Rechargeable Lithium Batteries. Chemistry of Materials. 2011 04;23(8):2027–2029. Available from: https://doi.org/10.1021/cm200441h. [11] Lu X, Jian Z, Fang Z, Gu L, Hu YS, Chen W, et al. Atomic-scale investigation on lithium storage mechanism in TiNb2O7,. Energy Environ Sci. 2011;4:2638–2644. Available from: http://dx.doi.org/10.1039/C0EE00808G. [12] J B Goodenough JTH. Niobium Oxide Compositions and Methods for Using

ro of

Same. Patent No PCT/US2011/045964. 2012;. [13] Fei L, Xu Y, Wu X, Li Y, Xie P, Deng S, et al. SBA-15 confined synthesis of

TiNb2O7 nanoparticles for lithium-ion batteries. Nanoscale. 2013;5:11102–11107. Available from: http://dx.doi.org/10.1039/C3NR03594H.

[14] Aravindan V, Sundaramurthy J, Jain A, Kumar PS, Ling WC, Ramakrishna S,

-p

et al. Unveiling TiNb2O7 as an Insertion Anode for Lithium Ion Capacitors with High Energy and Power Density. ChemSusChem. 2014 2019/06/12;7(7):1858–

re

1863. Available from: https://doi.org/10.1002/cssc.201400157. [15] Park H, Song T, Paik U. Porous TiNb2O7 nanofibers decorated with conductive Ti1-xNbxN bumps as a high power anode material for Li-ion batteries.

1039/C5TA00467E.

lP

J Mater Chem A. 2015;3:8590–8596. Available from: http://dx.doi.org/10.

[16] Song H, Kim YT. A Mo-doped TiNb2O7 anode for lithium-ion batteries with high

na

rate capability due to charge redistribution. Chem Commun. 2015;51:9849–9852. Available from: http://dx.doi.org/10.1039/C5CC02221E. [17] Wen X, Ma C, Du C, Liu J, Zhang X, Qu D, et al. Enhanced electrochemi-

ur

cal properties of vanadium-doped titanium niobate as a new anode material for lithium-ion batteries. Electrochimica Acta. 2015;186:58 – 63. Available from:

Jo

http://www.sciencedirect.com/science/article/pii/S0013468615307398.

[18] Noh H, Choi W. Preparation of a TiNb2O7 Microsphere Using Formic Acid and Wrapping with Reduced Graphene Oxide for Anodes in Lithium Ion Batteries. Journal of The Electrochemical Society. 2016;163(6):A1042–A1049. Available from: http://jes.ecsdl.org/content/163/6/A1042.abstract. [19] Park H, Shin DH, Song T, Park WI, Paik U. Synthesis of hierarchical porous TiNb2O7 nanotubes with controllable porosity and their application in high power

21

Li-ion batteries. J Mater Chem A. 2017;5:6958–6965. Available from: http: //dx.doi.org/10.1039/C7TA00597K. [20] Shen S, Guo W, Xie D, Wang Y, Deng S, Zhong Y, et al. A synergistic vertical graphene skeleton and S–C shell to construct high-performance TiNb2O7-based core/shell arrays. J Mater Chem A. 2018;6:20195–20204. Available from: http: //dx.doi.org/10.1039/C8TA06858E. [21] Inada R, Kumasaka R, Inabe S, Tojo T, Sakurai Y. Li+ Insertion/Extraction

ro of

Properties for TiNb2O7 Single Particle Characterized by a Particle-Current Collector Integrated Microelectrode. Journal of The Electrochemical Society. 2019;166(3):A5157–A5162.

Available from: http://jes.ecsdl.org/content/

166/3/A5157.abstract.

[22] Ulrich S, Szyszko C, Jung S, Verg¨ ohl M. Electrochromic properties of mixed

-p

oxides based on titanium and niobium for smart window applications. Surface

and Coatings Technology. 2017;314:41 – 44. Selected papers from the Society

re

of Vacuum Coaters 59th Annual Technical Conference. Available from: http: //www.sciencedirect.com/science/article/pii/S0257897216312105. [23] Cava RJ, Krajewski JJ, Peck WF, Roberts GL.

Dielectric properties of

lP

TiO2–Nb2O5 crystallographic shear structures. Journal of Materials Research. 1996;11(6):1428–1432.

[24] Lashtabeg A, Irvine JTS, Feighery A. Thermomechanical and conductivity studies

na

of doped niobium titanates as possible current collector material in the SOFC anode. Ionics. 2003 May;9(3):220–226. Available from: https://doi.org/10. 1007/BF02375970.

ur

[25] Xing W, Kalland LE, Li Z, Haugsrud R. erties in TiNb2O7.

