Enhanced piezoelectric properties of sodium bismuth titanate (Na0.5Bi4.5Ti4O15) ceramics with B‐site cobalt modification

The piezoelectric properties of the cobalt‐modified sodium bismuth titanate (Na_0.5Bi_4.5Ti₄O₁₅, NBT) piezoelectric ceramics were investigated. The piezoelectric properties of NBT ceramics were significantly enhanced by cobalt modification. The Curie temperature T_C and piezoelectric constant d₃₃ for the 0.3 wt% cobalt‐modified NBT ceramics (NBT‐C3) were found to be 663 °C and 30 pC/N, respectively. Thermal annealing studies presented that the cobalt‐modified NBT ceramics possess stable piezoelectric properties, demonstrating that the cobalt‐modified NBT‐based ceramics are promising candidates for high temperature piezoelectric applications. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Raman tensor analysis of (K0.5Na0.5)NbO3–LiSbO3 lead‐free ceramics and its application to study grain/domain orientation

A quantitative polarized Raman analysis of ferroelectric grain/domain orientation in LiSbO₃ (LS‐modified) (K_0.5Na_0.5)NbO₃ (KNN) ceramics is presented, based on the analysis of the complex orientation dependence in space of their Raman‐active modes. Complete sets of Raman tensor elements of A_g, and E_g phonon modes for orthorhombic/tetragonal structures of KNN have been determined. Through this spectroscopic algorithm, quantitative information could be extracted in terms of three Euler angles in space for KNN samples consisting of mixed phases, thus enabling quantitative visualization of the local distribution of grains/domains in the solid angle. As an application of the method, we quantitatively examined the unknown crystallographic grain orientation patterns on the surfaces of pure KNN and of KNN‐0.05LS ceramics. These two samples were useful to clarify a polymorphic phase transition from the orthorhombic to the tetragonal phase taking place in the LS‐modified KNN system. Thus, we demonstrated that polarized Raman spectroscopy is a valuable and efficient tool for nondestructive three‐dimensional assessments of grain/domain orientation in ferroelectric materials with complex polymorphic structures. We believe that the data shown here represent a typical scenario encountered in grain/domain orientation assessments of piezoelectric perovskites. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis and physical properties of Ca- and Ta-modified (K,Na)NbO3 lead-free piezoelectric ceramics

Polycrystalline samples of lead-free Ca and Ta co-substituted potassium sodium niobate (K_0.5Na_0.5NbO_3, KNN) ceramics have been prepared by solid state reaction technique. X-ray diffraction showed formation of a single-phase perovskite structure with orthorhombic symmetry. Substitution inhibits the grain growth, improves densification and decreases the ferro-paraelectric phase transition temperature. Temperature dependent dielectric permittivity studies demonstrate significant decrease in peak-permittivity values in the substituted samples. Bulk longitudinal piezoelectric coefficient is significantly enhanced, up to ～155 pC/N for (K_0.48Na_0.48Ca_0.02)(Nb_0.85Ta_0.15O₃) as compared to 95 pC/N for pristine KNN ceramic. Local piezoelectric properties have been observed by piezoresponse force microscopy (PFM) technique. Distinct piezocontrast was studied in both vertical and in-plane modes of PFM for all samples. The samples exhibit self-polarization effect in the unpoled state and effective local vertical piezoelectric coefficient was the largest in Ca and Ta co-substituted sample whereas the in-plane piezoelectric coefficient was maximum for Ca-substituted KNN sample. These studies are important for using substituted lead free KNN materials in various piezoelectric applications.     

