Indium-zinc-oxide thin films produced by low-cost chemical solution deposition: Tuning the microstructure, optical and electrical properties with the processing conditions.

Indium-zinc-oxide (IZO) films were prepared by spin coating an ethanol-ethylene-glycol precursor solution with a Zn/(In + Zn) ratio of 0.36 on glass. The effects of temperature on the structure, microstructure, electrical, and optical properties of the IZO thin films were investigated by thermal analysis, Fourier-transform infrared spectroscopy, X-ray diffraction, electron and atomic-force microscopy, X-ray photoelectron spectroscopy and variable-angle spectroscopic ellipsometry. The prepared IZO thin films heated at 500, 600, and 700 °C in air were transparent, without long-range ordering, and with an RMS surface roughness of less than 1 nm. The lowest electrical resistivity at room temperature, 0.0069 Ωcm, was observed for the 115-nm-thick IZO thin film heated at 600 °C in air and subsequently post-annealed in Ar/H2. The thin film exhibited a microstructure characterized by grains typically 20 nm in size and had no organic residues. This film exhibits uniaxial optical anisotropy due to its ultra-thin lamellae with a high electron density. The ordinary refractive index was fitted as a Tauc-Lorentz-Urbach function, which is typical of an indirect absorption edge occurring in amorphous semiconductor materials. The principal absorption peak with an onset at about 2.8 eV and a Tauc gap energy of ∼2.6 eV is similar to those observed for In2O3. The described process of chemical solution deposition and subsequent curing is promising for the low-cost fabrication of IZO thin films for transparent electronics, and can be used to tune the structure and microstructure of IZO thin films, as well as their electrical and optical properties.

Dielectric and energy storage properties of surface-modified BaTi0.89Sn0.11O3@polydopamine nanoparticles embedded in a PVDF-HFP matrix.

In the most recent electronic and electric sectors, ceramic-polymer nanocomposites with high dielectric permittivity and energy density are gaining popularity. However, the main obstacle to improving the energy density in flexible nanocomposites, besides the size and morphology of the ceramic filler, is the low interfacial compatibility between the ceramic and the polymer. This paper presents an alternative solution to improve the dielectric permittivity and energy storage properties for electronic applications. Here, the poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) matrix is filled with surface-modified BaTi0.89Sn0.11O3/polydopamine nanoparticles (BTS11) nanoparticles, which is known for exhibiting multiphase transitions and reaching a maximum dielectric permittivity at room temperature. BTS11 nanoparticles were synthesized by a sol-gel/hydrothermal method at 180 °C and then functionalized by polydopamine (PDA). As a result, the nanocomposites exhibit dielectric permittivity (εr) of 46 and a low loss tangent (tan δ) of 0.017 at 1 kHz at a relatively low weight fraction of 20 wt% of BTS11@PDA. This is approximately 5 times higher than the pure PVDF-HFP polymer and advantageous for energy storage density in nanocomposites. The recovered energy storage for our composites reaches 134 mJ cm-3 at an electric field of 450 kV cm-1 with a high efficiency of 73%. Incorporating PDA-modified BTS11 particles into the PVDF-HFP matrix demonstrates highly piezo-active regions associated with BTS11 particles, significantly enhancing functional properties in the polymer nanocomposites.

Direct Electrocaloric Characterization of Ceramic Films.

Reliable and accurate characterization of the electrocaloric effect is necessary to understand the intrinsic properties of materials. To date, several methods are developed to directly measure the electrocaloric effect. However, each of them has some limitations, making them less suitable for characterizing ceramic films, which rely almost exclusively on less accurate indirect methods. Here, a new approach is proposed to address the process of rapid heat dissipation in ceramic films and to detect the electrically induced temperature change before it thermally bonds with the surrounding elements. By using a polymer substrate that slows heat dissipation to the substrate and fast infrared imaging, a substantial part of the adiabatic electrocaloric effect in Pb(Mg1/3 Nb2/3 )O3 -based ceramic films is captured. Infrared imaging provides a robust technique to reduce the ratio between the adiabatic and the measured electrocaloric temperature change in micrometer-sized ceramic films to a single-digit number, ≈3.5. The obtained results are validated with another direct thermometric method and compared with the results obtained with an indirect approach. Despite different measurement principles, the results obtained with the two direct methods agree well. The proposed approach is timely and can open a door to verify the predicted giant electrocaloric effects in ceramic films.

Atomic-Level Response of the Domain Walls in Bismuth Ferrite in a Subcoercive-Field Regime.

