Weiwei Li

Oxygen-Vacancy-Induced Antiferromagnetism to Ferromagnetism Transformation in Eu0.5Ba0.5TiO3−δ Multiferroic Thin Films

Oxygen vacancies (VO) effects on magnetic ordering in Eu0.5Ba0.5TiO3−δ (EBTO3−δ) thin films have been investigated using a combination of experimental measurements and first-principles density-functional calculations. Two kinds of EBTO3−δ thin films with different oxygen deficiency have been fabricated. A nuclear resonance backscattering spectrometry technique has been used to quantitatively measure contents of the VO. Eu0.5Ba0.5TiO3 ceramics have been known to exhibit ferroelectric (FE) and G-type antiferromagnetic (AFM) properties. While, a ferromagnetic (FM) behavior with a Curie temperature of 1.85u2005K has been found in the EBTO3−δ thin films. Spin-polarized Ti3+ ions, which originated from the VO, has been proven to mediate a FM coupling between the local Eu 4f spins and were believed to be responsible for the great change of the magnetic ordering. Considering the easy formation of VO, our work opens up a new avenue for achieving co-existence of FM and FE orders in oxide materials.

Vertical-Interface-Manipulated Conduction Behavior in Nanocomposite Oxide Thin Films

Vertically aligned nanocomposites with vertical interfaces are a novel concept that show powerful advantages over conventional nanocomposites with lateral interfaces. However, significant obstacles to a systematic understanding of vertical interfaces still remain. Here, heteroepitaxial (BaTiO3)0.5:(Sm2O3)0.5 nanocomposite thin films have been fabricated and the conduction behaviors have been investigated. A spontaneous phase ordering with clear vertical interfaces has been found in the composite films. Because of the structural discontinuity as well as a large strain generated at the interfaces, the vertical interfaces are revealed to become the sinks to attract oxygen vacancies. The accumulated oxygen vacancies contributed to a largely reduced leakage current and a different leakage mechanism in the composite films compared to that of the pure BaTiO3 film. The present work represents a methodology to manipulate functionalities by designing configuration of the interfaces in oxide thin films.

Electronic Structure and Band Alignment at the NiO and SrTiO3 p-n Heterojunctions

Understanding the energetics at the interface, including the alignment of valence and conduction bands, built-in potentials, and ionic and electronic reconstructions, is an important challenge in designing oxide interfaces that have controllable multifunctionalities for novel (opto-)electronic devices. In this work, we report detailed investigations on the heterointerface of wide-band-gap p-type NiO and n-type SrTiO3 (STO). We show that despite a large lattice mismatch (∼7%) and dissimilar crystal structure, high-quality NiO and Li-doped NiO (LNO) thin films can be epitaxially grown on STO(001) substrates through a domain-matching epitaxy mechanism. X-ray photoelectron spectroscopy studies indicate that NiO/STO heterojunctions form a type II staggered band alignment. In addition, a large built-in potential of up to 0.97 eV was observed at the interface of LNO and Nb-doped STO (NbSTO). The LNO/NbSTO p-n heterojunctions exhibit not only a large rectification ratio of 2 × 103 but also a large ideality factor of 4.3. The NiO/STO p-n heterojunctions have important implications for applications in photocatalysis and photodetectors as the interface provides favorable energetics for facile separation and transport of photogenerated electrons and holes.

Vertical Interface Induced Dielectric Relaxation in Nanocomposite (BaTiO3)1-x:(Sm2O3)x Thin Films

Vertical interfaces in vertically aligned nanocomposite thin films have been approved to be an effective method to manipulate functionalities. However, several challenges with regard to the understanding on the physical process underlying the manipulation still remain. In this work, because of the ordered interfaces and large interfacial area, heteroepitaxial (BaTiO3)1-x:(Sm2O3)x thin films have been fabricated and used as a model system to investigate the relationship between vertical interfaces and dielectric properties. Due to a relatively large strain generated at the interfaces, vertical interfaces between BaTiO3 and Sm2O3 are revealed to become the sinks to attract oxygen vacancies. The movement of oxygen vacancies is confined at the interfaces and hampered by the misfit dislocations, which contributed to a relaxation behavior in (BaTiO3)1-x:(Sm2O3)x thin films. This work represents an approach to further understand that how interfaces influence on dielectric properties in oxide thin films.

Conduction mechanisms of epitaxial EuTiO3 thin films

To investigate leakage current density versus electric field characteristics, epitaxial EuTiO3 thin films were deposited on (001) SrTiO3 substrates by pulsed laser deposition and were post-annealed in a reducing atmosphere. This investigation found that conduction mechanisms are strongly related to temperature and voltage polarity. It was determined that from 50 to 150u2009K, the dominant conduction mechanism was a space-charge-limited current under both negative and positive biases. From 200 to 300u2009K, the conduction mechanism shows Schottky emission and Fowler-Nordheim tunneling behaviors for the negative and positive biases, respectively. This work demonstrates that Eu3+ is one source of leakage current in EuTiO3 thin films.

