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Direct Correlations of Grain Boundary Potentials to Chemical States and Dielectric Properties of Doped CaCu3Ti4O12 Thin Films Ahra Cho, Chan Su Han, Meenjoo Kang, Wooseok Choi, Jihwan Lee, Jaecheol Jeon, Sujae Yu, Ye Seul Jung, and Yong Soo Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02630 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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ACS Applied Materials & Interfaces

Direct Correlations of Grain Boundary Potentials to Chemical States and Dielectric Properties of Doped CaCu3Ti4O12 Thin Films Ahra Cho†, Chan Su Han†, Meenjoo Kang, Wooseok Choi, Jihwan Lee, Jaecheol Jeon, Sujae Yu, Ye Seul Jung, Yong Soo Cho* Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Korea

ABSTRACT: Colossal dielectric constant CaCu3Ti4O12 has been recognized as one of the rare materials having intrinsic interfacial polarization and thus unusual dielectric characteristics, in which the electrical state of grain boundary is critical. Here, the direct correlation between grain boundary potential and relative permittivity is proposed for the CaCu3Ti4O12 thin films doped with Zn, Ga, Mn and Ag as characterized by Kelvin probe force microscopy. The dopants are intended to provide the examples of variable grain boundary potentials that are driven by chemical states including Cu1+, Ti3+ and oxygen vacancy. Grain boundary potential is nearly linearly proportional to dielectric constant. This effect is attributed to the increased charge accumulation near the grain boundary, depending on the choice of dopant. As an example, 1 mol% Ag-doped CaCu3Ti4O12 thin films demonstrate the best relative permittivity as associated with a higher grain boundary potential of 120.3 mV compared to 82.6 mV for the reference film. The chemical states across grain boundaries were further verified by using spherical aberrationcorrected scanning transmission electron microscopy with the simultaneous electron energy-loss spectroscopy. KEYWORDS: CaCu3Ti4O12, grain boundary potential, dielectric constant, thin films, defects

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INTRODUCTION High-quality dielectric thin films with a higher dielectric constant εr have always been of interest as they determine the level of volume efficiency as well as the charge-storing capability of the passive components.1-4 In this regard, CaCu3Ti4O12 (CCTO), which has a perovskite crystal structure, is an excellent candidate owing to its unusual dielectric properties, e.g., an exceptionally high εr (greater than 104) at room temperature and good thermal stability without a structural phase transition in the temperature range of 100–600 K.5 Interestingly, the origin of the very large εr of CCTO has been recognized as notably different from that of other common perovskite materials such as BaTiO3 and Pb(Zr,Ti)O3 where dipolar polarization in the ferroelectric state is typically the main contributor for the high εr. In CCTO, on the contrary, the interfacial polarization of n-type semiconducting grains across the insulating grain boundaries has been widely accepted as the reason for its large εr; this can be explained by the internal barrier-layer capacitor model.6,7 According to this model, the presence of an electrically resistive barrier at grain boundary inhibits electron transport and establishes a large polarization from the accumulated charges. Therefore, defining the electrical properties of the grain boundary is crucial in explaining the unusually high εr of CCTO. Most of the studies on grain boundaries of CCTO materials have been based on the currentvoltage (I-V) measurement with the assumption of an insulator-semiconductor junction and the Cole-Cole plot which provides the electrical resistance of grain boundary. For example, the substitution of bulk CCTO with 10 mol% Nb was reported to decrease the resistivity of grain boundary with a thinner grain boundary according to the I-V measurement.8 Similarly, doping with Tb was shown to significantly enhance the nonlinear I-V characteristics and thereby elevate the internal resistive barrier at grain boundaries.9 Higher grain boundary resistivities relative to


