P. Ramos

Piezoelectric properties of Ca-modified PbTiO3 thin films

Low‐field piezoelectric coefficient and strain were investigated in Ca‐modified PbTiO3 thin films by means of optical interferometry. A remanent piezoelectric coefficient of 70 pm/V was observed that agrees well with the values previously reported for the bulk ceramics of the same composition. The efficient poling of the films can be achieved at room temperature under the field of 300 kV/cm. Electrically induced strains as high as 0.8% were observed using a bipolar driving field of 1 MV/cm. Strain response under a unipolar driving field exhibited a good linearity and small hysteresis. These properties combined with the low dielectric constant and high dielectric strength are shown to be attractive for micromechanical applications.

Size effect in morphotropic phase boundary Pb(Mg1∕3Nb2∕3)O3–PbTiO3

Phases, domain configuration, and properties of 0.65Pb(Mg1∕3Nb2∕3)O3–0.35PbTiO3 ceramics with grain sizes of 4 and 0.15μm have been studied. The average phase is monoclinic Pm in coexistence with tetragonal. An evolution from micron-sized lamellar domains towards submicron/nanometer sized crosshatched domains is found with the decrease in size, which results in electrical relaxor type behavior and hindered switching. This is proposed to be associated with the slowing down of the relaxor to ferroelectric transition that causes the long time presence of an intermediate domain configuration. Nevertheless, a high sensitivity piezoelectric submicron-structured material is obtained under tailored poling (d33∼300pCN−1).

Very high remnant polarization and phase-change electromechanical response of BiFeO3-PbTiO3 at the multiferroic morphotropic phase boundary

We have investigated the occurrence of phase-change functional responses in the BiFeO3-PbTiO3 perovskite solid solution, analogous to those anticipated by a recent first-principles study of BiFeO3-BiCoO3. Like the former system, BiFeO3-PbTiO3 shows a morphotropic phase boundary (MPB) between multiferroic polymorphs of rhombohedral and tetragonal symmetries. MPB BiFeO3-PbTiO3 is a high temperature ferroelectric with the phase transition around 900 K, and a room temperature square-shape hysteresis loop with remnant polarization as high as 62 μC cm−2. Strain under the electric field was studied, and a phase-change response was found. Analogous magnetoelectric effects are expected from the multiferroic nature of this MPB.

High-field electromechanical response of Bi0.5Na0.5TiO3?Bi0.5K0.5TiO3 across its morphotropic phase boundary

Lead-free (1 − x)Bi0.5Na0.5TiO3–xBi0.5K0.5TiO3 [(BNKTx); x = 0.18, 0.20, 0.22, 0.24] piezoelectric ceramics were prepared via conventional solid-state reactions. Phase characterization using x-ray diffraction showed coexistence of perovskite rhombohedral and tetragonal polymorphs as anticipated for compositions across the morphotropic phase boundary (MPB). Electrical and electromechanical properties were studied as a function of x. Permittivity and polarization figures were comparable to the best values previously reported. Largest piezoelectric and high field electromechanical responses were obtained for BNKT0.20 samples, considered to have optimum MPB phases. This material exhibited giant field-induced strains of 0.13% and 0.38% under electric fields of 1 and 2.4 kV mm−1 at room temperature by a phase-change mechanism. Figures are comparable to those reported for other BNT-based systems, but under significantly lower driving fields, which is a clear advantage for actuation.

Performance of Frozen Density Embedding for Modeling Hole Transfer Reactions.

We have carried out a thorough benchmark of the frozen density-embedding (FDE) method for calculating hole transfer couplings. We have considered 10 exchange-correlation functionals, 3 nonadditive kinetic energy functionals, and 3 basis sets. Overall, we conclude that with a 7% mean relative unsigned error, the PBE and PW91 functionals coupled with the PW91k nonadditive kinetic energy functional and a TZP basis set constitute the most stable and accurate levels of theory for hole transfer coupling calculations. The FDE-ET method is found to be an excellent tool for computing diabatic couplings for hole transfer reactions.

