Ni nanocrystals tuning low-frequency colossal permittivity of epitaxial BaTiO3 matrix

Journal of Alloys and Compounds 801 (2019) 460e464

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Ni nanocrystals tuning low-frequency colossal permittivity of epitaxial BaTiO3 matrix Rong Qiu a, Yuanyuan Ni a, Jun Li b, Zhengwei Xiong a, *, Zhen Yang b, Leiming Fang c, Yuanhua Xia c, Jian Gong c, Linhong Cao a, Ganghua Zhang d, Tao Zeng d, Zhipeng Gao a, b, ** a

Joint Laboratory for Extreme Conditions Matter Properties, Southwest University of Science and Technology, Mianyang, 621010, China Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang, 621900, China Institute of Physics Nuclear and Chemistry, China Academy of Engineering Physics, Mianyang, 621900, China d Shanghai Key Laboratory of Engineering Materials Application and Evaluation, Shanghai Research Institute of Materials, Shanghai, 200437, China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 April 2019 Received in revised form 23 May 2019 Accepted 12 June 2019 Available online 13 June 2019

The ceramic-metal composites (CMCs) with colossal permittivity have great potential to be widely applied into embedded capacitors. However, the colossal permittivity of these composites could not be explained in the low-frequency region (<100 Hz) due to the uncontrollable grains and grain boundaries in the polycrystalline ceramic matrix. In this work, Ni nanocrystals (NCs) were embedded in epitaxial BaTiO3 films. According to modulate the interfacial stress, the epitaxial growth of BaTiO3 was not disturbed by the NCs, avoiding the influence of grains and grain boundaries in polycrystalline oxides matrix. By tuning the spacing of NCs, the controllable distribution of grain (Ni NCs) and grain boundary (contact interface between the Ni and BaTiO3) was realized. The effects of grain and grain boundary on the low-frequency colossal dielectric response of CMCs were revealed. This study provides an ideal CMCs model for designing and application of new thin-layer capacitors. © 2019 Elsevier B.V. All rights reserved.

Keywords: Colossal permittivity Nanocrystals Grains Grain boundaries

1. Introduction Materials with a colossal dielectric constant (ε
103), such as BaTiO3 (BTO) [1] and CaCu3Ti4O12 (CCTO) [2], have been absorbed lots of attentions for years due to their ability to reduce the size of capacitive components, offering an opportunity to miniaturize electronic systems. There are usually three types to construct a capacitor [3]: (І) multilayer ceramic capacitor (MLCC), (II) internal barrier layer capacitors (IBLCs) or boundary layer capacitors (BLCs) and (III) ceramic-metal composites (CMCs). In comparison with the MLCCs, IBLCs and BLCs, CMCs provides several merits: (1) the increase of dielectric constant mainly depends on the metal particles instead of the ceramic matrix, so the temperature dependence of BTO can be avoided because the high dielectric constant of BTO usually occurs at the tetragonal-cubic phase transition at Curie

* Corresponding author. ** Corresponding author. Joint Laboratory for Extreme Conditions Matter Properties, Southwest University of Science and Technology, Mianyang, 621010, China. E-mail addresses: [email protected] (Z. Xiong), [email protected] (Z. Gao). https://doi.org/10.1016/j.jallcom.2019.06.154 0925-8388/© 2019 Elsevier B.V. All rights reserved.

temperature while high dielectric constant of CMCs is due to the metal conductivity; (2) unlike IBLCs or BLCs with relatively higher dielectric loss, the dielectric loss of CMCs can be tuned by optimizing the composition/configuration. As a typical colossal dielectric material, the composites consisting of oxide matrix and metal fillers, have great potential to be widely applied into embedded capacitors, gate dielectrics, catalysis and electric energy storage devices [3e11], due to its advantages of tunable colossal permittivity. In order to n et al. have reachieve the great dielectric properties, Pecharroma ported that high dielectric constants of 80,000 could be achieved with BaTiO3eNi CMCs due to the percolation phenomenon [5]. In a similar way, Saleem reported the enhancement of the dielectric properties of BaTiO3 ceramic by addition of Ni nanoparticles [6], while Qiao and Bi observed an improvement in the dielectric behavior of BaTiO3eNi composite ferroic films [7,8]. Studies on Ni as a composite filler have been performed because it leads to high dielectric constant, low frequency dependence, and a low cost [5e8]. Furthermore, two main theoretical models have been developed to describe these colossal dielectric behaviors. Most researchers attributed the dielectric enhancement to the percolation theory model, in which the metal particles were isolated by very thin