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the resistivity of grain were also reported in the CCTO bulk and thin films as results of the analysis of Cole–Cole plots obtained by impedance measurements, which depend strongly on microstructural features, e.g., grain size, and phase purity.8-12 However, there is very limited work on the direct detection of the surface potential at grain boundaries in CCTO materials, which is enabled by using Kelvin probe force microscopy (KPFM). To the best of our knowledge, only one study has measured the surface potential at grain boundary in pure bulk CCTO pellets, reporting a value of 50 mV.6 The grain boundary potential of CCTO in the form of thin film has not been reported yet using the KPFM. Here, we specifically examine the effects of grain boundary potential on the dielectric properties of CCTO thin films prepared by solution deposition. For the purpose of identifying the direct relations of grain boundary potential to the relative permittivity, various dopants such as Zn2+, Ga3+, Mn2+ and Ag1+ are used to induce different chemical states at grain boundary and thus cover an extensive range of εr while the grain size and film thickness are kept nearly constant in all samples to minimize the influence of microstructure. The different valence states of the dopants are hypothesized to incur unique chemical states in each doping case and thereby influence the dielectric properties substantially. Among the dopants, the impact of Ag is distinguishably noticeable in that the highest dielectric constant is achieved as associated with the maximum potential of grain boundary. The effect of Ag-doping has not been reported so far in any type of CCTO. In order to pursue the origin of electrostatic potential changes across the grain boundaries, detailed atomic-scale structures including local chemistry and valence changes of the grain and grain boundary regions in the Ag-doped CCTO film are studied with the example of Ag-doping.



EXPERIMENTAL SECTION CaCu3Ti4O12 thin films doped with 1 mol% of Ag, Mn, Ga, or Zn were deposited onto Pt/TiO2/SiO2/Si substrates via spin coating. Calcium nitrate tetrahydrate (Ca(NO3)2·4H2O, ≥99.0%, Sigma-Aldrich), copper nitrate trihydrate (Cu(NO3)2·3H2O, 99%–104%, SigmaAldrich), and titanium isopropoxide (Ti[OCH(CH3)2]4, 97%, Sigma-Aldrich) were dissolved in 2-methoxyethanol (C3H8O2, 99.8%, Sigma-Aldrich) to prepare a 0.2-M CCTO precursor solution. Either zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98%, Sigma-Aldrich), gallium(III) nitrate hydrate (Ga(NO3)3·xH2O, 99.9%, Sigma-Aldrich), manganese(II) nitrate tetrahydrate (Mn(NO3)2·4H2O, 97%, Sigma-Aldrich), or silver nitrate (AgNO3, 99.9999%, Sigma-Aldrich) was dissolved in the CCTO solution to obtain a homogeneous precursor solution. The final precursor solutions were spin-coated onto Pt/TiO2/SiO2/Si substrates at 3,000 rpm. The spin-coated films were dried at 200 °C for 1 min and then pyrolyzed at 400 °C for 10 min on a hot plate to remove the organics. The coating, drying, and pyrolysis processes were repeated three times to obtain approximately 600 nm-thick films. The deposited thin films were finally heat treated at 850 °C for 10 min under rapid thermal annealing in an ambient air atmosphere. The crystal structures of the annealed films were analyzed using Cu Kα XRD (Max-2500, Rigaku). Surface morphologies were observed via field-emission scanning electron microscopy (FE-SEM; JSM-7001F, JEOL), and surface chemical states were analyzed via high-resolution xray photoelectron spectroscopy (XPS; Escalab 220i-XL, VG Scientific Instrument) using Al Kα photons (1486.6 eV) at a base pressure of approximately 1.1 × 10−10 Torr in an ultrahigh-vacuum chamber. Shifts in the core-level spectra were corrected by calibrating the C 1s peak to 285 eV. To measure dielectric properties, a Pt top electrode was deposited using DC magnetron sputtering. Dielectric properties were measured using an impedance analyzer (HP4194A) while


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varying the frequency from 100 Hz to 1 MHz. KPFM measurements were performed using an atomic force microscope (Nanoscope V Multimode, Bruker) with a Pt/Ir-coated Si cantilever. The contact potential difference was detected by applying an AC bias of 1 V to the cantilever at its second resonance frequency of 270 kHz in order to generate topographical images. The detailed atomic structures of the grain and grain boundary, including local chemistry and valence changes, were investigated using spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM; JEM-ARM200F, JEOL) at 200 kV, which was equipped with GIF Quantum ER 965 (Gatan) for the electron energy loss spectroscopy (EELS) analysis. The convergence and collection semiangles of the electron beam were 19 mrad and 39.6322 mrad, respectively. The energy dispersion was 0.25 eV/channel.