The atomic-level response of zigzag ferroelectric domain walls (DWs) was investigated with in situ bias scanning transmission electron microscopy (STEM) in a subcoercive-field regime. Atomic-level movement of a single DW was observed. Unexpectedly, the change in the position of the DW, determined from the atomic displacement, did not follow the position of the strain field when the electric field was applied. This can be explained as low mobility defect segregation at the initial DW position, such as ordered clusters of oxygen vacancies. Further, the triangular apex of the zigzag wall is pinned, but it changes its shape and becomes asymmetric under electrical stimuli. This phenomenon is accompanied by strain and bound charge redistribution. We report on unique atomic-scale phenomena at the DW level and show that in situ STEM studies with atomic resolution are very relevant as they complement, and sometimes challenge, the knowledge gained from lower resolution studies.

Chemical Solution Deposition of Barium Titanate Thin Films with Ethylene Glycol as Solvent for Barium Acetate.

Chemical solution deposition (CSD) of BaTiO3 (BT) or BT-based thin films relies on using a carboxylic acid and alcohol as the solvents for alkaline-earth carboxylate and transition-metal alkoxide, respectively; however, the esterification reaction of the solvents may lead to in-situ water formation and precipitation. To avoid such an uncontrolled reaction, we developed a route in which ethylene glycol (EG) is used as the solvent for Ba-acetate. The EG-based BT coating solutions are stable for at least a few months. The thermal decomposition of the BT xerogel obtained by drying the EG-based solutions depends on the choice of the solvent for the Ti-alkoxide as well: in the case of EG and 2-methoxyethanol solvents carbon residues are removed at only about 1100 °C, while in the case of ethanol it is concluded at about 700 °C. About 100 nm thick BT films derived from the EG-ethanol solution deposited on platinized silicon reveal dense, crack-free columnar microstructure. They exhibit local ferro- and piezoelectric properties. The macroscopic polarization-electric field loops were obtained up to a quite high electric field of about 2.4 MV/cm. The EG-ethanol based CSD route is a viable alternative to the established acetic acid-alcohol route for BT and BT-based films.

Ferroelectric bismuth-titanate nanoplatelets and nanowires with a new crystal structure.

Two different morphologies of ferroelectric bismuth titanate (Bi4Ti3O12) nanoparticles, i.e., nanoplatelets and nanowires, were synthesized by changing the concentration of NaOH during a hydrothermal treatment of precipitated Ti4+ and Bi3+ ions. The nanoparticles' crystal structures were characterized using atomic-resolution imaging with a CS-probe-corrected scanning-transmission electron microscope in combination with X-ray diffractometry and Raman spectroscopy. The nanoplatelets (10 nm thick and from 50 nm to 200 nm wide) exhibit the Aurivillius-type layered-perovskite crystal structure that is characteristic of Bi4Ti3O12, whereas the nanowires (from 15 nm to 35 nm wide and from several hundreds of nm to several µm long) exhibit an entirely new structure with an orthorhombic unit cell (a = 3.804(1) Å, b = 11.816(3) Å, and c = 9.704(1) Å). The nanowire structure is composed of two structural layers alternating along the orthorhombic c-direction: a structural layer composed of two parallel layers of Bi atoms that resembles the (Bi2O2)2+ layer of the Aurivillius structure, and a structural layer composed of two parallel layers of Ti atoms, where every sixth Ti is replaced with Bi. Observations of the ferroelectric domains with transmission electron and piezo-response force microscopy indicated the ferroelectric nature of both nanostructures. The nanowire structure is a metastable polymorph of the bismuth titanate stabilized at the nanoscale. With annealing at temperatures above 500 °C the nanowire structure topotactically transforms into the Aurivillius structure.

Design of lead-free BCZT-based ceramics with enhanced piezoelectric energy harvesting performances.

