Xuejun Zheng

The fabrication of vanadium-doped ZnO piezoelectric nanofiber by electrospinning

Zn(1-x)V(x)O (x = 0.015, 0.02, 0.025, 0.03, V-ZnO) nanofibers were synthesized by electrospinning, and a butterfly-shaped piezoelectric response was measured by scanning force microscopy. The diameters of V-ZnO nanofibers with hexagonal wurtzite phase are in the range of 50-300 nm. The large piezoelectric coefficient d(33) of 121 pm V(-1) was obtained and the high piezoelectric property may be attributed to the switchable spontaneous polarization induced by V dopants and the easier rotation of V-O bonds under an electric field. The result shows that the V-ZnO nanofiber is a promising candidate for nanoscale piezoelectric devices.

A vacuum pressure sensor based on ZnO nanobelt film.

A vacuum pressure sensor was fabricated by assembling ZnO nanobelt film on the interdigital electrodes, and the current–voltage characteristics were measured with an Agilent semiconductor parameter tester. Under different pressures of 1.0 × 10(3), 6.7 × 10(−3), 8.2 × 10(−4) and 9.5 × 10(−5) mbar, the currents are 8.71, 28.1, 46.1 and 89.6 nA, and the pressure sensitive resistances are 1150, 356, 217 and 112 MΩ, respectively. In the range of 10(−5)–10(3) mbar the smaller the pressure is, the higher the current is. The pressure sensitive resistance of the vacuum pressure sensor increases linearly with the logarithmic pressure, and the measurement range is at least one order of magnitude wider than that of the previous sensors. Under the final pressure, the vacuum pressure sensor has maximum sensitivity (9.29) and power consumption of 0.9 μW. The sensitivity is larger than that of the previous sensor based on a ZnO single nanowire at that pressure, and the power consumption is much lower than that for the sensor based on a ZnO nanowire array. The pressure sensitive mechanism is reasonably explained by using oxygen chemisorption and energy band theory.

The dielectric relaxation behavior of (Na0.82K0.18)0.5Bi0.5TiO3 ferroelectric thin film

(Na1-xKx)0.5Bi0.5TiO3 (NBT-KBT-100 x) thin films were deposited on Pt/Ti/SiO2/Si(100) by metal organic decomposition, and the effects of potassium content (x = 0.15, 0.18, 0.20, 0.25) on ferroelectric, piezoelectric, dielectric properties of the thin films, and the temperature dependence of dielectric permittivity of NBT-KBT-18 thin film were investigated in detail. NBT-KBT-18 thin film is of the largest effective piezoelectric coefficient d33eff, remnant polarization 2Pr, spontaneous polarization 2 Ps, dielectric constant ɛr, and the lowest dielectric loss among the thin films. The dielectric constants decrease steeply with the increase of frequency, and there are a series of resonance peaks with Debye-like relaxation. In dielectric temperature spectra, two abnormal peaks corresponding to depolarization temperature and Curie temperature are at the range of 75-90 °C and 295-320 °C, and they are associated with the phase transitions. Based on the dielectric relaxation theory, Debye-like relaxation and diff...

Nanoscale mechanical behavior of vanadium doped ZnO piezoelectric nanofiber by nanoindentation technique

The nanoscale mechanical behavior of Zn0.975V0.025O (V-ZnO) piezoelectric nanofibers by electrospinning was investigated using a nanoindenter in detail. After being calcined at 700 °C, V-ZnO nanofibers are of hexagonal wurtzite phase crystal structure, and the diameter and length are in the range of 50–300 nm and several tens to several hundreds of micrometers. The statistical average values of reduced modulus and hardness are 58.7±4.2 and 3.3±0.2 GPa for the nanofibers, and they decrease by 47.2% and 34.0% in comparison with those of bulk ZnO. It indicates that size effect of the mechanical behavior was obviously observed for the nanofibers, and the mechanism is discussed in conjunction with their high surface-to-volume ratio. Indentation depth-dependent reduced modulus and hardness properties were observed at indentation depth less than 18 nm, and it is attributed to the strain gradient effect during nanoindentation.

Space-charge-limited leakage current in high dielectric constant and ferroelectric thin films considering the field-dependent permittivity

Distinguishing from the traditional characterization on high-field leakage current density-voltage relationship, the field-dependent permittivity from the polarization derivative is used to solve the space-charge-limited conduction, and the simulated leakage current densities are compared with the previous experimental observations. The influences of the mobility, ferroelectric parameters, and film thickness on the leakage current densities are discussed. The results verify that the high-field quasi-Ohmic region observed experimentally may result from the field-dependent permittivity, and that the leakage current can be influenced by the ferroelectric polarization.

