Ferroelectric and piezoelectric oxide nanostructured films for energy harvesting applications

Ferroelectric and piezoelectric oxide nanostructured films for energy harvesting applications

19

A. Datta*, D. Mukherjee†, S. Kar-Narayan* *University of Cambridge, Cambridge, United Kingdom, †Technical Research Center, Indian Association for the Cultivation of Science, Kolkata, India

19.1

Introduction

Electric charge polarization under the application of an external electric or mechanical stress field can give rise to ferroelectric (FE) and piezoelectric (PE) properties in certain perovskite crystals which make them attractive for various technological applications [1,2]. FE crystals are unique as their unit cells have a polar axis that results in a spontaneous electric dipole moment even in the absence of an electric field [3]. The existence of a spontaneous polarization implies that there is a preferred special orientation in the crystal. By virtue of this property, all FE materials are PE, but not all PE materials are FE, such as ZnO. Typically, these materials exhibit long-range alignment of electric dipoles resulting in a net polarization when a voltage or force is externally applied across the material [4,5]. FE materials have a phase-transition temperature above which they are paraelectric in nature. Owing to their unique polarization properties and other characteristics such as pyroelectricity and large dielectric constants, these materials have potentially become essential components in a wide spectrum of applications. FE thin films enable the design of integrated circuits for nonvolatile random access memory (NVRAM), microelectromechanical (MEM) devices, and high-frequency electrical components that result from distortions programmed into the very crystalline structure of the dielectric material and are understood to be sensitive to the length scale [4,5]. These memory and sensing effects make them act as nonlinear capacitors and give them sensitivity to environmental stimuli. In this regard, two-dimensional (2D) epitaxial or ultrathin oriented films, aligned/oriented 3D FE/PE nanostructures of comparable thickness enable greater tunability of the domain size, domain wall motion, and nonaxial polarization properties [6,7]. The orientation of polarization is extremely sensitive to not only the local morphology of the nanostructured ferroelectrics but also regarding how densely they are distributed (packing density), the thickness of the nanostructured layers on the substrates, and the substrate strain that finally determines the orientation of the nanostructures. Furthermore, the scaling of the domain size is correlated to the scaling in other domainrelated properties such as coercive fields and switching kinetics [1–6]. These potential applications have led to the extensive investigation of these materials in thin-film and Metal Oxide-Based Thin Film Structures. https://doi.org/10.1016/B978-0-12-811166-6.00019-4 © 2018 Elsevier Inc. All rights reserved.

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nanostructured forms. Most of the FE and PE materials that have been studied for practical applications are oxides because of their chemical stability and robustness. Oxide-based ferroelectric and piezoelectric ceramics have been in industrial use for a long time [1]. Research to find novel and environmentally safe compounds based on earth-abundant elements has witnessed tremendous growth over the past decade [3–5]. This is in part connected to the successful transfer of the multilayer capacitor technology to multilayer memories, sensors, and actuators [6,7]. Nowadays, displacements on the order of micrometers are possible in microseconds and PE actuators are reported to support stresses on the order of tens of megapascals [1–7]. Applications have diversified in the areas of fuel injection, printing machines, piezoelectrically controlled thread guides, and micropositioning systems, among others [2–6]. Ferroelectrics and PEs have now evolved into vast fields of application-based study, some of which are beyond the scope of this chapter. Keeping with the theme of this book, here we focus on some growth and properties of important FE and PE oxide thin films and nanostructures which have shown potential for emerging energy technologies. We highlight the importance of their structural engineering during device fabrication and associated interface-based phenomena, which can lead to enhanced properties in these materials.

19.2

Ferroelectric oxide nanostructures with enhanced properties

Since the late 1980s FE thin films have been developed and integrated into silicon chips [8] for applications in nonvolatile data storage with higher speeds, access times, and lower power consumption than the existing technologies [2,9]. FE thin-film technology for nonvolatile ferroelectric random access memory (FeRAM) devices is based on the principle of polarization reversal by an external applied electric field of FE capacitors. The computational “0” and “1” are represented by the nonvolatile storage of the negative or positive remnant polarization state, respectively [5]. Owing to their diverse technological applications in memory devices, infrared sensors, PE sensors, and actuators, FE thin films have attracted much attention and consequentially numerous books and exhaustive reviews have been written considering all aspects of their fabrications, properties, and applications [10–14]. Several thin-film deposition techniques such as sputtering, pulsed laser deposition (PLD), molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), and sol-gel processing have been extensively used to fabricate FE thin films and heterostructures [15–19]. Some of the major concerns which have remained under investigation are the proper control of stoichiometry, microstructure, grain size, porosity, crystallinity, and defects during their fabrication. The interface plays a crucial role in FE thin films as electrical measurements are made on a FE capacitor composed of a dielectric slab of material that is sandwiched between two electrodes. Interfacial effects at the FE film-electrode interface is complicated by the “screening” phenomena in the metallic electrodes, the existence of a “FE dead layer” of about 3 nm thickness, and leakage currents [2]. FE thin films with metal electrodes also suffer from