The design of lead-free ceramics for piezoelectric energy harvesting applications has become a hot topic. Among these materials, Ba0.85Ca0.15Zr0.10Ti0.90O3 (BCZT) and BaTi0.89Sn0.11O3 (BTSn) are considered as potential candidates due to their enhanced piezoelectric properties. Here, the structural, electrical, piezoelectric and piezoelectric energy harvesting properties of the (1 - x)Ba0.85Ca0.15Zr0.10Ti0.90O3-xBaTi0.89Sn0.11O3 (xBTSn, x = 0.2, 0.4 and 0.6) system are investigated. A systematic study of the structural properties of the xBTSn samples was carried out using X-ray diffraction, Raman spectroscopy, and dielectric measurements. The addition of BTSn allows a successive phase transition, which broadens the application temperature range. The enhanced piezoelectric energy harvesting properties were found in the 0.2BTSn ceramic, where the large-signal and small-signal piezoelectric coefficients, piezoelectric voltage and the piezoelectric figure of merit reached 245 pm V-1, 228 pC N-1, 16.2 mV m N-1 and 3.7 pm2 N-1, respectively. Consequently, the combination of BCZT and BTSn could provide suitable lead-free materials with enhanced piezoelectric energy harvesting performances.

Screen Printed Copper and Tantalum Modified Potassium Sodium Niobate Thick Films on Platinized Alumina Substrates.

We show how sintering in different atmospheres affects the structural, microstructural, and functional properties of ~30 µm thick films of K0.5Na0.5NbO3 (KNN) modified with 0.38 mol% K5.4Cu1.3Ta10O29 and 1 mol% CuO. The films were screen printed on platinized alumina substrates and sintered at 1100 °C in oxygen or in air with or without the packing powder (PP). The films have a preferential crystallographic orientation of the monoclinic perovskite phase in the [100] and [-101] directions. Sintering in the presence of PP contributes to obtaining phase-pure films, which is not the case for the films sintered without any PP notwithstanding the sintering atmosphere. The latter group is characterized by a slightly finer grain size, from 0.1 µm to ~2 µm, and lower porosity, ~6% compared with ~13%. Using piezoresponse force microscopy (PFM) and electron backscatter diffraction (EBSD) analysis of oxygen-sintered films, we found that the perovskite grains are composed of multiple domains which are preferentially oriented. Thick films sintered in oxygen exhibit a piezoelectric d33 coefficient of 64 pm/V and an effective thickness coupling coefficient kt of 43%, as well as very low mechanical losses of less than 0.5%, making them promising candidates for lead-free piezoelectric energy harvesting applications.

Investigating the Feasibility of Preparing Metal-Ceramic Multi-Layered Composites Using Only the Aerosol-Deposition Technique.

The preparation of metal-ceramic layered composites remains a challenge due to the incompatibilities of the materials at the high temperatures of the co-firing process. For densification, the ceramic thick-film materials must be subjected to high-temperature annealing (usually above 900 °C), which can increase the production costs and limit the use of substrate or co-sintering materials with a low oxidation resistance and a low melting point, such as metals. To overcome these problems, the feasibility of preparing dense, defect-free, metal-ceramic multilayers with a room-temperature-based method should be investigated. In this study, we have shown that the preparation of ceramic-metal Al2O3/Al/Al2O3/Gd multilayers using aerosol deposition (AD) is feasible and represents a simple, reliable and cost-effective approach to substrate functionalisation and protection. Scanning electron microscopy of the multilayers showed that all the layers have a dense, defect-free microstructure and good intra-layer connectivity. The top Al2O3 dielectric layer provides excellent electrical resistance (i.e., 7.7 × 1012 Ω∙m), which is required for reliable electric field applications.

Growth mode and strain effect on relaxor ferroelectric domains in epitaxial 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3/SrRuO3 heterostructures.

Controlling the growth of complex relaxor ferroelectric thin films and understanding the relationship between biaxial strain-structural domain characteristics are desirable for designing materials with a high electromechanical response. For this purpose, epitaxial thin films free of extended defects and secondary phases are urgently needed. Here, we used optimized growth parameters and target compositions to obtain epitaxial (40-45 nm) 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3/(20 nm) SrRuO3 (PMN-33PT/SRO) heterostructures using pulsed-laser deposition (PLD) on singly terminated SrTiO3 (STO) and ReScO3 (RSO) substrates with Re = Dy, Tb, Gd, Sm, and Nd. In situ reflection high-energy electron diffraction (RHEED) and high-resolution X-ray diffraction (HR-XRD) analysis confirmed high-quality and single-phase thin films with smooth 2D surfaces. High-resolution scanning transmission electron microscopy (HR-STEM) revealed sharp interfaces and homogeneous strain further confirming the epitaxial cube-on-cube growth mode of the PMN-33PT/SRO heterostructures. The combined XRD reciprocal space maps (RSMs) and piezoresponse force microscopy (PFM) analysis revealed that the domain structure of the PMN-33PT heterostructures is sensitive to the applied compressive strain. From the RSM patterns, an evolution from a butterfly-shaped diffraction pattern for mildly strained PMN-33PT layers, which is evidence of stabilization of relaxor domains, to disc-shaped diffraction patterns for high compressive strains with a highly distorted tetragonal structure, is observed. The PFM amplitude and phase of the PMN-33PT thin films confirmed the relaxor-like for a strain state below ∼1.13%, while for higher compressive strain (∼1.9%) the irregularly shaped and poled ferroelectric domains were observed. Interestingly, the PFM phase hysteresis loops of the PMN-33PT heterostructures grown on the SSO substrates (strain state of ∼0.8%) exhibited an enhanced coercive field which is about two times larger than that of the thin films grown on GSO and NSO substrates. The obtained results show that epitaxial strain engineering could serve as an effective approach for tailoring and enhancing the functional properties in relaxor ferroelectrics.

