Yuanjun Yang

Bio: Yuanjun Yang is an academic researcher from Hefei University of Technology. The author has contributed to research in topics: Thin film & Electric field. The author has an hindex of 15, co-authored 64 publications receiving 1320 citations. Previous affiliations of Yuanjun Yang include Tianjin University & Nanyang Technological University.

Electric-Field Control of Nonvolatile Magnetization in Co 40 Fe 40 B 20 / Pb ( Mg 1 / 3 Nb 2 / 3 ) 0.7 Ti 0.3 O 3 Structure at Room Temperature

Abstract: New iron selenide superconductors by intercalating smaller-sized alkali metals (Li, Na) and alkaline earths using high-temperature routes have been pursued ever since the discovery of superconductivity at about 30 K in KFe2Se2, but all have failed so far. Here we demonstrate that a series of superconductors with enhanced T-c=30 similar to 46 K can be obtained by intercalating metals, Li, Na, Ba, Sr, Ca, Yb, and Eu in between FeSe layers by the ammonothermal method at room temperature. Analysis on their powder X-ray diffraction patterns reveals that all the main phases can be indexed based on body-centered tetragonal lattices with a similar to 3.755-3.831 angstrom while c similar to 15.99-20.54 angstrom. Resistivities show the corresponding sharp transitions at 45 K and 39 K for NaFe2Se2 and Ba0.8Fe2Se2, respectively, confirming their bulk superconductivity. These findings provide a new starting point for studying the properties of these superconductors and an effective synthetic route for the exploration of new superconductors as well.

Growth mechanism of tubular ZnO formed in aqueous solution.

Abstract: Tubular ZnO microstructural arrays were fabricated by a hydrothermal decomposition method. The dependence of the morphologies on the growth time and temperature was investigated in detail. An experiment was carried out to determine the mechanism of tubular ZnO formation. Our results showed that ZnO microtubes originated from an ageing process from ZnO microrods at a lower temperature (compared to the temperature when hydrothermal deposition of ZnO microrods was dominant) due to the preferential chemical dissolution of the metastable Zn-rich (0001) polar surfaces. A growth model was proposed based on the coexistence of hydrothermal deposition and dissolution of ZnO in the fabrication process.

Investigation of the Hydrolysis of Perovskite Organometallic Halide CH3NH3PbI3 in Humidity Environment

Abstract: Instability of emerging perovskite organometallic halide in humidity environment is the biggest obstacle for its potential applications in solar energy harvest and electroluminescent display. Understanding the detailed decay mechanism of these materials in moisture is a critical step towards the final appropriate solutions. As a model study presented in this work, in situ synchrotron radiation x-ray diffraction was combined with microscopy and gravimetric analysis to study the degradation process of CH3NH3PbI3 in moisture, and the results reveal that: 1) intermediate monohydrated CH3NH3PbI3·H2O is detected in the degradation process of CH3NH3PbI3 and the final decomposition products are PbI2 and aqueous CH3NH3I; 2) the aqueous CH3NH3I could hardly further decompose into volatile CH3NH2, HI or I2; 3) the moisture disintegrate CH3NH3PbI3 and then alter the distribution of the decomposition products, which leads to an incompletely-reversible reaction of CH3NH3PbI3 hydrolysis and degrades the photoelectric properties. These findings further elucidate the picture of hydrolysis process of perovskite organometallic halide in humidity environment.

Suppression of Structural Phase Transition in VO2 by Epitaxial Strain in Vicinity of Metal-insulator Transition

Abstract: Mechanism of metal-insulator transition (MIT) in strained VO2 thin films is very complicated and incompletely understood despite three scenarios with potential explanations including electronic correlation (Mott mechanism), structural transformation (Peierls theory) and collaborative Mott-Peierls transition. Herein, we have decoupled coactions of structural and electronic phase transitions across the MIT by implementing epitaxial strain on 13-nm-thick (001)-VO2 films in comparison to thicker films. The structural evolution during MIT characterized by temperature-dependent synchrotron radiation high-resolution X-ray diffraction reciprocal space mapping and Raman spectroscopy suggested that the structural phase transition in the temperature range of vicinity of the MIT is suppressed by epitaxial strain. Furthermore, temperature-dependent Ultraviolet Photoelectron Spectroscopy (UPS) revealed the changes in electron occupancy near the Fermi energy EF of V 3d orbital, implying that the electronic transition triggers the MIT in the strained films. Thus the MIT in the bi-axially strained VO2 thin films should be only driven by electronic transition without assistance of structural phase transition. Density functional theoretical calculations further confirmed that the tetragonal phase across the MIT can be both in insulating and metallic states in the strained (001)-VO2/TiO2 thin films. This work offers a better understanding of the mechanism of MIT in the strained VO2 films.