RESULTS AND DISCUSSION As preliminary work, the annealing temperature was optimized in terms of the phase purity and microstructural evolution as shown in Figure S1 and S2. The pure-CCTO phase (JCPDS No. 075-2188; perovskite structure with the Im3 space group) was observed at the annealing temperature of 850 °C where relatively uniform grains are identified with clear grain boundaries. The unreacted secondary phase of CuO was observed at annealing temperatures of 750 °C and 800 °C. Accordingly, the annealing temperature of 850 °C was selected for the current study. Figure 1a shows the XRD patterns of CCTO thin films doped with Zn, Ga, Mn or Ag and annealed at 850 °C for 10 min. All the patterns clearly indicate phase-pure perovskite structure without a secondary phase. The polycrystalline nature of the films is clearly recognizable with no preferred orientation. The lattice parameter was calculated using the Bragg equation as being 7.381, 7.381, 7.381, 7.385, and 7.394 Å for the undoped and Zn-, Ga-, Mn-, and Ag-doped CCTO thin films, respectively; the average values are plotted as shown in Figure 1b. The range 5 Environment ACS Paragon Plus


of the lattice constants corresponds to the values reported in literature.13,14 The changes in the lattice constant associated with different dopants are attributed to the differences between the ionic radii of the dopants and the host cations. Based on their ionic sizes and valence states, Ga3+ (0.62 Å) is mostly likely substituted for Ti4+ (0.61 Å), Zn2+ (0.74 Å) and Mn2+ (0.83 Å) for Cu2+ (0.72 Å), and Ag1+ (1.15 Å) for Ca2+ (0.99 Å) in the CaCu3Ti4O12 perovskite structure. The observed variations in lattice constant according to the dopants are proportional to the difference in ionic sizes between the dopant and the host cation. For example, Ag1+ doping resulted in the largest change in lattice constant due to the greatest difference in ionic size compared with the host cation, Ca2+. Figure 1c shows the grain sizes in the thin films doped with different cations, which were estimated from the surface SEM images shown in Figure 1d. The average grain sizes in the CCTO thin films were very similar regardless of the dopant species. It is known that the graingrowth kinetics of CCTO is determined by the type and concentration of dopant species. According to the reported studies,14-16 Mn or Zn doping is likely to promote grain growth in CCTO, whereas Ga doping does not facilitate grain growth. There is no report on the effect of Ag in CCTO. The observed similar grain sizes in these samples may be due to the relatively low firing temperature, i.e., 850 °C, with the short soaking time of 10 min, which does not produce extra grain growth in the presence of certain dopants. The observed film quality appears to be very good, without detectable pores on the surface or cross-sectional microstructures with any of the dopants. The thicknesses of all the films were maintained at approximately 600 nm, as confirmed by the cross-sectional images; a cross-section of the pure CCTO film is shown as an example in Figure 1d. All cross-sectional images can be referred to Figure S3. High-resolution XPS spectra were obtained to trace the variations in the relative chemical


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states of Cu, Ti and O according to the type of dopant. The XPS spectra of the specific bindingenergy regions for each dopant confirm the expected chemical states of Ag1+, Mn2+, Ga3+ and Zn2+ in the doped thin films as shown in Figure S4. Figure 2 shows examples of the XPS spectra in the Cu 2p3/2, Ti 2p3/2, and O 1s regions of all the dopant cases, indicating the corresponding cation valence ratios of Cu1+/Cu2+ and Ti3+/Ti4+ and the ratio of the concentration of oxygen vacancies to that of the lattice oxygens (OV/OL). The binding-energy axes were calibrated by considering the C 1s peak at 285 eV as the internal standard. As shown in Figure 2a, the XPS spectra of the Cu valence state exhibit two peaks around 932.6 and 933.9 eV, which come from the binding energies of the Cu 2p3/2 states. The curves were fitted by Lorentzian–Gaussian functions with Shirley backgrounds, suggesting the coexistence of Cu1+ and Cu2+ valence states. The Cu1+/Cu2+ ratio was estimated from the respective peak areas. The XPS spectra of Ti 2p3/2, as shown in Figure 2b, include two peaks at approximately 457.8 and 458.3 eV after the identical curve fitting; these peaks arise from Ti3+ and Ti4+, respectively. The Ti3+/Ti4+ ratio was also estimated from the respective peak areas. Figure 2c shows the O 1s XPS spectra, which have three identified peaks at ~529.7, ~531.6 and ~530.5 eV; these peaks were attributed to the oxygen lattice (OL), the adsorbed oxygen (Oa) and the oxygen vacancy (OV), respectively.17,18 The relative concentration of oxygen vacancy in each sample was calculated from the fitted area of each curves, particularly for the OV and OL peaks. The ratios of Cu1+/Cu2+, Ti3+/Ti4+ and OV/OL depended on the type of dopant and increased from Zn to Ga to Mn to Ag doping as shown in Figure 2. It is known that oxygen vacancies act as donors and supply free electrons, thereby turning the grains into n-type semiconductors.19-22 The electrons released from an oxygen vacancy can enter the conduction band of either Cu2+ or Ti4+ and thereby reduce these ions to Cu1+ and Ti3+, respectively.23 Based on this mechanism, the