R. Qiu et al. / Journal of Alloys and Compounds 801 (2019) 460e464

dielectric layers (ceramic phase) and could serve as capacitor electrodes. In this case, the Debye law was commonly cited for the composites. However, it is noticeable that both the dielectric constant and dielectric loss of the composites can both be extremely high, especially when the frequency is very low [5e11], which could not be well addressed by the Debye law. Classically, four different polarization mechanisms contribute to the dielectric behaviors of materials, which are electronic, ionic, dipolar, and space charge or interfacial polarization. On this basis, some studies ascribe this abnormal dielectric behavior at low frequencies (<100 Hz) to the interfacial polarization in the CMCs system [5,6,9e11], in which the permittivity can be expressed by the Maxwell-Wagner (MW) functions. Practically, the interfacial polarization is strongly influenced by the microstructure of grains and grain boundaries, such as the typical CCTO with an internal barrier layer model. Unfortunately, the ceramic matrix in the CMCs system was all polycrystalline [5e11], inducing that the microstructures of grains and grain boundaries in the reported CMCs are uncontrollable. This results in that the mechanism for the colossal dielectric properties at low frequencies (<100 Hz) of CMCs is still under debate, especially the contributions of grains and grain boundaries to the dielectric constant was not clear. Here, we artificially designed an ideal CMCs structure, consisting of the single crystal BaTiO3 (BTO) matrix and the distributed Ni NCs. The integrated single crystal BTO is separated into many parts by the Ni NCs. The Ni NCs and contact interfaces between the NCs and BTO could be equivalent to the grains and grain boundaries, respectively. Based on this structure, it can quantitatively design the dielectric properties by controlling the Ni NCs and interface between the Ni and BTO. Importantly, our results give a clear description on the mechanism of the low-frequency colossal response. 2. Experimental section In this study, the Ni NCs were embedded in epitaxial BTO films with SrTiO3 (STO) buffer layer by laser molecular beam epitaxy (LMBE) method using rotating targets of STO, BTO and Ni, with an insitu monitoring of reflection high energy electron diffraction (RHEED). Subsequently, the Au top electrode was formed on fabricated composite films by the L-MBE. The 0.5 wt% Nb:SrTiO3 (001) substrates were used as the bottom electrode. The deposition process involved a number of pulses on the Ni target in ultra-high vacuum, followed by the epitaxial growth of BTO superlattice. After completing each BTO layer, oxygen gas was introduced into the chamber providing a background pressure of 10 Pa for annealing 20 min. The oxygen-rich atmosphere led to the formation of p-type BTO films, which reduced oxygen vacancies and generated holes in 1  our BTO films (V  O þ 2O2 /OO þ 2h ) [12e14]. The volume fraction (vol%) of Ni derived from the different laser pulsed number was confirmed by an X-ray fluorescence spectrometer (XRF). The nucleation and growth of the films was observed in-situ RHEED system [14]. The microstructure and interfacial dislocation was investigated by high resolution transmission electron microscopy (HRTEM). As a reference, the pure BTO thin films were set as the same thickness with the Ni NCs-BTO samples. The dielectric properties of films were measured using an impedance analyzer (HP 4294A, Agilent) in the frequency range from 0.1 to 107 Hz. 3. Results and discussion Fig. 1(a) shows a schematic diagram of the matrix-metal structure based on epitaxial BaTiO3 and Ni nanoparticles. With different preparation conditions such as laser frequencies, growth time and anneal atmosphere, the BaTiO3 thickness (d1) and the gap between the Ni NCs (d2) would be adjusted, which supply an ideal model to