Bipolar loop-like non-volatile strain in the (001)-oriented Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals.

Abstract: Strain has been widely used to manipulate the properties of various kinds of materials, such as ferroelectrics, semiconductors, superconductors, magnetic materials, and “strain engineering” has become a very active field. For strain-based information storage, the non-volatile strain is very useful and highly desired. However, in most cases, the strain induced by converse piezoelectric effect is volatile. In this work, we report a non-volatile strain in the (001)-oriented Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals and demonstrate an approach to measure the non-volatile strain. A bipolar loop-like S-E curve is revealed and a mechanism involving 109° ferroelastic domain switching is proposed. The non-volatile high and low strain states should be significant for applications in information storage.

Handbook of Chemistry and Physics

Abstract: There is a special reason for reviewing this book at this time: it is the 50th edition of a compendium that is known and used frequently in most chemical and physical laboratories in many parts of the world. Surely, a publication that has been published for 56 years, withstanding the vagaries of science in this century, must have had something to offer. There is another reason: while the book is a standard fixture in most chemical and physical laboratories, including those in medical centers, it is not as frequently seen in the laboratories of physician's offices (those either in solo or group practice). I believe that the Handbook can be useful in those laboratories. One of the reasons, among others, is that the various basic items of information it offers may be helpful in new tests, either physical or chemical, which are continuously being published. The basic information may relate

Hydrothermally grown oriented ZnO nanorod arrays for gas sensing applications

Abstract: C-axis vertically aligned ZnO nanorod arrays were synthesized on a ZnO thin film through a simple hydrothermal route. The nanorods have a diameter of 30–100 nm and a length of about several hundred nanometres. The gas sensor fabricated from ZnO nanorod arrays showed a high sensitivity to H2 from room temperature to a maximum sensitivity at 250 °C and a detection limit of 20 ppm. In addition, the ZnO gas sensor also exhibited excellent responses to NH3 and CO exposure. Our results demonstrate that the hydrothermally grown vertically aligned ZnO nanorod arrays are very promising for the fabrication of cost effective and high performance gas sensors.

Deterministic switching of ferromagnetism at room temperature using an electric field

Abstract: The technological appeal of multiferroics is the ability to control magnetism with electric field1, 2, 3. For devices to be useful, such control must be achieved at room temperature. The only single-phase multiferroic material exhibiting unambiguous magnetoelectric coupling at room temperature is BiFeO3 (refs 4 and 5). Its weak ferromagnetism arises from the canting of the antiferromagnetically aligned spins by the Dzyaloshinskii–Moriya (DM) interaction6, 7, 8, 9. Prior theory considered the symmetry of the thermodynamic ground state and concluded that direct 180-degree switching of the DM vector by the ferroelectric polarization was forbidden10, 11. Instead, we examined the kinetics of the switching process, something not considered previously in theoretical work10, 11, 12. Here we show a deterministic reversal of the DM vector and canted moment using an electric field at room temperature. First-principles calculations reveal that the switching kinetics favours a two-step switching process. In each step the DM vector and polarization are coupled and 180-degree deterministic switching of magnetization hence becomes possible, in agreement with experimental observation. We exploit this switching to demonstrate energy-efficient control of a spin-valve device at room temperature. The energy per unit area required is approximately an order of magnitude less than that needed for spin-transfer torque switching13, 14. Given that the DM interaction is fundamental to single-phase multiferroics and magnetoelectrics3, 9, our results suggest ways to engineer magnetoelectric switching and tailor technologically pertinent functionality for nanometre-scale, low-energy-consumption, non-volatile magnetoelectronics.