Zhenlin Luo

Strain Dynamics of Ultrathin VO2 Film Grown on TiO2 (001) and the Associated Phase Transition Modulation

Tuning the metal insulator transition (MIT) behavior of VO2 film through the interfacial strain is effective for practical applications. However, the mechanism for strain-modulated MIT is still under debate. Here we directly record the strain dynamics of ultrathin VO2 film on TiO2 substrate and reveal the intrinsic modulation process by means of synchrotron radiation and first-principles calculations. It is observed that the MIT process of the obtained VO2 films can be modulated continuously via the interfacial strain. The relationship between the phase transition temperature and the strain evolution is established from the initial film growth. From the interfacial strain dynamics and theoretical calculations, we claim that the electronic orbital occupancy is strongly affected by the interfacial strain, which changes also the electron-electron correlation and controls the phase transition temperature. These findings open the possibility of an active tuning of phase transition for the thin VO2 film through the interfacial lattice engineering.

Coexistence of ferroelectric triclinic phases in highly strained BiFeO3 films

The structural evolution of the strain-driven morphotropic phase boundary (MPB) in BiFeO3 films has been investigated using synchrotron x-ray diffractometry in conjunction with scanning probe microscopy. Our results demonstrate the existence of mixed-phase regions that are mainly made up of two heavily tilted ferroelectric triclinic phases. Analysis of first-principles computations suggests that these two triclinic phases originate from a phase separation of a single monoclinic state accompanied by elastic matching between the phase-separated states. These first-principle calculations further reveal that the intrinsic piezoelectric response of these two low-symmetry triclinic phases is not significantly large, which thus implies that the ease of phase transition between these two energetically close triclinic phases is likely responsible for the large piezoelectric response found in the BiFeO3 films near its MPB. These findings not only enrich the understandings of the lattice and domain structure of epitaxial BiFeO3 films but may also shed some light on the origin of enhanced piezoelectric response near MPB.

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

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.

Large anisotropic remnant magnetization tunability in (011)-La2/3Sr1/3MnO3/0.7Pb(Mg2/3Nb1/3)O3-0.3PbTiO3 multiferroic epitaxial heterostructures

A large anisotropic remnant magnetization tunability was observed in multiferroic (011)-La2/3Sr1/3MnO3/0.7Pb(Mg2/3Nb1/3)O3-0.3PbTiO3 (LSMO/PMN-0.3PT) epitaxial heterostructures. The remnant magnetization along [100] direction was suppressed by an electric field applied to the substrate while the remnant magnetization along [011¯] was enhanced. The tunabilities of the remnant magnetization along the [100] and [011¯] directions are about −17.9% and +157% under electric field of +7.27 kV/cm, respectively. This large anisotropic remnant magnetization tunability may find potential applications in the electrically written and magnetically read memories.

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

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.

Low symmetry monoclinic MC phase in epitaxial BiFeO3 thin films on LaSrAlO4 substrates

We reported that the tetragonal-like phase identified in strained epitaxial BiFeO3 films on a (001) LaSrAlO4 single crystal substrates is monoclinic MC, based on high resolution synchrotron x-ray studies and piezoresponse force microscopy measurements. This MC phase has different symmetry with the rhombohedral-like monoclinic MA phase found in BiFeO3 films grown on low mismatch SrTiO3 substrates. Transmission electron microscopy revealed that the films on LaSrAlO4 substrates have a high crystalline quality and coherent interface.

Piezo-strain induced non-volatile resistance states in (011)-La2/3Sr1/3MnO3/0.7Pb(Mg2/3Nb1/3)O3-0.3PbTiO3 epitaxial heterostructures

The non-volatile resistance states induced by converse piezoelectric effect are observed in ferromagnetic/ferroelectric epitaxial heterostructures of (011)-La2/3Sr1/3MnO3/0.7Pb(Mg2/3Nb1/3)O3-0.3PbTiO3 (LSMO/PMN0.7PT0.3). Three stable remnant strain states and the corresponding resistance states are achieved by properly reversing the electric field from the depolarized direction in ferroelectric PMN0.7PT0.3 substrate. The non-volatile resistance states of the LSMO film can be manipulated by applied electric-field pulse sequence as a result of the large coupling between the electronic states of LSMO film and the strain transferred from the ferroelectric substrate. The electrically tunable, non-volatile resistance states observed exhibit potential for applications in low-power-consumption electronic devices.

Electric-field-control of resistance and magnetization switching in multiferroic Zn0.4Fe2.6O4/0.7Pb(Mg2/3Nb1/3)O3―0.3PbTiO3 epitaxial heterostructures

Multiferroic (001)–Zn0.4Fe2.6O4/0.7Pb(Mg2/3Nb1/3)O3–0.3PbTiO3 (ZFO/PMN–PT) epitaxial heterostructures have been investigated to demonstrate the electric-field-controlled resistance and magnetization switching. The tunabilitiy of resistance of the ZFO film is about −0.1% under the in-plane strain −0.02% at 296 K and 0.2% for the electric field 1.0 kV/cm at 80 K, respectively, and the tunabilitiy of magnetization is about 1.1% under the in-plane strain −0.11% at 296 K, which is attributed to the controllable strain transferred into the ZFO film from the piezoelectric PMN–PT substrate. A possible microscopic mechanism of the manipulation of resistance and magnetization is the enhancement of hopping amplitude of electrons between mixed-valent Fe2+ and Fe3+ ions under the electric-field-induced in-plane compressive strain.

Combinatorial Discovery of Visible-Light Driven Photocatalysts Based on the ABO3-type (A= Y, La, Nd, Sm, Eu, Gd, Dy, Yb, B = Al and In) Binary Oxides

A combinatorial approach was used to systematically investigate the photocatalytic activities of different kinds of ABO(3)-type oxides (A = Y, La, Nd, Sm, Eu, Gd, Dy, Yb; B = Al, In). Two novel photocatalysts, cubic YInO(3) and perovskite YAlO(3), were identified rapidly. Scale-up experiments confirmed that the two photocatalysts, especially the YInO(3), had excellent photocatalytic activity for toluene oxidation and water splitting under visible-light irradiation.

Shape-controlled growth and single-crystal XRD study of submillimeter-sized single crystals of SnO

Submillimeter-sized single crystals of SnO exhibiting well-defined symmetric shapes of four-armed star and square plate were successfully synthesized through a mild surfactant-free hydrothermal reaction. Crystal structures, refined by single-crystal X-ray diffraction, and Raman scattering investigations performed on an individual SnO single crystal are reported.