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fatigue which limits their long-time performance as memory devices [2]. The use of metallic oxide electrode has been found to overcome fatigue by preventing the partial perovskite-pyrochlore conversion in FeRAMs. Interfacial effects in these FE thin films have been reviewed in detail [20]. With recent advances in the growth and characterization techniques of FE thin films there has been an unprecedented progress in the study of these materials [21–23], which has expanded both the fundamental understanding of novel phenomena and enabled precise control of their properties through strain engineering and discovery of new phases. Among the emerging FE thin-film technologies, the growth of nanostructured FE thin films has attracted considerable attention as multilevel 2D and 3D nanostructure engineering in FE thin films can offer advantages of different length scales using hierarchical nanostructures. Nanostructured thin films also allow the manipulation of dimensional effects in thin films to enhance their properties. From the materials perspective, the family of solid solutions of lead zirconium titanate, Pb(ZrxTi1
x)O3 (PZT) has remained as the state-of-the-art FE material for a wide range of applications such as transducers, MEMS devices, and nonvolatile memory devices in spite of the recent environmental concerns on the toxicity of lead [8,24]. This is primarily owing to its highest FE polarization and PE coefficients as well as its mechanical and chemical stability [25]. The realization of FE memory devices based on the polarization effects of PZT has led to the widespread investigation of this material in multiple dimensions and length scales [7,8,26]. Commercial FE memories (up to 64 Mb) in FeRAMs from Samsung, Fujitsu, and Matsushita have used PZT ceramics of varying thicknesses from 0.12 to 0.4 mm [2]. As the sample thickness decreases, however, they suffer from tunnel/leakage currents which limit their use in FeRAMs. Thus, research in FE thin films is limited due to intrinsic difficulties of structural engineering in the nano-scale. To this end, nano-heterostructured thin films of FE materials, in general, exhibit uniquely different properties from the nontextured homogeneous thin films due to the deliberate engineering of nanoscale features into the structure of the films [27–32]. Furthermore, it is commonly believed that high specific surface area achieved through multilevel 2D and 3D nanostructure engineering in thin films plays salient roles in achieving high structural and property efficiency compared to regular 1D nanostructured films [33–35]. Nanostructuring provides new insights into the size, shape, and surface effects on the charge-ordering in ferroelectric and PE perovskite oxide materials. The down-scaling effect results in an enhancement of the surface area of materials where surface charges play a dominant role in determining the magnitude and direction of polarization. As polarization properties are the cumulative phenomenon of crystal dimensions, orientation, and ordering, synthesis methodologies that can lead to the manipulation of size and dimensions of ferro- and piezoelectric nanostructures can offer great advantages. In recent years, there has been considerable enthusiasm in controlling arrangements of nanosized building blocks into hierarchical structures of perovskite materials and they have become a recent hot topic in the oriented-attachment and self-assembly research field [36–39]. Efforts to synthesize and understand FE properties of PbxZr1
xTiO3 (PZT) 1D nanostructured thin films have been undertaken because of the promise they show in the realization of nano- and microscale devices [40–44]. It has been observed

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that the properties of interest for PZT at the nanoscale range are directed by the crystal structure and orientation of the crystallized surface facets at the interface. A modified sol-gel technique was adopted to synthesize ultrathin-walled PZT nanotubes in porous alumina membranes (PAM) to form composite arrays as shown in Fig. 19.1A. The vertically aligned PZT nanotubes are shown schematically in the inset to Fig. 19.1A. Well-saturated FE hysteresis loops were obtained from the device fabricated using the 1D nanotubes embedded in a dielectric filler as shown in Fig. 19.1B [29]. This type of study manifested the potential of one-dimensional nanostructure arrays towards the development of three-dimensional FE capacitors for memory applications [29]. Thus, a pronounced dimensionality effect is observed in this material in ordered nanostructured form. To this end, the integration of FE PZT nanostructured thin films with ordered 2D and 3D periodic structures, controlled exposed facets, and crystal orientations of the nanostructures are of immense significance, although particularly challenging to synthesize. Furthermore, the large-scale synthesis and integration of ordered functional nanowires is a challenge, due to their complicated fabrication methodologies, high cost and difficulties with phase stability in many metastable perovskite compounds [30]. Facile solution-based synthesis processes can be utilized for fabricating state-ofthe art and novel perovskite nanostructured films where tuning crystallinity and strain engineering due to dimensional confinement and domain wall interaction due to grain boundaries in nanostructures may enhance the Curie temperature, permittivity, or polarizability, and induce “self-poling” effects, eventually enhancing the overall performance of the devices [45]. In this direction, the directed growth of PZT hierarchical 3D nano-heterostructures promoted by the PZT seed-assisted nucleation and controlled orientation is notable [46]. An innovative physical/chemical combinatorial synthetic strategy involving the PLD technique and hydrothermal process was adopted to obtain structure and orientation selective synthesis of novel oriented PZT hierarchical nano-heterostructures over large area substrates. In this approach, while on one hand, PLD guaranteed uniform deposition of PZT seed-layer thin films on lattice matched SrTiO3 substrates, chemical after-treatment of the seed-layer ensured controlled growth of the nanostructures bringing out the advantages of both the processes. The key aspect of this unique hierarchical 3D morphology of highly compact PZT nanostructured thin films is that the nano-heterostructures self-orient to form a dense network structure, which facilitates the measurement of the FE properties of these nanostructured thin films without using any insulating fill layer or template, which is a common practice to measure FE properties of nanostructured thin films [30]. The initiation of the growth of PZT hierarchical nanostructures was preceded by the growth of compact PZT seed layers on STO:Nb substrates by the controlled PLD process. The SEM images of the PZT seed-layer thin film, from an area of least coverage of film surface, indicates tooth-like geometry of the PZT seeds, shown in Fig. 19.1C, with clearly developed facets (inset to Fig. 19.1C). The average size of the PZT seeds varied from <5 to 50 nm from bottom to top, with the typical thickness of the seed-layer film being 50 nm. A cross-sectional TEM image of this seed-layer film shown in Fig. 19.1D provides in-depth insight into the distribution, morphology, and crystallinity of the PZT seeds on STO:Nb substrate. The closely spaced tooth-like

