Jiangtao Zhao

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.

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.

Phase coexistence and domain configuration in Pb(Mg1/3Nb2/3)O3-0.34PbTiO3 single crystal revealed by synchrotron-based X-ray diffractive three-dimensional reciprocal space mapping and piezoresponse force microscopy

The crystalline phases and domain configuration in the morphotropic phase boundary composition Pb(Mg1/3Nb2/3)O3-0.34PbTiO3 (PMN-0.34PT) single crystal have been investigated by synchrotron-based X-ray 3D Reciprocal Space Mapping (3D-RSM) and Piezoresponse Force Microscopy. The coexistence of tetragonal (T) and monoclinic MC phases in this PMN-0.34PT single crystal is confirmed. The affiliation of each diffraction spot in the 3D-RSM was identified with the assistance of qualitative simulation. Most importantly, the twinning structure between different domains in such a mixed phase PMN-PT crystal is firmly clarified, and the spatial distribution of different twin domains is demonstrated. In addition, the lattice parameters of T and MC phases in PMN-0.34PT single crystal as well as the tilting angles of crystal lattices caused by the interfacial lattice mismatch are determined.

Effect of Al addition on synthesis of the Ti3SiC2 by vacuum sintering

Abstract The effect of Al addition on the synthesis and resultant phase composition of Ti-Si-C ternary system was investigated. It was shown that Al addition decreased the synthesis temperature and shortened the soaking time of this system to promote the formation of Ti3SiC2. Resultant phases were identified by X-ray diffraction (XRD). The Ti-Al-C ternary compounds, Ti3AlC2 and Ti2AlC, were not found except in case of raw powders containing sufficient Al. Ti2AlC was obtained after sintered at 1350 °C for 120 min. However, this phase vanished when the soaking time was extended to 180 min. Further extended soaking time, i.e. 240 min yielded Ti3AlC2 at the same temperature. To detect the trace of Al in the products without Ti3AlC2 or Ti2AlC, scanning electronic microscope analysis (SEM) and plasma spectrochemical analysis were conducted. It was indicated that Ti3SiC2 might be a Al-containing solution as Ti3(Si,Al)C2.

Growth temperature-dependent metal–insulator transition of vanadium dioxide epitaxial films on perovskite strontium titanate (111) single crystals

Vanadium dioxide (VO2) epitaxial films were grown on perovskite single-crystal strontium titanate (SrTiO3) substrates by reactive radio-frequency magnetron sputtering. The growth temperature-dependent metal–insulator transition (MIT) behavior of the VO2 epitaxial films was then investigated. We found that the order of magnitude of resistance change across the MIT increased from 102 to 104 with increasing growth temperature. In contrast, the temperature of the MIT does not strongly depend on the growth temperature and is fairly stable at about 345 K. On one hand, the increasing magnitude of the MIT is attributed to the better crystallinity and thus larger grain size in the (010)-VO2/(111)-SrTiO3 epitaxial films at elevated temperature. On the other hand, the strain states do not change in the VO2 films deposited at various temperatures, resulting in stable V-V chains and V-O bonds in the VO2 epitaxial films. The accompanied orbital occupancy near the Fermi level is also constant and thus the MIT temperatur...

Facile synthesis of various epitaxial and textured polymorphs of vanadium oxide thin films on the (0006)-surface of sapphire substrates

Extensive attention has been paid to vanadium oxide polymorphs because of their potential to be used in applications in information and optoelectronic devices, as well as in energy harvesting technologies. However, vanadium oxides always form a very complex phase diagram; in particular, it is still challenging to synthesize pure vanadium oxide epitaxial polymorphs on low-cost, transparent and wafer-scale substrates. Here, we demonstrate the growth of epitaxial polymorphs of vanadium oxide (VO2 (R, M1 and their mixed phase), and V2O3) and (001)-textured VO2 (B) thin films on the (0006) surface of sapphire without selecting specific substrate orientations. This is achieved by controlling the vanadium arrival rate via sputtering power and oxidation of vanadium atoms through the partial pressure of oxygen using magnetron sputtering techniques, which enables wafer-scale production of the vanadium oxide thin films on the sapphire substrates. Growth phase diagrams of the various polymorphs are also developed for guiding device design based on the vanadium oxide thin films. This work paves a way towards practical applications of vanadium oxide thin films on chemically stable, transparent and low-cost sapphire substrates.

Effect of zirconia particle size distribution on the toughness of zirconia-containing ceramics

The zirconia particles in zirconia-containing ceramics have a size distribution similar to Gaussian distribution. The spontaneously martensitic start temperature (Ms) of different particles are not the same. The larger the particle, the higher its Ms. On cooling from high temperature to a value lower than the macro Ms of the materials, the stress-induced and spontaneous transformation will happen, so that the toughness of the materials increases at first, then gradually drops to a value slightly higher than that of the matrix. The size distribution plays an important role in affecting the toughness of the materials. When the average particle size increases, the maximum toughening (ΔKc)max and the temperature (Tmax) at which (ΔKc)max happens will all increase. The toughness at given temperature will increase at first and then drop also to a value slightly higher than that of matrix with increasing of average particle size. The stronger concentration of the size distribution, the higher (ΔKc)max will be. The weaker concentration for size distribution (either the range of zirconia particle sizes becomes wider or the scale parameter of the distribution increases), the lower (ΔKc)max, but the range of temperature in which the toughness is larger than certain value will become wider. Some suggestions of designing ceramics with high toughness at different temperatures are given.

Transformation and fracture of ZrO2-based ceramics at low-temperatures

Abstract Fracture experiments were done on 3 mol% Y-TZP ceramics from room temperature to 4.2 K. X-ray diffraction analysis was used to study the phase contents of the ceramics. The monoclinic phase content and fracture toughness, K Ic , all increase with decreasing temperature, which is mainly due to the stress-induced transformation. On the other hand, the spontaneous transformation also takes a part in toughening.

Non-thermal excitation and control of magnetization in Fe/GaAs film by ultrafast laser pulses

We present our recent study of non-thermal excitation and coherent control of spin reorientation in 10-nm epitaxially grown Fe thin films by low-energy femtosecond laser pulses. The magnetization dynamics and hysteresis curves were recorded by pump-probe differential magnetic Kerr (DMK) spectroscopy using linearly polarized laser beams. A sharp switching in DMK signal is observed when we rotated the pump polarization. This result indicates a non-thermal origin of magnetization excitation and reorientation in Fe films. We reveal that spins can interact coherently with the polarization induced by the pulsed laser field in magnetic metals. Such opto-magnetic interactions are instantaneous and are only limited in time by the properties of laser pulses. Our results suggest the feasibility of ultrafast optical control of both the magnetization and the demagnetization responses in magnetic films.

The Electric-Field Controllable Non-Volatile 35

Using in situ electric-field-modulated anisotropic magnetoresistance measurement, a large reversible and nonvolatile in-plane rotation of magnetic easy axis of ~35° between the positive and negative electrical poling states is demonstrated in Co40Fe40B20/(001)-cut Pb(Mg1/3Nb2/3)O3-0.25PbTiO3 (PMN-PT). The specific magnetoelectric coupling mechanism therein is experimentally verified to be related to the synchronous in-plane strain rotation induced by 109° ferroelastic domain switching in the (001)-cut PMN-PT substrate.