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Fig. 1. (a) A schematic diagram of the ideal matrix-metal structure based on BaTiO3 and Ni nanoparticles; (b) Cross-section TEM image of films with the separate BaTiO3 layer (the thickness of 40 nm) and the embedded Ni NCs (the red circle); (c) HRTEM image where the inset (upper left corner) shows the enlarged view for one Ni NC zone. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

study the relationships between microstructures and dielectric properties. TEM images in Fig. 1(b) reveals that the multilayer nanocomposite film consists of BTO layers alternating with 4.65 vol % Ni layers. The lattice of hetero-epitaxy BTO films is not disturbed by the embedded Ni NCs despite of the local dislocations. We further measured the lattice spacing of Ni (2.03 Å) and BTO (3.94 Å), corresponding to the planes of (111) and (100), respectively, as seen in the inset. This is consistent with the designed structure shown in Fig. 1(a). By measuring the diffraction spots (Ni NCs) and streaks (BTO) spacing of RHEED patterns, a real-time monitoring of surfacelattice parameter was obtained, as seen in Fig. S1 (supplementary material). Therefore, the changes of in-plane lattice parameters (aNi-aBTO)/aBTO or (aBTO-aNi)/aNi as a function of the deposition time, using the spot spacing for Ni NCs and the streak spacing for BTO, can be presented the lattice relaxation during the grown processes of Ni NCs deposited on BTO (Ni NCs/BTO) and BTO/Ni NCs (Fig. 2). Initially, the Ni NCs (or BTO) retain an in-plane lattice parameter identical to that of the BTO (or Ni NCs) layer in the first a few seconds, and then recover to the value of bulk Ni (or BTO) rapidly. On the BTO surface, the bright diffraction streaks of BTO indicate that a 2D surface is retained at the initial stage, meanwhile the deposited Ni lattice is clamped in plane. In this uniformly strained layer, the elastic strain energy increases with Ni amount. As reaching a critical thickness, the highly strained 2D layer tends to relax via coherently strained 3D islands (seen the bright spots in Fig. 2). As a result, the enlarged surface of the 3D islands can reduce the strain energy in order to get minimum surface energy and remain metallic property, so as to provide a smooth surface to the vicinal separation layer. After accomplishing the Ni NCs deposition, the next layer of BTO is still epitaxial growth, which the lattice relaxation is contrary to the Ni NCs. The sufficient stress relaxation provides an opportunity for the coexistence of 2D layer-by-layer

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R. Qiu et al. / Journal of Alloys and Compounds 801 (2019) 460e464

Fig. 2. The in-plane lattice relaxation during the grown processes of Ni NCs deposited on BTO layer (black) and BTO deposited on Ni NCs layer (red) in which the dark area shows the relaxation part. The inset displays the RHEED pattern measured for Ni NCs and BTO surface along the <100> azimuth. The surface-lattice parameter (a) can be deduced by the distance between the different diffraction spots or streaks measured from the RHEED patterns along the direction of vertical dotted lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

BTO and 3D islands Ni NCs in a perfect way, through which the ferromagnetic Ni NCs would be embedded in the ferroelectric matrix. Fig. 3(a) shows the ε0 -f curves at different temperatures for the composite films. Embedding 4.65 vol% Ni NCs into the single crystal BTO matrix induces a significant enhancement of ε0 at low frequencies (<100 Hz). For example, at 1 Hz the value of ε0 is increasing from room temperature values of 376 up to colossal values of 325510 at 613 K. Besides, a strong step was gradually presented with