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Fig. 19.1 (A) Highly ordered hexagonal array of PZT nanotubes (scheme shown in the inset) grown within alumina templates. (B) P-E hysteresis loops of PZT nanotube arrays obtained at 100 kHz. (C) PZT seed layer on STO:Nb substrate deposited by PLD. The inset shows the geometry of the PZT nano-seeds. (D) HRTEM image of the PZT seed layer on STO:Nb substrate showing the growth of PZT tooth-like seeds (preferential growth along [001]) showing a clean interface with the substrate. Locally other growth planes in the tetragonal PZT structure (shown by the dashed circle) and misfit dislocation planes (arrows) were observed. (E) Schematic diagram of the PZT nanostructured ferroelectric device. (F) P-E hysteresis loops for the nanostructured PZT thin film measured at varying driving voltages from 1 to 9 V. Part B: Reproduced with permission from Nano Lett. 8 (7) (2008) 1813–1818. Copyright 2008, American Chemical Society. Part F: Reproduced with permission from Adv. Funct. Mater. 24 (2014) 2638–2647. Copyright 2014, Wiley-VCH.

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growth of PZT seeds from the cross-sectional TEM image was observed, which were apparently formed with predominating growth of (00l) planes on the STO:Nb substrate as observed from the HRTEM image in Fig. 19.1D. The TEM analysis of the sample revealed a well-defined and relatively sharp interface of the PZT seed layer with STO:Nb substrate. However, as can be observed from Fig. 19.1D, some areas have misfit dislocations (shown by arrows in Fig. 19.1D), which might form due to relaxation of the lattice mismatch strain between the PZT nanocrystalline seeds and the substrate during PLD deposition. Lattice planes at other orientations were also observed as marked (circle) in Fig. 19.1D and selected area electron diffraction pattern (SAED) from the PZT seed layer shows preferentially oriented lattice planes with signatures of polycrystallinity (inset to Fig. 19.1D). While the seed layer of PZT facilitated the subsequent growth of PZT nanostructures, still, measurement of FE properties remained a challenge. While on one hand, the insulating polymer matrix can prevent the shorting of the device by filling the pores in the structure, on the other hand, it introduces huge dielectric losses during the polarization measurements, which significantly lowers the polarization values. In this case, however, the significantly reduced porosity and the increased conformity of the dense PZT hierarchical nanoheterostructures formed on the substrate allowed the successful deposition of the Pt-top electrodes without shorting the structures. This enabled the fabrication of a thin-film capacitor from the PZT nanostructures where the conducting STO:Nb substrate served as the bottom electrode and Pt pads deposited on top served a top electrode, as shown schematically in Fig. 19.1E. This allowed the successful polarization measurements of the nanostructured PZT films without the need for any polymeric host. Fig. 19.1F shows typical P-E curves measured at room temperature for varying driving voltages from 1 to 9 V for the nanostructured PZT thin film grown on STO:Nb substrate. For driving voltages of 7, 8, and 9 V the P-E curves show well-saturated square FE hysteresis loops with slightly increasing remanent polarization (Pr) and coercive field (Ec) values with the driving voltages. The highest remnant polarization value of 54 μC/cm2 at a coercive field of 237 kV/cm was recorded at a nominal applied driving voltage of 9 V. This value is much higher than the bulk remnant polarization for PZT of 26 μC/cm2 at a similar composition [26]. Moreover, given the fact that facile methods for preparing hierarchical PZT micro-/nanostructures are rare, this type of combinatorial physical/chemical approach represented an important advancement in this regard. These nanostructured PZT films can have a great impact in many applications such as in sensors, actuators, and electro-catalysts, where the use of hierarchical structures of FE materials are preferred. While on one hand, the family of Pb-based perovskites such as PZT has been extensively deliberated as feasible FE media, on the other hand, the use of toxic Pb in FE devices and the consequential environmental concerns have resulted in efforts to find novel environment-friendly FE materials with polarization properties comparable to or even better than PZT. Extensive research is therefore underway to discover new FE materials as well as to enhance the FE properties of state-of-the-art materials by structural manipulations [47–51]. As alternative materials, an increased interest has been developed around the noncentrosymmetric (NCS) complex oxides because of their symmetry-dependent properties including pyroelectricity, nonlinear optical behavior,