Li‐, Sb‐ and Ta‐modified (K,Na)NbO3 nanopowder prepared via a water‐based sol–gel method

Stable Li‐, Sb‐ and Ta‐modified (K, Na)NbO₃ (LTS‐KNN) sol and gel were successfully prepared via an economical water‐based sol–gel method. Simultaneous thermogravimetry and differential scanning calorimetry (TG‐DSC) and X‐ray diffraction showed that organic compounds were eliminated and a pure perovskite phase formed around 600 °C. Transmission electron microscopy showed that the LTS‐KNN particle size was in the range of 11–34 nm after decomposition at 600 °C. Moreover, high performance LTS‐KNN ceramic was successfully prepared at a low sintering temperature of 1000 °C by use of the nanopowder, and its room‐temperature d₃₃, K_p, K and loss are 311 pC/N, 46.8%, 1545 and 0.024, respectively. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of lead free (K0.5Na0.5)NbO3-LiSbO3 piezoceramic

(K_0.5Na_0.5)NbO₃ (KNN) based lead free ceramics have been fabricated by a solid state reaction. In this work, LiSbO₃ (LS) modified KNN based ceramics were sintered at atmospheric pressure and high density (>96% theoretical) was obtained. The detailed elastic, dielectric, piezoelectric and electromechanical properties were characterized by using the resonance technique combined with the ultrasonic method. The full set of material constants for the obtained polycrystalline ceramics were determined and compared to the pure hot pressed KNN counterpart. KNN-LS polycrystalline ceramic was found to have higher elastic compliance, dielectric permittivity and piezoelectric strain coefficients, but lower mechanical quality factor, when compared to pure KNN, exhibiting a “softening” behavior. However, a high coercive field (∼17 kV/cm) was found for the LS modified KNN material. The properties as a function of temperature were determined in the range of −50-250 ^°C, showing a polymorphic phase transition near room temperature, giving rise to improved piezoelectric behavior.     

Doping of Fullerene‐Like MoS2 Nanoparticles with Minute Amounts of Niobium

Inorganic fullerene‐like closed‐cage nanoparticles of MoS₂ and WS₂ (IF‐MoS₂; IF‐WS₂), are synthesized in substantial amounts and their properties are widely studied. Their superior tribological properties led to large scale commercial applications as solid lubricants in numerous products and technologies. Doping of these nanoparticles can be used to tune their physical properties. In the current work, niobium (Nb) doping of the nanoparticles is accomplished to an unprecedented low level (≤0.1 at%), which allows controlling the work function and the band gap. The Nb contributes a positive charge, which partially compensates the negative surface charge induced by the intrinsic defects (sulfur vacancies). The energy diagram and position of the Fermi level on the nanoparticles surface is determined by Kelvin probe microscopy and optical measurements. Some potential applications of these nanoparticles are briefly discussed.     

Piezoelectricity in two‐dimensional group III–V buckled honeycomb monolayers

We studied the elastic and piezoelectric properties of buckled honeycomb group III–V monolayers (GaP, GaAs, GaSb, InP, InAs and InSb) by DFT calculations. Those buckled monolayers are ferroelectric and have nonzero e₁₁, e₃₁, d₁₁ and d₃₁ piezoelectric coefficients. Our calculations show that those monolayers are good piezoelectric materials and a pronounced periodic trend of the piezoelectric coefficients e₁₁, e₃₁, d₁₁ and d₃₁ was found. Group III–V monolayers are promising candidates for future atomically thin piezoelectric applications such as transducers, sensors, and energy harvesting devices.     

Liquid‐Crystal‐Mediated Self‐Assembly of Porous α‐Fe2O3 Nanorods on PEDOT:PSS‐Functionalized Graphene as a Flexible Ternary Architecture for Capacitive Energy Storage

