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The confinement effect on the coercive field in relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene)[P(VDF-TrFE-CFE)] terpolymer nanowires with different diameters ranging from 30 to 170 nm was studied using piezoelectric force microscope. It was found that the coercive field increases significantly as the diameter of terpolymer nanowires decreases. The results can be explained by the conversion of molecule conformation from T3G to Tm>4 in the terpolymer nanowires, which is supported by Fourier transform infrared spectroscope.

Multilayers consisting of alternating ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer and relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) (P(VDF-TrFE-CFE)) terpolymer with different periodicities in thickness were prepared by the Langmuir-Blodgett method. Except for the dielectric and ferroelectric properties enhanced, the tunability also was enhanced in the multilayers with thinner periodic thickness. The polymer multilayers can be utilized for high energy density and low loss dielectrics, the multilayers with alternating 3 monolayers get the biggest high energy density. The reasons for the enhanced tunability and high energy density in the multilayer sample are discussed.

The leakage current mechanism of ferroelectric copolymer of polyvinylidene fluoride with trifluoroethylene prepared by Langmuir–Blodgett was investigated in the temperature range from 100 K to 350 K. The electron as the dominant injected carrier was observed in the ferroelectric copolymer films. The transport mechanisms in copolymer strongly depend on the temperature and applied voltage. From 100 K to 200 K, Schottky emission dominates the conduction. With temperature increasing, the Frenkel–Poole emission instead of the Schottky emission to conduct the carrier transport. When the temperature gets to 260 K, the leakage current becomes independent of temperature, and the space charge limited current conduction was observed.

Potassium tantalite niobate (KTa1−xNbxO3, KTN) single crystals with various compositions and sizes were grown by double crucible Czochralski method in KTaO3–KNbO3 solid solution system. All the crystals we have grown are in good transparence and are >1 cm in size. Growth habit, morphological, structural and homogeneity properties of five different compositions of KTN crystals, including KTaO3, KTa0·91Nb0·09O3, KTa0·81Nb0·19O3, KTa0·75Nb0·25O3 and KTa0·63Nb0·37O3, are investigated in this work. Electron probe microanalysis shows that the composition fluctuation of KTN crystal is

Ultrathin copolymer films of vinylidene fluoride and trifluoroethylene were deposited on Al-coated polyimide substrates, by the Langmuir–Blodgett method. A top Al electrode was evaporated onto the polymer film, to form an Al/polymer/Al structured infrared detector. The pyroelectric voltage response of the detector under various polarizing processes was characterized. The detector with only one transferred polymer layer exhibited a preferential polarization direction. This was considered to result from the self-polarization of the ultrathin polymer film. It was due to the preferred alignment of the dipoles on the Al substrates. This process can be applied for designing stable fast-response infrared detectors.

The β phase stability in poly(vinylidene fluoride/trifluoroethylene) [P(VDF-TrFE)] thin films was studied below 300 K using X-ray diffraction and polarization-electric-field (P-E) hysteresis loops measurements. On as-grown samples, an irreversible partial order-disorder transformation at T β ∼ 250 K, namely, the β relaxation temperature, was evidenced by the appearance of an additional X-Ray diffraction peak above T β as well as changes on the P-E loops on heating after the first cooling. This order-disorder-like transformation which is attributed to an all-trans order to helical disorder transition is suggested to take place in defect-rich regions like crystal-amorphous interphases and/or crystalline areas with randomly distributed TrFE defect-like units. © 2015 AIP Publishing LLC.

We study the effect of the AlGaN interlayer on structural quality and strain engineering of the GaN films grown on SiC substrates with an AlN buffer layer. Improved structural quality and tensile stress releasing are realized in unintentionally doped GaN thin films grown on 6H–SiC substrates by metal organic chemical vapor deposition. Using the optimized AlGaN interlayer, we find that the full width at half maximum of x-ray diffraction peaks for GaN decreases dramatically, indicating an improved crystalline quality. Meanwhile, it is revealed that the biaxial tensile stress in the GaN film is significantly reduced from the Raman results. Photoluminescence spectra exhibit a shift of the peak position of the near-band-edge emission, as well as the integrated intensity ratio variation of the near-band-edge emission to the yellow luminescence band. Thus by optimizing the AlGaN interlayer, we could acquire the high-quality and strain-relaxation GaN epilayer with large thickness on SiC substrates.

International audience; The temperature dependence of the dielectric and optical properties, crystal structure, and infraredspectra of the relaxor poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) terpolymerfilms obtained from Langmuir-Blodgett method have been comprehensively investigated. All theresults suggest that there exists a peculiar point at 370 K, which is attributed to the intermediatetemperature T* recently discovered in inorganic relaxors. Here, T* results from the change in thegrowth rate of the trans gauche T3GT3G0 chain conformations with temperature, which is similar tothe transformation from dynamic to static behavior of the so-called polar nano-regions in inorganicrelaxors.