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piezoelectricity and, most importantly, ferroelectricity [52–55]. Recently, among the family of Pb-free NCS oxides, special attention has been paid to LiNbO3 (LN) type ZnSnO3, a compound formed in the R3c space group and composed of only Zn2+ and Sn4+ cations with the electronic configuration of (n
1)d10ns0, primarily because of promising theoretical predictions of high spontaneous polarization of  59 μC/cm2 that can be obtained along the c-axis of the structurally ordered LN-type ZnSnO3 crystals [51,52,54–57]. Subsequently, the dielectric properties of polycrystalline ZnSnO3 have been experimentally explored on the samples synthesized by a solid-state reaction under high pressure at elevated temperatures [58]. The first ubiquitous ferroelectricity with a large remanent polarization  47 μC/cm2 was reported from a heteroepitaxially grown (111) ZnSnO3 thin film using PLD [59]. Significant work has also been done on the design and fabrication of nanogenerators (NGs) based on the enhanced PE properties of R3c-type ZnSnO3 nanostructures [60–63]. Nano-structuring provides new insights into the size, shape, and surface effects on the charge ordering in FE materials [29,64–69]. The down-scaling effect results in an enhancement of the materials surface area where surface charges play a dominant role in determining the FE polarization [29,64–69]. In recent years, progress in the direct growth of FE nanostructures homogeneously distributed on substrates with controlled morphologies, orientations, and dimensionalities and the capabilities to measure their polarization properties have further added to this technology [29,64–69]. Additionally, device miniaturization using bottom-up synthesized nanostructures has given a new direction to this research [29,49,67,69,70]. Understanding the FE behavior in low-dimensional LN-type ZnSnO3 is a major step in the realization of nanostructure-based Pb-free memory devices [46]. The other interesting aspect that demands attention is the implementation of industrially viable approaches to synthesize and integrate these nanostructures into devices and understanding their suitability for future applications in the FE industry. In this direction, a hybrid physical/chemical synthesis scheme that includes the PLD technique and a solvothermal process has been used to grow the NCS FE pervoskite material ZnSnO3 assisted by a conducting ZnO:Al template layer on a Si substrate [46]. The similar crystal symmetry between the doped ZnO crystals and LN-type ZnSnO3 facilitated the growth of well-aligned and self-supported ZnSnO3 NW arrays with a tightly packed distribution (packing density  0.8) as shown in Fig. 19.2A. Structural analyses revealed a unique “welding” mechanism along the wire lengths and their tips, thereby substantially decreasing the intrastructural porosity. This facilitated a direct measurement of the intrinsic FE polarization of ZnSnO3 from the resulting nanostructured device shown schematically in Fig. 19.2B. Superior FE properties from these ZnSnO3 NW arrays demonstrating a remnant polarization value as high as  30 μC/cm2 was reported as shown in the P-E loops in Fig. 19.2C [71,72]. This provides a new approach for searching Pb-free nanostructured FE materials. Nanostructured FE films can sometimes exhibit an anomalous shape of the FE hysteresis, which can provide deeper insight into their intrinsic properties such as microstructure, grain size, grain boundaries, phase, doping, and anisotropy as well as the external measurement conditions (applied field amplitude, frequency, fatigue, temperature, stress) and defect structures [73]. Abnormal shapes of FE hysteresis loops,

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Fig. 19.2 (A) Self-supported LN-type ZnSnO3 NW arrays on Si, assisted by a ZnO:Al template layer deposited by pulsed laser deposition (PLD) technique and (B) scheme of the device with top Au electrodes used for polarization measurement. (C) P-E hysteresis loops at varying applied voltages measured from the Pt/ZnSnO3 NW array/ZnO:Al capacitor device. (D) Cross-sectional SEM image of the NP-NW arrayed film on ZnO-seeded Si substrate. (E) TEM image shows ZnSnO3 NPs on the surface of the ZnSnO3 nanowires. (F) Schematic diagram of the interactions of these randomly distributed nanoparticle domains which may create internal fields when subject to external electric fields. (G) P-E loops of the samples with different concentrations of ZnSnO3 NPs, measured using the similar device geometry as for the nanowire arrays in (B). Part C: Reprinted from Small 10 (2014) 4093–4099. Copyright 2014, Wiley-VCH. Part G: Reprinted with permission from Appl. Phys. Lett. 105 (2014) 212903. Copyright 2014, American Physical Society.

