Vertical Interface Effect on the Physical Properties of Self-Assembled Nanocomposite Epitaxial Films

Autor: Haiyan Wang, Yongqiang Wang, Judith L. MacManus-Driscoll, D. M. Feldmann, Menka Jain, Hao Yang, Paul Dowden, Quanxi Jia, Jongsik Yoon
Rok vydání: 2009
Předmět:
Zdroj: Advanced Materials. 21:3794-3798
ISSN: 1521-4095
0935-9648
Popis: Composite 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. Metal oxides in nanocomposite form are particularly appealing as the interaction or coupling between the constituents can lead to enhanced or new functionalities. In a film-on-substrate geometry, epitaxial composite films can be created in two forms, horizontal and vertical. Nanocomposite films with a vertical architecture such as the nanopillar geometry offer numerous advantages over the conventional horizontal multilayers, such as a larger interfacial area and intrinsic heteroepitaxy in three dimensions. Moshnyaga et al. firstly showed vertical nanopillar films with a composition of (La0.7Ca0.3MnO3)1 x:(MgO)x. [3] The structural and magnetotransport properties of the La0.7Ca0.3MnO3 nanoclusters were tuned through the tensile stress originating from the MgO second phase. Zheng et al. reported vertical nanostructures consisting of magnetic spinel CoFe2O4 pillars epitaxially embedded into a ferroelectric BaTiO3 matrix. [4,8] This nanocomposite exhibited strong coupling of the ferroelectric and magnetic order parameters through the three-dimensional heteroepitaxy of the two lattices. For epitaxial metal oxide films, interfaces always play a critical role in controlling the structural and electrical properties. Both experimental and theoretical works have demonstrated the impact of lateral interface on the physical properties of either single-phase thin films or superlattices. For instance, Reyren et al. reported superconductivity in the electron gas formed at the interface between LaAlO3 and SrTiO3. [13] There have also been reports on the lateral interface effect on the physical properties of ferroelectric thin films, such as size effects, critical thickness, and strain and coupling enhanced ferroelectricity. Compared with the lateral interface, the effect of vertical interface on the physical properties of metal oxide films is profound. However, there are only a few reports because of the lack of vertical nanocomposites with ordered structures on a large scale. We have recently fabricated (BiFeO3)0.5:(Sm2O3)0.5 nanocomposite thin films, where both BiFeO3 (BFO) and Sm2O3 (SmO) phases are spontaneously self-assembled into a vertically ordered nanocolumnar structure. In this work, we demonstrate the vertical interface effect on lattice parameters, dielectric properties, and leakage current of BFO:SmO nanocomposite films. To illustrate the improved functionalities of our nanocomposites, we compare to the physical properties of pure BFO and SmO thin films. In order to reveal the microstructure of self-assembled BFO:SmO nanocomposites, plan-view and cross-sectional transmission electron microscopy (TEM) was performed. As can be seen from the plan-view TEM image (Fig. 1a), a checker-board structure is formed. In other words, an ordered alternative growth of BFO and SmO domains (marked as B and S, respectively) is self-assembled. Each of the domains has a lateral dimension of 15 nm. A low magnification bright-field cross-sectional TEM image (Fig. 1b) also shows that the BFO and SmO domains have grown alternately and vertically aligned with an average column size of 15 nm. A high resolution TEM (HRTEM) image (Fig. 1c) reveals excellent heteroepitaxial growth of the BFO and SmO nanocolumns on the STO substrate. It is interesting to note that there is a lattice matching relation along the vertical boundaries between the BFO and SmO. The matching spacing is about 1.3 nm. The corresponding fast Fourier transformed images from the areas of the BFO, SmO, and STO are shown as an insert of Figure 1c. The orientation relations of the BFO:SmO nanocomposite and the substrate are determined to be (002)BFO// (004)SmO//(002)STO and [200]BFO//[440]SmO//[200]STO, which is in accordance with the pure BFO and SmO thin films grown on STO substrates. The BFO:SmO nanocomposite thin film provides the opportunity to investigate the vertical interface effect on the physical properties of the individual BFO and SmO phases by comparison to the pure-phase films. Table 1 shows the out-of-plane lattice constants of individual BFO and SmO phases in the BFO:SmO nanocomposite. In comparison, lattice parameters of pure BFO and SmO thin films and bulk values of BFO and SmO are also included in the table. The out-of-plane lattice constant of the BFO phase in the nanocomposite is 3.905 A, compared with a value of 3.981 and 3.962 A for the pure BFO film and the bulk BFO, respectively. It C O M M U N IC A T IO N www.advmat.de
Databáze: OpenAIRE
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