Browsing by Author "Li, Yanxi"

Now showing 1 - 8 of 8
  • Thumbnail Image
    Anatomy of vertical heteroepitaxial interfaces reveals the memristive mechanism in Nb2O5-NaNbO3 thin films
    Li, Linglong; Lu, Lu; Wang, Zhiguang; Li, Yanxi; Yao, Yonggang; Zhang, Dawei; Yang, Guang; Yao, Jianjun; Viehland, Dwight D.; Yang, Yaodong (Springer Nature, 2015-03-18)
    Dynamic oxygen vacancies play a significant role in memristive switching materials and memristors can be realized via well controlled doping. Based on this idea we deposite Nb2O5-NaNbO3 nanocomposite thin films on SrRuO3-buffered LaAlO3 substrates. Through the spontaneous phase separation and self-assembly growth, two phases form clear vertical heteroepitaxial nanostructures. The interfaces between niobium oxide and sodium niobate full of ion vacancies form the conductive channels. Alternative I-V behavior attributed to dynamic ion migration reveals the memristive switching mechanism under the external bias. We believe that this phenomenon has a great potential in future device applications.
  • Thumbnail Image
    Controlled growth of epitaxial BiFeO3 films using self-assembled BiFeO3-CoFe2O4 multiferroic heterostructures as a template
    Li, Yanxi; Yang, Yaodong; Yao, Jianjun; Viswan, Ravindranath; Wang, Z. G.; Li, Jiefang; Viehland, Dwight D. (AIP Publishing, 2012-07-01)
    The growth mechanism of a BiFeO3 layer deposited on self assembled (0.65) BiFeO3-(0.35) CoFe2O4 (BFO-CFO) composite thin films was studied. Epitaxial and self-assembled BFO-CFO thin films were deposited on SrTiO3 (111) substrates by pulsed laser deposition and were subsequently used as a seed layer for the deposition of an additional BFO layer. x-ray line scans showed the heterostructures were highly epitaxial. Cross-sectional scanning electron microscopy and focused ion beam images revealed the top BFO layer grew preferentially from BFO nanopillars in the BFO-CFO thin films, thus, demonstrating controlled growth. The multiferroic properties of this new nanostructure were then studied. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4734508]
  • Thumbnail Image
    Effect of Mn substituents on the domain and local structures of Na1/2Bi1/2TiO3-BaTiO3 single crystals near a morphotropic phase boundary
    Yao, Jianjun; Yang, Yaodong; Monsegue, Niven; Li, Yanxi; Li, Jiefang; Zhang, Qinhui; Ge, Wenwei; Luo, Haosu; Viehland, Dwight D. (AIP Publishing, 2011-03-01)
    The ferroelectric domain and local structures of Na1/2Bi1/2TiO3-x% BaTiO3 (NBT-BT) and 0.14 at. % Mn substituted Na1/2Bi1/2TiO3-x% BaTiO3 (Mn:NBT-BT) single crystals with x=5.5 near a morphotropic phase boundary have been investigated by transmission electron microscopy. Increased ferroelectric ordering and enhanced in-plane octahedral tilting were observed for Mn: NBT-BT compared with NBT-BT. Bragg-filtered lattice images revealed that the size of the in-phase tilt domains of Mn:NBT-BT were on the order of 2 to 8 nm, with a tendency of alignment along {110}. (C) 2011 American Institute of Physics. [doi:10.1063/1.3573801]
  • Thumbnail Image
    Effects of Precursor-Substrate Distances on the Growth of GaN Nanowires
    Cheng, Hongbin; Li, Jia; Wu, Dongxu; Li, Yanxi; Wang, Zhiguang; Wang, Xianying; Zheng, Xuejun (Hindawi, 2015-08-12)
    GaN nanowires were synthesized through the Ni-catalyzed chemical vapor deposition (CVD) method using Ga2O3/GaN mixtures as gallium sources, and precursor-substrate distances were investigated as the important factor for the growth of GaN nanowires. The microstructure, composition, and photoluminescence property were characterized by X-ray diffraction, field emission scanning electron microscopy, high-resolution transmission electron microscopy, and photoluminescence spectra. The results showed that single crystalline GaN nanowires with the diameter of about 90 nm and the length up to tens of micrometers had been grown thickly across Si (100) substrates with uniform density. Moreover, the variations of the GaN nanowire morphology, density, and size were largely attributed to substrate positions which would influence Ga precursor density in the carrier gas, the saturation degree of gaseous reactants, and the catalyst activity, respectively, in the fabrication of GaN nanowires by the vapour liquid solid mechanism.