Metal Oxide-Based Thin Film Structures

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such as constriction or pinching at low fields [74–77] and asymmetricity, are often encountered in FE thin films and nanostructures [78–80]. These typically result from space charge effects [81], such as charged defects, trapped charges (oxygen vacancies), and other possible defect-dipole complexes (vacancy-acceptor dipoles) that could be possibly created during the growth of the FE structures, or with aging. The presence of the charged defects and defect-dipoles may create a pinning effect on the domain wall motion which in turn can distort the hysteresis shape from its ideal symmetric behavior. As hysteresis behavior is a cumulative phenomenon caused by the spatial arrangement of charged dipoles, it is often difficult to distinguish the possible trapped charge contributions from the real FE behavior. In nanostructures, complicated interplay among structures, surface charges, strain effects, and domain wall motion cause changes in the long- and short-range ordering of dipoles inside the FE material [31,46,64,66,67]. Composites consisting of different dimensions of nanostructures were reported to possess improved physical properties, thereby making them a desirable structural system [82–84]. Anomalous FE properties were reported in LN-type ZnSnO3 hybrid nanoparticle-nanowire (NP-NW) structured arrays (shown in the cross-sectional SEM image in Fig. 19.2D) [85]. Electron microscope studies revealed that the NPs were uniformly but randomly distributed along the length of the NWs with an estimated occupancy of 30–40 vol% in the sample. The diameters of the ZnSnO3 NWs were within 20–30 nm and the size of the NPs varied in the range of 5–15 nm as shown in the TEM image in Fig. 19.2E. A constricted FE hysteresis loop with a remanent polarization of  26 μC/cm2 intrinsic to the hybrid microstructure of the material has been recorded from the NP-NW LN-type ZnSnO3 nanostructured film. The physical mechanism of the constricted hysteresis was due to a strong interaction of the switchable dipoles emanating from the randomly distributed NPs with their surrounding in the hybrid nanostructured array [85]. The singlecrystalline NPs surrounding the LN-type ZnSnO3 NWs behaved as single-domain switchable units with unidirectional dipole alignments. The interactions of these randomly distributed dipoles create internal fields when subject to external electric fields, as shown schematically in Fig. 19.2F. The different local environments created as a result of these spatially dispersed internal fields lead to a distribution in the threshold fields required for domain switching. This consequently affects the domain wall motion and lead to the constriction in the FE hysteresis as shown in Fig. 19.2G. Low-dimensional FE materials can exhibit confinement-induced enhancement of polarization and strain-mediated depolarizing effect in nanodomains that can give rise to improved ferroelectric as well as piezoelectric properties [86–89]. Several studies have reported that strain engineering in FE epitaxial films grown on lattice-matched substrates can induce significant shifts in the Curie temperature (TC) and enhancement in the remanent polarization (Pr) in perovskite oxides [88–99]. These effects are especially useful in the case of lead-free ferroelectrics with low TC, which often limits their practical use. However, the use of substrate-induced epitaxial strain is limited to thin films with thicknesses of the order of tens of nanometers, beyond which strain relaxation occurs [98]. Recent studies have shown that nanostructuring could provide a new route to strain engineering in ferroelectrics [98] where size effect plays a dominant

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role in determining not only the ferroelectric polarization but also the PE responses from these materials. Induced strain due to dimensional confinement in nanowires (NWs) modifies the FE properties, such as the Tc, permittivity, or polarizability, thereby eventually extending the operational temperature range and overall performance of the devices. As ferroelectricity and piezoelectricity arise from cooperative effects of crystal dimensions, orientation, and ordering, scalable synthesis methodologies that can lead to the manipulation of size and dimensions of FE nanostructures can offer great advantages. With respect to lead-free ferroelectrics, studies concerning the strain-induced variation of FE properties in one-dimensional nanomaterials are rare. Amongst lead (Pb)free ferroelectrics in the bulk, xBa(Ti0.8Zr0.3)O3-(1
x)(Ba0.7Ca0.3)TiO3 where x ¼ 0.5 [100], otherwise known as (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3, or BCT-0.5BZT, is particularly promising as it has soft FE properties at room temperature (saturation polarization Pr  10–15 μC/cm2, and coercive field EC  1.5–3 kV cm
1) in combination with a large PE coefficient (620 pC N
1), and is comparable to Pb-based PbxZr1
xTiO3 (PZT), as well as Pb-free (K,Na)NbO3 [101]. Liu and Ren reported the phase diagram of the BCT-BZT system, in which they reported a “tilted morphotropic phase boundary” (MPB) separating rhombohedral (R) and tetragonal (T) phases. The presence of a tri-critical point between cubic (C)-R-T phases was reported as similar to that found in lead-based materials [102,103]. It was pointed out that along this MPB, the polarization has enhanced response to external stress or electric fields due to phase-transition instabilities within a narrow temperature range resulting from the low-energy barriers between different polar distortions allowing for easy rotation of the polarization direction [101,103]. More specifically, the properties can then be justified in terms of the interplay between the offcentering’s of Ti, Zr (mostly in the rhombohedral direction), and of Ca (mostly in the tetragonal direction) with chemical disorder. Regardless of the near room temperature MPB of BCT-0.5BZT, the bulk material exhibits a TC of about 90°C and a large temperature dependence of the properties [101], which limit the application in practical devices. To improve the TC a nanoconfinement-based approach to tune the strain and the FE properties in NWs of this material was undertaken. A novel and facile synthesis of highly crystalline BCT-0.5BZT NWs using a solgel assisted template wetting methodology was undertaken [104]. This approach is cost-effective and largely scalable expanding its viability to other Pb-free FE materials systems in the nanoscale. The combined sol-gel and template-assisted fabrication route results in well-controlled uniform growth of high aspect ratio, highly crystalline BCT-0.5BZT nanowires as shown in Fig. 19.3A, as compared to more complex multistep hydrothermal synthesis [105], or electrospinning [35]. The structural, ferroelectric, and dielectric properties of BCT-0.5BZT NW arrays were successfully measured on as-prepared template-freed nanowires and studied in terms of the effects of reduced dimensionality and strain due to lattice distortion as well as grain boundaries. Phasestabilized BCT-0.5BZT polycrystalline NWs with high aspect ratio were prepared using a simple, scalable sol-gel assisted template-wetting methodology. The room temperature P-E hysteresis loops measured at 20 V driving voltage as shown in