A novel aqueous‐based self‐assembly approach to a composite of iron oxide nanorods on conductive‐polymer (CP)‐functionalized, ultralarge graphene oxide (GO) liquid crystals (LCs) is demonstrated here for the fabrication of a flexible hybrid material for charge capacitive application. Uniform decoration of α‐Fe₂O₃ nanorods on a poly(3,4‐ethylene‐dioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)‐functionalized, ultralarge GO scaffold results in a 3D interconnected layer‐by‐layer (LBL) architecture. This advanced interpenetrating network of ternary components is lightweight, foldable, and possesses highly conductive pathways for facile ion transportation and charge storage, making it promising for high‐performance energy‐storage applications. Having such structural merits and good synergistic effects, the flexible architecture exhibits a high specific discharge capacitance of 875 F g^?1 and excellent volumetric specific capacitance of 868 F cm^?3 at 5 mV s^?1, as well as a promising energy density of 118 W h kg^?1 (at 0.5 A g^?1) and promising cyclability, with capacity retention of 100% after 5000 charge–discharge (CD) cycles. This synthesis method provides a simple, yet efficient approach for the solution‐processed LBL insertion of the hematite nanorods (HNR) into CP‐functionalized novel composite structure. It provides great promise for the fabrication of a variety of metal‐oxide (MO)‐nanomaterial‐based binder and current collector‐free flexible composite electrodes for high‐performance energy‐storage applications.     

Local piezoelectric response of ZnO nanoparticles embedded in a photosensitive polymer

Local piezoelectric properties of ZnO nanoparticles (NPs) embedded in a photo‐epoxy polymer are investigated by piezoresponse force microscopy (PFM). Integrating ZnO NPs into a photosensitive SU‐8 polymer matrix not only retains the highly desired piezoelectric properties of the ZnO, but also preserves photosensitivity and optical transparency of the SU‐8 polymer. These results have strong implications for simple photolithography based low‐cost fabrication of piezoelectric microelectromechanicalsystems (MEMS) and nanoelectromechanicalsystems (NEMS) in both sensing and energy harvesting applications. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and characterizations of BNT–BT–KNN ceramics for energy storage applications

Dielectric, ferroelectric and piezoelectric properties of the (0.94–x) Bi_0.5Na_0.5TiO₃–0.06BaTiO₃–xK_0.5Na_0.5NbO₃/BNT–BT–KNN ceramics with x = 0.02 and 0.05 (2KNN and 5KNN) were studied in detail. Dielectric study and temperature-dependent polarization hysteresis loops indicated a ferroelectric-to-antiferroelectric transition at depolarization temperature (T_d). The low T_d in both the ceramic samples suggested the dominant antiferroelectric ordering at room temperature (RT), which was also confirmed by RT polarization and strain hysteresis loops studies. Antiferroelectric-to-paraelectric phase transition temperature (T_m) was nearly same for both systems. The 5KNN ceramic samples showed the relaxor behaviour. The values of the dielectric constant, T_d, and maximum strain percentage increased, whereas the coercive field and remnant polarization decreased with the increase of the KNN percentage in the BNT–BT–KNN system. High-energy storage density ～0.5 J/cm³ at RT hinted about the suitability of the 5KNN system for energy storage applications.     

Molybdenum oxide thin films in CdTe‐based electronic and optoelectronic devices

This paper reports on the both possible applications of molybdenum oxide (MoO_x) thin films in combination with hole or electron conducting CdTe. The high quality ohmic contancts and strongly rectifying photodiodes were prepared by the DC magnetron sputtering of MoO_x thin films onto freshly cleaved p‐ and n‐type CdTe single crystal substrates. The analysis of DC and AC electrical properties of the MoO_x/ p‐CdTe ohmic contact was carried out. The dominating current transport mechanisms through the MoO_x/p‐CdTe heterojunction at forward and reverse bias were determined. The unoptimized heterojunction photodiode showed promising rectifying and photoelectrical characteristics for practical application in the photoconductive mode. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Effects of Mn‐doping on surface acoustic wave properties of ZnO films