variations in the Cu1+/Cu2+ and Ti3+/Ti4+ ratios may be directly associated with the differing oxygen vacancy concentration in each sample. The acceptor dopants, like Ga and Ag, may further contribute to the formation of oxygen vacancies and compensate for the charge imbalance. Another possible explanation for the formation of oxygen vacancies may be the lattice distortion induced by dopants with larger ionic radii compared with the host atoms. CCTO thin films doped with cations having large ionic radii may induce a high-energy lattice distortion, resulting in a more unstable state; this may lead to the occurrence of defects, including oxygen vacancies, during heat treatment.24,25 To gain further insight into the electrostatic properties across the grain boundaries, KPFM measurements were taken for all the samples. Figure 3 shows the topography and surface potential of the undoped and doped CCTO thin films. In the maps of the surface potential shown in the second row (Figures 3f–3j, the darker regions coinciding with the grain boundaries indicate a lower electronic potential compared with the brighter surrounding grain areas. This indicates that the grain boundaries have a lower potential than the neighboring grains.6 The absolute grain boundary potentials, taken as the average of multiple measurements, in the Ag-, Mn-, Ga-, Zn-doped, and undoped CCTO films were 120.3, 100.2, 93.5, 69.8, and 82.6 mV, respectively. Lower surface potentials at the grain boundaries indicate negatively charged states. Thus, an upward band-bending in the energy band diagram of the grain boundary is expected. Figure 4a,b shows the frequency dependence of εr and dielectric loss (tan δ) in the range of 100 Hz to 1 MHz at room temperature for the undoped and doped CCTO thin films. High εr values were observed in all the samples in the low-frequency range, and εr gradually decreased at higher frequencies (Figure 4a), which is a characteristic behavior of CCTO materials.10,14,15,26 The observed frequency-dependent behavior of tan δ is similar to the previously reported


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behavior, i.e., its value increases sharply above a frequency of approximately 105 Hz.10,14 The εr decreased from Ag to Mn to Ga to Zn dopants, and εr of the undoped thin film was higher than that of the Zn-doped thin film only. The varying tendency of tan δ according to the type of dopant is similar with that observed for εr. A higher εr was associated with a higher tan δ over the range of the frequencies tested. It has been widely recognized that the mechanism responsible for the large dielectric permittivity in polycrystalline CCTO is based on the internal barrier-layer capacitance model,6,7 which consists of the semiconducting grains and insulating grain boundaries. In this model, charge accumulation at grain boundaries resists the flow of charge carriers and the resulting separation of charges may cause a strong polarization across the grain boundary.26-28 Differences in the ionic radii between the dopants and the host atoms may also influence the magnitude of the dipole moments. It has been reported that εr can be enhanced by increasing the dipole moments through lattice stretching.29,30 However, the influence of ionic polarization may not be significant when compared with grain boundary-driven interfacial polarization. Dielectric constant of the samples with different dopants was plotted with the surface potential at grain boundary as shown in Figure 4c. It indicates that the correlation between the grain boundary potential and εr is approximately linear: a higher dielectric constant corresponds to a higher surface potential. This result is reasonable since the dielectric constant must be associated with the grain boundary potential according to the dominant interfacial polarization mechanism. We further investigated the case of Ag-doping, which exhibited the highest εr and surface potential, using spherical aberration-corrected scanning transmission electron microscopy with the simultaneous electron energy-loss spectroscopy across the grain boundary region. For dielectric materials which possess interface polarization across the grain boundaries, EELS has