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Fig. 19.3 (A) BCT-0.5BZT nanowire arrays prepared by sol-gel assisted growth inside a porous polyimide template. (B) P-E hysteresis loops from a BCT-0.5BZT nanowire network device at room temperature. (C) A PE d33 value of 35 pm V
1 could be obtained from the BCT-0.5BZT nanowire network device as shown by the SEM image in (D) [104].

Fig. 19.3B clearly indicated a measurable ferroelectric polarization of Pr  3 μC/cm2. Additionally, the PE coupling coefficient measured by piezo-response force microscopy (PFM) as shown in Fig. 19.3D gave a PE d33 coupling coefficient 35 pm V
1 at room temperature indicating the suitability of the BCT-0.5BZT NW device (shown in Fig. 19.3D) promising a Pb-free ferro/piezoceramic alternative. Further, BCT-0.5BZT NWs exhibited a largely increased TC of 300°C as compared to 95°C in bulk of similar composition. Volume expansion around the second TC  300°C is possibly due to the strain relaxation and phase change phenomena. Overall, the BCT-BZT NWs always possessed a contracted lattice volume with respect to the bulk and hence is more strained even at room temperature, which might result in the observed phasetransition shift to a large extent. The new synthesis technique has shown that it is possible to improve the temperature dependence of the ferroelectric properties by controlling morphology and strain.

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Metal Oxide-Based Thin Film Structures

Piezoelectric oxide nanostructures for energy harvesting

Energy derived from natural processes is being replenished at a faster rate than it is consumed and hence the demand for alternative resources is increasing at a greater rate. As quoted by International Energy Agency (IEA), in 2012, the world relied on renewable sources for around 13.2% of its total primary energy supply. In 2013, renewables accounted for almost 22% of global electricity generation, and the IEA Medium-Term Renewable Energy Report 2015 foresees that share reaching at least 26% increase in 2020. This is just one example of several surveys undertaken on the energy demand and crisis perceived for coming generations. As materials scientists we therefore must take this chance to assess what is scientifically possible, environmentally acceptable, and technologically promising to predict and implement materials technology to facilitate this change. Harvesting and harnessing commonly occurring energies from the ambient sources in our environment has drawn considerable attention in light of the demand for alternative energy solutions to “sustainably power” electronic applications that currently mostly rely on traditional power sources such as batteries. For example, ubiquitous and easily accessible ambient vibrations could provide a convenient means to power PE materials in devices such as wireless sensors, wearable and portable electronics, and even for biomedical implants [106–109]. Piezoelectricity promises to be one of the most potential alternative sources of energy with an associated fast growing investment market with a wide range of applications in information and communications, industrial automation, healthcare and medical monitoring, and automation and robotics [110]. PE nanostructures have demonstrated improved properties that enable new functionalities related to reduced dislocations and superior mechanical properties [6,109,111]. Harvesting energy from ambient sources in our environment through PE nanostructures offers a fundamental energy solution for small power applications. As ambient vibrations are ubiquitously available and easily accessible originating from ever-present sources such as moving parts of machines, fluid flow, and even body movements, PE nanostructures offer a plethora of opportunities to explore the structure-property relationships to tune the piezoelectricity for device structuring. For instance, in the pioneering work by the Wang group, aligned arrays and multilayer stacks of zinc oxide and lead zirconate titanate nanowires acting as nanogenerators (NGs) have been exploited to power light-emitting and wireless devices [112–114]. Lead-based ceramic PZT has been the efficient NG material choice for over several decades [115]. Despite its outstanding PE property, PZT is currently facing global restrictions due to its Pb toxicity; and therefore non-Pb substitutes such as non-Pb ceramics such as barium titanate [116], lead zirconium titanate [6], and zinc oxide [117], and polymers [108,118–120] that can compete with PZT is the focus of future NG research (Fig. 19.4A). However, non-Pb PE ceramics generally have inferior piezoelectricity (PE coefficient d33 < 150 pC/N in most cases) compared with PZT.