Domain-wall pinning and defect ordering in BiFeO3 probed on the atomic and nanoscale.

Electro-mechanical interactions between charged point defects and domain walls play a key role in the functional properties of bulk and thin-film ferroelectrics. While for perovskites the macroscopic implications of the ordering degree of defects on domain-wall pinning have been reported, atomistic details of these mechanisms remain unclear. Here, based on atomic and nanoscale analyses, we propose a pinning mechanism associated with conductive domain walls in BiFeO3, whose origin lies in the dynamic coupling of the p-type defects gathered in the domain-wall regions with domain-wall displacements under applied electric field. Moreover, we confirm that the degree of defect ordering at the walls, which affect the domain-wall conductivity, can be tuned by the cooling rate used during the annealing, allowing us to determine how this ordering affects the atomic structure of the walls. The results are useful in the design of the domain-wall architecture and dynamics for emerging nanoelectronic and bulk applications.

Investigations of ferroelectric polycrystalline bulks and thick films using piezoresponse force microscopy.

In recent years, ferroelectric/piezoelectric polycrystalline bulks and thick films have been extensively studied for different applications, such as sensors, actuators, transducers and caloric devices. In the majority of these applications, the electric field is applied to the working element in order to induce an electromechanical response, which is a complex phenomenon with several origins. Among them is the field-induced movement of domain walls, which is nowadays extensively studied using piezoresponse force microscopy (PFM), a technique derived from atomic force microscopy. PFM is based on the detection of the local converse piezoelectric effect in the sample; it is one of the most frequently applied methods for the characterization of the ferroelectric domain structure due to the simplicity of the sample preparation, its non-destructive nature and its relatively high imaging resolution. In this review, we focus on the PFM analysis of ferroelectric bulk ceramics and thick films. The core of the paper is divided into four sections: (i) introduction; (ii) the preparation of the samples prior to the PFM investigation; (iii) this is followed by reviews of the domain structures in polycrystalline bulks; and (iv) thick films.

Structure and Dynamics of Ferroelectric Domains in Polycrystalline Pb(Fe1/2Nb1/2)O3.

A complex domain structure with variations in the morphology is observed at ambient temperature in monoclinic Pb(Fe1/2Nb1/2)O3. Using electron microscopy and piezoresponse force microscopy, it is possible to reveal micrometre-sized wedge, lamellar-like, and irregularly shaped domains. By increasing the temperature, the domain structure persists up to 80 °C, and then starts to disappear at around 100 °C due to the proximity of the ferroelectric-paraelectric phase transition, in agreement with macroscopic dielectric measurements. In order to understand to what degree domain switching can occur in the ceramic, the mobility of the domain walls was studied at ambient temperature. The in situ poling experiment performed using piezoresponse force microscopy resulted in an almost perfectly poled area, providing evidence that all types of domains can be easily switched. By poling half an area with 20 V and the other half with -20 V, two domains separated by a straight domain wall were created, indicating that Pb(Fe1/2Nb1/2)O3 is a promising material for domain-wall engineering.

Domain-wall conduction in ferroelectric BiFeO3 controlled by accumulation of charged defects.

Mobile charged defects, accumulated in the domain-wall region to screen polarization charges, have been proposed as the origin of the electrical conductivity at domain walls in ferroelectric materials. Despite theoretical and experimental efforts, this scenario has not been directly confirmed, leaving a gap in the understanding of the intriguing electrical properties of domain walls. Here, we provide atomic-scale chemical and structural analyses showing the accumulation of charged defects at domain walls in BiFeO3. The defects were identified as Fe4+ cations and bismuth vacancies, revealing p-type hopping conduction at domain walls caused by the presence of electron holes associated with Fe4+. In agreement with the p-type behaviour, we further show that the local domain-wall conductivity can be tailored by controlling the atmosphere during high-temperature annealing. This work has possible implications for engineering local conductivity in ferroelectrics and for devices based on domain walls.

