J. S. Zhu

Doping effect on the dielectric property in bismuth titanate

The dielectric property complemented by the mechanical measurement (internal friction) in the doped Bi4Ti3O12 [Bi4−xLaxTi3O12 (x=0.5,0.75,1) and Bi4−y/3Ti3−yNbyO12 (y=0.015,0.03,0.06)] was systematically investigated from room temperature to 350 °C. In the plot of dielectric loss versus temperature for Bi4Ti3O12 (BiT), a relaxation peak was confirmed to be associated with the motion of the oxygen vacancy. It is found that the La doping at site A is in favor of improvement of the fatigue property, in contrast, the Nb doping at site B can mainly enhance the remanent polarization. Appropriate La doping at site A of perovskite-type unit in BiT enhances the chemical stability of oxygen vacancy by improving the height of the potential barrier for hopping and enhances the mobility of domain by the changing of domain structures. While the Nb doping at site B could induce the distortion of oxygen octahedral and reduce the oxygen vacancy concentration by a compensating effect so that it results in an enhancement of...

Intrinsic Dipole-Field-Driven Mesoscale Crystallization of Core−Shell ZnO Mesocrystal Microspheres

Novel uniform-sized, core-shell ZnO mesocrystal microspheres have been synthesized on a large scale using a facile one-pot hydrothermal method in the presence of the water-soluble polymer poly(sodium 4-styrenesulfonate). The mesocrystal forms via a nonclassical crystallization process. The intrinsic dipole field introduced by the nanoplatelets as a result of selective adsorption of the polyelectrolyte on some polar surfaces of the nanoparticles acts as the driving force. In addition, it plays an important role throughout the mesoscale assembly process from the creation of the bimesocrystalline core to the apple-like structure and finally the microsphere. Our calculation based on a dipole model confirms the dipole-field-driven mechanism forming the apple-like structure.

Why lanthanum-substituted bismuth titanate becomes fatigue free in a ferroelectric capacitor with platinum electrodes

Bi4Ti3O12, (BTO) a Bi-layered perovskite oxide, shows fatigue after repeated ferroelectric polarization reversals. On the other hand, Bi3.25La0.75Ti3O12 (BLT) is fatigue free. From an extensive transmission electron microscopy study, it was found that there is a high density of antiphase boundaries (APBs) in BLT like in the fatigue-free SrBi2Ta2O9 but not in BTO. It is proposed that the existence of APBs possibly plays a key role in the fatigue-free behavior of Bi-layered perovskite oxides.

Identification of Surface Structures on 3C-SiC Nanocrystals with Hydrogen and Hydroxyl Bonding by Photoluminescence

SiC nanocrystals (NCs) exhibit unique surface chemistry and possess special properties. This provides the opportunity to design suitable surface structures by terminating the surface dangling bonds with different atoms thereby boding well for practical applications. In this article, we report the photoluminescence properties of 3C-SiC NCs in water suspensions with different pH values. Besides a blue band stemming from the quantum confinement effect, the 3C-SiC NCs show an additional photoluminescence band at 510 nm when the excitation wavelengths are longer than 350 nm. Its intensity relative to the blue band increases with the excitation wavelength. The 510 nm band appears only in acidic suspensions but not in alkaline ones. Fourier transform infrared, X-ray photoelectron spectroscopy, and X-ray absorption near-edge structure analyses clearly reveal that the 3C-SiC NCs in the water suspension have Si-H and Si-OH bonds on their surface, implying that water molecules only react with a Si-terminated surface. First-principle calculations suggest that the additional 510 nm band arises from structures induced by H(+) and OH(-) dissociated from water and attached to Si dimers on the modified (001) Si-terminated portion of the NCs. The size requirement is consistent with the observation that the 510 nm band can only be observed when the excitation wavelengths are relatively large, that is, excitation of bigger NCs.

Tin Oxide Nanoribbons with Vacancy Structures in Luminescence-Sensitive Oxygen Sensing

Vacancy structures in tin oxide nanoribbons fabricated via thermal evaporation and post-processing are probed by luminescence spectroscopy, and interesting properties that bode well for oxygen sensing are observed. Besides a broad 620-nm band, the fabricated tin oxide nanoribbons show a photoluminescence band at 480 nm when the measurement temperature is <100 K. The blue band appears from nanoribbons synthesized under high oxygen pressure or annealed under oxygen. The dependence suggests that the oxygen interstitial and vacancy densities determine the electronic states that produce the blue band. Calculation of the electron structures based on the density functional theory shows that decreased oxygen vacancies or increased oxygen interstitials enhance the 480-nm band but suppress the 620-nm band. The results reported here indicate that the tin oxide nanoribbons with vacancy structures have potential applications in luminescence-sensitive oxygen sensing.