Defects and Transport Prop-

Journal of the American Ceramic Society. 2013

Jo

2019/06/12;96(12):3775–3781. Available from: https://doi.org/10.1111/jace. 12558.

[26] Chang MC, Huang CS, Ho YD, Huang CL. Sol-gel derived TiNb2O7 dielectric thin films for transparent electronic applications. Journal of the American Ceramic Society. 2018 2019/06/12;101(2):674–682. Available from: https: //doi.org/10.1111/jace.15221.

22

[27] Daramalla V, Krupanidhi SB. Growth and characterization of Titanium Niobium Oxide (TiNb2O7) thin films. MRS Proceedings. 2015;1805:mrss15–2128679. [28] Daramalla V, Penki TR, N M, B KS. Fabrication of TiNb2O7 thin film electrodes for Li-ion micro-batteries by pulsed laser deposition.

Materials Sci-

ence and Engineering: B. 2016;213:90 – 97. Li-ion batteries. Available from: http://www.sciencedirect.com/science/article/pii/S0921510716300265. [29] Dong BZ, Fang GJ, Wang JF, Guan WJ, Zhao XZ. Effect of thickness on struc-

ro of

tural, electrical, and optical properties of ZnO: Al films deposited by pulsed laser deposition. Journal of Applied Physics. 2007;101(3):033713. Available from: https://doi.org/10.1063/1.2437572. [30] Aziz MJ.

Film growth mechanisms in pulsed laser deposition.

Physics A. 2008 Jun;93(3):579.

Applied

Available from: https://doi.org/10.1007/

-p

s00339-008-4696-7.

[31] Jonscher AK. The ‘universal’dielectric response. Nature. 1977;267(5613):673–679.

re

Available from: https://doi.org/10.1038/267673a0.

[32] Hunt A. Transport in ionic conducting glasses. Journal of Physics: Condensed

lP

Matter. 1991 oct;3(40):7831–7842. Available from: https://doi.org/10.1088% 2F0953-8984%2F3%2F40%2F004.

[33] Hodge IM, Ingram MD, West AR. Impedance and modulus spectroscopy of

na

polycrystalline solid electrolytes. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry. 1976;74(2):125 – 143. Available from: http://www. sciencedirect.com/science/article/pii/S002207287680229X.

ur

[34] Jonscher AK. A new model of dielectric loss in polymers. Colloid and Polymer Science. 1975 Mar;253(3):231–250. Available from: https://doi.org/10.1007/ BF01470233.

Jo

[35] Almond DP, Hunter CC, West AR. The extraction of ionic conductivities and hopping rates from a.c. conductivity data. Journal of Materials Science. 1984 Oct;19(10):3236–3248. Available from: https://doi.org/10.1007/BF00549810.

[36] Nowick AS, Lim BS. Electrical relaxations: Simple versus complex ionic systems. Phys Rev B. 2001 Apr;63:184115. Available from: https://link.aps.org/doi/ 10.1103/PhysRevB.63.184115.

23

[37] Hill RM, Jonscher AK. DC and AC conductivity in hopping electronic systems. Journal of Non-Crystalline Solids. 1979;32(1):53 – 69. Electronic Properties and Structure of Amorphous Solids. Available from: http://www.sciencedirect. com/science/article/pii/0022309379900644. [38] NOWICK AS, MURCH GE, NOWICK AS. In: 3 - Atom Transport in Oxides of the Fluorite Structure. Academic Press; 1984. p. 143–188. Available from: http: //www.sciencedirect.com/science/article/pii/B978012522662250008X.

ro of

[39] Hsieh CH, Jain H. Are the low temperature-low frequency and high temperaturehigh frequency ac conductivity of glasses the same phenomenon?

Journal of

Non-Crystalline Solids. 1996;203:293 – 299. Optical and Electrical Propertias of Glasses. Available from: http://www.sciencedirect.com/science/article/ pii/0022309396003614.

-p

[40] Jain H, Krishnaswami S, Kanert O. ‘Jellyfish’ fluctuations of atoms in solids.

Journal of Non-Crystalline Solids. 2002;307-310:1017 – 1023. Available from:

re

http://www.sciencedirect.com/science/article/pii/S0022309302015673. [41] Anderson PW, Halperin BI, Varma CM. Anomalous low-temperature thermal properties of glasses and spin glasses. The Philosophical Magazine: A Journal of

lP

Theoretical Experimental and Applied Physics. 1972;25(1):1–9. Available from:

Jo

ur

na

https://doi.org/10.1080/14786437208229210.

24