GaN-based flip-chip light-emitting diodes (FC-LEDs) grown on nanopatterned sapphire substrates (NPSS) are fabricated using self-assembled SiO2 nanospheres as masks during inductively coupled plasma etching. By controlling the pattern spacing, epitaxial GaN can be grown from the top or bottom of patterns to obtain two different GaN/substrate interfaces. The optoelectronic characteristics of FC-LED chips with different GaN/sapphire interfaces are studied. The FC-LED with an antireflective interface layer consisting of a NPSS with GaN in the pattern spacings demonstrates better optical properties than the FC-LED with an interface embedded with air voids. Our study indicates that the two types of FC-LEDs grown on NPSS show higher crystal quality and improved electrical and optical characteristics compared with those of FC-LEDs grown on conventional planar sapphire substrates.

Thermal conductivity of single-crystal materials is crucial in the fields of lasers and nonlinear optics. Understanding the physical mechanism of thermal conductivity in such systems is therefore of great importance. In the present work, first principles calculations were employed to study the thermal conductivity of the infrared nonlinear optical materials, CdSiP2 and AgGaS2. These compounds crystallize in similar structures but with an order-of-magnitude difference in thermal conductivity. The average Grüneisen parameters are −0.8 and −2.6 for CdSiP2 and AgGaS2, respectively; these values are indicative of the soft-mode phenomenon of acoustic phonons. Crystal structures are considered unstable at low temperature through the whole Brillouin zone, especially in the region from K-point X to Γ. Acoustic phonon anharmonicity is concluded to be the main factor that determines the magnitude of thermal conductivity.

In this experiment, a flip-chip light-emitting diode with photonic crystal was fabricated at the interface of p-GaN and Ag reflector via nanospheres lithography technique. In this structure, photonic crystal could couple with the guide-light efficiently by reason of the little distance between photonic crystal and active region. The light output power of light emitting diode with embedded photonic crystal was 1.42 times larger than that of planar flip-chip light-emitting diode. Moreover, the embedded photonic crystal structure makes the far-field divergence angle decreased by 18° without spectra shift. The three-dimensional finite difference time domain simulation results show that photonic crystal could improve the light extraction, and enhance the light absorption caused by Ag reflector simultaneously, because of the roughed surface. The depth of photonic crystal is the key parameter affecting the light extraction and absorption. Light extraction efficiency increases with the depth photonic crystal structure rapidly, and reaches the maximum at the depth 80 nm, beyond which light extraction decrease drastically.

An actuator with a high piezoelectric response, the ferroelectric polymer multilayer actuator, is described. The ferroelectric polymer multilayers consisting of alternative ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer and relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) (P(VDF-TrFE-CFE)) terpolymer with different periodicities and fixed total thickness are prepared by the Langmuir-Blodgett technique. Both X-ray diffraction and Raman spectroscopic measurements indicate that the structure of the multilayer with thin alternating layer is similar to that of the ferroelectric copolymer. Compared with that of the copolymer, it is found that the piezoelectric coefficient of the multilayer could be improved by 57%. We attributed the enhanced piezoelectric response of the multilayers to the internal electric fields that arises from the electrostatic couplings between different layers.

Polyvinylidene fluoride homopolymer thin films have been prepared by the Langmuir-Blodgett technique, and their electrical properties have comprehensively been studied. The polyvinylidene fluoride homopolymer films show better ferroelectricity with higher polarization and higher breakdown electric field than that of the poly(vinylidene fluoride-trifluoroethylene) copolymer films. Inspection on the thickness dependence of the coercive field of the polyvinylidene fluoride films suggests an extrinsic polarization switching occurs in the thickness range from 200 to 45 nm, and a non-extrinsic switching is observed in the range between 45 and 11 nm, which is ascribed to the transition range from extrinsic to intrinsic switching.

Multilayers consisting of alternating ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer and relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) (P(VDF-TrFE-CFE)) terpolymer with different periodicities in thickness were prepared. A superlattice-like structure is shown in the polymer multilayer as the periodic thickness is lower than a critical value. The dielectric constant of the multilayer with a small periodic thickness is two times higher than that of the P(VDF-TrFE) copolymer over a temperature range between 300 K and 350 K. The multilayer also shows a good ferroelectricity in the same temperature range. The enhanced electrical properties of the multilayers are due to the long-range ferroelectric coupling.

The polarization switching behavior in poly(vinylidene fluoride-trifluoroethylene) thin films is studied by using a pulse transient current method. The dependence of the domain switching current on the coercive electric field was investigated. The charging current around the coercive field was found to be limited by domain switching instead of the series resistor in the measurement circuit because of the slow polarization switching in the films. The domain-switching process was explained by a creep model wherein the two-dimension domain walls motion in the transverse direction dominates the polarization switching process.