PbTiO3 Research Articles

The effect of domain walls on leakage current in PbTiO3 thin films

Combining nonequilibrium Green function's approach with density functional theory, the effects of the 180° domain walls on the leakage current of PbTiO3 (PTO) thin films have been investigated. It is found that the leakage current of the ultrathin PTO film with 180° domain walls is greatly increased compared with that of perfect PTO film. Moreover, we found that the effect of domain walls on the leakage current in TiO2-terminated PTO films is larger than that in PbO-terminated PTO films. The enhancement of the leakage current is mainly attributed to the extra transport channels introduced by the domain walls according to the calculated transmission coefficient and the band structures. These results suggest that the leakage current of ferroelectric thin films can be decreased by controlling the occurrence of domain walls and their orientation in the design of nanoscale electronic devices.

Local ferroelectric responses of epitaxial PbTiO3 thin films to heated atomic force microscopy

We report the dependence of ferroelectric domain size of epitaxially grown PbTiO3 (PTO) thin films on Nb-doped SrTiO3 substrate to temperature. PbTiO3 exhibited well-defined ferroelectricity with a high remnant polarization of about 60μC/cm2. Based on piezoelectric force microscopy (PFM) measurements, we confirmed that the ferroelectric domain size of PbTiO3 thin films depends on electric field and temperature. The minimum ferroelectric domain size could be reduced by the locally heated atomic force microscopy (h-AFM) technique.

Li+ion induced three-dimensional aggregation growth of single-crystal perovskite octahedrons

Li+ ions were introduced into a hydrothermal system as surfactants to tailor the growth of perovskite tetragonal PbTiO3 (PT) nanocrystals, giving rise to single-crystal octahedrons with {111} facets exposed. Systematic investigations reveal that single-crystal PT octahedrons were formed via a three-dimensional (3D) aggregation growth process. In the initial stage, the intermediate phase primary nanoparticles, with a size of 2–5 nm, aggregated to form an octahedral shaped particle agglomeration with a size of 20–50 nm; then, the particle crystallized via an oriented attachment (OA) mechanism, resulting in the observed single-crystal PT octahedrons. During this process, the interplay of Li+ ions and the electrostatic force of the primary nanoparticles are essential for octahedral aggregation and subsequent crystallization. Moreover, the concentration of Li+ ions has been demonstrated to significantly modify the growth of PT, leading to nanostructures with various shapes from hexahedral particles to cubic conglomerates to cuboids. These nanostructures demonstrate shape-dependent visible light photocatalytic activity.

Effects of Defect on Ferroelectric Stability in PbTiO3 Thin Films

ABSTRACTEffects of defect on ferroelectric stability in PbTiO3 (PTO) thin films have been investigated using molecular dynamics with first-principles effective Hamiltonian. In this paper, oxygen vacancy (Vo) has been considered to study the hysteresis loop, spontaneous polarization as a function of film thickness. Vo has been modeled as a charged point defect. Density functional theory (DFT) calculations are performed to determine the Vo-induced localized fields (both mechanical and electrical) and the calculated DFT results are used as inputs to molecular dynamics simulations in a large system. The strain field induced by the Vo is calculated by DFT calculations and fitted by the continuum strain modeling, and the electrostatic field is given by the superposition of the Vo-induced field and the external field. Vo significantly reduces the spontaneous polarization and increases the critical thickness.

Piezoelectric Sol-Gel Composite Film Fabrication by Stencil Printing.

Piezoelectric films using sol-gel composites could be useful as ultrasonic transducers in various industrial fields. For sol-gel composite film fabrication, the spray coating technique has been used often because of its adaptability for various substrates. However, the spray technique requires multiple spray coating processes and heating processes and this is an issue of concern, especially for on-site fabrication in controlled areas. Stencil printing has been developed to solve this issue because this method can be used to fabricate thick sol-gel composite films with one coating process. In this study, PbTiO3 (PT)/Pb(Zr,Ti)O3 (PZT) films, PZT/PZT films, and Bi4Ti3O12 (BiT)/PZT films were fabricated by stencil printing, and PT/ PZT films were also fabricated using the spray technique. After fabrication, a thermal cycle test was performed for the samples to compare their ultrasonic performance. The sensitivity and signal-to-noise-ratio (SNR) of the ultrasonic response of PT/PZT fabricated by stencil printing were equivalent to those of PT/PZT fabricated by the spray technique, and better than those of other samples between room temperature and 300°C. Therefore, PT/PZT films fabricated by stencil printing could be a good candidate for nondestructive testing (NDT) ultrasonic transducers from room temperature to 300°C.

