Critical Property in Relaxor-PbTiO(3) Single Crystals --- Shear Piezoelectric Response

The shear piezoelectric behavior in relaxor-PbTiO(3) (PT) single crystals is investigated in regard to crystal phase. High levels of shear piezoelectric activity, d(15) or d(24) >2000 pC N(-1), has been observed for single domain rhombohedral (R), orthorhombic (O) and tetragonal (T) relaxor-PT crystals. The high piezoelectric response is attributed to a flattening of the Gibbs free energy at compositions proximate to the morphotropic phase boundaries, where the polarization rotation is easy with applying perpendicular electric field. The shear piezoelectric behavior of pervoskite ferroelectric crystals was discussed with respect to ferroelectric-ferroelectric phase transitions and dc bias field using phenomenological approach. The relationship between single domain shear piezoelectric response and piezoelectric activities in domain engineered configurations were given in this paper. From an application viewpoint, the temperature and ac field drive stability for shear piezoelectric responses are investigated. A temperature independent shear piezoelectric response (d(24), in the range of -50°C to O-T phase transition temperature) is thermodynamically expected and experimentally confirmed in orthorhombic relaxor-PT crystals; relatively high ac field drive stability (5 kV cm(-1)) is obtained in manganese modified relaxor-PT crystals. For all thickness shear vibration modes, the mechanical quality factor Qs are less than 50, corresponding to the facilitated polarization rotation.     

Bifurcated Polarization Rotation in Bismuth‐Based Piezoelectrics

ABO₃ perovskite‐type solid solutions display a large variety of structural and physical properties, which can be tuned by chemical composition or external parameters such as temperature, pressure, strain, electric, or magnetic fields. Some solid solutions show remarkably enhanced physical properties including colossal magnetoresistance or giant piezoelectricity. It has been recognized that structural distortions, competing on the local level, are key to understanding and tuning these remarkable properties, yet, it remains a challenge to experimentally observe such local structural details. Here, from neutron pair‐distribution analysis, a temperature‐dependent 3D atomic‐level model of the lead‐free piezoelectric perovskite Na_0.5Bi_0.5TiO₃ (NBT) is reported. The statistical analysis of this model shows how local distortions compete, how this competition develops with temperature, and, in particular, how different polar displacements of Bi³⁺ cations coexist as a bifurcated polarization, highlighting the interest of Bi‐based materials in the search for new lead‐free piezoelectrics.     

One‐Pot Synthesis of Au25(SG)18 2‐ and 4‐nm Gold Nanoparticles and Comparison of Their Size‐Dependent Properties

A one‐pot synthesis of glutathione (denoted as ‐SG) capped gold nanoparticles, including Au₂₅(SG)₁₈ (ca. 1 nm in diameter) 2‐ and 4‐nm particles is reported. These nanoparticles are isolated by methanol‐induced precipitation with a controlled amount of added methanol. Except for their particle size, these nanoparticles have an identical chemical composition (i.e., gold and ‐SG content), synthetic history, and surface conditions, which allows for precise comparison of their size‐dependent properties, in particular the magnetic property as this could be attributed to contamination by trace iron impurities. Specifically, the structure, optical, and magnetic properties of these gold nanoparticles are compared. A trend from non‐fcc (fcc = face centered cubic) Au₂₅(SG)₁₈ nanoclusters (ca. 1 nm) to 2‐ and 4‐nm fcc‐crystalline Au nanocrystals is revealed. The Au₂₅(SG)₁₈ nanoparticles resemble molecules and exhibit multiple optical absorption peaks ascribed to one‐electron transitions, whereas the 4‐nm nanoparticles exhibit surface plasmon resonance at around 520 nm related to the collective excitation of conduction electrons upon optical excitation. The transition from the non‐fcc cluster state to the fcc crystalline state occurs at around 2 nm. Interestingly, both 2‐ and 4‐nm particles exhibit paramagnetism, whereas the Au₂₅(SG)₁₈ (anionic) clusters are diamagnetic. The information attained on the evolution of the properties of nanoparticles from nanoclusters to fcc‐structured nanocrystals is of major importance and provides insight into structure—property relationships.     

Direct Observation of Ferroelectric Domain Walls in LiNbO3: Wall‐Meanders,Kinks, and Local Electric Charges

Direct observations of the ferroelectric domain boundaries in LiNbO₃ are performed using high‐resolution high‐angle annular dark field scanning transmission electron microscopy imaging, revealing a very narrow width of the domain wall between the 180° domains. The domain walls demonstrate local side‐way meandering, which results in inclinations even when the overall wall orientation follows the ferroelectric polarization. These local meanders contain kinks with “head‐to‐head” and “tail‐to‐tail” dipolar configurations and are therefore locally charged. The charged meanders are confined to a few cation layers along the polarization direction and are separated by longer stretches of straight domain walls.     

