Bingyang Zhang

Electron acoustic imaging of BaTiO3 single crystals

Scanning electron acoustic microscopy (SEAM) has been used in the observation of ferroelectric domain configurations in barium titanate single crystals. The contribution to the image contrasts is related to signal generation through a piezoelectric coupling mechanism. The results prove that SEAM really has the ability to show not only the surface shape and domain structures but also the subsurface domain structures. Furthermore, SEAM is a convenient, fast and nondestructive way of observing the ferroelectric domains compared with other methods.

Splitting of the recombination zone in organic light emitting diodes by dye doping

In organic light emitting devices, doping of the electroactive organic layer with highly luminescent molecules has been shown to considerably increase device performance and lifetime. In most cases, the doping molecule does not act as a charge donor or an acceptor as in classical semiconductors, but is used to enhance light emission and to tune the emission color. By using the laser dye derivative 4-(dicyanomethylene)-2-methyl-6-{2-[(4-diphenylamino)phenyl]ethyl}-4H-pyran as dopant in a standard organic light emitting device, we have achieved highly efficient red to yellow emission depending on doping concentration. Furthermore the emission color changes with increasing current density. Using device model simulations, we have found that this color change is caused by the splitting of the recombination zone into two zones due to a decrease of the electron mobility inside the doped area.

Some New Applications of the Scanning Electron Acoustic Microscope for Materials Evaluation

Several different kinds of materials were investigated by an improved scanning electron acoustic microscope (SEAM). Residual stress distribution in aluminum metal and whisker-reinforced Al2O3–SiCw ceramic induced by Vickers indentation, domain structures in a ferroelectric Bi4Ti3O12 single crystal, dislocations and defects in the hetero-structure GaAs/GaAlAs epitaxial multilayer were successfully observed. Alternative contrast mechanisms were discussed. The advantages of the scanning electron acoustic microscope for materials evaluation were demonstrated.

Piezoelectric electron acoustic study of domain structures in ferroelectric ceramics BaTiO3

Abstract The domain structures of ferroelectric ceramics BaTiO3 have been observed by piezoelectric electron acoustic probe without any special processing to the sample. It is found that multi-distribution of domain structures could exist in a single grain and a domain structure could cross several grains. Experimental results show that the orientations of arrangements of domain structures on the boundary of two neighboring grains are the same as far as possible and the width of domain depends on the magnitude of grain. The origins of electron acoustic image contrasts of BaTiO3 were mainly determined by the electronic properties of ferroelectric domains.

Large magnetocaloric effect of HoxEr1-xNi (0 ≤ x ≤ 1) compounds

A secondary magnetic transition (spin reorientation transition) below Curie temperature in ErNi was observed via different characterization techniques. Ho-substitution for Er atoms has a great impact on the magnetic property and magnetocaloric effect. The two magnetic transitions change close to each other with 10% of Ho-substitution at the Er site. It is also found that 10% of Ho-substitution contributes up to ∼14.9% of enhancement on the maximal magnetic entropy change (ΔSM) and ∼21.9% of enhancement on the maximal adiabatic temperature change (ΔTad). The maximum value of ΔSM and ΔTad for Ho0.1Er0.9Ni compound is as high as 34u2009J/kg K and 8.9u2009K, respectively, under a field change of 0–5u2009T. The relationship between the maximal ΔSM and the refrigerant temperature width (δTFWHM) for HoxEr1-xNi (0u2009≤u2009xu2009≤u20091) compounds is analyzed. The enhancement of MCE for Ho0.1Er0.9Ni compound is considered to be resulted from the tendency of merging of spin reorientation transition and ferromagnetic to paramagnetic transition.

The physical mechanism of magnetic field controlled magnetocaloric effect and magnetoresistance in bulk PrGa compound

The PrGa compound shows excellent performance on the magnetocaloric effect (MCE) and magnetoresistance (MR). The physical mechanism of MCE and MR in PrGa compound was investigated and elaborated in detail on the basis of magnetic measurement, heat capacity measurement and neutron powder diffraction (NPD) experiment. New types of magnetic structure and magnetic transition are found. The results of the NPD along with the saturation magnetic moment (MS) and magnetic entropy (SM) indicate that the magnetic moments are randomly distributed within the equivalent conical surface in the ferromagnetic (FM) temperature range. PrGa compound undergoes an FM to FM transition and an FM to paramagnetic (PM) transition as temperature increases. The magnetizing process was discussed in detail and the physical mechanism of the magnetic field controlled magnetocaloric effect (MCE) and the magnetoresistance (MR) was studied. The formation of the plateau on MCE curve was explained and MR was calculated in detail on the basis of the magnetic structure and the analysis of the magnetizing process. The experimental results are in excellent agreement with the calculations. Finally, the expression of MRu2009=u2009β(T)X2 and its application conditions were discussed, where X is M(H)/Meff, and Meff is the paramagnetic effective moment.

Depolarization blueshift in intersubband transitions of triangular quantum wires

The depolarization effect (DE) in the intersubband transitions (ISBTs) of triangular cross-section quantum wires has been calculated in the framework of the effective-mass envelope-function theory and the self-consistent field approximation (Hartree approximation). Similar to quantum wells, the DE can bring an upward shift in ISBT. The shift quantities are affected significantly by apex angle but are insensitive to triangle size.

APPLICATION OF SEAM AND SAM TO FERROELECTRIC AND FERROELASTIC CRYSTALS

Abstract A scanning acoustic microscope (SAM) and a scanning electron acoustic microscope (SEAM) have been used to study domain structures in ferroelectric BaTiO3, Bi4Ti3O12 and ferroelastic NdP5O14 single crystals. In SAM, domain walls act as scattering boundaries for leaky surface acoustic waves and thus sources of image contrast; While in SEAM, different signal generation and image contrast mechanisms may arise, mainly thermal elastic and piezoelectric effects. Further insight into the properties of domains was approached by discussing and comparing the image contrasts involved in both techniques.

Crystalline state of InSb epilayers on GaAs substrates by metalorganic chemical vapor deposition

Plastic flows of a large lattice-mismatch InSb epilayer on GaAs substrate grown by metalorganic chemical vapor deposition (MOCVD) were first observed by scanning electron acoustic microscopy (SEAM), and crystalline state of the buried subsurfaces was discussed. From the SEAM images in two different positions a macroscopical heterogenous distribution of large compression stress fields was studied. It was a very important result to observe and study the plastic flows by SEAM uniquely imaging mechanism.

Acquisition of modulated PZT signal and application of SEAM

PZT transducers can be used to measure the characteristics of materials irradiated by laser or electron beams. In order to maintain high measurement resolution, the signal aroused on the transducer is modulated to produce a modulated PZT signal. Signal acquisition is accomplished with a lock-in amplifier and computer technologies. A typical application is SEAM (Scanning Electrical Acoustic Microscopy). The SEAM image reflects the thermal characteristics of materials. Furthermore, the SEAM image can supply subsurface information without destroying the sample.