Properties of morphotropic phase boundary Pb(Mg1/3Nb2/3)O3–PbTiO3films with submicrometre range thickness on Si-based substrates

The electrical properties of (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 films with composition in the morphotropic phase boundary region around x=0.35, submicron thickness and columnar microstructure, prepared on Si based substrates by chemical solution deposition are presented and discussed in relation to the properties of coarse and fine grained ceramics. The films show relaxor characteristics that are proposed to result from a grain size effect on the kinetics of the relaxor to ferroelectric transition. The transition is slowed down for grain sizes in the submicron range, and as a consequence intermediate polar domain configurations with typical length scales in the submicron- and nanoscales are stabilised. A high saturation polarisation can be attained under field, but fast polarisation relaxation occurs after its removal, and negligible remnant values are obtained. At the same time, they also show spontaneous piezoelectricity and pyroelectricity. Self polarisation is thus present, which indicates the existence of an internal electric field that is most probably a substrate effect. Films would be then in a phase instability, at an intermediate state between the relaxor and ferroelectric ones, and under a bias electric field, which would explain the very high spontaneous pyroelectric response found.

Piezoelectric ultrathin lead titanate films prepared by deposition of aquo-diol solutions

We report on the preparation of continuous ultrathin ferroelectric films of pure lead titanate by chemical solution deposition (CSD) methods. Aquo-diol solutions highly diluted have been used to obtain films with thickness down to 13 nm, the lowest reported for CSD films. The formation of islands instead of continuous coatings, which has been reported when CSD is used to prepare ultrathin films, is avoided here. The piezoelectric activity of the films has been characterized by piezoresponse force microscopy, showing that the thinnest film obtained retains a significant piezoelectric activity at the nanoscale, which is promising for their use as transducer elements in nanoelectromechanical systems (NEMS)

Quantifying Environmental Effects on the Decay of Hole Transfer Couplings in Biosystems.

In the past two decades, many research groups worldwide have tried to understand and categorize simple regimes in the charge transfer of such biological systems as DNA. Theoretically speaking, the lack of exact theories for electron-nuclear dynamics on one side and poor quality of the parameters needed by model Hamiltonians and nonadiabatic dynamics alike (such as couplings and site energies) on the other are the two main difficulties for an appropriate description of the charge transfer phenomena. In this work, we present an application of a previously benchmarked and linear-scaling subsystem density functional theory (DFT) method for the calculation of couplings, site energies, and superexchange decay factors (β) of several biological donor-acceptor dyads, as well as double stranded DNA oligomers composed of up to five base pairs. The calculations are all-electron and provide a clear view of the role of the environment on superexchange couplings in DNA-they follow experimental trends and confirm previous semiempirical calculations. The subsystem DFT method is proven to be an excellent tool for long-range, bridge-mediated coupling and site energy calculations of embedded molecular systems.

Structure and ferroelectric properties dependence on thermal treatment of modified lead titanate thin films

Calcium modified lead titanate thin films have been prepared from a sol-gel method. Different thermal conditions have been used for the crystallization of the as-deposited amorphous films. The influence of thermal treatment on the perovskite structure and strain of the films is studied by a grazing incidence X-ray diffraction technique (GIXRD). The ferroelectric response of the films is related to their crystalline structure and chemical composition.

Fabrication of continuous ultrathin ferroelectric films by chemical solution deposition methods

The integration of ferroelectrics in nanodevices requires firstly the preparation of high-quality ultrathin films. Chemical solution deposition is considered a rapid and cost-effective technique for preparing high-quality oxide films, but one that has traditionally been regarded as unsuitable, or at least challenging, for fabricating films with good properties and thickness below 100 nm. In the present work we explore the deposition of highly diluted solutions of pure and Ca-modified lead titanates to prepare ultrathin ferroelectric films, the thickness of which is controlled by the concentration of the precursor solution. The results show that we are able to obtain single crystalline phase continuous films down to 18 nm thickness, one of the lowest reported using these methods. Below that thickness, the films start to be discontinuous, which is attributed to a microstructural instability that can be controlled by an adequate tailoring of the processing conditions. The effect of the reduction of thickness on the piezoelectric behavior is studied by piezoresponse force microscopy. The results indicate that films retain a significant piezoelectric activity regardless of their low thickness, which is promising for their eventual integration in nanodevices, for example, as transducer elements in nanoelectromechanical systems.