Publications

Abstract Highly efficient piezoelectric nanogenerator operation is demonstrated based on dynamic bending of graphene‐like ZnO nanosheets. Energy is harvested by an external resistor by virtue of a strong time‐varying piezoelectric polarization component perpendicular to the graphene‐like ZnO plane. It is shown analytically and verified numerically using molecular dynamics simulations that the m2 point group of flat graphene‐like ZnO is reduced to monoclinic m symmetry for bent graphene‐like ZnO. The latter symmetry allows for a nonzero and large piezoelectric polarization component perpendicular to the plane of the 2D structure. The numerical results confirm that flexoelectric effects are negligible subject to graphene‐like ZnO bending operation.

Abstract A review is given on the techniques and applications of high‐energy reflection electron energy‐loss spectroscopy (REELS) and reflection electron microscopy (REM) for surface studies in scanning transmission electron microscopes (STEM) and conventional transmission electron microscopes (TEM). A diffraction method is introduced to identify a surface orientation in the geometry of REM. The surface dielectric response theory is presented and applied for studying α‐alumina surfaces. Domains of the α‐alumina (012) surface initially terminated with oxygen can be reduced by an intense electron beam to produce Al metal; the resistance to beam damage of surface domains initially terminated with Al +3 ions is attributed to the screening effect of adsorbed oxygen. Surface energy‐loss near‐edge structure (ELNES), extended energy‐loss fine structure (EXELFS), and microanalysis using REELS are illustrated based on the studies of TiO 2 and MgO. Effects of surface resonances (or channeling) on the REELS signal‐to‐background ratio are described. The REELS detection of a monolayer of oxygen adsorption on diamond (111) surfaces is reported. It is shown that phase contrast REM image content can be significantly increased with the use of a field emission gun (FEG). Phase contrast effects close to the core of a screw dislocation are discussed and the associated Fresnel fringes around a surface step are observed. Finally, an in situ REM experiment is described for studying atomic desorption and diffusion processes on α‐alumina surfaces at temperatures of 1,300—1,400°C.

Abstract Some properties of transition metal oxides are related to the presence of elements with mixed valences. In electron energy-loss spectroscopy (EELS), the L or M ionization edges of transition-metal, rare-earth and actinide elements usually display sharp threshold peaks known as white-lines. EELS experiments have shown that a change in cation valence state introduces a significant change in the White-line intensity ratio [1]. With the use of valence state information provided by the white lines, an experimental approach is demonstrated here to map the valence state distributions of Mn and Co using an energy-filtered transmission electron microscope (TEM). A spatial resolution of ˜ 2 nm has been achieved. This technique should be particularly useful in studying valence states of cations in magnetic oxides. To map the distribution of ionization states, an energy window of ˜ 10 eV in width is required to isolate the L3 from L2 white lines (Figure 1).

Superlattice-structured ZnO nanohelix is a structure that is made of a coiling nanobelt, which is composed of ZnO nanostripes oriented alternatively in two different orientations (Science 2005, 309, 1700). The nanostripes run almost in parallel to the nanobelt direction but at an offset angle of approximately 5 degrees . We have measured the transport properties of a nanohelix and found its abnormal nonlinear characteristic. In comparison to the Ohmic transport property of a single crystal ZnO nanobelt measured under the same experimental conditions, the symmetric "Schottky-type" I-V property of the nanohelix is suggested due to nanostripe boundaries and surfaces, where built-in periodic back-to-back energy barriers might occur across the nanostripe interfaces as a result of polar charges and interface-strain-induced piezoelectric effect. The effective potential barrier across the nanostripe boundary is estimated to be approximately 24 meV. With the increasing of bias voltage, electrons can effectively tunnel through and thermionic emission across nanostripe boundaries, leading to a fast increase in transport current. It is suggested that the ZnO nanohelix could form a new type of band structure modulated superlattice for fabricating novel electronic devices.

Dielectric elastomers (DEs) are a special material that deform responding to an electric field. The induced strain is known as actuated strain (AS). This phenomenon is totally different from electrostriction, for there is no crystal lattice in elastomers and the AS of DEs is much greater. The most accepted mechanism holds the view that the AS of DEs is induced by the Maxwell stress. According to this mechanism, materials exhibiting similar ratios of permittivity and Young's modulus should have similar ASs, while the experimental AS isn't relevant to the ideal value, contradicting this mechanism. The direction of uniaxial pre-strained DE's AS cannot be explained by this mechanism either. The electric field and DE are only regarded as a source of stress and a deformable body respectively in this mechanism, which ignores the interaction between those two. Recently, a new molecular mechanism for AS is proposed, in which the electric field first orient dipoles of chains, therefore the conformation of chains will be changed, finally leading to AS. With thermodynamical derivation and experiment, entropy-dominated elasticity is found to account for more during AS. This mechanism is systematically introduced in this perspective and presents current challenges and outlooks of DE.

Note: chercher Reference CIME-CONF-1998-012View record in Web of Science Record created on 2007-02-15, modified on 2017-05-12

Background: In rodent models, low-intensity extracorporeal shock wave therapy has been shown to negatively impact semen concentration after treatment on the penis, implying that the reproductive system in close proximity may be indirectly affected by this modality. We hypothesized that shock waves are detrimental to spermatogenesis, and the aim of this study was to evaluate the effect of shock waves on spermatogenesis after direct shockwave treatment on testes using different energy settings. Methods: Twenty-five male Sprague Dawley rats, 8 weeks old, were divided into five groups, including one control group and four treatment groups each treated using shock waves of different intensities. All rats in the treatment groups received 2000 shocks on the left testis twice a week for 4 weeks, with shock wave intensity and frequency varied by treatment group: 0.1 mJ/mm 2 at 4 Hz for Group A, 0.15 mJ/mm 2 at 4 Hz for Group B, 0.35 mJ/mm 2 at 4 Hz for Group C, and 0.55mJ/mm 2 at 3 Hz for Group D. At the end of the experiment, sperm collected from the epididymis was evaluated for concentration and motility. Testicular spermatogenesis, the apoptotic index of germ cells, and the expression of a meiotic-specific gene were also analyzed. Results: The treatment group receiving shock wave intensity at 0.55 mJ/mm 2 showed a significant decrease in sperm concentration, motility, and Johnsen score as compared to other groups. The apoptotic index of spermatogenic cells increased as the intensity of the shock wave treatment escalated, and reach a statistically significant difference at 4 weeks posttreatment. Treating testes with intensity levels of 0.55 mJ/mm 2 at 3 Hz interfere with the quality or quantity of spermatogenesis and also increases in spermatogenic cell apoptosis, whereas the expression of the SYCP3 gene significantly decreased after treatment with intensity levels of 0.10 mJ/mm 2 , 0.15 mJ/mm 2 , and 0.35 mJ/mm 2 at 4 Hz. Conclusion: Treating testes with intensity levels of 0.55 mJ/mm 2 at 3 Hz interfere with the quality or quantity of spermatogenesis and also increases spermatogenic cell apoptosis, whereas the expression of the SYCP3 gene significantly decreased after treatment with intensity levels of 0.10 mJ/mm 2 , 0.15 mJ/mm 2 , and 0.35 mJ/mm 2 at 4 Hz.

Self-attraction among vertical aligned Au/ZnO nanorods under the illumination of an electron beam has been observed. This phenomenon is attributed to the interaction between the accumulation charges near the metal-semiconductor junctions for two nanorods of different length. A model calculation is presented to explain the observed phenomenon.