The evolution of polarization inside ultrathin PbTiO3 films: a theoretical study

How to control the material properties by manipulating the unitcell thickness is crucial for applications of ferroelectric ultrathin films. To understand the polarization behaviour of ultrathin PbTiO3 (PTO) films grown on SrTiO3 (STO) substrate, we have systematically explored the strength and direction of polarization in each unitcell layer, using density functional theory combined with Born effective charge method. Strikingly, we find that the polar state of ultrathin PTO films is a composite result depending not only on thickness but also on boundary condition, initial polarization direction, etc. Besides, we also studied the surface effect on the polarization in the thicker PTO films for comparison with the ultrathin ones, which suggests that the surface effect is basically confined in a small range (3–5 unitcells thick at surface region) no matter what kinds of surface terminations and polarization directions.

Magnetism switching and band-gap narrowing in Ni-doped PbTiO3 thin films

Ions doping-driven structural phase transition accompanied by magnetism switching and band-gap narrowing effects has been observed in PbTi1−xNixO3−δ (xPTNO, x = 0.00, 0.06, and 0.33) thin films. With the increase of x, the xPTNO thin films exhibit not only a phase transition from the pseudotetragonal structure to a centrosymmetric cubic structure but also a drastic decrease of grain size. Moreover, the as-grown Ni-doped PbTiO3 (PTO) thin films show obvious room-temperature ferromagnetism and an increased saturation magnetization with increasing the Ni content, in contrast to undoped PTO, which shows diamagnetism. A bound magnetic polaron model was proposed to understand the observed ferromagnetic behavior of PTO-derived perovskite thin films. Furthermore, the 0.33PTNO thin film presents a narrowed band-gap, much smaller than that of PTO, which is attributed to new states of both the highest occupied molecular orbital and the lowest unoccupied molecular orbital in an electronic structure with the presence of Ni. These findings may open up a route to explore promising perovskite oxides as candidate materials for use in multiferroics and solar-energy devices.

Functional materials integrated on III–V semiconductors

The integration of ferroelectric oxides on semiconductors has been achieved through the use of molecular beam epitaxy and solution spin coating. The oxide layers were determined to be single phase and crystalline and were oriented with the (100) along the growth direction. Epitaxial SrTiO3 and BaTiO3 buffer layers were grown on GaAs and Si using a unique MBE process for nucleation that suppressed the formation of interfacial oxide. The deposition of ferroelectric Pb(Zr)TiO3, on STO buffered Si and GaAs was carried out using solution spin coating. The nucleation of the oxide growth by solution spin coating was promoted by the crystallinity of the surface structure of the SrTiO3 buffer layer. Ab initio Density Functional Theory calculations were used to determine the interfacial structure and band alignments for the STO/GaAs and BTO/GaAs interfaces. Initial electrical characterization of the PZT grown on GaAs was determined by measuring the polarization versus electric field which suggests evidence of ferroelectricity.

Facile synthesis and visible photocatalytic activity of single-crystal TiO2/PbTiO3 heterostructured nanofiber composites

For single-crystal TiO2/PbTiO3 nanofiber composites, anatase TiO2 nanorods grew on the surface of tetragonal perovskite (TP) PbTiO3 nanofibers to form TiO2/PbTiO3 heterostructures.

Strain effect on the visible emission in PbTiO3 nanotubes: Template and wall-thickness dependence

We investigated the strain effect on temperature-dependent photoluminescence property in the clamped (with template) and free-standing (without template) PbTiO3 (PTO) nanotubes. The wall-thickness of nanotubes was varied from 25 to 80 nm with the outer diameter fixed to 420 nm. While all nanotubes show sizable green/yellow emission, the temperature dependent shift of the emission energy is significantly suppressed in the clamped PTO nanotubes, which is attributed to the lattice strain driven by the template clamping. This clamping effect is more significant for thinner nanotubes. Even in the free-standing PTO nanotubes the temperature-dependence of emission is affected by the wall-thickness. Our finding is the clear manifestation of the template and geometrical shape effect on the optical property of the nanotubes.

Phase-field modeling of the influence of domain structures on the electrocaloric effects in PbTiO3 thin films

A phase-field model has been developed to study the influence of epitaxial strains and domain structures on the electrocaloric effects (ECEs) in PbTiO3 (PTO) thin films. The simulated domain-switching dynamics obtained from the proposed model was tallied with the existing experimental results. In the case of single-domain PTO thin film, the ECE gradually increased with the increasing epitaxial strain, whereas, in the case of poly-domain PTO thin film, the influence of epitaxial strain on ECE can be divided into three regions of stable domain structures, i.e., c-domains, a/c-domains, and a-domains regions. It is worth noting that the maximum ECE occurred under a tensile epitaxial strain in the a/c-domains region, which was the consequence of the competition between the enhancement of ECE and the reduction of c-domains volume fraction, both caused by the increasing tensile epitaxial strain. Although the change of temperature arising from the ECE may not be large enough to be viable for practical applications, it does illustrate the important influence of domain structures on the ECEs of thin films, which has not gained much attention of most researchers whose focus is still on single-domain thin films.