been found to be highly useful to probe the details of atomic structure of the grain boundary including local chemistry and valence changes affecting the surface potential at the grain boundary.31-32 Chemical states of the grain boundary was examined for the Ca L2,3, Cu L2,3 and Ti L2,3 edges as the STEM probe scans directly across the grain boundary with steps of 1.9 nm, as shown in Figure 5a. Figure 5b shows the corresponding high resolution image of the grain boundary region in the Ag-doped thin film. The (110) and (211) planes meet at the grain boundary. No amorphous phase is observed at the grain boundary. The Ca L2,3 edges from the grains exhibit two sharp white lines, which closely matches with the reported peaks for Ca2+ in bulk CCTO.31 Since Ca cation occupies the 12-fold coordination site in CCTO, the t2g-eg splitting of the d-states is not observed in the Ca L2,3 edge spectra.33 Interestingly, the Ca L2,3 edges in the grain boundary show weaker peak intensities than in the grain interiors. It indicates that the Ca2+ concentration decreased in the grain boundary. The Cu L2,3 and Ti2,3 edge spectra from the grain and grain boundary are also shown in Figure 5a. Unlike the result of Ca, no significant differences between the spectra obtained at the grain and grain boundary were observed. It directly indicates that there are no changes in the Cu-O and Ti-O coordinations at the grain boundary in the Ag-doped CCTO film. Therefore, the change in the chemical state of Ca at the grain boundary seems to contribute to the change in the surface potential at the grain boundary. The similar results have been reported that the electrostatic potential barrier height across the grain boundary can be controlled by doping.34,35 For example, doping with donor cations that substitute for Cu and Ti sites can significantly lower the electrostatic potential barrier height by compensating negatively charged acceptor states, leading to the electrically homogeneous microstructures with the negligible space charge across the grain boundaries. In this work, Ag acceptor doping on the Ca sites may introduce an additional acceptor states at the grain


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boundaries, resulting in the increase in the potential barrier height and the serious band bending across the grain boundary. The presence of Cu1+ and Ti3+ in the Ag-doped CCTO film could be confirmed from the shapes of both Cu L2,3 and Ti L2,3 spectra. The Cu L2,3 edges show white lines with shoulders to the high energy-loss side, corresponding to Cu1+.36 Therefore, it indicates the presence of mixed valence states of Cu2+/Cu1+ in the film. The Ti L2,3 edges exhibit the crystal-field splitting of the L2 and L3 edges, which is attributed to the spin-orbit coupling of t2g and eg molecular orbitals of Ti and O ions.33,36 The crystal-field splitting has been known to be arisen from the surrounding oxygen atom. In addition, the shift of L2,3 edges towards the lower energy loss suggests the lower oxidation state of Ti3+.31,37 As a result, the broad peaks seems to be composed of two crystal-field splitting sets arising from the Ti4+ and Ti3+, as demonstrated with straight and dashed lines in Figure 5a. In addition, the EELS spectra of O K-edge are shown in Figure. S5, indicating that there is no difference in oxygen state across the grain boundary. Figure 5c illustrates the band diagrams across the grain boundary region, suggesting a possible mechanism leading to the largest observed increase in grain boundary potential in the case of Ag doping as an example. The dominant presence of Cu1+, Ti3+ and oxygen vacancies may influence the chemical states of grain conductivity and lead to an increase in the number of electrons in the grain region. These electrons may become entrapped at the grain boundary, thereby inducing positive charges across the boundary. Such oxygen vacancies and other defects not only induce an accumulation of negative charges near the grain boundary but also act as electron acceptors.23,31,38 This separation and accumulation of charges may generate an electric field, resulting in the serious bending of the conduction band across the grain boundary.23 This bandbending may produce a resistive barrier and a depletion layer, both of which oppose the



conduction of electrons between grains and grain boundaries.39 Accordingly, a large difference between the electrical properties of grains and grain boundaries may increase the level of the barrier, thus increasing the grain boundary resistivity.