Large anisotropy of ferroelectric and dielectric properties for Bi3.15Nd0.85Ti3O12 thin films deposited on Pt∕Ti∕SiO2∕Si

Polycrystalline Bi3.15Nd0.85Ti3O12 (BNdT) thin films of a-axis preferential orientation [α(100)=62%] and high c-axis orientation [α(001)=96%] were fabricated directly on Pt∕Ti∕SiO2∕Si substrates through a sol-gel process. We observed strong dependences of ferroelectric and dielectric properties on the film orientation, with a remanent polarization 2Pr of 39μC∕cm2 and dielectric constant er of 343 at 100 kHz in the a-axis oriented film; 2Pr of 20μC∕cm2 and er of 331 in the film of random orientation; and 2Pr of 13μC∕cm2,er of 218 in the highly c-axis oriented film. Furthermore, the 2Pr value of a purely a-axis-oriented BNdT film can be predicted to be ∼51μC∕cm2. The large anisotropy of 2Pr and er values demonstrates that the polarization vector of BNdT is close to the a axis.

Fatigue characteristics of SrBi2Ta2O9 thin films prepared by metalorganic decomposition

Polycrystalline SrBi2Ta2O9 (SBT) ferroelectric thin films were synthesized on Pt/Ti/SiO2/Si substrates by metalorganic decomposition. Electric measurements demonstrate that fatigue increases with decreasing switching voltage and frequency, and the suppressed polarization caused at a lower switching voltage can be recovered by switching at a higher voltage. This suggests that the domain walls of SBT thin films are weakly pinned and easily depinned by a higher external field. The polarization of SBT thin films annealed in air shows more degradation than that annealed in oxygen, which indicates that the oxygen vacancy also plays an important role in fatigue behavior of SBT thin films.

Optical study on the size effects in BaTiO3 thin films

To study the size effects in ferroelectric thin film, we measured the optical transmittance and Raman spectra in BaTiO3 thin films deposited by the rf-magnetron sputtering technique on fused quartz and (111) Si substrates. A variation in the energy gap and Raman peaks with film thickness and grain size was observed and the possible origin was analyzed.

Optical properties of rocksalt and zinc blende AlN phases: First-principles calculations

We investigate the electronic structures and optical properties of the wurtzite, zinc blende, and rocksalt AlN phases at their structural phase transition pressures by the ultrasoft pseudopotential density functional method. We employ the Perdew–Burke–Eruzerhof form of the generalized gradient approximation available in the CASTEP code with the plane-wave basis sets to expand the periodic electron density. The dielectric function and optical properties such as reflectivity, absorption coefficient, refractive index, and electron energy-loss function are presented in a wide energy range between 0 and 50eV. Our results reveal that for rocksalt AlN phase, the intensity of N 2p orbitals in the conduction bands obviously decreases when the pressure increases from 15to17GPa. The calculated optical properties indicate that the zinc blende and wurtzite AlN phases have some similar features but the rocksalt AlN phase has other characteristics.

Study on ferroelectric and dielectric properties of niobium doped Bi4Ti3O12 ceramics and thin films prepared by PLD method

Abstract Nb doped bismuth titanate Bi 4− x (Ti 1− x Nb x ) 3 O 12 ( x =0%, 0.2%, 0.4%, 1%, 1.5%) ceramic samples were sintered and random oriented Bi 4− x (Ti 1− x Nb x ) 3 O 12 ( x =1%) thin films were fabricated on (111)Pt/TiO 2 /SiO 2 /Si with Pulsed Laser Deposition (PLD) method. The dielectric and ferroelectric properties of Nb doped Bi 4 Ti 3 O 12 (BTO) were investigated. A small amount of niobium doping results in a marked improvement in 2 P r . The 2 P r , 2 E c , dielectric constant and loss factor of Bi 4− x (Ti 1− x Nb x ) 3 O 12 ( x =1%) thin film are about 30 μC/cm 2 , 130 kV/cm, 175 and 0.02, respectively. However, after 3×10 9 switching cycles, 45% degradation of 2 P r was observed in the film.