Manipulating leakage behavior via distribution of interfaces in oxide thin films

Vertical interfaces have been approved to be able to effectively manipulate conduction behavior in oxide thin films. However, a systematic investigation is needed to understand the physical process underlying the interface effects. Here, epitaxial (BiFeO3)0.5:(Sm2O3)0.5 thin films have been fabricated and used as a model system. The microstructure and leakage behavior of (BiFeO3)0.5:(Sm2O3)0.5 composite films on (001) and (011) SrTiO3 substrates have been investigated. The different substrate orientation originated to different geometrical configurations of interfaces in the composite films. The leakage current has been significantly reduced when the complicated distribution of interfaces leads to a longer distance of conduction path. The present work represented an approach to reduce the leakage current and to further understand the way that how the interface contributes to the conduction behavior in oxide thin films.

Manipulating multiple order parameters via oxygen vacancies: The case of Eu0.5Ba0.5TiO3−δ

Controlling functionalities, such as magnetism or ferroelectricity, by means of oxygen vacancies (V-O) is a key issue for the future development of transition-metal oxides. Progress in this field is currently addressed through V-O variations and their impact on mainly one order parameter. Here we reveal a mechanism for tuning bothmagnetism and ferroelectricity simultaneously by using V-O. Combining experimental and density-functional theory studies of Eu0.5Ba0.5TiO3-delta , we demonstrate that oxygen vacancies create Ti3+ 3d(1) defect states, mediating the ferromagnetic coupling between the localized Eu 4f(7) spins, and increase an off-center displacement of Ti ions, enhancing the ferroelectric Curie temperature. The dual function of Ti sites also promises a magnetoelectric coupling in the Eu0.5Ba0.5TiO3-delta.

Manipulating redox reaction during pulsed laser deposition

Copper and copper oxide thin films have been synthesized on (001) SrTiO3 substrates using pulsed laser deposition. Three kinds of thin films can be achieved by changing deposition temperature and laser energy density: pure Cu, Cu:Cu2O composite, and pure Cu2O thin films. A phase diagram about the compositions of thin films has been represented and explained by redox reaction at the substrate surface during the deposition process. The present work represents a novel approach to further understand the surface reaction during deposition process and to fabricate oxide thin films with variable valence states using pulsed laser deposition.

Self-assembled Nanocomposite Oxide Films: Design, Fabrication, and Properties

Nanocomposite metal oxides have been extensively investigated in bulk and thin film forms because of their wide range of applications in microelectronics, magneto electronics, and optoelectronics (Ramesh and Spaldin, 2007). Oxide films in nanocomposite form are particularly appealing as the interaction or coupling between the constituents can lead to enhanced or new functionalities (Zheng et al., 2006). Furthermore, the nanocomposite oxide films could lead to their applications as building blocks for the assembly of nanostructures or molecular monolayers (Cheng et al., 2006). A currently primary goal is to fabricate nanocomposite oxide films using a simple and low-cost process. Self-assembly has been the focus of much research in the last four decades. It has proved its ability to offer a rich variety of periodic nanoscale patterns in an easy way (Guiton and Davies, 2007). Recent efforts have striven to bring these two fields together. A major challenge is to control a self-assembled nanocomposite oxide film to create a desired nanostructure (Moshnyaga et al., 2003). In a film-on-substrate geometry, epitaxial composite films can be divided into two forms: horizontal and vertical. These two structures can be reviewed as the building blocks for a desired nanostructure and they are also specific architectures which show lots of potential applications (Yang et al., 2009). This chapter outlines the recent results about the material design, fabrication, and physical properties of these two nanostructures.

Oxygen-vacancy-mediated dielectric property in perovskite Eu0.5Ba0.5TiO3-δ epitaxial thin films

Dielectric relaxation in ABO3 perovskite oxides can result from many different charge carrier-related phenomena. Despite a strong understanding of dielectric relaxations, a detailed investigation of the relationship between the content of oxygen vacancies (VO) and dielectric relaxation has not been performed in perovskite oxide films. In this work, we report a systematic investigation of the influence of the VO concentration on the dielectric relaxation of Eu0.5Ba0.5TiO3-δ epitaxial thin films. Nuclear resonance backscattering spectrometry was used to directly measure the oxygen concentration in Eu0.5Ba0.5TiO3-δ films. We found that dipolar defects created by VO interact with the off-centered Ti ions, which results in the dielectric relaxation in Eu0.5Ba0.5TiO3-δ films. Activation energy gradually increases with the increasing content of VO. The present work significantly extends our understanding of relaxation properties in oxide films.