Cationic Hybrid Hydrogels from Amino‐Acid‐Based Poly(ester amide): Fabrication,Characterization, and Biological Properties

A family of biodegradable, biocompatible, water soluble cationic polymer precursor, arginine‐based unsaturated poly (ester amide) (Arg‐UPEA), is reported. Its incorporation into conventional Pluronic diacrylate (Pluronic‐DA) to form hybrid hydrogels for a significant improvement of the biological performance of current synthetic hydrogels is shown. The gel fraction (G_f), equilibrium swelling ratio (Q_eq), compressive modulus, and interior morphology of the hybrid hydrogels as well as their interactions with human fibroblasts and bovine endothelial cells are fully investigated. It is found that the incorporation of Arg‐UPEA into Pluronic‐DA hydrogels significantly changes their Q_eq, mechanical strength, and interior morphology. The structure–property relationship of the newly fabricated hybrid hydrogels is studied in terms of the chemical structure of the Arg‐UPEA precursor, i.e., the number of methylene groups in the Arg‐UPEA repeating unit. The results indicate that increasing methylene groups in the Arg‐UPEA repeating unit increases Q_eq and decreases the compressive modulus of hydrogels. When compared with a pure Pluronic hydrogel, the cationic Arg‐UPEAs/Pluronic hybrid hydrogels greatly improve the attachment and proliferation of human fibroblasts on hydrogel surfaces. A bovine aortic endothelial cells (BAEC) viability test in the interior of the hydrogels shows that the positively charged hybrid hydrogels can significantly improve the viability of the encapsulated endothelial cell over a 2 week study period when compared with a pure Pluronic hydrogel.     

Controlling Interface Intermixing and Properties of SrTiO3‐Based Superlattices

By combining state‐of‐the‐art microscopy, spectrosccopy, and first‐principles calculations, atomic‐scale intermixing behavior at heterointerfaces in SrTiO₃‐based superlattices is investigated. It is found that Nb is confined to a unit‐cell thickness without intermixing, whereas Ba diffuses only to the adjoining Nb‐doped SrTiO₃ layer. It is revealed that the intermixing behaviors at the heterointerfaces are determined by not only the migration energy, but also by the vacancy‐formation energy and the Fermi energy of each layer. Based on these results, we find a method to control the atomic‐scale intermixing at the nonpolar heterointerfaces and clearly demonstrate the property improvements obtained by constructing an abrupt heterointerface.     

Systematic Control of Nucleation Density in Poly(3‐Hexylthiophene) Thin Films

While molecular ordering via crystallization is responsible for many of the impressive optoelectronic properties of thin‐film semiconducting polymer devices, crystalline morphology and its crucial influence on performance remains poorly controlled and is usually studied as a passive result of the conditions imposed by film deposition parameters. A method for systematic control over crystalline morphology in conjugated polymer thin films by very precise control of nucleation density and crystal growth conditions is presented. A precast poly(3‐hexylthiophene) film is first swollen into a solution‐like state in well‐defined vapor pressures of a good solvent, while the physical state of the polymer chains is monitored using in situ UV–vis spectroscopy and ellipsometry. Nucleation density is selected by a controlled deswelling of the film or by a self‐seeding approach using undissolved crystalline aggregates that remain in the swollen film. Nucleation densities ranging successively over many orders of magnitude are achieved, extending into the regime of spherulitic domains 10 to 100 μm in diameter, a length scale highly relevant for typical probes of macroscopic charge transport such as field‐effect transistors. This method is presented as a tool for future systematic study of the structure‐function relation in semicrystalline semiconducting polymers in a broad range of applications.     

Tunable Low‐Field Magnetoresistance in (La0.7Sr0.3MnO3)0.5:(ZnO)0.5 Self‐Assembled Vertically Aligned Nanocomposite Thin Films

Tunable and enhanced low‐field magnetoresistance (LFMR) is observed in epitaxial (La_0.7Sr_0.3MnO₃)_0.5:(ZnO)_0.5 (LSMO:ZnO) self‐assembled vertically aligned nanocomposite (VAN) thin films, which have been grown on SrTiO₃ (001) substrates by pulsed laser deposition (PLD). The enhanced LFMR properties of the VAN films reach values as high as 17.5% at 40 K and 30% at 154 K. They can be attributed to the spin‐polarized tunneling across the artificial vertical grain boundaries (GBs) introduced by the secondary ZnO nanocolumns and the enhancement of spin fluctuation depression at the spin‐disordered phase boundary regions. More interestingly, the vertical residual strain and the LFMR peak position of the VAN films can be systematically tuned by changing the deposition frequency. The tunability of the physical properties is associated with the vertical phase boundaries that change as a function of the deposition frequency. The results suggest that the tunable artificial vertical GB and spin‐disordered phase boundary in the unique VAN system with vertical ferromagnetic‐insulating‐ferromagnetic (FM‐I‐FM) structure provides a viable route to manipulate the low‐field magnetotransport properties in VAN films with favorable epitaxial quality.     