Ultra-thin platinum interfacial layer assisted-photovoltaic response of transparent Pb(Zr,Ti)O3 thin film capacitors

For transparent solar cells, Sn:In2O3/Pb(Zr, Ti)TiO3/Pt(⩽5nm)/Sn:In2O3 capacitor structures were fabricated using a cost-effective solution process. The insertion of ultra-thin Pt layer between the bottom Sn:In2O3 electrode and Pb(Zr, Ti)TiO3 plays a critical role in the photovoltaic characteristics of the capacitors. The Pb(Zr,Ti)O3 capacitors with a 5nm thick Pt layer showed excellent polarization–voltage curves with reduced leakage current due to both partial (111) orientation of Pb(Zr,Ti)O3 and alloy formation between Pt and the Sn:In2O3 bottom electrode, as confirmed using X-ray photoelectron spectra analysis. The capacitors with a 5nm thick Pt layer exhibit transmittance of 45–50% in the visible light region. The current density–voltage characteristics under light illumination (AM1.5G) exhibit an open circuit voltage value of −0.62V and short circuit current density of 0.6μA/cm2 after negative poling, with a maximum power conversion efficiency of 1.7×10−4%. The open circuit voltage of the capacitors with a 5nm thick Pt layer is larger with negative poling due to the higher net internal bias arising from the Schottky barrier.

Structure and Nanoscale Polarization Switching Induced by High Frequency Alternating Current in PbTiO<sub>3</sub> Polycrystalline Films

PbTiO3(PTO) films were successfully fabricated by sol-gel method. We studied the structure and the nanoscale polarization switching property using XRD and Piezoresponse Force Microscopy. The results reveal that the PTO films are single perovskite phase grain films with tetragonal structure, and are polycrystalline materials with no evidence of preferential orientation. The average grain size is about 150nm and a striped multi-domain structure is exhibited in individual grains. Under 15 kHz alternating current of PFM, a significant asymmetry of switching pattern was observed. We suggest that the cooperative action of the built-in electric field at the bottom interface and the PFM ac-voltage lead to the asymmetry switching.

Engineering 180° ferroelectric domains in epitaxial PbTiO3 thin films by varying the thickness of the underlying (La,Sr)MnO3 layer

Epitaxial ferroelectric thin films of PbTiO3 (PTO) grown on top of nominally La0.7Sr0.3MnO3 (LSMO) submicron hillocks on Nb-doped SrTiO3 (100) substrate were investigated by means of scanning transmission electron microscopy. 180° ferroelectric domains were observed in the c-axis oriented PTO films. The formation and configuration of ferroelectric domains and domain walls were found to exhibit strong correlation with the thickness of the underlying LSMO hillocks. The domain walls start at the locations of the hillocks where the LSMO layer has a thickness of about 3 nm. Our results demonstrate that controlling the thickness variation (shape) of the LSMO hillocks can manipulate the position and density of the ferroelectric domain walls, which are considered to be the active elements for future nanoelectronics.

Stress Modulation and Ferroelectric Properties of Nanograined PbTiO3 Thick Films on the Different Substrates Fabricated by Aerosol Deposition

Nanograined PbTiO3 (PT) thick films were deposited on Si, yttria‐stabilized zirconia (YSZ), and Ni substrates using an aerosol deposition (AD) method at room temperature. The AD PT thick films on each different substrate were annealed at 500°C and 700°C for 1 h to increase the crystallinity. The stresses in the PT film were modulated by controlling the difference in the coefficient of thermal expansion (CTE) between the films and substrates during the thermal annealing process. The morphology of the AD PT films was examined from the polycrystalline dense structure (thickness ~8 μm), and the changes in the crystallographic phase, in‐plane stresses, and ferroelectric properties in annealed films were investigated. In‐plane stress analysis showed that the PT films annealed at 500°C and 700°C on each substrate exhibited compressive stress. Owing to the effects of compressive stress in the PT film, the film showed less tetragonality (c/a ratio) and enhanced ferroelectric behaviors. The change in the polarization–electric field (P–E) hysteresis loop of the PT films was explained by the stress induced from CTE mismatch between the films and substrates.

