Li-Qian Cheng

Phase transition and piezoelectricity of sol–gel-processed Sm-doped BiFeO3 thin films on Pt(111)/Ti/SiO2/Si substrates

Bi1−xSmxFeO3 thin films (x = 0, 0.05, 0.10 and 0.15) on Pt(111)/Ti/SiO2/Si substrates were fabricated by the sol–gel process and the effect of Sm doping on their crystal structure was studied by synchrotron radiation X-ray diffraction and Raman spectroscopy. It is revealed that a phase transition from the rhombohedral to orthorhombic structure takes place with increasing Sm content, resulting in two-phase coexistence at x = 0.10, where the two phases are R3c and Pbam according to the refinement result. The phase transition can be ascribed to the difference between the smaller radius of substituted Sm3+ and Bi3+. Meanwhile, the composition-dependent dielectric, ferroelectric and piezoelectric properties were also investigated. PFM scanning and switching spectroscopy results confirmed the enhancement of the piezoresponse at x = 0.10 corresponding to the rhombohedral–orthorhombic morphotropic phase boundary (MPB) region. The ferroelectric properties of Sm-doped BiFeO3 films were found to decrease with increasing Sm content, indicating that the extrinsic piezoelectric response contributes more to the improved piezoelectricity at the MPB.

Structure and composition characterization of lead-free (K, Na)NbO3 piezoelectric nanorods synthesized by the molten-salt reaction

Lead-free (K, Na)NbO3 single crystalline nanorods (KNN NRs) with environmental compatibility and a high piezoelectric coefficient may find increasing applications including piezoelectric nanogenerators. To optimize the piezoelectric responses of KNN NRs, the K/Na ratio must be adjusted precisely to a critical value. In this work, composition-controlled KNN NRs were successfully synthesized via the molten-salt reaction, and some advanced measuring techniques, such as Raman, ICP, TEM, DSC, etc., were employed for the detailed characterization. The compositional homogeneity along the length direction of an individual NR was confirmed by confocal Raman spectroscopy. Furthermore, this work revealed the correlation between the characteristic frequencies of Raman spectra and K/Na ratio of KNN nanocrystals, which can be used as a simple and convenient method for non-destructive and quantitative analysis of the A-site composition in alkaline niobate perovskite compounds.

Determination of crystallographic orientation of lead-free piezoelectric (K,Na)NbO3 epitaxial thin films grown on SrTiO3 (100) surfaces

Crystallographic structure of sol-gel-processed lead-free (K,Na)NbO3 (KNN) epitaxial films on [100]-cut SrTiO3 single-crystalline substrates was investigated for a deeper understanding of its piezoelectric response. Lattice parameter measurement by high-resolution X-ray diffraction and transmission electron microscopy revealed that the orthorhombic KNN films on SrTiO3 (100) surfaces are [010] oriented (b-axis-oriented) rather than commonly identified c-axis orientation. Based on the crystallographic orientation and corresponding ferroelectric domain structure investigated by piezoresponse force microscopy, the superior piezoelectric property along b-axis of epitaxial KNN films than other orientations can be explained.