CONCLUSIONS We proposes the grain boundary potentials, which were measured by Kelvin probe force microscopy, as directly related to the measured dielectric constant in CCTO thin films doped with Zn2+, Ga3+, Mn2+ and Ag1+. The choices of dopants were carefully designed to collect the standard examples to cover a broad range of dielectric constant/surface potential, and the experimental condition was manipulated to exclude other influential physical factors like grain size and film thickness. As a highlight, we suggest a nearly linear relation between the dielectric constant and grain boundary potential, with experimental evidences. We believe that the chemical states of defects such as Cu1+, Ti3+ and oxygen vacancy are responsible for the dopantdependent dielectric performance. The case of Ag-doping exhibited the best dielectric constant as related to the highest grain boundary potential. A direct observation of the grain boundary region by using Cs-STEM with the simultaneous EELS confirms the changed chemical states across the grain boundary. As a result, the grain boundary potential is considered as a key parameter in determining the dielectric performance, which is related to the charge accumulation in the acceptor states across the grain boundary.


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Figure 1. (a) XRD patterns of the undoped and Zn-, Ga-, Mn- and Ag-doped (1 mol% each) CCTO thin films deposited by precursor solution and annealed at 850 °C for 10 min, with (b) lattice constant calculated from the XRD patterns and (c) average grain size of the annealed thin films, and (d) surface SEM images of the undoped and doped CCTO thin films annealed in the identical condition, with an example of cross-sectional SEM for the undoped case. The average grain size was estimated from the surface SEM images.



Figure 2. High-resolution XPS spectra of the (a) Cu 2p, (b) Ti 2p and (c) O 1s regions for the undoped and Zn-, Ga-, Mn- and Ag-doped (1 mol% each) CCTO thin films annealed at 850 °C for 10 min, with the resultant variations in the ratios of Cu1+/Cu2+, Ti3+/Ti4+ and OV/OL according to the type of dopant.


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Figure 3. (a-e) Topographic AFM images of the undoped and Zn-, Ga-, Mn- and Ag-doped (1 mol% each) CCTO thin films annealed at 850 °C for 10 min with (f-j) the corresponding relative potential field images. The potential versus position plots shown in the bottom row represent quantitative fluctuations in the surface z-potential along the line drawn in the individual image of (f-j) according to each type of dopant; the values beneath the plots indicate the average grain boundary potentials of each case.



Figure 4. Frequency dependence of (a) the dielectric constant and (b) tan δ of the undoped and Zn-, Ga-, Mn- and Ag-doped (1 mol% each) CCTO thin films annealed at 850 °C for 10 min, with (c) the plot of dielectric constant (at 1 kHz representing stable εr) and grain boundary potential for each dopant case.


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Figure 5. (a) EELS spectra recorded point-by-point across the grain boundary as obtained with an acquisition time of 1 s per spectrum and a resolution of 1.3 eV, (b) HRTEM image of a grain boundary region in the Ag-doped CCTO thin film, and (c) schematics illustrating the electrical barrier states around the grain boundary region, which are driven by the chemical states and defects, for the undoped and Ag-doped cases.



ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional XRD patterns, surface and cross-sectional SEM images, XPS spectra in full scale and EELS spectra of O K-edge (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: 82-2-2123-5848. ORCID Yong Soo Cho: 0000-0002-1601-6395 Author Contributions †

A.C. and C.S.H. contributed equally to this work.

Notes The authors declare no competing financial interest

ACKNOWLEDGEMENTS This work was financially supported by a grant (NRF-2016M3A7B4910151) of the National Research Foundation of Korea and also by the Industrial Strategic Technology Development Program (#10079981) funded by the Ministry of Trade, Industry & Energy (MOTIE) of Korea. This work was performed as a part of ‘creative design’ course at Yonsei University.

REFERENCES (1) Han, C. S.; Park, K. S.; Choi, H. J.; Cho, Y. S. Origin of in Situ Domain Formation of Heavily Nb-Doped Pb(Zr,Ti)O3 Thin Films Sputtered on Ir/TiW/SiO2/Si Substrates for Mobile Sensor Applications. ACS Appl. Mater. Interfaces, 2017, 9, 18904-18910. (2) Zhao, C.; Wu, J. Effcets of Secondary Phases on the High-Performance Colossal Permittivity in Titanium Dioxide Ceramics. ACS Appl. Mater. Interfaces, 2018, 10, 3680-3688. (3) Han, C. S.; Choi, H. R.; Choi, H. J.; Cho, Y. S. Origin of Abnormal Dielectric Behavior and Chemical States in Amorphous CaCu3Ti4O12 Thin Films on a Flexible Polymer Substrate. Chem. 18 Environment ACS Paragon Plus

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