Enhanced durability of designated polarization of PbTiO3 nanodot arrays investigated by piezoresponse force microscopy

We used piezoresponse force microscopy (PFM) to investigate local domain relaxation behavior of overhanging PbTiO3 (PTO) nanodot arrays on platinized silicon substrates, which were prepared by using PbO vapor phase reaction sputtering on micellar monolayer films of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) loaded with TiO2 sol–gel precursor. The overhanging PTO nanodot arrays (92% at a temperature of 100°C for 365min) showed better ferroelectric retention than the PTO thin films (80% at the same condition). The enhanced polarization states and the absence of depolarization field due to homogeneous electric field inside the overhanging nanodot allowed for the remarkable durability of designated ferroelectric polarization.

Reprint of “Overhanging ferroelectric nanodot arrays created by high surface energy seeds”

We demonstrated a facile route for the fabrication of overhanging PbTiO3 (PTO) nanodot arrays on platinized silicon substrates using PbO vapor phase reaction sputtering on micellar monolayer films of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) loaded with TiO2 sol–gel precursor. We also investigated that the high surface energy of TiO2 contributes to the formation of overhanging ferroelectric PTO nanodot arrays by means of high-resolution transmission electron microscopy. Additionally, the crystallographic orientation of the nanoseeds had a significant influence on the ferroelectric switching properties of the PTO nanodots, which were determined by piezoresponse force microscopy (PFM).

Electron density, d-p hybridization and dipole moment of PbTiO3 and BaTiO3

Present experiment first reveals electron density (ED) distributions and split atomic displacements of ferroelectrics PbTiO3 (PTO) and BaTiO3 (BTO) by high-resolution x-ray single crystal diffraction study under high pressures using maximum entropy method (MEM). Diffraction measurements were carried out using a four-circle diffractometer installed at BL-10A Photon Factory, KEK, Japan. *** ED observed by FMEM(h) illustrates the d-p pai-bond resulting from the hybridization of Ti 3d and O 2p orbitals. The split atom distribution is generated from the dynamical disorder (static time and space average) due to the thermal vibration of atoms in the [001] direction of these crystal structures. The split distributions of Ti and O are clearly disclosed in the tetragonal ferroelectric PTO and BTO. The ferroelectric pseudo-particle occupies double-well potential. The feature disappears in the cubic paraelectric phase and electron density is more localized with increasing pressure in the cubic phase. *** Effective charges of atoms in PTO and BTO are evaluated from the least-squares refinement based on diffraction intensities. Using the pseudopotential model, inner-core electrons are frozen with bonding effects, but only valence electron clouds are deformed due to coordination and thermal vibration of atoms. The pressure dependence of the dipole moment (l) is calculated by the effective charge (q) and atomic deformation Δr. The dipole moment of ferroelectric materials is derived from the spontaneous polarization Ps (= ρ l = ρNΔr). Pressure dependences of dipole moments of PTO and BTO are first experimentally determined by observed effective charge q and Δr from their structure refinements. The dipole moment of PTO is much larger than BTO, which implies the difference in the dielectricity of these ferroelectrics. The result is consistent with the dielectric measurement.

Impact of thermal stress on the piezoelectric and dielectric properties of PbTiO3 thick films on various substrates

The impact of thermal stress on the polarization, as well as dielectric and piezoelectric properties of (001) oriented PbTiO3 (PTO) thick films deposited on various substrates was investigated based on Landau-Devonshire thermodynamic model. The results showed that dielectric and piezoelectric properties of PTO films depend strongly on the thermal stress in PTO films decided by the deposition temperature TG and the thermal expansion coefficients' difference between PTO films and substrates. For IC-compatible substrates such as Si, c-sapphire, and a-sapphire that induce tensile in-plane thermal stresses, the dielectric and piezoelectric responses and tunabilities of PTO films were enhanced. Whereas for PTO films on MgO, compressive thermal in-plane stresses can degraded the dielectric and piezoelectric responses and tunabilities of the films.

The effects of strain relaxation on the dielectric properties of epitaxial ferroelectric Pb(Zr0.2Ti0.8)TiO3 thin films

We study the effects of strain relaxation on the dielectric properties of epitaxial 40 nm Pb(Zr0.2Ti0.8)TiO3 (PZT) films. A significant increase in the defect and dislocation density due to strain relaxation is observed in PZT films with tetragonality c/a < 1.07 grown on SrTiO3 (001) substrates, which results in significant frequency dispersion of the dielectric constant and strong Rayleigh type behavior in those samples. This combined structural-electrical study provides a framework for investigating strain relaxation in thin films and can provide useful insights into the mechanisms of fatigue in ferroelectric materials.