Self-Poling of BiFeO3 Thick Films.

Bismuth ferrite (BiFeO3) is difficult to pole because of the combination of its high coercive field and high electrical conductivity. This problem is particularly pronounced in thick films. The poling, however, must be performed to achieve a large macroscopic piezoelectric response. This study presents evidence of a prominent and reproducible self-poling effect in few-tens-of-micrometer-thick BiFeO3 films. Direct and converse piezoelectric measurements confirmed that the as-sintered BiFeO3 thick films yield d33 values of up to ∼20 pC/N. It was observed that a significant self-poling effect only appears in cases when the films are heated and cooled through the ferroelectric-paraelectric phase transition (Curie temperature TC ∼ 820 °C). These self-poled films exhibit a microstructure with randomly oriented columnar grains. The presence of a compressive strain gradient across the film thickness cooled from above the TC was experimentally confirmed and is suggested to be responsible for the self-poling effect. Finally, the macroscopic d33 response of the self-poled BiFeO3 film was characterized as a function of the driving-field frequency and amplitude.

A multicaloric material as a link between electrocaloric and magnetocaloric refrigeration.

The existence and feasibility of the multicaloric, polycrystalline material 0.8Pb(Fe1/2Nb1/2)O3-0.2Pb(Mg1/2W1/2)O3, exhibiting magnetocaloric and electrocaloric properties, are demonstrated. Both the electrocaloric and magnetocaloric effects are observed over a broad temperature range below room temperature. The maximum magnetocaloric temperature change of ~0.26 K is obtained with a magnetic-field amplitude of 70 kOe at a temperature of 5 K, while the maximum electrocaloric temperature change of ~0.25 K is obtained with an electric-field amplitude of 60 kV/cm at a temperature of 180 K. The material allows a multicaloric cooling mode or a separate caloric-modes operation depending on the origin of the external field and the temperature at which the field is applied.

Dual strain mechanisms in a lead-free morphotropic phase boundary ferroelectric.

Electromechanical properties such as d33 and strain are significantly enhanced at morphotropic phase boundaries (MPBs) between two or more different crystal structures. Many actuators, sensors and MEMS devices are therefore systems with MPBs, usually between polar phases in lead (Pb)-based ferroelectric ceramics. In the search for Pb-free alternatives, systems with MPBs between polar and non-polar phases have recently been theorized as having great promise. While such an MPB was identified in rare-earth (RE) modified bismuth ferrite (BFO) thin films, synthesis challenges have prevented its realization in ceramics. Overcoming these, we demonstrate a comparable electromechanical response to Pb-based materials at the polar-to-non-polar MPB in Sm modified BFO. This arises from 'dual' strain mechanisms: ferroelectric/ferroelastic switching and a previously unreported electric-field induced transition of an anti-polar intermediate phase. We show that intermediate phases play an important role in the macroscopic strain response, and may have potential to enhance electromechanical properties at polar-to-non-polar MPBs.

High-performance PMN-PT thick films.

This article describes some of our work on 0.65Pb(Mg1/3Nb(2/3)O3-0.35PbTiO3 (0.65PMN-0.35PT) thick films printed on alumina substrates. These thick films, with the nominal composition 0.65Pb(Mg1/3Nb(2/3)O3-0.35PbTiO3, were produced by screen-printing and firing a paste prepared from an organic vehicle and pre-reacted fine particles of avery chemically homogeneous powder. To improve the adhesion of the 0.65PMN-0.35PT to the platinized alumina substrate,a Pb(Zr0.53Ti0.47)O3 layer was deposited between the electrode and the substrate. The samples were then sintered at 950 °C for 2 h with various amounts of packing powder on the alumina (Al2O3) substrates. The sintering procedure was optimized to obtain dense 0.65PMN-0.35PT films. The films were then characterized using scanning electron microscopy as well as measurements of the dielectric and piezoelectric constants.The electrostrictive behavior of the 0.65PMN-0.35PT thick films was investigated using an atomic force microscope(AFM). Finally, substrate-free, large-displacement bending type actuators were prepared and characterized, and the normalized displacement (i.e., the displacement per unit length) of the actuators was determined to be 55 µm/cm at 3.6 kV/cm.