Surface acoustic wave (SAW) filters based on Mn‐doped ZnO films have been fabricated and effects of Mn‐doping on SAW properties are investigated. It is found that the electromechanical coupling coefficient (K²) of Zn_0.913Mn_0.087O films is 0.73 ± 0.02%, which is 73.8% larger than that of undoped ZnO films (0.42 ± 0.02%). Zn_0.913Mn_0.087O film filters also exhibit a lower absolute value of insertion loss (|IL|) of 16.1 dB and larger bandwidth (BW) of 5.9 MHz compared with that of undoped ZnO film filter. However, Zn_0.952Mn_0.048O film filters exhibit a smaller K² of 0.34 ± 0.02%, larger |IL| of 26.9 dB and smaller BW of 3.5 MHz. It is suggested that the SAW properties can be improved by appropriate Mn‐doping and Mn–ZnO/Si multilayer structure with large d₃₃ is promising for wide‐band and low‐loss SAW applications. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Selective fabrication of n‐ and p‐type SnO films without doping

Undoped n‐ and p‐type tin monoxide (SnO) films have been selectively fabricated by pulsed laser deposition with a Sn target and careful control of oxygen partial pressure. The films are epitaxially grown in optimal growth conditions on yttria‐stabilized zirconia (YSZ) substrates with out‐of‐plane and in‐plane orientation relationships of (001)_SnO//(001)_YSZ and [110]_SnO//[100]_YSZ, respectively. Both Seebeck and Hall measurements show consistent results on the carrier types of the films. The electron Hall mobility is approximately 11 cm²/Vs at room temperature and the carrier activation energy is 0.14 eV for the n‐type film. The growth at increased oxygen partial pressure yields p‐type films, demonstrating the selective fabrication of both n‐ and p‐type SnO films without doping. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

High‐performance In–Zn–O thin‐film transistors with a soluble processed ZrO2 gate insulator

Spin‐coated zirconium oxide films were used as a gate dielectric for low‐voltage, high performance indium zinc oxide (IZO) thin‐film transistors (TFTs). The ZrO₂ films annealed at 400 °C showed a low gate leakage current density of 2 × 10^–8 A/cm² at an electric field of 2 MV/cm. This was attributed to the low impurity content and high crystalline quality. Therefore, the IZO TFTs with a soluble ZrO₂ gate insulator exhibited a high field effect mobility of 23.4 cm²/V s, excellent subthreshold gate swing of 70 mV/decade and a reasonable I_on/off ratio of ～10⁶. These TFTs operated at low voltages (～3.0 V) and showed high drain current drive capability, enabling oxide TFTs with a soluble processed high‐k dielectric for use in backplane electronics for low‐power mobile display applications. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Correlation between the piezoelectric properties and the structure of lead‐free KNN‐modified ceramics,studied by Raman Spectroscopy

The structure of (Li,Na,K)(Nb,Ta,Sb)O₃ piezoelectric ceramics was thoroughly studied by X‐ray diffraction (XRD) and Raman spectroscopy for samples with different compositions and sintered for different times. A linear relationship is evidenced between the tetragonality ratio derived from XRD and the Raman shift. The Raman stretching modes are used to demonstrate the correlation of the structure and the piezoelectric properties of the materials, and to calculate the effective ionic displacement causing the piezoelectric polarization. The methodology proposed here offers invaluable insight into the nondestructive Raman analysis of perovskite structures based on the potassium sodium niobate systems, which are not amenable by studies of diffraction or other spectroscopic techniques. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of BaTiO<Subscript>3</Subscript> and SrTiO<Subscript>3</Subscript> as the buffer layers of epitaxial BiFeO<Subscript>3</Subscript> thin films

BiFeO₃ (BFO) thin films with BaTiO₃ (BTO) or SrTiO₃ (STO) as buffer layer were epitaxially grown on SrRuO₃-covered SrTiO₃ substrates. X-ray diffraction measurements show that the BTO buffer causes tensile strain in the BFO films, whereas the STO buffer causes compressive strain. Different ferroelectric domain structures caused by these two strain statuses are revealed by piezoelectric force microscopy. Electrical and magnetical measurements show that the tensile-strained BFO/BTO samples have reduced leakage current and large ferroelectric polarization and magnetization, compared with compressively strained BFO/STO. These results demonstrate that the electrical and magnetical properties of BFO thin films can be artificially modified by using a buffer layer.     