pH‐Based Regulation of Hydrogel Mechanical Properties Through Mussel‐Inspired Chemistry and Processing

The mechanical holdfast of the mussel, the byssus, is processed at acidic pH yet functions at alkaline pH. Byssi are enriched in Fe³⁺ and catechol‐containing proteins, species with chemical interactions that vary widely over the pH range of byssal processing. Currently, the link between pH, Fe³⁺‐catechol reactions, and mechanical function is poorly understood. Herein, it is described how pH influences the mechanical performance of materials formed by reacting synthetic catechol polymers with Fe³⁺. Processing Fe³⁺‐catechol polymer materials through a mussel‐mimetic acidic‐to‐alkaline pH change leads to mechanically tough materials based on a covalent network fortified by sacrificial Fe³⁺‐catechol coordination bonds. These findings offer the first direct evidence of Fe³⁺‐induced covalent cross‐linking of catechol polymers, reveal additional insight into the pH dependence and mechanical role of Fe³⁺‐catechol interactions in mussel byssi, and illustrate the wide range of physical properties accessible in synthetic materials through mimicry of mussel‐protein chemistry and processing.     

Exploiting Microphase‐Separated Morphologies of Side‐Chain Liquid Crystalline Polymer Networks for Triple Shape Memory Properties

We report a new strategy to achieve triple shape memory properties by using side‐chain liquid crystalline (SCLC) type random terpolymer networks (XL‐ TP‐n), where n is the length of flexible methylene spacer (n = 5, 10, and 15) to link backbone and mesogen. A lower glass transition temperature (T_g = T_low) and a higher liquid crystalline clearing temperature (T_cl = T_high) of XL‐TP‐n serve as molecular switches to trigger a shape memory effect (SME). Two different triple shape creation procedures (TSCPs), thermomechanical treatments to obtain temporary shapes prior to the proceeding recovery step, are used to investigate the triple shape memory behavior of XL‐TP‐n. The discrete T_g and T_cl as well as unique microphase‐separated morphologies (backbone‐rich and mesogen‐rich domains) within smectic layers of XL‐TP‐n enables triple shape memory properties. Motional decoupling between backbone‐rich and mesogen‐rich domains is also critical to determine the resulting macroscopic shape memory properties. Our strategy for obtaining triple shape memory properties will pave the way for exploiting a broad range of SCLC polymers to develop a new class of actively moving polymers.     

On the Relation between Morphology and FET Mobility of Poly(3‐alkylthiophene)s at the Polymer/SiO2 and Polymer/Air Interface

The influence of the interface of the dielectric SiO₂ on the performance of bottom‐contact, bottom‐gate poly(3‐alkylthiophene) (P3AT) field‐effect transistors (FETs) is investigated. In particular, the operation of transistors where the active polythiophene layer is directly spin‐coated from chlorobenzene (CB) onto the bare SiO₂ dielectric is compared to those where the active layer is first spin‐coated then laminated via a wet transfer process such that the film/air interface of this film contacts the SiO₂ surface. While an apparent alkyl side‐chain length dependent mobility is observed for films directly spin‐coated onto the SiO₂ dielectric (with mobilities of ≈10^?3 cm² V^?1 s^?1 or less) for laminated films mobilities of 0.14 ± 0.03 cm² V^?1 s^?1 independent of alkyl chain length are recorded. Surface‐sensitive near edge X‐ray absorption fine structure (NEXAFS) spectroscopy measurements indicate a strong out‐of‐plane orientation of the polymer backbone at the original air/film interface while much lower average tilt angles of the polymer backbone are observed at the SiO₂/film interface. A comparison with NEXAFS on crystalline P3AT nanofibers, as well as molecular mechanics and electronic structure calculations on ideal P3AT crystals suggest a close to crystalline polymer organization at the P3AT/air interface of films from CB. These results emphasize the negative influence of wrongly oriented polymer on charge carrier mobility and highlight the potential of the polymer/air interface in achieving excellent “out‐of‐plane” orientation and high FET mobilities.     