  • Thumbnail Image
    Magnetoelectric Oxide Nanocomposite Heterostructures
    Li, Yanxi (Virginia Tech, 2017-02-28)
    Multiferroics have attracted lots of research interest due to their potential in numerous multifunctional applications. The multiferroic materials could simultaneously exhibit two or more ferroic order parameters, and the coupling effects between ferroelectricity and ferromagnetism are named as magnetoelectric (ME) effect. Recently, with the development of thin film growth techniques, the multiferroics magnetoelectric composite heterostructures exhibit a very promising future prospects. This dissertation focused on the design, fabrication and characterization of new multiferroics magnetoelectric composite heterostructures. First, based on the specific phase architectures in BFO-CFO self-assembled thin films grown on variously oriented STO substrates and the epitaxial film growth knowledge, I designed two kinds of new film heterostructures: (i) I utilized self-assembled BFO nanopillars in a BFO-CFO two phase layer on (111) STO as a seed layer on which to deposit a secondary top BiFeO3 layer. The growth mechanism and multiferroic properties of these new heterostructures were investigated. (ii) I demonstrated the formation of a new quasi-(0-3) heterostructure by alternately growing (2-2) and (1-3) layers within the film. I proposed a new concept to overcome limitations of both the (2-2) and (1-3) phase connectivities and identified an indirect ME effect by the switching the characteristics of the piezoresponse for the new heterostructure. Second, for the option for candidates thin film materials with a high piezoelectric coefficient, which is a critical factor for ME composite films, I utilized the simple compositional BaSn0.11Ti0.89O3 bulk ceramic material as a target to grow films with the large piezoelectric properties. The grown high qualify lead-free epitaxial thin films had a chemical constituent similar to the reported giant piezoelectric ceramics near the MPB and with the QP. Both coherent and incoherent regions were observed in the interface and a larger piezoelectric coefficient d33 was achieved in this film. Finally, with respect to their characteristics and potential, I redirected from two-dimensional thin film materials to one-dimensional nanowire materials. By utilizing vertically aligned templates, I fabricated a new type of coaxial two-phase composite nanowires. Multiferroic properties of these new one-dimensional materials have been investigated. All these multiferroics magnetoelectric composite herterostructures would provide lots of potential in applications.
  • Thumbnail Image
    Magnetoelectric quasi-(0-3) nanocomposite heterostructures
    Li, Yanxi; Wang, Zhongchang; Yao, Jianjun; Yang, Tiannan; Wang, Zhiguang; Hu, Jia-Mian; Chen, Chunlin; Sun, Rong; Tian, Zhipeng; Li, Jiefang; Chen, Long-Qing; Viehland, Dwight D. (Nature Publishing Group, 2015-12-01)
    Magnetoelectric composites of magnetic and ferroelectric components are promising for their use in applications such as information storage. Here, the authors find that magnetic quasiparticles embedded in a ferroelectric film matrix show promising properties compared to the usual thin-film architectures.
  • Thumbnail Image
    Micro/Nanofabrication and Characterization of Advanced Materials and Devices
    Li, Yanxi; He, Liang; Yan, Mengyu; Wang, Zhengjun (2019-09-02)
  • Thumbnail Image
    Self-assembled NaNbO3-Nb2O5 (ferroelectric-semiconductor) heterostructures grown on LaAlO3 substrates
    Wang, Zhiguang; Li, Yanxi; Chen, Bo; Viswan, Ravindranath; Li, Jiefang; Viehland, Dwight D. (AIP Publishing, 2012-09-01)
    We deposited NaNbO3 (NNO)-Nb2O5 (NO) self-assembled heterostructures on LaAlO3 (LAO) to form ferroelectric-semiconductor vertically integrated nanostructures. The NNO component formed as nanorods embedded in a NO matrix. X-ray diffraction confirmed epitaxial growth of both NNO and NO phases. Phase distribution was detected by scanning electron microscopy. The NNO/NO volume ratio was strongly dependent on the deposition temperature due to the volatility of sodium. Piezoelectric force microscopy revealed a good piezoelectric response in the NNO component with a piezoelectric coefficient of D-33 approximate to 12 pm/V, with SrRuO3 (SRO) acting as bottom electrode. The current-voltage characterization of NNO-NO/SRO-LAO showed a typical diode rectifying behavior. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754713]