Yuanjun Yang

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.

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

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.

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.

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.

Surface-growth-mode-induced strain effects on the metal–insulator transition in epitaxial vanadium dioxide thin films

A series of high-quality vanadium dioxide (VO2) epitaxial thin films on (0001)-oriented sapphire substrates with various thicknesses were fabricated using radio frequency (RF) magnetron sputtering techniques. Structural analysis revealed that an out-of-plane tensile strain (∼+0.035%) in the thinner VO2 epitaxial films was induced by epitaxial lattice mismatch between the monoclinic VO2 films and Al2O3 substrates. However, an anomalous compressive strain (∼−0.32%) was accumulated along the out-of-plane direction in the thicker VO2 films. This result contradicts with the conventional epitaxial lattice-mismatch mechanism for strain formed in epitaxial films. We attribute this anomalous strain to the surface growth mode (island growth) in the thicker VO2 films, especially those sputtered from the metal target at low pressure. Furthermore, the metal–insulator transition (MIT) temperature shifted to lower temperature with decreasing thickness, which is attributed to modulation of the orbital occupancy through the epitaxial strain and growth-mode-induced strain in the VO2 epitaxial films. Moreover, the very large resistance change (on the order of magnitude ∼103) in the VO2/Al2O3 epitaxial heterostructures is promising for electrical switch applications.

Anomalous thickness-dependent strain states and strain-tunable magnetization in Zn-doped ferrite epitaxial films

A series of ZnxFe3−xO4 (ZFO, x = 0.4) thin films were epitaxially deposited on single-crystal (001)-SrTiO3 (STO) substrates by radio frequency magnetron sputtering. The anomalous thickness-dependent strain states of ZFO films were found, i.e., a tensile in-plane strain exists in the thinner ZFO film and which monotonously turns into compressive in the thicker films. Considering the lattice constant of bulk ZFO is bigger than that of STO, this strain state cannot be explained in the conventional framework of lattice-mismatch-induced strain in the hetero-epitaxial system. This unusual phenomenon is proposed to be closely related to the Volmer-Weber film growth mode in the thinner films and incorporation of the interstitial atoms into the islands boundaries during subsequent epitaxial growth of the thicker films. The ZFO/STO epitaxial film is found in the nature of magnetic semiconductor by transport measurements. The in-plane magnetization of the ZFO/STO films is found to increase as the in-plane compressi...

Periodic elastic nanodomains in ultrathin tetragonal-like BiFeO3 films

We present a synchrotron grazing incidence x-ray diffraction analysis of the domain structure and polar symmetry of highly strained BiFeO3 (BFO) thin films grown on LaAlO3 substrate. We reveal the existence of periodic elastic nanodomains in the pure tetragonal-like BFO ultrathin films down to a thickness of 6 nm. A unique shear strain-accommodation mechanism is disclosed. We further demonstrate that the periodicity of the nanodomains increases with film thickness but deviates from the classical square root law in the ultrathin thickness regime (6-30 nm). Temperature-dependent experiments further reveal the disappearance of periodic modulation above similar to 90 degrees C due to a M-C-M-A structural phase transition.

Resistance switching of epitaxial VO2/Al2O3 heterostructure at room temperature induced by organic liquids

We studied using organic liquids (cyclohexane, n-butanol, and ethylene glycol) to modulate the transport properties at room temperature of an epitaxial VO2 film on a VO2/Al2O3 heterostructure. The resistance of the VO2 film increased when coated with cyclohexane or n-butanol, with maximum changes of 31% and 3.8%, respectively. In contrast, it decreased when coated with ethylene glycol, with a maximum change of −7.7%. In all cases, the resistance recovered to its original value after removing the organic liquid. This organic-liquid-induced reversible resistance switching suggests that VO2 films can be used as organic molecular sensors.