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Fig. 19.4 (A) Comparison of d33 among BCT-0.5BZT and other non-Pb piezoelectrics and PZT family. (B) Piezoelectric response of the BCT-0.5BZT NW dense network as shown in the inset [104]. (C) Piezoelectric response of the BCT-0.5BZT 70 nm nanoparticles. (D) Power density plots with varying load resistors for different weight fractions of BCT-0.5BZT NWs in the NW-polymer nanocomposite energy harvester. Inset shows the device structuring. (E) Biocompatible BCT-0.5BZT NW-NG implanted in the rabbit’s back and (F) the current of the prepared NG in vivo. Part A: Reprinted from PRL 103 (2009) 257602. Copyright 2009, American Physical Society. Part C: Reprinted from J. Mater. Chem. C, 2015, 3, 4762. Copyright 2015, Royal Society of Chemistry. Part D: Reprinted from Nanoscale 8 (2016) 5098. Copyright 2016, Royal Society of Chemistry. Part F: Reprinted from Adv. Mater. 6 (2014) 7432. Copyright 2014, Wiley-VCH.

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Recently, their limit has been pushed to a higher level of d33  500 pC/N for (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCT-0.5 BZT) bulk at room temperature [100]. To improve the performance of BCT-0.5 BZT even higher strain engineering through nanostructuring was adopted as a new route to modify the PE coupling performance of the materials and devices. In our recent work, we have reported the PE response in BCT-0.5BZT polycrystalline NWs using a simple, scalable sol-gel assisted template-wetting methodology. The averaged PE displacement versus offset voltage and phase versus voltage plots obtained on the nanowire network device are shown in Fig. 19.4B. While the butterfly loops indicated polarization direction reorientation in the BCT-0.5BZT ferroelectric NWs, the observed phase shift clearly demonstrated the local polarization switching behavior in these NWs with a d33 value of 35 pm V
1 as discussed also in Section 2 [120]. A similar value of d33 was obtained from nanocrystalline BCT-0.5BZT powder with an average crystallite sizes of 70 nm synthesized via the complex oxalate precursor route at 800°C (Fig. 19.4c) [121]. Flexible nanocomposites utilizing dispersed BCT0.5BZT NWs are reported to demonstrate an energy harvesting application with an open circuit voltage of up to 6.25 V and a power density of up to 2.25 μW cm
3. The high electromechanical coupling coefficient and high power density demonstrated with the NWs produced via a scalable hydrothermal synthesis method showed the potential for high-performance NW-based NG devices [105]. This flexible energy harvester is fabricated by Zhou et al. [105] as a nanostructured cantilever is shown in Fig. 19.4D allowing the beam to freely vibrate at its resonant frequency. The fabrication process involved tape casting a thoroughly mixed slurry of BZT-BCT NWs and PDMS onto 35 μm-thick titanium foil, which functioned as the bottom electrode. One step forward, BCT-0.5BZT NWs with a high PE coefficient and the corresponding textile were synthesized through the electrospinning method and a post annealing process. The output voltage, current, and power density of 3.25 V, 55 nA, and 338 mW cm
3, respectively, were reported which were shown to power up a commercial LCD [122]. In a remarkable work by Yuan et al. [123], the biocompatibility of electrospun BCT-0.5BZT NWs was studied and a biocompatible NG was successfully developed as shown in Fig. 19.4E, which was implanted subcutaneously into a rabbit’s back to evaluate the in vivo biocompatibility. By periodically pressing the back of the rabbit, the partly packaged NG could give an output current of 0.13 nA (Fig. 19.4F) without causing any tissue damage even after 5 weeks thereby confirmed the biocompatibility of the entire NG. Among other important PE materials, NCS oxides have attracted considerable attention due to their unique symmetry dependent and spontaneous polarization properties. One of the simplest NCS oxides, ZnO, is an environmentally friendly and an attractive PE material. ZnO nanowires have extensively been demonstrated as NGs [124], solar cells [125], and strain sensors [126] due to their improved sensitivity to low amplitude ambient vibrations and reduced fragility in the nanoscale. PFM measurements performed on ZnO NWs reported d33 as 0.4–9.520 pm V
1. More recent studies have indicated a d33 increase of  18% in ZnO NWs as compared to