the increase of temperatures. This stepping behavior of ε0 also corresponds to the well pronounced peaks of loss tangent (tand) (Fig. 3(b)) and shifts to higher frequency with the increase of temperature. These are the typical signatures of a relaxation and the overall behavior qualitatively resembles that of CCTO.2 The relaxation frequency can be represented by: n ¼ n0 expðEa =kB TÞ, where n0, kB, and Ea represent the pre-exponential factor, Boltzmann constant, and activation energy for relaxation, respectively [15]. The imaginary part 00 of the dielectric constant (ε ) is proportional to nt=1 þ ðntÞ2 , and the 00 maxima of ε occurs when nt ¼ 1, where t is the dielectric relaxation time. So the relaxation temperatures at different frequencies can be 00 extracted from the maximums of ε , and Ea can be estimated. The values of n and T can be determined from Fig. 3(b) and the Arrhenius plots for the pure and 4.65 vol% Ni NCs/BTO films are shown in Fig. 3(c). The change in slope of the fitted curves for the Ni NCs/BTO clearly indicates that two different thermally activated polarization mechanisms exist. The activation energy of 0.65 eV is obtained for the pure BTO, in agreement with the potential barrier of Au/p-type epitaxial BTO/SrRuO3 [16]. Here the conductive mechanism can ascribe the hopping dipoles in the dielectric material by polaron hopping generated in the rich-oxygen atmosphere [17]. After embedding 4.65 vol% Ni NCs, two different activation energy of Ea ¼ 0.48 eV and 0.75 eV are calculated in the low and high temperature region, which is also consistent with the potential barrier of nanoscale Ni grain boundary/BTO and metal electrode/BTO interface, respectively [16,18]. Relative to a general Schottky interface of metalBTO (Ea ¼ 0.65 eV), the Ni NCs embedding reduces the potential barrier heights (Ea ¼ 0.48 eV) of BTO due to local field enhancement of NCs [18,19]. For the Ni NCs/BTO, the smaller Ea indicated the polaron hopping processes at low temperatures, while higher activation energy of the high temperature region implies interfacial polarization induced by the interfaces of Ni NCs and BTO. To further clarify this hypothesis, the ε0 in the universal dielectric response model can be written as: f ε0 ¼ AðTÞf s , where AðTÞ ¼

Fig. 3. (a) dielectric constant ε0 , (b) dielectric loss tand for the 4.65 vol% Ni NCs/BTO films, (c) activation energy of thermally activated relaxations, (d) Lg(ε0 f) vs lgf plot for the 4.65 vol % Ni NCs/BTO films at different temperatures (293e613 K).