Effect of Lanthanides on Acetone Sensing Properties of LnFeO 3 Nanofibers (Ln = La, Nd, and Sm)

LnFeO₃ (Ln = La, Nd, and Sm) nanofibers are synthesized by the electrospinning, and characterized by X-ray diffraction, field-emission scanning electron microscopy, and X-ray photoelectron spectroscopy. For investigating the effect of lanthanide on acetone sensing properties of LnFeO₃ nanofibers, the side-heating gas sensors based on LnFeO₃ nanofibers are fabricated and measured at different temperatures and acetone concentrations. The optimal working temperatures of the LnFeO₃ gas sensors are all 140 °C. When the LnFeO₃ gas sensors are exposed to 100 ppm and acetone at 140 °C, the SmFeO₃ sensor exhibits the largest sensing response (Response = 9.98) compared with LaFeO₃ and NdFeO₃ gas sensors, and the response and recovery times of SmFeO₃ sensor are about 17 and 16 s, respectively. The results indicate that the lanthanides have an impact on the acetone sensing properties of LnFeO₃, and the lanthanide with low binding energy of the Ln–O bond is beneficial for improving the acetone sensing properties of LnFeO₃.

Nanoscale domain switchings of Bi3.15Dy0.85Ti3O12 thin film under the simultaneous application of polarizing voltage and loading force

The nanoscale domain switchings of Bi3.15Dy0.85Ti3O12 thin film annealed at 700 °C were observed by piezoresponse force microscopy under the different DC polarizing voltages, loading forces, and simultaneously both of them. Then, the formation mechanisms of domain switchings were analyzed from the perspective of dynamics of ferroelectric nanodomain and equilibrium state free energy of ferroelectric nanodomain. First, the 180° domain switchings were observed under the single polarizing voltages ±12 V. However, there are small amounts of anti-parallel domains. Second, the 90° domain switching was detected under the different single loading forces (150 and 300 nN). Finally, both 90° and 180° domain switchings were obtained under the simultaneously applied fields of polarizing voltage (+10 V) and loading forces (100 and 200 nN). The formation mechanisms of typical electric-generated nanoscale domain switchings were analyzed by forward domain-growth mechanism and grain deformation, and the abnormal electric-ge...

Evaluation on residual stress in Bi3.15(Eu0.7Nd0.15)Ti3O12 polycrystalline ferroelectric thin film by using the orientation average method

We propose an orientation average method to evaluate residual stresses in polycrystalline thin films. Bi3.15(Eu0.7Nd0.15)Ti3O12 was used to verify our approach, with films prepared by metal organic decomposition at various annealing temperatures. The mechanical properties and microstructure were characterized by nanoindentation and X-ray diffraction. The thin film annealed at 600 °C has the largest residual compressive stress of 771 MPa among all thin films. The residual stresses are evaluated by the proposed method and traditional sin2ψ method, and the maximum distinction is less than 6.43%, demonstrating that the proposed method is reliable and convenient to evaluate residual stress in polycrystalline thin films.

The threshold electric field of 180° domain switching in the misfit strain-external electric field phase diagram

The single domain treatment on the selected single grain was performed by the negative DC bias in order to obtain the single-domain state, and the opposite color contrasts within the selected grain in piezoelectric phase images of Pb(Zr0.52Ti0.48)O3 ferroelectric thin film were observed by piezoelectric force microscopy. Based on nonlinear thermodynamic theory, the a1c– and r– phases with the negative P3 component are introduced to describe the electric-generated domain switching, and the external misfit strain-electric field phase diagram and the electric field-polarization components curve are simulated at the simplification of uniform stress/electric distribution for the single-domain state of a single grain. In phase diagram, the electric field at the misfit strain –0.002 evaluated by x ray diffraction is 139 kV/cm for the phase transition from a1c– phase to c+ phase, and it is corresponding to the threshold electric field for 180° domain switching observed by the piezoelectric phase images.

Photoinduced stiffening and photoplastic effect of ZnS individual nanobelt in nanoindentation

The photoinduced stiffening (PIS) and photoplastic effect (PPE) of ZnS individual nanobelt (NB) were observed by using a nanoindenter in conjunction with an incident ultraviolet (UV) light source system. The results show that the elastic modulus and hardness of ZnS individual NB under UV illumination are at least 32% and 20% larger than those in darkness. The mechanisms of PIS and PPE are interpreted by the increase in electronic strain and Peierls barrier due to the photogeneration of free carriers in ZnS individual NB. The research may offer useful guidelines to the application of optoelectronic devices based on individual nanostructures.