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bulk [127]. As the size, orientation, and surface geometry of nanowires largely control the PE properties, often it was seen that by adopting a synthesis process that imparts more control on the orientation of the nanostructures introduces large improvement in the PE properties in ZnO, which otherwise tailors the energyharvesting properties [128]. In our recent report, we used a template-assisted electrodeposition process to grow ZnO polycrystalline nanowire arrays inside the polycarbonate template that offer the higher flexibility of the device which allows the final NG device to be more compliant and less prone to mechanical fracture [129]. The presence of the template reduced the likelihood of electrical shorting when the electrodes are applied in the final NG device, as well as the likelihood of gases being adsorbed onto the surface of the NWs that externally screen polarization charges. A NG based on the ZnO NWs was measured for its electrical output in response to periodic impacting by an oscillating mechanical arm at a set frequency. The output power maximized under impedance-matched conditions across a resistor  20 MΩ. The output power density of 151  25 mW m
3 at an impedance-matched load, when subjected to a low-level periodic (5 Hz) impacting force, showed the superior fatigue characteristics under continuous impacting (Fig. 19.5A). The NG maintained a reasonably constant output voltage throughout the fatigue test with an energy conversion efficiency of  4.2%. The similar device prepared from ZnO NWs grown inside the polycarbonate template by a low temperature hydrothermal synthesis process generated a peak output power density of  1600 mW/m3 across a load resistance of  1 MΩ, and excellent fatigue performance with an energy conversion efficiency of again  4.2% (Fig. 19.5B). The as-grown ZnO NWs, well protected from detrimental environmental factors and mechanical failure within the PC template, proved a potential candidate for low power microenergy harvesting applications. Apart from ZnO, there are still many Pb-free PE materials in the NCS group that remain to be discovered. Among NCS oxides, ZnSnO3 is primarily important as it is characterized by a large displacement of Zn based on a strong covalent bond between three oxygen and zinc atoms, resulting in the ZnSnO3 having a strong PE response (Fig. 19.5C) and a high ferroelectric polarization. The successful synthesis of ZnSnO3 nanobelts was realized by high-temperature physical vapor deposition and an NG device was designed with a flexible polystyrene substrate and fixed to electrodes at both ends, as shown in Fig. 19.5D [60]. Such prepared energy harvesting devices could produce an output voltage and current exceeding 110 mV in parallel connection and 80 nA in serial connection, respectively. The total energy conversion efficiency could reach 6.6% based on the microbelt dimensions of  477  19  8 μm with 0.8% strain. In another report, ZnSnO3 nanowires were also demonstrated as an efficient piezotronic sensor [61]. To investigate how strain affects the electrical transport property of ZnSnO3 microwire-based devices, strain-dependent I-V characteristics revealed rectification behavior under all strain conditions, which were fully recoverable as the strains were released. These studies demonstrated the possibility of using the ZnSnO3 nanostructures for energy conversion and strain sensing applications.

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Fig. 19.5 (A) Root-mean-square voltage measured over different resistors and the corresponding power density of a single ZnO NW-polycarbonate NG device as shown in the inset. (B) Piezoelectric output voltage and power generated by a ZnO-polycarbonate-based NG across different load resistors RL, under the application of a periodic impacting force at a frequency of 75 Hz and amplitude of 1 mm. (C) Crystal structure of NCS ZnSnO3. (D) ZnSnO3 NG device prepared from microbelts which shows good stability with an increase in driving frequency as shown in (E). (F) ZnSnO3 nanowires prepared by a vapor-phase technique. (G) ZnSnO3 NG device prepared from the nanowires. (H) I-V characteristic of a strain sensor at different strains. Part A: Reprinted from Nanotechnology 27 (2016) 28LT02. Copyright 2016, Institute of Physics. Part B: Reprinted from ACS Appl. Mater. Interfaces 8 (2016) 13678. Copyright 2016, American Chemical Society. Parts D and E: Reprinted from ACS Nano 6 (2012) 4335. Copyright 2012, American Chemical Society. Part G: Reprinted from ACS Nano 6 (2012) 4369. Copyright 2012, American Chemical Society.

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Outlook

In summary, NGs are attracting more attention in meeting the increasing demands for energy in micro-nanosystems with the ultimate goal to integrate themselves as a part of self-power system and to harvest energy from the environment for sustainable and maintenance-free operation. Despite the great progress few issues still need to be considered such as unavailability of studies and reports of NG devices in vivo for practical implantable biodevices that can harness the biomechanical energy within the human body. Hence, the focus should be on nontoxic, biocompatible, and biodegradable PE materials. Future challenges also lie to integrate ferroelectric materials into devices, to sufficiently power nanoelectronics from a single device by collecting the energy from environment either from mechanical vibrations or from solar energy as realized by PE and ferroelectric materials, respectively, and storing it in a concurrent process. In this chapter, we have discussed some exciting new avenues for engineering the electrical properties of nanostructured thin-film devices by exploiting their properties in the nanoscale. By using appropriate facile growth conditions, it may be possible to directly control the structure of perovskite oxide interfaces in nanostructures and thus take advantage of the size effects described here. The work discussed in this chapter highlights how nanostructured film growth and interface processes, as well as appropriate materials integration strategies play critical roles in the development of a whole new generation of micro- and nanodevices.

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Further reading [1] J.P.B. Silva, et al., Ferroelectric phase transitions studies in 0.5Ba(Zr0.2Ti0.8)O3-0.5 (Ba0.7Ca0.3)TiO3 ceramics, J. Electroceram. 35 (2015) 135–140.