R. Qiu et al. / Journal of Alloys and Compounds 801 (2019) 460e464

tanðsp=2Þs0 ε0 , and f ¼ u=2p, s0 is the pre-exponential factor, ε0 is the permittivity of free space, s is the constant value between 0 and 1 [20]. So a slope of a straight line shown in lg(ε0  f) vs lgf plot at a given temperature express the value of s. As shown in Fig. 3(d), two different slopes do present at high temperatures and low temperatures. At 293 K, the slope of the line is 0.98. However, the straight line is deviated from the slope as frequency increases due to relaxation, and subsequently it decreases in a step-like behavior and displays another straight line at the high temperature region with s ¼ 0.91, corresponding to obvious changes in the dielectric permittivity and loss as shown in Fig. S2 (supplementary material). Significantly, the value of s as closer to 1 indicates that the polarization charges are more strictly localized [21]. Compared to high temperatures, the larger value of s at low temperatures implies that the charge carriers for polarization are more localized. Owing to certain energy to overcome the relatively small energy barrier (Ea ¼ 0.48 eV) for polarization, the hopping polarization is becoming active at low temperatures; while sufficient energy to overcome the large energy barrier (Ea ¼ 0.75 eV) at high temperatures, interfacial polarization associated with mobile electrons (e1) in the Ni NCs and holes (V  O ) in the BTO are activated. Therefore, the hopping polarization and interfacial polarization plays a dominant role in the low and high temperatures, respectively. In the CCTO, the colossal ε0 has been attributed to Maxwell Wagner (MW)-type contributions of the semiconducting grain and insulating grain boundaries, which can be explained by the internal barrier layer capacitance (IBLC) model [2]. On the assumption of IBLC model, the periodic Ni NCs/BTO nanostructure can be equivalent to a series connection of conductive Ni NCs and insulating BTO, which plays a role of grains and grain boundaries, respectively. From this view, this structure is much similar with the IBLC model. Based on the above analysis, the contribution of interfacial polarization to the colossal permittivity at 613 K can be calculated by using the IBLC model, which can be presented by：εeff ¼ εtg = tb , here εeff , ε, tg and tb is the effective permittivity, real permittivity of the material, grain size and thickness of grain boundary, respectively. As an example for the 4.65 vol% sample, the real permittivity of BTO is taken as ε ¼ 325510 at 1 Hz and 613 K. The grain size of Ni NCs is measured about 5 nm from Fig. 1(c), and the thickness of d2 (Fig. 1(a)) is estimated ~7 nm obtained from Fig. 1(c). Thus the effective permittivity of 232507 (equivalent to 71.43% of the experimentally observed colossal permittivity) is calculated. Therefore, the colossal permittivity at high temperature and low frequency could be attributed to the 71.43% interfacial and 28.57% polaron hopping polarizations. We further show the dielectric properties of the Ni NCs/BTO with a low content from 1.68 to 4.65 vol% at 613 K in Fig. 4(a). According to increase the pulsed laser numbers acted on the metal Ni target, we obtained the different content of Ni NCs in the BTO matrix, as shown in the inset. It is clear that the colossal dielectric constant at low frequencies is enhanced with the increase of Ni content, accompanying with the increase of dielectric loss. Based on the above estimation, the relative contribution of interfacial and polaron hopping polarizations were calculated for the 1.68, 2.13 and 3.23 vol% samples, in which the thickness of d2 (tb) was 13, 11 and 8 nm, respectively [22]. The values of grain size (Ni NC) are approximately unchanged, tg z 5 nm [23]. Thus the proportion of interfacial polarization is 38.46%, 45.46%, 62.50% and 71.43%, while the polaron hopping polarizations is 61.54%, 54.54%, 37.5% and 28.57% for the 1.68, 2.13, 3.23 and 4.65 vol% samples, respectively. With the increase of Ni NCs concentration, the interfacial polarization gradually plays a dominate role in the colossal dielectric response at low frequencies and high temperatures. The two contributions to the low-frequency colossal permittivity are schematically shown in Fig. 4(b). Therefore, the interfacial and polaron hopping polarizations could be

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Fig. 4. (a) dielectric properties of the Ni NCs-BTO with a low content from 1.68 to 4.65 vol% at 613 K where the inset shows the volume fraction of detected Ni by XRF, (b) interfacial and hopping polarization model for the Ni NCs/BTO films.

tuned by the artificial coexistence system of metal NCs and epitaxial dielectrics, in which the effective controlling of the grains and grain boundaries can be achieved. 4. Conclusions In summary, we clearly stated the contribution of grains and grain boundaries to the colossal dielectric constant at low frequencies (f < 100 Hz) by building a particular nanostructure of Ni NCs/epitaxial BTO. Based on this, the interfacial polarization and hopping polarization can be tuned quantitatively. Such nanocomposites are expected to exhibit great potential in dielectric capacitor and magnetoelectric nano-devices. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant No. 11704353), Scientific Research Fund of Sichuan provincial education department (Grant No. 18ZA0503), Longshan Academic Talent Research Supporting Program of Southwest University of Science and Technology (Grant No. 17LZX538), the LSD fund (Grant No. 6142A03010102) and CAEP Foundation (Grant No. YZJJLX2016001). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.06.154. References [1] B. Luo, X. Wang, E. Tian, H. Song, Q. Zhao, Z. Cai, W. Feng, L. Li, Giant permittivity and low dielectric loss of Fe doped BaTiO3 ceramics: experimental and first-principles calculations, J. Eur. Ceram. Soc. 38 (2018) 1562e1568. [2] L. Ni, X.M. Chen, Dielectric relaxations and formation mechanism of giant dielectric constant step in CaCu3Ti4O12 ceramics, Appl. Phys. Lett. 91 (2007) 122905. [3] H. Du, X. Lin, H. Zheng, B. Qu, Y. Huang, D. Chu, Colossal permittivity in percolative ceramic/metal dielectric composites, J. Alloy. Comp. 663 (2016) 848e861. [4] Z. Xiong, L. Cao, Tailoring morphology, enhancing magnetization and

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