Vertical Interface Effect on the Physical Properties of Self‐Assembled Nanocomposite Epitaxial Films
Vertical Interface Effect on the Physical Properties of Self‐Assembled Nanocomposite Epitaxial Films
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DOI：
10.1002/adma.200900781
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发表时间：
2009-10
影响因子：
29.4
通讯作者：
Hao Yang;Haiyan Wang;Jongsik Yoon;Yongqiang Wang;Menka Jain;D. Feldmann;P. Dowden;J. MacManus‐Driscoll-J.
中科院分区：
文献类型：
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作者：
Hao Yang;Haiyan Wang;Jongsik Yoon;Yongqiang Wang;Menka Jain;D. Feldmann;P. Dowden;J. MacManus‐Driscoll-J.
复合金属氧化物因其在微电子学、磁电子学和光电子学中的广泛应用，在块体和薄膜中得到了广泛的研究。纳米复合形式的金属氧化物特别吸引人，因为组分之间的相互作用或耦合可以导致增强或新的功能。在衬底上薄膜的几何结构中，外延复合薄膜可以以水平和垂直两种形式产生。与传统的水平多层膜相比，具有垂直结构的纳米复合薄膜具有许多优点，如更大的界面面积和三维的本征异质外延。Moshnyaga等人。首次制备出组成为(La0.7Ca0.3MnO3)1x：(MGO)x的垂直纳米晶薄膜。[3]La0.7Ca0.3MnO3纳米团簇的结构和磁输运性质是通过第二相产生的张应力来调节的。郑等人。报道了由磁性尖晶石CoFe2O4柱子外延嵌入到铁电BaTiO3基质中的垂直纳米结构。[4，8]这种纳米复合材料通过两个晶格的三维异质外延，表现出铁电和磁序参数的强烈耦合。对于外延金属氧化物薄膜，界面始终是控制其结构和电学性质的关键因素。实验和理论工作都证明了横向界面对单相薄膜和超晶格的物理性质的影响。例如，雷伦等人。报道了在LaAlO_3和SrTiO_3界面形成的电子气中的超导电性。[13]也有关于横向界面对铁电薄膜物理性质的影响的报道，例如尺寸效应、临界厚度以及应变和耦合增强的铁电性。与横向界面相比，垂直界面对金属氧化物薄膜物理性能的影响是深远的。然而，由于缺乏规模化的具有有序结构的垂直纳米复合材料，目前的报道还很少。我们最近制备了(BiFeO)0.5：(Sm_2O_3)0.5纳米复合薄膜，其中BiFeO_3(BFO)和Sm_2O_3(SMO)相自发地自组装成垂直有序的纳米柱结构。在这项工作中，我们展示了垂直界面对BFO：SMO纳米复合薄膜的晶格参数、介电性能和漏电流的影响。为了说明我们的纳米复合材料的改进功能，我们比较了纯BFO和SMO薄膜的物理性能。为了揭示自组装BFO：SMO纳米复合材料的微观结构，对其进行了平面图和横截面的透射电子显微镜观察。从平面图(图1a)可以看出，形成了一个棋盘结构。换句话说，BFO和SMO结构域(分别标记为B和S)的有序交替增长是自组装的。每个结构域的横向尺寸为15 nm。低放大倍明磁场横截面图像(图1B)也显示BFO和SMO结构域交替生长并垂直排列，平均柱尺寸为15 nm。高分辨率电子显微镜(HRTEM)图像(图1C)显示了BFO和SMO纳米柱在STO衬底上的良好异质外延生长。有趣的是，在BFO和SMO之间的垂直边界上存在晶格匹配关系。匹配间距约为1.3 nm。来自BFO、SMO和STO区域的相应快速傅立叶变换图像如图1C的插图所示。BFO：SMO纳米复合材料与衬底的取向关系为(002)BFO//(004)SMO//(002)STO和[200]BFO//[440]SMO//[200]STO，与在STO衬底上生长的纯BFO和SMO薄膜的取向关系一致。与纯相薄膜相比，BFO：SMO纳米复合薄膜提供了研究垂直界面对BFO和SMO单相物理性质影响的机会。表1显示了BFO：SMO纳米复合材料中各个BFO和SMO相的面外晶格常数。表中还包括了纯BFO和SMO薄膜的晶格参数以及BFO和SMO的体积值。纳米复合材料中BFO相的离面晶格常数为3.905 A，而纯BFO薄膜和块体BFO的晶格常数分别为3.981和3.962 A。IT C O M M U N IC A T IO N www.Advmat.de
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