Alkyl‐Chain‐Length‐Independent Hole Mobility via Morphological Control with Poly(3‐alkylthiophene) Nanofibers

The field‐effect transistor (FET) and diode characteristics of poly(3‐alkylthiophene) (P3AT) nanofiber layers deposited from nanofiber dispersions are presented and compared with those of layers deposited from molecularly dissolved polymer solutions in chlorobenzene. The P3AT n‐alkyl‐side‐chain length was varied from 4 to 9 carbon atoms. The hole mobilities are correlated with the interface and bulk morphology of the layers as determined by UV–vis spectroscopy, transmission electron microscopy (TEM) with selected area electron diffraction (SAED), atomic force microscopy (AFM), and polarized carbon K‐edge near edge X‐ray absorption fine structure (NEXAFS) spectroscopy. The latter technique reveals the average polymer orientation in the accumulation region of the FET at the interface with the SiO₂ gate dielectric. The previously observed alkyl‐chain‐length‐dependence of the FET mobility in P3AT films results from differences in molecular ordering and orientation at the dielectric/semiconductor interface, and it is concluded that side‐chain length does not determine the intrinsic mobility of P3ATs, but rather the alkyl chain length of P3ATs influences FET diode mobility only through changes in interfacial bulk ordering in solution processed films.     

Studies of the Room‐Temperature Multiferroic Pb(Fe0.5Ta0.5)0.4(Zr0.53Ti0.47)0.6O3: Resonant Ultrasound Spectroscopy,Dielectric, and Magnetic Phenomena

Recently, lead iron tantalate/lead zirconium titanate (PZTFT) was demonstrated to possess large, but unreliable, magnetoelectric coupling at room temperature. Such large coupling would be desirable for device applications but reproducibility would also be critical. To better understand the coupling, the properties of all 3 ferroic order parameters, elastic, electric, and magnetic, believed to be present in the material across a range of temperatures, are investigated. In high temperature elastic data, an anomaly is observed at the orthorhombic mm2 to tetragonal 4mm transition, T_ot = 475 K, and a softening trend is observed as the temperature is increased toward 1300 K, where the material is known to become cubic. Thermal degradation makes it impossible to measure elastic behavior up to this temperature, however. In the low temperature region, there are elastic anomalies near ≈40 K and in the range 160–245 K. The former is interpreted as being due to a magnetic ordering transition and the latter is interpreted as a hysteretic regime of mixed rhombohedral and orthorhombic structures. Electrical and magnetic data collected below room temperature show anomalies at remarkably similar temperature ranges to the elastic data. These observations are used to suggest that the three order parameters in PZTFT are strongly coupled.     

Nanostructured Alkaline‐Cation‐Containing δ‐MnO2 for Photocatalytic Water Oxidation

Oxygen evolution from water is one of the key reactions for solar fuel production. Here, two nanostructured K‐containing δ‐MnO₂ are synthesized: K‐δ‐MnO₂ nanosheets and K‐δ‐MnO₂ nanoparticles, both of which exhibit high catalytic activity in visible‐light‐driven water oxidation. The role of alkaline cations in oxygen evolution is first explored by replacing the K⁺ ions in the δ‐MnO₂ structure with H⁺ ions through proton ion exchange. H‐δ‐MnO₂ catalysts with a similar morphology and crystal structure exhibit activities per surface site approximately one order of magnitude lower than that of K‐δ‐MnO₂, although both nanostructured H‐δ‐MnO₂ catalysts have much larger Brunauer–Emmett–Teller (BET) surface areas. Such a low turnover frequency (TOF) per surface Mn atom might be due to the fact that the Ru²⁺(bpy)₃ sensitizer is too large to access the additional surface area created during proton exchange. Also, a prepared Na‐containing δ‐MnO₂ material with an identical crystal structure exhibits a TOF similar to that of the K‐containing δ‐MnO₂, suggesting that the alkaline cations are not directly involved in catalytic water oxidation, but instead stabilize the layered structure of the δ‐MnO₂.     

Influence of Line Defects on the Electrical Properties of Single Crystal TiO2

One‐dimensional defects are created in [001] and [110] oriented TiO₂ single crystals by uniaxial pressure. Transmission electron microscopy (TEM) characterization shows them to preferably lie on {110} planes. Electrical properties studied as a function of oxygen partial pressure reveal their influence on ionic and electronic charge carriers. At high oxygen partial pressures (1 bar–10^?5 bar) the conductivity due to positive charge carriers is strongly enhanced, e.g., the ionic conductivity is increased by more than two orders of magnitude, when the electrical measurement axis lies on the slip plane. In contrary, no changes are observed when the measurement axis does not lie on the slip planes. At low oxygen partial pressures (<10^?15 bar), irrespective of orientation and presence of dislocation, there is no change in the n‐type conductivity. The observed phenomena can be well explained within the space charge model, assuming the dislocation cores to exhibit an excess negative charge (increased titanium vacancy concentration). The present study gives a clear correlation between line defects and point defect concentrations in such an oxide for the first time.