Microstructure,dielectric, and piezoelectric properties of Ce‐modified Na0.5Bi4.5Ti4O15 high temperature piezoceramics

The cerium modified sodium bismuth titanate (Na_0.5Bi_4.5Ti₄O₁₅, NBT) piezoelectric ceramics have been prepared by using the conventional mixed oxide method. X‐ray diffraction analysis revealed that the cerium modified NBT ceramics have a pure four‐layer Aurivillius phase structure. The piezoelectric activity of NBT ceramics was found significantly improved by the modification of cerium. The Curie temperature T_c, and piezoelectric coefficient d₃₃ for the NBT ceramics with 0.50 wt% cerium modification were found to be 655 °C, and 28 pC/N respectively. The Curie temperature gradually decreased from 668 °C to 653 °C with the increase of cerium modification. The dielectric spectroscopy showed that the samples possess stable piezoelectric properties, demonstrating practical potential that for high temperature applications. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of polymer dielectric surface energy on thin‐film transistor performance of solution‐processed triethylsilylethynyl anthradithiophene (TES‐ADT)

This study investigates the correlation between surface energy of polymer dielectrics and the film morphology, microstructure, and thin‐film transistor performance of solution‐processed 5,11‐bis(triethylsilylethynyl) anthradithiophene (TES‐ADT) films. The low surface energy polyimide (PI) dielectric induced large grains with strong X‐ray reflections for spin‐cast TES‐ADT films in comparison to high surface en‐ ergy poly(4‐vinyl phenol) (PVP) dielectric. Furthermore, thin‐film transistors based on spin‐cast TES‐ADT films on PI dielectric exhibited enhanced electrical performance, small hysteresis, and high stability under bias stress with carrier mobility as high as 0.43 cm²/Vs and a current on/off ratio of 10⁷. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the Optical Properties of Thin‐Film Vanadium Dioxide from the Visible to the Far Infrared

The insulator‐to‐metal transition (IMT) in vanadium dioxide (VO₂) can enable a variety of optics applications, including switching and modulation, optical limiting, and tuning of optical resonators. Despite the widespread interest in VO₂ for optics, the wavelength‐dependent optical properties across its IMT are scattered throughout the literature, are sometimes contradictory, and are not available at all in some wavelength regions. Here, the complex refractive index of VO₂ thin films across the IMT is characterized for free‐space wavelengths from 300 nm to 30 µm, using broadband spectroscopic ellipsometry, reflection spectroscopy, and the application of effective‐medium theory. VO₂ films of different thicknesses are studied, on two different substrates (silicon and sapphire), and grown using different synthesis methods (sputtering and sol–gel). While there are differences in the optical properties of VO₂ synthesized under different conditions, these differences are surprisingly small in the ≈2–11 µm range where the insulating phase of VO₂ also has relatively low optical loss. It is anticipated that the refractive‐index datasets from this article will be broadly useful for modeling and design of VO₂‐based optical and optoelectronic components, especially in the mid‐wave and long‐wave infrared.     

New crystallographic dielectric phase boundary in K0.5Na0.5NbO3‐based lead‐free ceramics

0.979K_0.5Na_0.5Nb_1‐xSb_x O₃‐0.021Bi_0.5Na_0.5TiO₃ (KNNSx ‐BNT) lead‐free piezoelectric ceramics were fabricated by conventional solid state reaction technique, and their phase transition and electrical properties were studied. With the increase of x, the rhombohedral‐orthorhombic phase transition temperature of the ceramics increases. Finally, both the rhombohedral‐orthorhombic and orthorhombic‐tetragonal phase transitions of the ceramics were modified to be around room tempera‐ ture when about 6% Sb were substituted for the Nb site, resulting in the formation of a new phase boundary separating rhombohedral and tetragonal phases. The formation of the new phase boundary results in excellent properties for the ceramics, that is, the KNNS0.05‐BNT ceramic shows an enhancement in piezoelectric properties: d₃₃ = 380 pC/N and k_P = 0.438. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)